US3843250A

US3843250A - Imaging system employing ion-permeable control member

Info

Publication number: US3843250A
Application number: US00279205A
Authority: US
Inventors: R Fotland; V Straughan
Original assignee: Horizons Inc
Current assignee: Horizons Inc
Priority date: 1971-08-27
Filing date: 1972-08-14
Publication date: 1974-10-22
Anticipated expiration: 1991-10-22

Abstract

In the formation of visible copies of an optical image, the use of a conductive ion permeable member coated with a photoconductor which is employed to form an ion image corresponding to an optical image focused upon said ion permeable member. This ion image is caused to impinge upon a fine mesh screen which has been coated with neutral electrostatic toner particles, thereby forming a charged developer particle image and this charged developer particle image is then electrostatically attracted onto the surface of a plain paper receptor sheet.

Description

United States Patent 1191 Fotland et al.

1 1 IMAGING SYSTEM EMPLOYING ION-PERMEABLE CONTROL MEMBER Inventors: Richard A. Fotland, Warrensville Heights; Virgil E. Straughan, Euclid, both of Ohio Horizons Incorporated, a Division of Horisons Research Incorporated, Cleveland, Ohio Filed: Aug. 14, 1972 Appl. No.: 279,205

Related US. Application Data Continuation-impart of Ser. Nos. 178,521, Aug. 27, 1971, Pat. No. 3,797,926, and Ser. No, 275,674, Aug. 2, 1972, Pat. No. 3,761,173,

Assignee:

US. Cl. 355/3 DD, 96/1.3, 118/637 Int. Cl..; 603g 15/00 Field of Search 355/3 R, 16, 3 DD, 17;

96/1 R, l, 3; 346/74 ES; 118/637 References Cited UNITED STATES PATENTS York 118/637 3,610,205 10/1971 Rarey et a1. 118/637 3,613,638 10/1971 Solarek 118/637 3,645,614 2/1972 McFarlane et a1 355/3 3,680,954 8/1972 Frank 355/3 FOREIGN PATENTS OR APPLICATIONS 1,156,308 10/1963 Germany 96/1 R OTHER PUBLICATIONS Defensive Publication, T890,003, J. Y. Kaukeinen, Sept. 1971, Latitude in Photocond. Controlled Corona Charging.

Primary Examiner-Robert P. Greiner Attorney, Agent, or Firm-Lawrence I. Field [5 7] ABSTRACT In the formation of visible copies of an optical image, the use of a conductive ion permeable member coated with a photoconductor which is employed to form an ion image corresponding to an optical image focused upon said ion permeable member. This ion image is caused to impinge upon a fine mesh screen which has been coated with neutral electrostatic toner particles, thereby forming a charged developer particle image and this charged developer particle image is then electrostatically attracted onto the surface of a plain paper receptor sheet.

5 Claims, 7 Drawing Figures v r 1 I IMAGING SYSTEM EMPLOYING ION-PERMEABLE CONTROL MEMBER more particularly to a method and apparatus for the efficient formation of electrostatically developed ion current patterns corresponding to an optical image.

In conventional plain paper electrostatic photography, an insulating photoconductor is charged with a corona source of ions, exposed, the charge image developed with a toner, the developed toner image transferred to plain paper, and finally, the toned image is fixed, usually 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 fora number of different machine operations. In addition, the photoconductor suffers wear over a period of time, since the surface of the photoconductor 'is' repeatedly rubbed by toner particles, cleaning brushes and paper surfaces.

' A related process employs a photoconductively coated conducting paper. The photoconductor, usually zinc oxide (although organic photoconductors may be employed), is first charged, then exposed, and the charge image is then toned. Here the photoconductor is not reusable and thus the wear and tear restrictions in the aforementioned process are eliminated. In addition, the machine operation is simplified. One disadvantage of this process is 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 pounds per 3,000 ft ream), the papers are heavy and have a feel quite different from plain paper. I

A principal object of the present invention is to simplify the conventional plain paper electrophotographic process and the apparatus by which it is carried out.'

