NL8001632A - METHOD AND APPARATUS FOR ELECTROSTATIC COLOR PRINTING. - Google Patents

Description

♦

803096 / Ti / vL

Applicant: COULTER SYSTEMS CORPORATION 35 Wiggins Avenue Bedford, Mass. Q1J30 United States of America

Short designation: Method and device for electrostatic color printing

Known methods and apparatus for making color copies of colored originals using electrostatic techniques require the optical projection of color-decomposed parts of the original onto a single charged electrophotographic element or a plurality of charged electrophotographic elements. each color image, to form corresponding latent electrostatic images, developing each electrostatic image with its own color and then fixing the images on top of each other. The images 10 projected onto the electrophotographic elements are obtained by projecting the original thereon through a number of color separation filters, one for each color to be reproduced. Depending on the system used, the final 800 1 6 32 -2 composite image obtained is either formed directly on a single electrophotographic element or transferred from one or more electrophotographic elements to a layer of support material, such as plain paper. Although many different electrostatic systems have already been proposed and realized, they have all remained unsatisfactory. The reason is that the resulting device is too large, too complex in construction, expensive to manufacture and operate, unreliable, or unable to provide images with a quality or resolution comparable to that obtained with the photographic and / or lithographic techniques. In addition, it is not possible to make changes in color, size or shade in the final copy, except through difficult to perform and expensive processes.

An electrophotographic film is now commercially available which has properties and qualities that are considerably better than those of other known electrophotographic films. Said film has a photoconductive coating which is completely inorganic, microcrystalline, electrically anisotropic in nature, exhibits no reciprocity effects or effects resulting from intermittent operation, can be used at low voltages, is very sensitive and, based on analog or digital information, toner images with particularly high can deliver quality.

This electrophotographic film is well suited for the production of color originals of very high resolution color copies using electrostatic techniques and can be used in a manner not achievable with the hitherto known films.

Due to the very high response speed of the coating (which can be exposed in nanoseconds), a device in which such a film is applied can operate at a high speed.

For the prior art concerning electrostatic copying and printing techniques, reference is made to U.S. Patents 3,399-611, 3,690,756, 3,832,170, U.095-879, h.120,577 and h. 124,286.

The invention provides a method of electrostatic color printing in which the individual colors of a partial color image are digitally stored in a calculator and can be recalled separately for the purpose of laser-describing an electrophotographic element, using a rotating electrode -trophotographic drum and a transfer roller in which a receiving material interacts with the rotating electrophotographic drum, and the drum is loaded, exposed with an image of a partial color, this image is toned and the toner image is transferred to the receiving material according to The invention is characterized in that the exposure consists of retrieving and laser-writing the image and all these steps are performed in one revolution of the drum with a simultaneous single revolution of the transfer roller while loading, writing , with toner and transferring for each separate color of the color composite image is repeated during each subsequent revolution of drum and transfer roller until all colors have been transferred in the correct position relative to each other on the receiving material which is then removed from the transfer roller and is replaced by another, then the above steps are repeated.

Preferably, this method is performed such that each latent electrostatic image corresponding to one of the color components of the color original is formed during one revolution of the rotating element and each latent electrostatic image formed in this way is developed with the material whose color r ~ -4- corresponds to the color image component of the latent electrostatic image by successively moving a plurality of developing units, each containing a different coloring material, along the electrophotographic element and in a controlled time sequence, such that each developing unit is independently resp.

latent electrostatic image will develop during its revolution and each image so developed is transferred to a sheet of support material with the different images superimposed and with as many colors as there are rotations of the rotating element.

