[0001] This invention relates to the formation of toner images and, more specifically, to
the formation of toner images of two distinct toners, for example, toners of two different
colors.
[0002] U.S. Patent No. 5,001,028 to Mosehauer et al, issued March 19, 1991, is representative
of a large number of patents which show the creation of multicolor toner images by
creating two unfixed images on a single frame of a photoconductive image member. Color
printers have been marketed using this general approach, using discharged area development
(DAD) and electronic exposure for each image.
[0003] In the Mosehauer patent, the second and subsequent images are toned with a particular
toning process using high coercivity carrier and a rotating magnetic core. This process
provides a very soft magnetic brush which disturbs the earlier toner images less than
an ordinary magnetic brush, even though the brush strands may be allowed to contact
the image member.
[0004] "Scavenging" of first image toner into the second station is a well documented problem,
especially of repetitive DAD toning steps (DAD-DAD imaging). Many prior references
suggest projection toning for applying toner to second and subsequent images in DAD-DAD
processes. This reduces "scavenging" of the first image into the second toning station,
but it also reduces the speed at which the second image can be toned to reasonably
full density. U.S. Patent No. 5,409,791 to Kaukeinen et al, issued April 25, 1995,
demonstrates that the use of a toning process similar to that used in the Mosehauer
reference but with the brush strands separated from the image member can maintain
both the reduced scavenging of projection toning while still toning at high speed
(albeit not as high as contact toning as shown in Mosehauer).
[0005] Although projection toning reduces scavenging to tolerable limits and the use of
the Kaukeinen invention provides improved density at high speed compared to prior
projection systems, performance of projection toning is heavily dependent upon accurate
spacing between a toning applicator and the image member. Such spacing has been found
to be quite difficult to maintain in practice and, in fact, is accomplishable only
by putting more complexity and expense into the components themselves.
[0006] European Patent Application 94-107101.1, published November 23, 1994 (Publication
No. 0 625 730), describes a problem with a DAD-DAD system in which toner on the edges
of areas of the first image, which abut second images, have a tendency to move into
the second image as a result of the second exposure. Overlapping of the first image
with the second image corrects this problem in this process. The image overlapping
is facilitated by exposing the second image through the base.
[0007] A few references suggest a mixture of discharged area development and charged area
development (DAD and CAD). For example, see U.S. Patent No. 5,045,893 to Tabb, granted
September 3, 1991, in which the first toner image is made using DAD, the image member
is recharged and re-exposed with the second image made using CAD. See also in this
respect, U.S. Patent Nos. 5,208,636; 5,241,356; 5,049,949; and 5,258,820.
SUMMARY OF THE INVENTION
[0008] It is an object of the invention to improve on such prior multi-image systems.
[0009] This and other objects are accomplished by a method in which an image member is charged
to a first polarity and exposed to create an electrostatic image. The electrostatic
image is developed with toner also charged to the first polarity (DAD). The image
member is preferably not recharged, but is exposed to create a second electrostatic
image, still of the first polarity. This image is toned with a toner of a second polarity
opposite the first polarity (CAD). Preferably, the second toning step uses contact
development with a magnetic brush of the Mosehauer type. That is, it has "hard" magnetic
carrier and a rapidly rotating magnetic core with the developer moving into direct
contact with the image member during the development process. Because of the contact
development, applicator to image member spacing is no longer critical, and image density
at high speed is improved. Scavenging is far less than with a DAD-DAD process because
the development field in the second station is maintained at a level that rejects
toner of the first polarity. Also, because of the fields, there is very little "overtoning"
of the first image by the second station.
[0010] In experimenting with this process, we found that line images had a tendency to lose
their resolution. After some analysis, we concluded that this was due to movement
of toner in response to the second exposure step, somewhat similar to the movement
of toner in a DAD-DAD process toward which the prior Guth et al patent application
was directed (European Publication No. 0 625 730, cited above). We have demonstrated
that this problem is corrected when the second exposure is accomplished from the side
of the image member opposite that containing the first toner image. This allows us
to discharge the image member under the first image to a level below that of the untoned
portions of both images. Note that this is not a case of overlapping toner images,
as was done in the Guth application, since the image portion of the second image may
be quite separated from the first toner image. It is a solution based on assuring
thorough discharge of the area under the first toner image in the second exposure.
