Method and apparatus for improving a multi-color electrophotographic image using heat fusing

An improvement in the method and apparatus for producing a multi-color electrophotographic image wherein an electrophotographic medium is mounted on a carrier therefor and is translated over a predetermined path so that the medium is first charged, then exposed to a first light-borne image and then toned. Thus, the unexposed portions of the image bearing charged region of the medium are toned, preferably by a liquid toner, to produce a first color visible image. The carrier is then returned to the first position to repeat the foregoing steps to charge the medium and to expose it to a second light-borne image and to tone the second image with a second toning element to produce a second color visible image on the same sheet of medium. The improvement comprises means for heating the surface of the first toned image after the first toning step and prior to the second charging step to smooth and further fuse the previously toned image to prevent color contamination of subsequent imaging cycles.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The present invention relates to means for improving an electrophotographic 
image and is related to the improvements disclosed and claimed in 
copending applications Ser. No. 358,916, Apparatus for Improving a 
Multi-color Electrophotographic Image in the names of Lawrence C. Steele 
and Kenneth E. Rook, Ser. No. 358,918, Method of Improving a Multi-color 
Electrophotographic Image in the names of Lawrence C. Steele and Kenneth 
E. Rook, and Ser. No. 358,101, Method of Improving a Multi-color 
Electrophotographic Image by Buffing an Image Toned with an Improved 
Toner, in the names of Lawrence C. Steele, Kenneth E. Rook, Domenic 
Santilli, and Dennis R. Kamp, all filed on May 26, 1989, and Ser. No. 
500,430, Method and Apparatus for Improving a Multi-color 
Electrophotographic Image Using Vapor Fusing in the name of Domenic 
Santilli, filed on even date herewith. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electrophoretic liquid development and more 
particularly to an improved process of plural stage development. 
2. Description Relative to the Prior Art 
In the liquid development of electrostatically charged latent images, as in 
electrophotography and in other processes that form and develop 
electrostatic charge patterns, a substrate having a charge pattern on its 
surface is contacted with a liquid developer which is essentially a 
suspension of colloidal toner particles in an insulating liquid. Liquid 
developers normally also contain a stabilizer or charge control agent. The 
latter is an ionic compound which controls the magnitude of the charge on 
the toner particles and aids in maintaining a stable charge on the toner 
particles within the insulating carrier liquid. 
Liquid developers can be used in single stage or plural stage development 
processes. Examples of the latter may include the sequential development 
on a photoconductor of two or more color-separation images, the annotation 
of a previously developed image, or the repeated re-exposure and 
development of images on a reusable photoconductor, with transfer of 
images upon completion of a number of imaging cycles. 
In certain plural stage development processes which use liquid developers, 
a problem has been found which is especially significant in processes for 
the electrophotographic reproduction of multi-color images of graphic arts 
quality. In these processes electrostatic latent images are formed 
sequentially on a chargeable substrate such as an electrophotographic 
medium, with liquid development or toning of each latent image before the 
next image is formed. A leading example of this type of system involves 
processing an electrophotographic medium sequentially through a series of 
four imaging cycles including four sequential development or toning 
stages. 
While the present invention is useful in any electrostatic imaging process 
wherein a charge pattern is formed and developed with a liquid developer 
on a surface which has previously been developed with a liquid developer, 
it is particularly useful in combination with a recently developed 
electrophotographic process of making lithographic color proofs, such as 
described in U.S. Pat. No. 4,600,669. In that process a photoconductor, 
which has a uniformly charged thin transparent dielectric overlayer, is 
subjected to a series of exposures through registered color separation 
transparencies. After each exposure the dielectric layer is developed with 
a liquid developer, and the surface is again uniformly charged, exposed, 
and developed. The sequence is repeated for each of the color 
transparencies, usually four. It has been found that image defects occur 
in areas of the image which are toned areas of a previous imaging cycle. 
The defect appears as a color contamination of the previously toned areas 
which imparts a non-uniform density, thereby altering color rendition, a 
defect which, though perhaps acceptable in some kinds of add-on, plural 
stage imaging, is not acceptable for producing high quality images as 
required, for example, in the graphic arts field. 
