Abstract:
An endless belt circularly moves to convey a recording medium placed thereon in a conveying direction. A plurality of photosensitive bodies is juxtaposed in the conveying direction with confronting the endless belt. A plurality of process units supplies developer to the plurality of photosensitive bodies to form a developer image thereon, respectively. A first collecting member contacts the endless belt to collect the developer on the endless belt. A second collecting member is capable of collecting from the first collecting member the developer collected on the first collecting member. A control unit applies a bias between the first collecting member and the second collecting member to inhibit the developer collected on the first collecting member from being transferred to the second collecting member when the first collecting member rubs the endless belt with the toner collected on the first collecting member.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2010-063982 filed Mar. 19, 2010. The entire content of the priority applications is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an image forming apparatus. 
       BACKGROUND 
       [0003]    It has been well known a laser printer as a color image forming apparatus that is capable of forming a color image by electrophotographic method as disclosed in Japanese Patent Application Publication No. 2001-5360. For example, in a direct transfer type laser printer, an electrostatic latent image formed on a photosensitive body corresponding to respective colors is developed to form toner images by toner of the respective colors on a developing roller provided as a part of a developing device. 
         [0004]    Then, a sheet is conveyed by an endless conveying belt to face and contact each photosensitive body and the toner images are transferred onto the sheet to form a color image thereon. Note that, a foreign matter such as toner adhering to the conveying belt is collected by a collecting member provided to confront the surface of the conveying belt at a position different from positions at which each photosensitive drum contacts the conveying belt. 
       SUMMARY 
       [0005]    In the direct transfer type laser printer configured as described above, since the sheet is supported on the conveying belt to be conveyed, paper powder of the sheet adheres on the surface of the conveying belt when the sheet is conveyed. When the number of times of image forming operations increases, the conveying belt is covered by the accumulating paper powder to form a film. That is, a filming is occurred. With occurrence of filming by the paper powder, properties such as the electric resistance value of the surface of the conveying belt is changed to affect transferability for transferring the toner image from the photosensitive drums to the sheet. As a result, a print quality is adversely affected. 
         [0006]    In view of the foregoing, it is an object of the present invention to provide an image forming apparatus capable of eliminating filming arising from the paper powder. 
         [0007]    In order to attain the above and other objects, the present invention provides an image forming apparatus including an endless belt, a plurality of photosensitive bodies, a plurality of process units, a first collecting member, a second collecting member, and a control unit. The endless belt circularly moves to convey a recording medium placed thereon in a conveying direction. The plurality of photosensitive bodies is juxtaposed in the conveying direction with confronting the endless belt. The plurality of process units supplies developer to the plurality of photosensitive bodies to form a developer image thereon, respectively. The first collecting member contacts the endless belt to collect the developer on the endless belt. The second collecting member is capable of collecting from the first collecting member the developer collected on the first collecting member. The control unit applies a bias between the first collecting member and the second collecting member to inhibit the developer collected on the first collecting member from being transferred to the second collecting member when the first collecting member rubs the endless belt with the toner collected on the first collecting member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view showing a structure of a color laser printer according to an embodiment of the present invention; 
           [0010]      FIG. 2  is a schematic view showing a main section and an electrical structure for a process unit in the color laser printer shown in  FIG. 1 ; 
           [0011]      FIG. 3  is a timing chart of an image forming operation and a filming polish operation according to the embodiment of the present invention; 
           [0012]      FIG. 4  is a view showing a state that a toner for the polish operation is transferred from a cyan process unit to a conveying belt; 
           [0013]      FIG. 5  is a view showing a state that the toner held on a first collecting roller polishes the conveying belt in the polish operation; 
           [0014]      FIG. 6  is a view showing a state that a toner held on each holding roller is discharged to be collected by a cleaning unit; 
           [0015]      FIG. 7  is a timing chart of an image forming operation and a filming polish operation according to another embodiment of the present invention; 
           [0016]      FIG. 8  is a view showing a state that a toner for the polish operation is discharged from each holding roller to be held by the first collecting roller; and 
           [0017]      FIG. 9  is a graph showing a change in a gloss of the conveying belt between before and after conveying a number of sheets. 
