CA1167512A

CA1167512A - Electrophotographic color printing method and apparatus

Info

Publication number: CA1167512A
Application number: CA000347966A
Authority: CA
Inventors: Manfred R. Kuehnle; Jurgen Kruse; Robert M. Rose
Original assignee: Coulter Systems Corp
Current assignee: Coulter Systems Corp
Priority date: 1979-03-26
Filing date: 1980-03-19
Publication date: 1984-05-15
Also published as: IL59653A0; DE3010568A1; IL59653A; GB2046665A; AU531992B2; AT369562B; IT1146979B; AU5651280A; LU82270A1; SE8002164L; ATA148380A; CH638061A5; SE444736B; NL8001632A; IT8048205A0; GB2046665B; FR2452732A1; DK118780A

Abstract

ABSTRACT

This invention relates generally to color copying and particularly concerns electrostatic color copying technique.
An apparatus and method for making color copies of a color original in which color resolved latent electrostatic image components of the original are formed on the surface of a rotating electrophotographic drum, developed with color corresponding development material from a developing device and then transferred to a sheet of carrier material, such as paper, in superimposed relationship.
The color resolved latent electrostatic images are formed on the surface of the electrophotographic drum by converting the color resolved image of the original into digital data, feeding the data into a computer and then writing the images on the surface of the electrophotographic drum with a laser whose output is modulated by the digital data. The color resolved images are obtained by projecting an image of the color original through color separation filters.
The developing device includes a plurality of developing units, each containing developing material whose color corresponds to one of the color image components of the color original. The developing units are moved past the drum, seriatim in a stepwise cycle. The movement of developing device and the electrophotographic drum are coordinated so that each developing unit will apply developing material to its corresponding latent electrostatic image during the dwell period of the developing device when said developing unit is in toner applying relation with the drum.

Description

P~ior method~ and ~pparatus for making color copies o~ color origin~ls by electrostatic technlques has involved the steps of optically projectin~ color resolved images of the original onto a single charged eIectrophotvgxaphic me~ber or a plurality o~ charged eIectrophotographic ~embers, one for each color resolved lmage to form corre~pondin~ latent eIectrostatic images, developins each l~tent electrostatic image with its proper color and then fixing each image in superimposed relationships~ The images projected onto the electrophoto~raphic members were obtained by projecting an image o~ the original thereon through a plurality o colox 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 txansferred from one or more electrophotographic members to a sheet of carrier m~terial, such as plain paper. Although m~ny di~ferent types o electrostatic systems which have been proposed or constructed, these systems have proven to be generally unsatisfactory. Some of the reasons are that the resultin~ apparatus has been too large in size, co~plex in construction, expensive to build and operate, unreliable and/or not capable o 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 o~ the final copy, if such changes are either desired or required except by very difficult or expensive means involving changes in the process.

`~.

1 ~75 ~ 2 ~ n electrophotographic Eilm now is avallable which has ~ualities and properties which make it far superior to any other known eIectrophotographic film.
The said electrophoto~ra,phic film features a photoconductive coating tha~ is wholly inorganic, mi~rocrystalline, eIectrically anisotropic in nature, which does not have reciprocity or intexmit-tency effects, operates at low Yoltages~ has ~ery high sensitivity and can produce toned images of exceptionall~ high ~ualit~ from either analog ; 10 or digital information. This electrophotographic film is well suited for use in making high resolution color copies o~ color originals by electrostatic techniques/ and, in faGt, ha~ properties which enable'it to be used for this purpose in a manner that hitherto has not been achievable with other types of electrophotographic me~bers.
8ecause of the high speed of response of said co~ting (capable of being exposed in nanoseconds), the ~pparatus can comprise a high-speed machine~
Among the su~gestions made in the prior art, the following are believed pertinent to the field of the invention.

U.S. Patent to Lushex 3,399,611 '' Smith 3,690,756 Nagamatsu et al 3,832,170 Katayama et al 4,095,879 Watanabe et al 4,120,577 Barasch 4,124,286 No inference should be drawn that the above represents a comprehen5ive investigation of the prior art;
that ~ny or all are pe~tinent to the invention; that the apparatus therein is analogous to the inventionj that the invention hexein is the equivalent of any disclosed therein.

