JP6477560B2 - 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 in directions opposite to 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 the second magnetic pole, and a third magnetic pole disposed adjacent to the second magnetic pole on the downstream side in the first rotation direction with the facing position interposed therebetween, the second magnet comprising: A fourth magnetic pole disposed opposite to the second magnetic pole on the downstream side in the second rotational direction with respect to the opposed position and having the same polarity as the second magnetic pole, and upstream in the second rotational direction with respect to the fourth magnetic pole And a fifth magnetic pole arranged adjacent to the opposite position 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 third magnetic pole and the fifth magnetic pole. To do.

  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. Part of the toner is supplied from the developer to the photosensitive drum at the development position. And the 1st and 2nd magnetic pole of the same polarity is arrange | positioned in the 1st area | region of the 1st magnet of a developing roller. For this reason, due to the repulsive force of the first magnetic pole and the second magnetic pole, a stagnant portion in which the developer has partially stagnated is stably formed on the first sleeve. 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. Further, the fourth magnetic pole disposed opposite to the second magnetic pole is also composed of the same magnetic pole as the first and second magnetic poles. For this reason, the developer in the staying portion can be separated from the first sleeve by the repulsive force of the three homopolar magnetic poles. Accordingly, the developer that has passed through the development position is prevented from being delivered to the transport roller side. As a result, the delivery of the developer from the transport roller to the developing roller by the third magnetic pole and the fifth magnetic pole is stably realized, and the toner is prevented from scattering around the opposing position.

  In the above configuration, in the circumferential distribution of the radial component of the magnetic force of the first magnet, the peak magnetic force of the first magnetic pole is Tp (mT), the peak position of the first magnetic pole, and the peak of the second magnetic pole When the minimum value of the magnetic force arranged between the positions is Td (mT), it is desirable to satisfy the relationship of Tp−Td ≧ 25.

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

  In the above configuration, it is preferable that the third magnetic pole is a magnetic pole different from the second magnetic pole, and the fifth magnetic pole is a magnetic pole having the same polarity as the fourth magnetic pole.

  According to this configuration, the developer is stably transferred from the transport roller to the developing roller by the magnetic field formed by the third magnetic pole and the fifth magnetic pole.

  In the above configuration, the developer agitating unit conveys the developer in a predetermined direction along a horizontal direction and supplies the developer to the conveyance roller, and the horizontal direction in the horizontal direction A second conveying member that conveys the developer in a direction opposite to that of the first conveying member and collects the developer peeled from the developing roller, and includes the first conveying member and the second conveying member. It is desirable that the developer is circulated and conveyed by the conveying force of the conveying member.

  According to this structure, compared with the case where the 1st conveyance member collect | recovers the developer peeled from the developing roller, the 2nd conveyance member can be arrange | positioned in the position near a developing roller. For this reason, the scattering of the toner from the developer after peeling is suppressed.

  In the above configuration, the radial distribution of the magnetic force component of the first magnet is arranged to face the developing roller in a range from the peak position of the first magnetic pole to the peak position of the second magnetic pole in the circumferential distribution. It is desirable to further include a peeling member for peeling the developer from the first sleeve.

  According to this configuration, the developer can be stably peeled from the first sleeve, and the developer can be further prevented from adhering to the transport roller.

  In the above-described configuration, the developer is disposed on the upstream side of the fifth magnetic pole in the second rotation direction so as to face the transport roller, and the 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 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, the developer overflowing from the developer retention portion can be dropped and collected by the action of gravity.

  When viewed in a cross section perpendicular to the axis of the developing roller in the above configuration, the axis of the developing roller is arranged at a predetermined interval on one end side in the horizontal direction with respect to the axis of the photosensitive drum. It is desirable that the axis of the transport roller is disposed between the axis of the developing roller and the axis of the photosensitive drum in the horizontal direction.

  According to this configuration, the developer peeled from the developing roller is further suppressed from adhering to the transport roller.

  In the above configuration, it is preferable that the first magnetic pole, the second magnetic pole, and the fourth magnetic pole are arranged substantially in a straight line when viewed in a cross section orthogonal to the axis of the developing roller.

