[0001] The present invention relates to an image reproducing method and, more particularly,
to a multiplex image reproducing method of superposing toner images on an image retainer
having a photoconductive photosensitive member by repeatedly retaining and developing
electrostatic images on the image retainer.
[0002] As the above-specified image reproducing method, there are known in the art the methods
which are disclosed in Japanese Patent Laid-Open Nos. 144452/81, 116553/83 and 116554/83.
[0003] In any of these methods, the image retainer has on its surface a layer of a photoconductive
photosensitive material such as Se, and the development resorts to a reversal method
in which there is applied to an electrostatic image having a lower potential than
that of the background a toner for frictionally charging it with the same polarity.
According to this method, there arises a problem that the photoconductive photosensitive
surface layer is generally and relatively liable to have its electrostatic image retaining
performance changed by the charging step and to be subjected to the "toner filming"
or have its photosensitivity degraded. As compared with such a positive developing
method as in the ordinary electrophotographic reproducing machine, i.e., the developing
method in which the electrostatic image has a higher potential than the background
so that a toner charged with an opposite polarity is applied to that electrostatic
image, moreover, the development by the reversal developing method has a problem that
control of the toner application is so difficult that a sufficient development density
cannot be attained or that a reproducing apparatus is liable to have its inside blotted
by the toner scattered.
[0004] As the method in which an image retainer having a dielectric surface layer is used
to retain an electrostatic image on the dielectric surface layer, on the other hand,
there is known in the art a method using an electrostatic recording head, a method
using a screen photosensitive member (as is disclosed in Japanese Patent Publication
No. 34616/79) or a method using a screen control electrode (as in disclosed in Japanese
Patent Laid-Open No. 137363/81). The electro static image retaining methods thus disclosed
are superior in that the electrostatic image retainability and the toner image retainability
required of the photosensitive member are separated of each other. In these methods,
more specifically, it is deemed that the share of retaining the electrostatic image
is borne by the electrostatic recording head, the screen photosensitive member or
the screen control electrode, whereas the share of retaining the toner image is borne
by the dielectric surface layer. Those methods are featured by that color data are
retained consecutively and independently as the electrostatic image on the dielectric
surface layer. However, the toner image formed on the dielectric surface layer cannot
be other than a monochromatic one at all times.
[0005] This is because the developing method is conducted by the contact development so
that a previous toner image is disturbed or color mixing occurs upon the development
even if another electrostatic image could be recorded on the previously formed toner
image.
[0006] As the method in which an image retainer having a magnetic surface layer is used
to form a magnetic image on the magnetic layer, on the other hand, there is known
in the art methods which are disclosed in Japanese Patent Laid-Open Nos. 90342/75,
100732/76 and 106253/81. These reproducing methods resorting to that magnetic image
is excellent in that the retainability of AN electrostatic image by the corona discharge
or the like and the retainability of a toner image required of the photosensitive
member can be separated from each other. In the reproducing methods using the magnetic
image, more specifically, it is deemed that retention of the magnetic image makes
whole use of the inside of the magnetic layer while not having its surface state changed
as is different from the photosensitive member, and that the share of the toner image
retainability is borne by the surface of the magnetic layer. Those methods are featured
by that the color data can be newly retained as the magnetic image independently of
the toner image formed on the magnetic surface layer. However, the toner image retained
on the magnetic layer cannot be other than a monochromatic one at all times.
[0007] The method disclosed in Japanese Patent Laid-Open No. 144452
/81 retains a color image on an image retainer: by forming an electrostatic image on
the surface of an image retainer, which has been charged by a charger, by first exposure
means and developing it by first developing means; by forming an electrostatic image
on the same charged surface by second exposure means and developing it by second developing
means; and by forming an electrostatic image on the same charged surface by third
exposure means and developing it by third developing means. The method thus specified
has problems that the separate exposure means are required for the respective ones
of the repeated formations of the electrostatic images to enlarge the size of the
reproducing apparatus and to raise the cost of the same and that synchronizations
of the exposure of the respective exposure means with the image retainer have relationships
with the respective positions of the exposure means so that the synchronous control
is troublesome to make it liable to invite color shift. Moreover, each of the development
of that method is conducted by the forced method in which the electrostatic image
having a lower potential at its exposed portion than that of the background has such
toner applied thereto as is charged with the same polarity. In that forced developing
method, the toner for effecting the charge at the same polarity as that of the charge
of the image retainer is so used in the developer that it may not be applied to the
background. As a result, the reversal developing method has a problem that although
the toner is repulsed by the background potential so that it is reluctant to invite
any fog, it is also reluctant to be applied to the electrostatic images so that a
sufficient developed density can hardly be obtained.
[0008] Since the reversed image is obtained, according to this reversal developing method,
color reproduction of a positive image cannot be effected so that the coloring is
limited to the technique using the dot exposure of a printer or the like. In case
it is intended to obtain a positive image when an ordinary original is to be reproduced,
it is reversed, and the counter-measure for this reversal is difficult. Since the
potential at the photosensitive layer of the exposed portion is at the same polarity
as that of the developer although it is low, moreover, the reversal developing method
has a problem that the developer is reluctant to be applied to the electrostatic images
so that it is liable to be scattered to blot the inside of the reproducing apparatus.
[0009] On the other hand, the disclosed in Japanese Patent Laid-Open No. 144452/81 is one
conducted under a non-contact jumping developing condition in which the second and
later developments by the reversal developing method are conducted such that the layers
of the developers formed by the developing means are not in contact with the surface
of the image retainer. This method has problems that the development is reluctant
to have a sufficient density and is liable to be blotted with the toner scattered
unless a strong bias voltage is applied to the developing means to strongly apply
the toner to the electrostatic images, and that, the strong bias voltage is applied
to the developing means, it is liable to leak to the image retainer or the like, or
toner of another color is liable to stick to the toner image developed before or the
background.
[0010] The methods disclosed in Japanese Patent Laid-Open Nos. 116553/83 and 116554/83 are
substantially the same as that in Japanese Patent Laid-Open No. 144452/81 in that
the formations and developments of the electrostatic images are conducted by different
means for the respective repetitions. As a result, those methods also have problems
that the reproducing apparatus has its size enlarged to raise the cost, and that the
synchronous control of the exposures of the respectie exposing means is so difficult
as to invite the color shift. Here, the method disclosed in Japanese Patent Laid-Open
No. 116554/83 is different from the method disclosed in Japanese Patent Laid-Open
No. 144452 in that the respective developments by the reversal developing method are
conducted under the contact developing condition, in which the developer layers formed
by the developing means brush the surface of the image retainer, thereby to solve
the problems of the reversal developing method that the sufficient developing density
can hardly be obtained and that the toner is liable to be scattered. Moreover, the
method disclosed in Japanese Patent Laid-Open No. 116553/83 is different from the
same Japanese Patent Laid-Open No. 144452/81 in that, in the second and later retentions
of the electrostatic images, too, the surface of the image retainer is recharged before
exposure by the chargers, which are placed in front of the respective exposing means,
so that toner in another color may not be applied during a later development to the
portions having the toner adhered thereto after the previous development. Since the
second and later developments are conducted under the contact developing condition,
however, those methods have a serious problem that the toner adhered after the previous
development is liable to be shifted during the subsequent development or to be mixed
into the developer of the subsequent developing means.
[0011] A prototype in which an electrostatic latent image is expressed in a multi-color
image is concerned with a color image using an electophotog- raphic system. This system
of the prior art separates the colors of an original through an optical filter and
repeats the charging, exposing, developing and transferring steps by using the separated
colors. In order that respective images of- color particles such as yellow, magenta,
cyan and black colors may be retained, more specifically, those steps are repeated
four times by that system. There also exists the so-called "dichromatic developing
method", in which electrostatic latent images of different polarities are formed on
a common photosensitive member (or an image carrier) and are developed by particles
of black and red colors. These multi-color image retaining methods are desirable,
because they can add color data as compared with the data obtained from the dichromatic
images, but have the following problems:
(1) Transfer to a transfer member is required at each development of each color to
enlarge the size of the machine and to elongate the time period necessary for the
image retention; and
(2) It is necessary to ensure the accuracy of positional shifts resulting from the
repetitions.
[0012] In view of these problems, there has been conducted a trial in which a plurality
of toner images are developed in a superposed manner on a common photosensitive member
so that the transfer step may be finished by one time to reduce the size of the machine.
[0013] As the developer to be used in this machine, on the other hand, there exists a two-component
developer, which is composed of a toner and a carrier, and a one-component developer
which is composed only of a toner. The one-component developer has some problems in
the charge control of the toner but has advantages that no consideration is necessary
into the concentration and agitation of the toner and the carrier, and that the size
of the machine can be reduced.
[0014] The two-component developer requires control of the ratio of the toner to the carrier
but has an advantage that it is easy to control the frictional charges of the toner
particles. Since a magnetic material of black color need not be much contained in
the toner particles, on the other hand, the two-component developer composed of a
magnetic carrier and a non-magnetic toner can use a color toner having no color turbidity
by the magnetic material so that a clear color image can be formed.
[0015] In the multiplex development, incidentally, it is sufficient to repeat several times
the developments of the photosensitive member which has already been formed with the
toner image. However, the multiplex development has problems that the toner image
retained at a previous step on the photosensitive member is disturbed upon development
of a subsequent step, and that the toner having already been applied to the photosensitive
member is returned to a developing sleeve acting as a developer carrier until it steals
into the developing means at a subsequent step, in which a developer in a color different
from that of the developer of the previous step, thereby to cause color mixing. In
order to obviate those problems, there is disclosed in Japanese Patent Laid-Open No.
144452/81, for example, means for superposing an a.c. component upon a developing
bias while the photosensitive member being out of contact with the developer layer
on the developing sleeve acting as the developer carrier for developing an electrostatic
latent image, except the developing means for first forming the toner image on the
photosensitive member. However, there arises a problem that the image can neither
have a sufficient density nor be freed from the disturbance or color mixing.
[0016] The present invention has been conceived so as to solve the above problems which
are concomitant with the image reproducing method of the prior art. A first object
of the present invention is to a multiplex image reproducing method which is enabled
to reduce the size of and the cost for a recording apparatus and to make easy and
accurate the synchronous control of image exposures by using a common apparatus for
repeatedly retaining electrostatic images. A second object of the present invention
is to provide a multiplex image reproducing method which can facilitate control of
the adhesion of toners to electrostatic images so that a sufficient developing density
can be attained under a non-contact jumping developing condition to prevent any fog
and the mixture and application of the toners in different colors not only in the
case by the ordinary developing method for applicating the toners charged with an
opposite polarity to the electrostatic images but also in the case by the forced developing
method.
[0017] The present invention resides in a multiplex image reproducing method of the type,
in which toner images are superposed on an image retainer having a photoconductive
photosensitive surface layer by repeating the retentions and developments of electrostatic
images on the image retainer, characterized in that the retentions of said electrostatic
images are repeated by means of a common apparatus. The above-identified first object
is achieved by the above-specified construction, and the above-identified second object
is achieved by using in the developing means the two-component developer which contains
a mixture of a toner and an insulating carrier.
[0018] Another object of the present invention is to provide a novel multiplex image reproducing
method in which the retentions of electrostatic images are stabilized by using means
for retaining the electrostatic images on a dielectric layer and in which a method
of superposing developed images on the dielectric layer is devised.
[0019] The above-specified object of the present invention can be achieved by a multiplex
image reproducing method of the type, in which a plurality of color toner images are
superposed on an image retainer by repeating the retentions and developments of the
electrostatic images on the image retainer, characterized in that the retentions of
said electrostatic images are conducted on the dielectric surface layer.
[0020] Still another object of the present invention is to provide a multiplex image reproducing
method of the type, in which a number of magnetic recording heads are juxtaposed to
an image retainer formed of a magnetic layer and an insulating layer, if necessary,
so that a number of magnetized regions may be formed on the magnetic layer of the
image retainer thereby to write an image by sending an image signal current synchronized
with the movement of the image retainer to the respective ones of said magnetic recording
heads, while the image retainer being moved at a constant speed, whereby a toner image
is obtained by applying a magnetic toner to the written image to develop it, characterized
in that a plurality of toner images are formed in a superposed manner on the image
retainer by repeating the image writing and developing operations. The developments
are conducted under the condition in which the developer layers formed by the developing
means are out of direct contact with the surface of the image retainer. Thus, there
is no fear that the toner images once formed are damaged upon the subsequent development
so that the plural toner images can be superposed.
[0021] Moreover, the color images can be recorded by using the plural toner images in combination
with toners of different colors such as yellow, cyan, magenta and black colors.
[0022] A further object of the present invention is to provide a multiplex image reproducing
method which can not only obtain positive images and negative images but also resort
to the developing method, in which a developer charged with a polarity opposite to
that of an electrostatic image is applied to the electrostatic image by the coulomb
force, so that the application of the developer to the electrostatic image can be
made sufficient while reducing scattering of the developer.
[0023] The above-specified object of the present invention can be achieved by a multiplex
image reproducing method of the type, in which toner images are superposed on an image
retainer by repeating the retentions and developments of electrostatic images upon
the image retainer, characterized in that said electrostatic images are formed either
by conducting image exposures after an image retainer formed with a transparent insulating
surface layer on a photoconductive photosensitive layer has been charged primarily
and secondarily or by uniformly exposing that image retainer after the image retainer
has been subjected to an image exposure simultaneously with the secondary charge.
[0024] More specifically, the multiplex image reproducing method of the present invention
is different especially in the retentions of the electrostatic images from those methods
which are disclosed in Japanese Patent Laid-Open Nos. 144452/81, 116553/83 and 116554/83.
In other words, the method of the present invention is characterized in that the image
retainer is constructed to have the transparent insulating surface layer formed on
the photoconductive photosensitive layer, and in that the electrostatic images are
formed either by primarily and secondarily charging the image retainer and by subjecting
the charged surface of the same to the image exposure or by conducting the image exposure
simultaneously with the secondary charging treatment and by subsequently effecting
the uniform exposure. The above problems invited in the multiplex image reproducing
method by the reversal developing method of the prior art can be eliminated by using
the above-specified method of retaining the electrostatic images.
[0025] A further object of the present invention is to provide a multiplex image reproducing
method which can densely and finely reproduce a color image such as a landscape image,
construct a reproducing apparatus in a small size and at a low cost, and facilitate
the synchronous control of image exposures.
[0026] The above-specified object of the present invention can be achieved by a multiplex
image reproducing method of the type, in which a plurality of color toner images are
superposed on an image retainer by charging the surface of the image retainer, by
conducting the image exposures at least repeatedly, and by conducting a development
each time of the image exposure by developing means, characterized: in that said image
exposures are conducted such that the spot position of a previous image exposure and
the spot position of a subsequent image exposure are superposed as the spot distribution
exposure; and in that the image exposures thus repeated are performed by means of
a common apparatus.
[0027] A further object of the present invention is to provide a multiplex image reproducing
method of reproducing images having a desirable density but neither disturbance nor
color mixing by the use of a developer containing a plurality of components.
[0028] The above-specified object of the present invention can be achieved by a multiplex
image reproducing method of the type, in which an image is retained on an image carrier
by repeating a plurality of times both the step of forming a latent image on said
image carrier and the step of developing said latent image by the use of a developer
having a plurality components, characterized in that the following relationships are
satisfied:
0.2 5 VAC / (d•f); and
{(V Acid) - 1500}/f ≦ 1.0,
wherein, at each developing step: the amplitude of the a.c. component of a developing
bias is designated by V
AC (V); the frequency of the same by (Hz); and the gap between said image carrier and
a developer carrier for carrying said developer by d (mm).
[0029] More specifically, we, the Inventors, have researched the method of retaining an
image by conducting the development of the same while the a.c. component being superposed
on the developing bias, and have discovered that there is a region in which an image
of high quality can be obtained without incurring any disturbance and color mixing
of the image in accordance with the manner of selecting the developing conditions
such as the a.c. bias or the frequency.
[0030] The present invention contemplates to provide a novel method which is based upon
the above-specified discovery.
[0031] Moreover, the above-specified object of the present invention can be achieved by
the multiplex image reproducing method of the type, in which an image is retained
on an image carrier by repeating a plurality of times both the step of forming a latent
image on said image carrier and the step of developing said latent image, characterized
in that the following relationship is satisfied:
0.2≤ VAct(def);5 1.6,
wherein, at each developing step: the amplitude of the a.c. component of a developing
bias is designated by VA
C (V); the frequency of the same by f (Hz); and the gap between said image carrier
and a developer carrier for carrying said developer by d (mm).
Fig. 1 is a schematic view showing the construction of one embodiment of the reproducing
apparatus for practising the method of the present invention:
Fig. 2 is a schematic view showing the construction of a laser beam scanner for image
exposure;
Fig. 3 is a partially sectional view showing one example of developing means;
Figs. 4 to 7 are flow charts for practising the methods of the present invention,
respectively;
Fig. 8 is a partially sectional view showing the construction of an image retainer
in another reproducing apparatus for practising the method of the present invention;
Fig. 9 is a schematic view showing the construction of the reproducing apparatus;
Fig. 10 is a diagram schematically showing changes in the charged states of one example
of the process of electrostatic images;
Fig. 11 is a chart showing changes in the potential at the surface portion of the
image retainer in a manner to correspond to Fig. 10;
Figs. 12 to 17 are flow charts of practising the method of the present invention,
respectively;
Figs. 18 to 21 are flow charts showing another embodiment of the method of the present
invention to be practised by the reproducing apparatus of Fig. 1, respectively;
Figs. 22 to 25 are flow charts showing flow charts of an embodiment of the method
of the present invention to be practised by the recording apparatus of Fig. 9, respectively;
Figs. 26 and 27 are schematic views showing an example of the recording apparatus
to be used for practising another embodiment of the method of the present invention,
respectively; and
Figs. 28 to 30 are flow charts for practising the method of the present invention,
respectively.
[0032] In Figs. 31 to 38 showing a further embodiment of the present invention:
Fig. 31 is a sectional view showing developing means and a photosensitive drum;
Figs. 32 and 33 are diagrams showing changes in the image density when an a.c. current
is changed;
Fig. 34 is a diagram showing the density charac teristics when a field intensity and
a frequency are changed;
Figs. 35 and 37 are schematic views showing the essential portions of the multiplex
image reproducing apparatus which ,are equipped with a plurality of developing means;
Fig. 36 is a chart showing changes in the surface potential of the photosensitive
drum which is used in the multiplex image reproducing apparatus of Fig. 35; and
Fig. 38 is a chart showing changes in the surface potential of the photosensitive
drum which is used in the multiplex image reproducing apparatus of Fig. 37.
Figs. 39 and 40 are diagrams showing changes in an image density when an a.c. voltage
applied to the developing means is changed in a further embodiment of the present
invention;
Fig. 41 is a diagram showing the density characteristics when a field intensity and
a frequency are changed;
Figs. 42 and 43 are schematic views showing other examples of the recording means
which are used for practising the method of the present invention, respectively; and
Figs. 44 to 46 are flow charts for practising the method of the present invention,
respectively.
[0033] The present invention will be described in detail in the following in connection
with the embodiments thereof with reference to the accompanying drawings.
[0034] Fig. 1 is a schematic view showing the construction of one example of recording apparatus
for practising the method of the present invention; Fig. 2 is a schematic view showing
a laser beam scanner for image exposure; Fig. 3 is a partially sectional view showing
one example of developing means; and Figs. 4 to 7 are flow charts for practising the
method of the present invention, respectively.
[0035] In the recording apparatus of Fig. 1: reference numeral 1 is a drum-shaped image
retainer which is formed with a photoconductive photosensitive material such as Se
and which is made rotatable in the direction of arrow; numeral 2 is a charger for
uniformly charging the surface of the image retainer 1; numeral 3 is an exposing lamp
for uniformly exposing to a weak optical ray the surface of the image retainer which
is used in the example of the flow chart of Fig. 7; numeral 4 is an image exposing
ray of color images of different colors; numerals 5 to 8 are developing means using
as their developers toners of different colors such as yellow, magenta, cyan or black;
numerals 9 and 10 are a pre-transfer charger and a pre-transfer exposing lamp which
are provided, if necessary, respectively, so that a color image retained on the image
retainer 1 with its plural color toner images being superposed may be easily transferred
to a recording member P; numeral 11 is transfer means; numeral 12 is fixing means
for fixing the toner images transferred to the recording member P; numeral 13 is charge
eliminating means which is composed of a charge eliminator and/or a charge eliminating
corona discharger; and numeral 14 is cleaning means having a cleaning blade or a fur
brush which is adapted to come into contact with the surface of the image retainer
1 after transfer of the color images for eliminating the residual toners left on the
surface and to leave the surface of the iamge retainer 1 by the time the surface having
been subjected to a first development arrives.
[0036] Here, it is preferable to use as the charger 2 a corona discharger, as shown, which
can apply such a stable charge as is hardly affected by a previous charge, especially
in case the surface of the image retainer having already been charged is to be additionally
charged. In case the drum-shaped image retainer 1 is used as in that reproducing apparatus,
moreover, the image exposing ray 4 may be such an optical ray as has been prepared
by filtering a slit ray separately for colors, for example, the optical ray of an
ordinary monochromatic electrophotographic reproducing machine. In order to reproduce
a clear color image, however, an optical ray prepared by the laser beam scanner, as
shown in Fig. 2, is preferable.
[0037] The laser beam scanner of Fig. 2 is formed into the image exposing ray 4 for scanning
the surface of the image retainer 4 at a constant speed by turning on or off the laser
beam, which has emanated from a laser 21 such as a He-Ne laser, by means of an acoustic-optical
modulator 22 to deflect by means of a mirror scanner 23 composed of a rotating polygonal
or optagonal mirror thereby to guide it through a focusing f-0 lens 24. Incidentally,
reference numerals 25 and 26 indicate mirrors, and numeral 27 indicates a lens for
enlarging the diameter of a beam incident upon the focusing f-e lens 24 so as to reduce
the diameter of the beam on the image retainer 1. If such a laser beam scanner as
is shown in Fig. 2 is used for forming the image exposing ray 4, the electrostatic
images can be easily retained with a shift for different colors, as will be described
hereinafter, so that a clear color image can be reproduced. Despite of this fact,
the image exposing ray 4 is not limited to the slit exposing ray or a dot exposing
ray by the laser beam but may be one which is prepared by using an LED, a CRT, a liquid
crystal or an optical fiber transmitter, for example. In the reproducing apparatus
in which the image retainer can take a planar state such as a belt shape, moreover,
the image exposing ray may be a flash light.
[0038] As the developing means 5 to 8, on the other hand, there can be preferably used those
which have such a construction as is shown in Fig. 3.
