Patent Publication Number: US-2023152731-A1

Title: Image forming apparatus including drum cartridge having cushion member positioned between developing casing and developing shutter

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/330,806, filed May 26, 2021, which claims priority from Japanese Patent Application No. 2020-113268 filed Jun. 30, 2020. The entire contents of the aforementioned applications are incorporated herein by reference. 
    
    
     BACKGROUND ART 
     Technical Field 
     The present disclosure relates to an image forming apparatus. 
     Background 
     A conventional image forming apparatus including a drum cartridge and a toner cartridge attachable to the drum cartridge is known. The drum cartridge includes a photosensitive drum and a developing unit for supplying toner to the photosensitive drum. The toner cartridge is capable of supplying toner to the developing unit in a state where the toner cartridge is mounted on the drum cartridge. The developing unit includes a developing roller, a developing casing, and a developing shutter. The developing casing supports the developing roller. The developing casing has a toner inlet allowing toner from the toner cartridge to enter the developing casing therethrough in the state where the toner cartridge is attached to the drum cartridge. The developing shutter is configured to open and close the toner inlet. In the meantime, the toner cartridge includes a toner casing and a toner shutter. The toner casing can accommodate toner therein. The toner casing has a toner outlet allowing the toner in the toner casing to be discharged to an outside therethrough. The toner shutter is configured to open and close the toner outlet (refer to Japanese Patent Application Publication No. 2003-66815). 
     DESCRIPTION 
     Summary 
     In the image forming apparatus disclosed in Japanese Patent Application Publication No. 2003-66815, a configuration is conceivable where the developing roller and the developing casing supporting the developing roller are movable relative to the photosensitive drum in order to suppress variation in a state of contact of the developing roller with the photosensitive drum. 
     However, when the developing casing is moved, there is a likelihood that a position of the toner inlet relative to the toner outlet changes, which may cause leakage of the toner in the toner cartridge that is to be supplied into the developing casing. 
     In view of the foregoing, it is an object of the present disclosure to provide an image forming apparatus in which toner is supplied from a toner cartridge to a developing casing movable relative to a photosensitive drum, and the toner in the toner cartridge can be suppressed from leaking when supplied to the developing casing. 
     In order to attain the above and other object, according to one aspect, the present disclosure provides an image forming apparatus including: a main casing; a toner cartridge; and a drum cartridge. The toner cartridge is attachable to the main casing. The toner cartridge includes: a toner casing for accommodating toner therein; and a toner shutter. The toner casing is formed with a toner outlet allowing the toner accommodated in the toner casing to be discharged to an outside of the toner casing therethrough. The toner shutter is movable between a toner closed position where the toner shutter closes the toner outlet and a toner open position where the toner shutter opens the toner outlet. The drum cartridge is attachable to the main casing and for receiving the toner from the toner cartridge in a state where the drum cartridge and the toner cartridge are attached to the main casing. The drum cartridge includes: a photosensitive drum; and a developing unit for supplying toner to the photosensitive drum. The photosensitive drum is rotatable about a drum axis extending in a first direction. The developing unit includes: a developing roller; and a developing casing; a developing shutter; and a cushion member. The developing roller is rotatable about a developing axis extending in the first direction. The developing casing supports the developing roller and is movable relative to the photosensitive drum in a second direction crossing the first direction. The developing casing is formed with a toner inlet allowing the toner discharged from the toner casing through the toner outlet to enter the developing casing therethrough. The developing shutter is movable between a developing closed position where the developing shutter closes the toner inlet and a developing open position where the developing shutter opens the toner inlet. The developing shutter is in engagement with the toner cartridge in the state where the drum cartridge and the toner cartridge are attached to the main casing. The cushion member is positioned between the developing casing and the developing shutter. When the developing casing is moved relative to the photosensitive drum in a state where the drum cartridge and the toner cartridge are attached to the main casing, the developing shutter is in the developing open position, and the toner shutter is in the toner open position, the cushion member allows the developing casing to be moved relative to the developing shutter while the developing shutter is in engagement with the toner cartridge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the embodiment(s) as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG.  1    is a diagram schematically illustrating a configuration of an image forming apparatus according one embodiment of the present disclosure; 
         FIG.  2    is a diagram illustrating the image forming apparatus according to the embodiment, and illustrating a state where a drum cartridge to which a toner cartridge has been attached is detached from a main casing of the image forming apparatus; 
         FIG.  3    is a perspective view of the toner cartridge in the image forming apparatus according to the embodiment; 
         FIG.  4    is a plan view of the toner cartridge in the image forming apparatus according to the embodiment; 
         FIG.  5 A  is a cross-sectional view of the toner cartridge taken along a line VA-VA in  FIG.  4   ; 
         FIG.  5 B  is a side view of the toner cartridge in the image forming apparatus according to the embodiment; 
         FIG.  6    is a plan view of the drum cartridge in the image forming apparatus according to the embodiment; 
         FIG.  7    is a cross-sectional view of the drum cartridge taken along a line VII-VII in  FIG.  6   , and illustrating a state where a developing shutter is in its developing closed position; 
         FIG.  8    is a cross-sectional view of the drum cartridge taken along the line VII-VII in  FIG.  6   , and illustrating a state where the developing shutter is in its developing open position; 
         FIG.  9    is a view of the drum cartridge and toner cartridge attached thereto in the image forming apparatus according to the embodiment, and illustrating a state where the toner cartridge can be detached from the drum cartridge; 
         FIG.  10    is a view of the drum cartridge and toner cartridge attached thereto in the image forming apparatus according to the embodiment, and illustrating a state where the toner cartridge is locked relative to the drum cartridge; 
         FIG.  11    is a view of the drum cartridge and toner cartridge attached thereto in the image forming apparatus according to the embodiment, and illustrating a state where the developing shutter is in its developing closed position and a toner shutter is in its toner closed position; 
         FIG.  12    is a view of the drum cartridge and toner cartridge attached thereto in the image forming apparatus according to the embodiment, and illustrating a state where the developing shutter is in its developing open position and the toner shutter is in its toner open position; 
         FIG.  13    is a view illustrating an image forming apparatus according to a first modification, and illustrating a state where a drum cartridge and a toner cartridge are attached to a main casing of the image forming apparatus; 
         FIG.  14    is a view illustrating the image forming apparatus according to the first modification, and illustrating a state where the drum cartridge and the toner cartridge are detached from the main casing; 
         FIG.  15 A  is a view illustrating a configuration of a cushion member in an image forming apparatus according to a second modification; 
         FIG.  15 B  is a view illustrating a configuration of a cushion member in an image forming apparatus according to a third modification; and 
         FIG.  15 C  is a view illustrating a configuration of a cushion member in an image forming apparatus according to a fourth modification. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, one embodiment of the present disclosure will be described while referring to the accompanying drawings. 
