GB2141038A - Mixing device for particulate material - Google Patents

Abstract

A mixing device, of the kind known as a cross-mixer for particulate material. The device includes three rows (71, 72, 73) of chambers, with the chambers (e.g. 81, 82) of each row alternately diverting the particulate material, dropping under gravity through the device, to the left or to the right by the use of an exit aperture (90, 91) at the bottom of each chamber (81, 82) which is displaced to the left or to the right from a position vertically below the mouth (116, 104) of the chamber at the top of the device. The mouth and exit apertures of the chambers of different rows are aligned in such a fashion that on successive recirculations of an elemental volume of the material through the device there is systematic movement thereof from end to end of the device, with simultaneous movement of material from left to right and from row to row, and from right to left and from row to row. The mixing device, which in use is completely submerged in the particulate material comprises two inter-fitting castings, each having vanes (83, 84, 84) projecting from a dividing plate (77), the dividing plates separating the rows, and the vanes defining the chambers. <IMAGE>

Description

SPECIFICATION Mixing device for particulate material This invention relates to a mixing device for particulate material. Such a device is particularly, although not exclusively, useful in a magnetic brush development apparatus for the development of electrostatic latent images on an imaging surface. Such an apparatus commonly uses a twocomponent developer which comprises magnetically attractable carrier particles and toner particles adhering triboelectrically thereto.

A mixing device of the kind with which the present invention is concerned, and which is often used in a magnetic brush development apparatus, is that known as a cross-mixer. This comprises a plurality of sets of chambers arranged in parallel rows, each chamber being adapted to receive the material in a mouth at the top of the device and to release the material from an exit aperture at its lower end, each chamber having the exit aperture displaced from a position vertically below its mouth, and the mouths and exit apertures being aligned in such a way that on successive recirculations of an elemental volume of the material through the device there is systematic movement thereof from end to end of the device.

In one kind of magnetic brush development apparatus, for developing electrostatic latent images on an imaging surface, developer material is transported into developing engagement with the imaging surface by a magnetic brush developer roll, and is then transported upwardly away from the development roll by a magnetic brush transport roll. Near the top of the transport roll, the magnetic field causing the developer material to adhere to the roll vanishes, causing the developer material to be released from the roll and be either carried or projected towards the top of a cross-mixer of the kind described above.A magnetic brush development apparatus of this kind, including a cross-mixer, is described in GB 1,372,731. The cross-mixing device of that patent specification includes two parallel rows of chambers, in the form of chutes, with all the chutes in one row displacing material to the left, and all the chutes in the other row displacing material to the right. Although such a cross-mixer provides good mixing characteristics, it is desirable in certain applications to achieve even more thorough mixing. Furthermore, cross-mixing devices of this kind tend to be sensitive to tilt of the apparatus, as well as to an imbalance in the amount of developer material falling in the two rows of chutes.Either of these tendencies produces a pumping action which causes a large proportion of the developer material to accumulate at one end of the apparatus, with corresponding depletion at the other end.

The present invention is intended to overcome these problems, and provides a mixing device of the kind described which is characterised in that in use of the device it is completely submerged in the particulate material whereby material is caused to flow under gravity through the device as a result of demand for the material in a region adjacent said exit apertures.

A mixing device according to the invention, in a magnetic brush development apparatus, 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 magnetic brush development apparatus of the machine of Figure 1; Figure 3 is an enlarged cross-sectional view of part of the apparatus shown in Figure 2; Figure 4 is a plan view of the cross-mixing device of the develpment apparatus; Figures 5, 6 and 7 are diagrammatic cross sections, for explaining the operation of the crossmixer, taken through the rear, middle, and front rows of chambers respectively of the cross-mixer, and as viewed from the front of the developer housing;; Figure 8 is a perspective view of the two component parts of the cross-mixer, separated from each other, and viewed from the rear of the developer housing; and Figure 9 is a perspective view of the two component parts shown in Figure 8, but viewed from the front of the developer housing.

