Patent Publication Number: US-7907865-B2

Title: Image forming device capable of preventing wear to electrodes

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2005-375596 filed Dec. 27, 2005. The entire content of this priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The disclosure relates to an image bearing unit including a plurality of image forming units provided with respective image bearing members onto which electrostatic latent images are formed. The invention also relates to an image forming device including a unit accommodating section for detachably accommodating the image bearing unit. 
     BACKGROUND 
     Conventional color laser printers generally include a drum unit that is detachably mounted in a main casing of the printer. The drum unit has a plurality of image forming units provided with respective photosensitive drums on which electrostatic latent images are formed. 
     A plurality of receiving electrodes for receiving electric power for each of the image forming units is arranged, on a wall of the drum unit, in a direction parallel to a drum-mounting direction in which the drum unit is inserted into the main casing. Also, a plurality of supplying electrodes for supplying electric power to the respective receiving electrodes is arranged on the main casing at appropriate positions such that the supplying electrodes are brought into contact with the corresponding receiving electrodes when the drum unit is mounted in the main casing (see U.S. Pat. No. 6,944,415, for example). 
     In this type of color laser printer, the receiving electrodes gride or slide over the corresponding supplying electrodes each time the drum unit is inserted into or removed from the main casing. As a result, these electrodes are worn with time. 
     Particularly, the receiving electrode disposed on the downstream side in the drum-mounting direction and the supplying electrode disposed on the upstream side in the drum-mounting direction are also grided by electrodes other than those corresponding thereto, during the inserting or detaching operation of the drum unit. Thus, those electrodes are worn to a large extent to give rise to a problem of contact failures, and enough electric power may not be supplied to the image forming units. 
     SUMMARY 
     In view of the foregoing, it is an object of the invention to provide an image bearing unit and an image forming device including the image bearing unit capable of preventing electrodes for supplying electric power to image forming units from being worn. 
     In order to attain the above and other objects, the invention provides an image forming device including: an image-bearing unit including a plurality of image forming units each having an image bearing member on which an electrostatic latent image is formed; and a unit accommodating section. The image-bearing unit is detachably accommodated in the unit accommodating section by being inserted in a first direction. The image-bearing unit includes a first support member extending in a second direction orthogonal to the first direction and a first receiving electrode disposed on the first support member. The first receiving electrode receives electric power to be supplied to the image forming units. The unit accommodating section includes a first supplying electrode that contacts the first receiving electrode and supplies the electric power to the first receiving electrode. 
     The invention further provides an image-bearing unit that is detachably mountable on a section of an image forming device by being inserted in an inserting direction. The image-bearing unit including: a plurality of image forming units each including an image-bearing member on which an electrostatic latent image is formed; a first support member extending in a direction perpendicular to the inserting direction; and a first receiving electrode that is disposed on the first support member. The first receiving electrode is configured to receive electric power to be supplied to the image forming units. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects in accordance with the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a perspective view showing the exterior of a printer according to some aspects of the invention; 
         FIG. 2  is a perspective view of the printer in  FIG. 1  with an image forming unit being taken out from a main section of the printer; 
         FIG. 3  is a perspective view of a drum unit of the image forming unit with one of developing cartridges being detached from the drum unit; 
         FIG. 4  is a perspective view of the drum unit in  FIG. 3  with all the developing cartridges being detached therefrom; 
         FIG. 5  is a cross-sectional view of one of drum sections of the drum unit as viewed from the bottom; 
         FIG. 6  is an explanatory view showing a wiring system among LED substrates of the drum unit; 
         FIG. 7  is a perspective view of a front beam of the drum unit; 
         FIG. 8  is a perspective view of a front cover of the main casing; 
         FIG. 9  is a side cross-sectional view of the front cover, showing how the shutter member is displaced; 
         FIG. 10  is an enlarged view of the part encircled in  FIG. 9 ; 
         FIG. 11  is a side cross-sectional view of the printer with the image forming unit mounted therein; and 
         FIG. 12  is a detailed cross-sectional view of one of the drum sections. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming device according to some aspects of the invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description. 
       FIG. 1  is a perspective view of a printer  100 , to which the invention is applied, according to some aspects of the invention. Note that in the following description, the expressions “front”, “rear”, “left”, “right”, “above”, and “below” are used to define the various parts when the printer  100  is disposed in an orientation in which it is intended to be used. 
