PATENT DOCUMENT

Abstract:
An image forming apparatus includes: a casing; a belt unit including a first roller, a second roller positioned higher than the first roller and a belt; image bearing members; developer containers; a removing member; an accommodating member; and a conveying member. The image bearing members includes a first image bearing member and a second image bearing member. The removing member is positioned adjacent to the second image bearing member and below an imaginary plane. The accommodating member accommodates the waste toner removed by the removing member, and is positioned adjacent to the first image bearing member and above the imaginary plane. The accommodating member is mounted in and removed from the casing along with a first developer container corresponding to the first image bearing member. The conveying member conveys the waste toner removed by the removing member to the accommodating member.

Full Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application Nos. 2012-285874 filed Dec. 27, 2012, 2012-285905 filed Dec. 27, 2012, and 2012-285932 filed Dec. 27, 2012. The entire content of each of the priority applications is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus employing an electrophotographic system. 
     BACKGROUND 
     One electrophotographic image forming apparatus known in the art (hereinafter referred to as a first conventional image forming apparatus) includes a plurality of photosensitive members on which electrostatic latent images are formed. One such image forming apparatus known in the art includes four process cartridges and an image-forming unit to support the process cartridges. The process cartridges are provided for each of four colors used in the image forming apparatus and each include a photosensitive drum. In this image forming apparatus, a waste toner box serving to accommodate waste toner therein is provided in the image-forming unit at a leading end portion (i.e. downstream end portion) thereof in a direction in which the image-forming unit is pulled out of the apparatus. This conventional image forming apparatus also includes a belt-waste-toner conveying tube for conveying waste toner from a belt-cleaning device to the waste toner box. 
     Further, one electrophotographic image forming apparatus known in the art (hereinafter referred to as a second conventional image forming apparatus) is a printer including a device body that accommodates a sheet-feeding unit for supplying sheets of paper, and an image-forming unit for forming images on the sheets supplied by the sheet-feeding unit. The image-forming unit further includes a plurality of process cartridges arranged at intervals in a front-rear direction and respectively supporting photosensitive drums, and a transfer unit for transferring toner images formed on the photosensitive drums onto the sheets supplied from the sheet-feeding unit. One such color printer that has been proposed provides the sheet-feeding unit with conveying rollers for supplying the sheets to the image-forming unit, and provides the transfer unit with a conveying belt positioned in confrontation with the plurality of photosensitive drums for conveying the sheets. In this conventional color printer, the conveying rollers are disposed apart from and obliquely above and forward of the transfer unit so as to overlap the transfer unit when projected in the front-rear direction. 
     Further, one electrophotographic image forming apparatus known in the art (hereinafter referred to as a third conventional image forming apparatus) includes a plurality of photosensitive members on which electrostatic latent images are formed, and a plurality of developing units that supply developer to corresponding photosensitive members. One such image forming apparatus has a plurality of process cartridges corresponding to colors used in the image forming apparatus. Each process cartridge includes a photosensitive drum, a developing roller disposed on the upper rear side of the photosensitive drum, and a supply roller disposed on the upper rear side of the developing roller. The space in the process cartridge above the developing roller and the supply roller serves as a toner-accommodating chamber for accommodating toner. 
     SUMMARY 
     In the first conventional image forming apparatus described above, maintenance is performed separately on the waste toner box and the process cartridges. Here, there has been a proposal to provide the waste toner box integrally with one of the process cartridges so that maintenance could be performed on the waste toner box at the same time maintenance is performed on the process cartridge. 
     However, when the waste toner box is provided integrally with the process cartridge, the waste toner box is separated vertically from the belt-cleaning device, making it difficult to convey toner upward toward the waste toner box. A particular problem with this structure is that more image forming apparatus in recent years are employing spherical toner particles having excellent fluidity, such as polymer toner, in an effort to improve image quality. However, spherical toner has a low angle of repose and, thus, is difficult to convey upward. 
     In view of the foregoing, it is an object of the present invention to provide an image forming apparatus including an accommodating member that is mounted in and removed from the image forming apparatus integrally with a developer container and that can facilitate conveyance of deposited matter removed by a removing member to the accommodating member. 
     With the second conventional image forming apparatus described above, it is sometimes necessary to remove the transfer unit from the device body in order to perform maintenance on the transfer unit, such as replacing the conveying belt. However, since the conveying rollers in the sheet-feeding unit overlap the transfer unit when projected in the front-rear direction, the conveying rollers obstruct the transfer unit when the transfer unit is pulled forward out of the device body. Consequently, operations for mounting and removing the transfer unit relative to the device body are made more complex, making maintenance operation on the transfer unit complex. 
     In view of the foregoing, it is another object of the present invention to provide an image forming apparatus capable of facilitating smooth operations for mounting and removing a belt unit relative to a casing and thereby capable of facilitating maintenance operations on the belt unit. 
     In the third conventional image forming apparatus described above, toner supplied onto the developing roller tends to accumulate by its own weight near the region of contact between the developing roller and the supply roller. Consequently, the toner applies pressure to a thickness-regulating blade, sealing portions, and other members provided around the developing roller and can compromise the seal around the developing roller. If the periphery of the developing roller is insufficiently sealed, toner may leak out around the developing roller. 
     In view of the foregoing, it is still another object of the present invention to provide an image forming apparatus capable of restraining developer from leaking out around a developer carrying member. 
     In order to attain the above and other objects, the present invention provides an image forming apparatus including: a casing; a belt unit; a plurality of image bearing members; a plurality of developer containers; a removing member; an accommodating member; and a conveying member. The belt unit is configured to be accommodated in the casing. The belt unit including: a first roller; a second roller; and a belt. The first roller has a first rotation axis. The second roller has a second rotation axis parallel to the first rotation axis. The second roller is disposed spaced apart from the first roller and positioned higher than the first roller. The belt is looped around the first roller and the second roller and has an upper portion. The plurality of image bearing members is each configured to bear a developer image thereon. The plurality of image bearing members is arranged juxtaposed with each other and spaced apart from each other in an orthogonal direction that is orthogonal to the first and second rotation axes and that is oriented in a direction from the first roller to the second roller. The plurality of image bearing members each has a bottom portion contacting the upper portion of the belt. The plurality of image bearing members includes a first image bearing member positioned closer to the first roller than remaining image bearing members to the first roller and a second image bearing member positioned closer to the second roller than remaining image bearing members to the second roller. The plurality of developer containers is disposed in one-to-one correspondence with the plurality of image bearing members. The plurality of developer containers is each configured to accommodate developer therein and configured to be detached from and attached to the casing. The plurality of developer containers includes a first developer container corresponding to the first image bearing member. The removing member is configured to remove a waste toner deposited on the belt. The removing member is positioned adjacent to the second image bearing member and below an imaginary plane containing a top surface of the upper portion of the belt. The accommodating member is configured to accommodate the waste toner removed by the removing member. The accommodating member is positioned adjacent to the first image bearing member and above the imaginary plane. The accommodating member is configured to be mounted in and removed from the casing along with the first developer container. The conveying member is configured to convey the waste toner removed by the removing member to the accommodating member. 
     Specifically, in the first embodiment, the first roller corresponds to a drive roller and the second roller corresponds to a follow roller, and in the second embodiment, the first roller corresponds to a follow roller and the second roller corresponds to a drive roller. Further, in the first embodiment, the first image bearing member corresponds to a rearmost image bearing member and the second image bearing member corresponds to a frontmost image bearing member, and in the second embodiment, the first image bearing member corresponds to a frontmost image bearing member and the second image bearing member corresponds to a rearmost image bearing member. Further, in the first embodiment, the orthogonal direction corresponds to a direction from a lower rear side to an upper front side of a casing, and in the second embodiment, the orthogonal direction corresponds to a direction from a lower front side to an upper rear side of a casing. 
     According to another aspect, the present invention provides an image forming apparatus including: a casing; a plurality of photosensitive members; and a belt unit. The plurality of photosensitive members is arranged juxtaposed with each other in the casing in a juxtaposed direction and is each configured to transfer a developer image on a recording medium. The belt unit includes: a belt confronting the plurality of photosensitive members; and a pair of conveying rollers configured to convey a recording medium to a position between the plurality of photosensitive members and the belt. The pair of conveying rollers is disposed in proximity to each other. The belt unit is configured to move relative to the casing between a mounted position in which the belt unit is mounted in the casing and a removed position in which the belt unit is removed from the casing. 
     According to still another aspect, the present invention provides an image forming apparatus including: a casing; a belt unit; a plurality of image bearing members; a plurality of developer carrying members; and a plurality of supply members. The belt unit is configured to be accommodated in the casing. The belt unit including: a first roller; a second roller; and a belt. The first roller has a first rotation axis. The second roller has a second rotation axis parallel to the first rotation axis. The second roller is disposed spaced apart from the first roller and positioned higher than the first roller. The belt is looped around the first roller and the second roller, and has an upper portion. The plurality of image bearing members is each configured to bear an electrostatic latent image thereon. The plurality of image bearing members is arranged juxtaposed with each other and spaced apart from each other in an orthogonal direction that is orthogonal to the first and second rotation axes and that is oriented in a direction from the first roller to the second roller. The plurality of image bearing members each has a bottom portion contacting the upper portion of the belt. The plurality of image bearing members each has a third rotation axis and is each configured to rotate about the third rotation axis. The plurality of developer carrying members is disposed in one-to-one correspondence with the plurality of image bearing members. The plurality of developer carrying members is each configured to carry developer thereon and each contacts the corresponding image bearing member. The plurality of developer carrying members each has a fourth rotation axis parallel to the third rotation axis and is each configured to rotate about the fourth rotation axis. The fourth rotation axis is positioned upstream of the third rotation axis in the orthogonal direction. The plurality of supply members is disposed in one-to one correspondence with the plurality of developer carrying members. The plurality of supply members is each configured to supply developer to the corresponding developer carrying member and each contacts the corresponding developer carrying member. The plurality of supply members each has a fifth rotation axis parallel to the third rotation axis and is each configured to rotate about the fifth rotation axis. The fifth rotation axis is positioned upstream of the fourth rotation axis in the orthogonal direction and positioned lower than the fourth rotation axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings; 
         FIG. 1  is a schematic center cross-sectional view of a printer according to a first embodiment of the present invention; 
         FIG. 2  is a left side view of a process unit and a transfer unit shown in  FIG. 1 ; 
         FIG. 3A  is a cross-sectional view of the process unit and the transfer unit taken along a line A-A in  FIG. 2 ; 
         FIG. 3B  is a cross-sectional view of the process unit and the transfer unit taken along a line B-B in  FIG. 2 ; 
         FIG. 4A  is a left side view of the transfer unit shown in  FIG. 1 , showing a state where a frame-side shutter is in a closed position; 
         FIG. 4B  is a left side view of the transfer unit shown in  FIG. 1 , showing a state where the frame-side shutter is in an open position; 
         FIG. 5  is an explanatory diagram for performing maintenance operations on a developer cartridge of the printer, in which a front cover is open and the frame-side shutter is in the closed position; 
         FIG. 6  is an explanatory diagram, following  FIG. 5 , for performing the maintenance operations on the developer cartridge, in which a cleaner-side fitting part of a conveying tube is separated above a coupling portion of the transfer unit; 
         FIG. 7  is an explanatory diagram, following  FIG. 6 , for performing the maintenance operations on the developer cartridge, in which the process unit is in a withdrawn position; 
         FIG. 8  is an explanatory diagram for performing maintenance operations on the transfer unit of the printer, in which both the process unit and the transfer unit have been removed from a main casing; 
         FIG. 9  is a schematic center cross-sectional view of a printer according to a variation of the first embodiment of the present invention; 
         FIG. 10A  is a left side view of a transfer unit shown in  FIG. 9 , showing a state where a frame-side shutter is in a closed position; 
         FIG. 10B  is a left side view of the transfer unit shown in  FIG. 9 , showing a state where the frame-side shutter is in an open position; 
         FIG. 11  is a schematic center cross-sectional view of a printer according to a second embodiment of the present invention; 
         FIG. 12  is an explanatory diagram for conveyance of deposited matter from a belt-cleaning roller shown in  FIG. 11  to an accommodating portion; 
         FIG. 13  is a cross-sectional view of the process unit and the transfer unit shown in  FIG. 12  taken along a line C-C, showing a state where a conveying tube and the accommodating portion are coupled together; 
         FIG. 14  is a cross-sectional view of the process unit and the transfer unit shown in  FIG. 12  taken along the line C-C, showing a state where the conveying tube and the accommodating portion are uncoupled; 
         FIG. 15  is a cross-sectional view of the process unit and the transfer unit shown in  FIG. 12  taken along a line D-D; 
         FIG. 16  is an explanatory diagram for performing maintenance operations on a developer cartridge, in which the process unit is in a withdrawn position; 
         FIG. 17  is an explanatory diagram for performing maintenance operations on the transfer unit, in which both the process unit and the transfer unit have been removed from a main casing; and 
         FIG. 18  is an explanatory diagram for a variation of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     1. Overall Structure of Printer 
     Next, an overall structure of a printer as an image forming apparatus according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 8 . 