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 developing 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 1 6. 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 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 1 6 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 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 transpored, 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 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 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 roilers 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 roil 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 take-up 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 take-up 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 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 roll, 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 tranported 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 airfrom the region of the photoreceptor drum into the housing, which prevents the emission of ciouds 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 extrusion 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 cast from, for example, an aluminium alloy. The front casting 74 (nearest the developer rolls) has vanes projecting from both faces of a plate 75, while the rear casting 76 consists of a plate 77 with vanes projecting only from its rear face.The vanes on both castings are shaped so that the forward projecting vanes of the front casting abut the valance 60, and its rearward-projecting vanes abut the plate 77 of the rear casting. The rearward-projecting vane of the rear casting abut the inside face of the lower extrusion 45 of housing 23. In this way, the three rows of chambers are formed between the valance 60 and the lower extrusion 45, the rows being divided from one another by plates 75 and 77, and the chambers being divided from one another by the vanes projecting from the plates 75 and 77.

The chambers formed by the vanes on the front and rear castings each have a mouth at the top, and an exit aperture at the bottom, the exit aperture in every case being displaced either to the left or to the right of a line vertically below the mouth. These chambers constitute the main flow chambers for the passage of developer material through the device, as will be further described below. The chambers are grouped in pairs, and between one pair of chambers and the next pair, the vanes also define open topped hoppers which are provided with small exit apertures in their lowermost parts. In this way, the main flow of developer material through the cross mixer takes place through the main flow chambers, with a much smaller subsidiary flow through the intervening hoppers.

In each row of chambers, alternate chambers direct the developer material to the left or to the right of their mouths. Thus, in any given row, a given small volume of developer material entering one of the chambers will be displaced either to the left or to the right during its downward passage through the cross-mixer, and on recirculation by the magnetic brush rolls to the top of the device, which is a substantially vertical movement, the small volume of developer material will re-enter the top of the cross-mixer displaced either to the left or to the right of its position during its first passage. In addition, the main flow chambers are so arranged that on successive recirculations, developer material is also shifted from row to row, as will appear from the following description.

For an example of the arrangement of chambers and hoppers, reference will be made to Figures 4, and 8 and in particular to the central part of the rear row of chambers. The diagrammatic representation of the vanes shown in Figure 5 corresponds with the view of the rear casting 76 seen as the lower of the two castings depicted in Figure 8. However, the diagram of Figure 5 represents a cross section seen from the front of the developer housing, whereas the Figure 8 view is seen from the rear. Chambers 81 and 82 are formed between sloping vane 83 and vertical vane 84, and between vertical vane 84 and sloping vane 85, respectively. The mouth 11 6 of chamber 81 is formed by the upper edges of vanes 83 and 84, as well as by the plate 77 and lower extrusion 45 of the housing.Similarly, mouth 104 of chamber 82 is formed by the upper edges of vanes 84 and 85, as well as by plate 77 and lower extrusion 45. The bottoms of chambers 81 and 82 are formed by generally horizontal vanes 88 and 89, with the main exit aperture 90 of chamber 81 being formed in a position displaced to the left (as seen in Figure 5) of a position vertically below the mouth 11 6. Similarly, the main exit aperture 91 of chamber 82 is formed in a position displaced to the right of a position vertically below the mouth 104. Developer material entering mouth 11 6 of chamber 81 accordingly leaves the chamber through exit aperture 90, constituting a main flow path for the developer material.In order to prevent the formation of a static volume of developer material in the chambers, small exit apertures 92 and 93 are formed in the horizontal vanes 88 and 89 respectively, close to the vertical vane 84.

The vanes 83 and 85, as well as forming walls of the main flow chambers 81 and 82, also form the sloping floors of hoppers 94 and 95. The other sloping floors of each hopper are formed by the sloping vanes of the next main flow chambers, such as vane 96. In order to prevent the occurrence of a static volume of developer material in the hoppers, small exit apertures such as apertures 97, 98 are formed on either side of the lowest part of the hopper, just above the exit apertures of the main flow chambers.

In order to explain how the cross-mixer achieves a thorough mixing of developer material, the journey of a small sample volume of developer material will now be traced through a complete cycle. For ease of understanding, the successive mouths of the cross-mixer chambers encountered by the sample volume have been numbered consecutively in Figures 4, 5, 6 and 7, starting at 101 and finishing at 118. Turning first to Figures 4 and 5, the start point is taken to be the mouth 101 at the extreme left of the rear row. Developer material falls through this first chamber, and leaves it by the exit aperture 120 at the bottom of the cross-mixer. In passing through the crossmixer, by way of this first chamber, the developer material has been shifted to the right.The developer material is then recirculated by being carried vertically upwards by the magnetic brush rolls as described above, to be reintroduced into the cross-mixer directly above the exit aperture 120. The developer material accordingly falls partly into hopper 94 of the rear row, partly into mouth 102 (Fig. 6) of the middle row, and partly into hopper 121 of the front row. Hoppers 94 and 121 have only small exit apertures, and so maintain slow-flowing reservoirs of developer material. The main flow route at this point is provided by the chamber in the middle row which has mouth 102.