     &lt;1. Image Forming Apparatus  1 &gt; 
     An overview of an image forming apparatus  1  according to the embodiment will be described with reference to  FIGS.  1  and  2   . 
     The image forming apparatus  1  includes a main casing  2 , a sheet cassette  3 , a drum cartridge  4 , a toner cartridge  5 , an exposure unit  6 , a transfer unit  7 , and a fixing unit  8 . 
     &lt;1.1. Main Casing  2 &gt; 
     The main casing  2  accommodates therein the sheet cassette  3 , the drum cartridge  4 , the toner cartridge  5 , the exposure unit  6 , the transfer unit  7 , and the fixing unit  8 . 
     As illustrated in  FIG.  2   , the main casing  2  is formed with an opening  21 , and includes a cover  22 , a guide  23 , and a guide  24 . 
     The opening  21  is formed in one side wall of the main casing  2  in a first attachment direction in which the drum cartridge  4  is moved when attached to the main casing  2 . Specifically, the first attachment direction is also a direction in which the guide  23  extends. 
     The cover  22  is movable between a closed position (see  FIG.  1   ) in which the cover  22  closes the opening  21  and an open position (see  FIG.  2   ) in which the cover  22  opens the opening  21 . 
     The guide  23  is configured to guide a protrusion  45 A (described later) of the drum cartridge  4  (see  FIG.  6   ). The guide  23  extends in the first attachment direction. The guide  23  has a first end portion E 1  and a second end portion E 2  in the first attachment direction. The first end portion E 1  is interposed between the second end portion E 2  and the cover  22  in the first attachment direction when the cover  22  is in the closed position. The second end portion E 2  is spaced apart from the first end portion E 1  in the first attachment direction. Although the guide  23  is a groove in the present embodiment, the guide  23  may be a rib instead. 
     The guide  24  is configured to guide a protrusion  46 A (described later) of the drum cartridge  4  (see  FIG.  6   ). The guide  24  extends in the first attachment direction. The guide  24  has a first end portion E 11  and a second end portion E 12  in the first attachment direction. The second end portion E 12  is spaced apart from the first end portion E 11  in the first attachment direction. When the cover  22  is in the closed position, the first end portion E 11  is interposed between the second end portion E 12  and the cover  22  in the first attachment direction. The guide  24  is a groove, but may be a rib. 
     &lt;1.2. Sheet Cassette  3 &gt; 
     As illustrated in  FIG.  1   , the sheet cassette  3  can accommodate sheets S therein. The sheet S in the sheet cassette  3  is conveyed toward a photosensitive drum  41  (described later) of the drum cartridge  4 . 
     &lt;1.3. Drum Cartridge  4 &gt; 
     The drum cartridge  4  is attachable to the main casing  2 . Specifically, in a state where the cover  22  is in the open position, the drum cartridge  4  is attachable to the main casing  2  via the opening  21  (see  FIG.  2   ) in the first attachment direction. The drum cartridge  4  is fixed in position relative to the main casing  2  in a state where the drum cartridge  4  is attached to the main casing  2 . Specifically, in the state where the drum cartridge  4  is attached to the main casing  2 , the protrusion  45 A and the protrusion  46 A (see  FIG.  6   ) are fitted into the second end portion E 2  of the guide  23  and the second end portion E 12  of the guide  24 , respectively. In this state, the drum cartridge  4  is fixed in position relative to the main casing  2 . The drum cartridge  4  is attachable to the main casing  2  with the toner cartridge  5  being attached to the drum cartridge  4 . The drum cartridge  4  includes the photosensitive drum  41 , a charging unit  42 , and a developing unit  43 . 
     &lt;1.3.1. Photosensitive Drum  41 &gt; 
     The photosensitive drum  41  extends in a first direction. The first direction crosses the first attachment direction. Preferably, the first direction is orthogonal to the first attachment direction. The photosensitive drum  41  is rotatable about a drum axis A 1  extending in the first direction. 
     &lt;1.3.2. Charging Unit  42 &gt; 
     The charging unit  42  is in contact with a surface of the photosensitive drum  41  and configured to charge the surface of the photosensitive drum  41 . In the present embodiment, the charging unit  42  is a charging roller. Note that the charging unit  42  may be a scorotron charger. 
     &lt;1.3.3. Developing Unit  43 &gt; 
     The developing unit  43  is capable of supplying toner to the photosensitive drum  41 . The developing unit  43  includes a developing roller  431 , and a developing casing  432 . 
     The developing roller  431  extends in the first direction. The developing roller  431  is rotatable about a developing axis A 2  extending in the first direction. The developing roller  431  is capable of supplying toner in the developing casing  432  to the photosensitive drum  41 . In the present embodiment, the developing roller  431  is in contact with the photosensitive drum  41 . Alternatively, the developing roller  431  may be separated from the photosensitive drum  41  with a predetermined gap. 
     The developing casing  432  is configured to accommodate therein toner to be supplied to the photosensitive drum  41 . The developing casing  432  supports the developing roller  431 . 
     &lt;1.4. Toner Cartridge  5 &gt; 
     The toner cartridge  5  is attachable to the main casing  2 . In the present embodiment, the toner cartridge  5  is attachable to the drum cartridge  4  in a state where the drum cartridge  4  is detached from the main casing  2 . The toner cartridge  5  is attached to the main casing  2  as the drum cartridge  4  having the toner cartridge  5  thereon is attached to the main casing  2 . The toner cartridge  5  is positioned opposite to the photosensitive drum  41  with respect to the developing roller  431  in a state where the toner cartridge  5  is mounted on the drum cartridge  4 . The toner cartridge  5  includes a toner casing  51 . 
     The toner casing  51  is capable of accommodating therein toner to be supplied to the developing unit  43 . The toner cartridge  5  is capable of supplying toner to the developing unit  43  in a state where the drum cartridge  4  and the toner cartridge  5  are mounted on the main casing  2 . 
     &lt;1.5. Exposure Unit  6 &gt; 
     The exposure unit  6  is configured to expose the surface of the photosensitive drum  41  charged by the charging unit  42  in the state where the drum cartridge  4  and the toner cartridge  5  are mounted on the main casing  2 . Thereafter, the developing unit  43  is configured to supply toner to the surface of the photosensitive drum  41  which has been exposed by the exposure unit  6 . In the present embodiment, the exposure unit  6  is a laser scan unit. The exposure unit  6  may instead be an LED head. 
