Patent Publication Number: US-10317821-B2

Title: Intermediate transfer unit and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Patent Application No. PCT/JP2017/011113, filed Mar. 21, 2017, which claims the benefit of Japanese Patent Application No. 2016-080717, filed Apr. 14, 2016, both of which are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an intermediate transfer unit included in an image forming apparatus, and an image forming apparatus. Here, the image forming apparatus refers to, for example, an electrophotographic copying machine, an electrophotographic printer (for example, a laser beam printer, an LED printer, or the like), or the like. 
     BACKGROUND ART 
     Recently, along with an increase in the speed of image forming apparatuses, a configuration in which a plurality of image forming portions having developers of different colors are arranged side by side and the image forming processes of respective colors are processed in parallel is mainstream. 
     For example, in a full-color image forming apparatus in an electrophotographic type, there is an intermediate transfer tandem type in which toner images of the respective colors are sequentially overlapped and transferred onto a surface of an intermediate transfer belt by a primary transfer roller (transfer member), and after that, a full-color toner image is collectively transferred onto a recording material. This intermediate transfer tandem type has advantages that can cope with high productivity or conveyance of various media. The intermediate transfer belt is driven to travel in a state of being stretched by a plurality of stretching rollers including a driving roller. 
     In such an intermediate tandem type image forming apparatus, in order to reduce the abutting pressure of the primary transfer roller, a configuration in which the primary transfer roller is offset from a photosensitive member toward the downstream side in a conveyance direction of the intermediate transfer belt has been proposed (see Patent Literature 1). 
     In an intermediate transfer tandem type image forming apparatus, in the case of forming an image of a monochrome single color, a configuration in which a primary transfer roller included in a color image forming portion is separated from an intermediate transfer belt has been known. 
     Here, the tension of the intermediate transfer belt may change according to the change in the stretching layout of the intermediate transfer belt at the time of monochrome image formation and at the time of color image formation. In this case, if the primary transfer rollers are arranged as disclosed in Patent Literature 1, the force received by a black primary transfer roller which is not separated from the intermediate transfer belt may change and the following phenomenon may occur. 
     That is, along with the change from the stretching layout at the time of color image formation to the stretching layout at the time of monochrome image formation, when the tension of the intermediate transfer belt changes from a high state to a low state, the pressing force of the intermediate transfer belt to the primary transfer roller is weakened. The primary transfer roller  55  is applied with a biasing force by a primary transfer spring. Therefore, when the pressing force of the intermediate transfer belt  57  against the biasing force decreases, the primary transfer roller  55  changes from a position indicated by a solid line to a position indicated by a dashed line as illustrated in  FIG. 7 . In addition, as the position of the primary transfer roller  55  changes, a contact point P 2  between the primary transfer roller  55  and the intermediate transfer belt  57  changes to a contact point P 3 . Therefore, a contact point distance from a contact point P 1  between a photosensitive drum  51  and the intermediate transfer belt  57  to a contact point between the primary transfer roller  55  and the intermediate transfer belt  57  changes from L 1  to L 2 . Along with the change of the contact point distance, a primary transfer current flowing from the primary transfer roller  55  to the photosensitive drum  51  changes when a bias is applied to the primary transfer roller  55 , because the intermediate transfer belt  57  is a resistor. This may cause image defects. 
     Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to provide an intermediate transfer unit capable of suppressing a change in a current flowing from a transfer member to a photosensitive member in a configuration in which a part of a plurality of transfer members can be separated from an intermediate transfer belt. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2012-247756 
     SUMMARY OF THE INVENTION 
     A representative configuration of an intermediate transfer unit according to the present invention to achieve the above object is an intermediate transfer unit, which is supported by an image forming apparatus, the intermediate transfer unit including: an endless intermediate transfer belt onto which developer images respectively formed on surfaces of first and second photosensitive members are transferred; first and second transfer members which are respectively disposed corresponding to the first and second photosensitive members and abut on an inner peripheral surface of the intermediate transfer belt to transfer the developer image onto the intermediate transfer belt, wherein a first position which is a downstream side end portion of a region in which the photosensitive member and the intermediate transfer belt abut at the time of the transfer in a moving direction of the intermediate transfer belt is disposed upstream of a second position which is an upstream side end portion of a region in which the intermediate transfer belt and the transfer member corresponding to the photosensitive member abut; a moving mechanism which is capable of moving to an abutting position at which the first transfer member abuts against the intermediate transfer belt and a separating position at which the first transfer member is separated from the intermediate transfer belt; and a changing mechanism which changes a biasing force for abutting the second transfer member against the intermediate transfer belt, wherein the biasing force is changed such that the biasing force when the first transfer member is at the separating position becomes weaker than the biasing force when the first transfer member is at the abutting position. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus. 
