Abstract:
An image forming apparatus includes a main body, image formation units disposed in the main body, a moving member disposed in the main body, the moving member including a surface facing each image formation unit, a pair of first stoppers disposed in each image formation unit, and a pair of second stoppers disposed in the main body in correspondence with each image formation unit, each second stopper being configured to prevent the developer on the moving member from entering the image formation unit.

Description:
This application claims priority from Japanese Patent Application No. 2005-281146 filed on Sep. 28, 2005, the entire subject matter of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     Aspects of the invention relate to image forming apparatuses configured to form images on a recording medium using a powder-type developer. 
     BACKGROUND 
     A developing unit is contained in image forming apparatuses. The developing unit contains powder-type developer or toner. A developing roller is rotatably supported in a body casing of the developing unit. The developing roller is configured to have a cylindrical shape and hold the developer on a circumferential surface thereof. The developing roller is disposed such that a part of the circumferential surface is exposed external to the developing unit across its full length. 
     The developing unit is configured to develop an electrostatic latent image formed on a circumferential surface of a photosensitive member by facing the circumferential surface of the developing roller and the circumferential surface of the photosensitive member while rotating the developing roller. 
     A developing unit configured to be received in and removed from a main body of the image forming apparatus (or a developer cartridge) is known. As to the developer cartridge, disclosed are various kinds of configurations to prevent leakage of the developer from a gap between each end portion of the developing roller and the opening during the rotation of the developing roller. 
     For example, Japanese Laid-Open Patent Publication No. 2001-5287 discloses a developing unit in which side seals are provided to prevent leakage of the developer. The side seals are disposed to slidingly contact both ends of the circumferential surface of the developing roller, thereby preventing the developer from leaking from the gaps between each end of the developing roller and the opening. 
     Japanese Laid-Open Patent Publication No. 2001-60040 (corresponding to U.S. Pat. Nos. 6356723 B1 and 6496669 B2) discloses a developing cartridge in which safeguard members are provided in addition to the side seals. The safeguard members are disposed on both ends of the opening in which the developing roller is rotatably disposed, with respect to its length. According to this configuration, if a small amount of developer leaks from a sliding portion between each end portion of the developing roller and the side seal, the developer can be received by the safeguard member and thus prevented from leaking outside the developer cartridge. 
     SUMMARY 
     Aspects of the invention provide a color image forming apparatus that can prevent a chain reaction of sealing defects between developing units, an image formation process unit used for an image formation process in the image forming apparatus, and the developing units used for developing process in the image forming apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the invention will be described in detail with reference to various example structures and the following figures, wherein: 
         FIG. 1  is a sectional view showing an internal structure of a color laser printer according to an illustrative aspect of the invention; 
         FIG. 2  is a plan view of a developing unit shown in  FIG. 1 ; 
         FIG. 3  is a sectional view of the developing unit taken along the line A-A of  FIG. 2 ; 
         FIG. 4  is a sectional view of a developing unit according to another aspect; 
         FIG. 5  is a sectional view of a structure of a process cartridge according to another aspect; 
         FIG. 6  is a sectional view of a structure of a drum unit according to another aspect; 
         FIG. 7  is a sectional view of a structure of a process cartridge according to another aspect; and 
         FIG. 8  is a sectional view showing an internal structure of a color laser printer according to another aspect. 
     
    
    
     DETAILED DESCRIPTION 
     A first example according to aspects of the invention will be described with reference to the accompanying drawings. 
     A brief overview of a configuration of a color laser printer  100  will be described with reference to  FIG. 1 . In  FIG. 1 , a right side is referred to as a front side of the color laser printer  100 , and a left side is referred to as a back side of the color laser printer  100 . In addition, a direction from the top to the bottom is referred to as a height direction or vertical direction of the laser printer  100 , a direction from the right to the left is referred to as a front-back direction of the color laser printer  100 , and a direction perpendicular to the sheet of  FIG. 1  is referred to as a width direction of the color laser printer  100 . 
