JP6544296B2 - Transfer unit and image forming apparatus provided with the same - Google Patents

Description

  The present invention relates to a transfer unit having an endless belt and a plurality of transfer rollers for transferring a toner image, and an image forming apparatus including the same.

  Conventionally, various image forming apparatuses have been proposed. Among them, there is a tandem type color image forming apparatus in which toner images of respective colors are sequentially superimposed by a plurality of image forming units to form a full color image. Further, in a tandem-type color image forming apparatus, a direct transfer method for transferring a toner image formed by each image forming portion onto a recording medium conveyed by an endless conveyance belt, and a plurality of image forming portions There is an intermediate transfer method in which toner images are sequentially superimposed (primary transfer) on an endless intermediate transfer belt (belt), and then transferred (secondary transfer) on a recording medium at one time.

  In the intermediate transfer type tandem color image forming apparatus, image formation using only black toner is performed in the monochrome mode for outputting a monochrome image. If the image bearing members of each of the image forming portions other than black in the monochrome mode are kept in contact with the intermediate transfer belt, the intermediate transfer belt contacts the image bearing members of yellow, magenta and cyan. Or the drive torque of the intermediate transfer belt becomes unnecessarily large.

  Therefore, there is known a method of separating an image bearing member of an image forming portion other than black, yellow, magenta and cyan in the monochrome mode from the intermediate transfer belt. For example, a color mode in which the primary transfer roller (transfer roller) is in pressure contact with the yellow, magenta, cyan, and black image carriers across the intermediate transfer belt, and the primary transfer roller across the black image carrier only with the intermediate transfer belt. There is known an image forming apparatus capable of switching between a monochrome mode in which the other primary transfer rollers are separated and a standby mode in which all the primary transfer rollers are separated.

  Note that a color mode in which the primary transfer roller is in pressure contact with all the image carriers across the intermediate transfer belt, a monochrome mode in which the primary transfer roller is in pressure contact with the black image carrier only with the intermediate transfer belt, For example, Patent Document 1 discloses an image forming apparatus capable of switching to a standby mode in which the transfer roller is separated.

JP 2012-159571 A

  By the way, in the above-described conventional image forming apparatus, for example, when transitioning from the standby mode to the color mode, transition to the color mode is made via the monochrome mode or not via the monochrome mode. For example, when transitioning from the standby mode to the color mode via the monochrome mode, from the state where the primary transfer roller corresponding to black is in pressure contact with the intermediate transfer belt (monochrome mode), yellow, magenta, and cyan other than black are supported. The three primary transfer rollers are simultaneously brought into pressure contact with the intermediate transfer belt. When transitioning from the standby mode to the color mode without passing through the monochrome mode, all (four) primary transfer rollers are intermediately transferred from the state (standby mode) in which all the primary transfer rollers are separated from the intermediate transfer belt. Simultaneously press on the belt.

  As described above, when three or four primary transfer rollers are simultaneously brought into pressure contact with the intermediate transfer belt, a large load is rapidly applied to the intermediate transfer belt, so it takes time until the rotational behavior of the intermediate transfer belt is stabilized. There is a problem.

  The present invention has been made to solve the problems as described above, and it is an object of the present invention to provide a transfer unit capable of shortening the time until the rotational behavior of the belt is stabilized and the same. Providing an image forming apparatus.

  In order to achieve the above object, the transfer unit according to the first configuration of the present invention is driven by an endless belt moving along a plurality of image forming portions, a drive roller rotating the belt, and the belt A driven roller that rotates while applying a predetermined tension to the belt, a unit main body that rotatably supports the driving roller and the driven roller, and an image carrier disposed in each image forming unit with the belt interposed therebetween, A plurality of transfer rollers for transferring a toner image formed on an image carrier onto a belt, and both ends of the rotation shaft of each transfer roller are rotatably supported, and the transfer roller contacts and separates from the belt A plurality of bearing holders that can independently reciprocate independently in one direction, and a pressing member that presses the plurality of bearing holders in a pressing direction in which the transfer roller presses against the belt Obtain. The pressing member has a standby mode in which all the transfer rollers are separated from the belt, a first mode in which only the transfer roller disposed opposite to the image carrier disposed in a specific image forming unit is pressed against the belt, and all the transfer It is possible to switch between a second mode in which the roller is in pressure contact with the belt. The plurality of transfer rollers disposed opposite to the image carrier disposed in the image forming unit other than the specific image forming unit are pressed against the belt by two or more stages of pressure contact operations.

