Electroduplication Apparatus Employing A Conductive Porous Elastic Pressure Pad For Pressing The Recording Medium Against The Photoconductor

Abstract

In an electrophotographic apparatus having a photoconductive plate illuminated by a photon image, an elastic pressure pad holds the electrographic paper against the plate. The dielectric surface of the paper overlays the photoconductive plate. A potential is applied from the photoconductive plate through the paper to the pressure pad in order to form a charge image on the dielectric surface corresponding to the photon image. The charge image is subsequently developed by conventional methods employing electrographic toner. During deposition of the charge image, the paper is pressed against the photoconductive plate by means of a conductive, resilient, porous pressure pad. The pad is a carbon filled plastic foam (approximately 53 percent carbon and 47 percent foam) mounted on a firm backing member. The resilience of the foam is approximately 29 p.s.i. per inch of compression and provides good mechanical and electrical contact between the paper and the photoconductive plate. The conductivity of the foam is approximately 3,000 ohms/square unit and provides electrical continuity during the charge image formation. The porous nature of the foam pad permits air bubbles between the pad and the paper to escape through the pad, thus eliminating undesirable voids in the final print.

Description

DESCRIPTION OF THE PRIOR ART

Heretofore, relatively rigid backing members have been employed for pressing the charge retentive surface of the electrographic paper into contact with the photoconductive plate in electrophotographic devices. One difficulty with this structure is that small airgaps or bubbles form between the photoconductive plate and the dielectric surface of the electrographic paper. These gaps need only be 0.0005 inch or greater to prevent formation of a charge image on those portions of the paper proximate the bubbles. A flexible crowned pressure pad for preventing air bubbles is disclosed in application "Electrophotographic Apparatus Employing an Elastic Pressure Pad for Pressing the Film Against the Photoconductor," Ser. No. 615,257, filed Feb. 10, 1967 by Roy Uhlenberg et al., and assigned to the present assignee, now U.S. Pat. No. 3,510,217. The crown of the pad, and special grooves provided therein, displaced the troublesome air between the paper and the pad. The required conductivity was provided by a conductive layer over the pad.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to: provide an improved web-holding mechanism in an electroduplication device; provide more effective bubble prevention means in the web-holding mechanism in an electroduplication device; provide a pressure pad in an electroduplication device which is relatively insensitive to wear; provide a pressure pad which may be easily electrically connected to the backing plate; and provide a pressure pad partially applicable in a vacuum actuated, large area electrophotographic device.

Briefly, these and other objects are achieved by providing a porous, resilient, conductive pressure pad which is urged toward a selectively conducting member or plate to sandwich the dielectric paper or recording medium therebetween. The porous nature of the pad permits air displaced during the urging to pass therethrough, thus eliminating air bubbles on the backside of the recording paper. The resilient and conductive property of the pad provides good mechanical and electrical contact to the recording paper between the plate and pad. The image to be reproduced is formed on the selectively conductive plate as a photon pattern, thermal pattern, radiation pattern, etc. The plate is conductively responsive to the image to provide a corresponding conduction pattern therethrough. A voltage is applied between the plate and the pad causing an electric field through the paper corresponding to the conduction image. The conductive property of the pad provides electrical continuity and a charge image is formed on the charge retentive surface of the recording medium. The charge image is then developed to provide a copy of the original image to be reproduced.

BRIEF DESCRIPTION OF THE DRAWING

Further objects and advantages of the porous pad and the operation of the associated electrophotographic device will become apparent from the following detailed description taken in conjunction with the drawings in which:

FIG. 1 is a schematic sectional view of an electrophotographic camera using a foam pressure pad; and

FIG. 2 is a perforated embodiment of the backing plate of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown an electrophotographic camera 10 which includes a dark box 12 having a lens 13 and shutter 14 at one end thereof. A selectively conductive member, in this case a photoconductive plate 15, is disposed in the focal plane of lens 13, at the other end of box 12. Photoconductive plate 15 is supported by a glass plate 16 with an optically transparent conductive electrode 17 sandwiched therebetween. The plate assembly, comprising glass plate 16, electrode 17 and photoconductive plate 15, is cemented at its edges to an internal rim 18 of dark box 12.

