US3846020A

US3846020A - Electrostatic sheet transport system

Info

Publication number: US3846020A
Application number: US00421179A
Authority: US
Inventors: G Fletcher
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1973-12-03
Filing date: 1973-12-03
Publication date: 1974-11-05
Anticipated expiration: 1991-11-05
Also published as: GB1477534A; CA1038019A

Abstract

A transport for paper or other copy sheets consisting of a photoconductive belt which is corona charged and then optically exposed to a fine pattern of closely spaced alternating adjacent light and dark areas to provide a correspondingly fine electrostatic pattern of alternating charged and discharged areas on the belt. This provides a fine pattern of fringe fields for electrostatically holding either charged or uncharged paper on the belt. The charge pattern spacing is preferably that of the paper thickness or less so that the system may be utilized for the transport of a copy sheet through a zerographic transfer station without affecting image transfer.

Description

3,642,362 2/1972 Mueller ..226/94X Fletcher {451 Nov. 5,1974

' [54] ELECTROSTATIC SHEET TRANSPORT SYSTEM' Primary Examiner-Robert P. Greiner 75 inventor: Gerald M. Fletcher, Pittsford, NY. [73] Assignee: Xerox Corporation, Stamford, [57] ABSTRACT Conn. A transport for paper or other copy sheets conslstmg of a photoconductive belt which is corona charged [22] Flled: 1 and then optically exposed to a fine pattern of closely [2]] App]; M 421,179- spaced alternating adjacent light and dark areas to v v I provide a correspondingly fine electrostatic pattern of r I alternating charged and discharged areas on the belt. Cl SSS/3R 29,1 This provides a fine pattern ofofringe fields for electrostatically holding either charged or uncharged paper [51] B Cl 003g 15/00 on the belt. The charge pattern spacing is preferably [58] held f Search 355/3 3 l7; that of the paper thickness or less so that the system 7 96/l.4; 27l/DIG. 3; 226/94; 317/262 E may be utilized for the transport of .a copysheetthrou h a zero ra hic transfer station without affectv References C d ing im age trans%er. Y

' UNITED STATES PATENTS 6 Claims, 1 Drawing Figure ELECTROSTATIC SHEET TRANSPORT SYSTEM The present invention relates to a sheet transport system utilizing an optically controlled electrostatic sheet retaining charge.

The accurate and reliable transport of sheets, particularly cut paper or other copy sheets (or original documents) utilized in electrostatographic copying systems is a particular problem due to the variable nature of such materials. Various sheet transporting devices such as mechanical grippers, vacuum transport belts, feed rollers, etc., are well known.

It is also known that a copy sheet can be transported on a belt or other member which has been charged by an electrostatic charge pattern. The following U.S. Pats. are exemplary of this art of. electrostatic tacking of paper to a paper transport belt by uniform or nonuniform electrostatic charging of the belt or paper: No. 2,576,882 to P. Koole, et al, No. 3,357,325 to R. H. Eichorn, No. 3,642,362 to D. Mueller, No. 3,690,646 to J. A. Kolivis, No. 3,717,801 to M. Silverberg, and No. 3,765,957 to J.Weigl. However, these charged transport belts are dielectric or conductive rather than photoconductive. This presents serious practical problems in impressing thereon a sufficiently closely spaced electrostatic charge pattern of alternating charged and uncharged, or opposite polarity areas. Corona charging techniques are not practically capable of producing a fine (closely spaced) variable charge pattern. A fine charge pattern can be provided on a dielectric surface by a fine conductive grid therein or by direct contact of a conductive or semi-conductive biased electrode or roller which has a sufficiently finely textured or roughened surface to apply by electrical contact transfer a fine charge pattern on the dielectric surface. However, it will be appreciated that the placing of fine charge patterns on a dielectric surface by direct contact by an electrode is difficult, due to surface contact problems, variations in the surface to be charged, contamination, etc. Thus, a non-contacting method of charging of a copy sheet transport supporting surface is generally preferable.