     In the color laser printer  100 , a yellow image formation portion  100 Y, a magenta image formation portion  100 M, a cyan image formation portion  100 C, and a black image formation portion  100 K are accommodated in a main body  110 . The yellow image formation portion  100 Y, the magenta image formation portion  100 M, the cyan image formation portion  100 C, and the black image formation portion  100 K are arranged in this order in the front-back direction from the front side to the back side. 
     The main body  110  is covered with a body casing  111  made of a synthetic resin and having a box shape. An ejection tray  111   b  is formed on a top surface  111   a  of the body casing  111 . The ejection tray  111   b  is configured to receive a sheet P ejected from an ejection port  111   c , which is formed in an upper portion on the back side of the body casing  111 . A front-side opening  111   d  is formed on the front side of the body casing  111 . The front-side opening  111   d  is opened by opening a front cover  111   e  toward the front side so that maintenance of components disposed inside, such as the yellow image formation portion  100 Y, can be performed. The front cover  111   e  is pivotable about its lower end portion between open and closed positions. 
     A sheet supply cassette  120  is detachably attached to a bottom portion of the main body  110 . The sheet supply cassette  120  is configured to support a stack of sheets P (recording media) therein. 
     A sheet pressing plate  122  on which sheets are loaded is disposed inside a cassette case  121  which contains the sheet supply cassette  120 . The sheet pressing plate  122  is pivotable on a pressing plate rear end portion  122   a , which is disposed on the back side, so that a pressing plate front end portion  122   b  vertically swings as a free end. A pressing plate raising lever  123  is disposed under the pressing plate front end portion  122   b  so as to urge it upward. 
     A separation pad  125  is disposed near the front-side end of the cassette case  121  and on a downstream side of the pressing plate front end portion  122   b  in a sheet feeding direction. A separation surface  125   a  is formed on an upper surface of the separation pad  125 . A leading end of a sheet P being fed from the cassette case  121  in the sheet feeding direction contacts the separation surface  125   a . The separation surface  125   a  is made of a material having a higher coefficient of friction than that of paper, such as a rubber. The separation pad  125  is urged upward by a separation pad urging spring  126 . 
     A pinch roller  128  is disposed on an upper end of the cassette case  121  on the front side and on a downstream side of the separation pad  125  in the sheet feeding direction. The pinch roller  128  is rotatably supported by the cassette case  121 . 
     Inside the main body  110  and above the sheet supply cassette  120 , developing units  130  are detachably mounted. The developing units  130  include a yellow developing unit  130 Y, a magenta developing unit  130 M, a cyan developing unit  130 C, and a black developing unit  130 K, which are arranged in this order from the front side of the color laser printer  100  toward the back side. The yellow developing unit  130 Y contains yellow powder toner, the magenta developing unit  130 M contains magenta powder toner, the cyan developing unit  130 C contains cyan powder toner, and the black developing unit  130 K contains black powder toner. 
     Each developing unit  130  includes a developing unit case  131 , a developing roller  132 , a supply roller  133 , an agitator  134 , and a blade  135 . 
     The developing unit case  131  is configured to support the developing roller  132 , the supply roller  133 , the agitator  134 , and the blade  135 , and to hold toner to develop an electrostatic latent image. 
     The developing roller  132  is made of rubber, and is rotatably supported by the developing unit case  131 . The supply roller  133  is made of a sponge material, and is rotatably supported by the developing unit case  131 . The developing roller  132  and the supply roller  133  are disposed in parallel and contact each other. The developing roller  132  and the supply roller  133  are configured to be rotated in a direction of the arrow indicated in  FIG. 1 , to charge toner at their contact portion, and to cause the charged toner to be carried on the circumferential surface of the developing roller  132 . 
     The agitator  134  is configured to agitate toner in the developing unit case  131 , and feed toner toward the supply roller  133 . The agitator  134  is rotatably supported by the developing unit case  131 . 
     The blade  135  is configured to regulate the amount of toner on the circumferential surface of the developing roller  132  by contacting the circumferential surface of the developing roller  132  when it is rotated in the direction of the arrow. 
     Drum units  140  are disposed along the front-back direction so as to face the corresponding developing rollers  132  of each developing unit  130 . 