It has been discovered that the described image defects in plural-stage 
liquid development appear to be caused by the presence of forces, 
substances, or charge acceptance in the previously toned areas that 
interfere with the correct subsequent development in those areas. Although 
the nature of those forces or substances is not clear, the present 
invention provides apparatus that reduces or eliminates the problem. 
U.S. Pat. No. 4,660,503 describes the improvement in the process of the 
'669 patent of cleaning the image by brushing the surface of the toned 
image with a soft, smooth, and supple fibrous brush member between the 
development of an image and the exposure of the next image to remove 
counter-ion material from the untoned areas which interferes with 
subsequent toning cycles in the above-mentioned process. The improvement 
of the '503 patent affects only the untoned areas of the developed 
substrate or photoconductor and has been found to have no effect on the 
toned areas where the present problem has been found. 
Thus, the problem addressed by the present invention is that of preventing 
color contamination of previously toned areas rather than preventing 
contamination of the untoned areas. The solution must not adversely affect 
the desired electrophoretic deposition of toner particles in both 
previously toned and untoned areas to develop the latent electrostatic 
image areas of subsequent imaging cycles. The above-referenced, previously 
filed co-pending applications are directed to a method and apparatus for 
preventing the contamination of previously toned areas by physically 
smoothing the toned image before generating the subsequent image, such as 
by buffing with a rotating brush. 
Accordingly, the provision of an additional simple and relatively easily 
implemented solution to this problem, particularly if it can be done at a 
nominal cost and does not have any significant negative effects on the 
resulting image, would be very advantageous in providing a variety of 
solutions from which to choose. 
SUMMARY OF THE INVENTION 
The present invention thus provides a method and apparatus for producing a 
multi-color electrophotographic image that enhances the image quality by 
preventing the small toner particles of liquid developers from adhering to 
previously toned areas unless electrostatic latent image fields are 
present to support such deposition. 
According to one aspect of the present invention, apparatus for producing a 
multi-color electrophotographic image is provided which comprises means 
for providing an electrophotographic medium on a carrier therefor at a 
first position, means for translating the carrier and medium together over 
a predetermined path, means for charging the medium at a second position, 
and means for exposing the medium to a first light-borne image at a third 
position. Means is provided for toning the image to produce a first color 
visible image. Means is provided for returning the carrier and medium to 
the first position to repeat the foregoing steps to charge the medium and 
to expose it to a second light-borne image and to tone the second image 
with a second toning element to produce a second color visible image. The 
improvement of the present invention comprises means for heating the image 
with a source of heat following the first toning step and prior to the 
second charging step. 
According to another aspect of the present invention, it has been found 
that briefly heating the surface of the toner on the electrophotographic 
medium to a temperature of about 150.degree. F. will provide the desired 
level of smoothness and fusing characteristics. 
According to yet another aspect of the present invention, the source of 
heat comprises a radiant heater mounted transversely of the direction of 
movement of the carrier. 
According to still another aspect of the present invention, the heating 
means is arranged to direct heated air toward said image. 
According to a further aspect of the present invention, a method of 
producing a multi-color electrophotographic image is provided which 
comprises the steps of mounting an electrophotographic medium on the 
carrier and translating the carrier and medium together over a 
predetermined path, charging the medium and exposing the medium to a first 
light-borne image, and developing the image to produce a first color 
visible image. The carrier and the medium are then returned to the 
starting position to repeat the foregoing steps to charge the medium and 
to expose it to a second light-borne image and to tone the second image 
with a second toning element to produce a second color visible image. The 
surface of the image is cleaned following development and prior to the 
second charging. The improvement comprises the step of heating the surface 
of the image immediately following the cleaning step and prior to the 
second charging step to a temperature from about 150.degree. F. to about 
200.degree. F. thereby smoothing any previously toned areas and minimizing 
unwanted toner particle adherence. 
Various means for practicing the invention and other features and 
advantages thereof will be apparent from the following detailed 
description of an illustrative preferred embodiment of the invention, 
reference being made to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Although the present invention is applicable to various electrophotographic 
elements, methods and apparatus, the embodiment to be described is 
directed to a multi-color electrophotographic image producing apparatus 
employing an electrophotographic medium of the type disclosed in the 
above-identified '669 patent. 