  According to the first configuration of the present invention, the plurality of transfer rollers disposed opposite to the image carrier disposed in the image forming unit other than the specific image forming unit are pressed against the belt by two or more stages of pressure contact operation. Ru. Thus, when transitioning to the second mode, a time difference can be provided to press the transfer roller against the belt, so that the belt can be prevented from being subjected to an abrupt large load. For this reason, since the time until the behavior of the rotation of the belt stabilizes can be shortened, the printing waiting time can be shortened.

  In addition, the time until the behavior of the belt rotation is stabilized can be shortened, and the printing waiting time can be shortened, so that it is possible to shift to the standby mode frequently (for example, after every job is completed) . For this reason, since the load applied to the belt can be released frequently, the life extension of the belt can be realized.

FIG. 1 is a schematic cross-sectional view showing the configuration of an image forming apparatus on which an intermediate transfer unit according to an embodiment of the present invention is mounted. FIG. 3 is an external perspective view of the intermediate transfer unit according to the embodiment of the present invention as viewed from below. FIG. 5 is a perspective view showing a structure around a bearing holder and a slider in an intermediate transfer unit according to an embodiment of the present invention. FIG. 4 is a view of a slider and a bearing holder of an intermediate transfer unit according to an embodiment of the present invention as viewed from the front side of the paper surface of FIG. 3; FIG. 3 is a perspective view showing the structure of a bearing holder of an intermediate transfer unit according to an embodiment of the present invention. FIG. 8 is a side view showing the bearing holder and the slider of the intermediate transfer unit according to the embodiment of the present invention, in which the primary transfer roller is separated from the intermediate transfer belt. FIG. 8 is a side view showing the bearing holder and the slider of the intermediate transfer unit according to an embodiment of the present invention, showing the primary transfer roller in pressure contact with the intermediate transfer belt.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing the configuration of an image forming apparatus 100 on which an intermediate transfer unit 30 according to an embodiment of the present invention is mounted, and shows a tandem type color image forming apparatus here. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc and Pd are disposed in order from the upstream side in the transport direction (left side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow and black), and cyan, magenta and yellow are formed by the respective steps of charging, exposure, development and transfer. And black images are sequentially formed.

  In these image forming portions Pa to Pd, photosensitive drums (image carriers) 1a, 1b, 1c and 1d for carrying visible images (toner images) of respective colors are disposed, and drive means (see FIG. An intermediate transfer belt (belt) 8 which rotates counterclockwise in FIG. 1 is provided adjacent to each of the image forming portions Pa to Pd. After the toner images formed on the photosensitive drums 1a to 1d are sequentially primarily transferred and superimposed on the intermediate transfer belt 8 moving while being in contact with the photosensitive drums 1a to 1d, the secondary transfer roller is formed. After being secondarily transferred onto the transfer paper P as an example of the recording medium by the function 9 and further fixed on the transfer paper P in the fixing unit 13, the recording medium P is discharged from the apparatus main body. While rotating the photosensitive drums 1a to 1d clockwise in FIG. 1, an image forming process is performed on each of the photosensitive drums 1a to 1d.

  A transfer sheet P to which a toner image is transferred is accommodated in a sheet cassette 16 in the lower portion of the apparatus, and drives the secondary transfer roller 9 and an intermediate transfer belt 8 described later via the sheet feeding roller 12a and the resist roller pair 12b. It is conveyed to the nip with the roller 11. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt having no seam (seamless) is mainly used. Further, on the downstream side of the secondary transfer roller 9, a blade-like belt cleaner 19 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is disposed.

  Next, the image forming units Pa to Pd will be described. Around and below the photosensitive drums 1a to 1d rotatably disposed, charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d, and image information on the respective photosensitive drums 1a to 1d The developing device 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) and the like remaining on the photosensitive drums 1a to 1d. The cleaning units 7a, 7b, 7c and 7d are provided.

  When image data is input from a host device such as a personal computer, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging devices 2a to 2d, and then light is irradiated by the exposure device 5 according to the image data. An electrostatic latent image corresponding to the image data is formed on each of the photosensitive drums 1a to 1d. The developing devices 3a to 3d are each filled with a predetermined amount of a two-component developer including toners of respective colors of cyan, magenta, yellow and black. When the ratio of the toner in the two-component developer filled in the developing devices 3a to 3d falls below a specified value due to the formation of toner images described later, the developing devices 3a to 3d from the toner containers 4a to 4d Toner is replenished. The toner in the developer is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d, and is electrostatically attached, so that the electrostatic latent image formed by the exposure from the exposure device 5 is formed. The toner image is formed.