[0011] Thus, according to a preferred embodiment, high resolution two color toner images
are produced with little scavenging using a DAD-CAD process with contact developing
of the second image with a brush having "hard" magnetic carrier and a rotating magnetic
core and with a second exposure through the side of the image member opposite the
first toner image.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1-7 show seven steps in a method for forming a two color image.
[0013] FIGS. 8-14 show a submethod variation on the method shown in FIGS. 1-7.
[0014] FIGS. 15-18 show a four step submethod which is a variation to the submethod shown
in FIGS. 8-14.
[0015] FIG. 19 is a side schematic of an image forming apparatus.
[0016] FIG. 20 is a side schematic of a development station.
DETAILED DESCRIPTION OF THE INVENTION
[0017] FIGS. 1-7 illustrate a DAD-CAD method (sometimes herein called a "submethod") of
forming a two color toner image. According to FIG. 1, an image member 1 includes an
image side 2 and an opposite or base side 4. Preferably, the image member 1 is transparent.
For example, it can be a belt or web image member having a polyester or other suitable
transparent support well known and used in the art. Such image members include a transparent
conductive layer on the image side 2 upon which is coated suitable photoconductive
layers to make the image member 1 photoconductive. The conductive layer and photoconductive
layers are extremely thin and are not shown in the FIGS. The image member could also
be a glass drum with similar layers coated thereon.
[0018] The method will be described with respect to a negative charging image member. It
will be clear to those skilled in the art that a positive charging image member could
also be used.
[0019] The image member is uniformly charged to a negative potential. It is imagewise exposed
to create a negative electrostatic image and, as shown in FIG. 1, a negative toner
is applied to it in the presence of an electric field which encourages deposition
of the negative toner according to the amount of discharge of the electrostatic image.
That is, the toner shown in FIG. 1 adheres to areas of lowest potential in the electrostatic
image, creating a first toner image, as shown in FIG. 2, created by a DAD process.
[0020] According to FIG. 3, the image member is exposed through its base 4 (preferably without
recharging) to create a second electrostatic image. It is important that this exposure
be conducted through the base in order to discharge the charge underneath the first
toner image below that of exposed areas surrounding the first toner image. If this
charge is not dissipated, the negatively charged first toner has a tendency to be
pushed by that charge into the areas adjacent it that have been equally or more discharged,
thereby reducing the sharpness and resolution of the first toner image. With exposure
through the base, we have been able to reduce the charge underneath the first toner
image to 50-100 volts below that of areas surrounding the image that are also exposed.
This allows the adjacent negative charge to hold the negative toner of the first image
in place.
[0021] According to FIG. 4, the second electrostatic image is toned using a CAD toning process.
More specifically, a magnetic brush 11 containing positive toner applies toner to
image member 1 in the presence of an electric field that encourages deposition of
toner on the high potential portions of the second electrostatic image. Typically,
in CAD development, the electric field is biased so that magnetic brush 11 is slightly
more negative than the exposed portion of the second electrostatic image to inhibit
development of the nonimage areas in the second electrostatic image. This bias also
inhibits pickup of negative toner from the first image.
[0022] The two color toner image, shown in FIG. 4, contains both negatively charged and
positively charged toner. For electrostatic transfer, the image is treated so that
it responds more uniformly in an electrical field. According to FIG. 5, this is accomplished
by using a negatively biased corona charger 15 which sprays negative charges on the
image side of the image member 1. As is well known in the art, the charger 15 can
be assisted by an erase lamp 17 which, with a transparent image member 1, can be positioned
on the opposite or base side of the image member 1. As shown in FIGS. 6 and 7, the
two color image is transferred to a receiving sheet 30 in the presence of an electric
field created by a bias supplied from a source of potential 19 to a transfer backing
member or roller 23. The receiving sheet 30 is separated from the image member 1,
as shown in FIG. 7, with the two color image attached.
[0023] The use of CAD developing with opposite polarity toner for the second image greatly
reduces the tendency of first toner to scavenge into the second station or for the
second toner to overtone the first toner image. This permits the use of contact developing
rather than projection developing with its increased density at high speed and greatly
reduced sensitivity to spacing.