     Each drum unit  140  includes a photosensitive drum  141 , a scorotron charger  142 , a drum cleaner  143 , and a drum unit frame  144 . 
     The photosensitive drum  141  is configured so that an electrostatic latent image is formed on its circumferential surface. The photosensitive drum  141  is disposed to face the developing roller  132  of the developing unit  130 . The scorotron charger  142  is disposed to face the circumferential surface of the photosensitive drum  141  on an upstream side of a position where the photosensitive drum  141  faces the developing roller  132  in a rotation direction of the photosensitive drum  141  (a direction of the arrow indicated in  FIG. 1 , herein after referred to as a drum rotation direction). The scorotron charger  142  is configured to uniformly charge the circumferential surface of the photosensitive drum  141 . The drum cleaner  143  is disposed to face the circumferential surface of the photosensitive drum  141  on an upstream side of a position where the scorotron charger  142  faces photosensitive drum  141  in the drum rotation direction. The drum cleaner  143  is configured to clean the circumferential surface of the photosensitive drum  141  before being charged by the scorotron charger  142  along the longitudinal direction of the photosensitive drum  141  (or the width direction). 
     A scanner unit  150  is configured to generate and modulate a laser beam (indicated with a broken line in  FIG. 1 ) based on image data at a laser emitting portion (not shown), and emit the laser beam onto the circumferential surface of the photosensitive drum  141  which is charged uniformly by the scorotron charger  142 . That is, the scanner unit  150  is configured to scan the laser beam on the circumferential surface of the photosensitive drum  141  and form an electrostatic latent image on the circumferential surface of the photosensitive drum  141 . 
     Inside the main body  110 , a sheet feeding portion  160  for supplying a sheet P toward the developing units  130  and the drum units  140  is provided. The sheet feeding portion  160  is made up of a pickup roller  161 , a separation roller  162 , a sheet dust removing roller  163 , a feed roller  164 , and a sheet guide  165 . 
     The pickup roller  161  is rotatably supported in the main body  1   10 . The pickup roller  161  is configured to rotate in a direction of an arrow indicated in  FIG. 1  via a power transmission mechanism provided in the main body  110 . The pickup roller  161  is disposed to contact a sheet P, which is urged upward by the pressing plate front end  122   b  of the sheet pressing plate  122  and the pressing plate lifting lever  123 , during image formation. 
     The separation roller  162  is rotatably supported in the main body  110 . The separation roller  162  is configured to rotate in a direction of an arrow indicated in  FIG. 1  via the power transmission mechanism provided in the main body  110 . The separation roller  162  is disposed in contact with and applying pressure against the separation pad  125 . 
     The sheet dust removing roller  163  is rotatably supported in the main body  110 . 
     The sheet dust removing roller  163  is disposed ahead of the separation pad  125  in the sheet feeding direction so as to contact the pinch roller  128 . The sheet dust removing roller  163  is configured to remove foreign matter such as paper dust and dirt from a sheet P by sandwiching the sheet P being fed with the pinch roller  128 . 
     The feed roller  164  and the sheet guide  165  are disposed between the sheet dust removing roller  163  and the yellow image formation portion  100 Y. The feed roller  164  and the sheet guide  165  are configured to guide a sheet P past the sheet dust removing roller  163  toward between the yellow image formation portion  100 Y and a transfer portion  170 . 
     The transfer portion  170  is disposed inside the main body  110  and under the image formation portions  100 Y,  100 M,  100 C,  100 K. The transfer portion  170  is made up of a sheet conveyor belt  171 , transfer rollers  172 , a belt drive roller  173 , a belt support roller  174 , and a belt cleaner  175 . 
     The sheet conveyor belt  171  is formed as a circular belt made of a conductive plastic such as polycarbonate and polyimide, in which conductive particles, for example, carbon particles, are dispersed. The sheet conveyor belt  171  is stretched so that a conveyance-side surface  171   a , which is an outer surface of the conveyor belt  171 , faces the photosensitive drums  141 . 
