GB2046665A - Electrophotographic Color Copying Method and Apparatus - Google Patents

Abstract

Color resolved latent electrostatic images are formed by scanning an original through water separation filters, converting the color resolved images of the original into digital data, feeding the data into a computer 15 and then writing the images on the surface of a charged electrophotographic drum 21 with a laser 19 whose output is modulated by the digital data. During each revolution of drum 21 one color image is written, toned and transferred to receptor 5 on which the images are superimposed. The developing device 43 includes a plurality of developing units, each containing a different color toner, which are moved past the drum 21, seriatim in a stepwise cycle. Movements are coordinated so that each developing unit will apply toner to its corresponding latent electrostatic image during a dwell period of the developing device when said developing unit is in toner- applying relation with the drum. <IMAGE>

Description

SPECIFICATION Electrophotographic Color Printing Method and Apparatus Prior methods and apparatus for making color copies of color originals by electrostatic techniques has involved the steps of optically projecting color resolved images of the original onto a single charged electrophotographic member or a plurality of charged electrophotographic members, one for each color resolved image to form corresponding latent electrostatic images, developing each latent electrostatic image with its proper color and then fixing each image in superimposed relationships.

The images projected onto the electrophotographic members were obtained by projecting an image of the original thereon through a plurality of color separation filters, one for each color to be reproduced. Depending on the system employed, the final composite image has been either formed directly on a single electrophotographic member or transferred from one or more electrophotographic members to a sheet of carrier material, such as plain paper.

Although many different types of electrostatic systems which have been proposed or constructed, these systems have proven to be generally unsatisfactory. Some of the reasons are that the resulting apparatus has been too large in size, complex in construction, expensive to build and operate, unreliable and/or not capable of producing images having quality or resolution compatible with copies made by photographic and/or lithographic techniques. In addition, these systems have not included the capability for making changes in the color, size or hue of the final copy, if such changes are either desired or required except by very difficult or expensive means involving changes in the process.

An electrophotographic film now is available which has qualities and properties which make it far superior to any other known electrophotographic film. The said electrophotographic film features a photoconductive coating that is wholly inorganic, microcrystalline, electrically anisotropic in nature, which does not have reciprocity or intermittency "effects, operates at low voltages, has very high sensitivity and can produce toned images of exceptionally high quality from either analog or digital information. This electrophotographic film is well suited for use in making high resolution color copies of color originals by electrostatic techniques, and, in fact, has properties which enable it to be used for this purpose in a manner that hitherto has not been achievable with other types of electrophotographic members.

Because of the high speed of response of said coating (capable of being exposed in nanoseconds), the apparatus can comprise a high-speed machine.

Among the suggestions made in the prior art, the following are believed pertinent to the field of the invention.

U.S. Patent to Lusher3,399,61 1 U.S. Patent to Smith 3,690,756 U.S. Patent to Nagamatsu et al 3,832,1 70 U.S. Patent to Katayama et al 4,095,879 U.S. Patent to Watanabe et al 4,120,577 U.S. Patent to Barasch 4,124,286 No inference should be drawn that the above represents a comprehensive investigation of the prior art; that any or all are pertinent to the invention; that the apparatus therein is analogous to the invention; that the invention herein is the equivalent of any disclosed therein.

Accordingly, the invention provides a method of color printing electrostatically wherein the individual colors of a color composite are digitally stored in a computer and are capable of being called up separately for laser writing on an electrophotographic member, said method comprising the steps of: providing a rotary electrophotographic drum and a transfer roller arranged to carry a member of receptor material engaged against the rotary electrophotographic drum charging the drum, exposing the charged drum to an image of one color of the composite by laser writing said image on the drum, toning the image and transferring the toned image to the member of receptor material effecting said steps all in one revolution of the drum with a simultaneous single revolution of the transfer roller, the charging, writing, toning and transfer for each other color of the composite being repeated during each subsequent revolution of the drum and transfer roller until all colors have been transferred in superimposed relation on the member of receptor material and, then removing the member of receptor material from the transfer roller and replacing it with another to enable repetition of the process on said replacement member.

