Patent Publication Number: US-7899356-B2

Title: Developing device, image carrier device, and image forming apparatus

Description:
CROSS REFERENCE TO THE RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2006-235009 filed on Aug. 31, 2006, the disclosure of which is hereby incorporated into the present application by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus such as a laser printer, and a developing device such as a developer cartridge and an image carrier device such as a drum unit to be mounted in the image forming apparatus. 
     BACKGROUND 
     In an image forming apparatus such as a laser printer, an electrostatic latent image is formed on a surface of a photosensitive drum, and toner is supplied to the electrostatic latent image from a developer cartridge, whereby a toner image is carried on the surface of the photosensitive drum. Then, the toner image is transferred onto a sheet. Thus, an image is formed on the sheet. 
     The developer cartridge includes a casing which contains toner and has an opening facing toward the photosensitive drum. The developer cartridge further includes a developing roller rotatably provided in the casing as being exposed from the opening of the casing, a layer-thickness regulating blade provided in the casing for regulating the thickness of a toner layer carried on the developing roller, and seal members provided in the casing for preventing the toner from leaking from axially opposite end portions of the developing roller. 
     Referring to  FIG. 11 , a developer cartridge  151  is illustrated as such a developer cartridge. 
     In  FIG. 11 , the developer cartridge  151  includes a casing  152  having an opening  153  extending longitudinally thereof, and a layer-thickness regulating blade  154  provided in the opening  153  of the casing  152  as extending longitudinally of the opening  153 . 
     The layer-thickness regulating blade  154  includes a blade  155  and a pressure contact rubber  156 . The blade  155  is of a thin plate shape, and has a fixed downstream edge portion (an upper edge portion in  FIG. 11 ) on a downstream side with respect to the direction of the rotation of the developing roller  160  (see  FIG. 12 ). The pressure contact rubber  156  is provided on a free upstream edge portion (a lower edge portion in  FIG. 11 ) of the blade  155  on an upstream side with respect to the rotation direction of the developing roller  160  as extending to the vicinities of longitudinally opposite end portions of the blade  155 . 
     Further, seal members  157  are respectively provided on longitudinally opposite end portions of the opening  153 . The seal members  157  are disposed along the rotation direction of the developing roller  160 . Downstream end portions  158  of the seal members  157  located downstream with respect to the rotation direction of the developing roller  160  are respectively attached to surfaces of the longitudinally opposite end portions of the blade  155  in adjoining relation to longitudinally opposite ends of the pressure contact rubber  156 . Further, upstream end portions  159  of the seal members  157  located upstream with respect to the rotation direction of the developing roller  160  are respectively attached to surfaces of end portions of the casing  152 . 
     In the developer cartridge shown in  FIG. 11 , therefore, the developing roller  160  is kept in pressure contact with surfaces of the aforementioned seal members  157  as shown in  FIG. 12 . When the developing roller  160  is rotated in an arrow direction in a developing process, the toner is disadvantageously liable to leak through boundary portions  161  between the seal members  157  and the pressure contact rubber  156  in the rotation direction of the developing roller  160  as shown in  FIG. 11 . 
     SUMMARY 
     One aspect of the present invention may provide a developing device which is capable of effectively preventing a developing agent from leaking through a boundary portion between a leak preventing member and a layer-thickness regulating member with a simple construction, and to provide an image carrier device and an image forming apparatus including such a developing device. 