     As shown in  FIG. 1 , the printer  1  is a horizontal direct tandem-type color laser printer. The printer  1  has a main casing  2 . The main casing  2  is box-shaped and generally rectangular in a side view. 
     In the following description, the terms “upward”, “downward”, “upper”, “lower”, “above”, “below”, “beneath”, “right”, “left”, “front”, “rear” and the like will be used assuming that the printer  1  is disposed in an orientation in which it is intended to be used. That is, directions related to the printer  1  will be given based on the state of the printer  1  when the printer  1  is resting on a level surface. More specifically, in  FIG. 1 , a right side and a left side are a front side and a rear side, respectively. Further, a left side and a right side of the printer  1  will be based on the perspective of a user facing the front of the printer  1 . Hence, in  FIG. 1 , a near side and a far side are a left side and a right side, respectively. 
     A front cover  5  is provided on a front side of the main casing  2 . The front cover  5  can be opened in order to mount or remove a process unit  16  described later. Within the main casing  2 , the printer  1  is provided with a sheet-feeding unit  3  for feeding sheets P of paper to be printed, and an image-forming unit  4  for forming images on the sheets P fed by the sheet-feeding unit  3 . 
     (1) Sheet-Feeding Unit 
     The sheet-feeding unit  3  includes a paper tray  6  that accommodates the sheets P therein. The paper tray  6  is detachably mounted in a bottom section of the main casing  2 . 
     The sheet-feeding unit  3  also includes a pickup roller  7  disposed above a front end portion of the paper tray  6 , a feeding roller  8  disposed forward of the pickup roller  7 , a feeding pad  9  disposed below and in confrontation with the feeding roller  8 , and a guide roller  10  disposed in contact with a front portion of the feeding roller  8 . 
     As indicated by a solid line in  FIG. 1 , the pickup roller  7  rotates to convey the sheets P from the paper tray  6  to a position between the feeding roller  8  and the feeding pad  9 , and the rotation of the feeding roller  8  separates and feeds the sheets P one at a time. As the feeding roller  8  continues to rotate, each separated sheet P subsequently passes between the feeding roller  8  and the guide roller  10  and is supplied toward a pair of registration rollers  29  described later. The sheet-feeding unit  3  defines a path, serving as a first sheet conveying path, for conveying the sheets P from the paper tray  6  to the pair of registration rollers  29 . 
     The sheet-feeding unit  3  also includes a manual-feed tray  11  provided at the front cover  5 . The manual-feed tray  11  is pivotally movable to open and close a manual-feed insertion opening  12  formed in the front cover  5 . The manual-feed tray  11  in the open state is delineated by a two-dotted dashed line in  FIG. 1 . As indicated by a narrow dashed line in  FIG. 1 , the sheet-feeding unit  3  also defines a manual-feed path, serving as a second sheet conveying path, for conveying the sheets P from the manual-feed tray  11  to the pair of registration rollers  29 . 
     The sheet-feeding unit  3  also includes a manual-feed roller  13  disposed in confrontation with and on the rear side of the manual-feed insertion opening  12 , and a manual-feed pad  14  disposed in confrontation with and below the manual-feed roller  13 . When the sheets P are placed on the manual-feed tray  11 , leading edges of the sheets P are inserted through the manual-feed insertion opening  12  to confront a contacting part of the manual-feed roller  13  and the manual-feed pad  14 . 
     By rotating, the manual-feed roller  13  separates and feeds the sheets P resting on the manual-feed tray  11  one at a time toward the pair of registration rollers  29  described later. 
     (2) Image-Forming Unit 
     The image-forming unit  4  includes a scanning unit  15 , the process unit  16 , a transfer unit  17 , and a fixing unit  18 . 
     (2-1) Scanning Unit 
     The scanning unit  15  is disposed in a top section of the main casing  2 . The scanning unit  15  is adapted to emit laser beams toward respective photosensitive drums  21  (described later) based on image data, thereby exposing the photosensitive drums  21 . The paths of the laser beams are depicted using solid lines in  FIG. 1 . 
     (2-2) Process Unit 
     The process unit  16  is disposed beneath the scanning unit  15  and above the transfer unit  17 . The process unit  16  is inclined relative to the horizontal so that its front end is positioned higher than its rear end. The process unit  16  includes a process frame  19  and a plurality of developer cartridges  20  (four in the embodiment) corresponding to the four colors used in image formation. 
     The process frame  19  is movable relative to the main casing  2  in a juxtaposed direction X (described later) between an accommodated position (see  FIG. 1 ) and a withdrawn position (see  FIG. 7 ). In the accommodated position, the process frame  19  is accommodated inside the main casing  2 . In the withdrawn position, the process frame  19  is withdrawn to an outside of the main casing  2 . The process frame  19  can be removed from the main casing  2  in the withdrawn position. 
     The process frame  19  retains a plurality of photosensitive drums  21  (four in the embodiment), a plurality of scorotron chargers  22  (four in the embodiment), and a plurality of drum-cleaning rollers  23  (four in the embodiment). 
     The plurality of photosensitive drums  21  is disposed along a direction sloping downward toward the rear, that is, along the inclination of the process unit  16 . More specifically, the plurality of photosensitive drums  21  is arranged juxtaposed with each other and spaced at intervals along the juxtaposed direction X. The juxtaposed direction X includes a direction sloping downward toward the rear and a direction sloping upward toward the front. The plurality of photosensitive drums  21  is provided for the four colors of cyan, magenta, yellow, and black and is arranged at intervals in this order from the lower rear side to the upper front side of the main casing  2 . Each photosensitive drum  21  is generally cylindrical in shape and oriented with its axis in a left-right direction. That is, each photosensitive drum  21  is elongated in the left-right direction. 
     The plurality of scorotron chargers  22  is provided in one-to-one correspondence with the plurality of photosensitive drums  21 . Each scorotron charger  22  is positioned on the rear side of the corresponding photosensitive drum  21 , and also positioned to confront but not contact the corresponding photosensitive drum  21 . 
     The plurality of drum-cleaning rollers  23  is provided in one-to-one correspondence with the plurality of photosensitive drums  21 . Each drum-cleaning roller  23  is positioned so as to contact the corresponding photosensitive drum  21  from the lower rear side thereof. A prescribed drum-cleaning bias is applied to each drum-cleaning roller  23 . During an image-forming operation described later, the drum-cleaning rollers  23  electrostatically attract and temporarily retain toner particles and paper dust deposited on the surfaces of the photosensitive drums  21  after toner images have been transferred from the photosensitive drums  21  onto the sheet P. 
     The plurality of developer cartridges  20  is provided in one-to-one correspondence with the plurality of photosensitive drums  21 . Each developer cartridge  20  is detachably retained in the process frame  19  at a position obliquely above and forward of the corresponding photosensitive drum  21 . The plurality of developer cartridges  20  is provided for the four colors of cyan, magenta, yellow, and black, and is arranged in this order from the lower rear side to the upper front side. Each developer cartridge  20  has a box-like shape and is elongated in the left-right direction. 
     Each developer cartridge  20  includes a developing roller  24 . 
     The developing roller  24  is rotatably supported in a lower end portion of the developer cartridge  20 . The developing roller  24  is exposed in the lower rear side of the developer cartridge  20  and contacts the corresponding photosensitive drum  21  from the upper front side thereof. 
     Each developer cartridge  20  also includes a supply roller  25  that contacts the upper front side of the developing roller  24 , and a thickness-regulating blade  26  that contacts the top of the developing roller  24 . Each developer cartridge  20  defines an internal space formed above the supply roller  25  and the thickness-regulating blade  26  for accommodating toner therein. 
     (2-3) Transfer Unit 
     The transfer unit  17  is disposed in the main casing  2  at a position above the paper tray  6  and beneath the process unit  16 . 
     The transfer unit  17  is movable relative to the main casing  2  between a mounted position (see  FIG. 1 ) and a removed position (see  FIG. 8 ). In the mounted position, the transfer unit  17  is mounted in the main casing  2 . In the removed position, the transfer unit  17  has been removed from the main casing  2 . 
     In the mounted position, the transfer unit  17  is inclined relative to the horizontal so that its front end is positioned higher than its rear end. More specifically, the transfer unit  17  is inclined along the juxtaposed direction X. 
     The transfer unit  17  includes a registration portion  27 , and a transfer portion  28 . 
     The registration portion  27  is disposed at a front end portion of the transfer unit  17 . The registration portion  27  includes the pair of registration rollers  29 , and a paper-dust reservoir  30 . 
     The pair of registration rollers  29  opposes each other in the front-rear direction. A drive force is inputted into the rear registration roller  29  from a drive source (not shown) provided in the main casing  2 . The front registration roller  29  follows the rotation of the rear registration roller  29  and functions for removing paper dust from the surfaces of the sheets P. 
     The paper-dust reservoir  30  is disposed on the front side of the pair of registration rollers  29 . The paper-dust reservoir  30  has a box-like shape that is open on a rear end thereof. A paper-dust scraping member  35  and a grip portion  98  are provided at the paper-dust reservoir  30 . 
     The paper-dust scraping member  35  is disposed in a rear end portion of the paper-dust reservoir  30  and contacts the front side of the front registration roller  29 . The paper-dust scraping member  35  scrapes off paper dust that has been deposited on the front registration roller  29 . The paper dust scraped off the front registration roller  29  by the paper-dust scraping member  35  is collected inside the paper-dust reservoir  30 . 
     The grip portion  98  is formed on a top portion of a front end portion of the paper-dust reservoir  30  and protrudes forward from the front end portion of the paper-dust reservoir  30 . The grip portion  98  has a rectangular parallelepiped shape that is elongated in the left-right direction and substantially rectangular in a side view. In other words, the grip portion  98  is provided at the front end portion of the transfer unit  17 . 
     The transfer portion  28  includes a drive roller  31 , a follow roller  32 , a conveying belt  33 , and a plurality of transfer rollers  34  (four in the embodiment). 
     The drive roller  31  is disposed at a rear end portion of the transfer unit  17 . The drive roller  31  is generally cylindrical in shape and is elongated in the left-right direction. The drive roller  31  is capable of rotating about a center axis A 1  when a drive force is inputted from a drive source (not shown) provided in the main casing  2 . That is, the center axis A 1  extends in the left-right direction. 