The invention further provides an apparatus for carrying out the above-described method, comprising a rotating electrophotographic element, driving means for rotatingly rotating it in a cycle along successively a series of processing stations arranged along its periphery, namely a loading station, an exposure station , a developing station and an image transfer station, and with means for applying a uniform electrostatic charge to a part of the electrophotographic element in the charging station, which according to the invention is characterized by: A. means for successively forming a number of latent electrostatic images on the electrophotographic element in the exposure station during the rotation of this electrophotographic element, each of these electrostatic images corresponding to a different one of the color image components of the colored original and each image during a cycle is being formed; B. a developing device, adapted for movement through the developing station along a path substantially tangential to the rotating electrophotographic element, and which developing device comprises a plurality of spaced-apart 800-1 6 32 * -5- winding units arranged to move sequentially through the developing station, each developing unit being filled with developed and toner material, the color of which corresponds to the color image component of one of the latent electrostatic images; 5 C. driving means for moving the developing device stepwise according to the tangential path through the developing station in a time-controlled relationship with respect to the formation of the corresponding latent color images and for bringing the developing units into the toner transfer position with respect to the electrophotographic element when each unit is in the station the drive means step by step s bringing the developing unit into the developing station, keeping it there for a sufficiently long working period, and then moving the developing unit out of the developing station while simultaneously introducing the next unit into the station, while the portion of the electrophotographic image having a corresponding latent color image of each developing unit is moved entirely by the developing station during the working period in which the resp. developer unit is located in this station; and by D. a transfer device for transferring each developed image to a sheet of support material, the images superimposed when each developed image passes through the transfer station during the resp. cycle, and where there are as many cycles as colors, and there are as many prints on each sheet as there are cycles.

The invention is elucidated with reference to the drawing.

The drawing shows, partly in perspective and partly in block diagram form, an apparatus according to the invention for producing color prints from color originals.

The electrophotographic copying device according to the invention 80 0 1 fi 32 -6- is indicated in its entirety by the reference numeral 11.

The original to be copied is placed on a flat scanning bed 13 where the image is quickly scanned and decomposed into three or four color separations, recorded as digital information. The scanner 135 may be constructed with a white light source, an optical scanning unit, a red, a green, and a blue filter 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 can be formed simultaneously by projecting 10 images of the original simultaneously through the three filters; a special filter or scanning system can be used for black pigment.

The data obtained in this way is input into a computing device 15 where the data is stored, permanently or temporarily, and further processed. This further processing includes converting the red, green, blue and black digital data into resp. values for cyan, magenta, yellow and black. This further processing may also include operations such as linear scale change, magnification, density shift gamma modification, color balance change, color anchoring, designation, etc.

A graphic display terminal 17 connected to the computing device 15 makes it possible to constantly check the images stored therein.

A laser system 19 for writing images using the digital data is connected to the calculator 15. The laser system 19 may consist of a scanning prism, a laser and an acousto-optical modulator for modulating the output beam of the laser in accordance with digital information. The laser system 19 covers a certain angle. Instead of a single laser 800 1 6 32

• I

A number of lasers may also be used, each for covering part of the above angle.

The device includes an electrophotographic element in the form of a rotatable electrophotographic drum 21 having a 5 axis 23 rotatably supported in a frame 25 and rotating in the direction indicated by the arrows. The support frame 25 is shown broken away and the various bearings, supports and fixings are not shown for simplicity. The drum 21 is driven by a motor 27, the output shaft of which is mechanically coupled, as indicated at 22, to the shaft 23. The contour of the drum 21 carries a photoconductive coating on a conductive substrate. This conductive substrate may be a sleeve mounted on the drum 21, or be part of the drum 21. There may be a metal substrate with the photoconductive coating on the surface, or a sheet of insulating material with a conductive layer between this material and the photoconductive coating is applied. It must be possible to electrically connect the metal substrate or the conductive layer to a charging circuit having a corona voltage source. Latent electrostatic images can be formed on the photoconductive coating at very high speed by laser beam writing, which images can then be developed into prints of very good quality and high resolution. Such a coating is described in U.S. Pat. No. 1,025,339

Arranged around the path of the peripheral surface of the drum 21, a loading station 31, an exposure station 33, a developing station 35, a transfer station 37 and an ultrasonic or otherwise operating cleaning station 39 are arranged successively. The developing station 35 is located directly under the drum 21. The charging 800 1 6 32-8 station 31 contains a charging device by means of which a uniform electrostatic charge can be applied to the photoconductive surface of the drum 21. The charging device can be composed of a number of corona wires connected to a switchable corona voltage. Source k2 through a suitable connection 40. The wires (not shown) of the device Ui can be reciprocated or rotated to bring a uniform charge to the surface.

