WO2022005635A1

WO2022005635A1 - Developing device with magnetic poles for developing roller

Info

Publication number: WO2022005635A1
Application number: PCT/US2021/033062
Authority: WO
Inventors: Kiyofumi Morimoto
Original assignee: Hewlett-Packard Development Company, L.P.
Priority date: 2020-07-01
Filing date: 2021-05-19
Publication date: 2022-01-06
Also published as: JP2022012247A

Abstract

A developing device includes a developing roller having a plurality of magnetic poles. The plurality of magnetic poles include a pick-up pole to generate a magnetic force to transfer a developer to a surface of the developing roller, and a release pole to generate a magnetic force to release a developer from a surface of the developing roller. The magnetic force of the pick-up pole includes a pick-up-pole peak magnetic force. The magnetic force of the release pole includes a release-pole peak magnetic force. The pick-up-pole peak magnetic force is approximately 1.4 to 2.0 times greater than the release-pole peak magnetic force.

Description

DEVELOPING DEVICE WITH MAGNETIC POLES FOR DEVELOPING

ROLLER

BACKGROUND

[0001] Some developing devices in imaging apparatuses develop an electrostatic latent image formed on an image carrier using a developer that contains a toner and a carrier. Such a developing device includes a developing roller having a plurality of magnetic poles. Then, the developing device supplies the developer to the developing roller, regulates a thickness of the developer on the developing roller, develops the electrostatic latent image on the image carrier using the toner contained in the developer, and releases the developer from the developing roller in accordance with the rotation of the developing roller using a magnetic force of these magnetic poles.

BRIEF DESCRIPTION OF DRAWINGS

[0002] FIG. 1 is a schematic diagram of an example imaging apparatus. [0003] FIG. 2 is a schematic cross-sectional view of an example developing device, taken transversely.

[0004] FIG. 3 is a schematic cross-sectional view of the example developing device, taken longitudinally, illustrating an example developing chamber, a first conveyance path, and a second conveyance path.

[0005] FIG. 4 is a schematic diagram of the developing device illustrated in FIG. 2, illustrating an example pattern of magnetic fields generated by the developing device.

[0006] FIG. 5 is a diagram of an example developing roller of the example developing device, illustrating a pattern of magnetic fields generated by the developing roller.

[0007] FIG. 6 is a diagram illustrating a developer layer state in the developing device illustrated in FIG. 2, when an amount of the developer stored in the developing device is relatively low. [0008] FIG. 7 is a diagram illustrating an example of a developer layer state when the amount of the developer is relatively large in another example developing device.

[0009] FIG. 8 is a diagram illustrating an example of a developer layer state when the amount of the developer is relatively low in another example developing device.

[0010] FIG. 9 is a partially enlarged view of the schematic cross-sectional view of the developing device illustrated in FIG. 2.

[0011] FIG. 10 is a table showing experimental data.

DETAILED DESCRIPTION

[0012] In a developing device provided in an imaging system, when a developer is picked up (or transferred on) to a developing roller, the developer is carried by the developing roller. However, when the amount of the developer contained in the developing device is low, the developer carried by the developing roller may be uneven and/or the resulting printed image may have uneven density. Therefore, for example, in a developing device including a developing roller having a plurality of magnetic poles, the plurality of magnetic poles may include a pick-up pole to generate a magnetic force used to pick up (or transfer) a developer to a surface of the developing roller, in which the magnetic force of the pick-up pole includes a first peak magnetic force (or pick-up-pole peak magnetic force), and may further include a release pole to generate a magnetic force used to release a developer from the surface of the developing roller, in which the magnetic force of the release pole includes a second peak magnetic force (or a release-pole peak magnetic force) and the first peak magnetic force of the pick up pole is approximately 1 .4 to 2.0 times greater than the second peak magnetic force of the release pole. Accordingly, it is possible to prevent or inhibit an occurrence in which that the developer carried by the developing roller is uneven and/or in which the printed image has an uneven density.

[0013] Hereinafter, an example imaging system will be described with reference to the drawings. The imaging system may be an imaging apparatus such as a printer or a device used in the imaging apparatus or the like. In addition, in the description with reference to the drawings, the same components or similar components having the same function will be denoted by the same reference symbols and redundant description will be omitted. Further, the numerical range indicated by "to" indicates a range including the numerical values before and after "to" as the minimum value and the maximum value respectively. In addition, the value indicated by "approximately" indicates a range including the value indicated and the vicinity of the value.

[0014] With reference to FIG. 1 , an example imaging apparatus 1 may form a color image using toner of four colors including magenta, yellow, cyan, and black. In the present description, the characters “M”, Ύ”, “C” and Ύ” are appended to reference numbers to designate components that are associated with the toner colors of magenta, yellow, cyan, and black, respectively. The example imaging apparatus 1 includes a conveyance device 10 which conveys a sheet 3 (e.g., a sheet of paper) corresponding to a recording medium, image carriers 20M, 20Y, 20C, and 20K having respective surfaces to form electrostatic latent images, developing devices 30M, 30Y, 30C, and 30K which develop the electrostatic latent images to respective form toner images, a transfer device 40 which layers the toner images into a composite toner image and transfers the composite toner image onto the sheet 3, a fixing device 50 which fixes the toner image to the sheet 3, a discharge device 60 which discharges the sheet 3, and a control unit 70.

[0015] The conveyance device 10 conveys the sheet 3 corresponding to a recording medium on which an image is to be formed, along a conveyance path 11 . Prior to reaching the conveyance path 11 , the sheet 3 is stacked and stored in a cassette 12 and is picked up and conveyed by a feeding roller 13.

