JP2008209587A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a miniaturized image forming apparatus capable of switching between a color mode and a monochrome mode by making an intermediate transfer belt or a conveying belt contact with a color photoreceptor and separate from the color photoreceptor with much simpler constitution than before. <P>SOLUTION: The photoreceptor drums 1a to 1c on which a color image is formed are disposed on the upstream side of the photoreceptor drum 1d on which a black image is formed. A variable diameter roller 21 formed so that the outside diameter of its part inscribed with the intermediate transfer belt 8 may be varied is disposed between the photoreceptor drums 1d and 1c. Then, a driving roller 11 is disposed on the upstream side of the color photoreceptor drum 1a and a tension roller 23 is disposed on the downstream side of the black photoreceptor 1d. <P>COPYRIGHT: (C)2008,JPO&INPIT

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.

  FIG. 9 shows a schematic diagram of a conventional image forming apparatus. 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 transferred toner images are sequentially transferred (primary transfer) onto the intermediate transfer belt 8 that moves adjacent to each image forming portion while rotating clockwise in FIG. 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 dielectric resin is used for the intermediate transfer belt 8, and a belt in which the flange portions are overlapped and joined to each other 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.

  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 transferred, 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 transferred, and rotates. become. 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.

  Therefore, a technique has been proposed in which the intermediate transfer belt is brought into contact with all the photoconductors in the color mode and separated from the photoconductor for forming a color image (hereinafter referred to as “color photoconductor”) in the monochrome mode. 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.

For this reason, for example, Patent Document 3 discloses an image forming apparatus including a black photoconductor, a color photoconductor, an intermediate transfer belt, and a transfer roller in contact with each photoconductor through the intermediate transfer belt. A retract roll disposed at any position between the adjacent color photoreceptor and another color photoreceptor is provided, and the intermediate transfer belt and the color photoreceptor are brought into contact with and separated from each other by moving the retract roll. A technology is disclosed, and by this technology, 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, in order to move the retract roll up and down, a drive source, a gear and belt for driving transmission, an electric system, and the like are necessary, and the configuration becomes complicated, and it is difficult to reduce the size of the apparatus.

  In view of the above problems, the present invention can switch between a color mode and a monochrome mode by bringing a color photoconductor and an intermediate transfer belt or a conveyance belt into contact with and separating from each other with a simpler configuration than before. An object of the present invention is to provide a downsized image forming apparatus.

  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 toner image developed on the photoconductor In the image forming apparatus, comprising: a transfer unit that is stacked on a transfer belt; and a secondary transfer unit that transfers a toner image stacked on the intermediate transfer belt onto a recording medium at a time. A black photoreceptor on which the black toner image is disposed on the most upstream side or the most downstream side with respect to the rotation direction of the intermediate transfer belt, and one or more colors on which a toner image of a color other than black is formed The intermediate transfer belt is stretched around at least a driving roller, a driven roller, and a variable diameter roller in which an outer diameter of a portion inscribed in the intermediate transfer belt changes. To do.

  The present invention also provides a plurality of image forming units including a photoconductor and a developing device, an endless conveyance belt that conveys the recording medium to the image forming unit, and a toner image developed on the photoconductor as a recording medium. An image forming apparatus comprising: a transfer unit configured to transfer the image on the conveying belt; or a black toner disposed on the most upstream side or the most downstream side with respect to a rotation direction of the conveying belt. A black photosensitive member on which an image is formed and one or more color photosensitive members on which a toner image of a color other than black is formed. The conveying belt includes at least a driving roller, a driven roller, and the conveying belt. It is stretched around a variable diameter roller in which the outer diameter of the inscribed portion changes.

  Further, according to the present invention, the variable diameter roller has a cylindrical roller body and a flange portion that is disposed at both ends of the roller body and has a tapered outer peripheral surface that faces outward in the axial direction and is movable in the axial direction. The intermediate transfer belt or the conveyance belt is in contact with the flange in the color mode in contact with the color photoconductor, and in contact with the roller body in the monochrome mode in which the intermediate transfer belt is separated from the color photoconductor.

