US 3797926 A
In the formation of visible copies of an image, the use of an ion modulating array provided with an asymmetrical photosensitive coating together with means to electrostatically develop the ion image.
Claims available in
Description (OCR text may contain errors)
United States Patent 11 1 3,797,926 Fotland et a1. 1 Mar. 19, i974 1 1 IMAGING SYSTEM EMPLOYING IONS 3.603.790 9/1971 Cleare ass/17 x 5] Inventors: Richard A. Foland a e me 3.582.206 6/1971 Burdige 355/16 Heights; Virgil E. Straughan, FOREIGN PATENTS OR APPLICATIONS Eudid both of Ohio 1.156 308 10/1963 Germany 1. 96/1 R Assigneei Horizons Incorporated, a Division 1.152.308 V 5/1969 Great Britain 355/3 Horizons Research Incorporated, OTHER PUBLICATIONS Cleveland Ohm Defensive Publication, T879,010, L. F. Frank, 0m.  Filed: Aug. 27, 1971 1970, Exposure Latitude in Electrophotographic Sys- 211 App]. No.: 178,521
Primary Examiner-Robert P. Greiner  US. Cl 355/3, 355/16, 355/17, A t r y Agent; or Firm--Lawrence I, Field 96/1 R, l17/17.5  Int. CL... G03g 15/00  Field of Search 355/3, 16, 17; 96/1 R;  ABSTRACT 117/ 7 5 In the formation of visible copies of an image, the use of an ionmodulating array provided with an asymmet- 56 R fer n Ci rical photosensitive coating together with means to UNITED STATES PATENTS electrostatically develop the ion image. 3.645.614 2/1972 McFarlane et al 355/3 I 21 Claims, 17 Drawing Figures dimer PAIENIEDMAMSIQM 3.797.926
sum 1 or 2 FIG. I. 1 22 I 24 LJ INVENTORS Richard A. Foflond Virgil E. Sfraughon BY ida m yaw ATTORNEY Pmmmmwwn 3791.926
INVENTORS RichdrdA1FofIond Virgil E. Sfraughon I y w f u 75414 4 ATTORNEY This invention relates to image reproduction and ficient formation of ion current patterns corresponding to an optical image. 7
In conventional plain paper electrostatic photography, an insulating photoconductor is charged with a corona source of ions, exposed, the charge image developed, the developed image transferred to plain paper, and finally, the toned image is fixed, generally by fusing. After the transfer operation, the residual image is erased from the surface of the photoconductor and the photoconductor is cleaned in preparation of a repetition of the process. Although employing plain paper, this process is complicated by the requirement for a number of different machine operations. In addition, the photoconductor suffers wear over a period of time, since thesurface or the photoconductor is repeatedly rubbed by toner particles, cleaning brushes and paper surfaces.
A related process employs a photoconductively coated conducting paper. The photoconductor, generally zinc oxide (although organic photoconductors may be employed), is first charged, then exposed, and the image toned. Here'the photoconductor is not resuable and thus the wear and tear restrictions in the aforementioned process are eliminated. In addition, the machine operation, requiring four steps, is simplified; The disadvantage of this process is associated with the requirement for'coating the paper with a photoconductor. These photoconductively coated papers are significantly more expensive than plain uncoated paper. In addition, because of the heavy photoconductor coating (the coating weight generally amounting to pound s per 3,000 ft ream), the'papers are heavy and have a feel quite different'fro'm plain paper.
A principal object of the present invention is to simpiify the conventional plain paper electrophotographic process and the apparatusby'which it is carried out.
.Another object ofthe invention -is to provide an image reproduction method wherein there is no physimore particularly to a method and apparatus for the efcal contact of the photoconductor with either devel- I oper or paper. Y 4
In addition to having the advantages of eliminating photoconductor wear and simplifying the number of machine operations, the method and apparatus of this invention do not require a photoconductively coated paper. In comparison therefore to electrostatic copy processes employing conductive paper, the process of this invention has the advantage of lower paper cost and the advantage of a capability for employing "plain" (non-chargeable) paper orv a dielectric coated paper which has the feel, weight and appearance of a plain bond paper. v} m Another-object of the invention is to provide an image copying meansv whereby photoconductor defects, attracted dust and the like do not appear in the final copyi these defects being integrated out during an exposure. i 1 I a In the 'present'invention, a fine mesh screen or grid coated with a photoconductor is employed to spatially modulate the. flow of corona current in accordance with anoptical image projected onto said fine mesh screen or grid. A I
which is coated with alight sensitive material, is det The use of a fine wire meshor screen, the surface of scribed in US. Pat. No. 2,676,100 and in U.S. Pat. No.
In some respects thepresent invention is similar to that described in US. Pat. No. 3,220,324, which discloses an apparatus and method of forming an electroconductor is utilized. As a consequence of this change,
it is possible to obtain a contrast ratio of 6 to 11. One preferred embodiment of the present invention employs an insulating screen-woven from a Nylon or'Dacron monofilament which is coated on one side with an electrically conductive material and coated on the other side, in an asymmetrical fashion, with a photoconductor. By utilizing such a screen, contrast ratios as high as 10 or 12 are obtained. 7
The method of formingthe screen and the apparatus for utilizing the screen for the formation of. visible images will be more fully apparent from the description which follows taken with the drawings in which:
FIG. 1 is a schematic view of an apparatus for preparing electrostatic chargelimages corresponding to a projected optical image upon an image receptive surface;
FIG. 2 is a cross section view of a modulating conductive screen illustrating the asymmetric nature of the photoconductive coating thereon; i
FIG. 3 is a similar cross section illustrating the geometry of a dielectric or insulating screen coated asymmetrically with both a conducting layer and a photoconductor;
FIG. 4 is a fragmentary view of a section through a metal plate having a plurality of apertures and which is asymmetrically coated with a photoconductor;
FIGS. 2A, 3A .and 4A are enlarged views, in section, of portions of FIGS. 2, 3 and 4;
FIG. 5 illustrates schematically a means for moving the photoconductively coated screen and the corona wires during an exposure;
FIG. 6 illustrates a means for continuously supplying a fresh modulating screen during the operation of a copy device;
FIG. 7 is a schematic illustrating a means for generating full color copies of an original in a manner which eliminates registration problems;
FIGS. 8, 9 and 10 illustrate various modifications of apparatus including photoconductively coated screens in copy operations wherein the final image is formed upon plain paper, that is, paper which is not capable of sustaining a charge image. In FIG. 8, a toned image is fonned upon a plastic film, which is in the form of an endless belt, and is subsequently transferred to a plain paper. FIG. 9 employs a simultaneous charging and dev'eloping operation using a liquid or dry aerosol, while FIG. 10 employs liquid development, the charging and development once again being carried. out simultaneously; v FIG. 11 illustrates an embodiment of the invention which employs a screen grid to electrically isolate a 3 chargeable members surface potential from the screen potential; and
FIGS. l2, l3 and 14 schematically depict devices em- 7 ployingv means for simultaneously exposing, charging,
and developing a visible image upon plain" paper, i.e., non-chargeable members, and utilizing the modulating screen of the present invention.
Referring now to FIG. 1, illustrating an apparatus for preparing electrostatic images on an image receptor surface, the apparatus comprises an electrically conductive platen 10 upon which is supported a conducting paper 12, having a thin dielectric coating 14. A corona modulating screen, grid or aperture plate 16 controls the ion current reaching the surface of the dielectric paper in accordance with optical image projected onto element 16. A corona source is provided, which may comprise a fine wire 18. The corona operating potential is supplied by power supply 20. The paper support substrate 10 is maintained at a selected potential provided by power supply 21. Electronic controls 24 provide a means for simultaneously turning on power supplies 20, 21 and 24 and an illumination source for a projector 22. Projector 22 provides the image which is to be copied; this image being focused upon screen 16.