A particular sheet transport problem is the accurate and positive transporting of sheets into, through, and out of a xerographic transferstation. The copy sheet must be maintained in accurate registration with the toner image to be transferred from the imaging surface onto the copy sheet at the transfer station. The sheet typically acquires a tacking electrostatic charge and the imaging surface has a charge on it as well. Thus, the sheet must be either mechanically or electrostatically stripped from the imaging surface at the exit of the transfer station or process. The stripping device if it takes the conventional form of an air puffer, gripper, or stripper finger, can cause some disturbance of the transferred toner image, which is typically unfused at that point.

Thus, ideally, it is desirable to'fully support and positively retain the copy sheet on a transport through the transfer station. This can be done by a vacuum-belt as disclosed in U.S. Pat. No. 3,647,292 to D. J. Weikel, for example. The present invention provides a'different means for continuously positively retaining, and thereby stripping the copy sheet from the photoreceptor, in the transfer station. The present system utilizes purely electrostatic forces for this'function and does not require a vacuum system, although it will be appreciated that a vacuum may be additionally applied in combination therewith if so desired.

It will be noted that the use of a fine charge pattern produced on the imaging surface itself for increased toner retention by fringe field effects, e.g., for improved half-tone solid area image reproduction, is known. The fine charge pattern may be placed on the photoreceptor imaging surface by an optical screen, or by the photoreceptor construction itself, or by contact with a charging roller having a patterned or textured surface for transferring a fine electrical pattern to the photoreceptor. For example, the imaging surface may be pattern charged by a contacting electrically charged wire screen or knurled conducting rubber roller at a suitable voltage. However, this type of structure is utilized for increasing the quantity or uniformity of toner retained on a given area of the photoreceptor prior to its transfer to the copy sheet, and not for retention of a copy sheet. Thus, it affects the transfer by changing the image which is transferred. in contrast, the copy sheet transport system of the invention does not affect the imaging surface and does not affect the transfer process or the transferred image pattern.

The sheet transport system of the invention may be utilized in any desired path or configuration. It may be utilized for transfer with an imaging surface which has any desired configuration, such as a cylinder or a-belt. Belt imaging surface photoeonductors in electrographic copying systems are exemplified by U.S. Pat. Nos. 3,093,039 to Rheinfrank; 3,707,138 to Cartright, and 3,719,165 to Trachienberg, et al.

In the conventional transfer station in Xerography, toner is transferred from the photoreceptor (the original support and imaging surface) to the copy paper (the final support surface). Such development material transfers are also required in other electrostatographic processing steps, such as electrophoretic development. In Xerography, developer transfer is most commonly achieved by electrostatic force fields created by D. C. charges applied to the back of the copy paper (opposite from the side contacting the toner-bearing photoreceptor) sufficient to overcomethe charges holding the toner to the photoreceptor and to attract most of the toner to transfer onto the paper. These xerographic transfer fields are generally provided in one of two ways, by ion emission from a transfer corotron onto the paper, as in U.S. Pat. No. 2,807,233, or by a D. C. biased transfer roller or belt rolling along the back of the paper. Examples of bias roller transfer systems are described in U.S. Pat. Nos. 2,807,233; 3,043,684; 3,267,840; 3,598,580; 3,625,146; 3,630,591; 3,691,993; 3,702,482; and 3,684,364. The copy sheet is typically electrostatically tacked to the photoreceptor until it is subsequently mechanically or electrostatically stripped off as previously noted, i.e., the tacking forces must be overcome in some manner for the copy sheet to be removed from the imaging'surface.