Another object of the invention is to provide an image reproduction method wherein there is no physical contact of the photoconductor with either developer or paper.

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 paper coated with a photoconductor. In comparison therefore to electrostatic copy processes employing photoconductive paper, the process of this inventionhas the advantage of lower paper cost and the advantage of a capability for employing plain (nonchargeable) paper.

Another object of the invention is to provide an image copying means for generating full color copy.

In the present invention, a fine mesh screen or grid uniquely coated with a photoconductor is employed to spatially modulate the flow of corona current in accordance with an optical image projected onto said fine mesh screen or grid.

preparing electrostatic images on an image receptor The ion-permeable array which serves to modulate the flow of ions in accordance with an optical image is described in detail in co-pending United States Application Ser. No. 178,52l filed Aug. 27, 1971, of which the present application is a continuation in part and in United States Application Ser. No. 275,674 filed Aug. 2, 1972.

The method. of utilizing the photoconductor coated screen for the formation of visible images in the practice of the present invention 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 charge images corresponding to an optical image projected upon an image receptive surface.

FIG. 2 and 3 illustrate various 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. FIG. 2 employs a simultaneous charging and developing operation using a liquid or dry aerosol, while FIG. 3 employs liquid development, the charging anddevelopment once again being carried out simultaneously.

FIGS. 4, 5 and 6 schematically depict modifications of the device of FIG. 1 by means permitting simultaneously exposing, charging, and developing a visible image upon plain paper, i.e., non-chargeable members, and utilizing an ion current modulating screen.

FIG. 7 is a schematic illustrating a means for generating full color copies of an original in a manner which eliminates registration problems.

Referring now to FIG. 1, illustrating an apparatus for surface, the apparatus comprisesan 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 an optical image projected onto elementl6. 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 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 office copy, or any other suitable source of optical image depending upon the application of the apparatus. It should also be understood that, although the examples herein refer to an optical (light) exposure, the input to the screen may consist of other forms of energy to which the photoconductor employed exhibits sensitivity. These other radiations include x-rays, gamma rays, and alpha and beta particles.

A single corona wire 18 is shown in 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 20. 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 1 mil. A preferable wire diameter for this embodiment is 2 mils. Using a single corona wire spaced approximately 1 inch above modulating screen 16, uniform charging, in accordance with the projected optical image, of the dielectric paper occurs over an area equal to the length of the corona wire and a distance between 1 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 a variety 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 number 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 mechanical features being well known to those skilled in the art. FIG. 1 shows the corona modulating screen maintained at ground potential. In this event, the potential on the corona wire and backing plate must be opposite in polarity. Thus, ifthe 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 16. Alternately, the backing plate 10 might be maintained at ground potential, screen 16 ata 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 applied potential of 10 kv at a one-half inch spacing or 1 kv at a 50 mil spacing. At this electric field the corona current passing through screen 16 and onto the chargeable member follows the field line sufficiently well so that a resolution of 6 to 10 line-pairs/mm is readily obtained with screens having from 240 to 400 meshes per inch. At electric fields in the range of 50 to 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 photoconductor, 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 5 to 50 foot-candles of tunsten illumination and the exposure times range from 0.1 to 3 seconds.