         FIG. 2  is an explanatory diagram relating to the arrangement of a primary transfer roller. 
         FIGS. 3A and 3B  are perspective views of an intermediate transfer unit. 
         FIG. 4  is a schematic cross-sectional view of a separating slider. 
         FIGS. 5A and 5B  are cross-sectional views for explaining an operation in which a primary transfer roller and a separating roller move to a separating position. 
         FIGS. 6A and 6B  are cross-sectional views of an intermediate transfer unit in a monochrome mode and a color mode. 
         FIG. 7  is an explanatory diagram for explaining a problem of a conventional technique. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     &lt;Image Forming Apparatus&gt; 
     First, an overall configuration of an image forming apparatus A including an intermediate transfer unit according to a first embodiment of the present invention will be described with reference to the drawings together with an operation at the time of image formation. 
     The image forming apparatus A is an electrophotographic type or intermediate transfer tandem type color image forming apparatus which primarily transfers toners of four colors, that is, yellow Y, magenta M, cyan C, and black K, to an intermediate transfer belt, and then secondarily transfers the same onto an image forming medium to form an image. 
     As illustrated in  FIG. 1 , the image forming apparatus A includes an image forming portion which transfers a toner image onto a sheet, a sheet feeding portion which feeds the sheet to the image forming portion, and a fixing portion which fixes the toner image onto the sheet. 
     As illustrated in  FIG. 1 , the image forming portion includes a plurality of photosensitive drums  1  ( 1 Y,  1 M,  1 C,  1 K) serving as photosensitive members rotatably provided, and charging members  2  ( 2 Y,  2 M,  2 C,  2 K) which charge the photosensitive drums  1 . In addition, the image forming apparatus A includes an intermediate transfer unit  10 , laser scanner units  3  ( 3 Y,  3 M,  3 C,  3 K), developing devices  4  ( 4 Y,  4 M,  4 C,  4 K), cleaning blades  6  ( 6 Y,  6 M,  6 C,  6 K), and the like. 
     The intermediate transfer unit  10  is supported by the main body of the apparatus, and includes a plurality of primary transfer rollers  5  ( 5 Y,  5 M,  5 C,  5 K) as transfer members, a secondary transfer roller  9 , a secondary transfer counter roller  8 , and a cleaning device  11 . In addition, an intermediate transfer belt  7  (intermediate transfer member) which is an endless cylindrical belt is provided. The intermediate transfer belt  7  is installed on a belt frame (not illustrated) and is stretched by the secondary transfer counter roller  8 , a first tension roller  17 , a second tension roller  18 , and a separating roller  19 . That is, the intermediate transfer belt  7  is stretched by a plurality of stretching rollers. 
     Upon image formation, when a CPU  60  issues a print signal, a sheet S stacked and stored in a sheet stacking portion  12  by a feed roller  13  adopting a friction separation method is sent to a registration roller  14  via a conveyance path. After that, the sheet is fed to the image forming portion after skew feeding correction or timing correction are performed by the registration roller  14 . 
     On the other hand, in the image forming portion, the surface of the photosensitive drum  1  is first charged by the charging roller  2 . Then, a laser scanner unit  3  emits laser light from a light source (not illustrated) provided inside and irradiates the photosensitive drum  1  with the laser light according to image information. Therefore, an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum  1 . 
     The electrostatic latent image is developed by adhering a toner by the developing device  4 , thereby forming a toner image (developer image) on the photosensitive drum  1 . In the present embodiment, a two-component developer including a non-magnetic toner and a magnetic carrier is used as a developer, but a one-component developer including only a magnetic toner or a non-magnetic toner may also be used. 