     &lt;1.6. Transfer Unit  7 &gt; 
     The transfer unit  7  is configured to transfer the toner carried on the photosensitive drum  41  onto a sheet S. Specifically, a sheet S fed from the sheet cassette  3  passes through a portion between the transfer unit  7  and the photosensitive drum  41 . At this time, the toner on the photosensitive drum  41  is transferred onto the sheet S by the transfer unit  7 . In the present embodiment, the transfer unit  7  is a transfer roller. The transfer unit  7  is in contact with the photosensitive drum  41  in the state where the drum cartridge  4  is attached to the main casing  2 . However, the transfer unit  7  may be separated from the photosensitive drum  41  by a predetermined gap in the state where the drum cartridge  4  is mounted on the main casing  2 . 
     &lt;1.7. Fixing Unit  8 &gt; 
     The fixing unit  8  is configured to fix toner to a sheet S. In the present embodiment, the fixing unit  8  thermally fixes toner to a sheet S by applying heat and pressure to the sheet S onto which the toner has been transferred. The sheet S that has passed through the fixing unit  8  is then discharged onto an upper surface of the main casing  2 . 
     &lt;2. Details of Toner Cartridge  5 &gt; 
     A detailed configuration of the toner cartridge  5  will next be described with reference to  FIGS.  3  to  5 B,  11  and  12   . 
     As illustrated in  FIGS.  3  and  4   , the toner cartridge  5  includes the toner casing  51  described above, an auger screw  53 , a sealing member  52  (see  FIG.  12   ), a toner shutter  54 , and a handle  55 . 
     &lt;2.1. Details of Toner Casing  51 &gt; 
     The toner casing  51  extends in the first direction, and has one side surface S 1  in the first direction and another side surface S 2  in the first direction. 
     As illustrated in  FIG.  5 A , the toner casing  51  includes a toner container  512 , a receiving portion  516 , and a toner conveyance unit  514 . 
     &lt;2.1.1. Toner Container  512 &gt; 
     The toner container  512  can accommodate toner therein. 
     &lt;2.1.2. Receiving Portion  516 &gt; 
     The receiving portion  516  is configured to receive a developing shutter  437  (described later) through the toner shutter  54  in the state where the toner cartridge  5  is attached to the drum cartridge  4  (see  FIG.  11   ). The receiving portion  516  is provided to extend along an outer circumferential surface of the developing shutter  437 . 
     The receiving portion  516  and the toner container  512  are arranged in a direction crossing the first direction. The receiving portion  516  extends in a direction in which the toner shutter  54  is moved. 
     As illustrated in  FIG.  3   , the receiving portion  516  also extends in the first direction, and has one end portion E 21  and another end portion E 22  in the first direction. The one end portion E 21  of the receiving portion  516  protrudes in the first direction from the side surface S 1 , while the other end portion E 22  of the receiving portion  516  protrudes in the first direction from the side surface S 2 . 
     As illustrated in  FIG.  5 A , the receiving portion  516  is formed with a toner outlet  511 . In other words, the toner casing  51  is formed with the toner outlet  511 . The toner outlet  511  allows the toner accommodated in the toner casing  51  to be discharged to an outside of the toner casing  51  therethrough. 
     &lt;2.1.3. Toner Conveyance Unit  514 &gt; 
     The toner in the toner casing  51  is conveyed from the toner container  512  toward the toner outlet  511  through the toner conveyance unit  514 . The toner conveyance unit  514  has a hollow cylindrical shape and extends in the first direction. The toner conveyance unit  514  is positioned between the receiving portion  516  and the toner container  512 , and is connected to the toner container  512  and the receiving portion  516 . That is, the toner conveyance unit  514  has an inner space in communication with an inner space of the toner container  512  (see  FIG.  1   ) and the toner outlet  511 . 
     &lt;2.2. Auger Screw  53 &gt; 
     The auger screw  53  is positioned inside the toner conveyance unit  514 , and is configured to convey the toner in the toner conveyance unit  514  toward the toner outlet  511 . The auger screw  53  extends in the first direction. The auger screw  53  is rotatable about an axis extending in the first direction. 
     &lt;2.3. Toner Shutter  54 &gt; 
     As illustrated in  FIGS.  11  and  12   , the toner shutter  54  is movable along the receiving portion  516  between a toner closed position (see  FIG.  11   ) in which the toner shutter  54  closes the toner outlet  511  and a toner open position (see  FIG.  12   ) in which the toner shutter  54  opens the toner outlet  511 . 
     As illustrated in  FIG.  3   , the toner shutter  54  includes a toner shutter body  541  and a protrusion  542 . 
     &lt;2.3.1. Toner Shutter Body  541 &gt; 
     The toner shutter body  541  is provided to along the receiving portion  516 . The toner shutter body  541  extends in a direction in which the toner shutter  54  is moved. The toner shutter body  541  is a plate-like member having a circular arc shape. The toner shutter body  541  also extends in the first direction, and has one end portion E 31  and another end portion E 32  in the first direction. During movement of the toner shutter  54  between the toner closed position (see  FIG.  11   ) and the toner open position (see  FIG.  12   ), the one end portion E 31  and the other end portion E 32  of the toner shutter body  541  are guided by the one end portion E 21  and the other end E 22  of the receiving portion  516 , respectively. 
     The one end portion E 31  of the toner shutter body  541  has a U-like shape. The one end portion E 21  of the receiving portion  516  is fitted with the one end portion E 31  of the toner shutter body  541 . A shutter gear  544 A is provided on the one end portion E 31  of the toner shutter body  541  (see  FIG.  4   ). The shutter gear  544 A includes a plurality of gear teeth. 
     The other end portion E 32  of the toner shutter body  541  has a U-like shape. The other end portion E 22  of the receiving portion  516  is fitted into the other end portion E 32  of the toner shutter body  541 . A shutter gear  544 B is provided on the other end portion E 32  of the toner shutter body  541 . The shutter gear  544 B includes a plurality of gear teeth. 
     &lt;2.3.2. Protrusion  542 &gt; 
     The protrusion  542  is positioned opposite to the receiving portion  516  with respect to the toner shutter body  541 . The protrusion  542  is positioned between the one end portion E 31  and the other end portion E 32  of the toner shutter body  541  in the first direction. The protrusion  542  protrudes outward from the toner shutter body  541  in a radial direction of the toner shutter body  541 . Note that the protrusion  542  may be attached to the toner shutter body  541 . 
     &lt;2.4. Sealing Member  52 &gt; 
     The sealing member  52  seals a portion between the receiving portion  516  and the toner shutter  54  in a state where the toner shutter  54  is in the toner closed position, as illustrated in  FIG.  12   . On the other hand, a portion of the sealing member  52  is interposed between the receiving portion  516  and the toner shutter  54  in a state where the toner shutter  54  is in the toner open position. With this configuration, the toner in the toner cartridge  5  is prevented from leaking from the portion between the receiving portion  516  and the toner shutter  54  by virtue of the sealing member  52 . The sealing member  52  is positioned opposite to the toner conveyance unit  514  with respect to the receiving portion  516 . The sealing member  52  may be affixed to the receiving portion  516  with an adhesive, for example. The sealing member  52  is provided to surround the toner outlet  511 . In  FIGS.  5 A and  11   , the sealing member  52  is omitted for simplicity. 
