Patent Publication Number: US-7590371-B2

Title: Image carrying cartridge, process cartridge, and image-forming device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2005-130201 filed Apr. 27, 2005. The entire content of priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image-forming device, such as a laser printer, and to an image carrying cartridge and process cartridge mounted in the image-forming device. 
     BACKGROUND 
     In an image-forming device such as a laser printer, a charger, a developer, and a transfer roller are disposed around a photosensitive drum. When the photosensitive drum is rotated, the surface of the photosensitive drum is uniformly charged by the charger and then selectively exposed by a laser beam. As a result, the electric charge on the surface of the photosensitive drum is partially removed and a latent image is formed on the surface of the photosensitive drum. When the latent image formed on the surface of the photosensitive drum rotates opposite the developer, toner carried on the developer is supplied to the latent image and the latent image is developed into a toner image. Subsequently, when the toner image is rotated opposite a transfer roller, the toner image carried on the photosensitive drum is transferred onto a sheet of paper conveyed between the photosensitive drum and the transfer roller. 
     In order to form an excellent latent image and toner image on the surface of the photosensitive drum, as well as to transfer the toner image onto a paper with high accuracy, the relative positional relationship between the photosensitive drum and components disposed around the photosensitive drum, such as the charger, is very important in such image-forming device. 
     An image-forming device disclosed in Japanese Utility Model Publication No. HEI-7-19752 provides a first casing including a photosensitive drum and a second casing including a charger. In the image-forming device, a relative position between the photosensitive drum and the charger is positioned by attaching the first casing and the second casing. The second casing has an engagement portion to be engaged with a shaft of the photosensitive drum. The first casing has a claw portion to be locked a locking hole formed in the first casing. When the engagement portion is engaged with the shaft of the photosensitive drum, and the claw portion is locked the locking hole, engagement between the first and second casings is achieved. 
     SUMMARY 
     However, the engagement portion and the claw portion have errors in shape or dimension (manufacturing error) respectively. When the first and second casings are attached together, the errors in shape or dimension of the engagement portion and claw portion are summed together. Accordingly, There is a possibility that the claw portion cannot be locked into the locking hole. 
     Further, once the first and second casings are attached together, removal of the claw portion from the locking hole is made difficult. Therefore, replacement of components such as the photosensitive drum is troublesome after the first and second casings are attached together. 
     In view of the foregoing, it is an object of the present invention to provide an image-forming device and a image carrying cartridge and a process cartridge used in the image-forming device capable of positioning a relative position between a image carrying member and components disposed around the image carrying member with high accuracy, and reducing a work at the time of assembly and separation. 
     To achieve the above and other objects, one aspect of the present invention provides an image carrying cartridge including a shaft, an image carrying member, a first casing, and a second casing. 
     The shaft defines an axial direction. The image carrying member carries a developer image. The image carrying member is formed with a shaft insertion-hole extending in the axial direction through which the shaft is inserted and has one end and another end in the axial direction. The first casing includes two first walls arranged in parallel to each other and having surfaces perpendicular to the axial direction of the shaft. One first wall opposes the one end of the image carrying member in the axial direction, another first wall opposes the another end of the image carrying member in the axial direction. Each of the first walls is formed with a first through-hole. The second casing includes two second walls arranged in parallel to the two first walls. One second wall opposes the one first wall in the axial direction, another second wall opposing the another first wall in the axial direction. Each of the second walls is formed with a second through-hole. The first casing and the second casing are coupled by insertion of the shaft through the first though-hole and the second through-hole to provide an inner space in which the image carrying member is accommodated. 
     In another aspect of the invention, there is provided a process cartridge including above-described image carrying cartridge and a developer cartridge that supplies a toner to the image carrying member. 
     In another aspect of the invention, there is provided an image forming device including a main frame and above-described image carrying cartridge disposed in the main frame. 
     In another aspect of the invention, there is provided an image forming device including a main frame and above-described process cartridge provided in the main frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross-sectional view of a laser printer serving as the image-forming device of the present invention; 
         FIG. 2  is a side cross-sectional view of a process cartridge employed in the laser printer shown in  FIG. 1 ; 
         FIG. 3  is a side cross-sectional view of a process cartridge employed in the laser printer shown in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view from the rear side of the drum cartridge taken along the vertical plane extending parallel to a drum shaft according to the present invention; 
         FIG. 5  is a perspective view from above the rear side of the drum cartridge according to the present invention; 
         FIG. 6  is a perspective view from above the front side of a lower casing of the drum cartridge according to the present invention; 
         FIG. 7  is side cross-sectional view of the lower casing according to the present invention; 
         FIG. 8  is a view showing a process of assembling (upper casing is fitted to lower casing) the drum cartridge according to the present invention; 
         FIG. 9  is a view showing a process of assembling (insertion of drum shaft) the drum cartridge according to the present invention; 
         FIG. 10  is a view showing a process of assembling the drum cartridge (state after assembly work) according to the present invention; and 
         FIG. 11  is a view showing a process of removing a light-shielding paper from a photosensitive drum attached to the drum cartridge shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     Next, a laser printer as an image-forming device according to an embodiment of the present invention will be described while referring to the accompanying drawings. 
     As shown in  FIG. 1 , the laser printer  1  includes a main casing  2  and, within the main casing  2 , a feeding unit  4  for supplying sheets of a paper  3 , an image-forming unit  5  for forming images on the paper  3  supplied by the feeding unit  4 . 
     The laser printer  1  also includes an access opening  6  formed in one side wall of the main casing  2  for inserting and removing a process cartridge  20  described later, and a front cover  7  capable of opening and closing over the access opening  6 . The front cover  7  is rotatably supported by a cover shaft  8  inserted through a bottom edge of the front cover  7 . Accordingly, when the front cover  7  is rotated closed about the cover shaft  8 , the front cover  7  covers the access opening  6 , as shown in  FIG. 1 . When the front cover  7  is rotated open about the cover shaft  8 , the access opening  6  is exposed, enabling the process cartridge  20  to be mounted into or removed from the main casing  2  via the access opening  6 . 
