JP2011242521A - Transfer device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transfer device capable of suppressing a transfer failure, with a high degree of freedom in setting the position of a secondary transfer roller.SOLUTION: A transfer device 30 includes an intermediate transfer belt 311, a drive roller 312, a secondary transfer belt 331, a secondary transfer roller 332 and an auxiliary member 333. The drive roller 312 tensions the intermediate transfer belt 311. The secondary transfer belt 331 is in pressure contact with the intermediate transfer belt 311 in a secondary transfer region 92. The secondary transfer roller 332 is opposed to the drive roller 312 across the intermediate transfer belt 311 and the secondary transfer belt 331, and is electrically grounded. The auxiliary member 33 is arranged on the secondary transfer roller 332 side with respect to the secondary transfer belt 331 on a side closer to an upstream side of a paper conveying direction 93 than the secondary transfer roller 332. In secondary transfer, voltage having the same polarity as that of toner is applied to the drive roller 312 and the auxiliary member 33, respectively.

Description

  The present invention relates to a transfer device that transfers a developer image formed by an image forming body onto a sheet via an intermediate transfer body.

  In an electrophotographic image forming apparatus, a developer image formed by an image forming body is transferred to an intermediate transfer body, a primary transfer and a secondary transfer from the intermediate transfer body to a sheet, and the developer image is transferred to a sheet. There is an intermediate transfer type image forming apparatus provided with a transfer device for transferring to an image. Some transfer apparatuses include a secondary transfer belt that conveys a sheet to a secondary transfer region. The secondary transfer belt is in pressure contact with the intermediate transfer member, and the secondary transfer roller faces the intermediate transfer member with the secondary transfer belt interposed therebetween. The developer image carried on the intermediate transfer member is secondarily transferred to a sheet by an electric field formed in the secondary transfer region (see, for example, Patent Document 1).

  However, if a large electric field is applied to the developer on the intermediate transfer body before the paper is in close contact with the intermediate transfer body, the developer splatters or the developer charge is upstream of the secondary transfer area in the paper transport direction. Problems such as slipping out of the sheet occur, and these defects tend to cause a transfer defect in which an area where the developer is not sufficiently transferred onto the sheet is generated in the secondary transfer area.

  Therefore, a configuration is known in which the secondary transfer roller is offset from the intermediate transfer member on the downstream side in the sheet conveyance direction. If the secondary transfer roller is offset on the downstream side with respect to the intermediate transfer member, the electric field strength on the upstream side of the secondary transfer region in the paper transport direction is smaller than when the secondary transfer roller is not offset. It is possible to prevent a large electric field from being applied to the developer on the intermediate transfer member before the sheet comes into close contact with the intermediate transfer belt.

JP 2009-128735 A

  However, in the conventional transfer apparatus in which the secondary transfer roller is offset from the intermediate transfer member on the downstream side in the paper conveyance direction, the region where the electric field is formed is adjusted by the position of the secondary transfer roller. The degree of freedom in setting the roller position is reduced.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a transfer device that can suppress a transfer failure due to scattering of a developer and the like and has a high degree of freedom in setting a position of a secondary transfer roller.

  The transfer device of the present invention includes an intermediate transfer member, a secondary transfer belt, a secondary transfer roller, a bias member, and a power supply unit. The intermediate transfer member is primarily transferred with the developer image formed on the image forming member. The secondary transfer belt conveys a predetermined conveyance path so that a sheet on which the developer image carried on the intermediate transfer member is secondarily transferred is brought into close contact with the intermediate transfer member in the secondary transfer region. The secondary transfer roller is opposed to the intermediate transfer member with the secondary transfer belt interposed therebetween and is electrically grounded. The bias member is disposed on the secondary transfer roller side with respect to the secondary transfer belt on the upstream side in the sheet conveyance direction from the secondary transfer region. The power supply unit applies a voltage having the same polarity as the developer to each of the intermediate transfer member and the bias unit during the secondary transfer.

  In this configuration, an electric field is formed in the secondary transfer region by applying a voltage having the same polarity as the developer to the intermediate transfer member. Further, the bias member to which a voltage having the same polarity as that of the developer is disposed on the upstream side of the secondary transfer roller in the paper conveyance direction, so that the paper conveyance direction is compared with the case where no voltage is applied to the bias member. In this case, the electric field strength on the upstream side of the secondary transfer region becomes small. For this reason, regardless of the position of the secondary transfer roller, it is possible to prevent a large electric field from being applied to the developer on the intermediate transfer member before the sheet comes into close contact with the intermediate transfer member.

  In the above-described configuration, the intermediate transfer member has an endless belt-like intermediate transfer belt on which the developer image formed on the image forming member is primarily transferred, and the intermediate transfer belt is stretched to have the same polarity as the developer. And a tension roller to which a voltage of 1 is applied.