     The transfer rollers  172  are rotatably supported to face the respective photosensitive drums  141  via the sheet conveyor belt  171 . The transfer rollers  172  are electrically connected to an output terminal of high voltage. A bias voltage is applied between each transfer roller  172  and the corresponding photosensitive drum  141  to transfer toner on the circumferential surface of the photosensitive drum  141  onto a sheet P being fed on the sheet conveyor belt  171 . 
     The belt drive roller  173  is configured to rotate in a direction of an arrow in  FIG. 1  via the power transmission mechanism provided in the main body  110 . The belt drive roller  173  is disposed behind the photosensitive drum  141  facing the black developing unit  130 K, which is disposed backmost in the developing units  130 . 
     The belt support roller  174  is disposed before the photosensitive drum  141  facing the yellow developing unit  130 Y, which is disposed foremost in the developing units  130 . The sheet conveyor belt  171  is stretched between the belt drive roller  173  and the belt support roller  174  by a tension. When the belt drive roller  173  rotates in the direction of the arrow, the sheet conveyor belt  171  is moved around between the belt drive roller  173  and the belt support roller  174 , and the belt support roller  174  is rotated. 
     The sheet conveyor belt  171  is supported by the belt drive roller  173  and the belt support roller  174  so that the conveyance-side surface  171   a  is movable below the image formation units  100 Y,  100 M,  100 C, and  100 K along their arrangement. 
     The belt cleaner  175  is disposed under the sheet conveyor belt  171  that is stretched below the transfer rollers  172 . The belt cleaner  175  is configured to clean the conveyance-side surface  171   a  of the sheet conveyor belt  171  facing the image formation units  100 Y,  100 M,  100 C, and  100 K in the width direction. 
     A fixing portion  180  is disposed inside the main body  110  and downstream of the transfer portion  170  in the sheet feeding direction. The fixing portion  180  is configured to fix the toner image formed onto the sheet P. The fixing portion  180  includes a heat roller  181  and a pressure roller  182 . 
     The heat roller  181  includes a roller body made of a metal thin tube whose surface is mold free, and a halogen lamp set inside the roller body. The heat roller  181  is configured to rotate in a direction of an arrow in  FIG. 1  via the power transmission mechanism provided in the main body  110 . The pressure roller  182  can be a silicone rubber roller disposed to press against the heat roller  181  by a pressure. The pressure roller  182  is configured to sandwich a sheet P with the heat roller  181 , rotate along with the rotation of the heat roller  181 , fix the toner image onto the sheet P, and feed the sheet P toward the ejection port  111   c.    
     An ejection portion  190  is disposed backmost inside the main body  110  and above the fixing portion  180 . The ejection portion  190  is configured to eject the sheet P passing the fixing portion  180  outside the main body  110 . 
     The ejection portion  190  includes a feed roller  191 , a pinch roller  192 , sheet guides  193   a ,  193   b , an ejection roller  194 , and a driven roller  195 . 
     The feed roller  191  and the pinch roller  192  are disposed downstream of the heat roller  181  and the pressure roller  182  in the sheet feeding direction. The feed roller  191  is supported to be rotated in a direction of an arrow indicated with a broken line in  FIG. 1 . The pinch roller  192  is disposed facing the feed roller  191 . The pinch roller  192  is rotatably supported so as to follow the rotation of the feed roller  191 . The feed roller  191  and the pinch roller  192  are configured to feed the sheet P on which the toner image has been fixed toward the ejection portion  111   c  along the rotation of the feed roller  191  in the direction of the arrow indicated with the broken line in  FIG. 1 . 
     The sheet guides  193   a  and  193   b  are formed on the downstream side of the feed roller  191  and the pinch roller  192  in the sheet feeding direction. The sheet guides  193   a  and  193   b  are configured to guide the sheet P being fed by the feed roller  191  and the pinch roller  192 , toward a contact portion between the ejection roller  194  and the driven roller  195 . 