Further, the invention provides an apparatus for making one or more color copies of a color original electrophotographically comprising a rotary electrophotographic member, drive means for rotating said electrophotographic member sequentially in a cycle through a series of processing stations arranged along its periphery, said processing stations including in order, a charging station, an exposure station, a developing station and an image transfer station, means for applying a uniform electrostatic charge to a portion of the electrophotographic member at the charging station, means for forming in sequence a plurality of latent electrostatic images on said electrophotographic member at the exposure station during rotation of said electrophotographic member, each of said latent electrostatic images corresponding to a different one of the color image components of the color original and each image being formed during a respective cycle, a developing device capable of moving through the developing station on a path which is generally tangential to said rotary electrophotographic member, said developing device including a plurality of developing units arranged in a row and adapted to pass through the developing station seriatim, each developing unit having developing toner material whose color corresponds to the color image component of one of the latent electrostatic images, means for moving said developing device stepwise on its tangential path through said developing station in timed relation to the formation of the color corresponding latent images and bringing the developing units into toner applying relationship to the electrophotographic member seriatim as each unit is in said station, said moving means operating to step each developing unit into said developing station, then holding it there during a dwell period sufficiently long, and then stepping said developing unit out of the developing station while simultaneously moving the next following unit into the station, the portion of the electrophotographic member carrying a color corresponding latent image of each developing unit being fully moved through the developing station during the dwell period that its respective developing unit is in said station, and transfer means at said transfer station for transferring each developed image onto a sheet of carrier material in superimposed relationship as each developed image passes through said transfer station during its respective cycle, there being as many cycles as colors and as many impressions on each sheet as cycles.

The preferred embodiments of this invention now will be described, by way of example, with reference to the drawings accompanying this specification in which: The sole figure is a fragmentary view partially in perspective and partially in block diagram form of an apparatus for making color prints of color originals according to this invention, it being understood that some of the details are more or less symbolic or diagrammatic.

Referring now to the drawing, there is shown an embodiment of an electrophotographic copying apparatus constructed according to this invention and identified by reference numeral 11.

An original to be copied is deposited on a flat bed scanner 13 where its imagery is rapidly scanned and converted into three or four color separation digital information. Flat bed scanner 13 may include a white light source, an optical scanning unit, red, green and blue filter units for forming the three color separations and an analog to digital converter for converting the analog information into digital data. The three or four color separations may be formed simultaneously by directing images of the original through all three filters at the same time. A special filter or scanner system may be used for black pigment.

The digital data so obtained is fed into a computer 1 5 where it is stored, either permanently or temporarily, and further processed. The further processing includes converting the red, green, blue and black digital data respectively to cyan, magenta, yellow and black values. The further processing may also include such operations as linear scaling, cropping, density shifting, gamma modification, color balancing, color reversal, captioning and the like.

A graphic display terminal 17 which is connected to computer 1 5 enables the images stored in the computer to be monitored at any time.

A laser system 1 9 for writing images using the digital data is also connected to computer 1 5.

Laser system 19 may comprise a scanning prism, a laser and an acousto-optical modulator for modulating the output beam of the laser in accordance with digital information. Laser system 1 9 is arranged to scan over a particular angle.

Instead of a single laser, laser system 1 9 may include a plurality of lasers, each arranged to scan a portion of that particular angle.

There is an electrophotographic member in the form of a rotary electrophotographic drum 21 mounted on an axle 23 which is journalled for rotation on a support frame 25 in the direction shown by the arrows. The support frame 25 is broken away and bearings, journals and brackets are not illustrated in order to simplify the view.

Drum 21 is driven by a motor 27 whose output shaft is mechanically coupled as indicated at 22 to axle 23. The peripheral surface of drum 21 comprises a photoconductive coating on a conductive substrate. The conductive substrate may either be a sleeve mounted on drum 21 or an integral part of drum 21. Thus, it may comprise a metal substrate with the photoconductive coating on its surface or an insulative sheet material having an ohmic layer between the sheet material and the photoconductive coating. In either event, the metal substrate or ohmic layer is required to be connected electricaliy into a charging circuit comprising a corona voltage source. The photoconductive coating is one in which latent electrostatic images can be formed by laser writing at very high speed and then developed to provide prints having high quality and high resolution.An example of such a coating is the coating described in the above-noted U.S. Patent 4,025,339.

Positioned around the path of travel of the peripheral surface of drum 21 and in the following order are a charging station 31, an exposure station 33, a developing station 35, a transfer station 37 and an ultrasonic or other type of cleaning station 39. As can be seen, developing station 35 is located just below drum 21.