[0024] The reduced scavenging effect is optimized by the use of a brush similar to that
shown in the Mosehauer patent. This brush will be discussed in more detail with respect
to FIG. 20. It has a very soft nap which reduces the tendency of the brush to scrape
off the first image and also provides extremely high density development at high speed
with a relatively small development station.
[0025] Early resolution problems we encountered were caused by movement of toner in the
first image in response to the second exposure step. This movement is due to residual
charge of a negative polarity underneath the first toner which pushes the particles
into the background of the second image which has less remaining charge after the
second exposure. This problem has been virtually eliminated by exposing the second
image through the base, as shown in FIG. 3 and as explained above.
[0026] Any toner from the first image that has a tendency to be scavenged into the second
toning station is likely to be deposited in the background of subsequent images, assuming
it has maintained its original charge. The life of the developer in the second station
is dependent upon the extent of this scavenging. The point at which a noticeable amount
of the first, negative toner from the first image appears in the background of the
second image is the point at which the developer must be changed in the second station.
Because of the polarities of the toners in the DAD-CAD process, shown in FIGS. 1-7,
and the softness of the development brush, shown in FIG. 20, this effect is quite
small, and the life of the developer is relatively long. However, we have found two
submethods which can prolong the life of the second developer even further.
[0027] The first submethod is illustrated in FIGS. 8-14. These FIGS. show a method in which
there is a demand for second color only images. That is, if the color of the first
toner image is black and the color of the second toner image is red in the process
shown in FIGS. 1-7, the same apparatus will, from time to time, be asked to make only
red images for one or more runs. Image member 1 is shown in FIG. 8 without any charge
on it as it enters the process. According to FIG. 9, it is charged to a negative polarity
by the same charging device 9 used to form the first electrostatic image in FIG. 1.
According to FIG. 10, the image member 1 is imagewise exposed to create a negative
electrostatic image, and according to FIG. 11, that electrostatic image is toned by
application of positive toner from magnetic brush 11.
[0028] Magnetic brush 11 has been operating for many images without changing the developer
and contains some negatively charged toner that had been scavenged from first, black
images in earlier two-color image forming runs. Some of these negatively charged black
toner particles have a tendency to deposit in the background portions of the electrostatic
image, as shown in FIG. 11.
[0029] However, for the submethod shown in FIGS. 8-14 where only red images are desired,
the treatment step shown in FIG. 5 is eliminated and the red image with some black
toner particles in its background is fed into the transfer station, as shown in FIG.
12. However, the transfer field is reversed in FIG. 12 from that shown in FIG. 6 to
encourage the transfer of untreated positively charged (red) toner particles while
inhibiting the transfer of negatively charged (black) toner particles. Thus, as shown
in FIG. 13, the receiving sheet 30 is separated from image member 1 with the red toner
image adhering to receiving sheet 30 and the black toner adhering to image member
1. As shown in FIG. 14, the black toner is cleaned off image member 1 by a cleaning
device 29 at a cleaning station so that the image member 1 can be reused.
[0030] The submethod shown in FIGS. 8-14 provides background-free copy with an old developer
while actually getting rid of some of the contamination of magnetic brush 11. An extended
single color run of this nature would have a cleansing effect on the second development
station, thereby prolonging the life of its developer.
[0031] FIGS. 15-18 illustrate use of a similar submethod to that shown in FIGS. 8-14 to
clean the second development station without actually making images. According to
FIG. 15, the image member is discharged by some means, for example, the same exposure
means that forms the electrostatic image in FIGS. 3 and 10. According to FIG. 16,
the magnetic brush 11 is allowed to attempt to tone the discharged image member 1
in the presence of a field urging the application of negative toner to image member
1 while discouraging the application of any positive toner. This field is shown in
FIG. 16 as having a somewhat stronger negative bias on toning station 11 than is used
for development (FIGS. 4 and 11). The treating step shown in FIG. 5 can be again eliminated.