     &lt;2.5. Handle  55 &gt; 
     As illustrated in  FIG.  4   , the handle  55  is configured to be operated by a user to move the toner shutter  54 . The handle  55  includes a plate  553 A, a protrusion  552 A, a plate  553 B, a protrusion  552 B, and a handle body  551 . 
     &lt;2.5.1. Plate  553 A&gt; 
     The plate  553 A is disposed on the side surface  51 , and is supported by the toner casing  51 . As illustrated in  FIG.  9   , the plate  553 A has a circular shape as viewed in the first direction. The plate  553 A faces the one end portion E 31  of the toner shutter body  541  in a radial direction of the plate  553 A. The plate  553 A is rotatable about an axis A 3  extending in the first direction. A handle gear  554 A is provided on a portion of a peripheral surface of the plate  553 A. The handle gear  554 A includes a plurality of gear teeth, and in meshing engagement with the shutter gear  544 A. 
     &lt;2.5.2. Protrusion  552 A&gt; 
     The protrusion  552 A is provided on the plate  553 A. The protrusion  552 A is positioned opposite to the side surface  51  with respect to the plate  553 A in the first direction (see  FIG.  4   ). The protrusion  552 A protrudes in the first direction from the plate  553 A (see  FIG.  4   ). The protrusion  552 A may be attached to the plate  553 A. 
     The protrusion  552 A extends in the radial direction of the plate  553 A. In the state where the toner cartridge  5  is attached to the drum cartridge  4 , the protrusion  552 A is fitted with in a guide  441 A (described later) in the drum cartridge  4 . 
     In a state where the toner cartridge  5  is attached to the drum cartridge  4  and the toner shutter  54  is in the toner closed position, the protrusion  552 A extends in a second attachment direction. The second attachment direction denotes a direction in which the toner cartridge  5  is moved during an attachment process of the toner cartridge  5  to the drum cartridge  4 . The second attachment direction crosses the first direction, and preferably, is orthogonal to the first direction. The protrusion  552 A extends in a direction crossing both the first direction and the second attachment direction in a state where the toner cartridge  5  is attached to the drum cartridge  4  and the toner shutter  54  is in the toner open position (see  FIG.  10   ). 
     &lt;2.5.3. Plate  553 B&gt; 
     The plate  553 B will be described as similarly to the plate  553 A. As illustrated in  FIG.  5 B , the plate  553 B is disposed on the side surface S 2 . The plate  553 B faces the other end portion E 32  of the toner shutter body  541  in a radial direction of the plate  553 B. A handle gear  554 B is provided on a portion of a peripheral surface of the plate  553 B. The handle gear  554 B includes a plurality of gear teeth, and is in meshing engagement with the shutter gear  544 B. 
     &lt;2.5.4. Protrusion  552 B&gt; 
     The protrusion  552 B is provided on the plate  553 B. The protrusion  552 B is positioned opposite to the side surface S 2  with respect to the plate  553 B in the first direction. The protrusion  552 B protrudes in the first direction from the plate  553 B. The protrusion  552 B may be attached to the plate  553 B. The protrusion  552 B has a shape identical to that of the protrusion  552 A. The protrusion  552 B is fitted with a guide  441 B (described later) in the drum cartridge  4  in the state where the toner cartridge  5  is attached to the drum cartridge  4  (see  FIG.  6   ). 
     &lt;2.5.5. Handle Body  551 &gt; 
     As illustrated in  FIG.  3   , the handle body  551  has a U-like shape. The handle body  551  has a first end and a second end. The first end of the handle body  551  is connected to the plate  553 A (see  FIG.  9   ), while the second end of the handle body  551  is connected to the plate  553 B (see  FIG.  5 B ). 
     As the user operates the handle body  551 , each of the plate  553 A and the plate  553 B rotates. Then, the handle gears  554 A and  554 B and the shutter gears  544 A and  544 B transmit a driving force applied by the user to the toner shutter body  541 , thereby moving the toner shutter  54  between the toner closed position (see  FIG.  11   ) and the toner open position (see  FIG.  12   ). In this way, the user can move the toner shutter  54  easily by operating the handle  55 . 
     &lt;3. Details of Drum Cartridge  4 &gt; 
     Next, the detailed configuration of the drum cartridge  4  will be described with reference to  FIGS.  6  to  12   . 
     As illustrated in  FIG.  6   , the drum cartridge  4  includes the photosensitive drum  41 , the charging unit  42  (see  FIG.  7   ), the developing unit  43  those are described above, a drum frame  44 , two protrusions  45 A and  45 B, two protrusions  46 A and  46 B, a shaft  48 , and a pressing member  47  (see  FIG.  7   ). 
     &lt;3.1. Drum Frame  44 &gt; 
     The drum frame  44  includes a side plate  44 A on one side in the first direction, a side plate  44 B on the other side in the first direction, and a bottom plate  44 C. 
     &lt;3.1.1. Side Plate  44 A&gt; 
     The side plate  44 A constitutes one end in the first direction of the drum frame  44 . The side plate  44 A extends in the second attachment direction. The side plate  44 A has an inner surface S 11  and an outer surface S 12  in the first direction. The inner surface S 11  is provided between the outer surface S 12  and the side plate  44 B in the first direction. The side plate  44 A is formed with the guide  441 A. 
     As illustrated in  FIG.  7   , the guide  441 A is formed at a position opposite to the photosensitive drum  41  with respect to the developing unit  43 . The guide  441 A extends in the second attachment direction. The guide  441 A is a groove and configured to guide the protrusion  552 A (see  FIG.  4   ) during the attachment process of toner cartridge  5  to the drum cartridge  4 . The guide  441 A may be a rib. 
     The guide  441 A has a first end portion E 41  and a second end portion E 42  in the second attachment direction. The first end portion E 41  is positioned opposite to the developing unit  43  with respect to the second end portion E 42  in the second attachment direction. The second end portion E 42  is spaced apart from the first end portion E 41  in the second attachment direction. 
     As illustrated in  FIG.  9   , in the state where the toner cartridge  5  is attached to the drum cartridge  4 , the protrusion  552 A is positioned at the second end portion E 42  of the guide  441 A. In this state, the protrusion  552 A is fitted with the second end portion E 42  of the guide  441 A, whereby the toner casing  51  of the toner cartridge  5  is fixed in position relative to the drum cartridge  4 . In an attached state of the toner cartridge  5  to the drum cartridge  4 , the second end portion E 42  allows the protrusion  552 A to be rotated about the axis A 3  (see  FIG.  10   ). The second end portion E 42  has a first guide surface S 31 , a second guide surface S 32 , a first lock surface S 41 , and a second lock surface S 42 . 