     The follow roller  32  is disposed at the front end portion of the transfer unit  17 . More specifically, the follow roller  32  is disposed on the rear side of the pair of registration rollers  29  and positioned higher than the drive roller  31 . The follow roller  32  has a generally cylindrical shape and is elongated in the left-right direction. The follow roller  32  can rotate about a center axis A 2 . The center axis A 2  of the follow roller  32  is parallel to the center axis A 1  of the drive roller  31 . 
     The conveying belt  33  is looped around the drive roller  31  and the follow roller  32 , with an upper portion of the conveying belt  33  in contact with the bottom sides of the photosensitive drums  21 . In other words, the photosensitive drums  21  contact the conveying belt  33  from above. 
     The conveying belt  33  is disposed on the rear side of the pair of registration rollers  29  so as to overlap the pair of registration rollers  29  when projected in the juxtaposed direction X. 
     When the drive roller  31  is driven to rotate, the conveying belt  33  circulates so that its upper portion moves along a downward slope toward the rear (i.e. from the upper front side to the lower rear side), and the follow roller  32  rotates along with the circulating movement of the conveying belt  33 . 
     The plurality of transfer rollers  34  is provided in one-to-one correspondence with the plurality of photosensitive drums  21 . Each transfer roller  34  is disposed in confrontation with the corresponding photosensitive drum  21 , with the upper portion of the conveying belt  33  interposed between the top of the transfer roller  34  and the bottom of the corresponding photosensitive drum  21 . 
     (2-4) Fixing Unit 
     The fixing unit  18  is disposed on the rear side of the transfer unit  17 . The fixing unit  18  includes a heating roller  36 , and a pressure roller  37  that contacts the heating roller  36  with pressure from below. 
     (3) Image Forming Operation 
     Toner in each of the developer cartridges  20  is supplied onto the supply roller  25 , and the rotating supply roller  25  supplies the toner onto the developing roller  24  while the toner is tribocharged between the supply roller  25  and the developing roller  24 . 
     The thickness-regulating blade  26  regulates the thickness of the toner supplied to the developing roller  24  as the developing roller  24  rotates, maintaining the toner carried on the surface of the developing roller  24  at a thin uniform thickness. 
     In the meantime, the scorotron charger  22  applies a uniform charge to the surface of the corresponding photosensitive drum  21 . Subsequently, the photosensitive drum  21  is exposed to the laser beam emitted from the scanning unit  15 , forming an electrostatic latent image on the surface of the photosensitive drum  21  based on image data. 
     The toner carried on the developing roller  24  is then supplied to the electrostatic latent image formed on the surface of the photosensitive drum  21  to form a toner image, that is, a developer image, thereon. 
     Also during this time, the sheet P is supplied from the paper tray  6  to the pair of registration rollers  29 . The front registration roller  29  rotates to convey the sheet P to a position between the conveying belt  33  and the frontmost photosensitive drum  21  at a prescribed timing. 
     The conveying belt  33  subsequently conveys the sheet P rearward so that the sheet P passes sequentially between each photosensitive drum  21  and the corresponding transfer roller  34 . At this time, toner images carried on the photosensitive drums  21  are transferred to a first surface of the sheet P to form an image thereon. 
     Next, the sheet P is subjected to heat and pressure while passing between the heating roller  36  and the pressure roller  37  of the fixing unit  18 , thereby thermally fixing the image onto the first surface of the sheet P. 
     Subsequently, the sheet P is conveyed along a U-shaped path that changes the conveying direction of the sheet P from a rearward direction to a direction curved diagonally upward and forward. Discharge rollers  38  discharge the sheet P onto a discharge tray  39  formed on a top surface of the main casing  2 . 
     (4) Reversing Path 
     A reversing path  40  is defined in the main casing  2 , as indicated by a bold dashed line in  FIG. 1 . The reversing path  40  guides the sheet P from the fixing unit  18  back to the pair of registration rollers  29  after a toner image has been fixed on the first surface of the sheet P. The reversing path  40  serves as a third sheet conveying path. 
     The reversing path  40  extends vertically downward on the rear side of the fixing unit  18 , then extends in the front-rear direction forward toward the pair of registration rollers  29  above the paper tray  6  and beneath the fixing unit  18  and the transfer unit  17 . 
     In other words, the reversing path  40  is disposed opposite to the plurality of photosensitive drums  21  relative to the transfer unit  17 . 
     After the toner image has been fixed on the first surface of the sheet P, the discharge rollers  38  are driven in a reverse rotation prior to discharging the sheet P onto the discharge tray  39  in order to introduce the sheet P onto the reversing path  40 . 
     Subsequently, the sheet P is conveyed along the reversing path  40  from the rear side to the front side, with the first surface on which the toner image has been fixed facing upward. When the sheet P arrives at the pair of registration rollers  29 , the pair of registration rollers  29  supplies the sheet P to a position between the frontmost photosensitive drum  21  and the conveying belt  33  with a second surface of the sheet P opposite the first surface on which the toner image has been fixed facing upward. 
     Then, the sheet P is sequentially conveyed to positions between the plurality of photosensitive drums  21  and the plurality of transfer rollers  34 , as described above, at which time toner images are transferred from the photosensitive drums  21  onto the second surface of the sheet P to form an image thereon. The image formed on the second surface of the sheet P is thermally fixed to the second surface as the sheet P passes between the heating roller  36  and the pressure roller  37 . 
     2. Detailed Description of Transfer Unit 
     As shown in  FIGS. 3A and 4 , the transfer unit  17  also includes a transfer frame  41 , and a belt cleaner  42 . 
     (1) Transfer Frame 
     As shown in  FIG. 1 , the transfer frame  41  has a frame-like configuration with a closed bottom and an open top. 
     The transfer portion  28  described above is accommodated inside the transfer frame  41 . The registration portion  27  is integrally provided at the front end portion of the transfer frame  41 . 
     Accordingly, the bottom wall of the transfer frame  41  is positioned in confrontation with the lower portion of the conveying belt  33  of the transfer portion  28  so as to cover the lower portion of the conveying belt  33 . Further, when the transfer unit  17  is disposed in its mounted position, the bottom wall of the transfer frame  41  also defines the top of the front portion of the reversing path  40  described above. Hence, the reversing path  40  is adjacent to the lower portion of the conveying belt  33  in the vertical direction when the transfer unit  17  is disposed in its mounted position. 
     As shown in  FIG. 3A , the transfer frame  41  includes a coupling portion  43 . 
     The coupling portion  43  is provided on a left wall of the transfer frame  41  at a front end portion thereof. The coupling portion  43  has a generally cylindrical shape that extends in the left-right direction so as to penetrate the left wall of the transfer frame  41  and is closed on a left end thereof. The coupling portion  43  includes a support part  44 , a communication hole  45 , and a frame-side shutter  46 . 
     The support part  44  has a generally arcuate shape in a side view (see  FIGS. 4A ,  4 B) and follows a circumferential edge of the coupling portion  43 . The support part  44  protrudes leftward from a lower edge on the left wall of the coupling portion  43 . 
     The communication hole  45  is formed in the left wall of the coupling portion  43  at a lower end portion but above the support part  44 . The communication hole  45  has a sector shape in a side view (see  FIG. 4A ) and penetrates the left wall of the coupling portion  43 . 
     The frame-side shutter  46  is angularly rotatably supported on the left side of the left wall of the coupling portion  43  at a position above the support part  44 . The frame-side shutter  46  has a generally disc shape with substantial thickness in the left-right direction. The frame-side shutter  46  includes an operating part  47 , and an opening  48 . 
     The operating part  47  is provided at a top edge of the frame-side shutter  46  when the frame-side shutter  46  is in an open position described later (see  FIG. 4B ). The operating part  47  has an arcuate shape that follows a circumferential edge of the frame-side shutter  46 . The operating part  47  protrudes leftward from the top edge of the frame-side shutter  46 . A grip part  49  is provided at a rear end portion of the operating part  47  when the frame-side shutter  46  is in the open position described later. 
     The grip part  49  has a generally flat plate shape and protrudes outward in a radial direction of the frame-side shutter  46  from the rear end portion of the operating part  47 . 
     When the frame-side shutter  46  is in the open position described later, the opening  48  is positioned at a lower end portion of the frame-side shutter  46  at a position overlapping the communication hole  45 . The opening  48  has a sector shape in a side view (see  FIG. 4B ) and penetrates the frame-side shutter  46  in the left-right direction. 
     The frame-side shutter  46  is angularly rotatably movable between a closed position (see  FIG. 4A ) and an open position (see  FIG. 4B ). In the closed position, the opening  48  in the frame-side shutter  46  is disposed above the communication hole  45  so as not to be aligned with the communication hole  45 . In the open position, the opening  48  is aligned with the communication hole  45 . 
     (2) Belt Cleaner 
     As shown in  FIGS. 1 and 3A , the belt cleaner  42  is accommodated in the front end portion of the transfer frame  41  at a position rearward of the pair of registration rollers  29 . The belt cleaner  42  includes a cleaner frame  51 , a cleaning blade  52 , and a first screw  53 . 
     The cleaner frame  51  is formed in a generally cylindrical shape that is closed on both left and right ends thereof and is open on a lower rear end thereof. More specifically, the cleaner frame  51  integrally has a side wall  54 , a main body  55 , and a top wall  56 . 
     The side wall  54  has a flat plate shape and is generally rectangular in a side view. The side wall  54  is disposed on the right side of the cleaner frame  51 . 
     The main body  55  extends leftward from a front end portion of the side wall  54 . The main body  55  is generally columnar in shape, with a curved front end portion that gives the main body  55  a generally D-shaped cross-section. The main body  55  includes a recovery chamber  57 , and a fitting part  58 . 
     The recovery chamber  57  is a recessed groove elongated in the left-right direction. More specifically, the recovery chamber  57  is recessed frontward from a rear edge of the main body  55 . A cross-section of the main body  55  is generally U-shaped and open on a rear end thereof. 
     The fitting part  58  is generally cylindrical in shape and extends leftward from a left end portion of the main body  55 . The fitting part  58  has a right end portion in communication with a left end portion of the recovery chamber  57 . The fitting part  58  has an outer diameter smaller than an inner diameter of the coupling portion  43  of the transfer frame  41 . The fitting part  58  also includes a sealing member  59 . 
     The sealing member  59  has a generally flat plate shape and is formed of a resilient material, such as a sponge. The sealing member  59  is wrapped around the fitting part  58 , covering an outer circumferential surface of the fitting part  58 . 
     The top wall  56  has a generally flat plate shape and extends rearward from a top edge of the main body  55 . The top wall  56  has a rear edge extending to a position farther forward than the frontmost transfer roller  34 . 
     The cleaning blade  52  is provided at a rear edge of a top portion of the main body  55 . The cleaning blade  52  has a generally flat plate shape and is elongated in the left-right direction, with substantial thickness in the front-rear direction. The cleaning blade  52  has an upper half portion fixed to a top peripheral edge of the main body  55  defining the recovery chamber  57  and a lower half portion confronting an upper half portion of the recovery chamber  57 . 
     The first screw  53  is a left-handed auger screw extending in the left-right direction and aligned with a center axis of the fitting part  58 . The first screw  53  has a right end portion that is rotatably supported at the side wall  54  of the belt cleaner  42 . The first screw  53  includes a first screw drive gear  60 . 
     The first screw drive gear  60  is supported at the right end portion of the first screw  53  on the right side of the side wall  54  and is non-rotatable relative to the first screw  53 . A drive force is inputted into the first screw drive gear  60  from the main casing  2  through a gear train (not shown). 
     The belt cleaner  42  is supported at the transfer frame  41  by fitting the fitting part  58  into the coupling portion  43  of the transfer frame  41  from the right side thereof. In this state, the sealing member  59  is interposed between the fitting part  58  and the coupling portion  43  in a compressed state. 
