US 3801315 A
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United States Patent 191 Gundlach et al. Apr. 2, 1974  GRAVURE IMAGING SYSTEM 3,559,570 2/1971 Martel 101/170  In e ors: Robert w. Gundlach, victor; Joseph 3,561,358 2/1971 Weigl 101/170 Mammino; Alan B. Amidon, both of penfield; George can. Primary Exammer-Charles E. Van Horn Rochester, all of N.Y. I  Assignee: Xerox Corporation, Rochester, N.Y.  ABSTRACT  Filed; 27, 1971 A method of electrostatographic imaging utilizing a gravure member having a conductive backing with a PP N04 212,469 uniform pattern of lands and valleys thereon, with the lands having a surface of a photoconductive material. 52 US. Cl. 96/1.4, 96/1 LY, 117/37 LE A developing medium is applied the valleys of the  Int. Cl 603g 13/22 gravure member leaving the areas Substantially 58 Field of Search 96/1, 1.3, 1.4; 101/170; cleah- After application of the developing medium,
117/175 LE, 37'5 LY the gravure member is uniformly charged and then exposed to a light and shadow image The exposed mem-  References Cited bet is then brought into transfer configuration with a UNITED STATES PATENTS transfer 3,084,043 4/ 1963 Gundlach 96/1 11 Claims, 3 Drawing Figures GRAVURE IMAGING SYSTEM This invention relates to an electrostatographic imaging system, and more specifically, to an electrostatographic imaging system using a photoconductive gravure member. v
It is desirable to electrostatographically image using a gravure member as the photoconductive member. Several attempts have been made to base reproduction systems upon a gravure photoconductor. For example, US. Pat. No. 3,561,358 to J. W. Weigl and US. Pat. No. 3,559,570 to R. W. Martel, et al., discuss methods embodying this concept. In both of these patents a photoconductive plate or cylinder is provided, the surface of which is formed into a gravure pattern comprising uniformly spaced recessed areas or tiny cells. In each, the surface of the photoconductive gravure member is electrostatically charged and imaged in accordance with conventional xerographic tenchniques. In US. Pat. No. 3,559,570 the photoconductive surface of the gravure roller, which is formed of a hydrophobic photoconductive composition, is contacted with an aqueous base developer. The unexposed charged areas of the plate accept the ink while the exposed, discharged areas of the plate remain hydrophobic and repell the ink. The imaged or developed surface of the plate is then contacted with a copy sheet and the liquid developer is transferred thereto in an image pattern to produce a reproduction of the original. In US. Pat. No. 3,561,358 the electrostatic latent image on the gravure surface is developed in accordance with xerographic procedures so as to selectively occlude the cells of the gravure member. The developer particles are then fixed into the cells. A printing ink is applied to the resulting imaged member in such a manner that the ink fills the cells void of the developer particles. Upon contact of the surface of the inked member with a copy sheet, a print of the desired image is realized.
While these two methods provide attractive altermates to methods using dry developers, they suffer from the inherent disadvantage that the gravure member must be scrupulously cleaned after each pn'nting sequence. This of course necessitates the use of complex and costly cleaning equipment to assure a clean copy upon each cycle free from residual ink from the previous cycle.
Accordingly, it is an object of this invention to overcome this disadvantage while retaining the major advantages of systems of this type.
This and other objects are accomplished in accordance with the present invention which provides a method for electrostatographic imaging using a gravure member comprising: uniformly charging the surface of a gravure member having a uniform pattern of lands and valleys formed on a conductive support, wherein the valleys are filled with a developing medium and the lands have a photoconductive surface which is substantially free of developing medium; exposing the gravure member to a light and shadow image to cause development'of the image; and transferring the image to a receiving web.
The invention will become more apparent from the accompanying drawings and the ensuing discussion in which:
FIG. 1 represents a magnified cross-section through a gravure member having the valleys filled with developing medium, as it is thought to appear immediately after application of an electrostatic charge thereto;
FIG. 2 illustrates the gravure member of FIG. I after exposure; and,
FIG. 3 represents a side-sectional view of an exemplary continuous imaging process according to the present invention.
Referring now to FIG. I there is seen a gravure member generally designated 1 comprising a conductive support base 2 having disposed thereon a raised pattern which is comprised of a uniform pattern of lands 4 and valleys 3. The land areas 4 have a photoconductive surface.
