GB2141047A - Electrostatographic magnetic brush development apparatus - Google Patents

Abstract

Electrostatographic development apparatus comprises two magnetic brush rolls (41, 42) both adapted to develop an electrostatic latent image on the surface of a photoreceptor drum (1). The developer rolls (41, 42) are mounted one above the other for rotation in opposite senses (54, 55) so as to feed developer material between them towards the photo receptor surface. A third magnetic brush roll (43), which is a transport roll, is mounted above the upper developer roll (41) to carry the developer material upwards away from the upper developer roll, and to release the developer material onto the top of a passive cross-mixer (44) for mixing and recirculation under gravity to the lower developer roll (42). <IMAGE>

Description

SPECIFICATION Electrostatographic development apparatus This invention relates to an electrostatographic development apparatus for the development of an electrostatic latent image on an imaging surface.

The apparatus includes upper and lower magnetic brush developer rolls each arranged to apply developer material, comprising a toner and a magnetisable carrier, to the electrostatic latent image. The imaging surface is arranged for movement past the developer rolls, which are arranged for rotation in opposite senses so as to feed developer material through the nip between them towards the imaging surface.

Magnetic brush developer systems for electrostatographic development are well known, and commonly use a two-component developer material which comprises magnetically attractable carrier particles with toner particles triboelectrically adhering to them. The developer material is brought into engagement with an electrostatic latent image on the imaging surface, typically a photoconductive insulating material such as selenium, and toner particles are attracted from the carrier particles to the charged portions of electrostatic latent image. In order to bring the developer material into developing engagement with the latent image on the imaging surface, a magnetic brush roll is used. This typically takes the form of a magnetically transparent cylindrical shell, for example of aluminium, which is mounted for rotation about a stationary set of magnets.The magnetic field lines emanating from the magnets cause the magnetically attractable carrier particles to line up in a way resembling the bristles of a brush. As the cylindrical shell rotates, it causes the bristles to continuously collapse and reform, so that in operation the magnetic brush roll has the effect of a rotating brush which brushes toner particles onto the imaging surface.

In order to improve the development characteristics of magnetic brush developer systems, a number of two-roll systems have been proposed, in which two magnetic brush rolls each carry developer material into developing engagement with the imaging surface. Examples of such systems are found in GB 1,555,747 and US 3,640,248. In both cases the two magnetic roll developers are arranged for rotation in the same sense, in the former case both contrary to the direction of motion of the imaging surface, and in the latter case both in the same direction as the imaging surface.In both cases the developer rolls are situated below the imaging surface, and a blanket of developer material is carried from one developer roll to the other in contact with the imaging surface In another configuration of two-roll magnetic brush developer, the two developer rolls are situated adjacent an imaging surface, which is the cylindrical surface of a drum, with one developer roll above the other. In US 3,638,614, both developer rolls rotate in the same sense, carrying developer material downwardly over the imaging surface. A bucket-type conveyor system circulates developer material from the bottom of the developer housing to the top.In GB 1 ,489,435, the developer rolls rotate in opposite senses and are arranged such that the lower roll serves only to convey developer material from the bottom of the housing to the upper roll, whereby only the upper roll acts as a developer roll. In GB 1,529,048, the two developer rolls rotate in opposite senses, so as to feed developer material between them towards the imaging surface, with both rolls acting as developer rolls. In order to split the stream of developer material being fed towards the imaging surface into two streams, a mechanical stream splitting bar is located between the two rolls adjacent the imaging surface. Developer material is released from the top of the upper developer roll to fall into a rear chamber, from the bottom of which an auger conveys the developer material into the sump adjacent the lower developer roll.