     The same or different aspect of the present invention may provide a developing device including: a casing which contains a developing agent and has an opening extending longitudinally thereof; a developing agent carrier rotatably provided in the casing as being exposed from the opening and carrying the developing agent; leak preventing members disposed at longitudinally opposite ends of the opening for preventing the developing agent from leaking out of the casing; and a layer-thickness regulating member kept in pressure contact with a surface of the developing agent carrier for forming a thin layer of the developing agent on the surface of the developing agent carrier; wherein the layer-thickness regulating member includes a thin plate member having a thin plate shape extending longitudinally of the casing to positions such as to overlap with the leak preventing members, and having a downstream edge portion fixed to the casing with respect to a direction of rotation of the developing agent carrier that is the same direction as a direction perpendicular to a longitudinal direction of the casing and a free upstream edge portion with respect to the rotation direction, and a projection member provided on the free edge portion of the thin plate member as extending longitudinally of the casing and projecting toward the developing agent carrier; wherein the leak preventing members each include a first seal member and a second seal member each disposed along the rotation direction; wherein the first seal member includes a first downstream portion disposed adjacent, longitudinally of the casing, to the projection member on a front surface of the thin plate member opposed to the developing agent carrier, and a first upstream portion extending upstream of the first downstream portion with respect to the rotation direction continuously from the first downstream portion; wherein the second seal member includes a second downstream portion disposed on a rear surface of the thin plate member facing away from the developing agent carrier, and opposed to the first downstream portion and a portion of the projection member adjacent to the first downstream portion with intervention of the thin plate member, and a second upstream portion which extends upstream of the second downstream portion with respect to the rotation direction continuously from the second downstream portion and integrally includes a covered portion covered with the first upstream portion, an opposed portion disposed adjacent, longitudinally of the casing, to the covered portion and opposed to the adjacent portion of the projection member in the rotation direction, and an exposed portion extending upstream of the covered portion and the opposed portion with respect to the rotation direction. 
     One or more aspects of the present invention provide an image carrier device including: a developing device; and an image carrier which carries a developing agent image formed thereon by supplying a developing agent thereto from the developing device and developing an electrostatic latent image with the developing agent; wherein the developing device includes a casing which contains the developing agent to be supplied to the image carrier and has an opening extending longitudinally thereof, a developing agent carrier rotatably provided in the casing as being exposed from the opening and carrying the developing agent to be supplied to the image carrier, leak preventing members disposed at longitudinally opposite ends of the opening for preventing the developing agent from leaking out of the casing, and a layer-thickness regulating member kept in pressure contact with a surface of the developing agent carrier for forming a thin layer of the developing agent on the surface of the developing agent carrier; wherein the layer-thickness regulating member includes a thin plate member having a thin plate shape extending longitudinally of the casing to positions such as to overlap with the leak preventing members, and having a downstream edge portion fixed to the casing with respect to a direction of rotation of the developing agent carrier that is the same direction as a direction perpendicular to a longitudinal direction of the casing and a free upstream edge portion with respect to the rotation direction, and a projection member provided on the free edge portion of the thin plate member as extending longitudinally of the casing and projecting toward the developing agent carrier; wherein the leak preventing members each include a first seal member and a second seal member each disposed along the rotation direction; wherein the first seal member includes a first downstream portion disposed adjacent, longitudinally of the casing, to the projection member on a front surface of the thin plate member opposed to the developing agent carrier, and a first upstream portion extending upstream of the first downstream portion with respect to the rotation direction continuously from the first downstream portion; wherein the second seal member includes a second downstream portion disposed on a rear surface of the thin plate member facing away from the developing agent carrier, and opposed to the first downstream portion and a portion of the projection member adjacent to the first downstream portion with intervention of the thin plate member, and a second upstream portion which extends upstream of the second downstream portion with respect to the rotation direction continuously from the second downstream portion and integrally includes a covered portion covered with the first upstream portion, an opposed portion disposed adjacent, longitudinally of the casing, to the covered portion and opposed to the adjacent portion of the projection member in the rotation direction, and an exposed portion extending upstream of the covered portion and the opposed portion with respect to the rotation direction. 