     The ejection roller  194  and the driven roller  195  are disposed near the ejection port  111   c  to face the ejection port  111   c . The ejection roller  194  is supported to be rotated in a direction of an arrow shown in  FIG. 1 . The driven roller  195  is disposed facing the ejection roller  194 . The driven roller  195  rotatably supported so as to follow the rotation of the ejection roller  194 . The ejection roller  194  and the driven roller  195  are configured to eject the sheet P on which the toner image has been fixed, from the ejection port  111   c  outside the main body  110 , along with the rotation of the ejection roller  194  in the direction of the arrow indicated in  FIG. 1 . 
       FIG. 2  is a plan view of one of the developing units  130  shown in  FIG. 1 .  FIG. 3  is a sectional view of the developing unit  130  taken along the line A-A of  FIG. 2 . In  FIG. 3 , it is assumed that the developing unit  130  is mounted in the main body  110  of the color laser unit  100  as shown in  FIG. 1 . 
     Referring to  FIGS. 2 and 3 , the developing unit  130  includes a toner containing case  131 A, and a roller support portion  131 B. The toner containing case  131 A constitutes a toner chamber in which toner is contained. Inside the toner containing case  131 A, the agitator  134  is rotatably accommodated. The roller support portion  131 B is configured to support the developing roller  132  and the supply roller  133  rotatably. The roller support portion  131 B is formed with a developing roller exposure opening  131 C. The developing roller  132  is rotatably supported at the developing roller exposure opening  131 C and disposed so that more than half of the circumferential surface of the developing roller  132  is exposed outside of the developing unit case  131 , along the longitudinal direction or the width direction, that is, the left-right direction of  FIG. 2 . 
     As shown in  FIG. 3 , a side seal member  131 D 1  and a felt member  131 D 2  are disposed in a seal portion  131 D. The seal portion  131 D is a gap formed between each end portion of the developing roller exposure opening  131 C and each end portion of developing roller  132  in the width direction. The side seal member  131 D 1  may be made of a urethane sponge having relatively high rigidity. The side seal member  131 D 1  is bonded on a surface of the developing unit case  131  (or the roller support portion  131 B) facing the seal portion  131 D. The felt member  131 D 2  can be formed of a napped felt in which a fluorine-based synthetic resin is impregnated. The felt member  131 D 2  is bonded over the entire side seal member  131 D 1 . The developing roller  132  is disposed to press against the side seal member  131 D 1  from above the felt member  131 D 2  at each end of the developing roller  132  in the width direction. Referring to a double dotted line in  FIG. 3 , the side seal member  131 D 1 , the felt member  131 D 2 , and the developing roller  132  are in pressing contact with each other. 
     As shown in  FIG. 2 , safeguard members  131 E are disposed on both ends of the developing roller exposure opening  131 C in the width direction. As shown in  FIG. 3 , the safeguard member  131 E is fixed to a lower end of the roller support portion  131 B to form a lower end of the developing unit  130  when the developing unit  130  is mounted in the main body  110 . The safeguard member  131 E is configured to prevent toner from leaking outside the developing unit  130  when toner leaks from the seal portion  131 D during attachment to and detachment from the main body  110 . 
     Specifically, a base end portion  131 E 1  of the safeguard member  131 E is fixed to the lower end of the roller support portion  131 B so as to pinch an end of the felt member  131 D 2  against the roller support portion  131 B as shown in  FIG. 3 . An end portion  131 E 2  of the safeguard member  131 E is disposed to protrude from the base end portion  131 E 1  toward an upstream side in the rotation direction of the developing roller  132  as indicated with an arrow in  FIG. 3 . A hollow portion  131 E 3  provided between the base end portion  131 E 1  and the end portion  131 E 2  is configured to collect toner which leaks from the seal portion  131 D. 
     As shown in  FIG. 2 , toner intrusion stoppers  13 IF are disposed to correspond with both ends of the developing roller  132  in the width direction. As shown in  FIG. 3 , a toner intrusion stopper  131 F is fixed to the end portion  131 E 2  of the safeguard member  131 E. 
