US3885870A

US3885870A - Device for discharging charge accumulated between the recording medium and a photoconductive plate in an electrophotographic recorder

Info

Publication number: US3885870A
Application number: US412238A
Authority: US
Inventors: Raymond L Levy; Gene F Day
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1973-11-02
Filing date: 1973-11-02
Publication date: 1975-05-27
Anticipated expiration: 1992-05-27
Also published as: GB1483632A; DE2450130A1; CA1025929A; AU7482974A; JPS5085342A

Abstract

The CdS photoconductor in an electrophotographic recorder builds up a net negative charge during light exposure and charge transfer. This built-up charge induces a positive countercharge in the conductive base of recording medium. These charges must be neutralized before the recording web is completely separated from the photoconductor or else arcing may occur between the plate and the web. The arcing produces spurious charge transfer, causing undesirable dark areas on the final copy. After the exposure period, the charge is neutralized by disconnecting the photoconductor from the charging voltage and placing it in electrical communication with the conductive backing of the web via a conductive drive roller which engages the conductive backing of the web.

Description

[ 51 May 27,-1975 1 DEVICE FOR DlSCI-IARGING CHARGE ACCUMULATED BETWEEN THE RECORDING MEDIUM AND A Primary ExaminerRobert P. Greiner Attorney, Agent, or Firm-Stanley Z. Cole; Edward J. Radlo PHOTOCONDUCTIVE PLATE IN AN ELECTROPHOTOGRAPHIC RECORDER ct.e. .S hn l ua h fimwbnre Phebta a g s rg eea nawnnr k o.n e ur cc .md ubyyod p pd la UnSmmn l w l 22w t .lCm C u n.l l e mi d m om T nw w w C m nm mmm m m w .m R mtw s mb H m mwmgrw Bammflrmwe t e u 0 e n g u ddf OpI .1 n .nS m m w w m t ficw meoen O .l t h w .m t1mle b m mwpww fi m mpm hh e a ht mS%%b n newm rdmwu S xuf mwo TfedO.EfI fl 6 w i M .w B My v m n w 1 g .m t 0 a0 t 30 t l M A at 0 0 3m Mm m m A P m. W 5, NM 80 w 5" LL .m m a C 3 n L m 7 "u D S 9 m n A .l. 8 m 0F n 3 mm mefi .m n" ynuu r V 2 "u aea a 0 1 RGC V N 4 m s m w N .L h t n I C m N .c v P m A F A UI. .1 1]] 1:1 5 3 2 1 21. 7 7 2 2 55 .1 [[1 [.l.

on the final copy. After the exposure period, the charge is neutralized by disconnecting the photocon- [58] Field of Search....-.............. 355/3 R, l4, l6, 17;

96/] R ductor from the charging voltage and placing it in electrical communication with the conductive backing of the web via a conductive drive roller which engages the conductive backing of the web.

[56]. References Cited UNITED STATES PATENTS Goffe 3,185,051 5/1965 355/3 R 3,322,539 5/1967 Redington......................... 355/17 X 9 ClalmS, 3 Drawing Figures PATENTED HAYZY I975 SHEET DEVICE FOR DISCHARGING CHARGE ACCUMULATED BETWEEN THE RECORDING MEDIUM AND A PHOTOCONDUCTIVE PLATE IN AN ELECTROPI-IOTOGRAPHIC RECORDER FIELD OF THE INVENTION This invention relates to the simultaneous charge transfer process of electrophotography and more par ticularly, to this process wherein undesirable charges developed during charge transfer are eliminated.

PRIOR ART Heretofore, Se has been commonly used as the photoconductor in toned image transfer electrophotography. In this process, a charge pattern is produced on the surface of a previously charged Se layer by exposure to a light image. Then, in subsequent steps, the drum is toned and the toned image is transferred to copy paper. The Se layer must be able to retain its surface charge until after the toning step is complete. This charge retention is made possible by a blocking contact or Schottky barrier that exists between the Se and the area electrode immediately under the Se. The blocking contact is caused by an atomic property common along semiconductors which is exhibited between the Se and the substrate. No current can flow between the Se and the area electrode in this type of xerographic plate.

