Patent Publication Number: US-10317822-B2

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2017-157478 filed on Aug. 17, 2017, the entire subject-matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus including a photosensitive member capable of rotating in forward and reverse directions, and an image forming method of the image forming apparatus. 
     BACKGROUND 
     In the related art, an image forming apparatus has been known which is configured to rotate a photosensitive member in a reverse direction on re-conveyance of conveying a sheet with being inverted during a duplex printing. Specifically, according to this technology, when forming an image on the sheet, the photosensitive member is rotated in a forward direction and a transfer bias of a first polarity is applied to a transfer device. After forming an image on a first surface of the sheet, when a conveying direction of the sheet is switched, the photosensitive member is rotated in a reverse direction. 
     During a time period in which the photosensitive member is rotated in the reverse direction, the transfer bias is set to OFF or a bias of a second polarity in opposite to the first polarity so that developer is not to move from the photosensitive member to the transfer device. After switching the photosensitive member from the reverse rotation to the forward rotation, the transfer bias is set to the bias of the first polarity. 
     However, when switching the rotation direction of the photosensitive member from the reverse direction to the forward direction, the developer may be attached to a part, which faces a developing roller, of the photosensitive member. 
     SUMMARY 
     The specification discloses an image forming technology that enables to suppress developer from moving from a photosensitive member to a transfer device in a case where a rotation direction of the photosensitive member is switched from a reverse direction to a forward direction. 
     One illustrative aspect provides an image forming apparatus having: 
     a photosensitive member; 
     a charger configured to charge the photosensitive member; 
     a developing device configured to develop an electrostatic latent image on the photosensitive member; 
     a transfer device configured to transfer a developer image on the photosensitive member to a sheet; and 
     a controller configured to execute: 
     a printing control, in which the controller rotates the photosensitive member in a forward direction and applies a first transfer bias to the transfer device; 
     a reverse rotation control, in which the controller rotates the photosensitive member in a reverse direction and applies a second transfer bias to the transfer device after finishing the printing control, the second transfer bias being zero or having an opposite polarity to the first transfer bias and an absolute value smaller than the first transfer bias; and 
     a forward rotation control, in which the controller rotates the photosensitive member in the forward direction and applies a third transfer bias to the transfer device for a predetermined period of time including a time that a part of the photosensitive member reaches the transfer device after finishing the reverse rotation control, the part facing the developing device at a time of switching the rotating direction of the photosensitive member, and the third transfer bias having an opposite polarity to the first transfer bias and an absolute value larger than the second transfer bias. 
     The aspect provides an image forming method of an image forming apparatus that includes: 
     a photosensitive member; 
     a charger configured to charge the photosensitive member; 
     a developing device configured to develop an electrostatic latent image on the photosensitive member; and 
     a transfer device configured to transfer a developer image on the photosensitive member to a sheet, 
     the method having: 
     a printing step of rotating the photosensitive member in a forward direction and applying a first transfer bias to the transfer device; 
     a reverse rotation step of rotating the photosensitive member in a reverse direction and applying a second transfer bias to the transfer device after finishing the printing step, the second transfer bias being zero or having an opposite polarity to the first transfer bias and an absolute value smaller than the first transfer bias; and 
     a forward rotation step of rotating the photosensitive member in the forward direction and applying a third transfer bias to the transfer device for a predetermined period of time including a time that a part of the photosensitive member reaches the transfer device after finishing the reverse rotation step, the part facing the developing device at a time of switching the rotating direction of the photosensitive member, and the third transfer bias having an opposite polarity to the first transfer bias and an absolute value larger than the second transfer bias. 
     According to the above configuration, although the developer in the developing device may be attached to the photosensitive member in a case where the rotating direction of the photosensitive member is switched from the reverse direction to the forward direction, it is possible to suppress the developer attached to the photosensitive member from moving to the transfer device because the third transfer bias, which has an opposite polarity to the first transfer bias and the absolute value larger than the second transfer bias, is applied to the transfer device while the developer passes the transfer device. 
     It is therefore possible to suppress a surface potential of the photosensitive member from excessively increasing during the reverse rotation of the photosensitive member, and to suppress the developer attached to the photosensitive member from moving to the transfer device in a case where the rotation direction of the photosensitive member is switched from the reverse direction to the forward direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts a schematic configuration of a laser printer in accordance with an illustrative embodiment of the present disclosure. 
         FIG. 2  is a flowchart depicting operations of a controller. 
         FIG. 3  is a timing chart depicting controls of a motor and a transfer bias. 
         FIGS. 4A to 4E  depict a change in a surface potential of a photosensitive member, and the like after a first surface of a sheet is printed until the motor is rotated in a reverse direction and is stopped. 
         FIGS. 5A to 5F  depict the change in the surface potential of the photosensitive member, and the like after the motor is switched to a forward rotation until exposure starts. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an illustrative embodiment of the present disclosure will be described in detail with reference to the drawings. 