     The cleaning blade  52  has a bottom edge confronting the front side of the follow roller  32  with the conveying belt  33  interposed therebetween. Thus, the bottom edge of the cleaning blade  52  contacts the front side of the conveying belt  33  looped about the follow roller  32 . The cleaning blade  52  is also positioned near the lower front side of the frontmost photosensitive drum  21 . The cleaning blade  52  is also positioned below an imaginary plane I containing a top surface of the upper portion of the conveying belt  33 . 
     3. Detailed Description of Process Unit 
     (1) Process Frame 
     As shown in  FIGS. 1 ,  2 ,  3 A and  3 B, the process frame  19  has a frame-like configuration that is generally rectangular in a plan view. The process frame  19  includes a pair of side walls  71 , a front beam  72 , and a rear beam  73 . Hereinafter, the side wall  71  on the left side will be referred to as a left side wall  71 L and the side wall  71  on the right side will be referred to as the right side wall  71 R, when it is necessary to distinguish between the two. 
     The pair of side walls  71  is each generally rectangular in a side view (see  FIG. 2 ) and elongated in the front-rear direction. The pair of side walls  71  is arranged parallel to each other and separated in the left-right direction. As described above, the plurality of photosensitive drums  21 , the plurality of scorotron chargers  22 , and the plurality of drum-cleaning rollers  23  are supported between the pair of side walls  71 . The left side wall  71 L also includes an intermediary conveying portion  74  (see  FIGS. 3A and 3B ). 
     The intermediary conveying portion  74  is provided at a rear end portion of the left side wall  71 L at approximately a vertical center thereof. The intermediary conveying portion  74  has a generally cylindrical shape that is closed on a right end thereof and extends in the left-right direction, penetrating the left side wall  71 L. The intermediary conveying portion  74  includes a supply hole  75 , and a relay screw  76 . 
     The supply hole  75  is formed so as to penetrate a lower end portion of the intermediary conveying portion  74  at a right end portion thereof. 
     The relay screw  76  is a left-handed auger screw that extends in the left-right direction along a center axis of the intermediary conveying portion  74 . A right end portion of a rotational shaft in the relay screw  76  is rotatably supported at a right wall of the intermediary conveying portion  74 . The relay screw  76  also includes an engaging protrusion  77 . 
     The engaging protrusion  77  extends rightward from the right end portion of the relay screw  76  and protrudes in a radial direction of the relay screw  76 . 
     The front beam  72  bridges front end portions of the pair of side walls  71 . The front beam  72  has a generally flat plate shape and is elongated in the left-right direction, with substantial thickness in the front-rear direction. 
     The rear beam  73  bridges rear end portions of the pair of side walls  71 . The rear beam  73  has a generally flat plate shape and is elongated in the left-right direction, with substantial thickness in the front-rear direction. 
     (2) Conveying Tube 
     The process unit  16  also includes a conveying tube  81 . 
     The conveying tube  81  is supported on the left side of the left side wall  71 L of the process frame  19 . The conveying tube  81  is formed in a generally cylindrical shape that is closed on both front and rear ends thereof and is elongated in the front-rear direction. As shown in  FIGS. 2 ,  3 A and  3 B, the conveying tube  81  includes an accommodating-portion-side fitting part  83 , a cleaner-side fitting part  82 , and a second screw  84 . 
     As shown in  FIGS. 2 and 3B , the accommodating-portion-side fitting part  83  is provided at a rear end portion of the conveying tube  81 . The accommodating-portion-side fitting part  83  has a generally cylindrical shape that is closed on a left end thereof and is elongated in the left-right direction. The accommodating-portion-side fitting part  83  is fitted around a left end portion of the intermediary conveying portion  74  so as to be rotatable relative to the intermediary conveying portion  74 . 
     As shown in  FIGS. 2 and 3A , the cleaner-side fitting part  82  is provided at a front end portion of the conveying tube  81 . The cleaner-side fitting part  82  has a generally cylindrical shape that is closed on both left and right ends thereof and is elongated in the left-right direction. The cleaner-side fitting part  82  is fitted into the support part  44  of the transfer frame  41  from above. The cleaner-side fitting part  82  includes a receiving hole  85 , and a conveying-tube-side shutter  86 . 
     The receiving hole  85  penetrates a right wall of the cleaner-side fitting part  82  at a lower end portion thereof and has a shape the same as that of the communication hole  45  formed in the transfer frame  41 . 
     The conveying-tube-side shutter  86  is angularly rotatably supported on the right side of the right wall of the cleaner-side fitting part  82 . The conveying-tube-side shutter  86  has a generally disc shape, with substantial thickness in the left-right direction. The conveying-tube-side shutter  86  is engaged with the frame-side shutter  46  and non-rotatable relative to the frame-side shutter  46  when the cleaner-side fitting part  82  is fitted into the support part  44  of the transfer frame  41 . An opening  87  is also formed in the conveying-tube-side shutter  86 . 
     The opening  87  is formed in a shape the same as that of the opening  48  formed in the frame-side shutter  46 . The opening  87  penetrates the conveying-tube-side shutter  86  at a position aligned with the opening  48  of the frame-side shutter  46  when the conveying-tube-side shutter  86  is non-rotatably engaged with the frame-side shutter  46 . 
     The conveying-tube-side shutter  86  is angularly rotatably movable together with the frame-side shutter  46  between a closed position (see  FIG. 5 ) and an open position (see  FIG. 3A ). In the closed position, the opening  87  in the conveying-tube-side shutter  86  is disposed above the receiving hole  85  so as not to be aligned with the receiving hole  85 . In the open position, the opening  87  is aligned with the receiving hole  85 . 
     As shown in  FIG. 6 , the conveying tube  81  is pivotally movable about the accommodating-portion-side fitting part  83  so that the cleaner-side fitting part  82  moves upward and separates from the coupling portion  43  while the frame-side shutter  46  and the conveying-tube-side shutter  86  are in their closed positions. 
     The second screw  84  is a right-handed auger screw extending in the front-rear direction within the conveying tube  81 . The second screw  84  has a front end portion disposed inside the cleaner-side fitting part  82 . The second screw  84  has a rear end portion that is rotatably supported at a circumferential wall on the rear side of the accommodating-portion-side fitting part  83 . The second screw  84  includes a second screw drive gear  88 . 
     The second screw drive gear  88  is supported at the rear end portion of the second screw  84  on the rear side of the accommodating-portion-side fitting part  83  and is non-rotatable relative to the second screw  84 . A drive force is inputted into the second screw drive gear  88  from the main casing  2  through a gear train (not shown). 
     (3) Developer Cartridges 
     As shown in  FIGS. 3A and 7 , each developer cartridge  20  includes a developer frame  91 . 
     The developer frame  91  has a box-like shape and is elongated in the left-right direction. As described above, the developing roller  24 , the supply roller  25 , and the thickness-regulating blade  26  are supported at a bottom portion of the developer frame  91 , while an upper portion thereof serves to accommodate toner therein. 
     The rearmost developer cartridge  20  further includes an accommodating portion  92  (see also  FIG. 3B ). That is, the accommodating portion  92  is disposed adjacent to the rearmost photosensitive drum  21 . 
     The accommodating portion  92  is integrally provided on the rear side of the developer frame  91  and has a box-like shape that is elongated in the left-right direction. As shown in  FIG. 1 , the accommodating portion  92  is disposed above the imaginary plane I. 
     As shown in  FIGS. 3B and 7 , the accommodating portion  92  includes a receiving recessed part  93 , and an accommodating-portion screw  94 . 
     The receiving recessed part  93  has a generally cylindrical shape that is closed on a right end thereof. The receiving recessed part  93  extends rightward so as to be recessed rightward from a left wall of the accommodating portion  92 . A recovery hole  95  is formed in the receiving recessed part  93 . 
     The recovery hole  95  is formed so as to penetrate a right end portion of the receiving recessed part  93  at a lower end portion thereof, providing communication between the interior and exterior of the accommodating portion  92 . 
     The accommodating-portion screw  94  is disposed in the accommodating portion  92 . The accommodating-portion screw  94  is a left-handed auger screw that extends in the left-right direction. The accommodating-portion screw  94  has a right end portion that is rotatably supported at a right wall of the accommodating portion  92 . The right end portion of the accommodating-portion screw  94  protrudes rightward from the right wall of the accommodating portion  92 . The accommodating-portion screw  94  has a left end portion that is rotatably supported at a right wall of the receiving recessed part  93 . The left end portion of the accommodating-portion screw  94  protrudes into the receiving recessed part  93 . The accommodating-portion screw  94  includes an accommodating-portion screw drive gear  96 , and an engaging recessed part  97 . 
     The accommodating-portion screw drive gear  96  is supported at the right end portion of the accommodating-portion screw  94  on the right side of the accommodating portion  92  and is non-rotatable relative to the accommodating-portion screw  94 . A drive force is inputted into the accommodating-portion screw drive gear  96  from the main casing  2  through a gear train (not shown). 
     The engaging recessed part  97  is a recessed groove that is recessed rightward from the left end portion of the accommodating-portion screw  94 . 
     While the rearmost developer cartridge  20  is supported in the process frame  19 , the intermediary conveying portion  74  is fitted inside the receiving recessed part  93  of the accommodating portion  92 . 
     In this state, the engaging protrusion  77  of the relay screw  76  is engaged with the engaging recessed part  97  of the accommodating-portion screw  94 . With this configuration, the relay screw  76  is coupled to the accommodating-portion screw  94  and non-rotatable relative to the accommodating-portion screw  94 . Further, the recovery hole  95  of the receiving recessed part  93  confronts the supply hole  75  of the intermediary conveying portion  74 . 
     4. Recovery of Extraneous Matter 
     During the image-forming operation described above, a paper dust removing operation on the front registration roller  29  (paper-dust removing roller), a cleaning operation on the conveying belt  33  and cleaning operations on the respective photosensitive drums  21  are executed. Next, these cleaning operations will be described with reference to  FIGS. 1 ,  2 ,  3 A and  3 B. 
     When the image-forming operation described above is implemented on the printer  1 , paper dust deposited on the surface of the sheet P is captured by the front registration roller  29  when the sheet P is conveyed opposite the front registration roller  29 , and the paper dust is temporarily retained on the front registration roller  29 . The paper dust captured on the front registration roller  29  subsequently moves opposite the paper-dust scraping member  35  as the rear registration roller  29  rotates and is scraped off the front registration roller  29  by the paper-dust scraping member  35  and collected in the paper-dust reservoir  30 . 
     Further, a cleaning bias is applied to the drum-cleaning rollers  23 . Through this cleaning bias, residual toner, paper dust, and other extraneous matter deposited on the surfaces of the photosensitive drums  21  are attracted to and temporarily retained on circumferential surfaces of the corresponding drum-cleaning rollers  23  as the extraneous matter rotates opposite the drum-cleaning rollers  23 . 
     Extraneous matter deposited on the surface of the conveying belt  33  circulates together with the conveying belt  33  until reaching an area of contact between the conveying belt  33  and the cleaning blade  52 . At this time, the cleaning blade  52  scrapes the extraneous matter off the conveying belt  33 , and the extraneous matter is collected in the recovery chamber  57  of the cleaner frame  51 . 
     The rotating first screw  53  subsequently conveys the extraneous matter collected in the recovery chamber  57  of the cleaner frame  51  leftward toward a left end portion of the cleaner frame  51 . Then, the extraneous matter is sequentially conveyed from the left end portion of the cleaner frame  51  to the cleaner-side fitting part  82  of the conveying tube  81  through the communication hole  45  of the transfer frame  41 , the opening  48  of the frame-side shutter  46 , the opening  87  of the conveying-tube-side shutter  86 , and the receiving hole  85  of the conveying tube  81 . 
