BACKGROUND OF THE INVENTION
1. Field of the Invention:
[0001] The present invention relates to a toner to be used in image forming equipment such
as a copier, a laser printer and a facsimile, and to a non-contact developing method
using the same. More particularly, it relates to a toner applied to a non-contact
developing unit for visualizing an electrostatic latent image by flying the toner
to an electrostatic latent image holder facing to the toner on a toner carrier with
a gap by electrostatic force, and to a non-contact developing method using the same.
2. Description of Related Art:
[0002] Hitherto, there has been known an electrostatic copier in which charged toner is
carried on a toner carrier and the toner and an electrostatic latent image holder
are disposed in non-contact from each other to develop an electrostatic latent image
by electrostatic force acting between the toner and the electrostatic latent image
holder (see Japanese Patent Publication No. 41(1966)-9475). The publication No. 41-9475
teaches that the non-contact developing method allows a copied image having no background
fog to be obtained because the toner deposits only on the location which corresponds
to an image portion of the electrostatic latent image.
[0003] However, when the non-contact developing method is compared with a contact developing
method, the latter method can carry the toner to an electrostatic latent image portion
mechanically, while the non-contact developing method is required to fly the toner
by electrostatic force and is unable to assure sufficient development unless the electrical
property of the toner and the developing conditions of the developing unit are fully
optimized.
[0004] Accordingly, the above-mentioned publication No. 41(1966)-9475 teaches merely the
basic idea of the non-contact developing method and discloses nothing about the property
of the toner and the developing conditions, so that it is difficult to implement it.
[0005] As a case of color development in which a non-magnetic monocomponent toner is flied
in a DC electric field, an article (1) entitled "One Drum Color Superimposing Process
-DC Electric Field Flying-Development" has been published in the Journal of Society
of Electro-photograph of Japan, vol. 29, No. 1, 1990.
[0006] According to the article (1), the color development in which the non-magnetic monocomponent
toner is flying-developed in the DC electric field has been put into practical use
by reducing image-force, which is an adhesive force, acting on the toner laminated
on a toner carrier to increase the property of the toner for flying from the toner
carrier to an electrostatic latent image holder.
[0007] Further, in order to give a sufficient flying property, a non-magnetic monocomponent
toner having a relatively large particle size of 12µm was used and a charge-to-mass
ratio which is a quantity of charge per unit mass thereof was set at a low value of
1 to 5µC/g.
[0008] This is because a large toner charge-to-mass ratio was believed to increase image-force
Fi and to decrease the flying property of the toner, thus considerably decreasing
the developability, because the image-force Fi, which is an electrostatic adhesive
force of the toner, increases in proportion to the square of the charge-to-mass ratio.
Accordingly, it was necessary to increase the particle size of the toner because the
small toner particle size would increase the specific area of the toner, thereby increasing
the toner charge-to-mass ratio as well.
[0009] Further, because the non-contact development requires larger Coulomb force than the
contact development, the flying property of the toner having a small particle size
would be considerably decreased when it is applied to the non-contact development.
Due to that, there has been a problem that the toner having a small particle size
which should otherwise be very effective in improving an image quality cannot be used
in the non-contact developing method. The toner in the non-contact developing method
has been limited to those having a large particle size and having a small charge-to-mass
ratio.
[0010] The non-magnetic monocomponent toner is used in the DC electric field flying-development
because it allows toner images of a plurality of colors to be superimposed without
color mixture and is suited for color development.
[0011] Further, a method for increasing the flying property of the toner by giving mechanical
vibration other than the electrostatic force in a developing section has been proposed
as a method for reducing adhesive force of toner on a toner carrier.
[0012] In a developing unit described in Japanese Patent Laid-open No. Hei. 5(1993)-232802,
a method for increasing the flying property by providing a vibrating member in contact
with a belt-like toner carrier to reduce the adhesive force of the toner on the toner
carrier has been disclosed.
[0013] In a color image forming equipment described in Japanese Patent Laid-open No. Hei.
5(1993)-297711, a mechanical impact is applied to the developing unit when it begins
to fly the toner so that the toner having a small particle size can easily fly.
[0014] Further, when the non-magnetic monocomponent toner is used, the toner cannot be fully
conveyed unless the fluidity of the toner is good, because the toner cannot be conveyed
by magnetic force.
[0015] Then, there has been known a method of adding another kind of particles to the toner
for the purpose of improving the chargeability and fluidity of the non-magnetic monocomponent
toner as disclosed in, for example, Japanese Examined Patent Publication No. Sho.
59(1984)-7098 entitled "Electrostatic Latent Image Developing Method" and No. Hei.
2(1990)-45191 entitled "Developing Method".
[0016] In the above-mentioned publication No. 59(1984)-7098, a monocomponent developer containing
hydrophobic silica in toner is charged by triboelectric charging and is then supplied
to a developing section. Thereby the fluidity of the toner is enhanced to prevent
coagulation.
[0017] In the publication No. Hei. 2(1990)-45191, 1 to 50 parts by weight of granulating
silica powder having 1 to 100 µm of particle size is added into 100 parts by weight
of insulating toner particle to improve a triboelectric charging performance of the
toner.
[0018] Further, there has been known a method for carrying a toner having about 15 to 100
µm of thickness and 0.1 to 0.6 g/cm
3 of packing density on a toner carrier and flying-developing the toner through 100
to 500 µm of development gap as disclosed in, for example, US Patent No. 4,666,814
and Japanese Patent Laid-open No. Sho. 60(1985)-87347. Still more, there has been
known a method for carrying a toner having about 15 to 80 µm of thickness, 0.1 to
0.6 g/cm
3 of packing density and 3 × 10
-10≤|Q|≤10
-7 of charge density Q(C/m
2) on a toner carrier and flying-developing it through 100 to 500 µm of development
gap as disclosed in US Patent No. 4,666,815 and Japanese Patent Laid-open No. Sho.
60(1985)-87343 for example.
[0019] Further, there has been known a method for carrying a toner having about 30 µm of
thickness and 3 µC/g of charge-to-mass ratio on a toner carrier and flying-developing
it through 100 to 500 µm of development gap as published in an article (2) entitled
"Electrostatic Influence of the Toner Layer on the Photoconductor" in the Sixth International
Congress on Advances in Non-Impact Printing Technologies, 1990, p. 34.
[0020] However, the flying-development using the toner having the large particle size and
the low charge-to-mass ratio to improve the flying property thereof as described above
has had a problem that it is apt to produce wrong sign toners (reverse polarity toners)
and to cause background fog and a reduction of sharpness of edge, thus deteriorating
the image quality.
[0021] This problem is outstanding especially when monocomponent toner is used. It is because
the monocomponent toner is apt to produce a toner with the reverse polarity because
it uses no carrier, whereas two-component toner is charged by friction between the
carrier, having a charge polarity opposite to that of the toner, and the toner itself
can be charged with a normal polarity. In particular, when monocomponent toner having
a low charge-to-mass ratio is used the rate of the reverse polarity toner may reach
to 30 % in the toner to be developed.
[0022] Further, the method of developing the non-magnetic monocomponent toner in a DC electric
field has had a problem that the toner layer is apt to be flown apart, as common to
the non-magnetic toner. That is, while the non-magnetic toner is carried on the toner
carrier mainly by image-force (electrostatic adhesive force) because it cannot be
laminated and carried on the toner carrier by magnetic force like magnetic toner,
the toner is apt to be flown apart because the toner having a small charge-to-mass
ratio decreases the image-force, thus deteriorating the developability.
[0023] Although the method of developing the non-magnetic monocomponent toner in the DC
electric field is suitable for color development, it has a number of disadvantages
in terms of image quality as described above as compared to the conventional methods
such as a two-component magnetic brush development. While a method of developing a
black toner by the two-component magnetic brush development by using a toner having
a small particle size and of developing only color toners by the non-contact developing
method by using non-magnetic monocomponent toners having a relatively large particle
size has been adopted sometimes as practical means for putting into use, it has had
a problem that it complicates the equipment.
[0024] The toner having a large particle size has had a problem that a distance between
a position of the center of gravity of the toner at the outermost surface of the toner
carrier and an electrostatic latent image is separated, even though the development
gap is constant, so that an electric field pattern of the latent image acting on the
toner attenuates, thus decreasing a resolution of the image after the development.
[0025] Beside them, the non-contact development has had a problem of a phenomenon that a
density at edge is emphasized depending on a development pattern due to the relation
of the peripheral speed of the toner carrier with that of the electrostatic latent
image holder.
[0026] Although the method disclosed in Japanese Patent Laid-open Publications No. Hei.
5(1993)-232802 and No. Hei. 5(1993)-297711 allow the toner having a small particle
size to be used, they have problems such that the toner carrier is confined on a belt,
separate means for applying mechanical vibration or impact is necessary and the equipment
is complicated, thus increasing the cost.
[0027] Although the method disclosed in Japanese Examined Patent Publications No. Sho. 59(1984)-7098
and No. Hei. 2(1990)-45191 is effective in improving the chargeability and fluidity
of the non-magnetic monocomponent toner by externally adding silica to the toner,
it describes nothing about a correlation to the adhesive force acting on the toner,
which is an important factor in the non-contact development.
[0028] Although the developing methods disclosed in US Patent No. 4,666,814 (Japanese Patent
Laid-open No. Sho. 60(1985)-87347), US Patent No. 4,666,815 (Japanese Patent Laid-open
No. Sho. 60(1985)-87343) and in the article (2) in the Sixth International Congress
on Advances in Non-Impact Printing Technologies have a feature that the toner having
less charge-to-mass ratio or charge density is carried on the toner carrier with a
lower packing density and a thicker layer thickness, an experiment showed that the
prior art developing method without considering the inter-particle force of the toner
into account is not practical because its developability is remarkably inferior.
[0029] Actually, it has been found from the property of the toner, equations of the adhesive
force and flying experiments that the adhesive force which acts on the toner laminated
and carried on the toner carrier includes an adhesive force called the inter-particle
force Fv other than the electrostatic adhesive force called the image-force Fi and
that it is important to suppress the inter-particle force Fv, other than the image-force
Fi, which act on the toner in non-contact developing. It has been also found from
the experiment that the flying-development can be implemented fully with toner having
a large charge-to-mass ratio regardless of the particle size thereof by reducing the
inter-particle force Fv other than the electrostatic force.
[0030] Therefore, the toner having a small particle size which is very effective in improving
the image quality may be adopted in the non-contact developing method by suppressing
the inter-particle force and defining the size thereof, and thus the above-mentioned
problems of the prior art can be solved.
SUMMARY OF THE INVENTION
[0031] Accordingly, it is an object of the present invention to solve the aforementioned
problems by providing a toner and a non-contact developing method using the same which
allows an excellent image quality to be obtained by suppressing inter-particle force
Fv which is an adhesive force other than electrostatic force Fi acting on the toner.
[0032] The toner of the present invention can exhibit 5 nN or less of inter-particle force
which is calculated by the following equation (1) when it is laminated and carried
on a toner carrier:

where Fv is an inter-particle force, q·E is a Coulomb force calculated by the following
equation:

where Fi is an image-force calculated by the following equation (3):

where q is a quantity of charge [C] of the toner particle to be developed, E is an
electric field strength [V/m] acting on the toner layer, Q/M is a charge-to-mass ratio
[µC/g] of the toner, W
1 is an amount of the toner [mg/cm
2] separated by development among the toner which is laminated and carried on the toner
carrier, ε
o is a vacuum dielectric constant [C/(v·m)], ε
T is an apparent specific dielectric constant [C/(v·m)] of the toner layer, d is an
average particle size [µm] of the toner, δ is a true density [g/cm
3] of the toner, g is a gap [mm] between the outermost surface of the toner on the
toner carrier and the electrostatic latent image holder, dt
1 is a thickness [µm] of the toner layer on the toner carrier, Vb is a development
bias voltage [V] and P is a toner packing rate.
[0033] It is another object of the present invention to provide a toner which allows the
non-contact development even with the toner having a small particle size of 11 µm
or less by reducing the inter-particle force of the toner from 0.01 nN to 5 nN.
[0034] It is still another object of the present invention to provide a toner which allows
the non-contact development within a range in which the average particle size of the
toner is 5 µm to 11 µm and the charge-to-mass ratio thereof is 5 µC/g to 15 µC/g.
[0035] It is a further object of the present invention to provide a toner which allows the
non-contact development within the range in which a toner charge-to-mass ratio is
5 µC/g to 15 µC/g, the thickness of the toner laminated and carried on the toner carrier
is about 5 µm to 20 µm and the packing density thereof is about 0.4 g/cm
3 to 0.85 g/cm
3.
[0036] It is a further object of the present invention to provide a non-contact developing
method which can realize stable flying-development only by the means for controlling
electrostatic force and field strength acting on the toner by suppressing the inter-particle
force Fv which is an adhesive force other than the electrostatic force Fi acting on
the toner to 5 nN or less.
[0037] It is another object of the present invention to provide a non-contact developing
method in which the inter-particle force of the toner other than the electrostatic
force acting on the toner which is laminated on the toner carrier is 5 nN or less
and a charge-to-mass ratio thereof is controlled within a predetermined range.
[0038] It is still another object of the present invention to provide a non-contact developing
method which can avoid an edge enhancement which is a problem intrinsic to the non-contact
development.
[0039] It is a further object of the present invention to provide a non-contact developing
method which can avoid the edge enhancement and can assure a required amount to be
developed even if the toner charge-to-mass ratio is large and under a condition in
which an amount of toner separated by the development among the toner laminated and
carried on the toner carrier is small.
[0040] The above and other related objects and features of the present invention will be
apparent from a reading of the following description of the disclosure found in the
accompanying drawings and the novelty thereof pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041]
FIG. 1 is a section view showing a schematic structure of one embodiment of a developing
unit applied with a toner of the present invention and to a non-contact developing
method using the same;
FIG. 2 is an enlarged view showing a non-contact developing section applied with a
toner of the inventions and to the non-contact developing method using the same;
FIG. 3 is a graph showing a toner charge distribution of the invention;
FIG. 4 is a perspective view for explaining an area where an edge-emphasized image
is generated;
FIG. 5 is a drawing for explaining the edge-emphasized image on a recording sheet;
FIG. 6 is a drawing for explaining the directions of rotation and the peripheral speeds
of a toner carrier and an electrostatic latent image holder;
FIG. 7 is a graph for setting the range of a charge-to-mass ratio Q/M of a toner of
the invention;
FIG. 8 is a graph for setting the ratio of peripheral speed k in accordance with the
invention;
FIG. 9 is a graph showing relationship between the toner particle size d and the amount
to be developed M/A with respect to the value of inter-particle force Fv in accordance
with the invention;
FIG. 10 is a graph showing allowable ranges of the charge-to-mass ratio Q/M of a toner
with respect to the value of inter-particle force Fv in accordance wiht the invention;
FIG. 11 is a graph showing relationship between the charge-to-mass ratio of a toner
and the amount to be developed with respect to a toner particle size/inter-particle
force;
FIG. 12 is a table showing a result of a flying experiment of the toners of the present
invention;
FIG. 13 is a graph showing an actually measured example 1 of the density distribution
of a copied image developed by the inventive developing method; and
FIG. 14 is a graph showing an actually measured example 2 of the density distribution
of a copied image developed by the inventive developing method.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0042] The present invention provides a toner which can exhibit 5 nN or less of inter-particle
force calculated by the following equation (1) when it is laminated and carried on
a toner carrier:

where Fv is an inter-particle force, q·E is a Coulomb force calculated by the following
equation:

where Fi is an image-force calculated by the following equation (3):