The transfer step, shown in FIG. 6, is also eliminated. No receiving sheet need be
fed. As shown in FIG. 17, transfer backing member 23 is articulated away from image
member 1. Transfer to the backing member 23 can also be prevented by adjusting the
bias on transfer backing member 23 as is done in FIG. 12. If electrostatic transfer
is accomplished by a corona, the corona need only be turned off. The negative (black)
toner on image member 1 is then cleaned off by cleaning device 29, as shown in FIG.
18.
[0032] The submethod shown in FIGS. 15-18 can be employed any time the apparatus is otherwise
not in use, for example, it can be routinely used while the apparatus is warming up,
since the routine does not require use of a fuser which may require the longest warmup
time.
[0033] FIG. 19 shows an image forming apparatus 10 for carrying out all three submethods
shown in FIGS. 1-18. According to FIG. 19, an image member 1 in the form of a transparent
photoconductive belt is trained about a series of rollers for movement through a path
past a series of stations. Image member 1 is uniformly charged by a charging station
9 and imagewise exposed at a first exposure station, for example, an LED printhead
5, a laser, or the like, to form a first electrostatic image. The first electrostatic
image is of a first polarity as determined by the charge from charging station 9.
[0034] A first toner of a first color and a first polarity (for example, negative black
toner) is applied to the first electrostatic image by a suitable toning station 7
to form a first toner image. Because the toner is negative in polarity, it adheres
most densely to the most discharged portions of the first electrostatic image (DAD
development).
[0035] The image member with the first toner image can be recharged by a charging station
6, again, to a negative potential (a step that may well not be necessary). Whether
recharged or not, it is imagewise exposed by a second exposure station, for example,
an LED printhead 12, laser, or the like, positioned behind image member 1 to create
a second electrostatic image. This electrostatic image is toned by the application
of positively charged toner of a second color (for example, red) from the previously
shown second toning station 11. Alternatively, the second electrostatic image can
be toned by application of positively charged toner of a third color, for example,
yellow, from a third toning station 13.
[0036] Preferably, both toning stations 11 and 13 are constructed, as will be described
with respect to FIG. 20, to provide a soft magnetic brush having a tendency to cause
little disturbance to the first toner image and providing extremely high density at
high speed with a relatively small station.
[0037] If the colors of the toners in toning stations 7 and 11 (or 13) are of different
color, image member 1 now has a two color image. Obviously, the toners in stations
7 and 11 could be of the same color but different characteristics. For example, one
of the two toners could be a black magnetic toner and the other a black non-magnetic
toner, which arrangement would have certain advantages in certain processes. For purposes
herein, such a combination of non-magnetic and magnetic black toners is essentially
the same as a two color toner image.
[0038] The two color toner image proceeds to a treatment station made up of a corona charger
15 and an erase lamp 17. Corona charger 15 is biased to change the polarity of one
of the toners making up the two color toner image. For example, it may apply a negative
charge to the two color image, thereby changing the charge on the positive toner particles
applied at either toning station 11 or toning station 13. As is well known in the
art, this process can be assisted by use of an erase lamp 17 positioned at and/or
before the corona charger 15 which further reduces the affinity of the toner for image
member 1.
[0039] The treated two color image proceeds to the transfer station, including a transfer
backing roller 23, where a potential is applied from a reversible potential source
19, encouraging transfer of the toner from a receiving sheet fed from a receiving
sheet supply 21.
[0040] The receiving sheet is separated from image member 1 as image member 1 passes around
a small roller and is transported to a fuser 25 and eventually into an output tray
27. Cleaning station 29 cleans the image member 1 so that it can continue to be used
in the process.
[0041] The above description describes operation of image forming apparatus 10 in carrying
out the submethod described with respect to FIGS. 1-7. In carrying out the submethod
described in FIGS. 8-14, exposing LED printhead 5 and toning station 7 are turned
off while single color images are made using charging station 9, exposing LED printhead
12 and either of toning stations 11 or 13. Corona device 15 and erase lamp 17 are
also turned off, and the bias applied from reversible voltage supply 19 to backing
roller 23 is reversed so that the red or yellow toner applied by stations 11 and 13,
in fact, transfers to the receiving sheet, while any black toner contained in either
of those stations that is deposited in the background continues on on image member
1 to be cleaned off by cleaning device 29.