5 ~ ~

Accordingly~ the .inYention concerns a me-thod and apparatus ma~ing of color copies of a color original by forming color resolved latent electrostatic images of the original on a rotatiny electrophotoyrcphic member, developing each latent electrostatic image so formed with color corresponding developing material and then transferring each developed image to a sheet of carrier material in superimposed and registered relationship. The latent electrostatic images are formed on the electrophotographic member ~y means of a laser system whose output beam is modulated with digital information corresponding to each one of the color resolved images~ and developed with colored developing material from individual developing units.
Each color image is formed, deyeloped and transferred during one revolution of the electrophotographic member so that there are as many revolutions as colors; also there are as many transfers-to the same sheet of carrier material as coloxs bein~ printed on that sheet.

~ ~751 2 The preferred embodiments of this inventi.On now will be described, by way of example, with reference to the drawin~s accompanying this speci~ication in which:' The sole figure is a fragmentar,y view partially in perspective and partially in block diagram form of an apparatus for making color pr,ints of color originals accoxding to this invention, it being understood that some of the de-tails are more'or less symbolic or dia~rammatic.
Reerring now to the drawing, there'is shown an embodiment of an electxophoto~raphic co~yin~
app~ratus constructed according to this invention and ' identified by reference numeral ll.
~n oxiginal to be copied is deposited on a 1at 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, xed, green .
- and blue filter units for orming the three colo~
separations ~nd an ~nalog to digital converter for ~
converting the analog information into digital dat~. The - three or four color separations may be formed - simultaneously by airectins 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 dig.ital data so obtained is fed into a computer 15 where it is stored, either permanently or temporarily, and further processed. The further ~rocessing includes convertin~ the red, ~reen, blue and black digital 5 ~ 2 dat~ respecti~el~ t~ c~an, ma~enta, yellow and black ~alues.
The furthe.r processin~ ma~ also include such operations ~s linear scaling, croppin~, density shi.fting, ~all~a modiication, color balancin~, color reversal, c~ptioning and the like.
~ graphic display terminal 17 which is connected to compute~ 15 enables the imayes stored in the computer to be monitored at any time~
~ laser system 19 for writing images using the digital da~a is also connected to computer 15. Laser system 19 may comprise a scanning prism/ a laser and an - ~cousto-optical modulator for modulating the output beam of the laser in accordance ~ith digital information~
Laser system 19 is arran~ed to scan over a particular an~le. Instead of a single laser, laser s~stem 19 may include a pluralit~ o~ lasers, each arranged to sc~n a portion ofthat particular angle.
There is an electrophotographic member in the form o~ a rotar~ electrophotographic arum 21 mounted on an axle 23 ~hich is jouxnalled ~or rotation on a support frame 25 ~n the direction shown by the arrows~ The suppo frame 25 is broken aw~y and bearings, journals and bracXets are not illustrated in order to simplif~ 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 sur~ace of drum 21 comprises a photoconductive coaking on a conductive substrate~ The conductive substrate may either be a sleeve mounted on drum 21 or an inte~ral part of drum 21. Thus, it ~ay comprise a ~e~al substrate with the photoconductive coatin~ on its surface o.r an insulative ~ 1~'751 2 sheet material having an ohmic layer hetween the sheet material and the photoconductive coating. In either event, the metal substate or ohmic layer is required to be connected electrically into a charging circui-t 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 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 s-tation 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 ~1 for applying a uniform electrostatic charge to the photoconductive surface of drum 21. Charging device may comprise one or a series of corona wires eonneeted to a switehable eoronalvoltage souree 42 by a suitable eonnection 40. The wire or wires (not shown) of the deviee 41 may be reeiproeated and/or rotated to produce uniform charge.
As the portion of surface charged at charging station 31 passes through the exposure sta-tion 33, a latent electrostatic image corresponding to one of the four color image components of the original (i.e., cyan, ~ ,, . . ~, m~enta, yellow or black) is "wri-tten" on the sur~ace by the laser beams 20 ~rom the laser system l9~ (The beam or beams are illustrated only in symbolic fashion in the view.) As the l~tent eIectrostatic image is moved .
. through the .developin~ station-35, it is developed with its corxesponding colored developing material (toner) from a ~eveloping device 43~
Developing device 43 includes four indi~idual developing units 45, 47, 4g and 51 mounted in a row on a frame 53. Each developin~ unit includes a weIl having ~ quantity of di~ferent colored li~uid toner developing material and a ton~r applicator ro:Ller partially immersed in its well. The color of the liquid toner in each well corxesponds to one of the four colors of the color resolved image components of the original. Thus~ the colors of the toners in the ~oux wells are respectively cyan, - magentaj yellow and black~
Frame 53 is mounted on a rigid subcarriage 54 which is guided on tracks 55 for stepwise m~vement 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 dri~en by a screw and nut arrangement in which there is a programmed stepping motor 57 whose sha:~t is coupled to the screw 59 wh.ich engages a suitable nut mounted to the subcarriage 54. The nut could comprise a threaded bracket 61 dependent from the subcarriage 5~.
The ~otor 57 may be mounted upon and the screw shaft 5 ~'751~