     One or more aspects of the present invention provide an image forming apparatus including: an image carrier device; and a fixing unit which fixes a developing agent image carried by the image carrier device on a recording medium; wherein the image carrier device includes a developing device and an image carrier which carries a developing agent image formed thereon by supplying a developing agent thereto from the developing device and developing an electrostatic latent image with the developing agent; wherein the developing device includes a casing which contains the developing agent to be supplied to the image carrier and has an opening extending longitudinally thereof, a developing agent carrier rotatably provided in the casing as being exposed from the opening and carrying the developing agent to be supplied to the image carrier, leak preventing members disposed at longitudinally opposite ends of the opening for preventing the developing agent from leaking out of the casing, and a layer-thickness regulating member kept in pressure contact with a surface of the developing agent carrier for forming a thin layer of the developing agent on the surface of the developing agent carrier; wherein the layer-thickness regulating member includes a thin plate member having a thin plate shape extending longitudinally of the casing to positions such as to overlap with the leak preventing members, and having a downstream edge portion fixed to the casing with respect to a direction of rotation of the developing agent carrier that is the same direction as a direction perpendicular to a longitudinal direction of the casing and a free upstream edge portion with respect to the rotation direction, and a projection member provided on the free edge portion of the thin plate member as extending longitudinally of the casing and projecting toward the developing agent carrier; wherein the leak preventing members each include a first seal member and a second seal member each disposed along the rotation direction; wherein the first seal member includes a first downstream portion disposed adjacent, longitudinally of the casing, to the projection member on a front surface of the thin plate member opposed to the developing agent carrier, and a first upstream portion extending upstream of the first downstream portion with respect to the rotation direction continuously from the first downstream portion; wherein the second seal member includes a second downstream portion disposed on a rear surface of the thin plate member facing away from the developing agent carrier, and opposed to the first downstream portion and a portion of the projection member adjacent to the first downstream portion with intervention of the thin plate member, and a second upstream portion which extends upstream of the second downstream portion with respect to the rotation direction continuously from the second downstream portion and integrally includes a covered portion covered with the first upstream portion, an opposed portion disposed adjacent, longitudinally of the casing, to the covered portion and opposed to the adjacent portion of the projection member in the rotation direction, and an exposed portion extending upstream of the covered portion and the opposed portion with respect to the rotation direction. 
     One or more aspects of the present invention provide an image forming apparatus including: a developing device; an image carrier which carries a developing agent image formed thereon by supplying a developing agent thereto from the developing device and developing an electrostatic latent image with the developing agent; and a fixing unit which fixes the developing agent image carried by the image carrier on a recording medium; wherein the developing device includes a casing which contains the developing agent to be supplied to the image carrier and has an opening extending longitudinally thereof, a developing agent carrier rotatably provided in the casing as being exposed from the opening and carrying the developing agent to be supplied to the image carrier, leak preventing members disposed at longitudinally opposite ends of the opening for preventing the developing agent from leaking out of the casing, and a layer-thickness regulating member kept in pressure contact with a surface of the developing agent carrier for forming a thin layer of the developing agent on the surface of the developing agent carrier; wherein the layer-thickness regulating member includes a thin plate member having a thin plate shape extending longitudinally of the casing to positions such as to overlap with the leak preventing members, and having a downstream edge portion fixed to the casing with respect to a direction of rotation of the developing agent carrier that is the same direction as a direction perpendicular to a longitudinal direction of the casing and a free upstream edge portion with respect to the rotation direction, and a projection member provided on the free edge portion of the thin plate member as extending longitudinally of the casing and projecting toward the developing agent carrier; wherein the leak preventing members each include a first seal member and a second seal member each disposed along the rotation direction; wherein the first seal member includes a first downstream portion disposed adjacent, longitudinally of the casing, to the projection member on a front surface of the thin plate member opposed to the developing agent carrier, and a first upstream portion extending upstream of the first downstream portion with respect to the rotation direction continuously from the first downstream portion; wherein the second seal member includes a second downstream portion disposed on a rear surface of the thin plate member facing away from the developing agent carrier, and opposed to the first downstream portion and a portion of the projection member adjacent to the first downstream portion with intervention of the thin plate member, and a second upstream portion which extends upstream of the second downstream portion with respect to the rotation direction continuously from the second downstream portion and integrally includes a covered portion covered with the first upstream portion, an opposed portion disposed adjacent, longitudinally of the casing, to the covered portion and opposed to the adjacent portion of the projection member in the rotation direction, and an exposed portion extending upstream of the covered portion and the opposed portion with respect to the rotation direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side sectional view illustrating one embodiment of a color laser printer as an image forming apparatus of one or more aspects of the present invention. 