Although a preferred means of carrying out the teachings of the present invention, as shown in FIG. 1, is superficially similar to the apparatus described in Snelling US. Pat. No. 3,220,324, a number of very important differences exist. The disclosure in Snelling requires that the optical image incident upon the corona modulating screen be placed on the side of the screen opposite the source of corona. Thus, in a majority of Snellings examples, the optical image is presented to the screen either by transmission of said image through a transparent or translucent recording member or by) reflection from the surface of the recording member onto the side of the screen opposite the corona charging member. In the present invention, the optical image is projected onto the screen, on the same side of the screen as the source of ion current. It makes no difference whether the recording member is of high reflectivity or not, and at the large spacings employed with the present invention the influence of any reflected light would be to degrade resolution because of this relatively large spacing between the recording member and the corona modulating member. In addition, the teachings of Snelling indicate a requirement for employing low potentials, on the order of 100 to volts, between the recording member conducting backing and the ion current modulating screen. The spacings between these members are also indicated as being relatively small (one-sixteenth inch' or less) in order to preserve high resolution in the electrostatic latent image. Snelling further indicates that hisinvention appears to function because of an ion field component buildup between the ion modulating screen and recording member. In the present invention, the screen serves to modulate the flow of ions through the screen independently of any change in electric field between the screen and the recording member. This modulation permits high potentials and large spacings to be employed between the ion modulating member and the recording member.

British Patent Specification Nos. 1,149,901 and 1,152,308 which appear to correspond to US. Pat. No. 3,680,954 also describe the use of an ion-permeable screen with an optical image projected thereon to con-' trol the deposition of ions onto a chargeable recording member. Significant differences also exist between the inventions of these specifications and the method of employing the photoconductive coated screen which is the subject of the present invention. For example, in the apparatus described in these British specifications, coverage of the conductive grid with a photoconductive material is required to be complete, even microscopic cracks in the coating are said to be detrimental to the operation of the apparatus. For this reason, the British specifications indicate that fine woven wire mesh is not satisfactory as a support material for the photoconductive coating because of difficulties observed in completely covering the wires where they cross over each other. In the invention described herein, a fine woven wire mesh is one preferred conducting photoconductive support and with the asymmetrically coated screens of the present invention, actual bare screen is usually present.

The photoconductively coated screen in the British specification is operated in such a position as to be quite closely adjacent to the charge supporting recording member, it being indicated that spacing is not critical if it is within one hole diameter, and elsewhere that the spacing may be 0.1 mm or less.

In contrast, in the practice of the present invention, spacings of distances as great as 1 inch may be used, provided high potentials are maintained between the screen and the latest image receptor sheet conductive backing. In the description in the British specification it is indicated that it is preferable to employ potentials such that the conductive screen and the image receptor sheet conducting backing are at the same potential or even back biasing, i.e., maintaining the recording member potential at some value between the potential of the photoconductively coated screen and the corona source. It is further stated that the potential on the image receptor surface is limited to the maximum potential which may be sustained across the photoconductor. In British Patent Specification No. 1,152,308, provision is made for spacing the screen further away from the latent ion image recording member. Here, however, a second conducting screen must be interposed between the photoconductive screen and the recording member in such a manner that the second conducting grid is spaced very closely to the photoconductively coated screen. The closely spaced conducting screen is again back biased with respect to the photoconductively coated screen. From this discussion it will be apparent that the present invention is significantly different from the prior art taught in the British specifications.

Recently issued U.S. PATS. Nos. of Burdidge (3,582,206). McFarlin et al. (3,645,614) and Pressman et al. (3,647,29l all disclose screens containing an insulating layer for controlling the flow of ions or charged particles through the screen. In each of these patents, however, the method of operation involves first charging the screen on one side, next exposing the charged screen to an optical image, and finally projecting ions or charged particles through the screen but directed from-the opposite side. Thus, the method of employing the screen in these patents differs significantly from the practices of the present invention.

FIG. 2 is a schematic drawing of an apparatus for simultaneously charging, exposing and developing. The apparatus shown in FIG. 2 may be the apparatus in FIG. 1 with the addition thereto of means for injecting an aerosol into the region between corona modulating screen 16 and a paper image receptor sheet 81. The

image receptor sheet 81 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 orderto 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 a power supply 83, as shown in FIG. 2. Or else, 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 pass through 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. 2 involves the use of an air gun type atomizer 80, the invention is not restricted to this generation technique. Other means of forming a jet include directly atomizing a liquid through fine jets or thermally vaporizing a material to form an aerosol cloud.

