GB2039800A

GB2039800A - Squeegee Rollers

Info

Publication number: GB2039800A
Application number: GB8001291A
Authority: GB
Original assignee: Imaging Technology Ltd
Current assignee: Imaging Technology Ltd
Priority date: 1979-01-16
Filing date: 1980-01-15
Publication date: 1980-08-20
Also published as: GB2039800B

Abstract

Print quality in a wet developer photocopying machine is improved by 3) increasing the degree of electrical insulation of the resilient outer surface (2) of the "back-contacting roller" of a squeegee pair downstream of the developer station from the conducting frame (F) on which it is supported. The electrical insulation can be increased by the use of electrically non-conducting bearings (4). <IMAGE>

Description

SPECIFICATION Improvements in and Relating to Squeegee Rollers This invention relates generally to the development of electrostatic images using charged toner particles suspended in a liquid carrier and in particular to the squeegee rollers used for removing surplus toner/carrier from a photoconductive substrate in the production of a permanent copy.

In copying machines using photoconductive coated (e.g. zinc oxide coated) paper and liquid developer, it is normal to squeeze the paper as it leaves the developer to remove excess developer from the surface. The squeezing of the paper is usually effected by passing it through the nip formed between a pair of rollers (the squeegee rollers) and the composition and surface finish of such rollers is important having regard to the "dryness" of the copy produced and the quality of the reproduction of the copied image. Our British Patent 1450396 relates to the hardness and surface finish of at least one of a pair of squeegee rollers.

One of the recognised problems arising when using liquid development of images created by the electrostatic process, is the tendency for the toner to be deposited in the non-image areas, creating background marking to a greater or lesser degree. Background marking can arise for a variety of different reasons but heretofore it had not been appreciated that the electrical characteristics of the squeegee roller contacting the "back" of the copy was an important factor in the control of background marking.

According to one aspect of the invention a squeegee roller system in a copying machine, which system comprises an opposed pair of squeegee rollers, one of which rollers, the "backcontacting roller", has an electrically nonconducting outer layer and is rotatably supported from an electrically conducting frame, is characterised in that the back-contacting roller is supported in such a way that its outer layer is electrically insulated from the frame.

In the use of a squeegee roller system with zinc oxide coated paper, it is known to contact the "back" of the paper with the highly polished surface of a resilient, electrically non-conducting layer supported on an electrically conducting shaft or core, the shaft or core being carried in bearings at the electrical potential of the frame.

The "back" ofthe paper is rendered electrically conducting during its manufacture and suprisingly we have found that when the conducting coating of the paper contacts a non-conducting surface of a roller system, depending on the nature of the conducting coating (e.g. whether it is a salt base coating or an electro-conductive resin coating) the degree of background marking is affected by the electrical potential of the core or shaft of the back-contacting roller.

We have found therefore that an improved quality of copy can be obtained by the simple expedient of electrically insulating the core or shaft of the back contacting roller from the conducting frame supporting the roller pair.

The insulation of the core or shaft can, for example, be affected by using electrically nonconducting bearings for supporting the core or shaft from the frame or by making the core or shaft of an electrically non-conducting material.

Since replacement of one squeegee roller by another, or replacement of one set of bearings by another, are design modifications which can easily be made to existing machines, our invention is also seen to embrace a method of improving the print quality of an existing machine by a modification which involves increasing the degree of electrical insulation of the resilient outer surface of a squeegee roller from the conducting frame on which it is supported.

The improvement in print quality when employing a squeegee roller system in accordance with the invention is particularly marked when using zinc oxide paper whose conductive backing is resin-based and when using negatively charged zinc-oxide layers to produce positive prints from microfilm negatives in a single operation (i.e. using a liquid reversal (negative) toner. There is also some improvement when bi-charge zinc oxide papers are positively charged to produce positive prints with the same polarity toner, and when salt-based papers are used. The invention is thus thought to have utility in all cases where a roller having a nonconducting outer surface layer is used to contact the conductive backing of photoconductive copy material.

The accompanying drawing schematically indicates, by way of example, a squeegee roller system in accordance with the invention.

The upper roller of the pair shown in the drawing can be of wholly conventional design and accordingly has not been described or given a reference numeral in the drawing. It is supported (in a manner not shown) from a conducting frame F. The lower roller 1 has an annular outer layer 2 of electrically insulating material supported on an electrically conducting metal core 3. A part of the core 3 defines spindles 3a (only one of which is shown). Instead of the normal metal bearings, highly insulating bearings 4 are used for supporting the spindles 3a from the frame F. The bearings 4 increase the resistance between the frame F and the core 3 from a fraction of an ohm to at least 108 ohms.

The coated paper in the drawing has a zinc oxide coating 5, a paper substrate 6 and a conductive resin coating 7. The areas 8 represent printed images and the liquid developer is indicated at 9.

With the spindles 3a in good electrical contact with the frame F, the copy image could be described as of good density in the image areas but with noticeable background marking in the non-image areas, the background marking spreading over substantial parts of the non-image areas, but not necessarily all of them. With the spindles 3a well insulated from the frame F, the prints became of acceptable quality, with a marked decrease in background marking over the areas where it had previously appeared.

Whilst not attempting an explanation of the phenomenon we have discovered, we believe that the improvement of copy quality is in some way related to a reduction in capacitative coupling between the "back" of the copy paper and the "ground" potential of the frame via the core 3 when the "back" of the paper is not otherwise held at the same potential as the frame by contact with other components of the copy development system.

Claims

1. A squeegee roller system in a copying machine, which system comprises an opposed pair of squeegee rollers, one of which rollers, the "back-contacting roller", has an electrically nonconducting outer layer and is rotatably supported from an electrically conducting frame, characterised in that the "back-contacting roller" is supported in such a way that its outer layer is electrically insulated from the frame.

2. A squeegee roller system as claimed in claim 1, in which the "back-contacting roller" has a resilient, electrically non-conducting layer with a highly polished surface supported on an electrically conducting shaft or core, characterised in that the shaft or core is carried in electrically non-conducting bearings carried by the frame.

3. A squeegee roller system as claimed in claim 1, in which the "back-contacting roller" has a resilient electrically non-conducting layer with a highly polished surface supported from electrically conducting bearings at the electrical potential of the frame, characterised in that the said non-conducting layer is mounted on a core or shaft of an electrically non-conducting material.

4. A squeegee roller system substantially as hereinbefore described with reference to the accompanying drawing.

5. A method of improving the print quality of an existing copying machine which uses paper coated with a photoconductive material on its front face and a liquid developer for depositing toner particles on the said front face and which passes the paper, after contact with the developer, through a pair of squeegee rollers, one of which, the "back-contacting roller", presses against the back of the paper, and has a nonconducting resilient outer surface supported from a conducting frame, which improving method comprises increasing the degree of electrical insulation of the resilient outer surface of the "back-contacting roller" from the conducting frame on which it is supported.