     In the following description, the side of the laser printer  1  on which the front cover  7  is mounted and the corresponding side of the process cartridge  20  when the process cartridge  20  is mounted in the main casing  2  will be referred to as the “front side,” while the opposite side will be referred to as the “rear side.” 
     The main casing  2  is also provided with an exhausting fan  119  for exhausting air out of the main casing  2 , and a duct  120  for guiding air to the exhausting fan  119 . 
     The feeding unit  4  includes a paper tray  9  that can be inserted into or removed from a lower section of the main casing  2  in the front-to-rear direction, a separating roller  10  and a separating pad  11  disposed above a front end of the paper tray  9 , and a feeding roller  12  disposed on the rear side of the separating roller  10  upstream of the separating pad  11  with respect to the conveying direction of the paper  3  (hereinafter referred to as the “paper-conveying direction”). The feeding unit  4  also includes a paper dust roller  13  disposed above and forward of the separating roller  10  and downstream of the separating roller  10  in the paper-conveying direction, and a pinch roller  14  disposed in opposition to the paper dust roller  13 . 
     A paper-conveying path for the paper  3  reverses directions toward the rear side of the laser printer  1 , forming a substantial U-shape near the paper dust roller  13 . The feeding unit  4  also includes a pair of registration rollers  15  disposed below the process cartridge  20  farther downstream of the U-shaped portion of the paper-conveying path with respect to the paper-conveying direction. 
     A paper-pressing plate  16  is provided inside the paper tray  9  for supporting the paper  3  in a stacked state. The paper-pressing plate  16  is pivotably supported on the rear end thereof, so that the front end can pivot downward to a resting position in which the paper-pressing plate  16  rests on a bottom plate of the paper tray  9  and can pivot upward to a feeding position in which the paper-pressing plate  16  slopes upward from the rear end to the front end. 
     A lever  17  is provided in the front section of the paper tray  9  for lifting the front end of the paper-pressing plate  16  upward. The rear end of the lever  17  is pivotably supported on a lever shaft  18  at a position below the front end of the paper-pressing plate  16  so that the front end of the lever  17  can pivot between a level position in which the lever  17  lies along the bottom plate of the paper tray  9  and a sloped position in which the front end of the lever  17  lifts the paper-pressing plate  16  upward. When a driving force is inputted into the lever shaft  18 , the lever  17  rotates about the lever shaft  18  and the front end of the lever  17  raises the front end of the paper-pressing plate  16 , shifting the paper-pressing plate  16  into the feeding position. 
     When the paper-pressing plate  16  is in the feeding position, the topmost sheet of paper  3  stacked on the paper-pressing plate  16  is pressed against the feeding roller  12 . The rotating feeding roller  12  begins feeding the sheets of paper  3  toward a separating position between the separating roller  10  and separating pad  11 . 
     When the paper tray  9  is removed from the main casing  2 , the paper-pressing plate  16  settles into the resting position. While the paper-pressing plate  16  is in the resting position, the paper  3  can be stacked on the paper-pressing plate  16 . 
     When the feeding roller  12  conveys a sheet of the paper  3  toward the separating position and the sheet becomes interposed between the separating roller  10  and separating pad  11 , the rotating separating roller  10  separates and feeds the paper  3  one sheet at a time. Each sheet of paper  3  fed by the separating roller  10  passes between the paper dust roller  13  and pinch roller  14 . After the paper dust roller  13  removes paper dust from the sheet of paper  3 , the sheet is conveyed along the U-shaped paper-conveying path, thereby reversing directions in the main casing  2 , and is conveyed toward the registration rollers  15 . 
     After registering the paper  3 , the registration rollers  15  convey the paper  3  to a transfer position between a photosensitive drum  28  and a transfer roller  31  described later, at which position a toner image formed on the photosensitive drum  28  is transferred onto the paper  3 . 
     The image-forming unit  5  includes a scanning unit  19 , the process cartridge  20 , and a fixing unit  21 . 
     The scanning unit  19  is disposed in a top section of the main casing  2  and includes a laser light source (not shown), a polygon mirror  22  that can be driven to rotate, an fθ lens  23 , a reflecting mirror  24 , a lens  25 , and a reflecting mirror  26 . The laser light source emits a laser beam based on image data. As illustrated by a dotted line in  FIG. 1 , the laser beam is deflected by the polygon mirror  22 , passes through the fθ lens  23 , is reflected by the reflecting mirror  24 , passes through the lens  25 , and is reflected downward by the reflecting mirror  26  to be irradiated on the surface of the photosensitive drum  28  in the process cartridge  20 . 
     As shown in  FIG. 1 , the process cartridge  20  is provided in the main casing  2  beneath the scanning unit  19  and can be mounted in or removed from the main casing  2  through the access opening  6 . As shown in  FIG. 3 , the process cartridge  20  includes a drum cartridge  27  and a developer cartridge  30  that is detachably mounted on the drum cartridge  27 . As shown in  FIG. 2 , the drum cartridge  27  includes a drum side casing  76  described later in greater detail and, within the drum side casing  76 , the photosensitive drum  28 , a Scorotron charger  29 , the transfer roller  31 , and a cleaning member  32 . 
     The photosensitive drum  28  includes a main drum body  33  that is cylindrical in shape and has a positive charging photosensitive layer formed of polycarbonate or the like on its outer surface, and a metal drum shaft  34  extending through the center of the main drum body  33  along the axial direction thereof. The metal drum shaft  34  is supported in the drum side casing  76 , and the main drum body  33  is rotatably supported relative to the metal drum shaft  34 . With this construction, the photosensitive drum  28  is disposed in the drum side casing  76  and is capable of rotating about the metal drum shaft  34 . Further, the photosensitive drum  28  is driven to rotate by a driving force inputted from a motor (not shown). 