journalled in a block .62 that is connec-ted with the support 25. Its electrical connections ~re not shown in the dra~in~ .
The developing units ~5, 47, ~9 and 51 are ~rranged on the carria~e 53 in the ordex in whi~h it is desired to trans~er -the four developed images onto the receptor at the transfer s~-ation 37. Thus, if the . de~eloped images are transferred in the order ~an,' magenta, ~ello~ and black, these will be the tone~ colors in the ~ells of the developing units 45~ 47, 49 and 51, respeotively, assuming that the operatin~ stroke'of the subcarriage 54 is to the left. (See double-headed arro~ which indicates directions of moyement o~
.~ ' c~rriaga 53-54).
: In operation/ when a mu:Lticolor image is to - be xeproduced, the appar~tus is synchronized and programmed so that thè carriage 53 steps to the'left bringing the toner applicatox roller of the developing unit 45 into engagement with the bottom surface o~ the ,electrophotographic drum 21 a short time before arrival of the leading edge of the first latent image'which was .applied at the e,xposure station 33~ This gives ~he applicator roller of the unit 45 suf~icient time'to com~ence 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 bein~ overridden may be used ~o rotate the applicator rollers, ~ ~751 ~

The carriage 53 m~intains the applicator roller of deyel~pin~ unit 45 in en~agement with the dr~m 21 during a dwell pexiod until the complete applied laten't 'image of th~t particular color o~ the unit 45 has been toned, The'drum rotation causes ox accompanies rotatlon of the applicator roller~ 'In the blank area between latent images for ~ ~r~ction of a revolu-tion of the dr~um 21, the' carriage 53 is stepped once more to the left as viewed . in the.'figure, now bring.ing the applicator roller .l0 . of the second developing unit 47 into engagement with the bottom surface of the drum 21~ The applicator roller o~
developing unit 45 is now idle, having moved t~ the left .
' of the developing station~ This developin~ unit ~7 will be applying the magenta toner from its well to the drum 21 .
: 15 at the developing station to.tone the second latent image applied by the laser s~stem l9. It remains in pOSition for the same duration o~.dwell as before.
' ' '. . , The first to~ed im,age of c~an is carried on the drum 21'around to'the.transfer station 37 where it is transferred to the receptor sheet S carried on the transfex roller 65. An~ toner remaining on the surface'o~. drum 21 .
is cleaned of~ at station 39 by a cleanin~ device'67, such - as a vacuum manifold or an ultrasonic apparatus so that the photoconductive coating on the drum 21 is ready ~or the following image to be pxoduced auring the next revolution~ Thus, when the carriage 53 and subcarriage 54 are moved one step to the leXt to bring the appIica-tor roller o~ the develop.ing unit 47 into engagement with the botto}n of the roller 21, the~e may still be ~ ~ortion of ~ 1 --'7 5 ~ 2 the pxeYious toned image rotating towara the -transfer station 37 on the le~t side o~ the roller 21, and the nex-t follo~iny image may al.xeady be partially ~pplied in latent ~oxm on the x~ght side of the drum 21 and be moving toward the developing station 35.
~ n this ~anner, th.e carxi~ge 53 steps to the left and dwells once fox each revolution o~ the drum 21 . until it has carried all ~our of the developing st~tions 45,~47, 49 and 51 into en~agement with the drum 21 and four developed ima~es have been transfer~ed to the .
paper me~ber S.
This could be consi.dered one complete or overall cycle made up of foux 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 o~ any de~eloping unit~ In this period of time, the motor 57 may b~ programmed to reverse the screw 59 to return the subaarriage 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 be-~ween the applicator rollers o~ the developing units 45, 47, 49 and , 51 and the drum 21 is avoided duriny this return stroke~
For example, the entire carriage 53 could be mounted on springs which in turn are seated on the subcarriaye S4 which is directly movea by the screw 59. A simple cam, relay, le~er or the like can pull the carriage 53 slightly downw~rd ~o~rd the subcarri.age 55 and hold same while the subcarriaye 55 is mo~ing toward the right back to its ho~e position, releasin~ same therea~ter.

