Patent Publication Number: US-7917053-B2

Title: Image forming and toner cleaning apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic image forming apparatus and an electrophotographic image forming method. In particular, the present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and a multi-function peripheral with functions of such devices in combination and a method of forming images using the image forming apparatus. 
     2. Description of the Related Art 
     An electrophotographic, monochrome image forming apparatus forms single color toner images on a photosensitive member. The toner images are transferred onto a sheet material passing through a nipping region defined between the photosensitive member and a transfer roller. Not all the toner particles are transferred onto the sheet material and a part of the toner particles remains on the photosensitive member without being transferred. In order to remove the residual toner particles from the photosensitive member, a method is proposed in which a cleaning member is provided in contact with the surface of the photosensitive member to remove the toner particles therefrom. 
     A variety of full color image forming apparatuses have been proposed so far. Among other things, one of the proposed electrophotographic, full color image forming apparatus is designed to transfer the toner images on the photosensitive member onto an intermediate transfer belt passing through a nipping region defined between the photosensitive member and a first transfer roller. The toner images are then transferred onto the sheet material passing through a second nipping region defined between the intermediate transfer belt and a second transfer roller. The residual toner particles on the intermediate transfer belt are removed by a cleaning member provided in contact with the photosensitive member. 
     Conventionally, the cleaning member for removing residual toner particles from the photosensitive member and the intermediate transfer belt is made of rubber blade or rotatable brush. For example, JP 2004-310060 A discloses a cleaning device with a cleaning member made of rotatable brush for the cleaning of the intermediate transfer belt. 
     As shown in  FIG. 13 , the cleaning device disclosed in JP 2004-310060 A includes a cleaning brush  142  provided in contact with the intermediate transfer belt  130 , a charging brush  174  also provided in contact with the intermediate transfer belt  130  on the upstream side from the cleaning brush  142  with reference to the moving direction of the intermediate transfer belt (counterclockwise direction in the drawing), a collecting roller  177  provided in contact with the cleaning brush  142 , and a scraper  178  provided in contact with the collecting roller  177 . A power supply  184  is connected to the scraper  178 , and the charging brush  174  is grounded. With the arrangement, when the power supply  184  is turned on, electric current flows from the power supply  184  to the scraper  178  through the scraper  178 , the collecting roller  177 , the cleaning brush  142 , the intermediate transfer belt  130 , and the charging brush  174 . This results in that most of the toner particles on the intermediate transfer belt  177  are electrically charged into a negative polarity. The negatively charged toner particles are then transported by the rotation of the belt  130  in the contact region of the cleaning brush  142  and the intermediate belt  130  where they are electrically attracted by the cleaning brush  142  and then removed from the intermediate transfer belt  177 . 
     According to this arrangement, the toner particles not negatively charged between the intermediate transfer belt  130  and the charging brush  174  may be electrostatically and/or mechanically collected by and accumulated between the bristles of the brush  174 . The accumulated toner particles may be transferred from the brush  174  due to, for example, vibrations caused by the engagements of the bristles with the rotating belt  130  and then adhere to the outer periphery of the belt  130 . The toner particles adhered on the imaging region of the intermediate transfer belt can be transferred at the second transfer region onto the sheet material to deteriorate the resultant image quality. On the other hand, the toner particles adhered on the non-imaging region of the intermediate transfer belt can be transferred to the second transfer belt, which in turn is transferred onto the opposite surface of the sheet material. 
     In order to prevent the toner particles from being transferred onto the sheet material, before forming toner images, the intermediate transfer belt  130  may be circulated a full turn to transport the toner particles on the intermediate transfer belt into the contact region between the intermediate transfer belt  130  and the charging brush  174  where the toner particles are electrically charged and then removed by the subsequent contact with the cleaning brush, which disadvantageously delays the start of the image forming operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, a purpose of the present invention is to provide an image forming apparatus and an image forming method capable of preventing the recording medium from being stained by the transfer of the toner particles transferred from the image bearing member and also capable of starting the image forming operation without delay. 
     To this end, an image forming apparatus of the present invention comprises 
     an image bearing member, the image bearing member having an endless image bearing surface and supported for rotation; 
     a charging member made of a brush provided in contact with the image bearing surface to define a charging region; and 
     a controller for controlling 
     an image forming operation in which the toner image is provided to a recording medium; and 
     a transfer operation in which the rotation of the image bearing member is halted and then toner particles accumulated in the brush are transferred onto the image bearing surface at the charging region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view showing a schematic configuration of an image forming apparatus according to the present invention; 
         FIG. 2  is a schematic elevational view showing an intermediate transfer belt and members at the periphery thereof; 
         FIG. 3  is an elevational view showing a belt cleaning device; 
         FIG. 4  is a flowchart showing a program flow of process of the main routine; 
         FIG. 5  is a flowchart showing a program flow of process of a post-processing sequence according to a first embodiment; 
         FIG. 6  is a time chart of control of various operations of the post-processing sequence according to the first embodiment; 
         FIG. 7  is a flowchart showing the flow of process of a post-processing sequence according to a second embodiment; 
         FIG. 8  is a time chart of control of various operations of the post-processing sequence according to the second embodiment; 
         FIG. 9  is an enlarged view showing a belt cleaning device according to a third embodiment; 
         FIG. 10  is a flowchart showing the flow of process of a post-processing sequence according to the third embodiment; 
         FIG. 11  is a time chart of control of various operations of the post-processing sequence according to the third embodiment; 
         FIG. 12  is a graph showing distribution of charging amount of toner inside a charging brush and of toner transferred from the inside of the charging brush to the surface of a belt; and 
         FIG. 13  is a view showing one example of a configuration of a conventional intermediate transfer belt cleaning device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, several embodiments of the present invention will be described. In the following descriptions, terms indicating specific directions and positions (e.g., “up”, “down”, “left”, “right” and other terms including any one of such terms) are used as necessary, however, the use of such terms intends to facilitate better understanding of the invention in connection with the drawings and therefore the scope of the present invention should not be limited by such terms. 