     As shown in  FIG. 3 , the toner intrusion stopper  131 F is disposed to contact the circumferential surface of each end of the developing roller  132  by a pressure on an upstream side, in the rotation direction of the developing roller  132 , at a position where the side seal member  131 D 1  and the felt member  131 D 2  are in pressing contact with the developing roller  132 . A space corresponding to the hollow portion  131 E 3  is provided between an upstream-side end portion  131 D 3  of the seal portion  131 D in the rotation direction of the developing roller  132  and a downstream-side end portion  13 F 1  of the toner intrusion stopper  131  in the rotation direction of the developing roller  132 . The toner intrusion stopper  131 F can be formed of a urethane sponge. 
     Referring to  FIGS. 2 and 3 , the seal portion  131 D (the side seal member  131 D 1  and the felt member  131 D 2 ) and the toner intrusion stopper  131 F are disposed to overlap each other in the width direction. 
     Referring to  FIG. 1 , an image formation operation by the color laser printer  100  will be described. 
     When the pickup roller  161  is rotated in the direction of the arrow, a few of sheets P loaded in the cassette case  121  are fed toward the separation roller  162 . The leading ends of the sheets P are fed in between the separation roller  162  and the separation pad  125 . With the rotation of the separation roller  162  in the direction of the arrow, only an uppermost sheet P is fed toward the sheet dust removing roller  163 . The sheet P from which dust has been removed by the sheet dust removing roller  163  is fed toward the transfer portion  170  via the feed roller  164  and the sheet guide  165 . 
     When the agitator  134  is rotated, toner in the developing unit case  131  is agitated, and the toner is fed toward the supply roller  133 . The toner fed to the supply roller  133  is fed to the developing roller  132  with the rotation of the supply roller  133  in the direction of the arrow. The toner is frictionally charged at a position where the developing roller  132  and the supply roller  133  are in contact with each other, and adheres to the circumferential surface of the developing roller  132 . The toner adhering to the circumferential surface of the developing roller  132  is regulated to a specified density and charging amount by the blade  135 , and then supplied to a position where the developing roller  132  faces the photosensitive drum  141 . 
     The circumferential surface of the photosensitive drum  141  is charged uniformly by the scorotron charger  142 , and irradiated with laser light modulated according to image information. Thus, an electrostatic latent image based on the image information is formed on the circumferential surface of the photosensitive drum  141 . The circumferential surface of the photosensitive drum  141 , on which the latent image is formed, faces the circumferential surface of the developing roller  132 , and the toner adheres to the circumferential surface of the photosensitive drum  141 . Thus, the latent image on the circumferential surface of the photosensitive drum  141  is developed with the toner. 
     The sheet P fed to the transfer portion  170  is carried on the sheet conveyor belt  171 , and fed from the front to the back (from the right to the left in  FIG. 1 ). When the sheet P is fed between the photosensitive drum  141  and the transfer roller  172  (hereinafter referred to as a transfer area), a bias voltage is applied to the transfer area, and the toner on the circumferential surface of the photosensitive drum  141  is transferred to the sheet P. 
     The sheet P to which the toner has adhered in the transfer portion  170  is fed to the fixing portion  180 . The sheet P is pinched and heated between the heat roller  181  and the pressure roller  182 , and the toner on the sheet P is melted and fixed onto the sheet P. Then, the sheet P is ejected to the ejection tray  111   b  outside the main body  110  by the ejection roller  194 . 
     With reference to  FIGS. 1 and 3 , the following description will be made as to how to prevent toner that leaks (e.g., accidentally) from an end of one developing unit  130 , in the width direction, from intruding into the remaining developing units  130 . 
     In the following description, it is assumed that toner leaks from the seal portion  131 D of the developing unit case  131  of the yellow developing unit  130 Y. 
     In this case, the leaked toner adheres to an end of the circumferential surface of the developing roller  132  in the width direction. The leaked toner adhering to the end of the circumferential surface of the developing roller  132  adheres to an end of the circumferential surface of the photosensitive drum  141  in the width direction. The leaked toner adhering to the end of the circumferential surface of the photosensitive drum  141  is fed to the transfer area with the rotation of the photosensitive drum  141  in the direction of the arrow, and adheres to an end of the sheet conveyor belt  171  or the conveyance-side surface  171   a , in the width direction, in the transfer area. 
