ELECTROGRAPHIC GRAVURE PRINTING SYSTEM

Abstract: An electrostatic image transferring system for producing a copy of an electrostatic image on a sheet (27) is disclosed which incorporates gravure printing techniques. A toner roller (20) disposed adjacent to an electrostatic image-bearing dielectric drum (5) is uniformly coated with a toner solution. The toner roller (20) is arranged to retain a toner image that is the reverse of the electrostatic image on the drum (5) when the electrostatic image is passed over a coated portion of the toner roller (20). The toner image may then be printed. In the preferred embodiment, the toner roller (20) takes the form of a gravure printing cylinder having a multiplicity of pits for receiving the toner solution. The described system is particularly well suited for using an aqueous toner carrier. In an alternative embodiment of the invention, a negative image is electrostatically formed on a gravure roller (20) and hardened to provide engraved gravure printing cylinders for conventional gravure printing operations.

Description: ELECTROGRAPHIC GRAVURE PRINTING SYSTEM The present invention relates generally to an improved electrostatic image transferring system. More particularly, a system is disclosed for printing an electrostatic image using a toner roller which may take the form of a gravure printing cylinder to facilitate high speed good quality printing. BACKGROUND OF THE INVENTION The use of electrostatic methods for printing, short- run copying and duplicating jobs is well established. Electrophotographic copying machines of the nature currently being used in industry typically create an electrostatic image on a photoconductive surface. The electrostatic image is then developed using either a liquid or a dry developing composition. Conventional liquid developer compositions ("toners") are formed by dispersing pigment particles within an insulating carrier liquid, which dries after being applied to paper leaving the pigments to form the desired image. One drawback of such conventional photocopiers is that the toners and carriers used typically include long chain hydrocarbons which are at least mildly toxic and therefor are consider pollutants. Therefore, it would be desirable to provide an electrophotographic copying process provide an electrophotographic copying process capable of using a non-toxic compound such as water as its carrier. Dry developer compositions typically use heat or pressure fusible toner particles. Such copying machines typically include a mechanism for applying the toner particles to a sheet in the form of an image. The applied toner particles are then fused to the sheet by the application of heat or pressure, thereby transforming the sheet into a permanent copy. Again, a significant drawback to the use of such systems is the toxicity of the toner particles themselves. Gravure printing has been known for several centuries. Gravure is a process whereby the image area is etched or engraved below the surface of a printing plate. The surface is flooded with ink and wiped or scraped clean of excess ink while the etched or engraved image area holds ink in "pockets" beneath the plate's surface. In the 20th century, roto- gravure processes have been popularized which use an engraved cylinder to hold the image in place of a flat plate. Roto-gravure processes are particularly desirable for the high speeds at which they can print photographic and tonal art work materials (such as the comics section of most newspapers) . The principal drawback of gravure printing is the time and expenses involved in engraving the image on the gravure cylinder which traditionally has been formed of iron or steel. Due to the nature and expenses of the engraving process, roto-gravure printing has remained largely a process used for large-scale production runs. Indeed it is not uncommon to run as many as 1,000,000 copies by the gravure process. In response to the need for printers suited for office or personal used, the electronics industry has developed a large number of alternative printing mechanisms such as the ink jet printer. Although relatively low in cost, a major drawback to such printers is that they are relatively slow. Therefore, attempts have been recent efforts to combine electrostatic image forming with printing to provide low cost printers capable of attaining higher print speeds. However, such systems tend to be relatively expensive and have the pollutant problems experienced by conventional photocopier machines. The invention disclosed herein seeks to provide a roto-gravure type printing process that can be electrostatically charged. SUMMARY OF THE INVENTION In view of the foregoing, it is a primary objective of the present invention to provide a novel mechanism for developing electrostatic images. Another objective is to provide an electrostatic image development system capable of using water as its toner. A separate objective of the invention is to provide a mechanism for electrostatically forming images on a gravure roller. Yet another objective of the invention is to provide a relatively low cost, high speed printer design. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, an electrostatic image transferring system is disclosed. In a preferred aspect of the invention, a dielectric drum is provided to receive electrostatic image by a conventional means. A toner roller is disposed adjacent to the dielectric drum for receiving a reverse toner image of the electrostatic image. Specifically, the toner roller is uniformly coated with a selected toner solution. The toner roller rotates synchronously with the drum such that the electrostatic image on the dielectric media causes the toner carried by a coated portion of the toner roller to transfer to the dielectric media in the relatively charged portions of the electrostatic image, while leaving the toner solution on the toner roller in the relatively uncharged portions of the electrostatic image, thus forming a toner image on the toner roller that is the reverse of the electrostatic image formed on the drum. The toner image is then transferred to a sheet. In a preferred embodiment, the toner roller takes the form of a gravure roller having a multiplicity of pockets which receive the toner solution. Printing may then be accomplished using conventional gravure printing technique. The described system has several advantages one of which facilitates use of water as the toner carrier in a liquid toner system. In an alternative aspect of the invention, the toner solution is formed from a hardenable material and the toner image is retained on the gravure roller and hardened to form an electrostatically-engraved gravure roller. BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: FIGURE 1 is a diagrammatic cross sectional view of a prior art electrostatic photocopier. FIGURE 2 is a diagrammatic cross sectional view of a photocopier/printer designed in accordance with the present invention. FIGURE 3 is a diagrammatic side view of a gravure style toner roller. FIGURE 4 is a diagrammatic cross-sectional view of a gravure toner roller that is full of toner. FIGURE 5 is a diagrammatic cross-sectional view of a gravure toner roller having lands that are coated with a dielectric layer. FIGURE 6 is a diagrammatic cross-sectional view of a gravure engraving system in accordance with the invention. DETAILED DESCRIPTION OF THE DRAWINGS As illustrated in the drawings, the present invention is directed towards a high speed electrostatic image transferring device which is capable of utilizing roto-gravure type printing techniques. There are a wide variety of conventional systems that incorporate various means for transferring electrostatic images onto a sheet of paper. For example, as shown in Figure 1, a conventional photocopier might include a rotatable drum 5 coated with a photoconductive dielectric layer 7. A corona discharge device 8 is provided adjacent the drum 5 to uniformly charge the dielectric layer. The charged section of the drum is exposed to an image at imaging station 9, which effectively discharges the photoconductor in the regions that are struck by light. In this manner a latent electrostatic image is formed on the drum. Assuming that the image formed is a negative image, the image then passes through a toner station 11 which applies a toner solution to the regions of the drum which remain relatively charged. The toned image is then transferred to a sheet at a printing station 13 and fused using either temperature or pressure as the fusing mechanism. The drum may then be scraped of excess toner at cleaning station 15 before it is recharged by the corona discharge device 8. One way to form the toner station is to provide a toner roller 16 that nearly contacts the dielectric layer 7 on the drum 5. If the roller 15 is uniformly coated with a toner material, and a small air gap is provided between the toner roller 16 and the drum 5, then the toner will be attracted to the drum only in the regions that remain relatively charged. In this way, the negative electrostatic image is changed to a positive toner image. The toner image is then transferred to a sheet in the printing station 13 as described above. It has been observed that when the drum and the toner roller pass each other at the same surface speed, the latent reverse image appearing on a toner roller that is arranged as generally described above remains letter quality. Therefore, such an image can be used for printing as well. The present invention utilizes this characteristic to advantage in several novel arrangements. Referring initially to Figure 2, an electrostatic printing arrangement in accordance with one aspect of the present invention includes a drum 5 having a photoconductive dielectric layer 7 thereon. Conventional charging and imaging stations 8 and 9, respectively, may be provided to create an electrostatic image on the drum. Preferably, the image created is a negative image. A roller printing station 12 is provided in place of the toner and printing stations 11, 13 as described above and a conventional cleaning station may be provided to scrape toner from the drum after it passes the roller printing station 12. When appropriate, the scraped toner can be recycled. The roller printing station 12 includes a toner roller 20 that may take the form of a uniformly pitted gravure roller. Thus, no image is carved into the toner roller and, as can be seen in Figures 3 and 4, the roller has a multiplicity of pits 30 throughout its surface area. The regions of the roller's surface area between pits 30 are referred to as land areas 32. The toner roller is mounted to rotate in a single direction which is depicted as clockwise in the figures. A toner bath 22 is provided to load the toner roller with a selected liquid toner 24 and a scraping blade 26 wipes any toner away from the land areas 32. Thus as seen in Figure 4, after a particular portion of the toner roller passes the toner bath 22 and scraping blade 26, its pits 30 are loaded with a liquid toner solution 24, while its lands 32 are substantially bare. The toner solution 24 is retained within the pits primarily by surface tension, although it is contemplated that if the particular toner solution used does not have sufficient surface tension, a biasing voltage could be provided to help retain the toner. The filled portion