       
    
    
     DETAILED DESCRIPTION 
     Entire Configuration and Image Forming Operation 
       [0018]    An entire configuration of a color laser printer as an image forming apparatus will be described with reference to  FIGS. 1 and 2 , and an image forming operation performed on the color laser printer will be described with reference to  FIG. 3 . 
         [0019]    The color laser printer  1  shown in  FIG. 1  is a tandem type color laser printer. The color laser printer  1  includes a main body casing  2  formed in a box shape, and within the main body casing  2 , a sheet convey unit  3  for conveying a sheet P as a recording sheet, an image forming unit  4  for forming an image on the sheet P, a fixing unit  27 , and a discharge unit  5  for discharging the image fixed sheet P are provided. 
         [0020]    As shown in  FIG. 2 , the color laser printer  1  further includes a control unit  100 . This control unit  100  is configured of a microcomputer including, for example, a CPU, a RAM, and a ROM. The control unit  100  controls each of units of the color laser printer  1  according to a program read from the ROM or the RAM as a memory area. 
         [0021]    A first bias application circuit  200 A and a second bias application circuit  200 B provided in the main body casing  2  are connected to the control unit  100 . Each of the bias application circuits  200 A and  200 B is configured of an electric circuit applying a bias to each unit of the color laser printer  1  according to commands of the control unit  100 . 
         [0022]    The first bias application circuit  200 A is an electric circuit for applying a bias voltage to a first collecting roller  22  and a second collecting roller  23  described below. On the other hand, the second bias application circuit  200 B is an electric circuit applying a bias voltage to various rollers and a holding roller  13  of the image forming unit  4 . 
         [0023]    The sheet convey unit  3  includes a sheet tray  6  for accommodating therein a stack of sheets P and various rollers for conveying one of the sheets P in the sheet tray  6  at a time. The sheet P fed from the sheet tray  6  is conveyed toward the image forming unit  4  by the various rollers. 
         [0024]    The image forming unit  4  includes a plurality of process units  9 , an exposure device  15 , and a transfer unit  16 . The plurality of process units  9  includes a black process unit  9 K, a yellow process unit  9 Y, a magenta process unit  9 M, and a cyan process unit  9 C arrayed in this order in a sheet conveying direction of the sheet P. 
         [0025]    In the following description, if the black process unit  9 K, the yellow process unit  9 Y, the magenta process unit  9 M, and the cyan process unit  9 C do not need to be specially distinguished, these are referred to as “process unit  9 ”. 
         [0026]    Each process unit  9  includes a photosensitive drum  10 , a charger  11 , a developing device  12 , and the holding roller  13 . 
         [0027]    The photosensitive drum  10  is a cylindrical shape. When the image is formed, the photosensitive drum  10  is driven to rotate in a predetermined direction (clockwise direction in  FIGS. 1 and 2 ). 
         [0028]    The charger  11  is, for example, a scorotron type charger of a positive charging type. The charger  11  includes a wire and a grid, and generates corona discharge by applying a charging bias. 
         [0029]    The developing device  12  is positioned at the downstream side of the charger  11  in a rotational direction of the photosensitive drum  10 . The developing device  12  is filled with positively charging, non-magnetic, single-component toner as a developer for each color, and includes a developing roller  14  for supplying the toner onto the surface of the photosensitive drum  10 . The developing roller  14  has a circumferential surface in contact with the surface of the photosensitive drum  10 . When the image is formed, a developing bias is applied to the developing roller  14  by the second bias application circuit  200 B. 
         [0030]    The holding roller  13  is located at the upstream side of the charger  11  in the rotational direction of the photosensitive drum  10 . The holding roller  13  is positioned between the charger  11  and an endless conveyance belt  19  described below. The holding roller  13  has a circumferential surface in contact with the surface of the photosensitive drum  10 . 
         [0031]    As the photosensitive drum  10  rotates, the charger  11  applies a uniform positive polarity to the surface of the photosensitive drum  10 . Subsequently, the exposure device  15  irradiates a laser beam onto the surface of the photosensitive drum  10  in a high-speed scan, thereby forming an electrostatic latent image which corresponds to an image to be formed on the sheet P, on the surface of the photosensitive drum  10 . The electrostatic latent image on the photosensitive drum  10  is transformed into a visible image (a toner image) by supplying the toner from the developing roller  14 . 