  According to this configuration, the staying portion of the developer can be stably formed on the first sleeve of the developing roller by the repulsive magnetic field formed by the first, second, and fourth magnetic poles, and the first sleeve The developer can be stably peeled off.

  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 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. 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. FIG. 6 is a schematic cross-sectional view showing an internal structure of a developing device according to a modified 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 developing device 23 shown in FIG. 1 is schematic, and the arrangement of each member is different from the actual one. The detailed structure of the developing device 23 will be described in detail later.

  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 4 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. FIG. 4 is a schematic cross-sectional view showing the magnetic pole arrangement of the transport roller 232.

  Referring to FIG. 2, the developing device 23 includes a housing 23 </ b> H, a developing roller 231, a transport roller 232, two stirring screws 233 (developer stirring unit), a partition plate 234, and a layer thickness regulating member 235. And separator 236 (peeling member). 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 an electrostatic latent image is formed at a predetermined developing position NP (FIG. 2), and supplies toner to the photosensitive drum 20. The developing roller 231 includes a first magnet 231A and a first sleeve 231B. 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, as an example, 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. In other embodiments, the surface of the first sleeve 231B may be formed with a plurality of known groove shapes instead of being provided with a predetermined surface roughness (Rzjis).

  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 4), and carries a developer containing nonmagnetic toner and a magnetic carrier on the circumferential 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. Further, as shown in FIG. 2, 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 in directions facing 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 (first conveying member) and a second screw 233B (second conveying member). The first screw 233A conveys the developer in a predetermined direction (backward direction) along the horizontal direction. The second screw 233B transports the developer in the direction opposite to the first screw 233A (forward direction) in the horizontal direction, and collects the developer peeled from the developing roller 231.

  As shown in FIG. 2, the housing 23H includes a first transport unit 23S1 in which the first screw 233A is disposed and a second transport unit 23S2 in which the second screw 233B is disposed. The first transport unit 23S1 and the second transport unit 23S2 communicate with each other at both ends in the axial direction (longitudinal direction). Further, as shown in FIG. 2, the second transport unit 23S2 is disposed above the first transport unit 23S1. The developer is circulated and conveyed between the first conveyance unit 23S1 and the second conveyance unit 23S2 by the conveyance force of 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 transport unit 23S1 and the second transport unit 23S2 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.

  The separator 236 is disposed to face the developing roller 231 and peels the developer from the first sleeve 231B. The tip of the separator 236 is disposed with a predetermined gap with respect to the first sleeve 231B. The separator 236 is made of a metal or rubber plate member.

  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. Further, when viewed in a cross section orthogonal to the axis of the developing roller 231, the axis of the developing roller 231 is spaced at a predetermined interval on one end side (left side) in the horizontal direction with respect to the axis of the photosensitive drum 20. Has been placed. Further, the axis of the transport roller 232 is disposed between the axis of the developing roller 231 and the axis of the photosensitive drum 20 in the horizontal direction.

  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. After a part of the toner is supplied to the photosensitive drum 20 at the development position NP, the developer peeled off from the development roller 231 falls freely and is collected by the second screw 233B. Thereafter, the collected developer is circulated and conveyed by the stirring screw 233.

  2 and 3, in the present embodiment, the first magnet 231A of the developing roller 231 includes five magnetic poles along the circumferential direction. An S11 pole (third magnetic pole) is disposed downstream of the opposing position TP between the developing roller 231 and the transport roller 232 in the first rotation direction (D1). Further, the N11 pole is arranged downstream of the S11 pole in the first rotation direction. The N11 pole functions as a main pole that supplies toner to the photosensitive drum 20. The N11 pole is disposed in the vicinity of the development position NP.

  Further, the first magnet 231A has three magnetic poles (S12 pole, N12 pole, N12 pole, in the first region R downstream of the development position NP in the first rotation direction and upstream of the facing position TP in the first rotation direction). N13 pole). The N12 pole (first magnetic pole) is disposed at a substantially central portion of the first region R. The N13 pole (second magnetic pole) is arranged adjacent to the N12 pole on the downstream side in the first rotation direction, and has the same polarity as the N12 pole. The S12 pole is disposed adjacent to the N12 pole on the upstream side in the first rotational direction, and is a magnetic pole different from the N12 pole. The S12 pole has a function of transporting the developer that has passed through the development position NP toward the N12 pole. The S11 pole described above is arranged adjacent to the N13 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 N13 pole.