where q is a quantity of charge [C] of the toner particle to be developed, E is an
electric field strength [V/m] acting on the toner layer, Q/M is a charge-to-mass ratio
[µC/g] of the toner, W
1 is an amount of toner [mg/cm
2] separated by development among the toner laminated and carried on the toner carrier,
εo is a vacuum dielectric constant [C/(V· m)], ε
T is an apparent specific dielectric constant [C/(V·m)] of the toner layer, d is an
average particle size [µm] of the toner, δ is a true density [g/cm
3] of the toner, g is a gap [mm] between the outermost surface of the toner on the
toner carrier and the electrostatic latent image holder, dt
1 is a thickness [µm] of the toner layer on the toner carrier, Vb is a development
bias voltage [V] and P is a toner packing rate, and a non-contact developing method
using the same.
[0043] According to the present invention, the toner whose inter-particle force calculated
by the above equation (1) is 5 nN or less when it is laminated and carried on the
toner carrier is formed. The inter-particle force Fv expressed by the equation (1)
may be obtained by measuring numerical values to be substituted into the equations
(2) and (3) and by substituting those measured values into them.
[0044] It is noted that a developer of the present invention may be either a monocomponent
or a two-component developer.
[0045] While the monocomponent developer is composed of a toner only, the two-component
developer is composed of a toner and a carrier (e.g. Iron powder, ferrite powder,
magnetite powder, etc.).
[0046] In the case of the two-component developer, the electrical adhesive force includes
the Coulomb force between the toner and the carrier in addition to the image-force,
so that it can be calculated by Fv = q·E - Fi by defining the resultant force anew
as Fi.
[0047] Among them, the monocomponent developer is preferable from the aspect that it allows
toner images to be superimposed without color mixture and facilitates maintenance.
Hereinafter, the monocomponent developer will be explained.
[0048] The monocomponent developer usable in the present invention is composed of a toner
only which is mainly composed of a binder resin and contains optionally a colorant,
an internal additive and an external additive .
[0049] The binder resin usable in the present invention is not limited to specific ones
and any known materials such as those listed below may be used: styrene homopolymers
such as polystyrene, poly-p-chlorostyrene, polyvinyl toluene; styrene copolymers such
as styrene-p-chlorostyrene copolymer, styrene-propylene copolymer, styrene-vinyltoluene
copolymer, styrene-vinylnaphthalene copolymer, styrene-methyl acrylate copolymer,
styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl
acrylate copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate
copolymer, styrene-butyl methacrylate copolymer, styrene-α-methyl chloromethacrylate
copolymer, styrene-acrylonitrile copolymer, styrene-vinylmethylether copolymer, styrene-vinylethylether
copolymer, styrene-vinylmethylketone copolymer, styrene-butadiene copolymer, styrene-isopropylene
copolymer, styrene-maleic acid copolymer, styrene-maleate copolymer; and styrene terpolymers
such as styrene-acrylonitrile-indene terpolymer.
[0050] Besides them, polymethylmethacrylate, polybutylmethacrylate, polyvinylchloride, polyvinylacetate,
polyethylene, polypropylene, polyurethane, polyamide, epoxy resin, polyvinyl butyral,
polyacrylic acid resin, rosin, denaturated rosin, terpene resin, phenolic resin, aliphatic
or alicyclic hydrocarbon resin and aromatic petroleum resin may be listed up. Those
binder resins may be used solely or in a mixture.
[0051] The colorants usable in the present invention are not limited to specific ones and
any known materials such as those listed below may be used: carbon black, phthalocyanine
blue, indanthrene blue, peacock blue, permanent red, lake red, rhodamine lake, Hansa
yellow, permanent yellow and benzidine yellow.
[0052] The internal additives usable in the present invention include a charge control agent,
a filler and others. Among them, the charge control agent is not limited to a specific
one and any known agent such as those listed below may be used: negative charge control
agents such as metal complex salt compound and positive charge control agents such
as azine pigment and alkylammonium compound.
[0053] Examples of the external additives usable in the present invention include fluidizing
agents such as aliphatic carboxylates and cleaning agents such as higher fatty acids.
[0054] Further, in order to weaken the inter-particle force among the toner particles and
to reduce the inter-particle force (adhesive force) Fv at the section of flying portion
of the toner layer on the toner carrier to 5 nN or less, inactive micro particles
may be dispersed as spacers among the toner particles. The inactive micro particle
may be, for example, a silica powder. Preferably, the particle has 0.01 µm to 1 µm
of size. The particle having a size of less than 0.01 µm is not preferable because
it is less effective in reducing the inter-particle force among the toner. Also, the
particle having a size more than 1 µm is not preferable because it is the size close
to the toner particle and gives a bad influence on the image. It is noted that an
external additive may be dispersed in the toner in advance or at the developing stage.
[0055] The toner used in the present invention may be produced by using a known method.
That is, a homogeneously dispersed matter of the above-mentioned binder resin, the
colorant and the internal additive is formed under melting and kneading processes.
Then, the dispersed matter is cooled and is ground so as to have a predetermined particle
size in a grinding process. It is also subjected to a classification process to remove
big and fine particles to obtain a toner having the predetermined average particle
size.
[0056] Here, the average particle size of the toner is preferably within the range of 5
µm to 11 µm. Preferably, it is not less than 5 µm because otherwise the flying quality
of the toner will decrease and the developability will be lowered due to the reduction
of the Coulomb force acting on the toner and to the increase of the image-force. Preferably,
it is not more than 11 µm because otherwise the resolution and tone reproduction will
be lowered.
[0057] Accordingly, the toner having the following properties is preferable for the non-contact
development.
[0058] The inter-particle force of the toner is preferably within the range of 0.01 nN to
5 nN because the flying quality is increased thereby.
[0059] The average particle size of the toner is preferably within the range of 5 µm to
11 µm because less reverse polarity toner is produced thereby.
[0060] The toner preferably contains inactive particles having 0.01µm to 1µm of average
particle size as spacers because the inter-particle force is decreased thereby.
[0061] The toner charge-to-mass ratio is preferably within the range of 5 µC/g to 15 µC/g
because the optimum Coulomb force can be obtained thereby.
[0062] The toner preferably has an average particle size within the range of 5 µm to 11
µm and the charge-to-mass ratio within the range of 5 µC/g to 15 µC/g because the
optimum Coulomb force can be obtained thereby without producing the reverse polarity
toner.
[0063] The toner laminated and carried on the toner carrier preferably has a thickness within
the range of about 5 µm to 20 µm and a packing density thereof within the range of
about 0.4 g/cm
3 to 0.85 g/cm
3 because the developability is enhanced thereby.
[0064] The toner preferably has a charge-to-mass ratio within the range of 5 µC/g to 15
µC/g, the thickness of the toner laminated and carried on the toner carrier within
the range of about 5 µm to 20 µm and a packing density within the range of about 0.4
g/cm
3 to 0.85 g/cm
3 because the developability is enhanced thereby.
[0065] The toner is preferably an image forming toner mainly composed of a binder resin
and containing optionally a colorant, an internal additive and an external additive
because it can be produced by the known method.
[0066] The toner is preferably a non-magnetic monocomponent toner because it allows the
toner images to be superimposed without color mixture and facilitates maintenance.
[0067] The toner is preferably formed into the predetermined average particle size by melting,
kneading and grinding processes because it allows an image quality having good tone
reproduction to be obtained.
[0068] The invention also provides a non-contact developing method which comprises flying-developing
any one the toners described above to an electrostatic latent image holder so that
the toner exhibits an inter-particle force of 5 nN or less when it is laminated and
carried on a toner carrier, in a developing unit providing at least a toner carrier
for laminating and carrying a charged toner as a developer, an electrostatic latent
image holder disposed so as to face the toner carrier with a gap and an electric field
applying and controlling means for applying and controlling an electric field between
the toner carrier and the electrostatic latent image holder.
[0069] This non-contact developing method allows a stable flying-development to be realized
only by the means for controlling the electrostatic force and field strength acting
on the toner and allows an image quality having good tone reproduction to be obtained.
[0070] In the non-contact developing method in which the toner exhibits an inter-particle
force of 5 nN or less when it is laminated and carried on the toner carrier, it is
preferable that the field applying and controlling means is constructed so that it
controls the toner charge-to-mass ratio so as to satisfy the following inequality
(4):

where E is the electric field strength [V/m] acting on the toner layer, Q/M is the
charge-to-mass ratio [µC/g] of the toner, W
1 is an amount of toner [mg/cm
2] to be separated by the development among the toner laminated and carried on the
toner carrier, εo is the vacuum dielectric constant [C/(V·m)] and ε
T is the apparent specific dielectric constant [C/(V·m)].
[0071] Accordingly, when the field applying and controlling means described above is constructed
so that it controls the toner charge-to-mass ratio so as to satisfy the above inequality
(4), the optimum Coulomb force may be obtained, thus improving the developability
or the like.
[0072] For example, when the amount of toner to be separated by the development is W
1 = 0.3 [mg/cm
2] and when an electric field of E = 2.5 × 10
6 (V/m) is applied to a toner layer having an apparent specific dielectric constant
ε
T = 2, the range of the charge-to-mass ratio Q/M of the toner is found to be 5 ≤ Q/M
≤ 14.8 (µC/g) according to the inequality (4), so that the composition (property)
of the toner and a toner charging mechanism are designed targeting at those values.
Alternatively, it is also possible to determine the toner charge-to-mass ratio in
advance by setting the toner composition and the charging mechanism and then to set
the electric field strength E of the toner layer so as to satisfy the inequality (4).
The electric field strength E which acts on the toner layer varies depending on the
developing conditions such as the potentials of the latent image and the toner carrier,
the thickness of the toner layer laminated and carried on the toner carrier and the
gap between the toner carrier and the electrostatic latent image holder, so that those
values should be controlled so that an adequate field strength E is brought about.
[0073] Further, in the non-contact developing method in which the toner exhibits an inter-particle
force of 5 nN or less when the toner is laminated and carried on the toner carrier,
it preferably comprises peripheral speed ratio control means for controlling a ratio
of the peripheral speeds of the toner carrier and the electrostatic latent image holder
so that the ratio satisfies the following inequality (5) :