Although in this embodiment the optical image is provided by a projector such as might be employed in the projection of microfilm images to obtain hard copy, it will be understood that projector 22 could be replaced by a cathode-ray tube display using a projection lens system or by an original document support plus a projection system for conventional ofi' ce copy, or any other suitable source of optical image depending upon the application of the apparatus. v
A single corona wire 18 is shown iri FIG. 1. In order to provide a uniform corona over a large area, a plurality of corona wires may be utilized all connected in parallel to' power supply In order to provide sufficient corona current, the corona wire diameter should be less than 10 mils and to simplify handling of the wire, the wire diameter should be greater than l mil. A preferable wire diameter for this application is 3 mils. Using a single corona wire spaced approximately 1 inch above modulating screen 16, uniform charging, in accordancewith the projected optical image, of the dielectric paper occurs over an area equal to the length of the corona wire and a distance between l and 2 inches normal to the direction of the corona wire at the paper. In order to provide for more uniform charging, the corona wire(s) may be moved, in a plane parallel to the screen, during the exposure.
A dielectric paper is shown in FIG. 1, such papers being available from avariety of paper mills and being employed widely in high speed computer printers and recorders. The dielectric coated paper may be replaced with any of a wide variety of plastic films ranging in thickness from 0.1 to 5 mils. Images have been successfully formed on both polyester and acetate films; and, indeed, any film which has a dielectric relaxation time in excess of a few seconds and which falls within the aforementioned thickness range may be employed in the apparatus of FIG. 1.
Means for mechanically transporting the dielectric paper or plastic film under the corona modulating screen, maintaining said paper (film) stationary during the exposure, and then removing the paper from the imaging station are not shown in FIG. 1; these mechanica] features being well known to those skilled in the art.
FIG. It shows the corona modulating screen maintained at ground potential. In this event, the potential on the corona wire and backing plate 10 must be opposite in polarity. Thus, if the corona wire is maintained at a positive potential, the backing plate must be maintained at a negative potential so that positive ions emitted from the corona wire are accelerated to the dielectric paper after passing through the meshes of screen i6. Alternately, the backing plate 10 might be maintained at ground potential, screen 16 at a positive potential, and corona wire 18 at an even higher positive potential.
The potential required between corona wire 18 and screen 16 must be at least sufficient to initiate a corona current, i.e., at least 4 to 5 kv. The higher the potential the greater the ion current and hence the more rapidly dielectric paper may be charged and the lower the required exposure time. The upper limit of corona potential is realized when sparking occurs between corona wire 18 and screen 16. This is, of course, a function of the spacing between 16 and 18. Corona potentials as high as 25 kv have been employed in this invention successfully.
The potential required between screen 16 and backing plate 10 depends upon the spacing between said members and the required resolution of the electrostatic image formed on the charge supporting member. if the potential for a given spacing is too high, sparking will occur between the chargeable member and screen 16. Furthermore, at high potentials for a given spacing, the resolution of the charge image is sufficiently high so that a screen pattern corresponding to the screen 16 is observed in the charge pattern laid down on the chargeable member. A preferred electric field, in this region, is 20 kv per inch. This corresponds to an ap' plied potential dflO kv at a 6 inch spacing or i lav a 50 mil spacing. At this electric field the corona CL rent passing through screen 16 and onto the chargeabiq member follows the field line sufficiently well so that a resolution of 6 to 10 line-pairs/mm is readily obtained with screens havingfrom 240 to 325 meshes per inch. At electric fields in the range of 50 to I00 kv per inch, sparking occasionally occurs and the screen mesh pattern appears in the image. At fields below approximately 3 kv per inch, ion spreading is observed with subsequent degradation of image resolution.
The exposure times required are a complicated function of the corona voltage, corona-to-screen spacing, light intensity at the screen, nature of the photccoriductor, and also the nature of the charge receiving member and the type of development employed in converting the electrostatic image into a visible image. In general, the required screen illumination ranges from I to 50 ft.-candles of tungsten illumination and the exposure times range from 0.1 to 3 seconds.
FIG. 2 is a cross-sectional view of a wire mesh screen coatedwith a photoconductor. The wire mesh 30 may be formed of any available metal or alloy, typical materials including brass, stainless steel, aluminum or phosphor bronze. The mesh size, i.e., the nurnlzers of wires per linear inch, may range from to 1,009. A 100 mesh screen will provide a resolution of 2 to 4 linepairs/rnm while a 325 mesh screen is capable of providing 7 to 14 line-pairs/mm. The photoconductive coating 32 is shown here as being offset by an angle of 45 from the normal. By forming the photoconductor in an asymmetrical manner such as this, much higher contrast ratios, i.e., ion current transmissivity ratio be- One preferred way of applying the photoconductor I to the screen 30 is by vacuum vapor deposition. The materialto be vaporized is placed in a crucible or metal container which is electrically resistance heated. The
screen to be coated is supported above the crucible at an angle generally 45 with the normal. The angular orientation of this screen, while mounted 45, is not critical. Thus, either the warp or woof of the weave may be parallel to the ground or run at any angle to the ground without adversely affecting theincrease in contrast ratio. In addition, during the evaporation, the screen may be rotated .through some angle in order to form more complex asymmetrical patterns.
In FIG. 2A, a single elementof the array is shown, in section, showing the manner in whichthe photosensitive coating 32 is asymetrically disposed on the base 30. FIG. 3A isa similar view showing the disposition of both the photoconductive coating 37 and the electrically conductive coating36 on the insulating filament base 34. FIG. 4A is a similar view of the array in FIG. 4.
FIG. 3 shows a. preferred embodiment of the present invention; the screen 34 being fabricated from an insulating material. Typical insulating materials employed in this invention are woven fabrics consisting of monofilament nylon, polypropylene, polyester or polyamide. Such woven fabric screens'are available in mesh sizes to' over 325 mesh, are extremely strong andare much less expensive than corresponding metal woven screens. Furthermore, by employing an insulating screen having a photoconductor on one side and a continuous conducting layer on the other side, higher contrast ratios are obtained than with plain metal screens. In FIG. 3, the dielectric mesh 34 is shown having a conductivecoating 36 on the bottom anda photoconductive coating 37 on the top and offset from the normal by 45. Instead of offsetting the photoconductor, as
shown, thescreen may be formed by evaporating the photoconductor 37 normal to the surface, with a consequent reduction in contrast ratio. The conductive layer is preferably formed by vacuum vapor deposition of an FIG. 4, which is a cross sectional view of an aperture plate. The supporting plate'38, having a thickness in the range of l to Sriiils, may be fabricated from etching a plurality of holes through the surface and then coating the material with a photoconductor at an angle from the normal as shown in FIG. 4.'Alternately, the plate 38 may befabricated from a plastic sheet which also contains a plurality of holes etched in the surface. In the case of an insulating support sheet, a conductor would be deposited on the sides of the holes and bottom of the plate in amanner similar to that shown in FIG. 3.
The mesh screens may be woven with either a plain square weave or a twill-square weave. With a twill square weave, however, the resolution in one direction is degraded slightly.
' In all cases, a higher contrast ratio is observed if the screen is mounted so that the photoconductor coated side faces the corona wires. A somewhat lower contrast is obtained with the photoconductor coated side facing away from the corona wires.
One of the many advantages of the present invention in comparison to conventional electrostatic photographic systems involves a relaxation in the requirements for high dark resistivity of the photoconductor. A typical selenium xerographic plate or drum has a capacity close to 100 pf/cm. If such a plate is charged to 500 volts and the allowable voltage decay must be 100 volts or less in a period of 1 second (the minimum time interval between charging and image development), thena simple calculation will show that the dark current through the plate must be less than 10 amp/cm or the plate dark resistance must be in excess of 5 X 10' ohm/cm of plate area. In a typical screen modulation apparatus, as described herein, the corona current to the screen might be in the range of 3 l0 amp/cm". In order to provide effective modulation of the screen corona current, it is estimated that a voltage drop of at least I00 volts is required across the photoconductor coating of thescreen. Thus, the photoconductor resistance in the dark must be in excess of 3' X 10 ohm/cm of the screen area. This represents a 1,000 fold'reduction in the maximum dark resistance of the photoconductor coating the screen in comparison .to photoconductors employed in conventional electrostatic photographyThe relaxation of this constraint permits the utilization of a much wider range of photoconductor materials, particularly those having higher sensitivity and/or extended red response. Evaporated photoconductors such as zinc cadmium sulfide, zinc cadmium selenide and cadmium sulfide, which, in the vapor deposited form, have too low a resistivity for conventional electrostatic photography are suitable for preparing screens in the manner described in this in vention. In addition, the selenium alloys having extended red light response such as selenium-tellurium alloys containing more than 10 percent tellurium and selenium-arsenic alloys containing at least percent arsenic may also be employed in this invention.