     Alternatively, the leaked toner may drop to the end of the conveyance-side surface  171   a  of the sheet conveyor belt  171  disposed under the sealed portion  131 D by gravitation and deposit in the end. 
     The leaked toner that is deposited in the end of the conveyance-side surface  171   a  of the sheet conveyor belt  171  is fed to the transfer area of the magenta image formation portion  100 M as the conveyance-side surface  171   a  is moved in the direction of the arrow. In the transfer area, the end of the conveyance-side surface  171   a  where the leaked toner is deposited faces the photosensitive drum  141 , which results in the toner adhering to the end of the photosensitive drum  141 . 
     When the toner adheres to the end of the photosensitive drum  141  in the drum unit  140  that faces the magenta developing unit  130 M, the toner is fed to the magenta developing unit  130 M along with the rotation of the photosensitive drum  141  in the direction of the arrow. 
     The toner can be partially removed by the drum cleaner  143  provided in the drum unit  140 . As is well known, the seal portion  131 D is formed outside an image formation area of the photosensitive drum  141  in the width direction, and an area where the leaked toner adheres is determined outside the image formation area. 
     The drum cleaner  143  is designed on the assumption that it removes a small amount of toner or dust that is not transferred to the sheet P in the transfer area and remains in the image formation area. It is not assumed that the drum cleaner  143  completely removes all of a relatively large amount of toner adhering to the photosensitive drum  141  (especially outside the image formation area) such as the toner leaked as described above. 
     Thus, it can be difficult for the drum cleaner  143  to remove the leaked toner adhering to the end of the photosensitive drum  141  from the conveyance-side surface  171   a  of the conveyor belt  171  in the transfer area (that is, the toner inversely transferred from the conveyance-side surface  171   a  to the end of the photosensitive drum  141 ). As a result, a part of the leaked toner passes through the drum cleaner  143  and is fed to a position where the photosensitive drum  141  faces the end of the developing roller  143  in the magenta developing unit  130 M. In this position, the leaked toner adheres to the end of the developing roller  132  in the magenta developing unit  130 M. 
     In a configuration that does not include the toner intrusion stoppers  131 F, the leaked toner enters the sealed portion  131 D of the magenta developing unit  130 M. 
     That is, the toner goes in between the end of the circumferential surface of the developing roller  132  and the felt member  131 D 2  in the magenta developing unit  130 M, which causes a poor sealed state in the seal portion  131 D of the magenta developing unit  130 M. As a result, magenta toner leaks from the seal portion  131 D. 
     However, according to the above configuration, the leaked toner adhering to the end of the developing roller  132  of the magenta developing unit  130 M is removed by the toner intrusion stopper  131 F. Thus, the leaked toner can be prevented from intruding the seal portion  131 D of the magenta developing unit  130 M. 
     According to the above configuration, if a toner leakage happens at an end of one developing unit  130  in the width direction, the toner leakage can be prevented from bringing about a chain reaction of toner leakage at the neighboring developing unit  130 . A chain reaction of sealing trouble can be effectively contained. 
     While the invention has been described with reference to exemplary aspects, it is to be understood that the invention is not restricted to the particular forms shown in the foregoing exemplary aspects. Various modifications and alterations can be made thereto without departing from the scope of the invention. 
     While aspects of the invention are described in a color laser printer, it will be appreciated that these aspects may be applied to other image forming apparatus including, but not limited to, multi-function devices, scanners, facsimiles, copiers and the like. 
     The configuration of the toner intrusion stopper, which is configured to prevent toner from intruding in the seal portion  131 D of the developing unit case  131  from outside, is not limited to that shown in  FIG. 3 . As shown in  FIG. 4 , for example, a toner intrusion stopper  131 FF may be made of a plate member configured to scrape the leaked toner adhering to the end of the circumferential surface of the developing roller  132 . The toner intrusion stopper  131 FF is fixed to the end portion  131 E 2  of the safeguard member  131 E. When the edge of the toner intrusion stopper  131 FF contacts the end of the circumferential surface of the developing roller  132 , the leaked toner adhering thereto is scraped. 