     In below descriptions, the directions are described on the basis of directions shown in  FIG. 1 . That is, in  FIG. 1 , a right side of the drawing sheet is referred to as ‘front side’, a left side of the drawing sheet is referred to as ‘rear side’, an inner side of the drawing sheet is referred to as ‘right side’ and a front side of the drawing sheet is referred to as ‘left side’. Also, the upper and lower direction of the drawing sheet is referred to as ‘vertical direction’. 
     As shown in  FIG. 1 , a laser printer  100  includes a feeder unit  130 , an image forming unit  140 , a controller  200 , a motor  300 , and a re-conveyance mechanism  400 , in a main body housing  1 . 20 . A drive force of the motor  300  is transmitted to the feeder unit  130  and the image forming unit  140 . 
     The feeder unit  130  includes a supply tray  131  detachably mounted to a lower part of the main body housing  120 , and a conveyance mechanism  132  configured to convey a sheet S in the supply tray  131  toward the image forming unit  140 . The conveyance mechanism  132  includes a supply mechanism  133  configured to convey the sheet S in the supply tray  131  toward registration rollers  134  and the registration rollers  134  for aligning evenly each position of a leading end of the sheet S being conveyed. A first sheet sensor  101  is provided downstream of the registration rollers  134  with respect to a conveying direction of the sheet S. The first sheet sensor  101  is a sensor configured to detect the sheet S that is to be conveyed from the registration rollers  134  toward a transfer device  183 . The first sheet sensor  101  is arranged at a position closer to the registration rollers  134  than the transfer device  183 . 
     The first sheet sensor  101  includes a swing lever configured to swing as it is pushed by the sheet S being conveyed, and an optical sensor configured to detect the swinging of the swing lever. In the illustrative embodiment, while the sheet S is passing, i.e., when the swing lever is toppled by the sheet S, the first sheet sensor  101  becomes ON. 
     A third sheet sensor  103  is provided upstream of the registration rollers  134  with respect to the conveying direction of the sheet S. The third sheet sensor  103  is a sensor configured to detect the sheet S that is to be conveyed from the supply mechanism  133  or the re-conveyance mechanism  400  toward the registration rollers  134 . The third sheet sensor  103  has a configuration similar to the first sheet sensor  101 . The registration rollers  134  are configured so that the registration rollers are to contact the conveyed sheet S with rotation thereof being stopped and to start rotation after predetermined time since the sheet S is detected by the third sheet sensor  103 , thereby aligning evenly the leading end of the sheet S. 
     The image forming unit  140  has a function of forming an image on the sheet S. The image forming unit  140  includes an exposure device  150 , a process unit  160 , and a fixing device  170 . 
     The exposure device  150  is provided at an upper part in the main body housing  120 , and includes a laser light emitting unit, a polygon mirror, a lens, a reflector and the like, which are not shown. The exposure device  150  is configured to irradiate a laser beam to a surface of a photosensitive member  181  (which will be described later) by high-speed scanning. 
     The process unit  160  includes a photosensitive member  181 , a charger  182 , a transfer device  183 , and a process cartridge PC. In the process cartridge PC, developer is accommodated. The photosensitive member  181  is a circular cylinder-shaped photosensitive drum. The photosensitive member  181  is configured to rotate in forward and reverse directions. Here, the forward direction indicates a rotating direction on image formation, and the reverse direction indicates a direction opposite to the forward direction. 
     The charger  182  includes a charging wire and a grid electrode, and is configured to charge the photosensitive member  181  by corona discharge. The transfer device  183  is a circular column-shaped transfer roller, and is in contact with the photosensitive member  181 . The transfer device  183  is coupled to a rotary shaft of the photosensitive member  181  by a gear. Thereby, the transfer device rotates so that a contact part with the photosensitive member  181  is to move in the same direction. 
     The process cartridge PC is detachably mounted to the main body housing  120  through an opening  122  that is to he opened and closed by a front cover  123  rotatably provided to a front wall of the main body housing  120 . The process cartridge PC includes a developing device  1  and a developer cartridge  2 . 
     The developing device  1  includes a developing roller  12  configured to supply the developer with being in contact with the photosensitive member  181  and a supply roller  13  configured to supply the developer to the developing roller  12 . The developer cartridge  2  is detachably mounted to the developing device  1 . The developer cartridge  2  has the developer accommodated therein, and includes a mechanism for delivering the developer to the developing device  1 . 
     In the process unit  160 , a surface of the photosensitive member  181  being rotated is uniformly charged by the charger  182  and is then exposed by the high-speed scanning of the laser beam from the exposure device  150 . Thereby, an electrostatic latent image based on image data is formed on the surface of the photosensitive member  181 . 