Any suitable material can be used to prepare the support base 2 for the gravure member of the present invention. Generally, the preferred support material should have an electrical resistance less than the photoconductive layer so that it will act as a ground when the photoconductive layer is electrostatically charged and exposed to light. Typical materials are aluminum; brass; steel; copper; nickel; zinc; conductive rubber; conductive glass, e.g., tin oxide coated glass; and metalized plastic films such as aluminized polyethylene terephthalate or polycarbonate films. The selection of the particular support base material used may depend upon the desired use of the gravure member. For example, if the master is to take the shape of a fiat printing plate then it may be more desirable to select a support substrate which will add additional strength to the system. However, if the gravure member is to be prepared in the form of a roller or cylinder then it would be generally more desirable to select a material which will provide the necessary flexibility properties. When a rotogravure type of member is fabricated, the support surface can be in the nature of a hollow cylinder, or it can consist of a solid core such as a solid conductive rubber roller. The member can be rigid'or resilient.
The raised pattern on the surface of the support base can be formed by any suitable method so as to produce a spacing ranging from about 50 to about 300 cells per inch. In order to obtain maximum resolution, it is preferred that the number of cells per inch approach the upper limit of the specified operable range. The raised areas can be formed on the conductive support base and then coated with a photoconductive material, or a raised pattern of photoconductive material can be formed on a relatively smooth suppott base. It has been found to be desirable to have at-least the surface of the land areas 4 coated with a photoconductive material while the bottom of the recess 3 extends through to the conductive support base. Because the difficulty in forming a desired fine pattern of raised areas by applying a photoconductive material in the desired raised pattern to a smotth support base, it has been found desirable to coat the land areas of a support base having the desired raised pattern preformed thereon. The photoconductive layer may cover either the tips of the land areas along; or it can cover the tips of the land areas as well as the surfaces extending into the recessed areas. It is highly preferred that the tips of the land areas alone be covered. A trigngular-helix patterned gravure applicator roll has been found to be especially well suited to the present invention. However, other patterns having the desired spacing, uniformity and cell density are also acceptable. Trihelicoid-patterned rolls can be purchased from several commercial sources, or they can be made by knurling or photofabrication techniques. Any suitable photoconductive layer can be used as the charge carrier in conjunction with the process of the present invention. The photoconductive coating can comprise a photoconductor dispersed in an insulating binder composition, a solution of photoconductor and binder, or can consist of a homogeneous photoconductive composition. The photoconductive layer should have a thickness of from about 6 to about I microns, with from about to about 60 microns being preferred. Typical of the photoconductive compositions which are useful in the present invention are those listed in U.S. Pat. No. 3,561,358.
The developing medium is applied to the gravure member by any suitable means which will substantially fill the recessed areas 3 while leaving the land areas 4 substantially free of ink. Suitable inking techniques are discussed more fully in copending, commonly assigned U.S. application Ser. No. 838,133 filed July 1, 1969 by G. P. Carr. Typically, this can be accomplished by uniformly coating the surface of the gravure member and cleaning the land areas 4 by means of a doctor member, e.g., doctor blades, non-skidding rollers, skid-rollers, and the like. Skid-roll doctoring has been found to be an advantageous method of metering developing medium to rotogravure members. According to this technique a rotogravure member, which in this case has a photoconductive layer over at least the tops of the land areas, and is illustrated by 6 in FIG. 3, is rotated in contact with a resilient applicator roller 8 which is passed through a developing medium source 10. Both the rotogravure member 6 and the applicator roller 8 are rotated in the same direction; however, the rotogravure member 6 is rotated at a greater peripheral speed than applicator member 8. This causes application of developing medium to rotogravure member 6 by applicator roller 8 and a simultaneous cleaning of the land areas 4, shown exaggerated in relative size on member 6, due to the skidding action of resilient roller 8 on the surface of roller 6. Pressure between the two rolls is controlled by suitable adjustment devices. The member 8 is rotated just fast enough to maintain a bead of the developing medium. The skid roll 8 can be any elastomer, for example, a fluorosilicone rubber.
After suitable application of developing medium, a uniform electrostatic charge is applied to the surface of the gravure member in the dark. This may be done by corona discharge, and the applied charge can be either positive or negative. The gravure member is then exposed to a light and shadow pattern image by any suitable means.
