Patent Publication Number: US-7711299-B2

Title: Color image forming apparatus with discharge member

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority to Japanese Patent Application No. 2006-340073 filed on Dec. 18, 2006, the disclosure of which is hereby incorporated by reference into the present application. 
   TECHNICAL FIELD 
   The present invention relates to a color image forming apparatus, such as a color laser printer. 
   BACKGROUND 
   Conventionally, in color image forming apparatuses, such as a color laser printer, a method of transferring a toner image formed on a surface of a photosensitive drum to a sheet transported by a sheet transport belt, and a method of once transferring a toner image formed on a surface of a photosensitive drum to an intermediate transfer belt and then transferring it to a sheet therefrom have been known. 
   A belt, such as the sheet transport belt and the intermediate transfer belt, is wound around a plurality of rollers, and arranged so as to come in contact with the surface of the photosensitive drum. Therefore, the contact of the belt with the photosensitive drum causes a toner to be shifted and adhered to the surface of the belt, and the contact of the belt with a sheet causes a sheet dust to be adhered thereon. 
   A color image forming apparatus including such a belt is provided with a cleaning unit for removing the adherent on the surface of the belt. As the cleaning unit, one that makes a cleaning blade or a fur brush contact with the surface of the belt and then physically removes the adherent thereon has been known. 
   Another structure of the cleaning unit can be considered in which a cleaning roller is arranged in contact with the surface of the belt and a bias is applied to the cleaning roller, so that a potential difference is produced between the cleaning roller and the belt, whereby an electrostatic force causes the adherent on the surface of the belt to be shifted to the cleaning roller. 
   However, under the effect of the bias applied to the cleaning roller, the belt is charged, and due to the charging (uneven charging), a white patch (phenomenon in which an image is not locally printed) may appear in an image formed on a sheet. 
   SUMMARY 
   One aspect of the present invention may provide a color image forming apparatus capable of preventing a white patch resulting from uneven charging of a belt, while advantageously removing an adherent to the belt with a cleaning roller. 
   The same or different aspect of the present invention may provide a color image forming apparatus including: a belt that travels in a prescribed direction; a plurality of image carriers arranged in parallel in the prescribed direction, and opposed to the belt and carrying developing agent images of different colors respectively; a backup roller arranged in opposed relation to the belt; a cleaning roller arranged in opposed relation to the backup roller with the belt sandwiched therebetween, and adsorbing an adherent to the belt by a potential difference produced between the backup roller and the cleaning roller; and a discharge member opposed to the belt between a position more downstream in the prescribed direction than a position where the cleaning roller is opposed to the belt and a position more upstream in the prescribed direction than a position where the image carrier arranged on the most upstream side in the prescribed direction is opposed to the belt, for discharging the belt. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side sectional view of a color laser printer as an example of a color image forming apparatus of one or more aspects of the present invention; 
       FIG. 2  is a perspective view seen obliquely downwardly from the front left side of a transferring unit shown in  FIG. 1 ; 
       FIG. 3  is a perspective view seen obliquely upwardly from the rear right side of a transferring unit frame shown in  FIG. 2 ; 
       FIG. 4(   a ) is a partial front view of a discharge member shown in  FIG. 3 , and  FIG. 4(   b ) is a side view thereof; 
       FIG. 5(   a ) is a front view illustrating another structure (a structure in which edge portions are formed in spaced relation) of the discharge member, and  FIG. 5(   b ) is a side view thereof; 
       FIG. 6(   a ) is a front view illustrating yet another structure (a structure in which a brush is planted in a tip end portion thereof) of the discharge member, and  FIG. 6(   b ) is a side view thereof; and 
       FIG. 7(   a ) is a front view illustrating yet another structure (a structure in which the tip end portion thereof does not have the edge portion or the brush) of the discharge member, and  FIG. 7(   b ) is a side view thereof. 
   

   DETAILED DESCRIPTION 
   The embodiments of the present invention will be described below while referring to the drawings. 