  Then, by applying a predetermined transfer voltage to the primary transfer rollers (transfer rollers) 6a to 6d, the cyan, magenta, yellow and black toner images on the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. Be done. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full color image. Thereafter, in preparation for the formation of a new electrostatic latent image to be subsequently performed, the toner and the like remaining on the surfaces of the photosensitive drums 1a to 1d are removed by the cleaning units 7a to 7d.

  The intermediate transfer belt 8 is stretched around the driven roller 10 on the upstream side and the drive roller 11 on the downstream side, and the intermediate transfer belt 8 rotates counterclockwise as the drive roller 11 is rotated by the drive motor (not shown). When rotation starts, the transfer sheet P is conveyed from the resist roller pair 12b to the nip (secondary transfer nip) between the driving roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing. The full color image on the intermediate transfer belt 8 is transferred onto the transfer paper P. The transfer sheet P to which the toner image has been transferred passes through the sheet conveyance path 18 and is conveyed to the fixing unit 13.

  The transfer paper P conveyed to the fixing unit 13 is heated and pressed by the fixing roller pair 13a, and the toner image is fixed on the surface of the transfer paper P, whereby a predetermined full-color image is formed. The transfer paper P on which the full color image is formed is sorted in the transport direction by the branch portion 14 branched in a plurality of directions. When an image is formed on only one side of the transfer sheet P, the sheet is discharged to the discharge tray 17 by the discharge roller 15 as it is.

  On the other hand, in the case of forming an image on both sides of the transfer sheet P, the transfer sheet P having passed through the fixing unit 13 is once conveyed in the direction of the discharge roller 15 and the discharge roller after the rear end of the transfer sheet P passes the branch portion 14 15 is reversely rotated and the conveyance direction of the branch part 14 is switched, whereby the rear end of the transfer sheet P is distributed to the double-sided conveyance path 20 and is re-conveyed to the secondary transfer nip in a state where the image surface is reversed. . Then, the next image formed on the intermediate transfer belt 8 is transferred by the secondary transfer roller 9 to the surface on which the image of the transfer sheet P is not formed, and conveyed to the fixing unit 13 to fix the toner image. , And is discharged to the discharge tray 17.

  FIG. 2 is an external perspective view of the intermediate transfer unit 30 according to an embodiment of the present invention as viewed from below, and FIG. 3 is a perspective view of bearing holders 38a to 38h and a slider 37a in the intermediate transfer unit 30 according to an embodiment of the present invention. 37b are perspective views showing the structure around 37b.

  The intermediate transfer unit (transfer unit) 30 includes a unit main body 34 configured of two side frames 31 and 32 and an upper surface frame (not shown), and a primary transfer roller 6 a supported between the side frames 31 and 32. 6 d, a driven roller 10, a driving roller 11, and an endless intermediate transfer belt 8 stretched around these rollers.

  Sliders (pressing members) 37a and 37b are supported by the side frames 31 and 32 so as to be slidable in the horizontal direction (second direction). Both ends of the rotation shaft of the primary transfer rollers 6a to 6d are rotatably supported by bearing holders 38a to 38h, and the bearing holders 38a to 38h are vertically moved relative to the side frames 31 and 32 (first direction, primary transfer roller Each of 6a to 6d is supported so as to be movable independently in a direction in which it contacts and separates from the intermediate transfer belt 8. Further, the driven roller 10 and the drive roller 11 are rotatably supported by the side frames 31 and 32.

  A shaft 46 is disposed inside the intermediate transfer unit 30, and pinion gears 47a and 47b are fixed to both ends of the shaft 46. The rotational driving force is transmitted to the shaft 46 via a gear train (not shown) provided in the apparatus main body.

  The pinion gears 47a and 47b are disposed at positions engaged with the racks 48 formed on the lower surfaces of the sliders 37a and 37b, respectively, and reciprocate the sliders 37a and 37b in the horizontal direction by rotating the pinion gears 47a and 47b forward and reverse. It is movable. The shaft 46 and the pinion gears 47a and 47b constitute a slider drive mechanism (pressing member drive mechanism) for reciprocating the sliders 37a and 37b.

