JP5207636B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic method, and more particularly to contact / separation between a photoreceptor, an intermediate transfer belt or a conveyance belt, and a transfer roller in a tandem type color image forming apparatus.

  A schematic diagram of a conventional image forming apparatus is shown in FIG. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and magenta, cyan, and yellow are respectively subjected to charging, exposure, development, and transfer processes. And a black image is formed sequentially.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto the intermediate transfer belt 8 that moves adjacent to each image forming portion while rotating clockwise in FIG. 11 by a driving means (not shown). The image is transferred onto the paper P at a time (secondary transfer) by the next transfer roller 9 and further fixed on the paper P by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper P onto which the toner image is transferred is housed in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing units 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the start of image formation is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure units 4a to 4d. An electrostatic latent image corresponding to the image signal is formed on ˜1d. Each of the developing units 3a to 3d includes a developing roller (developer carrying member) disposed so as to face the photosensitive drums 1a to 1d, and each of magenta, cyan, yellow, and black toners is supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing units 3a to 3d, and electrostatically adheres to the electrostatic latent images formed by exposure from the exposure units 4a to 4d. A corresponding toner image is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred onto the intermediate transfer belt 8 by the transfer rollers 6a to 6d. The primary transfer. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet P is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet P is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto P. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  Further, the intermediate transfer belt 8, the transfer rollers 6 a to 6 d, the driven roller 10, the drive roller 11, and the tension roller 20 integrally form a transfer unit 50.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when forming images on both sides of the paper P, a part of the paper P that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  As described above, in the color image forming apparatus using the electrophotographic method, the surface of the photosensitive member is brought into contact with each photosensitive member carrying magenta, cyan, yellow, and black toner, and the intermediate transfer belt or the conveying belt. The toner images formed thereon are transferred sequentially.

  In the color mode for transferring a color image, a toner image is transferred by transfer from all the photoconductors. In the monochrome mode for transferring a monochrome image, a photoconductor carrying black toner (hereinafter referred to as “black photoconductor”). ) Only, a black image is transferred.

  However, when the color mode and the monochrome mode are selectively executed, in the monochrome mode, each photoconductor that forms a magenta, cyan, and yellow image is in contact with the intermediate transfer belt even though it is not used for transfer, and rotates. Will do. For this reason, these photoconductors are subject to wear due to unnecessary rotation, deterioration of the photosensitive layer due to the charge remaining on the belt, and generation of potential unevenness of the photoconductor due to the deterioration. Further, since the toner is agitated inside the developing device as the photosensitive member rotates, unnecessary charging occurs in the toner, and development on the photosensitive member becomes difficult. Further, the developing device is deteriorated due to unnecessary stirring. Further, the cleaning portion that comes into contact with the photosensitive member is also consumed due to friction. In addition, the intermediate transfer belt is consumed due to sliding with the photosensitive member.

  On the other hand, as described above, since the intermediate transfer belt 8 is disposed integrally with the transfer rollers 6a to 6d and the like, the transfer unit 50 needs to be detached from the apparatus for maintenance and the like.

  However, if the photosensitive member and the intermediate transfer belt are in contact with each other, the surface of the photosensitive member or the intermediate transfer belt is easily damaged when the transfer unit or the photosensitive member is attached or detached, and it becomes difficult to transfer the image with high accuracy.

  Therefore, the intermediate transfer belt is brought into contact with all the photoconductors in the color mode, and is separated from the photoconductor for color image formation (hereinafter referred to as “color photoconductor”) in the monochrome mode, and is brought into contact with only the black photoconductor. There has been proposed a technique for separating the transfer unit from all the photoconductors when the transfer unit is detached from the apparatus. For example, Patent Documents 1 and 2 disclose a technique in which a support body such as an arm or a crank gear is used and the belt and the photoconductor are brought into contact with and separated from each other by changing the circulation path of the intermediate transfer belt. However, if a support is provided to separate the intermediate transfer belt from the photoreceptor, the apparatus becomes large and the moving mechanism becomes complicated.

On the other hand, for example, in Patent Document 3, an image forming apparatus including a black photoconductor, a color photoconductor, an intermediate transfer belt, and a transfer roller that contacts the photoconductor via the intermediate transfer belt is adjacent to the black photoconductor. Technology that includes a retract roll disposed at any position between a color photoconductor and another color photoconductor, and moves the retract roll to contact and separate the intermediate transfer belt and the color photoconductor With this technique, the entire apparatus is reduced in size without using a complicated mechanism.
JP-A-9-274354 JP 2003-186313 A JP 2003-337454 A

  However, the technique of Patent Document 3 does not disclose anything about separating the intermediate transfer belt from the black photosensitive member.

  Further, in order to move the intermediate transfer belt away from all the color photoconductors by moving the retract rolls inserted between the color photoconductors, the color photoconductors arranged inside are arranged at both ends. It is necessary to dispose it above the color photoreceptor. For this reason, the optical path length of the laser irradiated from the exposure unit differs between color photoreceptors, and it becomes difficult to form an electrostatic latent image with high accuracy in the color mode. For this reason, if the exposure state is made the same, it is necessary to change the arrangement of the exposure unit corresponding to each photoconductor, or to make a complicated adjustment for each exposure unit, and the apparatus configuration is increased in size and complexity. There is a risk of becoming. Furthermore, since the trajectory of the intermediate transfer belt in contact with the black photosensitive member is different between the color mode and the monochrome mode, the transfer state of the black image is not stable.

  In view of the above problems, the present invention is further downsized and can obtain a stable transfer state regardless of the mode even when the monochrome mode and the color mode are selectively switched. An object of the present invention is to provide an image forming apparatus capable of separating the intermediate transfer belt or the conveyance belt from all the photosensitive members.