     Subsequently, the developer in the process cartridge PC is supplied to the electrostatic latent image on the photosensitive member  181 , so that a developer image is formed on the surface of the photosensitive member  181 . That is, the electrostatic latent image on the photosensitive member  181  is developed by the developing device  1 , so that the developer image is formed on the surface of the photosensitive member  181 . Then, the sheet S is conveyed between the photosensitive member  181  and the transfer device  183 , so that the developer image on the photosensitive member  181  is transferred to the sheet S. 
     The fixing device  170  includes a heating roller  171  and a pressing roller  172  to be pressed to the heating roller  171 . In the fixing device  170 , the developer image transferred to the sheet S is heat-fixed while the sheet S passes between the heating roller  171  and the pressing roller  172 . A second sheet sensor  102  configured to detect passing of the sheet S discharged from the fixing device  170  is provided downstream of the fixing device  170  with respect to the conveying direction of the sheet S. The second sheet sensor  102  has a configuration similar to the first sheet sensor  101 . 
     Rollers R capable of rotating in the forward direction and the reverse direction are provided downstream of the fixing device  170 . The rollers R are configured to rotate in the forward direction to thereby convey the sheet S in one direction and to rotate in the reverse direction to thereby convey the sheet S in the other direction. Here, one direction is a direction facing from the rollers R toward the discharge tray  121 , and the other direction is a direction facing from the discharge tray  121  toward the rollers R. The sheet S heat-fixed in the fixing device  170  is conveyed to the rollers R arranged downstream of the fixing device  170  and is sent toward the discharge tray  121  by the rollers R being rotated in the forward direction. 
     Here, during a duplex printing in which images are formed on both surfaces of the sheet S, before the sheet S is entirely discharged onto the discharge tray  121 , the rollers R are rotated in the reverse direction, so that the sheet S is pulled back into the main body housing  120 . The sheet S pulled back into the main body housing  120  passes a rear side of the fixing device  170  by a switching of a flapper  110  and is then sent to the re-conveyance mechanism  400 . 
     The re-conveyance mechanism  400  is a mechanism configured to invert a front surface and a back surface of the sheet S having the first surface on which the developer image is heat-fixed in the fixing device  170 , thereby re-conveying the sheet to an upstream side of the registration rollers  134 . The re-conveyance mechanism  400  is arranged between the image forming unit  140  and the supply tray  131 . The re-conveyance mechanism  400  includes a guide member  410 , and a plurality of return rollers  420 . 
     The guide member  410  is a guide for switching a direction of the sheet  5 , which passes the rear side of the fixing device  170  and is being conveyed downward, to a front direction. The return rollers  420  are rollers configured to return the sheet S guided by the guide member  410  to the upstream side of the registration rollers  134 . 
     The return rollers  420  are configured to rotate in the forward direction by the drive force of the motor  300 , irrespective of the rotating direction of the motor  300 . That is, the plurality of return rollers  420  is configured to rotate in a direction in which the sheet S is sent toward the registration rollers  134 , on both rotations of the forward rotation and the reverse rotation of the motor  300 . 
     The sheet S conveyed by the re-conveyance mechanism  400  is sent to the registration rollers  134  with the front and back surfaces being inverted. Thereby, after the leading end of the sheet S is aligned evenly by the registration rollers  134 , the sheet is again conveyed between the photosensitive member  181  and the transfer device  183 , so that the developer image on the surface of the photosensitive member  181  is transferred to a second surface of the sheet S. 
     The motor  300  is a motor configured to drive the photosensitive member  181 , the rollers R and the like, and is coupled to the photosensitive member  181 , the rollers R and the like. In a case where the motor  300  is rotated in the forward direction, the photosensitive member  181  and the rollers R are rotated in the forward direction. In a case where the motor  300  is rotated in the reverse direction, the photosensitive member  181  and the rollers R are rotated in the reverse direction. 
     Also, the motor  300  is coupled to the developing roller  12  via a one-way clutch. The one-way clutch is configured to transmit the drive force in a case where the motor  300  is rotated in the forward direction so that the developing roller  12  rotates, and not to transmit the drive force in a case where the motor  300  is rotated in the reverse direction so that the developing roller  12  does not rotate. 
     The controller  200  includes a CPU, a RAM, a ROM, a non-volatile memory, an ASIC, an input/output circuit, and the like. The controller  200  is configured to execute a variety of calculation processing on the basis of a printing command output from an external computer, signals output from the respective sheet sensors  101  to  103 , and programs and data stored in the ROM and the like, thereby controlling biases applied to the transfer device  183 , the rotation of the motor  300  and the like. A value of voltage applied to the transfer device  183  is referred to as “bias.” Meanwhile, in the below, only the control relating to the present disclosure will be described, and the descriptions of the other controls, for example, the controls of the feeder unit  130  and the fixing device  170  will be omitted. 
     The controller  200  is configured to execute a printing control of rotating the photosensitive member  181  in the forward direction and forming an image on the sheet S, and a reverse rotation control of rotating the photosensitive member  181  in the reverse direction. Also, the controller  200  is configured to execute a forward rotation control of rotating the photosensitive member  181  in the forward direction. 
