Patent Publication Number: US-6907217-B2

Title: Image forming device capable of suppressing distortion in output image

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming device for forming images on a recording medium using a developing agent. 
   2. Description of the Related Art 
   A laser printer is an example of a well-known image forming device for printing images on a recording medium. In such a laser printer, a laser device radiates a laser beam, based on image data, onto a photosensitive member to form electrostatic latent images thereon. Selectively transferring toner, which is a developing agent in powdered form, onto the photosensitive member develops the electrostatic latent images into visible toner images, which are then transferred onto a paper sheet or other recording medium. 
   Laser printers capable of forming color images are also well known. Such laser printers store different colors of toner, such as cyan (C), magenta (M), yellow (Y), and black (BK) toner. Electrostatic latent images are formed one after the other on the photosensitive member for each color, and then developed into visible toner images using toner in the corresponding colors. The toner images are transferred one after the other in a primary transfer operation onto an intermediate transfer member so that the different colored toner images overlap one on the other. As a result, a multicolor toner image is formed on the intermediate transfer member by the overlapping monochrome images. Afterward, the multicolor toner image is transferred onto the recording medium in a secondary transfer operation, to form a color image on the recording medium. 
   However, not all of the toner that forms the multicolor toner image is transferred from the intermediate transfer member to the recording medium. Therefore, a cleaning unit is normally provided to clean the intermediate transfer member by removing the residual toner that clings to the intermediate transfer member after the secondary transfer. 
   The cleaning unit can be switched between a non-cleaning mode, wherein the cleaning unit cannot remove toner from the intermediate transfer member, and a cleaning mode, wherein the cleaning unit can remove toner from the intermediate transfer member. By switching the mode of the cleaning unit at appropriate timings, residual toner alone can be selectively removed from the intermediate transfer member without damaging the multicolor toner image. Normally, the cleaning unit is in contact with the intermediate transfer member in the cleaning mode and separated from the intermediate transfer member in the non-cleaning mode. 
   When the cleaning unit is switched between these modes, vibration is generated in the laser printer and also the load on the rotating intermediate transfer member can fluctuate. These can warp or distort the image being output. 
   For example, when the cleaning unit is brought into or out of contact with the intermediate transfer member during the primary transfer, the intermediate transfer member vibrates due to the action of contact or separation. Further, the fluctuation in the load on the rotating intermediate transfer member produces temporary fluctuations in rotation speed. Therefore, the toner image that is in the process of being transferred in the primary transfer process can be distorted because of the mode switching operation of the cleaning unit. Accordingly, the corresponding portion of the image outputted after the secondary transfer will be distorted. There is also a problem particular to color laser printers because the different colored toner images are stacked one on top of the other on the intermediate transfer member. That is, the overlap between different colored toner images can be shifted when the rotational speed of the intermediate transfer member changes, resulting in distortion in the colors of the output image. 
   Japanese Patent-Application Publication (Kokai) No. HEI-10-48967 discloses an image forming device that does not perform the mode-switching operation of the cleaning unit during the primary transfer operation. Accordingly, distortion in the output image that results from generation of vibration in the image forming device and fluctuations of the rotation load during primary transfer that can be caused by contact and separation of the cleaning unit can be suppressed to a certain extend. 
   However, in the image forming devices such as laser printers that form images by forming an electrostatic latent image and performing a primary transfer and a secondary transfer, it is not possible to sufficiently suppress distortion in the output image by merely controlling the contact and separation operations of the cleaning unit by taking the primary transfer into consideration. 
   It is an objective of the present invention to provide an image forming device capable of suppressing distortion and shifts in outputted images due to the contact and separation operations of a cleaning unit. 
   In order to achieve the above and other objects, according to the present invention, there is provided an image forming device including an endless photosensitive member, an exposure unit, a developing unit, an endless image bearing member, a secondary transfer unit, a cleaning unit, and a control unit. The endless photosensitive member moves in a first direction. The exposure unit performs exposure operations for exposing the photosensitive member at an exposure position to form a latent image on the photosensitive member. The developing unit develops the latent image into a developing-agent image on the photosensitive member at a developing position that is downstream from the exposure position in the first direction. The endless image bearing member contacts the photosensitive member at a primary transfer position that is downstream from the developing position in the first direction. The image bearing member moves in a second direction. The developing-agent image is transferred from the photosensitive member onto the image bearing member at the primary transfer position in primary transfer operations. The secondary transfer unit performs secondary transfer operations for transferring the developing-agent image from the image bearing member onto a recording medium at a secondary transfer position that is downstream from the primary transfer position in the second direction. The cleaning unit is switched between a contact condition where the cleaning unit is in contact with the image bearing member at a cleaning position and a separation condition where the cleaning unit is separated from the image bearing member. The cleaning position is downstream from the secondary transfer position and upstream from the primary transfer position in the second direction. The cleaning unit in the contact condition removes residual developing agent from the image bearing member after the secondary transfer operations. The control unit switches the cleaning unit between the contact condition and the separation condition during a stopped period wherein no latent image is being formed during the exposure operations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a printer; 
       FIG. 2  is a block diagram showing electrical configuration of the printer of  FIG. 1 ; 
       FIGS. 3  ( a )- 3  ( b ) are cross-sectional views schematically showing a cleaning unit provided in the printer of  FIG. 1  for cleaning an intermediate transfer member of the printer; 
       FIG. 4  is a time chart representing timing of switching operations of the cleaning unit; 
       FIG. 5  is a schematic side view showing positional relationships between an exposure point, a primary transfer point, and a cleaning point; 
       FIG. 6  is a schematic side view showing configuration of a photosensitive belt mechanism and an intermediate transfer belt mechanism according to a modification of the first embodiment; 
       FIG. 7  ( a ) is a schematic side view showing configuration of a photosensitive belt mechanism and an intermediate transfer belt mechanism according to another modification of the first embodiment; 
       FIG. 7  ( b ) is a block diagram showing essential components of the modification of FIG.  7 ( a ); 
       FIG. 8  is a cross-sectional view showing configuration of a printer according to a second embodiment of the present invention; and 
       FIG. 9  is a time chart showing timing of switching operations of an intermediate transfer belt cleaning unit of the printer of FIG.  8 . 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Next, color laser printers will be described according to embodiments of the present invention with reference to the attached drawings. 
   First, a color laser printer  1  according to a first embodiment will be explained. 
   As shown in  FIG. 1 , the printer  1  includes a casing  2 , a sheet-supply unit  4 , an image forming unit  5 , a photosensitive belt cleaning unit  50 , and an intermediate transfer belt cleaning unit  60 . 