[0042] The submethod described with respect to FIGS. 15-18 is carried out by the image forming
apparatus 10, for example, during cycle-up of the apparatus 10 when it is initially
turned on for the day. During such cycle-up, while the fuser 25 is warming up, the
image member 1 is driven through its endless path. LED printhead 12 and either or
both of toning stations 11 and 13 are turned on. LED printhead 12 fully erases any
charge on image member 1 and bias is applied to stations 11 and 13 which encourages
the deposition of any black toner remaining in those stations onto image member 1.
The black toner, being negative in polarity, is repelled by a strong negative bias
on toning stations 11 and 13. Again, corona 15 and erase lamp 17 are turned off and
transfer backing member 23 is articulated away from image member 1 to allow the black
toner on image member 1 to continue to the cleaning device 29. Obviously, no sheet
is fed from receiving sheet supply 21 for this portion of the operation. As mentioned
above, backing member 23 can be maintained in place but biased to prevent the transfer
of toner to it.
[0043] Operation of the image forming apparatus 10 is controlled by logic and control 100
which turns on and off all of the subsystems in appropriate timing, as is well known
in the art. As shown in FIG. 19, it especially controls the bias applied to transfer
backing roller 23 and the actuation of corona 15 and erase lamp 17 for working of
the submethods shown in FIGS. 8-18. It also controls the bias on the development stations,
including the bias applied to stations 11 and 13 during the subprocess shown in FIGS.
15-18.
[0044] Another toning station 22 is shown in FIG. 19. This station could be used to provide
a second primary color alternative to that supplied by station 7 for the DAD image.
However, it can be used for a different purpose to further prevent scavenging of toner
applied by station 7. More specifically, for this purpose, station 22 contains clear
toner of the same polarity as the toner in station 7 (negative, in the example). This
station 22 is biased to apply a light topping of clear toner to the first toner image
created by stations 9, 5 and 7. This toner does not affect the density of the image
because it is clear. However, if any toner is scavenged from the image by stations
11 or 13, it is likely to be the toner on top, i.e., the clear toner supplied by station
22. Eventually, some toner from station 7, for example, black toner, will be scavenged
by station 22 since they are of both the same polarity.
[0045] The use of the clear toning station 22 greatly prolongs the life of the developers
in stations 11 and 13. However, the use of the clear toner also has another advantage.
Gradual scavenging of black toner into the clear station is quite obvious and can
be readily measured by either measuring light reflected or transmitted through toner
in station 22. This allows a monitor of the scavenging effect of the system which,
properly calibrated, can give the operator an indication when the developers in stations
11, 13 and 22 should be changed.
[0046] This feature associated with the use of clear toner in station 22 is shown with respect
to a DAD-CAD process. In fact, it is particularly useful in any process in which an
electrostatic image is toned in the presence of an unfixed toner image of any polarity.
Note that, because the station 22 involves DAD development, scavenging into it is
more likely than it is into stations 11 and 13, both of which are shown to be CAD
stations in FIG. 19. Thus, projection toning is preferred in station 22.
[0047] FIG. 20 shows in detail a preferred embodiment of toning station 11 (and also station
13). Toning station 11, as shown in FIG. 20, embodies technology first disclosed in
U.S. Patent No. 4,546,060, Miskinis et al, issued October 8, 1985, and further described
in U.S. Patent No. 5,001,028 to Mosehauer et al (cited above). Toning station 11 includes
a housing which defines a sump 39 in which a supply of developer is mixed by suitable
mixing augers 51. The developer includes "hard" magnetic carrier having a coercivity
of at least 300 gauss, preferably in excess of 800 gauss, when magnetically saturated.
The carrier exhibits an induced magnetic moment of at least 20 EMU per gram of carrier
when in an applied field of 1,000 gauss. The toner is a typical insulative toner and
is mixed thoroughly with the carrier (the combination generally being termed a "developer").
[0048] The developer is transported from sump 39 by a suitable transport device 53 to an
applicator 41 for application to an electrostatic image carried by image member 1.