   First Embodiment 
   1. General Structure of Color Laser Printer 
     FIG. 1  is a side sectional view of a color laser printer as an example of a color image forming apparatus of one or more aspects of the present invention. 
   The color laser printer  1  is of a tandem type adopting a direct transfer method. In a main body casing  2  having a box-like shape, a sheet feeding section  4  for feeding a sheet  3 , an image forming section  5  for forming an image on the sheet  3 , and a sheet ejecting section  6  for ejecting the sheet  3  formed with the image are arranged. 
   In the following description, the right side of  FIG. 1  is referred to as the front side, while the left side of  FIG. 1  is referred to as the rear side. A left and right direction is determined when the color laser printer  1  is viewed from the front side. 
   (1) Sheet Feeding Section 
   The sheet feeding section  4  includes a sheet feeding tray  7  for accommodating sheets  3  in a stacked state and a sheet feeding roller  8  for sending out the sheets  3  in the sheet feeding tray  7  on a sheet-by-sheet basis. The sheet  3  thus sent out from the sheet feeding tray  7  is transported toward the image forming section  5  on a sheet transport path  9 . 
   (2) Image Forming Section 
   The image forming section  5  includes a scanning section  10 , a processing section  11 , a transferring unit  12 , a cleaning unit  13 , and a fixing section  14 . 
   (2-1) Scanning Section 
   The scanning section  10  is arranged in the upper portion of the main body casing  2 . The scanning section  10  includes optical members such as a laser, a mirror, and a lens, and emits four laser beams toward four photosensitive drums  15  described later. As indicated by a broken line in  FIG. 1 , each of the laser beams is irradiated onto each of the surfaces of the corresponding photosensitive drums  15 . 
   (2-2) Processing Section 
   A plurality of the processing sections  11  are provided corresponding to toners of respective colors. Specifically, four processing sections  11  includes a black, yellow, magenta, and cyan processing sections  11 K,  11 Y,  11 M, and  11 C. These four processing sections  11  are arranged in parallel so as to be spaced away from one another from the front side to the rear side in the order of the black processing section  11 K, the yellow processing section  11 Y, the magenta processing section  11 M, and the cyan processing section  11 C. 
   Each of the processing sections  11  includes the photosensitive drum  15  as an example of an image carrier. The photosensitive drum  15  has a cylindrical shape, and its outermost surface layer is formed of a positively chargeable photosensitive layer of polycarbonate or the like. 
   A scorotron charger  16  and a developer cartridge  17  are arranged around the photosensitive drum  15 . 
   The developer cartridge  17  includes an agitator  19 , a feed roller  20 , a developing roller  21  and a layer-thickness regulating blade  22  in a casing  18 . 
   The casing  18  is formed in a box-like shape with its rear lower end portion opened. The upper portion in the casing  18  is a toner accommodating chamber  23 . The toner accommodating chamber  23  accommodates a toner of each color. Specifically, a yellow toner, a magenta toner, a cyan toner, and a black toner are accommodated in the corresponding toner accommodating chambers  23  of the developer cartridges  17  of the yellow processing section  11 Y, the magenta processing section  11 M, the cyan processing section  11 C, and the black processing section  11 K, respectively. As the toner of each color, a positively chargeable, non-magnetic, single-component polymerized toner is used, in which coloring agent of black, yellow, magenta, or cyan is mixed corresponding to each color. 
   The agitator  19  is rotatably provided in the toner accommodating chamber  23 . 
   The feed roller  20  is arranged in the lower portion of the toner accommodating chamber  23 . The feed roller  20  has a structure in which a metal roller shaft is covered with a roller portion made of an electrically-conductive sponge member. 
   The developing roller  21  is provided obliquely rearward below the feed roller  20 . The developing roller  21  has a structure in which a metal developing roller shaft is covered with a rubber roller made of an electrically-conductive rubber. A part of circumferential surface of the developing roller  21  is exposed from the casing  18  and is in pressure contact with the photosensitive drum  15  from the front side. 