A schematic illustration of a multi-color electrophotographic image 
processor 10 is illustrated in FIG. 1 and consists of a carrier or platen 
12 which is movable along the processing path, represented by dotted line 
14, past the respective processing stations of the apparatus, to be 
described hereinafter. The path 14 may be determined by guide rails or 
other structure of the apparatus in a manner well-known in the art whereby 
the platen may move from a starting position, illustrated, to the 
right-most position and then returned to the left to the starting 
position. The platen 12 is preferably transparent and is provided with 
means, not shown, for retaining an electrophotographic medium 16 on the 
lower surface thereof with an image-bearing transparency 18 disposed 
therebetween which is used to generate the image in the 
electrophotographic medium 16, in a manner more thoroughly described 
hereinbelow. 
As noted in the above-cited '669 patent, the electrophotographic medium 
comprises a photoconductive layer on an electrically conducting substrate 
which is capable of transmitting actinic radiation to which the 
photoconductive layer is responsive. A dielectric support is releasably 
adhered to the substrate and comprises the photoconductive layer or an 
overcoat thereof which forms an outer surface of the element capable of 
holding an electrostatic charge. To use the element, the surface of the 
dielectric support is charged and the photoconductive layer is image-wise 
exposed to the actinic radiation, thereby forming a developable 
electrostatic image on the dielectric surface. The electrostatic image in 
turn is developed with a toner to form a first color image. A composite 
color image can be formed on the element by repeating the sequence one or 
more times with image-wise exposure of the photoconductive layer to 
actinic radiation transmitted through the transparent support, and 
developing over each preceding image with a different color toner. The 
composite toned image is then transferred with the dielectric support to a 
receiving element to form a color copy which may be a color proof closely 
simulating the color print expected from a color print press. 
Accordingly, the electrophotographic medium 16 is mounted onto the platen 
12 with the transparency original 18, which may be a color separation 
representing a color to be printed, sandwiched therebetween. The 
electrophotographic medium 16 and the transparency original 18 may be held 
to the platen 12 by any suitable means known in the art such as a vacuum 
clamp whereby they are maintained in close proximity to assure 
satisfactory exposure, processing and register. Further, the 
electrophotographic medium must also be suitably grounded to the apparatus 
to enable the charging process to be satisfactorily carried out. A number 
of grounding means are known in the art and will not be described herein. 
As the platen 12, with the original and the electrophotographic medium 16, 
is translated to the right (in FIG. 1), the dielectric support of the 
electrophotographic medium is given an overall charge via a charging means 
20, such as a corona charger, to form a uniform potential on the surface 
of the dielectric support. Upon being so charged the electrophotographic 
medium is image-wise exposed by passing beneath an exposure lamp apparatus 
22 which projects light through the transparent platen 12, the 
transparency original 18, and through the transparent conductive substrate 
of the electrophotographic medium. When the photoconductive layer is thus 
image-wise exposed, mobile charge carriers, in this case positively 
charged holes, are formed in the photoconductive layer and migrate towards 
the interface of the photoconductive layer and the conducting layer as 
described in the '669 patent. Accordingly, the electric field strength in 
exposed regions is diminished while the field strength in unexposed 
regions remains approximately the same. As a result, an electrostatic 
differential pattern is formed on the dielectric support corresponding to 
the pattern on the transparency original. 
The platen continues its movement, to the right in FIG. 1, passing over a 
pre-rinse head 24 which is fixed in position whereby the fluid head 
provided thereat when activated contacts the lower surface of the 
electrophotographic medium as it passes in the processing direction, i.e., 
to the right, but does not contact the medium when the fluid head is 
inactivated as when the platen is moved to the left in FIG. 1, to the 
original position. The pre-rinse head prewets the medium with a dispersant 
dielectric liquid prior to the liquid toning step. Thereafter, the platen 
moves past a raised first liquid toning station 26 which is raised into 
operating position whereby the lower surface of the electrophotographic 
medium is contacted and a toner image is imparted thereto, in a manner 
well-known in the art. In this system, the liquid toner is deposited in 
the unexposed, still charged area of the electrophotographic medium 
thereby forming a positive image which is a duplicate of the image carried 
by the transparency 18. It is also well-known in the art to produce 
negative images with similar electrophotographic processes wherein the 
charges imparted to the electrophotgraphic medium and the toners are 
appropriately adjusted to give a negative image. The platen continues 
movement to the right in the illustration, past appropriate rinse heads 37 
and dryers, 38. The last station 28 at the right end of the apparatus is 
an erase lamp that exposes the electrophotographic medium after the toning 
operation to expose those parts of the photoconductive layer that were not 
exposed by the original image exposure so that the entire 
electrophotographic medium has substantially the same exposure history. 