     In a case of executing a printing control of developing the electrostatic latent image on the photosensitive member  181  and transferring the same to the sheet S, the controller  200  rotates the motor  300  in the forward direction to rotate the photosensitive member  181  in the forward direction, and applies a first transfer bias Vt 1  to the transfer device  183 . Also, in the case of executing the printing control, the controller  200  applies a first charging bias Vc 1  to the charger  182 , applies a first developing bias Vd 1  to the developing roller  12 , and rotates the developing roller  12  in the forward direction. In the case of executing the printing control, the developing roller  12  rotates in a direction opposite to the photosensitive member  181 . In other words, in the case of executing the printing control, the surfaces of the photosensitive member  181  and the developing roller  12  move in the same direction at contact parts and a peripheral speed of the developing roller  12  is higher. 
     Here, the first transfer bias Vt 1  may be set to −1.8 kV to −3.5 kV, for example. Also, the first charging bias Vc 1 , specifically, the bias to be applied to the grid electrode may be set to +610V, and the first developing bias Vd 1  may be set to a bias having an absolute value smaller than the first charging bias Vc 1 , for example, to +270V. In the meantime, in a case where the first transfer bias Vt 1 , the first charging bias Vc 1  and the first developing bias Vd 1  are set to the above values, a surface potential of the photosensitive member  181  immediately after the charging is about +600V, and the surface potential of the photosensitive member  181  immediately after passing the transfer device  183  is about +100V. 
     In a case where the controller  200  executes the reverse rotation control after finishing the printing control, the controller  200  rotates the motor  300  in the reverse direction to rotate the photosensitive member  181  in the reverse direction, and applies a second transfer bias Vt 2  to the transfer device  183 . The second transfer bias Vt 2  is a bias that has an opposite polarity to the first transfer bias Vt 1  and an absolute value smaller than the first transfer bias Vt 1 . The second transfer bias Vt 2  may be set to a bias having an absolute value larger than the first charging bias Vc 1 , for example, to +850V. Here, the second transfer bias Vt 2  has the same polarity as the surface potential of the photosensitive member  181  and the absolute value equal to or larger than the surface potential, so that it is possible to suppress current from flowing from the surface of the photosensitive member  181  toward the transfer device  183 . 
     In a case where the controller  200  executes the next printing control after finishing the reverse rotation control, the controller  200  executes the forward rotation control before executing the next printing control. In the forward rotation control, the controller rotates the motor  300  in the forward direction to rotate the photosensitive member  181  in the forward direction, and applies a third transfer bias Vt 3  to the transfer device  183  for a predetermined period of time T 1  including a time that a part Pd (refer to  FIG. 5A ), which faces the developing roller  12  at the time of switching the rotating direction, of the photosensitive member  181  reaches the transfer device  183 . The predetermined period of time T 1  is set to a time period within a length of time that the photosensitive member  181  rotates one turn. 
     The third transfer bias Vt 3  is a bias that has an opposite polarity to the first transfer bias Vt 1  and an absolute value larger than the second transfer bias Vt 2 . The third transfer bias Vt 3  may be set to a bias having an absolute value smaller than the first transfer bias Vt 1 , for example, to +1.6 kV. 
     When the predetermined period of time T 1  elapses after the controller  200  switches the rotating direction of the photosensitive member  181  from the reverse direction to the forward direction, the controller switches a transfer bias, which is to be applied to the transfer device  183 , from the third transfer bias Vt 3  to the first transfer bias Vt 1 . The controller  200  starts the exposure by the exposure device  150  after a part Pt (refer to  FIG. 5C ), which faces the transfer device  183  at the time of switching the transfer bias, of the photosensitive member  181  passes a position LP (refer to  FIG. 5F ) facing the exposure device  150 . 
     In a case of switching the conveying direction of the sheet S, the controller  200  changes the rotating direction of the motor  300 . Here, in a case where the motor  300  is rotated in the reverse direction to rotate the photosensitive member  181  in the reverse direction, the drive force of the motor  300  is not transmitted to the developing roller  12  by the one-way clutch. 
     Subsequently, operations of the controller  200  are described in detail. 
     As shown in FIG,  2 , the controller  200  determines whether a printing command is received (S 1 ). In a case where a printing command is not received (No) in step S 1 , the controller  200  ends the control. 
     In a case where a printing command is received (Yes) in step S 1 , the controller  200  rotates the motor  300  in the forward direction (S 2 ). Thereby, the developing roller  12  is rotated in the forward direction together with the photosensitive member  181 . After step S 2 , the controller  200  applies the first charging bias Vc 1  to the charger  182 , applies the first developing bias Vd 1  to the developing roller  12 , and applies the first transfer bias Vt 1  to the transfer device  183  (S 3 ). Thereby, in a case of transferring the developer to the first surface of the sheet S, the motor  300  is rotated in the forward direction and the transfer bias is set to the first transfer bias Vt 1  (for example, time t 1  in  FIG. 3 ). 