     As shown in  FIG. 1 , the printer  100  includes a main body  110  having a main casing  111  and a main frame  112  housed inside the main casing  111 . 
     The main casing  111  is formed of synthetic resin plates into a substantially box-shape. A sheet discharge tray  111   b  is formed in a top surface  111   a  of the main casing  111  so as to downwardly incline from the front side to the rear side of the main casing  111 . A sheet discharge opening  111   c  is formed in the upper section of the main casing  111  at a position above the lower rear end of the sheet discharge tray  111   b , for discharging sheets of paper onto the sheet discharge tray  111   b.    
     A front-side opening  111   d  is formed in the front surface of the main casing  111 . A front cover  111   e  is disposed at the front side of the main casing  111  for closing and exposing the front-side opening  111   d . The front cover  111   e  is supported at the lower end thereof by the main casing  111 . 
     The main frame  112  is for supporting various components that are accommodated in the main body  110  and that are used for image forming operations. The main frame  112  is also provided with a drive source and a drive power transmission mechanism for driving the various components to rotate. The main frame  112  is provided with a pair of upper guide rails  112   a  and a pair of lower guide rails  112   b . The upper guide rails  112   a  are formed to inwardly protrude from left and right inner surfaces of the main frame  112  to confront each other in a widthwise direction (left-to-right direction) of the printer  100 . Each of the upper guide rails  112   a  extends substantially horizontally in a front-to-rear direction S of the printer  100 . Similarly, the lower guide rails  112   b  are formed to inwardly protrude from the left and right inner surfaces of the main frame  112  to confront each other in the widthwise direction of the printer  100 . The pair of lower guide rails  112   b  is disposed substantially parallel to the pair of upper guide rails  112   a . A pair of guide grooves  112   c  is defined between the upper guide rails  112   a  and the lower guide rails  112   b.    
     An image forming unit  120  is disposed inside the main frame  112  such that the image forming unit  120  can be pulled out of the main frame  112  through the front side thereof, as shown in  FIG. 2 . More specifically, a user can remove the image forming unit  120  from the main body  110  by first opening the front cover  111   e , grabbing a front handle  131   a  provided to the front end of the image forming unit  120 , and pulling the image forming unit  120  toward the front. 
     As shown in  FIG. 2 , the image forming unit  120  includes a drum unit  130  and four developing cartridges  140 . The drum unit  130  includes a frame configured of a front beam  131 , a pair of support plates  132  and  133 , a rear beam  134 , and four pairs of side plates  135  and  136 , into a rectangular shape in a plan view. 
     The front beam  131  and the rear beam  134  are disposed to the front side and the rear side, respectively. The support plates  132  and  133  are linked to the widthwise ends of the front beam  131  and those of the rear beam  134 . The pairs of side plates  135  and  136  are supported by the support plates  132  and  133  so as to be located therebetween and extend therealong. The pairs of side plates  135  and  136  are arranged in the front-to-rear direction S. The developing cartridges  140  are arranged in a row in the front-to-rear direction S, such that each developing cartridge  140  is located between a corresponding one of pairs of side plates  135  and  136 . Each of the developing cartridges  140  is detachably supported by the drum unit  130  by means of the side plates  135  and  136 . 
     Flange sections  132   a  and  133   a  are formed at upper edges of the support plates  132  and  133 , respectively. The flange sections  132   a  and  133   a  are capable of engaging the guide grooves  112   c  of the main frame  112 . Thus, the drum unit  130  is guided by the flange sections  132   a  and  133   a  and the guide grooves  112   c  when being inserted into the main frame  112 . 
     An inverted U-shaped rear handle  134   a  is formed on an upper end of the rear beam  134 . Thus, the user can easily carry the image forming unit  120  (the drum unit  130 ) by holding the rear handle  134   a  and the front handle  131   a  that is formed on the front surface of the front beam  131 . 
       FIG. 3  is a perspective view of the drum unit  130  with one of the developing cartridges  140  detached therefrom.  FIG. 4  is a perspective view of the drum unit  130  with all the developing cartridges  140  are detached therefrom. As shown in  FIGS. 3 and 4 , guide grooves  135   a  and  136   a  (only the guide grooves  135   a  are shown in  FIG. 3 ) are formed in the inner surfaces of the side plates  135  and  136 , respectively, for guiding the developing cartridges  140  in the vertical direction A. 