FIG. 1 illustrates a uniformly, positively charged gravure member immediately after charging. FIG. 2 illustrates a gravure member similar to that of FIG. I, wherein the member has been selectively exposed such that the left-hand area remains unexposed while the right-hand area has been exposed. The member of FIG. 2 is shown as being ready for transfer of the image to a receiving web which is preferably backed by conductive pressure member. If the conductive pressure member exhibits no potential difference relative to the discharged background areas, i.e., where it is grounded, a positive image is printed. A negative image can be produced by applying a bias potential to the conductive pressure member of like polarity and degree to that originally applied to the gravure member. In FIG. 2, the charge on the developing medium has been fully dissipated by conduction to the conductive base 2, and the exposed areas of the photoconductive member have likewise been discharged.
Development is believed to occur by the combined effects of the electric field on the surface tension of the developing medium and the direct electrostatic attraction of the liquid developing medium toward the land areas 4. However, this theory is set forth for aid in explanation only and should not be taken as limiting the invention. In general, the preferred technique is to relax all fields in the exposed areas, while allowing fields between the developing medium and the land areas to build in the unexposed areas. There are two altemative approaches available here: (1) use a thermofluid, e.g., wax-like developing medium, that is applied in the liquid state, solidified during charging and imaging, and softened to the liquid state by heating after the latent image is completed by exposure; and (2) use a liquid developing medium which remains liquid throughout the processing. In either case, the developing medium should not be so conductive that it prevents proper charging of the photoconductive layer by conduction of the charge away from the land areas 4. If a thermofluid developing medium is used, it can be made to be relatively non-conductive in its nonflowable state, but relatively conductive after heating; or it can be made to lose its charge over a relatively long period of time, compared to charging time, while in its nonflowable state. For a liquid developing medium the light exposure level should be such that the rate of discharge of the photoconductor in background (illuminated) areas, and of the developing medium, should be mutually adjusted. Ideally, for sharpest development using a liquid developing medium, the charge on the developing medium should become fully dissipated at the same time that the charge on the exposed areas of the photoconductive material becomes fully dissipated. Too rapid dissipation of the charge on the liquid developing medium could result in development of the entire gravure member before exposure. And, although subsequent exposure of selective areas would mitigate the effects of a not-too-severe premature development, it would be difficult to impossible to achieve a final printed copy without a high background printing.
A suitable thermofluid developing medium may be made by adding dye or finely dispersed pigment in a wax or other easily melted solid. A preferred thermofluid developing medium can be made by adding 0.25 gm of crystal violet to 3.0 gm of Carbowax 1,500 polyethylene glycol and 6 gm Carbowax 6,000 polyethylene glycol (Carbowax is a product of Union Carbide Corporation). The admixture is then diluted in 25 cc. of methyl alcohol for coating purposes. After coating, the methanol is evaporated before use for imaging. This composition becomes fluid on heating to about C. If desired, nigrosine dye can be used in place of the crystal violet to give a more permanent, black image, and beeswax or parafin wax can be used in place of the Carbowax.
Suitable, liquid developing media can be those described in copending, commonly assigned U.S. application Ser. No. 839,801, filed on July 1, 1969 by A. B. Amidon, et a1. Typically these may include one or more liquid vehicles, colorants such as pigments and dyes, and dispersants. In addition, a variety of specialized agents may be employed for particular functions. For example, viscosity controlling additives or additives which contribute to fixing a pigment on copy paper may be employed. These developing media will generally exhibit a bulk resistivity of between about to 10 ohm centimeter. As previously discussed, the most important factor is to use a developing medium and light exposure level such that the discharge of the developing medium and the exposed areas are closely matched to minimize the fields therebetween. For example, a developing medium having a bulk resistivity (p) of 2 X 10 ohm-cm and a dielectric constant of 2 can be doctored into the cups or grooves between the photoconductive ridges of a gravure member. The discharge time constant (1) for the film of developing medium is given by the formula:
'r=p X 8.85 X 10.
Therefore, the relaxation time for this ink is:
7: 2 10 x 2 x 8.85 x 10- 3.5 seconds Using in-place, full-frame charging, with voltages set to charge the self-developing photoreceptor plate in onehalf second, followed immediately by full frame exposure adjusted by lighting and lens aperture to give maximum electrostatic contrast in 4 to 5 seconds, the discharge of the photoconductor in the background (lighted) areas closely coincides with that of the developing medium itself. By operating in this manner, potential gradients between the developing medium and the photoconductor are kept to negligible levels throughout the background areas, while the dark areas of the photoconductor retain their charge and high potential, thereby electrically attracting the adjacent developing medium. For slit charge and exposure systems, the exposure time is again about 5 to 10 times longer than the charging time and should match the relaxation time for the developing medium. Higher quality prints may be achieved by such systems by using a knife edge charging system, or, if rapid recycling is not necessary, by using a photoconductor which exhibits persistent conductivity after exposure.
