JP2014089389A - Transfer device, and image forming apparatus including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transfer device capable of stably maintaining equal magnification of images transferred on sheets even when a thickness of sheets passed through a transfer nip part changes, and an image forming apparatus including the transfer device.SOLUTION: A drive roller 142 circularly drives an intermediate transfer belt 141. A secondary transfer roller 145 is pressed by the drive roller 142 with the intermediate transfer belt 141 therebetween to form a secondary transfer nip N with the drive roller 142 to transfer a toner image onto a sheet P. A sheet sensor 62 detects a thickness of the sheet P on a sheet conveyance path. A determination unit determines the necessity of adjustment of the pressing force F of the secondary transfer roller 145 according to a detection result of the sheet sensor 62. A pressing force adjustment unit adjusts the pressing force according to a determination result of the determination unit.

Description

  The present invention relates to a transfer device including a secondary transfer roller for transferring a toner image from an intermediate transfer belt to a sheet, and an image forming apparatus including the transfer device.

  2. Description of the Related Art Conventionally, an image forming apparatus such as a printer or a copier employing an electrophotographic system includes a photosensitive drum that carries an electrostatic latent image and a transfer device that transfers a toner image from the photosensitive drum to a sheet. . In order to transfer an image composed of a plurality of colors typified by a full-color image onto a sheet, the transfer device includes an intermediate transfer belt, a primary transfer roller, and a secondary transfer roller. The intermediate transfer belt is circulated so as to face the plurality of photosensitive drums, and a toner image is transferred from each photosensitive drum onto the intermediate transfer belt by a primary transfer voltage applied to the primary transfer roller.

  In the transfer device as described above, the intermediate transfer belt is stretched around a plurality of rollers. A secondary transfer nip portion is formed between the tension roller facing the secondary transfer roller in the roller and the secondary transfer roller. Then, the toner images are collectively transferred from the intermediate transfer belt to the sheet by the secondary transfer voltage applied to the secondary transfer nip portion.

  In Patent Document 1, there is a difference between the speed of the intermediate transfer belt and the speed of the sheet depending on the thickness of the sheet that passes through the secondary transfer nip portion, and the equal magnification of the transferred image changes. It is disclosed. For this reason, Patent Document 1 discloses a technique in which the pressing force of the secondary transfer roller is adjusted according to the sheet setting of the sheet cassette input from the operation panel.

JP 2011-158613 A

  According to the configuration of Patent Document 1, when the sheet setting of the sheet cassette is different from the specifications of the sheets actually stacked on the sheet cassette, there is a problem that the equal magnification cannot be eliminated.

  The present invention has been made in view of the above problems, and even when the thickness of the sheet passing through the transfer nip changes, the equal magnification of the image transferred onto the sheet is stably maintained. It is an object of the present invention to provide a transfer device and an image forming apparatus including the transfer device.

  A transfer device according to one aspect of the present invention is driven in a single direction and has an intermediate transfer belt that carries a toner image on a surface thereof, and a driving roller that stretches the intermediate transfer belt and drives the intermediate transfer belt to rotate. And a transfer roller that forms a transfer nip portion that is pressed by the drive roller across the intermediate transfer belt and transfers the toner image to the sheet with the drive roller, and passes through the transfer nip portion. A thickness detection means for detecting the thickness of the sheet in the sheet conveyance path through which the sheet is conveyed, the sheet conveyance path upstream of the transfer nip portion in the sheet conveyance direction, and the thickness detection According to the detection result of the means, determination means for determining whether or not the adjustment of the pressing force of the transfer roller to the drive roller is necessary, and according to the determination result of the determination means, And having a pressing force adjusting means for adjusting the pressure, the.

  According to this configuration, the thickness detection unit detects the thickness of the sheet conveyed through the sheet conveyance path in the sheet conveyance path upstream of the transfer nip portion in the sheet conveyance direction. Then, the determination unit determines whether or not it is necessary to adjust the pressing force of the transfer roller with respect to the driving roller according to the detection result of the thickness detection unit. For this reason, whether or not it is necessary to adjust the pressing force of the transfer roller is determined according to the thickness of the sheet actually conveyed on the sheet conveyance path. Further, since it is determined whether or not the pressing force needs to be adjusted according to the actual thickness of the sheet, excessive adjustment of the pressing force is suppressed when adjustment is unnecessary. Accordingly, the pressing force of the transfer roller is unnecessarily changed, and the transfer conditions at the transfer nip portion are prevented from changing. Further, when the determination unit determines that the pressing force needs to be adjusted, the pressing force of the transfer roller is adjusted by the pressing force adjustment unit. Therefore, compared with the case where the pressing force of the transfer roller is adjusted according to the setting of the sheet in the sheet cassette on which the sheets are stacked, the pressing force is accurately applied according to the specification of the sheet conveyed on the sheet conveying path. It becomes possible to adjust. Further, when the toner image is transferred from the intermediate transfer belt to the sheet, it is possible to make the circumferential transfer speed of the intermediate transfer belt and the sheet conveyance speed close to each other in accordance with different sheet thicknesses. For this reason, the equal magnification of the toner image transferred to the sheet is maintained as much as possible.

  In the above configuration, the pressing force of the transfer roller with respect to the driving roller is set in advance to a first pressing force corresponding to the first sheet conveyed through the sheet conveying path, When the thickness detecting unit detects that the sheet conveyed through the sheet conveying path is a second sheet thinner than the first sheet, the adjustment of the pressing force is necessary, It is desirable that the pressing force adjusting means sets the pressing force to a second pressing force that is larger than the first pressing force in accordance with the determination result of the determining means.

  According to this configuration, the pressing force of the transfer roller is set to the first pressing force corresponding to the first sheet among the sheets transported through the sheet transport path. For this reason, by setting the general-purpose sheet as the first sheet, it is possible to transfer the toner image onto the sheet with the first pressing force without frequently adjusting the pressing force. Further, when a second sheet thinner than the first sheet is conveyed to the transfer nip portion, a second pressing force larger than the first pressing force is set by the pressing force adjusting means. For this reason, when the second sheet is inserted between the transfer roller pressed by the first pressing force and the intermediate transfer belt, the conveyance speed of the second sheet becomes excessively slow, and the toner image It is suppressed that the equal magnification is reduced.