         [0032]    The exposure device  15  may be configured of an LED array to be included in each of the process units  9  or may be located as a scanner unit including a light source and a polygon mirror above the image forming part  4 . 
         [0033]    The transfer unit  16  for transferring to the sheet P the toner image carried on the surface of each photosensitive drum  10  is provided below the process unit  9 . The transfer unit  16  includes a drive roller  17 , a follower roller  18 , the conveying belt  19  wound around the drive roller  17  and the follower roller  18 , and a metal roller  26  contacting the inner surface of the conveyance belt  19 . The follower roller  18  is in confrontation with the drive roller  17  with a space, and positioned at the upstream side of the drive roller  17  in the sheet conveying direction of the sheet P. The conveying belt  19  has an upper section and a lower section. 
         [0034]    The drive roller  17 , the follower roller  18 , and the conveying belt  19  are disposed such that a surface of the upper section of the conveyance belt  19  contacts the photosensitive drums  10  from below. The drive roller  17  is rotated in a direction opposite to the photosensitive drum  10  (counterclockwise direction in  FIGS. 1 and 2 ) by a drive force from a motor (not shown). With rotation of the drive roller  17 , the conveying belt  19  is circularly moved in the same direction as the drive roller  17 , and the follower roller  18  is rotated. At this time, the metal roller  26  also follows movement of the conveying belt  19  to rotate. 
         [0035]    Further, as shown in  FIG. 2 , the transfer unit  16  further includes four transfer rollers  20  positioned in confrontation with the photosensitive drums  10  interposing the conveying belt  19  therebetween and a cleaning unit  21  positioned in confrontation with the lower section of the conveyer belt  19  from below. A bias voltage is applied to the transfer roller  20  by the second bias application circuit  200 B. 
         [0036]    The cleaning unit  21  includes the first collecting roller  22 , the second collecting roller  23 , a urethane blade  24 , and a retaining portion  25 . 
         [0037]    The first collecting roller  22  extends in the horizontal direction (the width direction of the conveying belt  19 ) orthogonal to a moving direction of the conveying belt  19 , and has a circumferential surface in contact with the surface of the lower section (lower surface) of the conveying belt  19 . The first collecting roller  22  is driven to rotate in the same direction (counterclockwise direction in  FIGS. 1 and 2 ) as the moving direction of the conveying belt  19 . Hence, the outer surface of the upper portion of the first collecting roller  22  moves in a direction opposite to the moving direction of the lower section of the conveying belt  19 . When the conveying belt  19  is circularly moved and the first collecting roller  22  is rotated, the first collecting roller  22  is in slidingly contact with the conveying belt  19 . The first collecting roller  22  is positioned in confrontation with the metal roller  26  via the conveying belt  19 . In other words, the conveying belt  19  is nipped between the first collecting roller  22  and the metal roller  26 . 
         [0038]    The second collecting roller  23  extends parallel to the first collecting roller  22 , and contacts the circumferential surface of the first collecting roller  22  with pressure. Further, the second collecting roller  23  rotates in a direction (clockwise direction in  FIG. 2 ) opposite to the rotational direction of the first collecting roller  22 . 
         [0039]    The first bias application circuit  200 A applies a collecting bias to the first collecting roller  22  and the second collecting roller  23 . The potential differences are generated between the metal roller  26  and the first collecting roller  22  and between the first collecting roller  22  and the second collecting roller  23 . By the potential difference between the metal roller  26  and the first collecting roller  22 , a current flows between the metal roller  26  and the first collecting roller  22 . The toner on the surface of the conveying belt  19  is transferred to the first collecting roller  22  at a nip position of the first collecting roller  22  and the metal roller  26  by the current flowing therebetween. 
         [0040]    The fixing unit  27  is disposed downstream of the transfer unit  16  in the sheet conveying direction. The fixing unit  27  functions to fix a toner image formed on the surface of the sheet P to the sheet P. The fixing unit  27  includes a heating roller  28  and a pressure roller  29 . The heating roller  28  is configured of a metal cylinder whose surface has been treated by a release agent, and a halogen lamp accommodated inside the cylinder. The heating roller  28  is driven to rotate by a driving force. The pressure roller  29  is a roller formed of silicon rubber that is rotatably supported in confrontation with the heating roller  28 . The pressure roller  29  presses against the heating roller  28  with a pressure and follows the rotation of the heating roller  28 . 
