GB2127348A - Transferring toner images - Google Patents

Abstract

Electrostatographic apparatus including transfer means (7, 8, 42) for transferring a developed electrostatic latent image from an imaging surface (1) onto a copy sheet, the transfer means comprising a transfer corotron (7), a de-tack corotron (8) and a halo guide (42). The halo guide includes a halo limiting portion (43) adjacent the transfer corotron (7) and a copy sheet guide portion (44) for guiding the sheet onto the imaging surface. The copy sheet guide portion (44) carries an electrically biassed member, and by virtue of its contacting the copy sheet, maintains it at a potential suitable for efficient transfer of the developed image regardless of the electrical conductivity of the copy sheet. The biassed member may be provided by a thin metal plate (45) secured to the guide portion (44) by means of an insulating layer (46) such as a double sided adhesive tape. <IMAGE>

Description

SPECIFICATION Electrostatographic apparatus This invention relates to an electrostatographic apparatus, particularly, although not exclusively, a xerographic copying machine. Such an apparatus typically includes transfer means for transferring a developed electrostatic latent image from an imaging surface onto a copy sheet, the transfer means comprising a copy sheet guide for guiding the sheet onto the imaging surface, and means for creating an electric field for assisting the transfer of the developed image onto the sheet.

In an apparatus of the above kind, the imaging surface is usually the surface of a drum or an endless belt, and the means for creating an electric field is a corotron. The copy sheet guide directs a copy sheet, usually of paper, towards the image-carrying surface of the drum or belt, and the sheet adheres to the drum or belt electrostatically. The corotron creates a field which is such as to cause the charged toner particles forming the developed image to be released from the drum or belt and to adhere to the copy sheet. The copy sheet is then detached from the drum or belt, carrying the image with it.

A problem which arises with this arrangement is that for it to achieve a good transfer, the paper copy sheet must be of relatively low conductivity. If the conductivity of the paper is too high, charges on the paper immediately leak away via those parts of the machine which are in contact with the sheet, such as paper guides. Many papers suitable for xerographic copy paper have a conductivity which is satisfactory when the paper is dry, but which becomes too high for good transfer efficiency when the paper is damp.

Thus, under conditions of high ambient relative humidity, many copy papers become too conductive to work properly, and in extreme cases will receive virtually no transferred image at all during the transfer step of the copy cycle.

One known way of overcoming this problem is to provide an arrangement for heating the copy paper sheets before they are delivered to the transfer station. This is a cumbersome solution to the problem, and uses considerable amounts of power.

The present invention is intended to provide a more satisfactory solution to the problem and provides an electrostatog raphic apparatus of the kind defined above which is characterised by electrical biassing means adjacent the region where the copy sheet contacts the imaging surface for contacting the copy sheet and maintaining it at a potential suitable for efficient transfer of the developed image regardless of the electrical conductivity of the copy sheet.

The necessary biassing potential can easily and inexpensively be provided by means of non-moving parts which can easily be fitted to all machines, or only to those going to high relative humidity environments. The electrical potential can be provided from a separate source within the machine, or from the existing high voltage power supply of the machine.

An electrostatographic apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is, a diagrammatic cross-sectional view of a xerographic copying machine incorporating the invention; and Figure 2 is an enlarged view of the xerographic drum of Figure 1 together with its closely associated parts of the machine.

Referring first to Figure 1 there is shown a xerographic copying machine incorporating the present invention. The machine includes a photoreceptor drum 1 mounted for rotation (in the clockwise direction as seen in Figure 1) to carry the photoconductive imaging surface of the drum sequentially through a series of xerographic processing stations: 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 which deposits a uniform electrostatic charge on the photoreceptor. A document to be reproduced is positioned on a platen 13 and scanned by means of a moving optical scanning system to produce a flowing light image on the drum at 3. The optical image selectively discharges the photoconductor in image configuration, whereby an electrostatic latent image of the object is laid down on the drum surface. At the development station 4, the electrostatic latent image is developed into visible form by bringing into contact with it toner particles which deposit on the charged areas of the photoreceptor.Cut sheets of paper are moved into the transfer station 5 in synchronous relation with the image on the drum surface and the developed image is transferred to a copy sheet at the transfer station 5, where a transfer corotron 7 provides an electric field to assist in the transfer of the toner particles thereto. The copy sheet is then stripped from the drum 1, the detachment being assisted by the electric field provided by a de-tack corotron 8. The copy sheet carrying the developed image is then carried by a transport belt system 9 to a fusing station 10.