     The charger  29  is supported on the drum side casing  76  diagonally above and rearward of the photosensitive drum  28 . The charger  29  opposes the photosensitive drum  28  but is separated a prescribed distance from the photosensitive drum  28  so as not to contact the same. The charger  29  includes a discharge wire  74  disposed in opposition to but separated a prescribed distance from the photosensitive drum  28 , and a grid  75  provided between the discharge wire  74  and the photosensitive drum  28  for controlling the amount of corona discharge from the discharge wire  74  that reaches the photosensitive drum  28 . By applying a high voltage to the discharge wire  74  for generating a corona discharge from the discharge wire  74  at the same time a bias voltage is applied to the grid  75 , the charger  29  can charge the surface of the photosensitive drum  28  with a uniform positive polarity. 
     The transfer roller  31  is disposed in the drum side casing  76  below the photosensitive drum  28  and contacts the photosensitive drum  28  in a vertical direction from the bottom thereof so as to form a nip part with the photosensitive drum  28 . The transfer roller  31  is configured of a metal roller shaft  56  that is covered with a roller  57  formed of an electrically conductive rubber material. The roller shaft  56  is rotatably supported in the drum side casing  76 . The transfer roller  31  is driven to rotate by a driving force inputted from a motor (not shown). Further, a transfer bias is applied to the transfer roller  31  during a transfer operation. 
     The cleaning member  32  is mounted on the drum side casing  76  in a position confronting and contacting the photosensitive drum  28  from the rear side thereof. The cleaning member  32  includes a cleaning brush  65  that captures paper dust deposited on the photosensitive drum  28 , and a support plate  66  that supports the rear side of the cleaning brush  65  on the side opposite the photosensitive drum  28 . 
     The cleaning brush  65  is configured of a nonwoven fabric implanted with a plurality of fibrous brush bristles having electrical conductivity, and is fixed to the support plate  66 . The cleaning brush  65  is disposed so as to confront and contact the photosensitive drum  28 . The support plate  66  is supported on the drum side casing  76  while supporting the cleaning brush  65 . A cleaning bias is applied to the cleaning member  32  during cleaning operation. 
     As shown in  FIG. 3 , the developer cartridge  30  is detachably mounted to the drum side casing  76 . Accordingly, when the process cartridge  20  is mounted in the main casing  2 , the developer cartridge  30  can be mounted in the main casing  2  by first opening the front cover  7  and subsequently inserting the developer cartridge  30  through the access opening  6  and mounting the developer cartridge  30  on the process cartridge  20 . 
     The developer cartridge  30  includes a developer side casing  36  and, within the developer side casing  36 , a supply roller  37 , a developing roller  38 , and a thickness-regulating blade  39 . The developer side casing  36  has a box shape that is open on the rear side. A partitioning wall  40  is provided in the developer side casing  36  for partitioning the interior of the developer side casing  36  into a toner-accommodating chamber  41  and a developing chamber  42 . The partitioning wall  40  is disposed at a position in the developer side casing  36  midway in the front-to-rear direction for partitioning the interior of the developer side casing  36  in the front-to-rear direction. An opening  43  is formed through a midway region of the partitioning wall  40 . 
     The toner-accommodating chamber  41  occupies a space in the front side of the developer side casing  36  partitioned by the partitioning wall  40 . The toner-accommodating chamber  41  is filled with a nonmagnetic, single-component toner having a positive charge. The toner used in the preferred embodiment is a polymerized toner obtained by copolymerizing a polymerized monomer using a well-known polymerization method such as suspension polymerization. The polymerized monomer may be, for example, a styrene monomer such as styrene or an acrylic monomer such as acrylic acid, alkyl (C1-C4) acrylate, or alkyl (C1-C4) meta acrylate. The polymerized toner is formed as particles substantially spherical in shape in order to have excellent fluidity for achieving high-quality image formation. 
     This type of toner is compounded with a coloring agent, such as carbon black, or wax, as well as an additive such as silica to improve fluidity. The average diameter of the toner particles is about 6-10 μm. 
     Toner supply openings for filling the toner-accommodating chamber  41  with toner are formed in both side walls of the developer side casing  36  that define the toner-accommodating chamber  41 . The toner supply openings are sealed with caps  35 . 
     Toner detection windows  44  are provided in both side walls of the developer side casing  36  that define the toner-accommodating chamber  41  for detecting the amount of toner remaining in the toner-accommodating chamber  41 . The toner detection windows  44  are formed in the side walls near the partitioning wall  40  and oppose each other in the width direction W (the direction orthogonal to the front-to-rear direction and the vertical) across the toner-accommodating chamber  41 . The toner detection windows  44  are formed by embedding a transparent disc-shaped plate in each side wall. 
     An agitator  45  is disposed in the toner-accommodating chamber  41  for agitating toner accommodated therein. The agitator  45  includes a rotational shaft  46  and an agitating member  47 . 
     The rotational shaft  46  is rotatably supported in the side walls of the developer side casing  36  substantially in the center of the toner-accommodating chamber  41 . The agitating member  47  is provided on the rotational shaft  46 . A motor (not shown) produces a driving force that is inputted into the rotational shaft  46  for driving the rotational shaft  46  to rotate. Consequently, the agitating member  47  moves in a circular path about the rotational shaft  46  through the toner-accommodating chamber  41  and stirs toner accommodated in the toner-accommodating chamber  41 . When the agitating member  47  stirs the toner, some of the toner is discharged in the front-to-rear direction toward the supply roller  37  through the opening  43  formed in the partitioning plate  40 . 
     The agitator  45  also includes wipers  48  attached at both axial ends of the rotational shaft  46 . When the rotational shaft  46  rotates, the wipers  48  move in a circular direction about the rotational shaft  46  through the toner-accommodating chamber  41  in order to wipe the toner detection windows  44  provided in the side walls of the developer side casing  36 . Hence, the wipers  48  function to clean the toner detection windows  44 . 
     The developing chamber  42  occupies an interior space in the rear side of the developer side casing  36  partitioned by the partitioning wall  40 . The developing chamber  42  accommodates the supply roller  37 , the developing roller  38 , and the thickness-regulating blade  39 . 
