Developing apparatus

Abstract: A developing apparatus includes a developer container (200) for accommodating a developer having magnetic particles; a rotatable developer carrying member (220), provided in a opening of the the developer container, for carrying the developer; a magnet (210) in the developer carrying member (220); a magnetic member (160), provided at a end portion of the developer carrying member, being subjected to magnetic force of the the magnet; wherein the magnetic member (160) is disposed with a gap relative to the developer carrying member (220), along a circumference of the developer carrying member; a regulating member (230) for regulating the developer on the the developer carrying member; wherein magnetic confining force for the developer by the magnetic member is weaker toward a regulation portion of the regulating member in a circumferential direction of the developer carrying member.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to a developing device for developing a electrostatic image on a image bearing member usable with a image forming apparatus type a electrophotographic type or an electrostatic recording. Heretofore, in the developing device, sealing for preventing leakage of the developer at the end portions of a developing sleeve is widely used. As a sealing method in U.S. Patent No. 5177536, a member is disposed opposed to the developing sleeve with a gap at the end portion in the axial direction also called thrust direction of the developing sleeve to use magnetic confining force. Figure 2 shows an example of the developing device preventing the leakage of the developer from the end portion by the magnetic confining force. Figure 2, (a) is a sectional front view, and Figure 2, (b) is a sectional side view. In Figure 2, designated by 200 is a developer container, and a shaft 211 of a magnet roller 210 is fixed to the developer container 200 by a fixing member 212. Therefore, the magnet roller 210 is stationary relative to the developer container 200. In addition, designate by 220 is a developing sleeve of a non-magnetic material containing the above-described magnet roller 210, and is supported rotatably relative to the developer container 200, by the bearings 221. The above-described magnet roller 210, as shown in Figure 2, has magnetized 5 magnetic poles (S1 pole, S2 pole, N1 pole, N2 pole, N3 pole), and the two component developer including the non-magnetic toner and magnetic carrier is taken up on the outside surface of developing sleeve 220 by the function of N3 pole (take-up pole). The taken up developer is transported to S2 pole (transportation pole) in accordance with the rotation of the developing sleeve 220 in the direction of an arrow R in Figure 2, and thereafter is coated as the thin layer on the outside surface of the developing sleeve 220 by the function of N1 pole (cutting pole) and the blade 230 as a regulation means for regulating a layer thickness of the developer. The developer coated as the thin layer develops a latent image formed on a unshown latent image bearing member adjacent the opposing position of S1 pole (developing pole). The non-magnetic toner is consumed by the developing action,and toner content is lowered. Such a developer is transported to N2 pole (take-in pole),and thereafter is not transported toward N3 pole from N2 pole by the function of the repelling magnetic field constituted by N3 pole and N2 pole. The developer exceeding a amount of carrying limit of the developer by N2 pole falls by the gravity in a first stirring chamber 240 in the developer container 200. In the first stirring chamber 240, the first screw 241 is provided, and the developer having fallen from the developing sleeve 220 and the developer remaining without being taken up by (take-up pole) N3 pole are transported in the direction of an arrow S in Figure 2 while being stirred by the rotation of the screw. The developer transported up to the end portion of the first stirring chamber 240 by the first screw 241 is transported into the second stirring chamber 250 through the opening at an end portion of a partition wall 201. By an unshown toner content detection device and a toner supply device a proper amount of the non-magnetic toner is supplied, and thereafter is transported while being stirred by the rotation of the second screw 251, and is returned to the first stirring chamber through the opening at another end portion of the partition wall 201 to be used for the repeated developing action. In order to prevent the leakage of the circulated developer at the neighborhood of opposite end portions in the thrust direction of the developing sleeve 220, there is provided a magnetic member 260. Referring to Figures 3 and 4,the mechanism of the leakage prevention of the developer by the magnetic member 260 will be described. The same reference numerals as in Figure 2 are assigned to the elements having the corresponding functions, and the detailed description thereof are omitted for the sake of simplicity. As shown by the magnetic force line in Figure 3, when the magnetic member 260 is provided, the strong magnetic confining force is produced between the magnetic member 260 and magnet roller 210 so that the developer is confined between the magnetic member 260 and developing sleeve 220 along the magnetic force line by the magnetic confining force as shown in Figure 4. The developer is not moved in accordance with the rotation, even if the developing sleeve 220 is rotated. The developer per se is confined by the strong