JP6402736B2 - Developing device and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a developing device and an image forming apparatus including the developing device.

  Conventionally, image forming apparatuses such as printers and copiers that employ an electrophotographic method have a photosensitive drum carrying an electrostatic latent image, and toner is supplied to the photosensitive drum so that the electrostatic latent image becomes visible as a toner image. And a transfer device that transfers a toner image from the photosensitive drum to a sheet.

  Patent Document 1 discloses a developing roller that supplies toner to a photosensitive drum and a conveyance roller that supplies developer to the developing roller. Each of the developing roller and the transporting roller includes a magnet having a plurality of magnetic poles fixed and a sleeve that rotates around the magnet. The developer is supplied from the conveyance roller to the developing roller by a magnetic force generated between the first S pole on the conveyance roller side and the first N pole on the development roller side. The developer is collected from the developing roller to the transport roller by a magnetic force generated between the second N pole on the developing roller side and the second S pole on the transport roller side.

Japanese Patent Laid-Open No. 04-107586

  In recent years, a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller in order to improve the image quality of the toner image and to suppress the toner fog generated on the background portion of the photosensitive drum. When such a developing bias is applied to the developing roller of Patent Document 1, the toner is developed on the sleeve surface of the developing roller, and a toner layer is easily formed. In particular, in the background portion on the photosensitive drum, since the potential is set in a direction in which the toner is pulled back to the developing roller side, the toner layer on the sleeve surface of the developing roller becomes thick. On the contrary, in the image portion on the photosensitive drum, since the potential is set in the direction in which the toner moves to the photosensitive drum side, the toner layer on the sleeve surface of the developing roller becomes thin. Then, when the sleeve rotates once and development is performed again while the difference in thickness of the toner layer remains on the sleeve of the developing roller, the image density changes due to the influence of the toner layer. This phenomenon occurred particularly remarkably in the halftone image, and occurred in the form of an image history (ghost).

  The present invention has been made in view of the above problems, and in a developing device that includes a plurality of rollers each provided with a magnet therein and delivers a developer to each other, suppresses the occurrence of ghosts and includes the developing device. Another object of the present invention is to provide an image forming apparatus.

  A developing device according to one aspect of the present invention includes a first magnet fixed including a plurality of magnetic poles along a circumferential direction, and a toner and a magnetic carrier on a circumferential surface rotating around the first magnet in a first rotation direction. And a first sleeve that carries a developer containing the toner, and is disposed to face a photosensitive drum on which an electrostatic latent image is formed at a predetermined development position, and supplies the toner to the photosensitive drum. A developing roller, a second magnet fixed including a plurality of magnetic poles along the circumferential direction, a second sleeve rotating around the second magnet in the second rotational direction and carrying the developer on the circumferential surface; A conveying roller that is arranged to face the developing roller at a predetermined facing position, supplies the developer to the developing roller, and agitates the developer, and supplies the developer to the conveying roller. A developer stirring section to The first rotation direction and the second rotation direction are set so as to face each other at the facing position, and the first magnet is located downstream of the developing position in the first rotation direction and the facing direction. In the first region upstream of the first rotation direction with respect to the position, the first magnetic pole formed of a predetermined magnetic pole and the first magnetic pole are disposed adjacent to the first magnetic rotation direction downstream side, and the first magnetic pole A second magnetic pole having the same polarity as that of the first magnetic pole, adjacent to the first magnetic pole upstream of the first rotation direction, a third magnetic pole different from the first magnetic pole, and the second magnetic pole The second magnetic pole is disposed adjacent to the downstream side in the first rotational direction, and includes a fourth magnetic pole different from the second magnetic pole. In the circumferential distribution of the radial component of the magnetic force of the first magnet, The average value of the peak magnetic force of one magnetic pole and the peak magnetic force of the second magnetic pole ave (mT), where Td (mT) is the minimum value of the magnetic force arranged between the peak position of the first magnetic pole and the peak position of the second magnetic pole, the relationship of Tave−Td ≧ 15 is satisfied. It is characterized by that.

  According to this configuration, the developer is supplied from the developer stirring unit to the conveyance roller, and the developer is supplied from the conveyance roller to the development roller. When a part of the toner is supplied from the developer to the photosensitive drum at the developing position, the developer is collected from the developing roller to the transport roller. The first, second, third and fourth magnetic poles are arranged in the first region of the first magnet of the developing roller. Furthermore, the first magnetic pole and the second magnetic pole satisfy the relationship of Tave−Td ≧ 15, so that the developer is partially retained on the first sleeve by the repulsive force of the first magnetic pole and the second magnetic pole. Is formed stably. Therefore, even when the history of toner consumed at the development position remains on the first sleeve, the history of toner is eliminated by the magnetic brush of the developer staying at the staying portion. For this reason, a developing device in which the occurrence of ghosts is suppressed is provided.

  In the above configuration, the second magnet is disposed to face the fourth magnetic pole and includes a fifth magnetic pole different from the fourth magnetic pole, and is formed by the fourth magnetic pole and the fifth magnetic pole. It is desirable that the developer that has passed through the development position is transferred from the development roller to the transport roller by a magnetic field generated by the magnetic field.

  According to this configuration, the developer overflowing from the staying portion of the developer exceeds the second magnetic pole and is moved to the fourth magnetic pole, and is then transferred to the fifth magnetic pole side of the transport roller. For this reason, it is possible to stably recover the developer from the developing roller to the transport roller while suppressing the ghost.

  In the above configuration, the first magnet is disposed adjacent to the fourth magnetic pole on the downstream side in the first rotation direction with the facing position interposed therebetween, and a sixth magnetic pole different from the fourth magnetic pole is provided. The second magnet is disposed adjacent to the fifth magnetic pole on the upstream side in the second rotation direction with the facing position sandwiched therebetween, and opposed to the sixth magnetic pole, The developer roller having a seventh magnetic pole having a different polarity, and the developer supplied from the developer agitating unit to the transport roller from the transport roller by the magnetic field formed by the sixth magnetic pole and the seventh magnetic pole. It is desirable to be delivered to.

  According to this configuration, the developer delivery region between the developing roller and the transport roller is stably formed at a position across the facing position.

  In the above-described configuration, the developer is disposed on the upstream side of the seventh magnetic pole in the second rotation direction so as to face the transport roller, and has a layer thickness of the developer supplied to the transport roller from the developer stirring unit. It is desirable to further have a layer thickness regulating member for regulating.

  According to this configuration, the developer layer thickness can be stably regulated before the developer is transferred from the transport roller to the developing roller.

  In the above configuration, the housing further includes a housing that rotatably supports the developing roller and the transport roller, and the housing faces the first magnetic pole, the second magnetic pole, the third magnetic pole, and the fourth magnetic pole. And it is desirable to provide the 1st inner wall part extended so that the surrounding surface of the said 1st sleeve of the said developing roller might be met.

  According to this configuration, the toner in the developer is prevented from being scattered around the staying portion of the developer. In particular, the toner in the developer is prevented from scattering to the development position side. In addition, since the developer transport path is limited, the developer retaining portion can be formed stably.