         FIG. 2  is a sectional view of a drum subunit of the color laser printer shown in  FIG. 1 . 
         FIG. 3  is a perspective view of a developer cartridge. 
         FIG. 4  is a bottom view of the developer cartridge (with a developing roller attached thereto). 
         FIG. 5  is a bottom view of the developer cartridge (with the developing roller removed therefrom). 
         FIG. 6  is a perspective view of the developer cartridge (with the developing roller removed therefrom). 
         FIG. 7  is a sectional view taken along a line A-A in  FIG. 4 . 
         FIG. 8  is a sectional view taken along a line B-B in  FIG. 4 . 
         FIG. 9  is a sectional view taken along a line C-C in  FIG. 4 . 
         FIG. 10  is a sectional view taken along a line D-D in  FIG. 4 . 
         FIG. 11  is a front view of a developer cartridge (with a developing roller removed therefrom). 
         FIG. 12  is a sectional view of the developer cartridge shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will hereinafter be described with reference to the attached drawings. 
     First Embodiment 
     1. Overall Structure of Color Laser Printer 
       FIG. 1  is a side sectional view illustrating one embodiment of a color laser printer as an image forming apparatus of the present invention, and  FIG. 2  is a sectional view of a drum subunit of the color laser printer shown in  FIG. 1 . 
     The color laser printer  1  is a tandem color laser printer of a horizontal type, in which a plurality of drum subunits  28  to be described later are horizontally arranged. 
     The color laser printer  1  includes a sheet feeding section  4  for feeding a sheet  3  as an example of recording medium, an image forming section  5  for forming an image on the fed sheet  3 , and a sheet ejecting section  6  for ejecting the sheet  3  formed with the image, and these sections are provided in a main body casing  2  of the printer  1 . 
     (1) Main Body Casing 
     The main body casing  2  is of a box shape generally rectangular as seen from a lateral side, and has a drum receiving space  7  which accommodates a drum unit  25  as an example of image carrier device to be described later. 
     A front cover  8  is provided on one side of the main body casing  2 . With the front cover  8  being inclined outward, the drum receiving space  7  is opened, so that the drum unit  25  is detachably mountable in the drum receiving space  7 . With the front cover  8  being set upright, the drum receiving space  7  is closed, in which the drum unit  25  is accommodated. 
     In the following description, a side of the color laser printer  1  provided with the front cover  8  (right-hand side in  FIG. 1 ) is defined as a front side (forward side), and a side of the color laser printer  1  opposite from the front side (left-hand side in  FIG. 1 ) is defined as a rear side (rearward side). 
     Further, left and right sides of the color laser printer  1  are defined as seen from the front side of the printer  1 . That is, a front side of the paper face of  FIG. 1  is defined as the left side of the printer  1 , and a rear side of the paper face of  FIG. 1  is defined as the right side of the printer  1 . Further, it is herein defined that leftward and rightward directions are equivalent to transverse directions. 
     Directions to be herein used for explaining the drum unit  25  and developer cartridges  27  are based on the assumption that the drum unit  25  and the developer cartridges  27  are mounted in the main body casing  2 , unless otherwise specified. 
     (2) Sheet Feeding Section 
     The sheet feeding section  4  includes a sheet feeding tray  10  detachably mounted in a bottom portion of the main body casing  2 , a sheet feeding roller  11  disposed on an upper front side of the sheet feeding tray  10 , and a sheet feeding path  17  extending from the sheet feeding roller  11  to a transport belt  58  to be described later. 
     A separation roller  12  and a separation pad  13 , a sheet dust removing roller  14  and a pinch roller  15 , and registration rollers  16  are provided in the sheet feeding path  17 . 