     After step S 3 , the controller  200  executes exposure processing of exposing the photosensitive member  181  by the exposure device  150  (S 4 ). Specifically, after a predetermined time since the first sheet sensor  101  becomes ON, the controller  200  executes the exposure processing corresponding to an image of one page, and forms an electrostatic latent image corresponding to an image of one page on the photosensitive member  181 . 
     After step S 4 , the controller  200  determines whether it is necessary to re-convey the sheet S by determining whether image data for which the exposure processing is to be executed next time is data corresponding to the second surface of the sheet S (S 5 ). In a case where it is determined that it is necessary to re-convey the sheet S (Yes) in step S 5 , the controller  200  applies the second transfer bias Vt 2  to the transfer device  183  after the transfer of the developer image to the first surface of the sheet S is completed ( 56 ). That is, after the transfer of the developer image to the first surface of the sheet S is completed, the transfer bias is switched from the first transfer bias Vt 1  to the second transfer bias Vt 2  (time t 2  in  FIG. 3 ). In the meantime, the controller may determine whether the transfer of the developer image to the first surface of the sheet S is completed, based on elapse time after the first sheet sensor  101  becomes ON, for example. 
     After step S 6 , the controller  200  stops the motor  300  after the developer image on the sheet S is fixed by the fixing device  170 , specifically, when a part adjacent to a rear end portion of the sheet S is supported by the rollers R (S 7 , time t 3  in  FIG. 3 ). In the meantime, the stop timing of the motor  300  may be set on the basis of elapse time after the second sheet sensor  102  becomes OFF, for example. 
     After step S 7 , the controller  200  rotates the motor  300  in the reverse direction (S 8 , time t 4  in  FIG. 3 ). During the reverse rotation of the motor  300 , the photosensitive member  181  is rotated in the reverse direction and the drive force is not transmitted to the developing roller  12 . Also, in step S 6 , while the transfer bias is switched from the first transfer bias Vt 1  to the second transfer bias Vt 2  and the photosensitive member  181  is thus rotated in the reverse direction, the second transfer bias Vt 2  is applied to the transfer device  183 . 
     After step S 8 , the controller  200  stops the motor  300  after the rear end of the sheet S escapes from the rollers R and before the leading end of the sheet S reaches the registration rollers  134  (S 9 , time t 5  in  FIG. 3 ). In the meantime, the stop timing of the motor  300  may be set, as described above. After step  59 , the controller  200  rotates the motor  300  in the forward direction (S 10 , time to in  FIG. 3 ). Thereby, the developing roller  12  is rotated in the forward direction together with the photosensitive member  181 . 
     After step  510 , specifically, at substantially the same timing as step S 10 , the controller  200  applies the third transfer bias Vt 3  to the transfer device  183  (S 11 , time to in  FIG. 3 ). After step S 11 , the controller  200  applies the first transfer bias Vt 1  to the transfer device  183  after the predetermined period of time T 1  elapses since the applying of the third transfer bias Vt 3  starts (S 12 , time t 7  in  FIG. 3 ). 
     After step S 12 , the controller  200  causes the sheet S to stand by at the position of the registration rollers  134  for a prescribed time T 2  (S 13 ). After step S 13 , the controller  200  returns to step S 4  and executes the exposure processing. 
     Here, the prescribed time T 2  may be set on the basis of a following equation (1).
 
 T 2≥TA−( TB+TC )   (1)
 
     TA: time necessary for the part Pt, which faces the transfer device  183  at a time of switching the transfer bias from the third transfer bias Vt 3  to the first transfer bias Vt 1 , of the photosensitive member  181  to reach the position LP (the position to be exposed by the exposure device  150 ) facing the exposure device  150   
     TB: time after the first sheet sensor  101  becomes ON until the exposure processing starts 
     TC: time necessary for the leading end of the sheet S to move from a position at the time of switching the transfer bias from the third transfer bias Vt 3  to the first transfer bias Vt 1  it to the position of the first sheet sensor  101  (movement distance/conveying speed of the sheet S) 
     In a case where it is not possible to lengthen a distance of a re-conveyance path (refer to the broken line in  FIG. 1 ) due to miniaturization of the laser printer  100 , the first sheet sensor  101  becomes ON and a part (a part of a surface potential E 2  in  FIG. 5F ) at which the surface potential is increased is exposed before the part Pt, which faces the transfer device  183  at the time of switching the transfer bias from the third transfer bias Vt 3  to the first transfer bias Vt 1 , of the photosensitive member  181  reaches the position LP facing the exposure device  150 . In this case, the developer image becomes faint. However, in the illustrative embodiment, since the sheet S is caused to stand by at the position of the registration rollers  134  during the prescribed time T 2 , it is possible to expose a part (a part of a surface potential E 1  in  FIG. 5F ) of the proper surface potential. In the meantime, in a case where the distance of the re-conveyance path is sufficiently long, the standby time at the registration rollers  134  may be set to a normal time (a time enough to align evenly the leading end of the sheet S). That is, in the illustrative embodiment, before performing the printing on the second surface of the sheet S, the sheet S is caused to stand by at the position of the registration rollers  134  during the prescribed time T 2  longer than the normal standby time. However, in a case where the distance of the re-conveyance path is sufficiently long, it is not necessary to cause the sheet S to stand by for a time period longer than the normal standby time. 