A rear housing 19 covers over the rear side of photoconductive plate 15 and is hinged to dark box 12 at 21. Rear housing 19 includes a supply roll 22 of electrographic recording paper 23, a section of which is disposed overlaying photoconductive plate 15. Electrographic paper 23 comprises a thin film of dielectric (approximately 4 microns) coated on a conductive paper backing. The dielectric film forms a charge retentive surface. Roll 22 is mounted on a shaft 24, the ends of which are supported from the sidewalls of rear housing 19.

A pressure pad assembly 25 is disposed overlaying photoconductive plate 15 and recording paper 23. Pad assembly 25 is carried from a fixed shaft 26 by a plurality of leaf springs 27. Shaft 26 is supported at its ends from rear housing 19. A cam 28 is eccentrically mounted on shaft 26 and cam lever arm 29 extends from cam 28 out of housing 19 through a slot 31 therein. When cam lever arm 29 is moved to the upper position, as indicated by the dotted lines, cam 28 pushes pressure pad assembly 25 into engagement with recording paper 23 and presses the paper against the photoconductive plate 15.

A potential supply 46 supplies a negative potential, such as -500 v., to one side of photoconductive plate 15 via lead 47 and transparent electrode 17. A timing switch 48 is connected in lead 47 for controlling the closed time of the circuit. The other terminal of supply 46 is grounded as is pad assembly 25.

In operation, the operator pulls an unexposed length of paper 23 from roll 22 past photoconductive plate 15 such that the charge retentive dielectric film layer of electrographic paper 23 is disposed overlaying and facing photoconductive plate 15. The operator then moves cam lever 29 to the upper position, indicated by dotted lines, to press pressure pad assembly 25 against paper 23 causing the charge retentive surface of paper 23 to be in nominal contact with photoconductive plate 15.

According to the presently accepted theory for charge transfer to the charge retentive surface of paper 23, it is believed that a minute airgap, on the order of a few microns, is required between photoconductive plate 15 and the charge retentive surface of paper 23. Photoconductive plate 15 and the thin dielectric charge retentive film of paper 23 have surface irregularities on the order of a few microns. These surface irregularities are believed to provide the necessary airgap even though the two surfaces are pressed together. The term "nominal contact" has been employed herein to described this contact between the two surfaces which permits the existence of a minute airgap on the order of a few microns therebetween.

Once recording paper 23 has been pressed into nominal contact with photoconductive plate 15, shutter 14 is opened and a photon image to be reproduced illuminates photoconductive plate 15. Simultaneously, timer switch 48 is activated for a certain exposure time to apply the charge transfer potential across photoconductive plate 15 and the dielectric film of recording paper 23. This applied potential causes a charge image, corresponding to the photon image to form on the charge retentive film. After the charge image has been formed, timer 48 opens the circuit and shutter 14 is closed. The operator places cam lever 29 in the lower position to release the pressure on paper 23 so that the exposed paper may be pulled from housing 19 for subsequent development by conventional electrographic toner methods such as shown in U.S. Pat. No. 3,079,890 to Oliphant and U.S. Pat. No. 2,877,133 to Mayer. Suitable methods include either those employing liquid toner or dry powdered toner.

Pressure pad assembly 25 is formed by a conductive porous pad 50 mounted on a firm conductive backing member 52. Porous pad 50 is flexible and requires backing member 52 to match the contour of photographic plate 15 which in this case is planar. Backing member 52 also serves as an electric terminal in the charging circuit for photoconductive plate 15. Pad 50 is impregnated with a conductive substance such as carbon or silver which establishes electrical continuity from backing member 52 through paper 23 to photoconductive plate 15. The conductivity of pad 50 is determined by the percentage of conducting material in the composition thereof. This composition and resistivity is not critical because of the high resistance of paper 23 and photoconductive plate 15. The resistivity of pad 50 may vary over a wide range. The particular porous substance employed in the preferred embodiment is a carbon loaded plastic foam (47 percent vinyl or urethane foam and 53 percent carbon) having a resistivity of about 3,000 ohms per square unit. The thickness and resiliency of pad 50 is also not critical. The preferred embodiment employs a quarter-inch pad having a resiliency of about 0.29 p.s.i. per 10-mil deflection. A softer pad having a lower resiliency may be employed where only limited pressure forces are available or where the pad area is large. The porous or open-celled structure of pad 50 permits air in the pad and between the pad and paper 23 to be displaced through the four openside edges 54 of pad 50. This passage of air eliminates undesirable air bubbles on paper 23 and subsequent voids on the final print.