  Further, in the present invention, the flange portion includes a cavity portion that is inclined outward in the axial direction along the outer peripheral surface, and a weight member that is inserted into the cavity portion and movable within the cavity portion, The flange portion moves in the axial direction as the weight member moves due to a difference in belt speed between the color mode and the monochrome mode.

  In the invention, it is preferable that the variable diameter roller is disposed between the black photosensitive member and a color photosensitive member adjacent to the black photosensitive member.

  According to the first configuration of the present invention, it is not necessary to provide complicated members and mechanisms for contacting and separating the intermediate transfer belt and the color photoconductor from the photoconductor in the apparatus. The color mode and the monochrome mode can be switched. In addition, the apparatus can be miniaturized.

  According to the second configuration of the present invention, it is not necessary to provide a complicated member and mechanism for contacting and separating the conveyance belt and the color photoconductor from the photoconductor in the apparatus. It is possible to switch between the color mode and the monochrome mode. In addition, the apparatus can be miniaturized.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the contact portion of the variable diameter roller with respect to the intermediate transfer belt or the conveyance belt is changed to change the color photosensitive belt and the belt. Contact with and separation from the body is possible. For this reason, it is possible to switch between the color mode and the monochrome mode with a simpler configuration. In addition, the apparatus can be further downsized.

  According to the fourth configuration of the present invention, in the image forming apparatus having the third configuration, the intermediate transfer belt or the conveyance belt and the color photoconductor can be brought into contact with and separated from each other by using a mechanism that is originally provided in the apparatus. it can. Further, it is not necessary to provide a drive source or drive transmission means for changing the outer diameter of the variable diameter roller. Therefore, the color mode and the monochrome mode can be switched with a simpler configuration. In addition, the apparatus can be further downsized.

  According to the fifth configuration of the present invention, in the image forming apparatus having any one of the first to fourth configurations, the nip portion between each color photoconductor and the intermediate transfer belt or the conveyance belt is substantially on the same plane. Since the optical path length from the exposure unit to the color photoconductor can be made constant, it is not necessary to use a complicated mechanism to improve the image forming accuracy. In addition, waste of space can be reduced from the viewpoint of the arrangement of the photosensitive members and the arrangement of variable diameter rollers. Therefore, it is possible to simultaneously achieve further downsizing of the apparatus and improvement of image forming accuracy.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 and FIG. 2 are schematic views showing the configuration of the periphery of the image forming unit and the 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, and FIG. The configuration of the mode is shown. Portions common to those in FIG. 9 are denoted by the same reference numerals and description thereof is omitted.

  From the upstream side, a photosensitive drum 1a on which a cyan image is formed, a photosensitive drum 1b on which a magenta image is formed, and a photosensitive drum 1c on which a yellow image is formed. A photosensitive drum 1d on which an image is formed is rotatably arranged. Further, the nip portion between each of the photosensitive drums 1a to 1c and the intermediate transfer belt 8 is on substantially the same plane.

  Further, along the inner peripheral surface of the intermediate transfer belt 8, a driving roller 11 is provided upstream of the most upstream cyan photoreceptor drum 1a, and a tension roller 23 is provided downstream of the most downstream black photoreceptor 1d. A driven roller 10 is rotatably arranged on the downstream side of the roller 23 and the upstream side of the driving roller. A variable diameter roller 21 is rotatably disposed between the black photosensitive drum 1d on the most downstream side and the yellow photosensitive drum 1c closest to the upstream side of the photosensitive drum 1d.

  The intermediate transfer belt 8 is driven by a driving roller 11 so that the cyan photosensitive drum 1a, the magenta photosensitive drum 1b, the yellow photosensitive drum 1c, the diameter variable roller 21, the black photosensitive drum 1d, the tension roller 23, and the driven roller 10 are driven. Next, they pass through in a counterclockwise direction (in the direction of the arrow) in close proximity or contact. The intermediate transfer belt 8 is stretched by a variable diameter roller 21, a tension roller 23, a driven roller 10, and a driving roller 11.