     In the meantime, the motor  300  is coupled to the registration rollers  134  via a clutch (not shown). The controller  200  turns on/off the clutch, thereby rotating/stopping the registration rollers  134 . 
     In a case where it is determined that the re-conveyance is not necessary (No) in step  55 , the controller  200  determines whether the printing is over, specifically, whether the printing of the number of pages designated in the printing command is all completed (S 14 ). In a case where it is determined that the printing is not over (No) in step S 14 , the controller  200  returns to the processing of step S 4 . In a case where it is determined that the printing is over (Yes) in step S 14 , the controller  200  ends the control. 
     Subsequently, an example of the operation of the controller  200  is described in detail. 
     In a case of transferring the developer image to the first surface of the sheet S (for example, time t 1  in  FIG. 3 ), the controller  200  rotates the motor  300  in the forward direction to rotate the photosensitive member  181  in the forward direction and sets the transfer bias to the first transfer bias Vt 1 , which is a minus value. In this situation, the surface potential of the photosensitive member  181  is as shown in  FIG. 4A . Specifically, in a case of forming an image on the first surface of the sheet S, a part, which is located downstream of the charger  182  with respect to the rotating direction of the photosensitive member  181  and upstream of the transfer device  183  with respect to the rotating direction, of the photosensitive member  181  has a surface potential E 1  that is suitable for the exposure. Also, a part, which is located downstream of the transfer device  183  with respect to the rotating direction and upstream of the charger  182  with respect to the rotating direction, of the photosensitive member  181  has a surface potential E 0  close to zero (0) due to an influence of the first transfer bias Vt 1 . That is, the surface potential E 1  of the part charged by the charger  182  is reduced to the surface potential E 0  due to the influence of the first transfer bias Vt 1 . 
     After the transfer of the developer image to the first surface of the sheet S is completed, the controller  200  switches the transfer bias from the first transfer bias Vt 1  to the second transfer bias Vt 2  (time t 2  in  FIG. 3 ). Thereby, as shown in  FIG. 4B , the part of the photosensitive member  181  charged by the charger  182  is little influenced by the second transfer bias Vt 2 , which is a plus small value, and passes the transfer device  183  with keeping the surface potential E 1 . 
     Thereafter, at a predetermined timing before the sheet S having the first surface on which the developer image has been transferred passes through the fixing device  170  and is completely discharged to the discharge tray  121  by the rollers R, the controller  200  stops the motor  300  (time t 3  in  FIG. 3 ), as shown in  FIG. 4C . At this time, since the photosensitive member  181  has rotated one turn or more from the state of  FIG. 4B , the entire surface of the photosensitive member  181  has the surface potential E 1 . Here, the photosensitive member  181  rotates one turn or more, so that the part of the surface potential E 1  passes the charger  182 . However, the surface potential is kept at E 1  without being influenced by the charger  182 . 
     Thereafter, as shown in  FIG. 4D , the controller  200  rotates the motor  300  in the reverse direction (time t 4  in  FIG. 3 ) to rotate the photosensitive member  181  in the reverse direction. At this time, the drive force of the motor  300  is not transmitted to the developing roller  12  by the one-way clutch. The developing roller  12  is in a stationary state or in a state in which it rotates slightly in the reverse direction by a frictional force from the photosensitive member  181 . At this time, the surfaces of the photosensitive member  181  and the developing roller  12  move in the same direction at the contact parts and a peripheral speed of the photosensitive member  181  is set higher. 
     Thereafter, as shown in  FIG. 4E , the controller  200  stops the motor  300  (time t 5  in  FIG. 3 ) before the leading end of the sheet S reaches the registration rollers  134 , thereby stopping the photosensitive member  181 . Then, as shown in  FIG. 5A , the controller  200  rotates the motor  300  in the forward direction to rotate the photosensitive member  181  in the forward direction and switches the transfer bias from the second transfer bias Vt 2  to the third transfer bias Vt 3  (time t 6  in  FIG. 3 ). 
     In a case of switching the photosensitive member  181  from the reverse rotation to the forward rotation, the developer on the developing roller  12  is likely to be attached to the part Pd, which faces the developing roller  12 , of the photosensitive member  181 . When the developer is attached to the part Pd, which faces the developing roller  12 , of the photosensitive member  181 , the part Pd having the developer attached thereto moves toward the transfer device  183  in association with the forward rotation of the photosensitive member  181 , as shown in  FIG. 5B . 
