Abstract:
An image forming apparatus includes a main casing, a supporting frame movable relative to the main casing in a first direction between an internal position and an external position, and a developer container supported in the supporting frame. The main casing includes first and second walls extending in the first direction and opposing each other in a second direction orthogonal to the first direction. The supporting frame includes first and second plates opposing the first and second walls in the second direction respectively. The developer container includes: a first chamber disposed between the first plate and the second plate; a second chamber for storing developer and disposed between the first wall and the first plate; a third chamber for communicating between the first chamber and the second chamber; and a conveying portion for conveying the developer in the second chamber to the first chamber via the third chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application Nos. 2012-239998 filed Oct. 31, 2012 and 2013-151853 filed Jul. 22, 2013. The entire contents of the priority applications are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an electrophotographic image forming apparatus. 
       BACKGROUND 
       [0003]    One image-forming device known in the art is a horizontal tandem-type color printer provided with a plurality of photosensitive drums that are arranged parallel to each other and juxtaposed horizontally (hereinafter called the “juxtaposing direction”), and a plurality of developing cartridges for respectively supplying developer to the photosensitive drums. 
         [0004]    Of these types of color printers, a proposal has been provided for a printer having a support frame that integrally supports a plurality of photosensitive drums and a plurality of developing cartridges. The support frame can be moved with respect to the juxtaposed direction between an accommodated position and a withdrawn position. The printer also includes an intermediate transfer belt that contacts the photosensitive drums when the support frame is disposed in the accommodated position. Each developing cartridge includes a developing roller that contacts a corresponding photosensitive drum, and a toner-accommodating chamber for accommodating toner to be supplied onto the developing roller. The developing cartridges are detachably supported in the support frame and can be mounted therein and removed therefrom with respect to an axial direction of the developing rollers while the support frame is disposed in the withdrawn position. 
       SUMMARY 
       [0005]    In the conventional printer described above, the photosensitive drums are arranged in their juxtaposed direction so as to confront both the intermediate transfer belt and the corresponding developing rollers while the support frame is disposed in the accommodated position. Further, the toner-accommodating chambers of the developing cartridges that accommodated toner to be supplied onto the corresponding developing rollers are also aligned in the juxtaposed direction. Consequently, this arrangement increases the dimension of the printer in the juxtaposed direction, which leads to a desire to reduce this dimension of the printer. However, if the dimension of the conventional printer with respect to the juxtaposed direction is made more compact, sizes of the toner-accommodating chambers must be reduced. Consequently, the toner-accommodating chambers cannot be made large enough to accommodate a sufficient amount of toner. 
         [0006]    In view of the foregoing, it is an object of the present invention to provide an image forming apparatus that can be made more compact while allocating sufficient space to accommodate an adequate amount of developer. 
         [0007]    In order to attain the above and other objects, there is provided an image forming apparatus including: a main casing, a supporting frame and a developer container. The main casing includes a first wall and a second wall extending in a first direction and opposing each other in a second direction orthogonal to the first direction. The supporting frame is configured to move relative to the main casing in the first direction between an internal position accommodated in the main casing and an external position withdrawn from the main casing, the supporting frame comprising a first plate and a second plate extending in the first direction and configured to oppose the first wall and the second wall in the second direction respectively when the supporting frame is at the internal position. The developer container is configured to be supported in the supporting frame and configured to store developer therein, the developer container including: a first chamber configured to be disposed between the first plate and the second plate; a second chamber configured to store developer therein and be disposed between the first wall and the first plate when the supporting frame is at the internal position; a third chamber configured to establish communication between the first chamber and the second chamber; and a conveying portion configured to convey the developer stored in the second chamber to the first chamber via the third chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    In the drawings: 
           [0009]      FIG. 1  is a central cross-sectional view of a printer as an example of an image forming apparatus according to a first embodiment of the present invention, wherein the printer includes a process unit therein and the process unit is in an accommodated position; 
           [0010]      FIG. 2  is a right side view showing engagement of process-side positioning plates provided in the process unit and casing-side positioning plates provided in the printer, wherein parts other than essential portions are omitted in  FIG. 2  for explanatory purpose; 
           [0011]      FIG. 3  is a cross-sectional view of the printer according to the first embodiment taken along a line A-A in  FIG. 1 ; 
           [0012]      FIG. 4  is a central cross-sectional view of the printer according to the first embodiment when the process unit is in a withdrawn position, the process unit including a plurality of pivoting parts to receive corresponding developing cartridges; 
           [0013]      FIG. 5A  is a right side view explaining detachment and attachment of the developing cartridges relative to the process unit, wherein the pivoting parts are in their first position; 
           [0014]      FIG. 5B  is a right side view explaining detachment and attachment of the developing cartridges relative to the process unit, wherein the pivoting parts are in their second position; 
           [0015]      FIG. 6A  is a cross-sectional rear side view explaining detachment and attachment of the developing cartridges relative to the process unit, wherein the pivoting parts are in their first position; 
           [0016]      FIG. 6B  is a cross-sectional rear side view explaining detachment and attachment of the developing cartridges relative to the process unit, wherein the pivoting parts are in their second position; 
           [0017]      FIG. 7  is a vertical cross-sectional view of a process unit of an image forming apparatus according to a second embodiment of the present invention; 
           [0018]      FIG. 8  is a central cross-sectional view of a printer as an example of an image forming apparatus according to a third embodiment of the present invention, wherein a process unit of the third embodiment is in the accommodated position; 
           [0019]      FIG. 9  is a central cross-sectional view of the printer according to the third embodiment when the process unit of the third embodiment is in the withdrawn position; and 
           [0020]      FIG. 10  is a cross-sectional view of the printer according to the third embodiment taken along a line B-B shown in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
     1. Overall Structure of Printer 
       [0021]    A printer  1  is a direct horizontal tandem-type color printer, as shown in  FIG. 1 . The printer  1  is an example of an image forming apparatus according to a first embodiment of the present invention. 
         [0022]    In the following description, 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. Hence, the side of the printer  1  at the top of  FIG. 1  will be considered the upper side, and the side at the bottom will be considered the lower side. Further, the left side of the printer  1  in  FIG. 1  will be considered the front side, and the right side will be considered the rear side. Left and right sides of the printer  1  will be based on the perspective of a user facing the front of the printer  1 . Hence, the near side of the printer  1  in  FIG. 1  will be considered the right side, and the far side will be considered the left side. 
         [0023]    The printer  1  is provided with a main casing  2  within which provided are a sheet-feeding unit  3  for feeding sheets of paper P to be printed, an image-forming unit  4  for forming images on the sheets P fed by the sheet-feeding unit  3 , and a discharge unit  5  for receiving the sheets P discharged on which images have been formed. An image-reading unit  6  is provided above the main casing  2  for reading image data of originals. 
       (1) Main Casing 
       [0024]    The main casing  2  has a box-like shape. The main casing  2  is provided with a front cover  7 . 
         [0025]    The front cover  7  is provided on the front side of the main casing  2  and is capable of pivoting about a lower end portion of a front wall of the main casing  2 . When the front cover  7  is pivoted downward away from the main casing  2 , a process unit  16  described later can be slid into and out of the main casing  2 . 
