GB2141048A - 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 two 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 photoreceptor surface. A third magnetic brush roll (43) acts as a transport roll, to carry developer material into a cross-mixing arrangement (44) through which the developer material drops for mixing and recirculation. The developer material which is fed between the two developer rolls (41, 42) is split magnetically by the arrangements of stationary magnetic poles in magnets (51, 52) within the developer rolls so that the developer material is split into two streams of the desired proportions for optimum development of the electrostatic latent image. <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 a magnetic developer material 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 scases 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.

A serious problem encountered with the development system of the last-described kind is the difficulty of achieving accurate splitting of the developer material into two streams as it is fed between the two developer rolls. In GB 1,529,048, a mechanical splitting arrangement is used which, as can be seen in Figure 5 of that patent specification, is a relatively complex device. In particular, the device must be made adjustable in order to achieve the desired split.

Furthermore, considerable wear and tear of the developer material, especially the carrier, is caused by the mechanical splitting device.

It is an object of the present invention to provide an electrostatographic developing apparatus in which these disadvantages are overcome, and the apparatus of the present invention is characterised in that the magnetic fields of the developer rolls are such as to split the developer material being fed towards the imaging surface into two separate streams each capable of at least partially developing the electrostatic latent image.

The apparatus of the invention has the advantage of providing efficient development of electrostatic latent images, with accurately controlled splitting of the stream ofdeveloper material being fed between the developer rolls towards the photoreceptor, and with considerably reduced wear and tear of the developer material.

An electrostatographic 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, and Figure 4 is a cross-sectional view of the magnetic brush rolls showing the configuration of the magnetic poles therein.

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 1 3 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 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-rate 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 defiected by a fixed mirror 1 9. In operation, the full-rate mirror 1 6 and lamp 14 are moved across the machine at a constant speed, while at the same time the halfrate 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 2-component 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 ruser. 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 shet, 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 lefthand 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 roils, 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 cross-mixer 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 thorugh which passes developer material picked up by lower developer roll 42 from region 56.

The feed gap between the lower edge 62 of the valance 60 and the lower developer roll 42 is not so narrow as to form a strict metering gap (known as a trim gap) for the developer material, since the amount of developer material that is picked up by the lower developer roll is substantially self-controlling. It depends upon the strength and shape of the magnetic field of the developer roll in the pick-up region 56 in relation to the characteristics of the carrier particles of the developer material. Developer material handled in this way suffers much less wear and tear than when a trim gap is used.

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 roll, or by two developer rolls rotating in the same sense.

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.

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.

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 oils) 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.

Referring now to Figure 4, the configurations of the stationary magnets 51,52 and 53 are shown in more detail. Each magnet is of a ferrite material, magnets 51 and 53 within developer roll 41 and transport roll 43 respectively being almost semi-cylindrical in shape, and magnet 52 within lower developer roll 42 being a complete cylinder.

Magnetic poles are formed around the periphery of the magnets generally as shown, with each pole extending along the surface of the magnet parallel with the magnet axis. The magnetic polarities of the poles alternate around the circumference of each magnet, with an effective broadening of the poles of the magnets of the development rolls at their development positions adjacent the photoreceptor drum, this broadening being achieved by arranging two poles of the same polarity immediately adjacent to one another. In the discussion which follows, however, this broadened pole in each developer roll will be considered to be a single pole having its centre mid-way between the two constituent poles. Although the magnetic polarities have been shown with, for example, south poles at the development positions, it will be apparent that the same effect is achieved provided that all of the magnetic polarities are reversed from those shown.

The magnetic poles of the upper and lower developer rolls 41 and 42 are arranged such that like poles 81 and 82 respectively face one another in the region of the nip between them. In Figure 4, both of these poles are shown as north poles. The next pole 84 around the circumference of upper developer roll 41, in the direction of movement of the roll (clockwise as shown), is the broadened south pole of the upper development station. Similarly, the next pole 85 around the circumference of lower developer roll 42, in the direction of movement of the roll (anti-clockwise as shown), is the broadened south pole of the lower development station.