         [0041]    The discharge unit  5  is disposed for discharging the sheet P out of the color laser printer  1  after the sheet P has passed through the fixing unit  27 . The discharge unit  5  includes a sheet guide  30 , and a pair of discharge rollers  31 . The sheet guide  30  functions to guide the sheet P having passed through the fixing unit  27  to the discharge rollers  31 . A discharge tray  32  is provided downstream of the fixing unit  27  and at an upper surface of the main body casing  2 . Image fixed sheet P is discharged onto the discharge tray  32  by way of the sheet guide  30  and the discharge rollers  31 . 
         [0042]    Next, the image forming operation performed on the color laser printer  1  will be described.  FIG. 3  is a timing chart of the image forming operation and a filming polish operation described later. The control unit  100  begins executing the image forming operation when the control unit  100  receives image data. At the beginning of the image forming operation, the sheet P is conveyed from the sheet convey unit  3  to the image forming unit  4 . As shown in  FIG. 3 , the leading edge of the sheet P is supplied onto the conveying belt  19  and is conveyed so as to pass between the respective photosensitive drums  10  and the conveying belt  19  by the circular movement of the conveying belt  19 . 
         [0043]    On the other hand, the black process unit  9 K positioned at the most upstream side in the sheet conveying direction develops the toner image on the photosensitive drum  10 . Specifically, the control unit  100  applies 450V to each of the developing rollers  14 . Each of the developing rollers  14  selectively provides the toner onto the corresponding photosensitive drum  10  by a potential difference between the photosensitive drum  10  and developing roller  14 . Then, the toner image is formed on the photosensitive drum  10  of each process unit  9  in the order of yellow, magenta, and cyan. 
         [0044]    Then, the conveying belt  19  on which the sheet P has placed is circularly moved such that the sheet P passes between the photosensitive drums  10  of the respective process units  9  and the conveying belt  19  in the order of black, yellow, magenta, and cyan. The toner image formed on each photosensitive drum  10  is sequentially superimposed onto the sheet P by a transfer bias applied to each of the transfer roller  20  while the sheet P in conveyed on the conveying belt  19 . Specifically, the second bias application circuit  200 B applies 12, 10, 13, and 13 μA as transfer current to the transfer rollers  20  corresponding to the process units  9  (K, Y, M, and C), respectively. The toner image on the photosensitive drum  10  is attracted to the conveying belt  19  by the transfer current and is transferred on the sheet P. 
         [0045]    The toner remaining on the photosensitive drum  10  after the toner image has been transferred to the sheet P is removed from the photosensitive drum  10  to the holding roller  13  by electrostatic force when the remaining toner is in confrontation with the holding roller  13 . In other words, in the image forming operation, the second bias application circuit  200 B applies a collecting bias for removing the remaining toner on the photosensitive drum  10  to each of the holding rollers  13 , and the remaining toner removed to the holding roller  13  is accumulated on the circumferential surface of the holding roller  13  by electrostatic attraction. Specifically, the holding roller  13  collects and holds the remaining toner deposited on the photosensitive drum  10  while the collecting bias of −500 V is applied to the holding roller  13  as shown in  FIG. 3 . 
         [0046]    In this way, the sheet P having the toner image transferred from each process unit  9  leaves the conveying belt  19  and is furthermore conveyed to the fixing unit  27  provided at the downstream side in the sheet conveying direction. The fixing unit  27  fixes the toner image transferred on the sheet P to the sheet P by applying heat and pressure. The sheet P conveyed from the fixing part  27  is conveyed to the further downstream side to be ejected to the outside of the main body casing  2 , and the image forming operation is finished. 
         [0047]    After the image forming operation is finished, a toner collecting operation is performed. In the toner collecting operation, the cleaning unit  21  collects the toner held on the holding roller  13 . Specifically, the second bias application circuit  200 B applies a discharge bias having a polarity opposite to that of the collecting bias to each of the holding rollers  13 . The toner held on the holding roller  13  is discharged and transferred to the photosensitive drum  10 . Specifically, the discharge bias of +500 V is applied to the holding roller  13 . 