     Sheets  3  which are stacked in the sheet feeding tray  10  are fed by rotation of the sheet feeding roller  11 , and separated one from another between the separation roller  12  and the separation pad  13 . Then, the sheet  3  is transported to the registration rollers  16  after paper dust and the like are removed from the sheet  3  between the sheet dust removing roller  14  and the pinch roller  15 . The registration rollers  16  once stop the sheet  3  for registration, and then transport the sheet  3  to the transport belt  58  (to be described later). 
     (3) Image Forming Section 
     The image forming section  5  includes a scanning section  20 , a processing section  21 , a transferring section  22  and a fixing section  23  as an example of fixing unit. 
     (3-1) Scanning Section 
     The scanning section  20  is disposed in an upper portion of the main body casing  2 . Optical elements such as a light source, mirrors and lenses are provided in the scanning section  20 . Based on image data, surfaces of photosensitive drums  29  as an example of image carrier for respective colors to be described later are irradiated to be scanned at a high speed with a laser beam emitted from the light source by the mirrors and the lenses. 
     (3-2) Processing Section 
     The processing section  21  is disposed below the scanning section  20  above the sheet feeding section  4 . The processing section  21  includes a single drum unit  25  and four developer cartridges  27  (developing device) for the respective colors. 
     (3-2-1) Drum Unit 
     The drum unit  25  includes four drum subunits  28  for the respective colors, and a drawer frame  26  in which the drum subunits  28  are provided. 
     The four drum subunits  28  include a black drum subunit  28 K, a yellow drum subunit  28 Y, a magenta drum subunit  28 M and a cyan drum subunit  28 C. The drum subunits  28  are arranged in tandem in anteroposteriorly spaced relation. 
     All the drum subunits  28  are fixed to the drawer frame  26 , and unitarily mounted in and demounted from the drum receiving space  7 . 
     As shown in  FIG. 2 , the drum subunits  28  each support a photosensitive drum  29  as an example of image carrier, a scorotron charger  30  and a cleaning brush  31 . 
     The photosensitive drum  29  is of a hollow cylindrical shape, and includes a positively chargeable photosensitive outermost layer of polycarbonate. The photosensitive drum  29  is rotated by a driving force transmitted from a motor (not shown) provided in the main body casing  2  during an image forming process. 
     The scorotron charger  30  is disposed in opposed spaced relation to the photosensitive drum  29 . During the image forming process, a high voltage is applied to the scorotron charger  30 , whereby the surface of the photosensitive drum  29  is uniformly positively charged. 
     (3-2-2) Developer Cartridges 
     As shown in  FIG. 1 , the developer cartridges  27  are respectively provided in the drum subunits  28  for the respective colors in a detachable manner. 
     That is, the four developer cartridges  27  include a black developer cartridge  27 K, a yellow developer cartridge  27 Y, a magenta developer cartridge  27 M and a cyan developer cartridge  27 C. 
     As shown in  FIG. 2 , the developer cartridges  27  each include a developer frame  36  as an example of casing, and an agitator  37 , a feed roller  38 , a developing roller  39  as an example of developing agent carrier and a layer-thickness regulating blade  40  as an example of layer-thickness regulating member, which are provided in the developer frame  36 . 
     As will be detailed later, the developer frame  36  is of a box shape, and has an opening  41  provided in a lower portion thereof. The developer frame  36  is partitioned into an upper toner accommodating chamber  43  and a lower developing chamber  44  by a partition wall  42 . The partition wall  42  has a communication port  45  which permits communication between the toner accommodating chamber  43  and the developing chamber  44 . 
     The toner accommodating chamber  43  contains toner as an example of developing agent for each color. A positively chargeable nonmagnetic single-component polymer toner in which a black, yellow, magenta or cyan colorant is blended is used as the color toner. 
     The toner preferably has an average particle diameter (average primary particle diameter) of not greater than 10 μm, more preferably not greater than 8 μm, and usually not smaller than 6 μm, as determined by Coulter Multicizer II available from Beckman Coulter Incorporated. The toner preferably has a sphericity of not less than 0.95, more preferably not less than 0.98, as determined by FPIA3000 available from Sysmex Corporation. 