     The supply roller  37  is disposed rearward of the opening  43  and includes a metal roller shaft  50  covered by a sponge roller  51  formed of an electrically conductive foam material. The roller shaft  50  is rotatably supported within the developing chamber  42  in both side walls of the developer side casing  36 . The supply roller  37  is driven to rotate by a driving force inputted into the roller shaft  50  from a motor (not shown). 
     The developing roller  38  is disposed rearward of the supply roller  37  and contacts the supply roller  37  with pressure so that both are compressed. The developing roller  38  includes a metal roller shaft  52 , and a rubber roller  53  formed of an electrically conductive rubber material that covers the roller shaft  52 . The roller shaft  52  is rotatably supported in both side walls of the developer side casing  36  within the developing chamber  42 . The rubber roller  53  is more specifically formed of an electrically conductive urethane rubber or silicon rubber containing fine carbon particles, the surface of which is coated with urethane rubber or silicon rubber containing fluorine. The developing roller  38  is driven to rotate by a driving force inputted into the roller shaft  52  from a motor (not shown). Further, a developing bias is applied to the developing roller  38  during a developing operation. 
     The thickness-regulating blade  39  includes a main blade member  54  configured of a metal leaf spring, and a pressing part  55  provided on a distal end of the main blade member  54 . The pressing part  55  has a semicircular cross section and is formed of an insulating silicon rubber. A base end of the main blade member  54  is supported on the developer side casing  36  above the developing roller  38 , and the pressing part  55  contacts the developing roller  38  with pressure through the elastic force of the main blade member  54 . 
     Toner discharged through the opening  43  is supplied onto the developing roller  38  by the rotating supply roller  37 . At this time, the toner is positively tribocharged between the supply roller  37  and the developing roller  38 . As the developing roller  38  rotates, the toner supplied to the surface of the developing roller  38  passes between the rubber roller  53  of the developing roller  38  and the pressing part  55  of the thickness-regulating blade  39 , thereby maintaining a uniform thickness of toner on the surface of the developing roller  38 . 
     As the photosensitive drum  28  rotates, the charger  29  charges the surface of the photosensitive drum  28  with a uniform positive polarity. Subsequently, a laser beam emitted from the scanning unit  19  is scanned at a high speed over the surface of the photosensitive drum  28 , forming an electrostatic latent image corresponding to an image to be formed on the paper  3 . 
     Next, positively charged toner carried on the surface of the developing roller  38  comes into contact with the photosensitive drum  28  as the developing roller  38  rotates and is supplied to areas on the surface of the positively charged photosensitive drum  28  that were exposed to the laser beam and, therefore, have a lower potential. In this way, the latent image on the photosensitive drum  28  is transformed into a visible image according to a reverse developing process so that a toner image is carried on the surface of the photosensitive drum  28 . 
     Subsequently, as the registration rollers  15  convey a sheet of the paper  3  through the transfer position between the photosensitive drum  28  and the transfer roller  31 , the toner image carried on the surface of the photosensitive drum  28  is transferred onto the paper  3  by the transfer bias applied to the transfer roller  31 . After the toner image is transferred, the paper  3  is conveyed to the fixing unit  21 . 
     Toner remaining on the photosensitive drum  28  after the transfer operation is recovered by the developing roller  38 . Further, paper dust deposited on the photosensitive drum  28  from the paper  3  is recovered by the cleaning brush  58  of the cleaning member  32 . Further, the cleaning brush  65  of the cleaning member  32  physically scrapes off and electrically absorbs paper dust after the transfer process that was deposited from the paper  3  onto the surface of the photosensitive drum  28 . 
     The fixing unit  21  is disposed on the rear side of the process cartridge  20  and includes a fixed frame  59 , and a heating roller  60  and a pressure roller  61  provided within the fixed frame  59 . 
     The heating roller  60  includes a metal tube, the surface of which has been coated with a fluorine resin, and a halogen lamp disposed inside the metal tube for heating the same. The heating roller  60  is driven to rotate by a driving force inputted from a motor (not shown). 
     The pressure roller  61  is disposed below and in opposition to the heating roller  60  and contacts the heating roller  60  with pressure. The pressure roller  61  is configured of a metal roller shaft covered with a roller that is formed of a rubber material. The pressure roller  61  follows the rotational drive of the heating roller  60 . 
     In the fixing unit  21 , a toner image transferred onto the paper  3  at the transfer position is fixed to the paper  3  by heat as the paper  3  passes between the heating roller  60  and pressure roller  61 . After the toner image is fixed to the paper  3 , the heating roller  60  and pressure roller  61  continue to convey the paper  3  along a discharge end paper-conveying path toward a discharge tray  62  formed on the top surface of the main casing  2 . 
     The paper-conveying path on the discharge end leads from the fixing unit  21  to the discharge tray  62  and is substantially U-shaped for reversing the conveying direction of the paper  3  to a direction toward the front side of the laser printer  1 . A pair of conveying rollers  63  is disposed at a midpoint along the discharge end paper-conveying path, and a pair of discharge rollers  64  is disposed at a downstream end of the same path. 
     Hence, after passing through the fixing unit  21 , the paper  3  is conveyed along the discharge end paper-conveying path, where the conveying rollers  63  receive and convey the paper  3  to the discharge rollers  64 , and the discharge rollers  64  subsequently receive and discharge the paper  3  onto the discharge tray  62 . 
     Hot air generated from the heating roller  60  of the fixing unit  21  radiates and flows toward the photosensitive drum  28  of the process cartridge  20 . However, since a bottom end of the duct  120  is positioned between the fixing unit  21  and the process cartridge  20 , the duct  120  guides the hot air to the exhausting fan  119 , and the exhausting fan  119  exhausts the air out of the main casing  2 . 
     As shown in  FIG. 2 , the developer side casing  36  of the developer cartridge  30  has a box shape that is open on the rear side and is integrally provided with a top wall  70 , a front wall  72 , the two side walls  69  described above, and a bottom wall  71 . The open region on the rear side is an opening  94 . 