     First Embodiment 
       FIG. 1  schematically shows an image forming apparatus  2  according to a first embodiment of the present invention. The image forming apparatus  2  is an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction device with functions of such devices. Although various electrophotographic image forming apparatuses are currently available, the illustrated image forming apparatus is a so-called tandem type color image forming apparatus. The present invention may be applied not only to that image forming apparatus but also to a so-called four-cycle color image forming apparatus and a direct transfer color image forming apparatus in which the toner images on the electrostatic latent image bearing member are directly transferred onto the recording medium. In addition, the present invention is also applicable to the monochrome image forming apparatus with a single developing device. 
     The image forming apparatus  2  generally includes an image reading unit generally indicated by reference numeral  20  for reading a document image and a printing unit generally indicated by reference numeral  22  for printing the image. The image reading unit  20  is configured to perform a color separation of the document image into three color elements of red (R), green (G), and blue (B) by a well-known color separation technique and then generate image data of red (R), green (G), and blue (B). 
     The image forming apparatus may include a display device  24  for displaying various information relating to the printing and an operation panel  25  for allowing users to perform printing and various setting operations for printing. 
     The printing unit  22  has an image bearing member made of an endless intermediate transfer belt  30 , having an endless image bearing peripheral surface  30   a  ( FIGS. 2 and 3 ). Preferably, the belt  30  is made of a suitable material with an elevated transferring performance such as polyimide. More preferably, the belt  30  has a thickness of equal to or larger than 50 μm and equal to or less than 150 μm. 
     The belt  30  is entrained around a pair of rollers  32 ,  34  positioned on the left and right sides in the drawing. The right roller  32  is a drive roller drivingly coupled to a motor  33 , so that the rotation of the motor is transmitted to the drive roller  32 , which causes rotations of the belt  30  and the left roller  34  contacting the belt  30 , in the counterclockwise direction. 
     Preferably, the drive roller  32  has an outer diameter of equal to or larger than 12 mm and equal to or less than 30 mm to minimize the image forming apparatus. Also preferably, the peripheral surface of the drive roller  32  is made of material having a large friction coefficient such as rubber or urethane to attain an enlarged frictional force between the belt  30  and the roller  32  and thereby a reliable transmission of the drive force to the belt  30 . 
     Preferably, a suitable tensile force is introduced to the belt  30  by the rollers  32 ,  34  to ensure a sufficient frictional force between the drive roller  32  and the belt  30 . Preferably, the tensile force is adjusted to equal to or greater than 15N and equal to or less than 50N, for example. 
     A second transfer member made of transfer roller  40  is provided in a second transfer station  38  adjacent the belt portion supported by the right drive roller  32  so as to nip the recording medium  36  with the belt  30 . As shown in  FIG. 2 , the transfer roller  40  is supported by a mechanism  41  so that it can be moved between a contact position (indicated by solid line) where the roller  40  contacts the outer peripheral surface of the belt  30  to form a nipping region or a second transfer region  39  and a non-contact position (indicated by imaginary line) where the roller  40  is spaced away from the outer peripheral surface of the belt  30 . Preferably, the transfer roller  40  is made of an ion conductive roller or an electron conductive roller. 
     A cleaning device generally indicated by reference numeral  64  for cleaning the belt  30  is provided outside the belt portion supported by the left roller  34 , which will be described in detail later. 
     Referring back to  FIG. 1 , the image forming apparatus  2  has four first transfer stations  13  where four imaging units  3  ( 3 Y,  3 M,  3 C,  3 K) are mounted in this order below and along the lower belt portion running from the left roller  34  to the right roller  32  for forming toner images with developers of different colors, yellow (Y), magenta (M), cyan (C), and black (K). 
     Each of four imaging units  3  has an electrostatic latent image bearing member made of cylindrical photosensitive member  4  mounted for rotation in the clockwise direction. A charger  8 , an exposure device  10 , a developing device  18 , a first transfer roller  14 , and a cleaning member  16  are positioned around the photosensitive member  4  in this order with respect to the rotational direction thereof. 