       (2) Sheet-Feeding Unit 
       [0026]    The sheet-feeding unit  3  includes a paper tray  8 , a pickup roller  9 , a feeding roller  10 , a feeding pad  11 , a pair of pinch rollers  12 , and a pair of registration rollers  13 . 
         [0027]    The paper tray  8  is disposed in a bottom section of the main casing  2 . The paper tray  8  is detachably mounted in the main casing  2 . The paper tray  8  functions to accommodate sheets of paper P. 
         [0028]    The pickup roller  9  is disposed above a rear end of the paper tray  8 . 
         [0029]    The feeding roller  10  is disposed rearward of the pickup roller  9 . 
         [0030]    The feeding pad  11  is disposed beneath the feeding roller  10  so as to contact the bottom peripheral surface thereof. 
         [0031]    The pinch rollers  12  are juxtaposed vertically and positioned to contact the rear peripheral surface of the feeding roller  10 . 
         [0032]    The registration rollers  13  are disposed above the feeding roller  10  and confront each other in a front-rear direction. 
         [0033]    The pickup roller  9  rotates to pick up sheets of paper P in the paper tray  8  and convey the sheets P between the feeding roller  10  and feeding pad  11 , whereby the rotation of the feeding roller  10  separates and feeds the sheets P one sheet at a time. The rotating feeding roller  10  subsequently conveys each sheet of paper P upward toward the registration rollers  13  so that the sheets P pass sequentially between each pinch roller  12  and the feeding roller  10 . The registration rollers  13  rotate in order to convey the sheets P at a prescribed timing between an intermediate transfer belt  44  and a secondary transfer roller  41 , both described later. 
       (3) Image Forming Unit 
       [0034]    The image-forming unit  4  includes a scanning unit  15 , a process unit  16 , a transfer unit  17 , and a fixing unit  18 . 
         [0035]    (3-1) Scanning Unit 
         [0036]    The scanning unit  15  is disposed in a lower section of the main casing  2  above the paper tray  8 . The scanning unit  15  emits four laser beams toward respective photosensitive drums  29  (described later), the paths of which are depicted by solid lines in  FIG. 1 , thereby exposing the photosensitive drums  29 . 
         [0037]    (3-2) Process Unit 
         [0038]    The process unit  16  is disposed above the scanning unit  15  in an approximate vertical center of the main casing  2 . The process unit  16  includes a process frame  20 , and four developing cartridges  21 . 
         [0039]    The process frame  20  has a frame-like structure that is generally rectangular in a plan view. The process frame  20  includes four drum frames  23 . 
         [0040]    The drum frames  23  are provided inside the process frame  20  and are arranged at intervals in the front-rear direction. Each drum frame  23  is provided with a pair of side walls  26 , and a rear wall  27 . 
         [0041]    The side walls  26  are arranged parallel to each other and are spaced apart in a left-right direction. The side walls  26  have a flat plate shape that is generally rectangular in a side view. 
         [0042]    The rear wall  27  integrally bridges rear ends of the side walls  26 . The rear wall  27  has a general flat plate shape that is elongated in the left-right direction. 
         [0043]    Each drum frame  23  is further provided with a photosensitive drum  29 , and a scorotron charger  30 . 
         [0044]    The photosensitive drum  29  has a general cylindrical shape with its axis oriented in the left-right direction. The photosensitive drum  29  is rotatably supported to the side walls  26 . 
         [0045]    The scorotron charger  30  is embedded in a lower portion of the rear wall  27  at a position obliquely downward and rearward of the photosensitive drum  29 . The scorotron charger  30  is spaced away from the photosensitive drum  29 . 
         [0046]    Each drum frame  23  is also provided with a drum cleaner  31 . 
         [0047]    The drum cleaner  31  is disposed on a top end portion of the rear wall  27  and is positioned to contact a rear peripheral surface of the photosensitive drum  29 . The drum cleaner  31  functions to clean the peripheral surface of the photosensitive drum  29 . 
         [0048]    The process frame  20  is also provided with a belt cleaner  24 . 
         [0049]    The belt cleaner  24  is disposed in a front end portion of the process frame  20  at a position for contacting a lower portion of an intermediate transfer belt  44  of a belt unit  40  described later. The belt cleaner  24  functions to clean the intermediate transfer belt  44 . 
         [0050]    The four developing cartridges  21  are disposed at positions beneath the corresponding photosensitive drums  29 . Each developing cartridge  21  includes a developing frame  33 , and a developing roller  34 . 
         [0051]    The developing frame  33  has a box-like shape that is open on the top, and is elongated in the left-right direction. The developing frame  33  serves to accommodate toner. 
         [0052]    The developing roller  34  is disposed in an upper end portion of the developing frame  33 . A peripheral surface of the developing roller  34  is partially exposed from the developing frame  33  on its top side. The developing roller  34  is rotatably supported to the developing frame  33  by a developing-roller shaft  92  that penetrates side walls of the developing frame  33  and protrudes outward therefrom. The developing roller  34  contacts the lower peripheral surface of the photosensitive drum  29 . 
         [0053]    The developing cartridge  21  is further provided with a supply roller  35  for supplying toner to the developing roller  34 , and a thickness-regulating blade  36  for regulating the thickness of the toner layer carried on the developing roller  34 . 
         [0054]    (3-3) Transfer Unit 
         [0055]    The transfer unit  17  is disposed in an upper section of the main casing  2  above the process unit  16 . The transfer unit  17  includes the belt unit  40 , and a secondary transfer roller  41 . 
         [0056]    The belt unit  40  is elongated in the front-rear direction so as to be positioned above all the photosensitive drums  29 . The belt unit  40  includes a drive roller  42 , a follow roller  43 , the intermediate transfer belt  44 , and four primary transfer rollers  45 . 
         [0057]    The drive roller  42  is rotatably supported at a rear end of the transfer unit  17 . 
         [0058]    The follow roller  43  is rotatably supported at a front end of the transfer unit  17 . 
         [0059]    The intermediate transfer belt  44  is mounted on and looped around the drive roller  42  and follow roller  43  so that the lower portion of the loop contacts the top peripheral surfaces of all photosensitive drums  29 . When the drive roller  42  is driven to rotate, the intermediate transfer belt  44  circulates so that its lower portion moves rearward, and the follow roller  43  rotates along with the circulating movement of the intermediate transfer belt  44 . 
         [0060]    The primary transfer rollers  45  are arranged parallel to each other and spaced at intervals in the front-rear direction. More specifically, the primary transfer rollers  45  are disposed between the drive roller  42  and follow roller  43  within the loop of the intermediate transfer belt  44  at positions above the corresponding photosensitive drums  29 . The primary transfer rollers  45  contact the lower portion of the intermediate transfer belt  44  from above. 
         [0061]    The secondary transfer roller  41  is disposed on the rear side of the drive roller  42 , with the intermediate transfer belt  44  nipped therebetween. 
         [0062]    (3-4) Fixing Unit 
         [0063]    The fixing unit  18  is disposed obliquely above and forward of the secondary transfer roller  41 . The fixing unit  18  includes a heating roller  48 , and a pressure roller  49  that contacts a rear peripheral surface of the heating roller  48  with pressure. 