Although the asymmetrical photoconductor deposition onto either a conducting or nonconducting screen may be readily carried out by vacuum vapor deposition, it is also possible to prepare photoconductive coated screens using photoconductor binder layers. One preferred method of forming such a screen is to spray, using an air gun, the photoconductor binder layer material onto the screen; the spray being directed onto the screen at an angle. Either suitably doped and dye sensitized zinc oxide or doped cadmium sulfide dispersed in a suitable solvent with any of a number of appropriate binders may be sprayed onto the screen or mesh at the appropriate angle to form an effective ion control screen. The properties and nature of photoconductor binder layers are described in detail in the book, Xerography and Related Processes" by Dessauer and Clark (pages 119-168) publishedv 1965, Focal Press Limited. As previously indicated, because of the requirement for a relaxation of the dark resistivity requirement. the concentration of photoconductor pigment in the binder that may be employed on screens is significantly higher than that which must be utilized for the standard electrophotographic processes. This permits the fabrication of higher photosensitivity surfaces.
Organic photoconductors of this type are described in the previous reference (pages 169-199) may also be employed as photoconductors suitable for the present invention.
As previously mentioned, the screen pattern on the chargeable member may be eliminated by operating at electric fields sufficiently low that the screen is not resolved on the chargeable member. In inexpensive commercially available wire or plastic monofilament screens having very small mesh sizes or high mesh counts, the weave is found to be somewhat nonuniform, i.e., there are small random variations in mesh spacing which result in mesh irregularities appearing in the image developed upon the chargeable member. These small irregularities, which appear as mesh lines in the 'copy, may be eliminated by moving the screen over a very slight distance during the exposure. One manner of effecting this motion is illustrated schematically in FIG. 5. Here, the modulating screen 16 and a series of corona wires 18 are mounted together on a rigid framework 40. This framework is supported in such afmanner that it may be translated transversely from left to right. The frame is also spring loaded so that it is urged against cam 42 which is driven by low speed rriotor 44. Motor 44 is energized during the exposure sothatthe corona wires and screen move relative to the dielectric receptor sheet during theexposure. Motions as small as 0.1 inch are generally sufficient to eliminate all screens nonuniformities from the developed image. It has been found that the maximum screen velocity during an exposureis approximately 1 to 2 inches per second, depending upon the intensity of the corona current to the screen and the nature of the response time of the photoconductive coating. The screen may be advantageously moved in two directions as by a circular or figure 8 motion. In addition to eliminating screen variations from the developed image, the technique of moving the screen during an exposure also provides the advantage of eliminating the development of other screen defects such as random dirt and dust which settle upon the screen. This procedure which essentially integrates spatially over a photoconductor during exposure to eliminate photoconductor defects from appearing in the developed visible image is believed to be unique.
FIG. 6 illustrates a means for moving the screen during an exposure and for simultaneously providing for the replenishment of new screen within the exposure region. As shown screen 46, having a width of between 4 and 18 inches, depending upon the copy size desired, and coated with an asymmetrical photoconductor is supplied from supply drum 45. A takeup drum 437 collects the screen after it passes through the exposure area. In operation, during each exposure, the screen advances a distance of approximately 1/16 inch. In this way for every few hundred copies that are made, the screen is completely replaced with previously unused screen from drum 45. With many vacuum coated screens several thousands of copies have been formed from each screen with no degradation of the process. However it is possible that with certain very high photosensitivity screens some fatigue may be present. The screen motion, in order to eliminate irregularities in the direction of both screen wires or monofilaments, should be such that the motion is not in the direction of either wire or monofilament. A preferred direction of the motion is at an angle of 45 with each wire. Besides a linear motion, orbital motion or a zigzag motion may be employed to successfully eliminate screen nonuniformities from the image and mechanism in place of cam 42 to provide such motions is readily available.
FIG. '7 illustrates schematically an apparatus for obtaining full color prints employing the techniques of this invention. One of the major problems in generating full color prints, employing color separation principles, is associated wth registration of the three colors. A minor problem involves the complexity and expense in handling the copy sheet. The apparatus of FIG. 7 circumvents these difficultiesby sequentially generating a charge image pattern and developing the three primary colors without moving the charge receptor layer. In this figure, the projector 22 serves to project a color transparency onto the corona modulation screen 16. Three successive exposures are provided; one for blue, a second for red, and a third for green. The projected color is selected by placing a filter color wheel containing the three selected color filters-50 between the projection lens and the screen. The sequential operation of the three primary colors is carried out by indexing motor 52. The charge receptor sheet is developed in place employing a series of three liquid toners which are delivered in such a manner as to flow over the receptor sheet from manifold 54'. Solenoid activated valves 5o select the appropriate yellow, cyan or magenta liquid toners which are contained in gravity fed reservoirs The liquid developer,- after passing across the surface of the charge receptor sheet, is collected in reservoir 66 and discarded or else reclaimed for further use. v
In operation, the blue filter is indexed in front of projector 22 so that the image corresponding to the blue tones in a color print are projected upon modulating screen l6.'The required potentials are applied to the corona and backing plate to form a charge image on the receptor sheet, and the solenoid valve connecting the yellow toner reservoir to applicator manifold 54 is opened for a period of 2 to 4 seconds. The toner passingover the inclined surface of the charge receptor sheet deveiops the yellow components of the image. This process is then successively repeated with a red color filter using a cyan toner and the green coior filter employing a magenta toner. Effective liquid electrostatic colored toners for use in this apparatus are manufactured by the Day-Glo Corporation (Cieveland, Ohio).
An unexpected result obtained with this apparatus is the lack of a requirement for drying the charge image receptive paper between successive exposures. An image may be toned and, before the paper is dried, a second image placed on the surface.
When excess liquid remains at the surface of the charge image receiving layer after a developer has flowed over the surface this excess liquid may re moved by drawing a rubber squeegee or rolling a hard rubber roller over the surface of the paper. Alternately, excess liquid can be removed from the surface with the use of an air knife.
9 Thus far, in the description of this invention, the use of dielectric coated paper or plastic films have been indicated. In addition to these materials, papers fabricated from plastic (the so-called plastic papers) may also be employed in this invention. Conventional plain papers fabricated from cellulose generally contain a sufficient quantity of moisture and free ions so that these papers will not support an electrostatic charge image for the time intervals required to practice the present invention. By suitably treating plain papers, however, images maybe formed using this invention. Plain bond papers or plain blade-coated papers may be rendered sufficiently insulating by first heating the papers to a temperature of between 120 and 200C for a period of a few seconds. This may be carried outin a small oven. Immediately after removal from the oven and while the paper is cooling down, the paper is wet with a hydrocarbon. A preferred material for this application is the aliphatic hydrocarbon solvent known under the tradename of Isopar, manufactured and marketed by the Humble Oil & Refining Company. Paper so treated is capable of sustaining an electrostatic charge on the surface for long periods of time. A lateral surface conductivity is still present, however, so thatonce a charge image has been placed on the surface of the paper, the development must be carried out within a period of l to 2 seconds if excessive resolution degradation is to be avoided.
The latentelectrostatic image formed by the corona modulation screen may also be employed in recording an image using a deformable thermoplastic film composed of polystyrene, St'aybelite, Piccolastic or other deformable synthetic polymer material. After the formation of an electrostatic image on the film surface, the latent image isdeveloped by softening the film by known in the art.