     Next, the rotating second screw  84  conveys the extraneous matter conveyed in the cleaner-side fitting part  82  from the front end portion of the conveying tube  81  toward the rear end portion thereof, and introduces the extraneous matter into the left end portion of the intermediary conveying portion  74 . 
     Here, the rotating relay screw  76  conveys the extraneous matter introduced into the left end portion of the intermediary conveying portion  74  toward the right end portion of the intermediary conveying portion  74 . Then, the extraneous matter sequentially passes through the supply hole  75  and the recovery hole  95 , and is collected in the accommodating portion  92 . 
     The rotating accommodating-portion screw  94  then conveys the extraneous matter collected in the accommodating portion  92  rightward in the accommodating portion  92 . Thus, the extraneous matter conveyed to the accommodating portion  92  is uniformly distributed within the accommodating portion  92  with respect to the left-right direction. 
     Once the image-forming operation described above has completed, the drum-cleaning bias is controlled so that the extraneous matter temporarily retained on the drum-cleaning rollers  23  is expelled back onto the corresponding photosensitive drums  21 . 
     As the photosensitive drums  21  rotate, the extraneous matter returned to the photosensitive drums  21  rotates toward the conveying belt  33  and is thereby expelled onto the conveying belt  33 . 
     The extraneous matter expelled onto the conveying belt  33  is subsequently recovered by the belt cleaner  42  as the conveying belt  33  circulates, as described above, and is conveyed to the accommodating portion  92  through the conveying tube  81 . 
     5. Maintenance on Developer Cartridges 
     To perform maintenance on the developer cartridge  20 , the user initially pulls the process unit  16  out to the withdrawn position. 
     More specifically, first the user opens the front cover  5  on the main casing  2 , as shown in  FIG. 5 . 
     Next, the user grips the grip part  49  of the frame-side shutter  46  and angularly rotatably moves the frame-side shutter  46  clockwise in a left side view. Through this operation, the frame-side shutter  46  is moved from its open position to its closed position. At this time, both the frame-side shutter  46  and the conveying-tube-side shutter  86  are in their closed positions, blocking the communication hole  45  of the transfer frame  41  and the receiving hole  85  of the conveying tube  81 . 
     Next, as shown in  FIG. 6 , the user pivotally moves the conveying tube  81  counterclockwise in a left side view about the center axis of the accommodating-portion-side fitting part  83 , thereby moving the cleaner-side fitting part  82  upward away from the coupling portion  43 . 
     Subsequently, the user pulls the process unit  16  forward along the juxtaposed direction X into the withdrawn position. 
     In order to perform maintenance on the developer cartridge  20 , the user removes the developer cartridge  20  from the process unit  16 , as shown in  FIG. 7 . 
     To remove any of the developer cartridges  20  except the rearmost developer cartridge  20  from the process unit  16 , the user simply pulls the developer cartridge  20  upward from the process unit  16 . Accordingly, the developer cartridge  20  is separated from the process unit  16 . 
     To remove the rearmost developer cartridge  20 , on the other hand, the user first pulls up on the right end portion of the rearmost developer cartridge  20  to separate the intermediary conveying portion  74  of the transfer frame  41  from the receiving recessed part  93  of the accommodating portion  92 . 
     Through this operation, the rearmost developer cartridge  20  is separated from the process unit  16 . At the same time, the accommodating portion  92  is also separated from the process unit  16  together with the rearmost developer cartridge  20 . 
     Thereafter, the user can perform maintenance on the developer cartridge  20 . 
     The developer cartridge  20  is mounted in the main casing  2  by performing the operation to remove the developer cartridge  20  described above in reverse. 
     That is, when mounting any of the developer cartridges  20  except the rearmost developer cartridge  20  in the main casing  2 , the user first positions the developer cartridge  20  above the process unit  16  and inserts the developer cartridge  20  downward into the process unit  16  until the developer cartridge  20  is mounted therein. 
     When mounting the rearmost developer cartridge  20  in the main casing  2 , the user first inserts the left end portion of the rearmost developer cartridge  20  into the process unit  16  in order that the intermediary conveying portion  74  of the transfer frame  41  is fitted into the receiving recessed part  93  of the accommodating portion  92 . 
     Once the intermediary conveying portion  74  is fitted into the receiving recessed part  93  and the right end portion of the developer cartridge  20  is pushed downward, the developer cartridge  20  is completely mounted in the process unit  16 . 
     To finish mounting the developer cartridge  20  in the main casing  2 , the user pushes the process unit  16  rearward into the main casing  2 , as shown in  FIG. 6 . More specifically, the user moves the process unit  16  rearward along the juxtaposed direction X. As a result, the process unit  16  is accommodated in the main casing  2 . 
     Next, the user pivotally moves the conveying tube  81  clockwise in a left side view about the center axis of the accommodating-portion-side fitting part  83  until the cleaner-side fitting part  82  is fitted into the coupling portion  43 , as shown in  FIG. 5 . 
     Next, the user grips the grip part  49  of the frame-side shutter  46  and angularly rotatably moves the frame-side shutter  46  counterclockwise in a left side view from the closed position to the open position, as shown in  FIG. 2 . Through this operation, both the frame-side shutter  46  and the conveying-tube-side shutter  86  are in their open positions so as to uncover the communication hole  45  of the transfer frame  41  and the receiving hole  85  of the conveying tube  81 . 
     Lastly, the user closes the front cover  5  on the main casing  2 . 
     6. Maintenance on Transfer Unit and Paper Jam Resolution 
     In order to perform maintenance on the transfer unit  17  or to remove the sheet P that has become jammed in the reversing path  40 , the user removes the transfer unit  17  from the main casing  2  after removing the process unit  16  from the main casing  2 , as shown in  FIG. 8 . 
     To remove the transfer unit  17  from the main casing  2 , the user pulls the process unit  16  out to the withdrawn position, as described above, and removes the process unit  16  from the main casing  2 , as shown in  FIG. 8 . 
     Next, the user grips the grip portion  98  of the transfer unit  17  and pulls the transfer unit  17  in a direction sloping upward and forward. 
     With the transfer unit  28  and the registration unit  27  integrally provided in the transfer unit  17 , the transfer unit  17  moves along a removal direction Y sloping upward toward the front from the mounted position to the removed position. In other words, the grip portion  98  is provided on the downstream end portion of the transfer unit  17  with respect to the removal direction Y. 
     Through this operation, the transfer unit  17  is moved into the removed position outside of the main casing  2 . 
     By removing the transfer unit  17 , the bottom wall of the transfer frame  41  is no longer present in the front portion of the reversing path  40 , leaving an open region  100  in the top of the reversing path  40 . Accordingly, if a sheet P becomes jammed in the reversing path  40 , the user can hold and remove the jammed sheet P through the open region  100  formed in the reversing path  40 . 
     When performing maintenance on the transfer unit  17  to replace components or the like, for example, this maintenance is performed when the transfer unit  17  is in its removed position. 
     After clearing the paper jam or after completing maintenance on the transfer unit  17 , as described above, the user remounts the transfer unit  17  in the main casing  2 . 
     The transfer unit  17  is remounted by performing the operation to remove the transfer unit  17  described above in reverse. That is, the user grips the grip portion  98  of the transfer unit  17  and inserts the transfer unit  17  toward its mounted position along a mounting direction Z sloping downward toward the rear. 
     Once the transfer unit  17  arrives in its mounted position, the bottom wall of the transfer frame  41  is fitted into the open region  100  formed in the reversing path  40 , as shown in  FIG. 1 . Hence, the transfer unit  17  can move between its mounted position and its removed position while the transfer unit  28  and the registration unit  27  are integrally provided thereon. 
     Lastly, the user closes the front cover  5  on the main casing  2 . 
     7. Operational Advantages 
     (1) In the printer  1  according to the first embodiment described above, the belt cleaner  42  is disposed near the frontmost photosensitive drum  21 , which is the photosensitive drum  21  closest to the follow roller  32 , and the follow roller  32  is positioned higher than the drive roller  31 , as shown in  FIGS. 1 and 2 . The extraneous matter removed by the belt cleaner  42  can be conveyed to the accommodating portion  92  positioned near the rearmost photosensitive drum  21 , which is the photosensitive drum  21  closest to the drive roller  31 . 
     This configuration shortens the vertical distance between the belt cleaner  42  and the accommodating portion  92  by the vertical difference between the drive roller  31  and the follow roller  32 . 
     Thus, the slope of the conveying tube  81  relative to a horizontal plane can be made smaller than the angle of repose of the toner, making it easier to convey extraneous matter removed by the belt cleaner  42  to the accommodating portion  92 . In other words, this configuration facilitates the use of toner having greater fluidity for achieving higher quality images. 
     (2) In the printer  1  according to the first embodiment described above, as shown in  FIGS. 2 and 3 , the first screw  53  moves extraneous matter removed by the belt cleaner  42  leftward to collect the extraneous matter on the left end of the belt cleaner  42  so that the second screw  84  can efficiently convey the extraneous matter to the accommodating portion  92 . 
     (3) In the printer  1  according to the first embodiment described above, as shown in  FIG. 7 , the process frame  19  allows the plurality of developer cartridges  20  to be pulled out of the main casing  2  together in order to better facilitate maintenance on the developer cartridges  20  and the accommodating portion  92 . 
     (4) In the printer  1  according to the first embodiment described above, as shown in  FIG. 1 , the drive roller  31  is disposed on the rear end portion of the transfer unit  17 , and specifically the upstream end portion thereof with respect to the direction in which the process unit  16  is pulled out of the main casing  2 . 
     Accordingly, the rear end portion of the process unit  16  can be positioned lower than its front end portion. 
     Thus, when inserting the process unit  16  into the main casing  2 , the weight of the process unit  16  can be utilized to facilitate the mounting motion. 
     (5) In the printer  1  according to the first embodiment described above, as shown in  FIGS. 5 and 6 , the transfer unit  17  and the conveying tube  81  are coupled or uncoupled through the coupling portion  43  provided on the front end portion of the process unit  16 . 
     This arrangement facilitates operations for coupling and uncoupling the transfer unit  17  and the conveying tube  81  since the grip part  49  of the frame-side shutter  46  can be operated from the front side of the printer  1 . 
     (6) In the printer  1  according to the first embodiment described above, by positioning the belt cleaner  42  to the front side of the follow roller  32 , as shown in  FIG. 3 , the vertical distance between the belt cleaner  42  and the accommodating portion  92  can be made shorter than when the belt cleaner  42  is disposed beneath the follow roller  32 . 
     (7) In the printer  1  according to the first embodiment, as shown in  FIGS. 1 and 8 , the transfer unit  17  is moved between its mounted position and its removed position when it is necessary to perform maintenance on the transfer unit  17  and the like. During this movement, the conveying belt  33  and the pair of registration rollers  29  are integrally moved with the transfer unit  17 . 
     Therefore, the pair of registration rollers  29  is unlikely to interfere with the mounting and removal operations for the transfer unit  17 . 
     Thus, this configuration facilitates the smooth mounting and removal operations on the transfer unit  17  relative to the main casing  2 , thereby facilitating smooth maintenance operations on the transfer unit  17 . 
     (8) The front registration roller  29  shown in  FIG. 1  serves as a paper dust roller for capturing paper dust from the sheets P. 
     Hence, when the pair of registration rollers  29  conveys the sheet P during the image-forming operation, the front registration roller  29  captures paper dust from the surface of the sheet P. The paper dust captured on the front registration roller  29  is subsequently scraped off the front registration roller  29  by the paper-dust scraping member  35  and collected in the paper-dust reservoir  30 . 