  In this configuration, an electric field is formed in the secondary transfer region by applying a voltage having the same polarity as the developer to the stretching roller. Further, the bias member to which a voltage having the same polarity as that of the developer is disposed on the upstream side of the secondary transfer roller in the paper conveyance direction, so that the paper conveyance direction is compared with the case where no voltage is applied to the bias member. In this case, the electric field strength on the upstream side of the secondary transfer region becomes small. Therefore, regardless of the position of the secondary transfer roller, it is possible to prevent a large electric field from being applied to the developer on the intermediate transfer belt before the sheet comes into close contact with the intermediate transfer member.

  Further, the secondary transfer roller can be arranged so that the direction connecting the rotation shaft of the secondary transfer roller and the rotation shaft of the stretching roller is orthogonal to the paper transport direction.

  As a result, the secondary transfer roller is not offset with respect to the stretching roller in the paper conveyance direction, and the amount of winding of the secondary transfer belt around the intermediate transfer belt is reduced, so that the paper curls and is attracted to the intermediate transfer belt. It is possible to prevent the sheet from being transported from the secondary transfer region to the fixing device.

  Further, the intermediate transfer member and the secondary transfer belt are pressed against each other in the secondary transfer region, and the bias member is adjacent to the upstream side of the secondary transfer roller in the paper transport direction among a plurality of rollers that stretch the secondary transfer belt. It is an arranged roller, and can be arranged so that the secondary transfer belt does not protrude toward the intermediate transfer body with respect to the tangential plane of the intermediate transfer body at the center of the secondary transfer area in the sheet conveyance direction.

  In order to prevent a large electric field from being applied to the developer on the intermediate transfer member before the sheet comes into close contact with the intermediate transfer member, a bias member is provided so that the secondary transfer belt protrudes toward the intermediate transfer member with respect to the tangential plane. By arranging the secondary transfer belt, it is conceivable that the secondary transfer belt is more wound around the intermediate transfer body on the upstream side of the secondary transfer roller. In this configuration, the secondary transfer belt is wound more around the intermediate transfer member, so that the secondary transfer area becomes longer and the upstream end of the secondary transfer area in the paper transport direction moves more upstream. The vicinity region upstream of the transfer region moves to a region where the electric field strength is smaller. However, when the secondary transfer belt protrudes toward the intermediate transfer body from the tangential plane, the area where the sheet conveyance path can be arranged from the registration roller to the secondary transfer area becomes narrower and is arranged upstream of the secondary transfer area. The degree of freedom in setting the position of the registration roller and the sheet conveyance path from the registration roller to the secondary transfer region is reduced. On the other hand, by applying a voltage having the same polarity as the developer to the bias member on the upstream side of the secondary transfer roller in the paper transport direction, the upstream side of the position where the intermediate transfer member and the secondary transfer roller are opposed in the paper transport direction. Therefore, it is possible to prevent a large electric field from being applied to the developer on the intermediate transfer member before the sheet comes into close contact with the intermediate transfer member. For this reason, it is not necessary to project the bias member to the intermediate transfer member with respect to the tangential plane. In addition, the bias member is arranged so that the secondary transfer belt does not protrude toward the intermediate transfer body with respect to the tangential plane, so that the degree of freedom in setting the position of the sheet conveyance path from the registration roller and the registration roller to the secondary transfer region is set. Becomes higher.

  An image forming apparatus according to the present invention is an electrophotographic image forming apparatus, and includes a plurality of image forming bodies and a transfer device having any one of the above-described configurations.

  According to the present invention, transfer failure can be suppressed and the degree of freedom in setting the position of the secondary transfer roller can be increased.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus including a transfer device according to an embodiment of the present invention. It is a block diagram of the principal part of a transfer apparatus. It is a schematic diagram which shows a secondary transfer area | region and the electric field of the vicinity. It is a schematic diagram which shows the secondary transfer area | region in the structure of a comparative example, and the electric field of the vicinity. It is a one part block diagram of another comparative example. FIG. 10 is a partial configuration diagram of still another comparative example. FIG. 10 is a partial configuration diagram of still another comparative example. Electric field strength upstream of the secondary transfer area, degree of toner scattering upstream of the secondary transfer area, paper transportability after passing through the secondary transfer area, and paper transport disposed upstream of the secondary transfer area It is a figure which shows the relationship between each freedom degree of arrangement | positioning of a path | route, and the structure of a transfer apparatus. It is a block diagram of the principal part of the transfer apparatus which concerns on other embodiment.

  Embodiments of the present invention will be described below with reference to the drawings.

  As shown in FIG. 1, the image forming apparatus 10 forms a multicolor or single color image on a sheet based on image data. Examples of the paper include recording media such as plain paper, photographic paper, and OHP film. The image forming apparatus 10 includes image forming stations (image forming units) 21, 22, 23, 24, a transfer device 30, a fixing device 34, a paper conveyance path 35, a paper feed tray 36, a paper discharge tray 37, and a power supply unit 70. I have. The transfer device 30 includes an intermediate transfer belt unit 31, a plurality of primary transfer rollers 32 </ b> A to 32 </ b> D, and a secondary transfer belt unit 33.