     Pulverization toner may be used instead of the polymer toner, as long as the average particle diameter or the sphericity described above is satisfied. 
     The toner accommodating chamber  43  has detection windows  46  provided in opposite side walls  101  of the developer frame  36  (see  FIG. 3 ) as being opposed transversely to each other for detecting the amount of toner remaining in the toner accommodating chamber  43 . 
     The agitator  37  is provided in the toner accommodating chamber  43 . The agitator  37  includes an agitator rotating shaft  47  rotatably supported by the opposite side walls  101  of the developer frame  36 , and an agitating member  48  extending radially outward from the agitator rotating shaft  47 . 
     During the image forming process, a driving force is transmitted to the agitator rotating shaft  47  from a motor (not shown) provided in the main body casing  2 , whereby the agitating member  48  is circularly moved in the toner accommodating chamber  43 . 
     The feed roller  38  is disposed below the communication port  45  in the developing chamber  44 . The feed roller  38  includes a feed roller shaft  49  of a metal rotatably supported by the opposite side walls  101  of the developer frame  36 , and a sponge roller  50  of an electrically conductive sponge covering the feed roller shaft  49 . 
     During the image forming process, the feed roller  38  is rotated by a driving force transmitted thereto from a motor (not shown) provided in the main body casing  2 . 
     The developing roller  39  is disposed on an obliquely lower rearward side of the feed roller  38  in the developing chamber  44 . The developing roller  39  is exposed downward from the opening  41  of the developing chamber  44 . The developing roller  39  includes a developing roller shaft  51  of a metal rotatably supported by the opposite side walls  101  of the developer frame  36 , and a rubber roller  52  of an electrically conductive rubber covering the developing roller shaft  51 . 
     Further, the developing roller  39  is opposed to the feed roller  38  with the rubber roller  52  kept in pressure contact with the sponge roller  50 . 
     During the image forming process, the developing roller  39  is rotated by a driving force transmitted thereto from a motor (not shown) provided in the main body casing  2 . Further, a developing bias is applied to the developing roller  39 . 
     The layer-thickness regulating blade  40  is kept in pressure contact with the developing roller  39  from above in the developing chamber  44 . The layer-thickness regulating blade  40  includes a blade  53  as an example of thin plate member of a metal leaf spring having a thin plate shape, and a pressure contact rubber  54  as an example of projection member formed of an insulative elastic rubber provided on a free edge portion of the blade  53 . 
     A proximal edge portion (fixed edge portion) of the blade  53  is fixed to a fixing wall  105  (to be described later) by fixing screws  106  (to be described later), and the pressure contact rubber  54  provided on the free edge portion of the blade  53  is kept in pressure contact with the rubber roller  52  of the developing roller  39  from above by an elastic force of the blade  53 . 
     (3-2-3) Developing Operation in Processing Section 
     In each of the developer cartridges  27 , the corresponding color toner contained in the toner accommodating chamber  43  is agitated by the agitator  37  to be moved toward the communication port  45 , and discharged from the communication port  45  into the developing chamber  44 . 
     The toner discharged from the communication port  45  into the developing chamber  44  is fed to the feed roller  38 . The toner fed to the feed roller  38  is further fed to the developing roller  39  by rotation of the feed roller  38 . At this time, the toner is triboelectrically positively charged between the feed roller  38  and the developing roller  39  to which the developing bias is applied. 
     The toner fed to the developing roller  39  enters a boundary portion between the pressure contact rubber  54  of the layer-thickness regulating blade  40  and the rubber roller  52  of the developing roller  39  by rotation of the developing roller  39 , and is carried in the form of a thin layer having a predetermined thickness on a surface of the rubber roller  52 . 
     On the other hand, the scorotron charger  30  causes corona discharge in the drum subunit  28  provided in association with the developer cartridge  27 , whereby the surface of the photosensitive drum  29  is uniformly positively charged. 