     Thus, this configuration can reduce the amount of paper dust deposited on the conveying belt  33  and the photosensitive drums  21 , reducing the likelihood of image formation problems caused by paper dust deposits. 
     When the transfer unit  17  is moved between its mounted position and its removed position, not only the conveying belt  33  and the pair of registration rollers  29 , but also the paper-dust scraping member  35  and the paper-dust reservoir  30  move integrally with the transfer unit  17 , as shown in  FIG. 8 . Therefore, maintenance can be performed on both the paper-dust scraping member  35  and the paper-dust reservoir  30  at the same time maintenance is performed on the transfer unit  17 . Further, the paper-dust scraping member  35  and the paper-dust reservoir  30  are unlikely to interfere with the mounting and removal operations for the transfer unit  17 . 
     Thus, this configuration suppresses the occurrence of image formation problems while facilitating smooth mounting and removal operations of the transfer unit  17  relative to the main casing  2 . 
     (9) As shown in  FIG. 1 , the pair of registration rollers  29  overlaps the conveying belt  33  when projected in the juxtaposed direction X, which is the direction in which the photosensitive drums  21  are juxtaposed. 
     This arrangement reliably reduces the likelihood of the pair of registration rollers  29  interfering with the mounting and removal operations of the transfer unit  17  when the transfer unit  17  is moved between the mounted position and the removed position, as shown in  FIG. 8 . 
     (10) As shown in  FIGS. 1 and 7 , the printer  1  according to the first embodiment includes the process frame  19  for supporting the plurality of photosensitive drums  21 . The process frame  19  can move between the accommodated position in which the process frame  19  is accommodated in the main casing  2 , and the withdrawn position in which the process frame  19  is withdrawn to the outside of the main casing  2 . 
     Hence, the process frame  19  allows the plurality of photosensitive drums  21  to be pulled out of the main casing  2  together in order to facilitate maintenance on the photosensitive drums  21 . 
     In particular, the process frame  19  can move between the accommodated position and the withdrawn position along the juxtaposed direction X, while the pair of registration rollers  29  and the conveying belt  33  overlap each other when projected in the juxtaposed direction X. Put another way, the pair of registration rollers  29  is positioned so as not to overlap the process frame  19  when projected in the juxtaposed direction X. This configuration prevents the pair of registration rollers  29  from interfering with the movement of the process frame  19 . 
     (11) As shown in  FIG. 1 , the grip portion  98  is provided on the downstream end of the transfer unit  17  with respect to the removal direction Y. 
     Since the user grips the grip portion  98  to move the transfer unit  17  between the mounted position and the removed position, as shown in  FIG. 8 , this arrangement can ensure smooth mounting and removal operations of the transfer unit  17  relative to the main casing  2 . 
     (12) As shown in  FIG. 1 , the printer  1  according to the first embodiment is provided with the fixing unit  18  for fixing toner images onto the sheets P after the toner images have been transferred from the plurality of photosensitive drums  21 . Thus, toner images transferred from the plurality of photosensitive drums  21  onto the sheets P are reliably fixed to the sheets P with heat in the fixing unit  18 . 
     Further, the reversing path  40  is formed on the opposite side of the photosensitive drums  21  relative to the transfer unit  17  when the transfer unit  17  is in its mounted position. 
     After a toner image has been fixed to the first surface of the sheet P, the discharge rollers  38  are driven in a reverse rotation prior to discharging the sheet P onto the discharge tray  39  in order to introduce the sheet P onto the reversing path  40 . Subsequently, the sheet P is conveyed along the reversing path  40  to the pair of registration rollers  29 , after which the pair of registration rollers  29  again conveys the sheet P between the plurality of photosensitive drums  21  and the conveying belt  33  with the second surface of the sheet P facing upward. In this way, images can be formed on both first and second surfaces of the sheet P. 
     (13) As shown in  FIG. 1 , the reversing path  40  is positioned adjacent to the conveying belt  33  when the transfer unit  17  is in its mounted position. 
     Accordingly, the reversing path  40  and the transfer unit  17  can be more efficiently arranged than when other components are provided between the reversing path  40  and the conveying belt  33 . 
     This configuration allows the printer  1 , which can form images on both the first and second surfaces of the sheets P, to be made more compact. 
     (14) As shown in  FIG. 8 , the transfer unit  17  is provided with the transfer frame  41  at a position confronting the conveying belt  33  for covering the conveying belt  33 . This transfer frame  41  can prevent the conveying belt  33  from coming into contact with external components when the transfer unit  17  is moved between the mounted position and the removed position or placed outside the main casing  2  (removed position). Therefore, the transfer frame  41  can prevent the conveying belt  33  from being damaged. 
     Further, the bottom wall of the transfer frame  41  serves as the top wall defining the reversing path  40  in the front portion thereof when the transfer unit  17  is in its mounted position shown in  FIG. 1 . This configuration reliably positions the reversing path  40  adjacent to the conveying belt  33  to achieve a more efficient arrangement of the reversing path  40  and the transfer unit  17 . 
     Further, since the transfer unit  17  is provided with both the top wall of the reversing path  40  and the pair of registration rollers  29 , the sheets P can be more smoothly conveyed from the reversing path  40  to the pair of registration rollers  29 . 
     Further, since the front portion of the reversing path  40  is open on the top side when the transfer unit  17  is disposed in its removed position, as shown in  FIG. 8 , the user can easily remove the sheets P from the reversing path  40  through the open region  100  when the sheets P become jammed in the reversing path  40 . 
     8. Variations of First Embodiment 
     In the first embodiment described above, the grip portion  98  is provided at the front end portion of the transfer unit  17 . 
     In a variation of the first embodiment shown in  FIGS. 9 ,  10 A,  10 B, the grip portion  98  may be dispensed with. 
     The variation of the first embodiment can obtain the same operational advantages described for the first embodiment. 
     9. Second Embodiment 
     A printer  101  as an image forming apparatus according to a second embodiment of the present invention will be described while referring to  FIGS. 11 to 17 , wherein like parts and components are designated with the same reference numerals to avoid duplicating description. In the following description, only parts differing from those of the embodiment will be described in detail. 
     (1) Overview of Second Embodiment 
     In the first embodiment described above, the process unit  16  is disposed in an inclined orientation inside the main casing  2  such that the front end portion of the process unit  16  is higher than the rear end portion of the process unit  16 . Further, extraneous matter removed by the belt cleaner  42 , which is provided at the front end portion of the transfer unit  17 , is conveyed to the accommodating portion  92 , which is provided at the rearmost developer cartridge  20 . 
     In the second embodiment, a process unit  103  is disposed in an inclined orientation in the main casing  2  such that its rear end portion is higher than its front end portion. Further, extraneous matter removed by a belt-cleaning roller  108 , which is provided at a rear end portion of a transfer unit  104 , is conveyed to an accommodating portion  132  provided at a frontmost developer cartridge  122 . 
     (2) Detailed Description of Transfer Unit 
     As shown in  FIG. 11 , the transfer unit  104  is disposed in an inclined orientation in the main casing  2  such that the drive roller  31  is higher than the follow roller  32 . 
     The transfer unit  104  is movable relative to the main casing  2  between a mounted position (see  FIG. 11 ) and a removed position (see  FIG. 17 ). In the mounted position, the transfer unit  104  is mounted in the main casing  2 . In the removed position, the transfer unit  104  has been removed from the main casing  2 . 
     When the drive roller  31  is driven to rotate, the conveying belt  33  circulates so that its upper portion moves along an upward slope toward the rear (i.e. from the lower front side to the upper rear side). 
     As shown in  FIGS. 12 and 13 , the transfer unit  104  includes a transfer frame  105 . 
     The transfer frame  105  has a frame-like configuration with a closed bottom and an open top. The transfer frame  105  includes the belt-cleaning roller  108 , a relay roller  109 , a conveying tube  110 , and a plurality of sheet-conveying rollers  107 . 
     The belt-cleaning roller  108  is disposed at a position obliquely downward and forward of the drive roller  31  and contacts the lower portion of the conveying belt  33  from below. Thus, the belt-cleaning roller  108  is positioned in proximity to the lower rear side of the rearmost photosensitive drum  21 . Further, the belt-cleaning roller  108  is positioned lower than the imaginary plane I containing the top surface of the upper portion of the conveying belt  33 . 
     The relay roller  109  is disposed on the rear side of the belt-cleaning roller  108  and contacts the belt-cleaning roller  108  from the rear. 
     The conveying tube  110  includes a first conveying portion  111 , and a second conveying portion  112 . 
     The first conveying portion  111  is disposed on the rear side of the relay roller  109 . The first conveying portion  111  has a generally cylindrical shape and is elongated in the left-right direction. The first conveying portion  111  has a left end portion protruding leftward from a left wall of the transfer frame  105 . The first conveying portion  111  includes a recovery hole  113 , a scraping blade  114 , and a first screw  115 . 
     The recovery hole  113  extends approximately linearly in the left-right direction and penetrates a front circumferential wall of the first conveying portion  111 . 
     The scraping blade  114  is provided at a front edge of a top portion of the first conveying portion  111 . The scraping blade  114  has a generally flat plate shape and is elongated in the left-right direction with substantial thickness in the front-rear direction. The scraping blade  114  has an upper half portion fixed to a top peripheral edge of the first conveying portion  111  defining the recovery hole  113  and a lower half portion confronting an upper half portion of the recovery hole  113 . 
     The first screw  115  is a left-handed auger screw extending in the left-right direction. The first screw  115  is disposed inside the first conveying portion  111 . 
     The first screw  115  has a right end portion that is rotatably supported at a right wall of the first conveying portion  111 . The first screw  115  includes a first-screw drive gear  106 . 
     The first-screw drive gear  106  is supported at the right end portion of the first screw  115  on the right side of the first conveying portion  111  and is non-rotatable relative to the first screw  115 . A drive force is inputted into the first-screw drive gear  106  from the main casing  2  through a gear train (not shown). 
     The second conveying portion  112  has a generally cylindrical shape that extends continuously forward from a left end portion of the first conveying portion  111 . The second conveying portion  112  includes a second screw  116 , a supply hole  117 , and a conveying-tube-side shutter  118 . 
     The second screw  116  has a helical configuration and extends in the front-rear direction. The second screw  116  is disposed inside the second conveying portion  112 . The second screw  116  has a rear end portion that is formed continuously with the left end portion of the first screw  115  and a front end portion that is rotatably supported at a front end portion of the second conveying portion  112 . 
     The supply hole  117  has a generally circular shape in a side view (see  FIG. 12 ). The supply hole  117  is formed so as to penetrate a lower right peripheral wall of the second conveying portion  112  at a front end portion thereof. 
     The conveying-tube-side shutter  118  has a generally cylindrical shape. The conveying-tube-side shutter  118  is angularly rotatably fitted around the front end portion of the second conveying portion  112 . When an intermediary conveying tube  128  described later is in a coupled position, the conveying-tube-side shutter  118  is engaged with a relay-side shutter  131  described later and rotates together with the relay-side shutter  131 . An opening  119  is formed in the conveying-tube-side shutter  118 . 
     The opening  119  has a generally circular shape in a side view. The opening  119  is formed so as to penetrate a peripheral wall of the conveying-tube-side shutter  118 . When the conveying-tube-side shutter  118  is in an open position described later, the opening  119  is positioned on the lower right side of the conveying-tube-side shutter  118 , confronting the supply hole  117 . 
     The conveying-tube-side shutter  118  is angularly rotatably movable between an open position (see  FIG. 13 ) and a closed position (see  FIG. 14 ). In the open position, the opening  119  of the conveying-tube-side shutter  118  is aligned with the supply hole  117  of the second conveying portion  112 . In the closed position, the opening  119  is disposed obliquely below and leftward of the supply hole  117  so as not to be aligned with the supply hole  117 . 