         [0064]    (3-5) Image-Forming Operation 
         [0065]    (3-5-1) Developing Operation 
         [0066]    In an image-forming operation, the supply roller  35  rotates to supply toner from the developing cartridge  21  onto the developing roller  34 . The toner is tribocharged between the supply roller  35  and developing roller  34 . The thickness-regulating blade  36  regulates the thickness of toner supplied to the developing roller  34  as the developing roller  34  rotates, maintaining the layer of toner on the peripheral surface of the developing roller  34  at a thin uniform thickness. 
         [0067]    In the meantime, the scorotron charger  30  applies a uniform charge to the peripheral surface of the photosensitive drum  29  as the photosensitive drum  29  rotates. 
         [0068]    Subsequently, the scanning unit  15  exposes the surface of the photosensitive drum  29  based on prescribed image data, forming an electrostatic latent image on the surface of the photosensitive drum  29  based on the image data. Next, the toner carried on the surface of the developing roller  34  is supplied to the latent image formed on the surface of the photosensitive drum  29 , thereby developing the latent image into a toner image. 
         [0069]    (3-5-2) Transferring and Fixing Operations 
         [0070]    A primary transfer is performed by sequentially transferring toner images carried on the surfaces of the photosensitive drums  29  onto the lower portion of the intermediate transfer belt  44 . Through this primary transfer, the photosensitive drums  29  form a color image on the surface of the intermediate transfer belt  44 . As the intermediate transfer belt  44  passes through a contact position with the secondary transfer roller  41 , the color image formed on the surface of the intermediate transfer belt  44  is transferred in a secondary transfer onto a sheet of paper P supplied from the sheet-feeding unit  3 . Next, the color image transferred onto the sheet P is fixed to the sheet P by heat and pressure as the sheet P passes between the heating roller  48  and pressure roller  49  in the fixing unit  18 . 
       (4) Discharge Unit 
       [0071]    The discharge unit  5  protrudes upward from the top of the main casing  2  on a rear side thereof. The discharge unit  5  includes a discharge opening  52 , and three discharge rollers  53 . 
         [0072]    The discharge opening  52  is formed in a front surface of the discharge unit  5  and provides communication between interior and exterior of the main casing  2 . 
         [0073]    The discharge rollers  53  are positioned to guide the sheets of paper P being discharged through the discharge opening  52 . 
         [0074]    The top surface of the main casing  2  forward of the discharge unit  5  functions as a discharge tray  54 . After a toner image has been fixed to a sheet of paper P in the fixing unit  18 , the discharge rollers  53  discharge the sheet P through the discharge opening  52  onto the discharge tray  54 . 
       (5) Image-Reading Unit 
       [0075]    The image-reading unit  6  is disposed above the main casing  2  so as to cover the discharge unit  5 . The image-reading unit  6  has a general rectangular shape in a plan view, with approximately the same front-rear and left-right dimensions as the main casing  2 . The image-reading unit  6  includes a flatbed  56 , and an original cover  57 . 
         [0076]    The flatbed  56  has a box-like shape and a general rectangular shape in a plan view. The flatbed  56  includes a glass surface  58 . 
         [0077]    The glass surface  58  constitutes the top surface of the flatbed  56  and serves to support an original. 
         [0078]    The original cover  57  has a thick plate shape that is generally rectangular in a plan view. The original cover  57  is pivotably supported on the flatbed  56  and covers the top of the flatbed  56 . 
         [0079]    The image-reading unit  6  reads image data from an original placed between the glass surface  58  of the flatbed  56  and the original cover  57 . As described above, the image-forming unit  4  can form images on sheets of paper P based on image data read from an original. 
       2. Detailed Description of the Process Unit 
     (1) Process Frame 
       [0080]    The process frame  20  can slide along the front-rear direction between an internal position shown in  FIG. 1  in which the process frame  20  is positioned inside the main casing  2 , and an external position shown in  FIG. 4  in which the process frame  20  is withdrawn to the outside of the main casing  2 . As shown in  FIGS. 5 and 6 , the process frame  20  includes an outer frame  65 , a pair of left and right process-side positioning plates  66 , a process-side positioning shaft  67 , and the drum frames  23  described above. 
         [0081]    The outer frame  65  is formed of a hard resin material. The outer frame  65  has a frame-like structure with a general rectangular shape in a plan view. The outer frame  65  includes a pair of left and right side walls  70 , a front wall  71 , a rear wall  72 , and a pivoting part  73 . 
         [0082]    The side walls  70  are disposed at positions spaced apart in the left-right direction. The side walls  70  have a generally flat plate shape that extends both vertically and in the front-rear direction. When it is necessary in the following description to distinguish the left and right side walls  70  from each other, the side wall  70  on the left will be referred to as the left side wall  70 L, while the side wall  70  on the right will be referred to as the right side wall  70 R. 
         [0083]    Each side wall  70  is configured of a guide roller  76 , a guide rail  77 , and a notch  78 . 
         [0084]    The guide roller  76  has a general cylindrical shape and extends in the left-right direction. The guide roller  76  is rotatably disposed on the upper rear corner of the side wall  70 . 
         [0085]    The guide rail  77  is formed on the top edge of the side wall  70 , from the front end of the side wall  70  to a point forward of the guide roller  76 , and protrudes outward in the respective left-right direction to form a ridge-like shape. 
         [0086]    The notch  78  is cut out in an upper rear portion of the side wall  70 , forming a generally square U-shape in a side view that is recessed forward from the rear edge of the side wall  70  and is open on the rear side. 
         [0087]    The right side wall  70 R further includes a developing-cartridge extraction hole  79 . 
         [0088]    The developing-cartridge extraction hole  79  is cut out from the bottom edge of the right side wall  70 R so as to have a generally square shape in a side view with an open bottom. 
         [0089]    The front wall  71  bridges front ends of the side walls  70 . The front wall  71  has a generally flat plate shape and is elongated both vertically and in the left-right direction. 
         [0090]    The rear wall  72  bridges rear ends of the side walls  70 . The rear wall  72  has a generally flat plate shape and is elongated both vertically and in the left-right direction. 
         [0091]    The pivoting part  73  is provided on lower end portions of the side walls  70 . The pivoting part  73  includes four developing-cartridge support parts  81 , and a coupling rail  82 . 
         [0092]    The developing-cartridge support parts  81  are provided in a bottom portion of the outer frame  65  and are arranged at intervals in the front-rear direction to correspond to the positions of the developing cartridges  21 . The developing-cartridge support parts  81  have a tray-like structure that extends in the left-right direction. The developing-cartridge support parts  81  have a left-right dimension greater than the distance between the side walls  70 , i.e., the distance between the left side wall  70 L and right side wall  70 R (see  FIG. 3 ). The developing-cartridge support parts  81  are allowed to pivot about their left ends on the lower portion of the left side wall  70 L. 
         [0093]    The coupling rail  82  is disposed along the bottoms of the developing-cartridge support parts  81  and has a general strip-like shape that spans across the entire front-rear dimension of the developing-cartridge extraction hole  79 . The coupling rail  82  can move vertically along the edges of the right side wall  70 R defining the front and rear edges of the developing-cartridge extraction hole  79 . The coupling rail  82  includes a lever  83 . 
         [0094]    The lever  83  protrudes rightward from an approximate front-rear center region of the coupling rail  82 . 