In addition to providing a permanent image, the coexposure to either heat or solventl'vapor s'as is well rona modulating screens of this invention may also be employed with electric field sensitive cholesteric liquid crystal filmsin display applications. Here; the dielectric coated paper of FIG. I isreplaced by a liquid crystal film and the support platen 10 replaced by a glass sheet having a transparent conductive coating on the side adjacent the film. Under the influence of an electric field provided by ions reaching the free surface of the liquid crystalv film, the optical scattering and/or reflective properties of said film are modified, leading to the formation of a visible display on the film. Cholesteric materials suitable for this application are described in British Patents, 1,123,117 and l,l67,-486, and also by L. Melamed and D. Rubin, Appli. Phys. Lett. 16, 4, 149 (1970) and by .l. .l. Wysocki, J. Adams, and W. Haas, Phys. Rev. Lett. 20, I9, 1,024-(1968).
FIG.-8 illustrates an apparatus in which the toned image is first formed on an intermediate endless belt and subsequently transferred to a plain paper sheet or Web. In this drawing an endless plastic belt 62, preferably fabricated of polyseter and containing a conductive coating onthe inside surface issupported on rollers 64 and'7,0."An electrostatic image is formed on this belt using a corona modulation screen, corona wire and projection source in a manner similar to that shown in F IG. 1. After the electrostatic image has been formed, it is developedby immersion in liquid developer tank 66. A counterelectrode 68 is maintianed at a suitable potential inorder to minimize the development of background. In the region between roller 64 and W, the developed image is partially dried and then offset onto a paper sheet or web 72 as the paper and plastic film are held in contact by rollers 70. The image is fixed on thepaper and some residual solvent removed as the paper is heated by radiant heater 74-. Cleaning brush 76 removes residual toner from the plastic endless belt.
Rather than employ the endless plastic belt (as shown in FIG. 8), a conductive drum coated with a hard insulating surface, such as a glass-based enamel, may be employed. In apparatus employing a drum, the operational steps are the same. The electrostatic image is formed using a corona wire and a corona modulating screen; the electrostatic image is toned, employing either a dry or liquid electrostatic developer; the image transferred by offset to a plain paper sheet or web; and the drum cleaned. This apparatus is rather complicated but does possess several advantages over conventional plain paper electrostatic photography. A principal advantage is the fact that the photoconductor is never in physical contact with either a developer material or paper and, hence, is not subject to the usual wear which occurs in standard electrophotographicplain paper copies. An insulating surface -enameled drum possesses a hard abrasion resistant surface and hence has a life significantly greater than a typical selenium drum. Devices employing an endless plastic belt would be subject to a higher degree of wear; however, the belt may be readily changedandis relatively inexpensive compared to aselenium drum.
FIG. 9 is a schematic drawing of an apparatus for simultaneously charging, exposing and developing. The apparatus shown here is identical to the apparatus in FIG. 1 with the exception that provision for injecting an aerosol into the region between corona, modulating screen 16 and a paper image receptor sheet 81 l'lffi; been added. The image receptor sheet in this apparatus does not require a dielectric coating upon its surface. In order that uniform development occur, it is necessary that the development aerosol be injected with a high degree of uniformity into the region between screen 16 and receptor sheet 81. The air velocity of injection cannot be too high or a displacement and breakup of the image occurs. In addition, the aerosol must be initially uncharged or, if the aerosol particles are charged, the charge must be adjusted to some low value in order to minimize background. The charge potential of the aerosol may be controlled within certain limits by adjusting the potential of the conducting manifold from which the particles are ejected. This is accomplished with power supply 83, as shown in FIG. 9 Alternately, the particle charge may be controlled by induction, in which case the potential of power supply 83 is not connected directly to the conducting manifold, but is rather connected to an electrode immediately adjacent to the aperture or slit in the aerosol generating nozzle 82.
In operation, potentials are applied to appropriate electrodes, the image is projected onto the corona modulating screen, and the aerosol is injected into the region between screen and receptor sheet all processes occurring simultaneously. The aerosol particles, being essentially neutral, are not affected by the strong electric field and pass through the region defined by screen 16 and receptor sheet 81. As corona generated ions passthrough the screen, these ions interact with the aerosol, charging the aerosol particles which are subsequently drawn onto the receptor sheet.
While the aerosol generation embodiment shown in FIG, 9 involves the use of an air gun type atomizer 80, the invention is not restricted to this generation technique. Alternate means of forming a jet involve directly atomizing a liquid through fine jets or thermally volatizing a material to form an aerosol cloud.
In addition to using either a liquid aerosol or a thermally volatilized dyestuff, aerosol developmennemploying a solid powder, the so-called powder cloud development may be employed. Methods for generating powder clouds and details of powder cloud development are described in the Dessauer and Clark reference cited earlier, pages 309 through 340. An important difference between the use of a powder cloud in the present invention and powder clouds associated with conventional electrostatic photography involves the fact that, in the process of the present invention the aerosol powder cloud should be uncharged or the charge per particle should be maintained at a rather low value.
Dyestuffs which may be successfully vaporized from a hot surface to form a uniform aerosol cloud include brilliant oil blue, oil brown 0, and oil brown N.
FIG. 10 is a drawing of a modification of the apparatus shown in FIG. 1 which enables the process to be employed with plain paper. The electrostatic image development, employing a liquid toner, occurs essentially simultaneously with charging and exposing. A shallow metal tray 84, having rubber seals 86, contains a conventional liquid electrostatic toner 90 which is continuouslyrecirculated through the system by inlet and outlet tubes 91 and 92, respectively. A plain paper web 88 passes over the rubber seals 86. The liquid electrostatic toner level is maintained so that it is in contact with the paper web. The corona modulating screen 16 is spaced between 84 inch and 1 inch above the surface of the paper. The corona source, power supplies, and illumination source are similar to those shown in FIG. 1 and are not shown here.
Radiant heater 93 is provided for heating the paper, in order to drive residual moisture from the paper, prior to its being employed in the process of FIG. 10.
If conventional liquid electrostatic toners of the type employed in zinc oxide paper machines are utilized in the apparatus of FIG. 10, it is found that the paper picks up residual toner in uncharged areas, leading to an overall grey background. This problem has been overcome by diluting these commercially available liquid toners with carrier solvent in an amount of 8 parts solvent to 1 part toner. At this dilution, the solids content is near 0.1 percent. A majority of commercially .available liquid electrostatic toners employ aliphatic hydrocarbon solvents as the liquid carrier. Effective dilutions may, therefore, be carried out employing the aliphatic hydrocarbon solvent lsopar G, manufactured by Humble Oil and Refining Company.
The spacing between the bottom of developing pan 84 and the lower surface of web 88 is critical. If the spacing is too low, insufficient density is developed in the image, while if the spacing is too great, low density images are also observed. Optimum spacings appear to range from 0.050 inch to 0.300 inch. Optimum results are obtained under conditions such that the exposure time is short, generally one second or less. These conditions are realized by employing an illumination intensity at the corona screen of 5 ft.-candles or greater and high corona current which is obtained by running the corona wires at high potentials and spacing the wires reasonably close to control screen 16.
in certain highly absorbent papers, a background image is observed even at low toner dilutions. This is caused by the takeup of developer particles into the surface of the paper as the toner is absorbed by the paper. This background may be eliminated with the addition of auxiliary roller 87 which supplies a pure aliphatic hydrocarbon solvent to the web prior to the web contacting the liquid developer. The solvent, typically lsopar G, is fed to roller 87 as this roller revolves through a pan 8% containing the solvent. Since theweb is already saturated or prewet with pure solvent, no developer takeup occurs in the paper, thus resulting in cleaner backgrounds.
It has been found that, to a first approximation, the density of a toned image is roughly proportional to the charge per unit area that is developed. A charge density of approximately 0.15 [.LCOUL/Cfll is required to develop a dense image. The potential to which a charge supporting member must be charged in order to develop this charge density is inversely proportional to the capacity of the per unit area of the chargeable member. Dielectric papers, having a dielectric coating thickness of approximately 6 microns, develop dense images when charged to potentials of 300 volts, corresponding to a charge density close to 0.15 pcouL/crn? If the charge is developed across a 3 mil sheet of paper, the surface must be charged to potentials in the region of 3,000 to 4,000 volts to obtain this charge density. In view of this requirement, it has been found necessary to employ high potentials between screen 16 and developer container 84. Minimum potentials of 15 ltv are required with 20 kv resulting in higher resolution images having less distortion. It has further been-found that 13 rate of development is proportional to the surface voltage. Thus, for the apparatus of FIG. 10, even though the toner has been substantially diluted over that normally employed in electrostaticxerographic processes,
the development time is extremely short; generally less than 1 second, because of the high surface potentials developed due to the low capacity per unit area of paper compared to the thin dielectric coating utilized with dielectric coated paper.