     As shown in  FIG. 3 , each of the developing cartridges  140  is provided with a pair of collar members  147  outwardly protruding one from either ends of the developing cartridge  140  in the widthwise direction. The color members  147  are made of an electrically conductive material and are configured to engage the corresponding guide grooves  135   a  and  136   a  of the side plates  135  and  136 . A pair of leg sections  141   a   2  is formed one at either widthwise end of the lower surface of each of the developing cartridges  140 , enabling the user to stably place the developing cartridge  140  on a work bench or a table. 
     The support plate  133  is formed with four electrode through holes  133   b  arranged in the front-to-rear direction S so as to oppose the corresponding developing cartridges  140 . An electrode support section  136   b  is formed on each of the side plates  136  and supports a corresponding one of receiving electrodes  121 . The receiving electrodes  121  are for receiving power to be applied to the corresponding developing cartridges  140 . Both the electrode support sections  136   b  and the receiving electrodes  121  supported thereon project from the corresponding electrode through holes  133   b . When the image forming unit  120  is mounted in the main frame  112 , the receiving electrodes  121  respectively abut corresponding supplying electrodes  112   e  arranged in the main frame  112  and receive electric power from the supplying electrodes  112   e.    
     As will be described later, each developing cartridge  140  includes a developing roller  144  ( FIG. 11 ) having a metal rotary shaft  144   a , which penetrates through the corresponding color member  147 . When the developing cartridge  140  is fully inserted into the drum unit  130 , one of the collar members  147  of the developing cartridge  140  contacts the corresponding receiving electrode  121 , and also the rotary shaft  144   a  is electrically connected to the corresponding receiving electrode  121 . 
     As shown in  FIG. 4 , the support plate  132  is formed with four coupling through holes  132   b  arranged in the front-to-rear direction S so as to oppose the corresponding developing cartridges  140 . Each of the side plates  135  is formed with a coupling exposing hole  135   b  at a position opposing the corresponding coupling through hole  132   b . Each of the coupling through holes  132   b  and the corresponding coupling exposing hole  135   b  are provided for receiving a corresponding input shaft (not shown) of the main frame  112  that is engaged with a gear coupling (not shown) of each developing cartridge  140  for applying power to the gear coupling. 
     Four drum sections  150  are arranged in the front-to-rear direction S at the bottom section of the drum unit  130  so as to oppose the corresponding developing cartridges  140 . 
       FIG. 5  is a cross sectional view of one of the drum sections  150  as viewed from the bottom. As shown in  FIG. 5 , each of the drum sections  150  includes a photosensitive drum  151  and an LED substrate  153  disposed in opposition to the photosensitive drum  151 . A plurality of LEDs  153   a  is disposed on the LED substrate  153 . The LEDs  153   a  are arranged in a row in the widthwise direction and confront the photosensitive drum  151 . The LEDs  153   a  are electrically connected in parallel on the LED substrate  153 . 
       FIG. 6  is an explanatory view showing the wiring system among the LED substrates  153  of the drum unit  130 . As shown in  FIG. 6 , the LED substrates  153  are linked to each other by means of a harness  154 . More specifically, the harness  154  connects a connection substrate  155 , which is connected to a pair of receiving electrodes  131   b  and  131   c  (see  FIG. 7 ), and the LED substrate  153  disposed nearest the front, thereby connecting the receiving electrodes  131   b  and  131   c  to the anode side and the cathode side of the LEDs  153   a , respectively. The harness  154  also connects each of the LED substrates  153  to the adjacent one of the LED substrates  153  in the front-to-rear direction, so that groups of the LEDs  153   a  on the LED substrates  153  are connected to each other in parallel. Note that the harness  154  that is connected to the LED substrate  153  disposed nearest the rear has an open end. 
       FIG. 7  is a perspective view of the front beam  131  of the drum unit  130  as viewed from the front right side thereof. Note that the front handle  131   a  is omitted in  FIG. 7 . As shown in  FIG. 7 , the pair of receiving electrodes  131   b  and  131   c  mentioned above is provided at the left section of the front surface of the front beam  131  so as to project forward. 