  In the above configuration, the housing further includes a housing that rotatably supports the developing roller and the transport roller, and the housing faces the first magnetic pole, the second magnetic pole, the third magnetic pole, and the fourth magnetic pole. And a first inner wall portion extending along the peripheral surface of the first sleeve of the developing roller, connected to the first inner wall portion, facing the fifth magnetic pole, and And a second inner wall portion extending along the peripheral surface of the second sleeve of the transport roller.

  According to this configuration, the toner in the developer is prevented from being scattered around the staying portion of the developer. Further, the developer can be smoothly transferred from the developing roller to the transport roller by the magnetic field formed by the fourth magnetic pole and the fifth magnetic pole.

  In the above configuration, it is preferable that the first sleeve of the developing roller includes a base material having a surface blasted.

  According to this configuration, the developer retaining portion can be stably formed on the first sleeve, the adhesion force of the toner to the sleeve surface is reduced, and the toner layer can be easily eliminated.

  In the above configuration, it is preferable that the first sleeve of the developing roller includes a plating layer applied to the surface of the substrate.

  According to this configuration, the developer retaining portion can be more stably formed on the first sleeve.

  In the above configuration, in the circumferential distribution of the radial component of the magnetic force of the first magnet, an interpolar angle between the peak position of the first magnetic pole and the peak position of the third magnetic pole is X (degrees), The interpolar angle between the peak position of the second magnetic pole and the peak position of the fourth magnetic pole is Y (degrees), the sum of the peak magnetic force of the first magnetic pole and the peak magnetic force of the third magnetic pole is E (mT), When the sum of the peak magnetic force of the second magnetic pole and the peak magnetic force of the fourth magnetic pole is F (mT), it is desirable to satisfy the relationship of E / X> F / Y.

  According to this configuration, the developer retaining portion is more stably formed by the repulsive force of the first magnetic pole and the second magnetic pole. As a result, the toner in the developer is further prevented from scattering toward the development position.

  In the above configuration, the axis of the developing roller is disposed below the axis of the photosensitive drum, and the axis of the transport roller is disposed below the axis of the developing roller. Is desirable.

  According to this configuration, combined with the action of gravity, the developer overflowing from the staying portion of the developer moves beyond the second magnetic pole to the fourth magnetic pole, and is then transferred to the transport roller side.

  In the above configuration, it is preferable that a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller.

  According to this configuration, even when an oscillating electric field composed of an AC bias is formed at the development position and a ghost is likely to occur, the occurrence of the ghost is suppressed by the magnetic brush of the developer in the staying portion.

  An image forming apparatus according to another aspect of the present invention includes the developing device according to any one of the above, the photosensitive drum that is supplied with the toner from the developing device and carries a toner image on the peripheral surface, And a transfer unit that transfers the toner image from the photosensitive drum to a sheet.

  According to this configuration, it is possible to provide an image forming apparatus that can suppress the occurrence of ghosts and can form a stable image.

  According to the present invention, an image forming apparatus provided with a developing device that includes a magnet therein and includes a plurality of rollers that deliver a developer to each other while suppressing the occurrence of ghosts.

1 is a cross-sectional view illustrating an internal structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing the internal structure of the developing device according to the embodiment of the present invention. It is typical sectional drawing which shows magnetic pole arrangement | positioning of the developing roller which concerns on embodiment of this invention, and a conveyance roller. It is a typical sectional view showing magnetic pole arrangement of a development roller concerning an embodiment of the present invention. It is typical sectional drawing which shows magnetic pole arrangement | positioning of the conveyance roller which concerns on embodiment of this invention. FIG. 5 is a schematic cross-sectional view illustrating a state where a developer pool is generated in the developing device according to the embodiment of the present invention. 6 is a graph showing a magnetic force distribution of radial components between adjacent identical poles in the developing roller of the developing device according to the embodiment of the present invention. 6 is a graph showing a magnetic force distribution of a radial component (vertical component) and a tangential component (horizontal component) between adjacent identical poles in a developing roller of a developing device according to another embodiment of the present invention. It is a schematic diagram which shows a mode that the ghost generate | occur | produced on the print.

  Hereinafter, an image forming apparatus 10 according to an embodiment of the present disclosure will be described in detail with reference to the drawings. In this embodiment, a tandem color printer is illustrated as an example of an image forming apparatus. The image forming apparatus may be, for example, a copying machine, a facsimile machine, and a complex machine of these.

  FIG. 1 is a cross-sectional view showing the internal structure of the image forming apparatus 10. The image forming apparatus 10 includes an apparatus main body 11 having a box-shaped housing structure. In the apparatus main body 11, a sheet feeding unit 12 that feeds the sheet P, an image forming unit 13 that forms a toner image to be transferred to the sheet P fed from the sheet feeding unit 12, and the toner image are primarily transferred. Intermediate transfer unit 14, secondary transfer roller 145, toner replenishing unit 15 for replenishing toner to the image forming unit 13, and fixing unit for performing a process of fixing an unfixed toner image formed on the sheet P to the sheet P 16 are decorated. Further, a discharge unit 17 that discharges the sheet P that has been subjected to the fixing process by the fixing unit 16 is provided on the upper portion of the apparatus main body 11.

  In the apparatus main body 11, a sheet conveying path 111 extending in the vertical direction is further formed on the right side of the image forming unit 13. The sheet conveyance path 111 is provided with a conveyance roller pair 112 that conveys a sheet to an appropriate position. A registration roller pair 113 that corrects the skew of the sheet and feeds the sheet to a nip portion of secondary transfer described later at a predetermined timing is also provided on the upstream side of the nip portion in the sheet conveyance path 111. The sheet conveyance path 111 is a conveyance path that conveys the sheet P from the paper feeding unit 12 to the paper discharge unit 17 via the image forming unit 13 (secondary transfer nip unit) and the fixing unit 16.

  The paper feed unit 12 includes a paper feed tray 121, a pickup roller 122, and a paper feed roller pair 123. The sheet feeding tray 121 is detachably mounted at a lower position of the apparatus main body 11 and stores a sheet bundle P1 in which a plurality of sheets P are stacked. The pickup roller 122 feeds the uppermost sheet P of the sheet bundle P1 stored in the sheet feeding tray 121 one by one. The pair of paper feed rollers 123 sends out the sheet P fed out by the pickup roller 122 to the sheet conveyance path 111.

  The image forming unit 13 forms a toner image to be transferred to the sheet P, and includes a plurality of image forming units that form toner images of different colors. As this image forming unit, in this embodiment, a magenta (M) color which is sequentially arranged from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 141 described later (from the left side to the right side in FIG. 1). A magenta unit 13M using a developer, a cyan unit 13C using a cyan (C) developer, a yellow unit 13Y using a yellow (Y) developer, and a black (Bk) developer are used. A black unit 13Bk is provided. Each of the units 13M, 13C, 13Y, and 13Bk includes a photosensitive drum 20, and a charging device 21, a developing device 23, and a cleaning device 25 disposed around the photosensitive drum 20. An exposure device 22 common to the units 13M, 13C, 13Y, and 13Bk is disposed below the image forming unit.