FIG. 12 illustrates yet another process for employing a photoconductive coated screen together with devel opment apparatus to generate a visible image errlploying plain paper. In this drawing, screen 16, power supplies, illumination source, etc., are similar to FIG. 1. A paper web 98 is supported by paper drive rollers (not shown) so as to be spaced a very slight distance above conducting roller 94. This roller serves in a manner similar to paper backing plate 10 of PEG. 5, and is electrically connected to power supply 23. Rotter 94 revolves, the lower surface passing into tray 5 containing an ink 96 dispersed or dissolved in a polar liquid. During operation, roller 9d revolves carrying up a thin film of ink over its surface. The roller and paper web speeds are adjusted so that the web surface speed is equal to the velocity of the periphery of roller Since this is a dynamic process, means are provided for moving the image from left to right across screen It: at the same velocity as the paper moves from left to right. Thus there is speed correlation between the image projected on screen 16, paper web 98, and the motion of the ink film on roller 94 directly below the paper. As surface charge develops on the upper surface of paper web 98, an intense electrostatic field is developed between the paper and conducting roller 94. The high electrostatic forces generated in the gap between the bottom of the paper and the ink film cause the ink to jump from the roller to the surface of the paper, thereby forming a permanent visible image. A,.wide variety of inks are effective in this process, including alcohol and waterbase inks consisting of colloidal carbon dispersions, opaque dye pigments, or dissolved acid or basic dyesfFor effective operation, the gap spacing between the ink film and the lower surface of the paper must be maintained uniform and, for typical operating conditions, between the extremes of 2 mils and 50 mils. An operating gap of mils appears preferable. With certain inks and at certain velocities, it is difficult to establish a uniform ink film thickness on the surface of roller 94. In this event, thickness control attachments well known to the art, such as doctor blades or reverse rolls, may be added to establish the proper ink film thickness. This process has the advantage, in addition to using ordinary paper, of generating an extremely clean, background since no ink or developer touches the paper in areas which are not charged. For certain papers, under high hunidity environmental conditions, the paper web must again be preheated so that the charge placed upon the surface of the paper does not diffuse within a period of a few tenths of a second. This process functions effectively with standard weight plain papers since very high potentials, on the order of 2,000 to 3,000 volts, are placed on the surface paper and the spacing between the top surface of the paper and conducting roller 94 is only a few thousandths of an inch. These high potentials established across such a short distance result in high electrostatic'forces being developed at the surface of the ink film; such forces being sufficient to locally draw the ink film across the prints s p- It has been found that, when the surface of a charge receptor member is charged to voltages high in comparison to the voltage existing between screen 16 and backing plate I0, image distortion occurs. This distortion arises from fields at the surface of the chargeable member; these fields, existing between a charged and uncharged region. This results in corona generated ion beam bending in a manner so as to reduce the width of uncharged lines. Secondly, when a high potential is built up in a significantly large area, a reduction in the local field immediately below screen 16 occurs in this region with subsequent diffusion of ions passing through the screen in this region. This effect is not serious for low charging voltages, particularly when high potentials are applied between screen 16 and backing electrode 10. In charging relatively thick plastic films, which requires high surface voltage potentials (several thousand volts, for example, in case of 3 to 5 mil polyester or acetate film) these distortins are observed.
A means forcircumventing this problem is shown in FIG. 12. This apparatus is identical to that shown in FIG. I but includes a second fine mesh screen 100 whose potential. is established by power supply 102. This fine mesh conducting screen is spaced very close to the surface of'the chargeable member, generally within a distance of 5 to 25 mils. The screen-potential, as established by power supply 102, is maintained between the potential of backing plate and screen l6.
beam diffusion and distortions mentioned previously.
We have found that, because of the high system resolution, moire patterns are formed in the image corresponding to screen mesh overlap between screen 100 and screen 16. In order to eliminate this problem,
. screen 100 may be vibrated or caused to move by employing a motor and cam assembly 104 operating in a manner similar to that shown in FIG. 5.
FIG. 13 illustrates a further means of employing a corona current modulating screen to realize simultaneous charging, exposing, and developing while using a plain paper. Endless conducting belt 108, supported and driven by conducting rollers 110, is employed to support a layer of developer or toner particles and is positioned, with uniform spacing, immediately below paper web 88. The potential of the conducting rollers and the conducting endless belt is established by power supply 21. A uniform thin film of dry toner particles is continuously supplied to the endless belt from hopper 112 containing a reservoir of toner particles 114. After traversing the development area, toner particles falling off of the endless belt are collected, for reuse, in tray 116. The operation of the apparatus shown in this figure is similar to that of FIG. ll. The'rnotion of the endless belt andpaper may be continuous; in which case the image to be reproduced must be scanned across screen 16 to match velocityof paper web 88, or the machine 1 may operatein a stepand repeat" mode; the paperand .endless belt advancing between successive exposures.
' Eithera conducting or nonconducting toner may be employed with this apparatus; the toner being transferred from the endless belt to the underside of the paper webby virtue of the high electrostatic forces existing at charged regions of the paper. The same precautions regarding the endless belt paper spacing indicated for apparatus of FIG. 12 are pertinent for this apparatus.
FIG. 14 schematically illustrates an apparatus employing a corona modulation screen 16 in such a manner as to form a visible toned image on a plain paper sheet which is supported on backing plate 10. The image projection source, corona wire, screen, and backing plate are identical to the apparatus as shown in FIG. 1, and are connected to power supplies as shown in FIG. I. In this device, charging and toning of the paper image are carried out simultaneously by employing an open mesh screen 122 moving through -toner reservoir 130. The open mesh screen 122 consists of a fine mesh (generally I00 to 300 meshes per inch) formed as an endless belt and traveling over drive pulley 124, idler pulley 126, and pulley 128 which carries the open mesh-through a toner supply 132. In opera-' tion, the open mesh web is driven through toner to provide a toner laden mesh surface immediately adjacent the paper upon which the image is to be developed. During an exposure, ions passing through the ion corona current modulating screen impinge upon the open mesh screen carrying toner, charging the toner particles to a high potential, and the toner particles are subsequently electrostatically attracted onto the plain paper sheet 120. Toner is thus deposited on the paper in areas corresponding to regions in which corona current traverses modulating screen 16. Either liquid or dry toners may be employed in this apparatus, although best success has been realized with the use of dry toncrs. 'l'he toners are not. in general, electrostatically held onto the mesh screen 122 but are collected mechanicallyv It has been found advantageous, in instances wherelow toner pickup on the open mesh end-- less belt 122 is observed, to very slowly move the belt during the exposure. This provides an additional toner source as toner is depleted from the belt. Optimum belt drive speeds are-in the range of l-/32.to re inch per secnd Although the apparatus shown in FIGS. 7, 8, 9, l0, l2, l3, and 14 are described as being employed with corona modulated screens discussed in this invention, thesedevelopment means will also function with multilayer screens which are first charged and then exposed and then employed to modulate a subsequent corona discharge as described in US. Pat.v No. 3,582,206. When employed in this manner, an advantage of device simplicity is obtained over that described in the above referenced patent. v
The following examples illustrate the techniques of the method,'process and apparatus described in this disclosure. These examples are not meant to be restrictive in any way however.
EXAMPLE 1 A plain square weiiwiofimsh" phosphor bronze screen was stretched over a square brass frame whose inside dimension was 4inches on a side and whose out side dimension was 5 inches. The phosphor bronze screen was soft soldered onto the frame. The frame was mounted in a vacuum coater a distance 12 inches from a quartz crucible mounted in tantalum heater. The screen was inclined 45 from the normal. A charge of 30 grams of xerogrophic grade. selenium was placed in the evaporation crucible. The system was evacuated to a pressure of torr andlhe selenium e ap ra te d from the boat onto the screen over a periodof 45 minutes. During the evaporation, the screen was heated, with an electrical heater, to a temperature of 70C. The selenium coating thickness was found to be microns.