  3 and 4 are side views showing the positional relationship between the variable diameter roller used in the image forming apparatus of the first embodiment and the intermediate transfer belt in the color mode and the monochrome mode.

  As shown in FIGS. 3 and 4, the variable diameter roller 21 includes a rotating shaft 22 of the variable diameter roller 21, a roller body 23, flange portions 24 and 24 ′, and a weight member 25 (see FIGS. 5 and 6). And a cavity 26 (see FIGS. 5 and 6). The flange portions 24 and 24 'are urged by urging members 27 and 27' from the outside in the axial direction to the inside.

  Hereinafter, the variable diameter roller 21 will be described. Portions common to FIGS. 1, 2 and 9 are denoted by the same reference numerals and description thereof is omitted. Since the flange portions 24 and 24 'have the same configuration, only the flange portion 24 will be described. Since the urging members 27 and 27 'have the same configuration, only the urging member 27 will be described.

  The roller body 23 has a cylindrical shape and is fitted or integrally formed with the rotating shaft 22 and can contact the intermediate transfer belt 8 in the monochrome mode and support the rotation of the belt as shown in FIG. .

  The flange portion 24 has a shape having a tapered outer peripheral surface in the axial direction so that the outer diameter is larger than that of the roller body 23, and is fitted so as to rotate together with the roller body 23 and the rotating shaft 22. The outer side in the axial direction of the flange portion 24 is formed in a trapezoidal cone shape. As shown in FIG. 3, in the color mode, the outermost portion having the largest outer diameter is inscribed in the intermediate transfer belt 8 to rotate the belt. Can be supported. On the other hand, the roller body 23 side of the flange portion 24 has a cylindrical shape, and can contact the intermediate transfer belt 8 together with the roller body 23 and support the rotation of the belt 8 in the monochrome mode, as shown in FIG. it can. A chamfered portion 24a (and 24'a) is formed on the axially outer portion of the flange portion 24 so that the contact area with the intermediate transfer belt 8 can be increased and the rotation of the belt can be easily supported.

  In this way, the outer diameter of the flange portion 24 is made larger than that of the roller main body 23, and the intermediate transfer belt 8 is brought into contact with the flange main body 24 in the color mode and in contact with the roller main body 23 in the monochrome mode. By changing the outer diameter of the diameter variable roller 21 in contact, the intermediate transfer belt 8 and the color photosensitive drums 1a to 1c can be brought into contact with and separated from each other. The flange portion 24 of the variable diameter roller 21 only needs to have an outer diameter larger than that of the roller body 23, and is not limited to a trapezoidal cone as in the present embodiment. The movement of the belt 8 from the roller body 23 to the flange portion 24 can be made smooth and easy. Further, by providing a cylindrical portion on the roller body 23 side of the flange portion 24, the contact area between the intermediate transfer belt 8 and the diameter variable roller 21 can be increased in the monochrome mode, and the rotation of the intermediate transfer belt 8 can be performed. It can be stabilized.

  FIG. 5 is an enlarged sectional view (FIG. 5A) in the axial direction in the color mode around one flange portion of the variable diameter roller used in the image forming apparatus of the first embodiment, and FIG. FIG. 5B is an enlarged cross-sectional view perpendicular to the axial direction in A of FIG. Portions common to FIGS. 1 to 4 and 9 are denoted by the same reference numerals and description thereof is omitted. Inside the flange portion 24, a tapered cavity portion 26 is provided along the outer peripheral surface toward the outside in the axial direction. As shown in FIGS. 5A and 5B, the cavity 26 has two trapezoidal cross sections in the axial direction, and the cross section perpendicular to the axis is an annular shape. The inner diameter and the outer diameter are formed to increase toward the outer side in the direction. The hollow portion 26 may be a space in which a weight member 25 described later can be inserted so that the weight member 25 can be moved inward and outward in the axial direction. It is not limited. However, in order to move the weight member 25 to the outside, the cavity 26 needs to be inclined so as to be away from the shaft from the axial roller body 23 side toward the outside.