   The sheet-supply unit  4  is for supplying sheets, and includes a sheet-supply tray  6 , a sheet-supply roller  7 , a feed roller  8 , and a register roller  9 . The sheet-supply tray  6  holds a stack of sheets  3 . The sheet-supply roller  7  presses on the uppermost sheet  3  of the stack in the sheet-supply tray  6 . Rotation of the sheet-supply roller  7  pulls one sheet  3  at a time from the top of the stack and transports the same to the feed roller  8  and further to the register roller  9 . Then, the sheet  3  is transported to the image forming unit  5 . 
   The image forming unit  5  is for forming images onto supplied sheets  3 , and includes a scanner unit  10 , a developing unit  11 , a photosensitive belt mechanism  12 , a scorotron charge unit  13 , an intermediate transfer belt mechanism  14 , a transfer roller  15 , and a fixing unit  16 . 
   The scanner unit  10  is for performing exposure operations to form an electrostatic latent image on a photosensitive belt  22  (described later) based on image data. Although not shown in the drawings, the scanner unit  10  includes a laser emitting unit for emitting laser light, a polygon mirror for scanning the laser light following a scan direction perpendicular to the rotational direction of the photosensitive belt  22 , a reflection mirror for designating the light path of the laser light, and a lens for focusing the laser light. Laser light that was emitted by the laser emitting unit based on image data irradiates the surface of the photosensitive belt  22  at an exposure point A via the polygon mirror, the reflection mirror, the lens and the like, thereby forming an electrostatic latent image on the surface of the photosensitive belt  22 . 
   The developing unit  11  is disposed at the rear portion of the casing  2  and includes developing cartridges  11 C,  11 M,  11 Y, and  11 K, which are aligned vertically separated by a predetermined distance from each other. The developing cartridges  11 C,  11 M,  11 Y, and  11 K each store magnetic toner as a developing agent in the corresponding color of cyan (C), magenta (M), yellow (Y), and black (BK). 
   The developing cartridges  11 C,  11 M,  11 Y, and  11 K each includes a developing roller  18  and, although not shown in the drawings, a layer-thickness regulating blade, a supply roller, and a toner holding portion A cartridge drive mechanism (not shown) moves the developing cartridges  11 C,  11 M,  11 Y, and  11 K horizontally to selectively bring the developing roller  18  of the developing cartridges  11 C,  11 M,  11 Y, and  11 K into and out of contact with the surface of the photosensitive belt  22 . Each of the developing cartridges  11 C,  11 M,  11 Y, and  11 K operates in substantially the same manner. That is, rotation of the supply roller supplies the toner housed in the toner holding portion to the developing roller  18 , and the layer-thickness regulating blade regulates the thickness of the toner on the developing roller  18 . When the developing roller  18  contacts the surface of the photosensitive belt  22  in this condition, the toner borne on the surface of the developing roller  18  is selectively transferred onto the photosensitive belt  22 , thereby developing the electrostatic latent image into a visible toner image on the photosensitive belt  22 . 
   The photosensitive belt mechanism  12  is disposed in front of the developing unit  11  in confrontation with the developing unit  11 . The photosensitive belt mechanism  12  includes mainly a first photosensitive belt roller  19 , a second photosensitive belt roller  20 , a third photosensitive belt roller  21 , and the photosensitive belt  22 . 
   The first photosensitive belt roller  19  is disposed in substantial confrontation with the yellow developing cartridge  11 Y, which is at the lowest position in the stack of developing cartridges. The second photosensitive belt roller  20  is disposed vertically above the first photosensitive belt roller  19  in substantial confrontation with the black developing cartridge  11 K, which is at the highest position in the stack of developing cartridges. The third photosensitive belt roller  21  is disposed diagonally above the first photosensitive belt roller  19  and diagonally below the second photosensitive belt roller  20 . 
   The photosensitive belt  22  is an endless belt provided with an organic photosensitive layer on its surface. The photosensitive belt  22  is wound around the photosensitive belt rollers  19 ,  20 , to  21 . That is, the photosensitive belt  22  is mounted in contact with the outer surface of the photosensitive belt rollers  19  to  21 , which are disposed in a triangular arrangement. When a motor (not shown) drives the second photosensitive belt roller  20  to rotate, then the photosensitive belt  22  rotates around the photosensitive belt rollers  19  to  21  in the counterclockwise direction shown in FIG.  1 . 
   The scorotoron charge unit  13  includes a charge wire, made from tungsten for example, that generates a corona discharge to charge the surface of the photosensitive belt  22  to a uniform positive charge The scorotoron charge unit  13  is disposed below the photosensitive belt mechanism  12  at a position between the third photosensitive belt roller  21  and the first photosensitive belt roller  19  and separated from the photosensitive belt  22  by a predetermined distance It should be noted that the scorotron charge unit  13  charges the surface of the photosensitive belt  22  as a preprocess of the exposure operations by the scanner unit  10 . 
   The intermediate transfer belt mechanism  14  is disposed to the front of the photosensitive belt mechanism  12 , and includes mainly a first intermediate transfer belt roller  23 , a second intermediate transfer belt roller  24 , a third intermediate transfer belt roller  25 , and an intermediate transfer belt  26 . 
   The first intermediate transfer belt roller  23  is disposed in substantial confrontation with the second photosensitive belt roller  20  via the photosensitive belt  22  and the intermediate transfer belt  26 . The second intermediate transfer belt roller  24  is disposed to the front of and below the first intermediate transfer belt roller  23 . The third intermediate transfer belt roller  25  is disposed above the second intermediate transfer belt roller  24  and below and to the front of the first intermediate transfer belt roller  23 . 
   The intermediate transfer belt  26  is an endless belt made from a conductive resin, such as polyamide or polycarbonate, which is dispersed with conductive particles, such as carbon. The intermediate transfer belt  26  is wound around the intermediate transfer belt rollers  23  to  25 . 
   The intermediate transfer belt  26  is disposed to contact the photosensitive belt  22  at a primary transfer point B between the first intermediate transfer belt roller  23  and the second photosensitive belt roller  20 . This contact generates friction F between the intermediate transfer belt  26  and the photosensitive belt  22 . The friction F moves the intermediate transfer belt  26  to follow the rotational movement of the photosensitive belt  22 , so that the intermediate transfer belt  26  rotates around the periphery of the intermediate transfer belt rollers  23  to  25  in the clockwise direction of  FIG. 1  as the photosensitive belt  26  rotates in the counterclockwise direction. The intermediate transfer belt  26  includes a marker  26   a , which indicates an origin of the intermediate transfer belt  26 . As shown in  FIG. 2 , the marker  26   a  is a hole in the present embodiment. The marker  26   a  enables a control unit  31  to be described later to keep track of rotational movement of the intermediate transfer belt  26 . 
   The transfer roller  15  is disposed in substantial confrontation with the second intermediate transfer belt roller  24  through the intermediate transfer belt  26 , and driven into and out of contact with the intermediate transfer belt  26  at a position downstream from the primary transfer point B in the moving direction of the intermediate transfer belt  26 . The transfer roller  15  is applied with a predetermined transfer bias by a transfer bias application circuit (not shown) and presses a sheet  3  against the intermediate transfer belt  26 . 
   The fixing unit  16  is disposed to the front of the intermediate transfer belt mechanism  14  and at a position downstream in the sheet transport direction, and includes a thermal roller  27 , a pressing roller  28 , and a pair of transport rollers  29 . The thermal roller  27  is configured from an internal metal layer, an external silicone rubber layer, and a halogen lamp for heating up the metal and silicone rubber layers. The pressing roller  28  presses against the thermal roller  27 . The pair of transport rollers  29  are positioned downstream from the thermal roller  27  and the pressing roller  28  in the transport direction of the sheet  3 . After the image forming unit  5  forms a color image on a sheet  3 , the sheet  3  passes between the thermal roller  27  and the pressing roller  28  so that the color image is thermally fixed onto the sheet  3 . 