  FIG. 4 is a view of the sliders 37a and 37b and the bearing holders 38a to 38h of the intermediate transfer unit 30 according to the embodiment of the present invention as viewed from the front side of the sheet of FIG. Eight pressing ribs 51 to 58 are formed on the inner side surfaces of the sliders 37a and 37b. Each pressing rib 51 to 58 has one lower portion 51a to 58a extending in the horizontal direction, and one inclined portion 51b to 58b connected to the lower portion 51a to 58a.

  FIG. 5 is a perspective view showing the structure around the bearing holder 38 d of the primary transfer roller 6 b of the intermediate transfer unit 30 according to an embodiment of the present invention. The bearing holders 38a to 38c and 38e to 38h also have the same configuration as that of the bearing holder 38d, so the description thereof will be omitted.

  The bearing holder 38 d includes a holder body 55, a bearing portion 57 supported so as to be vertically slidable with respect to the holder body 55, and a coil spring (biasing member) disposed between the upper portion of the holder body 55 and the bearing portion 57 And 60). The bearing portion 57 rotatably supports the rotation shafts of the primary transfer rollers 6 a to 6 d and is biased downward by the biasing force of the coil spring 60.

  Further, at the upper end portion of the holder main body 55, a V-shaped (or inverted triangle) abutting portion 55a protruding toward the slider 37b is formed. As shown in FIGS. 5 and 6, when the contact portion 55a is separated from the pressing rib 54, the other end (the end on the bearing holder 38d side) of the primary transfer roller 6b is separated from the intermediate transfer belt 8. ing. On the other hand, when the slider 37b slides leftward from the state shown in FIGS. 5 and 6, the contact portion 55a is directed downward by the inclined portion 54b of the pressing rib 54 (the pressing direction, the primary transfer rollers 6a to 6d are in The bearing holder 38d is moved downwards by being pressed in the direction in which it is in pressure contact). Then, as shown in FIG. 7, in a state where the contact portion 55a is pressed by the lower portion 54a of the pressing rib 54, the other end (the end on the bearing holder 38d side) of the primary transfer roller 6b is an intermediate transfer belt It is pressed to 8.

  Here, as shown in FIG. 4, the horizontal separation distances W1 to W8 between the bearing holders 38a to 38h and the lower portions 51a to 58a of the pressing ribs 51 to 58 are configured to be different from each other. Specifically, the separation distances W1 to W8 satisfy W1> W2> W3> W4> W5> W6> W7> W8. Thus, when the sliders 37a and 37b are slid in the left direction in FIG. 4, the bearing holders 38a to 38h contact the pressing ribs 51 to 58 with a time lag and move downward. Further, the difference between the separation distances W1 to W8 is larger than the pitch of the rack teeth of the rack 48.

  Next, the operation of the primary transfer rollers 6a to 6d in the intermediate transfer unit 30 of the present embodiment will be described.

  In the standby mode, the sliders 37a and 37b are disposed at positions where the pressing ribs 51 to 58 do not press the bearing holders 38a to 38h. As a result, since the primary transfer rollers 6 a to 6 d are separated from the intermediate transfer belt 8, the tension of the intermediate transfer belt 8 is relaxed, and the deformation and the elongation of the intermediate transfer belt 8 are suppressed.

  In the monochrome mode (first mode), since the image formation is performed using only the black image forming unit (specific image forming unit) Pd, the primary transfer roller 6 d is connected to the photosensitive drum 1 d via the intermediate transfer belt 8. Need to be in pressure contact with

  Therefore, when transitioning from the standby mode to the monochrome mode, the sliders 37a and 37b are slid leftward from the state of FIG. 4 by a predetermined amount to lower the bearing holders 38g and 38h to the lower portions 57a and 58a of the pressing ribs 57 and 58. Place on As a result, the bearing holders 38g and 38h move downward, and the primary transfer roller 6d is in pressure contact with the intermediate transfer belt 8.

  At this time, since the separation distance W8 is smaller than the separation distance W7, after the other end (the end on the bearing holder 38h side) of the primary transfer roller 6d is in pressure contact with the intermediate transfer belt 8, a time difference is provided. One end (end on the bearing holder 38 g side) of 6 d is pressed against the intermediate transfer belt 8.

  In addition, since the primary transfer rollers 6a to 6c for color remain at a position separated from the intermediate transfer belt 8, the contamination of the intermediate transfer belt 8 with the residual toner on the surfaces of the photosensitive drums 1a to 1c can be suppressed. . Further, there is no possibility that the drive torque of the drive roller 11 for rotating the intermediate transfer belt 8 is unnecessarily increased.