     A pair of guide members  131   d  and  131   e  project forward from the front surface of the front beam  131  so as to interpose the pair of receiving electrodes  131   b  and  131   c  between the guide members  131   d  and  131   e.    
       FIG. 8  is a perspective view of the front cover  111   e  in the closed state, as viewed from inside the main body  110 . As shown in  FIG. 8 , a shutter member  113  is supported by the front cover  111   e  at a position opposing the receiving electrodes  131   b  and  131   c  of the front beam  131  when the image forming unit  120  is mounted in the main frame  112  and the front cover  111   e  is in the closed state. The shutter member  113  can swing upward. A pair of pressed members  113   a  is integrally formed to the shutter member  113 , one at either widthwise end thereof. The guide members  131   d  and  131   e  of the front beam  131  ( FIG. 7 ) abut and press the respective pressed members  113   a  when the front cover  111   e  is closed. 
       FIG. 9  is a cross-sectional view of the front cover  111   e , showing how the shutter member  113  is displaced.  FIG. 10  is an enlarged view of a part of the front cover  111   e  encircled in  FIG. 9 . 
     As shown in  FIG. 9 , a pair of supplying electrodes  114   a  and  114   b  is provided on the front cover  111   e . The supplying electrode  114   a  is connected to the anode of a power source (not shown) for supplying electric power to the LED substrates  153 , while the supplying electrode  114   b  is connected to the grounding of the power source. The supplying electrodes  114   a  and  114   b  are covered with the shutter member  113  when the front cover  111   e  is in the open state. This prevents the supplying electrodes  114   a  and  114   b  from being contacted by the user or other components of the printer  100 . However, the supplying electrodes  114   a  and  114   b  are exposed when the shutter member  113  swings upward. 
     Specifically, when the front-side opening  111   d  of the main casing  111  is closed by the front cover  111   e  with the image forming unit  120  being mounted in the main frame  112 , the guide members  131   d  and  131   e  of the front beam  131  respectively abut the pressed members  113   a  and  113   a  of the shutter member  113  to upwardly displace the shutter member  113 . As a result, as shown in  FIGS. 9 and 10 , the supplying electrodes  114   a  and  114   b  that have been covered with the shutter member  113  become exposed, and respectively contact the receiving electrodes  131   b  and  131   c  of the front beam  131 . 
     Although not shown in the drawings, a spring is attached to the shutter member  113 . When the user opens the front cover  111   e , the shutter member  113  automatically swings downward and covers the supplying electrodes  114   a  and  114   b  due to the urging force of the spring. Therefore, the supplying electrodes  114   a  and  114   b  are reliably prevented from being contacted by the user or other components of the printer  100  when the front cover  111   e  is opened. 
       FIG. 11  is a side cross-sectional view of the printer  100  with the image forming unit  120  mounted therein. As shown in  FIG. 11 , the four developing cartridges  140  ( 140 K,  140 C,  140 M,  140 Y) are arranged in a row in the front-to-rear direction within the main body  110  of the printer  100 . The four drum sections  150  are arranged in confrontation with the corresponding developing cartridges  140 . A scanner unit  160  and a transfer unit  170  are disposed respectively above and below both the developing cartridges  140  and the drum sections  150 . A feeder unit  180  is disposed below the transfer unit  170 . 
     The developing cartridges  140 K,  140 C,  140 M, and  140 Y respectively contain toners of different colors of black, cyan, magenta, and yellow. Note that the developing cartridges  140 K,  140 C,  140 M, and  140 Y are identical with each other except that they contain toners of different colors. 
     Each of the developing cartridges  140  includes a cartridge case  141  that accommodates toner. The developing cartridge  140  further includes an agitator  142 , a supply roller  143 , the developing roller  144 , and a blade  145 , all supported within the cartridge case  141 . 
     The agitator  142  is for agitating the toner accommodated in the cartridge case  141 , and is rotatably supported by the cartridge case  141 . The supply roller  143  is a sponge roller rotatably supported by the cartridge case  141 . The developing roller  144  is a rubber roller rotatably supported by the cartridge case  141 . The supply roller  143  and the developing roller  144  are disposed such that outer peripheral surfaces thereof contact with each other. As the supply roller  143  is driven to rotate counterclockwise in  FIG. 11 , electrically charged toner is supplied to the outer peripheral surface of the developing roller  144 . A developing bias voltage is applied to the rotary shaft  144   a  of the developing roller  144  from the corresponding supplying electrode  112   e  ( FIG. 3 ) for causing toner to adhere to the outer peripheral surface of the corresponding photosensitive drum  151 . 