[0016] The image carriers 20M, 20Y, 20C, and 20K may also referred to as electrostatic latent image carriers, photoconductive drums, or the like. The image carriers 20M, 20Y, 20C, and 20K form respective electrostatic latent images for forming a magenta toner image, a yellow toner image, a cyan toner image, and a black toner image, respectively. The image carriers 20M, 20Y, 20C, and 20K may have a similar configuration. For ease of reading, the image carrier 20M will be described as a representative image carrier, unless otherwise specified.

[0017] The developing device 30M, a charging roller 22M, an exposure unit 23, and a cleaning unit 24M are provided around the image carrier 20M. Similarly, the respective developing devices 30Y, 30C, and 30K, respective charging rollers, the exposure unit 23, and respective cleaning units are also provided around the image carriers 20Y, 20C, and 20K, respectively.

[0018] The charging roller 22M charges the surface of the image carrier 20M to a predetermined or targeted potential. The charging roller 22M rotates in accordance with the rotation of the image carrier 20M. The exposure unit 23 exposes a portion of the surface of the image carrier 20M having been previously charged by the charging roller 22M, in response to an image to be formed on the sheet 3, so as to change a potential of the portion of the surface exposed, which forms an electrostatic latent image. The cleaning unit 24M collects toner remaining on the image carrier 20M.

[0019] The developing device 30M develops the electrostatic latent image formed on the image carrier 20M with a magenta toner supplied from a toner tank 21 M, using a carrier, so as to form a magenta toner image. Similarly, the developing device 30Y develops the electrostatic latent image formed on the image carrier 20Y with a yellow toner supplied from a toner tank 21 Y, using a carrier, so as to form a yellow toner image. The developing device 30C develops the electrostatic latent image formed on the image carrier 20C with a cyan toner supplied from a toner tank 21 C , using a carrier, so as to form a cyan toner image. The developing device 30K develops the electrostatic latent image formed on the image carrier 20K with a black toner supplied from a toner tank 21 K, using a carrier, so as to form a black toner image. The developing devices 30M, 30Y, 30C, and 30K have similar configurations. Accordingly, for ease of reading, the developing device 30M will be described as a representative developing device unless otherwise specified.

[0020] The developing device 30M includes a developing roller 31 M which transfers the toner (e.g., the magenta toner) to the image carrier 20M. In the developing device 30M, a two-component developer containing the toner and the carrier is used as the developer. The toner and the carrier are adjusted in the developing device 30M, to a targeted mixture ratio and are further mixed and stirred to disperse the toner in the developer, to impart the developer with an optimal charge. The developer is carried by the developing roller 31 M. Then, when the developer is conveyed to a region facing the image carrier 20M by the rotation of the developing roller 31 M, the toner in the developer is transferred to the electrostatic latent image formed on the peripheral surface of the image carrier 20M so that the electrostatic latent image is developed, thereby forming the tone image (e.g., the magenta toner image).

[0021] The transfer device 40 conveys the toner image formed on each of the developing devices 30M, 30Y, 30C, and 30K so that the toner image is transferred to the sheet 3. The transfer device 40 includes a transfer belt 41 , suspension rollers 44, 45, 46, and 47 which suspend the transfer belt 41 , primary transfer rollers 42M, 42Y, 42C, and 42K which presses the transfer belt 41 against the respective image carriers 20M, 20Y, 20C, and 20K to primarily transfer the toner images from the respective image carriers 20M, 20Y, 20C, and 20K to the transfer belt 41 , and a secondary transfer roller 43 which presses the transfer belt 41 against the suspension roller 47 to secondarily transfer the toner images from the transfer belt 41 to the sheet 3.

[0022] The transfer belt 41 is an endless belt which is driven to rotate by a rotation of the suspension rollers 44, 45, 46, and 47. The suspension roller 47 may be a drive roller that drives a rotation of the suspension rollers 44, 45, and 46 which may be driven rollers. The primary transfer roller 42M presses against the image carrier 20M from the inner peripheral side of the transfer belt 41 to transfer a magenta toner image to the transfer belt. The primary transfer roller 42Y presses against the image carrier 20Y from the inner peripheral side of the transfer belt 41 to transfer a yellow toner image to the transfer belt. The primary transfer roller 42C presses against the image carrier 20C from the inner peripheral side of the transfer belt 41 to transfer a cyan toner image to the transfer belt. The primary transfer roller 42K presses against the image carrier 20K from the inner peripheral side of the transfer belt 41 to transfer a black toner image to the transfer belt. The magenta, yellow, cyan and black toner images are layered on the transfer belt 41 , to form a single composite toner image. The secondary transfer roller 43 is disposed in parallel to the suspension roller 47 with the transfer belt 41 interposed therebetween to press against the suspension roller 47 from the outer peripheral side of the transfer belt 41 . Accordingly, the secondary transfer roller 43 forms a transfer nip region 14 between the transfer belt 41 and the secondary transfer roller 43 where the composite toner image may be transferred from the transfer belt 41 to the sheet 3.

[0023] The fixing device 50 conveys the sheet 3 to pass through a fixing nip region for heating and pressing the sheet so that the toner image is attached and fixed to the sheet 3. The fixing device 50 includes a heating roller 52 which heats the sheet 3 and a pressing roller 54 which rotates in a driving manner while pressing against the heating roller 52. The heating roller 52 and the pressing roller 54 are formed in a cylindrical shape and the heating roller 52 includes a heat source such as a halogen lamp therein. A fixing nip region corresponding to a contact region is formed between the heating roller 52 and the pressing roller 54, and the sheet 3 is passed through the fixing nip region so that the toner image is melted and fixed to the sheet 3.

[0024] The discharge device 60 includes discharge rollers 62 and 64 which discharge the sheet 3 to which the toner image is fixed by the fixing device 50 to the outside of the apparatus.