  In order to achieve the above object, the present invention provides a plurality of image forming units including a photosensitive member and a developing device, an endless intermediate transfer belt in which toner images developed on the photosensitive member are sequentially stacked, and the photosensitive member. Transfer means for laminating the toner image developed on the body on the intermediate transfer belt by a transfer roller; and secondary transfer means for transferring the toner image laminated on the intermediate transfer belt onto a recording medium at a time; In the image forming apparatus, the photosensitive member includes a black photosensitive member on the most upstream side or the most downstream side with respect to the rotation direction of the intermediate transfer belt, on which a black toner image is formed, and the intermediate member. It is composed of one or more color photoconductors on which toner images of colors other than black are formed so that the nip portion with the transfer belt is substantially on the same plane. Located on the opposite side of the photoconductor And a first support roller movable to contact and separate the intermediate transfer belt and the color photosensitive member, and the black photosensitive member and a color photosensitive member adjacent to the black photosensitive member. A second support roller; and a third support roller disposed on the opposite side of the color photoconductor across the black photoconductor.

  The present invention also provides a plurality of image forming units including a photosensitive member and a developing device, an endless conveying belt that conveys a recording medium to the image forming unit, and a toner image developed on the photosensitive member. And a transfer unit that transfers onto the recording medium or onto the transport belt, the photosensitive member is disposed on the most upstream side or the most downstream side with respect to the rotation direction of the transport belt. A black photosensitive member on which a black toner image is formed, and at least one color photosensitive member on which a non-black toner image is formed, disposed so that a nip portion between the conveying belt and the conveying belt is substantially on the same plane; A first support roller that is disposed on the opposite side of the black photoconductor across the color photoconductor and is movable to contact and separate the transport belt and the color photoconductor; and the black photoconductor Photoreceptor and black sensitivity A second support roller disposed between the color photoreceptor and the third support roller disposed on the opposite side of the color photoreceptor with the black photoreceptor interposed therebetween. It is characterized by.

  Further, the invention is characterized in that a detection means for detecting that the intermediate transfer belt or the conveyance belt is separated from the photoconductor is provided.

  In the invention, it is preferable that the detection unit detects the movement of at least one of the first support roller, the second support roller, and the third support roller.

  Further, the present invention is characterized by comprising control means for controlling the transfer unit including the intermediate transfer belt or the conveyance belt and the transfer roller to be detachable based on the detection result of the detection means. To do.

  According to the present invention, the second support roller is arranged around the intermediate transfer belt or the conveyance belt that passes between the second support roller and the black photosensitive member in the selective switching between the monochrome mode and the color mode. It is possible to move in a direction parallel to the direction.

  Further, the present invention provides the second support roller side of a plane where the nip portion between the black photosensitive member and the intermediate transfer belt or the conveyance belt passes through the nip portion between the color photosensitive member and the intermediate transfer belt or the conveyance belt. It is arranged in that.

  According to the first configuration of the present invention, the nip portion between each color photoconductor and the intermediate transfer belt is substantially on the same plane with a simpler configuration than before, and the contact state between the belt and the black photoconductor is set. It is possible to switch between the color mode and the monochrome mode while keeping constant, and the intermediate transfer belt can be separated from all the photoconductors. Therefore, it is possible to further improve the stable transfer state regardless of the size and mode of the apparatus and the ease of detaching the transfer unit from the apparatus.

  According to the second configuration of the present invention, the nip portion between each color photoconductor and the conveyor belt is substantially on the same plane, and the contact state between the belt and the black photoconductor is constant. Thus, it is possible to switch between the color mode and the monochrome mode while keeping the distance between them, and the conveyance belt can be separated from all the photoconductors. Therefore, it is possible to further improve the stable transfer state regardless of the size and mode of the apparatus and the ease of detaching the transfer unit from the apparatus.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the intermediate transfer belt or the conveyance belt is in contact with or separated from the photoconductor. By detecting whether or not the transfer unit can be obtained, it is possible to obtain a material for determining whether or not the transfer unit is detachable, and it is possible to further avoid damage to the photoreceptor and the belt.

  According to the fourth configuration of the present invention, in the image forming apparatus having the third configuration, the separation of the intermediate transfer belt or the conveyance is detected by the movement of the first, second, and third support rollers. Therefore, the belt separation can be detected in more detail.

  Further, according to the fifth configuration of the present invention, when the belt is in contact with the photoconductor according to the detection result of the image forming apparatus detection means of the third or fourth configuration, the transfer unit is removed from the apparatus. If it cannot be detached and is separated from all the photoconductors, it can be detached. Therefore, damage to the photoreceptor and the belt can be avoided more reliably.

  According to the sixth configuration of the present invention, in the image forming apparatus having any one of the first to fifth configurations, even if the moving distance of the first support roller is reduced, the intermediate transfer belt or the conveyance belt The separation distance from the color photoreceptor can be increased. In the separation, the difference in clearance between each color photoconductor and the belt can be reduced. In particular, when the belts are spaced in parallel with respect to the nip portion between each color photoconductor and the belt, the clearance between each photoconductor and the belt becomes uniform, and the minimum release amount, that is, both the rollers described above. The photosensitive member and the belt can be separated from each other by the minimum movement. Therefore, in particular, the belt can be more easily separated from the color photoconductor, and the transfer unit can be easily detached from the apparatus, and the apparatus can be further miniaturized.

  According to the seventh configuration of the invention, in the image forming apparatus having the configuration of any one of the first to sixth configurations, even if the moving distance of the first support roller is further reduced, the intermediate transfer belt or the conveyance belt is used. The distance from the color photoconductor can be increased. Therefore, in particular, the separation of the belt from the color photoconductor can be further facilitated, the transfer unit can be easily detached from the apparatus, and the apparatus can be further miniaturized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1, 2, and 3 are schematic views showing configurations around an image forming unit and an intermediate transfer belt of the tandem type color image forming apparatus according to the first embodiment of the present invention. FIG. 1 is a color mode, FIG. 2 shows a configuration in a monochrome mode, and FIG. 3 shows a configuration in a state in which all the photosensitive members are separated from the intermediate transfer belt (hereinafter referred to as “all separation mode”). Portions common to those in FIG. 11 are denoted by the same reference numerals and description thereof is omitted.