     After the surface of the photosensitive drum  29  is uniformly positively charged by the scorotron charger  30  with the photosensitive drum  29  being rotated, the surface of the photosensitive drum  29  is scanned at a high-speed to be exposed to the laser beam (indicated by a broken line in  FIG. 1 ) from the scanning section  20  as shown in  FIG. 1 , whereby an electrostatic latent image corresponding to an image to be formed on a sheet  3  is formed on the surface of the photosensitive drum  29 . 
     While the photosensitive drum  29  is further rotated, the toner positively charged on the surface of the developing roller  39  is brought into contact with the photosensitive drum  29  by the rotation of the developing roller  39 . At this time, the toner is applied onto the electrostatic latent image formed on the surface of the photosensitive drum  29 , i.e., onto a portion of the surface of the photosensitive drum  29  which has been uniformly positively charged and exposed to the laser beam thereby having a reduced potential. Thus, the electrostatic latent image on the photosensitive drum  29  is developed to be made visible, whereby a toner image (developing agent image) of the corresponding color is carried on the surface of the photosensitive drum  29  by reversal development. 
     (3-3) Transferring Section 
     The transferring section  22  is anteroposteriorly disposed above the sheet feeding section  4  below the processing section  21  in the main body casing  2 . The transferring section  22  includes a driving roller  56 , a driven roller  57 , the transport belt  58  and transfer rollers  59 . 
     The driving roller  56  and the driven roller  57  are disposed in anteroposteriorly opposed spaced relation. The driving roller  56  is disposed rearward of the cyan drum subunit  28 C, and the driven roller  57  is disposed forward of the black drum subunit  28 K. 
     The transport belt  58  is an endless belt, which is stretched between the driving roller  56  and the driven roller  57 . 
     During the image forming process, the driving roller  56  is rotated by a driving force transmitted thereto from a motor (not shown) provided in the main body casing  2 . Then, the transport belt  58  is circularly moved between the driving roller  56  and the driven roller  57  in the same direction as the directions of the rotation of the photosensitive drums  29  at transfer positions at which the transport belt  58  contacts the photosensitive drums  29  in opposed relation. Further, the driven roller  57  is driven by the transport belt  58 . 
     The transfer rollers  59  are respectively disposed in opposed relation to the photosensitive drums  29  with the intervention of the transport belt  58  in a space surrounded by the transport belt  58  stretched between the driving roller  56  and the driven roller  57 . 
     The transfer rollers  59  are driven to be rotated in the same direction as the direction of the circular movement of the transfer belt  58  at the transfer positions at which the transfer rollers  59  contact the transport belt  58  in opposed relation. During the image forming process, a transfer bias is applied to the transfer rollers  59 . 
     The sheet  3  fed from the sheet feeding section  4  is transported from the front side to the rear side by the transport belt  58  circularly moved by the driving roller  56  and the driven roller  57 , thereby passing through the transfer positions associated with the respective photosensitive drums  29 . During the transportation of the sheet  3 , the color toner images respectively carried on the photosensitive drums  29  are successively transferred onto the sheet  3 , whereby a color image is formed on the sheet  3 . 
     (3-4) Fixing Section 
     The fixing section  23  is disposed behind the transferring section  22 , and includes a heating roller  65  and a pressure roller  66  which is pressed against the heating roller  65 . 
     While the sheet  3  passes through a nip between the heating roller  65  and the pressure roller  66  in the fixing section  23 , the color image transferred onto the sheet  3  is thermally fixed on the sheet  3  by application of heat and pressure. 
     (4) Sheet Ejecting Section 
     The sheet ejecting section  6  includes a sheet ejecting transport path  67 , a transport roller  69 , a pinch roller  70 , a pair of sheet ejecting rollers  71  and a sheet ejection tray  68 . 
     The sheet  3  transported from the fixing section  23  is further transported through the sheet ejecting transport path  67  by the transport roller  69  and the pinch roller  70 , and ejected onto the sheet ejection tray  68  by the sheet ejecting rollers  71 . 