The above-cited and other references teach details of various suitable exemplary xerographic structures, materials and functions to those skilled in the art. Further examples are disclosed in the books Electrophotography by R. M. Schaffert, and Xerography and Related Processes by John H. Dessauer and Harold E. Clark,

both first published in 1965 by Focal Press Ltd., Lon- Further objects, features and advantages of the present invention pertain to the particular apparatus, steps and details whereby the above-mentioned aspects of the invention are attained. Accordingly, the invention will be better understood by reference to the following description and to the drawing forming a part thereof,

Referring to the FIGURE, there is schematically I shown a sheet transport system l-which is an exemplary embodiment of the present invention. Since the conventional details thereof are well known and fully described in the above-cited and other references relatingto copy sheet handling, transfer and xerography,

these details, for improved clarity, will not be described herein.

The system here comprises a copy sheet transport belt 12 which is supported and rotatably driven between rollers 14 and 16. The transport belt 12, unlike conventional sheet transport belts, is photoconductive. It may be constructed of the same photoconductive materials and structure as other photoconductive belts utilized for xerographic imaging surfaces, such as those previously cited herein. Additional U.S. Pats. disclosing the construction-of photoconductive belts are No. 3,697,285 and No. 3,713,821.

The photoconductive transport belt 12 positively supports and carries the copy sheet 18 into and out of contact with an imaging surface 20 of a xerographic copying system 22 at a transfer station 24. Transfer is conventionally provided at the transfer station 24 by a bias transfer roller, here comprising the roller 19. The xerographic copying system 22 shown here also includes the conventional stations for optical imaging, cleaning, charging, and development of the imaging surface 20.

The transport belt 12, by the electrostatic fringe field chargepattern described herein, provides positive retention of the copy sheet 18 at all desired points along the path of the transport belt 12, until it is desired to strip the copy sheet therefrom by any suitable conventional sheet stripping means. With the disclosed system the copy sheet 18 can be positively retained through the entire transfer station 24 without affecting the normal xerographic transfer in any way.

To provide the copy sheet retaining charge pattern on the transport belt 12, the belt (which may be previously conventionally neutralized or grounded electrically or optically) is first uniformly conventionally corona charged by a noncontacting conventional corona charging device 26 extending laterally across the width of the transport belt 12, i.e., transversely of the direction of motion of the belt. Subsequent to this electrical charging of the copy sheet supporting surface of the transport belt 12, optical means are provided for selectively optically discharging discrete, but closely adjacent, areas of the transport belt surface across the belt, at a belt discharging station 30.

The discharging station 30 includes an optical system for optically imaging a fine light patternof closely alternating adjacent light and dark areas on the copy sheet supporting surface of the belt, so as to photoconductively discharge this supporting surface into a fine charge pattern of alternating closely adjacent charged and discharged areas which will provide copy sheet retaining electrical fringe fieldsThis may be provided as shown by sequentially flash imaging on the belt 12, as it advances, an original image pattern 32 of alternating closely adjacent light and dark areas through a conventional imaging lens 34 onto the supporting surface. This original image pattern 32 may be either an opaque or transparent pattern or any other suitable means for forming the desired image pattern on the transport belt, such as a scanning laser beam pattern or the like.

lt will be appreciated that the light utilized may be either visible or invisible radiant energy, depending on the radiant energy sensitivity of the photconductive material on the transport belt. it will also be appreciated that the image pattern 32 as shown is merely exemplary. The image pattern may be in the form of a pattern of closely spaced dots rather than lines. Alternatively, with a line of alternating closely spaced light and dark spots, or lines extending in the direction of motion of the belt, it may be possible with a stable belt support to utilize continuous imaging rather than intermittent flash exposure to form a continuous overall pattern.