Transport device 53 can include a valving feature, well known in the art, which can
turn off the station by cutting off the supply of developer to applicator 41. Applicator
41 includes a rotatable magnetic core 43 positioned inside an also rotatable non-magnetic
sleeve or shell 45. Although the core and shell can be rotated in either direction,
most commercial embodiments rotate the core 43 at speeds of 1,000 revolutions per
minute or higher in a clockwise direction to move the developer in a counter-clockwise
direction around shell 45. If shell 45 is moved in a counter-clockwise direction,
it also assists in the flow of developer. Alternatively, the developer can be driven
in its counter-clockwise direction primarily by rotation of sleeve 45, and core 43
can also be rotated in a counter-clockwise direction. In this last case, the rotation
of the core has primarily a mixing function as it actually encourages movement of
the developer against the counter-clockwise direction that the developer is driven
by the shell 45.
[0049] The rapidly rotating core causes rapid pole transitions on the surface of shell 45
which cause the high coercivity carrier to flip rapidly, usually in strings of carrier,
as the developer moves through a development zone 55 between applicator 41 and image
member 1. The flipping action moves the carrier from the shell to the image member
and back rapidly, replacing lost charge and toner. This device and process have been
in use for some years and provide highest quality development with a very soft magnetic
brush. The softness of this brush was pointed out in the Mosehauer patent as useful
in developing an electrostatic image on an image already carrying an unfixed toner
image of a different color. The height of the developer nap is controlled, in part,
by a skive 47, and the developer itself is skived off the sleeve 45 by another skive
49 so it can fall back into the sump 39 where it is mixed. The developer station itself
is mounted opposite a pair of skis 60 which control the position of image member 1
with respect to applicator 41. Similarly, printhead 12 also includes one or two skis
62 which control the location of image member 1 with respect to it.
[0050] Although the core 43 and shell 45 are shown to have the same axis of rotation, some
high quality commercial embodiments offset the core and shell axes. This strengthens
the magnetic field in the development zone 55 and weakens it opposite the development
zone where developer is returned to the sump 39. This construction works especially
well when the developer is being moved entirely by shell movement and the core is
only mixing, since the skive 49 is relatively ineffective in such an embodiment.
[0051] The development station 11, as shown in FIG. 20, provides the highest quality of
development available and accomplishes this with very high density at a very high
toning speed. Because of the pole transitions and flipping of the strings of carrier
themselves, this density and speed can be accomplished with a station of extremely
modest size. When the softness of this brush is combined with the scavenging suppressing
field associated with CAD developing, remarkably little scavenging occurs.
[0052] The invention has been described in detail with particular reference to a preferred
embodiment thereof, but it will be understood that variations and modifications can
be effected within the spirit and scope of the invention as described hereinabove
and as defined in the appended claims.
1. An image forming method comprising:
forming a first electrostatic image of a first polarity on a first side of a photoconductive
image member;
applying a first toner of the first polarity to the first electrostatic image to form
a first toner image on said first side;
imagewise exposing the image member from a second side of the image member opposite
the first side to form a second electrostatic image in registration with the first
toner image; and
applying a second toner of a second polarity opposite the first polarity to the second
electrostatic image to form a second toner image in registration with the first toner
image.
2. An image forming method according to claim 1 wherein the step of applying a second
toner to the second electrostatic image includes moving a developer containing a mixture
of hard magnetic carrier particles and toner particles through a development zone
in contact with the second electrostatic image.
3. An image forming method according to claim 2 wherein the step of applying a second
toner includes subjecting the developer to rapid magnetic pole transitions while it
is moving through the development zone.
4. An image forming method according to claim 3 wherein the developer is moved through
the development zone at least in part by the rotation of a magnetic core inside a
non-magnetic sleeve upon which the developer moves and said pole transitions are provided
by the rotating magnetic core.
5. An image forming method according to claim 1 wherein the imagewise exposing step reduces
the level of charge on the image member underneath the first toner image to a level
less than the level of charge in exposed areas outside the first toner image.
6. An image forming method according to claim 5 wherein the step of applying a second
toner includes applying the toner in the presence of an electric field of a direction
discouraging removal of toner in the first toner image and discouraging the deposit
of the second toner in the discharged portions of the second electrostatic image including
the first toner image.