  The image forming apparatus 10 uses an image data corresponding to four hues of black, cyan, magenta, and yellow, which are three subtractive primary colors obtained by color separation of a color image. The image forming process is performed. In the image forming stations 21 to 24, toner images of respective hues are formed. The image forming stations 21 to 24 are arranged in a row in the horizontal direction along the intermediate transfer belt unit 31.

  Hereinafter, the image forming station 21 will be mainly described. The image forming stations 22 to 24 are configured in substantially the same manner as the image forming station 21. The black image forming station 21 includes a photosensitive drum 41, a charging device 42, an exposure device 43, a developing device 44, and a cleaning unit 45. A black toner image (developer image) is obtained through an electrophotographic image forming process. ).

  The photoconductive drum 41 is an image forming body, has a cylindrical shape, and rotates in one direction when a driving force is transmitted from a first driving source (not shown). The photosensitive drum 41 has a cylindrical conductive substrate and a photosensitive layer provided on the peripheral surface of the conductive substrate.

  The charging device 42 is disposed so as to face the peripheral surface of the photoconductive drum 41, and uniformly charges the peripheral surface of the photoconductive drum 41 to a predetermined potential.

  The exposure device 43 has a semiconductor laser, and irradiates the peripheral surface of the photosensitive drum 41 with a laser beam modulated by black image data. An electrostatic latent image based on black image data is formed on the peripheral surface of the photosensitive drum 41.

  The developing device 44 contains a black developer. As the developer, for example, a two-component developer composed of a toner and a carrier is used. The developing device 44 charges the toner by stirring the toner and the carrier. The developing device 44 develops the electrostatic latent image into a toner image by supplying toner to the peripheral surface of the photosensitive drum 41 on which the electrostatic latent image is formed.

  The developing devices 44 of the image forming stations 22 to 24 contain developers of cyan, magenta, and yellow, and the photosensitive drums 41 store cyan, magenta, and yellow. A toner image of each hue is formed.

  The intermediate transfer belt unit 31 is an intermediate transfer member, and includes an intermediate transfer belt 311, a driving roller 312, and a driven roller 313. As the intermediate transfer belt 311, for example, a film having a predetermined thickness in the range of 100 to 150 μm is used. The intermediate transfer belt 311 is configured as an endless belt, is stretched around the driving roller 312 and the driven roller 313, and moves in one direction.

  The outer peripheral surface of the intermediate transfer belt 311 faces the respective photosensitive drums 41 of the image forming stations 21 to 24. Of the driving roller 312 and the driven roller 313 that stretch the intermediate transfer belt 311, the roller disposed on the downstream side of the image forming stations 21 to 24 in the moving direction 91 of the intermediate transfer belt 311 constitutes the stretching roller. To do. As an example, the driving roller 312 is disposed downstream of the image forming stations 21 to 24 in the moving direction 91 of the intermediate transfer belt 311 and constitutes a stretching roller, and the driven roller 313 is the image forming stations 21 to 24. It is arranged on the upstream side.

  The primary transfer rollers 32 </ b> A to 32 </ b> D are arranged to face the respective photosensitive drums 41 of the image forming stations 21 to 24 with the intermediate transfer belt 311 interposed therebetween. The primary transfer rollers 32B to 32D are configured in substantially the same manner as the primary transfer roller 32A. The primary transfer roller 32A has a shaft made of a metal (for example, stainless steel) having a diameter of 8 to 10 mm, and a conductive elastic material that covers the peripheral surface of the shaft. A high voltage is applied to the intermediate transfer belt 311 by a conductive elastic material. A region where the intermediate transfer belt 311 faces the respective photosensitive drums 41 of the image forming stations 21 to 24 is a primary transfer region.

  In the primary transfer rollers 32A to 32D, in order to primarily transfer the toner image carried on the peripheral surface of the photosensitive drum 41 onto the intermediate transfer belt 311, the polarity (for example, minus) of the toner is opposite to the polarity (for example, minus). (Plus) primary transfer bias is applied by constant voltage control. As a result, the toner images of the respective hues formed on the peripheral surfaces of the respective photosensitive drums 41 of the image forming stations 21 to 24 are sequentially transferred onto the outer peripheral surface of the intermediate transfer belt 311 (primary transfer), and the intermediate transfer belt. A full-color toner image is formed on the outer peripheral surface 311.