[0025] The control unit 70 is an electronic control unit which includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and the like. In the control unit 70, a instructions stored in the ROM may be loaded to the RAM and may be executed by the CPU to perform various controls. The control unit 70 may include a plurality of electronic control units or a single electronic control unit. The control unit 70 may control the above- described devices of the imaging apparatus 1 , based on an image signal received for forming the image on the recording medium (e.g., paper sheet 3).

[0026] FIG. 2 is a schematic cross-sectional view of an example developing device 30M that may be mounted in the imaging apparatus 1 illustrated in FIG. 1. The developing device 30M illustrated in FIG. 2 includes a developing roller 31 M, a housing 32M, a first stirring and conveying member 33M, a second stirring and conveying member 34M, and a layer thickness regulating member 35M.

[0027] The housing 32M is a container of the developing device 30M and includes a developing chamber 36M therein. A developer containing the toner (e.g., the magenta toner) and the carrier is stored in the developing chamber 36M of the housing 32M. Further, the developing chamber 36M of the housing 32M stores the developing roller 31 M and the layer thickness regulating member 35M. The housing 32M includes an opening at a position in which the developing roller 31 M faces the image carrier 20M and the toner in the developing chamber 36M is supplied from the opening to the image carrier 20M.

[0028] The developing roller 31 M is disposed to face the image carrier 20M so that a gap is formed between the image carrier 20M and the developing roller, and rotates to carry the developer stored in the housing 32M on the surface thereof. That is, the developing roller 31 M rotates in a rotation direction D1 and conveys the developer. The developing roller 31 M may be formed in a columnar shape (or cylindrical shape), for example. As illustrated in FIGS. 2 and 3, the developing roller 31 M has a rotation axis A1 extending in a longitudinal direction D2. The developing roller 31 M is disposed so that the rotation axis A1 of the developing roller 31 M is parallel to the rotation axis A2 of the image carrier 20M and a gap between the developing roller 31 M and the image carrier 20M is constant in the extension direction of the rotation axis A1 (or in the longitudinal direction of the developing roller 31 M and the image carrier 20M). The developing roller 31 M carries the developer stirred by the first stirring and conveying member 33M on the surface thereof. The developing roller 31 M conveys the carried developer to a developing region 37M to develop the electrostatic latent image of the image carrier 20M. The developing region 37M is a region located between the developing roller 31 M and the image carrier 20M where the developing roller 31 M faces the image carrier 20M. The developing region 37M may be a region in which the developing roller 31 M is closest to the image carrier 20M.

[0029] The developing roller 31 M includes a developing sleeve 311 M which forms a surface layer of the developing roller 31 M and a magnet 312M which is disposed inside the developing sleeve 311M. The developing sleeve 311 M is a tubular member that is formed of non-magnetic metal. The developing sleeve 311 M is rotatable around the rotation axis A1. The magnet 312M is fixed to, for example, a shaft 313M fixed to the housing 32M and disposed along the rotation axis A1. The magnet 312M includes a plurality of magnetic poles which are described further below. The developing sleeve 311 M is rotatably supported by, for example, the shaft 313M and is rotationally driven by a drive source such as a motor. The developer is carried on the surface of the developing sleeve 311 M due to the magnetic force of the magnet 312M. The developing roller 31 M conveys the developer in the rotation direction of the developing sleeve 311 M by the rotation of the developing sleeve 311 M. Additionally, the rotation of the developing roller 31 M refers to the rotation of the developing sleeve 311 M, and accordingly, the rotation direction D1 of the developing roller 31 M refers to the rotation direction of the developing sleeve 311 M.

[0030] A magnetic formation of developer (also referred to as a magnetic brush of developer) is formed on the developing sleeve 311 M due to the magnetic force of the magnetic poles of the magnet 312M. The magnetic brush of the developer may also be referred to as a spike formation of developer. As the developing roller 31 M rotates, the magnetic brush of the developer formed by the magnetic pole contacts or approaches the electrostatic latent image of the image carrier 20M in the developing region 37M. Accordingly, the toner in the developer carried on the developing roller 31 M transfers to the electrostatic latent image formed on the peripheral surface of the image carrier 20M so that the electrostatic latent image is developed.

[0031] As illustrated in FIGS. 2 and 3, the first stirring and conveying member 33M and the second stirring and conveying member 34M stir a magnetic carrier and a non-magnetic toner constituting the developer so that the carrier and the toner are frictionally charged. Further, the first stirring and conveying member 33M and the second stirring and conveying member 34M convey the developer while stirring the developer. The first stirring and conveying member 33M is disposed in a first conveyance path 381 M disposed below the developing chamber 36M (where the developing roller 31 M is located) and the second stirring and conveying member 34M is disposed in a second conveyance path 382M disposed adjacent to the first conveyance path 381 M. Each of the first conveyance path 381 M and the second conveyance path 382M extends in a direction parallel to the rotation axis A1 of the developing roller 31 M. A first end portion of the second conveyance path 382M is provided with a first opening 383M which is coupled (e.g., fluidly coupled) to the first conveyance path 381 M. A second end portion of the second conveyance path 382M, opposite the first end portion, is provided with a second opening 384M which is coupled to the first conveyance path 381 M. That is, the first conveyance path 381 M communicates with the second conveyance path 382M through the first opening 383M and the second opening 384M.

[0032] The first conveyance path 381 M is provided with an opening portion 385M which communicates with the developing chamber 36M. The opening portion 385M is an opening which opens the first conveyance path 381 M toward the developing chamber 36M. The first conveyance path 381 M has a length extending in the longitudinal direction D2 and the opening portion 385M extends longitudinally along a length of the first conveyance path 381 M. The developing roller 31 M faces the first conveyance path 381 M through the opening portion 385M.