     As shown in  FIG. 3 , the opening  94  is defined by the back support member  111  of the mounting member  109 , the rear edges of the side walls  69 , and the rear edge of the bottom wall  71 . When viewed from the rear side, the opening  94  is substantially rectangular and extends in the width direction W. The developing roller  38  is disposed in the opening  94  and is exposed therefrom. As shown in  FIG. 5 , the developing roller  38  is supported on the developer side casing  36  so as to protrude out of the developer side casing  36  via the opening  94  when viewed from the side. An insertion hole  121  is formed in the rear end of each side wall  69  of the developer side casing  36  at positions opposing each other in the width direction W. The roller shaft  52  of the developing roller  38  is inserted through the insertion holes  121  so that the developing roller  38  is rotatably supported on the developer side casing  36  with a vertical center portion of the rubber roller  53  protruding farthest out of the developer side casing  36  through the opening  94 . 
     Next, the drum side casing  76  of the drum cartridge  27  will be described in detail with reference to  FIGS. 1 through 7 .  FIG. 4  is a cross-sectional view from the rear side of the drum cartridge  27  taken along the vertical plane extending parallel to the drum shaft  34 .  FIG. 5  is a perspective view from above the rear side of the drum cartridge  27 .  FIG. 6  is a perspective view from above the front side of a lower casing  77  of the drum cartridge  27 .  FIG. 7  is a side cross-sectional view of the lower casing  77 . 
     As shown in  FIG. 2 , the drum side casing  76  includes the lower casing  77 , and an upper casing  78  formed separately from the lower casing  77  and assembled on top of the same. The upper casing  77  and the lower casing  78  is assembled to provide an inner space in which the photosensitive drum  28 , the transfer roller  31 , the charger  29 , and the cleaning member  32 . 
     As shown in  FIGS. 2 and 6 , the lower casing  77  is integrally provided with a developer cartridge mounting portion  79  disposed on the front side thereof for receiving the developer cartridge  30 , and a drum support portion  80  disposed on the rear side thereof and positioned to vertically confront the upper casing  78 . 
     As shown in  FIGS. 2 ,  5  and  6 , the developer cartridge mounting unit  79  is integrally provided with a front bottom wall  81  having substantially a rectangular plate shape for receiving the developer cartridge  30  ( FIG. 3 ), two front side walls  82  disposed at each widthwise end of the front bottom wall  81  and opposing each other across the width of the front bottom wall  81 , and a lower front wall  83  disposed on the front end of the front bottom wall  81 . The developer cartridge mounting unit  79  is formed in a frame shape having a bottom and an open top. A handle  84  is formed in a widthwise center region of the lower front wall  83  for gripping the drum cartridge  27  when mounting or removing the same. 
     As shown in  FIGS. 2 ,  5  and  6 , the drum support unit  80  is integrally provided with a rear bottom wall  85  having a curved shape on the bottom thereof for receiving the transfer roller  31 , two rear side walls  86  disposed on widthwise ends of the rear bottom wall  85  and opposing each other across the width of the rear bottom wall  85 , and a lower rear wall  87  disposed on the rear edge of the rear bottom wall  85 . The drum support unit  80  is formed in a frame shape having a bottom and an open top. 
     The front end of the rear bottom wall  85  is formed continuously with the rear edge of the front bottom wall  81 , sagging downward in the center from the front edge to the rear edge so as to be substantially fan-shaped in a side view. Transfer roller receiving parts  88  are formed in both widthwise ends of the rear bottom wall  85  for receiving the transfer roller  31 . 
     As shown in  FIG. 6 , two rear side walls  86  are bent upward from both widthwise edges of the rear bottom wall  85 . Lower drum insertion holes  89  are formed in the rear side walls  86  at positions opposing each other in the width direction W for inserting the drum shaft  34  of the photosensitive drum  28 . 
     As shown in  FIGS. 5 and 8 , a receiving groove  131  is formed on the outer surface of each rear side wall  86  in the width direction W. When the upper casing  78  is attached to the lower casing  77 , the receiving groove  131  receives a fitting portion  149  of the upper side wall  94  described later of the upper casing  78 . 
     As shown in  FIG. 7 , a pair of guide ribs  132  is integrally provided on the inner surface of one rear side wall  86  (left side in the front view). When the photosensitive drum  28  is mounted on the lower casing  77 , the pair of guide ribs  132  guides an edge of a bearing  143  of a drum gear  142  ( FIG. 4 ) described later to a position where the edge of the bearing  143  overlaps the lower drum insertion hole  89  in the width direction W. The guide ribs  132 , which are formed into a rib-shaped, protrude from the inner surface of the one rear side wall  86  toward the inside of the lower casing  77  in the width direction W, and extend in the vertical direction. The guide ribs  132  are disposed opposite to each other across the lower drum insertion hole  89  with a distance substantially equal to the outer diameter of the bearing  143  of the drum gear  142 . A restriction rib  133  is provided on the inner surface of the one rear side wall  86  at a position between the pair of guide ribs  132 . The restriction rib  133  prevents the bearing  143  of the drum gear  142  from being moved downward. 
     As shown in  FIG. 4 , a felt member  134  which a spring receiving member  148  described later contacts is disposed on the inner surface of the other rear side wall  86  (right side in the front view). A hole in communication with the lower drum insertion hole  89  is formed in the felt member  134 . As shown in  FIG. 2 , two lower restriction ribs  151  and a front restriction rib  152  are formed on the inner surface of the other rear side wall  86 . The two lower restriction ribs  151  contact a peripheral surface of a flange member  138  ( FIG. 4 ) describe later from lower side, respectively, in a state where the photosensitive drum  28  is attached to the lower casing  77 . The two lower restriction ribs  151  prevent the photosensitive drum  28  (flange member  138 ) from being moved downward. The front restriction rib  152  contacts the peripheral surface of the flange member  138  from the front side. The lower restriction ribs  151  and front restriction rib  152 , which are formed into a rib-shaped, protrude from the inner surface of the other rear side wall  86  toward the inside of the lower casing  77  and extend in the vertical direction. 