     Each of the first guide surface S 31  and the second guide surface S 32  extends in the second attachment direction. In the state where the toner cartridge  5  is attached to the drum cartridge  4 , the first guide surface S 31  is positioned opposite to the second guide surface S 32  with respect to the axis A 3 . In the state where the toner cartridge  5  is attached to the drum cartridge  4  and the toner shutter  54  is in the toner closed position, the protrusion  552 A extends in the second attachment direction, as described above. In this state, the protrusion  552 A is positioned between the first guide surface S 31  and the second guide surface S 32 , and faces both the first guide surface S 31  and the second guide surface S 32  in a direction crossing the second attachment direction. 
     With the above configuration, the toner cartridge  5  is allowed to be detached from the drum cartridge  4  along the guide  441 A in the state where the toner cartridge  5  is attached to the drum cartridge  4  and the toner shutter  54  is in the toner closed position. 
     Each of the first lock surface S 41  and the second lock surface S 42  extends in the direction crossing the second attachment direction. The first lock surface S 41  is positioned opposite to the second lock surface S 42  with respect to the axis A 3  in the attached state of the toner cartridge  5  to the drum cartridge  4 . The first lock surface S 41  is continuous with the first guide surface S 31 , and the second lock surface S 42  is continuous with the second guide surface S 32 . 
     As illustrated in  FIG.  10   , in the state where the toner cartridge  5  is attached to the drum cartridge  4  and the toner shutter  54  is in the toner open position, the protrusion  552 A extending in the direction crossing the second attachment direction is positioned between the first lock surface S 41  and the second lock surface S 42 , and faces the first lock surface S 41  and the second lock surface S 42  in the second attachment direction. 
     Accordingly, the toner cartridge  5  is locked relative to the drum cartridge  4  in a state where the toner cartridge  5  is attached to the drum cartridge  4 , the developing shutter  437  (described later) is in its developing open position, and the toner shutter  54  is in the toner open position. 
     “The toner cartridge  5  is locked relative to the drum cartridge  4 ” denotes that the toner cartridge  5  is fixed to the drum frame  44  in the second attachment direction so as not to be movable in the second attachment direction relative to the drum cartridge  4 . 
     The above configuration can prevent detachment of the toner cartridge  5  from the drum cartridge  4  in the state where the developing shutter  437  is in the developing open position and the toner shutter  54  is in the toner open position. 
     &lt;3.1.2. Side Plate  44 B&gt; 
     As illustrated in  FIG.  6   , the side plate  44 B constitutes another end in the first direction of the drum frame  44 . The side plate  44 B is spaced apart from the side plate  44 A in the first direction. The side plate  44 B extends in the second attachment direction. The side plate  44 B has an inner surface S 21  and an outer surface S 22  in the first direction. The inner surface S 21  is provided at a position between the outer surface S 22  and the side plate  44 A in the first direction. The side plate  44 B has the guide  441 B. 
     The guide  441 B has a shape identical to that of the guide  441 A. The guide  441 B is configured to guide the protrusion  552 B of the toner cartridge  5  (see  FIG.  4   ) during the attachment process of the toner cartridge  5  to the drum cartridge  4 . 
     &lt;3.1.3. Bottom Plate  44 C&gt; 
     The bottom plate  44 C is positioned between the side plate  44 A and the side plate  44 B in the first direction. The bottom plate  44 C extends in the first direction. The bottom plate  44 C has one end in the first direction connected to the side plate  44 A, and has another end in the first direction connected to the side plate  44 B. 
     &lt;3.2. Protrusions  45 A,  45 B,  46 A, and  46 B&gt; 
     The protrusion  45 A is provided on the outer surface S 12  of the side plate  44 A. The protrusion  45 A has a columnar shape and extends in the first direction. In the present embodiment, the protrusion  45 A serves as one end of a shaft that supports the photosensitive drum  41 . That is, the protrusion  45 A extends along the drum axis A 1 . 
     The protrusion  45 B is provided on the outer surface S 22  of the side plate  44 B. The protrusion  45 B has a columnar shape and extends in the first direction. Since the protrusion  45 B serves as another end of the shaft for supporting the photosensitive drum  41 , the protrusion  45 B extends along the drum axis A 1 . 
     The protrusion  46 A is disposed on the outer surface S 12  of the side plate  44 A. The protrusion  46 A is positioned to be spaced apart from the protrusion  45 A in a second direction D (see  FIGS.  10  and  12   ; described later). The protrusion  46 A has a columnar shape and extends in the first direction. 
     The protrusion  46 B is provided on the outer surface S 22  of the side plate  44 B. The protrusion  46 B is spaced apart from the protrusion  45 B in the second direction D. The protrusion  46 B extends in the first direction and has a columnar shape. 
     &lt;3.3. Shaft  48 &gt; 
     The shaft  48  has a columnar shape and extends in the first direction. The shaft  48  is supported by the side plate  44 A and the side plate  44 B so as to be rotatable about an axis A 4  illustrated in  FIG.  7   . The axis A 4  extends in the first direction. The shaft  48  is positioned opposite to the first end portion E 41  with respect to the second end portion E 42  of the guide  441 A in the second attachment direction. 
     &lt;3.4. Pressing Member  47 &gt; 
     The pressing member  47  is a member configured to press the developing unit  43  toward the photosensitive drum  41 . In the present embodiment, the pressing member  47  is a torsion spring. The pressing member  47  may instead be a compression coil spring. 
     &lt;3.5. Details of Developing Unit  43 &gt; 
     As illustrated in  FIGS.  6 ,  7  and  12   , in addition to the above-described developing roller  431  and developing casing  432 , the developing unit  43  further includes the developing shutter  437 , a sealing member  436 , and a cushion member  438 . 
     &lt;3.5.1. Details of Developing Casing  432 &gt; 
     The developing casing  432  is positioned between the side plate  44 A and the side plate  44 B in the first direction. The developing casing  432  extends in the first direction. 
     The developing casing  432  includes an accommodating portion  4321 , a tubular portion  4322  (see  FIG.  7   ), a flange portion  4323 , two connecting portions  4324  and  4325 . 
     &lt;3.5.1.1. Accommodating Portion  4321 &gt; 
     The accommodating portion  4321  accommodates the developing roller  431  therein. The accommodating portion  4321  includes a side wall  43211  on one side in the first direction of the developing casing  432 , and a side wall  43212  on another side in the first direction of the developing casing  432 . In other words, the developing casing  432  includes the side wall  43211  and the side wall  43212 . The side wall  43211  is spaced apart from the side wall  43212  in the first direction. The side wall  43211  and the side wall  43212  rotatably support the developing roller  431 . 