In addition to using either a liquid aerosol or a thermally vaporized dyestuff, aerosol development, employing 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 Dessauer and Clark Xerography and Related Processes, 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 maybe successfully vaporized from a hot surface to form a uniform aerosol cloud include Brilliant Oil Blue, Oil Brown 0, and Oil Brown N.

FIG. 3 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 which is continuously recirculated 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 one-fourth inch and l inch above the surface of the paper. The corona source, power supplies, and illumination source are similar to those shown in FIG. I 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. 3.

If conventional liquid electrostatic toners of the type employed in zinc oxide paper machines are utilized in the apparatus of FIG. 3, 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 & 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 little, 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 foot-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, 21 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 89 containing the solvent. Since the web 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/cm 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 ucouL/cm Ifthe 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 kv are required with kv resulting in higher resolution images having less distortion. It has further been found that the developed due to the low capacity per unit area of paper compared to the thin dielectric coatings utilized with dielectric coated paper.

FIG. 4 illustrates yet another apparatus employing a photoconductive coated screen together with development apparatus to generate a visible image employing plain paper. Inthis figure, 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 FIG. 1, and is electrically connected to power supply 21. Roller 94 revolves, the lower surface passing into tray 95 containing an ink 96 dispersed or dissolved in a polar liquid. During operation, roller 94 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 94. Since this is a dynamic process, means are provided for moving the image from left to right across screen 16 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 water base inks consisting of colloidal carbon dispersions, opaque dye pigments, or dissolved acid or basic dyes. For 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 5 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 humidity 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 printing gap.

FIG. 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 transversing 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. 4. The motion of the endless belt and paper may be continuous; in which case the image to be reproduced must be scanned across screen 16 to match velocity of paper web 88, or the machine may operate in a step and repeat mode; the paper and endless belt advancing between successive exposures. Either a conducting or nonconducting toner may be employed with this apparatus; the toner being transferred from the endless belt to the underside of the paper web by 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. 4 are pertinent for this apparatus.

FIG. 6 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 120 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. 1. 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 operation, 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 toners. The toners are not, in general, electrostatically held onto the mesh screen 122 but are collected mechanically. It has been found advantageous, in stances where low toner pickup on the open mesh endless 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 one thirty-second inch to one-half inch per second.

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 with 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 difficulties by 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 56 select the appropriate yellow, cyan or magenta liquid toners which are contained in gravity fed reservoirs 58. The liquid developed, after passing across the surface of the charge receptor sheet, is collected in reservoir 60 and discarded or else reclaimed for further use.

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 16. 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 passing over the inclined surface of the charge receptor sheet develops the yellow components of the image. This process is then successively repeated with a red color filter using a cyan toner and the green color filter employing a magenta toner. Effective liquid electrostatic colored toners for use in this apparatus are manufactured by the Day-Glo Corporation (Cleveland, 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 be removed 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.

The latent electrostatic image formed by the corona modulation screen may also be employed in recording crystal films in display applications. Here, the dielectric coated paper of FlG. l is replaced 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 crystal 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 Pats. Nos. 1,123,117 and 1,167,486, and also by L. Melamed and D. Rubin, Appli. Phys. Lett. 16, 4, 149 (1970) and by J. .l. Wysocki, .1. Adams, and W. Haas, Phys. Rev. Lett. 20, 19, 1024 (1968).

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 weave 400 mesh stainless steel screen was stretched over a square brass frame whose inside dimension was 6 inches on a side and whose outside dimension was 7 inches. The screen was soft soldered onto the frame. The frame was mounted in a vacuum coated an average 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 high purity selenium was placed in the evaporation crucible. The system was evacuated to a pressure of 10' torr-and the selenium evaporated from the boat onto the screen over a period of 45 minutes. During the I evaporation, the screen was heated, with an electrical heater, to a temperature of 90C. The selenium coating thickness was found to be 20 microns.