[0039] In Fig. 3: reference numeral 31 indicates a developing sleeve which is made of a
non-magnetic material such as aluminum or stainless steel; numeral 32 is a magnet
which is equipped in the circumferential direction with a plurality of magnetic poles
disposed inside of the developing sleeve 31; numeral 33 is a layer thickness regulating
blade for regulating the thickness of a developer layer formed on the developing sleeve
31; numeral 34 is a scraper blade for scraping the developer layer after development
from the surface of the developing sleeve 31; numeral 35 is an agitating rotor for
agitating the developer in a developer reservoir 36; numeral 37 is a toner hopper;
numeral 38 is a toner supply roller which is formed in its surface with a recess for
receiving the toner to supply the toner from the toner hopper 37 to the developer
reservoir 36; and numeral 39 is a power supply for applying a bias voltage containing
a vibratory voltage component, as the case may be, to the developing sleeve 31 through
a protecting resistor 40 to generate an electric field for controlling the movements
of the toner between the developing sleeve 31 and the image retainer 1. Fig. 3 shows
that the developing sleeve 31 and the magnet 32 are rotatable in the directions of
arrows, respectively. It is, however, sufficient that the developing sleeve 31 and
the magnet 32 be fixed, or that the developing sleeve 31 and the magnet 32 be rotatable
in a common direction. In case the magnet 32 is fixed, it is customary to strengthen
the magnetization or to dispose two magnetic poles of identical or different polarities
adjacent to each other so that the density of the magnetic flux of the magnetic pole
facing the image retainer 1 may be stronger than that of another magnetic pole.
[0040] In these developing means, the magnetic poles of the magnet 32 are usually magnetized
to a density of magnetic flux of 500 to 5,000 gausses to attract the developer in
the developer reservoir 36 to the surface of the developing sleeve 31 by that magnetic
force so that the attracted developer is formed, while having its thickness regulated
by the layer thickness regulating blade 33, into a developer layer. This developer
layer is moved in the same direction as or in the opposite direction (although Fig.
3 shows the same direction) to the rotating direction of the image retainer 1, as
indicated by the arrow, to develop the electrostatic image of the image retainer 1
in the developing region, in which the surface of the developing sleeve 31 faces the
surface of the image retainer 1, whereas the residual is scraped away from the surface
of the developing sleeve 31 by the scraper blade 34, until it is returned to the developer
reservoir 36. Moreover, the development, e.g., at least the second or subsequent developments,
which are repeated for superposing the color toner images, is preferred to be conducted
under the non-contact jumping developing condition so that the toner caught by the
image retainer 1 during the previous development may not be shifted by the later development.
Fig. 3 shows the state in which the development is executed under the non-contact
jumping developing condition.
[0041] Moreover, it is preferable to use in the developing means 5 to 8 the so-called "two-component
developer" which is composed of a non-magnetic toner and a magnetic carrier and which
is enabled to obtain a toner image of clear color without any necessity for containing
a black or brown magnetic material in the toner and to easily effect the control of
charging the toner. Specifically, the magnetic carrier may preferably be an insulating
carrier which has a resistivity of 10
8fl cm or more or, preferably, 10
13 or more and which is prepared either by dispersing and containing fine particles
of a ferromagnetic or paramagnetic material such as tri-ion tetroxide -y-ferric oxide,
chromium dioxide, manganese oxide, ferrite or manganese-copper alloy in a resin such
as a styrene resin, a vinyl resin, an ethyl resin, a denaturated rosin resin, an acrylic
resin, a polyamide resin, an epoxy resin or polyester resin, or by covering the surfaces
of the particles of those magnetic materials with the above-specified resins. If that
resistivity is low, there arises such a problem, in case the bias voltage is applied
to the developing sleeve 31, that the charges are caused to migrate into the carrier
particles so that they become liable to be trapped by the surface of the image retainer
1 and so that the bias voltage is not sufficiently applied. Especially, if the carriers
are trapped by the image retainer 1, the color image has its tone adversely affected.
[0042] Incidentally, the resistivity is a value which is obtained by tapping the particles
in a container having an effective sectional area of 0.50 cm
2, by subsequently loading the tapped particles with a load of 1 Kg/cm
2, and by reading out a current value when a voltage for generating an electric field
of 1.000 V/cm is applied across the load and the bottom electrode.
[0043] If the carriers have an average particle diameter less than 5 µm, on the other hand,
the magnetization obtainable becomes too weak. If the average particle diameter of
the carriers exceeds 50 u.m, there arise tendencies that the image is not improved,
and that a breakdown and a discharge become liable to occur so that a high voltage
cannot be applied. Therefore, the average particle diameter preferably has a value
more than 5 u.m and less than 50 um, and a fluidizer such as hydrophobic silica is
suitable added as an additive, if necessary.
[0044] The toner may preferably be prepared by adding a variety of pigments and, if necessary,
a charge controlling agent to a resin to have an average particle diameter of 1 to
20 µm and may preferably have an average charge of 3 to 300 µc/g or, especially, 10
to 100 ac/g. If the toner has an average particle diameter smaller than 1 um, it becomes
reluctant to leave the carrier. If the average particle diameter exceeds 20 um, on
the other hand, the image has its resolution degraded.
[0045] As the toner in the method of the present invention, there is used a magnetic or
non magnetic one which is used as an ordinary toner and which is prepared by dispersing
a coloring agent if necessary and a suitable amount of magnetic material in a known
resin. As the resin, there can be enumerated a synthetic resin such as: phenol, polystyrene,
alkyd, polyacryl or polyethylene; polycarbonate, polyester, polyamide, polyether,
polyolefin, polystyrene, a styrene-acrylate copolymer, a styrene-methacrylate copolymer,
an unsaturated styrene-ethylene monoolefin copolymer, styrene-vinylester copolymer,
a styrene-vinylester copolymer, a styrene-vinylether copolymer, a styrene-acrylonitrile
copolymer, a styrene-methacrylonitrile copolymer, a styrene-acrylamide copolymer,
a styrene-halogated vinylidene copolymer or polyvinyl acetate; a binary, ternary or
more copolymer of those; or a mixture of those copolymers.
[0046] As the coloring agent, in the other hand, there are enumerated a variety of inorganic
pigments, an organic pigment, a direct dye, an acid dye, a basic dye, a mordant, an
acid mordant dye, a dispersed dye, an oil-soluble dye and so on. As a black pigment,
more specifically, there can be enumerated carbon black, acetylene black, lamp black,
graphite, mineral black, anyline black, cyanine black and so on. As a yellow pigment,
there can be enumerated chrome yellow, zinc yellow, barium chromate, cadmium yellow,
lead cyanamide, calcium plumbate, Naphthol Yellow S, Hansa Yellow 10G, Hansa Yellow
5G, Hansa Yellow 3G, Hansa Yellow G, Hansa Yellow GR, Hansa Yellow A, Hansa Yellow
RN, Hansa Yellow R, Pigment Yellow L, Benzine Yellow, Benzine Yellow G, Benzine Yellow
GR, Permanent Yellow NCG, Vulcan Fast Yellow 5G, Vulcan Fast Yellow R, Tartrazine
Yellow Lake, Quinoline Yellow Lake, Anthragen Yellow 6GL, Permanent Yellow FGL, Permanent
Yellow H10G, Permanent Yellow HR, Anthrapyrimidine Yellow, and so on. As a red pigment,
there can be enumerated a red iron oxide, red lead, silver vermilon, Cadmium Red,
Permanent Red 4R, Para Red, polytun- gustophosphoriclacid, Fire Red, vermilion, Parach-
lor Orthonitroaniline Red, Lithol Fast Scarlet G. Brilliant Fast Scarlet, Brilliant
Carmine BS, Permanent Red F2R, Permanent Red F4R, Permanent Red FRL, Permanent Red
FRLL, Permanent Red F4RH, Fast Scarlet VD, Vulcan Fast Rubin B, eosine lake, Rhodamine
Lake, Rhodamine Lake Y, Alyzarin lake, Thioindigo Red B, Thioindigo maroon, Permanent
Red FGR, PV Carmine HR, and so on. As a blue pigment, there can be enumerated ultramarine,
prussian blue, cobalt blue, Alkali Blue Lake, Peacock Blue Lake, Victoria Blue Lake,
Metalless Phthalocyanine Blue, copper phthalocyanine, Fast Sky Blue, Indanthrene Blue
RS, Indanthrene Blue BS, Indigo, and so on. As a yellow dye, there can be enumerated
C.I. (i.e., Color Index) Direct Yellow 98, C.I. Direct Yellow 89 and C.I. Direct Yellow
88 (all of which are of direct type), C.I. Acid Yellow 1, C.I. Acid Yellow 3 and C.I.
Acid Yellow 7 (all of which are of acid type), C.I. Basic Yellow 1, C.I. Basic Yellow
2 and C.I. Basic Yellow 11 (all of which are of basic type), C.I. Modern Yellow 26
(which is of mordant or acid mordant type), C.I. Disperse Yellow 1, C.I. Disperse
Yellow 3 and C.I. Disperse Yellow 4 (all of which are of disperse type), C.I. Solvent
Yellow 2, C.I. Solvent Yellow 6 and C.I. Solvent Yellow 14 (all of which are of oil
soluble type), and so on. As a red dye, there can be enumerated C.I. Direct Red 1,
C.I. Direct Red 2 and C.I. Direct Red 4 (all of which are of direct type), C.I. Acid
Red 8, C.I. Acid Red 13 and C.I. Acid Red 14 (all of which are of acid type), C.I.
Basic Red 2, C.I. Basic Red 14 and C.I. Basic Red 27 (all of which are of basic type),
C.I. Modern Red 21 (which is of mordant or axid mordant type), C.I. Disperse Red 1,
C.I. Disperse Red 4 and C.I. Disperse Red 5 (all of which are of disperse type), C.I.
Solvent Red 1, C.I. Solvent Red 3 and C.I. Solvent Red 8 (all of which are of oil-soluble
type), and so on. As a blue dye, there can be enumerated C.I. Direct Blue 1, C.I.
Direct Blue 6 and C.I. Direct blue 22 (all of which are of direct type), C.I. Acid
Blue 1, C.I. Acid Blue 7 and C.I. Acid Blue 22 (all of which are acid type), C.I.
Basic Blue 7, C.I. Basic Blue 9 and C.I. Basic Blue 19 (all of which are of basic
type), C.I. Modern Blue 48 (which is of mordant or acid mordant type), C.I. Disperse
Blue 1, C.I. Disperse Blue 3 and C.I. Disperse Blue 5 (all of which are of disperse
type), C.I. Solvent Blue 2, C.I. Solvent Blue 11 and C.I. Solvent Blue 12 (all of
which are oil-soluble type), and so on. However, the coloring agent should not be
limited to those thus far enumerated. Moreover, the colors of the toners should not
be limited to the above-specified four but can be freely selected in accordance with
the object of use.
[0047] As the magnetizing material for magnetizing the toner, there can be used a material
which is similar to that used in the carrier. The amount of addition of the magnetic
material is preferred to be smaller than 60 wt. % of the toner and especially preferred
to be up to 30 wt. % so that the clearness of the color of the toner may not be deteriorated.
[0048] In order to improve the clearness of the color of the toner, moreover, it is possible
to use a coloring magnetic material or a transparent magnetic material using a rare
earth element. As a suitable one for the coloring magnetic material, there can be
enumerated: for a red color, an iron oxide (e.g. a red oxide), a material prepared
by covering the surface of Ni with a copper oxide, or a material prepared by causing
Ni to absorb Cadmium Red; for a blue color, cobalt or its compound; and for a yellow
color, aniron oxide or a material prepared by causing Ni to absorb Cadmium Yellow.
[0049] Moreover, it is quite natural that there can be applied to the above-specified toner
a variety of known additives, which are usually used in the toner, such as a charge
control agent.
[0050] On the other hand, the toner to be used in the present invention is preferred to
have an average particle diameter of 1 to 20 u.m and an average charge of 1 to 300
u.c/g or, especially preferably, 3 to 30 uc/g. If the toner has an average particle
diameter smaller than 1 u.m, it becomes reluctant to leave the carrier. If the average
particle diameter exceeds 20 u.m, on the other hand, the resolution of the image is
degraded.
[0051] If the developer composed of a mixture of the insulating carrier and the toner thus
for described, it is possible to easily set the bias voltage, which is to be applied
to the developing sleeve 31 of Fig. 3, without any fear of leakage such that the toner
is sufficiently applied to the electrostatic image but without any fog. Incidentally,
in order to make more effective the control of development and movement of the toner
by the application of such bias voltage, the magnetic material to be used in the magnetic
carrier may be contained in the toner within such a range as will not deteriorate
the color clearness.
[0052] The descriptions made hereinbefore are directed the constructions of to the developing
means and the developer which are preferably used in the method of the present invention.
However, the present invention is not limited to them but may use such developing
means and developer as are disclosed in Japanese Patent Laid-Open Nos. 30537/75, 18656
to 18659/80, 14445281, 116553/83, and 116554/83. More preferably, there may be resorted
to such a non-contact jumping developing condition by a two-component developer as
is disclosed in Japanese Patent Laid-Open Nos. 57446/83, 96900 to 96903/83 and 97973/83.
[0053] By the use of the recording apparatus thus far described, the method of the present
invention, as shown in Figs. 4 to 7, can be put into practice. Incidentally, all of
Figs. 4 to 7 show the step at which the development of Fig. 2 has been conducted.
[0054] Fig. 4 shows an embodiment of the present invention, in which an exposed portion
PH provides a background whereas an unexposed portion DA is formed with an electrostatic
image by an electrostatic image retaining method and in which the development is conducted
by applying to the electrostatic image a toner charged with an opposite polarity.
This is the embodiment having one color image reproducing cycle comprising, according
to the reproducing apparatus of Fig. 1, the steps of: uniformly conducting a first
charge of the surface of the image retainer 1 in its initial state, in which the image
retainer 1 is cleared of a charge by the charge eliminating means 13 and cleaned by
the cleaning means 14 to a potential 0, at its one rotation by means of the charger
2; executing a first image exposure of the charged surface by means of the image exposing
ray 4 of each of different colors so that the potential other than the electrostatic
image portion may be substantially 0; conducting a first development of the electrostatic
image, the resultant potential of which is substantially equal to that of the first
charge, by means of such one of the developing means 5 to 8 as uses a developer having
a color toner corresponding to the image exposing ray 4; uniformly eliminating the
charge to return again the surface potential of the image retainer 1 to zero by means
of the charge eliminating means 13 (or only its charge eliminating lamp) because the
potential of the electrostatic image, which is dropped as a result that it traps the
toner T charged with the opposite polarity, is still higher than that of the background;
uniformly conducting a second charge again at a second rotation by means of the charger
2; conducting such a second image exposure of the secondly charged surface by means
of the image exposing ray 4 having a color different from the previous one as will
drop the potential at other than the electrostatic image portion substantially to
zero; conducting a second development of the electrostatic image obtained with a toner
T' by means of another developing means using a developer containing a color toner
corresponding thereto; subsequently repeating third and fourth retentions and developments
of the electrostatic image in a similar manner; operating the pre-transfer charger
9 and the pre-transfer exposing lamp 10 until the color image, which has been subjected
to the fourth development so that the color torier images are superposed, moves therethrough
after that color image has been retained; next transferring the color image to the
recoridng member P, which is being fed in synchronism with the rotations of the image
retainer 1, by means of the transfer means 11; fixing the color image transferred
to the recording member P by means of the fixing means 12; eliminating the charge
from the surface of the image retainer 1 having the transferred color image by means
of charge eliminating means 13; and cleaning the charge-removed surface of the image
retainer 1 by means of the cleaning means 14 until the initial state is restored to
complete the one color image reproducing cycle. More specifically, the charging operation
for each retention of the electrostatic image is performed by the charger 2, and the
image exposure is performed by means of the common slit exposing means, which is equipped
with filter switching means, or the common exposing means which is constructed of
the laser beam scanner of Fig. 2, for example. This makes it unnecessary to use another
image exposing means for retaining the electrostatic image at each time so that the
reproducing apparatus can be constructed in a small size and at a low cost and so
that the synchronous control of the retention of the electrostatic image at each time
can be conducted with ease. Incidentally, the charge elimination by the charge eliminating
means 13, which is interposed between a previous development and a subsequent charging
operation, can be omitted.
[0055] Since the development is effected by the method of developing the electrostatic image
with the toner charged with the opposite polarity, the embodiment of Fig. 4 being
described can easily enhance the developed densities of the respective colors so that
a clear color image can be easily reproduced. Incidentally, in order to avoid the
color mixing, the d.c. biases at the developments may be set at consecutively higher
levels at the later steps. In a matter to correspond to this setting, moreover, the
charged potentials may be set at consecutively higher levels.
[0056] Figs. 5 to 7 show other embodiments of the present invention, in which electrostatic
images are retained by an electrostatic retaining method of forming the image exposed
portion PH into an electrostatic image at a lower potential than the background portion
and in which the developments are conducted by applying to the electrostatic images
the toners for charging the same with the same polarity as that of the background
potential.
[0057] The embodiment of Fig. 5 by the reproducing apparatus of Fig. 1 has one color image
reproducing cycle comprising the steps of: uniformly charging the surface of the image
retianer 1 in the same initial state as that of Fig. 4 at its one rotation by means
of the charger 2; projecting the image exposing rays 4 of different colors by the
laser beam scanner of Fig. 2 onto the charged surface to conduct a first image exposure
thereby to drop the potential of the electrostatic image portion substantially to
zero; conducting a first development of the obtained electrostatic image by such any
one of the developing means 5 to 8 as uses the developer (of which the toner charges
the image retainer 1 with the same polarity, as is different from the embodiment of
Fig. 4) having the color toner corresponding to the image exposing ray 4; conducting
a second image exposure projecting the image exposing ray 4 of a different color onto
a position, which is shifted from the projected position of the previous developing
ray 4, at a second rotation by means of the same laser beam scanner without any use
of the charger 2; developing the electrostatic image thus obtained to have a substantially
zero potential with a developer having a toner of corresponding color; and subsequently
repeating the third and fourth retentions and developments of the electrostatic image
in like manners thereby to subsequently complete the one color image reproducing cycle
similarly to Fig. 4. In this embodiment, incidentally, even if the electrostatic image
having the substantially zero potential is developed so that it traps the toner T
for effecting the charge with the same polarity as that of the charge of the image
retainer 1, the potential is not substantially equal to that of the background portion,
as shown. As a result, upon the development for applying the toner T' of different
color to the electrostatic latent image retained later, the toner T' is frequently
applied in a superposed manner to the electrostatic image portion having previously
trapped the toner T, although the portion is neither exposed nor written in yet. Since
the laser beam scanner is used for preparing the image exposing ray 4, however, the
projection position of the image exposing ray 4 at each time is so remarkably simple
that the charging operation can be finished at one time. Moreover, it can be prevented
by setting the d.c. biases of the respective developments at consecutively lower levels
that the electrostatic images of different colors are liable to be superposed, whereby
an excellently clear color image can be obtained.
[0058] The embodiment of Fig. 6 is one which is improved in such defects of the embodiment
of Fig. 5 that the electrostatic image cannot be positively retained in the superposed
manner at the position where the electrostatic image has been previously formed, and
that there is a fear, to the contrary, that a toner of different color may be applied,
even if a little, by the subsequent development to the electrostatic image portion
which has been previously developed. More specifically, the embodiment of Fig. 6 is
identical, at the steps from the initial time to the first development, to the steps
up to the first development of Fig. 5 but is different from the same in the steps
of: subsequently conducting or not the charge elimination by means of the charge eliminating
means 13 (or only the charge eliminating lamp); uniformly conducting the second charge
again at the second rotation by means of the charger 2; conducting the second image
exposure and the second development of the charged surface; and subsequently repeating
the third and fourth retentions and developments of the electrostatic image in a similar
manner. Thus, in the embodiment of Fig. 6, in which the surface of the image retainer
1 is uniformly charged again after the previous development and is then subjected
to the subsequent electrostatic image retentions and developments, there can be attained
the effects that the electrostatic image can be retained in the superposed manner
on the position where the electrostatic image has been previously retained, like the
embodiment of Fig. 4, and that, in case the position of the subsequent electrostatic
image is shifted from that of the previous one, the subsequent toner of different
color is hardly applied to the image position to which the previous toner is applied.
[0059] The embodiment of Fig. 7 is one which is especially devised to prevent a subsequent
toner of different color from being applied to an image position to which a previous
toner has been applied. This example is identical in the steps before a first development
to the embodiments of Figs. 6 and 7 but is different from the same in the steps of:
either previously uniformly exposing, after the first development, the surface of
the image retainer 1 by the use of the pre-transfer exposing lamp 10 or the charge-eliminating
lamp of the charge-eliminating means 13, and subsequently conducting a second charge
by means of the charger 2, or previously uniformly conducting a second charge by means
of the charger 2, and subsequently uniformly conducting a weak exposure by means of
the exposing lamp 3; then conducting a second image exposure and a second development;
and repeating third and fourth retentions and developments of electrostatic images
in a similar manner. Here, if the uniform exposure is previously conducted after the
development, the portion which has been developed to trap the toner has its charge
uneliminated so that it is held at a high potential, whereas the remaining portion
is dropped substantially to a zero potential, whereupon the potential at the portion
having the toner can be raised by conducting the second charge to a level slightly
higher than that at the other portion, where the electrostatic image is to be retained,
thereby to charge the surface of the image retainer 1. On the other hand, even if
the second charge is previously conducted after the development to uniformly charge
the surface of the image retainer 1 whereupon the weak exposure is uniformly conducted,
the charged state of the -surface of the image retainer 1 becomes similar to that
obtainable in case the uniform exposure has been previously conducted. As a result,
when the subsequent electrostatic image retained with a shift of position is to be
developed, the previous portion having the toner is at a higher potential so that
it can be effectively prevented from trapping the toner of different color.
[0060] In any of the foregoing embodiments, it is preferred that the developing means 5
to 8 use the developer which is composed of a mixture of the toner and the insulating
carrier, and that the development is conducted under the non-contact jumping developing
condition. As a result, as has also been described, it becomes possible to prevent
the mixing of the toner of different color and to easily apply the bias voltage suitable
for the toner control, whereby a color image having a high developed density and an
excellent clearness can be reproduced even in the case of the electrostatic image
retaining method and the developing method, as in the embodiments of Figs. 5 to 7,
in which the image exposing means such as the laser beam scanner can be advantageously
used.
[0061] Next, the embodiments of Figs. 4 to 7 will be described more specifically in the
following in connection with Examples 1 to 4, respectively.
Example 1 (i.e., Embodiment of Fig. 4)
[0062] The reproducing apparatus, as shown in Fig. 1, was used. However, the exposing lamp
3 was not used, but the image retainer 1 had a surface layer of a photosensitive material
such as CdS and a circumferential speed of 180 mm/sec. The surface of the above-specified
image retainer 1 was charged with a voltage of - 500 V by means of the charger 2 using
the corona discharger, and its charged surface was subjected to the slit exposure
through a blue filter. As a result, there was retained in the image retainer 1 the
electrostatic image in which the exposed portion PH had the background potential of
- 50 V whereas the unexposed portion DA had the potential of - 500 V. The electrostatic
image thus retained was firstly developed by the developing means 5, as shown in Fig.