     The blade  145  contacts the outer peripheral surface of the developing roller  144  so as to adjust the quantity of toner on the outer peripheral surface of the developing roller  144  while the developing roller  144  is driven to rotate counterclockwise in  FIG. 11 . 
     The four drum sections  150  have an identical configuration with each other.  FIG. 12  is a side cross-sectional view of one of the drum sections  150 . As shown in  FIG. 12 , each of the drum sections  150  has the photosensitive drum  151 , a Scorotron charger  152 , a cleaning roller  156 , a cleaning cabinet  157 , and the LED substrate  153 . 
     The photosensitive drum  151  has a photosensitive layer made of a photoconductor formed on the outer peripheral surface thereof. The photosensitive drum  151  is disposed such that the outer peripheral surface thereof opposes the outer peripheral surface of the developing roller  144 . A rotary shaft of the photosensitive drum  151  is supported by the drum unit  130  such that the photosensitive drum  151  can be driven to rotate clockwise in  FIG. 12 . The rotary shaft of the photosensitive drum  151  extends in the widthwise direction (that is, a direction perpendicular to the sheet surface of  FIG. 12 ) orthogonal to the mounting direction of the drum unit  130 , which is the front-to-rear direction. 
     The charger  152  is disposed above the photosensitive drum  151  so as to oppose the outer peripheral surface of the photosensitive drum  151 . The charger  152  applies a uniform charge across the entire surface of the photosensitive drum  151 . 
     The cleaning roller  156  is made of a sponge member and is disposed to the rear of the photosensitive drum  151  so as to press the photosensitive drum  151  to a predetermined pressure level. The cleaning roller  156  is supported by the drum unit  130  so as to rotate following the rotation of the photosensitive drum  151 . The cleaning cabinet  157  surrounds the cleaning roller  156 . 
     The LED substrate  153  is disposed below the cleaning roller  156 . The upper and lower sides of the LED substrate  153  are protected by the cleaning cabinet  157  and a film member  158 , respectively. The lower surface of the cleaning cabinet  157  and the upper surface of the film member  158  are coated with light reflecting paint in order to efficiently lead light emitted from the LED  153   a  to the outer peripheral surface of the photosensitive drum  151 . 
     As shown in  FIG. 11 , the scanner unit  160  includes a laser beam emitting section (not shown) that emits a laser beam based on image data for scanning the laser beam on the outer peripheral surface of the photosensitive drum  151  in the widthwise direction (the direction perpendicular to the sheet surface of  FIG. 11 ). 
     The transfer unit  170  includes a belt driving roller  171 , a follower roller  172 , a conveyer belt  173 , four transfer rollers  174 , and a belt cleaner  175 . The belt driving roller  171  is disposed to the rear of one of the drum sections  150  disposed nearest the rear. The follower roller  172  is disposed to the front of one of the drum sections  150  disposed nearest the front. The belt driving roller  171  and the follower roller  172  are rotatably supported by the main body  110 . 
     The conveyor belt  173  is an endless belt made of electrically conductive resin film, such as polycarbonate or polyimide, dispersed with electrically conductive particles of carbon or the like. The conveyor belt  173  is wound on the belt driving roller  171  and the follower roller  172 . As the belt driving roller  171  is driven to rotate counterclockwise in  FIG. 11 , the conveyor belt  173  rotates counterclockwise in  FIG. 11  and conveys a sheet of paper P supported on the conveyor belt  173  from the front side to the rear side. 
     Each of the transfer rollers  174  is disposed below the corresponding one of the photosensitive drums  151  such that the transfer roller  174  confronts the corresponding photosensitive drum  151  with the conveyor belt  173  interposed therebetween. Each transfer roller  174  is rotatably supported and rotates as the conveyor belt  173  rotates counterclockwise in  FIG. 11 . A transfer bias voltage for transferring toner from the photosensitive drum  151  toward the conveyor belt  173  (the sheet of paper P) is applied to the transfer roller  174 . The belt cleaner  175  is disposed below the conveyor belt  173  for cleaning the surface of the conveyor belt  173 . 