         [0095]    With this construction, the developing-cartridge support parts  81  can be pivoted together by operating the lever  83  in order to move the coupling rail  82  vertically. In this way, the pivoting part  73  can move between a first position shown in  FIG. 6A  in which the developing-cartridge support parts  81  are supported in a substantially horizontal orientation, and a second position shown in  FIG. 6B  in which the developing-cartridge support parts  81  are supported at an angle sloping downward toward the right. 
         [0096]    The process-side positioning plates  66  are configured to engage with casing-side positioning plates  101  described later in order to fix the positions of the photosensitive drums  29  relative to the main casing  2 . The process-side positioning plates  66  are respectively positioned inside the corresponding side walls  70  so as to be separated from the corresponding side walls  70  in the left-right direction (see  FIG. 3 ). The process-side positioning plates  66  are formed of a metal, such as stainless steel or steel. The process-side positioning plates  66  have a general L-shape in a side view (as indicated by broken lines in  FIGS. 2 ,  5 A and  5 B). Specifically, each process-side positioning plate  66  is configured of a process-side body part  86 , and a process-side extended part  87 . 
         [0097]    Each process-side body part  86  is positioned inward of the top edge of the corresponding side wall  70  with respect to the left-right direction. The process-side body part  86  has a flat plate shape that is generally rectangular in a side view and extends in the front-rear direction, spanning nearly the entire front-rear dimension of the side wall  70 . The process-side body part  86  includes a process-side fitting groove  88 . 
         [0098]    The process-side fitting groove  88  is cut out in a rear end portion of the process-side body part  86  to form a general rectangular shape in a side view that is open on the rear side. The process-side fitting groove  88  is similar in shape to the notch  78  formed in the right side wall  70 R but slightly smaller. When projected in the left-right direction, the process-side fitting groove  88  is positioned on the inside of the notch  78  formed in the right side wall  70 R and is exposed through the notch  78 . 
         [0099]    The process-side extended part  87  is positioned inward of a front end portion of the corresponding side wall  70  with respect to the left-right direction. The process-side extended part  87  has a flat plate shape that is generally rectangular in a side view. The process-side extended part  87  extends continuously downward from a lower edge on the front end of the process-side body part  86 , spanning nearly the entire vertical dimension of the side wall  70 . 
         [0100]    The process-side positioning shaft  67  is provided in a lower front corner of the process frame  20  and penetrates bottom end portions of the process-side extended parts  87  and lower front corners of the side walls  70  in the left-right direction. The process-side positioning shaft  67  is formed of metal, such as stainless steel or steel. The process-side positioning shaft  67  has a general rod shape and extends in the left-right direction. The left and right ends of the process-side positioning shaft  67  protrude outward in the corresponding left and right directions from outer left and right surfaces of the side walls  70 . 
         [0101]    The drum frames  23  are arranged between the pair of process-side body parts  86  in the left-right direction. The side walls  26  of the drum frames  23  have lower edge portions that are exposed through the developing-cartridge extraction hole  79  formed in the right side wall  70 R in a right side view. Each side wall  26  is provided with a developing-roller-shaft fitting groove  90 . The developing-roller-shaft fitting groove  90  is formed in the lower edge portion of the side wall  26  exposed through the developing-cartridge extraction hole  79 . The developing-roller-shaft fitting grooves  90  are cut out from a lower edge of the side walls  26  and have a general square shape in a side view with an open bottom. 
       (2) Developing Cartridges 
       [0102]    As shown in  FIG. 3 , each developing cartridge  21  is disposed on a top surface of each developing-cartridge support pars  81  provided in the pivoting part  73 . Each developing cartridge  21  is supported in the process frame  20  by fitting both ends of the developing-roller shaft  92  provided in the developing roller  34  into the corresponding developing-roller-shaft fitting grooves  90  formed in the side walls  26  from below. 
         [0103]    The developing frame  33  of the developing cartridge  21  includes a toner-accommodating section  93  that serves to accommodate toner. 
         [0104]    The toner-accommodating section  93  is provided with a first toner-accommodating section  94 , a second toner-accommodating section  95 , a third toner-accommodating section  96 , and a pair of auger screws  97 . 
         [0105]    The first toner-accommodating section  94  occupies approximately the rear half of the developing frame  33 . The first toner-accommodating section  94  has a generally square cylindrical shape and extends in the left-right direction. The first toner-accommodating section  94  is disposed inside the process frame  20 , and specifically between the left side wall  70 L and right side wall  70 R. Hence, the first toner-accommodating section  94  has a left-right dimension that is shorter than the distance between the side walls  70 , i.e., between the left side wall  70 L and right side wall  70 R. The interior of the first toner-accommodating section  94  is divided vertically by the border between the pair of auger screws  97 . 
         [0106]    The second toner-accommodating section  95  has a box-like shape and is elongated vertically. Hence, the second toner-accommodating section  95  has a vertical dimension greater than its left-right dimension. The second toner-accommodating section  95  is arranged such that its upper portion overlaps the photosensitive drum  29  in the left-right direction. When the process frame  20  is disposed in its internal position, the second toner-accommodating section  95  is disposed outside (rightward) of a right casing-side positioning plate  101  described later. More specifically, when the process frame  20  is disposed in its internal position, the second toner-accommodating section  95  is disposed between the right side wall  70 R and a right casing side wall  98 R described later. 
         [0107]    The third toner-accommodating section  96  is formed continuously with a right end of the first toner-accommodating section  94  and extends rightward therefrom, and is connected to a bottom end of the second toner-accommodating section  95  and is integral therewith. That is, the third toner-accommodating section  96  has a left end that is in communication with the right end of the first toner-accommodating section  94 , and an upper-right end portion that is in communication with the bottom end of the second toner-accommodating section  95 . The third toner-accommodating section  96  is positioned to penetrate, in the left-right direction, an opening  107  that is defined by the right process-side body part  86 , the right process-side extended part  87 , a right casing-side body part  103 , and a right casing-side extended part  104  when the process unit  20  is in the internal position, as will be described later. Put another way, the third toner-accommodating section  96  is aligned with the right end of the first toner-accommodating section  94  at the opening  107  in the left-right direction. As with the first toner-accommodating section  94 , the interior of the third toner-accommodating section  96  is divided vertically by the border between the pair of auger screws  97 . 
         [0108]    The auger screws  97  are disposed in the first toner-accommodating section  94  and third toner-accommodating section  96  and extend in the left-right direction. The auger screws  97  are arranged parallel to each other and juxtaposed vertically. The left ends of the auger screws  97  are rotatably supported in the left wall of the first toner-accommodating section  94 , and the right ends are rotatably supported in the right wall of the third toner-accommodating section  96 . Note that the lower auger screw  97  conveys toner from the second toner-accommodating section  95  into the first toner-accommodating section  94  via the third toner-accommodating section  96 , while the upper auger screw  97  conveys toner from the first toner-accommodating section  94  to the third toner-accommodating section  96 , thereby circulating toner in left and right directions within the toner-accommodating section  93 . 
         [0109]    With this configuration, when the process frame  20  is disposed in its internal position, each photosensitive drum  29  is positioned below the intermediate transfer belt  44 , and the corresponding first toner-accommodating section  94  is disposed below the photosensitive drum  29 . In other words, the first toner-accommodating section  94  is positioned at a side opposite to the intermediate transfer belt  44  with respect to the photosensitive drum  29 . 