where the toner carrier and the electrostatic latent image holder move in the same
direction, k is the ratio of peripheral speeds of the toner carrier and the electrostatic
latent image holder, W
R is a toner mass per unit area [mg/cm
2] on the toner carrier for carrying the toner, W
1 is an amount of toner [mg/cm
2] to be separated by development among the toners laminated and carried on the toner
carrier and W
D is a required amount to be developed [mg/cm
2].
[0074] Accordingly, when the peripheral speed ratio control means is controlled so that
the ratio of peripheral speed satisfies the above inequality (5), the development
density can be assured while preventing the edge enhancement.
[0075] A relation of W
1 < W
D means that the amount of toner W
1 separated from the toner carrier by the development is short from the required amount
to be developed W
D and it occurs when a toner is used whose average charge-to-mass ratio Q/M is large
or toner whose adhesive force is large, thus having an inferior developability.
[0076] When the toner having a large charge-to-mass ratio is used in the inequality (4)
for example, it is necessary to increase the right side of the inequality (4) = εo
ε
T · E/W
1. However, the enhancement of the specific dielectric constant ε
T and the field strength E which acts on the toner is limited in the right side of
the inequality (4) and therefore, the amount of toner W
1 separated by the development becomes small inevitably.
[0077] Accordingly, it is not enough to have the amount of toner W
1 separated from the toner carrier to assure the required amount to be developed by
using the toner having the large charge-to-mass ratio and it becomes necessary to
increase the total amount to be developed by increasing the peripheral speed of the
toner carrier more than that of the electrostatic latent image holder.
[0078] When the peripheral speed of the toner carrier is faster than that of the electrostatic
latent image holder (i.e. When the ratio of peripheral speed satisfies k > 1), the
ratio of peripheral speed k and the toner mass per unit area W
R need to be set so as to satisfy the inequality (5) to prevent the edge enhancement
and to assure the development density and a developing unit which satisfies both of
the inequalities (4) and (5) becomes necessary.
[0079] Then, it is preferable to arrange the field applying and controlling means so that
the toner charge-to-mass ratio satisfies the inequality (4) and to arrange the peripheral
speed ratio control means so that the ratio of the peripheral speeds of the toner
carrier and the electrostatic latent image holder satisfies the inequality (5) in
the non-contact developing method in which the inter-particle force of the toner exhibits
5 nN or less when the toner is laminated and carried on the toner carrier.
[0080] It is noted that the electric field applying and controlling means is composed of
a DC or AC high voltage generating circuit, an electrical field controlling circuit
and others. The electric field applied between the toner carrier and the electrostatic
latent image holder may be either DC or AC.
[0081] The peripheral speed ratio control means comprises a motor driving circuit, a speed
controlling circuit (including a speed detecting circuit and a peripheral speed setting
circuit) and others and is controlled by a microcomputer.
[0082] The present invention will now be explained in detail based on the preferred embodiment
shown in the drawings. It should be understood that the present invention is not limited
to the embodiment.
[0083] Fig. 1 is a section view schematically showing a structure of one embodiment of a
developing unit applied to the inventive toner and to the non-contact developing method
using the same. It is noted that the developing unit shown in FIG. 1 is used also
as a flying-development experimental equipment in the present invention. In the figure,
non-magnetic monocomponent toner 1 is filled in a hopper 7 and is supplied to a toner
carrier (developing roller) 2 by a toner supplying member 6 while being agitated by
a toner agitating member 5. The toner carrier 2 is made of an aluminum sleeve with
31.4 mm in diameter and 315 mm in length and is sandblasted with spherical particle
so as to have a surface roughness of Ra = 1 µm of center line average height.
[0084] The non-magnetic monocomponent toner 1 is charged by a contact and friction of the
supplying member 6 and aluminum. The toner is carried on the toner carrier 2 and is
charged again. A layer thereof is restricted when the toner passes through a blade
4 which charges and restricts the toner. A load of 1 kgf to 3 kgf is applied to the
blade 4 so as to abut against the toner carrier 2. The toner charge-to-mass ratio
is decided by the intrinsic chargeability of the toner, the material of the sleeve
of the toner carrier 2 and a degree of friction between the toner and the roller.
For example, the greater the load applied to the blade 4, the greater the charge-to-mass
ratio becomes.
[0085] A drum 3 which is selected as the electrostatic latent image holder and which is
80 mm in diameter and 320 mm in length is disposed facing to the toner carrier 2 while
keeping a certain gap (0.1 mm to 0.2 mm) therebetween. The toner on the toner carrier
2 is also kept in non-contact with the drum 3. The toner carrier 2 and the drum 3
rotate in a direction as indicated by an arrow in the figure with 175 mm/sec. of peripheral
speed. The toner carrier 2 is grounded and a bias voltage Vb = -700 V which corresponds
to a latent image potential is applied to the drum 3 only by time of one turn of the
drum 3 by field controlling means not shown.
[0086] Here, the development bias voltage Vb becomes 0-(-700) V = 700 V.
[0087] The experimental equipment comprises the field controlling means for applying a potential
or DC electric field between the toner carrier 2 and the drum 3 and the toner charging
means (the blade 4 and the toner supplying member 6) for charging the non-magnetic
monocomponent toner. It is noted that means for injecting charge from a conductive
electrode or corona discharge may be used as the means for charging the toner.
[0088] It further comprises peripheral speed ratio setting means (not shown) for setting
the ratio k of the peripheral speeds k of the toner carrier 2 and the drum 3 and a
motor speed controlling circuit (not shown) for driving the toner carrier 2 and the
drum 3 counterclockwise and clockwise, respectively, at a constant speed with the
set ratio of the peripheral speed. It also comprises adjusting means (not shown) for
finely adjusting the gap between the toner carrier 2 and the drum 3.
[0089] FIG. 2 is an enlarged view of a non-contact developing section applicable to the
inventive toner and to the non-contact developing method using the same. At a section
X of the inside of the toner layer la having a thickness dt
1 formed on the toner carrier 2 which is a metallic sleeve in the figure, the force
in the flying direction is a Coulomb force q · E and the force which impedes the flying
force is an image-force Fi and an inter-particle force Fv at the section X.
[0090] The section X can be found by measuring the thickness of the toner layer on the developing
roller after the flying or the amount of flied toner (amount to be developed). For
example, the use of a laser scanning microscope manufactured by Lasertec Corp. allows
the thickness of the toner layer on the developing roller before and after the flying
to be measured and then allows the section X to be found. The image-force Fi acting
on the section may be calculated when the section X can be found.
[0091] Accordingly, the inter-particle force Fv can be found as a difference between the
Coulomb force q·E and the image-force Fi at the section of the toner layer on the
toner carrier based on the actual measurement of the amount of toner W
1 mg/cm
2 separated by development among the toners laminated and carried on the toner carrier
from the following equations (1) through (3):



where Fv is the inter-particle force, q·E is the Coulomb force calculated by the
equation (2), Fi is the image-force calculated by the equation (3), q is a quantity
of charge [C] of the toner particle to be developed, E is an electric field strength
[V/m] acting on the toner layer, Q/M is a charge-to-mass ratio [µC/g] of the toner,
W
1 is an amount of toner separated by development among the toner laminated and carried
on the toner carrier, εo is a vacuum dielectric constant [C/(V·m)], ε
T is an apparent specific dielectric constant [C/(V·m)] of the toner layer, d is a
particle size [µm] of the toner, δ is a true density [g/cm
3] of the toner, g is a gap [mm] between the outermost surface of the toner on the
toner carrier and the electrostatic latent image holder, dt
1 is a thickness [µm] of the toner layer on the toner carrier, Vb is a development
bias voltage [V] and P is a toner packing rate. It is noted that the toner packing
rate P and the apparent specific dielectric constant ε
T can be found by using the equations (6) through (9) described below.
[0092] A method for obtaining the apparent specific dielectric constant ε
T of the toner layer will now be explained. First it is necessary to know the packing
rate of the toner layer P having a void in order to find the apparent specific dielectric
constant ε
T of the toner layer. The packing rate P can be obtained by using measurements of the
surface potential Vt, the toner charge-to-mass ratio Q/M and the toner mass per unit
area w as follows.
[0093] The surface potential Vt, the toner average charge-to-mass ratio Q/M and the toner
mass per unit area w of the toner layer la on the toner carrier 2 after passing through
the blade 4 were measured in the experimental equipment shown in FIG. 1.
[0094] The surface potential Vt of the toner layer can be expressed as follows:

Rearranging the equation (6) with respect to P gives the following equation as a
quadratic equation of P:

Solving the equation (7) with respect to P gives the following equation:

Accordingly, substituting the measurements of Vt, Q/M and w into the equation (8)
gives the packing rate P, thus allowing to obtain the apparent specific dielectric
constant ε
T from the following equation:

[0095] The actually measured values and the relational equations described above allow to
verify whether the inter-particle force of the toner laminated and carried on the
toner carrier is 5 nN or less.
[0096] Accordingly, it becomes possible to screen the toner having less inter-particle force
Fv by the toner flying experiment at a testing bench without performing any copying
test and to find the property and formula of the non-magnetic monocomponent toner
which attains the non-contact development efficiently.
[0097] It is noted that although the development bias voltage has been set at the same value
as the latent image potential in the present embodiment, the development bias voltage
may have a value different from that of the latent image potential. That is, because
the inter-particle force Fv can be found from the amount of separated toner W
1 with respect to the development bias voltage Vb, the development bias voltage Vb
can take a voltage value between a development starting voltage and a development
saturation voltage.
[0098] When the experiment of the non-contact development was performed by regulating the
inter-particle force of the toner which is an adhesive force other than the electrostatic
force to 5 nN or less in the experimental equipment described above, it was found
that there exist solutions which allow the development regardless of the toner particle
size and even with a toner having a large charge-to-mass ratio (see the Table in FIG.
12).
[0099] It was also found that the range of the charge-to-mass ratio Q/M which allows the
non-contact development at that time can be controlled only by the electrostatic force
so that the following inequality (4) is satisfied:

where E is a field strength [V/m] acting on the toner layer, Q/M is a charge-to-mass
ratio [µC/g] of the toner, W
1 is an amount of toner [mg/cm
2] to be separated by the development among the toner laminated and carried on the
toner carrier, εo is the vacuum dielectric constant [C/(V·m)] and ε
T is an apparent specific dielectric constant [C/(V·m)].
[0100] A process for deriving the inequality (4) will be explained below. It was found,
when the electric field acting on the toner layer and the development gap (the gap
between the outermost surface of the toner on the toner carrier and the electrostatic
latent image holder) was analyzed, that the flying amount (amount to be developed)
per unit area can be expressed by the following equation (10):

E in the equation (10) represents the field strength acting on the toner layer and
is expressed as follows:

where εo is the vacuum dielectric constant [8.85 × 10
-12 C/(V·m)], ε
T is an apparent specific dielectric constant of the toner layer, d is the particle
size of the toner, δ is a true density of the toner, Q/M is a toner charge-to-mass
ratio (quantity of charge per unit mass), Fv is an inter-particle force of the toner,
i.e., a flying restricting force other than the image-force at the flying section,
dt
1 is a thickness of the toner on the toner carrier, Vb is a development bias voltage,
P is a toner packing rate and g is the gap between the outermost surface of the toner
on the toner carrier and the electrostatic latent image holder.
[0101] The apparent specific dielectric constant ε
T of the toner layer within the equations (10) and (11) can be obtained by the specific
dielectric constant εt intrinsic to the toner and the packing rate P of the toner
layer from the equation described above:

[0102] By the way, the condition in which the amount to be developed M/A is greater than
W
1 can be expressed as follows:

[0103] Because it can be assumed that Fv ≅ 0 when the inter-particle force Fv is sufficiently
small, the minimum requirement in which the amount to be developed M/A is greater
than W
1 is expressed as follows by setting as Fv = 0 in the inequality (12):

Accordingly, the range of Q/M which satisfies the inequality (13) may be obtained
from the following inequality:

The inequality (14) is the requirement for obtaining the amount to be developed M/A
in the flying-development.
[0104] Because the electric field acting on the toner layer expressed by the equation (11)
can be approximated as follows when the thickness dt
1 of the toner layer is small as compared to the gap g:

(Where Eg is the electric field of the gap; Eg = Vb/g), the inequality (14) can be
simplified as follows:

It is noted that the equations described above are applicable regardless of the polarities
of the toner. That is, negatively charged toner may be used by letting the absolute
value thereof to satisfy the above-mentioned inequality.
[0105] Next, the lower limit value of the charge-to-mass ratio (Q/M) in the equation (4)
and the rate of the reverse polarity toner will be explained. The toner laminated
on the toner carrier has a charge distribution. FIG. 3 is a graph showing the toner
charge distribution. In the figure, the horizontal axis represents the charge-to-mass
ratio (q/m)
k of the toner particle and the vertical axis represents a rate of frequency p(k) of
the toner particle having the charge-to-mass ratio (q/m)
k.
[0106] Assume here a half-value width b of the distribution as a scale showing the divergence
of the charge distribution. The half-value width b is a difference between the values
of charge-to-mass ratio (q/m)
1 and (q/m)
2 when the rate of frequency p becomes half of the maximum rate of frequency p max,
i.e., (q/m)
1 - (q/m)
2.
[0107] While the half-value width b does not change so much when the average value Q/M (Q/M
= Σ((q/m)
k·p(k)) of the charge-to-mass ratio (q/m)
k of the toner changes, the distribution thereof is shifted in the X-axis direction
when Q/M changes. In such a distribution, the rate of the number of reverse polarity
toner R
N is (the total number of toner particles with reverse polarity)/(the total number
of all the toner particles), and a voluminal rate of reverse polarity toner R
V is (total volume of toner particle with reverse charge)/(the total volume of all
the toner particles).
[0108] In the toner whose Q/M is 5 (µC/g) or less, both the rate of the number of reverse
polarity toner R
N and the voluminal rate of the reverse polarity toner R
V reach around to 10 %, causing a background fog and producing images having less sharpness.
On the other hand, in toner whose Q/M is greater than 5 (µC/g), both the R
N and R
V take values less than 10 %, producing images less deteriorated. Accordingly, the
lower limit value of the Q/M is 5 (µC/g).
[0109] Setting the inter-particle force of the toner to 5 nN or less and the charge-to-mass
ratio Q/M of the toner within the range of the inequality (4) described above, i.e.,
5 (µC/g) ≤ Q/M ≤ (εo ε
T/W
1) · E, when the toner is laminated and carried on the toner carrier as described above
allows the development in non-contact because a desirable amount of toner among the
toners laminated on the toner carrier is desorbed from the toner carrier by the electrostatic
force, thus providing a non-contact developing unit in which the rate of the reverse
polarity toner is small and which provides images having excellent sharpness even
if the charge-to-mass ratio is more or less higher as compared to the past.
[0110] In the non-contact development, the toner laminated and carried on the toner carrier
does not fly to the electrostatic latent image holder by 100 %, so that there is a
method of increasing the peripheral speed of the toner carrier (developing roller)
more than that of the electrostatic latent image holder (photographic drum) as means
for increasing the amount of toner to be developed on the electrostatic latent image
holder.
[0111] However, when the peripheral speed of the toner carrier is increased, density of
the edge portion may be emphasized depending on the development pattern, so that it
is necessary to control the ratio of the peripheral speeds of the toner carrier and
the electrostatic latent image holder adequately.
[0112] The range of the ratio of the peripheral speed k which allows the non-contact development
in such a case and allows a predetermined amount of toner to be developed to be obtained
without causing the emphasis of edge density is obtained by controlling the speeds
so as to satisfy the following inequality:

where the toner carrier and the electrostatic latent image holder move in the same
direction, k is the ratio of peripheral speeds of the toner carrier and the electrostatic
latent image holder, W
R is a toner mass per unit area [mg/cm
2] on the toner carrier for carrying the toner, W
1 is an amount of toner [mg/cm
2] to be separated by development among the toner laminated and carried on the toner
carrier and W
D is a required amount to be developed [mg/cm
2].
[0113] When the moving speed of the toner carrier is twice that of the electrostatic latent
image holder, the amount of the toner W
1 [mg/cm
2] separated by the development among the toner laminated and carried on the toner
carrier is doubled and about 2 W
1 [mg/cm
2] of the toner can be obtained. However, in the case of the non-magnetic monocomponent
toner, the toner onto the toner carrier is apt to be depleted because the amount of
toner capable of adhering onto the toner carrier is less than that of the magnetic
toner or the two-component developer.
[0114] Especially when the latent image pattern changes from a non-developing portion to
a developing portion seen from the side of the toner carrier, there arises an edge-emphasized
development in which the development density is high in the developing portion to
be developed first (especially the boundary area of the non-developing portion and
the developing portion, i.e., the latent image edge portion) because sufficient toner
exists on the toner carrier and the density becomes low in the area other than that.
[0115] The experiment showed that the location where the edge enhancement arises and the
degree thereof are influenced by the orientation and the rate of the relative speed
of the electrostatic latent image holder with respect to the toner carrier. This mechanism
will be explained below.
[0116] FIG. 4 is a drawing showing an area where the edge emphasized image is created, FIG.
5 is a drawing showing the edge emphasized image on a recording sheet and FIG. 6 is
a drawing showing directions of the rotation and the relationships of the rates of
the peripheral speed of the toner carrier and the electrostatic latent image holder.
[0117] In FIG. 4, the reference character Si denotes the image portion (developed portion)
on the drum 3, and A and B denote non-image portions (non-developed portions). S
D in FIG. 5 denotes a toner-deposited portion when this development pattern is transferred
and fixed to the recording sheet 50.
[0118] When the drum 3 rotates clockwise and the toner carrier 2 rotates counterclockwise
as shown in FIG. 4, the moving directions of the both are the same and downward at
the developing section.
[0119] When the ratio of the peripheral speed satisfies k > 1 as shown in FIG. 6, i.e.,
when the peripheral speed V
D of the electrostatic latent image holder 3 is less than the peripheral speed V
R of the toner carrier 2, the orientation of the relative speed V
D - V
R of the electrostatic latent image holder 3 with respect to the toner carrier 2 is
counterclockwise as shown in FIG. 6 (1-b). As a result, the edge B1 which is a boundary
of the non-image portion B and the image portion Si encounters with the toner on the
developing roller first and the development density thereof is increased. Thereby,
an edge B2 on the recording sheet 50 is emphasized.
[0120] When the moving directions of the electrostatic latent image holder 3 and the toner
carrier 2 are the same and the ratio of the peripheral speed is k < 1 and when the
moving directions of the electrostatic latent image holder 3 and the toner carrier
2 are opposite, the relative speed V
D - V
R is clockwise as shown in FIG. 6 (2-b) and an edge A1 of the boundary of the non-image
portion A and the image portion Si is developed first. Accordingly, the development
density of the edge A1 is enhanced and the edge A2 on the recording sheet 50 is emphasized.
[0121] The developing condition should meet the inequality (5) to prevent the edge enhancement.
For example, when the required amount to be developed W
D is 0.5 mg/cm
2, the amount of the toner W
1 separated by development among the toners laminated and carried on the toner carrier
is 0.3 mg/cm
2 and the toner mass per unit area W
R on the toner carrier is 0.8 mg/cm
2, it follows:

and then

[0122] Accordingly, the ratio of the peripheral speed is set at a value between 1.67 and
2.67. Then, the edge enhancement can be prevented by defining a relational equation
among the required amount to be developed W
D, the amount of toner W
1 separated by development among the toners laminated and carried on the toner carrier,
the toner mass per unit area W
R on the toner carrier and the moving directions and the ratio of the peripheral speed
k of the toner carrier and the electrostatic latent image holder as described above.
[0123] FIG. 7 is a graph for setting the range of the charge-to-mass ratio Q/M of the toner.
In the figure, Y-axis represents W
1[mg/cm
2] and X-axis represents Q/M [µC/g].

[0124] When Vb = 700 V (development bias voltage) and the gap g = 0.15 mm, the graph is
expressed as follows:

When Vb = 900 V and g = 0.1 mm, the graph is expressed as follows:

When W
1 is 0.5 mg/cm
2 in the equation (1-2), the value of Q/M is 8.4 µC/g and the allowable width of the
charge-to-mass ratio is the range of 5 ≤ Q/M ≤ 8.4 µC/g indicated by (7a) in FIG.
7. Because the required amount to be developed is 0.5 mg/cm
2 to 0.6 mg/cm
2, the non-contact developing method which satisfies the equation (4) is provided.
[0125] Next, the parameter setting process for setting the charge-to-mass ratio at 10 µC/g
or more will be explained. The amount of the toner W
1 separated by development from the toner carrier under the condition of more than
10 mC/g of charge-to-mass ratio is less than 0.4 mg/cm
2 as can be seen from a curve (1-2) in the graph.
[0126] In a non-contact developing method wherein W
1 is set at 0.3 mg/cm
2, the upper limit of Q/M is 13.5 µC/g and the toner satisfying the inequality of 5
≤ Q/M ≤ 13.5 indicated by (7b) in FIG. 7 can be used, so that a developing unit having
a large allowable width can be realized. At this time, it is essential that the inequality
(5) is satisfied in order to supply the required amount to be developed.
[0127] FIG. 8 is a graph for setting the range of the ratio of the peripheral speed k. In
the figure, Y-axis represents W
1[mg/cm
2] and X-axis represents value of k.