At charging station 31 there is a charging device 41 for applying a uniform electrostatic charge to the photoconductive surface of drum 21. Charging device may comprise one or a series of corona wires connected to a switchable corona voltage source 42 by a suitable connection 40.

The wire or wires (not shown) of the device 41 may be reciprocated and/or rotated to produce uniform charge.

As the portion of surface charged at charging station 31 passes through the exposure station 33, a latent electrostatic image corresponding to one of the four color image components of the original (i.e., cyan, magenta, yellow or black) is "written" on the surface by the laser beams 20 from the laser system 19. (The beam or beams .are illustrated only in symbolic fashion in the view).

As the latent electrostatic image is moved ,through the developing station 35, it is developed with its corresponding colored developing material (toner) from a developing device 43.

Developing device 43 includes four individual developing units 45,47,49 and 51 mounted in a row on a frame 53. Each developing unit includes a well having a quantity of different colored liquid toner developing material and a toner applicator roller partially immersed in its well. The color of the liquid toner in each well corresponds to one of the four colors of the color resolved image components of the original. Thus, the colors of the toners in the four wells are respectively cyan, magenta, yellow and black.

Frame 53 is mounted on a rigid subcarriage 54 which is guided on tracks 55 for stepwise movement tangential to the drum 21 in which movement it will carry the developing units 45, 47, 49 and 51 seriatim to the developing station 35. The frame 53 and subcarriage 54 are driven by a screw and nut arrangement in which there is a programmed stepping motor 57 whose shaft is coupled to the screw 59 which engages a suitable nut mounted to the subcarriage 54. The nut could comprise a threaded bracket 61 dependent from the subcarriage 54. The motor 57 may be mounted upon and the screw shaft 59 journailed in a block 62 that is connected with the support 25. Its electrical connections are not shown in the drawing.

The developing units 45, 47, 49 and 51 are arranged on the carriage 53 in the order in which it is desired to transfer the four developed images onto the receptor at the transfer station 37. Thus, if the developed images are transferred in the order cyan, magenta, yellow and black, these will be the toner colors in the wells of the developing units 45, 47, 49 and 51, respectively, assuming that the operation stroke of the subcarriage 54 is to the left. (See double-headed arrow which indicates directions of movement of carriage 53-54).

In operation, when a multicolor image is to be reproduced, the apparatus is synchronized and programmed so that the carriage 53 steps to the left bringing the toner applicator roller of the developing unit 45 into engagement with the bottom surface of the electrophotographic drum 21 a short time before arrival of the leading edge of the first latent image which was applied at the exposure station 33. This gives the applicator roller of the unit 45 sufficient time to commence picking up the colored toner from its well to provide good development. Alternatively, the applicator rollers may be continuously rotating slowly to keep the toner flowing on the roller surfaces. Suitable friction drive means capable of being overridden may be used to rotate the applicator rollers.

The carriage 53 maintains the applicator roller of developing unit 45 in engagement with the drum 21 during a dwell period until the complete applied latent image of that particular color of the unit 45 has been toned. The drum rotation causes or accompanies rotation of the applicator roller. In the blank area between latent images for a fraction of a revolution of the drum 21, the carriage 53 is stepped once more to the left as viewed in the figure, now bringing the applicator roller of the second developing unit 47 into engagement with the bottom surface of the drum 21. The applicator roller of developing unit 45 is now idle, having moved to the left of the developing station. This developing unit 47 will be applying the magenta toner from its well to the drum 21 at the developing station to tone the second latent image applied by the laser system 19.It remains in position for the same duration of dwell as before.

The first toned image of cyan is carried on the drum 21 around to the transfer station 37 where it is transferred to the receptor sheet S carried on the transfer roller 65. Any toner remaining on the surface of drum 21 is cleaned off at station 39 by a cleaning device 67, such as a vacuum manifold or an ultrasonic apparatus so that the photoconductive coating on the drum 21 is ready for the following image to be produced during the next revolution.Thus, when the carriage 53 and subcarriage 54 are moved one step to the left to bring the applicator roller of the developing unit 47 into engagement with the bottom of the roller 21, there may still be a portion of the previous toned image rotating toward the transfer station 37 on the left side of the roller 21, and the next following image may already be partially applied in latent form on the right side of the drum 21 and be moving toward the developing station 35.