The screen was removed from the vacuum evapora- The contrast ratio,defined here as the ratio between the ion current to the conductivebacking plate 10 with the photoconductive screen in the dark and ioncurrent with the same screen illuminated, was determined by "connectinga Keithley Model 6m A electr ometer between papersupporting-electrode l0 and power supply 21. At a counterelectrode potential of 3. kv anda corona potential of +l0 kv, the dark current was 8.3
pamperes and the'current obtained whenthe screen was uniformly illuminated with tungsten illumination at a level of IO ft. -candles was 1.5 #amperes. The contrast ratio was thus 5.5. At a corona potential of +8 kv,
the dark current was 3.7 iamperes and the light current was 0.45 ampercs; yielding a contrast ratio of 5 8.2.
It may be seen from the aforementioned measure ments that a higher contrast potential is obtained at lower corona potentials. In this event, however, the col0 rona current is lower, and longer exposure times are required to charge the dielectric paper. At a screen-paper separation of inch, an applied potential of -3kv is sufficient to accelerate the ions to the surface of a dielectric coated paper and still maintain a resolution of l 3 line-pairs/mm in the developed image.
Copies of a projected image were obtained by placing sheets of dielectric coated paper on the paper counterelectrode' 10. An image having a high-light brightness of 10 ft.-candles was projected on the screen with a simultaneous application of corona and counterelectrode potentials; the total exposure time being 3 seconds. The paper was then removed from the counterelectrode, immersed in a beaker of liquid toner having a solids concentration of 1 percent in lsopar G. Since the paper surface was charged positively and since the liquid developer toner particles are also positively charged, a reversal imagewas obtained. After the paper was removed from the developer, excess liquid was squeegeed from the surface and the paper dried in an airstream which may or may not be heated. The
image was of high quality, having negligible background and a maximum density ofv 1.1. The develop ment time was 3 seconds. I
A selenium coated ion current modulating screen wasprepared in a manner identical to that of Example I l with the exception that the evapora fiodtilasTafiied out with the screen mounted normal to the line of evaporation. When evaluated in the apparatus of FIG. 2, it was found that, at a corona wire potential of +l0 kv, the dark current was 7.3 ,uamperes and the current with an illumination level of i0 ft.-candles was 4.6 pamperes; providing a contrast ratio of only 1.6. A number of attempts were made to obtain satisfactory copies of the light image in a manner described in Example 'lffln no'case was it possible to obtain a high contrast between light and dark areas on the paper.
;High background Ievelswere obtained together with low image density.
Example 2 illustrates results obtained employing the .teachings of US. Pat. No. 3,220,324. This example is t 5 0 included to indicate the advantages realized when practices the teachings of the present invention.
The following examples illustrate the diversity of photoconductor materials, deposition methods, and geometry of screen arrangements suitable for utilization in the present invention.
TABLE I Examples -Photnconductor Selenium Selenium Selenium Selenium Selenium-Tellurium D I Selenium-Arsenic* Deposition Vacuum Vacuum Vacuum Vacuum Vacuum Vacuum Evaporation Method Evaporation Evaporation Evaporation Evaporation Evaporation Photocondu'ctor 20 2O 20 20 20 Thickness (u) Angle of Deposit 45 90 60 30 45 45 (from normal)() Screen 325 mesh dacron 325 mesh dacron 325 mesh dacron 325 mesh dacron 325 mesh nylon 325 mesh nylon Conductor Layer Al evap. 90 to Al evap. 90 Al evap. 90 Alevap. 90 Al evap 90 normal Al evap. 45 normal normal opposite normal opposite normal opposite normal opposite opposite opposite photoconductor photoconductor photoconductor photoconductor photoconductor photoconductoi' Light Level 2 5 2 2 2 2 (ft-candles) Contrast Ratio 9.2 6.7 8.0 7.6 8.8 9.1
Evaporated alloy consisting of l2 atomic percent telluriurn and 88 atomic I a Q h prior to "Evapor'ated alloy consisting of 50 atomic percent selenium and 50 atomic percent arsenic formed by n L prior to r Photoconductor Cadmium sulfide Cadmium selcnide Zinc oxide-pliolite Selenium Zinc oxide-pliolite Selenium binder binder Deposition Vacuum Vacuum Air gun spray Vacuum Dip coat and air Vacuum evaporation Method evaporation evaporation evaporation blast to clear openings Photoconductor l0 I0 20 25 20 Thickness (11.)
Angle of Deposit 45 5 45 90 45"; screen rotated (from normal )t) during evaporation s r 325 mesh nylon 325 mesh nylon 325 mesh stainless 325 mesh dacron 250 mesh 325 mesh stainless steel Conductor Layer Al evap. 45 to Al evap. 45to None Al evap. 90 to None None nonnal opposite normal opposite normal on both photoconductor photoconductor sidesto completely 7 7 cover filament T Light Level 2 2 l0 2O 20 :(fL-Candles) Contrast Ratio"- 8.1 6.4 NJ 6.0 6.2 L6
5 l6 'corona assembly was mounted in a cross slide and spri- 4O r pho'oconducmcadmium Sulfide selenium Selenium rig loaded in such a manner that the assembly was free Polycarbonate to travel back and forth. A small. gear head motor operbinder ating at 60 rpm was employed to drive a cam having an eccentricity of /4 inch. When the motor was energized, Method screen immersed evaportion evaporation in dispersion the screen described a lateral motion having an ampliv rz tude of it inch. The corona wires were operated at a :2? potential of +l4kv, the conducting layer on the screen Photoconduc- I0 20 was grounded, and the paper support platen was operzza lilgcknm ated at a potential of l 5 kv. A microfiche image, hav- Angle f 45 45 45 ing clear letters on a darkened background, was imaged Denali! (from on the screen at a magnification ratio of approximately Screen ,325 mesh 7 2 mil copper Parallel wire grid e lg lg m e micro "P stainless steel sh eethaving 2 of 5 mil wires on was 4 ft. -candles. Exposures were made by placrng a sheet of dielectric coated paper upon the conducting Conductor None None None metal platen and simultaneously projecting the image t l lft Level s I 20 2o 55 on the screen and applying potential to both the corona mama) wires and paper support platen. The total exposure Contrast Ratio] l- 7.6 8.]
EX M LE l8 The screendescn bed i'nExample 3 was mounted in a frame as'sh own in FIG. 5. A Plexiglas arch mounted on opposite sides of the screen supported 35 mil diameter corona wires. The corona wire to screen spacing was 34 inch. The screen was supported t inch above a 6 inch square brass paper support platen. The screen time was second. After the exposure, the paper was then removed from the support platen and developed by immersion in a liquid toner consisting of a fine dispersion of carbon particles in lsopar G hydrocarbon fluid. A'small quantityof dissolved resin present in the developer provided a fixing action when the paper was heated to volatize the 'lsopar solvent. A variety of dielectric papers were evaluated in this apparatus and found to function effectively; high density images with clear backgrounds being obtained when the images were developed in a liquid toner having positively charged carbon particles. Such dielectric papers, available from a number of manufacturer'sycons'ist of a paperb ase rendered conducting through the incorporation of certain additives and over which is coated a thin plastic film generally having a thickness of from O.l to mil.
In all cases the images were crisp and sharp; a resolution of 6 to 10 lines-pairs/mm being routinely obtained.
Images were also developed employing transparent plastic. films. Polyester films of l to S'mil thickness were successfully employed as were acetate films in the same thickness range. For plastic films which do not possess a conductive. backing, an auxiliary backing platemust be employed. Such a plate was constructed of 15 mil thick aluminum. The polyester film was placed upon the conductive backing plate and the assembly placed together upon the brass support platen.
An exposure was made and the polyester film and conducting backing assembly removed together from the exposure apparatus and immersed in the liquidtoner together to develop the latent electrostatic image.
The utility of moving the screen corona assern bly during l'exposure was established with this apparatus.