3.
[0063] The developing means 5 used the developer, which was composed of: a carrier prepared
by dispersing and containing 50 wt. % of magnetite in a resin to have an average particle
diameter of 20 um, a magnetization of 30 emu/g and a resistivity of 10
,4f2 cm or higher; and a non-magnetic toner prepared by adding 10 wt. parts of a benzine
derivative as the yellow pigment and another charge controlling agent to the styrene-acryl
resin to have an average particle diameter of 10 urn, under a condition that the ratio
of the toner to the carrier was 25 wt. %. Moreover, the non-contact jumping developing
conditions was resorted to, under which the developing sleeve 31 had an external diameter
of 30 mm and a number of revolutions of 100 r.p.m., under which the magnet 32 has
its N and S magnetic poles of a magnetic flux density of 1,000 gausses and a number
of revolutions of 1,000 r.p.m., under which the layer of the developer in the developed
region had a thickness of 0.7 mm, under which the gap between the developing sleeve
31 and the image retainer 1 was 0.8 mm, and under which a superposed voltage containing
a d.c. voltage of - 100 V and an a.c. voltage of 3 kHz and 1.000 V was applied to
the developing sleeve 31.
[0064] While the developing image was being developed by the developing means 5, the remaining
developing means 6 to 8, as shown in Fig. 3, were held in their undeveloping state.
This was achieved by disconnecting the developing sleeve 31 from the power supply
39 into its floating state, by grounding the same to the earth, or by positively applying
the d.c. bias voltage, which had the same polarity as the charge of the image retainer
1 but the opposite polarity to the charge of the toner, to the developing sleeve 31.
Of these, it is preferred to apply the d.c. bias voltage. Since the developing means
6 to 8 were made to conduct their developments under the non-contact jumping developing
condition like the developing means 5, it was not necessary to especially eliminate
the layer of the developer from the developing sleeve 31. Of those developing means
6 to 8: the developing means 6 used a developer which was prepared by replacing the
toner of the developer of the developing means 5 by a toner containing polytungstate
as the Magenta pigment in place of the yellow pigment; the developer 7 used a developer
which was prepared by replacing the same toner by a toner containing copper phthalocyanine
as the cyan pigment; and the developing means 8 used a developer which was prepared
by replacing the same toner by a toner containing carbon black as the black pigment.
It is quite natural that a toner containing other pigment and dye could be used as
the color toner, and that the order of the colors to be developed and the order of
the developing means could be suitably selected.
[0065] The surface of the image retainer 1, which had been subjected to the first development,
was recharged to - 600 V by operating the charge eliminating means 13 and the charger
2 (although the former may be left inoperative). The charged surface was subjected
to the second iamge exposure by the slit exposure through a green filter, and the
developing sleeve 31 was then subjected to the second development with the Magenta
toner by the developing means under the non-contact jumping developing condition for
applying the superposed voltage of a d.c. voltage of - 200 V and an a.c. voltage of
3 kHz and 1,000 V. Likewise, the following steps were repeated: the charge, the slit
exposure through a red filter, and the third development of the cyan toner by the
developing means 7; and the charge, the slit exposure without any filtration, and
the fourth development of a black toner by the developing means 8. In the second and
later developments, incidentally, the amplitudes and frequencies of the d.c. bias
component and a.c. component of the voltage to be applied to the developing sleeve
31, the selecting time of a time selecting conversion, and so on were suitably changed
in accordance with the changes in the surface potential, the developing characteristics
and the color reproductivity of the image retainer 1. Especially, it was effective
for preventing the color mixing of the toner that the charge potentials were made
consecutively higher whereas the d.c. biases were made consecutively higher.
[0066] The color images of four colors were made liable, when they were retained on the
image retainer 1 as a result of the fourth development, to be transferred by the pre-transfer
charger 9 and the pre-transfer exposing lamp 10, and were then transferred to the
recording member P by the transfer means 11 until they were fixed by the fixing means
12. The image retainer 1 having the color images transferred thereto had its charge
eliminated by the charge eliminator 13 and its residual toner scraped off from its
surface by the contacts of the cleaning blade or fur brush of the cleaning means 14.
At the instant when the surface having the color images retained therein passed through
the cleaning means 13, the one color image reproducing cycle was completely ended.
[0067] The color image thus reproduced was so clear that the respective colors exhibited
sufficient densities, but a slight color mixing of the toners was found at the portion
in which the respective color toners are densely trapped.
Example 2 (i.e., Example of Fig. 5)
[0068] The reproducing apparatus, as shown in Fig.1, was used. However, the exposing lamp
3 was not used, but the image retainer 1 had a surface layer of a photosensitive materail
such as CdS and a circumferential speed of 180 mm/sec. The surface of the above-specified
image retainer 1 was charged with a voltage of + 500 V by means of the charger 2 using
the corona discharger, and its charged surface was subjected to the first image exposure
in a density of 16 dots/mm by the laser beam scanner of Fig. 2 using a He-Ne laser.
As a result, there was retained in the image retainer 1 the electrostatic image in
which the exposed portion PH had a potential of + 50 V contrary to the background
potential of + 500 V. The electrostatic image thus retained was firstly developed
by the developing means 5, as shown in Fig. 3.
[0069] Incidentally, the developing conditions by the developing means 5 were made identical
to those of Example 1 except that the carrier of the developer had an average particle
dize of 30 u.m, that the ratio of the toner to the carrier was 20 wt. %, and that
the superposed voltage of a d.c. voltage of + 400 V and an a.c. voltage of 1.5 kHz
and 1,000 V was applied to the developing sleeve 31. Moreover, the conditions of the
remaining developing means 6 to 8 were identical to those of the Example 1 except
the bias voltages. In this case, however, the bias voltages for holding the developing
means taking no part in the development in the state other than the development had
a polarity opposite to both those of the charge of the toner and the charge of the
image retainer 1.
[0070] The surface of the image retainer 1 having been subjected to the first development
was subjected again to the second image exposure without any change in the density
but with a shift of the dot position by means of the same laser beam scanner without
operating one of the pre-transfer charger 9, the pre-transfer exposing lamp 10, the
charge eliminating means 13, the cleaning means 14 and the charger 2, and was then
subjected to the second development with the Magenta toner by the developing means
6. Likewise, the third development with the cyano toner by the developer means 7 and
the fourth development with the black toner by the developing means 8 were repeated.
In the second and later developments, incidentally, the amplitudes and frequencies
of the d.c. bias component and a.c. component of the voltage to be applied to the
developing sleeve 31, the selecting time of a time selecting conversion, and so on
were suitably changed in accordance with the changes in the surface potential, the
developing characteristics and the color reproductivity of the image retainer 1. Especially,
in this case it was effective for preventing the color mixing of the toner that the
d.c. biases were made consecutively lower at the respective steps.
[0071] After the fourth development had been conducted to retain the color image of four
colors on the image retainer 1, it was transferred and fixed to the recording member
P like the Example 1, and the image retainer 1 had its charge eliminated and was cleaned,
thus ending the one color image reproducing cycle.
[0072] The color image thus reproduced was clear like that of the Embodiment 1.
Example 3 (i.e.. Embodiment of Fig. 6)
[0073] By using the same apparatus as that of the Example 2, the color image reproduction
was performed under the same conditions as those of the Example 2 except that the
voltage to be applied to the developing sleeve 31 of the developing means was a superposed
voltage of a d.c. voltage of + 400 V and an a.c. voltage of 500 Hz and 250 V, that
the charge elimination was conducted by means of the pre-transfer exposing lamp 10
before each image exposure on and after the second image exposing step, and that the
surface potential of the image retainer 1 was then recharged with + 500 V by the charger
2.
[0074] The color image reproduced was more clear than that obtained by the Example 2, because
the color mixture of the toners was reduced at the portion where the respective color
toners were densely trapped.
[0075] According to this Example, incidentally, the previous image exposed position and
the subsequent image exposed position can be superposed, as has been described hereinbefore,
and, in this case, the order of the colors to be developed imparts considerable influences
upon the clearness of the color image. It is, therefore, necessary to determine especially
carefully the order of the colors to be developed.
Example 4 (i.e., Embodiment of Fig. 7)
[0076] By using the same apparatus as that of the Example 2 except the provision of the
exposing lamp 3, the color image reproduction was conducted under the same conditions
as those of the Example 2 except that the voltage to be applied to the developing
sleeve 31 of the developing means was a superposed one of a d.c. voltage of + 450
V and an a.c. voltage of 2 kHz and 500 V, and that both the charge by the charger
2 for raising the surface potential of the image retainer 1 to + 600 V and the uniform,
weak exposure by the exposing lamp 3 for dropping the surface potential to + 500 V
were conducted before each image exposure on and after the second image exposure.
[0077] The color image thus reproduced had no color mixing of the respective color toners
even at a portion, where the toners were densely trapped, so that it was remarkably
clear.
[0078] Even in this Example, like the Example 3, the previous image exposed position and
the subsequent image exposed position can be superposed.
[0079] According to the present invention, the common apparatus can be used for the repeated
electrostatic image retentions to provide excellent effects that the reproducing apparatus
can be constructed in a small size and at a low cost, and that the synchronous control
of each image exposure can be conducted easily and accurately. Moreover, each development
can be conducted either by the developing method, in which there is applied to an
electrostatic image relatively easy to have its developed density controlled to a
toner for charging it with the opposite polarity, or by the developing method in which
the laser beam scanner can be used as the image exposing means and in which there
is applied to the electrostatic image a toner for charging it with the common polarity.
According to either of the developing methods, still moreover, there can be attained
an excellent effect that the development can be conducted under the non-contact jumping
developing condition to reproduce the color image having a sufficient developed density
and an excellent clearness.
[0080] As has been described hereinbefore, incidentally, the present invention should naturally
be limited neither to the method, in which the image retainer uses the drum-shaped
recording apparatus, but also to the method in which the color image is transferred
to the recording member. In other words, the present invention can be applied to the
method, in which the image retainer is mounted on a base such as that for electrofax
paper and in which the color image retained thereon is not transferred but fixed.
In this application, there can be dispensed with the pre-transfer charger, the pre-transfer
exposing lamp, the transfer means and the cleaning means. It is true, but the pre-transfer
charger, the pre-transfer exposing lamp or the charge eliminating means can be omitted
in the transfer case, too, and the transfer may be a direct pressure one or one using
an intermediate transfer member whereas the fixing operation should not be limited
to one using a heat roller.
[0081] In other embodiments of the present invention, the image retainer 1 is constructed,
as shown in Fig. 8, of: a conductive base 1 a made of aluminium or nickel; a photoconductive
photosensitive layer 1 b made of Se, CdS, Si or the like and formed on the conductive
base 1a; and a transparent insulating surface layer 1 made of a transparent resin
and formed on the photoconductive photosensitive layer 1 b, and said conductive substrate
1 a being grounded to the earth. Indicated at reference numeral 2 in Fig. 9 is a primary
charger which is composed, in combination, of a lamp 2a for irradiating the surface
of the image retainer 1 and a corona discharger 2b. Numeral 3' indicates a secondary
charger which is composed of a corona discharger. Here, the primary charger 2 need
not necessarily be equipped with the lamp 2a in case the photoconductive photosensitive
layer 1 b of the image retainer 1 has such semiconductor characteristics as exhibit
a rectifying action that charges can be implanted from the base 1a. Indicated at numeral
15 is a corona discharger for charging toners prior to transfers.
[0082] In the reproducing apparatus thus constructed, when the primary charger 2 conducts
the corona discharge by its corona discharger 2b while irradiating the surface of
the image retainer 1 by its lamp 2a (which may not be required as the case may be,
as has been described hereinbefore), the image retainer 1 is charged, as shown in
Fig. 10A, in its photoconductive photosensitive layer 1 and on the surface of its
transparent insulating surface layer 1c. When the secondary charger 3' conducts the
corona discharge of the charged surface of the image retainer 1, the charges on the
surface of the transparent insulating surface layer 1 are reduced because the photoconductive
photosensitive layer 1 has an insulating property in this case, so that the charged
state of the image retainer 1 charges as shown in Fig. 10B. When the image exposing
ray 4 is incident upon that surface of the image retainer 1 thus secondarily charged,
the surface charges of the photoconductive photosensitive layer 1b at the exposed
portion PH are reduced while being left as they are at the unexposed portion DA so
that the charges of the image retainer 1 change, as shown in Fig. 10C. The changes
of the surface potential of the image retainer 1 in this meanwhile are shown in Fig.
11, in which the potentials in states A, B and C correspond to the charged states
of Figs. 10A, B and C, respectively. More specifically, the potential of the exposed
portion PH exposed to the image exposing ray 4 takes such surface one as is indicated
at C(PH) in Fig. 11, whereas the potential of the unexposed portion having received
none of the image exposing ray 4 is at C (DA) which is substantially the same as the
surface potential indicated at B in Fig. 11. This results in that an electrostatic
image having a surface potential at C(PH) with respect to that background is retained
by the image exposing ray 4. The electrostatic image thus retained can be developed
like the ordinary electrophotographic re- produc ing machine with such a developer
as will charge the exposed portion PH with a polarity opposite to that of the latent
image, and this development is conducted by any one of such a color of the developing
means 5 to 8 as corresponds to the image exposing ray 4 having formed the electrostatic
image. When the development is conducted to apply the toner, the potential of the
electrostatic image is dropped in accordance with the trapped amount of the toner
having been charged in the opposite polarity. However, the charge of the unexposed
portion DA of the image retainer 1, which has received none of the image exposing
ray 4, remains in such a state DA of Fig. 10C as is identical to that of Fig. 10B,
and the surface potential of the same portion is at the same potential C-(DA) as that
of B of Fig. 11 even with a dark attenuation. As a result, if the image exposing ray
4 of another color is incident with a shift from the position of the previous image
exposing ray 4 at the second rotation of the image retainer 1, an electrostatic image
can be retained like the previous exposure with neither of the primary and secondary
charging operations. Thus, the second and later retentions of the electrostatic images
by making use of the first primary and secondary charging operations are easily effected
in case the previous development or developments are conducted by the developing method,
in which a developer for charging in an opposite polarity is applied, and under the
non-contact jumping developing condition. This is partly because the application of
the toners to the electrostatic images can be more easily conducted than the developing
method, in which the developing agent for charging in the same polarity to make it
unnecessary to apply such a high voltage to the developing sleeve 16 as to apply the
toners to the electrostatic images so that the charged state of the image retainer
1 is held stable, and partly because the developer layer of the developing means is
kept away from the surface of the image retainer 1 by resorting to the non-contact
jumping developing condition so that the charged state of the image retainer 1 is
held stable. Incidentally, the shift of the position of the subsequent image exposing
ray 4 with respect to the previous image forming ray 4 so as to retain the electrostatic
images of different colors can be easily conducted by using the laser beam scanner
of Fig. 3 for producing the image exposing ray 4.
[0083] The present invention should not be limited to the embodiment in which the second
and later retentions of the electrostatic images are conducted by making use of the
first primary and secondary charging operations. Generally speaking, however, the
primary and secondary charging operations may be conducted each time of the second
and later retentions of the electrostatic images, or therefore the previous charges
may be eliminated by the charge eliminating means 13 or only the secondary charging
operation may be so executed as to compensate the dark attenuation. Especially either
in case the primary and secondary charging operations are executed again without eliminating
the previous charges or in case only the secondary charging operation is executed,
the scorotron corona discharger may preferably for allowing stable charging operations
even in the presence of the previous charges may preferably be used as the corona
dischargers of the primary charger 2 and the secondary charger 3'. It is especially
desired when a graded reproductivity is stressed or in case the image exposing ray
4 is of slit or flash exposure type that the primary and secondary charging operations
are conducted again each retention of the electrostatic images. Moreover, it is possible
to adopt the NP or KIP method by which the electrostatic images are retained by conducting
the image exposure simultaneously with the secondary charging operation after the
primary charging operation and by conducting the exposure of the whole surface. In
the several methods thus far described, the electrostatic image potential can be so
controlled in dependence upon the relative strengths of the primary and secondary
charges that the exposed portion and the unexposed portion may be in an identical
or opposite polarity. Considering the feasibility of the development, however, it
is preferable that the exposed and unexposed portions take opposite polarities.
[0084] The electrostatic image at the second rotation thus retained is developed by such
one of a color different from the previous one of the developing means 5 to 8 as corresponds
to the image exposing ray 4 having retained that electrostatic image. Likewise, at
the third and fourth rotations of the image retainer 1, too, both the retentions of
the electrostatic images and the developments by the different developing means corresponding
to those electrostatic images are conducted to form a color image which has toner
images of different colors superposed on the image retainer 1. Moreover, the surface
of the image retainer 1 having been subjected to the last development has, if necessary,
charges applied to its toner image by means of the corona discharger 15 and is then
irradiated by the pre-transfer lamp 10 so that the color image can be easily transferred
to the recording member P by the transfer means 11. The color image thus transferred
to the recording member P is fixed to the recording member P by the fixing means 12.
The surface of the image retainer 1 having the color image transferred thereto has
its charges eliminated by the charge eliminating means 13 so that the residual toners
after the transfer are eliminated as a result that the cleaning blade of the cleaning
means 14 having been kept away until that time comes into abutment against the surface
of the image retainer 1. At the time the surface portion of the image retainer 1 having
retained the color image moves over the cleaning means 14, the cleaning blade leaves
the surface of the image retainer 1, thus completing the one color image reproducing
cycle.
[0085] Fig. 12 is a flow chart showing the changes of the surface potential of the image
retainer of the embodiment of the present invention, in which an electrostatic image
is retained each time with an image exposing ray by making use of the first primary
and secondary charging operations. Fig. 13 is a flow chart showing the changes of
the surface potential of the image retainer of the embodiment of the present invention,
in which only the secondary charging operations is conducted prior to the second and
later electrostatic image retentions. Fig. 14 is a flow chart showing the changes
of the surface potential of the image retainer of the embodiment of the present invention,
in which the primary and secondary charging operations are conducted after charges
are eliminated from the second and later retained electrostatic images like the first
one. All of these Figures show, like Figs. 10 and 11, the embodiments of the present
inventions for retaining the electrostatic images by the negative exposure, where
the image exposed portion retains the electrostatic images, to which the toners are
applied. On the contrary, Fig. 15 is a flow chart showing the changes of the surface
potential of the image retainer of the embodiment of the present invention for retaining
the electrostatic images by the positive exposure, where the exposed portion is formed
into the background portion whereas the unexposed portion is formed into the electrostatic
image, to which the toners are applied, by changing the conditions for the primary
and secondary charging operations in the embodiment of Fig. 14. On the other hand,
Figs. 16 and 17 are flow charts showing the changes of the surface potentials of the
image retainers of the embodiments of the present invention using the electro static
image retaining process called the "NP or KIP process". Fig. 16 corresponds to the
case of the positive exposure like Fig. 15, whereas Fig. 17 corresponds to the case
of the negative exposure like Figs. 12 to 14. The embodiments of Figs. 16 and 17 will
be briefly described in the following. Fig. 16 shows the embodiment comprising the
steps of: imparting charges to the photoconductive photosensitive layer 1 b by a primary
exposing operation to charge the image retainer 1 positive; subsequently conducting
an image exposure in a secondary charging operation to form an ion image on the transparent
insulating surface layer 1 thereby to charge the image retainer 1 negative; subjecting
the whole surface to an exposure to retain such an electrostatic image that the potential
of the unexposed portion not having been subjected to the image exposure during the
secondary charging operation exhibits a positive value; applying a toner charged in
an opposite polarity to that electrostatic image by a development; and subsequently
repeating a charge elimination (which may be omitted) and an electrostatic image retention
and development similar to the aforementioned ones. On the other hand, Fig. 17 shows
the embodiment comprising the steps of: charging the image retainer 1 negative by
a primary charging operation contrary to the fact that the image retainer 1 is charged
positive in the embodiment of Fig. 16; charging the image retainer 1 positive by a
subsequent second charging operation and a simultaneous image exposure; subjecting
the whole charged surface to an exposure so that the potential of the unexposed portion
other than the portion having been subjected the image exposure during the secondary
charging operation is negative like the primary charging operation; applying a toner
in an opposite polarity by a development to the electrostatic image at a portion which
has been charged positive by the image exposure during the secondary charging operation;
nad subsequently repeating a charge elimination (which may be omitted), and an electrostatic
image retention and a development like the aforementioned ones.
[0086] Incidentally, all Figs. 12 to 17 show the step, at which the second development has
been conducted, and the embodiments in which the second image exposure is shifted
from the first one. However, not only in the embodiments of Figs. 14 to 17, in which
the charge elimination and the primary and secondary charging operations are repeated
at each time, but also in the embodiments of Figs. 12 and 13, the subsequent image
exposing ray is incident upon the previous image exposed position, in which the electrostatic
image is formed, by means for increasing the quantity of light of the image exposing
ray the more for a later time so that the toner of a color different from the previous
one can be applied in a superposed manner by the developments. Indicated at reference
letters T and T' in Figs. 12 to 17 are toners in different colors, which are applied
to the surface of the image retainer.
[0087] In any of the embodiments of Figs. 12 to 17, moreover, each development is conducted
with the developer in a polarity opposite to that of the electrostatic image thereby
to provide an excellent effect that a remarkably clear color image can be reproduced
with little scattering of the developer, and the reproducing apparatus is so constructed
as is shown in Fig. 9 thereby to provide an advantage that the primary and secondary
chargers and the image exposing ray producing apparatus taking part in the respective
retentions of the electrostatic images can be commonly used. These embodiments of
Figs. 12 to 17 will be described more specifically in the following in connection
with Examples 5 to 10.
Example 5 (i.e., Embodiment of Fig. 12)
[0088] The reproducing apparatus shown in Fig. 9 was used. Despite of this fact, the image
retainer 1 was prepared by forming a transparent insulating film having a thickness
of 20 u.m on a CdS photosensitive layer having a thickness of 30 IJ.m and had a circumferential
speed of 180 mm/sec. The image retainer 12 thus prepared was so charged by means of
a d.c. scorotron corona discharger 2b, while being subjected to a uniform exposure
by the lamp 2a of the primary charger 2, that its surface potential took a level of
+ 1,000 V. Next, the image retainer 1 was charged to have a surface potential of -
100 V by means of the secondary charger 3 composed of the scorotron corona discharger
having an a.c. component. This charged surface was subjected to a writing exposure
with a density of 16 dots/mm by means of the laser beam scanner using the He-Ne laser,
as shown in Fig. 2, to retain an electrostatic image having a background potential
of - 100 V and an exposed portion potential of + 50 V. This electrostatic image was
developed by the developing means 5 shown in Fig. 3.