       3. Detailed Description of the Main Casing 
       [0110]    As shown in  FIGS. 3 and 4 , the main casing  2  is configured of a pair of casing side walls  98 , and a pair of guide plates  99 . 
         [0111]    The casing side walls  98  are arranged parallel to each other and are separated in the left-right direction. The casing side walls  98  have a generally flat plate shape that extends in the front-rear and vertical directions. When it is necessary to distinguish the left and right casing side walls  98  in the following description, the casing side wall  98  on the left side will be called the left casing side wall  98 L, while the casing side wall  98  on the right side will be called the right casing side wall  98 R. 
         [0112]    The guide plates  99  are positioned inside the respective casing side walls  98  in the left-right direction and are separated therefrom. The guide plates  99  have a generally flat plate shape and are elongated in the front-rear direction. When it is necessary to distinguish the left and right guide plates  99  in the following description, the guide plate  99  on the left side will be called the left guide plate  99 L, while the guide plate  99  on the right side will be called the right guide plate  99 R. 
         [0113]    The left guide plate  99 L is positioned between the left casing side wall  98 L and the left side wall  70 L of the process frame  20  when the process frame  20  is in its internal position. 
         [0114]    The right guide plate  99 R is positioned between the right casing side wall  98 R and the right side wall  70 R of the process frame  20  when the process frame  20  is in its internal position. The distance between the right guide plate  99 R and right casing side wall  98 R is greater than that between the left guide plate  99 L and left casing side wall  98 L. More specifically, the distance between the right guide plate  99 R and right casing side wall  98 R is greater than the left-right dimension of the second toner-accommodating section  95 . 
         [0115]    Each guide plate  99  includes a guide groove  100 . 
         [0116]    The guide groove  100  has a general linear shape that extends in the front-rear direction at a position lower than the belt unit  40 . The guide groove  100  is recessed into an inner left-right surface of the guide plate  99 . The guide grooves  100  slidably receive the guide rollers  76  and guide rails  77  of the process frame  20 . 
         [0117]    As shown in  FIGS. 2 and 3 , the main casing  2  is also provided with a pair of left and right casing-side positioning plates  101 , and a casing-side positioning shaft  102 . 
         [0118]    The casing-side positioning plates  101  function to position the photosensitive drums  29  within the main casing  2  when the process frame  20  is disposed in its internal position. The casing-side positioning plates  101  are respectively positioned within the main casing  2  outward of the scanning unit  15  in the left-right direction, respectively, and are positioned apart from the scanning unit  15 . The casing-side positioning plates  101  are formed of a metal, such as stainless steel or steel. The casing-side positioning plates  101  have a general L-shape in a side view. Each casing-side positioning plate  101  includes a casing-side body part  103 , and a casing-side extended part  104 . 
         [0119]    The casing-side body parts  103  are positioned outside of the respective left and right sides of the scanning unit  15  and are not in contact with the same. The casing-side body parts  103  have a flat plate shape that is generally rectangular in a side view and has approximately the same front-rear dimension as the process frame  20 . Each casing-side body part  103  includes a casing-side fitting groove  105 . 
         [0120]    The casing-side fitting groove  105  is formed in a front edge portion of the corresponding casing-side body part  103  so as to recess rearward from its front edge. The casing-side fitting groove  105  has a general rectangular shape in a side view and is open on the front side. When the process frame  20  is disposed in the internal position, the left and right ends of the process-side positioning shaft  67  of the process frame  20  are fitted inside the corresponding casing-side fitting grooves  105 . 
         [0121]    The casing-side extended parts  104  have a flat plate shape and are generally rectangular in a side view. The casing-side extended parts  104  extend continuously upward from top edges of the corresponding casing-side body parts  103  at the rear ends of the same. The casing-side extended parts  104  have top ends that are positioned lower than the rear ends of the guide grooves  100  formed in the corresponding guide plates  99 . 
         [0122]    The casing-side positioning shaft  102  bridges the upper ends of the casing-side extended parts  104 . The casing-side positioning shaft  102  is formed of a metal, such as stainless steel or steel. The casing-side positioning shaft  102  has a general rod shape and is oriented in the left-right direction. When the process frame  20  is in the internal position, the casing-side positioning shaft  102  is fitted inside the process-side fitting grooves  88  formed in the process-side positioning plates  66 . 
         [0123]    Thus, by engaging the casing-side positioning plates  101  with the corresponding process-side positioning plates  66 , the casing-side body parts  103  and corresponding process-side body parts  86  are in positions separated vertically, and the casing-side extended parts  104  and corresponding process-side extended parts  87  are in positions separated in the front-rear direction. Thus, the opening  107  is defined, on each of the left and right sides, by the casing-side body parts  103 , process-side body parts  86 , casing-side extended parts  104 , and process-side extended parts  87 . Hereinafter, for explanatory purpose, the opening on the right defined by the right casing-side body part  103 , right process-side body part  86 , right casing-side extended part  104 , and right process-side extended part  87  is referred to as the opening  107 . 
       4. Mounting and Removing Developing Cartridges 
       [0124]    The developing cartridges  21  are mounted in and removed from the main casing  2  by an operator. To remove a developing cartridge  21  from the main casing  2 , first the operator opens the front cover  7  and pulls the process unit  16  (process frame  20 ) forward toward the position shown in  FIG. 4 . Through this operation, the process-side positioning shaft  67  is removed from the casing-side fitting grooves  105  formed in the casing-side positioning plates  101  and the casing-side positioning shaft  102  is removed from the process-side fitting grooves  88  formed in the process-side positioning plates  66 . The operator pulls the process unit  16  (process frame  20 ) out to the external position as the guide rollers  76  and guide rails  77  on the process frame  20  are guided along the guide grooves  100  of the main casing  2 . 
         [0125]    Next, the operator operates the lever  83  on the process frame  20  to move the pivoting part  73  from its first position (see  FIGS. 5A and 6A ) to its second position (see  FIGS. 5B and 6B ). Through this operation, the right ends of the developing-roller shafts  92  on all developing cartridges  21  are extracted (removed) from the right developing-roller-shaft fitting grooves  90 . 
         [0126]    At this time, the developing cartridges  21  are supported on the top surfaces of the corresponding developing-cartridge support parts  81  at an angle sloping downward toward the right. Accordingly, the operator can remove a developing cartridge  21  from the process frame  20  by pulling the developing cartridge  21  diagonally downward and rightward along the top surface of the developing-cartridge support pars  81 , as illustrated by dashed lines in  FIG. 6B . 
         [0127]    This completes the operation for removing a developing cartridge  21  from the main casing  2 . To mount a developing cartridge  21  in the main casing  2 , the operation for removing a developing cartridge  21  described above is performed in reverse. 
         [0128]    More specifically, the operator places the process unit  16  in the external position shown in  FIG. 4  and places the pivoting part  73  in the second position shown in  FIGS. 5B and 6B . Next, the operator places the left end of the developing cartridge  21  on the top surface of the corresponding developing-cartridge support pars  81  and pushes the developing cartridge  21  diagonally upward and leftward into the process unit  16  along the surface of the developing-cartridge support pars  81 . As a result, the left end of the developing-roller shaft  92  becomes fitted into the left developing-roller-shaft fitting groove  90 . 