         [0048]    The toner transferred to the photosensitive drum  10  is transferred to the conveying belt  19  when the toner on the photosensitive drum  10  is in confrontation with the conveying belt  19 . At this time, the second bias application circuit  200 B applies current to the transfer roller  20 . This current (the transfer current) has the same value as current applied to the transfer roller  20  in the image forming operation. 
         [0049]    By the circularly movement of the conveying belt  19 , the remaining toner transferred to the conveying belt  19  is collected to the first collecting roller  22  having a lower potential at a position where the conveying belt  19  contacts the first collecting roller  22 . Specifically, the first bias application circuit  200 A sets a bias applied to the first collecting roller  22  to −1000V lower than the surface potential of the conveying belt  19 , and a bias applied to the second collecting roller  23  to −1400V. 
         [0050]    Then, the remaining toner held on the first collecting roller  22  is transfer to the second collecting roller  23  having a potential lower than the first collecting roller  22 . The toner collected on the second collecting roller  23  is scraped by the urethane blade  24  to be retained as waste toner in the retaining part  25 . 
       Filming Polish Operation 
       [0051]    Next, a polish operation will be described with reference to  FIGS. 3 through 6 . In the polish operation, the conveying belt  19  is polished by the toner supplied from the developing device  12 . Note that, in this embodiment, the image forming operation is sequentially performed for the plurality of sheets, and the polish operation is performed in the image forming operation for one sheet of paper. As shown in  FIG. 3 , the polish operation is performed while the image forming operation is performed on a second sheet P. 
         [0052]    When the polish operation starts, a toner T is supplied to the photosensitive drum  10  from the developing device  12  of the cyan process unit  9 C positioned at the most downstream side in a moving direction of the upper section of the conveying belt  19  as shown in  FIG. 4 . Specifically, the second bias application circuit  200 B applies 450V to the developing roller  14  of the cyan process unit  9 C. The toner T on the developing roller  14  selectively moves onto the photosensitive drum  10  by a potential difference between the photosensitive drum  10  and the developing roller  14 . 
         [0053]    The toner T is supplied throughout the whole of an image forming region in an axial direction of the photosensitive drum  10  (the toner T develops the electrostatic latent image into the toner image), and transferred to the conveying belt  19  by contacting the conveying belt  19 . This operation for transferring the toner T from the photosensitive drum  10  to the conveying belt  19  is referred to as a transfer operation. This developing and transfer operation of the cyan process unit  9 C and the conveying belt  19  is performed during a sheet interval described later. 
         [0054]    On the other hand, −1000V is applied to the first collecting roller  22 , and −600V is applied to the second collecting roller  23  as shown in  FIG. 3 . In other words, when the polish operation starts, the first bias application circuit  200 A applies the bias voltage to the second collecting roller  23  such that the relationship between the voltage of the first collecting roller  22  and the voltage of the second collecting roller  23  in the polish operation is opposite to that in the toner collecting operation. Thereby, the toner T collected on the first collecting roller  22  is inhibited from being transferred to the second collecting roller  23 . 
         [0055]    By applying the above bias voltage to each of the collecting rollers  22  and  23 , the toner T collected from the conveyance belt  19  to the first collecting roller  22  is not collected (transferred) to the second collecting roller  23  but is held on the first collecting roller  22  as shown in  FIG. 5 . The first collecting roller  22  moves in a direction opposite to the moving direction of the lower section of the conveying belt  19  at a position where the first collecting roller  22  contacts the conveying belt  19 . The first collecting roller  22  rubs the surface of the conveying belt  19  with the toner T held on the first collecting roller  22  while nipping the conveying belt  19  together with the metal roller  26 . 
         [0056]    The first collecting roller  22  holding the toner T rubs the surface of the conveying belt  19  at the nipping position to polish the surface of the conveying belt  19 , and eliminates the paper powder filming formed on the surface of the conveyance belt  19 . 
         [0057]    In order to perform the polish operation as described above, the control part  100  controls the first bias application circuit  200 A so as to apply the bias for the polish operation to the second collecting roller  23  at a timing during the sheet interval. 