The platen 12 is then reversed and is returned to the starting position 
illustrated in FIG. 1. At this point the electrophotographic medium 16, 
while still engaged along one edge to the platen to maintain registry for 
subsequent exposures, is moved away from the platen to a substantially 
vertical position and the first original transparency or color separation 
18 is removed and replaced by a second transparency or color separtion. 
The electrophotographic medium is returned to the platen in registry with 
the second transparency ready for the next pass through the apparatus to 
generate the second color image. When the electrophotographic medium 16 
and the next original sheet 18 are re-registered on the platen 12, the 
platen is moved to the right again for charging, exposure, and subsequent 
toning. However, the platen first passes over a cleaning station 30 which 
includes a rotating brush member 40. This cleaning station and its 
operation are described in the above-referenced '503 patent. When the 
cleaning station is raised into operative position with respect to the 
electrophotographic medium surface, the brush bristles engage the toned 
image and lightly clean it. The platen then moves to a heating station 31 
which forms the improvement of the present invention. The construction and 
operation of the heating station are more thoroughly described 
hereinbelow. 
The platen then moves to the charging station 20 where the 
electrophotographic medium is again charged and then moves to the exposure 
position 22 where light again is projected through the platen and the 
second color separation 18 to selectively discharge the photoconductive 
layer in accordance with the transparancy or color separation then in 
contact with the electrophotographic medium. Thereafter, the platen moves 
the electrophotographic medium to the pre-rinse station 24 and then to a 
second toning station 32 which is then in operative position to tone the 
surface of the electrophotographic medium with a second color toner to 
produce a second color visible image overlying the first image. The platen 
subsequently moves past the aforementioned rinse and drying stations and 
again past the erase exposure station 28 before being returned to the 
starting position at the left-hand end of the apparatus. Should it be 
desired to create a four color image (or a three color plus black image), 
the charging, exposing, and toning steps will be repeated for two more 
color separation originals with the platen and electrophotographic medium 
being moved into operative contact with an additional two toning stations 
34 and 36, one for each of the additional colors. Prior to each of these 
additional exposing and toning steps, the cleaning and heating stations 30 
and 31 are raised into operative position with respect to the surface of 
the electrophotographic medium as the platen begins its travel to the 
right to again clean and heat the toned medium to both remove toning 
materials from the untoned portions thereof without significantly altering 
the toned image and then to smooth the toned image, as will be further 
described hereinbelow. It will be appreciated that, as known in the art, 
the toning order may not necessarily be represented by the physical order 
of the toning stations in the apparatus, and the order given above is by 
way of example only. 
After the final toning, rinsing, and drying steps, the platen 12 is 
returned to the first position where the electrophotographic medium is 
removed. On the final return of the platen the cleaning brush 40 is again 
raised into the operative position to clean the final image. In keeping 
with the desire to rotate the cleaning brush in a direction counter to the 
direction of travel of the electrophotographic medium, the brush is 
rotated in a clockwise direction (as viewed in FIG. 1) for this operation. 
While there are no additional toning operations to be conducted on the 
medium at this point, it has been found that the final brushing step aids 
in the lamination of the image-bearing release layer of the film to the 
paper stock, minimizing artifacts that have been found to occur without 
this final brushing operation. 
While the other components of the electrophotographic apparatus described 
herein are known in the art, the heating station assembly 31 is described 
in further detail with respect to FIG. 2. As illustrated, the heating 
station assembly takes up little additional space along the travel length 
of the apparatus, being only wide enough to mount the heater itself and 
the ancillary components. In the embodiment illustrated, the heating 
station assembly is a self-contained assembly carrying the necessary 
structural members to support the heater assembly, and can be readily 
dropped into the allocated position in the electrophotographic apparatus. 