     The first transfer roller  14  is arranged within a space defined by the endless belt  30 . As shown in  FIG. 2 , the transfer roller  14  is supported by a support mechanism  14   a  for moving between a position where it is forced to the corresponding photosensitive member  4  through the belt  30  and a position where it is spaced away from the photosensitive member  4  and the belt  30 . A high voltage power supply (not shown) is connected to the transfer roller  14  so that a first transfer voltage is applied to the transfer roller  14  from the power supply during the formation of the toner images. 
     Referring again to  FIG. 1 , the printing unit  22  includes a control unit  70  for controlling various operations such as image forming operation. The printing unit  22  further includes a paper cassette  44  removably arranged in the lower part thereof so that, when printing, the recording mediums  36  stacked in the paper supply cassette  44  are fed out one by one to a transport passage  50  by the rotation of a feed roller  52  mounted on the paper cassette  44 . 
     A registration roller  54 , for transporting the paper  36  to the second transfer region  39  at a predetermined timing, is arranged adjacent the feed roller  52 . A paper detector  55  for detecting the front edge of the paper  36  being transported is arranged adjacent the registration roller  54 . 
     The transport passage  50  extends from the paper cassette  44  to a paper discharge tray  61  mounted at the upper portion of the printing unit  22  through the nipping regions defined by paired registration rollers  54 , the second transfer roller  40  and the belt  30 , paired fusing rollers  56 , and discharging rollers  60 . 
     Discussions will be made to a color image forming operation. In this operation, the image reading unit  20  reads the document image to generate image data of respective colors of red (R), green (G), and blue (B). The image data is transmitted to the control unit  70  where it is processed and transformed into color image data of yellow (Y), magenta (M), cyan (C), and black (K). The processed image data of yellow, magenta, cyan, and black colors is stored in an image memory  72  in the control unit  70 . The image date is corrected to remove possible misregistration of the images and then converted into drive signals for causing light emission of a light source (not shown) in the exposure device  10 . 
     Each photosensitive member  4  is rotated in the clockwise direction, during which its peripheral surface is electrically charged by the charger  8 . The charged peripheral surface is exposed to light emitted from the exposure device  10  in response to the drive signal from the control unit  70 , so that a corresponding electrostatic latent image is formed on the peripheral surface. The electrostatic image is then visualized by a developing material of toner particles supplied from the associated developing device  8 . The toner images of respective colors of yellow, magenta, cyan, and black on respective photosensitive members  4  are transported into respective first transfer regions  15  where they are transferred onto the belt  30  in this order and superimposed thereon. 
     Toner particles not transferred from each image bearing member  4  to the belt  30  are transported by the rotation of the image bearing member  4  into the contact region between the photosensitive member  4  and the cleaning member  16  where it is scraped off from the peripheral surface of the photosensitive member  4 . The superimposed four toner images are transported by the belt  30  into the second transfer region  39 . 
     The recording medium  36  accommodated in the paper cassette  44  is fed out by the rotation of the supply roller  52  into the nipping region of the paired registration rollers  54  and then into the second transfer region  39  while taking a suitable timing with the toner images being transported by the belt  30  into the second transfer region  39 . 
     Toner images are transferred onto the incremental portions of the recording medium  36  passing the second transfer region  39 . The recording medium  36  is further transported to the nipping region of the paired fusing rollers  56  where the toner images are fixed to the recording medium  36  and finally transported by paired the discharge rollers  60  onto the discharge tray  61 . 
     The toner particles without being transferred onto the recording medium and remaining on the peripheral surface of the belt  30  are removed therefrom by the cleaning device  64  which will be described below. As shown in  FIG. 3 , the cleaning device  64  has a charging brush  74  for electrically charging the toner particles on the peripheral surface of the belt  30  with a predetermined electric charge of negative polarity in this embodiment, a cleaning member made of brush  42  in the form of roll for removing the toner particles from the periphery of the belt  30 , a collecting roller  77  for collecting toner particles from the cleaning brush  42 , a scraper  78  for scraping off toner particles from the collecting roller  77 , and a housing  66  for housing those members  74 ,  42 ,  77 , and  78  therein. 
     The charging brush  74  and the cleaning brush  42  are mounted in contact with respective outer peripheral surface portions of the belt  30  supported by the roller  34 . The charging brush  74  has a base  75  in the form of plate, for example, and a number of bristles  76  planted in the base  75  so that distal ends thereof are in contact with the outer peripheral surface of the belt  30  to define a contact region or charging region  73  therebetween. The base  75  is securely mounted to a support  68  projected from and fixed to the inner surface of the housing wall. The base  75  is made of electrically conductive material such as metal. The bristles  74  are also made of electrically conductive material such as electrically conductive resin. 
     The cleaning brush  42  in the form of roll is positioned on the downstream of the charging brush  74  with respect to the rotational direction of the belt  30  in the image forming operation. Preferably, the cleaning brush  42  is designed to rotate in a direction so that the bristles  76  travel in a direction (i.e., counterclockwise direction) opposite to the moving direction of the belt  30  at the contact region  62  between the belt  30  and the bristles  42 . The contact region  62  of the brush  42  and the belt  30  defines a collecting region for collecting the untransferred toner particles from the belt  30 . In this embodiment, the cleaning brush  42  has a solid or hollow cylindrical central portion  44  and a number of bristles  46  planted in the entire outer periphery of the central portion  44  and extending radially outwardly from the central portion  44 . Preferably, the central portion  44  is made of metal such as iron, aluminum, and stainless and the bristles  76  are made of electrically conductive material such as conductive resin. 