  However, when image data of only a part of the hues of yellow, magenta, cyan, and black is input, only part of the four photosensitive drums 41 corresponding to the hue of the input image data is input. An electrostatic latent image and a toner image are formed. For example, during the monochrome image forming process, the electrostatic latent image and the toner image are formed only on the photosensitive drum 41 corresponding to the black hue, and only the black toner image is formed on the outer peripheral surface of the intermediate transfer belt 311. Transcribed.

  The cleaning unit 45 removes and collects toner remaining on the peripheral surface of the photosensitive drum 41 after development and primary transfer.

  The toner image transferred to the outer peripheral surface of the intermediate transfer belt 311 in each of the primary transfer regions is conveyed to the secondary transfer region 92 by the circumferential movement of the intermediate transfer belt 311.

  The secondary transfer belt unit 33 includes a secondary transfer belt 331 and a secondary transfer roller 332.

  The secondary transfer belt 331 is pressed against the intermediate transfer belt 311 with a predetermined nip pressure. The secondary transfer roller 332 is disposed so as to face the intermediate transfer body with the secondary transfer belt 331 and the intermediate transfer belt 311 interposed therebetween. In this embodiment, the secondary transfer roller 332 constitutes a tension roller among the driving roller 312 and the driven roller 313 that stretch the intermediate transfer belt 311 with the secondary transfer belt 331 and the intermediate transfer belt 311 interposed therebetween. It arrange | positions so that the drive roller 312 which may be.

  A region where the sheet on which the toner image carried on the intermediate transfer belt 311 is secondarily transferred is in close contact with the intermediate transfer belt 311 is a secondary transfer region 92. In the secondary transfer region 92, the intermediate transfer belt 311 and the secondary transfer belt 311 are pressed against each other.

  The paper feed tray 36 stores paper. The paper fed from the paper feed tray 36 is transported through the secondary transfer region 92 with an electric field formed between the intermediate transfer belt 311 and the secondary transfer belt 331. As a result, the toner image is secondarily transferred from the outer peripheral surface of the intermediate transfer belt 311 to the sheet.

  Of the toner primarily transferred from one or all of the four photosensitive drums 41 to the intermediate transfer belt 311, the toner remaining on the intermediate transfer belt 311 without being transferred to the paper prevents color mixing in the next step. Therefore, the intermediate transfer belt cleaning unit 51 collects the toner.

  The fixing device 34 includes a heating roller 341 and a pressure roller 342. The sheet on which the toner image has been transferred is guided to the fixing device 34 and is heated and pressed by passing between the heating roller 341 and the pressure roller 342. As a result, the toner image is firmly fixed on the paper. The sheet on which the toner image is fixed is discharged onto the discharge tray 37 by the discharge roller pair 52.

  The image forming apparatus 10 is provided with a paper conveyance path 35. The paper transport path 35 is configured to guide the paper stored in the paper feed tray 36 to the paper discharge tray 37 via the secondary transfer region 92 and the fixing device 34. As an example, the paper transport path 35 extends in a substantially vertical direction and then curves in a substantially horizontal direction before the paper discharge roller pair 52.

  In the paper conveyance path 35, a pickup roller, conveyance roller pairs 54A, 54B, 54C, a registration roller pair 55, and the above-described paper discharge roller pair 52 are arranged. The pickup roller feeds the paper in the paper feed tray 36 to the paper transport path 35 one by one. The conveyance roller pairs 54A, 54B, and 54C convey the fed paper along the paper conveyance path 35. The registration roller pair 55 guides the conveyed paper to the secondary transfer area 92 at a predetermined timing.

  In the full color image forming process in which the image forming process is performed in all of the image forming stations 21 to 24, the four primary transfer rollers 32A to 32D press the intermediate transfer belt 311 against all the photosensitive drums 41. On the other hand, at the time of monochrome image forming processing in which image forming processing is performed only at the image forming station 21, only the primary transfer roller 32 </ b> A facing the image forming station 21 presses the intermediate transfer belt 311 against the photosensitive drum 41 of the image forming station 21. .

  As shown in FIG. 2, the secondary transfer belt unit 33 includes an auxiliary member 333 and a plurality of rollers 334, 335, and 336 in addition to the secondary transfer belt 331 and the secondary transfer roller 332.

The secondary transfer belt 331 is configured as an endless belt, and is stretched by a secondary transfer roller 332, an auxiliary member 333, and a plurality of rollers 334, 335, and 336. The outer peripheral surface of the secondary transfer belt 331 is in pressure contact with the outer peripheral surface of the intermediate transfer belt 311. The secondary transfer belt 331 has a predetermined outer peripheral surface so that a sheet on which the toner image carried on the outer peripheral surface of the intermediate transfer belt 311 is secondarily transferred is brought into close contact with the intermediate transfer belt 311 in the secondary transfer region 92. It is comprised so that a conveyance path | route may be conveyed. As the secondary transfer belt 331, a belt having a predetermined electrical resistance value (for example, 1 × 10 ^{10 to} 5 × 10 ¹⁰ Ωcm) is used.