     The feeder unit  180  includes a feeder case  181 , a feed roller  183 , a separation roller  184 , a separation pad  185 , and a paper dust removing roller  187 . 
     The feeder case  181  accommodates a stack of large number of sheets of paper P. A sheet pressing plate  182  is disposed inside the feeder case  181 . The sheet pressing plate  182  is pivotably supported at its rear end  182   a  so that its front end  182   b  can move upward and downward. 
     The feed roller  183  is made of synthetic rubber member. The feed roller  183  is supported by the main body  110 , at a position above the front end  182   b  of the sheet pressing plate  182 , so as to be driven to rotate counterclockwise in  FIG. 11 . As the feed roller  183  is driven to rotate counterclockwise in  FIG. 11 , the feed roller  183  conveys the uppermost sheet of paper P stacked in the feeder case  181  toward the front. 
     The separation roller  184  is made of synthetic rubber material, and is supported by the main body  110  to the front of the feed roller  183  so as to be driven to rotate counterclockwise in  FIG. 11 . The separation pad  185  is disposed in confrontation with the separation roller  184 . A separation surface  185   a  of the separation pad  185  that faces the separation roller  184  is made of a material having a high frictional coefficient, such as synthetic rubber or felt. An urging spring  186  is disposed below the separation pad  185  for urging the separation pad  185  toward the separation roller  184 , pressing the separation pad  185  against the separation roller  184 . 
     The paper dust removing roller  187  is for removing paper dust adhering to sheets of paper P. The paper dust removing roller  187  is disposed above the separation roller  184  to confront a pinch roller  188 . 
     A sheet conveyor roller  191  and a sheet guide member  192  are disposed to the front of the follower roller  172 . The sheet of paper P that is conveyed from the feeder unit  180  is conveyed further toward the conveyor belt  173  by the sheet conveyor roller  191  and the sheet guide member  192 . 
     A fixing unit  193  is disposed to the rear of the belt driving roller  171 . The fixing unit  193  includes a heating roller  193   a  and a pressure roller  193   b . The heating roller  193   a  includes a metal cylinder, whose surface is processed for mold releasing, and a halogen lamp disposed inside the cylinder. 
     The heating roller  193   a  is supported by the main body  110  so as to be driven to rotate clockwise in  FIG. 11 . The pressure roller  193   b  is made of silicon rubber. The pressure roller  193   b  presses against the heating roller  193   a  with a predetermined pressure. The pressure roller  193   b  is supported by the main body  110  so as to be rotatable counterclockwise in  FIG. 11 , following the rotary motion of the heating roller  193   a.    
     With this configuration, while the rotation of the heating roller  193   a  conveys the sheet of paper P toward the sheet discharge opening  111   c , the toner on the sheet of paper P is fused and rigidly fixed to the sheet of paper P. 
     A fixed sheet conveyor roller  194  and a pinch roller  195  are disposed to the rear of and diagonally above the fixing unit  193 . The fixed sheet conveyor roller  194  is supported by the main body  110  so as to be driven to rotate clockwise in  FIG. 11 . On the other hand, the pinch roller  195  is disposed in confrontation with the fixed sheet conveyor roller  194  and is supported by the main body  110  so as to be able to rotate following the rotary motion of the fixed sheet conveyor roller  194 . The fixed sheet conveyor roller  194  and the pinch roller  195  together convey the sheet of paper P with images fixed thereon toward the sheet discharge opening  111   c  as the fixed sheet conveyor roller  194  is driven to rotate clockwise in  FIG. 11 . 
     Fixed sheet guides  196   a  and  196   b  are disposed above the fixed sheet conveyance roller  194  and the pinch roller  195 . The fixed sheet guide rollers  196   a  are  196   b  guide the sheet of paper P conveyed by the fixed sheet conveyor roller  194  and the pinch roller  195  toward a contact area of a sheet discharge roller  197  and a sheet discharge follower roller  198 . 