[0089] The developing means 5 used the developer, which was composed of: a carrier prepared
by dispersing and containing 50 wt. % of magnetite in a resin to have an average particle
diameter of 20 u.m, a magnetization of 30 emu/g and a resistivity of 10
140 cm or higher; and a non-magnetic toner prepared by adding 10 wt. parts of a benzine
derivative as the yellow pigment and another charge controlling agent to the styrene-acryl
resin to have an average particle diameter of 10 u.m, under a condition that the ratio
of the toner to the carrier was 20 wt. %. Moreover, the non-contact jumping developing
condition was resorted to, under which the developing sleeve 31 had an external diameter
of 30 mm and a number of revolutions of100 r.p.m., under which the magnet 32 has its
N and S magnetic poles of a magnetic flux density of 900 gausses and a number of revolutions
of 1,000 r.p.m., under which the layer of the developer in the developed region had
a thickness of 0.7 mm, under which the gap between the developing sleeve 31 and the
image retainer 1 was 0.8 mm, and under which a superposed voltage containing a d.c.
voltage of - 50 V and an a.c. voltage of 2.5 kHz and 2,000 V was applied to the developing
sleeve 31.
[0090] While the developing image was being developed by the developing means 5, the remaining
developing means 6 to 8, as shown in Fig. 3, were held in their undeveloping state.
This was achieved by disconnecting the developing sleeve 31 from the power supply
39 into its floating state, by grounding the same to the earth, or by positively applying
the d.c. bias voltage, which had the same polarity as the charge of the image retainer
1 but the opposite polarity to the charge of the toner, to the developing sleeve 31.
Of these, it is preferred to apply the d.c. bias voltage. Since the developing means
6 to 8 were made to conduct their developments under the non-contact jumping developing
condition' like the developing means 5, it was not necessary to eliminate the layer
of the developer layer from the developing sleeve 31. Of those developing means 6
to 8: the developing means 6 used a developer which was prepared by replacing the
toner of the developer of the developing means 5 by a toner containing polytungstate
as the Magenta pigment in place of the yellow pigment; the developer 7 used a developer
which was prepared by replacing the same toner by a toner containing copper phthalocyanine
as the cyan pigment; and the developing means 8 used a developer which was prepared
by replacing the same tone by a toner containing carbon black as the black pigment.
It is quite natural that a toner containing other pigment and dye could be used as
the color toner, and that the order of the colors to be developed and the order of
the developing means could be suitably selected. Especially in case the positions
of the image exposing rays were superposed, the order of the colors to be developed
had to be carefully determined because it had a serial relationship with the clearness
of the color image.
[0091] When the surface of the image retainer 1 developed by the developing means 5 arrived
again at the position, where it was to be exposed to the image exposing ray 4, after
it had passed through the positions of the corona discharger 15 and the pre-transfer
lamp 10, both of which were not necessary until the final transfer was conducted,
the charge eliminating means 13 and the cleaning means 14, both of which are held
in their inoperative states, and the primary charger 2 and the secondary charger 3'
which came into their paused states after having conducted the primary and secondary
charging operations, a second writing operation was conducted with a shift of the
dot positions, in a doubled light quantity and in the same dot density by means of
the same laser beam scanner as the previous one. The electrostatic image thus obtained
had a potential of + 200 V for the background portion potential of - 100 V. The resultant
electrostatic image was developed by the developing means 6 under the same conditions
as those of the developing means 5 except that a voltage having a d.c. component of
100 V and an a.c. component of 2.5 kHz and 2,000 V was applied to the developing sleeve
31.
[0092] Likewise, at the third rotation of the image retainer 1, a writing operation was
conducted in a tripled light quantity by means of the laser beam scanner to retain
an electrostatic iamge having a potential of + 350 V for a background portion potential
of - 100 V. This electrostatic iamge was developed by the developing means 7 under
the same conditions as those of the developing means 5 except that a voltage having
a d.c. component of 250 V and an a.c. component of 2.5 kHz and 2,000 V was applied
to the developing sleeve 31. Likewise, moreover, a writing operation was conducted
at the fourth rotation of the image retainer 1 in a quadrupled light quantity by means
of the laser beam scanner to retain an electrostatic iamge of + 500 V for a background
potential of - 100 V. This electrostatic image was developed by the developing means
8 under the same conditions as those of the developing means 5 except that a voltage
having a d.c. component of 400 V and an a.c. component of 2.5 kHz and 2,000 V was
applied to be developing sleeve 31.
[0093] At the stage when that fourth development was conducted so that the color image of
four colors was retained on the image retainer 1, the corona discharger 15 and the
pre-transfer lamp 10 were operated to make the color image liable to be transferred,
and this color image was transferred to the recording member P by the transfer means
11 and fixed by the fixing means 12.
[0094] The image retainer 1 having the color image thus transferred thereto had its charges
eliminated by the charge eliminating means 13 and its surface cleared of the residual
toners by the abutment against the cleaning blade of the cleaning means 14. As a result,
the one color image reproducing cycle was completely finished at the time the surface
having the color image retained therein passed over the cleaning means 14.
[0095] The color image thus reproduced was freed from any color mixing not only at the portion,
to which the respective color toners were coarsely applied, but also at the portion,
to which the same were densely applied, so that it was remarkably clear.
Example 6 (i.e., Embodiment of Fig. 13)
[0096] The same reproducing apparatus as that of the Example 5 was used, and primary and
secondary charging operations were conducted under the same conditions as those of
the Example 5 by means of the primary charger 2 and the secondary charger 3'. After
that, the writing operations was conducted by the same laser beam scanner as that
of the Example 5 but in a light quantity four times as large as that of the first
writing operation of the Example 5 and with the same dot density as that of the Example
5 to retain an electrostatic image having an exposed portion potential of + 500 V
for a background portion potential of - 100 V in the image retainer 1 under the same
conditions as those of the Example 5.
[0097] This electrostatic image was developed by the developing means 5 under the same conditions
as those of the Embodiment 5 except that a superposed voltage composed of a d.c. voltage
of 50 V and an a.c. voltage of 1.5 kHz and 1,000 V was applied to the developing sleeve
31.
[0098] Moreover, when the surface of the image retainer 1 thus developed by the developing
means 5 arrived again at the position of the secondary charger 3', the image retainer
1 is so charged again by the secondary charger 3' that its surface potential took
a level of - 100 V. That surface was subjected to second writing and exposing operations
under the same conditions as the previous ones except the shift of the dot positions
by means of the same laser beam scanner to retain again an electrostatic image. This
electrostatic image was developed by the developing means 6 using the same developer
as that of the Example 5 and under basically the same conditions as the developing
conditions of the developing means 5.
[0099] Like the second operation, moreover, the charging and exposing operations were repeated
to develop a third electrostatic image by the developing means 7 and to develop a
fourth electrostatic image by the developing means 8 in a similar manner. In this
case, each development was conducted by suitably changing the d.c. bias component
and the amplitude, frequency, duty ratio and so on of the a.c. component of the voltage,
which was to be applied to the developing sleeve 31, in accordance with the changes,
developing characteristics and color reproducing state of the surface potential of
the image retainer 1.
[0100] After the fourth development had been conducted to retain a color image of four colors
on the image retainer 1, the operations of reproducing the color image was completed
like the Example 5.
[0101] The reproduced image thus obtained was remarkably clear like that of the Example
5.
Example 7 (i.e., Embodiment of Fig. 14)
[0102] The reproducing apparatus shown in Fig. 9 was used. Despite of this fact, the image
retainer 1 was prepared by forming a transparent insulating film having a thickness
of 10 µm on an a-Si photo sensitive layer having a thickness of 10 αm and had a circumferential
speed of 180 mm/ sec. The image retaienr 1 thus prepared was charged with a voltage
of + 700 V by the primary charger 2, while being uniformly exposed to the lamp 2a
like the Example 5, and was then charged with a voltage of 0 V by the secondary charger
3'. This charged surface was subjected to a writing operation by the laser beam scanner
like the Example 5. The electrostatic image thus obtained had a potential of + 300
V with respect to the background portion potential of 0 V. The resultant electrostatic
image was developed by the developing means 5 under the same conditions as those of
the Example 5 except that a voltage composed of a d.c. component of 100 V and an a.c.
component of 500 Hz and 400 V was applied to the developing sleeve. A second development
was conducted by the developing means 6 using the same developer as that of the Examle
5 and under the same conditions as those of the developing means 5 by executing the
primary and secondary charging operations and the image exposure under the same conditions
as those of the first development after the charge eliminating means 13 had been operated
to eliminate the charges (although this charge eliminating step might be omitted).
Third and fourth develop ments were repeated in the same manner as that of the second
one by means of the developing means 7 and 8, respectively. The developers of the
developing means 7 and 8 were the same as those of the Example 5, respectively. Although
substantially unnecessary for practical purposes, however, the change of the conditions
of the voltage to be applied to the developing sleeve was conducted like the Example
6 in accordance with the potential change, the developing characteristics and the
color reproductivity of the image retainer 1. Thus, the color image reproducing operations
were completed like the Example 5.
[0103] The reproduced image thus obtained was remarkably clear like the Example 5.
Example 8 (i.e., Embodiment of Fig. 15)
[0104] The reproducing apparatus shown in Fig. 9 was used. Despite of this fact, the image
retainer 1 was prepared by forming a transparent insulating film having a thickness
of 10 /J.m on an a-Si photosensitive layer having a thickness of 10 /J.m and had a
circumferential speed of 180 mm/sec. The image retainer 1 thus prepared was charged
with a voltage of - 700 V by the primary charger 2, while being uniformly exposed
to the lamp 2a like the Example 5, and was then charged with a voltage of + 300 V
by the secondary charger 3'. This charged surface was subjected to a slit exposure
through the blue filter of an ordinary color reproducing machine. As a rusult, there
was obtained at a potential of + 300 V an electrostatic image which corresponds to
the unexposed portion with respect to the background portion of 50 V corresponding
to the exposed portion. The resultant electrostatic image was developed by the developing
means 5 under the same conditions as those of the Example 5 except that a voltage
composed of a d.c. component of 50 V and an a.c. component of 500 Hz and 400 V was
applied to the developing sleeve. A second development was conducted by the developing
means 6 using the same developer as that of the Example 5 and under the same conditions
as those of the developing means 5 by executing the primary and secondary charging
operations and the image exposure through a green filter under the same conditions
as those of the first development after the charge eliminating means 13 had been operated
to eliminate the charges (although this charge eliminating step might be omitted).
Third and fourth developments were repeated in the same manner as that of the second
one but by changing the exposing filters into a red filter and an neutral filter,
respectively, to form an electrostatic image, and this electrostatic image was developed
by means of the developing means 7 and 8, respectively. The developers of the developing
means 7 and 8 were the same as those of the Example 5, respectively. Although substantially
unnecessary for practical purposes, however, the change of the conditions of the voltage
to be applied to the developing sleeve was conducted like the Example 6 in accordance
with the potential change, the developing characteristics and the color reproductivity
of the image retainer 1. Thus, the color image reproducing operations were completed
like the Example 5.
[0105] The reproduced image thus obtained was remarkably clear like the Example 5.
Example 9 (i.e., Embodiment of Fig. 16)
[0106] The reproducing apparatus shown in Fig. 9 was used. Despite of this fact, the image
retainer 1 was prepared by forming a transparent insulating film having a thickness
of 10 u.m on an a-Si photosensitive layer having a thickness of 10 u.m and had a circumferential
speed of 180 mm/sec. The image retainer 1 thus prepared was charged with a voltage
of + 700 V by the primary charger 2, while being uniformly exposed to the lamp 2a
like the Example 5, and was then charged with a voltage of - 100 V by the secondary
charger 3'. This charged surface was simultaneously subjected to a slit exposure through
the blue filter of an ordinary color reproducing machine, and then to a uniform exposure.
As a result, there was retained at a potential of + 300 V an electrostatic image which
corresponds to the unexposed portion with respect to the background portion of - 100
V corresponding to the exposed portion. The resultant electrostatic image was developed
by the developing means 5 under the same conditions as those of the Example 5 except
that a voltage composed of a d.c. component of 0 V and an a.c. component of 500 Hz
and 400 V was applied to the developing sleeve. A second development was conducted
by the developing means 6 using the same developer as that of the Example 5 and under
the same conditions as those of the developing means 5 by executing the primary and
secondary charging operations, the image exposure through a green filter, and the
uniform exposure under the same conditions as those of the first development after
the charge eliminating means 13 had been operated to eliminate the charges (although
this charge eliminating step might be omitted). Third and fourth developments were
repeated in the same manner as that of the second one but by changing the exposing
filters into a red filter and an neutral filter, respectively, to form an electrostatic
image, and this electrostatic image was developed by means of the developing means
7 and 8, respectively. The developers in the developing means 7 and 8 were the same
as those of the Example 5, respectively. Although substantially unnecessary for practical
purposes, however, the change of the conditions of the voltage to be applied to the
developing sleeve was conducted like the Example 6 in accordance with the potential
change, the developing characteristics and the color reproductivity of the image retainer
1. Thus, the color image reproducing operations were completed like the Example 5.
[0107] The reproduced image thus obtained was remarkably clear like the Example 5.
Example 10 (i.e., Embodiment of Fig. 17)
[0108] The reproducing apparatus shown in Fig. 9 was used. Despite of this fact, the image
retainer 1 was prepared by forming a transparent insulating film having a thickness
of 10 u.m on an a-Si photosensitive layer having a thickness of 10 αrn and had a circumferential
speed of 180 mm/sec. The image retainer 1 thus prepared was charged with a voltage
of - 700 V by the primary charger 2, while being uniformly exposed to the lamp 2a
like the Example 5, and was then charged with a voltage of + 300 V by the secondary
charger 3'. Simultaneously with this secondary charging operation, the charged surface
was subjected to a writing operation by means of the laser beam scanner and then to
a uniform exposure. As a result, there was retained at a potential of + 300 V an electrostatic
image which corresponds to the exposed portion with respect to the background portion
of - 100 V corresponding to the unexposed portion. The resultant electrostatic image
was developed by the developing means 5 under the same conditions as those of the
Example 5 except that a voltage composed of a d.c. component of 50 V and an a.c. component
of 500 Hz and 400 V was applied to the developing sleeve. A second development was
conducted by the developing means 6 using the same developer as that of the Example
5 and under the same conditions as those of the developing means 5 by executing the
primary and secondary charging operations, the image exposure, and the uniform exposure
under the same conditions as those of the first development after the charge eliminating
means 13 had been operated to eliminate the charges. Third and fourth developments
were repeated in the same manner as that of the second one, and this electrostatic
image was developed by means of the developing means 7 and 8, respectively. The developers
in the developing means 7 and 8 were the same as those of the Example 6, respectively.
Although substantially unnecessary for practical purposes, however, the change of
the conditions of the voltage to be applied to the developing sleeve was conducted
like the Example 6 in accordance with the potential change, the developing characteristics
and the color reproductivity of the image retainer 1. Thus, the color image reproducing
operations were completed like the Example 5.
[0109] The reproduced image thus obtained was remarkably clear like the Example 5.
[0110] According to the Examples of the present invention thus far described, it is possible
to change the potentials and polarities of the elctrostatic images in dependence upon
the relative strengths of the primary and secondary charging operations and to easily
conduct the positive or negative exposure. It is also possible to superpose the developers
under the non-contact jumping developing conditions and especially to make the electrostatic
images and the developers have opposite polarities. As a result, the developers are
easily applied to the electrostatic images so that the adjustment of the developing
conditions of the image retainer for the changes in the potential can be facilitated
to reproduce a color image having a sufficient density and an excellent clearness
while having little scattering of the developers. Since the common charger and image
exposing ray producing means are used for each retention of the electrostatic images,
moreover, there can be attained an effect that the recording apparatus can be reduced
in size and cost without being troubled by registration.
[0111] Incidentally, the present invention can be applied to such an image retainer as is
applied to a base, as in electrofax paper, and that the color image retained thereon
is not transferred but fixed. In this case, the pre-transfer lamp, the transfer means,
the cleaning means and so on can be dispensed with. It is true but the pre-transfer
lamp 10 and the charge eliminating means 15 can be omitted in the case of the transfer,
and this transfer may be conducted not only the corona transfer one but also a bias
roller one, an adhesion transfer, a direct pressure one or on using an intermediate
transfer member, whereas the fixture should not be limited to the heat roller.
[0112] On the other hand, it is a great advantage that the polarities of the potentials
of the latent images of the image portion and the non-image portion can be reversed
by the balance in the strength between the primary and secondary charging operations.
Even if the polarities are common, however, the development can naturally be conducted
by changing the developing bias conditions. As to the potentials at this time, the
relationship, in which the zero potential in Figs. 12 to 17 is shifted up or down,
holds as it is. A similar development can be conducted if the developing bias is accordingly
changed.
[0113] Figs. 18 to 21 show other embodiments of the method of the present invention. Incidentally,
Figs. 18 to 21 all show the steps after the second development has already been conducted.
[0114] Fig. 18 shows the embodiment of the reproducing method of the present invention,
comprising the steps of: uniformly subjecting the surface of the image retainer 1
in the initial state, which has had its charges eliminated by the charge eliminating
means 13 of the reproducing apparatus of Fig. 1 and has been cleaned by the cleaning
means 14 to have a zero potential, to a first charging operation by means of the charger
2; subjecting the charged surface to a first image exposure of different color by
the image exposing means 4, as shown in Fig. 2, to retain an electrostatic image in
which the potential of the exposed portion PH is dropped with respect to the background
potential of the unexposed portion DA; firstly developing the electrostatic image
by any one of the developing means 5 to 8, which uses as its developer the color toner
corresponding to the first image exposure, so that the potential of the electrostatic
image by the first image exposure is elevated up to the background potential as a
result that the color toner having been frictionally charged with the same polarity
as that of the charges of the image retainer 1 is applied; secondly charging the image
retainer 1 uniformly again at the second rotation by means of the charger 2 with or
without the charge elimination by the charge eliminating means 13, because that portion
cannot retain the electrostatic image even if it is subjected again to the image exposure;
conducting a second image exposure by the same image exposing means 4 as the previous
one with the same spot density as that of the previous image exposure and in a manner
that the positions are superposed at least partially; subsequently conducting a second
development by another development means using as its developer the color toner corresponding
to the second image exposure; subsequently repeating in a like manner third and fourth
charging operations and image exposures to retain a color image having a plurality
of color toner images superposed on the image retainer 1; operating one or both of
the pre-transfer charger 9 and the pre-transfer exposing lamp 10, if necessary, from
the step at which the fourth development is conducted to the step at which the color
image finishes its passage; transferring the color image by the transfer means 11
to the recording member P being fed in synchronism with the rotations of the image
retainer 1; fixing the color image transferred to the recording member P by the fixing
means 12; eliminating the charges from the surface of the image retainer 1, from which
the color image has been transferred, by the charge eliminating means 13; and cleaning
the image retainer 1 by the cleaning means 14 until the initial state is restored,
thus completing the one cycle of the color image reproduction. Thus, by conducting
the image exposure of 'each time such that the spot positions are superposed at least
partially, it is possible to prevent the color image retained from having its picture
element density dropped so that the color image can be reproduced densely and finely
in its colors. Incidentally, the retention of such an electrostatic image by the second
image exposure at a position different from that having previously trapped the toner
T as has a lower potential at its exposed portion PH than the background potential
at its unexposed portion DA is intended to show the case in which the color image
has in that particular position none of the picture elements of the color of the toner
T. Indicated at reference letter T' is the toner which has been applied at the second
development. Although Fig. 18 shows the case in which the spot positions of the image
exposures of the respective times are completely superposed, the spot positions may
be partially superposed. In the method of the present invention, moreover, since the
retentions of the electrostatic images at the respective times can be conducted by
the common apparatus, as has been described hereinbefore, the reproducing apparatus
can be constructed in a small size and at a low cost, and the synchronous control
of the image exposures can be easily conducted.
[0115] Fig. 19 shows the embodiment of the present invention, which is the same as the embodiment
of Fig. 18 until the first development but different therefrom in that, after the
first development, the surface of the image retainer 1 is either secondly charged
by the charger, after it has been uniformly exposed by the pre-transfer exposing lamp
10 or the charge eliminating lamp of the charge eliminating means 13, or uniformly
but weakly exposed by means of the exposing lamp 3 after it has been secondly charged
by the charger 2, so that the electrostatic image retaining portion having trapped
the previous toner T is made to have such a slightly higher potential than that of
the background portion that it is liable to retain the electrostatic image, whereupon,
like the embodiment of Fig. 18, the second image exposure and the second development
are executed, followed by repeating the third and fourth image exposures and developments
in a similar manner to retain the color image. The embodiment of Fig. 19 is suitable
in case it is undesirable to apply the subsequent toner T' to the previous toner T.
Generally speaking, more specifically, most of the images are required to have a reproductivity
of black letters. According to the method being described, the light is not passed
fully at the subsequent writing and the color toner to be subsequently applied in
a superposed manner can be prevented from being applied by firstly conducting the
writing operation of a black letter portion so that the vagueness of colors and the
shift of positions can be prevented. Thus, it is possible to obtain an image in which
a preferential color is stressed. In combination with the embodiment of Fig. 18, on
the other hand, an image of a selected color can be stressed or weakened. It is quite
natural that the charge eliminating step and the charging step can be introduced after
the first development and after the subsequent process is entered.
[0116] Figs. 20 and 21 show the embodiments of the present invention, which are improved
in the problem of mixing by applying the subsequent toner T' in a manner to surround
the previous toner T, because the mixing state of a color toner is changed in dependence
upon the fixing method or the . color superposing order to change the color balance
if the subsequent toner T' is superposed upon the previous toner T. The embodiments
of Figs. 20 and 21 correspond to those of Figs. 18 and 19, respectively, but are different
therefrom in that the spot of the second image exposing ray, i.e., the subsequent
image exposing ray is made so large by the image exposing means 4, i.e., by means
of the lens 27 of the laser beam scanner of Fig. 2 as to enclose the spot of the first
image exposing ray, i.e., the previous image exposing ray. Incidentally, the boundaries
between the unexposed portion DA and the exposed portion PH in the second image exposure
are shown in two ways, i.e., the upper portion shows the spot of the second image
exposing ray, and the lower portion shows the exposing area shield by the center toner
image. In the embodiments of Figs. 20 and 21, too, the color image retained can be
prevented from having its picture element density dropped so that it can have its
colors retained densely and finely, and the problem of toner mixing is eliminated
so that a color image having clearer colors can be retained in the embodiments of
Figs. 20 and 21. Especially in the embodiment of Fig. 21, as compared with the embodiment
of Fig. 20, the portion, in which the previous spot and the subsequent spot are superposed,
does not trap or is reluctant to trap the subsequent toner T' on the previous toner
T thereby to prevent the toners T and T' from being mixed to exhibit a mixed color
at the portion having the spots superposed because the subsequent toner T' is applied
only around the previous spot, so that a beautiful blended color can be attained.
[0117] Incidentally, in the embodiments of Figs. 18 and 19, the toners are mixed so that
the color developing sequence exerts great influences upon the clearness of the color
image. It is, therefore, important to determine the sequence of the developments.