  Table 1 shows the angles of the five magnetic poles and the peak magnetic force of the radial component as an example of the first magnet 231A according to the present embodiment. 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. 3 as the starting point (angle 0 °). 3 shows a straight line CL (a straight line connecting the rotation axis of the developing roller 231 and the rotation axis of the transport roller 232) connecting the facing position TP and the rotation axis of the developing roller 231.

  On the other hand, referring to FIGS. 2 and 4, second magnet 232 </ b> A of transport roller 232 includes five magnetic poles along the circumferential direction. An N3 pole (fourth magnetic pole) is arranged on the downstream side in the second rotational direction (D2) from the position TP where the developing roller 231 and the transport roller 232 face each other. Further, the S2 pole is disposed downstream of the N3 pole in the second rotation direction. The S2 pole functions as a pumping pole that pumps up the developer from the first screw 233A. N1 pole, S1 pole, and N2 pole (fifth magnetic pole) are arranged downstream of the S2 pole in the second rotation direction. The N1 pole and the S1 pole function as transport poles that transport the developer pumped up by the S2 pole toward the N2 pole. As shown in FIG. 2, the layer thickness regulating member 235 is the second of the transport roller 232 between the S1 pole and the N1 pole (near the N1 pole) on the upstream side in the second rotational direction from the N2 pole. The sleeve 232B is disposed to face the sleeve 232B at a predetermined interval. 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.

  Note that the N3 pole is disposed opposite to the N13 pole of the first magnet 231A on the downstream side in the second rotational direction from the facing position TP, and is the same pole as the N13 pole. Further, the N2 pole is disposed adjacent to the N3 pole on the upstream side in the second rotation direction with the opposing position TP interposed therebetween, and is the same pole as the N3 pole. The developer supplied from the first screw 233A to the conveyance roller 232 and regulated by the layer thickness regulating member 235 is caused by the magnetic field formed by the S11 pole of the first magnet 231A and the N2 pole of the second magnet 232A. 232 to the developing roller 231. Further, when viewed in a cross section perpendicular to the axis of the developing roller 231 (FIG. 2), the N12 pole and N13 pole of the first magnet 231A and the N3 pole of the second magnet 232A are downward and rightward from above and from the left. It is arrange | positioned on the substantially straight line along the straight line extended toward. Further, on the above straight line and on the extension line of the N3 pole, the S2 pole as the pumping pole is arranged.

  Further, the second screw 233B is arranged above the first screw 233A so as to follow the inclined arrangement of the magnetic poles. In other words, the rotational axis of the second screw 233B and the rotational axis of the first screw 233A are also arranged with steps along a straight line extending from the upper side to the lower side and to the right side.

  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. 4 as the starting point (angle 0 °). In FIG. 4, a straight line CL (a straight line connecting the rotation axis of the developing roller 231 and the rotation axis of the transport roller 232) that connects the facing position TP and the rotation axis of the transport roller 232 is shown.

  Referring to FIG. 2, the partition plate 234 extends upward from the bottom of the housing 23H along the peripheral surface of the second screw 233B, and then curves and extends to the left. The distal end portion of the partition plate 234 supports the proximal end portion of the separator 236. In the present embodiment, the separator 236 extends in the vertical direction. For this reason, the developer is prevented from being deposited on the separator 236. Further, the front end of the separator 236 is disposed to face the developing roller 231 in the range from the peak position of the N12 pole to the peak position of the N13 pole in the circumferential distribution of the radial component of the magnetic force of the first magnet 231A. Has been. For this reason, as shown in FIG. 2, the storage space 23S of the housing 23H is divided into right and left by the separator 236 and the partition plate 234 at a position below the developing roller 231.

  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 thin toner layer (toner film) is likely to be formed on the first sleeve 231B at the root of the magnetic brush made of toner and carrier.