  In the above configuration, the determination unit detects that the sheet conveyed on the sheet conveyance path is a third sheet thicker than the first sheet by the thickness detection unit. The pressing force adjusting means determines that the pressing force needs to be adjusted, and the pressing force adjusting means reduces the pressing force to a third pressing force smaller than the first pressing force according to the determination result of the determining means. It is desirable to set the pressure.

  According to this configuration, when a third sheet thicker than the first sheet is conveyed to the transfer nip portion, a third pressing force smaller than the first pressing force is set by the pressing force adjusting unit. . For this reason, when the third sheet is inserted between the transfer roller pressed by the first pressing force and the intermediate transfer belt, the conveyance speed of the third sheet becomes excessively high, and the toner image The expansion of the equal magnification is suppressed.

  In the above configuration, the pressing force of the transfer roller with respect to the driving roller is set in advance to a first pressing force corresponding to the first sheet conveyed through the sheet conveying path, When the thickness detecting unit detects that the sheet conveyed through the sheet conveying path is a second sheet thinner than the first sheet, the adjustment of the pressing force is necessary, The pressing force adjusting means sets the pressing force to a second pressing force larger than the first pressing force according to the determination result of the determining means, and the determining means further includes the sheet conveying path. When the thickness detecting unit detects that the sheet being conveyed is a third sheet thicker than the first sheet, the pressing force adjustment is determined to be necessary. The means is the judgment hand Depending on the result of the determination, it is desirable to set the pressing force to the third pressing force is smaller than said first pressing force.

  According to this configuration, the pressing force of the transfer roller is set to the first pressing force corresponding to the first sheet among the sheets transported through the sheet transport path. For this reason, by setting the general-purpose sheet as the first sheet, it is possible to transfer the toner image onto the sheet with the first pressing force without frequently adjusting the pressing force. Further, when a second sheet thinner than the first sheet is conveyed to the transfer nip portion, a second pressing force larger than the first pressing force is set by the pressing force adjusting means. For this reason, when the second sheet is inserted between the transfer roller pressed by the first pressing force and the intermediate transfer belt, the conveyance speed of the second sheet becomes excessively slow, and the toner image It is suppressed that the equal magnification is reduced. Further, when a third sheet thicker than the first sheet is conveyed to the transfer nip portion, a third pressing force smaller than the first pressing force is set by the pressing force adjusting means. For this reason, when the third sheet is inserted between the transfer roller pressed by the first pressing force and the intermediate transfer belt, the conveyance speed of the third sheet becomes excessively high, and the toner image The expansion of the equal magnification is suppressed.

  In the above-described configuration, the pressing force adjusting unit is configured such that when the second sheet passes through the transfer nip portion, the conveyance speed of the second sheet and the rotation speed of the intermediate transfer belt are constant. Thus, it is desirable to set the second pressing force.

  According to this configuration, the second pressing force is set so that the conveyance speed of the second sheet and the rotation speed of the intermediate transfer belt are equal. For this reason, the equal magnification of the toner image transferred to the second sheet is stably maintained.

  In the above-described configuration, the pressing force adjusting unit causes the third sheet conveyance speed and the rotation speed of the intermediate transfer belt to be constant when the third sheet passes through the transfer nip portion. Thus, it is desirable to set the third pressing force.

  According to this configuration, the third pressing force is set so that the conveyance speed of the third sheet and the rotation speed of the intermediate transfer belt are equal. For this reason, the equal magnification of the toner image transferred to the third sheet is stably maintained.

  In the above configuration, it is desirable that the thickness detection unit detects the thickness of the sheet in a non-contact state with respect to the sheet in the sheet conveyance path.

  According to this configuration, the thickness of the sheet is suitably detected without hindering the conveyance of the sheet.

  In the above configuration, it is desirable that the thickness detecting means is an optical displacement sensor.

  According to this configuration, the thickness of the sheet is suitably detected by the optical displacement sensor without hindering the conveyance of the sheet.

  In the above configuration, it is desirable that the transfer roller has a crown shape in the axial direction.

  According to this configuration, since the transfer roller has a crown shape, the sheet conveyance speed is likely to vary according to the thickness of the sheet conveyed to the transfer nip portion. Even in such a case, when the toner image is transferred from the intermediate transfer belt to the sheet, the rotation speed of the intermediate transfer belt and the conveyance speed of the sheet can be made close to each other according to the thickness of the different sheets. it can. For this reason, the equal magnification of the toner image transferred to the sheet is maintained as much as possible.

  In the above configuration, it is preferable that the transfer roller is pressed against the drive roller in a state where the transfer roller is offset to the upstream side in the circumferential direction of the intermediate transfer belt.

  According to this configuration, the transfer roller is arranged offset with respect to the drive roller. Therefore, the transfer nip portion is formed with a predetermined width by partially winding the intermediate transfer belt around the peripheral surface of the transfer roller. As a result, when a difference occurs between the rotation speed of the intermediate transfer belt and the sheet conveyance speed in the transfer nip portion, the equal magnification of the toner image is likely to be changed. Even in such a case, when the toner image is transferred from the intermediate transfer belt to the sheet, the rotation speed of the intermediate transfer belt and the conveyance speed of the sheet can be made close to each other according to the thickness of the different sheets. it can. For this reason, the equal magnification of the toner image transferred to the sheet is maintained as much as possible.

  In the above configuration, a housing that movably supports the transfer roller with respect to the drive roller, a motor disposed in the housing, a cam member rotated by the motor, a fulcrum portion, and the fulcrum portion A first arm portion that is extended and pressed against the cam member; and a second arm portion that extends from the fulcrum portion to the opposite side of the first arm portion and presses the transfer roller, and the fulcrum portion. An arm member supported by the housing so as to be capable of swinging around, and the pressing force adjusting unit changes a rotation angle of the cam member to cause the transfer arm to press the second arm unit. It is desirable to adjust the pressing force of the transfer roller.