The screen was removed from the vacuum evaporator and mounted in'the apparatus shown in FIG. 1. A 6 inch corona wire comprised of a 2 mil thick diameter platinum wire was supported a distance of 1 inch above the screen. The screen to conducting platen spacing was one-half inch.

The contrast ratio, defined here as the ratio between the ion current to the conductive backing plate with the photoconductive screen in the dark and ion current with the same screen illuminated, was determined by connecting a Keithley Model 600A electrometer between an electrically isolated portion of paper supporting electrode 10 and power supply 21. At a counterelectrode potential of 5 kv and a corona potential of +16 kv, the dark current was 24 tamperes and the current obtained when the screen was uniformly illuminated with tungsten illumination at a level of 10 footcandles was 0.3 uamperes. The contrast ratio was thus 80. At a corona potential of +12 kv, the dark current was 1 1 uamperes and the light current was 0.15 uamperes; yielding a contrast ratio of 75.

It may beseen from the aforementioned measurements that a higher contrast potential is obtained at lower corona potentials. In this event, however, the corona current is lower, and longer exposure times are required to charge the dielectric paper. At a screen-paper separation of one-half inch, an applied potential of 3 kv is sufficient to accelerate the ions to the surface of a dielectric coated paper and still maintain a resolution of 6 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-limit brightness of 10 foot candles was projected on the screen with a simultaneous application of corona and counterelectrode potentials; the total exposure time being 2 seconds. The paper was then removed from the'counterelectrode and immersed in a beaker of liquid electrostatic toner having a solids concentration of 1 percent. Since the paper surface was charged positively and since the liquid developer toner particles are also positively charged, a reversal image was obtained. When a positive line copy image was projected on the screen and the resulting latent electrostatic image developed in liquid toner with negatively charged particles, then the developed image was a positive image, i.e., black characters on a white background as exhibited bythe positive original. After the paper was removed from the developer, excess liquid was squeegeed from the surface and the paper dried in an air stream which may be heated. The image was of high quality, having negligible background and a maximum density of 1.1. The development time was 3 seconds.

EXAMPLE 2 Apparatus as shown in FIG. 2 was assembled. The backing plate-photoconductor coated screen-corona projection source, shown in FIG. 1, was used in this example with the screen and backing plate mounted in a vertical direction.

A stainless steel 400 mesh screen was coated with a 20 micron layer of selenium by vacuum vapor deposition. During the coating operation, the screen temperature was maintained at C and the screen was angled such that the coating was deposited upon thescreen at an angle of 45 to the normal.

The apparatus of FIG. 1 was further modified to include one additional power supply and a means for generating an aerosol 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 other than 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. a

One technique involved the use of a nichrome wire 15 mils in diameter. This wire was precoated with a film of DuPont Oil Brown 0, adark-brown anthraquinone dye. The wire was positioned midway between the photoconductive 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 nichrome wire, 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 screen and the plain paper receptor sheet. The potential of the wire, neglecting the small potential drop required to heat the 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. I

In another modification, 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. 2, 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 output was 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. Conductive aerosols, ejected through the distribution manifold ports, were thus charged by induction; the potential of the particles depending upon the power supply 83 potential.

EXAMPLE 3 The apparatus of FIG. 3 was assembled. The corona modulating screen 16 was employed to modulate the corona current to a continuous receptor web 88. The corona wire assembly, means for moving the screen, etc., were identical to that employed in previous examples. The liquid toner reservoir 84 was 5 inches square and contained inlet and

outlet tubes

91 and 92 through which a 1 percent solids content, conventional, electrostatic toner was circulated. A large O-ring seal 86 was cemented to the top of toner reservoir 84 to confine the liquid 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 foot-candles, ranged from one-half to 2 seconds.