[0033] The first stirring and conveying member 33M is a supply auger for stirring the developer and supplying the developer to the developing roller 31 M. The first stirring and conveying member 33M includes a rotation shaft 331 M and a spiral blade 332M which protrudes in a spiral shape from the rotation shaft 331 M. The first stirring and conveying member 33M conveys the developer of the first conveyance path 381 M from the first opening 383M toward the second opening 384M. In the first stirring and conveying member 33M, when the rotation shaft 331 M is rotationally driven to rotate (e.g., by a drive device), the developer is conveyed from the first opening 383M toward the second opening 384M by the rotation of the spiral blade 332M. The developer is then supplied to the developing roller 31 M through the opening portion 385M and is supplied from the first conveyance path 381 M to the second conveyance path 382M through the second opening 384M.

[0034] The second stirring and conveying member 34M is an admix auger for stirring the developer and circulating the developer between the first conveyance path 381 M and the second conveyance path 382M. The second stirring and conveying member 34M includes a rotation shaft 341 M and a spiral blade 342M which protrudes in a spiral shape from the rotation shaft 341 M. The second stirring and conveying member 34M conveys the developer of the second conveyance path 382M from the second opening 384M toward the first opening 383M. The first opening 383M is disposed on the downstream side of the second opening 384M in the developer conveyance direction of the second conveyance path 382M. In the second stirring and conveying member 34M, when the rotation shaft 341 M is driven to rotate (e.g., by a drive device), the developer is conveyed from the second opening 384M toward the first opening 383M by the rotation of the spiral blade 342M. The developer is then supplied from the second conveyance path 382M to the first conveyance path 381 M through the first opening 383M.

[0035] As illustrated in FIG. 2, the layer thickness regulating member 35M limits the thickness of the developer carried by the developing roller 31 M. Namely, the layer thickness regulating member 35M limits an amount of the developer conveyed by the developing roller 31 M. The layer thickness regulating member 35M is disposed in the vicinity of the developing region 37M on the upstream side of the developing region 37M, in the rotation direction D1 of the developing roller 31 M. Further, the layer thickness regulating member 35M is disposed above the rotation axis A1 of the developing roller 31 M in the vertical direction. In some examples, the layer thickness regulating member 35M may be disposed at a position that is aligned with the rotation axis A1 of the developing roller 31 M, in a top plan view of the developing device 30M (e.g., directly above or below the developing roller 31 M in FIG. 2). In other examples, the layer thickness regulating member 35M may be disposed at a position different from the rotation axis A1 of the developing roller 31 M, when viewed in a top plan view of the developing device 30M, such as a position that is offset from the rotation axis A1 of the developing roller 31 M. In the drawings, an example is illustrated in which the layer thickness regulating member 35M is offset from the rotation axis A1 of the developing roller 31 M, when viewed from the top plan view.

[0036] The layer thickness regulating member 35M forms a gap (e.g., of a predetermined size) between the developing roller 31 M and the layer thickness regulating member. Consequently, the layer thickness regulating member 35M limits the layer thickness of the developer carried on the peripheral surface of the developing roller 31 M by the rotation of the developing roller 31 M. By adjusting a size of the gap between the layer thickness regulating member 35M and the developing roller 31 M, the amount of the developer conveyed to the developing region 37M using the developing roller 31 M can be adjusted. The layer thickness regulating member 35M may have a columnar shape (a rod shape), a plate shape, or the like depending on examples. In the drawings, an example is illustrated in which the layer thickness regulating member 35M is formed in a columnar shape (a rod shape).

[0037] With reference to FIGS. 4 and 5, the plurality of magnetic poles of the magnet 312M of the developing roller 31 M may include a developing pole 81 M, a conveyance pole 82M, a release pole 83M, a pick-up pole 84M, and a regulation pole 85M.

[0038] The developing pole 81 M is disposed at a position facing the image carrier 20M and the polarity is, for example, S. The developing pole 81 M generates a magnetic force for developing the electrostatic latent image of the image carrier 20M with the developer carried on the developing roller 31 M in the developing region 37M. The developing pole 81 M forms a magnetic field 811 M capable of developing the electrostatic latent image of the image carrier 20M with the developer carried on the developing roller 31 M, using the magnetic force of the developing pole 81 M.

[0039] The conveyance pole 82M is disposed on the downstream side of the developing pole 81 M, in the rotation direction D1 of the developing roller 31 M, and may have a polarity that is designated by N, for example. The conveyance pole 82M is a magnetic pole that generates a magnetic force to convey the developer (or a remaining developer) that remains on the developing roller 31 M after having moved past the developing region 37M (e.g., the remaining developer that has not been transferred to the image carrier 20M), to a given position. The conveyance pole 82M forms a magnetic field 821 M capable of conveying the remaining developer due to the magnetic force of the conveyance pole 82M.

[0040] The release pole 83M is disposed on the downstream side of the conveyance pole 82M in the rotation direction D1 of the developing roller 31 M, and may have a polarity that is designated by S, for example. The release pole 83M generates a magnetic force to release (separate) the developer from the surface of the developing roller 31 M. The release pole 83M forms a magnetic field (a second magnetic field or a release-pole magnetic field) 831 M capable of releasing the developer from the surface of the developing roller 31 M, due to the magnetic force of the release pole 83M. A peak magnetic force of the release pole 83M in the normal direction of the developing roller 31 M (the radial direction of the developing roller 31 M) is referred to as a second peak magnetic force (or a release-pole peak magnetic force) S2. The second peak magnetic force S2 is formed along a second radial direction B2 of the developing roller 31 M.