  According to this configuration, the pressing force adjustment unit changes the rotation angle of the cam member by the motor. The cam member presses the first arm portion of the arm member. As a result, the arm member is swung around the fulcrum portion. The second arm portion presses the transfer roller by swinging the arm portion. Therefore, the movement of the transfer roller and the adjustment of the pressing force are suitably realized by the motor, the cam member, and the arm member.

  An image forming apparatus according to another aspect of the present invention, the transfer device described above, a plurality of image carriers that are disposed to face the intermediate transfer belt and transfer the toner image to the intermediate transfer belt, It is characterized by providing.

  According to this configuration, when the toner image is transferred from the intermediate transfer belt to the sheet, the circumferential speed of the intermediate transfer belt and the sheet conveyance speed can be made close to each other in accordance with the thicknesses of the different sheets. For this reason, the equal magnification of the toner image transferred to the sheet is maintained as much as possible. As a result, an image in which the equal magnification is stably maintained is formed on the sheet.

  According to the present invention, there is provided a transfer device capable of stably maintaining the same magnification of an image transferred onto a sheet even when the thickness of the sheet passing through the transfer nip portion changes. An image forming apparatus is provided.

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 periphery of an intermediate transfer unit according to an embodiment of the present invention. It is an expanded sectional view of the secondary transfer nip part concerning the embodiment of the present invention. FIG. 2 is a perspective view of a secondary transfer unit according to an embodiment of the present invention. It is an electrical block diagram of the control part which concerns on embodiment of this invention. It is a graph which shows the relationship between the thickness of a paper, and a magnification. 6 is a graph showing a relationship between a pressing force at a secondary transfer nip portion and a magnification. It is a flowchart of the pressing force control which concerns on embodiment of this invention.

  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. FIG. 2 is a schematic cross-sectional view showing the periphery of the intermediate transfer unit 14 inside 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. An intermediate transfer unit 14 (transfer device), a secondary transfer roller 145 (transfer roller), a toner replenishing unit 15 for replenishing toner to the image forming unit 13, and an unfixed toner image formed on the sheet P. A fixing unit 16 for performing a fixing process is internally provided. 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.

  At an appropriate position on the upper surface of the apparatus main body 11, an unillustrated operation panel for inputting an output condition for the sheet P and the like is provided. This operation panel is provided with a power key, a touch panel for inputting output conditions, and various operation keys. 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 paper feeding unit 12 includes a manual paper feeding unit attached to the left side surface of the apparatus main body 11 shown in FIG. The manual paper feed unit includes a manual feed tray 124, a pickup roller 125, and a paper feed roller pair 126. The manual feed tray 124 is a tray on which the manually fed sheet P is placed, and is opened from the side surface of the apparatus main body 11 as shown in FIG. 1 when the sheet P is manually fed. The pickup roller 125 feeds out the sheet P placed on the manual feed tray 124. The pair of paper feed rollers 126 sends out the sheet P fed out by the pickup roller 125 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 (image carrier), 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. As shown in FIG. 2, the photosensitive drums 20M, 20C, 20Y, and 20Bk 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. As the charging device 21, a charging device using a contact charging method including a charging roller and a charging cleaning brush for removing toner attached to the charging roller can be used. 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 photosensitive drum 20 is rotationally driven by a drum motor 61 (FIG. 2). In the present embodiment, the photosensitive drums 20 corresponding to the image forming units of a plurality of colors are rotationally driven by the drum motor 61 in synchronization with each other. 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 a toner and a carrier, and includes two stirring rollers 23A, a magnetic roller 23B, and a developing roller 23C. The stirring roller 23A circulates and conveys the two-component developer while stirring to charge the toner. A two-component developer layer is carried on the peripheral surface of the magnetic roller 23B, and toner formed by transferring toner to the peripheral surface of the developing roller 23C due to a potential difference between the magnetic roller 23B and the developing roller 23C. The layer is supported. The toner on the developing roller 23C is supplied to the peripheral surface of the photosensitive drum 20, and the electrostatic latent image is developed. In this embodiment, the toner has a characteristic of being charged to a positive polarity.

  The intermediate transfer unit 14 is disposed in a space provided between the image forming unit 13 and the toner supply unit 15. With reference to FIG. 2, 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 intermediate transfer belt 141 is a conductive soft belt having a laminated structure including a base layer, an elastic layer, and a coat layer. The base layer constitutes the lowermost layer of the intermediate transfer belt 141. For the base layer, for example, PVDF (polyvinylidene fluoride), polyimide resin, or the like is preferably used. The elastic layer imparts moderate elasticity to the intermediate transfer belt 141. As the material of the elastic layer, for example, hydrin rubber, chloroprene rubber, polyurethane rubber or the like is used. The coat layer constitutes the uppermost layer of the intermediate transfer belt 141 and comes into contact with the photosensitive drum 20 (FIG. 1). The coat layer protects the elastic layer, and as the material, a fluororesin such as acrylic, silicon, PTFE or the like is used.

  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. A rotational driving force is applied to the driving roller 142 from a belt motor 60 described later. The driving roller 142 is a metal roller. More specifically, the drive roller 142 has a two-layer structure in the cross-sectional direction. The inner layer of the drive roller 142 is made of an aluminum layer. Further, an alumite layer constituting the surface layer of the driving roller 142 is provided on the upper layer of the aluminum layer. In this embodiment, an alumite film obtained by anodizing the aluminum layer is used as the alumite layer. By supporting the intermediate transfer belt 141 from the inside while the alumite layer is in contact with the inner peripheral surface of the intermediate transfer belt 141, the rotational driving force is suitably transmitted from the drive roller 142 to the intermediate transfer belt 141. The

  As a method for forming the alumite film constituting the alumite layer, a known film forming method can be employed. In this embodiment, an element tube made of an aluminum tube is used, and an alumite film is formed on the surface of the element tube by an anodic oxidation method using dilute sulfuric acid as an electrolyte. After the coating is formed, a porous coating composed of a barrier layer and numerous fine pores is formed on the surface of the alumite coating. If these fine holes are left open, foreign substances and corrosive substances are absorbed, and therefore, preferably, a sealing process is performed as a post-process. Thereby, the micropores on the surface of the coating are closed, and the corrosion resistance, weather resistance, and contamination resistance are improved.