     After that, in primary transfer nip portions N (NY, NM, NC, NK) in which a primary transfer electric field is formed by applying a primary transfer bias to the primary transfer roller  5 , the toner images formed on the photosensitive drums  1  are primarily transferred onto the intermediate transfer belts  7 , respectively. 
     Here, as illustrated in  FIG. 2 , each primary transfer roller  5  is disposed to be offset from each photosensitive drum  1  by a distance L 3  on the downstream side in a moving direction of the intermediate transfer belt  7 . In addition, in the moving direction of the intermediate transfer belt  7 , a contact point P 4  is a downstream side end portion of a region at which each photosensitive drum  1  and the intermediate transfer belt  7  abut, and is a position (first position) at which each photosensitive drum  1  and the intermediate transfer belt begin to be separated from each other. In addition, a contact point P 5  is an upstream end portion of a region at which the intermediate transfer belt  7  and each primary transfer roller  5  abut, and a position (second position) at which the intermediate transfer belt  7  and each primary transfer roller  5  begin to abut. 
     In addition, in the present embodiment, the distance L 3  is within the range of 3.5 mm to 7.0 mm. In addition, although the distances L 3  are the same in the respective image forming portions, at least one of the distances L 3  may be set to a different value. In addition, the primary transfer roller  5  is made of a conductor such as a metal or a conductive resin having a volume resistance of 600Ωcm or less. 
     Next, as illustrated in  FIG. 1 , the intermediate transfer belt  7  rotates in a direction of an arrow X when the secondary transfer counter roller  8  receives a driving force from a driving source and rotates in a direction of an arrow R 1 . The primarily transferred toner image reaches the secondary transfer portion formed by the secondary transfer counter roller  8  and the secondary transfer roller  9  positioned downstream in the rotation direction by the rotation of the intermediate transfer belt  7 , and the toner image is transferred onto the sheet. 
     The sheet onto which the toner image is transferred is sent to the fixing device  15 , heated, and pressed to fix the toner image on the sheet, and then discharged to a discharge tray  16 . 
     The toner remaining on the photosensitive drum  1  after the primary transfer is removed by the cleaning blade  6 . In addition, the toner remaining on the intermediate transfer belt  7  after the secondary transfer is removed by the cleaning device  11 . 
     In addition, when the toner amount in the developing device  4  of each color decreases, the toners of the respective colors stored in toner storage containers  21  ( 21 Y,  21 M,  21 C,  21 K) are supplied to the developing devices  4  by toner replenishing devices  22  ( 22 Y,  22 M,  22 C,  22 K), respectively. Although not illustrated, the toner replenishing devices  22 M,  22 C, and  22 K have the same shape as the toner replenishing device  22 Y. 
     &lt;Intermediate Transfer Unit&gt; 
     Next, the configuration of the intermediate transfer unit  10  will be described.  FIGS. 3A and 3B  are perspective views of the intermediate transfer unit  10 .  FIG. 3A  illustrates a state in which the intermediate transfer belt  7  is installed and stretched, and  FIG. 3B  illustrates a state in which the intermediate transfer belt  7  is detached. 
     As illustrated in  FIGS. 3A and 3B , the intermediate transfer unit  10  includes a front frame  23 F and a rear frame  23 R. The front frame  23 F is disposed on the side illustrated in  FIG. 1  in the rotational axis direction of the primary transfer roller  5 , and the rear frame  23 R is disposed on the opposite side thereof. 
     In addition, the front frame  23 F and the rear frame  23 R rotatably and axially support both end portions of the secondary transfer counter roller  8 , the second tension roller  18 , the separating roller  19 , the separating shaft  28 , and the like in the rotational axis direction in a sandwiched form. 
     In addition, a driving coupling  24  is attached to one end portion of the rotational shaft of the secondary transfer counter roller  8 . The driving coupling  24  is connected to an output shaft of a belt driving unit (not illustrated) for driving the intermediate transfer belt  7 , and a driving force is transmitted to the driving coupling  24 . When the driving force is transmitted, the secondary transfer counter roller  8  having the surface with a relatively high friction coefficient such as a rubber rotates, thereby rotating the intermediate transfer belt  7  in a direction of an arrow X. In the present embodiment, the driving coupling  24  is used as a driving transmission unit, but may be connected by using a gear. 