     Also, since the third transfer bias Vt 3  is a plus large value, after the photosensitive member  181  is rotated in the forward direction from the state of  FIG. 5A  to the state of  FIG. 5B , the part, which has passed the transfer device  183 , of the photosensitive member  181  has a surface potential E 2  larger than the surface potential E 1  due to the influence of the third transfer bias Vt 3 . 
     After applying the third transfer bias Vt 3  for the predetermined period of time T 1 , the controller  200  switches the transfer bias from the third transfer bias Vt 3  to the first transfer bias Vt 1  (time t 7  in  FIG. 3 ), as shown in  FIG. 5C . At this time, the part Pd having the developer attached thereto has already passed the position facing the transfer device  183 . That is, when the part Pd having the developer attached thereto faces the transfer device  183 , since the third transfer bias Vt 3 , which is a plus large value, is applied to the transfer device  183 , it is possible to favorably suppress the developer from moving from the part Pd toward the transfer device  183 . 
     After the photosensitive member  181  is rotated in the forward direction from the state of  FIG. 5C  to the state of  FIG. 5D , the surface potential of the part, which has passed the transfer device  183 , of the photosensitive member  181  is reduced from the surface potential E 1  to the surface potential E 0  due to the influence of the first transfer bias Vt 1 , which is a minus large value. Also, at this time, a part, which has the largest surface potential E 2 , of the photosensitive member  181  passes the charger  182  but the part having the large surface potential E 2  keeps the surface potential E 2  without being influenced by the charger  182 . 
     As shown in  FIG. 5E , after the photosensitive member  181  is further rotated in the forward direction and the part of the surface potential E 0  passes the charger  182 , the part of the surface potential E 0  is charged and has the surface potential E 1 . That is, although the surface of the photosensitive member  181  is influenced by the charger  182  in a case where the surface potential is small, the surface of the photosensitive member  181  is little influenced by the charger  182  in a case where the surface potential increases to some extent. 
     As shown in  FIG. 5F , after the part, which has the largest surface potential E 2 , of the photosensitive member  181  passes the position LP that is exposed by the exposure device  150 , the controller  200  starts the exposure processing. Meanwhile, in the illustrative embodiment, since the part Pd having the developer attached thereto has reached the position facing the developing roller  12  on the start of the exposure processing, the developer on the part Pd is collected by the developing roller  12 . The timing at which the exposure processing starts and the timing at which the developer on the part Pd is collected by the developing roller  12  may not substantially the same timings, unlike the illustrative embodiment, and may be different timings. 
     According to the above illustrative embodiment, it is possible to accomplish following effects. 
     The laser printer  100  may include the photosensitive member  181 , the charger  182 , the developing device  1 , the transfer device  183  and the controller  200 . In the case of executing the printing control, the controller  200  rotates the photosensitive member  181  in the forward direction and applies the first transfer bias Vt 1  to the transfer device  183 . In the case of executing the reverse rotation control, the controller  200  rotates the photosensitive member  181  in the reverse direction and applies the second transfer bias Vt 2  to the transfer device  183 . Also, in a case where the rotating, direction of the photosensitive member  181  is switched from the reverse direction to the forward direction, the third transfer bias Vt 3 , which has an opposite polarity to the first transfer bias Vt 1  and an absolute value lager than the second transfer bias Vt 2 , may he applied to the transfer device  183  for the predetermined period of time including the time that the part, which faces the developing device  1  at the time of switching the rotating direction, of the photosensitive member  181  reaches the transfer device  183 . According to this configuration, in a case where the rotating direction of the photosensitive member  181  is switched from the reverse direction to the forward direction, it is possible to suppress the developer from moving to the transfer device  183 . Also, in a case of rotating the photosensitive member  181  in the reverse direction, since the small second transfer bias Vt 2 , which has an opposite polarity to the first transfer bias Vt 1 , is applied to the transfer device  183 , it is possible to suppress the surface potential of the photosensitive member  181  from excessively increasing. 
     The predetermined period of time T 1  in which the third transfer bias Vt 3  is to he applied can he set to a short time period within a length of time that the photosensitive member  181  rotates one turn. According to this configuration, it is possible to suppress the surface potential of the photosensitive member  181  from excessively increasing by the third transfer bias Vt 3 . 
     The laser printer  100  may include the exposure device  150 , and the controller  200  may start the exposure after the part, which faces the transfer device  183  at the time of switching the transfer bias from the third transfer bias Vt 3  to the first transfer bias Vt 1 , of the photosensitive member  181  passes the position facing the exposure device  150 . According to this configuration, since it is possible to suppress the part, which has the surface potential increased by the third transfer bias Vt 3 , of the photosensitive member  181  from being exposed, it is possible to suppress the developer image from being faint. 