  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. In the vicinity of the driven roller 143, a belt cleaning device 144 (FIG. 1) that removes toner remaining on the peripheral surface of the intermediate transfer belt 141 is disposed.

  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. As shown in FIG. 2, primary transfer rollers 24M, 24C, 24Y, and 24Bk are arranged to face the photosensitive drums 20 of the respective colors. As a result, a primary transfer nip portion for each color is formed between each photoconductor drum 20 and the primary transfer roller 24.

  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 N. 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 N. In the present embodiment, the secondary transfer roller 145 is composed of epichlorohydrin. The secondary transfer roller 145 has a crown shape in which the outer diameter of the central portion is large and the outer diameters of both end portions are small in the axial direction. Further, in the present embodiment, the pressing position of the secondary transfer roller 145 against the driving roller 142 is arranged offset to the upstream side in the rotation direction of the intermediate transfer belt 141.

  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. These toner containers 15M, 15C, 15Y, and 15Bk store replenishment toners for each color of MCYBk, respectively, and image forming units 13M, 13C, and 13C corresponding to each color of MCYBk from a toner discharge port 15H formed on the bottom surface of the container. The 13Y and 13Bk developing devices 23 are supplied with toner of each color through a toner transport unit (not shown).

  The fixing unit 16 includes a heating roller 161 having a heating source therein, a fixing roller 162 oriented with the heating roller 161, a fixing belt 163 stretched between the fixing roller 162 and the heating roller 161, and a fixing belt A pressure roller 164 which is disposed to face the fixing roller 162 via the H.163 and forms a fixing nip portion. The sheet P supplied to the fixing unit 16 is heated and pressurized by passing through the fixing nip portion. 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 intermediate transfer unit 14 according to the present embodiment will be described in more detail with reference to FIGS. 3 and 4 in addition to FIG. FIG. 3 is an enlarged view showing the periphery of the secondary transfer nip portion N of the intermediate transfer unit 14. FIG. 4 is a perspective view of the secondary transfer unit 145A. The intermediate transfer unit 14 further includes a first backup roller 146, a second backup roller 147, a belt motor 60, a sheet sensor 62 (thickness detection means), and a secondary transfer unit 145A.

  The first backup roller 146 is disposed between the primary transfer roller 24Bk located on the most downstream side in the circumferential direction of the intermediate transfer belt 141 among the plurality of primary transfer rollers 24 and the driving roller 142, and the intermediate transfer belt 141 is stretched. Mount. More specifically, the intermediate transfer belt 141 is stretched around the peripheral surface of the first backup roller 146 at a predetermined angle. Similarly, the second backup roller 147 is disposed between the primary transfer roller 24M and the driven roller 143 located on the most upstream side in the circumferential direction of the intermediate transfer belt 141 among the plurality of primary transfer rollers 24, and the intermediate transfer belt. 141 is stretched. When the first backup roller 146 and the second backup roller 147 support the intermediate transfer belt 141, a plurality of primary transfer nip portions are linearly arranged. When the image forming apparatus 10 is not used, the primary transfer roller 24 and the second backup roller 147 of each color are movable toward the inside of the intermediate transfer belt 141. That is, the primary transfer roller 24 can be separated from the photosensitive drum 20. At this time, the intermediate transfer belt 141 is stretched between the first backup roller 146 and the driven roller 143. By separating the primary transfer roller 24 from the photosensitive drum 20, the intermediate transfer belt 141 or the photosensitive drum 20 can be attached to and detached from the apparatus main body 11 without the intermediate transfer belt 141 and the photosensitive drum 20 being in contact with each other. .

  The belt motor 60 generates a driving force for rotating the intermediate transfer belt 141. The rotational driving force is transmitted from the belt motor 60 to the driving roller 142.

  The sheet sensor 62 is disposed on the upstream side of the registration roller pair 113 so as to face the sheet conveyance path 111. The sheet sensor 62 detects the thickness D of the sheet P conveyed through the sheet conveyance path 111. Specifically, the sheet sensor 62 is configured by a combination of a light emitting element and a light receiving element (not shown) based on a triangulation method. A semiconductor laser is used for the light emitting element. The light of the semiconductor laser is condensed through a light projection lens (not shown) and irradiated onto the sheet P. A part of the light beam diffusely reflected from the sheet P forms a spot on the light receiving element through a light receiving lens (not shown). As a result, the amount of displacement from the sheet sensor 62 to the object is detected. By fixing the position where the sheet sensor 62 is mounted in advance, the thickness D of the sheet P is detected from the amount of displacement.

  The secondary transfer unit 145 </ b> A (housing) is disposed on the right side of the driving roller 142 so as to face the driving roller 142. The secondary transfer unit 145A includes the secondary transfer roller 145 described above. Referring to FIG. 4, bearings 145 </ b> B are disposed at both ends of secondary transfer roller 145. The bearing 145B rotatably supports the rotation shaft of the secondary transfer roller 145. Further, the bearing 145B is pressed by an arm 70 which will be described later, and can move in the direction of arrow DP in FIG.

  The secondary transfer unit 145A includes an adjustment mechanism 50. The adjustment mechanism 50 includes an adjustment motor 63 (motor), a first gear 64, a second gear 65, a shaft 66, a pulse plate 67, a rotation sensor 68, a cam 69 (cam member), and an arm 70 ( Arm member).

  The adjustment motor 63 is disposed on the back side of the secondary transfer unit 145A. The adjustment motor 63 generates a driving force for moving the secondary transfer roller 145. The first gear 64 is connected to a drive shaft (not shown) protruding from the adjustment motor 63. The second gear 65 is connected to the first gear 64. The shaft 66 is a shaft member that extends in the front-rear direction on the back side (right side) of the secondary transfer unit 145A. Although not appearing in FIG. 4, a worm gear that is engaged with the second gear 65 is disposed in the central portion of the shaft 66 in the front-rear direction. The rotational driving force of the adjustment motor 63 is transmitted to the shaft 66 by the first gear 64, the second gear 65, and the worm gear.