  A photosensitive drum 1a (hereinafter referred to as “photosensitive member 1a”) on which a cyan image is formed, a photosensitive drum 1b (hereinafter referred to as “photosensitive member 1b”) on which a magenta image is formed, and a yellow image are formed. The nip portion between the photosensitive drum 1c (hereinafter referred to as “photosensitive member 1c”) and the intermediate transfer belt 8 is defined as X, Y, and Z. Each of the color photoconductors 1a to 1c is arranged to be rotatable in the horizontal direction so that the above X to Z are on substantially the same plane. Further, the photosensitive drum 1d (hereinafter referred to as “photosensitive member 1d”) on which the black image on the most downstream side is formed is such that the nip portion with the intermediate transfer belt 8 is substantially on the same plane as the above X to Z. It is arranged.

  Further, along the inner peripheral surface of the intermediate transfer belt 8, a first support roller 21 is disposed further upstream of the most upstream cyan photoreceptor 1a, and the most downstream black photoreceptor 1d and the upstream side of the photoreceptor. The second support roller 22 is movable between the yellow photoconductor 1 c and the third photoconductor 1 d and the driving roller 11, and the third support roller 24 is movable between the black photoconductor 1 d and the downstream side. And it is rotatably arranged. A tension roller 23 is rotatably disposed further upstream of the first support roller 21. A driven roller 10 is rotatably disposed on the downstream side of the driving roller 11 and the upstream side of the tension roller 23. The transfer unit 50 includes the transfer rollers 6a to 6d, the intermediate transfer belt 8, the first support roller 21, the second support roller 22, the third support roller 24, the driven roller 10, the drive roller 11, and the tension roller 23. And is detachable from the image forming apparatus 100.

  The intermediate transfer belt 8 is driven by a driving roller 11 to drive a driven roller 10, a tension roller 23, a first support roller 21, a cyan photoconductor 1a, a magenta photoconductor 1b, a yellow photoconductor 1c, a second support roller 22, and a black color. The photosensitive member 1d and the third support roller 24 are sequentially brought close to or in contact with each other and rotated clockwise (in the direction of the arrow). The intermediate transfer belt 8 is stretched by a driven roller 10, a tension roller 23, a first support roller 21, a third support roller 24 and a drive roller 11.

  The arrangement of the photoreceptors 1a to 1c is not particularly limited as long as the above X to Z are on substantially the same plane. If these photoconductors 1a to 1c are not arranged on substantially the same plane, it is difficult to equalize the optical path lengths of the laser beams irradiated from the exposure units 4a to 4d to the corresponding photoconductors 1a to 1c. It becomes difficult to form an electrostatic latent image accurately in the color mode, for example, the exposure of the photoconductor varies in the mode. Further, if the exposure state is made the same, the arrangement of the corresponding exposure unit needs to be changed, or complicated adjustments must be made for each exposure unit, and the apparatus configuration may be increased in size and complexity. is there. The arrangement of the color photoconductors 1a to 1c is not limited to the horizontal plane shown in the present embodiment as long as the nip portion with the intermediate transfer belt 8 is substantially on the same plane. The apparatus can be further miniaturized and complicated. When there is one color photoconductor, the plane where the nip portion exists is only one plane, which satisfies the above requirements.

  The black photosensitive drum 1d is arranged so that the nip portion with the intermediate transfer belt 8 is substantially on the same plane as X to Z as in the present embodiment, so that the laser light path from the exposure unit is provided. The length can be made the same as the other photoconductors 1a to 1c, and an electrostatic latent image can be formed with higher accuracy. However, the present embodiment is not particularly limited. For example, as will be described later, the second support roller 22 is formed on the nip portion between the black photoreceptor 1d and the intermediate transfer belt 8 from the plane passing through X to Z. It can also be arranged on the side.

  In the color mode, as shown in FIG. 1, the first support roller 21, the second support roller 22, and the third support roller 24 have the intermediate transfer belt 8 between the first support roller 21 and the second support roller 24. It moves on the same plane as X to Z and moves upward until it contacts the photoreceptors 1a to 1d. As the intermediate transfer belt 8 moves upward, the transfer rollers 6 a to 6 d move upward and come into contact with the photoreceptors 1 a to 1 d via the intermediate transfer belt 8. Thus, the intermediate transfer belt 8 is horizontal from the first support roller 21 to the second support roller 22. The intermediate transfer belt 8 is stretched by a driven roller 10, a tension roller 23, a first support roller 21, a third support roller 24, and a drive roller 11, contacts the photoreceptors 1a to 1d, and is transferred by the transfer rollers 6a to 6d. Moves clockwise while receiving primary transfer. Further, the tension roller 23 moves in the inner direction (right direction) of the belt circulation in order to relieve the tension caused by the movement of the belt.

  On the other hand, in the monochrome mode, as shown in FIG. 2, by switching from the color mode, the first support roller 21 moves downward until the intermediate transfer belt 8 is separated from all the color photoreceptors 1a to 1c (from FIG. 1). (State of FIG. 2). As the intermediate transfer belt 8 moves, the intermediate transfer belt 8 moves away from the photoreceptors 1a to 1c while the transfer rollers 6a to 6c are pushed down by the tension between the first support roller 21 and the second support roller 22. The intermediate transfer belt 8 contacts only with the black photosensitive drum 1d and moves clockwise while receiving primary transfer by the transfer roller 6d. The tension roller 23 moves in the outward direction (left direction) of the belt circulation in order to compensate for the tension attenuation caused by the movement of the belt.

  In the all separation mode, as shown in FIG. 3, the second support roller 22 and the third support roller 24 are moved downward until the intermediate transfer belt 8 is separated from the black photosensitive member 1d (FIG. 3). 2 to 3). As the intermediate transfer belt 8 moves, the intermediate transfer belt 8 is separated from the photoreceptor 1d while the transfer roller 6d is pushed down by the tension between the second support roller 22 and the third support roller 24. Thus, all the photoconductors 1 a to 1 d are separated from the intermediate transfer belt 8. As a result, the transfer unit 50 can be detached without damaging the surfaces of the photoreceptors 1a to 1c and the intermediate transfer belt 8.