magnetic confining force to form a wall between the magnetic member 260 and the developing sleeve 220 so that the leakage of the developer from developer container 200 at the neighborhood of opposite end portions in the thrust direction of the developing sleeve 220 is prevented. However, when the length in the thrust direction of the developing sleeve 220 is shortened as much as possible in an attempt to meet the recent demand for the downsizing of the image forming apparatus, and therefore, the length of the magnet roller 210 in the thrust direction of is also shortened as much as possible, then the following problems arise. Referring to Figures 5 and 6, the problems will be described. In Figures 5 and 6, the same reference numerals as in Figure 2 are assigned to the elements having the corresponding functions, and the detailed description thereof are omitted for the sake of simplicity. As shown in Figures 5 and 6, when the length of the magnet roller 210 in the thrust direction is shortened, the position of the magnetic member 260 necessarily approaches to the end portion of the magnet roller 210. This is because, when the position of the magnetic member 260 is also deviated in the thrust direction simultaneously the width, in the thrust direction, of the developer applied in the thin layer on the developing sleeve 220 is decreased, with the result that the entire area of the image of the maximum width in the thrust direction which is determined in accordance with the specifications of the image forming apparatus using the developing device, is now unable to be developed. On the other hand, when the position of the magnetic member 260 further approaches the end portion of the magnet roller 210, as shown in Figure 5, the magnetic force line formed between the magnet roller 210 and the magnetic member 260 becomes further sparse and broad, since the magnetic flux density at the end portion of magnet roller 210 is small as compared with the position other than end portion. In addition,the magnetic flux density of the magnet roller 210 is larger inside in the thrust direction, and therefore, the magnetic force line is pulled inwardly in the thrust direction into the state as shown in Figure 5. Therefore, the configuration of the wall of the developer formed between the developing sleeve 220 and the magnetic member 260 becomes as shown in Figure 6. When the wall of the developer becomes as shown in Figure 6 adjacent the blade, the end portion of the developer to be coated as the thin layer on the developing sleeve 220 is obstructed by the wall of the developer stretched widely in thrust direction so that the developer coating width on the developing sleeve in the thrust direction is narrowed, and therefore, the entire area of the image of the maximum width in the thrust direction is now unable to be developed. SUMMARY OF THE INVENTION A principal function of the present invention is to provide a developing device wherein the uniform layer thickness of the developer is provided over the developing width on the developer carrying member. Another concern of the present invention is to provide a developing device wherein the length in the thrust direction of the developer carrying member is shortened. According to an aspect of the present invention, there is provided a developing apparatus comprising: a developer container for accommodating a developer having magnetic particles; a cylindrical rotatable developer carrying member, provided in an opening of the said developer container, for carrying the developer; a magnet in said developer carrying member; a magnetic member, provided at an end portion of said developer carrying member and being subjected to magnetic force of the said magnet to counteract axial leakage of developer; wherein said magnetic member is disposed with a radial gap relative to said developer carrying member, along a part of the circumference of said developer carrying member; and a regulating member extending axially of the developer carrying member for regulating the thickness of a layer of developer on the said developer carrying member; characterised in that the magnetic confining force for the developer between the magnet and said magnetic member is weaker toward said regulating member in a circumferential direction of said developer carrying member. These and other 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. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1, (a) is a sectional front view of a developing device according to an embodiment of the present invention. Figure 1, (b) is a sectional side view of an apparatus of Figure 1, (a). Figure 2, (a) is a sectional front view of the developing device as a background of the present invention. Figure 2, (b) is a sectional side view of the apparatus of Figure 2, (a). Figure 3,Figure 4 illustrate prevention of leakage of the developer. Figure 5,Figure 6 illustrate task of to be solved by the present invention. Figure 7,Figure 8 illustrate effects according to an embodiment of the present invention. Figure 9, (a) is a sectional front view of the developing device according to another a embodiment of the present invention. Figure 9, (b) is a sectional side view of an apparatus of Figure 9, (a). Figure 10 is a partial sectional view of a developing device of another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention of the embodiments in conjunction with the accompanying drawings described will be described. The same reference numerals as in Figure 2 are assigned to the elements having