  The weight member 25 is inserted into the hollow portion 26 so as to be movable around the rotation shaft 22 and inward and outward in the axial direction. The rotation of the variable diameter roller 21 causes a force in the centrifugal direction around the central shaft 22 and the outward direction of the shaft to act on the weight member 25. The weight member 25 moves to the outside of the cavity portion 26 by the outward force, that is, inertia force. As the rotational speed of the diameter variable roller 21 increases, the inertial force also increases. When the outermost portion of the cavity portion 26 is reached, the flange portion 24 is further moved outward in the axial direction. When the inertial force of the weight member 25 becomes larger than the urging force of the urging member 27 that urges the flange portion 24 from the outside in the axial direction to the inside, the flange portion 24 moves outward in the axial direction.

  As the weight member 25, a conventionally known member can be used as long as it has a mass that allows sufficient inertial force to act, and examples thereof include stainless steel balls. In addition to a spherical shape, the shape may be an elliptic rotating body, a cube or a rectangular parallelepiped, a polyhedron such as a tetrahedron, a pentahedron, or a hexahedron. By using a spherical shape as in the present embodiment, friction between the weight member 25 and the cavity 26 can be reduced. One or more weight members 25 need only be provided, and are not particularly limited. However, if there are many weight members 25, a larger inertial force can be easily applied, but the weight of the variable diameter roller 21 is increased due to its weight. May affect rotation. Therefore, for example, the determination may be made in consideration of these factors.

  As the urging member 27, the flange portion 24 is brought into contact with the roller body 23 when the variable diameter roller 21 stops rotating and at a low rotation speed in the color mode described later. On the other hand, in the monochrome mode, by the inertia force of the weight member 25. Although it will not specifically limit if the urging | biasing which moves the flange part 24 to the outer side can be provided to the flange part 24, For example, elastic bodies, such as a conventionally well-known spring, can be used.

  Usually, the image forming speed is determined by the moving speed of the intermediate transfer belt 8. The rotational speed of the diameter variable roller 21 that is in contact with the intermediate transfer belt 8 is determined by the moving speed of the intermediate transfer belt 8. Therefore, if the image forming speed is high, the weight member 25 can apply a large inertial force to the flange portion 24.

  Here, in the color mode, transfer of cyan, magenta, yellow and black images is executed, whereas in the monochrome mode, only transfer of black images is executed. The color mode requires image formation accuracy, whereas the monochrome mode tends to require image formation speed. For this reason, the image forming speed is usually about 1.5 to 2 times larger in the monochrome mode than in the color mode. Accordingly, the moving speed of the intermediate transfer belt 8 and the rotating speed of the variable diameter roller 21 that rotates in contact with the belt are about 1.5 to 2 times larger in the monochrome mode than in the color mode.

  For this reason, in the color mode, since the rotation speed of the variable diameter roller 21 is small and the inertial force of the weight member 25 is small, the urging force of the urging member 27 is shown in FIGS. 5 (a) and 5 (b). Therefore, the flange portion 24 cannot be moved outward in the axial direction. Therefore, as shown in FIG. 3, the intermediate transfer belt 8 comes into contact with the flange portion 24 having an outer diameter larger than that of the roller body 23 and is supported for rotation. As a result, as shown in FIG. 1, the intermediate transfer belt 8 moves upward and contacts the color photosensitive drums 1a to 1c. In this way, the intermediate transfer belt 8 that has contacted all the photosensitive drums 1a to 1d is rotated by receiving the primary transfer by the transfer rollers 6a to 6d.

  FIG. 6 is an enlarged cross-sectional view (FIG. 6A) in the axial direction in the monochrome mode around one flange portion of the variable diameter roller used in the image forming apparatus of the first embodiment, and FIG. FIG. 6B is an enlarged cross-sectional view perpendicular to the axial direction in B of FIG. Portions common to FIGS. 1 to 5 and 9 are denoted by the same reference numerals and description thereof is omitted.