  The downward movement of the first support roller 21 is sufficient if all the color photoconductors 1a to 1c can be separated from the intermediate transfer belt 8, and the degree thereof is determined in consideration of other device configurations. do it. Further, the color mode state (the state shown in FIGS. 3 to 2 or FIG. 4 to FIG. 2) can be obtained by switching from the monochrome mode to the color mode or from the full separation mode to the color mode.

  Since the second support roller 22 does not move in the color mode and the monochrome mode as in this embodiment, the contact state of the intermediate transfer belt 8 with the black photosensitive member 1d does not change in both modes. Therefore, a stable black image can be transferred regardless of the mode. However, if the nip portion between the intermediate transfer belt 8 and the black photosensitive member 1d does not change by switching between the two modes, the second support roller 22 may be movably disposed. For example, the second support roller 22 can be moved in a direction parallel to the circumferential direction of the intermediate belt 8 passing between the roller and the black photoreceptor 1d.

  Referring to FIGS. 1 and 2, the second support roller 22 is moved horizontally in the right direction on the X to Z plane by switching the mode from the color mode to the monochrome mode, and the second support roller 22 is switched by switching from the monochrome mode to the color mode. 2 The support roller 22 is moved horizontally in the left direction on the XZ plane. At this time, the contact state of the intermediate transfer belt 8 with the black photosensitive member 1d does not change by mode switching. Even if the moving distance of the first support roller 21 is reduced by the movement of the second support roller 22, the separation distance of the intermediate transfer belt 8 from the photoconductors 1 a to 1 c can be increased. Separation is further facilitated. Note that the second support roller 22 may be moved simultaneously with the first support roller 21 or before or after the roller, and is not particularly limited. Moving at the same time will lead to a reduction in waiting time.

  In the present embodiment, the second support roller 22 moves downward by switching to the full separation mode (thick arrow). However, if the black photosensitive member 1d can be separated by the movement of the third support roller 24, the second support roller 22 is moved to the second separation roller 22 in the second separation mode. It can also be set as the structure which does not move a support roller. However, if the second support roller moves downward, the separation distance between the intermediate transfer belt 8 and the black photoreceptor 1d can be increased even if the movement distance of the third support roller 24 is reduced. It becomes easier. Such movement of the second support roller is sufficient if the intermediate transfer belt 8 can be separated from the black photoreceptor 1d, and may be determined in consideration of the movement of the third support roller 24 and other configurations.

  In the present embodiment, the third support roller 24 does not move in the color mode and the monochrome mode, but moves downward by switching to the full separation mode. Accordingly, the stable transfer of the black image is maintained without affecting the transfer state in the color mode and the monochrome mode. Further, the downward movement of the third support roller 24 is sufficient as long as the black photosensitive member 1d can be separated from the intermediate transfer belt 8, and the extent thereof is the movement of the second support roller 22 and other devices. What is necessary is just to determine by balance with a structure. The third support roller 24 may be moved simultaneously with the first support roller 21 and the second support roller 22 by switching between the color mode and the full separation mode, or may be before or after the roller, and is not particularly limited. Further, switching between the monochrome mode and the all separation mode may be performed simultaneously with the second support roller 22 or before or after the roller, and is not particularly limited. If both move at the same time, the waiting time will be shortened.

  In this embodiment, the first support roller 21, the second support roller 22, and the third support roller 24 are moved by using one drive motor (not shown), and a drive gear 61 (see FIG. 4 described later). The drive gear 61 is connected to the three support rollers by a gear and a belt (not shown). Further, the gear structure is such that the drive gear 61 transmits the drive to the second support roller 22 and the third support roller 24 after the drive gear 61 rotates 90 ° clockwise. The initial state is the color mode, and the first support roller 21 is moved downward by rotating the drive gear 61 by 90 ° clockwise by switching from the color mode to the monochrome mode.

  Next, by switching from the monochrome mode to the full separation mode, the drive gear 61 is further rotated 90 ° clockwise (a total rotation of 180 °) to move the second support roller and the third support roller downward. The drive for moving the first support roller 21, the second support roller 22, and the third support roller 24 is not limited to the present embodiment. For example, three drives corresponding to each support roller are used. A motor can be provided and moved by independent driving. In addition, two drive motors are provided, and the first support roller 21 is moved by one drive motor when switching to the monochrome mode, and the second support roller 22 and the third support are supported by the other drive motor when switching to the full separation mode. The roller 24 can also be moved.

  The tension roller 23 is not particularly limited as long as it can adjust the tension of the intermediate transfer belt 8 when the mode is switched by a biasing member made of a conventionally known elastic body such as a spring.

  For example, the transfer rollers 6a to 6d are moved by a biasing member made of an elastic body such as a spring, and the rollers move according to the tension of the intermediate transfer belt 8. Can be done using technology.

  The selective switching between the color mode, the monochrome mode, and the total separation mode can be performed by using a conventionally known technique. The initial state may be any mode. Further, by switching the mode contrary to the above, it is possible to change from the fully separated mode to the monochrome mode (the state shown in FIGS. 3 to 2), and from the fully separated mode to the color mode, from the fully separated mode. The color mode (the state shown in FIGS. 3 to 2 and the state shown in FIG. 1) can be set via the monochrome mode. In this embodiment, for convenience of explanation, the switching from the color mode to the all-separation mode is performed through the monochrome mode, but the first support roller 21, the second support roller 22, and the third support roller 24 are moved. The direct separation mode can be set directly or vice versa.