         [0058]    The sheet interval indicates an interval between a first timing and a second timing. The first timing indicates a timing that the trailing edge of the first sheet P is passed through a confrontation position where the photosensitive drum  10  of the cyan process unit  9 C positioned at the most downstream side in the sheet conveying direction is in confrontation with the conveying belt  19  after the image forming operation for the first sheet P is completed. The second timing indicates a timing that the leading edge of the second sheet P reaches the confrontation position. In  FIG. 3 , the sheet interval indicates an interval between a timing that the trailing edge of the first sheet P is separated from the conveying belt  19  and a timing that the leading edge of the second sheet P contacts the conveying belt  19 . A conveying timing of the sheet P is detected by using such known manners that, for example, a sheet detecting sensor is provided on a sheet conveying path around the end portions of the conveying belt  19  or that the timing is calculated on the basis of the conveying speed of the sheet P. 
         [0059]    Next, timings of starting and finishing the polish operation will be described. As shown in  FIG. 3 , in the case of performing the image forming operation sequentially, the first bias application circuit  200 A sets to −1000V the bias voltage applied to the first collecting roller  22  and to −600V the bias voltage applied to the second collecting roller  23  as the bias voltage for the polish operation after the image forming operation for the first sheet P is finished and after the trailing edge of the first sheet P separates from the conveying belt  19 . 
         [0060]    After the bias voltages for the polish operation are applied to the first collecting roller  22  and the second collecting roller  23 , the developing device  12  supplies the toner T for polishing the conveying belt  19  to the photosensitive drum  10  of the cyan process unit  9 C at the timing during the sheet interval as the transfer operation. The toner T supplied to the photosensitive drum  10  is preferably supplied on the whole of the photosensitive drum  10  in the width direction thereof. The toner T supplied to the photosensitive drum  10  is furthermore transferred to the conveying belt  19  at a position where the photosensitive drum  10  is in confrontation with the conveying belt  19  by the transfer bias applied to the transfer roller  20 . Note that, after the toner for polish is supplied to the surface of the photosensitive drum  10  of the cyan process unit  9 C and the supplied toner is transferred to the conveying belt  19 , the toner image to be transferred to the sheet P is developed on the photosensitive drum  10 . 
         [0061]    After the toner T transferred to the conveying belt  19  in the transfer operation is transfer to the first collecting roller  22 , the polish operation is performed by the first collecting roller  22 . Note that, the polish operation is performed in parallel with the image forming operation. In other words, while the first collecting roller  22  and the conveying belt  19  rub each other, the image forming operation is performed for the second sheet P. This polish operation is performed preferably until at least the entire region of the surface of the conveying belt  19 , in other words, at least the amount corresponding to one round of the conveying belt  19  is polished by the toner T. Obviously, the polish operation may also be performed throughout the image forming operation. 
         [0062]    The polish operation is finished at a time when the image forming operation is finished. In more detail, in the case of performing the image forming operation for two sheets sequentially, the toner of each color (K to C) is transferred onto the sheet P, the trailing edge of the second (last) sheet P leaves the conveying belt  19 , and after that, the bias voltage of −600V applied to the second collecting roller  23  is changed to the bias voltage of −1400V for the collecting operation as shown in  FIG. 3 . 
         [0063]    Thus, at the same time as finishing the polish operation, the bias voltage applied to the holding roller  13  is changed to +500 V, and the collecting operation is started. When the collecting operation is started, the toner T held on the first collecting roller  22  in the polish operation is transferred to the second collecting roller  23  as shown in  FIG. 6  and after that, is retained in the retaining part  25 . Note that, when the collecting operation finishes, the color laser printer  1  is kept in a standby mode until a next image forming instruction is inputted to the control part  100 . The control unit  100  may also perform the above polish operation during this standby mode. Additionally, the polish operation may also be performed by performing the transfer operation before the image forming operation for the first sheet P is performed. 
         [0064]    As described above, since the control unit  100  controls the bias voltage for the second collecting roller  23  so as to inhibit the toner T collected on the first collecting roller  22  from being transferred to the second collecting roller  23  and the conveying belt  19  is rubbed with the toner T on the first collecting roller  22 , the surface of the conveying belt  19  is polished by the toner. T Thereby, the filming of the paper powder formed on the surface of the conveyance belt  19  can be properly eliminated. 