     The collecting roller  77  is positioned in contact with the cleaning brush  42 . The rotational direction of the collecting roller  77  is so determined that the peripheral portions of the cleaning brush  42  and the collecting roller  77  move in the same direction in the contact region thereof. In this embodiment, the collecting roller  77  is mounted to rotate in the clockwise direction. The collecting roller  77  is made of electrically conductive material such as iron, aluminum, and stainless. 
     The scraper  78  is made of elongate plate and is positioned so that it extends substantially parallel to the axial direction of the collecting roller  77  with its distal end in contact with the outer peripheral surface of the collecting roller  77 . Although not limited thereto, a suitable metal plate such as stainless plate is used for the scraper  78 . 
     Preferably, a filming protection and sealing member  80  is filled in a gap defined on the downstream side of the cleaning brush  42  and between the belt and the opposing housing portion to prevent generations of film of toner, i.e., filming, and toner scattering, which would otherwise be caused by toner particles passing through the contact region between the belt  30  and the cleaning brush  42 . 
     A first voltage apply device made of power source  82  is connected to the base  75  of the brush roller  74  and a second voltage apply device made of power source  84  is connected to the collecting roller  77 . The belt  34  to which the charging brush  74  and the cleaning brush  42  are forced is connected to the ground. In the case that the scraper  78  is made of electrically conductive material, it may be connected to the power source  84 . 
     The power source  82  is designed to apply a charging voltage Vc to the charging brush  74  in order to electrically charge the toner particles being transported by the belt  30  at the contact region  73  between the belt  30  and the charging brush  74 . Preferably, the voltage Vc is controlled under the constant current between −100 μA and −10 μA. The charging voltage Vc has the same polarity (negative polarity in the embodiment) as the properly charged toner particles and is set to be about −5,000 volts to −500 volts, for example. 
     The power source  84  is designed to apply a cleaning voltage Vr to the collecting roller  77  so as to flow a certain electric current from the power source  84  through the collecting roller  77  and the cleaning brush  42 , causing the toner particles on the belt  30  to be electrically collected from the belt  30  to the cleaning brush  42 . This results in a voltage gap between the collecting roller  77  and the cleaning brush  42  so that the voltage of the collecting roller  77  is higher than that of the cleaning brush  42 . The cleaning voltage Vr has a different polarity (positive polarity in the embodiment) than the properly charged toner particles and is set to be about 500 volts to 5,000 volts, for example. 
     Discussions will be made to the operation in which the untransferred toner particles are collected from the outer peripheral surface of the belt  30  by the use of the cleaning device  64 . In this discussion, the toner particles are normally charge with negative polarity and the cleaning voltage Vr has positive polarity. 
     As seen from  FIG. 2 , during a time period from the formation of the toner images to the second transfer thereof onto the recording medium  36 , the untransferred toner particles on the belt  30  are transported into the downstream side of the second transfer region  39  with the movement of the belt  30 . The properly charged toner particles have a negative charge. The untransferred toner particles consist essentially of the one with insufficient charge and the one with positive charge in different amounts per toner particle. 
     As shown in  FIG. 3 , since the charging brush  74  is applied with the negative voltage Vc, a large part of the untransferred toner particles transported by the rotation of the belt  30  into the contact region  73  with charging brush  74  are negatively charged by the contact with the charging brush  74 . The negatively charged untransferred toner particles are further transported into the downstream side by the rotation of the belt  30 . 
     In the contact region  73  of the charging brush  74 , the positively charged untransferred toner particles may be electrically attracted to the bristles of the charging brush  74  and the insufficiently charge untransferred toner particles may be caught by the mechanical contact with the bristles of the charging brush  74 . The toner particles collected by the charging brush  74  are accumulated within the charging brush  74 . 
     The toner particles negatively charged by the charging brush  74  is transported by the rotation of the belt  30  into the next contact region  62  between the belt  30  and the cleaning brush  42  where they are electrically attracted and collected by the cleaning brush  42  with the positive voltage Vr applied thereto. 
     The toner particles collected by the cleaning brush  42  is then transported by the rotation of the brush  42  into the contact region between the brush  42  and the collecting roller  77  where, since the voltage of the collecting roller  77  is higher than that of the brush  42 , the toner particles are electrically attracted and collected onto the collecting roller  77 . 
     The toner particles collected by the collecting roller  77  are then transported by the rotation of the roller  77  into the contact region between the roller  77  and the scraper  78  where they are mechanically collected by the scraper  78 . 