CdS, however, exhibits an injecting or ohmic contact with the area electrode on certain substrates. Charging current flows freely therebetween. As a consequence, CdS has a low charge retention and is especially suited for simultaneous charge transfer electrophotography. Here the CdS and the web are in contact during exposure, and current flows from the CdS plate to the dielectric coating of the web simultaneously as the CdS is exposed to the light image. Current flow during exposure produces photoconductive gain amplification of the image.

In addition to the ohmic contact, CdS is also characterized by internal electron traps caused by bulk defects, certain dopants, and surface effects. During exposure, electrons fall into these traps and the CdS builds up a fixed net negative charge which induces a positive mobile charge in the conductive backing of the web. After the exposure step, as the web is being separated from the CdS plate, arcing may occur between the CdS and the web causing spurious charge transfer .and dark areas in the finished copy. In order to prevent these dark areas, it is necessary to provide means for bleeding off this mobile positive charge from the web backing during the separation step.

In the Se toned image transfer process of electrophotography, it is necessary that the photoconductor substrate form ablocking contact in order to prevent neutralization of the charge image and allow for good image contrast. It is also desirable that there be very few internal electron or hole traps in the photoconductor material. It is, in fact, preferable that there be no such traps at all in the toned image transfer process. Such internal traps tend to inhibit charge flow during image exposure and also serve to reduce image contrast. In CdS simultaneous charge transfer electrophotography, the existence of an injecting or ohmic contact actually introduces the necessity for internal traps in the photoconductor. If these internal traps did not exist, chargebwould flow to the charge retentive web very readily, even in the dark, and no image could be produced at all. There is actually an optimum number of such traps. If there are too few traps, the image contrast is poor. If there are too many, the sensitivity of the process is reduced. In practical applications, this last effect also reduces the available contrast. Thus, two characteristics which would be destructive in Se toned image transfer electrophotography combine in CdS simultaneous charge transfer electrophotography to produce a highly sensitive process with good contrast. Unfortunately, the injecting contact and the bulk trapping in the CdS photoconductor cause the buildup of trapped space charge in the photoconductor together with a corresponding mobile countercharge in the conductive backing of the web. If no means is provided for allowing neutralization of these two charge layers with each other during separation of the web from the photoconductor, they can produce spurious arcing and charge transfer, resulting in defects on the finished copy.

SUMMARY OF THE INVENTION It is therefore an object of this invention to prevent spurious charge transfer between the CdS and the recording web in the simultaneous charge process of electrophotography.

It is another object of this invention to prevent charge build-up on the CdS to the point of arcing to the recording web in the simultaneous charge transfer process.

It is still another object of this invention to provide for eliminating the excess net charge built-up during the simultaneous charge transfer process of electrophotography.

Briefly, these and other objects are achieved by providing an electrophotographic device having a photoconductive member which is exposed to light or selectively illuminated to form a charge image and providing a recording web for receiving the charge image formed by the photoconductive member. A resilient conductive pressure pad is mounted proximate the photoconductive plate to retain the recording web therebetween during the exposure period. Simultaneously, a voltage is applied across the photoconductive member and the web. The charge image is caused to be transferred to the web by a current flow through the photoconductive member. During the charge transfer process, the photoconductive plate builds-up a net negative charge due to fixed electron traps within the photoconductive material. This negative charge induces a mobile positive charge in the web. The electric field established by the two charges must be eliminated prior to complete separation of the web from the photoconductive plate. In order to eliminate this field, electrical communication is established between the plate and the web during web separation which allows the mobile web charge to neutralize the fixed plate charge. A cycling device is provided for defining a web-to-photographic member engagement period, an image exposure and voltage application period, a web-to-photographic member disengagement period, and a discharge period during which the accumulated charge is neutralized prior to complete disengagement of the web.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the electrophotographic device and the operation of the discharging structure will become apparent from the following detailed description taken in conjunction with the drawings in which:

FIG. 1 is an isometric schematic view, partly in section, showing an electrophotographic recorder system employing the present discharge circuit;

FIG. 2 is a sectional view showing the pressure pad disengaging the photoconductive plate and the recording web peeling away from the plate; and

FIG. 3 is an enlarged fragmentary view of the web separation of FIG. 2 showing the undesirable charge buildup, and the discharging circuit.

