Magnet roller and developing device

Abstract: A magnet roll (1) includes a columnar magnet; a through bore (2) having a cross-section proving a thickness of the columnar magnet which is uneven along a circumference thereof; wherein a most thick portion of the columnar magnet has a strongest magnetic pole.

Description: The present invention relates to a developing apparatus for developing an electrostatic image on an image bearing member of an image forming apparatus or process cartridge and a magnet roll usable for the developing apparatus. It also relates to a cartridge including such developing apparatus; an image forming apparatus including such developing apparatus or cartridge, and methods of developing an electrostatic image and forming an image on a paper recording medium. In an image forming apparatus using an electrophotographic type process, an electrostatic latent image formed on the surface of an image bearing member such as a photosensitive member, is visualised with a developer using a developing apparatus, and the visualised image is transferred onto a recording paper from the image bearing member by a transfer device, and is fixed by a fixing device; and finally discharged from the image forming apparatus. The developing apparatus in the image forming apparatus comprises a developer carrying member extended close to and in parallel with the image bearing member, a magnet roll disposed in the developer carrying member, a developer and a developer layer thickness regulation member for regulating the amount of the developer on said developer carrying member. Usually, the developer is a one component developer using magnet toner or two component developer containing magnetic carrier and non-magnetic toner. In order to carry the developer on the developer carrying member surface and to feed it to the neighbourhood of the image bearing member, the developer carrying member is in the form of a sleeve rotatable relative to the magnet roll which is stationary therein. The magnet roll usable with such a developing apparatus, is produced by extruding a mixture of resin material and magnetic powder and magnetising it. It is mounted on a metal shaft. In another example, shaft and magnet portions are of the same material which are injection moulded, and magnetised (integral type). In both of these cases, the shaft portions at the opposite ends of the magnet roll are thinnest, and at least one of the ends is formed into a non-circular shape, i.e. square, D-shaped, for example, to permit engagement and support. When the size of the magnet roll is reduced, the following problems arise. In the case of magnets mounted around a metal shaft, the magnet portions are too small to provide the desired magnetic force. When the magnetic force is insufficient, a foggy background tends to be produced on the image. Particularly, one magnetic pole nearest to the image bearing member, is required to be the strongest magnetic pole. This requires that the size of the magnet roll is determined by this largest magnet, so that the desired downsizing is limited. In the shaft integral type, the mechanical strength of the shaft portion is low because of the property of material (resin), and therefore, it is relatively easily broken. In the conventional type having such shaft, erroneous mounting is liable to occur upon assembling the magnet roll into the developing apparatus. Therefore, as described above, the cross-sectional configuration of the end portion of the shaft is made D-shaped for example. In this case, the position of the magnet has to be aligned to the desired magnetic pole position in the rotational direction of the shaft. Upon the downsizing of the magnet roll, there is a tendency of decrease of the magnetic force with the result that the developer leakage tends to occur in the developing device, as described above, and therefore developer leakage tends to occur. It is a principal object of the present invention to provide a small diameter magnet roll having sufficient strength at its supporting portion. It is another object of the present invention to provide a magnet roll having a small diameter but which can also provide a high magnetic force. According to one aspect of the present invention, as defined in claim 1 attached, there is provided a magnet roll comprising a columnar magnet having a through-bore of cross-section providing a thickness of said columnar magnet which is uneven along a circumference thereof; characterised in that: the thickest portion of said columnar magnet includes the strongest magnetic pole of said columnar magnet. According to another aspect of the present invention as defined in claim 6 attached, there is provided a developing apparatus comprising a developer container for containing a developer; a developer carrying member for carrying the developer; a magnet roll, as above defined, arranged in said developer carrying member, and a supporting member inserted into one end of the through-bore of said magnet roll to fixedly support said magnet roll. It is acknowledged that a magnetic roll of the type recited in the preamble of claim 1 attached is disclosed in each of the following references: JP-A-3-189667 and JP-A-61-115303. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. In the accompanying drawings: Figure 1(a) is a perspective view of a magnet roll according to an embodiment of the present invention; Figure 1(b) is a partially sectional view of a developing device according to an embodiment of the present invention; Figure 2(a) is a sectional view of a process cartridge using a magnet roll type developing device according to an embodiment of the present invention; Figure 2(b) is a sectional view of an image forming apparatus according to an embodiment of the present invention; Figures 3(a) and (b) are sectional views of a magnet roll according to an embodiment of the present invention; and Figure 3(c) is a perspective view of the magnet roll of Fig. (a), (b). Referring to Figure 1, a first embodiment of the present invention will be described. Figure 1(a) is a perspective view of the magnet roll in the first embodiment. Magnet roll 1 in this embodiment, as shown in Figure 1(a) is of columnar shape having a D-shape through-bore or hole 2. Said magnet roll 1 is produced by extrusion moulding of a mixture (magnetic material) of a magnetic powder and a binder such as resin material, rubber or the like. Two to eight parallel longitudinal poles are formed by magnetisation with alternating polarities in the circumferential direction. No supporting shaft or another shaft for the magnet roller 1 is formed, and it is of the same material and of the same sectional cross-section over the entire length. The thus constructed magnet roll 1 is supported in the. developing apparatus shown in Figure 1(b). The developer carrying member 3, fixed on the driving gear 7 (which is a rotation transmission means) is supported on a bearing 5 and therefore is rotatable, but the magnet roll 1 is fixed to the developing apparatus by engagement between the D-shaped through-hole 2 of the magnet roll 1 and a projection of the supporting member 4 which is fixed on a developing device frame 8 and which is integral with bearing 5 for the developer carrying member 3. As regards the driving gear 7 side, a sliding contact member 6 is provided between the inside of the developer carrying member 3 and the magnet roll 1 to support it smoothly so as to prevent vibration of the magnet roll 1 due to rotation of the developer carrying member 3. Therefore, the magnet roll 1 and the developer carrying member 3 are kept from contact from each other. In this embodiment, the cross-sectional configuration of the through-hole 2 of the magnet roll 1 is not rotational-symmetry engageable with the supporting member 4, so that the magnet roll 1 can be fixed assuredly and easily to the developing apparatus, and the orientation can be easily assured to permit correct positioning of the poles. The extension portion of the supporting member 4 of the magnet roll 1 (indicated by reference numeral 4 in Figure 1) is smaller than the diameter of the magnet roll 1 except for the engaging portion, and the total length thereof is shorter than the total length of the developer carrying member 3. By doing so, lateral leakage of the developer at the region of the developer carrying member 3 can be prevented. Because of the structure of the supporting member 4, the diameter of the magnet roll 1 can be maximised relative to the inside diameter of the developer carrying member 3, and therefore, sufficient magnetic force can be provided. The engagement or locking can be made firmer by using a magnetic member such as one of iron for the supporting member 4 for the magnet roll 1. As described above, the magnet roll 1 is columnar and has a non-rotational-symmetry through-hole, and therefore, a small size magnet roll without a shaft or a supporting shaft. The total length of the magnet roll 1 is made shorter than the total length of the developer carrying member 3, and the outer diameter of the upper portion of the extension of the magnet roll 1 except for the engaging portion of the supporting member 4 of the magnet roll 1 is made smaller than the outer diameter of the magnet roll 1, and therefore leakage of the developer can be prevented, and it can be downsized, and a low cost magnet roll 1 without meaningless portion can be accomplished. The outer diameter of the magnet roll 1 is maximum so that even when the same magnetic force as in a conventional apparatus is desired, the size of the developer carrying member 3 and the size of the developing apparatus can be reduced. The moulding or manufacturing method described above is not limiting. Figure 2 shows a second embodiment of the present invention. In this embodiment, the above-described magnet roll 1 is incorporated in a process cartridge detachably mountable to a main assembly of an image forming apparatus such as a copying machine or printer. Figure 2(a) is a sectional view of the process cartridge. The developing apparatus D comprises a developer container 10 as developer supply means, a developer layer thickness regulation member 44, a magnet roll 1 and a developer carrying member 3. The process cartridge 50 is constituted by the developing device D, the image bearing member 11, a cleaning device 12, charging device 14 and a cover C, as a unit. Figure 2(b) is a view when the process cartridge 50 is mounted on a main assembly 100 of the image forming apparatus. The device such as a process cartridge detachably mountable relative to image forming apparatus main assembly is desirably small in size, and therefore, the magnet roll 1 of this invention is particularly preferred in this case. Referring to Figure 3, a third embodiment of the present invention will be described. The same reference numerals as in embodiment 1 are assigned to the elements having the corresponding functions, and