One of the problems encountered with those of the known two-roll magnetic brush developer systems which do not include a cross-mixing arrangement is that of partial depletion of the developer material. Such depletion might take place in the following circumstances. If a region requiring a particularly large amount of toner is developed, for exampie a large solid black patch, then for several copy cycles thereafter, there may be less than the optimum amount of toner available for development in the developer material. This is because recirculation of the developer material brings the developer material back to the same position on the imaging surface, and the rate of supply of new toner material is often insufficient to bring the toner concentration back up to the optimum level before the next development takes place.In GB 1,529,048, this problem is alleviated by the use of an auger arrangement to shift the toner material longitudinally of the developer rolls. This achieves a certain measure of cross-mixing, because the developer material is moved along the developer roll, and has the chance to acquire fresh toner material during its travels before recirculation to a position in which it is developing an image.

Furthermore, the developer material is distributed along the length of the developer roll during transportation by the auger. However, such a development system suffers from the disadvantage that there is considerable wear and tear of the developer material, especially the carrier, due to the relatively harsh mechanical action of the augers, as well as by the mechanical splitting of the developer material into two streams which is achieved by the splitting bar.

It is an object of the present invention to provide an electrostatographic development apparatus in which these disadvantages are overcome, and the apparatus of the present invention is characterised by means for removing developer material from the upper developer roll, after the application of the developer material to the imaging surface, and for releasing the developer material into a static cross-mixer for mixing and recirculation under gravity to the lower developer roll.

The apparatus of the invention has the advantages of providing efficient development, due to the use of counter-rotating magnetic brush rolls, with effective cross-mixing which prevents local depletion, and with very low wear and tear of the carrier particles due to the gentle action of the magnetic brush rollers, and the lack of active mechanical cross-mixers or stream splitters.

An electrostatograph ic 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, Figure 3 is an enlarged cross-sectional view of part of the apparatus shown in Figure 2.

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 directon 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 iamp 14 that is mounted on a scanning carriage 1 5 which also carries a mirror 1 6. Mirror 1 6 is the full-rate scanning mirror of a full and half-rates scanning system. The full-rate mirror 1 6 reflects an image of a strip of the document to be copied onto the half-rate scanning mirror 1 7. The image is focussed by a lens 1 8 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 half-rate mirrors 1 7 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 2component developer mixture comprising a magnetically attractable carrier and the toner, which is brought into developing engagement with drum 1 by a three-roll 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 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 Figures 2 and 3, the developer system 20 is a non-conducting magnetic brush developer, the essential components of which are: upper and lower development rolls 41 and 42, a single transport roll 43, and a cross-mixer 44. The developer mixture comprises magnetisable carrier particles, and toner particles. The carrier particles are recirculated within the developer housing 23, and the toner particles some of which are consumed during development, 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 45, and an upper extrusion 46. The left hand extremity 47 of the lower extrusion 45, as viewed in Figure 2, and the lower extremity 48 of the front extrusion 46 define an opening adjacent the photoreceptor drum 1. The extrusions 45 and 46 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 49.

Mounted within the housing 23 are the three magnetic brush rollers 41,42 and 43. Rolls 41 and 42 are the developer rolls, and roll 43, above the upper developer roll 41, is the transport roll.

The rolls 41, 42 and 43 are flow formed or extruded aluminium or aluminium alloy tubes surrounding fixed multi-pole rubber magnets 51, 52 and 53 respectively. The magnets are held in position by flats on respective spindles about which the rolls 41 to 43 rotate by means of bearings in the end caps.

The operation of the three-roll developer arrangement of the present invention will now be described in more detail with reference to Figure 3. Upper developer roll 41 and lower developer roll 42 are mounted for rotation in opposite senses, as indicated by arrows 54 and 55.

Developer material is picked up by the lower developer roll 42 in the region 56, which is a region near the bottom of the housing 23, adjacent the bottom of the cross-mixer 44. As indicated by the arrows in Figure 3, developer material is carried upwards on the portion of lower developer roll 42 which is furthest from the photoreceptor drum 1, and is carried into the gap between the lower developer roll 42 and the upper developer roll 41. Upper developer roll 41 is rotating in the opposite sense to lower developer roll 42, so the top of the lower roll and the bottom of the upper roll are moving in the same direction, i.e. towards the photoreceptor drum 1.The magnetic poles within stationary magnets 51 and 52 are arranged to cause splitting of the stream of developer material into substantially equal streams, one of which is carried upwards against the photoreceptor surface by the upper developer roll 41, and the other of which is carried downwards against the photoreceptor by the lower developer roll 42. During passage of the developer material over the photoreceptor surface adjacent both developer rolls, development of the electrostatic latent image on the photoreceptor takes place by the electrostatic deposition of some of the toner particles in the developer material.