Upon the next recirculation, developer material falls into the cross-mixer displaced to the right of mouth 102, i.e. in a position corresponding with mouth 103 (Fig. 7) of the front row. Once again, developer material which falls into the hoppers of the rear and middle rows at this point forms a very minor part of the flow, with most of the flow taking place through mouth 103. The next recirculation produces a main flow through mouth 104 in the rear row, and successive recirculations give a main flow through mouth 105 (middle row), 106 (front row), 107 (rear row), 108 (middle row) and 109 (front row).Mouth 109 is the last mouth which causes developer material movement to the right, and upon recirculation at this position (i.e. at the right of the cross-mixer) developer material enters mouth 110 at the extreme right of the rear row. The next main flow mouth is mouth 111 in the front row, and successive recirculations continue the movement to the left, by way of mouth 112 (middle row), 113 (rear row), 114 (front row), 11 5 (middle row), 116 (rear row), 117 (front row) and 11 8 (middle row). After passing through the left-hand end chamber of the middle row (mouth 11 8), the developer material is once again in position for recirculation and re-entry into mouth 101 to repeat the cycle.As will be clear, developer material is passing through all of the mouths at any one time, causing simultaneous movements of developer material from left to right and from row to row, and from right to left and from row to row of the cross-mixer.

The cross-mixer described produces a thorough mixing of the developer material, without any tendency to imbalances of any kind. The entire cross-mixer is kept full of developer material at all times, with flow being produced as a result of demand for developer material in the take-up region 56 at the bottom of the cross-mixer.

Although the cross-mixer has been described as having three tows of chambers, it is clear that the same mixing principles can be applied to a crossmixer having two or more rows of chambers.

The volume taken up by the material which forms the cross-mixer is minimal, there being no cavity or chamber which is not accessible to the developer material, and all the walls of the crossmixer castings being of relatively small thickness.

This enables a maximum quantity of developer material to reside in the development apparatus, thereby reducing wear and tear on each carrier particle and so prolonging the useful working life of the developer material.

Claims (7)

1. Mixing device for particulate material comprising a plurality of sets of chambers arranged in parallel rows, each chamber being adapted to receive the material in a mouth at the top of the device and to reiease the material from an exit aperture at its lower end, each chamber having the exit aperture displaced from a position vertically below its mouth, and the mouths and exit aperatures being aligned in such a way that on successive recirculations of an elemental volume the material through the device there is systematic movement thereof from end to end of the device, characterised in that in use of the device it is completely submerged in the particulate material whereby material is caused to flow under gravity through the device as a result of demand for the material in a region adjacent said exit aperatures.

2.The mixing device of claim 1 wherein, on successive recirculations of material through the device, there is simultaneous movement of material from left to right and from row to row, and from right to left and from row to row.

3. The mixing device of claim 2 wherein there are three rows of chambers.

4. The mixing device of claim 2 or claim 3 wherein said chambers are grouped in pairs, with the two chambers of each pair having their exit apertures spaced in opposite directions from their mouths, and with intervening hoppers between one pair and the next pair of chambers, the chambers forming the main route for the flow of developer material through the device, and the hoppers having aperatures that are small compared with the exit aperatures of said chambers to provide a subsidiary, slower-moving flow through the device.

5. The mixing device of any one of claims 1 to 4 wherein the mixing device is situated in a walled enclosure such that all the particulate material entering the enclosure from above passes through the mixing device.

6. The mixing device of any one of claim 1 to 5 wherein said chambers are formed between vanes projecting from one or more dividing plates.

7. The mixing device of claim 6 comprising two components, the first component comprising a dividing plate with vanes projecting from both faces thereof and the second component comprising a dividing plate adapted to have one face in abutment with one set of vanes of the first component, and having a set of vanes projecting from its other face, the three sets of vanes defining three rows of chambers.