  The pulse plate 67 is disposed in the rear portion of the shaft 66 with respect to the second gear 65. The pulse plate 67 is rotated integrally with the shaft 66. The pulse plate 67 is provided with a plurality of slits opened at intervals in the circumferential direction. The rotation sensor 68 is arranged so as to sandwich the outer periphery of the pulse plate 67 in the front-rear direction. The rotation sensor 68 includes a light emitting unit and a light receiving unit (not shown). Detection light is emitted from the light emitting unit, and the detection light passing through the slit is received by the light receiving unit. The rotation angle of the shaft 66 is detected based on the pulse signal output from the light receiving unit corresponding to the slit.

  The cam 69 is disposed at the rear end portion of the shaft 66. The cam 69 is fitted on the shaft 66. The cam 69 includes a maximum outer diameter portion and a minimum outer diameter portion in the circumferential direction of the shaft 66.

  The arm 70 is rotatably disposed on the secondary transfer unit 145A between the cam 69 and the rear bearing 145B. The arm 70 includes a load portion 70A (first arm portion), a fulcrum portion 70B, and a pressing portion 70C (second arm portion). The fulcrum part 70B is pivotally supported by the secondary transfer unit 145A and serves as a fulcrum for the rotation of the arm 70. The weight part 70A and the fulcrum part 70B are projecting pieces that protrude from the fulcrum part 70B to the opposite side. The weight portion 70 </ b> A is disposed so as to face the cam 69 and is in contact with the cam 69. Further, the pressing portion 70C is disposed to face the bearing 145B, and can contact the bearing 145B.

  When the adjustment motor 63 is rotated, the shaft 66 is rotated via the first gear 64 and the second gear 65. When the cam 69 disposed at the end of the shaft 66 is rotated, the maximum outer diameter portion of the cam 69 contacts the load portion 70A. As a result, the arm 70 is rotated with the fulcrum part 70B as a fulcrum. Then, when the pressing portion 70C of the arm 70 contacts the bearing 145B, the secondary transfer roller 145 can be moved in the direction of the arrow DP in FIGS. That is, the pressing force F of the secondary transfer roller 145 against the driving roller 142 is increased. Further, as the cam 69 is further rotated, the contact position between the cam 69 and the load portion 70A changes from the maximum outer diameter portion to the minimum outer diameter portion of the cam 69, so that the secondary transfer roller 145 is changed to FIG. 4 is moved in the direction opposite to the arrow DP direction in FIG. That is, the pressing force F of the secondary transfer roller 145 against the driving roller 142 is reduced. In addition, by detecting the rotation angle of the shaft 66 by the pulse plate 67 and the rotation sensor 68, the pressing force F can be set in a fine range.

  Further, the intermediate transfer unit 14 includes a control unit 90. The control unit 90 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores a control program, a RAM (Random Access Memory) that is used as a work area of the CPU, and the like. FIG. 5 is an electrical block diagram of the control unit 90. The above-described belt motor 60, drum motor 61, sheet sensor 62, adjustment motor 63, and rotation sensor 68 are electrically connected to the control unit 90. When the CPU executes a control program stored in the ROM, the control unit 90 causes the drive control unit 91, the determination unit 92 (determination unit), the pressing force adjustment unit 93 (pressing force adjustment unit), and the storage unit 94 to be stored. To function.

  The drive controller 91 controls the drum motor 61 to rotate the photosensitive drum 20. Further, the drive control unit 91 controls the belt motor 60 to rotate the intermediate transfer belt 141 via the drive roller 142.

  The determination unit 92 determines whether it is necessary to adjust the pressing force F of the secondary transfer roller 145 according to the detection result of the sheet sensor 62.

  The pressing force adjustment unit 93 adjusts the pressing force F of the secondary transfer roller 145 according to the determination result of the determination unit 92. Specifically, the pressing force adjusting unit 93 drives the adjusting motor 63 to rotate, and changes the contact position of the cam 69 with respect to the weighted portion 70A, thereby pressing the pressing portion 70C to the bearing portion 145B. Then, the pressing force adjusting unit 93 changes the pressing force F of the secondary transfer roller 145 against the driving roller 142.

  The storage unit 94 stores threshold values D1 and D2 regarding the thickness D of the sheet P. The threshold values D1 and D2 are compared and referred to by the determination unit 92 with the sheet thickness D detected by the sheet sensor 62.

<Regarding fluctuations in equal magnification>
Next, with reference to FIG. 6, the change in the equal magnification of the toner image in the secondary transfer nip N will be described. FIG. 6 is a graph relatively showing changes in the equal magnification when the thickness D of the sheet P is changed. The equal magnification is preferably 1.0. That is, it is desirable that the length of the toner image having a length of 1 on the intermediate transfer belt 141 is 1 when it is transferred onto the sheet P. On the other hand, when the length of the toner image having a length of 1 on the intermediate transfer belt 141 is transferred onto the sheet P exceeds 1, the equal magnification is greater than 1. Further, when the length of the toner image having a length of 1 on the intermediate transfer belt 141 is less than 1 when transferred onto the sheet P, the equal magnification becomes smaller than 1. In FIG. 6, the pressing force F of the secondary transfer roller 145 against the driving roller 142 is fixed to a predetermined value. When the thickness D of the sheet P is 70 μm to 120 μm (referred to as plain paper in this embodiment), the equal magnification of the toner image formed on the sheet P is maintained substantially constant.

  On the other hand, when the thickness D of the sheet P is less than 70 μm (referred to as thin paper in this embodiment), the equal magnification of the toner image is reduced. This is because, when the thickness D of the sheet P is thin, the virtual effective diameter of the secondary transfer roller 145 including the thickness D of the sheet P becomes small. As a result, the conveyance speed of the sheet P is relatively slow with respect to the circumferential speed of the intermediate transfer belt 141. Therefore, when the image is transferred from the intermediate transfer belt 141 to the sheet P, the toner image shrinks and the equal magnification is reduced.