Referring now to FIG. 2, there is shown a modified backing member 60 having perforations 62 for further assisting air passage through pad 50 in applications involving high pressures or large pad areas. When pad 50 is pressed under high pressures, the pores or cells partially close inhibiting somewhat the airflow and speed of air displacement. This constriction is more noticeable in large area pads. Perforations 62 provide passages for the displaced air in addition to the passage through the sides 54 of pad 50. Such high pressures are easily established and maintained in vacuum pressure systems wherein the pad housing is partially evacuated and atmospheric pressures force contact between the web, the plate and backing member. These vacuum established pressures are normally sufficient to easily operate large area pads, such as 22 by 34 inches. The pad may also be thicker, such as three-eighths inch or greater in these vacuum applications. The thicker pads are even more insensitive to wear and are less constricting to airflow under high pad pressures. If desired, the back plates for the larger pads may be bowed slightly in one or more planes to iron out or prevent wrinkles in paper 23 as it is pressed between pad 50 and photoconductive plate 15. In the embodiment of FIG. 1, pad 50 was bowed approximately 15 mils along the 10 inch longitudinal dimension.

Clearly, various changes may be made in the structure and embodiments shown herein without departing from the concept of the present invention. For example, other mediums besides light, such as heat or pressure, may be used to form the image to be projected onto plate 15. Conductive plate 15 would have to be selectively conductive in response to the particular image medium employed. The pad 50 may be any porous conductive resilient substance such as metal-filled sponge rubber.

Although the pressure pad feature of the present invention has been described, for simplicity of explanation, as it is employed in an electrophotographic camera 1, it is equally applicable to other electrophotographic devices such as, for example, microfilm printers and radiographic cameras.

Claims

What is claimed is:

1. In an electroduplication device for reproducing a charge image on the charge retentive surface of a recording medium, the combination comprising:

imaging means for providing the image which is to be reproduced;

a selectively conductive member responsive to the image for providing a conductive pattern therethrough corresponding to the image;

a porous, resilient member impregnated with a conductive material and positioned proximate to the selectively conductive member with the recording medium therebetween;

means for urging the conductor impregnated porous member and the selectively conductive member closer together to establish electrical continuity from the selectively conductive member through the recording medium to the porous member, the porous nature of the porous member permitting air displaced during the urging to pass therethrough; and

voltage means in electrical communication with the selectively conductive member and the porous member for establishing an electric field thereacross through the recording medium for causing a charge image to form on the charge retentive surface of the recording medium corresponding to the conductive image on the selectively conductive member.

2. The device of claim 1, wherein the selectively conductive member is photoconductive.

3. The device of claim 1, wherein the porous member is a plastic foam substance selected from the group consisting of vinyl and urethane impregnated with a conducting material.

4. The device of claim 1, wherein the porous member is plastic foam impregnated with carbon particles.

5. The device of claim 1, wherein the means for urging includes a firm backing member upon which the porous member is mounted.

6. The device of claim 5, wherein the firm backing member is perforated to permit additional air open-celled through the porous member.

7. The device of claim 5, wherein the firm backing member is conductive and is in electrical contact with the conductor impregnated porous member to facilitate connection of the voltage means to the porous member.

8. The device of claim 1, wherein the conductor impregnated porous member is formed of an open plastic foam substance for facilitating the displacement of air from within the porous member as the porous member is urged toward the selectively conductive member to contact the recording medium.