  Depending on the positions of the secondary transfer roller 332, the auxiliary roller 333, and the rollers 334 and 335, a sheet conveyance path including the secondary transfer region 92 is determined.

  The secondary transfer roller 332 is arranged so that the direction connecting the rotation shaft of the secondary transfer roller 332 and the rotation shaft of the stretching roller (in this embodiment, the driving roller 312) is orthogonal to the sheet conveyance direction 93. Yes. In other words, in the secondary transfer roller 332, the direction in which the rotation shaft of the secondary transfer roller 332 and the rotation shaft of the stretching roller (in this embodiment, the drive roller 312) are connected is the sheet transfer direction of the secondary transfer region 92. The intermediate transfer belt 311 is disposed so as to be orthogonal to the tangential plane 94 of the intermediate transfer belt 311. As described above, the secondary transfer roller 332 is not offset with respect to the drive roller 312 in the sheet conveyance direction 93 in the secondary transfer region 92 and faces the drive roller 312. The secondary transfer roller 332 is electrically grounded.

  The auxiliary member 333 constitutes a bias member, and is disposed adjacent to the upstream side of the secondary transfer roller 332 in the paper transport direction 93. As an example, the auxiliary member 333 is a bias disposed adjacent to the upstream side of the secondary transfer roller 332 in the paper transport direction 93 among the plurality of rollers 332, 333, 334, 335, and 336 that stretch the secondary transfer belt 331. Laura. The auxiliary member 333 is not limited to being a roller.

  The auxiliary member 333 is disposed on the secondary transfer roller 332 side with respect to the secondary transfer belt 331, that is, on the side facing the secondary transfer region 93 across the secondary transfer belt 331. The auxiliary member 333 is arranged so that the secondary transfer belt 331 does not protrude toward the driving roller 312 with respect to the tangential plane 94 of the intermediate transfer belt 311 at the center of the secondary transfer region 92 in the sheet conveyance direction 93. ing. As described above, the secondary transfer area 92 is an area where the intermediate transfer belt 311 and the secondary transfer belt 331 are in pressure contact with each other.

  During the secondary transfer, a high voltage having the same polarity (for example, minus) as the charging polarity (for example, minus) of the toner is applied to the driving roller 312 by the power supply unit 70. In addition, a high voltage having the same polarity (for example, minus) as the toner charging polarity (for example, minus) is also applied to the auxiliary member 333 by the power supply unit 70 during the secondary transfer. As an example, a voltage of −3 kV is applied to the drive roller 312 and the auxiliary member 333, respectively. The voltage values applied to the drive roller 312 and the auxiliary member 333 are not limited to being the same as each other.

  In FIG. 3, the distribution of the electric field in the secondary transfer region 92 of the transfer device 30 and the vicinity thereof is indicated by a bold solid line, and the distribution of the electric field when the auxiliary member 333 is not provided is indicated by a thick dashed line. In FIG. 3, the electric field value is higher toward the upper side, and the position of the secondary transfer belt 331 indicates 0. In the image forming apparatus 10, a voltage having the same polarity (for example, minus) as the toner charging polarity (for example, minus) is applied to the auxiliary member 333 in addition to the drive roller 312. And the secondary transfer roller 332 are formed so that the maximum value is distributed downstream of the position facing the secondary transfer roller 332, and the electric field strength on the upstream side of the secondary transfer region 92 is smaller than when the auxiliary member 333 is not provided. Become.

  Comparative examples are shown in FIGS. 4 to 7, the same reference numerals are used for the same configurations as those of the image forming apparatus 10 for convenience of explanation. In FIG. 4, the solid thick line indicates the electric field distribution in a configuration in which no voltage is applied to the auxiliary member 333 and the auxiliary member 333 is electrically grounded, and the alternate long and short dashed line indicates that the auxiliary member 333 is electrically connected. Shows the distribution of the electric field in a configuration that is not grounded.

  As shown in FIG. 4, in the transfer device 60 </ b> A of the comparative example configured similarly to the transfer device 30 except that no voltage is applied to the auxiliary member 333, the drive roller 312 and the secondary transfer roller 332 in the paper transport direction 93. An electric field is formed so that the maximum value is distributed on the upstream side of the position opposite to. Therefore, in the transfer device 60A of the comparative example, a large electric field is applied to the toner on the intermediate transfer belt 311 before the sheet comes into close contact with the intermediate transfer belt 311, and the toner is scattered on the upstream side of the secondary transfer roller 332. There is a problem that transfer defects tend to occur.

  Therefore, it is conceivable that the secondary transfer roller 332 is offset by a predetermined value L downstream of the drive roller 312 in the paper transport direction 93 as in the transfer device 60B of another comparative example shown in FIG.