     &lt;3.5.1.2. Tubular Portion  4322 &gt; 
     The tubular portion  4322  constitutes one end portion in the first direction of the developing casing  432 . The tubular portion  4322  is positioned opposite to the side wall  43212  with respect to the side wall  43211  in the first direction. Also, the tubular portion  4322  is positioned between the side wall  43211  and the side plate  44 B in the first direction. In other words, the tubular portion  4322  is positioned at the side wall  43211 . The tubular portion  4322  extends in the first direction. The tubular portion  4322  has a hollow cylindrical shape. 
     As illustrated in  FIG.  7   , the tubular portion  4322  is formed with a toner inlet  433 . Put another way, the developing casing  432  is formed with the toner inlet  433 . The toner inlet  433  allows the toner discharged from the toner cartridge  5  through the toner outlet  511  to enter the developing casing  432  therethrough in the state where the drum cartridge  4  and the toner cartridge  5  are mounted on the main casing  2  (see  FIG.  12   ). 
     &lt;3.5.1.3. Flange Portion  4323 &gt; 
     As illustrated in  FIG.  6   , the flange portion  4323  is provided on an outer circumferential surface of the tubular portion  4322 . The flange portion  4323  protrudes outward from the outer circumferential surface of the tubular portion  4322  in a radial direction of the tubular portion  4322 . The flange portion  4323  is positioned away from the side wall  43211  in the first direction. 
     &lt;3.5.1.4. Two Connecting Portions  4324  and  4325 &gt; 
     The connecting portion  4324  connects the side wall  43211  to the shaft  48 . The connecting portion  4325  is spaced apart from the connecting portion  4324  in the first direction. The connecting portion  4325  connects the side wall  43212  to the shaft  48 . With this configuration, the developing casing  432  is pivotally movable about the axis A 4  of the shaft  48 , as illustrated in  FIG.  10   . In other words, the developing casing  432  is movable relative to the photosensitive drum  41  in the second direction D in which the developing casing  432  is pivotally moved. The second direction D crosses the first direction. 
     &lt;3.5.2. Developing Shutter  437 &gt; 
     The developing shutter  437  is movable between a developing closed position (see  FIG.  7   ) and the developing open position (see  FIG.  8   ). Specifically, the developing shutter  437  is pivotally movable between the developing closed position and the developing open position about an axis A 5  extending in the first direction. As illustrated in  FIG.  7   , the developing shutter  437  closes the toner inlet  433  when the developing shutter  437  is in the developing closed position. As illustrated in  FIG.  8   , the toner inlet  433  is open when the developing shutter  437  is in the developing open position. 
     As illustrated in  FIG.  6   , the developing shutter  437  extends in the first direction. The developing shutter  437  has a hollow cylindrical shape. Accordingly, the developing shutter  437  is movable between the developing closed position and the developing open position by rotation thereof, thereby reducing a space required for the developing shutter  437  to be moved. 
     As illustrated in  FIGS.  7  and  8   , the developing shutter  437  is positioned outside the tubular portion  4322 . In other words, the tubular portion  4322  is disposed inside the developing shutter  437 . The tubular portion  4322  is spaced apart from the developing shutter  437  in a radial direction of the developing shutter  437 . Further, the developing shutter  437  is positioned between the side wall  43211  and the flange portion  4323  in the first direction. Therefore, the developing shutter  437  can be prevented from moving in the first direction, which may cause unintentional detachment of the developing shutter  437  from the tubular portion  4322 . The developing shutter  437  is formed with a developing shutter opening  4371  and a hole  4372 . 
     &lt;3.5.2.1. Developing Shutter Opening  4371 &gt; 
     The developing shutter opening  4371  is away from the toner inlet  433  when the developing shutter  437  is in the developing closed position (see  FIG.  7   ). The developing shutter opening  4371  is not in communication with the toner inlet  433  at all when the developing shutter  437  is in the developing closed position. 
     As illustrated in  FIG.  8   , when the developing shutter  437  is in the developing open position, the developing shutter opening  4371  has at least a portion in communication with the toner inlet  433 . 
     As illustrated in  FIG.  12   , the developing shutter  437  is fitted with the receiving portion  516  of the toner cartridge  5  in the attached state of the toner cartridge  5  to the drum cartridge  4 . The toner outlet  511  is in communication with the developing shutter opening  4371  in a state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position. Thus, the toner discharged from the toner cartridge  5  through the toner outlet  511  is allowed to enter the tubular portion  4322  of the developing casing  432  through the developing shutter opening  4371  and the toner inlet  433 . 
     In the state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the toner outlet  511  is positioned higher than the toner inlet  433 . With this arrangement, the toner in the toner cartridge  5  can be supplied through the toner outlet  511  to the developing unit  43  by virtue of the gravitational force. Thereafter, the toner discharged through the toner outlet  511  enters the developing casing  432  through the developing shutter opening  4371  and the toner inlet  433 . 
     &lt;3.5.2.2. Hole  4372 &gt; 
     The hole  4372  is positioned away from the developing shutter opening  4371  in a circumferential direction of the developing shutter  437 . In the state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the hole  4372  receives the protrusion  542  of the toner shutter  54 . Accordingly, the developing shutter  437  is in engagement with the toner cartridge  5  in the state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 . Specifically, in this state, the developing shutter  437  is in engagement with the toner shutter  54 . By virtue of this engagement between the developing shutter  437  and the toner shutter  54  of the toner cartridge  5 , the developing shutter  437  can be fixed to the toner cartridge  5 . Also with this engagement, the developing shutter  437  is movable between the developing open position and the developing closed position in accordance with the movement of the toner shutter  54  between the toner open position and the toner closed position. 
     “The developing shutter  437  is fixed to the toner cartridge  5 ” denotes that movement of the developing shutter  437  relative to the toner cartridge  5  in the second direction D is prevented even when the developing casing  432  is moved relative to the photosensitive drum  41  in the second direction D. 
     Accordingly, even when the developing casing  432  is moved relative to the photosensitive drum  41  in the second direction D while the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position, deviation in position of the developing shutter opening  4371  relative to the toner outlet  511  in the second direction D can be restrained. 
     Consequently, in a configuration in which toner is supplied from the toner cartridge  5  to the developing casing  432  that is movable relative to the photosensitive drum  41 , leakage of toner supplied from the toner cartridge  5  to the developing casing  432  can be more reliably suppressed. 
     The developing shutter  437  is pivotally movable together with the toner shutter  54  about the axis A 5  in a state where the developing shutter  437  is fixed to the toner cartridge  5 . 