FIG. 3 represents a simple, exemplary apparatus for carrying out the imaging technique of the present invention. In this apparatus, there is seen a rotary gravure roller 6 having a trihelicoid patterned roller 12 of a conductive material such as copper whereon the land areas are coated with a photoconductive material such as polyvinylcarbazole and 2,4,7-trinitro-9-fluorenone at a weight ratio of 4:1. The roller, when in operation, is generaly rotated at a uniform velocity in the direction indicated by the arrow so that the surface of the roller passes in pressure contact with applicator roller or skid roller 8. Roller 8 is rotated in the direction shown by the arrow at a peripherial speed less than that of roller 6, such that developing medium is supplied to the re cesses of the gravure member and the land areas are wiped clean. Roller 8 can have a surface of fluorosilicone rubber, having a shore A-scale hardness of about 60, which is well bonded to a steel core to a thickness of about one-eighth inch. The fluorosilicone roll surface is ground smooth and polished to remove any imperfections. This material is desirable primarily because of its inertness to many oils and solvents commonly used as developing medium vehicles or components, and its resistance to compression set. Developing medium is supplied to the skid roll 8 by passing it through supply 10. The gravure roller 6, having valleys filled with and land areas substantially free of developing medium, is uniformly charged in the dark by continuously rotating it at uniform velocity past charging unit 14, which can be a high voltage corona discharge electrode adapted to supply ions or electric charges to the surface of the roller. The charged gravure roller passes beneath a scanning image mechanism 16 or other means for exposing the charged plate according to the desired image. The exposed roller surface then continues around so as to come into transfer configuration with copy web 18 which is fed from supply roll 20 and passed up against the gravure roller surface by a grounded conductive transfer roller 22 which travels at the same peripherial speed as the surface of the gravure roller. The desired image is thereby transferred to the copy web 18. Following transfer of the image to the copy web 18 the surface of the gravure roller may be immediately processed for the next cycle of printing of the same or a different image without requiring cleaning of the surface thereof.
Where the transfer roller 22 is grounded and therefore has no charge relative to the uncharged areas of gravure roller 6, positive, white-for-white and blackfor-black, copying of an original can be achieved. Where negative or reversal, that is, white-for-black and black-for-white, printing is desired, a potential can be applied to transfer roller 22, equal in polarity and degree to the charge initially applied to the surface of gravure roller 6. The initial charge applied to the gravure roller can of course be either positive or negative.
To further explain the details of the present invention, the following examples are presented to illustrate but not to limit the particulars of the present invention. Parts and percentages are be weight unless othewise indicated.
EXAMPLE 1 A copper letter press plate bearing a ISO-line pattern of to percent tonal half-tone dots raised 35 microns above the base plane is coated with a solution of polyvinylcarbazole (PVK) and 2,4,7-trinitro-9- fluorenone (TNF) at a weight ratio of 4:1. The solution is prepared in a 1:1 toluene-cyclohexanone solvent mixture which is used in sufficient amounts to provide a coating composition that is so viscous that it will not completely coat the bottoms of the recesses in the halftone pattern but sufficiently fluid so that a uniform coating of the land areas can be obtained. The plate is inverted during coating so that the coating solution will drain away from the bottom of the recesses to the land areas of the plate. The coating is metered by a number 30 wire-wound bar and forced air dried at C. for about 15 minutes. The coating is cooled to room ambient conditions and electrically tested. The plate at saturation potential is found to accept 300 volts either positive or negative potential, and to discharge to about 20 volts residual potential when exposed to white light (15 fcs). A developing medium consisting of about 45 percent light mineral oil, 27 percent alcolated polyvinylpyrrolidene and 28 percent of a resinated carbon black pigment, is applied to the photoconductive letter press plate. The excess developing medium on the lands or top of the photoconductive dots is squeegeed away with a rubber wiper so that the ink is solely in the depressions of the plate. In the dark the plate is corona charged using a positive potential (V equals 280 volts) and exposed by projection to an image pattern (15 fcs). The image is then allowed to develop in the dark. The
developed image is then printed, still in the dark, by placing a sheet of Xerox 100 bond paper in face contact to the photoconductive plate andpassing a grounded conductive roll over the paper to achieve pressure transfer. The paper or receiver sheet is removed from the photoconductive plate and yields a positive image corresponding to the original, that is white-for-white and black-for-black. The plate is reapplied with developing medium as described above, exposed, and allowed to develop as described above. Several images are obtained by this procedure, reusing the same plate each time. In each case the plate is charged and exposed in the inked stage. Charging is possible because the developing medium is not so highly conductive as to divert the corona current from the photoconductive lands (p is equivalent to about 2 X ohm-cm.) Exposure is no problem because the developing medium does not cover the photoconductive areas.