After transfer of the developed image from the drum, some toner particles usually remain on the drum, and these are removed at the cleaning station 6. After cleaning, any electrostatic charges remaining on the drum are removed by an erase corotron 11. The photoreceptor is then ready to be charged again by the charging corotron 2, as the first step in the next copy cycle.

The optical image at imaging station 3 is formed by optical system 12. A document (not shown) to be copied is placed on platen 13, and is illuminated by a lamp 14 that is mounted on a scanning carriage 15 which also carries a mirror 16. Mirror 16 is the full-rate scanning mirror of a full and half-rate scanning system. The full-rate mirror 16 reflects an image of a strip of the document to be copied onto the half-rate scanning mirror 17. The image is focussed by a lens 18 onto the drum 1, being deflected by a fixed mirror 19. In operation, the full-rate mirror 16 and lamp 14 are moved across the machine at a constant speed, while at the same time the half-rate mirrors 17 are moved in the same direction at half that speed. At the end of a scan, the mirrors are in the position shown in a broken outline at the left hand side of Figure 1.These movements of the mirrors maintain a constant optical path length, so as to maintain the image on the drum in sharp focus throughout the scan.

At the development station 4, a magnetic brush developer system 20 develps the electrostatic latent image. Toner is dispensed from a hopper 21 by means of a rotating foam roll dispenser 22, into developer housing 23. Housing 23 contains a 2component developer mixture comprising a magnetically attractable carrier and the toner, which is brought into developing engagement with drum 1 by a two-roller magnetic brush developing arrangement.

The developed image is transferred, at transfer station 5, from the drum to a sheet of copy paper (not shown) which is delivered into contact with the drum by means of a paper supply system 25. Paper copy sheets are stored in two paper trays, an upper, main tray 26 and a lower, auxiliary tray 27. The top sheet of paper in either one of the trays is brought, as required, into feeding engagement with a common, fixed position, sheet feeder 28. Sheet feeder 28 feeds sheets around curved guide 29 for registration at a registration point 30. Once registered, the sheet is fed into contact with the drum in synchronous relation to the image so as to receive the image at transfer station 5.

The copy sheet carrying the transferred image is transported, by means of vacuum transport belt 9, to fuser 10, which is a heated roll fuser. The image is fixed to the copy sheet by the heat and pressure in the nip between the two rolls of the fuser. The final copy is fed by the fuser rollers along output guides 31 into catch tray 32, which is suitably an offsetting catch tray.

After transfer of the developed image from the drum to the copy sheet, the drum surface is cleaned at cleaning station 6. At the cleaning station, a housing 33 forms with the drum 1 an enclosed cavity, within which is mounted a doctor blade 34.

Doctor blade 34 scrapes residual toner particles off the drum, and the scraped off particles then fall into the bottom of the housing, from where they are removed by an auger 35.

Referring now to Figure 2, the housing 33 of the cleaning system is a rigid structure to which end plates are secured. The photoreceptor drum 1 is mounted on a shaft which carries its own bearings, the bearings being supported in the end plates by means of spring clips. The ends of the corotrons 2,7,8 and 11 are also supported by spring clips on the end plates.

The whole assembly including the cleaning system, photoreceptor and corotrons constitutes a module which is mounted in the machine by means of dowels, two at the front of the machine, and two at the rear, which engage the end plates. A slide arrangement is provided to aid in the removal and replacement of the module, and includes two channels 41 formed as part of the cleaning system housing 33. These channels 41 are on top of the housing 33, the channels being defined by members which stand up from the generally flat top of the housing and then extend towards each other. These inward extensions engage over suitable support rails which extend from the rear to the front of the main machine frame. When the module is fully 'home' in the machine, and located on the dowels, the channels 41 are out of contact with the support rails.A clip arrangement on the machine frame secures the module in place.

The photoreceptor drum 1 consists of an aluminium cylinder which has an oxide barrier layer grown onto the surface by baking it in an oven before a selenium coating is evaporated onto it by vacuum deposition. The photoreceptor is mounted on a shaft by way of end bells. The drum is driven by a gear wheel on the rear end of its shaft, driven by the main machine drive system. The photoreceptor is earthed to the drum shaft by a spring clip in the end bells.