When the portion of the surface loaded in the charging station 31 passes through the exposure station 33, a latent electrostatic image corresponding to one of the four color components of the original, for example, cyan, magenta, yellow or black, is written on the area by the laser beams 20 from the laser 19. These beams are indicated by dashed lines.

When the latent electrostatic image moves through the unwinding station 35, it is developed with the corresponding colored developing material (toner) from the developing device.

The developing device U3 comprises a four individual developing units ^ 5, ^ 7, ^ 9 and 51 arranged in a row on a frame 53. Each developing unit has a container in which an amount of differently colored liquid toner developing material is arranged, and has a toner supply roller partially submerged in this container. The color of the liquid toner in the container space corresponds to one of the four colors of the original image to be decomposed. Thus, the colors of the toner in the four containers will be cyan, magenta, yellow and black.

The frame 53 is mounted on a sturdy auxiliary carriage 5 ^ · which is guided on lines 55 so that the whole can be moved stepwise tangentially with respect to the drum 21, at which movement the 800 1 6 32 -9- / \ • »de-skin units 1 5, 1 + 7, 1 + 9 and 51 are successively fed to the developing station 35. The frame 53 and the auxiliary carriage 5¾ are driven by a screw and nut combination via a programmed stepper motor 57, the shaft of which is coupled to the screw 59 which cooperates-5 like a nut on the carriage 5 * + · The nut can be designed as a tab-threaded lip 61 protruding from the carriage 53.

The motor 57 can be mounted on, and the shaft 59 can be guided in a block 62 connected to the carrier 25. The electrical connection are not shown in the drawing.

The developing units 1 + 5, 1 + 7, 1 + 9 and 51 are mounted on the carriage 53 in the order in which the four developed images are to be transferred to the receiver in the transfer station 37.

When the developed images are transferred in the order: cyan, magenta, yellow and black, these will also be the toner colors in the 15 resp. Holders of the developing units 1 + 5 »1 + 7» 1 + 9 and 51, assuming the working stroke of the carriage 5¾ is from right to left: see the arrow indicating the directions of movement of the carriage 53-5¾.

If a multi-color image is to be reproduced, the device must be synchronized and programmed so that the carriage 53 always moves one step to the left, whereby the toner supply roller of the developing unit 1 + 5 is already in contact with the bottom surface of the electrophotographic drum 51 for a short time before the arrival of the leading edge of the first latent image applied in the exposure station 33. This gives the feed roll of the unit 1 + 5 sufficient time to pick up colored toner from the container and to give it a good development.

The feed rolls can also rotate slowly continuously to allow the toner to flow over the roll surfaces. A suitable friction drive 80 0 1 6 32 -10- in which slip may possibly occur can be used for the rotary driving of the feed rolls.

The carriage 53 keeps the feed roller of developing unit 1 * 5 in contact with the drum 21 for a certain period of time until the complete 5 applied latent image of that particular color of unit 1 * 5 is supplied with toner. The drum rotation also rotates the application roller. In the free surface with no latent image, during a fraction of the rotation of the drum 21, the carriage 53 is moved stepwise to the left, whereby the application roller of the second developing unit 1 * 7 comes into contact with the bottom surface of the drum 21. The application roller of the developing unit 1 * 5 is now inactive and is located to the left of the developing station. The developing unit 1 * 7 will now apply the magenta toner from the container to the drum 21 for toner supplying the second latent image applied by the laser system 19. The development unit 1 * 7 remains in this place for the same work period.

The first, cyan, toner image is brought on the drum 21 to the transfer station 37 where it is transferred to the receiving sheet S present on the transfer roller 65. Toner remaining on the surface of the drum 21 is transferred into the station 39 20 by a cleaning device 67 removed; here, for example, use can be made of a vacuum suction nozzle or of an ultrasonically operating device so that the photoconductive coating on the drum 22 is ready for the next image to be produced during the next revolution. When the carriage 53 and the auxiliary carriage 5 ^ are moved one step to the left to bring the applicator roll of developing unit 1 * 7 into contact with the bottom of the roller 21, a portion of the previously toned image may rotate to the transfer station 37 to the left of the roll 21, and the next image may already have 800 1 6 32 partially arranged in latent form on the right side of the drum 21 and moving to the developing station 35 · -11-

Thus, the carriage 53 moves stepwise to the left and certain periods remain with each revolution of the drum 21 until all four 5 developing stations 1 + 5, 1 + 7, 1 + 9 and 51 have cooperated with the drum 21 and four developed images have been transferred to the paper S.