  Further, when the thickness D of the sheet P exceeds 120 μm (referred to as “thick paper” in this embodiment), the equal magnification of the toner image is increased (elongated). This is because when the thickness D of the sheet P is thick, the virtual effective diameter of the secondary transfer roller 145 including the thickness D of the sheet P becomes large. As a result, the conveyance speed of the sheet P becomes relatively faster than the rotation speed of the intermediate transfer belt 141. Therefore, when the image is transferred from the intermediate transfer belt 141 to the sheet P, the toner image is stretched and the equal magnification is increased. As described above, when the pressing force F of the secondary transfer roller 145 against the driving roller 142 is constant, the equal magnification of the toner image varies depending on the thickness D of the sheet P conveyed to the secondary transfer nip portion N.

<Adjustment of pressing force>
In order to solve the above-described problems, in this embodiment, the determination unit 92 and the pressing force adjustment unit 93 adjust the pressing force F of the secondary transfer roller 145 against the driving roller 142. Hereinafter, the pressing force control performed by the determination unit 92 and the pressing force adjustment unit 93 in the intermediate transfer unit 14 according to the present embodiment will be described. FIG. 7 is a graph showing the relationship between the pressing force F of the secondary transfer roller 145 and the equal magnification. FIG. 8 is a flowchart showing a control mode of the pressing force control according to this embodiment.

  Referring to FIG. 8, in the present embodiment, when the image forming apparatus 10 is turned on (step S001), the plain paper (first sheet) among the sheets P conveyed through the sheet conveyance path 111 is used. Correspondingly, the pressing force F of the secondary transfer roller 145 against the driving roller 142 is set in advance to the first pressing force F1. By setting the pressing force F of the secondary transfer roller 145 to the first pressing force F1, the conveyance speed of the plain paper passing through the secondary transfer nip portion N and the rotation speed of the intermediate transfer belt 141 are constant. Set to For this reason, in the image forming apparatus 10, a pressing force F that maintains the same magnification of the toner image is set corresponding to the most commonly used plain paper.

  When an image forming operation is actually executed after the image forming apparatus 10 is turned on, the sheet sensor 62 detects the thickness D of the sheet P. Then, the determination unit 92 compares the thickness D of the sheet P detected by the sheet sensor 62 with the threshold value D1 stored in advance in the storage unit 94 (step S002). In the present embodiment, the threshold value D1 is set to 70 μm. When the detected thickness D of the sheet P is less than the threshold value D1 (YES in step S002), the determination unit 92 determines that the sheet P transported through the sheet transport path 111 is thin paper (second sheet) and is pressed. It is determined that the pressure F needs to be adjusted (step S003). The pressing force adjusting unit 93 sets the pressing force F of the secondary transfer roller 145 to a second pressing force F2 that is larger than the first pressing force F1. The pressing force adjustment unit 93 rotates the adjustment motor 63 and moves the secondary transfer roller 145 so as to approach the driving roller 142. As a result, as shown in FIG. 7, the effect of increasing the magnification is exhibited as the pressing force of the secondary transfer roller 145 increases. More specifically, the pressing force adjustment unit 93 is configured to adjust the second paper so that the thin paper conveyance speed and the rotation speed of the intermediate transfer belt 141 are constant when the thin paper passes through the secondary transfer nip portion N. The pressing force F2 is set. As a result, the toner image having the same magnification maintained as 1.0 as possible is transferred onto the thin paper conveyed through the sheet conveyance path 111 and passing through the secondary transfer nip portion N.

  On the other hand, when the detected thickness D of the sheet P is equal to or greater than the threshold value D1 (NO in step S002), the determination unit 92 stores the thickness D of the sheet P detected by the sheet sensor 62 in advance in the storage unit 94. The threshold value D2 is compared (step S004). In the present embodiment, the threshold value D2 is set to 120 μm. When the detected thickness D of the sheet P is less than the threshold value D2 (YES in step S004), the determination unit 92 adjusts the pressing force F because the sheet P conveyed through the sheet conveyance path 111 is plain paper. It is determined that it is unnecessary (step S005). As a result, the toner image is transferred onto the plain paper with the first pressing force F1 preset as the pressing force F of the secondary transfer roller 145. Therefore, a toner image with the same magnification maintained as 1.0 as possible is transferred onto the plain paper.

  Furthermore, when the detected thickness D of the sheet P is equal to or greater than the threshold value D2 (NO in step S004), the determination unit 92 determines that the sheet P conveyed through the sheet conveyance path 111 is a thick sheet (third sheet). It is determined that the pressing force F needs to be adjusted (step S006). The pressing force adjusting unit 93 sets the pressing force F of the secondary transfer roller 145 to a third pressing force F3 that is smaller than the first pressing force F1. The pressing force adjustment unit 93 rotates the adjustment motor 63 to move the secondary transfer roller 145 away from the driving roller 142. At this time, the pressing force adjusting unit 93 causes the third paper so that the thick paper transport speed and the rotational speed of the intermediate transfer belt 141 are equal when the thick paper is passed through the secondary transfer nip portion N. A pressing force F3 is set. As a result, the toner image having the same magnification as 1.0 as possible is transferred onto the thick paper that is transported through the sheet transport path 111 and passed through the secondary transfer nip portion N.

  When a plurality of sheets P are continuously conveyed through the sheet conveying path 111, the second pressing force F2 to the first pressing force F1 or the third pressing force F2 is determined according to the determination result of the determination unit 92. The pressing force F may be changed to the pressing force F3 or from the third pressing force F3 to the first pressing force F1 or the second pressing force F2.

  Further, as shown in FIG. 1, in the present embodiment, the sheet sensor 62 is disposed directly below (just upstream) the registration roller pair 113. In the registration roller pair 113, the leading end position of the sheet P is aligned, in other words, the skew of the sheet P is corrected, so that the leading end of the sheet P is aligned with the registration roller pair 113 whose rotation is stopped. There is a temporary collision. As described above, the sheet sensor 62 is arranged directly below the registration roller pair 113, so that it is possible to stably detect the thickness D of the sheet P whose conveyance has been temporarily stopped. Further, it is possible to adjust the pressing force F of the secondary transfer roller 145 by using the time from the leading end of the sheet P to the secondary transfer roller 145 from the registration roller pair 113.