[0041] The pick-up pole 84M is disposed on the downstream side of the release pole 83M in the rotation direction D1 of the developing roller 31 M, and may have a polarity that is designated by S, for example. The pick-up pole 84M generates a magnetic force to pick up the developer stirred by the first stirring and conveying member 33M onto the surface of the developing roller 31 M. The pick up pole 84M forms a magnetic field (a first magnetic field or a pick-up-pole magnetic field) 841 M capable of picking up the developer onto the surface of the developing roller 31 M, due to the magnetic force of the pick-up pole 84M. A peak magnetic force of the pick-up pole 84M in the normal direction of the developing roller 31 M (the radial direction of the developing roller 31 M) is referred to as a first peak magnetic force (or pick-up-pole peak magnetic force) S1. The first peak magnetic force S1 is formed along a first radial direction B1 of the developing roller 31 M.

[0042] The regulation pole 85M is disposed to face the layer thickness regulating member 35M on the downstream side of the pick-up pole 84M in the rotation direction D1 of the developing roller 31 M, and may have polarity that is designated by N, for example. The regulation pole 85M generates a magnetic force to limit the layer thickness of the developer carried by the developing roller 31 M using the layer thickness regulating member 35M. The regulation pole 85M forms a magnetic field 851 M capable of limiting the layer thickness of the developer carried by the developing roller 31 M using the layer thickness regulating member 35M, due to the magnetic force of the regulation pole 85M. [0043] Along the periphery of the developing roller 31 M having such a configuration, the layer thickness of the developer carried by the developing roller 31 M is limited in the magnetic field 851 M of the regulation pole 85M, the toner in the developer is used and consumed for the development in the magnetic field 811 M of the developing pole 81 M, a part of the developer is released from the developing roller 31 M in the magnetic field 831 M of the release pole 83M, and the developer stirred by the first stirring and conveying member 33M is picked up onto the developing roller 31 M in the magnetic field 841 M of the pick-up pole 84M. [0044] With reference to FIG. 7, when a sufficient amount of the developer is stored in the developing device 30M, that is, the developer is stored to a sufficient height with respect to the first stirring and conveying member 33M, a sufficient amount of the developer is picked up onto the developing roller 31 M in the magnetic field 841 M of the pick-up pole 84M. With reference to FIG. 8, when the amount of developer stored is not sufficient, namely, when the developer is not stored to a sufficient height with respect to the first stirring and conveying member 33M, then the magnetic field 841 M of the pick-up pole 84M of the developer roller 31 M does not pick-up a sufficient amount of developer. Consequently, an unevenness corresponding to the spiral blade 332M of the first stirring and conveying member 33M may be formed in the developer carried by the developing roller 31 M, which may deteriorate a printing quality of the example imaging apparatus 1 . Such an unevenness is referred to as an auger mark. [0045] Accordingly, the first peak magnetic force S1 of the pick-up pole 84M may be set to be greater than the second peak magnetic force S2 of the release pole 83M, in order to increase the amount of the developer picked up to the developing roller 31 M by the pick-up pole 84M with respect to the amount of the developer released from the developing roller 31 M by the release pole 83M, so as to prevent or inhibit the occurrence of an auger mark formed in the developer carried by the developing roller 31 M even when an insufficient amount of the developer is stored in the developing device 30M. [0046] However, when the first peak magnetic force S1 of the pick-up pole 84M is excessively high, relative to the second peak magnetic force S2 of the release pole 83M, namely, when the second peak magnetic force S2 of the release pole 83M is excessively low relative to the first peak magnetic force S1 of the pick-up pole 84M, then the developer in which the toner density decreases due to the toner consumed during the development operation remains on the developing roller 31 M without being released in the magnetic field 831 M of the release pole 83M, and is subsequently used for the next development operation. As a result, since the toner density of the developer carried by the developing roller 31 M decreases excessively, printing defects such as uneven density of printed images and reduction of optical density (OD) may occur.

[0047] On the other hand, when the first peak magnetic force S1 of the pick-up pole 84M is not sufficiently greater than the second peak magnetic force S2 of the release pole 83M, namely, when the second peak magnetic force S2 of the release pole 83M is not sufficiently low relative to the first peak magnetic force

51 of the pick-up pole 84M, then the amount of the developer released in the magnetic field 831 M of the release pole 83M tends to increase excessively and an auger mark may be formed in the developer carried by the developing roller 31 M as illustrated in FIG. 8.

[0048] Therefore, the first peak magnetic force S1 of the pick-up pole 84M is approximately 1 .4 to 2.0 times greater than the second peak magnetic force S2 of the release pole 83M. In other words, a magnetic force ratio (S1/S2) of the first peak magnetic force S1 of the pick-up pole 84M with respect to the second peak magnetic force S2 of the release pole 83M satisfies a relationship defined by: 1.4 < S1/S2 < 2.0. In this case, the first peak magnetic force S1 of the pick-up pole 84M may be approximately 1.4 to 1.9 times greater than the second peak magnetic force S2 of the release pole 83M in some examples, approximately 1.5 to 2.0 times greater than the second peak magnetic force S2 in other examples, or approximately 1.5 to 1.9 times greater than the second peak magnetic force