   The layer-thickness regulating blade  22  includes a leaf-spring member  24  one end of which is fixed to the casing  18 , and a pressure contact rubber  25  provided at the tip end portion (distal-end portion) of the leaf-spring member  24 . The layer-thickness regulating blade  22  is provided so that the pressure contact rubber  25  is brought into pressure contact with the circumferential surface of the developing roller  21  from above. 
   In each of the developer cartridges  17 , a toner in the toner accommodating chamber  23  is supplied to the feed roller  20  while being agitated, by rotation of the agitator  19 . During an image forming operation (during development), the developing roller  21  and the feed roller  20  are rotationally driven in the reverse direction (counterclockwise in the figure) to the photosensitive drum  15  so as to rub against each other at the roller portions thereof. Further, a developing bias is supplied to the developing roller  21 . Thus, a toner positively charged is carried on the circumferential surface of the developing roller  21 . Along with the rotation of the developing roller  21 , the toner thus carried on the circumferential surface of the developing roller  21  enters between the pressure contact rubber  25  of the layer-thickness regulating blade  22  and the developing roller  21 , thereby forming a thin layer having a uniform thickness. 
   On the other hand, the photosensitive drum  15  is rotationally driven, and along with this rotation, the surface of the photosensitive drum  15  is uniformly positively charged by corona discharge from the scorotron charger  16 . Then, as the portion thus positively charged is irradiated with the laser beams from the scanning section  10 , an electrostatic latent image of each color corresponding to the image to be formed on the sheet  3  is formed on the surface of the photosensitive drum  15 . When the electrostatic latent image is opposed to the surface of the developing roller  21  by rotation of the photosensitive drum  15 , the toner carried on the developing roller  21  is shifted to a portion having a lower potential due to the exposure to the laser beams on the surface of the photosensitive drum  15 . Thus, the electrostatic latent image on the photosensitive drum  15  is transformed into a visible image, whereby a toner image corresponding to each color is carried on the surface of the photosensitive drum  15 . 
   (2-3) Transferring Unit 
   The transferring unit  12  is arranged below the four processing sections  11 . The transferring unit  12  includes a transferring unit frame  26  as an example of a frame, a driving roller  27  as an example of a second belt roller, a driven roller  28  as an example of a first belt roller, a transport belt  29  as an example of a belt, a backup roller  30 , and four transfer rollers  31 . 
   The transferring unit frame  26  is anteroposteriorly extended. The front end portion of the transferring unit frame  26  is positioned forward of the photosensitive drum  15  of the black processing section  11 K, while the rear end portion of the transferring unit frame  26  is positioned rearward of the photosensitive drum  15  of the cyan processing section  11 C. 
   The driving roller  27  has a structure in which a shaft having a shape of a round tube made of aluminum or stainless steel is covered with a roller portion made of rubber or the like. The driving roller  27  is extended in the right and left direction, with both the end portions thereof supported on the rear end portion of the transferring unit frame  26 . 
   The driven roller  28  is a roller having a shape of a round tube made of aluminum or a stainless steel. The circumferential surface of the driven roller  28  (roller portion) is plated in order to prevent wear due to contact with the transport belt  29 . The driven roller  28  is extended in the right and left direction, with both the end portions thereof supported on the front end portion of the transferring unit frame  26 . Further, the driven roller  28  is attached to the transferring unit frame  26  with an anteroposterior play. The driven roller  28  is always urged to a direction spaced away from the driving roller  27 , that is, forward, by a spring  32  provided on the transferring unit frame  26 . The driven roller  28  is grounded via the transferring unit frame  26 . 
   The transport belt  29  is wound between the driving roller  27  and the driven roller  28 . The transport belt  29  is made of resins, such as polycarbonate, and has a resistance value of 1.0×10 10  Ω·cm or more. A moderate tension is given to the transport belt  29  by urging the driven roller  28  in the direction spaced away from the driving roller  27 . 