     The plurality of sheet-conveying rollers  107  is rotatably supported at a lower end portion of the transfer frame  105 . Each of the plurality of sheet-conveying rollers  107  is disposed at a position confronting the top of corresponding main-casing-side paper-conveying roller  141  provided at a lower wall defining the reversing path  40 . 
     (3) Detailed Description of Process Unit 
     The process unit  103  is inclined relative to the horizontal so that its front end is positioned lower than its rear end. 
     The process unit  103  includes a process frame  121  and a plurality of developer cartridges  122  (four in the embodiment). 
     As shown in  FIGS. 12 and 13 , the process frame  121  has a frame-like structure that is generally rectangular in a plan view. The process frame  121  includes the pair of side walls  71 , and the rear beam  73 . 
     The process frame  121  can move relative to the main casing  2  in the front-rear direction between an accommodated position (see  FIG. 11 ) and a withdrawn position (see  FIG. 16 ). In the accommodated position, the process frame  121  is accommodated inside the main casing  2 . In the withdrawn position, the process frame  121  is withdrawn to an outside of the main casing  2 . The process frame  121  can be removed from the main casing  2  in the withdrawn position. 
     The pair of side walls  71  is each generally rectangular in a side view (see  FIG. 2 ) and elongated in the front-rear direction. The pair of side walls  71  is arranged parallel to each other and separated in the left-right direction. 
     The plurality of photosensitive drums  21  is arranged juxtaposed with each other and spaced at intervals along a direction sloping upward toward the rear. The plurality of photosensitive drums  21  is provided for the four colors of black, yellow, magenta, and cyan and is arranged at intervals in this order from the lower front side to the upper rear side of the main casing  2 . Each photosensitive drum  21  is generally cylindrical in shape and is elongated in the left-right direction. Each photosensitive drum  21  is capable of rotating about a center axis A 3  extending in the left-right direction. 
     Incidentally, although not illustrated in  FIG. 1 , the photosensitive drum  21  in the first embodiment inherently has the center axis A 3 . 
     The plurality of scorotron chargers  22  is positioned on the upper rear side of the corresponding photosensitive drum  21 . The plurality of drum-cleaning rollers  23  is positioned so as to contact the photosensitive drum  21  from the rear side thereof. 
     The process frame  121  includes a registration roller  123 , and the intermediary conveying tube  128 . 
     The registration roller  123  is supported at the process frame  121  at a position between the pair of side walls  71 . More specifically, the registration roller  123  is rotatably supported at lower front end portions of the side walls  71 . 
     The intermediary conveying tube  128  is provided at the left side wall  71 L constituting the process frame  121  at a front end portion thereof. The intermediary conveying tube  128  has a generally cylindrical shape that is closed on both left and right ends thereof and is elongated in the left-right direction when in the coupled position described later. The intermediary conveying tube  128  (in the coupled position shown in  FIG. 13 ) has a left wall that is formed in a curve that conforms to a circumferential surface of the second conveying portion  112  of the conveying tube  110 . The intermediary conveying tube  128  includes a receiving hole  129 , a discharge hole  130 , and the relay-side shutter  131 . 
     The receiving hole  129  penetrates the left wall of the intermediary conveying tube  128  and confronts the opening  119  of the conveying-tube-side shutter  118  when the intermediary conveying tube  128  is in the coupled position. 
     The discharge hole  130  penetrates a right wall of the intermediary conveying tube  128  and confronts an accommodating hole  134  formed in the accommodating portion  132  described later when the intermediary conveying tube  128  is in the coupled position. 
     The relay-side shutter  131  is disposed on the left side of the left wall of the intermediary conveying tube  128 . The relay-side shutter  131  has a generally flat plate shape that is curved to conform to the curved left wall of the intermediary conveying tube  128 . The relay-side shutter  131  is vertically slidingly movable between an open position (see  FIG. 13 ) and a closed position (see  FIG. 14 ). In the open position, the relay-side shutter  131  is disposed above the receiving hole  129 . In the closed position, the relay-side shutter  131  confronts the receiving hole  129  from the left side thereof. 
     The intermediary conveying tube  128  is pivotally movable about its center between a coupled position (see  FIG. 13 ) and an uncoupled position (see  FIG. 14 ). In the coupled position, the intermediary conveying tube  128  extends in the left-right direction. In the uncoupled position, the intermediary conveying tube  128  extends vertically. When the intermediary conveying tube  128  is in the uncoupled position, the receiving hole  129  is separated from and moved below the opening  119  of the conveying-tube-side shutter  118 , and the discharge hole  130  is separated from and moved above the accommodating hole  134  of the accommodating portion  132  described later. 
     The rear beam  73  bridges the rear end portions of the pair of side walls  71 . The rear beam  73  has a generally flat plate shape and is elongated in the left-right direction, with substantial thickness in the front-rear direction. 
     As described above and as shown in  FIGS. 12 and 15 , each developer cartridge  122  includes the developing roller  24 , the supply roller  25 , and the thickness-regulating blade  26 . The developing roller  24  is rotatably supported at a lower end portion of the developer cartridge  122  and is exposed in the lower rear side of the developer cartridge  122 . The developing roller  24  contacts the photosensitive drum  21  from the upper front side thereof. 
     The developing roller  24  has a generally cylindrical shape and is elongated in the left-right direction. The developing roller  24  is capable of rotating about a center axis A 4  extending in the left-right direction. The center axis A 4  of the developing roller  24  is parallel to the center axis A 3  of the photosensitive drum  21  and occupies a position obliquely above and forward of the center axis A 3  of the photosensitive drum  21 . 
     Incidentally, although not illustrated in  FIG. 1 , the developing roller  24  in the first embodiment inherently has the center axis A 4 . 
     The supply roller  25  is disposed slightly obliquely below and forward of the developing roller  24  and contacts the developing roller  24  from the lower front side thereof. 
     The supply roller  25  has a generally cylindrical shape and is elongated in the left-right direction. The supply roller  25  is capable of rotating about a center axis A 5  extending in the left-right direction. The center axis A 5  of the supply roller  25  is parallel to the center axis A 3  of the photosensitive drum  21  and occupies a position forward of and slightly lower than the center axis A 4  of the developing roller  24 . 
     Incidentally, although not illustrated in  FIG. 1 , the supply roller  25  in the first embodiment inherently has the center axis A 5 . 
     As shown in  FIGS. 11 and 15 , the developer cartridge  122  also includes a toner-accommodating chamber  124  for accommodating toner therein. 
     The toner-accommodating chamber  124  is disposed obliquely above and forward of the supply roller  25 . The toner-accommodating chamber  124  has a box-like shape that is elongated in the left-right direction. The toner-accommodating chamber  124  has a front end portion protruding downward. 
     Toner in each of the toner-accommodating chambers  124  is supplied onto the corresponding supply roller  25 , and the rotating supply roller  25  supplies the toner onto the corresponding developing roller  24  while the toner is tribocharged between the supply roller  25  and developing roller  24 . 
     The toner-accommodating chamber  124  includes a partitioning wall  125 , a toner supply screw  126 , and a toner recovery screw  127 . 
     The partitioning wall  125  is disposed in a lower front portion of the toner-accommodating chamber  124 . The partitioning wall  125  has a generally flat plate shape and is elongated in the front-rear direction. The partitioning wall  125  divides the lower front portion of the toner-accommodating chamber  124  vertically. Within the partitioning wall  125  are formed a toner supply hole  135 , and a toner recovery hole  136  (see  FIG. 15 ). 
     The toner supply hole  135  is formed so as to penetrate a left end portion of the partitioning wall  125 . 
     The toner recovery hole  136  is formed so as to penetrate a right end portion of the partitioning wall  125 . 
     The toner supply screw  126  is disposed inside the toner-accommodating chamber  124  above the partitioning wall  125 . The toner supply screw  126  is a left-handed auger screw that extends in the left-right direction. The toner supply screw  126  has left and right end portions that are rotatably supported at respective left and right walls of the toner-accommodating chamber  124 . The toner supply screw  126  includes a drive gear  137 . 
     The drive gear  137  is supported at the right end portion of the toner supply screw  126  on the right side of the toner-accommodating chamber  124  and is non-rotatable relative to the toner supply screw  126 . A drive force is inputted into the drive gear  137  from the main casing  2  through a gear train (not shown). 
     The toner recovery screw  127  is disposed inside the toner-accommodating chamber  124  below the partitioning wall  125 . The toner recovery screw  127  is a left-handed auger screw that extends in the left-right direction. The toner recovery screw  127  has left and right end portions that are rotatably supported at the respective left and right walls of the toner-accommodating chamber  124 . The toner recovery screw  127  has a drive gear  138 . 
     The drive gear  138  is supported at the right end portion of the toner recovery screw  127  on the right side of the toner-accommodating chamber  124  and is non-rotatable relative to the toner recovery screw  127 . The drive gear  138  engages with the drive gear  137  provided at the toner supply screw  126  from below. 
     The toner supply screw  126  conveys toner from right to left in the toner-accommodating chamber  124  and supplies the toner through the toner supply hole  135  to the left end portion of the toner recovery screw  127 . The toner recovery screw  127  conveys the toner supplied onto its left end portion toward the right side and supplies the toner onto the supply roller  25 . The toner recovery screw  127  subsequently conveys any excess toner not carried on the surface of the supply roller  25  rightward and supplies this toner through the toner recovery hole  136  onto the right end portion of the toner supply screw  126 . 
     As shown in  FIGS. 12 and 13 , the frontmost developer cartridge  122  also includes the accommodating portion  132 , and an opposing roller  133 . 
     The accommodating portion  132  is integrally disposed on the front side of the toner-accommodating chamber  124  provided in the frontmost developer cartridge  122 . That is, the accommodating portion  132  is disposed adjacent to an upper front portion of the frontmost photosensitive drum  21 . The accommodating portion  132  is also positioned above the imaginary plane I containing the top surface of the upper portion of the conveying belt  33 . The accommodating portion  132  has a box-like shape that has a generally L-shaped cross-section. The accommodating hole  134  is formed in the accommodating portion  132 . 
     The accommodating hole  134  is formed so as to penetrate a left wall of the accommodating portion  132 . The accommodating hole  134  has a generally circular shape in a side view. 
     The opposing roller  133  is rotatably supported at the developer cartridge  122  below the toner-accommodating chamber  124 . The opposing roller  133  contacts the registration roller  123  of the process frame  121  from above. 
     The sheets P accommodated in the paper tray  6  are conveyed by the feeding roller  8  one at a time toward a position between the registration roller  123  and the opposing roller  133 . 
     (4) Recovery of Extraneous Matter 
     During the image-forming operation described above, a cleaning operation on the conveying belt  33  and cleaning operations on the respective photosensitive drums  21  are executed. Next, these cleaning operations will be described with reference to  FIGS. 12 and 13 . 
     When the image-forming operation described above is executed in the printer  101 , a cleaning bias is applied to the drum-cleaning rollers  23 . Through this cleaning bias, residual toner, paper dust, and other extraneous matter deposited on the surfaces of the photosensitive drums  21  are attracted to and temporarily retained on circumferential surfaces of the corresponding drum-cleaning rollers  23  as the extraneous matter rotates opposite the drum-cleaning rollers  23 . 
     Extraneous matter deposited on the surface of the conveying belt  33  circulates together with the conveying belt  33  until reaching an area of contact between the conveying belt  33  and the belt-cleaning roller  108 . At this time, the extraneous matter is attracted to and retained on the belt-cleaning roller  108  by static electricity. The extraneous matter is subsequently passed to the relay roller  109 , where it is scraped off the relay roller  109  by the scraping blade  114  and collected in the first conveying portion  111  through the recovery hole  113 . 