  As described above, by switching to the all separation mode, the intermediate transfer belt 8 is separated from all the photoconductors 1 a to 1 d, and the transfer unit 50 is formed without contacting the photoconductors 1 a to 1 d and the intermediate transfer belt 8. It can be detached from the apparatus 100. Therefore, it is possible to suppress damages on the surfaces of the photoreceptors 1a to 1d and the surface of the intermediate transfer belt 8 when the transfer unit 50 is attached and detached, and to obtain excellent maintainability. Also, the photoconductors 1a to 1d can be easily detached.

  As described above, according to the first embodiment, the color photoreceptors 1a to 1c have substantially the same nip portion with the intermediate transfer belt 8, and the contact state between the intermediate transfer belt 8 and the black photoreceptor 1d is constant. It is possible to transfer while switching between the color mode and the monochrome mode while maintaining. In addition, the intermediate transfer belt 8 can be separated from all the photoreceptors 1a to 1d. Therefore, it is possible to reduce the size of the image forming apparatus with a simpler configuration than before, and to have a stable transfer state and excellent maintainability regardless of the mode.

  In the first embodiment, an intermediate transfer belt separation detection unit that detects that the intermediate transfer belt 8 is separated from all the photoconductors 1a to 1d by switching to the all separation mode can be further provided. According to this aspect, it can be used as a material for determining whether or not the transfer unit 50 can be detached from the image forming apparatus 100, and damage to the surfaces of the photoreceptors 1a to 1d and the intermediate transfer belt 8 can be avoided more reliably. Further, based on the detection result, a transfer unit lock that makes it impossible to attach or detach the transfer unit 50 when it is in contact with any one or more photoconductors is further provided, and the intermediate transfer belt 8 includes all the photoconductors 1a. The transfer unit lock release control can also be performed when it is separated from ˜1d. Such control makes it possible to more reliably avoid damage to the surfaces of the intermediate transfer belt 8 and the photoconductors 1a to 1d when the transfer unit 50 is detached.

  Accordingly, an example of transfer unit lock release control in the first embodiment will be described. 4A and 4C are enlarged sectional views around the intermediate transfer belt separation detection unit, and FIGS. 4B and 4D are views around the intermediate transfer belt separation detection unit shown in FIG. It is the side view seen from the arrow C direction of a). Portions common to FIGS. 1 to 3 and 11 are denoted by the same reference numerals, and description thereof is omitted.

  As described above, the drive gear 61 is provided in one drive motor (not shown), and is connected to each support roller by a gear and a belt. A flag 25 is disposed on the side surface of the drive gear 61. A unit 26 that detects the separation of the intermediate transfer belt 8 is fixedly disposed below the drive gear 61. The separation detection unit 26 includes a separation detection sensor 27 of the intermediate transfer belt 8 and a reflection facing the sensor 27. And a plate 28. Further, when the flag 25 moves to the 6 o'clock position of the timepiece by the rotation of the driving gear 61, the flag 25 is inserted into the gap between the separation detection sensor 27 and the reflection plate 28.

  The flag 25 only needs to be capable of blocking the light emitted from the separation detection sensor 27, and a conventionally known member can be appropriately designed and disposed. Further, the separation detection sensor 27 irradiates the reflection plate 28 with measurement light, and detects the amount of reflected light from the reflected light 28. A detection result is transmitted to the control part 32 mentioned later as a light reception output signal. As the separation detection sensor 27, generally, an optical sensor including a light emitting element made of an LED or the like and a light receiving element made of a photodiode or the like is used. When detecting the separation, when the measurement light is emitted from the separation detection sensor 27 to the reflection plate 28, the measurement light is incident on the light receiving element by the light reflected by the reflection plate.

  In the color mode in which the first support roller 21, the second support roller 22, and the third support roller 24 are moved upward to bring the intermediate transfer belt 8 into contact with the photoreceptors 1a to 1d, the flag 25 is set at 12 o'clock of the watch. Arranged in position. By switching from the color mode to the monochrome mode, the drive gear 61 moves 90 ° clockwise, and the flag 25 moves to the 3 o'clock position (FIGS. 4A and 4B). As described above, in the color mode and the monochrome mode, the flag 25 does not exist in the gap between the separation detection sensor 27 and the reflection plate 28, so that the light reception amount of the light receiving element in the separation detection sensor 27 is large. Then, by switching to the full separation mode, the flag 25 further moves 90 ° clockwise and moves to the 6 o'clock position (FIGS. 4C and 4D). In this case, since the flag 25 is inserted into the gap between the separation detection sensor 27 and the reflection plate 28, the amount of light received by the light receiving element at the separation detection sensor is reduced by light shielding. Therefore, it is possible to detect whether the intermediate transfer belt 8 is in contact with or separated from the photoreceptors 1a to 1d based on the output value of the received light signal based on the received light quantity. As described above, detailed detection can be performed by detecting the movement of the first support roller 21, the second support roller 22, and the third support roller 24 as the separation detection means.

  The same flag and separation detection unit as those described above are directly provided for the gears that transmit the drive from the drive gear, which are arranged on the first support roller 21, the second support roller 22, and the third support roller 24, respectively. It is also possible to detect whether the support roller is moving in the direction in which the intermediate transfer belt 8 is brought into contact with the black photosensitive member 1d or in the direction in which it is separated. With such a configuration, further detailed separation detection can be performed, and malfunction of each support roller can be detected.

  FIG. 5 is an enlarged cross-sectional view of the transfer unit lock. The transfer unit lock 51 is provided with a detachment preventing member 52, and the detachment preventing member 52 protrudes from the housing of the transfer unit 50 in the color mode and the monochrome mode so that the transfer unit 50 cannot be detached. On the other hand, by switching to the full separation mode, the attachment / detachment prevention member 52 is stored in the housing in accordance with a control signal described later, the transfer unit lock 51 is released, and the transfer unit 50 can be attached / detached. As the desorption prevention member 52, a conventionally known flag, pin, or the like can be used. In addition, conventionally known techniques can be used as long as the transfer unit 50 can be switched according to the control signal.