  Further, the sheet sensor 62 is disposed between the registration roller pair 113 and the conveyance roller pair 112. The distance on the sheet conveyance path 111 between the registration roller pair 113 and the conveyance roller pair 112 is set to be shorter than the length in the conveyance direction of the minimum size sheet P conveyed on the sheet conveyance path 111. For example, the distance is set to be shorter than the length of the postcard in the conveyance direction, corresponding to the postcard size. Therefore, the sheet sensor 62 can stably detect the thickness D of the sheet P in a state where the sheet P is sandwiched between the registration roller pair 113 and the conveyance roller pair 112.

  As described above, according to the above-described embodiment, the sheet sensor 62 has the thickness of the sheet P conveyed through the sheet conveyance path 111 in the sheet conveyance path 111 upstream of the secondary transfer nip portion N in the conveyance direction of the sheet P. Detect D. Then, the determination unit 92 determines whether or not it is necessary to adjust the pressing force F of the secondary transfer roller 145 with respect to the driving roller 142 according to the detection result of the sheet sensor 62. For this reason, whether or not the pressing force F of the secondary transfer roller 145 needs to be adjusted is determined according to the thickness D of the sheet P actually conveyed through the sheet conveyance path 111. As a result, when the adjustment of the pressing force F is not necessary, the pressing force F is prevented from being erroneously adjusted. Therefore, the pressing force F of the secondary transfer roller 145 is unnecessarily changed, and the transfer conditions at the secondary transfer nip portion N are prevented from changing. Further, when the determination unit 92 determines that adjustment of the pressing force F is necessary, the pressing force F of the secondary transfer roller 145 is adjusted by the pressing force adjustment unit 93. Therefore, compared with the case where the pressing force F of the secondary transfer roller 145 is adjusted according to the setting of the sheet P in the sheet feeding tray 121 on which the sheet P is stacked, the sheet is actually conveyed through the sheet conveyance path 111. The pressing force F can be adjusted with high accuracy according to the specifications of the sheet P. Further, when the toner image is transferred from the intermediate transfer belt 141 to the sheet P, the rotation speed of the intermediate transfer belt 141 and the conveyance speed of the sheet P can be made close to each other in accordance with the thickness D of the different sheet P. . For this reason, the equal magnification of the toner image transferred to the sheet P is maintained as much as possible.

  Further, according to the above-described embodiment, the pressing force F of the secondary transfer roller 145 is set to the first pressing force F <b> 1 corresponding to plain paper among the sheets P conveyed through the sheet conveying path 111. For this reason, the pressing force F is set corresponding to the general-purpose plain paper, so that the toner image is transferred to the sheet P with the first pressing force F1 without frequently adjusting the pressing force F. It becomes possible. When thin paper thinner than plain paper is conveyed to the secondary transfer nip portion N, the pressing force adjusting unit 93 sets a second pressing force F2 larger than the first pressing force F1. For this reason, the thin paper is inserted between the secondary transfer roller 145 pressed by the first pressing force F1 and the intermediate transfer belt 141, so that the transport speed of the thin paper becomes excessively slow, and the toner image and the like. Reduction of the magnification is suppressed.

  Further, according to the above-described embodiment, when thick paper thicker than plain paper is conveyed to the secondary transfer nip portion N, the pressing force adjusting unit 93 causes the third pressing force that is smaller than the first pressing force F1. F3 is set. For this reason, when the thick paper is inserted between the secondary transfer roller 145 and the intermediate transfer belt 141 pressed by the first pressing force F1, the transport speed of the thick paper is excessively increased, and the toner image is It is prevented that the magnification is extended.

  Further, according to the above-described embodiment, the second pressing force F2 is set so that the conveyance speed of the thin paper and the rotation speed of the intermediate transfer belt 141 are equal. For this reason, the equal magnification of the toner image transferred to the thin paper is stably maintained. The third pressing force F3 is set so that the transport speed of the thick paper and the rotation speed of the intermediate transfer belt 141 are equal. For this reason, the equal magnification of the toner image transferred to the thick paper is stably maintained.

  Further, according to the above embodiment, the sheet sensor 62 detects the thickness D of the sheet P in the non-contact state with respect to the sheet P in the sheet conveyance path 111. For this reason, the thickness D of the sheet P is suitably detected without hindering the conveyance of the sheet P.

  Further, according to the above embodiment, since the secondary transfer roller 145 has a crown shape, the conveyance speed of the sheet P is changed according to the thickness D of the sheet P conveyed to the secondary transfer nip portion N. Cheap. Even in such a case, when the toner image is transferred from the intermediate transfer belt 141 to the sheet P, the circumferential speed of the intermediate transfer belt 141 and the conveyance of the sheet P correspond to the thickness D of the different sheet P. You can get close to speed. For this reason, the equal magnification of the toner image transferred to the sheet P is maintained as much as possible.

  Further, according to the above-described embodiment, the secondary transfer roller 145 is arranged offset with respect to the drive roller 142. For this reason, the secondary transfer nip N is formed with a predetermined width by partially winding the intermediate transfer belt 141 around the peripheral surface of the secondary transfer roller 145. Therefore, when a difference occurs between the rotation speed of the intermediate transfer belt 141 and the conveyance speed of the sheet P in the secondary transfer nip portion N, the equal magnification of the toner image is likely to be changed. Even in such a case, when the toner image is transferred from the intermediate transfer belt 141 to the sheet P, the circumferential speed of the intermediate transfer belt 141 and the conveyance of the sheet P correspond to the thickness D of the different sheet P. You can get close to speed. For this reason, the equal magnification of the toner image transferred to the sheet P is maintained as much as possible.

  Further, according to the above embodiment, the pressing force adjusting unit 93 changes the rotation angle of the cam 69 by the adjusting motor 63. The cam 69 presses the load portion 70 </ b> A of the arm 70. As a result, the arm 70 is swung around the fulcrum part 70B. As the arm 70 swings, the pressing portion 70 </ b> C presses the secondary transfer roller 145. Accordingly, the movement of the secondary transfer roller 145 and the adjustment of the pressing force F are suitably realized by the adjustment motor 63, the cam 69, and the arm 70.