     The untransferred toner collecting operations by the cleaning device  64  described above causes an accumulation of the toner particles within the charging brush  78 . The accumulated toner particles within the charging brush  74  may cause in the subsequent image forming operations that the accumulated toner particles be transferred onto the belt  30  due to vibrations generated by the repetitional and frictional contacts of the bristles of the brush  74  with the rotating belt  30 . As described above, the most of the accumulated toner particles within the charging brush  74  have less charge and/or positive charge, so that the toner particles transferred from the charging brush  74  onto the belt  30  are unlikely to be electrically attracted by the bristles of the cleaning brush  42  from the belt  30  and transported by the belt into the second transfer region  39  where they would be transferred directly or by way of the second transfer roller  40  onto the recording medium  36 . 
     In order to eliminate such drawbacks, according to the present invention the controller  70  causes the charging brush  74  to transfer the accumulated toner particles from the charging brush  74  onto the belt  30  at the final stage of the image formation, in particular, in the post post-process sequence. 
     The transfer operation is accompanied by an additional rotational movement of the belt performed after the completion of the rotational movement of the belt  30  for the image formation. 
     The additional rotational movement of the belt  30  is performed after turning off the application of the negative voltage Vc to the charging brush  74 . This eliminates the electrical attraction between the positively charged toner particles within the brush  74  and the bristles of the brush  74 , causing the accumulated toner particles to be transferred easily from the brush  74 . 
     In the transfer operation, the second transfer roller  40  is maintained away from the belt  30 . This prevents the transferred toner particles from being transferred onto the second transfer roller  40  when passing through the opposing region between the belt  30  and the second transfer roller  40  with the rotational movement of the belt  30 . 
     The additional rotational movement of the belt  30  is so limited that the toner particles transferred from the brush  74  onto the belt at the contact region  73  between the belt  30  and the brush  74  are transported and maintained in a region extending from the second transfer region  39  to the contacting region  73  when the rotation of the belt  30  is halted, which allows that the transferred toner particles on the belt  30  are further transported by the rotational movement of the belt  30  immediately after the starting of the subsequent image forming operation into the contact region  73  where they are negatively charged by the charging brush  74  and then into the contact region  62  where they are collected by the cleaning brush  42 , prohibiting the untransferred toner particles from being transferred from the belt  30  to the recording medium  36  during the subsequent image forming operation. 
       FIG. 12  shows a relationship between an amount of electric charge of the toner particles accumulated within the charging brush  74  and the number of toner particles transferred from the brush  74  to the belt  30 . The amount of electric charge was measured for each of 3,000 toner particles using the analyzer commercially available from Hosokawa Micron Co. under the tradename “E-SPART”. The graph shows that the toner particles accumulated within and transferred from the brush  74  have various amount of electric charges with positive and negative polarities. Also, the total amount of electric charge of the accumulated toner particles is substantially zero. Further, the total amount of electric charge of the transferred toner particles is likely to have a slight positive polarity. Furthermore, each of the accumulated and transferred toner includes slightly charged particles. 
     Referring to the flowcharts, an embodiment of the transfer operation will be described in detail. 
     Main Routine 
     As shown in  FIG. 4 , when the main switch of the image forming apparatus  2  is turned on, the main routine is initiated. In this routine, it is determined at step  1  whether a pre-processing operation is required. 
     If it is determined at step  1  that the pre-processing operation is required, this operation is performed at the next step  2 . Otherwise, the program proceeds to step  3 . 
     At step  3  it is determined whether the printing is required. If yes, the toner-image forming operations including development and first and second transfer operations are performed at step  4 . Otherwise, the program proceeds to step  5 . 
     It is then determined at step  5  whether the post-processing operations is required. If yes, the post-processing operation is performed at the next step  6 . Otherwise, the program proceeds to step  7 . 
     If it is determined at step  7  that the image forming apparatus is disconnected from the power source. If yes, various operational settings in the controller  70  are reset at step S 8  and then the program completes the main routine. Otherwise, the program returns to step S 1 . 
     Post-Processing Operation 
     The post-processing operation is performed after the toner-image forming operations including development and first and second transfer operations. When entered the post-processing operation, as shown in  FIG. 6 , the first and second transfer rollers,  14  and  40 , are forced to the belt  30  while the belt  30  and the cleaning brush  42  are rotating. Also, the charging brush  74  is applied with the charging voltage Vc, and the cleaning brush  42  is applied with the toner cleaning voltage Vr. 
     As shown in  FIG. 5 , in the post-processing operation, in particular at step  11 , it is determined whether the image formation of the images to be printed has been completed. If yes, the first and second transfer rollers,  14  and  40 , are spaced away from the belt  30  at step  12  (see  FIGS. 6A and 6B ) and then the program proceeds to step  13 . Otherwise, the program proceeds to step  15 . 
     At step  13 , the rotational movement of the belt  30  and the cleaning brush  74  are halted (see  FIGS. 6C and 6D ). Subsequently, a counter T A  of the timer A starts counting at step  14 . 
     If it is determined at step  15  that the counter T A  counts up a predetermined time T 1 , the counter T A  is reset and the program proceeds to step  17 . Otherwise, the program proceeds to step  19 . 
