GB2126925A

GB2126925A - Magnetic brush development apparatus

Info

Publication number: GB2126925A
Application number: GB08325193A
Authority: GB
Inventors: Philip Roger Thompson
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1982-09-21
Filing date: 1983-09-21
Publication date: 1984-04-04
Also published as: GB8325193D0; GB2126925B

Abstract

A magnetic brush development apparatus using two-component developer includes a developer roller (41) of aluminium or an alloy of aluminium for transporting the developer into developing contact with the imaging surface. The surface of the developer roller is hard anodised, and is textured to provide, for example, a series of axially extending ridges and valleys on its outer surface. <IMAGE>

Description

SPECIFICATION Magnetic brush development apparatus This invention relates to a magnetic brush development apparatus for the development of electrostatic latent images on an imaging surface.

The apparatus uses a two-component developer which comprises magnetically attractable carrier particles and toner particles adhering triboelectrically thereto. The apparatus includes a developer roller of aluminium or an aluminium alloy for transporting the developer into developing contact with the imaging surface.

A magnetic brush development apparatus of this kind is commonly used in a xerographic copying machine. With certain kinds of developers, however, which have been selected for their excellent imaging qualities, the problem has arisen that if any defects occur in the surface of the photoreceptor, these are clearly developed by the magnetic brush development apparatus, and as the developer ages, it is sometimes the case that a black band is developed across the copies, the width of the band being dependent upon the size of the defect in the photoreceptor.

The present invention is intended to overcome this problem, and provides a magnetic brush development apparatus which is characterised in that the roller surface is hard anodised.

A magnetic brush development apparatus according to 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 present invention; Figure 2 is a cross-sectional view of the development apparatus; and Figure 3 is an enlarged partial perspective view of a developer roll suitable for use in the apparatus of the invention.

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 1 2. 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 1 5 which also carries a mirror 16. Mirror 16 is the full-rate scanning mirror of a full and half-rate scanning system. The full-rats mirror 1 6 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 1 9. 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 halfrate 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 develops 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 2-component 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 24.

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 regristration 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 1 0, which is 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 to Figure 2, the developer system 20 is a conventional non-conducting magnetic brush developer, the essential components of which are: a single development roll 41, a single transport roll 42, and a cross-mixer 43. The developer mixture comprises magnetisable carrier particles, and toner particles. The carrier particles are recirculated within the developer housing 23, and toner particles are replenished from a supply contained in a toner hopper 21, from which they are dispensed when required by a rotating foam roller 22.

The developer housing 23 consists of a lower extrusion 44, and an upper extrusion 45. The left hand extremity 46 of the lower extrusion 44, as viewed in Figure 2, and the lower extremity 47 of the front extrusion 45 define an opening adjacent the photoreceptor drum 1. The extrusions 44 and 45 are mounted between end plates (not shown) at the front and rear of the machine, the whole assembly forming a substantially sealed chamber which is closed at the top by the toner hopper 21, and by a negative pressure chamber 48.

Mounted within the housing 23 are magnet rolls 41 and 42. Roll 41 , the lower roll, is the developer roll, and roll 42, the upper roll, is the transport roll. The rolls 41 and 42 are flow formed or extended aluminium or aluminium alloy tubes surrounding fixed multi-pole rubber magnets 49 and 50 respectively. The magnets are held in position by flats on respective spindles about which the rolls 41 and 42 rotate by means of bearings in the end caps. Lower roll 41 presents developer material to the photoreceptor drum 1, in order to develop the latent image, and the upper roll 42 transports material up towards the top of the cross-mixer 43.The magnet 50 inside the transport roll 42 is designed to allow the developer material to be projected from the top of roll 42, over a baffle plate 51, and into the cross mixer 43, where the toner is cross-mixed front to rear in order to prevent non-uniformity of toner concentration across the housing.

Rolls 41 and 42 rotate clockwise as seen in Figure 2, as does the photoreceptor drum 1.

Accordingly, at their points of closest approach, the two rolls are counter-rotating, and likewise at the point of closest approach between roll 41 and drum 1 the surfaces are counter-rotating.