   The photosensitive belt cleaning unit  50  is for cleaning the photosensitive belt  22 . The photosensitive belt cleaning unit  50  is fixedly disposed on the opposite side of the photosensitive belt mechanism  12  than the developing unit  11  and at a position downstream from the primary transfer point B with respect to the rotational direction of the photosensitive belt  22 . 
   The intermediate transfer belt cleaning unit  60  is for cleaning the intermediate transfer belt  26 , and is disposed in confrontation with the third intermediate transfer belt roller  25  through the intermediate transfer belt  26 . 
   As shown in  FIG. 2 , the printer  1  further includes the control unit  31  for performing overall control of the components described above. The control unit  31  includes internal components such as a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). The control unit  31  includes an image forming process main controller  31   a , a secondary transfer processor  31   e , a latent image forming processor  31   c , a development processor  31   d , a cleaning processor  31   f , and a counter  31   b . When image forming processes start, the image forming process main controller  31   a  is started up. 
   The image forming process main controller  31   a  performs initialization operations on all components that are subject to control operations during the image forming processes, and also performs controls for various components, with the exception of controls relating to latent image formation, development, secondary transfer, and cleaning of the intermediate transfer belt  26 . For example, the image forming process main controller  31   a  is connected to the photosensitive belt mechanism  12  through a main drive portion  33 . When the image forming process main controller  31   a  inputs a control signal to the main drive portion  33 , then the main drive portion  33  drives the photosensitive belt mechanism  12  using a motor (not shown) provided to the main drive portion  33 . 
   The secondary transfer processor  31   e  is connected to the transfer roller  15  through a secondary transfer mechanism driver, and the latent image forming processor is connected to the scanner unit  10 . The development processor  31   d  is connected to the developing unit  11  through the cartridge driver  31 , and the cleaning processor  31   f  is connected to the intermediate transfer belt cleaning unit  60  through an intermediate transfer belt cleaning unit drive portion  34 . 
   The counter  31   b  is connected to an origin sensor  35  that detects the marker  26   a  of the intermediate transfer belt  26  and output detection signals. Based on the detection signals form the origin sensor  35 , the counter  31   b  measures the time that elapses from the time when the marker  26   a  passes by a predetermined location. Because the intermediate transfer belt  26  of the present embodiment is configured to rotate at a predetermined speed, the elapsed time measured by the counter  31   b  can be used as a parameter that represents a coordinate position of the intermediate transfer belt  26 , wherein the marker  26   a  serves as a reference point. 
   Next, an explanation will be provided for the image forming operations of the printer  1 . 
   When the printer  1  starts image forming operations, the scorotron charge unit  13  starts charging the surface of the photosensitive belt  22  to the positive uniform charge. The latent image forming processor  31   c  starts driving the scanner unit  10  at a predetermined timing to form an electrostatic latent image that corresponds to a cyan image on the surface of the photosensitive belt  22 . 
   Explained in more detail, laser light from the scanner unit  10  irradiates based on input image data the positively charged surface of the photosensitive belt  22  at the exposure point A. This changes the electric potential at the surface of the photosensitive belt  22  that has been positively and uniformly charged, thereby forming an electrostatic latent image on the surface of the photosensitive belt  22 . Rotation of the photosensitive belt  22  transports thus formed electrostatic latent image toward the developing unit  11  located downstream from the exposure point A with respect to the rotational direction of the photosensitive belt  22 . 
   The development processor  31   d  inputs a control signal to a cartridge driver  32  at a predetermined timing before the electrostatic latent image reaches the developing unit  11 . In response to the control signal, the cartridge driver  32  drives the above-mentioned drive cartridge drive mechanism to bring the developing roller  18  of the developing cartridge  11 C into contact with the photosensitive belt  22 . At this time, the developing rollers  18  of the magenta, yellow, and black developing cartridges  11 M,  11 Y, and  11 K are kept separated from the photosensitive belt  22 . As a result, the electrostatic latent image is developed into a cyan toner image on the photosensitive drum  22  when passing by the developing unit  11 . After the development is completed, the developing roller  18  is separated from the photosensitive belt  22 . 
   Rotation of the photosensitive belt  22  transports the cyan toner image to the primary transfer point B, where the toner image is transferred from the photosensitive belt  22  onto the intermediate transfer belt  26 . This transfer is referred to as “primary transfer operations”. 
   The photosensitive belt cleaning unit  50  removes from the surface of the photosensitive belt  22  any residual toner that was not transferred onto the intermediate transfer belt  26  during the primary transfer operation. In this way, the photosensitive belt  22  is cleaned up by the photosensitive belt cleaning unit  50 . 
   Next, the photosensitive belt  22  is again charged to a uniform charge by the scorotron charge unit  13  and formed with an electrostatic latent image corresponding to a magenta image. The developing roller  18  of the magenta developing cartridge  11 M is brought into contact with the photosensitive belt  22 , and the developing rollers  18  of the cyan, yellow, and black developing cartridges  11 C,  11 Y, and  11 K are maintained separated from the photosensitive belt  22 . As a result, the electrostatic latent image that corresponds to a magenta image is developed on the photosensitive belt  22  into a magenta toner image, which is then transferred at the primary transfer point B onto the cyan toner image that was transferred onto the intermediate transfer belt  26  during the previous operation. 
   The same operations are performed for the other colors of yellow and black so that a multicolor toner image made from cyan (C), magenta (M), yellow (Y), and black (BK) toner is formed on the surface of the intermediate transfer belt  26 . 
   Afterward, the secondary transfer processor  31   e  controls a secondary transfer mechanism driver  36  to move the transfer roller  15  into contact with the intermediate transfer belt  26 . Also, a sheet  3  that was transported from the sheet-supply tray  6  passes between the transfer roller  15  and the intermediate transfer belt  26  at the same time that the multicolor toner image passes between the transfer roller  15  and the intermediate transfer belt  26 . As a result, the multicolor image is transferred onto the sheet  3 , thereby forming a color image on the surface of the sheet  3 . This transfer is referred to as “secondary transfer operations”. 
   After the secondary transfer is completed, the sheet  3  is transported to the fixing unit  16 , which fixes the color image onto the sheet  3 . Then, the pair of transport rollers  29  transport the sheet  3  to a pair of sheet discharge rollers  42 , which discharges the sheet  3  onto a sheet discharge tray  43  formed on the top of the casing  2 . 
   Next, the photosensitive belt cleaning unit  50  will be explained. As shown in  FIG. 1 , the photosensitive belt cleaning unit  5 D includes a photosensitive belt cleaning box  51 , a photosensitive belt cleaning roller  52 , a secondary photosensitive belt cleaning roller  53 , and a photosensitive belt cleaning blade  54 . 
   The photosensitive belt cleaning box  51  has a box shape with an opening at the side that confronts the photosensitive belt  22 . The space at the bottom of the photosensitive belt cleaning box  51  forms a waste-toner accumulation portion for accumulating toner that is scraped off by the photosensitive belt cleaning blade  54 . 
   The photosensitive belt cleaning roller  52  is a resilient member made from silicone rubber, for example, and is rotatably supported at the opening in the photosensitive belt cleaning box  51  at a position near the third photosensitive belt roller  21 . The photosensitive belt cleaning roller  52  is constantly in contact with the photosensitive belt  22  and rotates in the same direction as the photosensitive belt  22 . Although not shown in the drawings, a cleaning bias application circuit is provided to apply a predetermined cleaning bias to the photosensitive belt cleaning roller  52  with respect to the photosensitive belt  22 . 