     At step  17 , the application of the charging voltage Vc to the charging brush  74  is turned off (see FIG.  6 ) and then the program proceeds to step  18 . When the negative charging voltage Vc is turned off, the electrical attraction between the toner particles within the charging brush  74  and the bristles of the brush  74  is eliminated. In addition, the voltage difference between the belt  30  and the charging brush  74  becomes substantially zero, which completely eliminates the electrical attraction between the positive toner particles and the charging brush  74 . 
     A counter T B  of the timer B starts counting at step  18  and then it is determined at step  19  whether the counter T B  counts up a predetermined time T 2 . The time T 2  is so determined that the charging voltage V c  fully established within the time T 2  in response to the instruction from the controller  70 . The time may be 0.1-2.0 seconds, for example. 
     If it is determined that the counter T B  counts up the time T 2  at step  19 , the counter T B  is reset at the subsequent step  20  and then program proceeds to step  21 . Otherwise, the program proceeds to step  23 . 
     At step  21  the rotational movements of the belt  30  and the cleaning brush  42  are started for the transfer of the toner particles (see  FIGS. 6C and 6D ) and then the program proceeds to step  22 . This results in the vibrations of the bristles of the brush  74 , causing the toner particles accumulated within the charging brush  74  to be transferred onto the outer peripheral surface of the belt  30 . 
     A counter T C  of the timer C starts counting at step  22  and then it is determined at step  23  whether the counter T C  counts up the time T 3 . The time T 3  is determined so that the toner particles transferred from the brush onto the belt at the contact region between the belt  30  and the brush  74  are transported by the rotation of the belt  30  and, as a result, stay within a belt portion extending from the second transfer region  39  to the contacting region b 73  when the rotation of the belt  30  is halted. 
     If it is determined at step  23  that the counter T c  counts up the time T 3 , the counter Tc is reset at step  24  and the program proceeds to step  25 . Otherwise, the program returns to the main routine. 
     At step  25 , the rotations of the belt  30  and the cleaning brush  42  are halted and the application of the voltage Vr is turned off (see  FIGS. 6C ,  6 D, and  6 E). Afterwards, the program returns to the main routine. 
     When the rotation of the belt  30  is halted, the portions of the belt  30  bearing the transferred toner particles stay between the opposing region of the belt  30  and the second transfer roller  40  and another opposing region of the belt  30  and the charging brush  74 . The toner particles on the belt portions will be transported by the rotation of the belt  30  in the subsequent image forming operation into the contact region  73  between the belt  30  and the charging brush  74  where they are charged into the negative polarity by the charging brush  74 . The charged toner particles are then transported into the subsequent contact region  62  between the belt  30  and the brush  42  where they are collected by the cleaning brush  42 . This prohibits the transferred toner particles from being transferred from the belt  30  onto the recording medium  36  in the subsequent image forming operation. 
     Although the rotation of the cleaning brush  42  is halted simultaneously with the halt of the rotation of the belt  30  in the post-processing sequence, it may still be in the state of rotation when the belt  30  is halted. 
     Second Embodiment 
     According to the second embodiment of the present invention, the controller  70  drives the belt  30  in the opposite direction (i.e., clockwise direction in  FIG. 2 ) in the toner transfer operation. 
     For this purpose, although not limited thereto, the motor  41  (see  FIG. 1 ) of the drive mechanism uses a motor capable of being driven to rotate in opposite directions. Other structures and the resultant advantages are substantially the same as those described in the first embodiment. 
     Referring to  FIGS. 7 and 8 , the sequence flow of the post-processing operation will be described below. The operations in the main routine are the same as those in the first embodiment. 
     As shown in  FIG. 7 , when entered the post-processing operation, it is determined at step  31  whether the image forming operation has completed. If yes, the first and second transfer rollers,  14  and  40 , are spaced away from the belt  30  at step  31  (see  FIGS. 8A and 8B ), and the program proceeds to step  33 . Otherwise, the program proceeds to step  35 . 
     The rotations of the belt  30  and the cleaning brush  42  are halted at step  33  and the application of the voltage Vr to the cleaning brush  42  is turned off (see  FIGS. 8C ,  8 E, and  8 F). A counter T D  of the timer D then starts counting at step  34 . 
     It is determined at step  35  whether the counter T D  of the timer D counts up a predetermined time T 4 . The time T 4  is determined so that times required for the belt  30  and brush  42  to halt completely after an issuance of an instruction from the controller  70  for halting the belt  30  and brush  42 , respectively, and time required for the voltage Vr to be removed completely after an issuance of the instruction from the controller  70  for turning off the voltage, whichever is the longest. For example, the time is set to be equal to or more than 0.5 seconds and equal to or less than five seconds. 
     If it is determined at step  35  that the counter T D  counts up the predetermined time T 4 , the counter T D  is reset to zero at step  36  and the program proceeds to step  37 . Otherwise, the program proceeds to step  39 . 
     At step  37 , the voltage Vc to the charging brush  74  is turned off (see  FIG. 8G ), and the program proceeds to step  38 . As described above at step  17  in the first embodiment, this causes that the electric attraction between the accumulated positive toner particles within the brush  74  and the bristles of the brush  74  is eliminated. 