In this manner, the carriage 53 steps to the left and dwells once for each revolution of the drum 21 until it has carried all four of the developing stations 45, 47, 49 and 51 into engagement with the drum 21 and four developed images have been transferred to the paper member S.

This could be considered one complete or overall cycle made up of four step and dwell cycles because a complete multicolored image has been made. One revolution of the transfer roller 65 may be used to remove the receptor member S while the drum 21 rotates free of any developing unit. In this period of time, the motor 57 may be programmed to reverse the screw 59 to return the subcarriage 54 to its home position on the right as shown in the views. This need not be done by stepping. Preferably, contact and/or transfer of toner between the applicator rollers of the developing units 45,47,49 and 51 and the drum 21 is avoided during this return stroke. For example, the entire carriage 53 could be mounted on springs which in turn are seated on the subcarriage 54 which is directly moved by the screw 59.A simple cam, relay, lever or the like can pull the carriage 53 slightly downward toward the subcarriage 55 and hold same while the subcarriage 55 is moving toward the right back to its home position, releasing same thereafter.

Alternatively, the drum 21 and transfer roller 65 can be moved slightly upward during the return movement.

Another alternative would be to permit contact of applicator rollers with the drum 21 during the return movement and provide means such as 63 to clean the drum 21 of any toner which may be transferred onto the blank part of the drum 21 during this movement.

Another alternative, but one which is not as preferred, is to reverse the order of transfer of colored images on alternate sheets of paper S.

Thus, the laser system 1 9 would be commanded by the computer 1 5 to reverse the order of writing the colored images and the latent images would be developed in the order 51,49,47 and 45 on the return stroke of the subcarriage 54 and carriage 53 from left to right, step by step.

In any event, the various components are required to be synchronized in movement and in operation so that when a particular color image is written by the laser system 1 9 onto the drum 21, it will reach the developing station 35 at the time that the applicator roller of the developing unit of that same color is engaged against the bottom surface of the drum 21. Thus each latent electrostatic image is developed with its proper colored toner and in the proper order.

After thenlatent electrostatic image is developed at the developing station, excess toner is removed from the surface of drum 21 as the developed image is moved lo the transfer station 37 by a vacuum knife 63 located along the path of travel between these two stations. At the transfer station 37, the developed image is transferred to a sheet of carrier material S, such as plain paper derived from a supply of such sheets indicated at 64.

Located at transfer station 37 is the transfer roller 65 which is mounted for rotation in the direction shown by the arrows and driven in any suitable manner at the same linear surface rate of speed as drum 21. A sheet of carrier material S is fed through guides 67 by a feed mechanism whose vacuum cups are shown at 68 and brought into contact with transfer roller 65. By any means known in the art, such as electrostatic attraction, sheet S is wrapped around and held securely on the peripheral surface of transfer roller 65 as transfer roller 65 is rotated.

After each developed image has been transferred to the receptor or sheet S which is enwrapped around the transfer roller 65, the surface of the drum 21 moves in a clockwise direction through the area which is designated the cleaning station 39. At this station, excess toner which may have remained upon the surface of the drum 21 after transfer is removed by the cleaning device 67 which, as mentioned above, can be in the form of a vacuum manifold or ultrasonic apparatus that removes and/or dissipates the remaining toner.

The charging, image forming, developing, transferring and cleaning steps are repeated for each one of the color resolved images. If there are three, the total number of images transferred to the receptor sheet is three, if there are four color images, the total number of images transferred is four. These images are required to be perfect" registration which is a simple matter to be achieved with the apparatus of the invention. The color mixing to achieve the reproduction through the use of toners of different basic colors such as cyan, magenta, yellow and black must consider their transparency and perhaps other characteristics which, of course, are known to those skilled in this art. Obviously, the toner character must be taken into consideration to achieve the most realistic results.

As the final composite color image of the individual color images is being formed on the receptor sheet through a series of seriatim transfers of the multiple different colors making up the final color image, the rollers 21 and 65 are rotating together at the same speed. These rollers are identical in diameter; hence, the number of revolutions of each in order to produce a single composite color image on the receptor S is equal and consists of the number of colors making up the final composite color image. Normally, this is four, and the receptor sheet S is held in place on the periphery of the roller while this is taking place.