In the embodiments of Figs. 20 and 21, however, the importance of the developing order
is not so high as that of the embodiments of Figs. 18 and 19. Despite of this fact,
how the colors are arranged from the central portion to the outer side still influences
upon the tone of the color image.
[0118] The foregoing are the embodiments in which all the developments by the developing
means 5 to 8 are conducted by the developing method of applying the toner charged
with the same polarity as that of the background potential to the electrostatic image
corresponding to the exposed portion having a lower potential than that of the background
potential. According to the reproducing apparatus shown in Fig. 9, however, the developments
of the method of the present invention can be conducted by the developing method of
applying the toner charged with the polarity opposite to that of the electrostatic
image to the electrostatic image.
[0119] Fig. 22 shows the embodiment of the reproducing method of the present invention,
comprising the steps of: subjecting like the above embodiment the surface of the image
retainer 1, which has had its charges eliminated by the charge eliminating means 13
of the reproducing apparatus of Fig. 9 and has been cleaned by the cleaning means
14 to have a zero potential, to a primary charging operation by means of the charger
2; subjecting the surface to a secondary charging operation by means of the secondary
charger 3'; subsequently subjecting the charged surface to a first image exposure
by the image exposing means 4 like that of Fig. 2; firstly developing the electrostatic
image, in which the absolute value of the potential of the exposed portion PH retained
is higher than the background potential of the unexposed portion DA, by that one of
the developing means 5 to 8, which uses as its developer the toner for effecting the
changing operation with a polarity opposite to that of the electrostatic image of
the color corresponding to the first image exposure, so that the surface potential
of the electrostatic image retaining portion is dropped by the toner having the opposite
polarity applied; discharging again the surface of the image retainer 1 before the
image exposure at the second rotation by means of the second charger 3' to retain
the electrostatic image in the previously developed electrostatic image retaining
portion, too; conducting again a second image exposure by the same image exposing
means 4 as the previous one with the same spot density as that of the previous image
exposure and in a manner that the positions are superposed at least partially; subsequently
conducting a development by another developing means using as its developer the color
toner corresponding to the second image exposure having the same charging characteristics
as those of the first development; subsequently repeating in a like manner third and
fourth electrostatic image retentions and developments, thus completing the one cycle
of the color image reproduction like the embodiments of Figs. 18 to 21 after the color
image has been retained. According to this embodiment, all the developments are conducted
by applying the toners for charging the electrostatic images with an opposite polarity
so that the control of the developing densities of the respective colors is easier
than the embodiments of Figs. 18 to 21.
[0120] Fig. 23 shows an embodiment of the present invention, which is different from the
embodiment of Fig. 22 in that not only the secondary charging operation by the secondary
charger 3' is conducted but also the primary charging operation is conducted beforehand
by the primary charger 2 during the time period between the first development and
the second image exposure, and in that the charge elimination is also conducted by
the charge eliminating means 13 prior to the primary charging operation. According
to the embodiment of Fig. 23, it becomes possible to apply the toner T' in the same
density to the toner T having been previously applied.
[0121] Figs. 24 and 25 show embodiments of the method of the present invention, which are
different from the embodiments of Figs. 22 and 23 in that the second image exposure
is conducted in a manner to change the spot diameter like the embodiments of Figs.
20 and 21. According to the embodiments of Figs. 24 and 25, there can be attained
a result that the vagueness due to the color mixing can be eliminated like the embodiments
of Figs. 10 and 21.
[0122] Next, the embodiments of Figs. 18 to 21 and Figs. 22 to 25 thus far described will
be described in more detail in the following as Examples 11 to 18, respectively.
Example 11
[0123] The reproducing apparatus of Fig. 1 was used. However, the exposing lamp 3 was not
used, but the image retainer 1 had a photosensitive surface layer of Se and a circumferential
speed of 180 mm/sec. This image retainer 1 had its surface charged to + 500 V by means
of the charger 2 using the scorotron corona discharger, and the charged surface was
subjected to a first image exposure in a density of 16 spots/mm by means of the laser
beam scanner of Fig. 2 using the He-Ne laser.
[0124] As a result, there was retained in the image retainer 1 an electrostatic image which
had a background potential of + 500 V but an exposed portion potential of + 30 V.
The resultant electrostatic image was subjected to a first development by the developing
means 5 shown in Fig. 3.
[0125] The developing means 5 used the developer, which was composed of: a carrier prepared
by dispersing and containing 50 wt. % of magnetite in a resin to have an average particle
diameter of 30 am, a magnetization of 30 emu/g and a resistivity of 10140 cm or higher;
and a non-magnetic toner prepared by adding 10 wt. parts of a benzine derivative as
the yellow pigment and another charge controlling agent to the styrene-acryl resin
to have an average particle diameter of 10 u.m, under a condition that the ratio of
the toner to the carrier was 20 wt. %. Moreover, the non-contact jumping developing
condition was resorted to, under which the developing sleeve 31 had an external diameter
of 30 mm and a number of revolutions of 100 r.p.m., under which the magnet 32 has
its N and S magnetic poles of a magnetic flux density of 1,000 gausses and a number
of revolutions of 1,000 r.p.m., under which the layer of the developer in the developed
region had a thickness of 0.7 mm, under which the gap between the developing sleeve
31 and the image retainer 1 was 0.8 mm, and under which a superposed voltage containing
a d.c. voltage of + 400 V and an a.c. voltage of 1.5 kHz and 1,000 V was applied to
the developing sleeve 31.
[0126] While the developing image was being developed by the developing means 5, the remaining
developing means 6 to 8, as shown in Fig. 3, were held in their undeveloping state.
This was achieved by disconnecting the developing sleeve 31 from the power supply
39 into its floating state, by grounding the same to the earth, or by positively applying
the d.c. bias voltage, which had the polarity opposite to that of the charge of the
image retainer 1 i.e., the opposite polarity to the charge of the toner, to the developing
sleeve 31. Of these, it is preferred to apply the d.c. bias voltage. Since the developing
means 6 to 8 were made to conduct their developments under the non-contact jumping
developing condition like the developing means 5 it was not necessary to especially
eliminate the layer of the developer from the developing sleeve 31. Of those developing
means 6 to 8: the developing means 6 used a developer which was prepared by replacing
the toner of the developer of the developing means 5 by a toner containing polytungstate
as the Magenta pigment in place of the yellow pigment; the developer 7 used a developer
which was prepared by replacing the same toner by a toner containing copper phthalocyanine
as the cyan pigment; and the developing means 8 used a developer which was prepared
by replacing the same toner by a toner containing carbon black as the black pigment,
It is quite natural that a toner containing other pigment and dye could be used as
the color toner, and that, as has been touched hereinbefore, the sequence of the colors
to be developed and accordingly the sequence of the developing means could be suitably
selected.
[0127] The surface of the image retainer 1 thus having been subjected to the first development
was subjected, after it had been secondly charged with + 500 V at the second rotation
by means of the charger 2 while the pre-transfer exposing lamp 10 being operated but
the charge eliminating means 13 and the cleaning means 14 being left inoperative,
to a second image exposure again in the superposed spot positions and in the same
spot density by means of the same laser beam scanner and then to a second development
using the Magenta toner by the developing means 6. Likewise, a third development using
the cyan toner by the developing means 7 and a fourth development using the black
toner by the development means 8 were repeated. In each of the developments, incidentally,
the developing density of each color can be adjusted in accordance with the changes
of the surface potential of the image retainer 1, the developing characteristics,
the color reproductivity and so on by changing the d.c. bias component and the amplitude
and frequency of the voltage to be applied to the developing sleeve 31, and the selecting
time of the time selecting conversion.
[0128] After the fourth development was conducted so that the four-color image was retained
on the image retainer 1, it was made liable to be transferred by the pre-transfer
charger 9 and the pre-transfer exposing lamp 10 so that it was transferred to the
recording member P by the transfer means 11 until it was fixed by the fixing means
12. The image retainer 1 having the color image transferred thereto had its charges
eliminated by the charge eliminating means 13 and its surface cleared of the residual
toners by its abutment against the cleaning blade or fur brush of the cleaning means
14. The one cycle of the color image reproduction was completely ended at the time
when the surface having retained the color image therein passes over the cleaning
means 14.
[0129] The color image thus reproduced had the vagueness in color due to the color mixing
but had a high density of spot picture elements and a finely expressed pattern.
Example 12
[0130] The same reproducing apparatus of Fig. 1 as that of the Example 11 was used. In this
case, however, the apparatus is equipped with the exposing lamp 3. And, a first development
was conducted under absolutely the same conditions as those of the Example 11 except
that a superposed voltage of a d.c. voltage of + 40 V and an a.c. voltage of 2 kHz
and 1,000 V was applied in the development to the developing sleeve 31. Next, at the
second rotation, the surface of the image retainer 1 having been subjected to the
first development was secondly charged with + 600 V by means of the charging means
2 and was then subjected to a uniformly and weak exposure by the exposing means 3
to take a surface potential of + 500 V. As a result, the surface potential of the
portion having trapped the toner T by the first development came into a slightly higher
state than + 500 V. Therefore, this image retainer 1 had its surface subjected to
a second image exposure and a second development like the Example 11. The exposure,
uniform and weak exposure and development described above were repeated thirdly and
fourthly thereby to conduct the color image reproduction like the Example 11.
[0131] The color image thus reproduced was finely expressed not differently of that of the
Example 11 except that the tone of the mixed-color portion is stressed slightly better
in its previously color than that of the Example 11.
[0132] Incidentally, in this Example, too, similar effects can be attained even if the charging
operation is conducted by means of the charger 2 after a uniform exposure using the
pre-transfer exposing lamp 10 or the exposing lamp of the charge eliminating means
13 in place of the charging operation and the uniform and weak exposure.
Example 13
[0133] The same reproducing apparatus as that of the Example 11 was used. The reproduction
of a color image was conducted absolutely similarly to the Example 11 except that
a first image exposure using a spot having a diameter of 20 um, a second image exposure
using a spot having a diameter of 30 am, a third image exposure using a spot having
a diameter of 40 u.m, and a fourth image exposure using a spot having a diameter of
50 u.m in the same spot position and in the same density of 16 spots/mm were conducted
by the switching operation of the lens 27 of the laser beam scanner thereby to retain
an electrostatic image having a potential of + 50 V with respect to the background
potential of + 600 V, that a superposed voltage of a d.c. voltage of + 450 V and an
a.c. voltage of 1.5 kHz and 1,000 V was applied for the development to the developing
sleeve 31, and that the colors were superposed in the order of the black, cyan, red
and yellow toners.
[0134] The color image thus reproduced was substantially cleared of any vagueness by the
color mixing so that it has a fine and clear tone.
[0135] Incidentally, in this Example, an identical color image could be attained even if
the charging operation between the first development and the second image exposure,
i.e., the charging operation between the previous development and the subsequent image
exposure was omitted.
Example 14
[0136] The same reproducing apparatus as that of the Example 12 was used. The reproduction
of a color image was conducted under absolutely the same conditions as those of the
Example 12 except that the image exposures were conducted in the same manner as the
Example 13 to retain the same electrostatic image, and that a superposed voltage of
a d.c. voltage of + 450 V and an a.c. voltage of 2 kHz and 500 V was applied for the
development to the developing sleeve 31.
[0137] The color image thus reproduced had a fine and clear color tone which was hardly
different from that of the Example 13.
Example 15
[0138] The reproducing apparatus of Fig. 9 was used. The image retainer 1 was prepared by
laying a transparent insulating surface layer having a thickness of 20 µm on a photosensitive
layer of CdS having a thickness of 30 um, and had a circumferential speed of 180 m/sec.
The image retainer 1 thus prepared was primarily charged to have a surface potential
of + 1,000 V by means of the d.c. scorotron corona discharger while being uniformly
exposed by the exposing lamp of the primary charger 2. Next, the image retainer 1
was charged to have a surface potential of - 100 V by means of the secondary charger
3' which is constructed of the scorotron corona discharger having an a.c. component.
The resultant charged surface was subjected to a first image exposure in a density
of 16 spots/mm by means of the laser beam scanner of Fig. 2 using the He-Ne laser
to retain an electrostatic image exhibiting a potential of + 200 V with respect to
the background potential of - 100 V. The resultant electrostatic image was firstly
developed by the developing means 5 under the same conditions as those of the Example
11 except that only an a.c. voltage component of 1.5 kHz and 1.000 V was applied to
the developing sleeve 31, and that the charging polarity of the toner was opposite
to that of the electrostatic image. In this Example, moreover, a second image exposure
and a second development were conducted like the Example 11 after a secondary charging
operation by the secondary charger 3' was conducted again at the second rotation of
the image retainer 1. Likewise, a secondary charging operation, an image exposure
and a development were subsequently repeated thirdly and fourthly, and reproduction
of the color image was then conducted like the Example 11.
[0139] Since, in this Example, the developments were effected by the coulomb attractive
force, the density adjustment of the color image reproduced could be conducted more
easily than the cases of the Examples 11 to 14, by which the toners for charging in
the same polarity were applied to the electrostatic image, so that the color image
obtained had the same color tone as that by the Example 11.
Example 16
[0140] The reproduction of a color image was conducted under the same conditions as those
of the Example 15 except that the image retainer 1 was prepared by placing a transparent
insulating surface layer having a thickness of 10 µm on an a-Si photosensitive layer
having a thickness of 10 u.m, that a primary charging operation was effected to +
700 V by means of the primary charger 2, that a secondary charging operation was effected
to 0 V by the secondary charger 3' to retain electrostatic images, the first one of
which had a potential of + 300 V and the second and later of which had a similar potential
with respect to the background potential of 0 V, in the image exposures by the laser
beam scanner, that the voltage to be applied to the developing sleeve 31 for the development
was a superposed one composed of a d.c. voltage of + 100 V and an a.c. one of 500
Hz and 400 V, and that a charge elimination by the charge eliminating means 13, the
primary charging operation by the primary charger 2, and the secondary charging operation
by the secondary charger 3' were conducted prior to the second and later image exposures.
[0141] In this Example, the color tone of the color image reproduced were similar to that
of the Example 11, and the densities of the respective colors could be better adjusted.
Example 17
[0142] The reproduction of the color image was conducted under absolutely the same conditions
as those of the Example 15 except that the image exposure by the laser beam scanner
was conducted, like the Example 13, firstly with a spot having a diameter of 20 µm,
secondly with a spot having a diameter of 30 am, thirdly with a spot having a diameter
of 40 um, and fourthly with a spot having a diameter of 50 um, in the same spot position
and in the same density of 16 spots/mm to retain an electrostatic image having a potential
of + 400 V with respect to the background potential of - 100 V, and that the colors
were superposed in the order of the black, cyan, red and yellow toners.
[0143] The color image thus reproduced had a color tone similar to that by the Example 13
but had a clearer tone.
[0144] In this Example, too, the density adjustments of the respective colors could naturally
be easily effected.
Example 18
[0145] The reproduction of the color image was conducted under absolutely the same conditions
of those of the Example 16 except that the image exposure by the laser beam scanner
used the same spot and spot density as those of the Example 17 to retain an electrostatic
image having a potential of + 300 V at each time with respect to the background potential
of 0 V.
[0146] The color image thus reproduced was substantially the same as that by the Example
17.
[0147] In this Example, too, the density adjustments of the respective colors could naturally
be easily effected.
[0148] According to the Examples of the present invention thus far described, it is possible
to make the spot densities fine thereby to reproduce a fine color image and to prevent
any color mixing thereby to reproduce a color image having a clear color tone. Since
the retentions of the electrostatic images are conducted by the common apparatus,
moreover, there can be attained excellent effects that the reproducing apparatus can
be constructed in a small size and at a low cost, and that the synchronous control
of the image exposure as to the image retainer is facilitated.
[0149] Incidentally, the present invention can be applied to the case, in which the image
retainer has a belt or sheet shape, or to such an image retainer, e.g., electrofax
paper as is placed on a base as can fix without any transfer the color image retained
thereon by the toners. In this case, it is highly necessary to consider the superposing
sequence of the color toners, but there arises an advantage that the pre-transfer
lamp, the transfer means and the cleaning means can be dispensed with. Despite of
this fact, the pre-transfer lamp and the charge eliminating means can be omitted in
case the toners have predetermined polarities and quantities of charges so that they
can be transferred. On the other hand, the transfer may be not only the corona type
but also a bias roller type, an adhesion type and a pressure type through an intermediate
transfer member. It is quite natural that the fixing operation should not be limited
to a heat roller type.
[0150] The methods of the Examples 15 to 18 according to the present invention are highly
advantageous in that the polarities of the potentials at the image portion and the
non-image portion can be reversed by the balance between the strengths of the primary
and secondary charging operations. However, the development can be effected even by
using the same polarities and by changing the developing bias conditions. As to the
potentials at this time, there holds as it is the relationship in which the zero potential
of Figs. 21 to 23 is shifted up and down. If the developing bias is accordingly changed,
a like development can be made. Moreover, those methods can naturally be applied even
to the NP-or KIP-method.
[0151] Figs. 26 and 27 are schematic views showing the constructions of the embodiments
of the reproducing apparatus which are used for the method of the present invention,
respectively. Figs. 28 to 30 are flow charts for the method of the present invention,
respectively.
[0152] In Fig. 26, reference numeral 41 indicates a drum-shaped image retainer which is
constructed by laying a dielectric layer such as a resin on a metal base and which
is made rotatable in the direction of arrow, and numeral 43 indicates an electrostatic
recording head which is equipped with needle discharge poles. The remaining portions
are identical to those of the example of Fig. 1.
[0153] The pre-transfer charger 9 may be omitted in case the transfer can be sufficiently
effected merely by the transfer means 11. The electrostatic recording head 43 is used
to form an electrostatic image having a charged spot distribution on the dielectric
layer of the image retainer 41 by means of the needle discharge poles which are arrayed
in one or plural rows.
[0154] Of the toners: the black toner is similar to that of the two-component developer
of the prior art; the cyan toner is prepared by adding copper phthalocyanine in place
of carbon black having a black color; the Magenta is prepared by similarly adding
polytungstophosphate; and the yellow toner is prepared by similarly adding a benzidine
derivative. However, those toners should not be limited to those color toners made
of such pigments, but it is naturally possible to use color toners made of dyes and
to add an electrification controlling agent or the like, if necessary. On the other
hand, the sequence of the colors to be developed by the developing means 5 to 8 using
the developers of different color toners has to be carefully determined because it
exerts influences upon the tone of the color image.
[0155] The method of the present invention can be practised by the reproducing apparatus
of Fig. 26 described above but can also be carried out by the reproducing apparatus
shown in Fig. 27.
[0156] The reproducing apparatus of Fig. 27 is one in which a series of recording members
are formed with dielectric layers on their surfaces to provide an image retainer 41'.
The retentions and developments of electrostatic images are repeated while the image
retainer 31' is being linearly conveyed. Along the conveyor passage of the image retainer
41', the pre-writing charger 2, the electrostatic recording head 43 and the developing
means 5 to 8 are repeatedly arranged side by side, and the fixing means 12 for fixing
the color image to the image retainer 41' is disposed at the last position. The reproducing
ap paratus under consideration does not require the pre-transfer charger, the transfer
means, the charge eliminating means and the cleaning means but can reproduce a series
of color images. In order that the image retainer 41' may not depend, however, it
is necessary to strengthen the tension or to provide such a supporting roller midway
as to prevent the toners applied to the image retainer 41' from being offset, although
not shown.
[0157] In the reproducing apparatus shown in Fig. 26, too, the pre-transfer charger 9, the
transfer means 11, the charge eliminating means 13 and the cleaning means 14 can be
dispensed with if the image retainer 41 is prepared by rolling an image retainer similar
to the image retainer 41', which is used in the reproducing apparatus of Fig. 27,
on a drum.
[0158] The method of the present invention, as is exemplified by the embodiments of Figs.
28 to 30, can be practised by the reproducing apparatus thus far described. Incidentally,
Figs. 28 to 30 all show the steps after a second development has been finished.
[0159] The embodiment of Fig. 28 shows the method of the present invention, comprising the
steps of: subjecting the surface of the image retainer 41 to a first writing operation
by means of the electrostatic recording head 43, either from the initial state (which
is shown to be a charged state), in which the surface of the image retainer 41 has
its charges eliminated by one or both of the charge eliminating means 13 and 13, cleaned
by the cleaning means 14 and charged to be positive or negative by the pre-writing
charger 2, if necessary, according to the reproducing apparatus of Fig, 26 or from
the initial state, in which the image retainer 41' is conveyed from the left and charged
to be negative or positive by the first pre-writing charger 2, if necessary, according
to the reproducing apparatus of Fig. 27, thereby to retain an electrostatic image
at a potential having a polarity different from that of the background potential;
firstly developing that electrostatic image by the developing means 5; conducting
a second writing operation by the electrostatic recording head 43 after the uniform
charging operation by the charger 2, if necessary, either when the image retainer
41 comes into its second rotation, according to the reproducing apparatus of Fig.
26, or when the image retainer 41' advances to the position of the next charger, according
to the reproducing apparatus of Fig. 27; secondly developing the electrostatic image
thus retained by the developing means 6; subsequtnely repeating third and fourth writing
and developing operations in a similar manner so that a color image having superposed
color images is retained on the image retainer 41 or 41 and either fixing the resultant
color image to the recording member P by means of the fixing means 12, after the color
image has been made reluctant to be transferred by the pre-transfer charger 9 so that
it is transferred to the recording member P by the transfer means 11, according to
the reproducing apparatus of Fig. 26, or directly fixing the same color image to the
image retainer 41' by the fixing means 12 according to the reproducing apparatus of
Fig. 27. According to the reproducing apparatus of Fig. 26, moreover, the surface
of the image retainer 41 thus having the color image transferred thereto has its charges
eliminated by the charge eliminating means 13, and cleared of the residual toners
by the cleaning means 14, and further has its charges eliminated, if necessary, by
the charge eliminating means 13, thus ending one cycle of the color image reproduction.
According to the reproducing apparatus of Fig. 27, on the other hand, the portion
of the image retainer 41', which has been formed with the color image, ends its steps
of reproducing the color image when it completely passes the fixing means 12.
[0160] Moreover, the embodiment of Fig. 29 uses the reproducing apparatus of Fig. 26 and
is similar to that of Fig. 28 except that the image retainer 41 having the toner images
retained thereon has its charges eliminated by the charge eliminating means 13 before
a subsequent image retaining stage is entered after each development.
[0161] The embodiment of Fig. 30 resorts to the reproducing apparatus of Fig. 26 and is
different from that of Fig. 28 in that the pre-writing charger 2 is operated before
each writing operation.
[0162] Incidentally, reference letters T and T' appearing in Figs. 28 to 30 indicate toners
of different colors, which are applied to the image retainer 41 or 41'.