     In the meantime, the present disclosure is not limited to the illustrative embodiment, and can be diversely used, as exemplified below. 
     In the above illustrative embodiment, the rotating speed of the photosensitive member  181  is constant. However, the present disclosure is not limited thereto. For example, in a case of performing the printing with a predetermined first image quality, the rotating speed of the photosensitive member  181  may be set to a first rotating speed, and in a case of performing the printing with a second image quality higher than the first image quality, the rotating speed of the photosensitive member  181  may be set to a second rotating speed higher than the first rotating speed. 
     Also, in this case, the controller  200  may be configured to increase an absolute value of the current flowing through the transfer device  183  as the rotating speed of the photosensitive member  181  becomes higher, for the predetermined period of time T 1 . Specifically, for example, in the processing of step S 11 , it is determined whether the rotating speed of the photosensitive member  181  is higher than the first rotating speed. In a case where it is determined that the rotating speed of the photosensitive member  181  is equal to or lower than the first rotating speed, the controller  200  applies the third transfer bias Vt 3  to the transfer device  183  such that the absolute value of the current flowing through the transfer device  183  is a predetermined value. Also, in a case where the rotating speed of the photosensitive member  181  is higher than the first rotating, speed, the controller  200  increases a value of the third transfer bias Vt 3  such that the absolute value of the current flowing through the transfer device  183  is higher than the predetermined value. According to this configuration, it is possible to suppress a situation where a charge amount per unit time to be applied to the photosensitive member  181  from the transfer device  183  becomes largely different in correspondence to the rotating speed of the photosensitive member  181 . 
     Also, in a case of rotating the photosensitive member  181  in the forward direction, the controller  200  may rotate the photosensitive member  181  at a first peripheral speed, and in a case of rotating the photosensitive member  181  in the reverse direction, the controller  200  may rotate the photosensitive member at a second peripheral speed equal to or lower than the first peripheral speed. Specifically, for example, the controller  200  may control the rotating speed of the motor  300  so that the peripheral speed of the photosensitive member  181  is to be the first peripheral speed, in steps S 2  and S 10 , and so that the peripheral speed of the photosensitive member  181  is to be the second peripheral speed, in step S 8 . 
     In the above illustrative embodiment, the second transfer bias Vt 2  is set to the value slightly larger than zero (0). However, the present disclosure is not limited thereto. For example, the second transfer bias may be zero (0). 
     In the above illustrative embodiment, the charging bias and the developing bias are constant, irrespective of the rotating direction of the photosensitive member  181 . However, the present disclosure is not limited thereto. For example, the charging bias and the developing bias may be set to different values on the forward rotation and on the reverse rotation. For example, the charging bias on the forward rotation may he set to the first charging bias Vc 1 , and the charging bias on the reverse rotation may be set to the second charging bias Vc 2  smaller than the first charging bias Vc 1 . The second charging bias Vc 2  may be set to +550V, for example. Also, for example, the developing bias on the forward rotation may he set to the first developing bias Vd 1 , and the developing bias on the reverse rotation may be set to the second developing bias Vd 2  smaller than the first developing bias Vd 1 . 
     In the above illustrative embodiment, the photosensitive drum has been exemplified as the photosensitive member  181 . However, the present disclosure is not limited thereto. For example, a belt-shaped photosensitive member may also be used. 
     In the above illustrative embodiment, the drive force is not transmitted to the developing roller  12  by the one-way clutch in a case where the motor  300  is rotated in the reverse direction. However, the present disclosure is not limited thereto. For example, a clutch may be provided between the motor  300  and the developing roller  12 , and the clutch may he cut off by the controller  200  so that the drive is not to be input to the developing roller  12  in a case where the reverse rotation control is executed. Alternatively, the developing roller  12  may be configured to rotate in the same direction, irrespective of the rotating direction of the motor  300 . 
     In the above illustrative embodiment, the charger  182  including the charging wire and the grid electrode has been exemplified. However, the present disclosure is not limited thereto. For example, the charger may be a circular cylinder-shaped charging roller or the like. 
     In the above illustrative embodiment, the contact-type developing device  1  including the developing roller  12  in contact with the photosensitive member  181  has been exemplified. However, the present disclosure is not limited thereto. For example, a contactless type developing device that is not in contact with the photosensitive member  181  may also be used. 
     In the above illustrative embodiment, the present disclosure has been applied to the laser printer  100 . However, the present disclosure is not limited thereto. For example, the present disclosure can be applied to the other image forming apparatuses such as a copier, a complex machine and the like. 
     The sheet S may be a thick sheet, a postcard, a thin sheet, an OHP sheet or the like. 
     In the above illustrative embodiment, the scanner configured to emit the laser has been exemplified as the exposure device  150 . However, the present disclosure is not limited thereto. For example, an exposure device having an LED head may also be used. 
     Also, the respective elements described in the above illustrative embodiment and modified embodiments may be implemented with being arbitrarily combined.