   The secondary photosensitive belt cleaning roller  53  is formed from a metal roller and disposed so as to contact the photosensitive belt cleaning roller  52  from the opposite side of the photosensitive belt cleaning roller  52  than the photosensitive belt  22 . The secondary photosensitive belt cleaning roller  53  is applied with a predetermined bias with respect to the photosensitive belt cleaning roller  52 . 
   The photosensitive belt cleaning blade  54  is formed from a thin plate-shaped blade, and contacts the secondary photosensitive belt cleaning roller  53  at a side opposite from the photosensitive belt cleaning roller  52  to scrape toner from the surface of the secondary photosensitive belt cleaning roller  53 . 
   With this configuration, the photosensitive belt cleaning roller  52  electrically picks up toner that remains on the photosensitive belt  22  after the primary transfer operation. Then, the secondary photosensitive belt cleaning roller  53  electrically picks up the toner that clings to the photosensitive belt cleaning roller  52 . Further, the photosensitive belt cleaning blade  54  removes the toner from the secondary photosensitive belt cleaning roller  53 , whereupon the toner is collected in the waste toner accumulation portion. In this way, toner that remains after the primary transfer operation is removed as it passes by the photosensitive belt cleaning unit  50  so that the photosensitive belt  22  can be cleaned. 
   Next, the intermediate transfer belt cleaning unit  60  will be explained. The intermediate transfer belt cleaning unit  60  is for cleaning the intermediate transfer belt  26  by removing residual toner that remains on the intermediate transfer belt  26  after the secondary transfer operation in order to swingably supported to the casing  2 . As shown in FIG.  3 ( a ), the intermediate transfer belt cleaning unit  60  includes an intermediate transfer belt cleaning box  61 , an intermediate transfer belt cleaning roller  62 , a secondary transfer belt  63 , an intermediate transfer belt cleaning blade  64 , a protrusion  65 , and an oval rotation portion  66 . 
   The intermediate transfer belt cleaning box  61  has a box shape formed with an opening at the side in confrontation with the intermediate transfer belt  26 . The space at the bottom of the intermediate transfer belt cleaning box  61  forms a waste-toner accumulation portion for accumulating toner that is scraped off by the intermediate transfer belt cleaning blade  64 . 
   The intermediate transfer belt cleaning roller  62  is made from a metal roller that is rotatably supported at the opening of the intermediate transfer belt cleaning box  61  at a position in confrontation with the third intermediate transfer belt roller  25 . Also, the intermediate transfer belt cleaning roller  62  is applied with a predetermined cleaning bias with respect to the intermediate transfer belt  26 . 
   The secondary transfer belt  63  has substantially the same configuration as the secondary photosensitive belt cleaning roller  53  of the photosensitive belt cleaning unit  50  and is disposed in contact with the intermediate transfer belt cleaning roller  62 . The intermediate transfer belt cleaning blade  64  has substantially the same configuration as the photosensitive belt cleaning blade  54  and is disposed in contact with the secondary transfer belt  63 . 
   The protrusion  65  protrudes from a side of the intermediate transfer belt cleaning box  61  opposite from the intermediate transfer belt  26 . The oval rotation portion  66  contacts the protrusion  65  and is supported on the casing  2  so as to be rotatable around a rotational axis that is shifted from the oval center. 
   With this configuration, the intermediate transfer belt cleaning roller  62  can be brought into and out of contact with the intermediate transfer belt  26  as the oval rotation portion  66  rotates. 
   That is, to switch the intermediate transfer belt cleaning unit  60  from a contact condition shown in FIG.  3 ( a ) to a separation condition shown in FIG.  3 ( b ), the cleaning processor  31   f  shown in  FIG. 2  drives the intermediate transfer belt cleaning unit drive portion  34  to rotate the oval rotation portion  66 . As a result, the protrusion  65  is raised upward and the intermediate transfer belt cleaning roller  62  tilts downward toward the intermediate transfer belt  26 . This separates the intermediate transfer belt cleaning roller  62  from the intermediate transfer belt  26 . 
   To switch the intermediate transfer belt cleaning unit  60  from the separation condition shown in FIG.  3 ( b ) into the contact condition shown in FIG.  3 ( a ), the cleaning processor  31   f  controls the intermediate transfer belt cleaning unit drive portion  34  to drive the oval rotation portion  66  to rotate. As a result, the protrusion  65  moves downward from its raised position so that the intermediate transfer belt cleaning roller  62  is raised upward toward the intermediate transfer belt  26 . This brings the intermediate transfer belt cleaning roller  62  into contact with the intermediate transfer belt  26 . 
   When intermediate transfer belt cleaning unit  60  is in the contact condition as shown in FIG.  3 ( a ), the intermediate transfer belt cleaning roller  62  electrically catches residual toner clinging to the intermediate transfer belt  26 , and the secondary transfer belt  63  electrically catches the toner that was caught by and that clings to the intermediate transfer belt cleaning roller  62 . Then, the intermediate transfer belt cleaning blade  64  scrapes the toner off the secondary transfer belt  63  whereupon the toner accumulates in the waste toner accumulation portion. 
   Here, the cleaning processor  31   f  controls the intermediate transfer belt cleaning unit drive portion  34  at a predetermined timing that meets the following conditions. That is, the intermediate transfer belt cleaning unit  60  is maintained at its separation condition, where the intermediate transfer belt cleaning roller  62  is separated from the intermediate transfer belt  26  by a predetermined distance, until all of the four colors of toner images are primarily transferred onto the surface of the intermediate transfer belt  26 . Then, the intermediate transfer belt cleaning unit  60  is brought into its contact condition, where the intermediate transfer belt cleaning roller  62  is in contact with the intermediate transfer belt  26 , before the residual toner clinging on the intermediate transfer belt  26  after the secondary transfer operation reaches a cleaning point D, which is the position where the intermediate transfer belt cleaning roller  62  abuts against the intermediate transfer belt  26 . 
   Next, the timing to switch the intermediate transfer belt cleaning unit  60  between the contact condition and the separation condition will be described with reference to FIG.  4 .  FIG. 4  is a timing chart representing the relationships among timing of latent-image forming operation, timing of primary transfer operations, and timing of switching the intermediate transfer belt cleaning unit  60  between the contact condition and the separation condition. It should be noted that an operation for switching the intermediate transfer belt cleaning unit  60  from the separation condition to the contact condition will be referred to as “contact operation”, and an operation for switching the intermediate transfer belt cleaning unit  60  from the contact condition to the separation condition will be referred to as “separation operation”, hereinafter. 
     FIG. 5  is a schematic view for explaining the positional relationship of the exposure point A, the primary transfer point B, and the cleaning point D. 
   The following explanation will be provided assuming that during image forming operations, the main drive portion  33  rotates the photosensitive belt  22  and thus the intermediate transfer belt  26  at a fixed rotational speed v, and that the printer  1  is forming the maximum-sized image that the printer  1  is capable of forming. Also, operations to be described below are executed by the latent image forming processor  31   c  and the cleaning processor  31   f  of the control unit  31  at the timings shown in  FIG. 4  while referring to the elapsed time measured by the counter  31   b.    
   When the image forming operations are started, as shown in  FIG. 4  the latent image forming processor  31   c  executes latent image forming operations for cyan, magenta, yellow, and black in this order at a predetermined cycle T 0  to form the electrostatic latent images for these colors on the photosensitive belt  22 . Because the cycle T 0  equals to a rotation cycle T 0  of the intermediate transfer belt  26 , the cycle T 0  is expressed by a formula:
 