     &lt;3.5.3. Sealing Member  436 &gt; 
     As illustrated in  FIG.  12   , the sealing member  436  seals a portion between the developing shutter  437  and the receiving portion  516  in cooperation with the sealing member  52  in the state where the toner shutter  54  is in the toner open position and the developing shutter  437  is in the developing open position. The sealing member  436  is provided on the outer circumferential surface of the developing shutter  437 . The sealing member  436  may be affixed to the developing shutter  437  with an adhesive, for example. The sealing member  436  surrounds the developing shutter opening  4371 . In  FIGS.  7  to  11   , the sealing member  436  is omitted for simplicity. 
     &lt;3.6. Cushion Member  438 &gt; 
     The cushion member  438  is positioned between the tubular portion  4322  of the developing casing  432  and the developing shutter  437 . 
     &lt;3.6.1. Material of Cushion Member  438 &gt; 
     The cushion member  438  is made of an elastic material. In the present embodiment, the cushion member  438  is made of sponge. The cushion member  438  is positioned between the tubular portion  4322  and the developing shutter  437  while the cushion member  438  is being slightly compressed. 
     When the developing casing  432  is moved relative to the photosensitive drum  41  in the state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position, the tubular portion  4322  of the developing casing  432  is moved relative to the developing shutter  437  while the developing shutter  437  is fixed to the toner cartridge  5 . At this time, the cushion member  438  comes into elastic deformation. 
     Specifically, the tubular portion  4322  is moved in a direction away from the photosensitive drum  41  in accordance with movement of the developing casing  432  in the direction away from the photosensitive drum  41 . Meanwhile, the developing shutter  437  is fixed to the toner cartridge  5  and thus is immovable. 
     In this state, a portion P 1  of the cushion member  438  that is positioned between the photosensitive drum  41  and the tubular portion  4322  is restored from its compressed state and fills a space between the developing shutter  437  and the tubular portion  4322  generated due to movement of the developing casing  432 . Meanwhile, a portion P 2  of the cushion member  438  that is positioned opposite to the photosensitive drum  41  with respect to the tubular portion  4322  becomes more compressed between the developing shutter  437  and the tubular portion  4322 . 
     In the meantime, when the developing casing  432  is moved in a direction toward the photosensitive drum  41 , the portion P 1  of the cushion member  438  positioned between the photosensitive drum  41  and the tubular portion  4322  becomes compressed between the developing shutter  437  and the tubular portion  4322 , and the portion P 2  of the cushion member  438  positioned opposite to the photosensitive drum  41  with respect to the tubular portion  4322  is restored from its compressed state to fill a space generated between the developing shutter  437  and the tubular portion  4322  due to the movement of the tubular portion  4322 . 
     In this way, the cushion member  438  allows the tubular portion  4322  of the developing casing  432  to be moved relative to the developing shutter  437  while the developing shutter  437  is engaged with the toner cartridge  5 , when the developing casing  432  is moved relative to the photosensitive drum  41  in the state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position. 
     &lt;3.6.2. Arrangement and Shape of Cushion Member  438 &gt; 
     In the present embodiment, the cushion member  438  has a hollow cylindrical shape. The cushion member  438  is positioned between the side wall  43211  and the flange portion  4323  in the first direction (see  FIG.  6   ). Therefore, detachment of the cushion member  438  from a portion between the tubular portion  4322  and the developing shutter  437  due to movement of the cushion member  438  in the first direction can be restrained. The cushion member  438  has a first surface S 51  and a second surface S 52 . 
     The first surface S 51  is an inner surface of the cushion member  438 . The first surface S 51  makes contact with the outer circumferential surface of the tubular portion  4322  of the developing casing  432 . In other words, the cushion member  438  makes contact with the tubular portion  4322 . The cushion member  438  may be affixed to the outer circumferential surface of the tubular portion  4322  with an adhesive, for example. 
     The second surface S 52  is an outer surface of the cushion member  438 . The second surface S 52  makes contact with an inner circumferential surface of the developing shutter  437  to support the developing shutter  437 . When the developing shutter  437  is moved between the developing closed position and the developing open position, the developing shutter  437  maintains contact with the second surface S 52  and is slidingly moved on the second surface S 52 . 
     The cushion member  438  does not cover the toner inlet  433  and allows the toner to pass through the toner inlet  433 . Meanwhile, the cushion member  438  surrounds an outer periphery of the toner inlet  433 . As illustrated in  FIG.  11   , the cushion member  438  seals a portion between the tubular portion  4322  of the developing casing  432  and the developing shutter  437  when the developing shutter  437  is in the developing closed position, as the cushion member  438  is in contact with the outer circumferential surface of the tubular portion  4322  at the first surface S 51  and in contact with the inner surface of the developing shutter  437  at the second surface S 52  while surrounding the toner inlet  433 . Accordingly, leakage of the toner from the portion between the developing casing  432  and the developing shutter  437  can be restrained. 
     &lt;4. Advantageous Effects&gt; 
     (1) In the image forming apparatus  1  according to the present embodiment, the cushion member  438  is positioned between the tubular portion  4322  of the developing casing  432  and the developing shutter  437 , as illustrated in  FIG.  12   . 
     Accordingly, the developing casing  432  is movable relative to the developing shutter  437  while the developing shutter  437  is engaged with the toner cartridge  5 , when the developing casing  432  is moved relative to the photosensitive drum  41  while the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position. 
     This configuration can restrain the deviation in position of the developing shutter  437  relative to the toner cartridge  5  even when the developing casing  432  is moved relative to the photosensitive drum  41 . 
     As a result, in a configuration in which toner is supplied from the toner cartridge  5  to the developing casing  432  which is movable relative to the photosensitive drum  41 , the toner supplied from the toner cartridge  5  to the developing casing  432  can be kept from leaking. 
     (2) According to the image forming apparatus  1 , the developing shutter  437  has the developing shutter opening  4371 , as illustrated in  FIG.  11   . The developing shutter opening  4371  is not in communication with the toner inlet  433  when the developing shutter  437  is in the developing closed position. On the other hand, as illustrated in  FIG.  12   , the developing shutter opening  4371  is in communication with the toner inlet  433  when the developing shutter  437  is in the developing open position. 
     Further, the developing shutter opening  4371  is brought into communication with the toner outlet  511  as the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is moved to the developing open position, and the toner shutter  54  is moved the toner open position. 
     With the above arrangement, a position of the developing shutter opening  4371  relative to the toner outlet  511  can be kept from changing when the developing casing  432  is moved relative to the photosensitive drum  41  while the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position. 
     Consequently, in a configuration in which toner is supplied from the toner cartridge  5  to the developing casing  432  that is movable relative to the photosensitive drum  41 , the toner supplied from the toner cartridge  5  to the developing casing  432  can be kept from leaking more reliably. 
     (3) According to the image forming apparatus  1 , the cushion member  438  seals the portion between the tubular portion  4322  of the developing casing  432  and the developing shutter  437 . 