7 5 1 ~

~ ltern~ti~ely, the drum 21 and transfer rolle~ 65 can be moved slightly upward during the return movement~
Another alternative would be to permi-t contac-t S of applicator xollers with the drum 21 during the xeturn movement and provide me~ns such as 63 to clean the drum 21 of any toner which may be -transferred onto the blank part of the drum 21 durin~ this movement.
: Another alternative, but one which is not as pre$erred, is .to reverse the order of transfer of colored . images on alternate sheets vf paper S. Thus, the laser system 19 would be commanded by the computer 15 to reverse the order of writing the colored images and the . . latent images would be developed in the order 51~49, 47 ; 15 a~d 45 on the re.turn stroke of the subcarriage 54 and carriage - 53 from left to right, step b~ step Xn~any event~ the ~arious components are re~uired to be synchronized in movement and in operation : so that when a particular color image is written ~y the laser s~s-tem 19 onto the drum 21, it will reach the .
developing station.35 at the time that the applicator roller o~ the developing unit of th~t same color is engaged against the bottom suxface of the drum 21- Thus each latent electrostatic image is developed with its proper colored toner and in the proper order.
After the latent electrostatic image is developed at the developing station, excess to~er is removed ~rom the surface of drum 21 as the developed image i5 moved to the transfer station 37 bya vacuu~ knife 63 located along the path of travel be-tween these two stations. ~t 1~7512 the transfer st~tion 37~ the developed ~nage i~
transferred t~ a sheet of ca,rrier material S, SUCh as plain p~per deri~ed ~rom a supply of such sheets indicated at 64v - 5 Loc~ted at tr~nsfer s-tation 37 is the transfer - roller 65 which is mounted for rotation in the directi,on shown b~ the arrows ~nd driven in any suitable manner at the same'linear surface rate of speed as drum 21. ~ sheek~
. of carrier materi~l S is fed through guides 67 by a feed mechRnis~ whose vacuum cups are shown ~t 68 ana brought . into contact ~rith transfer roller 65. By an~ means known in the art~ such ~s electrostatic attraction, sheet S is wrapped around and held securel~ 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 651. the surface of the drum 21 moves in a clockwise direction throuyh'the area w~ich is'designated the cleaning ~tation 39. At this station, excess toner which ma~ have remained upon the surface'of the drum 21 af-ter tr~nsfer is removed by the cleaning device 67 which, as mentioned abo~e, can be in the form of a ~acuum manifold or ultrasonic app~ratus that removes and/or ~issipates the remaining toner.
The charging, image forming, developing, txansferring ana cleaning steps are repeated for each one of the color resolved images, If there are three, the total number of imayes transferred to -the receptor sheet is three; if there are four color images, the total number of imayes transferred is foux. These i.ma,~es are required 7 'j ~ 2 to be in per~ect recJistration which is ~ simple' matter to be'achieved with the apparatus of the invention. The color mixin~ to'achieve the reproduction through 'the ~se of toners of different basic colors such'as cyan, magenta, ~ellow 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 ima~e of the ' individual co:lor 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 5ame speed.
These rollers are identical in diameter; hence, the number of revolutions of each in order ~o pxoduce 5 single composite color image on the receptor S is equal and consists of the nu~ber of colors making up the final composite color image. Normall~, this is four, and the receptor sheet S is held in place on the peripher~ of the rollex while this is taking place.
When the composite ima~e has been comple-ted and all of the colors makirlg 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 thereaEter, the toned composite color image is fixed to the sheet by passing same past a fixing device 69. ~his ma~ be a heatex or other suitable fixing 5 ~ ~