 T   0   =Lc/v 
 
   wherein Lc is a total length around the periphery of the intermediate transfer belt  26 ; and 
   v is the rotational speed of the photosensitive belt  22 . 
   That is, the latent image forming processor  31   c  controls the scanner unit  10  to perform the exposure operations. During this exposure operations, the latent image forming processor  31   c  controls scanner unit  10  to perform the latent image forming operations for a latent-image forming time T 1 , which corresponds to the size of the image to be formed, and not to perform the latent image forming operations for a no-image forming time T 2  until the next latent image forming operation starts. The latent image forming processor  31   c  repeats this control of performing and not performing the latent image forming operations for a plurality of times, that is, for four times in the present embodiment. Here, the no-image forming time T 2  is expressed in the following formula:
 
 T   2   =T   0   −T   1 
 
   wherein T 0  is the rotation cycle of the intermediate transfer belt  26 ; and 
   T 1  is the latent-image forming time, which is the maximum latent-image forming time of the printer  1  in this example. 
   Also, the latent-image forming time T 1  is expressed in the following formula:
 
 T   1   =L   1   /v 
 
   wherein L 1  is a length of the maximum-sized toner image that the printer  1  can form with respect to the peripheral direction of the photosensitive belt  22 ; and 
   v is the rotational speed of the photosensitive belt  22 . 
   The primary transfer operation starts at the primary transfer point B when a fixed delay time ΔT(AB) has elapsed after the corresponding latent image forming operation was started. The fixed delay time ΔT(AB) is calculated by a formula:
 
Δ T ( AB )= d ( AB )/ v 
 
   wherein d(AB) is a movement distance of the photosensitive belt  22  from the exposure point A to the primary transfer point B; and 
   v is the rotational speed of the photosensitive belt  22 . 
   Then, the toner image transferred onto the intermediate transfer belt  26  reaches the cleaning point D after a fixed delay time ΔT(BD), which is calculated by a formula:
 
Δ T ( BD )= d ( BD )/ v 
 
   wherein d(BD) is a movement distance of the intermediate transfer belt  26  from the primary transfer point B to the cleaning point D; and 
   v is the rotational speed of the intermediate transfer belt  26 . 
   Then, the toner image on the intermediate transfer belt  26  again reaches the primary transfer point B after a time ΔT(DB), whereupon a primary transfer operation is again performed for a next color image. The time ΔT(DB) is calculated by a formula:
 
Δ T ( DB )= d ( DB )/ v 
 
   wherein d(DB) is a movement distance of the intermediate transfer belt  26  from the cleaning point D to the primary transfer point B; and 
   v is the rotational speed of the intermediate transfer belt  26 . 
   Accordingly,
 
Δ T ( DB )= T   0   −ΔT ( BD )
 
   As shown in  FIG. 4 , the intermediate transfer belt cleaning unit  60  is maintained at the separation condition until the primary transfer operations for all of the cyan to black toner images that correspond to image data are completed. Afterward, the intermediate transfer belt cleaning unit  60  is switched into the contact condition at a predetermined timing that is before the leading edge of residual toner from the secondary transfer operations reaches the cleaning point D and that is during a period where neither a latent image forming operation nor a primary transfer operation is being performed. 
   The intermediate transfer belt cleaning unit  60  is again switched into the separation contacting condition at a predetermined timing that is after all of the residual toner from the secondary transfer operation is collected, that is, after the tail edge portion of the residual toner passes through the cleaning point D, and that is during a period wherein neither a latent image forming operation nor a primary transfer operation is being performed. 
   If the latent image forming operations for a subsequent set of image data are not started after a multicolor toner image for a previous set of image data is formed onto a sheet  3  until the intermediate transfer belt cleaning unit  60  completes cleaning of the intermediate transfer belt  26 , then the time required for processing a plurality of consecutive image data sets would increase. Therefore, in the present embodiment, as shown in  FIG. 4  the latent image forming operation that corresponds to a subsequent set of image date is started before the intermediate transfer belt cleaning unit  60  completes cleaning of the intermediate transfer belt  26 . In concrete terms, when latent image forming operations for a black image for a previous set of image data is completed, latent image forming operations for a cyan image for a subsequent set of image data is stated after a next-image movement time T 3  elapses. In the present embodiment, the next-image movement time T 3  is set equal to the no-image forming time T 2 . 
   According to the embodiment, the primary transfer point B and the like are set in the following manner so that the intermediate transfer belt cleaning unit  60  can be switched between the contact condition and the separation condition at the timing described above. 
   Firstly, the primary transfer point B is designated so as that the following relationship is fulfilled:
 
 T   1 &lt;Δ T ( AB )&lt; T   0 
 
or, in terms of distance;
 
 L   1 &lt; d ( AB )&lt; Lc 
 
   wherein Lc is the peripheral length of the intermediate transfer belt  26 . When these relationships are fulfilled, primary transfer operations will start during a period wherein no latent image forming operation is being performed, but not for a fixed time after latent image forming operations are stopped. That is, a time wherein neither a latent image forming operation nor a primary transfer operation is being performed can be designated. 
   Secondly, the cleaning point D is set at a position to fulfill the following relationship:
 