     In addition, a gear  33  is provided at one end portion of the separating shaft  28  in an axial direction. In addition, a separating coupling  30  and gears  31  and  32  connected to an output shaft of a separating driving unit (not illustrated) are provided in the vicinity of the gear  33 . Then, the driving force transmitted from the separating driving unit (not illustrated) to the separating coupling  30  is transmitted to the gear  33  via the gears  31  and  32 , such that the separating shaft  28  rotates. 
     In addition, the front frame  23 F and the rear frame  23 R are provided with a pair of tension bearings  25 . The tension bearings  25  rotatably and axially support both end portions of the first tension roller  17  in a rotational axis direction in a sandwiched form. In addition, in a state in which the intermediate transfer belt  7  illustrated in  FIG. 3A  is installed, a tension spring  26  (biasing member) which is a compression spring is provided between the front frame  23 F and the rear frame  23 R and the tension bearing  25  in a contracted state. Then, a tension force acting on the inner periphery of the intermediate transfer belt  7  is applied to the first tension roller  17  by the biasing force of the tension spring  26 . 
     In addition, the intermediate transfer unit  10  has feeding portions  27  ( 27 Y,  27 M,  27 C,  27 K). The feeding portions  27  feed the primary transfer bias to the corresponding primary transfer rollers  5 , respectively. A description of a main feeding configuration from a high-voltage board (not illustrated) provided in the image forming apparatus A to the feeding portion  27  will be omitted. 
     &lt;Movement of Transfer Member&gt; 
     Next, the movement of the primary transfer roller  5  as the transfer member will be described. When the monochrome mode and the color mode are switched, the primary transfer rollers  5 Y,  5 M, and  5 C (first transfer member) other than black K move from an abutting position which abuts with the intermediate transfer belt  7  to a separating position which is separated from the intermediate transfer belt  7  by a separating slider  40 . 
       FIG. 4  is a schematic cross-sectional view of the separating slider  40 . As illustrated in  FIG. 4 , the separating slider  40  has elevating surfaces  41  ( 41 Y,  41 M,  41 C) for lifting and lowering the primary transfer rollers  5  ( 5 Y,  5 M,  5 C) other than the primary transfer roller  5 K of black K. In addition, the separating slider  40  has an elevating surface  42  for lifting and lowering the separating roller  19 , a spring seat elevating surface  43 , and a slide biasing surface  44 . The separating sliders  40  are provided in the front frame  23 F and the rear frame  23 R, respectively, and have a symmetrical shape in the rotational axis direction of the primary transfer roller  5 . 
       FIG. 5  is a cross-sectional view for explaining an operation in which the primary transfer roller  5  and the separating roller  19  move to a separating position. Here,  FIG. 5A  illustrates a state of the color mode and  FIG. 5B  illustrates a state of the monochrome mode. 
     As illustrated in  FIG. 5 , both end portions of each primary transfer roller  5  are rotatably and axially supported by a pair of primary transfer bearings  34  ( 34 Y,  34 M,  34 C,  34 K). In addition, while the movement of the intermediate transfer belt  7  in the moving direction is regulated by the front frame  23 F and the rear frame  23 R, the primary transfer bearing  34  is fitted and held to be elevatable vertically upwardly and downwardly. The primary transfer bearing  34  is biased together with the primary transfer roller  5  downward in the vertical direction by primary transfer springs  35  ( 35 Y,  35 M,  35 C,  35 K) as a spring member, and as a biasing unit. That is, each of the primary transfer rollers  5  is biased against the intermediate transfer belt  7  by the biasing force of the primary transfer spring  35 . 
     In addition, both end portions of the separating roller  19  (stretching member) disposed at a position adjacent to the upstream side of the primary transfer roller  5 Y at the most upstream position with respect to the moving direction of the intermediate transfer belt  7  and stretching the intermediate transfer belt  7  are rotatably and axially supported by the separating roller bearing  36 . In addition, while the movement of the intermediate transfer belt  7  in the moving direction is regulated by the front frame  23 F and the rear frame  23 R, it is fitted and held to be elevatable vertically upwardly and downwardly. In addition, it is biased together with the separating roller  19  downwardly in the vertical direction by the separating roller spring  39 . 
     In addition, separating cams  37  as cam members are provided at both end portions of the separating shaft  28 . 