     At step  38 , the counter T B  of the timer B starts counting. It is then determined at step  39  whether the counter counts up T B . The time T 2  is determined to be the same as that in the first embodiment. 
     At step  39 , if it is determined whether the counter T B  counts up the predetermined time T 2 , it is reset to zero at step  40  and the program proceeds to step  41 . Otherwise, the program proceeds to step  43 . 
     The belt  30  is driven to rotate for the transfer operation (see  FIG. 8D ) at step  41 , and then the program proceeds to step  42 . In the second embodiment, the belt  30  is rotated in another direction (clockwise direction in  FIG. 2 ) which is opposite to that in the image forming operation. This allows that the toner particles accumulated within the charging brush  74  are transferred onto the outer peripheral surface of the belt  30 , as described in the first embodiment. 
     The counter T E  of the timer E stats counting at step  42 , and then it is determined at step  43  whether the counter T E  counts up the predetermined time T 5 . The time T 5  is determined so that the toner particles transferred from the brush onto the belt at the contact region between the belt  30  and the brush  74  are transported by the rotation of the belt  30  and, as a result, stay within a belt portion extending from the second transfer region  39  to the contacting region  73  when the rotation of the belt  30  is halted. Since the rotational direction of the belt  30  is opposite to that in the image forming operation (i.e., clockwise direction in  FIG. 2 ), which requires a reduced rotational displacement of the belt  30  than that in the first embodiment. This in turn means that the time T 5  is less than the corresponding time T 3  in the first embodiment and, as a result, the total time required for the post-operation is decreased. 
     If it is determined at step  43  that the counter T E  counts up the predetermined time T 5 , it is reset to zero at step  44 . The program then proceeds to step  45 . Otherwise, the program returns to the main routine. 
     At step  45 , the rotation of the belt  30  is halted (see  FIG. 8D ). Then, the program proceeds to the main routine. 
     This causes that, as described in the first embodiment, the toner particles transferred on the belt  30  exist on the belt portion extending from the second transfer region between the belt and the second transfer roller to another contacting region between the belt and the charging brush  74  when the rotation of the belt  30  is halted. Therefore, when the subsequent image forming operation is started, the transferred toner particles are transported immediately by the rotation of the belt  30  into the contact region  73  of the belt  30  and the charging brush  74  where they are negatively charged by the charging brush  74 . The negatively charged toner particles are then collected by the cleaning brush  74 , which prevents the transferred toner particles from being transferred from the belt  30  to the recording medium  36 . 
     Third Embodiment 
     The voltage Vr to be applied to the cleaning brush  42  for collecting toner particles from the belt  30  may take different levels in the image forming and the transfer operations, respectively. Specifically, the controller  70  controls the voltage Vr so that it has a first level with positive polarity in the image forming operation and a second level with negative polarity in the transfer operation. 
     More specifically, as shown in  FIG. 9 , the image forming apparatus of this embodiment has a second voltage application means made of two voltage supplies  84  and  86  selectively connected to the collecting roller  77  on the basis of the instruction from the controller  70 , so that the voltage Vr with the positive polarity is applied to the brush  42  when the voltage supply  84  is connected to the collecting roller  77  and the voltage Vr with the negative polarity is applied to the brush  42  when the voltage supply  86  is connected to the collecting roller  77 . 
     The controller  70  connects the collecting roller  77  to the voltage supply  84  in the image forming operation to apply the voltage Vr with the positive polarity to the cleaning brush  42  and connects the collecting roller  77  to another voltage supply  84  in the transfer operation to apply the voltage Vr with the negative polarity to the cleaning brush  42 . 
     This allows that the toner particles negatively charged by the brush  74  are collected by the cleaning brush  42  during the image forming operation. 
     Also, the transferred, positively charged toner particles on the belt  30  are electrically attracted and collected by the bristles of the brush  42  during the toner transfer operation. As shown in  FIG. 12 , most of the transferred toner particles have positive polarity and therefore the substantial part thereof are collected by the cleaning brush  42  in the transfer operation. 
     A small part of the transferred toner particles may not be collected by the cleaning brush. The uncollected toner particles are then transported into the region from the opposing portion of the belt  30  and the second transfer roller  40  to another opposing portion of the belt  30  and the charging brush  74  without being transferred onto the recording medium or the second transfer roller during the transfer operation, which will be collected by the cleaning brush  42  during the subsequent image forming operation. 
     Other structures and advantages relating to the third embodiment are substantially the same as those described in the previous embodiments. 
     Post-Processing Sequence of Third Embodiment 
     Referring to  FIGS. 10 and 11 , the program flow of the post-processing sequence of the third embodiment will be described. It could be understood that the each of the operations in the main routine is the same as that of the first embodiment. 
     As shown in  FIG. 10 , when the post-processing operation is started, it is determined at step  51  whether the image forming operation has been completed. If yes, the first and second transfer rollers,  14  and  40 , are spaced away from the belt  30  at step  52  (see  FIGS. 11A and 11B ), and the program proceeds to step  53 . Otherwise, the program proceeds to step  55 . 