     The lower rear wall  87  is formed continuously from the rear edge of the rear bottom wall  85  across the entire width between the rear side walls  86 . The lower rear wall  87  is an elongated rectangular plate in the front view that is erected vertically and extends in the width direction W. As shown in  FIGS. 2 and 6 , a receiving plate  90  is formed continuously with the lower rear wall  87  and extends from the bottom edge of the lower rear wall  87  forward in a slightly downward slope. 
     An inner rear wall  91  and a positioning wall  92  are vertically erected from the receiving plate  90  forward of the lower rear wall  87  and are parallel to each other but separated a prescribed distance in the front-to-rear direction. 
     The inner rear wall  91  is disposed in front of and separated a prescribed interval from the lower rear wall  87 . The inner rear wall  91  extends in the width direction W and is parallel to the lower rear wall  87 . The positioning wall  92  also extends in the width direction W and is parallel to the inner rear wall  91 . The positioning wall  92  is disposed in front of the inner rear wall  91  and is separated a prescribed distance therefrom. The positioning wall  92  functions to position the cleaning member  32 . 
     As shown in  FIG. 2 , a film member  117  is disposed on the front end of the receiving plate  90  for contacting the photosensitive drum  28 . The film member  117  is adhesively fixed to the end face on the front end of the receiving plate  90  and protrudes upward therefrom. The film member  117  extends in the width direction W and serves as a barrier between the receiving plate  90  and the photosensitive drum  28 . 
     As shown in  FIG. 4 , the photosensitive drum  28  is disposed between both the rear side walls  86  of the lower casing  77 . Flange members  135  and  138  are attached at both ends of the main drum body  33  of the photosensitive drum  28  respectively. The drum shaft  34  passes through the flange members  135  and  138  and rotatably supports the main drum body  33  through the flange members  135  and  138 . 
     Each of the flange members  135  and  138  is formed of an insulating resin material. The flange member  135  is fixed to one end (left side end in the front view, but right side end in  FIG. 4 ) of the main drum body  33 . The flange member  135  is integrally formed with an insertion portion  136  to be inserted into the main drum body  33  and an exposed portion  137  which is exposed from the main drum body  33 . The exposed portion  137  is formed in substantially a double cylindrical shape. The inner cylinder of the exposed portion  137  serves as a coupling portion for coupling to the drum gear  142 . An outer teeth portion is formed on the peripheral surface of the outer cylinder of the exposed portion  137 . The outer teeth portion is engaged with a transfer gear  139  attached to one end of the transfer roller shaft  56  of the transfer roller  31 . Since the insertion section  136  of the flange member  135  is pressed into the main drum body  33 , the flange member  135  is incapable of rotating relative to the main drum body  33 . 
     The drum gear  142  is fixed to the flange member  135 . A rotational drive force of the photosensitive drum  28  is transmitted to the drum gear  142 . The drum gear  142  is integrally provided with the bearing  143 , an input gear  144 , and a coupling portion  145 . The drum shaft  34  is inserted into the bearing  143 . The bearing  143  has an inner diameter substantially equal to the outer diameter of the drum shaft  34 , is formed in a tubular shape, and is fixed to the outer peripheral surface of the drum shaft  34 . The bearing  143  slightly protrudes from the outer surface of the coupling portion  145  toward the outside in the width direction W. 
     The input gear  144  is engaged with a drive transmission gear (not shown). The input gear  144  is formed in a cylindrical shape and provided with a plurality of outer teeth protruding outward in the radial direction thereof. The plurality of outer teeth are engaged with the drive transmission gear (not shown). The coupling portion  145  couples the bearing  143  and input gear  144 . The coupling portion  145  is integrally provided with a gear side coupling portion  146 . The gear side coupling portion  146  protrudes from the middle portion of the coupling portion  145  in the axial direction of the coupling portion  145  toward the flange member  135  and is coupled to the inner cylinder of the exposed portion  137  of the flange member  135 . 
     When a driving force is transmitted to the input gear  144  from the motor (not shown) provided in the main body casing  2  through a gear line (not shown), the photosensitive drum  28  is rotated together with the drum gear  142 . 
     The flange member  138  is fixed to the other end (right side end in the front view) of the main drum body  33  and integrally provided with a cylindrical insertion portion  140  and a press fitting portion  141 . The drum shaft  34  is inserted into the cylindrical insertion portion  140 . The press-insertion portion  141  has a U-shape cross-section, which protrudes outward in the radial direction of the drum shaft  34  from the middle of the insertion portion  140  and is bent toward the outside in the width direction W. Since the press fitting section  141  of the flange member  138  is pressed into the main drum body  33 , the flange member  138  is incapable of rotating relative to the main drum body  33 . 
     A spring receiving member  148  is provided on outer side of the flange member  138  in the width direction W and contacts the felt member  134 . A spring  147  is provided on the peripheral surface of the insertion portion  140 , and interposed between the spring receiving member  148  and the press fitting section  141 . 
     The spring receiving member  148  has a U-shape cross-section that is open toward the flange member  138 . The flange member  138  is urged by an elastic force of the spring  147  toward the flange member  135  in a state where the spring receiving member  148  contacts the felt member  134  disposed on the inner surface of the rear side wall  86 . As a result, the edge of the bearing  143  of the drum gear  142  contacts the inner surface of the rear side wall  86 , thereby positioning the photosensitive drum  28  in the axial (width) direction thereof. 
     As shown in  FIGS. 2 and 5 , the upper casing  78  is integrally provided with a top wall  93  having a substantially rectangular plate shape, two upper side walls  94  disposed at both widthwise ends of the top wall  93  and opposing each other across the width thereof, and an upper rear wall  95  disposed on the rear edge of the top wall  93 . This construction forms a substantially rectangular shape in a front view that is open on the bottom and on the front and rear sides. 
