GB2126913A - Mixing device for particulate material - Google Patents

Abstract

The device (43) comprises two sets of chutes (65, 67 Fig. 4) arranged in parallel rows and adapted to receive the material in their mouths at the top of the device (43) and to drop the material from their lower ends, the chutes (65, 67) being inclined to the vertical such that each chute (65) of one set drops the material to the left of its mouth whereas each chute (67) of the other set drops the material to the right of its mouth. An apertured plate (68) is provided on the bottoms of the chutes (65, 67) to restrict the flow of material falling from each chute (65, 67) whereby there is no predominating movement of the material falling from the chutes (65, 67) to either left or right. The plate (68) substantially closes the bottoms of the chutes, with exit slots (69) beneath the chutes. The device is for use in a xerographic machine. <IMAGE>

Description

SPECIFICATION Mixing device for particulate material This invention relates to a mixing device for particulate material. Such as 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 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 commonly used in a magnetic flash development apparatus, comprises two sets of chutes arranged in parallel rows and adapted to receive the material in their mouths at the top and to drop the material from their lower ends, the chutes being inclined to the vertical such that each chute of one set drops the material to the left of its mouth whereas each chute of the other set drops the material to the right of its mouth.

In one kind of magnetic brush development apparatus, developer material is transported into developing engagement with the imaging surface by a lower magnetic brush roller, and is then transported upwardly away from the development zone by an upper magnetic brush roller. Near the top of the upper roller, the magnetic field causing the developer material to adhere to the roller vanishes, causing the developer material to be released from the roller and be either carried or projected towards the top of a mixing device of the kind described above, often called a crossmixer. One of the problems encountered with a mixing device of this kind is its tendency 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 produce 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 kind described which is characterised in that restrictor means are provided on the bottoms of the chutes to restrict the flow of material falling from each chute whereby there is no predominating movement of the material falling from the chute to either left or right.

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; Figures 3 and 4 are perspective views of part of a mixing device according to the invention, viewed from opposite sides; Figure 5 shows side and plan views illustrating one embodiment of a bottom plate for the device of Figures 3 and 4; and Figure 6, in views corresponding with Figure 5, shows another embodiment of bottom plate.

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 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 1 6. Mirror 16 is the full-rate scanning mirror of a full and half-rate scanning system. The full-rate mirror 16 reflects an image of a strip of the document to be copied onto the half-rate scanning mirror 17. The image is focussed by a lens 18 onto the drum 1, being deflected by a fixed mirror 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 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 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 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 isfixed 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 crossmixer 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 leftto-right direction so that the required gap can be formed between the edge 53 and the lower roll 41. The carrier, of course, transports tone with it, and toner particles are deposited on the electrostatic latent image, in image configuration, and are carried 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 within 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 cross-mixer, 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, which is shown in partial perspective view in Figures 3 and 4 is a solid casting secured between the baffle plate 51 and a rear plate 57 (Figure 2). Figure 3 shows the crossmixer from its face which is secured to the baffle plate 51, and Figure 4 shows the cross-mixer seen from its face which is fixed to the rear plate 57.

As viewed in Figure 3, the cross-mixer has two alternating series of compartments 62, 63.

Compartments 62 have a downwardly sloping surface 64 which extends just over half-way across the compartment 62, allowing developer to drop off its further edge. Compartment 63 consists of a chute 65 nearest to the baffle plate 51, and a sloping face 66, which slopes downwardly towards the further edge of compartment 63. As developer material is thrown off the upper roll 50, that which falls into the cross-mixer moves in the direction indicated generally by the arrows on Figure 3 to take one of the three routes indicated. Thus, some of the developer material drops over the further edge of sloping face 64, some of its descends to the left, as shown in Figure 3, down chute 65, and some of it drops over the further edge Of sloping face 66.In addition, some of the developer is projected over the top of the cross-mixer, and falls directly into the lower part of housing 33.

The developer materials which drops over the further edge of sloping face 64 falls into a chute 67 (Figure 4) which is similar to chute 65, but which has a downwardly sloping surface which goes to the right as viewed in Figure 3, as opposed to chute 65 which goes to the left as viewed in Figure 3.

Referring again to Figure 3, it will be seen that developer material failing into a compartment 62 will drop out of the bottom of the cross-mixer beneath the next compartment 62 to its right, whereas developer material falling into a compartment 63 will generally drop out of the bottom of the cross-mixer beneath the next compartment 63 to its left. In addition, some of the toner which falls into compartment 63 is allowed to fall directly over the top of the crossmixer.

Secured beneath the cross-mixer is a restrictor plate 68, which has slots 69 in it, and although Figures 3 and 4 show two slots underneath each chute, it is in some cases preferable to use only a single slot. The two forms of restrictor plates are shown in Figures 5 and 6, with Figure 5 showing the two-slot form, and therefore corresponding with Figures 3 and 4, and with Figure 6 showing the single-slot form.

Figures 5 and 6 show the relationship of the slots to the chutes with which they are associated.

As can be seen, the two slots in the Figure 5 embodiment are substantially symmetrically placed about the centre line of the bottom opening of the chute. For the single slot embodiment, however, the best results are obtained when, as shown, the slot is not in the centre of the bottom opening of the chute, but is closer to the wall of the chute down which the developer falls.

In use of the apparatus, developer material falls into the top of the cross-mixer, and the slots in the restrictor plate are of such dimensions as to permit the chutes to stay full of toner. Excess toner, of course, simply falls over the cross-mixer into the lower part of the developer housing. By maintaining the chutes full, constant and equal flows of developer fall from each set of chutes. It is found that such an arrangement also solves the problem of sensitivity of the apparatus to tilt. It is also found beneficial to leave a few of the apertures in the centre of the cross-mixer unrestricted, as indicated by the apertures 70 in the battle plates of Figures 5 and 6.

Claims (6)

1. Mixing device for particulate material comprising two sets of chutes arranged in parallel rows and adapted to receive the material in their mouths at the top of the device and to drop the material from their lower ends, the chutes being inclined to the vertical such that each chute of one set drops the material to the left of its mouth whereas each chute of the other set drops the material to the right of its mouth, characterised in that restrictor means are provided on the bottoms of the chutes to restrict the flow of material falling from each chute whereby there is no predominating movement of the material falling from the chute to either left or right.

2. The device of claim 1 wherein the restrictor means comprises a plate substantially closing the bottoms of the chutes, with exit slots beneath the chutes.

3. The device of claim 2 wherein the plate has two exit slots per chute.

4. The device of claim 2 wherein the plate has one exit slot per chute.

5. The device of claim 4 wherein the exit slot for each chute is closest to the wall of the chute down which the material falls.

6. The device of any one of claims 2 to 5 wherein less than all of said chutes are closed by the plate.