The developer material on the lower developer roll 42, after developing the latent image on the photoreceptor, is carried down to the bottom of housing 23, and back into region 56, where it joins and mixes with developer material that is dropping under gravity out of the bottom of crossmixer 44. Developer material on the upper developer roll 41, after developing the latent image on the photoreceptor, is carried upwards towards the transport roll 43. Transport roll 43 is mounted for rotation in the same rotational sense as upper developer roll 41, and the magnetic poles in magnet 51 of upper developer roll 41 are such that its magnetic field substantially disappears in the nip between the upper developer roll 41 and the transport roll 43, so developer material is substantially prevented from entering the nip between the two rolls.Transport roll 43 accordingly carries developer material upwardly away from the photoreceptor drum, and towards the rear of the housing 23. The magnetic field of magnet 53 in transport roll 43 substantially disappears just beyond the top of the transport roll 43, so developer material leaves the transport roll to fall into the reservoir 58 of developer material which fills the cross-mixer 44 at all times. A valance 60 divides the region of housing 23 containing the magnetic brush rolls from the region containing the cross-mixer 44. The upper edge 61 of the valance assists in deflecting developer material into the reservoir 58 of developer material, and the lower edge 62 of the valance defines a feed gap adjacent the lower developer roll 42 through which passes developer material picked up by lower developer roll 42 from region 56.

As indicated in a general way in Figure 3 by the dots representing developer material, and by the accompanying arrows, the developer material is circulated around the developer housing, and comes into developing engagement with the photoreceptor at two separate places. The first development takes place adjacent the upper developer roll 41, when the developer material is carried in a direction against the direction of movement of the photoreceptor. The second development takes place adjacent the lower developer roll 42, where the developer material is carried in the same direction as the photoreceptor.

The different development characteristics encountered by development in each of these two modes are found to produce in combination a pronounced improvement in the quality of developed images as compared with development by a single magnetic brush developer roil, or by two developer rolls rotating in the same sense.

During development the developer material loses a certain proportion of its toner particles, and this loss is made good by adding fresh toner particles. The addition 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 transport roll 43 towards the reservoir 58.

In order to contain developer material in the housing 23, a magnetic strip seal 77 is provided along the edge of the housing formed by the lower extremity 48 of the front extrusion 46. A groove or ledge is provided along the edge of extremity 48 to accommodate a magnetic strip of rectangular cross-section. The magnetic strip may be a flexible strip of a ferrite material, and the desired length of strip may be secured by adhesive into the groove or ledge in extremity 48. The magnetic poles of the magnetic strip 77 are arranged so that its face adjacent to the photoreceptor is of one polarity, that polarity being selected so as to repel carrier particles being carried towards it by the upper developer roll 41. A similar magnetic strip seal (not shown) may be provided along the edge of the housing formed by the left-hand extremity 47 of the lower extrusion 45.

Mounted on top of the housing 23 to the right of the toner hopper 21 is the negative pressure chamber housing 49. An outlet 63 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 64 in its top face, the lid 64 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 65 is mounted, to assist the toner within hopper 21 to flow smoothly to the roller 22.

The cross-mixer 44 is located between the valance 60 and the lower extension 45 of housing 23, with the lowermost part of the cross-mixer adjacent the developer take-up region 56. The cross-mixer consists of three parallel rows of chambers; a front row 71 closest to the developer rolls, a middle row 72, and a rear row 73 furthest from the developer rolls. The three rows of chambers are formed by sets of vanes projecting from dividing walls, and in a preferred embodiment the cross-mixer consists of two component parts, which are suitably aluminium alloy castings. The front casting 74 (nearest the developer rolls) has vanes projecting forwardly to abut the valance 60 and rearwardly to abut the rear casting 76, while the rear casting 76 has vanes projecting only rearwardly to abut the lower extension 45 of housing 23.