     The rotating first screw  115  subsequently conveys the extraneous matter collected in the first conveying portion  111  leftward toward a left end portion of the first conveying portion  111 . Then, the rotating second screw  116  conveys the extraneous matter forward from a rear end portion of the second conveying portion  112 . The extraneous matter sequentially passes through the supply hole  117  of the second conveying portion  112 , the opening  119  of the conveying-tube-side shutter  118 , and the receiving hole  129  of the intermediary conveying tube  128 , and is introduced into a left end portion of the intermediary conveying tube  128 . 
     The extraneous matter introduced into the left end portion of the intermediary conveying tube  128  sequentially passes through the intermediary conveying tube  128  and through the discharge hole  130  of the intermediary conveying tube  128  and the accommodating hole  134  of the accommodating portion  132  to be collected in the accommodating portion  132 . 
     The cleaning operation on the conveying belt  33  is completed after the extraneous matter deposited on the surface of the conveying belt  33  is recovered in the accommodating portion  132 . 
     Once the image-forming operation described above has completed, the drum-cleaning bias is controlled so that the extraneous matter temporarily retained on the drum-cleaning rollers  23  is expelled back onto the corresponding photosensitive drums  21 . 
     As the photosensitive drums  21  rotate, the extraneous matter returned to the photosensitive drums  21  rotates toward the conveying belt  33  and is thereby expelled onto the conveying belt  33 . 
     The extraneous matter expelled onto the conveying belt  33  is subsequently recovered by the belt-cleaning roller  108  as the conveying belt  33  circulates, as described above, and is conveyed to the accommodating portion  132  through the conveying tube  110 . 
     The cleaning operations on the photosensitive drums  21  are completed after the extraneous matter deposited on the surfaces of the photosensitive drums  21  is collected in the accommodating portion  132 , as described above. 
     (5) Maintenance on Developer Cartridges 
     To perform maintenance on a developer cartridge  122 , the user initially pulls the process unit  103  out to the withdrawn position. 
     More specifically, first the user opens a front cover  102  on the main casing  2 , as shown in  FIG. 16 . 
     Next, the user slides the relay-side shutter  131  clockwise in a front view from its open position toward its closed position, as shown in  FIGS. 13 and 14 . Through this operation, both the relay-side shutter  131  and the conveying-tube-side shutter  118  are in their closed positions, blocking the receiving hole  129  of the intermediary conveying tube  128  and the supply hole  117  of the second conveying portion  112 . 
     Next, the user pivotally moves the intermediary conveying tube  128  counterclockwise in a front view into its uncoupled position. At this time, the relay-side shutter  131  is disengaged from the conveying-tube-side shutter  118 . 
     Subsequently, the user pulls the process unit  103  forward, as shown in  FIG. 16 . 
     In order to perform maintenance on the developer cartridge  122 , the user pulls the developer cartridge  122  upward until the developer cartridge  122  separates from the process unit  103 . When pulling the frontmost developer cartridge  122 , the accommodating portion  132  separates from the process unit  103  together with the frontmost developer cartridge  122 . 
     This completes the operation to remove the developer cartridge  122  from the main casing  2 . 
     Thereafter, the user can perform maintenance on the developer cartridge  122 . 
     The developer cartridge  122  is mounted in the main casing  2  by performing the operation to remove the developer cartridge  122  described above in reverse. 
     That is, when mounting the developer cartridge  122  in the main casing  2 , the user first positions the developer cartridge  122  above the process unit  103  and inserts the developer cartridge  122  downward into the process unit  103  until the developer cartridge  122  is mounted therein. 
     Next, the user pushes the process unit  103  rearward into the main casing  2 . 
     Next, the user pivotally moves the intermediary conveying tube  128  clockwise in a front view until the intermediary conveying tube  128  is in its coupled position, as shown in  FIGS. 13 and 14 . At this time, the relay-side shutter  131  and the conveying-tube-side shutter  118  are engaged with each other. 
     Subsequently, the user slides the relay-side shutter  131  counterclockwise in a front view from its closed position toward its open position. Through this operation, both the relay-side shutter  131  and the conveying-tube-side shutter  118  are in their open positions so as to uncover the receiving hole  129  of the intermediary conveying tube  128  and the supply hole  117  of the second conveying portion  112 . 
     Lastly, the user closes the front cover  102  on the main casing  2 . This completes the operation to mount the developer cartridges  122  in the main casing  2 . 
     (6) Maintenance on Transfer Unit and Paper Jam Resolution 
     To perform maintenance on the transfer unit  104  or to remove the sheet P that has become jammed in the reversing path  40 , the user first pulls the process unit  103  out to the withdrawn position, as described above, and removes the process unit  103  from the main casing  2 . 
     Thereafter, the user removes the transfer unit  104  from the main casing  2 , as shown in  FIG. 17 , and performs maintenance on the transfer unit  104  or clears the jammer sheet P from the reversing path  40 . Note that the conveying tube  110  is removed together with the transfer unit  104  at this time. 
     (7) Operational Advantages of Second Embodiment 
     (7-1) In the printer  101  according to the second embodiment, the belt-cleaning roller  108  is disposed near the rearmost photosensitive drum  21 , which is the developer cartridge  122  closest to the drive roller  31 , and the drive roller  31  is positioned higher than the follow roller  32 , as shown in  FIG. 12 . The extraneous matter removed by the belt-cleaning roller  108  can be conveyed to the accommodating portion  132  positioned near the frontmost photosensitive drum  21 , which is the photosensitive drum  21  closest to the follow roller  32 . 
     This configuration shortens the vertical distance between the belt-cleaning roller  108  and the accommodating portion  132  by the vertical difference between the drive roller  31  and the follow roller  32 . 
     Thus, the slope of the second conveying portion  112  relative to a horizontal plane can be made smaller than the angle of repose of the toner, making it easier to convey extraneous matter removed by the belt-cleaning roller  108  to the accommodating portion  132 . In other words, this configuration facilitates the use of toner having greater fluidity for achieving higher quality images. 
     (7-2) In the printer  101  according to the second embodiment, as shown in  FIGS. 14 and 17 , the conveying tube  110  (i.e., the first conveying portion  111  and the second conveying portion  112 ) can be mounted and removed together with the transfer unit  104 . 
     Hence, this configuration enables more efficient maintenance of the conveying tube  110  since this maintenance can be performed at the same time as maintenance on the transfer unit  104 . 
     (7-3) In the printer  101  according to the second embodiment, as shown in  FIG. 11 , the follow roller  32  is disposed on the front end portion of the transfer unit  104 , and specifically the downstream end portion thereof with respect to the direction in which the process unit  103  is pulled out of the main casing  2 . 
     Accordingly, the front end portion of the process unit  103  can be positioned lower than its rear end portion. 
     Thus, when removing the process unit  103  from the main casing  2 , the weight of the process unit  103  can be utilized to facilitate the withdrawal motion. 
     In the printer  101  according to the second embodiment, as shown in  FIGS. 13 and 14 , the accommodating portion  132  and the conveying tube  110  are coupled or uncoupled through the intermediary conveying tube  128  provided at the front end portion of the process unit  103 . 
     This arrangement facilitates operations for coupling and uncoupling the accommodating portion  132  and the conveying tube  110  since the relay-side shutter  131  can be operated from the front side of the printer  1 . 
     (7-4) In the printer  101  according to the second embodiment, as shown in  FIG. 11 , the center axis A 5  of the supply roller  25  is lower than the center axis A 4  of the developing roller  24 . Thus, the supply roller  25  confronts the peripheral surface of the developing roller  24  on the lower side thereof. 
     This arrangement reduces the likelihood of toner being excessively supplied from the supply roller  25  to the developing roller  24  due to gravity. 
     As a result, this configuration suppresses accumulation of toner about the developing roller  24 , reducing the likelihood that toner will leak around the developing roller  24 . 
     Further, the photosensitive drums  21  provided closer to the front side are lower than the photosensitive drums  21  provided relatively rearward. 
     Further, the developing rollers  24  are provided on the front sides of the corresponding photosensitive drums  21 , and the supply rollers  25  are provided on the front sides of the corresponding developing rollers  24 . 
     In this way, the developing roller  24  and the supply roller  25  corresponding to the photosensitive drum  21  disposed to the rear side thereof can be efficiently arranged in the space above the photosensitive drum  21  disposed to the front side thereof. 
     Thus, this configuration can reduce the gap between adjacent photosensitive drums  21  more than a structure that arranges the developing roller  24  and the supply roller  25  between adjacent photosensitive drums  21 . 
     Hence, this arrangement prevents the printer  101  from becoming larger. 
     (7-5) In the printer  101  according to the second embodiment, as shown in  FIG. 11 , each of the developing rollers  24  contacts the corresponding photosensitive drum  21  such that the center axis A 4  of the developing roller  24  is higher than the center axis A 3  of the photosensitive drum  21 . 
     Accordingly, the developing roller  24  and the supply roller  25  corresponding to the photosensitive drum  21  disposed to the rear side thereof can be more efficiently arranged in the space above the photosensitive drum  21  disposed to the front side thereof. 
     Hence, this arrangement can further reduce the gap between adjacent photosensitive drums  21 , suppressing an increase in the size of the printer  101 . 
     (7-6) In the printer  101  according to the second embodiment, as shown in  FIG. 11 , the toner recovery screw  127  in the toner-accommodating chamber  124  confronts the front circumferential surface of the supply roller  25 . 
     This configuration reduces the likelihood of toner being supplied excessively to the supply roller  25  due to gravity. 
     Thus, this construction further reduces the accumulation of toner about the developing roller  24 , thereby further suppressing leakage of toner around the developing roller  24 . 
     (7-7) In the printer  101  according to the second embodiment, as shown in  FIG. 15 , the toner recovery screw  127  can convey excess toner supplied to the supply roller  25  through the toner recovery hole  136  to be recovered in the toner-accommodating chamber  124 . 
     This configuration provides a uniform amount of toner to the supply roller  25  while suppressing an excessive amount of toner from being supplied to the supply roller  25 . 
     Accordingly, this configuration can better suppress toner from being supplied excessively from the supply roller  25  to the developing roller  24 . 
     As a result, this arrangement better suppresses the accumulation of toner about the developing roller  24  and, hence, better suppresses toner from leaking around the developing roller  24 . 
     (7-8) In the printer  101  according to the second embodiment, as shown in  FIG. 11 , the conveying belt  33  conveys the sheets P along the plurality of photosensitive drums  21 . 
     This configuration is that of a direct tandem-type printer in which toner images are directly transferred from the photosensitive drums  21  onto the sheet P. 
     (7-9) The printer  101  according to the second embodiment can obtain the same operational advantages described for the printer  1  according to the first embodiment. 
     (8) Variations of Second Embodiment 
     In the second embodiment described above, both the toner supply screw  126  and the toner recovery screw  127  of the developer cartridge  122  are configured as left-handed auger screws, further the toner supply hole  135  is formed in the left end portion of the partitioning wall  125 , and the toner recovery hole  136  is formed in the right end portion of the partitioning wall  125 . 
     In a variation of the second embodiment shown in  FIG. 18 , the right half portions of both the toner supply screw  126  and the toner recovery screw  127  are configured as left-handed auger screws, while the left half portions are configured as right-handed auger screws. Further, the toner supply hole  135  is formed in the left-right center of the partitioning wall  125 , while two toner recovery holes  136  are formed respectively in the left and right end portions of the partitioning wall  125 . 
     The variation of the second embodiment can obtain the same operational advantages described for the second embodiment. 
     While the present invention has been described in detail with reference to the embodiments 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 present invention.