When the composite image has been completed and all of the colors making same up have been transferred to the sheet S, the image is a series of, for example, four toned but unfixed images placed on top of one another. At this point in the operation of the machine, either before the sheet S is released from the transfer roller 65 or immediately thereafter, the toned composite color image is fixed to the sheet by passing same past a fixing device 69. This may be a heater or other suitable fixing means. For example, instead of or in addition to applying heat, the device 69 may roll or spray a protective overcoat or resinous glaze onto the completed composite image. There are many different forms of material which are transparent and relatively abrasion-resistant and which are used in protecting inked or toned images that can be applied. Preferably, a glossy or glazed finish is desired to give the resulting image some depth of appearance. The completed Sheet S is carried to the left in the view and deposited in an output tray 73.

After the developing device 43 has been moved back to the right to its "home" position which is that illustrated in the view, the apparatus 11 has completed its overall cycle and is ready for making another composite copy, with the same information derived from the computer store or different information carried in said store.

Additional toner for the developing units may be stored in reservoirs 75, 77, 79 and 81 mounted on the bottom of frame 25 which may be provided with pumps and flexible connections to enable transfer to the respective wells of the developing units.

Although the invention has been described with reference to making color copies of color originals, it can be appreciated that the apparatus can also be used, if desired, to make black and white copies from black and white originals or to make black and white copies of color originals.

In the operation of the apparatus 11, the programming of the functions of the various components can be achieved by suitable programming means such as banks or relays, shafts, cams and the like. Preferably, the operation is programmed by the computer with suitable control and/or enabling signals being transmitted on the lines 100 to the several motors, drive means, etc., of the apparatus.

Achieving registration of the superimposed images on the paper member S is a matter of careful construction and synchronization of the moving parts of the apparatus 11 but adjustments are feasible either by manually operated mechanical means, automatic sensing devices, laser pattern adjustment and/or a combination thereof.

Reference in the claims to a "cycle" is generally intended to mean a revolution of the drum 21 and transfer roller 65 during which one color impression is produced and transferred. For a four-color complete composite, it takes four such "cycles", the overall process or overall cycle being complete after the sheet S has been released, its composite image fixed and/or overcoated, deposited in its output tray, the developing device moved back to "home" position and the apparatus made ready for the next series of cycles or revolutions to begin.

Claims (22)

Claims

1. A method of color printing electrostatically wherein the individual colors of a color composite are digitally stored in a computer and are capable of being called up separately for laser writing on an electrophotographic member, said method comprising the steps of: providing a rotary electrophotographic drum and a transfer roller arranged to carry a member of receptor material engaged against the rotary electrophotographic drum, charging the drum, exposing the charged drum to an image of one color of the composite by laser writing said image on the drum, toning the image and transferring the toned image to the member of receptor material effecting said steps all in one revolution of the drum with a simultaneous single revolution of the transfer roller, the charging, writing, toning and transfer for each color of the composite being repeated during each subsequent revolution of the drum and transfer roller until all colors have been transferred in superimposed relation on the member of receptor material and, then removing the member of receptor material from the transfer roller and replacing it with another to enable repetition of the process on said replacement member.

2. The method according to claim 1 in which each latent electrostatic image corresponding to one of the color image components of the color original, each image being formed during one revolution of the rotary member, developing each latent electrostatic image so formed with material whose color corresponds to the color image component of the latent electrostatic image by moving a plurality of developing units each containing a different color material past said electrophotographic member in succession and in a controlled timing sequence so that each developing unit will independently develop a respective latent electrostatic image during its revolution and transferring each image as so developed in sequence onto a sheet of carrier material in superimposed relationship in as many colors as revolutions of the rotary member.

3. The method according to claims 1 or 2 wherein the step of forming the plurality of latent electrostatic images comprises charging said electrophotographic member and then writing the latent electrostatic images thereon in succession using a laser modulated with digital information derived from the color original.

4. The method according to claims 1, 2 or 3 and wherein the plurality of latent electrostatic images comprises at least three latent electrostatic images, each adapted to provide a developed image of a different color.

5. The method according to any one of claims 1 to 4 wherein the step of developing each one of the latent electrostatic images comprises applying a liquid toner to each one of the latent electrostatic images.