EXAMPLE 4 The apparatus as shown in FIG. 4 was assembled. The photoconductive screen of Example 1 was employed in this modification. A single corona wire, 4 inches in length and 2 mils in diameter, was positioned l inch above the photoconductive coated screen 16. The photoconductively coated side of the screen was positioned up. The spacing between photoconductive screen 16 and plain tape web 98 was one-fourth inch. A solid brass roller 94, 2 inches in diameter, was employed to carry ink to a position immediately adjacent but not touching the paper web. This roller 94 revolved through a trough of ink which was formulated by dissolving 2 percent (by weight) crystal violet dye in water. The apparatus was adjusted so that a gap of 10 mils was effected between the surface of the ink covered roller and the bottom of the paper web 98. In operation, the paper web was run at speeds of one-fourth to 3 inches per second while the brass roller was revolved so that the peripheral speed of the brass roller was identical to the velocity of the paper web. At the same time, a positive optical image was projected onto the screen and caused to move across the screen at the same velocity and in the same direction as the paper. Thus, there was no relative motion between the optical image and the paper. During operation, the brass roller 94 was maintained at ground potential, the photoconductively coated screen 16 was maintained at +5 kv, and the corona wire was operated at a potential of +20 kv.

At an unilluminated region of the screen, corona current was transmitted to the top surface of the plain paper and the intense local field caused the ink to jump from the roller onto the back surface of the paper. Effective printing operation was obtained at paper web velocities up to I inch per second.

EXAMPLE 5 In this example the apparatus of FIG. 4 was again employed. One modification involved replacing the aqueous ink developer with a commercially available electrostatic toner in this case a Hunt Chemical Corporation reversal electrostatic toner. This toner was diluted, over the recommended concentration generally employed in electrostatic development, by mixing 1 part reversal toner to 8 parts of lsopar G, a hydrocarbon solvent manufactured by Humble Oil & Refining Company. A second difference from the experiments of Example 4 involved reducing the paper to roller spacing so that a liquid developer meniscus was formed between the brass roller and the bottom of the paper web. Again, during operation, developed images in unexposed areas were obtained at web velocities approaching 2 inches per second. It was found possible in this example to run the brass roller at angular velocities such that the peripheral speed of the roller was several times the paper velocity.

' EXAMPLE 6 The apparatus of FIG. 5 was assembled. The same screen, corona wire, applied potentials, optical scanning system, etc. that were employed in Example 4 were utilized. Two brass rollers were employed to support a 400 mesh stainless steel screen formed into a continuous belt. During operation, the spacing between the plain paper web 88 and the surface of screen 108 was maintained at a distance of approximately 20 mils. Dry electrostatic toner powder 114 was continuously supplied onto the endless belt from a hopper spaced directly above the screen over one of the brass rollers. During operation, positive images were obtained at web velocities up to 2 inches per second. It was found in that the dry toner would sometimes fall off the web after the web had passed through the toner deposition region. This was minimized by preheating the paper, using a blast of hot air from a hot air gun to a temperature of approximately C. This served the function of partially fusing the toner to the paper surface almost immediately after the toner was deposited upon the surface.

EXAMPLE 7 The apparatus of FIG. 2 was modified with the addition of a continuous belt composed of a fine mesh screen which was mechanically supported so as to be driven across the surface of the paper as shown in FIG. 6. The screens evaluated were formed into a continuous belt 5 inches wide and 20 inches long. The screen was driven by a metal cylinder drive roller 124, 2 inches in diameter.

Idler rollers

126 and 128 positioned the screen and provided support for the screen as the screen was driven through toner 132 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 were evaluated; the screen mesh sizes ranging from 200 mesh to 325 mesh. it was found that both the nonconducting and conducting screens were equally satisfactory informing images corresponding to the image projected onto the corona modulating screen on the plain paper sheet 120.