   The backup roller  30  is made of an electrically-conductive material, such as metal. The backup roller  30  is extended in the right and left direction, with both the end portions thereof supported on the transferring unit frame  26 . The backup roller  30  is arranged in a space surrounded by the transport belt  29 , and is opposed to a primary cleaning roller  33  described later of the cleaning unit  13  while sandwiching the transport belt  29  therebetween. 
   The four transfer rollers  31  are arranged in the space surrounded by the transport belt  29 , and are opposed to the respective photosensitive drums  15  while sandwiching the transport belt  29  therebetween. Each of the transfer rollers  31  has a structure in which a metal roller shaft is covered with a roller portion made of an elastic member, such as an electrically-conductive rubber material. The respective transfer rollers  31  are in parallel to one another while being extended in the width direction, and both the end portions thereof are supported on the transferring unit frame  26 . During an image forming operation, a transfer bias is applied to each of the transfer rollers  31 . 
   When the driving roller  27  is rotationally driven in the reverse direction (clockwise in the figure) to the rotation direction of the photosensitive drum  15  by a driving force from a motor (not shown) provided in the main body casing  2 , the transport belt  29  circumferentially travels in the same direction as the rotation direction of the driving roller  27 . Specifically, the driving force from the motor causes the transport belt  29  to circumferentially travel so that the upper portion (portion extending between the upper circumferential end of the driving roller  27  and that of the driven roller  28 ) of the transport belt  29  moves rearward. Along with the travel of the transport belt  29 , the driven roller  28  is driven and rotates in the same direction as the driving roller  27 . 
   The sheet  3  transported from the sheet feeding section  4  to the image forming section  5  is fed onto the transport belt  29 . Then, as the transport belt  29  circumferentially travels, the sheet  3  thus fed is transported so as to sequentially pass the spaces between the respective photosensitive drums  15  and the transport belt  29 . During such transportation, the toner images carried on the respective photosensitive drums  15  are overlapped in color and transferred onto the sheet  3  by the transfer bias applied to each of the transfer rollers  31 . Thus, a color image is formed on the sheet  3 . At this time, each of the transfer rollers  31  is driven to rotate in the same direction as the circumferentially moving direction of the transport belt  29  at a corresponding transfer position where it is opposed to and contacts the transport belt  29 . 
   (2-4) Cleaning Unit 
   The cleaning unit  13  is arranged between the transferring unit  12  and the sheet feeding tray  7  of the sheet feeding section  4 . The cleaning unit  13  includes the primary cleaning roller  33  as an example of a cleaning roller, a secondary cleaning roller  34 , a urethane blade  35 , and a receiving section  36 . 
   The primary cleaning roller  33  is arranged so as to extend in the right and left direction, and the circumferential surface thereof is in contact with the surface (under surface) of the lower portion (portion extending between the lower circumferential end of the driving roller  27  and that of the driven roller  28 ) of the transport belt  29 . Further, the primary cleaning roller  33  is opposed to the backup roller  30  while sandwiching the upper portion of the transport belt  29  therebetween. The primary cleaning roller  33  is formed by covering a shaft made of an electrically-conductive material (e.g., a Ni (nickel)-plated iron material or a stainless steel material) with a foam material of silicone. The primary cleaning roller  33  is rotationally driven in the reverse direction (counterclockwise in the figure) to the moving direction of the transport belt  29  at a position where it is in contact with the transport belt  29 . 
   The secondary cleaning roller  34  is arranged so as to extend in parallel to the primary cleaning roller  33 , and is in contact with the circumferential surface of the primary cleaning roller  33 . The secondary cleaning roller  34  is comprised of a bar-like member (shaft) made of an electrically-conductive material, such as iron material. 
