GB2206535A - Supporting an endless electrophotographic belt - Google Patents

Abstract

A xerographic copying machine has a flexible, endless photoreceptor belt (1) arranged to be driven in a loop around two support rollers (1a, 1b). In order to prevent the belt rippling-and hence to ensure uniform intimate contact with a copy sheet at the transfer station (5)-a fixed support member (40) is provided between the rollers and the fixed support member (40) has a convex surface (41) in the plane orthogonal to the direction of motion of the belt. <IMAGE>

Description

Electrophotoqraphic Copier This invention relates to an electrophotographic copier comprising an imaging member in the form of a flexible endless belt arranged to be driven in a loop around support members.

Such a copier is disclosed in our copending European Patent Appiication No.

87 303 037.3 (our reference R/86003), wherein the imaging member is provided in a process unit adapted to be removably mounted in the main assembly of the copier. The imaging member is loosely retained in the process unit when the process unit is removed from the main assembly, and is adapted to be held in tension in an operative position by support members forming part of the main assembly when the process unit is inserted into the main assembly. At least one of the support members is a drive roll for driving the image member in a loop around the support members.

It has been found in practice that when the imaging member is tensioned the surface of the belt tends to become rippled with ribs which extend in the direction of motion of the belt. This rippling effect prevents intimate contact between a copy sheet and the belt surface in the transfer region of the copier which can cause undesirable linear deletions in the copy output.

US Patent No. 4 647 177 to Yoshito et al discloses an electrophotographic copier comprising an endless belt photoreceptor. Figure 1 appears to show a support for the belt directly opposite the transfer corotron (25) and this support member appears to have a slightly convex profile in the plane parallel to the direction of movement of the belt.

US Patent No. 4 368 339 (see especially Figure 4) discloses charge deposition apparatus for an electrographic copier wherein an endless dielectric belt photoreceptor is supported in the charge transfer region by a surface which is generally convex in the plane parallel to the direction of movement of the belt.

US Patent No. 3 984 183 discloses a crowned support roller for a belt photoreceptor at the transfer station of a copier in order to promote self-stripping of a copy sheet from the photoreceptor after image transfer.

According to the present invention there is provided an electrophotographic copier comprising an imaging member in the form of a flexible endless belt, two rollers about which the belt is entrained, and a fixed member supporting the belt between said rollers. wherein the bearing surface of the fixed member is convex in the plane transverse to the direction of movement of the belt.

The curved profile of the support member across the width of the belt in accordance with the invention prevents the belt from rippling and so avoids the problem of linear image deletion.

It is particularly important that the belt is not rippled at the transfer station of the copier, i.e. where the copy sheet is brought into contact with the imaging member for an image to be transferred therefrom to the copy sheet, and therefore it is preferable for the support member having the convex bearing surface to be disposed in the vicinity of the transfer station.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a xerographic copier in accordance with the invention, and Figure 2 is an enlarged section taken on the line ll-ll' in Figure 1.

Referring first to Figure 1, there is shown schematically a xerographic copying machine incorporating the present invention The machine includes an endless flexible photoreceptor belt 1 mounted for rotation (in the clockwise direction as shown in Figure 1) about support rollers la and ib to carry the photosensitive imaging surface of the belt 1 sequentially through a series of xerographic processing stations, namely a charging station 2, an imaging station 3, a development station 4, a transfer station 5, and a cleaning station 6.

The charging station 2 comprises a corotron 2a which deposits a uniform electrostatic charge on the photoreceptor belt 1.

An original document D to be reproduced is positioned on a platen 13 and is illuminated in known manner a narrow strip at a time by a light source comprising a tungsten halogen lamp 14. Light from the lamp is concentrated by an elliptical reflector 15 to cast a narrow strip of light on to the side of the original document D facing the platen 13.

Document D thus exposed is imaged on to the photoreceptor 1 via a system of mirrors M1 to M6 and a focussing lens 18. The optical image selectively discharges the photoreceptor in image configuration, whereby an electrostatic latent image of the original document is laid down on the belt surface at imaging station 3. In order to copy the whole original document the lamp 14, the reflector 15, and mirror M1 are mounted on a full rate carriage (not shown) which travels laterally at a given speed directly below the platen and thereby scans the whole document. Because of the folded optical path the mirrors M2 and M3 are mounted on another carriage (not shown) which travels laterally at half the speed of the full rate carriage in order to maintain the optical path constant.The photoreceptor 1 is also in motion whereby the image is laid down strip by strip to reproduce the whole of the original document as an image on the photoreceptor.