  FIG. 6 is a block diagram showing a control path of the image forming apparatus of the present invention. Portions common to FIGS. 1 to 5 and 11 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, a fixing unit 7, an intermediate transfer belt 8, a secondary transfer roller 9, a first support roller 21, a second support roller 22, a third support roller 24, and an intermediate transfer belt separation. The configuration includes a detection unit 26, an image input unit 30, an AD conversion unit 31, a control unit 32, a storage unit 33, an operation panel 34, a transfer unit lock 51, and the like.

  When the image forming apparatus 100 is a color copying machine, the image input unit 30 includes a scanning lamp mounted with a scanner lamp that illuminates the original during copying and a mirror that changes the optical path of reflected light from the original, and reflection from the original. 11 is an image reading unit including a condensing lens that collects light and forms an image, and a CCD that converts the formed image light into an electric signal. In the case of a printer, the receiving unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 30 is converted into a digital signal by the AD conversion unit 31 and then sent to the image memory 40 in the storage unit 33.

  The storage unit 33 includes an image memory 40, a RAM 41, and a ROM 42. The image memory 40 stores an image signal input from the image input unit 30 and digitally converted by the AD conversion unit 31, and is stored in the control unit 32. Send it out. The RAM 41 and the ROM 42 store a processing program, processing content, and the like of the control unit 32. The RAM 41 (or ROM 42) stores the relationship between the output value of the separation detection sensor 27 and the separation distance between the intermediate transfer belt 8 and the photoreceptors 1a to 1d, and the transfer unit lock according to the output value. A control program for canceling is also stored.

  The operation panel 34 includes an operation unit composed of a plurality of operation keys, and a display unit (none of which is shown) for displaying various setting conditions, apparatus states, and the like. For example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 33 and reading or rewriting the registered transmission destination.

  The control unit 32 is, for example, a central processing unit (CPU), and in accordance with a set program, the image input unit 30, the image forming units Pa to Pd, the fixing unit 7, the first support roller 21, the second support roller 22, and the third The overall control of the support roller 24, the intermediate transfer belt separation detection sensor 27, the transfer unit lock 51, the conveyance of the sheet P from the sheet cassette 16 (see FIG. 11), and the like, and the image signal read by the image input unit 30 Are converted into image data by scaling processing or gradation processing as necessary. The exposure units 4a to 4d irradiate laser light based on the processed image data to form latent images on the photosensitive drums 1a to 1d.

  Further, when the all separation mode is input by operating the key on the operation panel 34, the control unit 32 receives the light reception signal detected by the separation detection sensor 27, and based on the light reception amount data stored in the storage unit 33. It has a function of releasing the transfer unit lock 51.

  Next, transfer unit lock release control in the image forming apparatus according to the first embodiment will be described. FIG. 7 is a flowchart illustrating a transfer unit lock release control procedure of the image forming apparatus according to the first embodiment. The release control procedure will be described in accordance with the steps in FIG. 7 with reference to FIGS.

  First, when the all separation mode is set by an input operation from the operation panel 34 by the user (step S1), the received light amount is detected by the separation detection sensor 27 and output to the control unit 32 (step S2). Next, the controller 32 detects whether or not the separation detection sensor 27 is shielded from light by the flag 25 (step S3). When it is confirmed that the shield is shielded, the transfer unit lock 51 is released and the transfer unit 50 is moved. It can be detached from the image forming apparatus 100 (step S4). On the other hand, when it is detected that the light is not shielded, the transfer unit lock 51 is not released, and the transfer unit cannot be detached. In this case, of the first support roller 21, the second support roller 22, or the third support roller 24, the support roller that is not shielded from light is moved (step 5), and when the transfer unit lock 51 is released, the process ends.

  According to the above procedure, the transfer unit 50 cannot be detached when the intermediate transfer belt 8 is in contact with the photoreceptors 1a to 1d, and the transfer unit 50 can be detached only when they are separated from each other. Therefore, the transfer unit 50 can be detached and attached without damage to the surfaces of the photoreceptors 1a to 1d or the intermediate transfer belt 8.

  Note that a detector and a unit lock release control mechanism similar to the separation detector 27 and the transfer unit lock 51 are provided in the image apparatus 100, and whether or not the transfer unit 50 is attached to the image forming apparatus 100 is determined by the transfer unit 50. It can also be controlled by the presence or absence of light shielding.

  FIGS. 8, 9 and 10 are schematic views showing the configuration around the image forming unit and the intermediate transfer belt of the tandem color image forming apparatus according to the second embodiment of the present invention. Parts common to FIGS. 1 to 3 and FIG.

  In the present embodiment, the black photoreceptor 1d has a nip portion with the intermediate transfer belt 8 from a plane through which the nip portions X to Z of the color photoreceptors 1a to 1c arranged horizontally on substantially the same plane. Is also disposed below (on the second support roller 22 side). In the color mode and the monochrome mode, the second support roller 22 is disposed so as to be movable and rotatable as will be described later. Except for this configuration, the configuration is the same as that of the first embodiment, and a description thereof will be omitted.

  A nip portion between the black photosensitive member 1d and the intermediate transfer belt 8 is disposed closer to the second support roller side 22 than the plane through which the lines X to Z pass as in the present embodiment. Even if the moving distance of the support roller 21 and the second support roller 22 is further reduced, the separation distance of the intermediate transfer belt 8 from the photoreceptors 1a to 1c can be increased, and the separation of the intermediate transfer belt 8 can be made easier. In addition, the apparatus can be made smaller. The arrangement of the nip portion is not particularly limited as long as the intermediate transfer belt 8 can contact and separate from the color photoconductors 1a to 1c. However, if the distance is too close to the second support roller side, the moving distance of the first support roller 21 and the second support roller 22 can be shortened, but the apparatus tends to be large and complicated. The travel distance increases. Therefore, for example, the determination may be made in consideration of these factors.