  When the toner layer as described above is formed on the first sleeve 231B of the developing roller 231, a history between the image portion and the background portion of the toner consumed at the developing position NP tends to remain. FIG. 7 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 N12 pole and N13 pole. Further, the transport roller 232 includes an N3 pole facing the N13 pole. In this case, referring to FIG. 2, the developer conveyed from the S12 pole to the N12 pole of the developing roller 231 does not easily move to the N13 pole, and therefore partially stays on the N12 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 portion TD can be suitably formed by the repulsive force of the N12-N13 poles that the developing roller 231 has. Further, the N3 pole disposed opposite to the N13 pole is also composed of magnetic poles having the same polarity as the N12 pole and the N13 pole. For this reason, the excess developer that can no longer be held in the stay portion TD can be peeled off from the first sleeve 231B by the repulsive force. Therefore, the developer that has passed through the development position NP is prevented from being delivered to the transport roller 232 side. As a result, the delivery of the developer from the transport roller 232 to the developing roller 231 by the N2 pole and the S11 pole is stably realized, and the toner is prevented from scattering around the facing position TP. Furthermore, in this embodiment, the separator 236 is arrange | positioned between N12 pole and N13 pole. For this reason, the developer can be stably peeled from the first sleeve 231B together with the action of the repulsive force. Further, the developer is further prevented from adhering to the transport roller 232.

  When the transport roller 232 temporarily collects the developer on the developing roller 231, the magnetic brush flows from the developing roller 231 toward the transport roller 232 and the magnetism from the transport roller 232 toward the developing roller 231 around the facing position TP. The brush flow is close. In this case, toner scattering is likely to occur due to the collision of both magnetic brushes. In this embodiment, the occurrence of such toner scattering is preferably suppressed.

  Furthermore, in this embodiment, when viewed in a cross section orthogonal to the axis of the developing roller 231, the N12 pole, the N13 pole, and the N3 pole are arranged on a substantially straight line. Therefore, the rebound magnetic field formed by the N12, N13, and N3 poles can stably form the developer retaining portion TD on the first sleeve 231B of the developing roller 231 and develop from the first sleeve 231B. The agent can be peeled stably.

FIG. 5 is a graph showing the magnetic force distribution of the radial component between the N12 pole and the N13 pole, which are adjacent homopolar poles, in the developing roller 231 of the developing device 23 according to the present embodiment. In this embodiment, in the circumferential distribution of the radial component (Radial component) of the magnetic force of the first magnet 231A, the peak magnetic force of the N12 pole is Tp (mT), and the peak position of the N12 pole and the peak position of the N13 pole are When Td (mT) is the minimum value of the magnetic force disposed between them (the trough portion of the magnetic force)
Tp−Td ≧ 25 (Formula 1)
The relationship is satisfied. Thus, by setting the difference between the peak magnetic force Tp of the N12 pole and the magnetic force Td of the valley portion between the N12 and N13 poles, the repulsive magnetic force formed by the N12 pole and the N13 pole is further increased. Therefore, the history on the toner layer generated at the development position NP can be more stably removed. Such a stay portion TD cannot be formed sufficiently with a known developer peeling electrode. As described above, the three magnetic poles having the same polarity are sequentially arranged, and when Formula 1 is satisfied, it is possible to form the staying portion TD that can polish the toner layer on the first sleeve 231B.

  In the present embodiment, the N12 pole and the N13 pole are each formed from a fan-shaped ferrite magnet. The magnetic force difference between the peak value of the N12 pole and the peak value of the N13 pole is set to a range of 30 mT or less. Within this range, it is possible to determine the formation state of the staying portion TD by comparing the peak value Tp of the N12 pole on the upstream side with the magnetic force Td of the valley portion. In order to further stabilize the formation of the retention portion TD, the peak value of the N12 pole is preferably set to be equal to or higher than the peak value of the N13 pole.

  In the present embodiment, the S11 pole is disposed downstream of the N13 pole in the first rotation direction, and the N2 pole is disposed upstream of the N3 pole in the second rotation direction. Then, the developer can be stably supplied from the transport roller 232 to the developing roller 231 by the magnetic field formed by the N2 pole and the S11 pole having a different polarity relationship. At this time, since the N2 pole and the N3 pole having the same polarity are formed at the position sandwiching the facing position TP, the developer on the transport roller 232 can be pushed up to the developing roller 231 side by the repulsive magnetic field of both. Further, the developer that has moved to the S11 pole side is quickly conveyed to the N11 pole side along with the rotation of the first sleeve 231B by the repulsive magnetic field formed by the N2, N3, and N13 poles. In other words, the developer that has moved to the S11 pole side is prevented from being clogged at the opposing position TP. 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. 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.