As the lower roll 41 rotates, it transports carrier particles in magnetic brush formation from the lowermost part of the housing 23 up into contact with the photoreceptor drum 1. The rate of supply of carrier particles into the lowermost part of the housing 23 is governed by a trim bar 52 which extends from front to rear of the housing, with one chamfered edge 53 adjacent the lower roll 41. The position of the trim bar 52 is adjustable in the left-to-right direction so that the required gap can be formed between the edge 53 and the lower roll 41. The carrier, of course, transports toner with it, and toner particles are deposited on the electrostatic latent image, in image configuration, and are arried away from the development zone on the drum 1.Depleted carrier particles continue to move upwards on the lower roll 41, until, at the uppermost part of roll 41, the magnetic field inside roll 41 disappears, and particles are attracted to the lower part of the upper roll 42. This continues to transport the carrier particles into the uppermost region of the housing 23, until they approach the upper edge of baffle plate 51. Before this point, the magnetic field with upper roll 42 disappears, and the particles are projected generally towards the top of cross-mixer 43 (to be described below), some of the particles falling into the crossmixer, and some overshooting to fall directly into the lower part of the housing 23.Replenishment of toner takes place in the upper regions of the housing 23 by means of foam roller 22 which is arranged to rotate in response to demand for more toner, thus dropping toner particles onto the carrier particles being transported by the upper roll 42 towards the cross-mixer 43.

Mounted on top of the housing 23 to the right of the toner hopper 21 is the negative pressure chamber housing 48. An outlet 54 on the top of this chamber is connected by a tube to a vacuum system which creates a small negative pressure inside the developer housing. This causes a general flow of air from the region of the photoreceptor drum into the housing, which prevents the emission of clouds of toner from the housing, and reduces contamination in the machine.

The toner housing 21 is a relatively tall, narrow container with a generally horizontal lid 55 in its top face, the lid 55 being accessible from the top of the machine. The housing 21 is so shaped as to fit around the right-hand part of the optical system of the machine, and is shaped at its lower extremity to accommodate the foam roll 22. The neck of the hopper is arranged to slightly pinch the foam roller so as to assist in dislodging toner from the roller, and drop it into the housing 23.

Just above the roller 22, a stirrer 56 is mounted, to assist the toner within hopper 21 to flow smoothly to the roller 22.

The cross-mixer 43 is a solid casting secured between the baffle plate 51 and a rear plate 57.

The cross-mixer casting has two alternating series of compartments which are arranged such that developer material falling into one of the compartments of one series will drop out of the bottom of the cross-mixer beneath the next compartment to its right, whereas developer material falling into one of the compartments of the other series will drop out of the bottom of the cross-mixer beneath the next compartment to its left. In addition, as mentioned above, some of the developer material is allowed to fall directly over the top of the cross-mixer and into the lower part of the housing 23 without cross-mixing.

Referring now to Figure 3, the developer roller 41 comprises an aluminium alloy tube which has a textured surface, such as a knurled or roughened surface, for assisting the transportation of developer material into developing contact with the photoreceptor drum 1. This surface is hard anodised to provide a thin, hard surface layer of aluminuim oxide, which is both insulative and dielectric, between the developer roller and the photoreceptor drum. One way of forming the textured surface of the developer roller is to flow-form the tube. In this case, the surface of the tube has a series of axially extending ridges and valleys as indicated in Figure 3. The surface profile of a tube formed in this way tends to be somewhat angular, and further treatment is needed to smooth out the angularities before anodising. This further treatment may consist of electropolishing or sandblasting with glass beads or bauxite. The anodised layer preferbaly has a minimum thickness of about 50 microns.

Claims

1. Magnetic brush development apparatus for the development of electrostatic latent images on an imaging surface using a two-component developer which comprises magnetically attractable carrier particles and toner particles triboelectrically adhering thereto, the apparatus comprising a developer roller of aluminium or an alloy of aluminium for transporting the developer into developing contact with the imaging surface, characterised in that the roller surface is hard anodised.

2. The apparatus of claim 1 wherein the surface of the roller is textured.

3. The apparatus of claim 2 wherein the roller comprises a flow-formed or extended tube having on its outer surface a series of axially extending ridges and valleys.