         [0129]    Next, the operator operates the lever  83  to move the pivoting part  73  from the second position back to the first position shown in  FIGS. 5A and 6A . This operation fits the right end of the developing-roller shaft  92  into the right developing-roller-shaft fitting groove  90  so that the developing cartridge  21  is supported in the process frame  20 . 
         [0130]    Next, the operator pushes the process unit  16  into the main casing  2  from the state shown in  FIG. 4 . At this time, the process unit  16  slides rearward, with the guide rollers  76  and guide rails  77  of the process frame  20  guided in the guide grooves  100  formed in the main casing  2 . As a result of this operation, the process-side positioning shaft  67  becomes fitted into the casing-side fitting grooves  105  of the casing-side positioning plates  101  and the casing-side positioning shaft  102  becomes fitted into the process-side fitting grooves  88  of the process-side positioning plates  66 , placing the process unit  16  in its internal position shown in  FIG. 1 . Lastly, the operator closes the front cover  7 , thereby completing the operation to mount a developing cartridge  21  in the main casing  2 . 
       5. Operational Advantages 
       [0131]    (1) In the printer  1  according to the first embodiment, the toner-accommodating section  93  includes the first toner-accommodating section  94 , second toner-accommodating section  95 , and third toner-accommodating section  96 . When the developing cartridge  21  is supported in the process frame  20 , the first toner-accommodating section  94  is positioned between the left side wall  70 L and right side wall  70 R, the second toner-accommodating section  95  is positioned between the right side wall  70 R and right casing side wall  98 R, and the third toner-accommodating section  96  is configured to connect the first toner-accommodating section  94  and the second toner-accommodating section  95 , as shown in  FIG. 3 . 
         [0132]    Hence, with respect to the left-right direction, the first toner-accommodating section  94  of the toner-accommodating section  93  is positioned in the process frame  20  between the left side wall  70 L and right side wall  70 R, and the second toner-accommodating section  95  of the toner-accommodating section  93  is positioned between the right side wall  70 R and right casing side wall  98 R, i.e., between the main casing  2  and process frame  20 . Hence, the second toner-accommodating section  95  can accommodate a sufficient amount of toner, while the developing cartridge  21  can be made more compact within the process frame  20 . Thus, the overall printer  1  can be made smaller. 
         [0133]    (2) With the printer  1  according to the first embodiment, the second toner-accommodating sections  95  having a greater vertical dimension than left-right dimension can be arranged in the space between the right casing side wall  98 R and right side wall  70 R, i.e., between the main casing  2  and process frame  20 , as illustrated in  FIG. 3 . Hence, the toner capacity of the second toner-accommodating sections  95  can be increased, even though the space between the main casing  2  and process frame  20  is narrow. As a result, the toner capacity of the first toner-accommodating sections  94  positioned inside the process frame  20  can be reduced in order to reduce the size of the developing cartridges  21  positioned within the process frame  20 . 
         [0134]    (3) By pulling the process frame  20  out to the external position and placing the pivoting part  73  in the second position as illustrated in  FIGS. 6A and 6B , the developing cartridge  21  can easily be pulled out of the process frame  20  along the top surface of the developing-cartridge support pars  81  in a direction diagonally downward and rightward. In other words, the developing cartridge  21  is pulled in a direction with the second toner-accommodating section  95  on the downstream end. 
         [0135]    While conventionally developing cartridges  21  accommodated in the process frame  20  have been difficult to be extracted therefrom when the process frame  20  is in the external position shown in  FIG. 4 , the developing cartridges  21  are easy to be extracted in the printer  1  of the depicted embodiment because the second toner-accommodating sections  95  protrude from the right side of the process frame  20 . 
         [0136]    (4) Since the developing cartridges  21  can be made more compact within the process frame  20 , as shown in  FIG. 3 , the configuration according to the first embodiment prevents the printer  1  from becoming larger when the photosensitive drums  29  are supported in the process frame  20 . 
         [0137]    (5) Since the developing cartridges  21  can be made more compact within the process frame  20 , as shown in  FIG. 3 , the configuration according to the preferred embodiment prevents the printer  1  from becoming larger when the developing cartridges  21  are provided with the developing rollers  34 . 
         [0138]    (6) Since the intermediate transfer belt  44 , photosensitive drum  29 , and toner-accommodating section  93  are arranged vertically within the main casing  2 , as shown in  FIG. 3 , there is a danger that the vertical dimension of the printer  1  will become larger. However, in addition to the first toner-accommodating section  94  disposed in the process frame  20 , the toner-accommodating section  93  also includes the second toner-accommodating section  95  disposed in the space between the right side wall  70 R and right casing side wall  98 R, i.e., between the main casing  2  and process frame  20 . Therefore, the first toner-accommodating section  94  can be made more compact while enabling the toner-accommodating section  93  to hold a sufficient quantity of toner due to the provision of the second toner-accommodating section  95 . This configuration prevents the developing cartridge  21  from becoming too large in its vertical dimension achieving the low-profile printer  1 . 
         [0139]    (7) As shown in  FIG. 3 , the second toner-accommodating sections  95  are accommodated in the printer  1  in the space between the right side wall  70 R positioned to the right of the photosensitive drums  29 , and the right casing side wall  98 R. By using the space in the printer  1  to the right of the photosensitive drums  29  to accommodate toner, the vertical dimension of the printer  1  can be made more compact. 
         [0140]    (8) As shown in  FIGS. 2 and 3 , the casing-side positioning plates  101  can reliably position the photosensitive drums  29  supported in the process frame  20  relative to the main casing  2  while also supporting the second toner-accommodating sections  95  positioned further right of the right casing-side positioning plate  101 . Hence, this configuration ensures that the photosensitive drums  29  are positioned accurately relative to the main casing  2 , while allowing the vertical dimension of the printer  1  to be made more compact. 
         [0141]    (9) By fitting the left and right ends of the process-side positioning shaft  67  into the casing-side fitting grooves  105  and fitting the casing-side positioning shaft  102  in the process-side fitting grooves  88  as shown in  FIGS. 2 and 3 , a vertical gap can be formed between the casing-side body parts  103  of the casing-side positioning plates  101  and the respective process-side body parts  86  of the process-side positioning plates  66  (on both left and right sides) when the casing-side positioning plates  101  are engaged with the process-side positioning plates  66 . The third toner-accommodating section  96  is then disposed between the casing-side body part  103  and process-side body part  86  on the right side. Accordingly, by disposing the third toner-accommodating section  96  in this gap between the casing-side body part  103  and process-side body part  86 , it is possible to provide the second toner-accommodating section  95  on the outside (right side) of the right casing-side positioning plate  101 . 
         [0142]    (10) As illustrated in  FIG. 2 , the casing-side positioning plates  101  and process-side positioning plates  66  can be reliably engaged to each other by engaging the process-side positioning shaft  67  penetrating the lower edges of the process-side extended part  87  with the casing-side fitting grooves  105  formed in the casing-side body parts  103  and by engaging the casing-side positioning shaft  102  bridging the upper ends of the casing-side extended parts  104  with the process-side fitting grooves  88  formed in the process-side body parts  86 . This structure not only ensures sufficient rigidity of the printer  1 , but also provides the printer  1  with the opening  107  defined by the casing-side positioning plate  101  and process-side positioning plates  66  on the right side. As a result, the photosensitive drums  29  can be accurately positioned relative to the main casing  2 , and the toner-accommodating sections  93  can be extended (enlarged) rightward through the opening  107 . 