         [0065]    Further, since the control unit  100  controls the toner T so as to be transferred from the photosensitive drum  10  to the conveying belt  19 , the toner T for rubbing the conveying belt  19  can be supplied to the first collecting roller  22 . Particularly, in this embodiment, since the toner T is supplied from the developing device  12  of the image forming unit  4  to the first collecting roller  22 , any specific supply source of the toner T for eliminating the paper powder filming does not need to be provided. Therefore, without complicating the entire configuration of the apparatus, the paper powder filming can be eliminated. 
         [0066]    Further, since the transfer operation is performed by the photosensitive drum  10  of the cyan process unit  9 C before the sheet P passes (reaches) a position where the photosensitive drum  10  of the cyan process unit  9 C in which the toner image has been formed is in confrontation with the conveying belt  19 , the polish operation can be performed while the image forming operation is performed on the sheet P. Particularly, since the transfer operation is performed during the sheet interval, the sheet interval can be effectively utilized. Hence, time loss due to the polish operation in the image forming operation can be reduced to the minimum. Moreover, since the polish operation is performed in parallel with the image forming operation, a waste of time does not arise in the image forming operation. 
         [0067]    Further, since the toner T is supplied from the photosensitive drum  10  positioned at the most downstream side in the sheet conveying direction, the toner T transferred from the photosensitive drum  10  to the conveying belt  19  does not adhere to another photosensitive drums  10  to dirty another color photosensitive drums  10 . 
         [0068]    Further, in this embodiment, the polish operation is performed in parallel with the image forming operation, but the polish operation may be performed when the image forming operation does not executed (for example, when the color laser printer  1  is kept in the standby mode). In this case, the toner T may also be supplied from not only the developing device  12  positioned at the most downstream side in the sheet conveying direction but also another developing device  12  positioned at the upstream side of the developing device  12  of the cyan process unit  9 C, or the toner T may also be supplied from the developing devices  12  for all process units  9 K,  9 Y,  9 M, and  9 C. 
       Another Embodiment 
       [0069]    Next, a polish operation in accordance with another embodiment will be described with reference to  FIGS. 7 and 8 . Note that, only parts different from the embodiment described above will be described hereinafter, same parts and components are denoted by the same reference numerals, and detailed descriptions thereof will be omitted. 
         [0070]    A point different from the embodiment described above is that the toner used for performing the polish operation is supplied from each of the holding rollers  13  provided in the corresponding process unit  9 . Further, in this embodiment, the polish operation is performed after the image forming operation is performed for all the sheets P to be printed. 
         [0071]    As shown in  FIG. 7 , in the image forming operation, the collecting bias of −500V is applied to each of the holding rollers  13  so that each holding roller  13  collects the toner remaining on the photosensitive drum  10  and holds the collected toner on the surface after the toner image has been transferred to the sheet P. On the other hand, −1000V and −1400V are applied to the first collecting roller  22  and the second collecting roller  23 , respectively, and the toner adhering onto the conveying belt  19  is retained in the retaining portion  25  through the first collecting roller  22  and the second collecting roller  23 . 
         [0072]    When it is determined that the image forming operation is completed after the trailing edge of the sheet P leaves the conveying belt  19 , the second bias application circuit  200 B applies to the holding roller  13  of each process unit  9  the discharge bias of +500V for transferring (discharging) the toner to the photosensitive drum  10  as the transfer operation. The toner on the holding roller  13  is transferred to each of the photosensitive drums  10  and is furthermore transferred to the conveying belt  19  by the transfer bias. 
         [0073]    On the other hand, at the same time as changing the bias voltage applied to the holding roller  13  for the transfer operation as described above, the bias voltage applied to the second collecting roller  23  is set to −600V from −1400V. Thereby, since the second collecting roller  23  has the bias voltage (−600V) higher than the first collecting roller  22  (−1000V), the toner transferred from the conveying belt  19  to the first collecting roller  22  is inhibited from being transferred to the second collecting roller  23  and is held on the first collecting roller  22  as shown in  FIG. 8 . The first collecting roller  22  holding the toner rubs the conveying belt  19  to thereby polish the paper powder filming. The polish operation is continuously performed until the next image forming operation starts. However, the polish operation may be finished at a time that the conveying belt  19  goes one round. 