detailed description thereof is omitted for simplicity. The magnet roll 1 has a different cross-section, as shown in Figure 3. In the first embodiment, the through-hole of D-shape is provided substantially at the centre of the circle of cross-section. But in this embodiment, a through-hole is formed so as to provide non-uniform thickness of the magnetic material portion around the hole. In Figure 3(a) the internal through-hole 2 is eccentric relative to the centre of the outer circular surface. By doing so, there are provided a larger thickness portion and a smaller thickness portion. The larger thickness portion can be given a strong magnetic force. This thick portion can be magnetised as the development pole which is faced to the image bearing member 11 and which requires stronger magnetic force, by which the cross-sectional area can be minimised, thus permitting reduction of weight and cost. It is added that in this embodiment, the through-hole is not of necessity of a rotation-asymmetrical shape. It may be a right square as shown in Figure 3(a). This is advantageous in that if an attempt is made to engage it with the supporting member 4 with 90 degrees deviation from correct angular position, the magnet roll 1 and the developer carrying member 3 are interfered with each other, so that erroneous engagement is prohibited in effect. As shown in Figure 3(b), the outer periphery can be made non-circular, i.e. oval. With this shape, equivalent advantageous effects are provided. In the case that the magnet roll is supported in the developer carrying member 3 as in the first embodiment, a sliding member 6 is mounted on the outer periphery of the magnet roll 1 to provide a circular outer sliding surface, as shown in Figure 3(c). As described in the foregoing, the section of the magnet roll 1 is made rotation-asymmetrical or irregular, and the thickness difference is made to correspond to the strength of the magnetic pole, by which the cross-sectional area of the magnet roll 1 can be minimised so that the cost and weight thereof can be reduced. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: A magnet roll (1,2) comprising: a columnar magnet (1) having a through-bore (2) providing a thickness of said columnar magnet which is uneven along a circumference thereof; characterised in that : the thickest portion of said columnar magnet (1) includes the strongest magnetic pole of said columnar magnet. A magnet roll according to claim 1, wherein the through-bore (2) of said columnar magnet (1) is of non-circular cross-section. A magnet roll according to claim 2, wherein the through-bore (2) of said columnar magnet (1) is of D-shaped cross-section. A magnet roll according to claim 2, wherein the through-bore (2) of said columnar magnet (1) is of a rectangular cross-section. A magnet roll according to any preceding claim, wherein said columnar magnet (1) is of resin material containing magnetic powder. A developing apparatus comprising: a developer container (10) for containing a developer; a developer carrying member (3) for carrying the developer; a magnet roll (1,2) as claimed in any preceding claim, arranged in said developer carrying member, and a supporting member (4) inserted into one end of the through-bore (2) of said magnet roll to fixedly support said magnet roll (1,2). A developing apparatus according to claim 6, further comprising an outer supporting member (6) at the end of said columnar magnet (1) which is opposite said one end into which said supporting member (4) is inserted. A developing apparatus according to either of claims 6 or 7, wherein the strongest magnetic pole is a developing magnetic pole provided at a developing zone where said developer carrying member (3) is to be arranged opposite an image bearing member (11) for bearing an electrostatic image. A process cartridge (50) for developing an electrostatic latent image, said cartridge including a developing apparatus according to any of claims 6 to 8. An image forming apparatus (100) comprising: an image bearing member (11) for bearing an electrostatic image; image forming means for forming the electrostatic image on said image bearing member (11); a developing apparatus (1-4,10) for developing the electrostatic image on said image bearing member (11), which developing apparatus is as claimed in any of claims 6 to 8 and which is arranged relative to said image bearing member (11) with said thickest portion of said columnar magnet nearest to said image bearing member (11); and transfer means for transferring the image developed by said developing apparatus (1-4,10) onto a recording paper. An image forming apparatus (100) according to claim 10 including a detachable process cartridge (50) incorporating said image bearing member (11) and said developing apparatus (1-4,10). A method of developing an electrostatic latent image which comprises applying toner to the image by means of a developing apparatus as claimed in any of claims 6 to 8. A method of forming an image on a paper recording medium wherein an electrostatic latent image on an image bearing member (11) is developed by the method of claim 12 and the developed image is transferred onto the paper recording medium.

Inventors: HIBI TAKASHI, KONISHI GAKU, SHOJI TAKEO, HIBI, TAKASHI, KONISHI, GAKU, SHOJI, TAKEO, HIBI, TAKASHI, C/O CANON K.K., KONISHI, GAKU, C/O CANON K.K., SHOJI, TAKEO, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G03G15
IPC codes: G03G15, G03G21, H01F7