EXAMPLE 2 Other images are obtained using the same procedure as in Example 1 above except that the photoconductive plate is charged using a negative potential. Here again, direct images are developed. These images are equal in quality to those obtained when the plate is corona charged positive as in Example 1.
EXAMPLE 3 Further images are obtained using the same procedure as described above in Example 1 except that a bias potential of +300 volts is applied to the conductive pressure transfer roll at the time of image transfer. The developing medium used has a resistivity of 2 X 10 ohm-cm. In this instance, reversal images, i.e., whitefor-black and black-for-white of the original projected image are obtained.
EXAMPLE 4 Further images are obtained using the same procedure as that of Example 2 except that a bias potential of -300 volts is applied to the conductive pressure transfer roll at the time of image transfer. Here as in Example 3, reversal images are obtained.
EXAMPLE 5 Still further images are obtained using the same procedure as that of Example 1 above except that a negative bias potential of 300 volts is applied to the conductive pressure transfer roll at the time of image transfer. Here positive images with somewhat high background are obtained.
EXAMPLE 6 Yet further images are obtained using the same procedure as that described above in Example 2 except that a positive bias potential is applied to the pressure transfer roll at the time of image transfer. Here, as in Example 5, positive images with somewhat high background are obtained.
In an alternative embodiment of this invention, the photoconductive layer can completely cover the surface of the gravure member. Thus, in this embodiment, the recessed areas 3 and lands 4 would be made of or coated with one continuous layer of photoconductive material such that the developing medium supplied to the recesses would be maintained out of contact with the conductive base 2. In this embodiment it is also important that the developing medium be opaque to the actinic energy for the photoconductor. Operation according to this embodiment would be much the same as in the case where the recessed areas are not coated with a photoconductor; however, here the images prepared in accordance with the procedures outlined in Examples 1 and 2 would be reversal or negative images. This is so because the charge on the developing medium is not dissipated, and the exposed areas become developed due to the potential difference between the charged developing medium and the discharged photoconductor. Likewise, there will be no potential difference between the charged photoconductor and the charged developing medium in the unexposed areas. The developing media used in this embodiment can be those disclosed above, but here the upper limit on the bulk resistivity of the developing medium is not as critical. However, the developing medium should have sufficient conductivity that its charge can be dissipated after each printing cycle by contact with a grounded developing medium source or other suitable charge dissipation means.
It will be apparent to those skilled in the art that many modifications and changes may be made without departing from the spirit of this invention which has as a principal feature the charging and exposing of a gravure photoconductive member after application of a deformable developing medium.
What is claimed is:
l. A method for electrostatographic imaging using a photoconductive gravure member comprising:
uniformly charging the surfce of a gravure member having a substantially uniform pattern of lands and valleys formed on a conductive support, said pattern having a cell spacing ranging from about 50 to about 300 cells per inch, wherein the valleys are substantially filled with a developing medium and the lands have a photoconductive surface which is substantially free of developing medium; exposing the gravure member to a light and shadow image to cause development of the image; and transferring the image to a receiving web.
2. The method according to claim 1 wherein the gravure member has a cylindrical surface with a trihelicoid pattern thereon.
3. The method according to claim 1 wherein the developing medium is supplied to the valleys by applying a layer of developing medium to the entire surface of the gravure member and then wiping the developing medium from the land areas.
4. The method according to claim 3 wherein the developing medium is applied by a skid roller.
5. The method according to claim 1 wherein the developing medium is applied in the liquid state and remains liquid through the steps of charging, exposing, and transferring.
6. The method according to claim 5 wherein the developing medium has a bulk resistivity of between about 10 to 10 ohm-centimeters.
7. The method according to claim 1 wherein the receiving web is backed by a grounded conductive member while in transfer configuration with the gravure member.
8. The method according to claim 1 wherein the receiving web is backed by a device which creates a charge equal in polarity and degree to the charge inileys are substantially devoid of the photoconductive material.
11. The method according to claim 1 wherein the lands and valleys have a substantially uniform coating of photoconductive material.