52 of the release pole 83M in yet other examples. The first peak magnetic force S1 may be, for example, 400 to 700 mT in some examples, 450 to 660 mT in other examples, or 500 to 630 mT in yet other examples. [0049] Accordingly, the first peak magnetic force S1 of the pick-up pole 84M may be approximately 1.4 to 2.0 times greater than the second peak magnetic force S2 of the release pole 83M, in order to prevent or inhibit the occurrence of an auger mark from being formed in the developer carried by the developing roller 31 M and to prevent or inhibit the toner density of the developer carried by the developing roller 31 M from decreasing excessively as with reference to FIG. 6, even when the developer is not stored to a sufficient height with respect to the first stirring and conveying member 33M. Accordingly, the occurrence of an auger mark may be prevented or inhibited from being formed in the developer carried by the developing roller 31 M, so as to prevent or reduce printing defects such as uneven density of printed images and reduction of OD. [0050] With reference to FIG. 5, a first half value width (or pick-up-pole half value width) C1 corresponds to a half value width of the first peak magnetic force S1 of the pick-up pole 84M, and a second half value width (or release-pole half value width) C2 corresponds to a half value width of the second peak magnetic force S2 of the release pole 83M. The first half value width C1 of the pick-up pole 84M is the half value width of the magnetic field 841 M defined based on the first peak magnetic force S1 . Namely, the first half value width C1 is a dimension of the magnetic field 841 M of the pick-up pole 84M, taken at a position corresponding to the half-value of the magnetic force of the first peak magnetic force S1 , and extending along the rotation direction D1 of the developing roller 31 M (e.g., a width substantially parallel to a tangent of the developer roller 31 M). In some examples, the first half value C1 width may correspond to a substantially widthwise dimension of the magnetic field 841 M, extending along the rotation direction D1 , and taken at the position corresponding to the half-value of the magnetic force of the first peak magnetic force S1 . The first half value width C1 can be expressed, for example, by a center angle taken from the rotation axis A1 of the developing roller 31 M. The second half value width C2 of the release pole 83M is the half value width of the magnetic field 831 M defined based on the second peak magnetic force S2. Namely, the second half value width C2 is a dimension of the magnetic field 831 M of the release pole 83M, taken at a position corresponding to half-value of the magnetic force of the second peak magnetic force S2, and extending along the rotation direction D1 of the developing roller 31 M (e.g., a width substantially parallel to a tangent of the developer roller 31 M). In some examples, the second half value C2 width may correspond to a substantially widthwise dimension of the magnetic field 831 M, extending along the rotation direction D1 , and taken at the position corresponding to the half-value of the magnetic force of the second peak magnetic force S2. The second half value width C2 can be expressed, for example, by a center angle taken from the rotation axis A1 of the developing roller 31 M.

[0051] In order to prevent or inhibit an excessive deviation between the amount of the developer released from the developing roller 31 M and the amount of the developer picked up to the developing roller 31 M, a ratio (C2/C1 ) of the first half value width C1 associated with the pick-up pole 84M with respect to the second half value width C2 associated with the release pole 83M may be approximately 12% or less in some examples, approximately 10% or less in other examples, or approximately 8% or less in yet other examples, of a ratio (S1/S2) of the first peak magnetic force S1 with respect to the second peak magnetic force S2. For example, when the magnetic force ratio (S1/S2) of the first peak magnetic force S1 with respect to the second peak magnetic force S2 is 1.4, the ratio (C2/C1 ) of the first half value width C1 with respect to the second half value width C2 may be 1.4 ± 12% in some examples, 1.4 ± 10% in other examples, or 1.4 ± 8% in yet other examples. Further, when the magnetic force ratio (S1/S2) of the first peak magnetic force S1 with respect to the second peak magnetic force S2 is 2.0, the ratio (C2/C1 ) of the first half value width C1 with respect to the second half value width C2 may be 2.0 ± 12% in some examples, 2.0 ± 10% in other examples, or 2.0 ± 8% in yet other examples.

[0052] Further, an angle Q formed between the first radial direction B1 where the first peak magnetic force S1 of the pick-up pole 84M is generated and the second radial direction B2 where the second peak magnetic force S2 of the release pole 83M is generated may be approximately 50 to 110° (approximately 80° ± 30°) in some examples, approximately 52 to 108° (approximately 80° ± 28°) in other examples, or approximately 54 to 106° (approximately 80° ± 26°) in yet other examples. The angle Q is an angle from the second radial direction B2 to the first radial direction B1 in the rotation direction of the developing roller 31 M. [0053] As illustrated in FIGS. 4 and 9, the closest horizontal distance from the housing 32M to a portion of the developing roller 31 M located inside the magnetic field 851 M of the regulation pole 85M on the downstream side of the layer thickness regulating member 35M in the rotation direction D1 of the developing roller 31 M may be set to an upper distance E1 (also referred to herein as a second distance). Additionally, the closest horizontal distance from the housing 32M to a portion of the developing roller 31 M located inside the magnetic field 841 M of the pick-up pole 84M may be set to a lower distance E2 (also referred to herein as a first distance). The closest horizontal distance refers to a closest distance between two components, in the horizontal direction.

[0054] In some examples, the lower distance E2 associated with the pick up pole 84M may be equal to or greater than the upper distance E1 associated with the regulation pole 85M. In other examples, the lower distance E2 is greater than the upper distance E1 .

[0055] Further, the upper distance E1 may be approximately 1.5 mm or more in some examples, approximately 1.8 mm or more in other examples, or approximately 2.0 mm or more in yet other examples.

[0056] Further, the housing 32M may extend downward and away from the developing roller 31 M, from the closest position 321 M with respect to the developing roller 31 M in the magnetic field 841 M of the pick-up pole 84M. In this case, the housing 32M may extend downward from the closest position 321 M in a planar shape in some examples, a curved surface shape in other examples, or a stepped shape in yet other examples. In the drawings, an example is illustrated in which the housing 32M extends downward from the closest position 321 M in a planar shape.

[0057] Experimentation was carried out on various samples to evaluate the amount of developer not generating the auger mark, the uneven density and the OD reduction when the developer amount was 100 g, and the uneven density and the OD reduction when the developer amount was 150 g. Namely the experimentation was carried out on Samples 1 to 11 which were obtained by changing the magnetic force ratio (S1/S2) of the first peak magnetic force S1 of the pick-up pole 84M with respect to the second peak magnetic force S2 of the release pole 83M, the upper distance E1 , the lower distance E2, and the axial diameter of the rotation shaft 331 M of the first stirring and conveying member 33M.