2 T   1   +T   3 −Δ T ( AB )&lt;Δ T ( BD )&lt; T   0   &lt;T   1   +T   2   +T   3 .
 
   By this, it is possible to perform the contact and separating operations of the intermediate transfer belt cleaning unit  60  at timing wherein no latent image forming operation or primary transfer operation is performed in order to remove only residual toner that remains from a secondary transfer operation. 
   Thirdly, the next-image movement time T 3  is set equal to the no-image forming time T 2  as mentioned above. Also, the peripheral length L 0  of the photosensitive belt  22  is set to less than the peripheral length of the intermediate transfer belt  26  (L 0 &lt;Lc). The primary transfer point B is set at a position that satisfies the relationship L 1 &lt;d(AB)&lt;L 0 , and the cleaning point D is set to a position that satisfies the relationship of L 1 &lt;Lc+L 1 −d(AB)&lt;d(BD)&lt;Lc. With this, the leading edge of residual toner remaining from a secondary transfer operation will reach the cleaning point D during a time wherein no latent image forming operation or primary transfer operation is being performed. This makes possible to perform the contact operation of the intermediate transfer belt cleaning unit  60  during the period wherein neither a latent image forming operation nor a primary transfer operation is being performed. Further, by setting the cleaning point D to fulfill the relationship of T(BD)&gt;T 1 , that is, d(BD)&gt;L 1  , the leading edge of residual toner from a secondary transfer operation will still have not reached the cleaning point D by the time that the primary transfer operation of the black toner image is completed. Accordingly, it is possible to to perform the separation operation of the intermediate transfer belt cleaning unit  60  during the period wherein neither a latent image forming operation nor a primary transfer operation is being performed. 
   An elapsed time Ts from when the latent image forming operation corresponding to the black toner image starts until the contact operation to bring the intermediate transfer belt cleaning unit  60  into toe contact condition is set to fulfill the following relationship:
 