   During cleaning process for removing an adherent such as a toner or a sheet dust adhered to a surface of the transport belt  29 , appropriate cleaning biases are applied to the primary cleaning roller  33  and the secondary cleaning roller  34  respectively. On the other hand, the backup roller  30  is grounded. Thus, potential differences are respectively produced between the backup roller  30  (transport belt  29 ) and the primary cleaning roller  33 , and between the primary cleaning roller  33  and the secondary cleaning roller  34 . The potential difference between the backup roller  30  and the primary cleaning roller  33  causes the adherent on the surface of the transport belt  29  to be shifted to the primary cleaning roller  33 . The potential difference between the primary cleaning roller  33  and the secondary cleaning roller  34  then causes the adherent thus shifted to the primary cleaning roller  33  to be shifted to the secondary cleaning roller  34 . The adherent thus shifted to the secondary cleaning roller  34  is scraped off by the urethane blade  35 , thereby falling off from the secondary cleaning roller  34 . As a result, the adherent thus fallen is stored in the receiving section  36 . 
   (2-5) Fixing Section 
   The fixing section  14  is arranged behind the transferring unit  12 . The fixing section  14  includes a heating roller  37  and a pressure roller  38 . The pressure roller  38  is in pressure contact with the heating roller  37  from below. The sheet  3  transported by the transport belt  29  is sent between the heating roller  37  and the pressure roller  38 . While the sheet  3  passes between the heating roller  37  and the pressure roller  38 , the toner image transferred on the sheet  3  is fixed thereto by heat and pressure. 
   (3) Sheet Ejecting Section 
   The sheet ejecting section  6  includes a sheet ejecting transport path  39  having a generally C-shape opening frontward. The sheet  3  transported from the fixing section  14  passes along the sheet ejecting transport path  39 , and is then ejected by a sheet ejecting roller  40  onto a sheet ejecting tray  41  formed on the upper surface of the main body casing  2 . 
   2. Transferring Unit Frame 
     FIG. 2  is a perspective view seen obliquely downwardly from the front left side of the transferring unit  12  shown in  FIG. 1 .  FIG. 3  is a perspective view seen obliquely upwardly from the rear right side of the transferring unit frame  26  shown in  FIG. 2 . 
   The transferring unit frame  26  includes a left side plate  51  and a right side plate  52  opposed in spaced relation to each other in the right and left direction. The left side plate  51  and the right side plate  52  are made of metal, and each extended anteroposteriorly. Both the end portions of the driving roller  27 , the driven roller  28 , the backup roller  30 , and each of the four transfer rollers  31  are rotatably supported on the left side plate  51  and the right side plate  52  respectively. 
   As shown in  FIG. 3 , a connecting member  53  made of resin is extended between the left side plate  51  and the right side plate  52 . The connecting member  53  integrally includes a transfer-roller accommodating section  54  that encloses each of the transfer rollers  31  from the front, rear, and lower sides, a closed portion  55  that closes a space between the upper end portions of the transfer-roller accommodating section  54 , and a plurality of ribs  56  that are extended between the side surfaces of the transfer-roller accommodating section  54 . 
   A discharge member  57  made of an electrically-conductive material, such as stainless steel, is provided on the under surface of the transfer-roller accommodating section  54  on the foremost side, that is, the transfer-roller accommodating section  54  that accommodates the transfer roller  31  opposed to the photosensitive drum  15  of the black processing section  11 K. The discharge member  57  is electrically conductively connected with, for example, an electrically conductive member (not shown) provided on the left side plate  51  and/or the right side plate  52 , and is grounded via the electrically conductive member. 
   The backup roller  30  is arranged between the transfer-roller accommodating section  54  positioned on the foremost side and the transfer-roller accommodating section  54  (that accommodates the transfer roller  31  opposed to the photosensitive drum  15  of the yellow processing section  11 Y) adjacent thereto. Thus, the discharge member  57  is opposed to; the rear surface of the transport belt  29  between a position where the backup roller  30  and the primary cleaning roller  33  are opposed to each other and the driven roller  28 . 
   As shown in  FIG. 2 , a front wall  58  made of resin is extended between the respective front end portions of the left side plate  51  and of the right side plate  52 . It should be noted that the illustration of the front wall  58  is omitted in  FIG. 3 . 