  The photosensitive drum 20 is driven to rotate about its axis, and an electrostatic latent image and a toner image are formed on its peripheral surface. As this photosensitive drum 20, a photosensitive drum using an amorphous silicon (a-Si) material can be used. The photosensitive drums 20 are arranged corresponding to the image forming units of the respective colors. The charging device 21 uniformly charges the surface of the photosensitive drum 20. The charging device 21 includes a charging roller and a charging cleaning brush for removing toner attached to the charging roller. The exposure device 22 has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror, and irradiates light that has been modulated based on image data onto the circumferential surface of the uniformly charged photoreceptor drum 20. Thus, an electrostatic latent image is formed. The cleaning device 25 cleans the peripheral surface of the photosensitive drum 20 after the toner image is transferred.

  The developing device 23 supplies toner to the peripheral surface of the photosensitive drum 20 in order to develop the electrostatic latent image formed on the photosensitive drum 20. The developing device 23 is for a two-component developer composed of toner and carrier. In this embodiment, the toner has a characteristic of being charged to a positive polarity.

  The intermediate transfer unit 14 is disposed in a space provided between the image forming unit 13 and the toner supply unit 15. The intermediate transfer unit 14 includes an intermediate transfer belt 141, a driving roller 142, a driven roller 143, and a primary transfer roller 24.

  The intermediate transfer belt 141 is an endless belt-like rotating body, and is stretched between the driving roller 142 and the driven roller 143 so that the circumferential surface side thereof is in contact with the circumferential surface of each photosensitive drum 20. The intermediate transfer belt 141 is driven to circulate in one direction, and carries the toner image transferred from the photosensitive drum 20 on the surface.

  The drive roller 142 stretches the intermediate transfer belt 141 on the right end side of the intermediate transfer unit 14 and drives the intermediate transfer belt 141 to rotate. The driving roller 142 is a metal roller. The driven roller 143 stretches the intermediate transfer belt 141 on the left end side of the intermediate transfer unit 14. The driven roller 143 applies tension to the intermediate transfer belt 141.

  The primary transfer roller 24 forms a primary transfer nip portion with the photosensitive drum 20 with the intermediate transfer belt 141 interposed therebetween, and primarily transfers the toner image on the photosensitive drum 20 onto the intermediate transfer belt 141. A primary transfer roller 24 is disposed to face the photosensitive drum 20 of each color.

  The secondary transfer roller 145 is disposed to face the driving roller 142 with the intermediate transfer belt 141 interposed therebetween. The secondary transfer roller 145 is pressed against the peripheral surface of the intermediate transfer belt 141 to form a secondary transfer nip portion. The toner image primarily transferred onto the intermediate transfer belt 141 is secondarily transferred to the sheet P supplied from the paper feeding unit 12 at the secondary transfer nip portion. The intermediate transfer unit 14 and the secondary transfer roller 145 of this embodiment constitute a transfer unit of the present invention. The transfer unit transfers the toner image from the photosensitive drum 20 to the sheet P.

  The toner replenishing unit 15 stores toner used for image formation, and includes a magenta toner container 15M, a cyan toner container 15C, a yellow toner container 15Y, and a black toner container 15Bk in this embodiment. The toner containers 15M, 15C, 15Y, and 15Bk replenish toner of each color to the developing devices 23 of the image forming units 13M, 13C, 13Y, and 13Bk corresponding to the respective colors of MCYBk through a toner transport unit (not shown).

  The sheet P supplied to the fixing unit 16 is heated and pressed by passing through the fixing nip. As a result, the toner image transferred to the sheet P at the secondary transfer nip is fixed to the sheet P.

  The paper discharge unit 17 is formed by recessing the top of the apparatus main body 11, and a paper discharge tray 171 for receiving the discharged sheet P is formed at the bottom of the concave portion. The sheet P on which the fixing process has been performed is discharged toward the discharge tray 151 via the sheet conveyance path 111 extending from the upper part of the fixing unit 16.

  Next, the developing device 23 according to the present embodiment will be described in more detail with reference to FIGS. 2 to 6 in addition to FIG. FIG. 2 is a schematic cross-sectional view showing the internal structure of the developing device 23 according to this embodiment. In FIG. 2, the rotation direction of each rotation member of the developing device 23 is indicated by an arrow. FIG. 3 is a schematic cross-sectional view showing the magnetic pole arrangement of the developing roller 231 and the transport roller 232 according to this embodiment. FIG. 4 is a schematic cross-sectional view showing the magnetic pole arrangement of the developing roller 231. FIG. 5 is a schematic cross-sectional view showing the magnetic pole arrangement of the transport roller 232. FIG. 6 is a schematic cross-sectional view showing a state where a developer reservoir (retention portion TD) is generated in the developing device 23 according to the present embodiment.

  1 to 6, the developing device 23 includes a housing 23H, a developing roller 231, a transport roller 232, two stirring screws 233 (developer stirring unit), a partition plate 234, and a layer thickness. A restricting member 235. The housing 23H is a housing portion that supports each member of the developing device 23.

  The developing roller 231 is arranged to face the photosensitive drum 20 on which the electrostatic latent image is formed at a predetermined developing position NP (FIG. 3), and supplies toner to the photosensitive drum 20. The developing roller 231 includes a first magnet 231A and a first sleeve 231B (FIG. 3). In the present embodiment, the development position NP includes the closest position between the photosensitive drum 20 and the development roller 231. The first magnet 231A is a columnar magnet including a plurality of magnetic poles along the circumferential direction and fixed to the housing 23H. The first sleeve 231B rotates around the first magnet 231A in the first rotation direction (the direction of arrow D1 in FIGS. 2 and 3), and carries a developer containing toner and a magnetic carrier on the peripheral surface. In the present embodiment, the first sleeve 231B is made of an aluminum circular pipe member (base material). The circumferential surface of the circular pipe member of the first sleeve 231B is subjected to sandblasting (blasting) and further includes a Ni plating layer applied on the circumferential surface. The surface of the Ni plating layer of the first sleeve 231B has a predetermined surface roughness. In the present embodiment, the surface roughness Rzjis of the first sleeve 231B is set in the range of 4.0 μm to 14.0 μm. The first sleeve 231B of the developing roller 231 is rotatably supported by the housing 23H.

  The transport roller 232 is disposed so as to face the developing roller 231 at a predetermined facing position TP (FIG. 3), and supplies the developer to the developing roller 231. In the present embodiment, the facing position TP includes the closest position between the transport roller 232 and the developing roller 231. The transport roller 232 includes a second magnet 232A and a second sleeve 232B. The second magnet 232A includes a plurality of magnetic poles along the circumferential direction, and is fixed to the housing 23H. The second sleeve 232B rotates around the second magnet 232A in the second rotation direction (the direction of arrow D2 in FIGS. 2 and 3), and carries a developer containing toner and carrier on the peripheral surface. The second sleeve 232B of the transport roller 232 is rotatably supported by the housing 23H.