[0163] In the method of the present invention, the developing means other than that conducting
each development under the non-contact jumping developing conditions can be easily
held in an inoperative state, even if the developer layer is not removed from the
developing sleeve 31, by disconnection of the developing sleeve 31 from the power
supply 39 into a floating state, by grounding the developing sleeve 31 to the earth,
or positively applying such a d.c. bias voltage to the developing sleeve 31 as has
a polarity identical to that of the electrostatic image, i.e., opposite to that of
the charges of the toners. Of these, the ap plication of the bias voltage having the
opposite polarity to that of the toners may be preferably used to hold the developing
means in the inoperative state.
[0164] Next, the embodiment of Figs. 28 to 30, which are practised by the reproducing apparatus
of Fig. 26, will be described in more detail as the following Examples 19 to 21, respectively.
Example 19
[0165] The reproducing apparatus shown in Fig. 26 was used. The image retainer 41 was prepared
by laying an insulating layer having a thickness of 20 j.J.m on an aluminum base and
had a circumferential speed of 180 mm/sec. The image retainer 41 thus prepared had
its surface charged to - 100 V by means of the pre-writing charger 2 using the scorotron
corona discharger and then subjected a writing operation in a distribution density
of 10 spots/mm by means of the electrostatic recording head 43 the needle electrodes
of which had their tips spaced by about 30 j.J.m from the surface of the image retainer
41. As a result, there was retained on the image retainer 41 an electrostatic image
which had a written portion potential of + 200 V with respect to the background portion
potential of - 100 V. The resultant electrostatic image was firstly developed by the
developing means 6 shown in Fig. 3. This develop ing means 6 used the developer, which
was composed of: a carrier having 50 wt. % of magnetite dispersed and contained in
a resin and having an average particle diameter of 20 um, a magnetization of 30 emu/g
and a resistivity of 10
140 or higher; and a nonmagnetic toner prepared by adding 10 wt. % of copper phthalocyanine
and another electrification control agent as the cyan pigment to the styrene-acryl
resin and which had an average particle diameter of 10 am, under the condition of
the ratio of 10 wt. % of the toner to the carrier. Moreover, the non-contact jumping
developing conditions were resorted to under which the developing sleeve 31 had an
external diameter of 30 mm and a number of revolutions of 100 r.p.m., under which
the magnet 32 had a magnetic flux density of its N and S magnetic poles of 1,000 gausses
and a number of revolutions of 1,000 r.p.m., under which the developer layer had a
thickness of 0.7 mm at its developed portion, under which the gap between the developing
sleeve 31 and the image retainer 1 was 0.8 mm, and under which a bias voltage having
a d.c. voltage component of 0 V and an a.c. voltage component of 1.5 kHz and 1,000
V was applied to the developing sleeve.
[0166] The surface of the image retainer 41 having been firstly developed was subjected
to such a second writing operation with a spot position shift from the first writing
operation but in the same spot density again by the same electrostatic recording head
43 but without operating the pre-transfer charger 9, the charge eliminating means
13 and 13, the cleaning means 14 and the pre-writing charger 2 that the written portion
took a potential of + 300 V. Next, a second development was conducted by the developing
means 6 under the same conditions as those of the developing means 5 except that the
toner of the developer used one which was prepared by adding polytungstophosphate
as the Magenta pigment in place of the cyan pigment, and that a bias voltage having
a d.c. voltage component of 100 V and an a.c. voltage component of 1.5 kHz and 1,000
V was applied. Likewise, a third writing operation for elevating the potential of
the written portion to + 400 V and a third development were conducted by the developing
means 7 under the same conditions as those of the developing means 5 except that the
toner of the developer used one which was prepared by adding a benzidine derivative
as the yellow pigment, and that the developing bias was composed of a d.c. component
of 200 V and an a.c. component of 1.5 kHz and 1,000 V. Moreover, a fourth writing
operation for raising the potential of the written portion to + 500 V and a fourth
development were conducted by the developing means 8 under the same conditions as
those of the developing means 5 except that the toner of the developer used one which
was prepared by adding carbon black as the black pigment, and that the developing
bias had a d.c. component of 300 V and an a.c. component of 1.5 kHz and 600 V. The
color image thus retained on the image retainer 41 was transferred to and fixed on
the recording member P. Moreover, the surface of the iamge retainer 41 thus having
the color image transferred thereto had its charges eliminated by the charge eliminating
means 13 and cleared of the residual toners by the cleaning means 14.
[0167] The reproduced image thus obtained had little mixing of the color toners and was
a remarkably clear color image.
[0168] Incidentally, in this Example, the spot position of the subsequent writing operation
may be superposed of that of the previous writing operation, or, the discharge voltage
of the electrostatic recording head 43, and the voltage value, frequency and selected
time of the d.c. or a.c. component of the voltage to be applied to the developing
sleeve may be so changed in the writing and/or developing operations as to adjust
the developed densities of the respective colors. If the spot positions of the writing
operations are superposed, the color mixing occurs to make the colors liable to be
vague. However, the tone can be enhanced by increasing the spot density. In this case,
moreover, especially the sequence of colors to be developed plays an important role.
By adjusting the developed densities of the respective colors in the manner thus far
described, moreover, it is possible to obtain a color image which has a changed tone.
Example 20
[0169] The color image reproduction was conducted by the use of the same reproducing apparatus
as that of the Example 19 and under the same conditions as those of the Example 19
except that the charging operation of the Example 19 by the pre-writing charger 2
prior to the first writing operation was not conducted to form an electrostatic image
having a potential of + 150 V with respect to the background potential of 0 V by a
first writing operation, that a superposed voltage having a d.c. voltage of + 50 V
and an a.c. voltage of 3 kHz and 2,000 V was applied as the bias voltage upon the
development to the developing sleeve 31, and that charge elimination was conducted
before second and later writing operations by the charge eliminating means 13 to retain
an electrostatic image having a potential of + 150 V with respect to the background
potential of 0 V even in the second and later writing operations. The reproduced image
thus obtained was a color image having an excellent clearness like that of the Example
19.
Example 21
[0170] The color image reproduction was conducted by the use of the same reproducing apparatus
as that of the Example 19 under the same conditions of those of the Example 19 except
that the charge of - 300 V was conducted by the pre-writing charger 2 so that an electrostatic
image having a potential of + 50 V with respect to the background potential of - 300
V was retained by a first writing operation, that a superposed voltage composed of
a d.c. voltage of - 200 V and an a.c. voltage of 2 kHz and 1 kV was applied as a bias
for the development to the developing sleeve 31, and that the pre-writing charger
2 was used before second and later writing operations. The reproduced image obtained
was a color image having an excellent clearness like that of the Example 19.
[0171] By using the image retaining means having its electrostatic retainability and toner
image- retainability separated, according to the foregoing Examples of the present
invention, there can be attained ex cellent effects that the color tone and so on
of the color image can be easily changed, and that the color image having the excellent
clearness and a high tone can be reproduced so that the reproduction can be stably
effected.
[0172] Incidentally, the present invention can be applied to the case, in which the image
retainer has a belt or sheet shape, or to such an image retainer, e.g., electrofax
paper as is placed on a base as can fix without any transfer the color image retained
thereon by the toners. In this case, it is highly necessary to consider the superposed
order of the color toners, but there arises an advantage that the pre-transfer lamp,
the transfer means and the cleaning means can be dispensed with. Despite of this fact,
the pre-transfer lamp and the charge eliminating means can be omitted in case the
toners have predetermined polarities and quantities of charges so that they can be
transferred. On the other hand, the transfer may be not only the corona type but also
a bias roller type, an adhesion type and a pressure type through an intermediate transfer
member. It is quite natural that the fixing operation should not be limited to a heat
roller type.
[0173] Although the Examples of the present invention thus far described used the electrostatic
recording head as the writing means, moreover, other means can be similarly used if
it can retain the electrostatic charge image on the dielectric layer. More specifically,
there can be likewise applied either a method in which the passage rate of a corona
ion flow is controlled by control electrodes so that an electrostatic image may be
retained on the dielectric layer, or a method in which a screen photosensitive member
is used so that the electrostatic charge pattern retained thereon may be used for
controlling the passage rate of the corona ion flow to retain the electrostatic image
on the dielectric layer.
[0174] Other Examples of the present invention will be described in the following. In the
method of consecutively superposing toner images by repeating the step of retaining
a latent image on an image carrier and the step of developing the retained latent
image, as has been described as the prior art, a development in a suitable density
has to be conducted without disturbing the toner image which was retained in the image
carrier at the previous step. Here, the term "superposition" means not only that the
toner images are formed plural times in an identical position of the developing regions
of the image carrier but also that the toner images are retained in plural times in
another portion of the image region. The result of our investigations has revealed
that an excellent image cannot be obtained even if the values such as the gap d (mm)
(which may be simply called the "gap" in the following) between the image carrier
and a developer carrier and the voltage V
AC and frequency f of the a.c. component of the developing bias are satisfied so as
to satisfy the above-specified conditions, and that those parameters have close relationships
to each other. Therefore, experiments have been conducted by the developing means
16, as shown in Fig. 31, with the parameters such as the voltage and frequency of
the a.c. component of the developing bias being changed, so that the results, as shown
in Figs. 32 and 33, have been obtained. Incidentally, the toner image is previously
formed on the photosensitive drum 1 acting as the image carrier drum. The developing
means 16 carries a developer D in the direction of arrow B on the circumference of
the sleeve 31 to supply the developer D to a developing region E as a result that
the sleeve 31 acting as the developer carrier and the magnetic roll 32 are rotated.
Incidentally, the developer D is a two-component developer composed of a magnetic
carrier and a non-magnetic toner. Said carrier is composed of ball-shaped particles
which have an average particle diameter of 30 u.m (which is a weight-averaged value
measured by means of the Omnicon Alpha (manufactured by Bausch & Lomb Inc.) or the
Caulter Counter (manufactured by Caulter Inc.), a magnetization of 50 emu/g and a
resistivity of 10
,40 or more and which are coated with a resin. Incidentally, the resistivity is a value
which is obtained by reading out a current value when a load of 1 kgicm
2 is applied to the tapped particles so that the carrier particles have a thickness
of 1 mm after the particles have been tapped in a container having an effective sectional
area of 0.50 cm
2 and when a voltage for establishing an electric field of 1,000 V/cm is applied between
the load and the bottom electrodes. Said toner is prepared by adding a small quantity
of an electrification controlling agent to 90 wt. % of a thermoplastic resin and 10
wt. % of a pigment (e.g., Carbon Black) and by blending and pulverizing the mixture
so that the particles may have an average particle diameter of 10 µm. The developer
D is carried in the direction of arrow B by rotating the magnetic roll 32 in the direction
of arrow A and the sleeve 31 in the direction of the arrow B. The developer D has
its thickness regulated in its carrying course by means of the head regulating blade
33. A developer reservoir 47 is equipped therein with an agitating screw 35 so that
the developer D may be sufficiently agitated. When the developer D in the developer
reservoir 47 is consumed, its supply is made from the toner hopper 37 by rotating
the toner supply roller 38.
[0175] Between the sleeve 31 and the photosensitive drum 1, moreover, there is connected
a d.c. power supply 45 for applying the developing bias. In order that the developer
D may be vibrated in the developing region E to be sufficiently supplied to the photosensitive
drum 1, an a.c. power supply 46 is connected in series with the d.c. power supply
45. Reference numeral 40 is the protecting resistor.
[0176] Fig. 32 shows the relationship between the amplitude of the a.c. component, when
the gap d between the photosensitive drum 1 and the sleeve 31 is set at 1.0 mm; the
thickness of the developer at 0.5 mm; when the charged potential of the photosensitive
drum at 600 V; and the developing bias has its d.c. component at 500 V and its a.c.
component at a frequency of 1 kHz, and the image density of a toner image which is
formed by the reverse phenomenon on the exposed portion (at a potential of 0 V) of
the photosensitive drum 1. The amplitude E
AC of the intensity of the a.c. electric field takes a value which is made by dividing
the a.c. voltage of the developing bias by the gap d. Curves A, B and C appearing
in Fig. 32 are the results obtained in case the toners used are controlled to have
average charges of 30 µc/g, 20 ac/g and 15 µc/g, respectively. It is observed from
the three curves A, B and C that the effect of the a.c. component appears for the
amplitude of the a.c. component of the electric field of 200 V/mm or larger, and that
the toner image retained in advance on the photosensitive drum is partially broken
for the amplitudes of 2,500 V/mm or larger.
[0177] Fig. 33 shows the changes in the image density when the frequency of the a.c. component
of the developing bias is set at 2.5 kHz and when the a.c. field intensity E
AC is changed under the same conditions of those of the experiments of Fig. 32.
[0178] According to these experiments, the image density is high when the amplitude E
AC of the a.c. field intensity exceeds 500 V/mm, and the toner image retained in advance
on the photosensitive drum 1 is partially broken when that amplitude exceeds 4 KV/mm,
although not shown.
[0179] Incidentally, as being seen from the results of Figs. 32 and 33, the image density
highly changes across a certain amplitude, which has a value obtainable hardly in
dependence upon the average- charges of the toners, as seen from the curves A, B and
C. The reason therefor can be thought, as follows. In the two-component developer,
specifically, it is predicted that the toners are charged by the friction with the
carrier or by the mutual frictions with one another, and that the charges of the toners
distribute over a wide range, and it is thought that toners having a large quantity
of charges are preferably developed. Even if the average charges are controlled by
the electrification controlling agent, the ratio occupied by those toners having the
large quantity of charges does not change so much. As a result, it is thought that
the changes in the developing characteristics are found more or less but not highly
observed.
[0180] Now, experiments similar to those of Figs. 32 and 33 were conducted under changing
conditions to pigeonhole the relationship between the amplitude E
Ac and frequency f of the a.c. field inten- tisy so that the results shown in Fig. 34
could be obtained.
[0181] In Fig. 34: indicated at is a region where a developing unevenness is liable to
occur; indicated at is a region where the effect of the a.c. component does not appear;
indicated at @ is a region where the toners are liable to return, i.e., where the
color mixing is liable to occur; and indicated at @ and @ are regions where the effect
of the a.c. component appears so that no color mixing occurs.
[0182] These results indicate that a proper region for the amplitude and frequency of the
intensity of the a.c. electric field exists so that a next (or subsequent) toner image
may be developed in a proper density without breaking the toner image which was retained
at the previous step on the photosensitive drum 1. This is thought to be explained
by the following reasons.
[0183] In the region where the image density has a tendency to increase for the amplitude
E
AC of the a.c. field intensity, e.g., for the density curve of Fig. 32, i.e., where
the amplitude of E
AC of the a.c. field intensity ranges from' 0.2 to 1.2 KV/mm, the a.c. component of
the developing bias acts to make it liable to jump a threshold value at which the
toners fly from the sleeve. As a result, even the toner having a small quantity of
charges is trapped by the photosensitive drum 1 so that it can be used for rhe development.
As a result, the image density is increased to the higher level as the amplitude of
the a.c. field intensity becomes the larger.
[0184] For the region where the image density is saturated for the amplitude E
AC, i.e., where the amplitude E
AC exceeds 1.2 KV/mm in the curve A of Fig. 32, this phenomena can be explained as follows.
In this region, more specifically, the toners are the more intensely vibrated as the
amplitude of the a.c. field intensity becomes the larger, and the cluster formed as
a result of the aggregation of the toners becomes liable to be broken so that only
the toners having high charges are selectively applied to the photosensitive drum
1 whereas the toner particles having low charges become reluctant to be developed.
Moreover, the toners having low charges are liable to be returned to the sleeve 31
by the a.c. bias because they have a weak image forming force even if they are once
trapped by the photosensitive drum 1. Since the charges on the surface of the photosensitive
drum 1 leak because the amplitude of the field intensity of the a.c. component, still
moreover, the phenomenon that the toners become reluctant to be developed become liable
to occur. As a matter of fact that, it is thought that those causes are overlapped
to make the image density constant for the increase in the a.c. component.
[0185] If the a.c. field intensity is raised to have an amplitude exceeding 2.5 KV/mm under
the condition of obtaining the curve A of Fig. 32, for example, it is found that the
toner image retained in advance on the photosensitive drum 1 is broken, and that the
degree of this breakage is the higher for the higher a.c. component. This is thought
to be caused by the fact that such a force is applied on the toners trapped by the
photosensitive drum 1 as to return to the sleeve 31 by the a.c. component.
[0186] In case the development is conducted by consecutively superposing toner images on
the photosensitive drum 1, it is a fatal problem that the toner image or images having
already been retained are broken at a subsequent developing step.
[0187] As is seen by comparing the results of Figs. 32 and 33, on the other hand, the experiments
conducted by changing the frequency of the a.c. component have revealed that the image
density becomes the lower for the higher frequency. This is caused by the fact that
the toner particles have their vibrating range narrowed, because they cannot follow
the changes in the electric field, so that they become reluctant to be trapped by
the photosensitive drum 1.
[0188] On the basis of the experimental results thus far described, the Inventors have attained
a conclusion that a later development can be conducted in a proper density without
disturbing the toner image already having been retained on the photosensitive drum
1, if each development is conducted under the conditions satisfying the following
relationship when the amplitude of the a.c. component of the developing bias is designated
at V
AC (V); the frequency of the same at f (Hz); and the gap between the photosensitive
sleeve 1 and the sleeve at d (mm).
0.2 ≤ VAc/(d * f); and
{(VAc/d) - 1500}/f ≦ 1.0.
In order to ensure a sufficient image density and not to disturb the toner image having
been retained by the previous step, it is preferable that the relationships of the
above-specified conditions be satisfied:
0.5 ≤ VAC/(d f); and
{(VAC/d) - 1500}/f ≦ 1.0.
If especially the following relationships of the above are satisfied, it is possible
to obtain a multi-color image having a better clearness but no color vagueness and
to prevent the toner of another color from being mixed into the developing apparatus
even with a number of operations:
0.5 ≤ VAC/(d f); and
{(VAC/d) - 1500}f ≦ 0.8.
[0189] Moreover, it is further preferable to set the frequency of the a.c. component at
200 Hz or higher so as to prevent the developing unevenness due to the a.c. component
and to set the frequency of the a.c. component at 500 Hz or higher so as to eliminate
the influences from the beats, which are caused by the a.c. component and by the rotations
of the magnetic roll in case the rotating magnetic roll is used as the means for supplying
the developer to the photosensitive drum 1.
[0190] According to the construction of the present invention thus far described, in order
to consecutively develop the subsequent toner images in predetermined densities on
the photosensitive drum without breaking the toner images retained on the photosensitive
drum 1, it is further preferable to use either solely or in suitable combination the
following methods in accordance with the repetitions of the developments:
(1) toners having consecutively higher -charges are used;
(2) the amplitude of the field intensities of the a.c., component of the developing
bias are made consecutively smaller; and
(3) the frequencies of the a.c. component of the developing bias are made consecutively
higher.
[0191] In other words, the toner particles having the higher charges are the more susceptible
to the influences of the electric field. As a result, the toner particles having high
charges may return to the sleeve at the step of the subsequent development if they
are trapped by the photosensitive drum 1 at an early development. Therefore, the method
(1) is intended to prevent the toners having low charges from returning to the sleeve
at a later development by using those toner particles at the early development. The
method (2) is intended to prevent the toner particles, which have already been trapped
by the photosensitive drum 1, from returning by making the field intensities consecutively
the smaller in accordance with the repetitions of the development (i.e., at the later
steps of developments). As the specific method of consecutively weakening the electric
field intensity, there is either a method of consecutively dropping the voltage of
the a.c. component or a method of making the larger the gap d between the photosensitive
drum 1 and the sleeve 31 at the later steps of developments. On the other hand, the
method (3) is intended to prevent the toner particles, which have already been trapped
by the photosensitive drum 1 from returning by raising the frequency of the a.c. component
consecutively to a higher level as the developments are repeated. Some effect can
be obtained if those methods (1 (2) and (3) are solely used, but a better effect can
be attained, if they are used in combination, for example by consecutively increasing
the toner charges in accordance with the repetitions of the developments with the
a.c. bias being consecutively dropped. In case those three methods are adopted, moreover,
proper image density and color balance can be held by adjusting the d.c. biases, respectively.
[0192] Other specific Examples practised by the use of the construction thus far described
will be explained in the following with reference to Figs. 35 and 37.
Example 22
[0193] Fig. 35 is a schematic view showing an essential portion of a color image reproducing
apparatus. The photosensitive drum 1 having been uniformly charged by means of the
scorotron charger was exposed to the ray, which had been guided from the He-Ne laser
light source (although not shown) through a rotary polygonal mirror 51 and a focusing
lens 52, to retain an electrostatic latent image. This electrostatic latent image
was developed by the first developing means 5 so that a first toner image was retained
on the photosensitive drum 1. And, this first toner image was charged again by the
scorotron charger 2 and exposed without being transferred to the recording paper so
that a second toner image was then retained by the second developing means 6. This
is repeated until a fourth toner image is retained. In other words, the steps of the
charging operation (the second and later ones of which are not always required) -
the exposure - the development were repeated four times in the form containing no
transfer step. After the toner images had been wholly retained on the photosensitive
drum 1, the pre-transfer exposing lamp 10 irradiated the region, in which the toner
image had been retained on the photosensitive drum 1, to transfer the toner image
to the recording paper (the path of which is indicated by a broken line), which was
fed from the paper feeder (although not shown) by the transfer means 11. The recording
paper was heated and fixed by the fixing means 12, which was composed of at least
one heated roller, until it was discharged to the outside of the machine.
[0194] On the other hand, the photosensitive drum 1 having ended its transferring operation
had its charges eliminated by the charge eliminating means 13, which had not been
used during the toner image retention, and was then cleared of the spare toners, which
had been left on the surface thereof, by the cleaning means 14 which had been left
inoperative during the toner image retention.
[0195] The color image reproducing apparatus thus far described were caused to repeat the
above operations each time its operation button was depressed. Incidentally, in the
present Example, the photosensitive material used was selenium, and the photosensitive
drum 1 had a diameter of 120 mm, a circumferential speed of 120 mm/sec and a charged
potential of 600 V. To the developing means 5 and 6 used, there was applied at each
developing time a developing bias which was composed of a d.c. component of 500 V
and an a.c. component having an amplitude of 1 KV and a frequency of 1 kHz. The gap
d between the photosensitive drum 1 and the sleeve of each of the developing means
was set at 0.8 mm. Moreover, the developer used was a two-component developer which
is composed of a magnetic carrier and a non-magnetic toner. As this carrier, there
was used a ball-shaped one which had an average particle size of 30 u.m, a magnetization
of 50 emu/g and a resistivity of 10
140 or more and which was coated with a resin. The toner was prepared by adding a small
quantity of an electrification controlling agent to 90 wt. % of a thermoplastic resin
and 10 wt. 5 of a pigment. In the developing means 5, 6, 7 and 8, respectively, there
were used the yellow, Magenta, cyan and black pigments, all of which had an average
quantity of charges of 20 u.c/g and an average particle diameter of 10 u.m. The developer
used was a mixture which was composed of 80 wt. % and 20 wt. % of the above-specified
carrier and toner, respectively. Moreover, at each developing time the sleeve 31 and
the magnetic roll 32 were rotated in each of the developing means in directions opposite
to each other and had their heads regulated by the magnetic blade so that the developer
layer had a thickness of 0.4 mm.