No screen patterns due to screenweaving irregularities were observed-when the screen was moved during'an exposure. In addition, defects due to dusta nd'dirt on the screen were eliminated from the final copy. If the screen was not moved during exposure,'a series of very 40 faint lines running in both directions corresponding to the screen filaments were observed'in the developed image.
Excellent continuous tone images were obtained by 5 projecting a continuous tone transparency onto the screen. Good-solid area coverage after development was observed. It is thought that the solid area coverage is obtained since the photoconductive coated screens serves a function of screening the image. A very fine screen pattern is typically observed in large dark developed areas. 1 I
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i The apparatus of the previous examplewas modified by tilting the whole apparatus at an angle of 30? to the horizontal undudding a developer manifold, solenoid operated valves and developer supply tanks as shown in FIG. 7. In addition, provisionswere made for placing color filters in front of theslide projector 22. The developer manifold consistedof a /4 inch diameter copper tube (6 inches long) sealed at one end. Holes 0.020 inches in diameter were drilled in a line along the distribution manifold at a spacing of /4- inch. The open end of the copper distribution manifold was connected to electrically operated solenoid valves 56 withsuitable unions. Each of the three solenoid valves were connected in turn to'a liquid developer reservoir. A collection pan 60 was provided to collect the liquid developer after it had flowed over the surface-of the paper being developed. I
"Afull color positive transparency was placed in projector 22 and a yellow filter placed in front of the projector. The high-light intensity at the screen was 20 ft. -candles'. The first exposure was made in 1 second, employing a negative corona potential, and immediately after exposing, thesolenoid valve connecting the developer distribution manifold to the tank containing the yellow liquid. toner was opened for a period of 2 seconds. During" this period, the yellow toner flowed across the dielectric paper which had previously been placed on the brass paper support platen. Immediately after the yellow development operation was completed, the red filter was placed in front of the slide projector. The light intensity was increased until a high-light intensity of 60 ft. -candles was incident upon the screen and the image exposed with the potentials applied to screen and backing electrode for a period of 1 second. The solenoid valve connecting the developer manifold to the cyan liquid toner reservoir was opened again for a period of 2 seconds. The process was continued a third time employing a green filter with a high-light intensity of 20 ft. -candles and a 1 second exposure followed by a 2 second magenta development. The paper was then removed from the conducting brass platen and dried in a warm air stream. 'A positive print was thus obtained havinga good color balance and a high degree of registration. I
. In several experiments it was found that the charged colors had runsomewhat. This problem waseliminated by removing excess liquid developer from the surface of the paper aftereachdevelopment operation. It was found that this liquid removal could be effectively carried out by employing either a rubber squeegee, a /4 inch diameter, rubber roller which was run over the dielectric paper, or by directing a high velocity jet of air over the surface of the paper to blow excess developer from the paper surface.
"EXAMPLE 20 The schematic drawing of FIG. 8 illustrates a means for employing this invention with any plain paper. The apparatus, as shown schematically in FIG. 8, was constructed employing a 6 inch wide Mylar endless belt 30 inches in circumference. The 5 mil thick polyester film was rendered conducting on the inner surface by vacuumdepositing a layer of aluminum thereon. Drive puileys 64 were constructed of solidbrass 3 inches in diameter. A spring loaded idler pulley (Zinche's in di- 55 ameter) loaded the endless belt to maintain tautness.
T l1e p l otoc c nd uc tor coated screen, corona wire, and projector setup describe in Exariiplei tiwasemployed and mounted in a vertical direction as shown in FIG. 8; the screen to endless belt separation being is inch. A conducting development immersion tray 66 was electrically connected to drive pulleys 64 which, irrturn, ter'ssasieasrs'saiehssrar T5 kv. outin exposure, the corona wireswere connected toa potential of +15 kv. A negative image was projected on the screen and the tray 66 was filled with a liquid developer containing positively charged carbon colloidal particles. The toner image formed by sequentially exposing a portion of the Mylar ball belt to the modulated corona image followed by liquid development was offset onto a plain paper web 72 with the aid of pressure roller. 70. The image was fixed on the surface of the paper by heating with radiant heater 74. This heater consisted of a General Electric Quartzline infrared 'lamp mounted in a polished aluminum reflector. The residual image remaining on the endless polyester belt was cleaned with the soft fur brush 76, revolving at a speed of 500 rpm in contact with the surface of the belt. in operation, the drive motor is turned off andan image projected upon the photoconductor screen while the proper high voltage is applied to form a latent electrostatic charge image onthe surface of the polyester belt. The drive motors are then energized, providing an and less belt speed of 3 inches per second. The electrostatic latent image is developed as it passes through developer bath and the image is offset onto the paper by means of pressure roller 70. Image transfer may be assisted by operating pressure backing rollers 70 behind the paper at a potential positive with respect to the pressure roller behind the polyester web. After the image has been transferred to the paper, the polyester belt drive is halted and a second exposuremay be generated andthe process repeated.
' Under certain operating conditions it is found that a slight residual charge imageis present on the polyester surface. This residual image was eliminated by employing a polonium radioactiveantistatic device. The radioactive source was positioned a fraction of an inch above the web and extended across theweb'inthe region between cleaning brush,76 and the photoconductor coated screen. I j j i I n EXAMPLE 21 I Apparatus as shown in FIG. 9 was assembled. The backing plate-photoconductor coated screen-corona projection source employed in Example 18 was used in this experimental arrangement with the screen ind backing plate mounted in a vertical direction. The apparatus of FIG. 1 was modified to include one additional power supply and a means forgenerating anaerosol which would traverse across the face of a receptor sheet during an exposure,.thus'providing for simultaneously charging, exposure, and development. By this means it is possible to employ ordinary plain paper as a receptor sheet for charged aerosol particles. No further processing is required otherthan the heating of the receptor sheet to fix the toned image. A number of approaches were employed for generating a neutrally charged aerosol suitable for employment in this example.
One technique, involved the use of a Nichrome wire vmils in diameter. This wire was precoated with a film of DuPont Oil Brown 0, a dark-brown anthraquinone dye. The wire was positioned midway between the 556- toconductive screen and the receptor sheet near the bottom of. the screen. During the exposure and while potentials were applied to the corona wires and the paper conductive backing current was'passed through the nichromewire, raising its temperature just below red heat. The .anthraquinone dye rapidly volatized without decomposition from the'wire and, because of the thermal currents generated, traversed-the space between the screenand the plain paper receptor sheet. The potential of the wire, neglecting the small potential drop required to'heatthe wire, was at ground. it was found that positive ions traversing the screen in unexposed areas deposited a charge upon the aerosol. resulting in the formation of a visible print as the oil dye deposited upon the receptor sheet due to the electrostatic forces present.
In another experiment, an aerosol of dry powder was generated employing a distributor manifold having small apertures and extending across the bottom of the opening defined by the photoconductive coating screen and the plain paper backing electrode. A number of both wet and dry aerosols were employed in this apparatus, pressurized gas driving the aerosol through the distributor manifold. Carbon blacks, colored pigment particles, and both conducting and nonconducting inks were employed. The potential of the pigment particle system, as shown in FIG. 9, is controlled by power supply 83. Depending upon the triboelectric charging properties, this potential was adjusted to minimize background density.
in additional experiments, the power supply outputwas not connected directly to the conductive manifold (which was maintained at ground potential) but was connected to a bar electrode mounted immediately above and to the side of the aerosol ejection manifold. Conductiveaerosolsyejected through the distribution manifold ports, were thus charged by induction; the potential of the particles depending upon the power supply 83 potential.
EXAMPLE}? Th apparatus of FIG. 10. was'assembled. The corona modulating screen of Example 3 v was employed to modulate the corona current to a continuous receptor web 88. The corona wire assembly imea'ris formoving the screen, etc.,. were identical to that employed in previous examples. The liquid toner reservoir M was 5 inches square and contained inlet and outlet tubes ill and 92 through which a 1 percent solids content, conventional, electrostatic tonerwas circulated. A large O-ring seal 86 was cemented to the top of toner reservoir 84 to confine theliquid toner to the region immediately below web 88.