This is considered to be a complete cycle consisting of four steps and four rest cycles after which a complete multi-color image has been obtained. One revolution of the transfer roller 65 can be used to remove the receiving sheet S while the drum 21 rotates free from the developing units. The motor 57 may be programmed to reverse the screw 59 during this time period to return the auxiliary carriage 5l + to the home position on the right. This does not have to be a stepwise movement. Preferably, during this return stroke, contact and / or toner transfer between the application rollers of the developing units 1 + 5, 1 + 7, 1 + 9 and 51 on the one hand and the drum 21 on the other is prevented. For example, the entire carriage 53 can be mounted on springs that rest on the auxiliary carriage 5I +, which in turn is driven directly by the screw 59. A simple cam, 20 relay, lever and the like can be used to move the carriage 53 slightly down to the auxiliary carriage 5I + and keep it there while the latter moves to the right, i.e. to the starting position.

It is also possible to raise the drum 21 and the transfer roller 65 slightly during the return movement.

Another possibility is to allow contact between the application rollers and the drum 21 during the return movement and the use of means 63 for removing toner from the drum 21 which would have been transferred to any part of the drum 21 during this movement.

o η π 4 & ίο -12-

Yet another alternative, but less favorable, is to reverse the order of transfer of colored heights to different sheets of cm cm. The laser system 19 must then be controlled by the computing device 15 such that the order of recording 5 colored images and developing latent images becomes: 51, 9, 1 + 7 and 1 + 5 on the return stroke of the auxiliary car 5 ^ and the carriage 53 from left to right and step by step.

In any case, the different components must perform mutually synchronized movements while their operation must also be synchronized such that, when a certain color image is inscribed by the laser system 19 on the drum 21, it reaches the developing station 35 at the moment when the application roller of the developing unit of this same color is in contact with the bottom surface of the drum 21. Thus, each latent electrostatic image is developed with its own toner of the correct color and in the correct order.

After the latent electrostatic image is developed on the developing station, excess toner is removed from the surface of the drum 21 when the developed image is moved to the transfer station by a vacuum knife 63 arranged along the path of travel between these two stations. At the transfer station 37, the developed image is transferred to a sheet of support material S, which may be plain paper, from a stock indicated by 6b.

On the transfer station 37 there is the transfer roller 65 which can rotate in the direction indicated by the arrows and is suitably driven at the same linear pulling speed as the drum 21. A sheet of support material S is fed via the guide 67 by means of a feed mechanism, the vacuum heads of which are marked with 63 and contacted with the transfer roller 65. E) e known per se. By means, for example electrostatic stretching, the sheet S is wrapped around the surface of the transfer roller 65 and held thereon during the rotation of this roller.

After each developed image has been transferred to a sheet S disposed around the transfer roller 65, the surface of the drum 21 moves clockwise through that portion forming the cleaning station 39.

In this station, any excess toner remaining on the surface of the drum 21 after the transfer is removed by a cleaning device 67 which may be, for example, a vacuum nozzle or an ultrasonically operating device that removes the remaining toner.

Loading, imaging, developing, transfer and cleaning are steps repeated for each of the color-decomposed images. When there are three, the total number of images transferred to the receiving sheet is also three; when there are four color images, a total of four images must be transferred. These images must be accurately reported, which can easily be realized with the device according to the invention. Color mixing to obtain a reproduction by using toner of different base colors such as cyan, magenta yellow and black must take into account the transparency and any other important properties; this is known per se. Of course, the properties of the toner must be taken into account in order to achieve the most realistic results.

Since the final composite color image from the individual color images is formed on the receiving sheet through a series of transfer operations of the different color images until the final color image is obtained, the rollers 21 and 65 rotate at the same 80 0 1 6 32 -1U speed. These rolls are of equal diameter so that the number of revolutions of each of the rolls to obtain a single composite color image on the sheet S is equal and equal to the number of colors that make up the final image. Normally this is four and the sheet S is held on the circumference of the roll until the whole process is complicated.