  In contrast to the color mode, in the monochrome mode, the rotational speed of the variable diameter roller 21 is high, and therefore, as shown in FIGS. 6A and 6B, the weight member 25 has a large inertial force. The flange portion 24 can be moved outward in the axial direction against the urging force of the urging member 27 by moving to the outermost side of the cavity portion 26. Therefore, as shown in FIG. 4, the intermediate transfer belt 8 comes into contact with the roller body 23 having a smaller outer diameter than the flange portion 24 and is supported for rotation. As a result, as shown in FIG. 2, the intermediate transfer belt 8 moves downward and is separated from the color photosensitive drums 1a to 1c. Thus, the intermediate transfer belt 8 that is in contact with only the photosensitive drum 1d is rotated by receiving the primary transfer only by the transfer roller 6d.

  The rotational speed of the variable diameter roller 21 in the color mode and the monochrome mode, the weight and quantity of the weight member 25, and the urging force of the urging member are subjected to a preliminary test. In the color mode, the urging force of the urging member 27 is the weight member. The flange portion 24 cannot move outward against the inertial force 25 (FIGS. 5a and 5b). On the other hand, in the monochrome mode, the flange portion 24 is axially moved against the biasing member due to the inertial force of the weight member 25. What is necessary is just to set suitably so that it may move outside (FIG. 6 (a) and FIG.6 (b)).

  As described above, the variable diameter roller 21 is arranged as long as the intermediate transfer belt 8 and the color photosensitive drums 1a to 1c can be brought into contact with and separated from each other by changing the outer diameter of the roller. It is not limited to between the photosensitive drum 1d and the color photosensitive drum 1c closest to the upstream side of the black photosensitive drum 1d. For example, it can be disposed between any of the color photosensitive drums 1a to 1c. When arranged between the color photosensitive drums 1b and 1c, by arranging the photosensitive drums 1a and 1d below the photosensitive drums 1b and 1c in FIG. 1 (on the variable diameter roller 21 side), The photosensitive drums 1a to 1c and the intermediate transfer belt 8 can be brought into contact with and separated from each other. However, by arranging the variable diameter roller 21 as in this embodiment, the arrangement of the roller 21 and the photosensitive drums 1a to 1d can be made more compact, and the image forming apparatus can be made smaller. Further, the nip portion of each of the photosensitive drums 1a to 1c with the intermediate transfer belt 8 can be substantially on the same plane.

  In the present embodiment, the photosensitive drums 1a to 1c are arranged so that the nip portions with the intermediate transfer belt 8 are on substantially the same plane. However, the arrangement of the photosensitive drums 1a to 1c is not limited as long as the outer diameter of the variable diameter roller 21 is changed as described above so that the intermediate transfer belt 8 and the color photosensitive drums 1a to 1c can be brought into contact with and separated from each other. What is necessary is just to set suitably according to the arrangement | positioning position of the said diameter variable roller 21. FIG. In the present embodiment, as described above, since the nip portion between the intermediate transfer belt 8 and the photosensitive drums 1a to 1c is on substantially the same plane, the apparatus can be miniaturized. In addition, since the optical path lengths of the laser beams irradiated from the exposure units 4a to 4c to the corresponding photosensitive drums 1a to 1c can be made equal, it is difficult for the photosensitive drums to be exposed to variations, and the color mode and the monochrome mode. Thus, an electrostatic latent image for forming a color image can be accurately formed. For this reason, it is possible to avoid the need to change the arrangement of the corresponding exposure units or to make complicated adjustments for each exposure unit in order to make the exposure state the same, further increasing the size and complexity of the apparatus configuration. This can be avoided and contributes to miniaturization of the apparatus.

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

  The selective switching between the color mode and the monochrome mode can be performed by using a conventionally known technique. The initial state may be any mode. However, when the variable diameter roller 21 is not rotating, the weight member 25 and the flange portion 24 remain on the inner side in the axial direction. Further, by switching the mode contrary to the above, the rotational speed of the variable diameter roller 21 decreases and the flange portion 24 comes into contact with the roller body 23, so that the monochrome mode is changed to the color mode (the state shown in FIGS. 2 to 1). It can be.