     Therefore, the cushion member  438  can suppress leakage of the toner from the portion between the developing casing  432  and the developing shutter  437 . 
     (4) According to the image forming apparatus  1 , the developing shutter  437  is slidingly moved on the second surface S 52  of the cushion member  438  while maintaining contact with the second surface S 52  during the movement of the developing shutter  437  between the developing closed position (see  FIG.  11   ) and the developing open position (see  FIG.  12   ). 
     Accordingly, leakage of toner from the portion between the developing shutter  437  and the cushion member  438  when the developing shutter  437  is moved between the developing closed position and the developing open position can be restrained. 
     (5) According to the image forming apparatus  1 , the developing shutter  437  extends in the first direction, as illustrated in  FIGS.  11  and  12   . Moreover, the developing shutter  437  has a hollow cylindrical shape. 
     Therefore, the developing shutter  437  is movable between the developing closed position and the developing open position by rotating. 
     Consequently, the space required for the movement the developing shutter  437  can be made small. 
     Moreover, this configuration can reduce a possibility that the developing casing  432  interferes with the developing shutter  437  even when the developing casing  432  is moved relative to the photosensitive drum  41 . 
     (6) According to the image forming apparatus  1 , the developing shutter  437  is positioned between the side wall  43211  and the flange portion  4323  in the first direction (see  FIG.  6   ). 
     This arrangement can restrain the movement of the developing shutter  437  in the first direction which may cause detachment of the developing shutter  437  from the tubular portion  4322 . 
     (7) According to the image forming apparatus  1 , the toner cartridge  5  includes the handle  55  for moving the toner shutter  54 , as illustrated in  FIGS.  9  and  10   . 
     Therefore, the user can move the toner shutter  54  easily by operating the handle  55 . 
     (8) According to the image forming apparatus  1 , the toner outlet  511  is positioned higher than the toner inlet  433  in the state where the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , as illustrated in  FIG.  12   . 
     Therefore, even if a thickness of the cushion member  438  becomes greater, the toner accommodated in the toner cartridge  5  can be supplied to the developing unit  43  by use of the gravitational force regardless of the thickness of the cushion member  438 . 
     (9) According to the image forming apparatus  1 , the toner cartridge  5  is locked relative to the drum cartridge  4  in the state where the toner cartridge  5  is attached to the drum cartridge  4 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position, as illustrated in  FIG.  10   . 
     This configuration can prevent unintentional detachment of the toner cartridge  5  from the drum cartridge  4  when the developing shutter  437  is in the developing open position and the toner shutter  54  is in the toner open position. 
     &lt;5. Modifications&gt; 
     Hereinafter, various modifications to the embodiment will be described with reference to  FIGS.  13  to  15 C , wherein like parts and components are designated by the same reference numerals as those shown in the above-described embodiment to avoid duplicating description. 
     (1)  FIGS.  13  and  14    illustrate a first modification to the embodiment employing an image forming apparatus  101  including a main casing  102 . In this modification, a toner cartridge  105  and a drum cartridge  104  is attachable to the main casing  102  independently from each other. That is, the toner cartridge  105  is attachable directly to the main casing  102  without being attached to the drum cartridge  104 . Further, an opening  121  is formed in an upper wall of the main casing  102 , and a cover  122  is configured to open and close the upper opening  121 . Still further, the image forming apparatus  101  illustrated in  FIGS.  13  and  14    includes an exposure unit  106  which is an LED head. The exposure unit  106  may be supported by the cover  122 . 
     (2)  FIG.  15 A  illustrates a second modification to the embodiment in which a cushion member  2438  is employed instead of the cushion member  438 . In this second modification, the cushion member  2438  includes a first layer  24381  and a second layer  24382 . 
     The first layer  24381  constitutes an outer portion of the cushion member  2438 , i.e., positioned radially outward of the second layer  24382 . The first layer  24381  may be affixed to the second layer  24382  with an adhesive, for example. The first layer  24381  makes contact with the developing shutter  437 . The first layer  24381  is made of a hard resin, for example. The first layer  24381  is not deformed when the developing casing  432  is moved relative to the photosensitive drum  41  while the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position. Accordingly, the toner shutter  54  can be supported more stably by using the first layer  24381 . 
     The second layer  24382  is interposed between the first layer  24381  and the tubular portion  4322  of the developing casing  432 . The second layer  24382  may be affixed to the outer circumferential surface of the tubular portion  4322  with an adhesive, for example. The second layer  24382  has a flexibility greater than that of the first layer  24381 . The second layer  24382  is made of an elastic material, such as sponge, for example. The second layer  24382  is elastically deformed when the developing casing  432  is moved relative to the photosensitive drum  41  while the drum cartridge  4  and the toner cartridge  5  are attached to the main casing  2 , the developing shutter  437  is in the developing open position, and the toner shutter  54  is in the toner open position. With the above configuration, the second layer  24382  allows the movement of the tubular portion  4322  of the developing casing  432  relative to the developing shutter  437 . 
     (3) The cushion member  438  may not have a cylindrical shape. Further, the toner cartridge  5  may include a plurality of cushion members  438 . 
     Specifically,  FIG.  15 B  illustrates a third modification to the embodiment in which the drum cartridge  4  includes two cushion members  3438 A and  3438 B. The cushion member  438 A is provided to surround the toner inlet  433 . The cushion member  3438 B is positioned opposite to the cushion member  3438 A with respect to the tubular portion  4322 . In this case, each of the cushion members  3438 A and  3438 B has a stick-like shape extending in the first direction. 
     Further,  FIG.  15 C  illustrates a fourth modification to the embodiment in which the drum cartridge  4  includes three cushion members  4438 A,  4438 B, and  4438 C. The cushion member  4438 A surrounds the toner outlet  511 . Each of the cushion members  4438 B and  4438 C are positioned to be spaced apart from the cushion member  3438 A in a circumferential direction of the tubular portion  4322 . In this modification, each of the cushion members  4438 A,  4438 B, and  4438 C has a stick-like shape extending in the first direction. 
     (4) The tubular portion  4322  need not have a cylindrical shape. Instead, the tubular portion  4322  may have a polygonal tubular shape, such as a rectangular tubular shape, a hexagonal tubular shape, and an octagonal tubular shape. In a case where the tubular portion  4322  has a polygonal tubular shape, the cushion member  438  may be a tubular member with a hole that has a shape identical to an outer shape of the polygonal tubular portion  4322 . In this case, the cushion member  438  may not be affixed to a peripheral surface of the polygonal tubular portion  4322 . 
     (5) The developing casing  432  may be slidingly movable in the second attachment direction relative to the photosensitive drum  41 . In this case, the second attachment direction serves as a second direction. 
     While the description has been made in detail with reference to the embodiments, it would be apparent to those skilled in the art that many modifications and variations may be made thereto.