By varying the speed of the scan carriages relative to the photoreceptor belt 1 it is possible to alter the size of the image along the length of the belt, i.e. in the scanning direction. In full size copying, that is to say with unity magnification, the speed of the full rate carriage and the speed of the photoreceptor belt are equal. Increasing the speed of the scan carriage makes the image shorter, i.e. reduction, and decreasing the speed of the scan carriage makes the image longer, i.e. magnification.

The image size can also be varied in the direction orthogonal to the scan direction by moving the lens 18 along its optical axis closer to the original document i.e. closer to mirrors M2 and M3, for magnification greater than unity, and away from the mirrors M2 and M3 for reduction, i.e. magnification less than unity. When the lens 18 is moved, the length of the optical path between the lens and the photoreceptor, i.e. the image distance, is also varied by moving mirrors M4 and M5 in unison to ensure that the image is properly focused on the photoreceptor 1. For this purpose mirrors M4 and M5 are suitably mounted on a further carriage (not shown).

At the development station 4, a magnetic brush developer system 20 develops the electrostatic latent image into visible form. Here, toner is dispensed from a hopper (not shown) into developer housing 23 which contains a two-component developer mixture comprising a magnetically attractable carrier and the toner, which is deposited on the charged area of belt 1 by a developer roll 24.

The developed image is transferred at transfer station 5 from the belt to a sheet of copy paper which is delivered into contact with the belt in synchronous relation to the image from a paper supply system 25 in which a stack of paper copy sheets 26 is stored on a tray 27.

The top sheet of the stack in the tray is brought, as required, into feeding engagement with a top sheet separator/feeder 28. Sheet feeder 28 feeds the top copy sheet of the stack towards the photoreceptor around a 18û path via two sets of nip roll pairs 29 and 30. The path followed by the copy sheets is denoted by a broken line in Figure 1. At the transfer station 5 a transfer corotron 7 provides an electric field to assist in the transfer of the toner particles thereto.

In the vicinity of the transfer station 5 directly opposite transfer corotron 7 the photoreceptor belt 1 is supported by a fixed support member or so called skid 40. As can be seen from Figure 2, the skid 40 has a convex or crowned surface 41 in the plane orthogonal to the direction of motion of the belt, and the belt 1 bears against this surface 41. The profile of the curved surface is suitably an arc of a circle, or otherwise arcuate. In practice the curvature is only very slight. For example, in the case of the belt 1 being 300mm wide, the distance d between the lowest and highest point of the curved surface 41 need only be approximately 0.5mm. This slight curvature is sufficient to prevent the belt 1 from rippling, thereby ensuring uniform intimate contact with the copy sheet during image transfer.

The copy sheet bearing the developed image is then stripped from the belt 1 and subsequently conveyed to a fusing station 10 which comprises a heated roll fuser to which release oil may beapplied in known manner. The image is fixed to the copy sheet by the heat and pressure in the nip between the two rolls 10a and lOb of the fuser. The final copy is fed by the fuser rolls into catch tray 32 via two further nip roll pairs 31a and 31 b.

After transfer of the developed image from the belt some toner particles usually remain on the suface of the belt, and these are removed at the cleaning station 6 by a doctor blade 34 which scrapes residual toner from the belt. The toner particles thus removed fall into a receptacle 35 below. Also, any electrostatic charges remaining on the belt are discharged by exposure to an erase lamp 11 which provides an even distribution of light across the photoreceptor surface. The photoreceptor is then ready to be charged again by the charging corotron 2a as the first step in the next copy cycie.

The photoreceptor belt 1, the charge corotron 2a, the developer system 20, the transfer corotron 7, and the cleaning station 6 may all be incorporated in a process unit 12 adapted to be removably mounted in the main assembly 100 of the xerographic copier.

Claims (4)

Claims:

1. An electrophotographic copier comprising an imaging member in the form of a flexible endless belt, two rollers about which the belt is entrained, and a fixed member supporting the belt between said rollers, wherein the bearing surface of the fixed member is convex in the plane transverse to the direction of movement of the belt

2. An electrophotographic copier as claimed in claim 1, comprising a transfer station whereat an image may be transferred from the imaging member to a copy sheet, wherein the fixed member having the convex bearing surface is disposed in the vicinity of the transfer station.

3. An electrophotographic copier as claimed in claim 2, wherein the transfer station comprises a transfer corotron, and the fixed member having the convex bearing surface is disposed opposite the transfer corotron.

4. An electrophotographic copier substantially as herein described with reference to Figures 1 and 2 of the accompanying drawings.