FIG. 1 shows a photoconductive plate which is preferably CdS suitably doped with copper or silver halides mounted on a glass substrate 12 contained within a dark box 14 having a transparent upper surface. The image to be copied 16 is projected onto plate 10 by an optical imaging system depicted in FIG. 1 as light source 18. A web 20 from a supply reel 22 is pressed into mechanical and electrical engagement with photoconductive plate 10 by a pivoting assembly 24. A voltage source 26 applies an exposure or charge transfer voltage serially across plate 10 and web 20. One lead is connected to a suitable cycling device 30 such as a cam operated or rotary switch with a timer 31. Cycling device 30 applies the charging voltage to a thin transparent area electrode 28 sandwiched between glass substrate 12 and plate 10. The other lead of voltage source 26 is preferably grounded and is connected to conductive pivoting assembly 24. Timer device 31 is preferably activated by start switch and operates cycling switch for determining the application of charge transfer voltage 26, the exposure of image 16 by light source 18, the engagement and disengagement of web 20 against plate 10, and the discharge or neutralization of unwanted charge accumulation.

Current flowing through the thickness of the photoconductor 10 in response to the image exposure causes a latent charge image 32 (shown in dotted lines) corresponding to image 16 to be formed on web 20. Web 20 is disengaged from photoconductor 10 by a peeling process explained more fully in connection with FIG. 2 and in U.S. Pat. No. 3,715,156. Web 20 is then moved to a toning station 34 by a pair of

opposed drive wheels

36 and 37 activated by a web transport motor 38 where charge image 32 is developed to form a visible image 40.

FIG. 2 shows an electrophotographic apparatus having a modified cycling device which employs limit switches and a simple timer switch 48. Pivoting assembly 24 is shown pivoted slightly open to partially engage web 20 against plate 10. At the beginning of the imaging cycle, pivoting assembly 24 is clear of plate 10, and fresh copy web is in the exposure position directly beneath photoconductor 10. A gear motor is then energized by start switch 25 to rotate crank 52 and operate toggle linkage 54 in the expanding mode, causing pivoting assembly 24 to pivot about axis 56 and swing toward photoconductor l0. Pivoting assembly 24 includes a porous resilient conductive flexible pressure pad 58 mounted on a conductive back-up electrode 60 which is mounted on pivoting frame 62. The pivoting action causes pressure pad 58 to initially engage web 20 along leading edge 58a as shown in FIG. 2. As the area of engagement expands across pressure pad 58 in the direction of arrow 59 toward lagging edge 58b, web 20 is smoothed out toward lagging edge 58b preventing wrinkle formation. The air immediately above web 20 and next to photoconductor 10 is displaced towards the lagging edge 58b and towards the sides of pressure pad 58. The air immediately under web 20 and next to pad 58 escapes through the pores of pad 58. As the increasing area of engagement expands to the full area of pad 58 and web 20 becomes fully engaged, toggle linkage 54 approaches full toggle and activates an upper limit switch 64 which stops gear motor 50, and energizes timer 48 which triggers light source 18 initiating the web exposure-charging cycle. Also. as pivoting assembly approaches full engagement. a pawl contacts upper striker 77 and is pushed into engagement with ratchet 72. This engagement locks a pair of opposed non-slip guide rollers 66 and 68 preventing movement of web 20.

After the exposure period, which is determined by timer 48, gear motor 50 is re-energized and operates toggle linkage 54 in the contracting mode. Pivoting assembly 24 pivots away from photoconductor l0 causing pad 58 to progressively disengage web 20 from photoconductor 10. Web 20 is peeled away from plate 10 along the boundary of the progressively decreasing area of engagement. Web 20 is held at lagging edge 58b during the peeling disengagement by rotably locked opposed guide rollers 66 and 68. Web 20 is held taunt at leading edge 58a by the gradually decreasing web-plate engagement and by