         [0143]    (11) As shown in  FIG. 3 , the third toner-accommodating section  96  of the toner-accommodating section  93  is inserted through the opening  107  formed by the casing-side body part  103 , casing-side extended part  104 , process-side body part  86 , and process-side extended part  87  on the right side, enabling the second toner-accommodating section  95  to be provided rightward from the right end of the photosensitive drum  29 . This construction ensures the rigidity of the printer  1  and enables the printer  1  to be made more compact in its vertical dimension. Moreover, by accommodating toner in the second toner-accommodating section  95 , the printer  1  can be provided with a sufficient amount of toner. 
       6. Second Embodiment 
       [0144]    A toner-accommodating section  93  according to a second embodiment will be described with reference to  FIG. 7 , wherein like parts and components are designated with the same reference numerals with those of the first embodiment to avoid duplicating description. 
         [0145]    In the developing cartridge  21  according to the first embodiment described above, the toner-accommodating section  93  is integrally configured of the first toner-accommodating section  94 , second toner-accommodating section  95 , and third toner-accommodating section  96 . 
         [0146]    In contrast, the toner-accommodating section  93  of the second embodiment includes a first toner-accommodating section  110 , a third toner-accommodating section  112  integrally configured with the first toner-accommodating section  110 , and a second toner-accommodating section  111  detachably mounted on the third toner-accommodating section  112 . 
         [0147]    More specifically, the first toner-accommodating section  110  and third toner-accommodating section  112  are integrally configured so as to be in communication with each other internally. The third toner-accommodating section  112  is provided with a first communication opening  114 , and the second toner-accommodating section  111  is provided with a second communication opening  115 . 
         [0148]    The first communication opening  114  is formed on a top wall of the third toner-accommodating section  112  and vertically penetrates the same so as to provide communication between the interior and exterior of the third toner-accommodating section  112 . 
         [0149]    The second communication opening  115  is formed on a bottom wall of the second toner-accommodating section  111  and vertically penetrates the same so as to provide communication between the interior and exterior of the second toner-accommodating section  111 . 
         [0150]    The second toner-accommodating section  111  is disposed above the third toner-accommodating section  112 . The second toner-accommodating section  111  is engaged with the third toner-accommodating section  112  through engaging parts (not shown) so that the first communication opening  114  and second communication opening  115  are in communication with each other. 
         [0151]    When the process frame  20  is disposed in the external position, the second toner-accommodating section  111  alone can be separated from the third toner-accommodating section  112  by disengaging the engaging parts. Hence, only the second toner-accommodating section  111  can be removed from the toner-accommodating section  93 . 
         [0152]    As illustrated by dashed lines in  FIG. 7 , the toner-accommodating section  93  according to the second embodiment can be replenished with toner through the simple operation of replacing only the second toner-accommodating section  111  on the third toner-accommodating section  112 , rather than replacing the entire developing cartridge  21 . 
         [0153]    The printer according to the second embodiment can obtain the same operational and technical advantages described in the first embodiment. 
       7. Third Embodiment 
       [0154]    While the printer  1  according to the first embodiment described above is configured as an intermediate transfer-type color printer, a printer  120  according to a third embodiment of the present invention is configured as a direct tandem-type color printer. 
         [0155]    Next, the printer  120  of the third embodiment will be described with reference to  FIGS. 8 through 10 , wherein like parts and components are designated with the same reference numerals with those of the first embodiment to avoid duplicating description. 
       (1) Printer According to the Third Embodiment 
       [0156]    The printer  120  shown in  FIG. 8  is a horizontal direct tandem-type laser printer. The printer  120  includes the main casing  2  and, within the main casing  2 , provided are the sheet-feeding unit  3  for feeding sheets of paper P, and the image-forming unit  4  for forming images on the sheets of paper P supplied by the sheet-feeding unit  3 . 
         [0157]    The sheet-feeding unit  3  conveys the sheets of paper P accommodated in the paper tray  8  between the photosensitive drums  29  and a conveying belt  128  described later at a prescribed timing. 
         [0158]    The image-forming unit  4  is disposed above the sheet-feeding unit  3 . The image-forming unit  4  includes the scanning unit  15 , the process unit  16 , and a transfer unit  123 . 
         [0159]    The scanning unit  15  is disposed in the top section of the main casing  2 . 
         [0160]    The process unit  16  is disposed beneath the scanning unit  15  in the approximate vertical center of the main casing  2 . The process unit  16  of the third embodiment is provided with the process frame  20 , and the plurality of developing cartridges  21 . 
         [0161]    The process frame  20  includes the plurality of photosensitive drums  29 , a plurality of charging rollers  125 , and a plurality of drum cleaning unit  126 . 
         [0162]    The photosensitive drums  29  are arranged at intervals in the front-rear direction. 
         [0163]    The charging rollers  125  are disposed in positions for contacting the upper rear surfaces of the corresponding photosensitive drums  29 . 
         [0164]    The drum-cleaning units  126  are disposed in positions for contacting rear surfaces of the corresponding photosensitive drums  29 . 
         [0165]    The developing cartridges  21  of the third embodiment are disposed above the corresponding photosensitive drums  29 . Each developing cartridge  21  includes the developing frame  33  and the developing roller  34 . 
         [0166]    The developing frame  33  has a box-like shape that is open on the lower rear side. The developing frame  33  is elongated in the left-right direction. 
         [0167]    The developing roller  34  is disposed in the bottom end of the corresponding developing frame  33 , with a portion of its peripheral surface exposed obliquely below and rearward of the developing frame  33 . The developing roller  34  is rotatably supported in the developing frame  33  and is in contact with upper front surface of the corresponding photosensitive drum  29 . 
         [0168]    The transfer unit  123  is disposed in the main casing  2  at a position above the sheet-feeding unit  3  and below the process unit  16 . The transfer unit  123  includes the drive roller  42 , the follow roller  43 , a conveying belt  128 , and a plurality of transfer rollers  129 . 
         [0169]    The drive roller  42  is rotatably supported in the rear end of the transfer unit  123 . 
         [0170]    The follow roller  43  is rotatably supported in the front end of the transfer unit  123 . 
         [0171]    The conveying belt  128  is mounted on and looped around the drive roller  42  and follow roller  43  such that the upper portion of the loop opposes and contacts the bottom surfaces of all photosensitive drums  29 . When the drive roller  42  is driven to rotate, the conveying belt  128  circulates so that its upper portion moves rearward, and the follow roller  43  follows the circulation of the conveying belt  128 . 
         [0172]    The transfer rollers  129  are arranged parallel to each other and are spaced at intervals in the front-rear direction. Specifically, the transfer rollers  129  are disposed inside the loop of the conveying belt  128  between the drive roller  42  and follow roller  43  at positions beneath the corresponding photosensitive drums  29 . The transfer rollers  129  contact the upper portion of the conveying belt  128  from below. 
         [0173]    When the sheet-feeding unit  3  feeds a sheet of paper P onto the conveying belt  128 , the conveying belt  128  conveys the sheet rearward so that the sheet P passes sequentially between the photosensitive drums  29  and corresponding transfer rollers  129 . As the sheet P is conveyed on the conveying belt  128 , toner images carried on the surfaces of the photosensitive drums  29  are sequentially transferred onto the sheet P to form a color image thereon. 