[0128] When W
D = 0.5 mg/cm
2 and W
R = 0.8 mg/cm
2,


When W
1 = 0.3 mg/cm
2, 1.67 ≤ k ≤ 2.67. Accordingly, the toner having a large charge-to-mass ratio can
be applied to the developing unit by satisfying the development condition of the inequality
(5).
[0129] Therefore, while only the inequality (4) needs to be satisfied when the amount of
toner W
1 separated by the development among the toners laminated and carried on the toner
carrier has reached the required amount to be developed W
D, the inequality (5) should also be satisfied at the same time when the amount of
toner W
1 is less than the required amount to be developed W
D. That is, the required amount to be developed can be assured even with the toner
whose average charge-to-mass ratio is large or the toner whose adhesive force is relatively
large and whose developability is bad by satisfying the inequalities (4) and (5) at
the same time.
[0130] Then, it becomes possible to provide a developing unit which can avoid the edge enhancement
and can assure the required amount to be developed even under the condition in which
the amount to be developed per toner carrier is small.
[0131] In the non-contact developing method of the equation (4), the method for increasing
the charge-to-mass ratio Q/M includes methods of controlling it by controlling the
amount of the charge control agent added to the toner, methods of enhancing the degree
of the friction of the toner in the frictional charging mechanism or methods of injecting
charge to the toner forcibly from the outside.
[0132] When the upper limit of the Q/M is to be increased by enhancing the electric field,
a method of increasing Vb (development bias voltage: charge potential of photographic
drum - potential of developing roller) or of reducing the developing gap g may be
adopted.
[0133] For example, the curve of the equation (1-3) when Vb = 900 V and the gap g = 0.1
mm in FIG. 7 is shifted from the curve of the equation (1-2) when the Vb = 700 V (development
bias voltage) and g = 0.15 mm to the side where the charge-to-mass ratio is larger.
Accordingly, it allows the developing unit having a larger allowable width to be constructed.
[0134] One of the purpose of the present invention is to provide a method which allows a
non-contact development even with a small size toner whose particle size is 11 µm
or less. While it has been mentioned that the developability of the small size toner
is low, the reason thereof will be explained below. While the condition required for
the flying-development described above is a condition in which the inter-particle
force Fv other than the image-force acting on the toner is reduced to zero and the
equation (11) becomes independent of the particle size of the toner, the inter-particle
force Fv actually has a certain value and the flying quality of the toner depends
on the particle size.
[0135] For example, when the amount to be developed M/A is calculated by using the equation
(10), the result turns out as shown in FIG. 9. Here, the smaller the particle size
is the lower the flying property is. FIG. 9 is a graph showing a relationship between
the particle size of the toner d and the amount to be developed M/A with respect to
the inter-particle force Fv of the toner. When the allowable range of the charge-to-mass
ratio which allows the development is calculated with respect to the inter-particle
force Fv, the result turns out as shown in FIG. 10, which is a graph showing the allowable
range of the charge-to-mass ratio Q/M with respect to the inter-particle force Fv.
[0136] In FIG. 10, the allowable ranges of the charge-to-mass ratio when the inter-particle
force Fv = 0, 2 and 5 nN are A, B and C (A > B > C) and it can be seen that the greater
the value of the inter-particle force Fv is, the lower the flying quality of the toner
for assuring the required amount to be developed and the narrower the allowable range
of the charge-to-mass ratio is. It hampers the improvement of the image quality as
described before. It can be seen that it is important to reduce the inter-particle
force Fv of the toner in order to make it possible to develop even with the small
size toner and with a relatively high charge.
[0137] FIG. 11 is a graph showing a relationship between the toner charge-to-mass ratio
and the amount to be developed with respect to the particle size/inter-particle force
of the toner. For example, while toner having 12 µm of particle size can assure 0.25
mg/cm
2 of amount to be developed M/A even when the inter-particle force Fv is 6 nN, the
developed amount decreases considerably in case of a toner having 7 µm of particle
size when the Fv is 6 nN as shown in FIG. 11. Meanwhile, the toner with 7 µm of particle
size can have the same or higher flying property as the toner with 12 µm of particle
size under the condition of Fv = 1 nN. It can be then understood from FIGs. 10 and
11 that Fv must be kept at 5 nN or less in order to assure more than 0.25 mg/cm
2 of toner amount separated by the development among the toners whose particle size
is less than 11 µm and laminated and carried on the toner carrier.
[0138] Reducing, by this way, the inter-particle force Fv which is an adhesive force of
the toner means that it can be controlled only by the electrostatic force. Because
the inter-particle force Fv is susceptible to the influence of the environment such
as temperature and humidity from the beginning, the flying property of the toner is
swayed, rendering it impossible to obtain a stable flying-development. However, the
use of a toner having a small Fv value allows the toner having a relatively high charge-to-mass
ratio to be used and allows an electrical control to be implemented readily. Then,
the present invention provides a non-magnetic monocomponent toner having a small inter-particle
force Fv. When Fv was evaluated by the above-mentioned method by producing, in a trial,
various non-magnetic monocomponent toners having an average particle size of 11 µm
or less, it was found that the effect of reducing the value of Fv is significant when
particles of 0.01 µm to 1 µm in diameter are added.
[0139] The method of adding another kind of particles to the toner for the purpose of improving
the characteristics of the toner has been described, for example, in Japanese Examined
Patent Publications No. Sho. 59(1984)-7098 and No. Hei. 2(1990)-45191 as described
before. In the Publication No. Sho. 59(1984)-7098, a hydrophobic silica is contained
in the toner to improve the fluidity of the toner and to prevent coagulation. In the
Publication No. Hei. 2(1990)-45191, a granulating silica powder having 1 µm to 100
µm of particle size is contained to improve frictional charging performance of the
toner.
[0140] In the present invention, the charging performance of the toner is controlled by
adding a CCA (a toner charge control agent) and particles having 0.01 µm to 1 µm of
diameter are contained as a factor for controlling the adhesive force of the toner.
[0141] The inter-particle force of the toner can be reduced and the inter-particle force
Fv at the flying section of the toner layer on the toner carrier can be reduced to
5 nN or less by including the particles having 0.01 µm to 1 µm of diameter in the
toner having an average particle size of 11 µm or less.
[0142] As a result, the amount to be developed can be assured in the flying-development
even with the small size toner of 11 µm or less. The effect of reducing the adhesive
force becomes low when particles whose diameter is less than 0.01µm are added to the
toner whose average particle size is 11 µm or less. Further, when particles larger
than 1 µm are added, it gives a bad influence to the image quality because their size
is close to that of the toner particle.
[0143] As described above, the amount to be developed can be assured in non-contact even
with the small size toner whose diameter is 11 µm or less by finding the inter-particle
force Fv other than the image-force Fi which acts on the section of the toner layer
on the toner carrier and by reducing the value to 5 nN or less.
[0144] FIG. 12 is a table showing results of the flying experiment of the inventive toners.
As shown in the Figure, the results of the experiment carried out with respect to
the toners (toners A through F) each having different average particle size d, average
charge-to-mass ratio Q/M and inter-particle force Fv are shown in the table form.
Among the items [1] through [21] in the figure, the measured values in the items from
[7] to [14] are average values taken by carrying out the same measurement by three
times.
[0145] The toner A is a toner having an average particle size of 12.3 µm and a small charge-to-mass
ratio of 2.1 µC/g. No external additive is added to this toner, so that the inter-particle
force Fv is 6.77 nN and is relatively large.
[0146] The toner B is a toner having a small average particle size of 7.3 µm and a very
large chargeability such that a charge-to-mass ratio thereof is 31.9 µC/g. No external
additive is added to this toner. The inter-particle force Fv at this time is 8.28
nN.
[0147] The toner C has an average particle size equal to that of the toner B, which is 7.3
µm, and has 14 µC/g of charge-to-mass ratio which is the intermediate value between
the toners A and B. Silica particles having 0.02 µm of average particle size are added
externally as an external additive. The inter-particle force Fv at this time is 0.79
nN.
[0148] The toner D has 7.3 µm of average particle size and 14 µC/g of charge-to-mass ratio
and contains conductive particles having 0.5 µm of average particle size added to
it as an external additive. The inter-particle force at this time is 0.47 nN.
[0149] The toners E and F have the respective values as shown in the table.
[0150] When the upper limit of the charge-to-mass ratio is calculated by the equation (14)
assuming that the required amount to be developed would be 0.3 mg/cm
2 with respect to the toners A through E, it can be seen from the table that the resultant
values are 17.3, 14.3, 20.0, 21.8 and 20.6 µC/g, respectively, and that although the
toners A, C, D and E stay within the range of the equation (14), the toner charge-to-mass
ratio B is out of the adequate range.
[0151] In the experiment, while the flying amount W
1 of the toner A is 0.36 mg/cm
2, that of the.toner C is 0.30 mg/cm
2, that of the toner D is 0.28 mg/cm
2 and that of the toner E is 0.30 mg/cm
2, which are close to the required amount to be developed, the flying amount W
1 of the toner B is 1/100 of the target value and almost nothing is developed.
[0152] The adequacy of the equation (14) could be proved from the above.
[0153] Next, the analysis of the rate of the reverse polarity toner will be explained. When
the toner charge distribution flied on the drum 3 assumed to be the electrostatic
latent image holder was measured by a simple harmonic oscillatory air current method
by using a laser Doppler method (E-Spart Analyzer of Hosokawa Micron Co.), while a
voluminal rate Rv of the reverse polarity toner of the toner A whose average charge-to-mass
ratio Q/M is 2.1 µC/g was 28.5 %, Rv of the toner D whose Q/M is 5.1 µC/g was 9.8
%, Rv of the toner E whose Q/M is 7.3 µC/g was 5.2 % and Rv of the toner F whose Q/M
is 8.2 µC/g was 3.0 %. The flying amount of the toner B was so small that no measurement
could be implemented. From above, the rate of the reverse polarity toner of the toners
whose average charge-to-mass ratio exceeds 5 µC/g could be reduced to less than 10
%. It was also confirmed that the greater the Q/M is, the smaller the rate of the
reverse polarity toner after the development is.
[0154] From the results of the toner flying experiment, it was proven that the required
amount to be developed W
1 (the amount of the toner separated by development among the toners laminated and
carried on the toner carrier) can be assured by reducing the inter-particle force
Fv of the toner of of µm or less to 5 nN or less. The inter-particle force Fv can
be obtained by substituting the measured values into the equations (1) through (3).
[0155] It was also proved that the inter-particle force Fv can be further reduced by adding
the particles of 0.01 µm to 1 µm to the toner of 11 µm or less.
[0156] Further, the effectiveness of the lower and upper limits of the charge-to-mass ratio
(Q/M) in the inequality (4): 5 µC/g ≤ Q/M ≤ (εo ε
T/W
1)·E, was proved.
[0157] While it has been considered in the past that the essence of non-contact development
is to carry a toner having a lower charge-to-mass ratio Q/M (3 µC/g) or a lower charge
density Q/A (3 x10
-10 ≤ | Q/A (C/m
2) | ≤ 10
-7) on the toner carrier with, for example, a lower packing density δP (0.1 g/cm
3 to 0.6 g/cm
3) and with a thicker toner layer dt
1 (15 µm to 100 µm), it is not practical because it contains much reverse polarity
toner as can be seen from the result of the flying experiment of the toner A carried
out under the conditions which are close to the above-mentioned developing conditions.
[0158] However, as the result of the flying experiment of the toners C through F shows,
the toner having a higher charge-to-mass ratio Q/M (5.1 µC/g to 14.0 µC/g) can be
carried on the toner carrier and flying-developed with a higher packing density δP
(0.51 g/cm
3 to 0.82 g/cm
3) and with a thinner toner layer dt
1 (8.5 µm to 15.5 µm) and an excellent image quality having a smaller voluminal rate
Rv of the reverse polarity toner can be obtained by suppressing the inter-particle
force Fv of the toner to 0.79 nN to 2.79 nN by the means for controlling the electrostatic
force and the field strength.
[0159] Here, packing density (δP) = true density (δ) × packing rate (P), and