opposed drive rollers

36 and 37. This tauntness in web 20 helps to keep web 20 in mechanical and electrical contact with pad 58. Nonetheless, the electrostatic attraction between photoconductor 10 and web 20 may cause the web to pull away from the pad resulting in loss of electrical contact to web 20. The disruption in electrical contact between web 20 and pad 58 is compensated by a discharging circuit shown in FIG. 3, and spurious charge transfers between plate 10 and web 20 are eliminated. As the disengagement is completed, toggle linkage 54 approaches fully collapsed position and activates lower limit switch 74 which de-energizes gear motor 50 and activates web drive motor 38. Meanwhile, the downward movement of pivoting assembly 24 has caused pawl 70 to abut striker plate 76 and become disengaged from ratchet 72 to release web 20 allowing fresh web to be drawn into the exposure position by drive motor 38. Drive motor 38 and gear motor 50 may be replaced by a single motor with appropriate clutch mechanisms to drive the web and pivot assembly 24.

FIG. 3 shows in detail the web separation and the discharging circuit which provides electrical communication between plate 10 and web 20 during web separation. A fixed negative charge 80 accumulates within the interior of photoconductive plate 10 during exposure because mobile electrons within the photoconductor become trapped within the body of plate 10. Fixed charge 80 induces a mobile positive charge 82 into web 20. Web 20 consists of a dielectric coating or charge retaining layer 84 adjacent to plate 10 and a conductive backing 86. Layer 84 retains the charge image 32 (not shown in FIG. 3) formed during exposure until the image can be developed in toning station 34. Backing 86 provides an area electrical contact spaced from area electrode 28 required to apply charging voltage 26 across plate 10 and layer 84. Prior to disengagement of web 20, induced charge 82 is dispersed throughout conductive backing 86 proximate the dielectricbacking interface 88 in response to fixed charge 80. During web separation, mobile charge 82 becomes progressive ly removed from fixed charge 80 and is free to flow from backing 86 through pad 58 and frame 62, through timer 30 (shown in the discharge position in FIG. 3 and throughout area electrode 28 to neutralize fixed charge 80. As described in connection with FIG. 2, the electrostatic attraction between web 20 and plate may cause web to lift and lose electrical contact with conductive pad 58. As an alternative means of maintaining electrical contact between backing 86 and plate 10, drive roller 36 is made of conductive rubber which engages backing 86 and is periodically electrically connected to plate 10 through timer and area electrode 28. Conductive roller 36 is preferably grounded and in direct electrical contact with pad 58 and backing 86 as is shown by a roller lead 90 connected to a pad lead 91. Opposed drive roller 37 is preferably made of metal and is in direct electrical contact with rubber roller 36 and pad 58 through a lead 92. During exposure, cycling switch 30 applies charging potential 26 across plate 10 by connecting a lead 94 of source 26 to lead 96 of plate 10. During separation, cycling switch 30 disconnects voltage source 26 and connects pad 58 and conductive roller 36 directly to plate 10 by switching plate lead 96 to a lead 98 which is connected to roller lead 90. In short, plate 10 is switched to voltage source 26 during exposure and switched to ground during web separation.

The grey scale capabilities of the electrophotographic recorder may be enhanced if during web separation, voltage source 26 is continuously applied across plate l0 and web 20. In this case, plate lead 96 is not grounded until the final stages of separation. When this is done, however, the electrostatic attraction between web 20 and plate 10 during separation is enhanced and it is yet more difficult to maintain electrical contact between pad 58 and web 20. Thus, web tension must be greater and separation performed more slowly in order to prevent spurious charge transfer (arcing).

While the preferred embodiment has been illustrated and described, it will be understood by those skilled in the art that changes and modifications may be resorted to without departing from the spirit and scope of the invention.

What is claimed is:

1. An electrophotographic recorder responsive to an image to be recorded for providing an image on the charge retentive surface of a recording medium having a conductive backing, comprising:

cycling means for defining a recording medium engagement period, an image exposure period, a recording medium disengagement period, and a discharge period for discharging accumulated net charge prior to the complete disengagement of the recording medium;

a photoconductive member which is selectively illuminated by the image to be recorded during the exposure period while engaging the charge retentive surface of the recording medium;

means for applying a charge transfer voltage across the photoconductive member and the recording medium during the exposure period in response to the cycling means, which voltage is sufficient to cause current to flow from the illuminated areas of the photoconductive member to the charge retentive surface and form a charge image thereon, the current flow causing a fixed space charge to build up throughout the photoconductive member which induces a mobile charge in the conductive backing of the recording medium proximate the charge retentive surface:

means movable with respect to the photoconductive member for urging the charge retentive surface of the recording medium into engagement with the photoconductive member during the engagement period, and for maintaining this engagement throughout the exposure period, and for disengaging the recording medium from the photoconductive plate during the disengagement period; and

conductive means responsive to the cycling means for establishing electrical communication between the conductive backing of the recording medium and the photoconductive member during the discharge period for permitting the induced mobile charge on the recording medium to flow into the photoconductive member and neutralize the fixed charge in the photoconductive member; said conductive means comprising:

a first conductive drive roller which engages the conductive backing of the recording medium for moving the recording medium with respect to the photoconductive plate after completion of the disengagement period; and

a switch responsive to the cycling means for electrically connecting said first conductive drive roller to the photoconductive member.

2. The electrophotographic recorder of claim 1,

wherein said conductive means further comprises:

a first electrical short circuit connecting said first conductive drive roller to electrical ground; and

a second electrical short circuit connecting said first conductive drive roller to said conductive backing of said recording medium.

3. The electrophotographic recorder of claim 2 wherein said conductive means are not activated until the latter stages of the discharge period.

4. The electrophotographic recorder of claim 2 further comprising a second conductive drive roller situated opposite said first conductive drive roller and spaced apart from said first roller so that said recording medium is gripped between said two rollers, wherein said second roller is electrically connected to said first roller.

5. The electrophotographic recorder of claim 2, wherein the switch is additionally responsive to the cycling means for applying the charge transfer voltage across the photoconductive member and the recording medium.

6. The electrophotographic recorder of claim 2 wherein the photoconductive member is mounted on a substrate with a thin transparent electrode therebetween which is connected to the switch for facilitating the application of the charge transfer voltage across the photoconductive member and the recording medium, and for facilitating electrical communication between the photoconductive member and said first conductive drive roller.

7. The electrophotographic recorder of claim 6 wherein a means for toning the charge image on the dielectric coating is provided for producing a visible charge image.

8. The electrophotographic recorder of claim 1 wherein the photoconductive member is cadmium sulphide.

9. The electrophotographic recorder of claim 8 wherein the fixed charge built-up in the photoconductive member is negative and in which the induced mobile charge on the conductive backing of the recording medium is positive.

Claims

1. An electrophotographic recorder responsive to an image to be recorded for providing an image on the charge retentive surface of a recording medium having a conductive backing, comprising: cycling means for defining a recording medium engagement period, an image exposure period, a recording medium disengagement period, and a discharge period for discharging accumulated net charge prior to the complete disengagement of the recording medium; a photoconductive member which is selectively illuminated by the image to be recorded during the exposure period while engaging the charge retentive surface of the recording medium; means for applying a charge transfer voltage across the photoconductive member and the recording medium during the exposure period in response to the cycling means, which voltage is sufficient to cause current to flow from the illuminated areas of the photoconductive member to the charge retentive surface and form a charge image thereon, the current flow causing a fixed space charge to build up throughout the photoconductive member which induces a mobile charge in the conductive backing of the recording medium proximate the charge retentive surface; means movable with respect to the photoconductive member for urging the charge retentive surface of the recording medium into engagement with the photoconductive member during the engagement period, and for maintaining this engagement throughout the exposure period, and for disengaging the recording medium from the photoconductive plate during the disengagement period; and conductive means responsive to the cycling means for establishing electrical communication between the conductive backing of the recording medium and the photoconductive member during the discharge period for permitting the induced mobile charge on the recording medium to flow into the photoconductive member and neutralize the fixed charge in the photoconductive member; said conductive means comprising: a first conductive drive roller which engages the conductive backing of the recording medium for moving the recording medium with respect to the photoconductive plate after completion of the disengagement period; and a switch responsive to the cycling means for electrically connecting said first conductive drive roller to the photoconductive member.

2. The electrophotographic recorder of claim 1, wherein said conductive means further comprises: a first electrical short circuit connecting said first conductive drive roller to electrical ground; and a second electrical short circuit connecting said first conductive drive roller to said conductive backing of said recording medium.