  The nip portions of the second support roller 22 and the third support roller 24 with the intermediate transfer belt 8 are A and B, respectively. The second support roller 22 moves in a direction parallel to the circumferential direction of the intermediate transfer belt 8 passing between the roller and the black photoreceptor 1d (FIGS. 8 and 9). That is, the second support roller 22 moves on a plane formed by the nip portions A and B. The plane formed by the nip portions A and B is not changed by mode switching.

  In the color mode, as shown in FIG. 8, along with the upward movement of the first support roller 21, the second support roller 22 moves in the upper left direction until A is substantially flush with X to Z. The intermediate transfer belt 8 is in contact with the photoreceptors 1a to 1c. As a result, the intermediate transfer belt 8 becomes horizontal from the first support roller 21 to the second support roller 22 and comes into contact with the photoconductors 1a to 1c, and then comes into contact with the black photoconductor 1d to perform primary transfer by the transfer rollers 6a to 6d. Move clockwise while receiving.

  On the other hand, in the monochrome mode, as shown in FIG. 9, the second support roller 22 is formed at the nip portions A and B as the first support roller 21 moves downward by switching the mode from the color mode. The intermediate transfer belt 8 which moves in the lower right direction (the state shown in FIGS. 8 to 9) on the same plane as the plane and is separated is parallel to the above X to Z between the first support roller 21 and the second support roller 22. It becomes. As a result, the intermediate transfer belt 8 contacts only with the photoreceptor 1d, and moves clockwise while receiving primary transfer by the transfer roller 6d.

  In the present embodiment, switching from the color mode and the monochrome mode to the full separation mode (states in FIGS. 8 to 10 and FIGS. 9 to 10) is exactly the same as in the first embodiment, and the description thereof is omitted. Further, a flag 25 and a separation detection sensor 27 are provided for the drive gear 61, and it is detected that all the support rollers have moved in a direction in which the intermediate transfer belt 8 is separated from the photoreceptors 1a to 1d, and the transfer unit is locked. As for the control for releasing the driving force, the second support roller 22 moves in the lower right direction by rotating the driving gear 61 by 90 ° from the initial state, and moves downward by moving further by 90 °. Except for the above, the configuration is exactly the same as in the first embodiment, and a description thereof will be omitted.

  According to the present embodiment, by moving the second support roller 22, even if the moving distance of the first support roller 21 is further reduced, the separation distance of the intermediate transfer belt 8 can be increased. The transfer belt 8 can be further easily separated from the color photoreceptors 1a to 1c, and the apparatus can be further miniaturized. In addition, since the nip portion of the black photoreceptor 1d with the intermediate transfer belt 8 is disposed on the second support roller 22 side with respect to the plane passing through X to Z, even if the moving distance of both the rollers is further reduced, The separation distance of the intermediate transfer belt 8 can be increased, the separation of the intermediate transfer belt 8 is further facilitated, and the apparatus can be further miniaturized.

  In the first embodiment and the second embodiment, the intermediate transfer that transfers the full color image formed by sequentially stacking the toner images of the respective colors onto the paper P at a time on the intermediate transfer belt 8 which is an example of the toner carrier. Although the tandem type color image forming apparatus of the type has been described, the present invention is also applied to the tandem type color image forming apparatus of the direct transfer type that sequentially transfers the toner images of each color onto the paper P carried and conveyed on the conveyance belt. It is equally applicable.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the configuration of the developing device shown in the above embodiment is merely an example, and the secondary transfer roller 9 may be arranged at a position in contact with the tension roller 23 via the intermediate transfer belt 8. .

  In each of the above embodiments, the black photoconductor 1d is the most downstream side, but the black photoconductor 1d may be the most upstream side. For example, each photoconductor can be arranged in the order of a black photoconductor 1d, a yellow photoconductor 1c, a magenta photoconductor 1b, and a cyan photoconductor 1a. In this case, along the inner peripheral surface of the intermediate transfer belt or the conveyance belt, the first support roller 21 is disposed further downstream of the most downstream cyan photoreceptor 1a, the most upstream black photoreceptor 1d and the photoreceptor. A second support roller 22 is disposed between the closest yellow photoreceptor 1c, and an idle roller 24 is disposed further upstream of the black photoreceptor 1d and between the photoreceptor 1d and the driving roller 11. The tension roller 23 can be disposed further downstream of the support roller 21, and the driven roller 10 can be disposed upstream of the drive roller 11 and downstream of the tension roller 23.

    The present invention includes a plurality of image forming units including a photosensitive member and a developing device, an endless intermediate transfer belt on which the toner images developed on the photosensitive member are sequentially stacked, or a conveying belt for conveying a recording medium, Means for transferring the toner image developed on the photoconductor onto the intermediate transfer belt or the conveyance belt, wherein the photoconductor is relative to the rotation direction of the intermediate transfer belt or the conveyance belt. A black photoconductor disposed on the most upstream side or the most downstream side, on which a black toner image on which a black toner image is formed, and the intermediate transfer belt or the conveyance belt are arranged on a substantially same plane. And one or more color photoreceptors on which toner images of other colors are formed, disposed on the opposite side of the black photoreceptor with the color photoreceptor interposed therebetween, and the intermediate transfer belt or the conveyance bell A first support roller movable to contact and separate the color photosensitive member, and a second support roller disposed between the black photosensitive member and the color photosensitive member adjacent to the black photosensitive member. And a third support roller disposed on the opposite side of the color photoconductor across the black photoconductor.

  As a result, a stable color image transfer state in the color mode and a stable black image transfer state in the color and monochrome modes can be obtained with a simpler configuration and a transfer unit with a smaller device than the conventional one. It is possible to avoid damage to the photosensitive member and the intermediate transfer belt when the toner is removed.

  In addition, a means for detecting the separation of the intermediate transfer belt or the conveyance belt from the photosensitive member is provided, and the damage is more reliably controlled by controlling the attachment / detachment of the transfer unit from the image forming apparatus based on the detection result of the detection means. Can be avoided. Further, the arrangement of the black photoconductor is arranged on the second support roller side from the nip portion between the color photoconductor and the intermediate transfer belt or the conveyance belt, and the second support roller is moved together with the first support roller, thereby Further downsizing of the apparatus is possible.