[0058] In Samples 1 to 11 , the first peak magnetic force S1 of the pick-up pole 84M was kept constant and the second peak magnetic force S2 of the release pole 83M was changed to change the magnetic force ratio (S1/S2) of the first peak magnetic force S1 with respect to the second peak magnetic force S2. Further, in Samples 1 to 11 , the maximum diameter of the first stirring and conveying member 33M was set to 12 mm. The uneven density was evaluated by visually observing the printed image of the halftone image. The evaluation indicator A was given when no uneven density was observed and the evaluation indicator B was given when some uneven density was observed. The OD reduction was evaluated by measuring the optical density of a printed image of a solid image with an OD measuring device (manufactured by KONICA MINOLTA, INC., automatic scanning spectrophotometer, FD-9). This was evaluated by the degree of OD reduction with respect to the reference OD corresponding to the optical density when the developer was sufficient. In the evaluation, the optical density measured when the developer was sufficient was set to the reference optical density of 1 .3, a case in which the reduction of the optical density from the reference optical density was 0.2 or less was evaluated as A, and a case in which the reduction of the optical density from the reference optical density exceeded 0.2 was evaluated as B. The evaluation results for Samples 1 to 11 are shown in FIG. 10.

[0059] Based on FIG. 10, in Samples 1 to 7, the magnetic force ratio (S1/S2) was different and the other conditions were the same.

[0060] In Samples 1 and 2 in which the magnetic force ratio (S1/S2) was 1.3 or less, the auger mark was generated when the developer was completely separated from the developing roller 31 M in the magnetic field 831 M of the release pole 83M and the developer amount was less than 200 g. For this reason, the uneven density and the OD reduction could not be evaluated in both cases of the developer amounts of 100 g and 150 g. [0061] On the other hand, in Samples 6 and 7 in which the magnetic force ratio (S1/S2) was 2.1 or more, the auger mark was not generated until the developer amount was less than 80 g. However, since the developer with decreased toner density (due to the toner consumed for the development remained on the developing roller 31 M without being released in the magnetic field 831 M of the release pole 83M) was used for the next development operation, the uneven density and the OD reduction were observed in both cases of the developer amounts of 100 g and 150 g.

[0062] In contrast, in Samples 3 to 5 in which the magnetic force ratio (S1/S2) was of 1.5 to 1.9, the auger mark was not generated until the developer amount was less than 80 g and the uneven density and the OD reduction were not observed in both cases of the developer amounts of 100 g and 150 g. Accordingly, the generation of the auger mark and the generation of printing defects could be prevented when at least the magnetic force ratio (S1/S2) was of 1.5 to 1.9.

[0063] In Samples 8 and 9, the magnetic force ratio (S1/S2) was set to 1 .5 to be the same as the ratio of Sample 3, but the upper distance E1 and the lower distance E2 were set to be shorter than those of Sample 3. In Samples 8 and 9, the developer amount not generating the auger mark slightly increased in comparison to Sample 3, but in any case, the developer amount not generating the auger mark was less than 100 g. Further, in both cases of the developer amounts of 100 g and 150 g, the uneven density and the OD reduction were not observed. In this way, when the upper distance E1 and the lower distance E2 were set to be less than those of Sample 3, the degree of the generation of the auger mark and the degree of the generation of the printing defects slightly increased in comparison to Sample 3, but the results were nevertheless suitable. [0064] In Sample 10, although the magnetic force ratio (S1/S2) was set to 1 .5 to be the same as the ratio of Sample 3, the upper distance E1 and the lower distance E2 were set to be less than those of Sample 3 and the upper distance E1 was the same as the lower distance E2. In Sample 10, the developer amount not generating the auger mark increased in comparison to Sample 3 and the developer amount not generating the auger mark became 150 g. For this reason, the uneven density and the OD reduction could not be evaluated when the developer amount was 100 g, and the uneven density and the OD reduction were not observed when the developer amount was 150 g. In this way, since the upper distance E1 and the lower distance E2 were less than those of Sample 3 and the upper distance E1 was the same as the lower distance E2, the degree of the generation of the auger mark and the degree of the generation of the printing defects were greater than those for Sample 3, but the results were improved over those of Samples 1 and 2.

[0065] In Sample 11 , although the magnetic force ratio (S1/S2) was set to 1 .5 to be the same as that of Sample 3, the upper distance E1 was less than the lower distance E2 and the upper distance E1 was set to 1 mm. In Sample 11 , the developer amount not generating the auger mark increased in comparison to Sample 3 and the developer amount not generating the auger mark became 180 g. For this reason, the uneven density and the OD reduction could not be evaluated in both cases of the developer amounts of 100 g and 150 g. Since the upper distance E1 was less than the lower distance E2 and the upper distance E1 was set to 1 mm, the degree of the generation of the auger mark increased in comparison to Sample 3, but the results were improved over those of Samples 1 and 2.

[0066] It is to be understood that not all aspects, advantages and features described herein may necessarily be achieved by, or included in, any one particular example. Indeed, having described and illustrated various examples herein, it should be apparent that other examples may be modified in arrangement and detail is omitted.

Claims

1. A developing device comprising: a developing roller having a plurality of magnetic poles, wherein the plurality of magnetic poles include: a pick-up pole to generate a magnetic force to transfer a developer to a surface of the developing roller, the magnetic force of the pick-up pole including a pick-up-pole peak magnetic force; and a release pole to generate a magnetic force to release the developer from the surface of the developing roller, the magnetic force of the release pole including a release-pole peak magnetic force, wherein the pick-up-pole peak magnetic force is approximately 1.4 to 2.0 times greater than the release-pole peak magnetic force.

2. The developing device according to claim 1 , wherein the pick-up- pole peak magnetic force is approximately 1.5 to 1.9 times greater than the release-pole peak magnetic force.