  As described above, according to the first embodiment, the intermediate transfer belt and the photoconductor can be brought into contact with and separated from each other without using complicated members and mechanisms for moving the intermediate transfer belt up and down. Therefore, it is possible to switch between the color mode and the monochrome mode with a simpler configuration than before. In addition, the apparatus can be miniaturized.

  FIG. 7 is an enlarged sectional view (FIG. 7A) in the axial direction in the color mode around one flange portion of the variable diameter roller used in the image forming apparatus according to the second embodiment of the present invention. It is an expanded sectional view (Drawing 7 (b)) of the direction perpendicular to the direction of an axis in C of 7 (a). FIG. 8 is an enlarged sectional view (FIG. 8A) in the axial direction in the monochrome mode around one flange portion of the variable diameter roller used in the image forming apparatus of the second embodiment, and FIG. It is an expanded sectional view (Drawing 8 (b)) of the direction perpendicular to the direction of an axis in D of (a). Portions common to FIGS. 1 to 6 and 9 are denoted by the same reference numerals and description thereof is omitted.

  In the present embodiment, the configuration is the same as that of the first embodiment except that a cubic member as shown in FIGS.

  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 directed to the direct transfer type tandem that sequentially transfers the toner images of each color onto the paper P carried and conveyed on the conveyance belt as shown in FIG. The present invention can be applied to the mold color image forming apparatus in exactly the same manner.

  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. .

  Further, in each of the above embodiments, the black photosensitive drum 1d is the most downstream side, but the black photosensitive drum 1d may be the most upstream side. For example, each photoconductor can be disposed in the order of a black photoconductor drum 1d, a yellow photoconductor drum 1c, a magenta photoconductor drum 1b, and a cyan photoconductor drum 1a. In such a case, along the inner peripheral surface of the intermediate transfer belt or the conveyance belt, the drive roller 11 is further downstream of the most downstream cyan photoreceptor drum 1a, and further upstream of the most upstream black photoreceptor drum 1d. The tension roller 23 is provided with a driven roller 10 upstream of the tension roller 23 and downstream of the driving roller 11, and the most upstream black photosensitive drum 1 d and the yellow photosensitive member 1 c closest to the photosensitive drum are arranged. A variable diameter roller can be disposed between them.

  The present invention includes a plurality of image forming units including a photosensitive member and a developing device, an intermediate transfer belt on which the developed toner images are sequentially stacked, or a conveying belt for conveying a recording medium, and the photosensitive member on the photosensitive member. Means for transferring the toner image developed on the intermediate transfer belt or the conveyance belt to the intermediate transfer belt or the conveyance belt. Alternatively, the intermediate transfer belt or the conveyance belt is composed of a black photoreceptor on which the black toner image disposed on the most downstream side is formed and one or more color photoreceptors on which toner images of colors other than black are formed. The belt is stretched around at least a driving roller, a driven roller, and a variable diameter roller that changes an outer diameter of a portion inscribed in the intermediate transfer belt or the conveyance belt.

  This eliminates the need to add complicated members and mechanisms to contact and separate the color photoconductor from the intermediate transfer belt or the conveyance belt, so it is possible to switch between the color mode and the monochrome mode with a simpler configuration than before. Is possible. In addition, the apparatus can be miniaturized.

  Further, flange members having tapered outer peripheral surfaces facing outward in the axial direction are provided at both ends of the diameter variable roller, and a weight member is provided by providing a cavity portion inclined along the outer peripheral surface of the flange portion in the flange portion. By changing the outer diameter of the variable diameter roller in contact with the intermediate transfer belt or the conveying belt according to the difference in the belt speed between the color mode and the monochrome mode, the apparatus configuration can be further simplified. If the variable diameter roller is disposed between the black photoconductor and the color photoconductor nearest to the photoconductor, the optical path length from the exposure unit to the photoconductor can be made constant. It is also possible to avoid adding a complicated mechanism for improving the above.