  In the transfer device 60B of the comparative example, an electric field is formed so that the maximum value is distributed at the position where the driving roller 312 and the secondary transfer roller 332 face each other. In the transfer device 60B, the secondary transfer roller 332 is offset from the downstream side in the paper transport direction 93, so that an electric field is formed so that the maximum value is distributed on the downstream side as shown by the one-dot chain line in FIG. The electric field intensity on the upstream side of the secondary transfer region 92 is smaller than that indicated by the alternate long and short dash line and the transfer device 60A. Therefore, transfer failure due to toner scattering or the like on the upstream side of the secondary transfer region 92 is suppressed as compared with the transfer device 60A.

  However, in the transfer device 60B of the comparative example, in order to sufficiently reduce the electric field strength on the upstream side of the secondary transfer region 92, the offset amount of the secondary transfer roller 332 needs to be considerably increased. For this reason, in the transfer device 60B, the amount of winding of the secondary transfer belt 331 around the intermediate transfer belt 311 increases, so that the paper that has passed through the secondary transfer region 92 is easily curled. In addition, since the contact area between the intermediate transfer belt 311 and the sheet is increased, the peelability of the sheet from the intermediate transfer belt 92 is reduced. For this reason, in the transfer device 60B of the comparative example, the paper after passing through the secondary transfer region 92 is easily attracted to the intermediate transfer belt 311 and the paper transportability in the transport path from the secondary transfer region 92 to the fixing device 34 is reduced. There is a problem of doing.

  Further, as in the transfer device 60C of still another comparative example shown in FIG. 6, the secondary transfer belt 331 with respect to the tangential plane 94 of the intermediate transfer belt 311 at the center of the secondary transfer region 92 in the sheet conveying direction 93. It is conceivable that the auxiliary member 333 is disposed so as to protrude toward the drive roller 312 side.

  Accordingly, in the transfer device 60C of the comparative example, the amount of winding of the secondary transfer belt 331 around the intermediate transfer belt 311 increases on the upstream side of the secondary transfer roller 332. For this reason, the region upstream of the secondary transfer region 92 is moved to a region where the electric field strength is small, and in the transfer device 60C of the comparative example, the toner on the intermediate transfer belt 311 is transferred to the sheet before the sheet closely contacts the intermediate transfer belt 311. A large electric field is prevented from being applied, and transfer defects due to toner scattering or the like on the upstream side of the secondary transfer roller 332 are suppressed as compared with the case of the transfer device 60A.

  However, a pre-transfer charging device 56 and a registration sensor 57 are arranged on the upstream side of the secondary transfer region 92 in the moving direction 91 of the intermediate transfer belt 311 to equalize the charge of the toner image before the secondary transfer. Therefore, when the auxiliary member 333 is arranged so that the secondary transfer belt 331 protrudes toward the driving roller 312 with respect to the tangential plane 94, the pre-transfer charging device 56, the registration sensor 57, and the registration roller 55 collide with each other. There is a risk of doing. Therefore, it is practically difficult to dispose the auxiliary member 333 so that the secondary transfer belt 331 protrudes toward the driving roller 312 with respect to the tangential plane 94.

  Further, even if the auxiliary member 333 can be arranged so that the secondary transfer belt 331 protrudes toward the driving roller 312 with respect to the tangential plane 94, in the transfer device 60C of the comparative example, the auxiliary member 333 is separated from the tangential plane 94. Since it protrudes toward the drive roller 312, the area in which the sheet conveyance path 35 can be arranged to the secondary transfer area 92 is narrowed, and the position setting of the sheet conveyance path 35 from the registration roller 55 and the registration roller 55 to the secondary transfer area 92 is set. There is a problem that the degree of freedom decreases.

  Further, as shown in FIG. 7, the drive roller 312 is electrically grounded, and a voltage having a polarity opposite to the toner charging polarity (for example, minus) and the polarity (for example, plus) is applied to the secondary transfer roller 332. The transfer device 60D of another comparative example can be considered as follows.

  That is, in the transfer device 60D of the comparative example, since the potential difference between the secondary transfer roller 332 and the auxiliary member 333 is large, a part of the current that should flow from the secondary transfer roller 332 to the driving roller 312 via the paper is generated. If the secondary transfer roller 332 leaks current to the auxiliary member 333 via the secondary transfer belt 331, or if the secondary transfer roller 332 and the auxiliary member 333 are disposed close to each other, the secondary transfer roller 332 There is a concern that current may flow directly between the secondary transfer roller 332 and the auxiliary member 333 through the gap between the auxiliary member 333 and the auxiliary member 333. For this reason, in the transfer device 60D of the comparative example, the voltage value applied to the auxiliary member 333 cannot be increased as compared with the transfer device 30 according to the embodiment. In addition, it is necessary to increase the voltage applied to the secondary transfer roller 332 in consideration of leakage from the auxiliary member 333. Further, when the secondary transfer roller 332 and the auxiliary member 333 are separated from each other, the transfer device 60D is increased in size.