     In this case, a collecting member  131   g  may be provided to collect the leaked toner scraped by the toner intrusion stopper  131 FF. The toner intrusion stopper  131 FF is fixed to the safeguard member  131 E. The collecting member  131 g is formed with a recessed portion  131   g   1 . The toner intrusion stopper  131 FF is disposed so that the recessed portion  131   g   1  is open toward a contact portion between the developing roller  132  and the toner intrusion stopper  131 FF. 
     According to the above configuration, if a large amount of toner leaks, a chain reaction of sealing trouble can be effectively prevented. In addition, the toner intrusion stopper  131 FF and the collecting member  131   g  are provided in the developing unit  130  that is replaceable and can be attached to and removed from the main body  110  of the color laser printer  100 . Thus, the leaked toner that is collected can be removed from the inside of the main body  110  at a specified time. 
     Each of the yellow image formation portion  100 Y, the magenta image formation portion  100 M, the cyan image formation portion  100 C, and the black image formation portion  100 K that are shown in  FIG. 1  may be configured as a process cartridge  200  that is attachable to and detachable from the color laser printer  1  as shown in  FIG. 5 . Namely, the process cartridge  200  including the developing unit  130  and the drum unit  140  may be configured to be attached to and removed from the color laser printer  1 . 
     In the process cartridge shown in  FIG. 5 , the developing unit  130  and the drum unit  140  may be separated from each other. 
     As shown in  FIG. 6 , the drum unit  140  may include a drum end cleaner  145 . The drum end cleaner  145  may be provided independently from a mechanism (the drum cleaner  143  in the above description) for removing toner adhering to the image formation area on the photosensitive drum  141  after toner has been transferred. 
     The drum end cleaner  145  is disposed in contact with the end of the circumferential surface of the photosensitive drum  141  on a downstream side of the transfer position in the rotation direction of the photosensitive drum  141 . More specifically, the drum end cleaner  145  is supported by the drum unit frame  144  via a support member  146  that is made of a flexible plate member. The drum end cleaner  145  is disposed in contact with the end of the circumferential surface of the photosensitive drum  141  on an upstream side of a position where the photosensitive drum  141  faces the scorotron charger  142 , in the rotation direction of the photosensitive drum  141 . 
     The drum end cleaner  145  may be disposed in contact with the end of the circumferential surface of the photosensitive drum  141  on the upstream side of a position where the drum cleaner  143  faces the photosensitive drum  141  in the rotation direction of the photosensitive drum  141 , as shown in  FIG. 6 . Alternatively, the drum end cleaner  145  may be disposed in contact with the end of the circumferential surface of the photosensitive drum  141  on a downstream side of the position where the drum cleaner  143  faces the photosensitive drum  141  in the rotation direction of the photosensitive drum  141 . 
     The above modification may be applied to a configuration of a cleaner-less type, which omits the drum cleaner  143 . 
     As shown in  FIG. 7 , the toner intrusion stopper  131 F may be used in conjunction with the drum end cleaner  145 . As described above, the developing unit  130  including the toner intrusion stopper  131 F and the drum unit  140  including the drum end cleaner  145  may be configured as the process cartridge  200  ( FIG. 5 ) that is attachable to and detachable from the main body  110  ( FIG. 1 ) of the color laser printer  1 . 
     As shown in  FIG. 8 , the transfer portion  170  may include belt end cleaners  176 . 
     The belt end cleaners  176  are disposed between the adjacent image formation portions so as to face each end of the sheet conveyor belt  171  in the width direction. The belt end cleaners  176  are configured to remove the leaked toner adhering to the conveyance-side surface  171   a  of the conveyor belt  171 . 
     According to the above configuration, the leaked toner can be removed from the conveyance-side surface  171   a  of the conveyor belt  171  before the leaked toner intrudes in the neighboring image formation portion disposed on a downstream side in the rotation direction of the sheet conveyor belt  171 . Thus, a chain reaction of sealing trouble can be effectively prevented.