2 T   1   +T   3   =T   0   +T   1   &lt;Ts&lt;ΔT ( AB )+Δ T ( BD )
 
   Also, the separating operation is performed after a time Tc=T 0  from the contact operation. 
   With this configuration, the separation operation is performed at a timing that is after the trailing edge of the residual toner from a secondary transfer operation passes by the cleaning point D, before the next cyan-toner image that was transferred to the intermediate transfer belt  26  in a primary transfer operation reaches the cleaning point D, and also during a period wherein no latent image forming operation or primary transfer operation is being performed. 
   As mentioned above, the intermediate transfer belt  26  vibrates when the intermediate transfer belt cleaning roller  62  is switched between the contact condition and the separation condition. Also, the resultant temporary fluctuation in rotational load of the photosensitive belt  22  and the intermediate transfer belt  26  can temporarily change the rotational speed of the photosensitive belt  22  and intermediate transfer belt  26 . These can result in image distortion during latent image forming operations to form an electrostatic latent image and shift in position where different colored images are transferred during primary transfer operations. However, these problems can be suppressed because the intermediate transfer belt cleaning unit  60  of the present embodiment can be switched into and out of contact with the intermediate transfer belt  26  while no latent image forming operation or primary transfer operation is being performed. Therefore, distortion in the output image formed on the sheet  3  after the secondary transfer operation can be sufficiently suppressed. 
   In particular, in contrast to conventional image forming devices, the image forming device according to the present embodiment prevents distortion of the latent image that can be caused by vibration during formation of the electrostatic latent image, which is a process that can easily influence the image output after the secondary transfer operation. Therefore, distortion in the output image can be efficiently suppressed. 
   Also, because the above timing control of the intermediate transfer roller cleaning unit  60  is performed in the cleaning processor  31   f  of the control unit  31 , residual toner removal can be performed at the desired timing by merely performing a simple control operation in the cleaning processor  31   f.    
   In the present embodiment, a friction force f that is generated between the intermediate transfer belt  26  and the intermediate transfer belt cleaning roller  62  at the time of when the intermediate transfer belt cleaning unit  60  is switched into the contact condition is set smaller than a friction force F that is generated between the intermediate transfer belt  26  and the photosensitive belt  22  at the primary transfer point B (f&lt;F). Therefore, the Intermediate transfer belt  26  will not slide across the surface of the photosensitive belt  22  even if the rotational load on the intermediate transfer belt  26  increases for the instant that the intermediate transfer belt cleaning roller  62  first contacts the intermediate transfer belt  26 . As a result, the intermediate transfer belt  26  can be rotated at the same speed as the photosensitive belt  22 . Accordingly, toner images from the primary transfer operation will not be shifted out of position or distorted from the action of the intermediate transfer belt cleaning roller  62  contacting the intermediate transfer belt  26 , so that the different colored images can be prevented from being shifted out of alignment with each other when stacked on top of each other at the primary transfer point B. 
   Here, it is possible to configure a bearing of the second photosensitive belt roller  20  to be movable and connecting a bearing of the first intermediate transfer belt roller  23  using a spring such that the photosensitive belt  22  can be set to contact the intermediate transfer belt  26  with the friction force F that is greater than the friction force f. With this configuration, fluctuation in the friction force F can be suppressed even if the printer  1  is vibrated, and fluctuation in the rotational load on the photosensitive belt  22  and the intermediate transfer belt  26  can be suppressed. 
   In the above embodiment, the rotational force of the photosensitive belt  22  is transmitted to the intermediate transfer belt  26  by the friction force F so as to rotate the intermediate transfer belt  26  in linked association with the photosensitive belt  22 . However, this configuration is not a limitation of the present invention. When the intermediate transfer belt  26  and the photosensitive belt  22  are rotated in linked association by the friction force generated by their mutual contact, the intermediate transfer belt  26  and the photosensitive belt  22  can easily slide at their contact surfaces due to fluctuation in the rotational load of the intermediate transfer belt  26  generated by the contact or separation operation of the intermediate transfer belt cleaning unit  60 . However, this slippage can be easily prevented by the following configuration. 
   For example, as in  FIG. 6 , gears  71  and  73  could be provided on the outer surfaces of the second photosensitive belt roller  20  and the first intermediate transfer belt roller  23  to connect the second photosensitive belt roller  20  and the first intermediate transfer belt roller  23  by meshing engagement between the gears  71 ,  73 . The gear  73  follows rotation of the gear  71  that is driven by drive force of a drive motor (not shown) to which the motor  71  is connected, so that the intermediate transfer belt  26  and the photosensitive belt  22  can be rotated at the same speed. 
   With this configuration, the intermediate transfer belt  26  can be reliably prevented from slipping across the surface of the photosensitive belt  22 . Accordingly, positional shifts between the different colored images, which can be caused by the contact and separation condition of the intermediate transfer belt cleaning roller  62  when the different colored toner images are transferred on top of each other by the primary transfer operations, can be prevented. 
   In an alternative example, the intermediate transfer belt  26  and the photosensitive belt  22  could be driven to rotate by different motors as shown in FIG.  7 ( a ), and the rotational speed of the motors could be controlled using the same reference signal oscillator  99  shown in FIG.  7 ( b ). 
   Described in more detail, the photosensitive belt mechanism  12  shown in FIG.  7 ( a ) is additionally provided with gears  82 ,  83 , and  84 . The gear  82  is connected to a direct current (DC) motor  81 . The gear  83  is connected to the second photosensitive belt roller  20 . The gear  84  is for transmitting drive force from the gear  82  to the gear  83 . Rotational force generated by the DC motor  81  is transmitted to the second photosensitive belt roller  20  through the gear  84  so drive the second photosensitive belt roller  20 . 
   The intermediate transfer belt mechanism  14  shown in FIG.  7 ( a ) is additionally provided with gears  86 ,  87 ,  88 , and  89 . The gear  86  is connected to a DC motor  85 . The gear  87  is connected to the first intermediate transfer belt roller  23 . The gears  88 ,  89  are for transmitting drive force from the gears  86  to  87 . Rotational force generated by the DC motor  85  is transmitted to the first intermediate transfer belt roller  23  through the gears  86 ,  87 ,  88 ,  89  to drive the first intermediate transfer belt roller  23 . 
   As shown in FIG.  7 ( b ), the main drive portion  33  includes the DC motor  81  connected to the gear  82 , a drive circuit  91  connected to the DC motor  81 , a comparing circuit  93 , the DC motor  85  connected to the gear  86 , a drive circuit  95  connected to the DC motor  85 , the comparing circuit  97 , and the reference signal generating oscillator  99 . 
   With this configuration, the comparing circuit  93  receives the rotation pulse from a sensor (not shown) that is incorporated in the DC motor  81  and outputs to the drive circuit  91  a drive signal based on the rotation pulse and on a reference signal from the reference signal generating oscillator  99 . The drive circuit  91  supplies drive power based on the drive signal to the DC motor  81  to drive the drive motor  81  at a fixed rotational speed. 
   In the same way, the comparing circuit  97  receives a rotation pulse from a sensor of the drive motor  85  and outputs to the drive circuit  95  a drive signal based on the rotation pulse and on the reference signal from the reference signal generating oscillator  99 . The drive circuit  95  supplies drive power based on the drive signal to the DC motor  85  to drive the DC motor  85  at the same rotational speed as the DC motor  81 . 
   With this configuration both the DC motors  81 ,  85  are maintained at a fixed rotational speed by the same reference signal generating oscillator  99 , Therefore, the rotational speeds of the DC motors  81 ,  85  can be properly matched and the intermediate transfer belt  26  and the photosensitive belt  22  can be rotated at the same speed. Accordingly, the above-described problems of distorted image because of the contact and separation operations of the intermediate transfer belt cleaning unit  60  and color shifts in the multicolor image by positional shift during the primary transfer operations can be prevented. 
   Next, configuration and operation of a printer  101  according to a second embodiment of the present invention will be explained with reference to  FIGS. 7 and 8 . Components that are the same or similar to those in the first embodiment are assigned with the same numberings, and their explanation will be omitted to avoid duplication of explanation. 
   As shown in  FIG. 8 , the printer  101  includes a charge unit  103 , a photosensitive drum  105 , a developing unit  110 , an intermediate transfer member mechanism  120 , a photosensitive drum cleaning unit  130 , and an intermediate transfer member cleaning unit  150 . 
   The intermediate transfer member mechanism  120  includes an intermediate transfer belt  121  and a plurality of intermediate transfer belt rollers  123  to  127  for rotating the intermediate transfer belt  121  while supporting the intermediate transfer belt  121  from the inside. The photosensitive drum cleaning unit  130  has substantially the same configuration as the photosensitive belt cleaning unit  50  and the intermediate transfer member cleaning unit  150  has substantially the same configuration as the intermediate transfer belt cleaning unit  60 . The photosensitive drum cleaning unit  130  is disposed downstream from the primary transfer point B in the rotational direction of the photosensitive drum  105 . The charge unit  103  is disposed further downstream than the photosensitive drum cleaning unit  130 . 
   The developing unit  110  is a rotating type developing unit that has a plurality of developing cartridges, that is, a cyan developing cartridge storing cyan toner, a magenta developing cartridge storing magenta toner, a yellow developing cartridge storing yellow toner, and a black developing cartridge storing black toner. One of four developing rollers  111  to  114  is provided to each of the developing cartridges. 
   When images are to be formed, first the photosensitive drum  105  is driven to rotate by a motor (not shown). Friction force between the photosensitive drum  105  and the intermediate transfer belt  121  rotates the intermediate transfer belt  121  in linked association with rotation of the photosensitive drum  105 . 
   Also, the charge unit  103  charges the surface of the photosensitive drum  105  to a uniform charge, and the developing unit  110  rotates to bring the developing roller  112 , which bears cyan toner, into contact with the photosensitive drum  105 . An electrostatic latent image for cyan is formed at the exposure point A by using laser light. Rotation of the photosensitive drum  105  transports the electrostatic latent image to a position in confrontation with the developing roller  112 , which develops the electrostatic latent image into a cyan color toner image. Further rotation of the photosensitive drum  105  transports the cyan toner image to the primary transfer point B, whereupon the cyan toner image is transferred onto the intermediate transfer belt  121  in a primary transfer operation. Afterward, rotational movement of the intermediate transfer belt  121  moves the toner image to the secondary transfer point C and the cleaning point D in this order and back to the primary transfer point B. 
   Before the cyan toner image reaches the primary transfer point B again, the photosensitive drum cleaning unit  130  removes all residual toner remaining from the primary transfer operation off the photosensitive drum  105 . 
   Next, the charge unit  103  again charges the surface of the photosensitive drum  105 . An electrostatic latent image for magenta color is formed at the exposure point A at timing that matches the rotation cycle T 0  of the intermediate transfer belt  121 . At timing that matches this, the developing unit  110  rotates until the developing roller  111 , which bears magenta toner, abuts the photosensitive drum  105  to develop the electrostatic latent image into a magenta toner image on the surface of the photosensitive drum  105  as the electrostatic latent image passes by the developing roller  111 . 
   Afterward, the magenta toner image is transferred on top of the cyan toner image at the primary transfer point B in a primary transfer operation. These operations are repeated for yellow and black to form a multicolor toner image by overlapping all four colors of toner image on the surface of the intermediate transfer belt  121 . 
   As the multicolor toner image passes by the secondary transfer point C, the multicolor toner image is transferred onto the sheet  3  that passes between the transfer roller  140  and the intermediate transfer belt roller  126 . Before the residual toner from the secondary transfer operation reaches the cleaning point D, the intermediate transfer member cleaning unit  150  is switched into a contact condition where the intermediate transfer member cleaning unit  150  abuts against the surface of the intermediate transfer belt  121  to start cleaning the surface of the intermediate transfer belt  121 . 
   Because the printer  101  of the present embodiment uses the photosensitive drum  105 , taking the size of the printer  101  into consideration, the distance d(AB) from the exposure point A to the primary transfer point B can only be made so long. For this reason, the distance d(AB) is shorter than the maximum-sized toner image that the printer  101  can form, that is, as determined by the maximum sized sheet that the printer  101  can print on. 
   To cope with this difference, the exposure point A, the primary transfer point B, and the cleaning point D are located at positions that fulfill the relationship explained below and latent image forming, primary transfer, and cleaning operations are performed at a predetermined timing to be described below. It should be noted that the following example will be explained assuming that the next-image movement time T 3  and the no-image forming time T 2  are equal to each other (T 3 =T 2 ). Also, the no-image forming time T 2  is equal to the rotation cycle T 0  of the intermediate transfer belt  121  less the latent-image forming time T 1  (T 2 =T 0 −T 1 ). 
   As shown in  FIG. 9 , the primary transfer operation for the leading edge of the cyan toner image, for which latent image forming and development processes have been completed, starts while latent image forming operations are still being performed for later parts of the cyan toner image. That is, the fixed delay time ΔT(AB) is less than the latent-image forming time T 1  (i.e., ΔT(AB)&lt;T 1 ). Therefore, in the present embodiment the primary transfer point B is set to a location so that the fixed delay time ΔT(AB) is less than the no-image forming time T 2  (i.e., ΔT(AB)&lt;T 2 ). As a result, the primary transfer operation can be completed during a period wherein latent image forming is not being performed so that a period wherein neither latent image forming nor primary transfer operation is performed can be secured. It should be noted that the fixed delay time ΔT(AB) is the time required to move any particular point on the surface of the photosensitive drum  105  from the exposure point A to the primary transfer point B, and is calculated by a formula:
 