the corresponding functions, and the detailed description thereof are omitted for the sake of simplicity. Embodiment 1. In Figure 1, a sectional view of a developing device according to an embodiment of the present invention is shown. The magnetic member 160 comprising the ferromagnetic member such as iron, cobalt, nickel is disposed opposed, with the gap from a surface of the developing sleeve, at the end portion in the thrust direction of the developing sleeve 220 comprising a non-magnetic metal such as aluminum, SUS as a developer carrying member for carrying the developer comprising a magnetic carrier and a non-magnetic toner. The magnetic member 160 is extended over approx. a half of the circumference in the circumferential direction of the developing sleeve 220. In the thrust direction, the magnetic member 160 is positioned inwardly beyond the end surface of the magnet roller 210 producing the magnetic force for carrying the developer, and the magnetic force by the magnet roller 210 is concentrated, and the magnetic brush of the carrier is formed similarly to Figure 4, by the magnetic confining force so as to prevent the leakage of the developer. In addition, in this embodiment, the magnetic member 160 includes a magnetic force reduction portion 160a having a large gap between the surface of developing sleeve 220 adjacent, in the circumferential direction of the developer carrying member, a blade 230 as a developer regulation member for regulating an amount of the developer on the developer carrying member and comprising non-magnetic metal such as SUS aluminum. From the demand for downsizing of the image forming apparatus using the present developing device, the lengths of the developing sleeve 220 and the magnet roller 210 in the thrust direction are shortened, and therefore, the position of the magnetic member 160 is closer to the end portion of the magnet roller 210. Figures 7 and 8 shows magnetic force lines between the magnetic member 160 and the magnet roller 210 adjacent the blade 230, and a wall constituted by the developer between magnetic member 160 and the developing sleeve 220, with this structure. The same reference numerals as in Figure 1 are assigned to the elements having the corresponding functions, and the detailed description thereof are omitted for the sake of simplicity. As shown in Figure 8, the width of the wall of the developer adjacent the blade is narrowed by decrease of the magnetic confining force, and the developer is hardly confined inside of the magnetic member 160 in the thrust direction. Therefore, the width of the developer in the thrust direction applied as the thin layer on the developing sleeve 220 can be assured, so that the entire area of the image having the maximum thrust direction width in the image forming apparatus using the present developing device can be developed. The developer blocking effect of the wall of the above-described developer per se is weakened, but as a result of the durability test, the problem such as the rotation load increase for the developing sleeve 220 due to the leakage of the developer is not produced, up to the lifetime of the developing device. Test conditions are as follows: Developing sleeve diameter: 24.5mm Rotational frequency of the developing sleeve: 186rpm Cutting pole magnetic flux density of the magnet roller at magnetic member portion: 0,36T (360G) Cutting pole half-peak width of the magnet roller at magnetic member portion: 50° Cutting pole magnetic flux density of the magnet roller other than magnetic member portion: 0,65T (650G) Cutting pole half-peak width of the magnet roller other than magnetic member portion: 50° Magnetic member / developing sleeve clearance at the blade contact portion: 6 mm Magnetic member / developing sleeve clearance other than the neighborhood of the blade: 1.5 mm Average particle size of the magnetic carrier: 50 µm The maximum magnetization of the magnetic carrier: 60A·m 2 /kg (60 emu/g) Magnetic material: Fe ( Ni plating) Developing device lifetime: 60000 sheets (A4 size image output) A point ( A in Figure 1) where the clearance between the magnetic material 160 and the developing sleeve 220 starts to widen is positioned downstream (direction of arrow R in Figure 1) with respect to the movement direction of the periphery of developing sleeve 220, of the point where the magnetic flux density between the pole N1 and the pole S2 is 0. The magnet roller has the magnetic pole N1 in the region opposed to the magnetic force reduction portion 160a, and particularly, the magnetic flux density of the region opposed to the magnetic force reduction portion 160a of the magnet roller is not zero. Therefore, even if the magnetic force is reduced adjacent the magnetic material the high developer leakage prevention effect can be provided. Embodiment 2. Figure 9 schematically shows arrangement of this embodiment. The same reference numerals as in Figure 1 are assigned to the elements having the corresponding functions, and the detailed description thereof are omitted for the sake of simplicity. In Figure 9 designated by 900 is a magnet provided outside the magnetic material 160 in the thrust direction. The magnet is magnetized to S-pole at the side opposed to the developing sleeve 220 and N-pole at the opposite side therefrom. The component of magnetic flux density toward the center of the magnet roller 210 is 0,2T (200G) and is arcuated. By confining the very small amount of developer having passed the wall of the developer formed by the function of the magnet roller 210 and magnetic material 160, by said magnet, the lifetime of the developing device of this embodiment, can be extended to approx. 