       (2) Process Unit According to the Third Embodiment 
       [0174]    (2-1) Process Frame 
         [0175]    The process frame  20  can slide with respect to the front-rear direction between an internal position shown in  FIG. 8  in which the process frame  320  is disposed inside the main casing  2 , and an external position shown in  FIG. 9  in which the process frame  20  is withdrawn to the outside of the main casing  2 . 
         [0176]    The process frame  20  includes the pair of process-side positioning plates  66 , and the process-side positioning shaft  67 . 
         [0177]    Each process-side positioning plate  66  includes the process-side body part  86  and the process-side extended part  87 . 
         [0178]    The process-side body parts  86  are disposed inward of the corresponding side wall  70  constituting the outer frame  65  with respect to the left-right direction, and specifically inward of the lower portions of the side walls  70 . 
         [0179]    The process-side extended parts  87  extend continuously upward from the top edges on the front end portions of the corresponding process-side body parts  86 . 
         [0180]    The process-side positioning shaft  67  is provided in the upper front corner of the process frame  20  and penetrates top ends of the process-side extended parts  87  and upper front corners of the side walls  70  in the left-right direction. 
         [0181]    (2-2) Developing Cartridges 
         [0182]    As shown in  FIG. 10 , each developing cartridge  21  includes a toner-accommodating section  132 . 
         [0183]    The toner-accommodating section  132  is provided with a first toner-accommodating section  133 , a second toner-accommodating section  134 , and a third toner-accommodating section  135 . 
         [0184]    The first toner-accommodating section  133  occupies approximately the upper half of the developing frame  33 . The first toner-accommodating section  133  has a generally square cylindrical shape and extends in the left-right direction. The first toner-accommodating section  133  is disposed inside the process frame  20 , and specifically between the left side wall  70 L and right side wall  70 R. Hence, the first toner-accommodating section  133  has a left-right dimension shorter than the distance between the side walls  70 , i.e., between the left side wall  70 L and right side wall  70 R. 
         [0185]    The second toner-accommodating section  134  has a box-like shape and is elongated vertically. Hence, the second toner-accommodating section  134  has a vertical dimension greater than its left-right dimension. The second toner-accommodating section  134  is arranged such that its upper portion overlaps the scanning unit  15  in the left-right direction when the process frame  20  is disposed in the internal position. Also, when the process frame  20  is disposed in its internal position, the second toner-accommodating section  134  is disposed outside (rightward) of the right casing-side positioning plate  101 . More specifically, when the process frame  20  is disposed in its internal position, the second toner-accommodating section  134  is disposed between the right side wall  70 R and the right casing side wall  98 R. 
         [0186]    The third toner-accommodating section  135  is formed continuously with a right end of the first toner-accommodating section  133  and extends rightward therefrom, and is connected to a bottom end of the second toner-accommodating section  134 . That is, the left end of the third toner-accommodating section  135  is in communication with the right end of the first toner-accommodating section  133 , and the top portion on the right end of the third toner-accommodating section  135  is in communication with the bottom end of the second toner-accommodating section  134 . 
         [0187]    The third toner-accommodating section  135  is positioned to penetrate in the left-right direction the opening  107  that is surrounded by the process-side body part  86 , process-side extended part  87 , casing-side body part  103 , and casing-side extended part  104  disposed on the right side. In other words, the third toner-accommodating section  135  is connected to the right end of the first toner-accommodating section  133  at the opening  107  in the left-right direction. 
         [0188]    With this configuration, when the process frame  20  is disposed in its internal position, each photosensitive drum  29  is positioned above the conveying belt  128 , and the corresponding first toner-accommodating section  133  is disposed above the photosensitive drum  29 . In other words, the first toner-accommodating section  133  is positioned at a side opposite to the conveying belt  128  with respect to the photosensitive drum  29 . 
       (3) Main Casing According to the Third Embodiment 
       [0189]    The main casing  2  is provided with the pair of casing-side positioning plates  101 , and the casing-side positioning shaft  102 . 
         [0190]    Each casing-side positioning plate  101  includes the casing-side body part  103 , and the casing-side extended part  104 . 
         [0191]    The casing-side body parts  103  are positioned outside of the respective left and right sides of the scanning unit  15  disposed in the top section of the main casing  2  and are not in contact with the scanning unit  15 . 
         [0192]    The casing-side extended parts  104  extend continuously downward from the bottom edges of the corresponding casing-side body parts  103  at the rear ends of the same. 
         [0193]    The casing-side positioning shaft  102  bridges the lower ends of the casing-side extended parts  104 . 
       (4) Mounting and Removing Developing Cartridges According to the Third Embodiment 
       [0194]    The developing cartridges  21  are mounted in and removed from the main casing  2  by an operator. To remove a developing cartridge  21  from the main casing  2 , first the operator opens the front cover  7  and pulls the process unit  16  forward toward the position shown in  FIG. 9 . Through this operation, the process-side positioning shaft  67  is removed from the casing-side fitting grooves  105  formed in the casing-side positioning plates  101  and the casing-side positioning shaft  102  is removed from the process-side fitting grooves  88  formed in the process-side positioning plates  66 . The operator pulls the process unit  16  out to the external position as the guide rollers  76  and guide rails  77  on the process frame  320  are guided along the guide grooves  100  of the main casing  2 . 
         [0195]    Next, the operator lifts the developing cartridge  21  upward relative to the process frame  20  so that the developing cartridge  21  is removed from the process frame  20 , as indicated by dashed lines in  FIG. 9 . 
         [0196]    This completes the operation for removing a developing cartridge  21  from the main casing  2 . To mount a developing cartridge  21  in the main casing  2 , the operation for removing a developing cartridge  21  described above is performed in reverse. 
         [0197]    More specifically, the operator places the developing cartridge  21  above the process frame  20  while the process unit  16  is in the external position shown in  FIG. 9  and pushes the developing cartridge  21  downward into the process frame  20 . Next, the operator pushes the process unit  16  into the main casing  2  from the state shown in  FIG. 9 . At this time, the process unit  16  slides rearward, with the guide rollers  76  and guide rails  77  of the process frame  20  guided in the guide grooves  100  formed in the main casing  2 . As a result of this operation, the process-side positioning shaft  67  becomes fitted into the casing-side fitting grooves  105  of the casing-side positioning plates  101  and the casing-side positioning shaft  102  becomes fitted into the process-side fitting grooves  88  of the process-side positioning plates  66 , placing the process unit  16  in its internal position shown in  FIG. 8 . Lastly, the operator closes the front cover  7 , thereby completing the operation to mount a developing cartridge  21  in the main casing  2 . 
       (5) Operational and Technical Advantages of the Third Embodiment 
       [0198]    In the printer  120  according to the third embodiment shown in  FIG. 10 , the second toner-accommodating sections  134  of the toner-accommodating sections  132  can be accommodated in the space between the right side wall  70 R and right casing side wall  98 R to the right of the scanning unit  15  in the printer  120 . Accordingly, the vertical dimension of the printer  120  can be made smaller by using the space in the printer  120  to the right of the scanning unit  15  disposed in the upper section of the main casing  2  to accommodate toner. 
         [0199]    The printer  120  according to the third embodiment can obtain the same operational and technical advantages described in the first embodiment. 
         [0200]    While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.