An important desired feature of the optical image pattern formed on the photoreceptor belt is the alternating light and dark areas are sufficiently closely spaced, i.e., sufficiently fine, such that the corresponding alternating adjacent charged and discharged areas on the belt surface form a very fine fringe field electrostatic pattern which will not affect the image transfer at the transfer station. Preferably the spacing of the alternating charged and discharged areas is not substantially greater than the thickness of the copy sheet. Such close or fine spacing will cause the fringe fields to extend mainly inside the copy sheet from the supported back surface thereof, and not extend appreciably outside of the front or image receiving surface of the copy sheet. For most conventional copy sheet thicknesses the preferred charge pattern is thus approximately 0. l 3 millimeters (5 mils) in spacing between the charged and uncharged areas. With this spacing the fringe fields on the underlying transport belt 12 will not significantly affect the transfer fields in the transfer nip of the transfer station, and thereby will not affect the transfer of toner to the upper or exposed surface of the copy sheet 18. Further, they will not disturb the toner once it is transferred to the copy sheet. This substantially eliminates the chances for any observable toner print-out of the transport belt charge pattern onto the copy sheet.

It will be noted that the exposed or discharged areas of the transport belt 12 do not have to be fully discharged, nor do the unexposed'or undischarged areas have to be fully unexposed (i.e., remain at their total initial charge). his only necessary for this system that the discharged areas be discharged to a different, i.e., lower, voltage level than the undischarged areas, so as to create fringe fields between different adjacent voltphotoconductive belt is used. This is basically a different way of making an electrically relaxable" belt material, but with a less critical resistivity specification. The requirement is a dark relaxation time for charge flow near the transfer nip much longer than the time spent near the nip. The minimum corresponding dark resistivity of the photoconductor to ensure this depends on process speed, belt thickness, and system geometry but is typically near ohm-cm. The light resistivity must be such that the relaxation time for charge flow in the nip is less than the nip time (typically 10 ohmcm) but dependent-on process speed, belt thickness, and geometry.

The sheet transport system disclosed herein is presently considered to be preferred; however, it is contemplated that further variations and modifications within the purview of those skilled in the art can be made herein. The following claims are intended to cover all such variations and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an electrostatographic copying system in which an image is formed on an imaging surface and transferred at a transfer station to a copy sheet, the improvement comprising:

copy sheet transport means including a copy sheet supporting surface,

said supporting surface being photoconductively electrically dischargable;

support means for said transport means for moving said supporting surface intimately past said imaging surface; charging means for electrically charging said sup- 6 porting surface, and

optical means for optically imaging a fine light pattern of closely alternating adjacent light and dark areas on said supporting surface, subsequent to said electrical charging thereof by said charging means, for photoconductively discharging said sup porting surface into a fine charge pattern of alternating closely adjacent charged and discharged areas on said supporting surface providing copy sheet retaining electrical fringe fields.

2. The copying system of claim 1 wherein said optical means includes an original image pattern of alternating closely adjacent light and dark areas, and lens means for imaging said original image pattern over said supporting surface.

3. The copying system of claim 1 wherein said copy sheet is moved on said support surface into and out of engagement with said imaging surface at said transfer station, retained on said support surface by said fine charge pattern of alternating closed adjacent charged and discharged areas.

4. The copying system of claim 3 wherein said spacing of said alternating charged and discharged areas is sufficiently fine that said image transfer at said transfer station is unaffected.

5. The copying system of claim 3 wherein said spacing of said alternating charged and discharged areas is not substantially greater than the thickness of said copy sheet.

6. The copying system of claim 3 wherein said spacing of said alternating charged and uncharged areas is less than approximately 0.13 millimeters.

Claims

1. In an electrostatographic copying system in which an image is formed on an imaging surface and transferred at a transfer station to a copy sheet, the improvement comprising: copy sheet transport means including a copy sheet supporting surface, said supporting surface being photoconductively electrically dischargable; support means for said transport means for moving said supporting surface intimately past said imaging surface; charging means for electrically charging said supporting surface, and optical means for optically imaging a fine light pattern of closely alternating adjacent light and dark areas on said supporting surface, subsequent to said electrical charging thereof by said charging means, for photoconductively discharging said supporting surface into a fine charge pattern of alternating closely adjacent charged and discharged areas on said supporting surface providing copy sheet retaining electrical fringe fields.