Each chamber in each row has a mouth at the top and an exit aperture at the bottom of the cross-mixer. The exit aperture in each chamber is displaced from a position vertically below the mouth of that chamber. The chambers in each row are arranged with their exit apertures displaced alternately to the left and to the right of their mouths. Thus a quantity of developer material entering the mouth of one of the chambers will be displaced to the left or to the right (along a direction parallel with the axes of the developer rolls), so that on recirculation by the developer rolls to the top of the cross-mixer, which is a substantially vertical movement, the developer material will re-enter the top of the cross-mixer displaced to the left or the right of the position where it entered on the previous passage through the cross-mixer.In any one position along the cross-mixer, the mouth of only one chamber is available for entry of developer material, with the mouths and exit apertures being arranged such that on each recirculation the next mouth entered by the developer material is in a different row, with the developer material displaced in the same direction as on the previous recirculation, until the end of a row is reached. The direction of displacement is then reversed, and the developer material is recirculated with displacement in the opposite direction to that just described by way of a set of chambers alternating with the first set.

The cross-mixer is.maintained full of developer material at all times, and it will accordingly be clear that developer material is passing through all of the mouths at any one time, causing simultaneous movements of developer from left to right and from row to row, and from right to left and from row to row.

A more detailed description of a cross-mixer of the kind outlined above will be found in our copending patent application no.

Claims (9)

1. Electrostatographic development apparatus including upper and lower magnetic brush developer rolls each arranged to apply developer material, comprising a toner and a magnetisable carrier, to an electrostatic latent image on an imaging surface that is arranged for movement past the developer rolls, the developer rolls being arranged for rotation in opposite senses so as to feed developer material through the nip between them towards the imaging surface, characterised by means for removing developer material from the upper developer roll, after the application of the developer material to the imaging surface, and for releasing the developer material into a static cross-mixer for mixing and recirculation under gravity to the lower developer roll.

2. The apparatus of claim 1 wherein the two developer rolls are arranged with one above the other, and wherein the imaging surface is arranged to move downwardly past the developer rolls, whereby the lower developer roll moves developer material in the same direction as the imaging surface at the point of closest approach between them, and the upper developer roll moves developer material in a direction contrary to the direction of motion of the imaging surface at the point of closest approach between them.

3. The apparatus of claim 1 or claim 2 wherein said means for removing developer material from the upper developer roll comprises a transport roll in the form of a third magnetic brush roll positioned above the upper developer roll and arranged to carry developer material upwards away from the upper developer roll for release into the cross-mixer.

4. The apparatus of claim 3 wherein the transport roll is mounted for rotation in the same rotational sense as the upper developer roll.

5. The apparatus of claim 3 or claim 4 wherein the magnetic brush rolls and the cross-mixer are contained within a housing, and the housing contains a dividing plate for dividing the housing into a first compartment containing the magnetic brush rolls and a second compartment containing the cross-mixer.

6. The apparatus of claim 5 wherein the upper edge of the dividing plate is adjacent the upper part of the transport roll to assist in transfering developer material from the transport roll into the cross-mixer.

7. The apparatus of claim 5 or claim 6 wherein the lower edge of the dividing plate is adjacent the lower part of the cross-mixer, and adjacent the portion of the lower developer roll furthest from the imaging surface, said lower edge defining flow channels for developer material between itself and the housing and between itself and the lower developer roll.

8. The apparatus of any one of claims 5 to 7 wherein said second compartment containing said cross-mixer, comprises a reservoir for developer material, whereby said cross-mixer is always full of developer material.

9. The apparatus of any one of claims 1 to 8 including a toner dispensing arrangement adapted to dispense toner particles onto developer material being released into the cross-mixer.