The heater comprises, in the preferred embodiment, a radiant heater 
element 43 and a reflector element 45 focused at the film plane. The power 
of the heater is selected so as to provide sufficient energy to the film 
to heat the surface thereof to a temperature of between about 150.degree. 
F. and about 200.degree. F. as the film and platen pass the heating 
station. It has been found that the heating of the toned surface 
apparently causes the toner to fuse and, as a result, become smoother, 
resulting in less unwanted toner from subsequent toning steps from 
adhering thereto. 
ALTERNATIVE EMBODIMENTS 
Although the heater power and the conductivity of the platen can be 
selected to provide the desired heating of the toned surface, it may be 
desirable to heat the toned surface when the film is not secured to the 
platen. Thus, as illustrated in FIG. 3, a radiant heater 47 can be 
provided at the loading station in lieu of the heating station assembly 
31. The radiant heater at the loading station is arranged to direct 
radiant energy to the toned surface of the electrophotographic medium 16 
when it is moved to the vertical position to permit one color separation 
to be replaced by another. This alternative embodiment may be desirable to 
minimize the heater power necessary to adequately smooth the toner surface 
by eliminating the conduction of heat away from the toner by the platen. 
In this embodiment, the sequence of operation is changed to operate the 
cleaning station 30 as the platen is moved left to the starting position 
so that the cleaning step is completed before the toned surface is heated. 
Other alternate embodiments include the substitution of a source of heated 
air for the radiant heaters of the previously described embodiments. In 
these embodiments, the heated air is directed toward the toner surface and 
is adequate to provide the degree of surface smoothing necessary to 
minimize unwanted toner adherence. 
A further alternative embodiment employs a heated platen surface in lieu of 
the other heaters that can be selectively activated to provide the desired 
heating of the toner surface. 
By heating the developed image in accordance with the present invention, it 
has been found that the toned image areas are prevented from becoming 
contaminated, such as by having their color rendition altered, as 
subsequent images are produced, apparently by preventing the small toner 
particles of the liquid developer from adhering to the portions of the 
previously toned areas which are not imaged in the current imaging step. 
Although we do not wish to be bound by any theoretical explanation of the 
mechanism of the present invention, a possible explanation for the results 
observed is that the heating of the developed photoconductor smoothes any 
previously toned areas thereby minimizing unwanted toner particle 
adherence. It may be that such heating of the toned image areas fuses them 
and smoothes the microstructure of the toned image so that there is less 
surface roughness to which stray toner particles can adhere. 
The brushing and heating steps together provide for the removal of 
counter-ions and backround density in the untoned areas by the brushing 
step and the prevention of color contamination in the toned image areas by 
smoothing the toner surface with the heating step. While the heating 
station has been disclosed as being preferably located between the 
cleaning station and the charging station, it will be appreciated that it 
can also be located along with the cleaning station to the right of the 
erase lamp 28. The only necessity is that the heating step be accomplished 
after the cleaning step, if the cleaning step is used. The location of the 
cleaning and heating stations between the loading position and the 
charging station provides the advantage that the toned electrophotographic 
medium has had the maximum opportunity to dry prior to cleaning and 
heating without necessitating an unduly long waiting time. However, if 
time is not of the essence then the platen can be arranged to stop prior 
to the brushing and heating assemblies to the right of the erase exposure 
lamp, so that the toned image is sufficiently dry to permit brushing and 
heating at that location. 
It will be appreciated that the present invention thus provides an improved 
method and apparatus for producing a multi-color electrophotographic image 
that enhances the image quality by preventing small toner particles of the 
liquid developers from adhering to previously toned areas, which causes 
altered color rendition, unless that area has been imaged during the 
current imaging cycle. Moreover, the present invention provides a simple 
and relatively easily implemented solution to this problem at a nominal 
cost and without any significant negative effects on the resulting image. 
Still further, it has been found that the use of the present invention 
does not adversely affect the ability to transfer the final multi-color 
image to a paper substrate. 
The invention has been described with reference to specific embodiments and 
variations, but it should be apparent that other modifications and 
variations can be made within the spirit and scope of the invention, which 
is defined by the following claims.