     The upper side walls  94  are bent downward from the widthwise edges of the upper rear wall  95  and includes fitting portions  149 . Fitting portions  149  protrude downward from lower edges of the upper side walls  94  and has a substantially square shape in a side view. A through-hole  116  is formed in each fitting portion  149  at positions opposite each other in the width direction W. The through-holes  116  accept the insertion of the drum shaft  34  of the photosensitive drum  28 . The upper rear wall  95  is bent diagonally downward and toward the rear side from the rear edge of the top wall  93 . The upper rear wall  95  forms an obtuse angle with the top wall  93 . 
     A charger support unit  96  is disposed in a front-to-rear midpoint of the top wall  93  for supporting the charger  29 . A beam injection part  97  is disposed on the top wall  93  in front of the charger support unit  96  for allowing passage of a laser beam emitted from the scanning unit  19 . A cleaning support part  98  is also provided on the top wall  93  and the upper rear wall  95  to the rear of the charger support unit  96  for supporting the cleaning member  32 . 
     As shown in  FIG. 2 , the charger support unit  96  includes a front holding plate  99  and a rear holding plate  100  disposed at a front-to-rear midpoint of the top wall  93 . The front holding plate  99  and rear holding plate  100  oppose each other in the front-to-rear direction with the grid  75  interposed therebetween. 
     The front holding plate  99  protrudes downward from an inner wall surface of the top wall  93  and extends in the width direction W of the top wall  93 . The front holding plate  99  abuts the grid  75  on the front side. 
     The rear holding plate  100  protrudes downward from the inner wall surface of the top wall  93  and extends in the width direction W of the top wall  93  parallel to the front holding plate  99 . The rear holding plate  100  abuts the grid  75  on the rear side thereof. The amount that the rear holding plate  100  protrudes from the top wall  93  is less than the protruding length of the front holding plate  99 . 
     Hence, with the grid  75  interposed between the front holding plate  99  and rear holding plate  100 , the front holding plate  99  and rear holding plate  100  grip the grid  75  in the front-to-rear direction. The discharge wire  74  spans between both upper side walls  94  and between the front holding plate  99  and rear holding plate  100 . 
     The beam injection part  97  includes a beam injection opening  101  formed in front of the charger support unit  96  and having a substantially elongated rectangular shape in a plan view, extending in the width direction W, and a front injection plate  102  and a rear injection plate  103  opposing each other in the front-to-rear direction with the beam injection opening  101  formed therebetween. 
     The front injection plate  102  protrudes at a slant downward and to the rear from the inner wall surface of the top wall  93  and extends in the width direction W of the top wall  93 . The front injection plate  102  is disposed on the front side of the beam injection opening  101 . 
     The rear injection plate  103  protrudes downward from the inner wall surface of the top wall  93  and extends in the width direction W of the top wall  93 . The rear injection plate  103  is disposed on the rear side of the beam injection opening  101 . Further, the rear injection plate  103  protrudes a shorter distance than the front injection plate  102 . 
     In a side view, the beam injection part  97  is substantially triangular-shaped, growing narrower in the downward direction. When the scanning unit  19  emits a laser beam, the laser beam enters the beam injection opening  101 , passing between the front injection plate  102  and rear injection plate  103 , and is scanned in a high speed over the surface of the photosensitive drum  28 . 
     Next, an assembling method of the drum cartridge  27  will be described with reference to  FIGS. 8 through 10 . 
     Firstly, the photosensitive drum  28  is mounted on between both the rear side walls  86  of the lower casing  77 , after the transfer roller  31  ( FIG. 2 ) is mounted on the transfer roller receiving portion  88  ( FIG. 6 ). At this time, the edge of the bearing  143  of the drum gear  142  attached to the photosensitive drum  28  is positioned between the pair of guide ribs  132  from above and the peripheral surface of the flange member  138  is positioned to contact the front restriction rib  152 . Then, the photosensitive drum  28  is pressed downward. The edge of the bearing  143  and the flange member  138  are moved downward while being guided by the pair of guide ribs  132  and the front restriction rib  152  respectively. Thus, the edge of the bearing  143  and the insertion portion  140  of the flange member  138  are lead to a position where the edge of the bearing  143  and the insertion portion  140  are aligned with the lower drum insertion holes  89  of both the rear side walls  86  in the width direction W respectively. When the edge of the bearing  143  and the insertion portion  140  of the flange member  138  are aligned with the lower drum insertion holes  89  of both the rear side walls  86  in the width direction W respectively, the restriction rib  133  and the flange member  138  contact the bearing  143  and the lower restriction rib  151  respectively, thereby preventing the photosensitive drum  28  from being moved downward further. 
     Next, as shown in  FIG. 8 , the upper casing  78  on which the charger  29  and the cleaning member  32  are mounted is assembled on the lower casing  77  on which the photosensitive drum  28  is mounted, in such a manner that the upper casing  78  covers the lower casing  77  from the upper side. At this time, the fitting portions  149  of both the upper side walls  94  of the upper casing  78  are fitted to the receiving grooves  131  formed on the outer surfaces of both the rear side walls  86  of the lower casing  77 . As a result, the upper casing  78  is lead to a predetermined position relative to the lower casing  77 , so that the through-holes  116  formed in the upper side walls  94  are aligned and communicate with the lower drum insertion holes  89  formed in the rear side wall  86  in the width direction W. 
     Subsequently, as shown in  FIG. 9 , the drum shaft  34  is inserted through the main drum body  33  of the photosensitive drum  28 , the through-holes  116 , and lower drum insertion holes  89 . In this way, as shown in  FIG. 10 , the upper casing  78  and lower casing  77  are attached via the drum shaft  34  and assembly work of the drum cartridge  27  is completed. 
     While the upper casing  78  is attached to the lower casing  77 , the support plate  66  contacts the positioning wall  92  provided on the receiving plate  90  of the lower casing  77  (see  FIG. 2 ), thereby fixing the position of the cleaning brush  65  relative to the photosensitive drum  28 . 