  FIG. 8 shows the electric field strength on the upstream side of the secondary transfer area, the degree of toner scattering on the upstream side of the secondary transfer area, the paper transportability after passing through the secondary transfer area 92, and the upstream side of the secondary transfer area. A relationship between each degree of freedom of arrangement of the sheet conveyance path to be arranged and the configuration of the transfer device is shown.

  As described above, the transfer devices 60 </ b> A to 60 </ b> D of the comparative example have the above-described problems, respectively, but according to the transfer device 30, toner is added to the auxiliary member 333 in addition to the drive roller 312. Since a voltage having the same polarity (for example, minus) as the charging polarity (for example, minus) is applied, the electric field strength on the upstream side of the secondary transfer region 92 in the sheet conveyance direction 93 is reduced. Therefore, it is possible to prevent a large electric field from being applied to the toner carried on the intermediate transfer belt 311 before the sheet comes into close contact with the intermediate transfer belt 311. Thus, even if the secondary transfer roller 332 is not offset to the downstream side in the paper transport direction 93, transfer defects due to toner scattering on the upstream side of the secondary transfer roller 332 are suppressed. Accordingly, the degree of freedom in setting the position of the secondary transfer roller 332 is increased.

  Further, since the secondary transfer roller 332 is directly opposed to the drive roller 312 without being offset downstream, the sheet is less likely to curl, so that the adsorption to the intermediate transfer belt 311 is suppressed, and the secondary transfer region 92 is suppressed. Transportability of the sheet from the toner to the fixing device 34 is improved.

  Further, in the transfer device 30, a voltage having the same polarity (for example, minus) as the charging polarity (for example, minus) of toner is applied to the auxiliary member 333, so that the intermediate transfer is performed before the sheet comes into close contact with the intermediate transfer belt 311. Since a large electric field is prevented from being applied to the toner on the belt 311, it is not necessary to arrange the auxiliary member 333 so that the secondary transfer belt 331 protrudes toward the drive roller 312 with respect to the tangential plane 94. Therefore, there is no risk of collision between the pre-transfer charging device 56 or the registration sensor 57 and the registration roller 55. Further, by arranging the auxiliary member 333 so that the secondary transfer belt 331 does not protrude toward the driving roller 312 with respect to the tangential plane 94, the sheet conveyance path from the registration roller 55 and the registration roller 55 to the secondary transfer region 92 is provided. The degree of freedom of position setting 35 is increased.

  As shown in FIG. 9, in the transfer device 30A in which the secondary transfer roller 332 is offset downstream in the paper conveyance direction, the roller 334 adjacent to the downstream side of the secondary transfer roller 332 in the paper conveyance direction 94 can be disposed so as to protrude toward the intermediate transfer belt 311.

  By offsetting the secondary transfer roller 332, the amount of winding of the secondary transfer belt 331 around the intermediate transfer belt 311 increases. Since the sheet is curved in the secondary transfer region, the sheet is placed on the secondary transfer belt 331 by arranging the roller 334 in the direction along the curve so as to arrange the sheet conveyance path on the downstream side of the secondary transfer region. It becomes easy to be conveyed along. This suppresses paper jams. In particular, a remarkable effect can be obtained when the paper is thick paper. Further, when the driving roller 312 is hard and the secondary transfer roller 332 is soft, a more remarkable effect can be obtained.

  Further, as the intermediate transfer member, a drum-shaped member carrying a toner image on the peripheral surface can be used. In this case, the secondary transfer belt 331 is pressed against the peripheral surface of the intermediate transfer member. A region where the intermediate transfer member and the secondary transfer belt 331 are in pressure contact with each other is a secondary transfer region 92. The secondary transfer roller 332 is disposed so as to face the intermediate transfer body with the secondary transfer belt 331 interposed therebetween. A voltage having the same polarity (for example, minus) as the charging polarity (for example, minus) of the toner is applied to the intermediate transfer member by the power supply unit 70.

  The present invention is not limited to being applied to a transfer device that transfers toner images formed on a plurality of photosensitive drums corresponding to different hues onto a sheet via an intermediate transfer belt. The present invention can also be applied to a transfer device that transfers a toner image formed on a photosensitive drum to a sheet via an intermediate transfer belt.

  Further, the present invention is not limited to being applied to the transfer device 30 that transfers toner images formed on a plurality of photosensitive drums 41 to a sheet via an intermediate transfer belt. The present invention can also be applied to a transfer device that transfers a formed toner image onto a sheet via an intermediate transfer belt.

  Further, the auxiliary member 333 is not necessarily limited to the configuration in which the secondary transfer belt 331 is stretched as long as the auxiliary electric field 333 can reduce the intensity of the transfer electric field on the upstream side of the secondary transfer region 92 in the paper conveyance direction 93.