[0160] The toner whose flying amount is the largest among the toners shown in the table
in FIG. 12 is the toner F whose average particle size is the largest next to the toner
A and 0.5 g/cm
2 of developed amount can be obtained per one turn of the toner carrier (the developing
roller). When the developed amount is more than 0.5 g/cm
2, an optical reflection density of more than 1.3 can be obtained, so that the desirable
performance can be assured with the developing unit in which the ratio of peripheral
speed k of the developing roller = 1 with respect to the toner F. Meanwhile, considering
the developing unit using the toner C, the toner C is a toner whose particle size
is the smallest, whose charge-to-mass ratio is higher and whose voluminal rate Rv
of the reverse polarity toner is 1.4 % which is sufficiently small. Accordingly, although
the toner C is expected to give an image quality having an excellent sharpness with
the synergetic effect of improving the image quality by reducing the particle size,
the developed amount per one turn of the developing roller is 0.3 mg/cm
2 and is not reaching the required amount of 0.5 mg/cm
2 to be developed.
[0161] Accordingly, the total amount to be developed on the electrostatic latent image holder
must be increased by increasing the ratio of the peripheral speed k of the developing
roller to more than one. However, because the edge enhancement is caused as described
before under the condition of a large ratio of the peripheral speed, the inequality
(5), i.e., W
D ≤ W
1 ·k ≤ W
R, must be further satisfied in order to realize the developing unit which causes no
edge enhancement.
[0162] Therefore, an arrangement which satisfies the both developing conditions of the inequalities
of (4) and (5) becomes important.
[0163] In order to confirm the effectiveness of the inequality (5) of the present invention,
a developing unit having the same arrangement as the experimental developing unit
in FIG. 1 was incorporated into an actual copying process to carry out a copying test
using the toner C. A type of copier having a copying rate of 20 sheets/minute and
a processing speed of 175 mm/second was used.
[0164] When the required amount to be developed W
D is 0.5 mg/cm
2, substituting 0.3 mg/cm
2 of developed amount W
1 per one turn of the developing roller of the toner C and 0.6 mg/cm
2 of toner mass per unit area W
R in the inequality (5) gives the following results:

Then, the ratio of the peripheral speed k was set at 1.7.
[0165] That is, the drum 3, i.e., the electrostatic latent image holder, was rotated clockwise
with 175 mm/second of the peripheral speed and the developing roller 2 was rotated
counterclockwise with 300 mm/second of the peripheral speed. The gap between the drum
3 and the developing roller 2 was set at 0.13 mm.
[0166] A potential of the latent image at the image portion of the drum 3 was set at -700
V and the developing roller 2 was grounded. Copied images were taken under these developing
conditions. They were then photographed by a CCD camera, moved in the Y-axis direction
on the recording sheet 50 shown in FIG. 5 and were taken in by setting the output
level i of the CCD camera as the data of one pixel with 256 gradations. When these
data were translated into density data by using the relationship of reflection density
D = -ln (i/256), a density distribution as shown in FIG. 13 could be obtained.
[0167] FIG. 13 is a graph showing an actually measured example 1 of the density distribution
of the copied image flying-developed by the inventive developing method. As shown
in the figure, a good image quality having no edge enhancement and no background fog
can be obtained with more than 1.4 of optical reflection density when the ratio of
the peripheral speed k (peripheral speed of developing roller/peripheral speed of
drum) is set at 1.7.
[0168] Further, when a repetitive pattern of black and white stripes was copied and the
copied image was evaluated by the CCD camera to determine the resolution, the resolution
which enables to reproduce 5 lp/mm was obtained.
[0169] Further, in order to verify the effectiveness of the inequality (5), a copying test
was carried out by using the same developing unit as that used in the embodiment described
above and by changing the ratio of the peripheral speed k of the developing roller
for comparison.
[0170] FIG. 14 is a graph showing an actually measured example 2 of the density distribution
of the copied image flying-developed by the inventive developing method. It shows
results of the density distribution of the toner deposit portion S
D with respect to the Y-axis direction (direction from the edge A2 to the edge B2)
when k = 3 and k = 0.5.
[0171] When k = 3, density of the edge B2 was emphasized and when k = 0.5, the edge A2 was
emphasized, disallowing to obtain a homogeneous density distribution.
[0172] It is noted that the density distribution when the developing roller and the drum
are rotated in the opposite direction from each other as shown in (3-b) in FIG. 6
was such that the edge A2 was highly emphasized.
[0173] The copying tests described above proved that a developing unit in which the developed
amount is increased without having any edge enhancement can be provided by arranging
so that the developed amount per one turn of the developing roller, the ratio of the
peripheral speed of the developing roller and the required amount to be developed
satisfy the inequality (5).
[0174] While it has been explained in the present embodiment that the desirable developing
unit can be realized by combining an inventive arrangement for setting the developing
condition of the inequality (4) with an inventive arrangement for setting the developing
condition of the inequality (5) in the case when the toner such as the toner C whose
particle size is small, whose charge-to-mass ratio is relatively large and whose developed
amount is small is used, it is possible to realize the developing unit which satisfies
the developing condition of the inequality (4) or of the inequality (5) without combining
those two inventions when the developed amount exceeds the developed amount per one
turn of the developing roller.
[0175] That is, the developing unit which can satisfy the developing condition of the inequality
(4) can be constructed by setting the ratio of the peripheral speed k at 1 or an arrangement
which does not satisfy the two developing conditions at the same time (means of setting
the ratio of the peripheral speed k to a value smaller than 1), though it is included
in the invention satisfying the developing condition of the inequality (5), is also
possible.
[0176] The inventive non-magnetic monocomponent non-contact development allowed linear Gamma
characteristics (tone reproduction) to be obtained without any offset in the development
starting potential owing to the features thereof that there is no magnetic restraint
at the developing section, that it allows the development only by the control of the
electrostatic force and the electric field strength acting on the toner and that the
mechanical adhesive force of the toner is small. Due to that, it allowed the development
faithful to a latent image potential to be realized and good images containing half-tones
like a photograph to be copied.
[0177] It is noted that although the toner supplying member 6 which contacts the toner carrier
2 has been used as means to charge and to supply the non-magnetic monocomponent toner
1, i.e., it charges the non-magnetic monocomponent toner by the friction with the
toner carrier 2 and applies it onto the toner carrier 2, in the embodiment of the
present invention, it is possible to use means of injecting charge from a conductive
electrode or of corona discharge as means for charging the non-magnetic monocomponent
toner.
[0178] Although the change of the toner charging and applying means may change the toner
charge-to-mass ratio and the toner mass per unit area on the developing roller even
if the same toner is used, the developing unit which allows an excellent image quality
having less reverse polarity toner and no background fog to be obtained and to avoid
the edge enhancement may be provided by controlling the electrostatic force and the
electric field strength acting on the toner.
[0179] Further, although the developing roller has been provided as a toner carrier in the
present invention, it is also possible to use means other than the roller. For example,
the developing unit which has been applied to the embodiment of the present invention
may be provided even when a turning developing belt is used as a toner carrier.
[0180] As described above, according to the present invention, the stable non-contact development
can be realized only by means for controlling the electrostatic force and the electric
field strength acting on the toner by suppressing the inter-particle force of the
toner other than the image-force which acts on the toner to 5 nN or less. As a result,
the present invention brings about the following effects:
a) It provides a stable flying-development having a large allowable width of the toner
charge-to-mass ratio to be realized;
b) It can provide the non-contact developing method which provides images having less
reverse polarity toner, having no background fog and having an excellent sharpness;
and
c) It provides a non-contact development even with the small size toner whose particle
size is 11 µm or less, thus providing a non-contact developing method excellent in
resolution and gradation. Accordingly, it is not necessary to develop images in such
a manner that a monochromatic image is contactingly developed in order to secure the
resolution and a color image is non-contactingly developed aiming at gradation and
convenience of color superimposition as in a conventional method.
[0181] The applicable range of the present invention is broadened further by controlling
the ratio of the peripheral speeds of the toner carrier and the electrostatic latent
image holder. That is,
d) It can provide a sufficient development density even with a toner having a low
developed amount per one turn of the developing roller;
e) At that time, it prevents the edge enhancement which might be caused by setting
the ratio of the peripheral speed and thus allows a homogeneous density distribution
to be obtained; and
f) As a result, toners with a high charge-to-mass ratio which could not be put into
practical use in the past because of its low developability can be actively used.
That is, the toner having a high charge-to-mass ratio can be held on the developing
roller even when the roller rotates at a high-speed and is thus applicable to a high-speed
process.
[0182] As described above, the present invention can provide a developing unit which can
be applied to the non-contact development without setting any particular restriction
on the charge-to-mass ratio and the particle size of the toner, which can realize
development in a low-speed through high-speed process, whether in monochrome or color,
and which can be widely utilized as an image forming unit of copiers and printers.
[0183] While the preferred embodiments have been explained, it is to be understood that
various modifications thereto will occur to those skilled in the art within the specific
scope of the present inventive concepts which are exhibited by the following claims.