  A developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller 231 and the transport roller 232. As shown in FIG. 3, the first rotation direction D1 in which the developing roller 231 rotates and the second rotation direction D2 in which the transport roller 232 rotates are set so as to face each other at the facing position TP (counter). direction).

  The stirring screw 233 charges the toner by circulating and conveying the two-component developer while stirring. The stirring screw 233 includes a first screw 233A and a second screw 233B. Although not shown in FIG. 2, the housing 23H includes a first agitation unit (not shown) in which the first screw 233A is arranged and a second agitation unit (not shown) in which the second screw 233B is arranged. Provided (see developing device 23 in FIG. 1). The developer is circulated and conveyed between the first screw 233A and the second screw 233B. Then, the first screw 233 </ b> A supplies the developer to the transport roller 232. The partition plate 234 is a plate-like member provided in the housing 23H. The partition plate 234 partitions the first stirring portion and the second stirring portion along the axial direction of the first screw 233A and the second screw 233B. Further, the toner replenished from the toner replenishing portion 15 flows into the housing 23H from one end side in the axial direction of the second screw 233B, and is agitated with the other developer.

  The layer thickness regulating member 235 is a plate-like member made of a nonmagnetic metal that is disposed so as to face the peripheral surface of the transport roller 232. In another embodiment, a magnetic member may be fixed to the upstream side surface of the layer thickness regulating member 235. The layer thickness regulating member 235 regulates the layer thickness of the developer supplied from the first screw 233 </ b> A of the stirring screw 233 to the transport roller 232.

  As shown in FIG. 2, the axis of the developing roller 231 is disposed below the axis of the photosensitive drum 20, and the axis of the transport roller 232 is further below the axis of the developing roller 231. Has been placed.

  Referring to FIG. 2, the developer composed of toner and carrier and circulated and conveyed by stirring screw 233 is supplied from first screw 233 </ b> A to conveying roller 232. Thereafter, after the layer thickness of the developer is regulated by the layer thickness regulating member 235, the developer is supplied to the developing roller 231. When a part of the toner is supplied to the photosensitive drum 20 at the development position NP (FIG. 3), the developer is collected from the development roller 231 to the transport roller 232. Thereafter, the developer collected on the transport roller 232 again flows into the first stirring unit around the first screw 233A.

  3 and 4, in the present embodiment, the first magnet 231A of the developing roller 231 includes seven magnetic poles along the circumferential direction. The N1 pole is disposed on the downstream side in the first rotation direction (D1) from the facing position TP between the developing roller 231 and the transport roller 232. The S1 pole is disposed downstream of the N1 pole in the first rotation direction. The S1 pole functions as a transport pole for transporting the developer received from the transport roller 232 to the photosensitive drum 20 side. Further, an N2 pole that functions as a main pole that supplies toner to the photosensitive drum 20 is disposed downstream of the S1 pole in the first rotation direction. The N2 pole is disposed in the vicinity of the development position NP.

  Further, the first magnet 231A has four magnetic poles (S2, N3, N4, S4) in the first region R downstream of the developing position NP in the first rotational direction and upstream of the facing position TP in the first rotational direction. S3). The N3 pole (first magnetic pole) is disposed at a substantially central portion of the first region R. The N4 pole (second magnetic pole) is arranged adjacent to the N3 pole on the downstream side in the first rotation direction, and has the same polarity as N3. The S2 pole (third magnetic pole) is arranged adjacent to the N3 pole on the upstream side in the first rotation direction, and is a magnetic pole different from the N3 pole. The S3 (fourth magnetic pole) pole is disposed adjacent to the N4 pole on the downstream side in the first rotation direction, and is a magnetic pole different from the N4 pole.

  Table 1 shows a magnet 1 and a magnet 2 exemplifying angles of seven magnetic poles as the first magnet 231A according to the present embodiment. The magnet 1 and the magnet 2 are different from each other in the magnitude of the magnetic force (peak value of the radial component) of some of the poles. Further, the angles of the magnetic poles shown in Table 1 are shown along the first rotation direction with the opposing position TP in FIG. 4 as the starting point (angle 0 °). In FIG. 4, a straight line CL connecting the facing position TP and the rotation axis of the developing roller 231 is shown.

  On the other hand, referring to FIGS. 3 and 5, second magnet 232 </ b> A of transport roller 232 includes five magnetic poles along the circumferential direction. The N11 pole is disposed on the downstream side in the second rotational direction (D2) from the facing position TP between the developing roller 231 and the transport roller 232. Further, the S11 pole is disposed downstream of the N11 pole in the second rotation direction. Further, the S12 pole is disposed downstream of the S11 pole in the second rotation direction. The S11 pole functions as a peeling pole for peeling the developer from the transport roller 232. The S12 pole functions as a pumping pole that pumps up the developer from the first screw 233A. An N12 pole and an S13 pole are disposed downstream of the S12 pole in the second rotation direction. As shown in FIG. 5, the layer thickness regulating member 235 is spaced apart from the S13 pole by the second magnet 232A of the transport roller 232 at a predetermined interval between the S12 pole and the N12 pole. It is arranged facing each other. For this reason, the developer layer thickness can be stably regulated before the developer is transferred from the transport roller 232 to the developing roller 231. The S13 pole is disposed adjacent to the N11 pole across the facing position TP.

  Table 2 shows angles and magnetic forces (peak values of radial components) of five magnetic poles as an example of the second magnet 232A according to the present embodiment. The angles of the magnetic poles shown in Table 2 are shown along the second rotation direction with the opposing position TP in FIG. 5 as the starting point (angle 0 °). In FIG. 5, a straight line CL connecting the facing position TP and the rotation axis of the transport roller 232 is shown.

  With reference to FIGS. 2 and 3, the arrangement and function of the magnetic poles of the first magnet 231 </ b> A of the developing roller 231 and the second magnet 232 </ b> A of the transport roller 232 will be further described. The N11 pole (fifth magnetic pole) of the second magnet 232A is disposed opposite to the S3 pole of the first magnet 231A, and is a magnetic pole different from the S3 pole. Then, the developer passing through the development position NP is transferred from the development roller 231 to the transport roller 232 by the magnetic field formed by the N11 pole and the S3 pole.

  Further, the N1 pole (sixth magnetic pole) of the first magnet 231A is arranged adjacent to the S3 pole on the downstream side in the first rotation direction with the opposing position TP interposed therebetween, and is a magnetic pole different from the S3 pole. . Further, the S13 pole (seventh magnetic pole) of the second magnet 232A is adjacent to the N11 pole on the upstream side in the second rotational direction with the opposing position TP interposed therebetween, and is opposed to the N1 pole. Is a magnetic pole of a different polarity. The developer supplied from the first screw 233A of the stirring screw 233 to the transport roller 232 is regulated by the layer thickness regulating member 235, and then developed from the transport roller 232 by the magnetic field formed by the N1 pole and the S13 pole. Delivered to the roller 231.