  The intermediate transfer unit 14 and the image forming apparatus 10 including the intermediate transfer unit 14 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, the threshold values D1 and D2 have been described as being set to 70 μm and 120 μm, respectively, but the present invention is not limited to this. The expressions thin paper, plain paper, and thick paper are relative names used for the description of the present embodiment, and do not limit the present invention.

  (2) In the above embodiment, the non-contact displacement sensor is used as the sheet sensor 62, but the present invention is not limited to this. The sheet sensor 62 may be a sensor that detects the thickness of the sheet from the torque fluctuation of the conveying roller when the sheet P enters the nip portion of the conveying roller pair that conveys the sheet P.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 11 Apparatus main body 111 Sheet conveyance path 13 Image forming part 14 Intermediate transfer unit (transfer apparatus)
141 Intermediate transfer belt 142 Drive roller 143 Driven roller 144 Belt cleaning device 145 Secondary transfer roller (transfer roller)
145A Secondary transfer unit (housing)
146 First backup roller 147 Second backup roller 20 Photosensitive drum (image carrier)
24 Primary transfer roller 50 Adjustment mechanism 60 Belt motor 61 Drum motor 62 Sheet sensor (thickness detection means)
63 Adjustment motor (motor)
64 First gear 65 Second gear 66 Shaft 67 Pulse plate 68 Rotation sensor 69 Cam (cam member)
70 Arm (arm member)
70A Weighting part (first arm part)
70B fulcrum part 70C pressing part (second arm part)
90 control unit 91 drive control unit 92 determination unit (determination means)
93 Pressure adjusting part (Pressure adjusting means)
94 Memory

Claims (12)

An intermediate transfer belt that is driven to rotate in one direction and carries a toner image on its surface;
A driving roller that stretches the intermediate transfer belt and drives the intermediate transfer belt to rotate,
A transfer roller that is pressed by the drive roller across the intermediate transfer belt and forms a transfer nip portion that transfers the toner image to the sheet with the drive roller;
A sheet conveyance path through which the sheet is conveyed so as to pass through the transfer nip portion;
A thickness detecting means for detecting the thickness of the sheet in the sheet conveying path upstream of the transfer nip portion in the sheet conveying direction;
A determination unit that determines whether or not it is necessary to adjust the pressing force of the transfer roller with respect to the drive roller, according to a detection result of the thickness detection unit;
A pressing force adjusting means for adjusting the pressing force according to a determination result of the determining means;
A transfer device comprising: The pressing force of the transfer roller against the driving roller is set in advance to the first pressing force corresponding to the first sheet conveyed through the sheet conveying path,
The determination means adjusts the pressing force when the thickness detection means detects that the sheet conveyed through the sheet conveyance path is a second sheet thinner than the first sheet. Judging that it is necessary,
The said pressing force adjustment means sets the said pressing force to the 2nd pressing force larger than a said 1st pressing force according to the said determination result of the said determination means, The Claim 1 characterized by the above-mentioned. Transfer device. The pressing force of the transfer roller against the driving roller is set in advance to the first pressing force corresponding to the first sheet conveyed through the sheet conveying path,
The determination means adjusts the pressing force when the thickness detection means detects that the sheet conveyed through the sheet conveyance path is a third sheet thicker than the first sheet. Judging that it is necessary,
The said pressing force adjustment means sets the said pressing force to the 3rd pressing force smaller than a said 1st pressing force according to the said determination result of the said determination means, The Claim 1 characterized by the above-mentioned. Transfer device. The pressing force of the transfer roller against the driving roller is set in advance to the first pressing force corresponding to the first sheet conveyed through the sheet conveying path,
The determination means adjusts the pressing force when the thickness detection means detects that the sheet conveyed through the sheet conveyance path is a second sheet thinner than the first sheet. The pressing force adjusting means determines that it is necessary, and sets the pressing force to a second pressing force larger than the first pressing force according to the determination result of the determining means,
Further, the determination means detects the pressing force when the thickness detection means detects that the sheet conveyed through the sheet conveyance path is a third sheet thicker than the first sheet. It is determined that adjustment is necessary, and the pressing force adjusting unit sets the pressing force to a third pressing force smaller than the first pressing force according to the determination result of the determining unit. The transfer device according to claim 1.   The pressing force adjusting means is configured so that when the second sheet passes through the transfer nip portion, the conveyance speed of the second sheet and the rotation speed of the intermediate transfer belt are equal. The transfer device according to claim 2, wherein a second pressing force is set.   The pressing force adjusting means is configured so that when the third sheet passes through the transfer nip portion, the conveyance speed of the third sheet and the rotation speed of the intermediate transfer belt are equal. The transfer device according to claim 3, wherein a third pressing force is set.   The transfer according to any one of claims 1 to 6, wherein the thickness detection unit detects the thickness of the sheet in a non-contact state with respect to the sheet in the sheet conveyance path. apparatus.   The transfer apparatus according to claim 7, wherein the thickness detection unit is an optical displacement sensor.   The transfer device according to claim 1, wherein the transfer roller has a crown shape in the axial direction.   10. The transfer roller according to claim 1, wherein the transfer roller is pressed against the drive roller while being offset to the upstream side in the circumferential direction of the intermediate transfer belt with respect to the drive roller. The transfer apparatus according to 1. A housing that movably supports the transfer roller with respect to the drive roller;
A motor disposed in the housing;
A cam member rotated by the motor;
A fulcrum part, a first arm part extending from the fulcrum part and pressed against the cam member, and a second arm extending from the fulcrum part to the opposite side of the first arm part and pressing the transfer roller And an arm member supported by the housing so as to be swingable around the fulcrum part,
The said pressing force adjustment part adjusts the pressing force of the said transfer roller by changing the rotation angle of the said cam member, and making the said 2nd arm part press the said transfer roller. The transfer apparatus according to any one of 10. A transfer device according to any one of claims 1 to 11,
An image forming apparatus, comprising: a plurality of image carriers that are arranged to face the intermediate transfer belt and transfer the toner image to the intermediate transfer belt.

Images