   3. Discharge Member 
     FIG. 4(   a ) is a partial front view of the discharge member  57  shown in  FIG. 3 , and  FIG. 4(   b ) is a side view thereof. 
   With reference to  FIGS. 3 and 4 , the discharge member  57  is provided on the under surface of the transfer-roller accommodating section  54  positioned on the foremost side and along the lengthwise direction (right and left direction) thereof. The discharge member  57  has in the right and left direction a length of the width or more of the maximum-sized sheet  3  usable in the color laser printer  1 . Further, the discharge member  57  has a height with which the tip end portion thereof does not contact the rear surface of the transport belt  29 . 
   The tip end portion of the discharge member  57  is formed in a sawtooth-like shape by continuously forming a number of edge portions  59  each having a triangular shape in front view. The distal end of each of the edge portions  59  has a radius of curvature of 0.2 mm or less. Particularly, the distal end thereof has preferably a radius of curvature of 0.1 mm or less. The discharge member  57  is placed so that a distance between the distal end of each of the edge portions  59  and the belt is in the range from 1 to 10 mm. 
   4. Effects 
   As described above, the backup roller  30  and the primary cleaning roller  33  are opposed to each other while sandwiching the transport belt  29  therebetween. A potential difference produced between the backup roller  30  (transport belt  29 ) and the primary cleaning roller  33  causes the adherent adhering to the transport belt  29  to be adsorbed onto the primary cleaning roller  33 . Thus, the adherent can be advantageously removed from the transport belt  29 . 
   The discharge member  57  for removing charges from the transport belt  29  is arranged between the position where the primary cleaning roller  33  is opposed to the transport belt  29  and a position where the photosensitive drum  15  arranged on the most upstream side (on the foremost side) in the traveling direction of the transport belt  29  is opposed to the transport belt  29 . More specifically, the discharge member  57  is opposed to the transport belt  29  between the position where the primary cleaning roller  33  is opposed to the transport belt  29  and a position where the driven roller  28  contacts the transport belt  29 . 
   When the transport belt  29  is electrically charged during cleaning by the primary cleaning roller  33 , if a portion where charges are concentrated is produced on the transport belt  29 , the charges are discharged from the portion, which may result in uneven charging of the transport belt  29  in some cases. Particularly, with the structure in which the driven roller  28  is grounded, the charges partially escape from the transport belt  29  to the driven roller  28  in the right and left direction, so that uneven charging tends to occur in the transport belt  29 . Such uneven charging in the transport belt  29  can locally produce a portion having a small potential difference between the photosensitive drum  15  and the sheet  3  when a toner image is transferred from the photosensitive drum  15  onto the sheet  3 . As a result, a so-called white patch where the toner image is not transferred onto the sheet  3  may be produced at the portion. 
   In the color laser printer  1 , the discharge member  57  is opposed to the rear surface of the transport belt  29  between the position where the primary cleaning roller  33  is opposed to the transport belt  29  and the position where the driven roller  28  contact the transport belt  29 . Therefore, even if the charge-concentrated portion exists on the transport belt  29 , the concentrated charges can be discharged toward the discharge member  57 , so that the charges on the transport belt  29  can be removed. This can prevent discharging from the transport belt  29  to the driven roller  28 , thereby preventing uneven charging of the transport belt  29  due to such discharging from being produced. As a result, the white patch resulting from the uneven charging of the transport belt  29  can be prevented. 
   Since the discharge member  57  is grounded, the electric charges captured from the transport belt  29  by the discharge member  57  can be advantageously discharged. 
   In addition, the discharge member  57  is provided in a non-contact state with the transport belt  29 . For this reason, it is possible to prevent the transport belt  29  from being damaged due to contact with the discharge member  57 . 