         [0074]    When the circumferential surface corresponding to one round of the conveying belt  19  has been polished, the control unit  100  controls the first bias application circuit  200 A so as to finish the polish operation. Specifically, the first bias application circuit  200 A sets to −1400V the bias voltage applied to the second collecting roller  23 , and controls the toner on the first collecting roller  22  so as to transfer the toner to the second collecting roller side  23 . The toner transferred to the second collecting roller  23  is scraped by the urethane blade  24  to be retained as waste toner in the retaining portion  25 . On the other hand, the bias voltage applied to the holding roller  13  is set to −500V by the second bias application circuit  200 B such that the holding roller  13  collects the remaining toner on the photosensitive drum  10  and holds the collected remaining toner. 
         [0075]    In this way, the color laser printer  1  in this embodiment is controlled such that the toner held on the holding roller  13  is transferred to the first collecting roller  22  through the conveying belt  19  to polish the surface of the conveying belt  19 . Therefore, the next image forming operation can be performed in a state that the paper powder filming is eliminated. Further, since the toner temporarily collected from the surface of the photosensitive drum  10  is used for the polish operation, the new toner for the polish operation does not need to be supplied. Accordingly, it is economical on the color laser printer  1 . 
         [0076]    Since the transfer operation is performed after finishing the image forming operation, the transfer operation does not interfere with the image forming operation. Further, since the polish operation is performed by using the toner discharged from the holding roller  13  to the photosensitive drum  10 , the transfer operation can be incorporated into a part of time for performing the collecting operation. 
         [0077]    Note that, the toner is discharged from all the holding rollers  13  in the polish operation in this embodiment, but as described above, the toner may also be supplied from only the holding roller  13  provided in the cyan process unit  9 C located at the most downstream side in the sheet conveying direction. Additionally, the control unit  100  may control the first bias application circuit  200 A to supply the bias voltage to each of the collecting rollers  22  and  23 . Further, the transfer operation may be performed by supplying the toner from both the holding roller  13  and the developing device  12 . 
       Experimental Example 
       [0078]    The experimental example about the above-described embodiments will be described below with reference to  FIG. 9 . In this experimental example, a change in the gloss of the surface of the conveying belt with the polish operation and without the polish operation is evaluated. 
         [0079]    Now, conditions for an evaluation experiment of the gloss level will be described. HL-4050 produced by Brother Industries, Ltd. was used as the laser printer. The conveying belt was a nylon resin belt. The used toner mainly includes styrene-acrylic non-magnetic single-component toner. Additionally, the experiment was performed under environment at a temperature of 10° C. and a relative humidity (RH) of 20%. 
         [0080]    The evaluation on a change in the gloss of the surface of the conveying belt was conducted as follows. First, the gloss of the surface of the conveying belt is measured before the conveying belt conveys the sheet (no sheet is conveyed by the conveying belt.). After that, ten thousand sheets are conveyed by the conveying belt, and thereafter the gloss is measured. After the conveying belt conveys the sheets, the surface of the conveying belt is covered with the paper powder. Hence, the gloss level of the conveying belt is reduced. Thus, the more the gloss level on the surface of the conveying belt changes between before and after conveying the sheets, the more the belt filming is formed. 
         [0081]    The gloss level after conveying the sheets was evaluated immediately after conveying ten thousand sheets and immediately after performing the above-described polish operation for the conveying belt while ten thousand sheets are conveyed. Note that, GLOSS CHECKER IG-320 produced by HORIBA was used as an evaluation instrument for measuring the gloss level of the conveying belt, and the gloss level was measured twice under each of the conditions. The value of the gloss level is an average value of when the gloss level is measured at four positions in the circumferential direction of the conveying belt with the gloss checker. 
         [0082]      FIG. 9  shows the result of the experiment. The gloss level of the conveying belt is found to be deteriorated between before and after conveying ten thousand sheets. This is caused by coating the conveying belt with the paper powder. On the other hand, in comparison with the conveying belt without the polish operation, the gloss level of the conveying belt with the polish operation is inhibited from being deteriorated. Thus, as a result of polishing the belt with the toner, the paper powder filming on the belt can be eliminated to recover the gloss level.