3. The developing device according to claim 1 , comprising: a housing to house the developing roller; and a layer thickness regulating member disposed adjacent the developing roller to regulate a thickness of a layer of the developer carried by the developing roller, wherein the plurality of magnetic poles of the developing roller include a regulation pole disposed to face the layer thickness regulating member, and wherein a closest horizontal distance from the housing to a portion of the developing roller located inside a magnetic field of the regulation pole on a downstream side of the layer thickness regulating member in a rotation direction of the developing roller, is approximately 1.5 mm or more.

4. The developing device according to claim 1 , comprising: a housing to house the developing roller; and a layer thickness regulating member disposed adjacent to the developing roller to regulate a thickness of a layer of the developer carried by the developing roller, wherein the pick-up pole forms a pick-up-pole magnetic field and wherein a first distance corresponds to a closest horizontal distance from the housing to a portion of the developing roller located inside the pick-up-pole magnetic field, wherein the plurality of magnetic poles of the developing roller include a regulation pole disposed to face the layer thickness regulating member, wherein the regulation pole forms a regulation-pole magnetic field and wherein a second distance corresponds to a closest horizontal distance from the housing to a portion of the developing roller located inside the regulation-pole magnetic field on a downstream side of the layer thickness regulating member in a rotation direction of the developing roller, and wherein the first distance associated with the pick-up pole is equal to or greater than the second distance associated with the regulation pole.

5. The developing device according to claim 4, wherein the housing extends downward and away from the developing roller, from a closest position of the housing with respect to the developing roller inside the pick-up-pole magnetic field.

6. The developing device according to claim 1 , wherein a magnetic field of the pick-up pole has a pick-up-pole half value width defined based on the pick-up-pole peak magnetic force, wherein a magnetic field of the release pole has a release-pole half value width defined based on the release-pole peak magnetic force, and wherein a ratio of the pick-up-pole half value width with respect to the release-pole half value width is approximately 12% or less of a ratio of the pick- up-pole peak magnetic force with respect to the release-pole peak magnetic force.

7. The developing device according to claim 1 , wherein the pick-up-pole peak magnetic force is generated by the pick-up pole along a first radial direction of the developing roller, wherein the release-pole peak magnetic force is generated by the release pole along a second radial direction of the developing roller, and wherein an angle formed between the first radial direction and the second radial direction is approximately 50 to 110°.

8. The developing device according to claim 1 , comprising: a layer thickness regulating member disposed adjacent the developing roller to regulate a thickness of a layer of the developer carried by the developing roller, wherein the layer thickness regulating member is disposed above a rotation axis of the developing roller in a vertical direction.

9. An imaging apparatus comprising: a developing device to develop an image, wherein the developing device includes a developing roller having a plurality of magnetic poles, wherein the plurality of magnetic poles include: a pick-up pole to generate a magnetic force to transfer a developer to a surface of the developing roller, wherein the magnetic force of the pick up pole includes a pick-up-pole peak magnetic force; and a release pole to generate a magnetic force to release the developer from the surface of the developing roller, the magnetic force of the release pole including a release-pole peak magnetic force, and wherein the pick-up-pole peak magnetic force is approximately 1.4 to 2.0 times greater than the release-pole peak magnetic force.

10. The imaging apparatus according to claim 9, wherein the developing device includes a housing to house the developing roller and a layer thickness regulating member disposed adjacent the developing roller to regulate a thickness of a layer of the developer carried by the developing roller, wherein the plurality of magnetic poles of the developing roller include a regulation pole disposed to face the layer thickness regulating member, and wherein a closest horizontal distance from the housing to a portion of the developing roller located inside a magnetic field of the regulation pole on a downstream side of the layer thickness regulating member in a rotation direction of the developing roller is approximately 1.5 mm or more.

11. The imaging apparatus according to claim 9, wherein the developing device includes a housing to house the developing roller and a layer thickness regulating member disposed adjacent to the developing roller to regulate a thickness of a layer of the developer carried by the developing roller, wherein the pick-up pole forms a pick-up-pole magnetic field and wherein a first distance corresponds to a closest horizontal distance from the housing to a portion of the developing roller located inside the pick-up-pole magnetic field, wherein the plurality of magnetic poles of the developing roller include a regulation pole disposed to face the layer thickness regulating member, wherein the regulation pole forms a regulation-pole magnetic field and wherein a second distance corresponds to a closest horizontal distance from the housing to a portion of the developing roller located inside the regulation-pole magnetic field on a downstream side of the layer thickness regulating member in a rotation direction of the developing roller, and wherein the first distance associated with the pick-up pole is equal to or greater than the second distance associated with the regulation pole.

12. The imaging apparatus according to claim 11, wherein the housing extends downward and away from the developing roller, from a closest position with respect to the developing roller inside the pick-up-pole magnetic field.

13. The imaging apparatus according to claim 9, wherein a magnetic field of the pick-up pole has a pick-up-pole half value width defined based on the pick-up-pole peak magnetic force, wherein a magnetic field of the release pole has a release-pole half value width defined based on the release-pole peak magnetic force, and wherein a ratio between the pick-up-pole half value width and the release- pole half value width is approximately 12% or less.

14. The imaging apparatus according to claim 9, wherein the pick-up-pole peak magnetic force is generated along a first radial direction of the developing roller by the pick-up pole, wherein the release-pole peak magnetic force is generated along a second radial direction of the developing roller by the release pole, and wherein an angle formed between the first radial direction and the second radial direction is approximately 50 to 110°.

15. The imaging apparatus according to claim 9, wherein the developing device includes a layer thickness regulating member disposed adjacent the developing roller to limit a thickness of a layer of the developer carried by the developing roller, and wherein the layer thickness regulating member is disposed above a rotation axis of the developing roller in a vertical direction.