  Further, the secondary transfer roller 332 is not limited to being arranged so that the direction connecting the rotation axis of the secondary transfer roller 332 and the rotation axis of the drive roller 312 is orthogonal to the sheet conveyance direction 93. Even in a configuration in which the secondary transfer roller 332 is offset with respect to the driving roller 312 in the paper transport direction 93 in the secondary transfer region 92, it is necessary to suppress transfer defects by applying the present invention. Since the offset amount of the secondary transfer roller 332 can be reduced, an effect that the degree of freedom in setting the position of the secondary transfer roller 332 is improved is achieved.

  The above description of the embodiment is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is shown not by the above embodiments but by the claims. Furthermore, the scope of the present invention is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 21-24 Image forming station 30 Transfer apparatus 31 Intermediate transfer belt unit (intermediate transfer body)
311 Intermediate transfer belt 312 Driving roller (stretching roller)
33 Secondary transfer belt unit 331 Secondary transfer belt 332 Secondary transfer roller 333 Auxiliary member (bias member)
41 Photosensitive drum (image forming body)
92 Secondary transfer area 93 Paper transport direction 94 Tangent plane

The transfer device of the present invention includes an endless belt-like intermediate transfer belt , a tension roller, a secondary transfer belt, a secondary transfer roller, a bias member, a power supply unit , and a secondary transfer belt tension roller . The intermediate transfer belt is primarily transferred with the developer image formed on the image forming body. The tension roller stretches the intermediate transfer belt. The secondary transfer belt is conveyed along the predetermined path so as to contact the intermediate transfer belt sheet the developer image carried on the intermediate transfer belt is secondarily transferred in the secondary transfer region. The secondary transfer roller is opposed to the stretching roller across the secondary transfer belt and is electrically grounded. The bias member is disposed on the secondary transfer roller side with respect to the secondary transfer belt on the upstream side in the sheet conveyance direction from the secondary transfer region. The power supply unit applies a voltage having the same polarity as the developer to each of the stretching roller and the bias member during the secondary transfer. The secondary transfer belt stretching roller is disposed adjacent to the downstream side of the secondary transfer roller in the paper transport direction among the plurality of rollers that stretch the secondary transfer belt. The secondary transfer roller is offset downstream in the paper transport direction. The secondary transfer belt stretching roller is disposed so as to protrude in a direction approaching the intermediate transfer belt with respect to the tangential plane of the intermediate transfer belt at the center of the secondary transfer region in the sheet conveyance direction.

Further , an electric field is formed in the secondary transfer region by applying a voltage having the same polarity as the developer to the stretching roller. Further, the bias member to which a voltage having the same polarity as that of the developer is disposed on the upstream side of the secondary transfer roller in the paper conveyance direction, so that the paper conveyance direction is compared with the case where no voltage is applied to the bias member. In this case, the electric field strength on the upstream side of the secondary transfer region becomes small. Therefore, regardless of the position of the secondary transfer roller, it is possible to prevent a large electric field from being applied to the developer on the intermediate transfer belt before the sheet comes into close contact with the intermediate transfer member.

Claims (5)

An intermediate transfer member to which a developer image formed on the image forming member is primarily transferred;
A secondary transfer belt that conveys a predetermined conveyance path so that a sheet on which a developer image carried on the intermediate transfer member is to be secondarily transferred is in close contact with the intermediate transfer member in a secondary transfer region;
A secondary transfer roller that faces the intermediate transfer member across the secondary transfer belt and is electrically grounded;
A bias member disposed on the secondary transfer roller side with respect to the secondary transfer belt on the upstream side in the paper conveyance direction from the secondary transfer region;
And a power supply unit that applies a voltage having the same polarity as the developer to each of the intermediate transfer member and the bias member during secondary transfer. The intermediate transfer member is
An intermediate transfer belt configured as an endless belt to which a developer image formed in the image forming body is primarily transferred;
The transfer device according to claim 1, further comprising: a tension roller that stretches the intermediate transfer belt and is applied with a voltage having the same polarity as the developer.   The transfer device according to claim 2, wherein the secondary transfer roller is disposed so that a direction connecting a rotation shaft of the secondary transfer roller and a rotation shaft of the stretching roller is orthogonal to the sheet conveyance direction. In the secondary transfer region, the intermediate transfer member and the secondary transfer belt are pressed against each other,
The bias member is a roller disposed adjacent to the upstream side of the secondary transfer roller in the paper transport direction among a plurality of rollers that stretch the secondary transfer belt, and transports the paper in the secondary transfer region. 4. The transfer device according to claim 1, wherein the secondary transfer belt is disposed so as not to protrude toward the intermediate transfer body with respect to a tangential plane of the intermediate transfer body at a central portion in the direction. An electrophotographic image forming apparatus,
The image forming body;
An image forming apparatus comprising: the transfer device according to claim 1.

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