[0196] With the construction thus far described, as has been described above, the toner
images were consecutively superposed to form a multi-color image. As a result, a visible
image having a sufficient density was obtained with neither breaking the toner images,
which had already been retained on the photosensitive drum 1 at the subsequent development,
nor any toner of another color being mixed into each of the developing means.
[0197] The resultant superposed toner images were transferred to and fixed to the recording
paper so that a clearly reproduced image could also be attained. - Even after the
toner images had been reproduced on a number of sheets of the transfer paper, moreover,
none of other colors were not mixed into each of the developing means. Incidentally,
a small quantity of magnetic material was contained in the toner of each developing
means so that the fog of the image could be further prevented by the magnetic force.
Example 23
[0198] This Example was practised by the color image reproducing apparatus shown in Fig.
35, too. The difference from the Exmaple 22 was that both the gap d between the photosensitive
drum 1 and the sleeve and the d.c. component of the developing bias to be applied
at the developing time were different among the developing means. The gaps and the
d.c. components were set at 0.5 mm and 450 V, at 0.7 mm and 500 V, at 0.8 mm and 500
V, and at 1.0 mm and 550 V in the developing means 5, 6, 7 and 8, respectively. The
average quantities of the charges of the toners and the amplitude and frequency of
the a.c. biases were common among the developing means like the Example 22 and were
set at 20 acig, 1 KV and 1 kHz, respectively.
[0199] In the present Example, the return of the toners on the photosensitive drum 1 was
prevented by constructing the photosensitive drum 1 and the sleeves, of the respective
developing means such that the gaps d inbetween were widened the more in the developing
sequence, and the balance of the densities of the respective color toner images was
held by raising the d.c. biases in the developing order.
[0200] According to this Example, a clearer image was obtained, and another color was not
mixed into each of the developing means even after the reproductions of the multiple
sheets.
Example 24
[0201] This Example was practised by the color image reproducing apparatus shown in Fig.
35, too. The difference from the Example 22 was that the a.c. component and d.c. component
of the developing bias to be applied at the developing time were different among the
developing means. The amplitudes of the a.c. components and the d.c. components were
set at 1.5 KV and 450 V, at 1.2 KV and 500 V, at 1.0 KV and 520 V, and at 0.8 KV and
550 V in the developing means 5, 6, 7 and 8, respectively. The average quantities
of the toners, the frequencies of the a.c. biases, and the gaps between the photosensitive
drum 1 and the sleeve were common among the developing means like the Example 22 and
were set at 20 ac/g, 1 kHz and 0.8 mm, respectively.
[0202] In the present Example, the return of the toners on the photosensitive drum 1 was
prevented by setting the a.c. components at lower levels in the developing order,
and the balance of the densities of the respec tive color toner images was held by
consecutively raising the d.c. biases.
[0203] According to the present Example, a clear multicolored image could be obtained without
any mixing of another color into each developing means even after the reproducing
operations of the multiple sheets.
Example 25
[0204] This Example was also practised by the color image reproducing apparatus shown in
Fig. 35.
[0205] The developing conditions were such that the amplitudes of the a.c. components of
the developing bias applied at the developing time were all 1 KV for the respective
developing means, and such that the frequencies and the d.c. components of the same
were set at 800 Hz and 450 V, at 1 kHz and 500 V, at 1.5 kHz and 550 V, and at 2 kHz
and 600 V in the developing means 5, 6, 7 and 8, respectively.
[0206] In each developing means, moreover, at the developing time only the sleeve was rotated
to supply the developer whereas the internal magnets were fixed. The head height regulations
were conducted by the magnetic blade to provide a gap of 0.5 mm so that the developer
had a thickness of 0.2 mm.
[0207] The average quantities of the charges of the toners and the gaps between the photosensitive
drum 1 and the sleeve were common among the respective developing means and were set
at 20 µc/g and 0.8 mm, and the remaining developing conditions and developers were
the same as those of the Example 22.
[0208] In the present Example, the return of the toners on the photosensitive drum 1 was
prevented by increasing the frequencies of the a.c. components in the developing sequence,
and the balance of the densities of the respective color toner images was held by
consecutively raising the d.c. biases.
[0209] A clear multi-colored image could also be obtained by the present Example, and another
color was not mixed into each developing means even after the reproductions of multiple
sheets.
[0210] Fig. 36 is a flow chart showing the changes in the potential on the photosensitive
drum 1 when the developments are conducted by the color image reproducing apparatus
of Fig. 35. Reference letters PH and DA indicate the exposed portion and the unexposed
portion, respectively.
[0211] The photosensitive drum 1 holds a predetermined potential when it is charged by the
scorotron charger 2, and the portion having been optically irradiated has its potential
dropped when the image exposure is conducted. Next. by applying a bias, which has
its d.c. component substantially equal to the potential of that of the unexposed portion,
to the developing means, the toner charged positively in the developing means is trapped
by the exposed portion having a lower potential so that a development is conducted
to retain a first visible image. The potential at that particular portion rises a
little (as indicated at DUP in the drawing) as a result it traps the positive toners.
Next, the potential on the photosensitive drum 1 is so uniformly charged again by
the charger 2 that it is raised to a predetermined potential (as indicated at CUP
in the same drawing). Next, if a second image exposure is conducted and if a development
is similarly conducted, the toners are applied to the exposed portion to retain a
second visible image. By repeating these steps four times, four color visible images
are retained in' a superposed manner on the photosensitive drum 1.
[0212] In the methods thus far described, the second and later charging operations can be
omitted. In case these charging operations are not omitted, on the other hand, a charge
eliminating step may be inserted before each of the charging operations.
[0213] All of the three Examples described hereinbefore conduct the reversal developing
methods but can be practised by the normal developing method, i.e., the method in
which the toners are applied to the unexposed portion to retain toner images. In case
the superposed developments are conducted by the normal method, however, it is necessary
to introduce the charging step at each time.
Example 26
[0214] Next, the description to be made in the following is directed to the case in which
the developments were conducted by means of the color image reproducing apparatus
shown in Fig. 37.
[0215] The photosensitive drum 1 was made of a CdS photosensitive member which had its surface
covered with an insulating layer and had a diameter of 120 mm, a circumferential speed
of 120 mm/sec, an insulating layer thickness of 20 u.m and a photosensitive layer
thickness of 30 u.m.
[0216] First of all, the photosensitive drum 1 had its surface charged to + 1,000 V by means
of the primary charger 2 while being exposed all over its surface by the action of
a lamp L mounted in that charger 2. This exposure was conducted so as to facilitate
injection of charges into the photosensitive layer of the photosensitive drum 1. Next,
the surface of the photosensitive drum 1 was charged to - 100 V to reduce the positive
charges on the surface of its insulating lay by means of the secondary charger 3'
having an a.c. component. The photosensitive drum 1 thus charged to - 100 V was subjected
to an image exposure with a ray which was reflected from the rotary polygonal mirror
51. The portion thus exposed took a plus potential and was developed by the first
developing means 5 so that a first visible image was retained. Next, the photosensigive
drum 1 was uniformly charged again to - 100 V by the secondary charger 3' and was
then subjected to an image exposure so that a second visible image was retained by
the second developing means 6. These operations were repeated four times to retain
all the visible images on the photosensitive drum 1. After that, the pre-transfer
exposing lamp 10 irradiated the region, in which the visible images of the photosensitive
drum had been retained, and these visible images were transferred by the transfer
means 11 to the recording paper (the path of which is shown by the broken line), which
was fed from the paper feeder (although not shown). The recording paper was heated
and fixed by the fixing means 12, which was composed at least one heated roller, until
it was discharged to the outside of the machine.
[0217] On the other hand, the photosensitive drum 1 having its transferring operation completed
had its charges eliminated by the charge eliminating means 10 which had not been used
during the toner image retention. After that, the photosensitive drum 1 was cleared
of the spare toners, which were left on its surface, by the action of the cleaning
means 14 which had been left inoperative during the toner image retention.
[0218] The color image reproducing apparatus thus far described repeated the foregoing operations
each time its operating button was depressed. The developing conditions of each developing
step were such that the developing bias to be applied at the developing time had its
a.c. component set at 1.5 KV and having a frequency of 2 kHz and its d.c. component
set at 0 V, and such the gap d between the photosensitive drum 1 and the sleeve of
each developing means was 0.5 mm. In each developing means, at the developing time
the sleeve and the magnetic roll were rotated in the same common direction to carry
the developer, and this developer had its layer thickness regulated to 0.3 mm by the
action of the magnetic blade.
[0219] Each of the developers had the same composition as that of the Example 22 except
that its charge was controlled to - 20 nc/g.
[0220] With the construction thus far described, the multi-color images were retained to
form a visible image having a sufficient density with neither breakage of the tone
images, which had already been retained on the photosensitive drum 1, nor any mixing
of the toner of another color into each developing means.
Example 27
[0221] This example was likewise practised by the color image reproducing apparatus shown
in Fig. 37. The difference from the Example 26 is located in that the average quantities
of the developers used and the d.c. component of the developing bias applied at the
developing time were different among the developing means and were set at - 10 u.c/g
and 0 V, at - 15 uc/g and 0 V, at - 20 µcg and 20 V, and at - 40 u.c/g and 50 V in
the developing means 5, 6, 7 and 8 respectively. On the contrary, the amplitudes and
frequencies of the a.c. bias and the gaps between the photosensitive drum 1 and the
sleeve were common among the respective developing means like the Example 26 and were
set at 1.5 KV, 2 kHz and 0.5 mm, respectively.
[0222] In the present Example, the return of the toners on the photosensitive drum 1 was
prevented by controlling the electrifications such that the average quantities of
the charges of the developers had their absolute values increased in the developing
sequence, and the balance of the densities among the respective color toner images
was held by consecutively increasing the values of the d.c. biases.
[0223] According to the present Example, too, a clear multi-color image was obtained, and
another color was not mixed into each developing means.
Example 28
[0224] This Example was likewise practised by the color image reproducing apparatus shown
in Fig. 37. The difference from the Example 26 was found in that the average quantities
of the developers used and the amplitudes of the a.c. components of the developing
biases applied at the developing time were different among the developing means and
were set at - 10 µc/g and 1.6 KV, at - 15 µc/g and 1.4 KV, at - 20 u.c/g and 1.2 KV,
and at - 40 µc/g and 1.0 KV in the developing means 5, 6, 7 and 8, respectively. The
frequencies of the a.c. biases, the potentials of the d.c. biases, and the gaps d
between the photosensitive drum 1 and the sleeve were shared among the respective
developing means and were set at 2 kHz, 0 V and 0.5 mm, respectively.
[0225] In the present Example, the return of the toners on the photosensitive drum 1 was
prevented, and at the same time the balance among the densities of the respective
color toner images was held partly by controlling the electrifications such that the
average quantities of the charges of the developers had their absolute values increased
and partly by consecutively setting the a.c. biases.
[0226] According to the present example, a clearer multi-color image was obtained, and no
color was mixed into each developing means even after reproductions of multiple sheets.
[0227] Fig. 38 shows the changes in the potentials on the photosensitive drum when the developments
are conducted by the color image reproducing apparatus of Fig. 37.
[0228] After has been charged positive by the primary charger 2, the photosensitive drum
1 is charged negative so that its surface potential is dropped substantially to 0
V. Next by conducting the image exposure, the portion optically irradiated has its
potential raised to trap the toners, which have been charged negative in the developing
means, so that the portion having trapped the toners has its potential dropped (as
indicated at DDW in the drawing). Next, a uniform charging operation is so conducted
by the secondary charger that the surface potential is dropped substantially to 0
V, and the image exposure and the development are repeated. After the visible images
of all the colors have been formed on the photosensitive drum 1, the resultant toner
images are transferred to the recording paper, and the phtosensitive drum 1 has its
charged eliminated and is then cleaned until the step advances to a subsequent image
reproduction.
[0229] In the methods described hereinabove, the second and later secondary charging operations
can be omitted. On the other hand, the primary and secondary charging operations may
be conducted each time, and in this case the charge eliminating step may be introduced
prior to the charging step.
[0230] In the respective Examples thus far described, the corona transfer is used as the
toner image transfer, but another type may be used. If the adhesion transfer disclosed
in Japanese Patent Publication 41679/71, 22763/73 or the like, for example,is used,
the transfer can be conducted without considering the polarities of the toners. Moreover,
it is possible to adopt the method of effecting direct fixture to the photosensitive
member as in the electrofax method.
[0231] The two-component developer used in the present invention may especially preferably
be composed of a magnetic carrier as its carrier and a non-magnetic toner as its toner.
[0232] The compositions of the toners are generally, as follows:
(1) Thermoplastic Resin: 80 to 90 wt. % of binder
Examples: polystyrene, styrene-acryl polymer, polyester, polyvinyl butyral, epoxy
resin, polyamide resin, polyethrene, and ethylene-vinyl acetate copolymer, which are
frequently used in a mixed form;
(2) Pigment: 0 to 15 wt. % of coloring agent
Examples:
Black: Carbon Black;
Blue: copper phthalocyanine, derivative dye of sulfonamide;
Yellow: benzine derivative; and
Magenta: polytungstophosphate, Rhodamine Lake, Carmine 6B;
(3) Electrification Controlling Agent: 0 to 5 wt. %
Examples:
Plus: Nigrosine (i.e., electron donor); and
Minus: organic complex (i.e., electron acceptor);
(4) Fluidizer:
Examples: coloidal silica or hydrophobic silica as representative, silocone varnish,
metallic soap, nonionic active agent;
(5) Cleaning Agent: intended to prevent the filming of the toners of photosensitive
member
Examples:
fatty acid metal salt, oxidized silicate having a surface radical, surface active
agent containing fluorine; and
(6) Filler: intended to improve the surface gloss of images and to reduce the cost
for raw materials
Examples:
calcium carbonate, clay, talc, pigment.
[0233] In addition to the above-enumerated materials, a magnetic material may be contained
so as to prevent a fog and a toner dispersion.
[0234] As the magnetic powders, there are proposed such powders of tri-iron tetraoxide,
y-ferric oxide, chromium dioxide, nickel ferrite or iron alloy as have a diameter
of 0.1 to 1 u.m. At present, however, the tri-iron tetraoxide is frequently used and
is contained in 5 to 7 wt. % with respect to the toners. The resistances of the toners
are variable in dependence upon the kinds and quantities of the magnetic powders.
In order to provide a sufficient resistance, however, it is preferred to contain 55
wt. % or less of the magnetic material. Moreover, the quantity of the magnetic material
is desired to be contained in 30 wt. % or less so that it may hold a clear color as
the color toner.
[0235] In addition, as the resin suitable for the pressure fixing toner, an adhesive resin
such as wax, polyorefines, ethylene-vinyl acetate copolymer, polyurethane or rubber
is selected so that it may be plastically deformed and adhered to paper by a force
of about 20 kg/cm. A capsule toner may also be used.
[0236] The toners can be made of the above-enumerated materials and prepared by the method
known in the prior art.
[0237] In order to obtain a more preferable image in the construction of the present invention,
the particle diameters of those toners are desired to be no more than 50 microns in
their ordinary average values in relation to the resolution. In the present invention,
the toner diameters of about 1 to 30 microns may preferably be used in relation to
the resolution, the toner scattering and the carriage, although they are not restricted
on principle.
[0238] In order to reproduce fine points and lines and to enhance the gradation, moreover,
the magnetic carrier particles may preferably be particles composed of magnetic particles
and a resin, for example, a resin-dispersed system of magnetic powders and a resin
or resin-coated magnetic particles and may more preferably be rounded to have an average
particle diameter of 50 u.m or smaller, especially preferably, a particle diameter
no more than 30 u.m and no less than 5 u.m.
[0239] Moreover, in order to prevent the problems that the carrier particles for providing
an obstruction against the satisfactory image reproduction are made liable to receive
the charges by the bias voltage so that they become liable to be trapped by the surface
of the image carrier and that the bias voltage is not applied to a sifficient level,
the carrier may have such an insulating property of a resistivity no less than 10
8{2, preferably, 1013{2, more preferably, 101'[1. Moreover, the carrier particles may
have this resistivity and the above- mentioned diameter.
[0240] The carrier particles described above can be prepared either by coating the surface
of the magnetic materials described as to the toners with the thermoplastic resin
or by making the particles of a resin having fine magnetic particles dispersed and
contained therein and by selecting the resultant particles by the well-known average
diameter selecting means. Moreover, in order to improve the agitating characteristics
of the toners and the carriers and the carrying characteristics of the developers
and to improve the electrification controlling characteristics of the toners thereby
to make the toner particles reluctant to aggregate or the toner particles and the
carrier particles to aggregate, it is desirable to round the carriers. Of these rounded
magnetic carrier particles, the resin-coated ones are prepared by selecting magnetic
particles as round as possible and by coating the particles selected with a resin,
and the carriers having fine magnetic powders dispersed therein are prepared either
by rounding fine particles of a magnetic material, if possible, by hot wind or water
after making the dispersed resin particles or by directly forming the rounded dispersed
resin particles by the spray dry method.
[0241] Incidentally, the present invention can be further modified on the basis of the technical
concept thereof. In the Examples, the description has been made as to the case in
which the two-component developer composed of the toner and the carrier was used as
the developer having a plurality of components. However, the developer may additionally
contain a third component.
[0242] In the Examples, the description is limited to the development of the color image.
However, the present invention can be applied to the case in which toners of the same
color are developed in plural times. In this case, a toner having an excellent gradation
can be retained on the photosensitive drum.
[0243] Still moreover, the present invention can be applied not only to the reproducing
apparatus by electrophotography but also to the non-impact printer making use of the
electrostatic reproducing method or the magnetic reproduding method.
[0244] According to the Examples of the present invention, an image at a subsequent step
can be retained on an image carrier without disturbing an image retained at a previous
step even if the step of retaining a latent image on the image carrier and the step
of developing the latent image with a developer having a plurality of component are
repeated a plurality of times.
[0245] In other words, a clear image can be retained on the image carrier if the amplitude
V
AC and the frequency f of the a.c. component and the gap d between the developer carrier
and the image carrier are so set as to satisfy the following relathin- ships:
0.2 ≤ VAci(d f); and
((VAC/d) - 1500}/f ≦ 1.0.
[0246] In the other Examples, the developer D used was a one-component magnetic developer
which was prepared by blending and pulverizing 70 wt. % of a thermoplastic resin,
10 wt. % of a pigment (e.g., Carbon Black), 20 wt. % of a magnetic material and an
electrification controlling agent to have an average particle diameter of 10 µm. The
quantity of the charges is controlled by the electrification controlling agent.
[0247] In case the development is conducted with a one-component developer using only the
magnetic or non-magnetic toner, there can be used developing means which is disclosed
in U.S.P. Nos. 3,866,574 and 3,893,418. On the other hand, developing means having
two or more magnetic rollers may be used. The electric bias containing vibratory components
and applied upon the development has to be set under such a condition that the toner
image retained already on the image retainer may neither be disturbed nor have a color
mixing. Under the bias condition used in the non-contact jumping development, e.g.,
the condition as is disclosed in Japanese Patent Laid-Open Nos. 18656 to 18659/80
and 106253/81, the toner images having already been retained may be damaged by the
vibrations of the toners, which are caused by the intense a.c. electric field. In
case the developments according to the present invention are repeated to superpose
the toner images, the intensity of the a.c. component of the bias has to be set within
such a proper range without deteriorating the retained toner images that a subsequent
toner image can be completely retained.
[0248] Fig. 39 shows the relationship between the amplitude of the a.c. component, when
the gap d between the photosensitive drum 1 and the sleeve 31 is set at 0.7 mm; the
thickness of the developer at 0.3 mm; the developing bias to be applied to the sleeve
31 has its d.c. component at 500 V and its a.c. component at a frequency of 1 kHz;
and the charged potential of the photosensitive drum at 600 V, and the image density
of a toner image which is formed by the reverse phenomenon on the exposed portion
(at a potential of 0 V) of the photosensitive drum 1. The amplitude E
AC of the intensity of the a.c. electric field takes a value which is made by dividing
the a.c. voltage of the developing bias by the gap d. Curves A, B and C appearing
in Fig. 39 are the results obtained in case the magnetic toners used are controlled
to have average charges of 5 uc/g, 3 µc/g and 2 µc/g, respectively. It is observed
from the three curves A, B and C that the effect of the a.c. component appears for
the amplitude of the a.c. component of the electric field of 200 V/mm or higher and
1.5 KWmm or lower, and that the toner image retained in advance on the photosensitive
drum is partially broken for the amplitude of 2,500 V/mm or larger.
[0249] Fig. 40 depicts the changes in the image density when the frequency of the a.c. component
of the developing bias is set at 2.5 kHz and when the a.c. field intensity is changed
under the same conditions of those of the experiments of Fig. 32.
[0250] According to these experiments, the image density is high when the amplitude E
Aα of the a.c. field intensity is 500 V/mm or higher and 3.8 KV/mm or lower (although
not shown in Fig. 39), and the toner image retained in advance on the photosensitive
drum 1 is partially broken when that amplitude exceeds 3.2 KV/mm (although not shown
in Fig. 39).
[0251] Incidentally, as being seen from the results of Figs. 39 and 40, the image density
highly changes across a certain amplitude, which has a value obtainable hardly in
dependence upon the average charges of the toners, as seen from the curves A, B and
C. The reason therefor can be thought, as follows. Specifically, it is predicted that
the one-component developer has its charge quantities distributed widely across the
positive and negative ranges because of the mutual frictions of the toner particles.
As a result, the average quantities of the charges take a small value, but in fact
toners having a large quantity of charges, e.g., 20 u.c/g or larger exist at a predetermined
ratio and are thought to be mainly developed. Even if the average charge quantity
is controlled by the electrification controlling agent, the ratio occupied by the
toners having that large charge quantity is not varied so much, so that it is thought
that the change in the developing characteristics is not substantially observed.
[0252] Now, experiments similar to those of Figs. 39 and 40 were conducted under changing
conditions to pigeonhole the relationship between the amplitude E
Ac and frequency f of the a.c. field intensity so that the results shown in Fig. 41
could be obtained.
[0253] In Fig. 41: indicated at is a region where a developing unevenness is liable to occur;
indicated at is a region where the effect of the a.c. component does not appear; indicated
at Ⓒ is a region where the toners are liable to return; and indicated at Ⓓ and Ⓔ are
regions where the effect of the a.c. component appears so that no toner return occurs.
[0254] These results indicate that a proper region for the amplitude and frequency of the
intensity of the a.c. electric field exists so that a next (or subsequent) toner image
may be developed in a proper density without breaking the toner image which was retained
previously (at the previous step) on the photosensitive drum 1. This is thought to
be explained by the following reasons.