Excellent results were obtained in a step and repeat mode using a 3 mil polyester film as web 88. In the case of the polyester film, preheating and presolvent wetting was not required. Exposure times, at a screen illumination intensity of 10 ft.- candles, ranged from we to 2 seconds. I 7
T we of x le fiw mgs ifisqw the addition of a continuous belt cornfised of a fine mesh screen which was mechanically supported so as to be driven across the surface of the paper as shown in FIG. 14. The screens evaluated were formed into a continuous belt 5 inches wide a'nd20 inches long. The screen was driven by a metal cylinder drive roller 12$, 2 inches in diameter. ldler rollers 126 and 128 positioned the screen and provided support for the screen as the screen was driven through toner, l32, contained in toner reservoir 130. Samples were prepared under a variety. of conditions ranging from the case in which a screen was stationary in front of the paper during an exposure to situations in which the screen was driven at speeds to 2 inches per second past the surface of the paper during "the exposure. Both metal, phosphor bronze, and stainless steel as well as dacron and nylon screens wereevaluated; the screen mesh sizes ranging from 200 mesh to 325 .mesh. It was found that both the nonconducting and conducting scre'ens wereequally satisfactory in fon'ningimages corresponding to the image projected onto the corona modulating screen on the plain paper sheet 120. y
In operation, the corona modulating "screen was maintained at'ground potential and the plain paper'support platen'was maintained at kv. A number ofexperime'nts were carried out in which the tonercontai'ning-screen to paper spacing was varied, and no substantial difference in image quality was found over spacings from a few mils to as inchi Both conventional liquidelectrostatic toners and dry carbon powders were employed sa'tisfactorily in this apparatus. A number of finely divided carbon particles were evaluated and excellent results were'obtained by Van Dyke Corporation gold seal toner for 3,600 Copier. The light intensitiesv and exposure timeslrequired were, in most cases, similar to that of Example 18 V. i I
In the case of the metal screen, good results were obtainedwhen the screenwas operated at potentials in the region of 5 to 8 kv. Equally good results were obtained when the s'creen-roller-toner reservoir assembly was left floating, i.e., not connected directly to any potential. In this case, the screen assembly probably was stabilized to a potential near -1 0 kv due to capacitive coupling between the screen and paper support platen 10. I v Y We claim: I I I I ,I
1. In an apparatus for preparing electrostatic images conforming to an optical image on a'ch'argea'ble, image receptor surface, which includes: 7
an electrically conductive platen adapted to support an image receiving member;
a corona source adapted tospray ions on saidimage receiving member; and I a corona modulating screen disposed adjacent said image receiving "member between "said corona source and said image receiving member; the improvement which comprises providing as said corona modulating screen, a screen consisting of a plurality of strands and a photoconductive coating on at least a portion of the outer surfaces of said strands for spatially modulating the flow of ion current from said corona source to said chargeable image receiving member, said coating being asymmetrical with respect to said strands by being offset from a plane passing through said strand, and perpendicular to said screen, when saidscreenis viewed in cross sectionff I I I 2. The apparatus of claim 1- including, in addition, means to maintain said electrically conductive platen at a selected potential; a power supply electrically connected to said corona source and means for simultaneously providing-electricity to saidplatemsaidscreen and said corona source concurrently with projection-of a light image onto said image receiving member.
3. The apparatus of claim including means to project and to focus said, image on said image receiving member. I I I I 4. The apparatus of claim 1. including means to move said corona source in a plane parallel to said ion modu-f lating screen during projection of said'imageonto said image receiving member.
5. The apparatus of claim 1 including means to move both said corona source and said ion modulating screen in planes parallel to said image receiving member during projection of said image onto said image receptor surface.
6. The apparatus of claim 1 including means'to sup ply fresh array within an exposure region in order to provide for continued and repetitive operation of said apparatus. i
' 7. The apparatus of claim 1 including, in addition, means to develop a visible image on said image receptor surface. I I
8. The apparatus of claim 7 wherein said means to develop a visible image'is a means to develop a colored image. 1
9. Theapparatus of claim 1 including, in addition, means for heating said charge receiving member and for wetting said charge receiving member while it retains someof said heat prior to projection of an image thereon. v v I I 7 10. The apparatus of claim 7 including, in addition, means to transfer the visible image developed on said charge receptor surface onto a permanent record member. I 11'. The apparatus of claim 1 including in addition, a second screen positioned very closely adjacent to the surface of said image receptor member, and means to maintain said second screen at a suitable potential.
12. The apparatus of claim 1 wherein said charge receiving member isa conductive drum coated with an insulating layer. I I
13. The apparatus of claim 1 wherein said ion modulating screen is an endless belt.
14.- The apparatus of Claim 1 wherein said ion per-, meable membe'r'is an apertured plate.
ln a n appa r atu s for preparing visible images on I receptor sheet conforming to an optical image projected onto an ion permeable member having a photosensitive coating thereon which includeszanion source; t an imagereceptor sheet'disposed so as to receive ions from said source; I anion permeable member disposed between said source and-said receptor sheet and being coated at least in part with a photosensitive material, said coating being asymmetrical with respect to said member -by being offset from a plane passing through'said member and perpendicular to the plane of said member, whensaid member is viewed incro'ss'sectiom I v II II means to project an optical imageonto said ion permeablernernber; I
means for inaintaining desired electrical potentials between said. receptor sheet, said ion source and said ion permeabie member during projection of said p al'ima e; an
.m eans to develop a visible imag e on said image renter h t a 1 6 The apparatus of claim 15 whereinisaid means to develop ajyisible image includes means. for projecting an aerosol into the region between said ionrnodulating member and said image receptor vmember. simulta neously with the operation of the remainderofsaid apparatus, W I
-17. The apparatus of claim wherein the means for, projecting an aerosol is a means to project a fine cloud 25 26 of solid particles having substantially no electrostatic tive surface being on side of image receptor sheet opcharge thereon. posite the ion permeable array.
18. The apparatus of claim 16 wherein the means to project an aerosol projects a fine cloud of liquid particles having substantially no electrical charge thereon.
20. The apparatus of claim 19 wherein the conductive surface is on a roller at least partly immersed in a liquid ink bath.
19. The apparatus of claim including means to The apParatuS of chum 'P f mage support an image receptor Shaet in close proximity to ceptor sheet is contacted, on the side of said sheet opbut not in physical contact with a conductive surface Posite the Permeable member, with? liquid electrocoated with a thin film of liquid ink, the ink having a 10 Stalic eveloper.
conductivity less then 10 ohm-cm and saidconduc- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,797,926 Dated March 19, 197
Inventor(s) Richard A. Fotland and Virgil E. Straughan It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Columns 17 and 18 in Table in Example 4, sixth line down "90" should read--80-- In Examples 3, l, 5, 6 and 7 in Table I, the conductor layer should read--Al evap. perpendicular to the plane of the screen onto the side opposite the photoconductor--instead of "Al evap. 90 to normal opposite photoconductor" In Example 12 the conductor layer should read--Al evap. perpendicular to the plane of the screen on both sides to completely cover filament--1nstead of "A1 evap. 90 to normal on both sides to completely cover filament" Signed and sealed this 6th day of May 1975.
C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 'RM PO-IOSO (10-69) USCOMM'DC O37Q P69 U.S. GOVIINIIINI' PRINTING OFFICI "I! O-lOl-lll,
Patent No. 3,797,926 Dated March 19, 197 4 Inventor-(s) Richard A. Fotland and Virgil E. Straughan It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Columns 17 and 18 in Table I, in Example L Sixth line down "90" should read--80....
In Examples 3, '4, 5, 6 and 7 in Table I, the conductor layer should read--Al evap. perpendicular to the plane of the screen onto the side opposite the photoconductor--instead of "Al evap. 90 to normal opposite photoconductor" In Example 12 the conductor layer should read--Al evap. perpendicular to the plane of the screen on both sides to completely cover filament--instead of "Al evap. 90 to normal on both sides to completely cover filament" Signed and sealed this 6th day of May 1975.
C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks FORM PO-1050 (10-69) uscoMM-oc 60376-P69 ".5. GOVERNMENT PRINTING OFFICE I90! O-SiG-IM,