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. 4 is a side view showing an arrangement relationship in a color mode between a variable diameter roller and an intermediate transfer belt used in the image forming apparatus of the first embodiment. FIG. 4 is a side view showing a positional relationship in a monochrome mode between a variable diameter roller and an intermediate transfer belt used in the image forming apparatus of the first embodiment. FIG. 5 is an enlarged cross-sectional view (FIG. 5A) in the axial direction in the color mode around one flange portion of the variable diameter roller used in the image forming apparatus of the first embodiment and a direction perpendicular to the axial direction. It is an expanded sectional view (FIG.5 (b)). FIG. 6 is an enlarged cross-sectional view (FIG. 6A) in the monochrome direction around the one flange portion of the variable diameter roller used in the image forming apparatus of the first embodiment and a direction perpendicular to the axial direction. FIG. 6 is an enlarged sectional view (FIG. 6B). FIG. 7 is an enlarged cross-sectional view (FIG. 7A) in the axial direction in the color mode around one flange portion of the variable diameter roller used in the image forming apparatus of the second embodiment and a direction perpendicular to the axial direction. It is an expanded sectional view (FIG.7 (b)). [Fig. 8A] is an enlarged sectional view (FIG. 8A) in the monochrome direction around the one flange portion of the variable diameter roller used in the image forming apparatus of the second embodiment, and is perpendicular to the axial direction. It is an expanded sectional view (FIG.8 (b)). FIG. 1 is a schematic diagram illustrating an overall configuration of a conventional intermediate transfer type image forming apparatus. FIG. 1 is a schematic view showing an overall configuration of a conventional direct transfer type image forming apparatus.

Explanation of symbols

Pa to Pd Image forming unit 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 Drive roller 11 Drive roller 21 Diameter variable roller 22 Rotating shaft 23 Roller body 24 Flange part 25 Weight member 26 Cavity part 27 Energizing member 30 Conveying 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 toner image developed on the photoconductor In an image forming apparatus comprising: a transfer unit that is stacked on a transfer belt; and a secondary transfer unit that transfers a toner image stacked on the intermediate transfer belt onto a recording medium at once.
The photosensitive member is formed with a black photosensitive member on which a black toner image is disposed on the most upstream side or the most downstream side with respect to the rotation direction of the intermediate transfer belt, and a toner image of a color other than black. One or more color photoreceptors,
The image forming apparatus, wherein the intermediate transfer belt is stretched between at least a driving roller, a driven roller, and a variable diameter roller that changes an outer diameter of a portion inscribed in the intermediate transfer belt. A plurality of image forming units including a photosensitive member and a developing device, an endless conveying belt for conveying a recording medium to the image forming unit, and a toner image developed on the photosensitive member on the recording medium or on the conveying belt An image forming apparatus comprising:
The photoconductor is formed with a black photoconductor 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 a toner image of a color other than black. Consisting of one or more color photoreceptors,
The image forming apparatus, wherein the conveying belt is stretched around at least a driving roller, a driven roller, and a variable diameter roller that changes an outer diameter of a portion inscribed in the conveying belt.   The variable diameter roller includes a cylindrical roller body, and a flange portion that is disposed at both ends of the roller body and has a tapered outer peripheral surface facing outward in the axial direction and movable in the axial direction, 3. The intermediate transfer belt or the conveyance belt is in contact with the flange portion in a color mode in which the intermediate transfer belt or the conveyance belt is in contact with the color photoconductor, and in contact with the roller body in a monochrome mode in which the intermediate transfer belt is separated from the color photoconductor. The image forming apparatus described.   The flange portion includes a cavity portion that is inclined outward in the axial direction along the outer peripheral surface, and a weight member that is inserted into the cavity portion and is movable within the cavity portion. 4. The image forming apparatus according to claim 3, wherein the weight member moves in the axial direction due to a difference in belt speed between the color mode and the monochrome mode.   The image forming apparatus according to claim 1, wherein the variable diameter roller is disposed between the black photosensitive member and a color photosensitive member nearest to the black photosensitive member.