[0255] In the region where the image density has a tendency to increase for the amplitude
E
Ac of the a.c. field intensity, e.g., for the density curve of Fig. 39, i.e, where the
amplitude of E
AC of the a.c. field intensity ranges from 0.2 to 1.0 KV/mm, the a.c. component of the
developing bias acts to make it liable to jump a threshold value at which the toners
fly from the sleeve. As a result, even the toner having a small quantity of charges
is trapped by the photosensitive drum 1 so that it can be used for the development.
As a result, the image density is increased to the higher level as the amplitude of
the a.c. field intensity becomes the larger.
[0256] On the other hand, the reason, for which the image density is dropped in accordance
with the increase in the amplitude of the a.c. electric field (e.g., the region in
which the amplitude E
AC of the a.c. field intensity is no less than 1 KV for the density curve A of Fig.
29), can be thought in several ways. The toners are the more intensely vibrated as
the amplitude E
AC of the a.c. field intensity becomes the larger, and the cluster formed as a result
of the aggregation of the toners becomes liable to be broken so that only the toners
having high charges are selectively applied to the photosensitive drum 1 whereas the
toner particles having low charges become reluctant to be developed. Moreover, the
toners having low charges are liable to be returned to the sleeve 31 by the a.c. bias
because they have a weak image forming force even if they are once trapped by the
photosensitive drum 1. Since the charges on the surface of the photosensitive drum
1 leak if the amplitude of the field intensity of the a.c. component is too large,
still moreover, the phonomenon that the toners become reluctant to be developed become
liable to occur. As a matter of fact that, it is thought that those causes are overlapped
to make the image density constant for the increase in the a.c. component.
[0257] If the amplitude E
AC of the a.c. field intensity is enlarged, as has been described hereinbefore, on the
other hand, the toner image retained in advance on the photosensitive drum 1 is broken,
and the degree of this breakage is the higher for the higher a.c. component. This
is thought to be caused by the fact that the toners trapped by the photosensitive
drum 1 are acted by a force for returning it to the sleeve 31 by the a.c. component.
In case the development is conducted by consecutively superposing toner images on
the photosensitive drum 1, it is a fatal problem that the toner image or images having
already been retained are broken at a subsequent developing step.
[0258] As seen by comparing the results of Figs. 39 and 40, on the other hand, the experiments
conducted by changing the frequency of the a.c. component have revealed that the image
density becomes the lower for the higher frequency. This is caused by the fact that
the toner particles have their vibrating range narrowed, because they cannot follow
the changes in the electric field, so that they become reluctant to be trapped by
the photosensitive drum 1.
[0259] On the basis of the experimental results thus far described, the Inventors have attained
a conclusion that a later development can be conducted in a proper density without
disturbing the toner image already having been retained on the photosensitive drum
1, if each development is conducted under the conditions satisfying the following
relationships when the amplitude of the a.c. component of the developing bias is designated
at V
AC (V); the frequency of the same at f (Hz); and the gap between the photo sensitive
sleeve 1 and the sleeve at d (mm):
0.2≦VAC/d·f ≦ 1.6. In order to obtain a sufficient image density but not to disturb the toner
images having been retained until the previous stpe, the following condition, i.e.,
the region of Figs. 29 and 30, in which the image density has a tendency to increase
for the a.c. electric field, is desirably satisfied:
0.4≦VAC/d·f≦1.2. Of this region, it is preferable to satisfy the following region corresponding
to a slightly lower electric field in which the image density takes its maximum:
0.6≦VAC/d·f ≦ 1.0.
[0260] Moreover, it is further preferable to set the frequency f of the a.c. component at
200 Hz or higher so as to prevent the developing unevenness due to the a.c. component
and to set the frequency of the a.c. component at 500 Hz or higher so as to eliminate
the influences from the beats, which are caused by the a.c. component and by the rotations
of the magnetic roll in case the rotating magnetic roll is used as the means for supplying
the developer to the photosensitive drum 1.
[0261] On the other hand, not only the magnetic toner but also a non-magnetic toner can
be used. As the developing method using the non-magnetic toner, there is known a method
which is disclosed in Japanese Patent Laid-Open No. 30537/75 or 22926/77, for example.
In order to easily transfer the visible image on the photosensitive drum 1 to the
recording paper, moreover, the specific resistance of the toner is desired to be no
less than 10
13(2 cm. Incidentally, the resistivity is a value which can be obtained by reading out
a current value when a load of 1 Kgicm
2 is applied to the particles tapped in a container having an effective area of 0.5
cm
2 and when a voltage for establishing an electric field of 1,000 V/cm is applied between
the load and the bottom electrodes.
[0262] Moreover, the materials composing the developer except the magnetic material are
similar to those of the foregoing Examples.
[0263] These materials may be simply blended and pulverized, but the following additional
devices are made, as the case may be:
1. An insulating material is added to the inside or surface of the toner.
2. The toner is prepared either by coating in advance the surfaces of magnetic powders
with a surface active agent, an organic dye or a specified resin or by activating
in advance the same surfaces to form cover films by polymerizations and by mixing
the magnetic powders with a resin or the like. This device is intended to facilitate
uniform dispersion into the resin and to improve the image quality in a high humidity.
3. The developing quality is improved to prevent the toner scatter, as the case may
be, by selecting the magnetic characteristics of the magnetic powders such as the
shape, the axial ratio or the retaining force of the same.
4. The fluidity is enhanced to improve the developing property by mixing magnetic
toners which have different particle diameters, quantities of magnetic powders contained,
magnetic characteristics and electric resistances.
[0264] On the other hand, most of the magnetic powders are black so that they can be used
in place of the black pigment.
[0265] In addition, as the resin suitable for the pressure-sensitive toner, wax, polyorefines,
ethylene-vinyl acetate copolymer, polyurethan, rubber and so on are selected such
that they are elastically deformed and adhered to the paper by a force of about 20
Kglcm
2. Capsulated toners may also be used.
[0266] The particle diameters of those toners may preferably be no more than 50 microns
on an average value in relation to the resolution. In the present invention, the toner
particle diameters are not limited on principle but may be ordinally about 1 to 30
microns in relation to the resolution and the scattering and carriage of the toners.
[0267] Incidentally, the present invention can be further modified on the basis of the technical
concept thereof. In the foregoing Examples, the description is restricted to the development
of the color image. The present invention can also be applied to the case in which
toners of the same color are developed in plural times. In this case, a toner iamge
having an excellent gradation can be retained on the photosensitive drum.
[0268] Moreover, the present invention can be applied not only to the recording method for
electrophotography but also the non-impact printer which makes use of the electrostatic
reproducing method or the magnetic producing method.
[0269] Even both the step of retaining the latent image on the same image carrier and the
step of developing the latent image with the one-component developer are repeated
plural times, according to the Examples of the present invention, an image at a subsequent
step can be retained on the image carrier without disturbing the image which has been
retained at a previous step.
[0270] In other words, a clear image can be retained on the image carrier if the amplitude
V AC and the frequency f of the a.c. component and the gap d between the developer
carrier and the image carrier are so set as to satisfy the following relationships:
0.2≦VAC/d·f≦1.6.
[0271] Fig. 42 shows a reproducing apparatus according to another embodiment of the present
invention, in which: reference numeral 61 indicates an image retainer which is constructed
of such a magnetic layer and an insulating layer as is prepared by evaporating or
sputtering a magnetic material on a metal base or by applying a magnetic material
dispersed in a binder to the metal base and which is placed on a drum rotating in
the direction of arrow; numeral 63' indicates a magnetic erasing head; and numeral
63 indicates a magnetic recording head. The remaining portions are identical to the
embodiment of Fig. 1.
[0272] The recording head 63 composed of one or more rows of recording heads for retaining
a magnetic image on the magnetic layer of the image retainer 61. Magnetic force of
the magnet 32 is arranged so as not to disturb a magnetic image on the retainer 61.
[0273] The bias conditions for practising the method of the present invention are preferred
to satisfy the following inequalities:
for the two-component developer used:
0.2 5 VAri(d !): and
{(VAε d) - 1500}/f ≦ 1.0; and
for the one-component developer used:
0.2≦VAC/d·f≦1.6.
[0274] In the above inequalities: V
AC indicates the amplitude (V) (although not an effective value) of the a.c. component
of the developing bias; f indicates the frequency (Hz); and d indicates the gap (mm)
between the image retainer, e.g., the sleeve and the developer carrier.
[0275] Moreover, the order of the colored toners for image superpositions has to be determined
to be the most proper for the object because it exerts influences upon the tone of
the color image.
[0276] Although the foregoing description is directed to the reproducing apparatus of Fig.
42, the method of the present invention can also be practised by the reproducing apparatus
shown in Fig. 43. In Fig. 43, parts having the same functions as those of Fig. 42
are indicated by the same reference numerals as those of Fig. 42.
[0277] Shown in Fig. 43 is the reproducing apparatus in which a series of recording members
are prepared by placing a magnetic layer and a colorless insulating layer on the surface
of a conductive base to provide an image retainer 61'. While this image retainer 61'
is being fed straight, the retentions and developments of the magnetic images are
repeated. Along the passage for the image retainer 61', more specifically, the pre-writing
charger 2, the magnetic erasing head 63', the magnetic recording head 63 and developing
means 5 to 8 are juxtaposed in a repeated manner, and the fixer 12 for fixing the
color image on the image retainer 61' is disposed at the last position. This reproducing
apparatus can reproduce a series of color images without any provision of the pre-transfer
charger, the transfer means, the charge eliminating means and the cleaning means.
In order that the image retainer 61' may not depend, it is necessary to increase the
tension or to provide such a supporting roller midway, although not shown, as providing
the toners trapped by the image retainer 61' from being offset.
[0278] In the recording apparatus shown in Fig. 42, too, the pre-transfer charger 9, the
transfer means 11, the charge eliminating means 13 and the cleaning means 14 can be
omitted if the image retainer 61 is constructed by winding on the drum an image retainer
which is similar to the image retainer 61' used in the reproducing apparatus of Fig.
43. In order to hide the color of the magnetic layer, moreover, it is desired to provide
a conductive layer or an insulating layer having a white or desirable color.
[0279] In order to practise the method of the present invention, it is preferable to use
the image retainer having a highly insulating layer as the image retainer. Once the
developer is trapped by the image retainer, generally speaking, it is remarkably difficult
to remove, because not only the van der Waals' force but also the image forming force
acts, to cause troubles such as the fog and the reduction in the transfer ratio. These
phenomena can be prevented by suitably charging the image retainer in the same polarity
as that of the charges of the toners. However, the ordinary magnetic image retainer
constructed of a conductive base and a magnetic layer has such a low insulating property
that it is difficult to charge. Despite of this fact, this ordinary image retainer
can be charged by forming an insulating layer on the surface of the magnetic layer.
By using the image retainer having the insulating layer on the surface of the magnetic
member, the unnecessary trap of the toners can be prevented to enhance the transfer
efficiency, and a charger is placed in front of the magnetic image writing operation
to effect the charging operation so that the fog can be prevented. Moreover, the insulating
layer is also effective for protecting the magnetic layer and for preventing the toner
filming. If this toner filming occurs, there arises no practical problem if it takes
place on the insulating layer. Moreover, the magnetic layer may also act as the conductive
base if it is conductive. In case the thickness of the insulating layer is excessive,
it drops the density and magnetization of the magnetic image recorded. Therefore,
that thickness is preferred to be no more than 50 /.l.m or, preferably, no more than
10 µm.
[0280] Incidentally, Fig. 42 shows an embodiment of the reproducing apparatus which uses
an image retainer having the insulating layer, but the pre-writing charger 2 and the
charge eliminating means 13 can be omitted in case an image retainer having no insulating
layer is used.
[0281] In the embodiment of Fig. 42, the writing operation of the magnetic image is conducted
by the reproducing method of parallel magnetization type using a ring head. However,
a perpendicular magnetization type method can be likewise used as the magnetically
writing means. In this case, the magnet 32 is fixed, and its opposed magnetic poles
are made different from the magnetizing direction by the writing operation so that
the toners may be reluctant to jump to the non-image portion but liable to jump to
the image portion. In this case, it is needless to say that the magnetizing direction
and the magnetization facilitating direction of the magnetic layer should be aligned.
[0282] On the other hand, the magnetic erasing head 63' and the magnetic recording head
63 may be disposed in front of, in front of and at the back of, or at the back of
the pre-writing charger as shown in Figs. 42 and 43.
[0283] The image reproducing process using the reproducing apparatus according to the method
of the present invention will be described in the following with reference to Figs.
44 to 46.
[0284] In the embodiment of Fig. 44, by the reproducing apparatus of Fig. 42: (1) the surface
of the image retainer 61 has its charges eliminated by the image eliminating means
13 and cleaned by the cleaning means 14; and the initial state, in which the surface
of the image retainer 61' is charged to a suitable potential e (in which the broken
lines indicate the presence of the charges) in the same polarity as that of the toners,
is established by the pre-writing charger 2 so as to prevent the fog.
[0285] Next, after the residual magneticism m has been demagnetized by the demagnetizing
head 63' (shown at (3) ), that surface is subjected to a first writing operation by
the recording head 63 to retain a magnetic image M, (shown at (4) ) which is firstly
developed by the developing means 5 to obtain a first image T, (shown at (5) ). Moreover,
the image retainer 1 enters its second rotation in the reproducing apparatus of Fig.
42 so that it is demagnetized by the erasing head 63' (shown at (6) ) and is subjected
to a second writing operation by the recording head 63 (shown at (7) ). The magnetic
image M2 thus formed is secondly developed by the developing means 6 to provide a
second image T2 (shown at (8) ). Then, third and fourth demag- netizating, writing
operations and developments are likewise repeated so that a color image having its
color toner images superposed is retained on the image retainer 61. The resultant
color image is made liable to be transferred by the pre-transfer charger 9 in the
reproducing apparatus of Fig. 42 so that it is fixed on the recording member P by
the fixing means 12 after it has been transferred to the recording member P by the
transfer means 11. In the reproducing apparatus of Fig. 43, on the other hand, that
color image is fixed directly on the image retainer 61 by the fixing means 12. In
the reproducing apparatus of Fig. 42, moreover, the surface of the image retainer
61 having the color image transferred thereto has its charges eliminated by the charge
eliminating means 13 and is cleared of the residual toners by the cleaning means 14
until the one cycle of the color image reproduction is ended by further eliminating
the charges, if necessary. In case the reproducing apparatus of Fig. 43 is used, too,
the image retaining process is not changed except for the shape of the image retainer
61'.
[0286] Fig. 45 shows a process which is simplified by omitting the uniformly charging step
from the process of Fig. 44. Fig. 46 shows a process which is difference from that
of Fig. 44 in that the charger 2 is operated to effect the uniform charging operation
before each writing step. However, their basic operations are all common.
[0287] Incidentally, reference letters T, and T2 indicate the toners of different colors,
which are trapped by the image retainer 61 or 61'.
[0288] By conducting the developments under the non-contact jumping developing conditions,
according to the method of the present invention, the developing means other than
that conducting the development of each time can be easily held in an inoperative
state, even if the developer layer is not removed from the developing sleeve 31, by
disconnecting the developing sleeve 31 from the power supply 39 into a floating state,
by grounding the same to the earth, or by positively applying a d.c. bias voltage
having a polarity opposite to that of the charges of the toners to the developing
sleeve 31. Of these means, it is preferable that the developing means are held inoperative
by applying the bias voltage having a polarity opposite to that of the toners.
[0289] Next, the embodiments of Figs. 44 to 46, which are practised by the reproducing apparatus
of Fig. 42, will be described in more detail in the following in connection with Examples
29 to 31.
[0290] The reproducing apparatus shown in Fig. 42 was used. The image retainer 61 was prepared
by forming a Co alloy having a thickness of 10 u.m on an aluminum base by the electron
beam heating operation and by forming the insulating layer having a thickness of 5
µm on the surface of the Co alloy and which had a circumference speed of 180 mm/sec.
The surface of the image retainer 61 thus prepared was charged to +50 V by the pre-writing
charger 2 using the scorotron corona discharger and was demagnetized by means of the
magnetic erasing head which had its leading end spaced at a distance of about 30 Ilm
from the surface of the image retainer 61. Next, a first image writing operation was
conducted in a distribution density of 10 spotsimm by means of the recording head
63 which had a similar spacing. As a result, a first magnetic image was retained on
the image retainer 61. This magnetic image was firstly developed by the developing
means 6 shown in Fig. 3. This developing means 6 uses the developer, which was composed
of: a carrier having 50 wt. % of magnetite dispersed contained in a resin and having
an average particle diameter of 30 u.m, a magnetization of 30 emu/g and a resistivity
of 10
,4 Qcm or more; and a positive magnetic toner prepared by adding 25 wt. % of magnetite,
10 wt. % of copper phthalocyanine as the cyan pigment and an electrification controlling
agent to a styrene-acryl resin and having an average particle diameter of 10 u.m,
under the condition that the ratio of the toner to the carrier was 10 wt. %. Moreover,
there were resorted to the non-contact jumping developing conditions under which:
the developing sleeve 31 had an external diameter of 30 mm and a number of revolutions
of 100 r.p.m.; the magnet 32 had its N and S magnetic poles having a magnetic flux
density of 500 gausses and had a number of revolutions of 1,000 r.p.m.; the developer
layer in the developing region had a thickness of 0.7 mm; the gap between the developing
sleeve 31 and the image retainer 1 was 0.8 mm; and a bias voltage having a d.c. voltage
component of - 50 V and an a.c. voltage component of 1.5 kHz and 1,000 V was applied
to the developing sleeve 31. In the following Examples, the a.c. component has a sine
wave, and its exemplified values are effective ones.
[0291] The surface of the image retainer 61 having been subjected to the first development
was subjected again to an erasure by the same magnetic erasing head 63' without operating
the pre-transfer charger 9, the charge eliminating means 13 and 13, the cleaning means
14 and the pre-writing charger 2, and a second writing operation was conducted in
the same spot density but with the spot position being shifted from that of the first
writing operation by means of the recording head 63. Next, a second development was
conducted by the developing means 6 which was under the same conditions as those of
the developing means 5 except that it used as its toner a toner prepared by adding
polytungstophosphate as the Magenta pigment in place of the cyan pigment. Likewise,
a demagnetization and a third writing operation were conducted. A third development
was then conducted by the developing means 7 which was under the same conditions as
those of the developing means 5 except that it used as its toner a toner prepared
by adding a bendizine derivative as the yellow pigment. Moreover, demagnetization
and a fourth writing operation were conducted. A fourth development was conducted
by the developing means 8 which was under the same conditions as those of the developing
5 except that it used a toner prepared by adding Carbon Black as the black pigment.
The color image thus retained on the image retainer 61 was transferred to and fixed
on the recording member P, as has been described with reference to Fig. 42. Moreover,
the surface of the image retainer 61 having the color image transferred thereto had
its charged eliminated by the charge eliminating means 13 and was cleared of the residual
toners by the cleaning means 14.
[0292] The reproduced image thus obtained had little color toner mixing and was a remarkably
clear color image.
[0293] Incidentally, in the present Example, the spot position of a subsequent writing operation
may be overlapped upon that of a previous writing operation. In the writing and developing
operations, moreover, the recording current of the recording head 3, and the voltage
value, frequency and time selecting period of the d.c. or a.c. component of the voltage
to be applied to the developing sleeve may be so changed as to adjust the developed
densities of the respective colors. If the writing spot positions are superposed,
the color mixing becomes liable to occur to invite color vagueness but not to drop
the resolution. In this case, moreover, especially the sequence of the colors to be
developed is important. By adjusting the developing densities of the respective colors
in the aforementioned manner, on the other hand, it is possible to attain a color
image which has its tones changed.
Example 30
[0294] The same reproducing apparatus as that of the Example 29 was used. The color image
reproduction was conducted under the same conditions as those of the Example 29 except:
that a magnetic image was for a background potential of 0 V by a first writing operation
without any of the charging operation of the Example 29 by the pre-writing charger
2 before the first writing operation, after the charge elimination for demagnetization
by the charge eliminating means 13; that a superposed voltage composed of a d.c. voltage
of - 50 V and an a.c. voltage of 3 kHz and 2,000 V was applied as the bias voltage
before development to the developing sleeve 31; and that charge elimination and demagnetization
were effected by the charge eliminating means 13 before second and later writing operations
so that a magnetic image was retained for the backgound potential of 0 V even during
the second and later writing operations.
[0295] The reproduced image which is excellent in clearness like that of the Example 29
was thus obtained.
Example 31
[0296] The color image reproduction was conducted by the use of the same reproducing apparatus
as that of the Example 29 and under the same conditions as those of the Example 29
except: that a charging operation is conducted to + 300 V before a first writing operation
by the pre-writing charger 2 so that a magnetic image was retained for the backgound
potential of + 300 V by the first writing operation after it had been demagnetized;
that a superposed voltage composed of a d.c. voltage of +. 300 V and an a.c. voltage
of 2 kHz and 1 KV was applied as the bias before the development to the developing
sleeve 31; and that the pre-writing charger 2 was used before the demagnetization
and the second and later writing operations. The reproduced image obtained was a color
image which was excellent in cleaness like that of the Example 29.
[0297] According to the Examples of the present invention, there can be attained an excellent
effect that the tone or the like of the color image can be easily changed thanks to
the use of the image retaining means having its image retainability and toner image
formability separated so that a color image having excellent clearness and a high
tone can be reproduced while stabilizing the reproduction.
[0298] Incidentally, the present invention can be applied not only to an image retainer
having a belt or sheet shape, but also to an image retainer such as electrofax paper,
which is placed on a base so that the color image formed thereon by the toners is
fixed without being transferred. In this case, the sequence of superposing the color
toner images has to be taken into consideration, but the transfer means, the cleaning
means and so on can be omitted. It is true, but the the charge eliminating means can
be omitted, too, in case the toners are transferred with a predetermined polarity
and a charge quantity. On the other hand, the transfer should not be limited to the
corona type but may be exemplified by the bias roller type, the adhesion type, the
direct pressure type or other means using an intermediate transfer member, and the
fixture is not limited to the heat roller type.
[0299] In the foregoing embodiments of the present invention, moreover, the magnetic recording
head is used as the writing means, but another means may be likewise used if the magnetic
image is to be retained on the magnetic layer. More specifically, the present invention
can be applied to the method, in which the magnetic image is retained by heating imagewise
a demagnetized magnetic layer, while passing through a unidorm magnetic field, by
the heating means such as a laser and by cooling the heated magnetic layer in a magnetic
field.
[0300] Although the foregoing description has been directed only to the reproduction of
the color image, furthermore, the method of the present invention can also be applied
to the superposition of an image of identical colors. In addition, the electrophotographic
image and the magnetic image can be reproduced in combination if an electrophotog-
taphic photosensitive layer is provided on the magnetic layer.