In this way, when the composite image is complicated and all the image-forming colors have been transferred to the sheet S, the image consists of a series of, for example U, toner images that are not yet fixed and superimposed. At this stage of the operation of the device, either before the sheet S is released from the roller 65 or immediately thereafter, the composite color toner image is fixed by passing the sheet along a fixing device 69.

It can consist of a heating device or other suitable toner fixing device. It is thus possible not only to supply heat, but also to apply a protective coating consisting of a plastic layer to the complicated composite image. There are many different forms of materials that are translucent and scratch resistant and can be used to protect toner images. Preferably, one will form a glossy surface to give the resulting image some depth. The completed sheet S is then moved to the left and deposited in the scale 73.

After the developing device U3 has been moved back to the starting position, i.e. to the right, a complete work cycle has been completed and another composite copy can be formed with the same information derived from the computing device or from other information.

Additional toner for the developing units can be stored in the reservoirs 75, 77, 79 and 81 mounted on the bottom of the frame 25; 800 1 6 32 «· · φ -15- they can be connected to the resp. holders of development units.

It is clear that the device according to the invention can also be used for making black and white copies.

Programming the functions of the different components is possible using suitable programming means such as programming relays, shafts, cams and the like. Preferably, the entire operation is programmed by the computing device, with appropriate control and / or enabling signals being applied to the various members of the device via the lines 100 schematically indicated.

Reporting the superimposed partial images on the paper S is a matter of accurate construction and synchronization of the moving parts of the device 11, an adjustment being conceivable or, for example, by means of manually operated mechanical means, automatic scanning devices , laser pattern adjustments and / or a combination thereof.

By the term "cycle" is generally meant one revolution of the drum 21 and the transfer roller 65 during which a 2Q color print is formed and transferred. Thus, for a four-color image, four such cycles are required; the entire process or total cycle is then completed after the sheet S is released, the composite image is fixed and / or topped, the sheet is deposited in the take-up tray, the developing device is moved back to its initial position and the device is ready for the next series of cycles or revolutions.

800 1 6 32

Claims (21)