Apart from the poles already described in the upper developer roll 41, i.e. the north pole 81 in the nip region and the broadened south pole 84 at the upper development station, there are three further poles in the clockwise direction around the upper developer roll. Their polarities, moving clockwise, are north, south, north. The final north pole 86 is substantially vertically above the north pole 81 in the nip between the upper and lower developer rolls. The angular separation of the five poles of the upper developer roll, starting at the north pole 81 in the nip between the upper and lower developer rolls, and moving clockwise, is as follows: 540, 500, 37.50, 360. The relative field strengths of the poles, starting in the same place and moving clockwise, are 355, 425, 326, 253 and 302.

In the lower developer roll 42, in addition to the north pole 82 in the nip region and the broadened south pole 85 at the lower development station, there are six further poles of alternating polarity around the roll. Starting with the north pole 82 in the nip and moving clockwise, the angular separation of the poles is as follows: 400, 560, 34.5a, 41.50, 370, 440, 60.50, 46.50. The relative field strengths, starting again with the north pole in the nip and moving clockwise, are 335, 427, 377, 320, 404, 285, 455, 423.

The transport roll 43 has six poles around its circumference, with a south pole 87 nearest to the north pole 86 at the top of upper developer roll 41. There are two poles below this south pole and three poles above it, the poles being of alternating polarity throughout. The two lowermost poles, of which the final pole is the south pole 88, are located on the opposite side of the nip (between the upper developer roll 41 and the transport roll 43) from the developertransporting portions of the rolls. This ensures that any developer material not thrown off the transport roll in the region of the upper edge 61 of the valance 60 is captured and recirculated around the transport roll 43.This prevents depleted developer material from dropping into the region of the developer rolls, and ensures that the developer material, each time it has been used to develop an image, is recirculated via the cross-mixer before it is re-used. Starting with the south pole (88) at the bottom of the transport roll and moving clockwise, the angular separation of the poles is as follows: 350,440,31 0, 270 and 31.50. The relative field strengths are 266,411, 380,180,258,305.

The orientations of the magnetic poles of the developer rolls are set with respect to the lines joining the axes of the developer rolls to the axis of the photoreceptor drum. The lower developer roll 42 has the south pole 85 at its development station displaced by 12.50 in the clockwise direction from the line 91 joining the axes of the drum and the lower developer roll. The upper developer roll 41 has the south pole 84 at its development station displaced by 90 in the anticlockwise direction from the line 92 joining the axes of the drum and the upper developer roll.

The transport roll 43 has its south pole 87 nearest the top of the upper developer roll displaced by 230 in the clockwise direction from the line 93 joining its axis to the axis of the upper developer roll 41.

The arrangement described ensures that developer material fed by the lower developer roll 42 into the nip between the lower developer roll and the upper developer roll is magnetically split into two streams of desired proportions. Typical proportions achieved by the configuration described above are about 57% of developer material transferring to the upper developer roll, with about 43% remaining on the lower developer roll.

Claims (6)

1. Electrostatographic development apparatus including upper and lower magnetic brush developer rolls each arranged to apply a magnetic developer material 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 in that the magnetic fields of the developer rolls are such as to split the developer material being fed towards the imaging surface into two separate streams each capable of at least partially developing the electrostatic latent image.

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 the magnets within the developer rolls are arranged with like poles facing one another in the region of the nip between the two rolls.

4. The apparatus of claim 3 wherein the magnetic poles within the lower developer roll are spaced apart around the circumference of a complete cylinder and the magnetic poles within the upper developer roll are spaced apart around the circumference of approximately half a cylinder with its curved surface facing the imaging surface, the poles of each developer roll being of alternating polarity around its circumference.

5. The apparatus of claim 4 wherein the next pole of each of the magnets within the developer rolls, in the direction of movement of the respective roll, is of opposite polarity to that of said poles which face each other in the nip, and is located in the region of closest approach of the roll to the imaging surface to constitute a development station.

6. The apparatus of claim 5 wherein said poles in said development stations are of greater circumferential extent than the other poles of said magnets.