FIG. 2 is a side cross-sectional view showing a configuration around an image forming unit and an intermediate transfer belt in a color mode of the tandem color image forming apparatus according to the first embodiment. FIG. 3 is a side cross-sectional view illustrating the configuration of the periphery of the image forming unit and the intermediate transfer belt in the monochrome mode of the tandem color image forming apparatus according to the first embodiment. FIG. 3 is a side cross-sectional view illustrating the configuration of the periphery of the image forming unit and the intermediate transfer belt in the full separation mode of the tandem type color image forming apparatus according to the first embodiment. FIG. 4 is an enlarged view (FIGS. 4A and 4C) of an intermediate transfer belt separation detection unit used in the image forming apparatus of the first embodiment, and an arrangement relationship between the intermediate transfer belt separation detection unit and the drive gear. It is a side view (Drawing 4 (b) and 4 (d)) shown. FIG. 2 is an enlarged view of a transfer unit lock used in the image forming apparatus according to the first embodiment. FIG. 3 is a block diagram showing a control path of the color image forming apparatus of the first embodiment. FIG. 5 is a flowchart illustrating transfer unit lock release control in the image forming apparatus of the first embodiment. FIG. 5 is a side cross-sectional view illustrating a configuration around an image forming unit and an intermediate transfer belt in a color mode of a tandem color image forming apparatus according to a second embodiment. FIG. 5 is a side cross-sectional view illustrating a configuration around an image forming unit and an intermediate transfer belt in a monochrome mode of a tandem color image forming apparatus according to a second embodiment. FIG. 5 is a side cross-sectional view illustrating a configuration around an image forming unit and an intermediate transfer belt in a fully separated mode of a tandem color image forming apparatus according to a second embodiment. FIG. 1 is a schematic diagram illustrating an overall configuration of a conventional image forming apparatus.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum 2a to 2d Charger 3a to 3d Developing unit (developing device)
4a to 4d Exposure unit 6a to 6d Transfer roller (transfer means)
7 Fixing Section 8 Intermediate Transfer Belt 9 Secondary Transfer Roller (Secondary Transfer Unit)
DESCRIPTION OF SYMBOLS 10 Follower roller 11 Drive roller 21 1st support roller 22 2nd support roller 23 Tension roller 24 3rd support roller 25 Flag 26 Intermediate transfer belt separation detection unit 27 Intermediate transfer belt separation detection sensor 28 Reflector 50 Transfer unit 51 Transfer unit lock 52 Desorption prevention member 61 Drive gear X to Z Nip portion between photosensitive drum and intermediate transfer belt A Nip portion between second support roller and intermediate transfer belt B Nip portion between third support roller and intermediate transfer belt

Claims (5)

A plurality of image forming units including a photoconductor and a developing device; an endless intermediate transfer belt on which the toner images developed on the photoconductor are sequentially stacked; and a transfer roller for transferring the toner image developed on the photoconductor An image forming apparatus comprising: transfer means for laminating on the intermediate transfer belt, and secondary transfer means for transferring a toner image laminated on the intermediate transfer belt onto a recording medium at once.
The photoreceptor has a nip portion between the intermediate transfer belt and a black photoreceptor on which a black toner image disposed on the most upstream side or the most downstream side with respect to the rotation direction of the intermediate transfer belt is formed. And one or more color photoreceptors on which toner images of colors other than black are formed, which are arranged on the same plane,
A first support roller disposed on the opposite side of the black photoconductor across the color photoconductor, and movable to contact and separate the intermediate transfer belt and the color photoconductor;
A second support roller disposed between the black photoreceptor and a color photoreceptor immediately adjacent to the black photoreceptor;
A third support roller disposed on the opposite side of the color photoconductor across the black photoconductor,
A nip portion between the black photosensitive member and the intermediate transfer belt is disposed closer to the second support roller than a plane passing through the nip portion between the color photosensitive member and the intermediate transfer belt;
The second support roller is movable in a direction parallel to the circumferential direction of the intermediate transfer belt passing between the second support roller and the black photosensitive member in selective switching between the monochrome mode and the color mode. an image forming apparatus wherein there. A plurality of image forming units including a photoconductor and a developing device, an endless conveying belt that conveys a recording medium to the image forming unit, and a toner image developed on the photoconductor on a recording medium or by the transfer roller In an image forming apparatus comprising a transfer means for transferring onto a conveyor belt,
In the photoconductor, the nip portion between the black belt on which the black toner image disposed on the most upstream side or the most downstream side with respect to the rotation direction of the transport belt is formed and the transport belt is substantially flush. Comprising one or more color photoreceptors disposed on top to form a non-black toner image;
A first support roller disposed on the opposite side of the black photoconductor across the color photoconductor and movable to contact and separate the transport belt and the color photoconductor;
A second support roller disposed between the black photoreceptor and a color photoreceptor immediately adjacent to the black photoreceptor;
A third support roller disposed on the opposite side of the color photoconductor across the black photoconductor ,
A nip portion between the black photosensitive member and the transport belt is disposed closer to the second support roller than a plane passing through the nip portion between the color photosensitive member and the transport belt;
The second support roller is movable in a direction parallel to a circumferential direction of the transport belt passing between the second support roller and the black photosensitive member in selective switching between the monochrome mode and the color mode. An image forming apparatus.   The image forming apparatus according to claim 1, further comprising a detection unit configured to detect that the intermediate transfer belt or the conveyance belt is separated from the photosensitive member.   The image forming apparatus according to claim 3, wherein the detecting unit detects the movement of at least one of the first support roller, the second support roller, and the third support roller.   4. A control unit for controlling the transfer unit including the intermediate transfer belt or the conveyance belt and the transfer roller to be detachable based on a detection result of the detection unit. 5. The image forming apparatus according to 4.

Images