     At step  53 , the rotations of the belt  30  and the cleaning brush  42  are halted (see  FIGS. 11C and 11D ). Then, at step  54 , a counter T A  of the timer A starts counting. 
     It is determined at step  55  whether the counter T A  of the timer A counts up a predetermined time T 1 . The time T 1  is determined as described in the first embodiment. 
     If it is determined at step  55  whether the counter T A  counts up the time T 1 , the counter T A  is reset to zero at step  56  and then the program proceeds to step  57 . Otherwise, the program proceeds to step  59 . 
     At step  57 , the voltage Vr to be applied to the cleaning brush  42  is switched from the positive voltage level to the negative voltage level (see  FIG. 11E ), and the program proceeds to step  58 . 
     The counter T F  of the timer F starts counting at step  58 , and it is determined at step  59  whether the counter T F  counts up a predetermined time T 6 . The time T 6  is set to be the one required for the voltage Vr to be substantially switched after the issuance of the instruction from the controller  70 . 
     If it is determined at step  59  that the counter T F  counts up the time T 6 , the counter T F  is reset to zero at step  60  and then the program proceeds to step  61 . Otherwise, the program proceeds to step  63 . 
     The application of the voltage Vc to the charging brush  74  is turned off (see  FIG. 11F ) at step  61 , and then the program proceeds to step  62 . This causes that the positively charged toner particles accumulated within the charging brush  74  lose electrical attraction force with the bristles of the brush  74 , as described in the first embodiment. 
     The counter T B  of the timer B starts counting at step  62  and then it is determined at step  63  whether the counter T B  counts up the predetermined time T 2  which is determined as described in the first embodiment. 
     If it is determined at step  63  that the timer T B  counts up the predetermined time T 2 , the program proceeds to step  64  where the counter T B  is reset to zero and then the program proceeds to step  65 . Otherwise, the program proceeds to step  67 . 
     For toner transfer operation, the rotations of the belt  30  and the cleaning brush  42  are started at step  65  (see  FIGS. 11C and 11D ), and then the program proceeds to step  66 . This allows that the toner particles accumulated within the charging brush  74  are transferred onto the belt  30  as described in the first and second embodiments. Most of the transferred toner particles have positive polarity and therefore, when transported to the contact region of the belt  30  and the cleaning brush  42 , they are collected by the brush  42  and thereby removed from the belt  30 . 
     The counter T C  of the timer C starts counting at step  66 . It is then determined at step  67  whether the counter T C  counts up the predetermined time T 3 . The time T 3  is determined as described in the first embodiment. 
     If it is determined at step  67  that the counter T C  counts up the time T 3 , the counter T C  is reset to zero at step  68  and then the program proceeds to step  69 . Otherwise, the program returns to the main routine. 
     At step  69 , the rotations of the belt  30  and the cleaning brush  42  are halted and the voltage Vr to the cleaning brush  42  is turned off ( FIGS. 11C ,  11 D, and  11 E). Afterwards, the program returns to the main routine. 
     The toner particles uncollected by the cleaning brush  42  stay on the belt portion extending from the opposing region of the belt  30  and the transfer roller  40  to another opposing region of the belt  30  and the charging brush  74  with respect to the rotational direction of the belt  30  when the rotation of the belt is halted. The uncollected, transferred toner particles on the belt  30  are then transported by the rotation of the belt  30  into the contact region of the belt and the charging brush  74  during the subsequent image forming operation, where they are charged with negative polarity by the contact with the charging brush  74  and therefore collected by the cleaning brush  42 . This prevents the toner particle from being transferred onto the recording medium  36  from the belt  30  during the subsequent image forming operation. 
     Although the present invention has been fully described with the embodiments, it is not limited thereto. 
     For example, although the transfer operation is described in connection with the embodiments in each of which the endless intermediate belt  30  is used, the present invention is equally applicable to other embodiments in which the toner particles are transferred onto and collected from another type of image bearing members, rather than the intermediate belt  30 , such as cylindrical drum-type intermediate transfer member and cylindrical and endless-belt type photosensitive member. 
     Further, although the charging brush  74  is electrically connected to the power supply  82  so as to apply the charge voltage to the charging brush  74 , the present invention is not limited thereto. For example, the charging brush  74  may be grounded as described in JP 2004-310060 A, the entire disclosure of which being incorporated herein by reference. In this instance, simply by turning off the power supply  84  connected to the collecting roller  77  after the halt of the transfer belt  30 , electric current to the charging brush  74  is turned off and, as a result, the negative voltage to be applied to the charging brush  74  is eliminated. 
     Furthermore, although the displacement of the belt  30  in the transfer operation is controlled by the use of the timer counter, it may be controlled in another way. For example, the displacement may be controlled by the use of an output of a pulse encoder mounted on the rotational portion or shaft of the roller or rollers supporting the belt  30 . Alternatively, the displacement may be controlled by the use of a mark or indication provided on the outer periphery of the belt  30  and a detector for detecting the mark so that the controller controls the displacement upon receiving a signal from the detector indicative of the detection of the mark.