1. A method of electrostatic color printing in which the individual colors of an image made up of partial colors are digitally stored in a computer and can be separately called up for describing an electrophotographic element by means of a laser, using a rotating electrophotographic drum and a transfer roller in which a receiving material interacts with the rotating electrophotographic drum, and the drum is loaded, exposed with an image of a partial color, this image is toned and the toner image is transferred to the receiving material 10, characterized in that the exposure consists of recalling and laser inscribing the image, and all these steps in one revolution of the drum are performed with a simultaneous single revolution of the transfer roller while loading, writing, toning and transferring for each individual color of the combined colors The image is repeated during each subsequent rotation of the drum and transfer roller until all the colors have been transferred to each other in the correct position relative to each other, which is then removed from the transfer roller and replaced by another, after which the above-mentioned steps be repeated. 2. A method according to claim 1, characterized in that each latent electrostatic image corresponding to one of the color components of the color original is formed during one revolution of the rotating element and each latent electrostatic image formed in this way is developed with the material of which the color corresponds to the color image component of the latent electrostatic image by $ 0 0 1 6 32 \ 17- successively "moving a number of developing units, each containing a different color material" along the electrophotographic element and in a "controlled time sequence, and otherwise such that each developing unit will independently develop a respective electrostatic image during its revolution and each image so developed is transferred to a sheet of support material with the different images superimposed and with as many colors as there are rotations of the rotating element. 3. Method according to claim 1 or 2, characterized in that the number of latent electrostatic images is formed by charging the electrostatic element and subsequently writing successively the latent electrostatic images thereon, using a laser modulated with digital information derived from the color original. A method according to claims 1-3, characterized in that each of the latent electrostatic images is developed by applying liquid toner to each of the latent electrostatic images. Method according to claim 1, characterized in that the number of latent electrostatic images is at least three, each for providing a developed image of mutually different color. 6. Device for carrying out the method according to claims 1-5 for obtaining one or more color copies of a multi-color original, comprising a rotating electrophotographic element, driving means for rotatingly driving it in a cycle along the successive series of processing stations which the periphery thereof are provided, namely a charging station, an exposure station, a developing station and an image transfer station, and with means for applying a uniform electrostatic charge to a part of the electrophotographic element in the charging station, characterized by: A. means for successively forming a plurality of latent electrostatic units on the electrophotographic element 5 in the exposure station during the rotation of this electrophotographic element, each of these electrostatic images corresponding to different color image components of the colored .original and each image is formed during a cycle; B. a developing device adapted for movement through the developing station along a path substantially tangential to the rotating electrophotographic element, and which developing device comprises a plurality of development units arranged in a row arranged to move successively through the developing station each developing unit being filled with developed and toner material, the color of which corresponds to the color image component of one of the latent electrostatic images; C. drive means for stepwise moving the developing device along the tangential path through the developing station in a time-controlled relationship with respect to the formation of the corresponding latent color images and for bringing the developing units into the toner transfer position with respect to the electrophotographic element when each unit is in the station, the drive means step-wise bringing the developing unit into the developing station, keeping it there for a sufficiently long working period and then moving the developing unit out of the developing station while simultaneously bringing the next unit into the station, while the portion of the electrophotographic element which is a corresponding latent color image of each developing unit 800 1 6 32 k-S & -19- is moved entirely through the developing station during the working period in which the resp. developer unit is located in this station; and by D. a transfer device for transferring each developed image to a sheet of support material, the images superimposed as each developed image passes through the transfer station during the resp. cycle, and where there are as many cycles as colors, and there are as many prints on each sheet as there are cycles. Device according to claim 6, characterized by an encoding converter for converting the colored original to color-coded digital information and by a laser whose output is modulated in accordance with the digital information for the. writing each of the latent electrostatic images onto the electrophotographic element. An apparatus according to claim 6 or 7, characterized in that each developing unit comprises a container and a toner application device cooperating with the electrophotographic element and the developing material of which toner the color corresponds to the color component of the latent electrostatic image and a is liquid toner located in the container in contact with the applicator. Device according to claim 8, characterized in that the application device consists of a roll. 10. Device according to claims 6-9, characterized in that there are at least three developing units, each for a different color. Device according to claims 6-10, characterized in that the developing device consists of a carriage mounted on a guide and a stepper motor coupled thereto for driving it. Device according to claim 11, characterized in that the unwinding device is moved stepwise in one direction according to the tangential cock during the execution of the cycles and the carriage is returned in the opposite direction to the starting position after completing the cycles for a fully developed image such that the carriage is ready for a renewed step-wise motion for a subsequent cycle series. Device according to claims 6-12, characterized in that the tangential path is rectilinear. I **. Device according to claims 1-13, characterized in that the transfer device comprises a rotary transfer trramel. Device according to claim 1H, characterized in that the transfer traum is provided with means for selectively holding and releasing a sheet of support material and moving it along a path away from the transfer tram. Device according to claim 1¼ or 15, characterized in that the rotating electrophotographic element and the rotating transfer drum are in contact with each other according to a line contact on the surface of each of them, both have the same diameter and are driven at the same speed. Device according to claims 6-16, characterized by a cleaning station adapted to remove toner residues from the electrophotographic element after the developed image on the electrophotographic element has passed the image transfer station, which cleaning station is also located along the circumference of the electrophotographic element between the image transfer station and the exposure station. Device as claimed in claim 17, characterized in that the cleaning station comprises an ultrasonic cleaning device. 800 1 6 32 -21- 19. Device according to claim 15 ». characterized by a fixation station adapted to fix the developed heals on the sheet of support material after all developed heals of a complete series of image impressions have been applied thereto. 20. Device as claimed in claim 19, characterized in that the fixing station is located after the cock which deposits the sheet after it has been released. 21. Device according to claim 19, characterized in that the fixing station is provided with a covering device for applying a protective coating to the developed images. 22. Device as claimed in claim 19, characterized in that a covering station is arranged near the fixing station, adapted to apply a coating to the sheet after it has passed through the fixing station. 800 1 6 32