  In the color mode (second mode), four primary transfer rollers 6a to 6d are connected to the photosensitive drums 1a to 1d via the intermediate transfer belt 8 in order to form an image in four image forming portions Pa to Pd. Need to be in pressure contact with

  Therefore, when transitioning from the monochrome mode to the color mode, the bearing holders 38a to 38f are disposed at the lower portions 51a to 56a of the pressing ribs 51 to 56 by sliding the sliders 37a and 37b further to the left than in the monochrome mode. Do. Thereby, the bearing holders 38 a to 38 f move downward, and the primary transfer rollers 6 a to 6 c are in pressure contact with the intermediate transfer belt 8. Thus, all the primary transfer rollers 6 a to 6 d are in pressure contact with the intermediate transfer belt 8.

  At this time, since the separation distances W1 to W6 satisfy W1> W2> W3> W4> W5> W6, the other end of the primary transfer roller 6c (the end on the bearing holder 38f side), one end (bearing holder 38e) End), the other end of primary transfer roller 6b (end on bearing holder 38d), one end (end on bearing holder 38c), the other end of primary transfer roller 6a (bearing holder 38b) And one end (the end on the side of the bearing holder 38a) are in pressure contact with the intermediate transfer belt 8 with a time lag.

  In the transition from the color mode to the standby mode, the sliders 37a and 37b and the bearing holders 38a to 38h operate reversely to the above.

  In the present embodiment, as described above, the plurality of primary transfer rollers 6a to 6c for colors disposed opposite to the photosensitive drums 1a to 1c disposed in the image forming portions Pa to Pc other than black have two or more stages. The intermediate transfer belt 8 is pressure-contacted by (in this case, six steps) pressure contact operation. Thereby, when transitioning to the color mode, since the primary transfer rollers 6a to 6c can be pressed against the intermediate transfer belt 8 by providing a time difference, it is possible to prevent the intermediate transfer belt 8 from being suddenly subjected to a large load. Can. For this reason, since the time until the behavior of the rotation of the intermediate transfer belt 8 is stabilized can be shortened, the printing waiting time can be shortened.

  In addition, since the time until the rotational behavior of the intermediate transfer belt 8 becomes stable can be shortened and the printing waiting time can be shortened, it is possible to shift to the standby mode frequently (for example, after completion of each job) It becomes possible. Therefore, since the load applied to the intermediate transfer belt 8 can be released frequently, it is possible to realize the long life of the intermediate transfer belt 8.

  Further, as described above, all the primary transfer rollers 6a to 6d are in pressure contact with the intermediate transfer belt 8 at different timings. As a result, the application of a large load to the intermediate transfer belt 8 can be further suppressed.

  Further, as described above, the primary transfer rollers 6a to 6d are in pressure contact with the intermediate transfer belt 8 sequentially (here, from the right side in FIG. 3) along the moving direction (horizontal direction) of the intermediate transfer belt 8. As a result, the application of a large load to the intermediate transfer belt 8 can be further suppressed.

  Further, as described above, one end and the other end of the primary transfer rollers 6a to 6d are in pressure contact with the intermediate transfer belt 8 at different timings. As a result, the application of a large load to the intermediate transfer belt 8 can be further suppressed.

  Further, as described above, by moving the sliders 37a and 37b in the horizontal direction, the pressing ribs 51 to 58 press the contact portion 55a, and the bearing holders 38a to 38h move in the pressure contact direction. Thereby, the primary transfer rollers 6 a to 6 d can be easily brought into pressure contact with the intermediate transfer belt 8.

  Further, as described above, at least two (here, all) of the bearing holders 38a to 38f for supporting the rotation axes of the primary transfer rollers 6a to 6c other than black are the contact portion 55a in the horizontal direction and the lower portion 51a to The separation distances W1 to W6 between 58a and 58a are arranged to be different from each other. Thus, the end portions of the primary transfer rollers 6a to 6c supported by the bearing holders 38a to 38f can be easily brought into pressure contact with the intermediate transfer belt 8 with a time lag.

  Further, as described above, the difference between the separation distances W1 to W8 is larger than the pitch of the rack teeth of the rack 48. As a result, it is possible to prevent the occurrence of vibrations at the time of pressing the primary transfer rollers 6a to 6d against the intermediate transfer belt 8 twice or more in one pitch, so that large vibrations occur in the intermediate transfer unit 30. It can be suppressed.