6. Apparatus for making one or more color copies of a color original electrophotographically comprising a rotary electrophotographic member, drive means for rotating said electrophotographic member sequentially in a cycle through a series of processing stations arranged along its periphery, said processing stations including in order, a charging station, an exposure station, a developing station and an image transfer station, means for applying a uniform electrostatic charge to a portion of the electrophotographic member at the charging station, means for forming in sequence a plurality of latent electrostatic images on said electrophotographic member at the exposure station during rotation of said electrophotographic member, each of said latent electrostatic images corresponding to a different one of the color image components of the color original and each image being formed during a respective cycle, a developing device capable of moving through the developing station on a path which is generally tangential to said rotary electrophotographic member, said developing device including a plurality of developing units arranged in a row and adapted to pass through the developing station seriatim, each developing unit having developing toner material whose color corresponds to the color image component of one of the latent electrostatic images, means for moving said developing device stepwise on its tangential path through said developing station in timed relation to the formation of the color corresponding latent images and bringing the developing units into toner applying relationship to the electrophotographic member seriatim as each unit is in said station, said moving means operating to step each developing unit into said developing station, then holding it there during a dwell period sufficiently long, and then stepping said developing unit out of the developing station while simultaneously moving the next following unit into the station, the portion of the electrophotographic member carrying a color corresponding latent image of each developing unit being fully moved through the developing station during the dwell period that its respective developing unit is in said station, and transfer means at said transfer station for transferring each developed image onto a sheet of carrier material in superimposed relationship as each developed image passes through said transfer station during its respective cycle, there being as many cycles as colors and as many impressions on each sheet as cycles.

7. The apparatus according to claim 6 and wherein the means for forming the plurality of latent electrostatic images comprise means for converting the color original to be copied into color encoded digital information and a laser whose output is modulated according to said digital information for writing each of said plurality of latent electrostatic images on said electrophotographic member.

8. The apparatus as claimed in claims 6 or 7 and wherein each developing unit comprises a well and an applicator in toner transfer relation with the electrophotographic member and the developing material whose color corresponds to the color image component of the latent electrostatic image is a liquid toner dispersed in the well in contact with the applicator.

9. The apparatus according to claim 8 wherein the applicator is a roller.

10. The apparatus according to any one of claims 6 to 9 wherein there are at least three developing units, each for a different color.

11. The apparatus according to any one of claims 6 to 10 wherein the developing device comprises a carriage mounted on track means and the means for moving the carriage is a stepping motor coupled to the carriage.

12. The apparatus according to any one of claims 6 to 11 in which the means for moving the developing device on said tangential path steps the developing device in one direction while said cycles are being executed and returns said carriage in an opposite direction to a home position after the cycles for any one fully developed image has been completed so that the carriage is poised for again stepping in said one direction for the next following series of cycles.

13. The apparatus according to any one of claims 6 to 12 in which the tangential path is rectilinear.

14. The apparatus according to any one of claims 6 to 13 wherein the transfer means include a rotary transfer drum.

1 5. The apparatus according to claim 14 wherein the transfer drum includes means for selectively holding and releasing the sheet of carrier material and moving same on a path away from said transfer drum.

1 6. The apparatus according to claims 14 or 1 5 in which the rotary electrophotographic member and the rotary transfer drum are in substantial contact on an axial line on the surface of each, are of the same diameter and are driven at the same speed.

17. The apparatus according to any one of claims 6 to 1 6 and further including a cleaning station adapted to remove residual toner from said electrophotographic member after the developed image on said electrophotographic member has passed through said image transfer station, said cleaning station being located also along the periphery of said electrophotographic member between said image transfer station and said exposure station.

18. The apparatus according to claim 1 7 in which said cleaning station includes an ultrasonic cleaner.

19. The apparatus according to any one of claims 6 to 1 8 in which there is a fixing station arranged to fix the developed images to said sheet of carrier material after all of the developed images of a complete series of impressions have been applied thereto.

20. The apparatus according to claim 19 in which the fixing station is located adjacent the path of movement of said sheet after its release.

21. The apparatus according to claims 19 or 20 in which the fixing station comprises means for applying a protective coating to said developed images.

22. The apparatus according to claims 1 9 to 21 in which an overcoating station is located adjacent the fixing station to apply an overcoat to said sheet after it has passed through the fixing station.