of the toner roller is then rotated past the drum 5 in synchrony therewith. A small air gap is provided between the toner roller 20 and the drum 5 to prevent inadvertently discharging the drum. The drum is charged sufficiently such that it attracts the toner from the toner roller within its charged regions, thereby creating a residual toner image equivalent to the negative electrostatic image on the drum 5. The positive image on the toner roller 20 may then be printed using a wide variety of techniques many of which are similar to conventional roto-gravure printing. In the embodiment shown in Figure 2, a sheet 27 is passed over the toner roller by a roller 28 and the natural wicking action of the paper tends to draw the toner onto the paper. Additionally, an electric field may be added to draw the toner from the pits. A suitable field can be created by a corona 29. In a purely mechanical system, a wheel can be provided to press the sheet against the toner roller. After printing, the toner roller again passes through the toner bath 22. The described system can be used with a wide variety of toner compositions. One particularly interesting aspect of the invention is that it allows the use of water as the toner's carrier liquid. Such a use in place of the conventional hydrocarbons currently in use would virtually eliminate the pollutants created by photocopiers. To facilitate the use of water as the principal toner carrier, a non-corrodible photoconductive dielectric material should be used to coat the drum 5. Appropriate dielectrics include organic and silicon photoconductors. Selenium does not work well due to its corrosive nature. The actual charging of the drum and the critical dimensions of the parts discussed may all be widely varied. By way of example, for an aqueous toner, an air gap on the order of 3 mils would be appropriate between the toner roller and drum, together with a drum charge potential on the order of 1000 volts. Conventional roto-gravure techniques may be used to print the toner image. A smooth toner roller could be used in place of the gravure roller if certain hydrocarbon toners are used. In such a system, the scraping blade or other toner applicator is arranged to provide a uniform coat of toner material on the toner roller. The toner roller is then slightly charged to retain the toner. In an alternative aspect of the invention as shown in Figure 6, the described arrangement can be modified to electronically create permanent images on gravure rollers for roto-gravure printing applications. Specifically, if the toner 24 is replaced with a photoresist 40 (i.e. a photopolymer that will harden with the application of light) and the imaging station 9 is arranged to create a positive electrostatic image on the drum 5 (and therefore a negative toner image on the toner roller) , then the resultant image on the gravure roller 42 will be a negative of the desired image. The negative is then hardened by exposure to an intense light source 44. The hardened gravure roller may then be removed and used in conventional roto-gravure printing operations. The described arrangement would significantly reduce the time and costs associated with etching or engraving gravure rollers. In another aspect of the invention, a composite gravure roller is formed wherein the outer surface of the roller (i.e. the lands 32) are covered with a dielectric material 50. This would allow the air gap between the drum 5 and the toner roller 20 to be eliminated since the danger of inadvertently discharging the drum through contact with the toner roller is eliminated. As seen in Figure 5, the dielectric material 50 covers only the land areas 32. There are numerous methods of fabricating such a device including coating the roller with a dielectric material prior to peaning the pits. Alternatively, a shrink fit polyester mesh could be placed over a conventional metallic gravure roller, and shrunk over the roller. The mesh would form a dielectric layer that covers portions of both the land and pit areas. However, when a relatively fine mesh is used, the mesh will not interfere with the transfer of toner to or from the toner roller. Still another fabrication process would involve electroless plating a metallic material into the pits of a roller formed of a dielectric material. It should be appreciated that the electrostatic image transferring system described can also be used in small scale printers frequently associated with computers. A system using a gravure roller as described can readily operate at speeds in the neighborhood of 8-15" vertical inches of print a second, which is significantly faster that the ink jet printers commonly used today. in printer applications, a laser could be used to inscribe images onto the drum or other photoconductive surface. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention may be embodied in many other specific forms without departing from -li¬ the spirit or scope of the invention. Particularly, it should be appreciated that the actual toner compositions and dielectric materials used to store an electrostatic charge can be varied widely within the scope of the claims. Further, the actual construction of the drum, as well as the charging, imaging and cleaning stations used in conjunction with the image transferring system described may be widely varied as well. Indeed, the drum could be replace with a stationary or flat dielectric material in certain embodiments. It should be apparent that the electrostatic image transferring system described has uses in a wide variety of photocopying and printing applications beyond those described above. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.