  Further, as described above, when transitioning from the color mode to the monochrome mode (or the standby mode), the primary transfer rollers 6a to 6c are separated from the intermediate transfer belt 8 with a time difference, and therefore the load applied to the intermediate transfer belt 8 It is possible to suppress rapid change. As a result, even when shifting to the monochrome mode (or standby mode), it is possible to shorten the time until the behavior of the rotation of the intermediate transfer belt 8 is stabilized.

  It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is indicated not by the description of the embodiments described above but by the claims, and further includes all modifications within the meaning and scope equivalent to the claims.

  For example, although the example which applied this invention to the color printer was shown, this invention is not limited to this. Needless to say, the present invention can be applied to various image forming apparatuses such as color copying machines and digital multi-functional machines provided with an intermediate transfer unit having an intermediate transfer belt and a plurality of primary transfer rollers.

  Moreover, in the said embodiment, although the example which presses all the bearing holders 38a-38h at a mutually different timing was shown, it is not necessary to make all the timings which press bearing holders 38a-38h differ.

1a to 1d Photosensitive drum (image carrier)
6a to 6d Primary transfer roller (transfer roller)
8 Intermediate Transfer Belt (Belt)
10 driven roller 11 drive roller 30 intermediate transfer unit (transfer unit)
34 Unit Body 37a, 37b Slider (Pressing Member)
38a to 38h bearing holder 47a, 47b pinion gear 48 rack 51 to 58 pressing rib 51a to 58a lower portion 51b to 58b inclined portion 55 holder main body 55a abutting portion 57 bearing portion 60 coil spring (biasing member)
100 Image forming apparatus Pa to Pd Image forming unit W1 to W8 Distance

Claims (5)

An endless belt moving along the plurality of image forming units;
A driving roller for rotating the belt;
A driven roller that rotates following the belt and applies a predetermined tension to the belt;
A unit body rotatably supporting the driving roller and the driven roller;
A plurality of transfer rollers disposed opposite to the image bearing member disposed in each of the image forming units with the belt interposed therebetween, and transferring a toner image formed on the image bearing member onto the belt;
A plurality of bearing holders rotatably supporting both end portions of the rotation shaft of each of the transfer rollers, and capable of independently reciprocatingly moving in the first direction in which the transfer rollers contact and separate from the belt;
A pressing member for pressing the plurality of bearing holders in a pressing direction in which the transfer roller presses against the belt;
Equipped with
The pressing member is in a standby mode for separating all the transfer rollers from the belt, and presses only the transfer roller disposed opposite to the image bearing member disposed in a specific image forming unit against the belt. It is possible to switch between one mode and a second mode in which all the transfer rollers are pressed against the belt,
The plurality of transfer rollers disposed opposite to the image bearing member disposed in the image forming unit other than the specific image forming unit are pressed against the belt by two or more pressing operations .
All the transfer rollers are pressed against the belt at different timings,
The transfer roller is in pressure contact with the belt in order along the moving direction of the belt;
The transfer roller disposed opposite to the image carrier disposed in the black image forming unit is first brought into pressure contact with the belt;
A transfer unit characterized in that one end and the other end of the transfer roller are pressed against the belt at different timings . A pressing member drive mechanism configured to reciprocate the pressing member in a second direction intersecting the first direction;
The pressing member has a pressing rib for pressing the bearing holder,
The bearing holder includes a holder body having a contact portion pressed against the pressing rib of the pressing member in the pressing direction, a bearing portion slidably supported by the holder body, the bearing portion, and the holder body And a biasing member disposed between the
When the pressing member drive mechanism moves the pressing member to one side in the second direction, the pressing rib presses the contact portion, and the bearing holder moves in the pressing direction. Item 2. A transfer unit according to item 1 . The pressing rib is connected to the lower end of the inclined portion to move the abutting portion in the pressure contact direction by moving the pressing member in one of the second directions, and a lower end extending in the second direction. Have a department,
The transfer roller is brought into pressure contact with the belt by arranging the contact portion of the bearing holder at the lower portion.
At least two of the bearing holders for supporting the rotation shafts of the plurality of transfer rollers disposed opposite to the image carrier disposed in the image forming unit other than the specific image forming unit are the ones in the second direction. The transfer unit according to claim 2 , wherein separation distances between the contact portion and the lower portion are different from each other. The pressing member drive mechanism has a rotatably provided pinion gear.
The pressing member has a rack that engages with the pinion gear and extends in the second direction,
The transfer unit according to claim 3 , wherein a difference between at least two of the separation distances of the bearing holders is larger than a pitch of rack teeth of the rack. An image forming apparatus comprising the transfer unit according to any one of claims 1 to 4 .

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