  In the present embodiment, the developer peeled from the developing roller 231 is collected by the second screw 233B instead of the first screw 233A. In this case, as compared with the case where the first screw 233 </ b> A collects the developer peeled from the developing roller 231, the second transport member 233 </ b> B is disposed near (above) the developing roller 231 as shown in FIG. 2. be able to. For this reason, the scattering of the toner from the developer after peeling is suppressed. Accordingly, the toner is also prevented from scattering into the image forming apparatus 10 from the gap of the housing 23H of the developing device 23.

  In the case where the developer peeled off from the developing roller 231 is collected by the first screw 233A, the arrangement of the first screw 233A needs to be shifted to the left from FIG. In this case, the function of supplying the developer from the first screw 233A to the transport roller 232 is likely to be lowered. Also, the first screw 233A can have two functions of supplying and collecting the developer by increasing the outer diameter of the first screw 233A. In this case, the entire developing device 23 is increased in size. .

  Furthermore, in this embodiment, 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 disposed below the axis of the developing roller 231. Therefore, the developer overflowing from the developer retention part TD can be dropped and collected by the action of gravity. Further, referring to FIG. 2, the outer diameter of developing roller 231 and the outer diameter of conveying roller 232 are substantially the same, and the axis of developing roller 231 is predetermined on the left side with respect to the axis of photosensitive drum 20. The axis of the conveying roller 232 is arranged between the axis of the developing roller 231 and the axis of the photosensitive drum 20 in the horizontal direction. For this reason, since the transport roller 232 is not disposed directly below the developing roller 231, a space where the transport roller 232 does not exist is formed below the N12 pole and the N13 pole. Therefore, the developer peeled from the developing roller 231 is further suppressed from adhering to the transport roller 232.

Next, based on an Example, this invention is further demonstrated. In addition, this invention is not limited to a following example.
<Experiment>
In this experiment, 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: 300 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 the experiment 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, groove number 120)
(Condition 2) Sandblasting (Rzjis 10 μm)
Further, the magnetic pole distribution of the developing roller 231 used in Experiment 1 is as 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. GAUSS METER Model GX-100.

Moreover, the conditions of the conveyance roller 232 used for experiment 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: 1.05
-Gap between transport roller 232 and developing roller 231: 250 μm
Further, the magnetic pole distribution of the transport roller 232 used in the experiment is shown in Table 2 above.

  In this experiment, under the above-described conditions, the relationship of the magnetic force difference of Tp-Td was changed in the first magnet 231A. Table 3 shows a list of examples and comparative examples of this experiment, 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. 7 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, the difference in magnetic force between Tp and Td is small, and the developer retention portion TD (FIG. 2) is not sufficiently formed, and therefore development ghost is generated. In Examples 1, 2, and 3, there is a sufficient difference in magnetic force between Tp and Td, and transport of the developer from the N12 pole to the N13 pole is temporarily hindered. Well formed. As a result, the occurrence of development ghost was suppressed. On the other hand, in Comparative Example 2, although the conveyance performance of the first sleeve 231B of the developing roller 231 is lower than that in Comparative Example 1 and the retention portion TD is easily formed, the magnetic force difference between Tp and Td is still small. The portion TD was not sufficiently formed, resulting in the development ghost. In Examples 4, 5, and 6, the conveyance performance of the first sleeve 231B is reduced compared to Examples 1, 2, and 3, and the developer retention portion TD is significantly formed, and the development ghost is generated. Further improvements have been made.

  As for condition 2 of the first sleeve 231B, it was found that the same result as above was obtained as a result of performing the same evaluation in the range of Rzjis 4 μm to 14 μm.

  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, the two adjacent magnetic poles having the same polarity are described as the N12 pole and the N13 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-described embodiment, the layer thickness regulating member 235 is described as being disposed facing 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.