     Next, a separating operation when a part of the primary transfer roller  5  moves from the abutting position which abuts with the intermediate transfer belt  7  to the separating position which is separated from the intermediate transfer belt  7  will be described. 
     When switching from the color mode to the monochrome mode, the driving power is input to the separating coupling  30  from a separating driving unit (not illustrated) which has received a signal from the CPU  60 . Therefore, the driving power is transmitted to the separating shaft  28 , and the separating cam  37  is rotated by 90 degrees in a direction of an arrow R 2  (a state changes from a state of  FIG. 5A  to a state of  FIG. 5B ). 
     When the separating cam  37  rotates in the direction of the arrow R 2 , the separating cam  37  biases the slide biasing surface  44  of the separating slider  40 . Therefore, the separating slider  40  slides in a direction of an arrow K 1 . 
     When the separating slider  40  slides in the direction of the arrow K 1 , protrusions  34 Ya,  34 Ma, and  34 Ca of the primary transfer bearings  34 Y,  34 M, and  34 C are pushed up by the elevating surfaces  41 Y,  41 M, and  41 C. Therefore, the primary transfer rollers  5  ( 5 Y,  5 M,  5 C) other than the primary transfer roller  5 K of black K move from the abutting position which abuts with the intermediate transfer belt  7  to the separating position which is separated from the intermediate transfer belt  7 . 
     That is, in the present embodiment, the separating slider  40  and the separating cam  37  are moving mechanisms capable of moving the primary transfer rollers  5 Y,  5 M, and  5 C to the abutting position and the separating position, and the movement thereof is controlled by the rotation of the separating cam  37 . In addition, as the elevating surfaces  41 Y,  41 M, and  41 C (guide portions) of the separating slider  40  guide the protrusions  34 Ya,  34 Ya, and  34 Ma (guided portions), the primary transfer rollers  5 Y,  5 M, and  5 C move. 
     In addition, as the separating slider  40  slides, the protrusion  36   a  of the separating roller bearing  36  is pushed up by the elevating surface  42 , and the separating roller  19  moves in a direction away from the intermediate transfer belt  7  from a position (third position) before the movement. At this time, the moving amount of the separating roller  19  is set to be smaller than the moving amounts of the primary transfer rollers  5 Y,  5 M, and  5 C. Therefore, a position (fourth position) of the separating roller  19  after the movement is a position closer to the intermediate transfer belt  7  than the positions of the primary transfer rollers  5 Y,  5 M, and  5 C after the movement, and the intermediate transfer belt  7  is stretched along the separating roller  19  (the state of  FIG. 5B ). 
       FIGS. 6A and 6B  are cross-sectional views of the intermediate transfer unit  10  in the monochrome mode and the color mode. Here,  FIG. 6A  illustrates the state of the color mode and  FIG. 6B  illustrates the state of the monochrome mode. 
     The surplus length of the intermediate transfer belt  7 , which is caused by the change in the stretching layout of the intermediate transfer belt  7  according to the mode change, is absorbed by the movement of the first tension roller  17  from the position indicated by a dashed line in  FIG. 6B  to a position indicated by a solid line. At this time, since the biasing force of the tension spring  20  against the intermediate transfer belt  7  decreases, the tension of the intermediate transfer belt  7  decreases, and the tension of the intermediate transfer belt  7  at the primary transfer nip portion NK also decreases. 
     At this time, since the tension from the intermediate transfer belt  7  against the biasing force of the primary transfer spring  35 K decreases, the position of the primary transfer roller  5 K changes like the position change of the primary transfer roller  55  from the solid line to the dashed line illustrated in  FIG. 7 . Therefore, the distance L 3  from the contact point P 4  between the photosensitive drum  1 K and the intermediate transfer belt  7  illustrated in  FIG. 2  to the contact point P 5  between the primary transfer roller  5 K and the intermediate transfer belt  7  is reduced, and the primary transfer current may change and image defect may occur. 
     In the present embodiment, the biasing force of the primary transfer spring  35 K is reduced by the following configuration to suppress the change in the position of the primary transfer roller  5 K due to the change in the stretching layout. Therefore, the change in the primary transfer current is suppressed, and the occurrence of image defect is suppressed. 