  With reference to FIG. 6, the housing 23 </ b> H includes a plurality of inner wall portions facing the developing roller 231 and the transport roller 232. Specifically, the housing 23H includes a first inner wall portion 23H1, a second inner wall portion 23H2, a third inner wall portion 23H3, and a fourth inner wall portion 23H4. The first inner wall portion 23H1 faces the S2, N3, N4, and S3 poles and extends along the peripheral surface of the first sleeve 231B of the developing roller 231. The second inner wall portion 23H2 is connected to the first inner wall portion 23H1, is opposed to the N11 pole and the S11 pole, and extends along the peripheral surface of the second sleeve 232B of the transport roller 232. Similarly, the third inner wall portion 23H3 extends so as to face the S1 pole and the N1 pole, and to extend along the peripheral surface of the first sleeve 231B of the developing roller 231. Between the first inner wall portion 23H1 and the third inner wall portion 23H3, the first sleeve 231B of the developing roller 231 is partially exposed and arranged to face the photosensitive drum 20. The fourth inner wall portion 23H4 is connected to the third inner wall portion 23H3, is opposed to the S13 pole and the N12 pole, and extends along the peripheral surface of the second sleeve 232B of the transport roller 232. As shown in FIG. 6, a substantially uniform gap H (developer transport path) is formed between each inner wall and the first sleeve 231B of the developing roller 231 and the second sleeve 232B of the transport roller 232. Has been. In this embodiment, the height of the gap H is smaller than the radii of the developing roller 231 and the transport roller 232, and is set in the range of 0.5 mm to 2.0 mm.

  As described above, a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller 231 and the transport roller 232 in the developing operation for developing the electrostatic latent image on the photosensitive drum 20. As a result, an oscillating electric field due to an AC bias is formed at the development position NP (development nip), so that the fog toner adhered to the background portion on the photosensitive drum 20 can be collected. However, such an oscillating electric field attracts toner also onto the first sleeve 231B of the developing roller 231. As a result, a toner layer (toner film) is easily formed on the surface of the first sleeve 231B.

  The thickness of the toner layer formed on the first sleeve 231B of the developing roller 231 differs between the image portion and the background portion, and the difference in thickness remains as a history. FIG. 9 is a schematic diagram showing a ghost image generated on a halftone image due to such toner consumption history. A history of ring-shaped images formed on the upstream side in the process direction (sheet conveyance direction) appears on the subsequent halftone image. Such a history is based on the difference in toner consumption in the toner layer, and in the next halftone image, the potential difference between the first sleeve 231B and the photosensitive drum 20 is equal to the remaining toner charge. Is due to a partial shift.

  In the present embodiment, a toner layer is formed on the developing roller 231 that is one roller facing the photosensitive drum 20 in the developing device 23 in which two magnetic rollers (the developing roller 231 and the transport roller 232) are arranged. Then, the occurrence of the ghost image as described above is preferably suppressed. That is, in order to suppress such a ghost image, the developing roller 231 of the developing device 23 includes the aforementioned N3 pole and N4 pole.

FIG. 7 is a graph showing the magnetic force distribution of the radial component between the N3 pole and the N4 pole that are adjacent homopolar poles in the developing roller 231 of the developing device 23 according to the present embodiment. In FIG. 7, the peak magnetic force of the N3 pole and the peak magnetic force of the N4 pole are illustrated as substantially equal values. In this embodiment, in the circumferential distribution of the radial component (Radial component) of the magnetic force of the first magnet 231A, the average value of the peak magnetic force of the N3 pole and the peak magnetic force of the N4 pole is Tave (mT), and the peak of the N3 pole When the minimum value of magnetic force (magnetic trough portion) arranged between the position and the peak position of the N4 pole is Td (mT),
Tave−Td ≧ 15 (Formula 1)
The relationship is satisfied.

  Thus, the repulsive magnetic force formed by the N3 pole and the N4 pole is increased by setting a large difference between the average value Tave of the peak magnetic forces of the N3 pole and the N4 pole and the magnetic force Td of the valley portions of both. In this case, with reference to FIG. 6, the developer conveyed from the N2 pole of the developing roller 231 to the S2 pole and further to the N3 pole is difficult to move to the N4 pole, and therefore partially stays on the N3 pole. As a result, a developer retention portion TD is formed on the first sleeve 231B of the developing roller 231. In the stay part TD, the magnetic brush of the developer stays while slipping on the first sleeve 231B. Therefore, even if the history of the toner consumed at the development position NP remains in the toner layer on the first sleeve 231B, the toner history is eliminated (polished) by the magnetic brush of the developer staying in the stay portion TD. Is done. For this reason, the developing device 23 in which the occurrence of the ghost as described above is suppressed is provided. In particular, when the magnetic pole peak position does not exist at the opposing position TP between the developing roller 231 and the transport roller 232 as in the present embodiment, the polishing force (scraping power) of the developer magnetic brush on the transport roller 232 is high. It is difficult to reach the surface of the first sleeve 231B. Even in such a case, the staying part TD can be suitably formed by the repulsive force of the N3-N4 pole that the developing roller 231 has. Such a stay portion TD cannot be formed sufficiently with a known developer peeling electrode. When Expression 1 is satisfied as described above, a retention portion TD that can polish the toner layer on the first sleeve 231B can be formed. In this embodiment, one developing roller 231 is disposed to face the photosensitive drum 20 and develops the electrostatic latent image on the photosensitive drum 20. Therefore, it is necessary to stably expose the electrostatic latent image at one development position NP as compared with other development devices in which a plurality of development rollers are arranged adjacent to each other along the circumferential surface of the photosensitive drum 20. There is. In other words, when a plurality of developing rollers are arranged along the rotation direction of the photosensitive drum 20 as described above, the downstream developing roller removes the density-decreasing portion of the ghost image formed by the upstream developing roller. It can be corrected. On the other hand, in the present embodiment, correction becomes difficult when the history of toner consumption formed on the first sleeve 231B of the developing roller 231 becomes a ghost image in the next round. Therefore, by satisfying the above expression 1, it is possible to suitably suppress the history of the toner layer from going around toward the next development position NP. As a result, the structure of the developing device 23 is prevented from becoming complicated, and the cost of the developing device 23 is prevented from increasing.

  In the present embodiment, the N3 pole and the N4 pole are made of one fan-shaped ferrite magnet, and a notch is partially formed in the central portion of the fan-shaped circumferential direction to form two homopolar magnetic poles. Has been. The magnetic force difference between the peak value of the N3 pole and the peak value of the N4 pole is set to a range of 25 mT or less. If it is this range, it will become possible to judge the formation condition of the retention part TD by the average value Tave of the peak value of both poles. Similarly, when two magnetic poles of the same polarity are formed by two adjacent magnet pieces, if the magnetic difference between the peak value of the N3 pole and the peak value of the N4 pole is set to a range of 25 mT or less, It is possible to determine the formation state of the staying portion TD based on the average value Tave of the peak values of both poles.