  (3) In the above embodiment, the developing device 23 is described as having the separator 236 (FIG. 2). However, the present invention is not limited to this. FIG. 6 is a schematic cross-sectional view showing the internal structure of the developing device 23M according to a modified embodiment of the present invention. In FIG. 6, members having the same functions and structures as those of the developing device 23 according to the previous embodiment are denoted by the same reference numerals as those in FIG. The developing device 23M does not include the separator 236 as compared with the developing device 23 according to the previous embodiment. Further, the partition plate 234 of the developing device 23M has a function of partitioning between the first screw 233A and the second screw 233B. Even in such a configuration, the ghost image is suppressed by the developer retention portion TD formed between the N12 pole and the N13 pole. Further, the surplus developer in the staying portion TD can be stably peeled from the first sleeve 231B by the repulsive pole formed by the N12 pole, the N13 pole, and the N3 pole.

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 (first conveying member)
233B Second screw (second conveying member)
234 Partition plate 235 Layer thickness regulating member 236 Separator (peeling member)
23H Housing 24 Primary transfer roller NP Development position TP Opposite position

Claims (9)

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 in directions facing 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 disposed adjacent to the second magnetic pole on the downstream side in the first rotational direction with the facing position interposed therebetween;
With
The second magnet is
A fourth magnetic pole disposed opposite to the second magnetic pole on the downstream side in the second rotational direction from the opposing position, and having the same polarity as the second magnetic pole;
A fifth magnetic pole disposed adjacent to the fourth magnetic pole on the upstream side in the second rotational direction with the facing position interposed therebetween;
With
The developer supplied from the developer stirring unit to the transport roller is transferred from the transport roller to the developing roller by a magnetic field formed by the third magnetic pole and the fifth magnetic pole ,
The developer stirring unit is
A first conveying member that conveys the developer in a predetermined direction along a horizontal direction and that supplies the developer to the conveying roller;
A second conveying member that conveys the developer in a direction opposite to the first conveying member in the horizontal direction and collects the developer peeled from the developing roller;
The developer is circulated and conveyed by the conveying force of the first conveying member and the second conveying member,
In the circumferential distribution of the radial component of the magnetic force of the first magnet, the peak magnetic force of the first magnetic pole is Tp (mT), and the peak position of the first magnetic pole is between the peak position of the second magnetic pole. When the minimum value of the arranged magnetic force is Td (mT),
Tp−Td ≧ 25
A developing device satisfying the relationship: The developing device according to claim 1 , wherein the third magnetic pole is a magnetic pole different from the second magnetic pole, and the fifth magnetic pole is a magnetic pole having the same polarity as the fourth magnetic pole. The first sleeve is disposed to face the developing roller in a range from the peak position of the first magnetic pole to the peak position of the second magnetic pole in the circumferential distribution of the radial component of the magnetic force of the first magnet, the developing device according to claim 1 or 2, further comprising a peeling member for peeling the developer from. Layer thickness regulation for regulating the layer thickness of the developer disposed on the upstream side in the second rotation direction from the fifth magnetic pole so as to face the conveyance roller and supplied from the developer stirring unit to the conveyance roller an apparatus according to any one of claims 1 to 3, further comprising a member. The axial center of the developing roller is disposed below the axial center of the photosensitive drum,
The axis of the conveying roller, a developing apparatus according to any one of claims 1 to 4, characterized in that it is disposed below the axis of the developing roller. When viewed in a cross section perpendicular to the axis of the developing roller,
The axial center of the developing roller is disposed at a predetermined interval on one end side in the horizontal direction with respect to the axial center of the photosensitive drum,
The developing device according to claim 5 , wherein an axis of the conveying roller is disposed between the axis of the developing roller and the axis of the photosensitive drum in a horizontal direction. When viewed in cross section perpendicular to the axis of said developing roller, said first magnetic pole, the second pole and the fourth pole, according to claim 6, characterized in that it is arranged on a substantially straight line Development device. Wherein the developing roller, a developing apparatus according to any one of claims 1 to 7, characterized in that the developing bias AC bias on a DC bias is superposed is applied. A developing device according to any one of claims 1 to 8 ,
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:

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