   Further, the discharge member  57  is attached to the transferring unit frame  26  retaining the transfer roller  31 . For this reason, it is not necessary to provide any member for supporting the discharge member  57 , separately from the transferring unit frame  26 . Thus, the number of component in the apparatus can be reduced. Further, since the transferring unit frame  26  and the transport belt  29  are arranged in proximity to each other, the discharge member  57  and the transport belt  29  can be arranged in proximity to each other by attaching the discharge member  57  to the transferring unit frame  26 . As a result, the discharge member  57  can advantageously remove charges from the transport belt  29 . 
   The transport belt  29  has a resistance value of 1.0×10 10  Ω·cm or more. Higher resistance of the transport belt  29  can produce a larger potential difference between the primary cleaning roller  33  and the transport belt  29 , so that the adherent on the transport belt  29  can be advantageously shifted to the primary cleaning roller  33 . On the other hand, the higher resistance of the transport belt  29  tends to cause discharging from the transport belt  29  to other members (e.g., the driven roller  28  or a frame). However, since the discharge member  57  is provided, the discharging from the transport belt  29  can be prevented. As a result, uneven charging of the transport belt  29  can be prevented. 
   Since the discharge member  57  has the edge portions  59  each having a radius of curvature of 0.2 mm or less, charges can be advantageously shifted from the transport belt  29  to the discharge member  57 . Therefore, the transport belt  29  can be effectively discharged by the discharge member  57 . 
   In addition, the discharge member  57  has a number of the edge portions  59  in the right and left direction that is a direction intersecting the traveling direction of the transport belt  29 . This can uniformly remove charges from the transport belt  29  in the right and left direction. As a result, white patch resulting from the uneven charging of the transport belt  29  can be reliably prevented. 
   Second Embodiment 
     FIG. 5(   a ) is a front view illustrating another structure of the discharge member, and  FIG. 5(   b ) is a side view thereof. 
   As shown in  FIG. 5 , in the discharge member  57 , the edge portions  59  each having a triangular shape in front view do not need to be continuously formed and may be formed in spaced relation in the right and left direction. 
   Third Embodiment 
     FIG. 6(   a ) is a front view illustrating yet another structure of the discharge member, and  FIG. 6(   b ) is a side view thereof. 
   As shown in  FIG. 6 , a number of bristles of the brushes  60  may be planted in the tip end portion of the discharge member  57 . 
   Fourth Embodiment 
     FIG. 7(   a ) is a front view illustrating yet another structure of the discharge member, and  FIG. 7(   b ) is a side view thereof. 
   As shown in  FIG. 7 , the discharge member  57  may be formed in a thin plate and may not have the edge portions  59  in the tip end portion thereof. However, the belt-side end portion of the discharge member  57  has a radius of curvature of 0.2 mm or less. 
   Fifth Embodiment 
   The discharge member  57  may be, for example, opposed to the surface of the transport belt  29  in a non-contact state between the position where the primary cleaning roller  33  is opposed to the transport belt  29  and the position where the photosensitive drum  15  arranged on the foremost side is opposed to the transport belt  29 . 
   Sixth Embodiment 
   The discharge member  57  may be arranged in a contact state with the surface or the rear surface of the transport belt  29 . 
   Seventh Embodiment 
   In the color laser printer  1  of a tandem type, the case where the present invention is applied to the transport belt  29  for transporting a sheet  3  has been considered. However, the present invention can also be applied to an intermediate transfer belt in a color laser printer of an intermediate transfer type in which toner images for respective colors are transferred from respective image carriers to an intermediate transfer belt, and thereafter, transferred onto a sheet by one operation. 
   The embodiments described above are illustrative and explanatory of the invention. The foregoing disclosure is not intended to be precisely followed to limit the present invention. In light of the foregoing description, various modifications and alterations may be made by embodying the invention. The embodiments are selected and described for explaining the essentials and practical application schemes of the present invention which allow those skilled in the art to utilize the present invention in various embodiments and various alterations suitable for anticipated specific use. The scope of the present invention is to be defined by the appended claims and their equivalents.