     The sheet discharge roller  197  and the sheet discharge follower roller  198  are disposed near the sheet discharge opening  111   c  so as to confront the same. The sheet discharge roller  197  is rotatably supported by the main body  110  so as to be able to be driven to rotate counterclockwise in  FIG. 11 . The sheet discharge follower roller  198  is disposed in confrontation with the sheet discharge roller  197  and is rotatably supported by the main body  110  so as to be able to rotate following the rotary motion of the sheet discharge roller  197 . 
     With this configuration, the sheet discharge roller  197  and the sheet discharge follower roller  198  together discharge the sheet of paper P to the outside the main body  110  through the sheet discharge opening  111   c , as the sheet discharge roller  197  is driven to rotate counterclockwise in  FIG. 11 . 
     An image forming operation of the printer  100  will be described next. 
     As the feed roller  183  is driven to rotate counterclockwise in  FIG. 11 , the sheets of paper P stacked in the feeder case  181  are fed toward the separation roller  184 . Subsequently, the leading ends of the sheets of paper P are conveyed to a position between the separation roller  184  and the separation pad  185 . As the separation roller  184  is driven to rotate counterclockwise in  FIG. 11 , only the uppermost sheet of paper P is fed toward the paper dust removing roller  187  where the paper dust is removed from the sheet of paper P. The sheet of paper P is then fed to the transfer unit  170  by way of the sheet conveyor roller  191  and the sheet guide member  192 . 
     As the agitator  142  is driven to rotate in each of the developing cartridges  140 , the toner in the cartridge case  141  of each developing cartridge  140  is agitated and supplied to the supply roller  143 . The toner supplied to the supply roller  143  is then conveyed to the developing roller  144  by the counterclockwise rotation of the supply roller  143  in  FIG. 11 . The toner is electrically charged by friction at the position between the supply roller  143  and the developing roller  144  and adheres onto the outer peripheral surface of the developing roller  144 . The toner adhered to the outer peripheral surface of the developing roller  144  is adjusted for density and the amount of electric charge to predetermined respective levels by means of the blade  145 , and then is conveyed to the position between the developing roller  144  and the photosensitive drum  151  by the counterclockwise rotation of the developing roller  144 . 
     Meanwhile, the outer peripheral surface of each photosensitive drum  151  is uniformly electrically charged by the charger  152 , and is subsequently irradiated with a laser beam corresponding to image data. As a result, an electrostatic latent image corresponding to the image data is formed on the outer peripheral surface of the photosensitive drum  151 . When the toner on the outer peripheral surface of the developing roller  144  contacts the outer peripheral surface of the photosensitive drum  151  where the electrostatic latent image is formed, the electrostatic latent image on the outer peripheral surface of the photosensitive drum  151  is developed by the toner, forming a toner image. 
     The sheet of paper P conveyed to the transfer unit  170  is supported on the conveyor belt  173  and conveyed toward the rear. When the sheet of paper P reaches a position between the photosensitive drum  151  and the transfer roller  174 , the toner image on the outer peripheral surface of the photosensitive drum  151  is transferred onto the sheet of paper P due to the transfer bias voltage between the transfer roller  174  and the photosensitive drum  151 . In this manner, a plurality of toner images of different colors are sequentially transferred onto the sheet of paper P, thereby forming a full-color image thereon. After the transfer operation, the outer peripheral surface of each photosensitive drum  151  is discharged by light emitted from the LEDs  153   a  on the LED substrate  153 . 
     After passing through the transfer unit  170 , the sheet of paper P with the full-color image formed thereon is conveyed to the fixing unit  193 . As the sheet of paper P passes between the heating roller  193   a  and the pressure roller  193   b , the toner on the surface of the sheet of paper P is fused and fixed onto the surface. Thereafter, the sheet of paper P is discharged onto the discharge tray  111   b  outside the main body  110 . 
     As shown in  FIGS. 7 and 9 , the receiving electrodes  131   b  and  131   c  are disposed on the front surface of the front beam  131 , and the supplying electrodes  114   a  and  114   b  are disposed on the front cover  111   e , as described above. Thus, the receiving electrodes  131   b  and  131   c  do not gride or slide over the supplying electrodes  114   a  and  114   b  when mounting or removing the image forming unit  120  (the drum unit  130 ) in or from the main frame  112 . Thus, the electrodes  131   b ,  131   c ,  114   a , and  114   b  are not worn. 