7. An image forming method comprising:
applying a uniform charge of a first polarity to a first side of a photoconductive
image member;
imagewise exposing the image member to form a first electrostatic image of the first
polarity on the image member;
applying a first toner of the first polarity to the first electrostatic image to form
a first toner image on the first side of the image member;
imagewise exposing the image member to form a second electrostatic image in registration
with the first toner image; and
applying a second toner of a second polarity opposite the first polarity to the second
electrostatic image to form a second toner image in registration with the first toner
image, said step of applying a second toner including moving a developer of hard magnetic
carrier particles and toner particles through contact with the image member while
subjecting the developer to rapid magnetic pole transitions which flips the developer
to and away from the second electrostatic image.
8. An image forming method comprising:
forming a first electrostatic image on an image member;
applying toner to said electrostatic image to form a first toner image,
imagewise exposing the image member to form a second electrostatic image; and
applying a toner to the second electrostatic image to form a second toner image in
registration with the first toner image;
characterized by applying a clear toner to said first electrostatic image after the
application of the first toner to form a light coating of clear toner on top of the
first toner image, thereby inhibiting scavenging of the first toner image during application
of the second toner.
9. An image forming method which forms a two toner image by a first submethod including:
forming a first electrostatic image of a first polarity on an image member;
applying a first toner to the electrostatic image to form a first toner image on the
image member;
imagewise exposing the image member to form a second electrostatic image in registration
with the first toner image;
applying a second toner to the second electrostatic image to form a second toner image
in registration with the first toner image; one of the toner images being made up
of toner of the first polarity and the other toner image being made up of toner of
a second polarity opposite the first polarity;
treating the two toner images to make the toners substantially of one polarity; and
transferring the two toner image to a receiving surface by application of an electrical
field of a direction which urges transfer of toner of the one polarity toward the
receiving surface;
said method being characterized by a second submethod including the following steps;
determining that a single toner image is to be made in only the toner of a polarity
opposite the one polarity;
forming the single toner image using the steps of the first submethod applicable to
formation of that toner image except that the treating step is eliminated and the
transferring step includes application of an electrical field of a second direction,
opposite the first direction, urging transfer of the untreated single toner image
to a receiving surface.
10. Image forming method according to claim 1 wherein said treating step applies a charge
to the two toner image that is the same as the charge on the first toner and the direction
of the field applied during the transferring step of the second submethod is reversed
of that of the first submethod.
11. An image forming method which forms a two toner image by a first submethod including:
forming a first electrostatic image of a first polarity on an image member;
applying a first toner to the electrostatic image to form a first toner image on the
image member;
imagewise exposing the image member to form a second electrostatic image in registration
with the first toner image;
applying a second toner to the second electrostatic image to form a second toner image
in registration with the first toner image, said applying step including moving a
developer containing said second toner into contact with the image member in the presence
of an electrical field urging toning of the second electrostatic image, one of the
toner images being made up of toner of the first polarity and the other toner image
being made up of toner of a second polarity opposite the first polarity;
treating the two toner image to make the toner substantially of one polarity; and
transferring the two toner image to a receiving surface by application of an electrical
field of a first direction which urges transfer of toner of the one polarity toward
the receiving surface;
said method being characterized by a second submethod including the following steps
to be carried out when no images are to be made:
moving said developer into contact with the image member in the presence of an electric
field urging the deposit on the image member of any of the first toner that is mixed
in said developer;
allowing any such deposited first toner to pass through the transfer station on the
image member; and
cleaning such deposited toner off the image member.
12. An image forming apparatus comprising:
means for forming a two color toner image made up of two toner images of different
color and opposite polarities, said means including first and second toning stations,
said first station containing toner of the first polarity and said second station
containing toner of the second polarity, said second station being positioned to apply
toner to an electrostatic image on the image member after formation of a first toner
image by the first toning station, where there is a tendency to scavenge toner applied
by the first toning station into the second toning station;
means for treating the two color toner image to make the toner substantially of one
polarity; and
means for transferring the two color toner image to a receiving sheet in the presence
of an electric field urging the transfer of toner of said one polarity;
said apparatus being characterized by means for passing the image member through the
second toning station in the presence of an electric field urging the deposit on the
image member of toner of the first polarity present in the second toning station,
means for disabling the transfer means to permit such deposited toner of the first
polarity to pass through the transfer means on the image member; and
means for cleaning such toner of the first polarity off the image member.