  Further, in the present embodiment, the first sleeve 231B is made of an aluminum circular pipe member (base material). Further, the circumferential surface of the circular pipe member of the first sleeve 231B is subjected to sand blasting (blasting) and further includes a Ni plating layer applied on the circumferential surface. For this reason, the developer tends to slip due to the surface property of the first sleeve 231B subjected to the blasting process, and the developer retention portion TD is stably formed. Furthermore, the charge amount of the positively charged toner is likely to be reduced by the plating layer on the developing roller 231. As a result, the charge amount of the developer is reduced, the developer slip at the staying part TD is promoted, and the force with which the toner adheres to the sleeve surface is reduced. Therefore, the generation of ghost images is further suppressed. As will be described later, in other embodiments, the first sleeve 231B of the developing roller 231 is not subjected to blasting and may have a known groove shape. In this case, by performing peripheral polishing such as centerless processing before or after the groove formation, or by performing blasting after the groove formation, the adhesion force of the toner to the sleeve surface is reduced, and the ghost image is simply generated. Suppresses more than just the groove shape.

  Referring to FIG. 6, when the amount of developer in the retention portion TD on the developing roller 231 gradually increases, the overflowed developer (surplus developer) that cannot be held by the magnetic force of the N3 pole jumps over the N4 pole. And moved to the S3 pole. Thereafter, the developer is transferred from the developing roller 231 to the transport roller 232 by the magnetic field formed by the S3 pole and the N11 pole. When the developer jumps from the N3 pole to the S3 pole, the toner in the developer may be scattered. Therefore, paying attention to the first region R (FIG. 3) of the developing roller 231, in the circumferential distribution of the radial component of the magnetic force of the first magnet 231A, the pole between the peak position of the N3 pole and the peak position of the S2 pole. The angle between the peak position of the N4 pole and the peak position of the S3 magnetic pole is Y (degree), the sum of the peak magnetic force of the N3 pole and the peak magnetic force of the S2 pole is E (mT), When the sum of the peak magnetic force of the N4 pole and the peak magnetic force of the S3 pole is F (mT), it is desirable that the relationship of Formula 2 is satisfied.

E / X> F / Y (Formula 2)
In Formula 2, the holding power of the developer formed by the N3 pole and the S2 pole and the holding power of the developer formed by the N4 pole and the S3 pole are defined by inequalities. The above formula 2 is easily satisfied when the pole angle is smaller or the sum of the peak magnetic forces is larger on the upstream side in the first rotation direction across the N3 pole and N4 pole of the same pole. When the developer holding force on the upstream side in the first rotation direction is large as described above, the developer retention portion TD shown in FIG. 6 is formed to be more stable and raised. As a result, the top part of the retention part TD is likely to come into contact with the first inner wall part 23H1 of the housing 23H. In this case, the developer conveyance path is partially blocked by the developer in the retention portion TD. Therefore, even when the toner is scattered around the N4 pole, the toner is prevented from reaching the development position NP. For this reason, the occurrence of toner fog or toner contamination on the image is suppressed. Moreover, when the relationship of Formula 2 is satisfied, pressure is applied to the developer in the staying portion TD between the first inner wall portion 23H1 and the first sleeve 231B. For this reason, the polishing power of the surface of the first sleeve 231B by the developer in the staying portion TD is increased.

  Furthermore, in this embodiment, as shown in FIG. 6, the S3 pole different from the N4 pole is disposed downstream of the N4 pole in the first rotation direction. For this reason, the developer detached from the N3 pole can be stably captured at the S3 pole. Furthermore, by arranging the N11 pole different from the S3 pole, the developer can be stably delivered from the developing roller 231 to the transport roller 232. At this time, since the first inner wall portion 23H1 and the second inner wall portion 23H2 are connected to each other while being formed along the peripheral surfaces of the developing roller 231 and the conveying roller 232, the developer is supplied from the developing roller 231 to the conveying roller 232. It can be delivered smoothly. Referring to FIG. 6, the axis of the developing roller 231 is arranged below the axis of the photosensitive drum 20, and the axis of the transport roller 232 is arranged below the axis of the developing roller 231. Yes. For this reason, coupled with the action of gravity, the developer overflowing from the developer retention portion TD is transferred to the S3 pole after passing over the N4 pole, and then stably delivered to the transport roller 232 side.

  In the present embodiment, the N1 pole is disposed downstream of the S3 pole in the first rotation direction, and the S13 pole is disposed upstream of the N11 pole in the second rotation direction. The developer can be stably supplied from the transport roller 232 to the developing roller 231 by the magnetic field formed by the N1 pole and the S13 pole. Therefore, a developer transfer area between the developing roller 231 and the transport roller 232 is stably formed in a different direction at a position sandwiching the facing position TP. Further, the developer jumping from the S3 pole beyond the N4 pole from the developer retention part TD hardly moves to the S13 pole side. This is because the S13 pole is composed of the same pole as the S3 pole. In this embodiment, as an example, the gap (development position NP) between the developing roller 231 and the photosensitive drum 20 is set to be larger than 0.25 mm and equal to or smaller than 0.40 mm. On the other hand, the gap (opposing position TP) between the developing roller 231 and the transport roller 232 is set to 0.18 mm or more and 0.25 mm or less. In other words, the gap between the developing roller 231 and the transport roller 232 is set to be narrower than the gap between the developing roller 231 and the photosensitive drum 20. Then, the developer is transferred between the developing roller 231 and the transport roller 232 so as to sandwich the opposing position TP set so narrow. As described above, the peak position of any magnetic pole does not face the facing position TP. For this reason, even if the gap at the facing position TP is set narrow as described above, the developer is prevented from sticking to the facing position TP due to the presence of the developer. Further, since two magnetic brushes for the developer to be transferred are formed so as to sandwich the facing position TP in the circumferential direction, even when the toner is scattered at the facing position TP, the toner can be contained.

Next, based on an Example, this invention is further demonstrated.
<Experiment 1>
In Experiment 1, the experiment was performed under the following experimental conditions.
<Experimental conditions>
Photosensitive drum 20: Amorphous silicon photosensitive member (a-Si), diameter φ30 mm, surface potential Vo = 270 V, peripheral speed = 300 mm / sec
A gap between the layer thickness regulating member 235 and the second sleeve 232B: 200 to 600 μm
-Developer transport amount on developing roller 231 (after regulation of layer thickness): 250 g / m ²
Carrier: volume average particle size 35 μm, magnetic force 80 emu / g
Toner: Volume average particle size 6.8 μm, toner concentration 7%

The conditions of the developing roller 231 used in Experiment 1 are as follows.
・ Developing roller 231: Diameter φ20 mm
A peripheral speed ratio of the developing roller 231 to the photosensitive drum 20: 1.6
A gap between the developing roller 231 and the photosensitive drum 20: 300 μm
Development bias: DC bias = 170 V, AC bias = Vpp 1.4 kV, frequency f4.7 kHz, Duty 50%, rectangular wave (note that the development bias of the transport roller 232 is also at the same potential)
-Surface condition of the first sleeve 231B:
(Condition 1) Knurled V groove (groove depth 80 μm, groove width 0.2 mm, number of grooves 120), Ni plating treatment (Condition 2) Sand blasting (Rzjis 8 μm before plating), Ni plating treatment (Condition 3) Sand blasting (Rzjis 8 μm) ), No plating treatment, and the magnetic pole distribution of the developing roller 231 used in Experiment 1 is the magnet 1 and the magnet 2 shown in Table 1 above. In addition, the magnetic force measurement of the following developing roller 231 and the conveyance roller 232 was performed using the Japan Electromagnetic Instrument Co., Ltd.