4 times the lifetime of the developing device of the first embodiment with the effect which is similar to the first embodiment maintained. The durability test conditions are the same as in the first embodiment. Embodiment 3. In the first and the second embodiment, the present invention is applied to the developing device using two component developer including the non-magnetic toner and magnetic carrier. The present invention is not limited to the developing device using two component developer. It is effective also in the case that it is applied to a developing device using magnetic one component toner. However, generally, the average particle size of the magnetic one component toner is smaller as compared with the magnetic carrier, e.g. substantially 10 µm, and therefore, in this embodiment, the clearance between the developing sleeve and magnetic material end portion is 2 mm, adjacent the blade, and 0.5 mm in the other portion. Figure 10 schematically shows arrangement of the major part of this embodiment. In this Figure, reference numeral 21 designates a magnet roller, 22 designates a developing sleeve, 23 designates a blade, and 16 designates a magnetic member, and the other structure is the same as in Figure 1, and the detailed description is omitted. As described in the foregoing, according to the present invention, even if the length in the thrust direction of the magnet roller is shortened as much as possible in accordance with the length of the developing sleeve in the thrust direction to meet the demand for downsizing of the image forming apparatus, the width of the developer wall constituted by the magnetic material provided in order to prevent the leakage of the developer toward the outside in the thrust direction can be narrowed as much as possible, and therefore, the entire width of the maximum image of the image forming apparatus in the thrust direction can be developed. 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 claims.

Claims: A developing apparatus comprising: a developer container (200) for accommodating a developer having magnetic particles; a cylindrical rotatable developer carrying member (220), provided in an opening of the said developer container, for carrying the developer; a magnet (210) in said developer carrying member (220); a magnetic member (160), provided at an end portion of said developer carrying member (220) and being subjected to magnetic force of the said magnet (210) to counteract axial leakage of developer; wherein said magnetic member (160) is disposed with a radial gap relative to said developer carrying member (220), along a part of the circumference of said developer carrying member (220); and a regulating member (230) extending axially of the developer carrying member (220) for regulating the thickness of a layer of developer on the said developer carrying member (220); characterised in that the magnetic confining force for the developer between the magnet (210) and said magnetic member (160) is weaker toward said regulating member (230) in a circumferential direction of said developer carrying member (220). An apparatus according to claim 1 wherein the magnetic confining force is substantially constant excluding a neighbourhood of the regulating member (230). An apparatus according to claim 1 or claim 2, wherein the gap between said developer carrying member (220) and said magnetic member (160) is wider toward the regulating member (230) in said circumferential direction. An apparatus according to claim 3, wherein the gap is substantially constant excluding a neighbourhood of the regulating member (230). An apparatus according to claim 3, wherein in a region of said magnet (210) opposed to a portion (160a) of said magnetic member (160) having a wide gap and adjacent the regulating member, the magnetic flux density is not zero. An apparatus according to claim 1, wherein said magnetic member (160) is of iron. An apparatus according to claim 6, wherein the iron is plated with nickel. An apparatus according to claim 1, wherein a brush of the magnetic particles is formed by the magnetic force of the said magnet (210) between said developer carrying member (220) and said magnetic member (160), the brush being effective to prevent leakage of the developer. An apparatus according to claim 1, further comprising an auxiliary seal member (900) outside said magnetic member (160) in the axial direction of said developer carrying member (220). An apparatus according to claim 9, wherein said auxiliary seal member (900) includes a magnet. An apparatus according to claim 1, wherein said magnet (210) is extended along the axial direction of said developer carrying member (220), and said developer carrying member (220) produces magnetic force for carrying the developer for development. An apparatus according to claim 11, wherein said magnetic member (160) is disposed inwardly beyond an end portion of said magnet (210) in the axial direction of said developer carrying member (220). An apparatus according to claim 1, wherein the developer contains non-magnetic toner. An apparatus according to claim 3, wherein said magnet (210) has a magnetic pole in a region (160a) opposed to a portion of the magnetic member (160) having a wide gap and adjacent the regulating member (230).

Inventors: DANZUKA TOSHIMITSU, DANZUKA, TOSHIMITSU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G03G15
IPC codes: G03G15