     That is, as illustrated in  FIGS. 5A and 5B , while the movement of the intermediate transfer belt  7  in the moving direction is regulated by the front frame  23 F and the rear frame  23 R, a variable spring seat  38  (first support portion) on which the primary transfer spring  35 K is installed is fitted and held to be elevatable upwardly and downwardly in the vertical direction. In addition, the variable spring seat  38  is biased in the K 2  direction which is upward in the vertical direction by the primary transfer spring  35 K (elastic member). 
     Then, when switching from the color mode to the monochrome mode, the protrusion  38   a  of the variable spring seat  38  is pushed up by the spring seat elevating surface  43  of the separating slider  40  by the sliding of the separating slider  40  in the direction of the arrow K 1  due to the rotation of the separating cam  37 . That is, the variable spring seat  38  moves to a position away from the intermediate transfer belt  7  by interlocking with the movement of the primary transfer rollers  5 Y,  5 M, and  5 C to the separating position separated from the intermediate transfer belt  7 . In addition, the movement thereof is performed by guiding the protrusion  38   a  as the guided portion to the spring seat elevating surface  43  as the guide portion. That is, the separating slider  40  and the separating cam  37  form an interlocking mechanism for moving the variable spring seat  38  to a position away from the intermediate transfer belt  7  by interlocking with the movement of the primary transfer rollers  5 Y,  5 M, and  5 C to the separating position separated from the intermediate transfer belt  7 . 
     Due to this, the biasing force of the primary transfer spring  35 K as the spring member when the primary transfer rollers  5 Y,  5 M, and  5 C are at the separating position is smaller than that when the primary transfer rollers  5 Y,  5 M, and  5 C are at the abutting position. Therefore, the biasing force of the primary transfer roller  5 K (second transfer member) against the intermediate transfer belt  7  by the primary transfer spring  35 K is reduced. At this time, the biasing force of the primary transfer spring  35 K and the movement amount of the variable spring seat  38  are set such that the distance L 3  from the contact point P 4  to the contact point P 5  illustrated in  FIG. 2  during the image formation does not change in the monochrome mode and the color mode. At the time of the image formation, the biasing force of the primary transfer roller  5 K against the intermediate transfer belt  7  changes according to the magnitude of the primary transfer bias, in addition to the stretching layout switching of the intermediate transfer belt  7 , and thus the setting is also performed in consideration of this. That is, the primary transfer bearing  34 K (second support portion), the primary transfer spring  35 K, the variable spring seat  38 , the separating slider  40 , and the separating cam  37  form a changing mechanism for changing the biasing force for biasing the primary transfer roller  5 K against the intermediate transfer belt  7 . The changing mechanism also includes the interlocking mechanism. 
     With such a configuration, in the monochrome mode and the color mode, it is possible to suppress the change in the primary transfer current flowing from the primary transfer roller  5 K to the photosensitive drum  1 K, and it is possible to suppress the occurrence of image defect. 
     The switching from the monochrome mode to the color mode is performed by an operation opposite to the above separating operation when the separating cam  37  rotates by 90 degrees in the direction opposite to the direction of the arrow R 2  illustrated in  FIG. 5A . 
     In this embodiment, the compression spring is used as the biasing unit for biasing the primary transfer roller  5 K, but the present invention is not limited thereto. That is, it may be any configuration as long as the biasing force for biasing the primary transfer roller  5  not separated against the intermediate transfer belt  7  is reduced in a state in which a portion of the plurality of primary transfer rollers  5  is separated from the intermediate transfer belt  7 . Further, even when the primary transfer roller  5  is pressed against the intermediate transfer belt  7  with a configuration other than biasing the primary transfer roller  5 , the effect of the present invention can be obtained. 
     In addition, although the configuration in which the primary transfer roller  5 K of black K is not separated from the intermediate transfer belt  7  has been described in the present embodiment, the present invention is not limited thereto. That is, even when the primary transfer roller for transferring a developer of a color having a high use frequency to the intermediate transfer belt  7  is not separated, or even when the primary transfer rollers of a plurality of colors are not separated, the effect of the present invention can be obtained. 
     According to the present invention, it is possible to suppress the change in the primary transfer current in the configuration in which a part of the plurality of transfer members can be separated from the intermediate transfer belt. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to an image forming apparatus and has industrial applicability.