     As described above, the upper casing  78  and the lower casing  77  of the drum cartridge  27  are attached to each other by the drum shaft  34  of the photosensitive drum  28 . Accordingly, the charger  29  and cleaning member  32  supported by the upper casing  78  and transfer roller  31  supported by the lower casing  77  are arranged on the basis of the position of the drum shaft  34 , so that the relative position between the photosensitive drum  28  and the charger  29 , cleaning member  32 , and transfer roller  31  can be positioned with high accuracy. 
     The drum shaft  34  is inserted through the main drum body  33  of the photosensitive drum  28 , the through-holes  116 , and lower drum insertion holes  89  after the upper casing  78  is fitted to the lower casing  77 . Accordingly, the photosensitive drum  28  can be positioned relative to the lower casing  77 . Further, the charger  29 , cleaning member  32 , and transfer roller  31  disposed around the photosensitive drum  28  can be positioned relative to the lower casing  77  with high accuracy at a time. Therefore, a work for assembling the drum cartridge  27  can be reduced. 
     Further, by inserting the drum shaft  34  through the main drum body  33  of the photosensitive drum  28 , the through-holes  116 , and lower drum insertion holes  89 , the upper casing  78  and lower casing  77  can be attached via the drum shaft  34 . By drawing out the drum shaft  34  from the main drum body  33  of the photosensitive drum  28 , the through-holes  116 , the attaching between the upper casing  78  and lower casing  77  can be released (separated). Therefore, the work for assembling and separating the drum cartridge  27  can be reduced. 
     The pair of guide ribs  132  is provided on the inner surface of the one rear side wall  86  of the lower casing  77 . When the photosensitive drum  28  is mounted on the lower casing  77 , the pair of guide ribs  132  guides the edge of the bearing  143  of the drum gear  142  attached to the photosensitive drum  28  to the position where the edge of the bearing  143  overlaps the lower drum insertion hole  89  formed on the rear side wall  86  in the width direction W. Further, the front restriction rib  152  is provided on the inner surface of the other rear side wall  86  of the lower casing  77 . When the photosensitive drum  28  is mounted on the lower casing  77 , the front restriction rib  152  guides the insertion portion  140  of the flange member  138  attached to the photosensitive drum  28  to the position where the insertion portion  140  overlaps the lower drum insertion hole  89  formed on the rear side wall  86  in the width direction W. Therefore, the photosensitive drum  28  can smoothly be disposed between both the rear side walls  86 . Further, the guide ribs  132  and front restriction rib  152  guide the photosensitive drum  28  such that the bearing  143  of the drum gear  142  and the insertion portion  140  are aligned with the lower drum insertion holes  89  in the width direction W respectively. Accordingly, the drum shaft  34  can be smoothly inserted through the bearing  143 , the insertion portion  140 , and lower drum insertion holes  89  after the upper casing  78  is fitted to the lower casing  77 . 
     Further, the pair of guide ribs  132  and front restriction rib  152  are provided on the rear side walls  86 . Thus, as shown in  FIG. 11 , when a light-shielding paper  150  wound on the surface of the main drum body  33  for protecting the surface thereof is pulled for removal from the surface of the main drum body  33  after the photosensitive drum  28  is disposed between both the rear side walls  86 , the front side guide ribs  132  and the front restriction rib  152  prevent the photosensitive drum  28  from being moved to the front side, thereby preventing displacement of the position of the photosensitive drum  28 . 
     The receiving groove  131  is formed on the outer surface of each rear side wall  86  of the lower casing  77 . When the upper casing  78  is mounted on the lower casing  77 , the receiving groove  131  receives a fitting portion  149  of the upper side wall  94  of the upper casing  78 . Then, the upper casing  78  is lead to the predetermined position relative to the lower casing  77 , so that the through-holes  116  formed in the upper side walls  94  are aligned and communicate with the lower drum insertion holes  89  formed in the rear side wall  86  in the width direction W. Therefore, the upper casing  78  can smoothly be attached to the lower casing  78 . Further, the receiving portion  131  guides the upper casing  78  such that the through-holes  116  are aligned with the lower drum insertion holes  89  in the width direction W. Accordingly, the drum shaft  34  can be smoothly inserted through the through-holes  116  and the lower drum insertion holes  89  after the upper casing  78  is fitted to the lower casing  77 . 
     Further, each receiving groove  131  is formed on the outer surface of each rear side wall  86  and each fitting portion  149  of each upper side wall  94  of the upper casing  78  is provided opposite side of the photosensitive drum  28  with respect to each inner surface of the rear side walls  86  of the lower casing  77 , that is, provided outer side of the rear side wall  86  in the width direction W. This configuration prevents the fitting portions  149  of the upper side walls  94  from contacting the main drum body  33  or the drum gear  142  of the photosensitive drum  28  when the upper casing  78  is fitted to the lower casing  77 . 
     Further, the guide rib  132  and receiving groove  131  are both integrally provided on the rear side wall  86 , thereby reducing the number of parts and simplifying the structure of the lower casing  77 . 
     Further, in the process cartridge  20  including the drum cartridge  27  described above, the relative position between the photosensitive drum  28  and the components disposed around the photosensitive drum  28 , such as the charger  29 , cleaning member  32 , and transfer roller  31  can be positioned with high accuracy. Further, the work for assembling and separating the process cartridge  20  including the drum cartridge  27  can be reduced. 
     Further, the laser printer  1  including the drum cartridge  27  capable of positioning the relative position between the photosensitive drum  28  and the components disposed around the photosensitive drum  28 , such as the charger  29 , cleaning member  32 , and transfer roller  31  can achieve high-quality developer image formation, thereby achieving high-quality image formation on the paper  3 . 
     While the developer cartridge  30  can be mounted into or removed from the drum cartridge  27 , the developer cartridge  30  may be integrally formed with the drum cartridge  27  (i.e., the developer cartridge  30  cannot be mounted into or removed from the drum cartridge  27 ).