In operation, the corona modulating screen was maintained at ground potential and the plain paper support platen was maintained at l kv. A number of experiments were carried out in which the tonercontaining-screen to paper spacing was varied, and no substantial difference in image quality was found over spacings from a few mils to one-eighth inch.

Both conventional liquid electrostatic toners and dry carbon powders were employed satisfactorily in this apparatus. A number of finely divided carbon particles were evaluated and excellent results were obtained using Van Dyke Corporation gold seal toner. The light intensities and exposure times required were, in most cases, similar to that of Example 5.

in the case of the metal screen, good results were obtained when the screen was operated at potentials in the region of to 8 kv. Equally good results were obtained when the screen-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 -l0 kv due to capacitive coupling between the screen and paper support platen 10.

EXAMPLE 8 The apparatus of FIG. 1 was modified by tilting the whole apparatus at an angle of 30 to the horizontal and addinga developer manifold, solenoid operated valves and developer supply tanks as shown in FIG. 7. In addition, provisions were made for placing color filters in front of the slide projector 22. The developer manifold consisted of a /4 inch diameter copper tube (6 inches long) sealed at one end. Holes 0.020 inch in diameter were drilled in a line along the distribution manifold at a spacing of one-fourth inch. The open end of the copper distribution manifold was connected to electrically operated solenoid valves 56 with suitable unions. Each of the three solenoid valves were connected in turn to a liquid developer reseroir. A collection pan 60 was provided to collect the liquid developer after it had flowed over the surface of the paper being developed.

A full 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 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 foot-candles was incident upon the screen and the image exposed with the potentials applied to 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 highlight intensity of 20 foot-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 having a good color balance and a high degree of registration.

in several runs it was found that the charged colors had run somewhat. This problem was eliminated by removing excess liquid developer from the surface of the paper after each development operation. It was found that this liquid removal could be effectively carried out by employing either a rubber squeegee, a /1 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.

While but a limited number of embodiments of the present invention have been here' disclosed, it will be apparent that many variations may be made therein without departing from the spirit of the invention as defined in the following claims.

We claim:

1. In an apparatus for preparing visible images on a receptor sheet conforming to and simultaneous with an optical image projected onto an ion-permeable member having a photosensitive coating thereon which includes:

on ion source;

a photosensitive coated ion-permeable member;

an image receptorsheet;

optical image projection means;

an open mesh web adapted to transport neutral electrostatic toner particles, said web being positioned between said image receptor sheet andsaid ionpermeable member; and

means for supplying electrical potential between said ion source and said ion-permeable member, and means for supplying electrical potential between said ion-permeable member and said image receptor sheet during exposure said electrical potentials serving respectively (1) to direct ions which traverse the ion-permeable member onto the toner laden web and (2) to direct toner particles onto said image receptor sheet.

2. The apparatus of claim 1 includes a toner supply through which said open mesh web is passed and then caused to move past the surface of said image receptor member.

3. The apparatus of claim 2 wherein said open mesh web is electrically conductive.

4. The apparatus of claim 2 wherein said open mesh web consists of a woven screen of synthetic polymer filaments.

5. The apparatus of claim 2 wherein said toner conthereon.

Claims

1. In an apparatus for preparing visible images on a receptor sheet conforming to and simultaneous with an optical image projected onto an ion-permeable member having a photosensitive coating thereon which includes: on ion source; a photosensitive coated ion-permeable member; an image receptor sheet; optical image projection means; an open mesh web adapted to transport neutral electrostatic toner particles, said web being positioned between said image receptor sheet and said ion-permeable member; and means for supplying electrical potential between said ion source and said ion-permeable member, and means for supplying electrical potential between said ion-permeable member and said image receptor sheet during exposure said electrical potentials serving respectively (1) to direct ions which traverse the ionpermeable member onto the toner laden web and (2) to direct toner particles onto said image receptor sheet.

5. The apparatus of claim 2 wherein said toner consists of finely divided carbon particles having no charge thereon.