Moreover, the conditions of the conveyance roller 232 used for Experiment 1 are as follows.
・ Conveyance roller 232: Diameter φ20mm
-Surface condition of the second sleeve 232B: Knurled V groove (groove depth 80 μm, groove width 0.2 mm, number of grooves 120)
-Peripheral speed ratio of transport roller 232 to developing roller 231: 1.0
-Gap between transport roller 232 and developing roller 231: 250 μm
Further, the magnetic pole distribution of the transport roller 232 used in Experiment 1 is shown in Table 2 above.

  In Experiment 1, under the above conditions, the relationship between the magnetic waveforms of the N3 pole and the N4 pole was changed in the two first magnets 231A of the magnetic force 1 and the magnet 2. Table 3 shows a list of Examples and Comparative Examples of Experiment 1 and evaluation results of development ghosts (ghost images) under the respective conditions.

The development ghost occurrence level is visually ranked based on the following criteria after a pattern image as shown in FIG. 9 is printed, and level 3 or higher is determined as an OK level.
5: Not at all (no problem in actual use)
4: Level that can be confirmed if you look closely but is not worrisome (no problem in actual use)
3: Level that is generated but not worrisome (no problem in actual use)
2: Can be confirmed 1: Can be clearly confirmed

  In Comparative Example 1-1, the difference in magnetic force between the N3 and N4 poles was small, and the developer retention portion TD (FIG. 6) was not sufficiently formed, so that development ghost was generated. In Examples 1-1, 1-2, and 1-3, there is a sufficient difference in magnetic force between the N3 and N4 poles, and transport of the developer from the N3 poles to the N4 poles is temporarily hindered. The developer retention part TD was sufficiently formed. As a result, the occurrence of development ghost was suppressed. On the other hand, in Comparative Example 1-2, the conveyance performance of the first sleeve 231B of the developing roller 231 is lower than that of Comparative Example 1-1 and the staying portion TD is easily formed, but the magnetic force difference between the N3-N4 poles is small. Due to the small size, the retention portion TD was not sufficiently formed, and a development ghost was generated. In Examples 1-4, 1-5, and 1-6, the conveyance performance of the first sleeve 231B is lower than that in Examples 1-1, 1-2, and 1-3. The staying portion TD is remarkably formed, and the development ghost is further improved. In Example 1-7, although the effect of suppressing the development ghost is slightly reduced by the amount that the blast surface of the first sleeve 231B is not plated, it is at a level that does not cause any problem in actual use. . Further, in Example 1-8, the condition is that the developer is only a knurled groove that is difficult to slip on the first sleeve 231B. However, since the magnetic force difference between the N3-N4 poles is a minimum of 15 mT, the same applies. In practical use, it is at a level where there is no problem. Thus, the result of suppressing the development ghost was obtained by satisfying the relationship of the above-mentioned formula 1.

  As for the conditions 2 and 3 of the first sleeve 231B, it was found that the same results as described above were obtained as a result of performing the same evaluation in the range of Rzjis 4 μm to 14 μm. Further, it was confirmed that there is no difference in Rzjis of the first sleeve 231B before and after plating when the Ni plating film thickness is in the range of 3 μm or more and 5 μm or less.

<Experiment 2>
Next, the surface condition of the first sleeve 231B of Experiment 1 is set to (Condition 1) the knurled V groove, and the magnetic field difference between the N3 pole and the N4 pole (Tave-Td) is 15 mT. The toner scattering under each condition was evaluated while adjusting the magnetic force (peak magnetic force) and angle (peak position) of the N2, N3, and S3 poles under different conditions. Further, the evaluation of the toner scattering was determined by the toner contamination of the housing 23H of the developing device 23. The confirmed place is the wall surface of the housing 23H on the downstream side in the rotation direction of the photosensitive drum 20 with respect to the developing region NP, and the degree of contamination in the durability test of 10,000 sheets was confirmed. The evaluation of toner contamination on the housing 23H is performed based on the following criteria, and ◎ and ○ correspond to levels that do not cause any problem in actual use.
×: Very dirty ○: Dirty but slight ◎: Almost dirty

Tables 4 and 5 show the evaluation results obtained under the above experimental conditions.

  From the experimental results of Table 4 and Table 5, in the circumferential distribution of the radial component of the magnetic force of the first magnet 231A, the interpolar angle between the peak position of the N3 pole and the peak position of the S2 pole is X (degrees), N4 The interpolar angle between the peak position of the pole and the peak position of the S3 magnetic pole is Y (degrees), the sum of the peak magnetic force of the N3 pole and the peak magnetic force of the S2 pole is E (mT), the peak magnetic force of the N4 pole and the S3 pole When the sum of the peak magnetic force and F (mT) is satisfied, if the relationship of E / X> F / Y (Formula 2) is satisfied, the retention portion TD of the developer increases, and the toner scattering is suppressed. became. When the relationship of E / X−F / Y> 0.5 is satisfied, toner scattering is further suppressed.

  FIG. 8 shows the distribution of the magnetic force (vertical distribution) of the radial component and the distribution of the magnetic force (horizontal distribution) of the tangential component around the N3 and N4 poles of the first magnet 231A when the above equation 2 is satisfied. It is the graph which showed an example. The maximum value of the horizontal magnetic force located between the S2 and N3 poles is determined by the sum and angle of the magnetic forces of the S2 and N3 poles. The maximum value of the horizontal magnetic force located between the N4 and S3 poles is determined by the sum and angle of the magnetic forces of the N4 and S3 poles. Since the position of the maximum value of the horizontal magnetic force overlaps the point where the vertical magnetic field becomes zero, the point where the horizontal magnetic force becomes zero is approximated as a straight line connecting the maximum values of the horizontal magnetic force. In this case, the position G of the point where the horizontal magnetic force becomes 0 is the position of the maximum value of the horizontal magnetic force located between the S2-N3 poles and the position of the maximum value of the horizontal magnetic force located between the N4-S3 poles. The value of horizontal magnetic force is closer to the lower one. Therefore, when the relationship of Expression 2 is satisfied, the position G at which the horizontal magnetic force is 0 is located on the N4 pole side with respect to the center position PL between the N3 pole peak and the N4 pole peak.

In each of Experiment 1 and Experiment 2, the gap (blade gap) between the layer thickness regulating member 235 and the second sleeve 232B is adjusted, and the toner conveyance amount on the first sleeve 231B is 100 g / m ² or more and 400 g / m ² or more. ^When the same evaluation was performed in the range of ² or less, the amount of the developer stagnant portion TD was not changed, and similar results were obtained with respect to the toner scattering and the development ghost suppressing effect. Further, in each of Experiment 1 and Experiment 2, when the same evaluation as described above was performed in a toner concentration range of 5% to 12%, there was no change in the amount of staying portion TD of the developer, and the toner scattering. Similar results were obtained with respect to the effect of suppressing the development ghost. Further, when the same evaluation is performed in the range where the diameter of the developing roller 231 and the transport roller 232 is 16 mm or more and 35 mm or less and the peripheral speed of the photosensitive drum 20 is 200 mm / sec or more and 400 mm / sec or less, toner scattering and Similar results were obtained with respect to the development ghost suppression effect.