     Also, because the receiving electrodes  131   b  and  131   c  are disposed to the upstream section of the drum unit  130  with respect to the mounting direction of the image forming unit  120  (the drum unit  130 ), it is possible to bring the receiving electrodes  131   b  and  131   c  into abutment with the supplying electrodes  114   a  and  114   b  with a simple configuration. Also, because the receiving electrodes  131   b  and  131   c  do not contact the supplying electrodes  114   a  and  114   b  when mounting the image forming unit  120  in the main frame  112 , the electrodes  131   b ,  131   c ,  114   a , and  114   b  can be prevented from being damaged. 
     Because the developing cartridges  140  and the drum sections  150  are arranged in a direction perpendicular to the rotary shafts of the photosensitive drums  151 , a gear mechanism for the developing cartridges  140  and the drum sections  150  can be disposed remote from the receiving electrodes  131   b  and  131   c . Thus, the configuration of the main frame  112  and the drum units  130  can be simplified, and the printer  100  and the drum units  130  can be assembled and serviced efficiently. That is, since such a gear mechanism is usually disposed on a side of rotary shafts of the photosensitive drums  151 , it is difficult to dispose the receiving electrodes  131   b  and  131   c  on the front beam  131  or the rear beam  134  if the developing cartridges  140  and the drum sections  150  are arranged in a direction parallel to the rotary shafts of the photosensitive drums  151 . 
     Because each of the LED substrates  153  relays power supplied from the receiving electrodes  131   b  and  131   c  to the other LED substrate  153 , the lengths of the parts of the harness  154  can be the same for all the LED substrates  153 . Therefore, the work load for manufacturing the LED substrates  153  and assembling the drum unit  130  can be reduced. 
     Because the receiving electrodes  121  ( FIG. 3 ) are disposed through the support plate  133 , and the supplying electrodes  112   e  to contact the corresponding receiving electrodes  121  are disposed inside main frame  112 , all power receiving sections for receiving power to be supplied to the developing cartridges  140  and the LED substrates  153  are no longer necessary to be arranged on the front beam  131  in a concentrated manner. Thus, the degree of freedom for designing the drum unit  130  and the main frame  112  is remarkably increased. 
     Because each of the LED substrates  153  is disposed between the transfer unit  170  and the corresponding cleaning roller  156  when the image forming unit  120  is mounted in the main frame  112 , the photosensitive drum  151  can be discharged immediately after the completion of the transfer operation. 
     Also, because the upper side and the lower side of each LED substrate  153  are protected by the cleaning cabinet  157  and the film member  158 , toner scattered inside the main body  110  is prevented from adhering to the LEDs  153   a  and from blocking light emitted from the LEDs  153   a . Thus, the photosensitive drums  151  can be reliably discharged. 
     Because the lower surface of the cleaning cabinet  157  and the upper surface of the film member  158  are coated with light reflecting paint, even if the amount of light emitted from the LED  153   a  is small, the light can be efficiently led to the outer peripheral surface of the photosensitive drum  151 , and the photosensitive drum  151  can be reliably discharged. 
     While the invention has been described in detail with reference to the above aspects thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     For example, in the above-described printer  100 , the receiving electrodes  131   b  and  131   c  are disposed on the front beam  131  disposed on the upstream side with respect to the mounting direction of the image forming unit  120 . However, the receiving electrodes  131   b  and  131   c  may be disposed on the rear beam  134  disposed on the downstream side, or on both the front beam  131  and the rear beam  134 . 
     In the above-described printer  100 , the receiving electrodes  131   b  and  131   c  are electrically connected to the LED substrates  153 . However, the receiving electrodes  131   b  and  131   c  may be electrically connected to the rotary shafts  144   a  of the development rollers  144 . 
     The invention can also be applied to a printer that includes a photosensitive-member unit formed integrally with a plurality of image forming units, each having a photosensitive drum. The photosensitive-member unit includes a charging unit for charging the photosensitive drums, a developing unit for developing electrostatic images formed on the photosensitive drums, a transfer unit for transferring toner on the photosensitive drums onto a sheet of paper, and a discharging unit for discharging the electric charges of the photosensitive drums after the developing operation. In this case, the charging section, the developing section, the transfer unit, and the discharging unit can be replaced simultaneously.