Δ T ( AB )= d ( AB )/ v )
 
   wherein d(AB) is a moving distance of the photosensitive drum  105  from the exposure point A to the primary transfer point B; and 
   v is the rotational speed of the photosensitive drum  105 . 
   By switching the intermediate transfer member cleaning unit  150  between the contact and separation conditions during a period when neither latent image forming nor primary transfer operations are performed, shift and distortion during image formation can be prevented. 
   Further, the cleaning point D is located at a position to fulfill the following relationship:
 
 T   1 &lt;Δ T ( BD )&lt; T   0   &lt;T   1   +T   2   +T   2 −Δ T ( AB )
 
   wherein ΔT(AB) is the time required for the toner image that was transferred in a primary transfer operation to the surface of the intermediate transfer belt  121  to reach the cleaning point D via the secondary transfer point C, and is calculated by a formula:
 
Δ T ( BD )= d ( BD )/ v 
 
   wherein d(BD) is a moving distance of the photosensitive drum  105  from the primary transfer point B to the cleaning point D; and 
   v is the rotational speed of the intermediate transfer belt  121 , which equals the rotational speed of the photosensitive drum  105 . 
   This insures that only residual toner from a secondary transfer operation is removed from the intermediate transfer belt  121  without damaging a toner image that has not yet been transferred in a secondary transfer operation. 
   With this configuration, the intermediate transfer member cleaning unit  150  can be switched into the contact condition to clean the intermediate transfer belt  121  during a period wherein no latent image forming or primary transfer operation is being performed and before the leading edge of the multicolor toner image that was transferred in a secondary transfer operation reaches the cleaning point D. Further, the intermediate transfer member cleaning unit  150  can be switched into the separation condition during a period wherein no latent image forming or primary transfer operation is being performed, after the trailing edge of residual toner from a secondary transfer operation reaches the cleaning point D, and before a next cyan toner image reaches the cleaning point D. Therefore, image shift and distortion caused by the contact and separation operations of the intermediate transfer member cleaning unit  150  can be prevented. 
   It should be noted that in the second embodiment, the primary transfer point B is set to a position that fulfills the following relationship:
 
 d ( AB )&lt; Lc−L   1 
 
   wherein Lc is a total length around the periphery of the intermediate transfer belt  121 ; and 
   L 1  is a maximum length of the maximum-sized toner image that the printer  101  can form with respect to the peripheral direction of the photosensitive drum  105 . 
   As a result, a period will exist wherein neither latent image forming nor primary transfer operations are performed. Also, the cleaning point D is set to a position that fulfills the following relationship:
 
 L   1 &lt; d ( BD )
 
   wherein d(BD) is a moving distance of the intermediate transfer belt  121  from the primary transfer point B to the cleaning point D. 
   As a result, the contact operation can be performed at a time when neither a latent image forming operation nor a primary transfer operation is being performed, and at the same time, residual toner from the secondary transfer operation can be removed without damaging toner images before they are transferred in a secondary transfer operation. 
   While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
   For example, in the same manner as the modification of the first embodiment, the printer  101  of the second embodiment can be provided with separate motors for driving the intermediate transfer belt  121  and the photosensitive drum  105  to rotate such that the intermediate transfer belt  121  and the photosensitive drum  105  do not slide against each other at the primary transfer point B. 
   Also, rotational shafts of the intermediate transfer belt  121  and the photosensitive drum  105  can be connected by gears so that the intermediate transfer belt  121  is rotated in linked association with rotation of the photosensitive drum  105  by rotational force transmitted by the gears. 
   When the gears are used in this manner, there will be no image distortion from the surfaces where the intermediate transfer belts  26 ,  121  contact the photosensitive belt  22  or the photosensitive drum  105  sliding against each other at the primary transfer point B. Therefore, the intermediate transfer belt cleaning unit  60  or the intermediate transfer member cleaning unit  150  can be switched into the contact or the separation condition without concern as to whether a primary transfer operation is being performed, as long as it is during a period wherein latent image forming operations are not being performed. Because contact and separation operations can be performed even if a primary transfer operation is being performed, the printer can be more freely designed with respect to location of the intermediate transfer belt cleaning unit  60  or the intermediate transfer member cleaning unit  150 . 
   In more concrete terms, if the condition that contact and separation operations of the intermediate transfer member cleaning unit  150  can only be performed when no primary transfer operations are being performed is removed from the second embodiment, then in order to insure that contact and separation operations of the intermediate transfer member cleaning unit  150  are performed when no latent image forming operations are being performed, then the intermediate transfer member cleaning unit  150  merely needs to be located at a position that fulfills the following relationship:
 
 T   1 &lt;Δ T ( AB )+Δ T ( BD )&lt; T   0 ,
 
or in terms of distance:
 
 L   1   &lt;d ( AB )+ d ( BD )&lt; Lc. 
 
   Although the above embodiments described the relationship of the exposure point A, the primary transfer point B, and the cleaning point D when latent image forming operations and primary transfer operations are performed at the timings showing in the time chart of FIG.  4  and the time chart of  FIG. 9 , the present invention can be applied to image forming devices that perform latent image forming operations and primary transfer operations at other timings as well. In order to perform contact and separation operations of an image bearing member cleaning unit of such image forming devices at timing when neither latent image forming nor primary transfer operations are being performed while performing latent image forming operations at timing to match the rotation cycle T 0  of an image bearing member, the exposure point A, the primary transfer point B, and the cleaning point D can be set at positions that fulfill the following relationship:
 
 d ( AB )&lt; Lc−L   1  and  d ( BD )&gt; L   1 
 
   or at positions that fulfill the following relationship: 
     L   1 &gt; d ( AB ) and  L   1   &lt;d ( BD ) and  d  ( AB )+ d ( BD )&gt; Lc+L   1   
   wherein Lc is a total length around the periphery of the image bearing member; and 
   L 1  is a length of a maximum-sized image that the image forming device can form.