means. For example~ instead of or in addition to applyiny heat, the device 69 ~lay ~oll or spray a protective overcoat or resinous glaze onto the completed composi-te'ima~e. There are ~any different forms oE material which a.re transparent and rel~tively abrasion-resistant and which are used in protecting inked or toned imac~es that can be applied~
Preferably, a glossy or glazed finish is desired to give the resulting image some dep~h'of appearance~ The comple-t'ed sheet S is carried to the left in the view and deposited in ~n output tra~ 73.
After ~he de~eloping .device 43 has been moved back to the xight to its "home" position which is -that illustrated in the view, thQ apP~ratus 11 has completed its o~erall -' cycle and is read~ for making another composite'copy, with the same informa-tion derived from the computex store or dif~erent information carried in said store.
Additional toner for the de~eloping-uni~s may be stored in reservoirs 75, 77, 79 and 81 mountea on the botto~ of frame 25 which may be provided with pumps and flexible connections to enable transfer to the respective wells of the develop~ngunits~
Although the invention has been described with ' re~erence to making color copies of color originals, it can be appreciated that:the apparatus can also be usedr if desired, to make black and white copies ~rom black and white original.s or to make black and white copies o~ color ~riginals.
In the operation of the apparatus 11, the programming of the functions of the various co~ponents can be achieved by suitable programmin~ means such as banks ~ 1~ 6 ~

or relays, shafts, cams a~d the like. Pre~erably, the operation is pro~rammed by the computer with suitable control and/or enablin~ signals bein~ tran5mitted on the lines 100 to the several mokors, drive means, etc , of the apparatus.
Achieving reyistration of the superimposed images on the paper member S is a matter of care~ul construction and synchronization of the moving parts of the apparatus 11 but adjustments are easible either by manually operated mechanical means, automatic sensing devices, laser pattern adjustment and/or a combination ~hereof.
Reference in the claims to a "cycle" is generally intended to mean a revolution o~ the drum 21 and transfer roller 65 during which one color irnpression 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 imaye ~ixed and/or overcoated, deposited in its output tra~, the developin~ device moved ba~k to "home" position and the apparatus made ready for the next series of cycles or revolutions to begin.

.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

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 electro-photographic 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 tile 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 as defined in claim 1 in which each latent electrostatic image corresponds 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 as defined in 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 as defined in claim 1 or 2 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 as defined in 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 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.

6. The method as defined in claim 1 or 2 wherein there are formed at least three latent images, each one of the latent electrostatic images being developed by applying a liquid toner of a different color to the respective latent electrostatic image for providing developed images of respective different colors.

7. The method as defined in 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 and 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 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.

8. 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.

9. The apparatus as defined in claim 8 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.

10. The apparatus as defined in claim 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.

11. The apparatus as defined in claim 10 in which the fixing station is located adjacent the path of movement of said sheet after its release.

12. The apparatus as defined in claim 10 in which the fixing station comprises means for applying a protective coating to said developed images.

13. The apparatus as defined in claim 10, 11 or 12 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.

14. The apparatus as defined in claim 8, 9 or 10 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.

15. The apparatus as defined in claim 8, 9 or 10 wherein the transfer means include a rotary transfer drum.

16. The apparatus as defined in claim 8, 9 or 10 wherein the transfer means include a rotary transfer drum and means for selectively holding and releasing the sheet of carrier material and moving same on a path away from said transfer drum.

17. The apparatus as defined in claim 8, 9 or 10 wherein the transfer means include a rotary transfer drum and means for selectively holding and releasing the sheet of carrier material and moving same on a path away from said transfer drum and in which transfer means comprise a rotary transfer drum, the rotary electrophotographic member and the rotary transfer drum being in substantial contact on an axial line on the surface of each being of the same diameter and being driven at the same speed and means for selectively holding and releasing the sheet of carrier material and moving same along a path away from the transfer drum.

18. The apparatus as defined in claim 9 in which said cleaning station includes an ultrasonic cleaner.

19. The apparatus as defined in claim 9 wherein the applicator is a roller.

20. The apparatus as defined in claim 8, 9 or 10 wherein there are at least three developing units, each for a different color.

21. The apparatus as defined in claim 8, 9 or 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.

22. The apparatus as defined in claim 8, 9 or 10 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.

23. The apparatus as defined in claim 8, 9 or 10 in which the tangential path is rectilinear.