The corotrons consist of semi-cylindrical aluminium extrusions, with insulating end blocks and corotron wires extending between the end blocks.

The charging corotron 2 and the erase corotron 11 are separate units, whereas the transfer corotron 7 and de-tack corotron 8 share a common extrusion having two semi-cylindrical channels in it. The charging corotron 2 has a positive potential applied to the wire which is such as to cause the photoreceptor surface to retain a net positive charge with a potential in the region of 900 volts depending on corotron current and radial spacing of the wire.

As the drum 1 rotates, the charged region of the photoreceptor passes through the imaging station 3 where photons of light from the non-image areas of the original discharge the photoreceptor to about 200 volts (depending on exposure level), thus leaving an electrostatic latent image on the photoreceptor. Potentially superfluous development may be precluded by appropriate flood exposure from a flood exposure system (not shown).

The electrostatic latent image is developed with toner at the developer station 4, and the toner image passes to the transfer station 5 where it is met by a sheet of paper which has been registered at registration position 30 in the paper feed mechanism to ensure that the image is transfered in correct alignment onto the paper. The transfer corotron 7 applies a charge to the paper which causes it to stick to, and be transported by, the photoreceptor drum.

The paper must be presented at the correct angle and position in order to ensure good transfer without smudges or smears, image halo, or misregistration. The angle and position at which the lead edge of the copy paper hits the photoreceptor drum is achieved by means of the halo guide 42.

The halo guide 42 is another aluminium extrusion, with a dielectric coating, mounted between the end plates close to the transfer corotron 7, with a generally horizontal field limiting portion 43, extending into the gap between the transfer corotron 7 and the photoreceptor drum 1, to accurately limit the field of the transfer corotron to a predetermined angular extent around the drum, and with a sloping guide portion 44 for guiding copy paper sheets into contact with the drum at the correct location and at the correct angle.

In order to apply an electrical bias to the copy sheet, a suitable conductive member is provided adjacent the region where the copy sheet contacts the photoreceptor drum. The conductive member must be so placed as to be in contact with the copy sheet for as long as possible during the passage of the sheet through the transfer station. Furthermore, the conductive member must be electrically isolated from the surrounding parts of the machine so that it can support the potential applied to it. With the machine configuration described above, an ideal location for the conductive member is on the sloping guide portion 44 of the halo guide. As shown in Figure 2, a thin flat conductive plate, such as a metal foil 45, is secured to the guide portion 44 by means of an insulating, double-sided adhesive tape 46.The copy paper sheet is in contact with this conductive member 45 from the time when the leading edge of the copy sheet passes over it until the time when the trailing edge of the copy sheet leaves the upper edge of the conductive member 45. Once the trailing edge has left the conductive member 45, there is no longer a potential applied to the copy sheet, so a narrow band at the trailing edge of the copy sheet (corresponding to the distance between the upper edge of the conductive member 45 and the line of contact between the copy sheet and the drum) may be rather inefficient at receiving the developed image. The whole of the rest of the copy sheet, however, will be at a suitable potential to effect good transfer to it of the image.

Although the use of a conductive member on the halo guide is described, other constructions are possible. For example, a conductive member may be a separate structure, such as a rod, bar or sheet of conductive material, extending the length of the drum.

With the xerograhic system described above, a typical bias voltage to be applied to the conductive member 45 is around 1500 volts. This bias voltage may be supplied by a separate power supply, or may be supplied by tapping the existing high voltage power supply of the machine.

Claims (3)

1. Electrostatographic apparatus including transfer means for transferring a developed electrostatic latent image from an emitting surface onto a copy sheet, the transfer means comprising a copy sheet guide for guiding the sheet onto the imaging surface, and means for creating an electric field for assisting the transfer of the developed image onto the sheet, characterised by electrical biassing means adjacent the region where the copy sheet contacts the imaging surface, for contacting the copy sheet and maintaining it at a potential suitable for efficient transfer of the developed image regardless of the electrical conductivity of the copy sheet.

2. The apparatus of claim 1 wherein the electrical biassing means comprises a conductive member and means to apply an electrical potential thereto.

3. The apparatus of claim 2 wherein the conductive member is carried by said copy sheet guide.