Claims: N THE CLAIMS ; 1. A printer for printing an image on a sheet, the printer comprising: a dielectric drum for receiving an electrostatic image; imaging means for forming an electrostatic image on the dielectric drum; a toner roller disposed adjacent said dielectric drum for receiving a reverse toner image of the electrostatic image; and transfer means for transferring said toner image from said toner roller to a sheet to form a permanent copy of the image. 2. A printer as recited in Claim 1 wherein said toner roller is a gravure roller having a multiplicity of pits and land areas, said pit areas being arranged to receive a toner solution or dispersion. 3. A printer as recited in Claim 2 wherein said toner includes an aqueous carrier. 4. An electrostatic image transferring system for printing a copy of an electrostatic image on a sheet, the image transferring system comprising: a dielectric media for receiving an electrostatic image, the image bearing dielectric media including relatively charged portions and relatively uncharged portions; a toner solution including an aqueous toner carrier; a toner roller disposed adjacent said dielectric media for receiving a reverse toner image of the electrostatic image, the toner roller including a multiplicity of pits for holding the toner solution; means for coating the toner roller with the toner solution by filling said pits with the toner solution; means for passing a coated portion of the toner roller over the electrostatic image on the dielectric media to cause the toner solution carried by the coated portion of the toner roller to transfer to the dielectric media in the relatively charged portions of the electrostatic image leaving a reverse toner image on the toner roller; and transfer means for transferring said toner image from the toner roller to a sheet to form a permanent copy of the image. 5. An electrostatic image transferring system as recited in Claim 4 wherein said transfer means includes means for creating an electronic field in the vicinity of the sheet to draw the toner solution from the toner roller onto the sheet. 6. An electrostatic image transferring system as recited in Claim 4 wherein said transfer means includes means for pressing the sheet against the toner roller, to wick the toner solution from said pits. 7. An engraving system for creating a permanent image on a gravure roller, the engraving system comprising: a dielectric media for receiving an electrostatic image, the image bearing dielectric media including relatively charged portions and relatively uncharged portions; a hardenable toner solution; a gravure roller disposed adjacent to said dielectric media for receiving a reverse toner image of the electrostatic image, the gravure roller including a multiplicity of pits for holding the toner solution; mean for coating the gravure roller with the toner solution by filling said pits with the toner solution; means for passing a coated portion of the toner roller over the electrostatic image on the dielectric media to cause the toner solution carried by the coated portion of the toner roller to transfer the toner solution to the dielectric media in the relatively charged portions of the electrostatic image leaving a reverse toner image on the gravure roller; and hardening the toner image to form a engraved gravure roller. 8. A system as recited in Claim 7 wherein said toner solution is photoresistive. 9. A system as recited in Claim 7 wherein the toner solution hardens under the influence of heat. 10. An electrostatic image transferring system for producing a copy of an image on a sheet comprising: a dielectric drum for receiving an electrostatic image; imaging means for forming an electrostatic image on the dielectric drum, the electrostatic image including a relatively charged portion and a relatively uncharged portion; a toner material; a toner roller disposed adjacent the dielectric drum and means for coating the toner roller with the toner material, the toner roller being arranged to receive a toner image that is the reverse of the electrostatic image wherein the toner image is formed by passing a coated portion of the toner roller over the electrostatic image on the dielectric drum to cause the toner to transfer to the dielectric drum in the charged portion of the electrostatic image leaving a negative toner image on the toner roller; and means for transferring said toner image to a sheet to form a permanent copy of the image. 11. An electrostatic image transferring system as recited in Claim 10 wherein the toner material is an aqueous solution having pigments dissolved therein. 12. An electrostatic image transferring system as recited in Claim 10 wherein the toner material includes a liquid hydrocarbon carrier. 13. An electrostatic image transferring system as recited in Claim 10 wherein said toner roller is a substantially cylindrical gravure roller. 14. An electrostatic image transferring system as recited in Claim 10 wherein said toner roller is substantially cylindrical and has a multiplicity of pits and land areas. 15. An electrostatic image transferring system as recited in Claim 14 wherein said toner roller is metallic and said lands are coated by a dielectric layer. 16. An electrostatic image transferring system as recited in Claim 14 wherein said pits are plated with a conductive material. 17. An electrostatic image transferring system as recited in Claim 10 wherein said transferring means includes a field generating means for drawing the toner image from the toner roller onto the sheet. 18. An electrostatic image transferring system as recited in Claim 17 wherein said field generating means includes a corona discharge device. 19. An electrostatic image transferring system as recited in Claim 10 wherein said transferring means includes a pressure roller for impressing the toner image from the toner roller onto the sheet.

Inventors: SWIDLER RONALD, SWIDLER, RONALD
Applicants: STANFORD RES INST INT, SRI INTERNATIONAL
Application year: 1991
Main classification: G03G15
IPC codes: B41C1, G03G9, G03G13, G03G15