  The developing device 23 and the image forming apparatus 10 including the developing device 23 according to an embodiment of the present invention have been described in detail above, but the present invention is not limited to this. The present invention can take, for example, the following modified embodiments.

  (1) In the above embodiment, in the first magnet 231A, two adjacent magnetic poles having the same polarity are described as the N3 pole and the N4 pole, but the present invention is not limited to this. Two adjacent magnetic poles of the same polarity may be composed of S poles. In this case, the other magnetic poles may be reversed between the S pole and the N pole.

  (2) In the above embodiment, the S2 pole is arranged on the upstream side of the N3 pole of the first magnet 231A, and the S3 pole is arranged on the downstream side of the N4 pole. It is not limited. In the first region R of FIG. 3, another magnetic pole may be arranged so as to sandwich the S2-N3-N4-S3 pole.

  (3) In the above-described embodiment, the layer thickness regulating member 235 has been described as being disposed so as to face the transport roller 232, but the present invention is not limited to this. The layer thickness regulating member 235 may be disposed to face the periphery of the S1 pole of the developing roller 231 and the like. In this case, another magnetic pole for transporting the developer may be added to the first magnet 231A.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 11 Apparatus main body 13 Image forming part 14 Intermediate transfer unit (transfer part)
145 Secondary transfer roller (transfer section)
20 Photosensitive drum (image carrier)
23 Developing device 231 Developing roller 231A First magnet 231B First sleeve 232 Conveying roller 232A Second magnet 232B Second sleeve 233 Stirring screw 233A First screw 233B Second screw 234 Partition plate 235 Layer thickness regulating member 23H Housing 23H1 First inner wall Part 23H2 Second inner wall part 23H3 Third inner wall part 23H4 Fourth inner wall part 24 Primary transfer roller NP Development position TP Opposite position

Claims (12)

A first magnet including a plurality of magnetic poles fixed along the circumferential direction; a first sleeve that rotates around the first magnet in a first rotational direction and carries a developer containing toner and a magnetic carrier on the circumferential surface; A developing roller that is arranged opposite to a photosensitive drum on which an electrostatic latent image is formed at a predetermined development position and supplies the toner to the photosensitive drum;
A second magnet that includes a plurality of magnetic poles fixed along the circumferential direction, and a second sleeve that rotates around the second magnet in a second rotational direction and carries the developer on the circumferential surface, A transport roller that is arranged to face the developing roller at a predetermined facing position, and that supplies the developer to the developing roller;
A developer agitating unit for agitating the developer and supplying the developer to the transport roller;
With
The first rotation direction and the second rotation direction are set to face each other at the facing position,
In the first region, the first magnet is downstream in the first rotation direction from the development position and upstream in the first rotation direction from the facing position.
A first magnetic pole composed of a predetermined magnetic pole, a second magnetic pole disposed adjacent to the first magnetic pole downstream of the first rotation direction, and having the same polarity as the first magnetic pole;
A third magnetic pole arranged adjacent to the first magnetic pole upstream of the first rotation direction, and having a different polarity from the first magnetic pole;
A fourth magnetic pole disposed adjacent to the second magnetic pole on the downstream side in the first rotational direction, and having a different polarity from the second magnetic pole;
With
In the distribution in the circumferential direction of the radial component of the magnetic force of the first magnet, the average value of the peak magnetic force of the first magnetic pole and the peak magnetic force of the second magnetic pole is Tave (mT), and the peak position of the first magnetic pole is When the minimum value of the magnetic force arranged between the peak position of the second magnetic pole is Td (mT),
Tave-Td ≧ 15
A developing device satisfying the relationship: The second magnet is disposed to face the fourth magnetic pole, and includes a fifth magnetic pole different from the fourth magnetic pole,
2. The development according to claim 1, wherein the developer that has passed through the development position is transferred from the development roller to the transport roller by a magnetic field formed by the fourth magnetic pole and the fifth magnetic pole. apparatus. The first magnet is disposed adjacent to the fourth magnetic pole on the downstream side in the first rotation direction with the facing position sandwiched therebetween, and includes a sixth magnetic pole different from the fourth magnetic pole,
The second magnet is disposed adjacent to the fifth magnetic pole on the upstream side in the second rotation direction with the facing position sandwiched therebetween and opposed to the sixth magnetic pole, and is different from the fifth magnetic pole. The seventh magnetic pole
The developer supplied from the developer stirring unit to the transport roller is transferred from the transport roller to the development roller by a magnetic field formed by the sixth magnetic pole and the seventh magnetic pole. The developing device according to claim 2.   Layer thickness regulation that is disposed opposite to the conveyance roller and upstream of the seventh magnetic pole in the second rotation direction and regulates the layer thickness of the developer supplied from the developer stirring unit to the conveyance roller. The developing device according to claim 3, further comprising a member. A housing that rotatably supports the developing roller and the transport roller;
The housing is a first extending opposite to the first magnetic pole, the second magnetic pole, the third magnetic pole, and the fourth magnetic pole and extending along the peripheral surface of the first sleeve of the developing roller. The developing device according to claim 1, further comprising an inner wall portion. A housing that rotatably supports the developing roller and the transport roller;
The housing is
A first inner wall portion facing the first magnetic pole, the second magnetic pole, the third magnetic pole, and the fourth magnetic pole and extending along the peripheral surface of the first sleeve of the developing roller;
A second inner wall connected to the first inner wall, facing the fifth magnetic pole, and extending along the peripheral surface of the second sleeve of the transport roller;
The developing device according to claim 2, further comprising:   The developing device according to claim 1, wherein the first sleeve of the developing roller includes a base material having a surface blasted.   The developing device according to claim 7, wherein the first sleeve of the developing roller includes a plating layer applied to the surface of the base material. In the circumferential distribution of the radial component of the magnetic force of the first magnet, the pole angle between the peak position of the first magnetic pole and the peak position of the third magnetic pole is X (degrees), and the peak of the second magnetic pole is The angle between the position and the peak position of the fourth magnetic pole is Y (degrees), the sum of the peak magnetic force of the first magnetic pole and the peak magnetic force of the third magnetic pole is E (mT), and the peak of the second magnetic pole When the sum of the magnetic force and the peak magnetic force of the fourth magnetic pole is F (mT),
E / X> F / Y
The developing device according to claim 1, wherein the relationship is satisfied. The axial center of the developing roller is disposed below the axial center of the photosensitive drum,
The developing device according to claim 1, wherein an axis of the transport roller is disposed below an axis of the developing roller.   The developing device according to claim 1, wherein a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller. A developing device according to any one of claims 1 to 11,
The photosensitive drum that is supplied with the toner from the developing device and carries a toner image on the peripheral surface;
A transfer unit for transferring the toner image from the photosensitive drum to a sheet;
An image forming apparatus comprising:

Images