     2. Major Construction of Developer Cartridge 
       FIG. 3  is a perspective view of the developer cartridge, and  FIG. 4  is a bottom view of the developer cartridge (with the developing roller attached thereto).  FIG. 5  is a bottom view of the developer cartridge (with the developing roller removed therefrom), and  FIG. 6  is a perspective view of the developer cartridge (with the developing roller removed therefrom).  FIG. 7  is a sectional view taken along a line A-A in  FIG. 4 , and  FIG. 8  is a sectional view taken along a line B-B in  FIG. 4 .  FIG. 9  is a sectional view taken along a line C-C in  FIG. 4 , and  FIG. 10  is a sectional view taken along a line D-D in  FIG. 4 . The major construction of the developer cartridge  27  will hereinafter be described with reference to  FIGS. 3 to 10 . 
     In the following description, the rotation direction of the developing roller  39  is such that a portion of the developing roller  39  initially opposed to the feed roller  38  is brought into opposed relation to the layer-thickness regulating blade  40  and then to the photosensitive drum  29 . The terms “upstream” and “downstream” with respect to the rotation direction of the developing roller  39  are herein used to define a positional relationship between two positions along the circumference of the developing roller  39 , and the positional relationship is determined so that the two positions are spaced a shorter distance clockwise or counterclockwise circumferentially of the developing roller  39 . 
     2-1. Developer Frame 
     The developer frame  36  of the developer cartridge  27  is shown in  FIG. 3 . As described above, the developer frame  36  is of a box shape, and has the opening  41  provided in the lower portion thereof as extending transversely (longitudinally thereof). 
     As shown in  FIGS. 3 and 6 , the developer frame  36  has shaft support grooves  102  which are respectively formed in the opposite side walls  101  disposed on transversely opposite sides of the opening  41  by cutting out the portions of the side walls  101  upward from lower ends thereof. 
     Seal pedestal portions  103  (see  FIG. 7 ) are provided on upper and front peripheral portions of the shaft support grooves  102  on the opposite side walls  101  as projecting transversely inward from the opposite side walls  101 . 
     As shown in  FIG. 7 , the seal pedestal portions  103  are curved along the outer circumference of the developing roller  39  on the respective side walls  101  as seen from the lateral side. The seal pedestal portions  103  are each divided into a front portion and an upper portion along a periphery thereof by a shaft insertion portion  104  for the feed roller  38 . 
     As shown in  FIGS. 6 and 8 , the fixing wall  105  on which the layer-thickness regulating blade  40  is fixed is provided along a rear edge of the opening  41  of the developer frame  36 . 
     The fixing wall  105  extends transversely along the rear edge of the opening  41 . As shown in  FIG. 7 , the fixing wall  105  has screw thread holes  107  formed in transversely opposite end portions thereof to be respectively threadingly engaged with the fixing screws  106  to be described later. 
     As shown in  FIGS. 3 and 4 , the developer frame  36  further includes receiving members  114  provided below transversely opposite ends of the opening  41  for preventing the toner from falling out of the developer roller  39 . 
     2-2. Layer-Thickness Regulating Blade 
     As shown in  FIGS. 5 and 6  and described above, the layer-thickness regulating blade  40  includes the blade  53  of the leaf spring extending transversely, and the pressure contact rubber  54  of the insulative elastic rubber provided on the free edge portion of the blade  53 . 
     2-2-1. Blade 
     The blade  53  has a generally rectangular elongated thin plate shape as seen from the bottom side, and extends transversely to positions such as to overlap with side seals  116  (to be described later) provided at the transversely opposite ends of the opening  41 . 
     The blade  53  has screw holes  108  formed in transversely opposite end portions of the proximal edge portion thereof which corresponds to a rear edge portion or a downstream edge portion with respect to the rotation direction of the developing roller  39  that is the same direction as a direction perpendicular to the longitudinal direction of the opening  41  (or transverse directions). 
     The blade  53  further has notched portions  109  formed in the vicinity of transversely opposite end portions of the free edge portion thereof which corresponds to a front edge portion or an upstream edge portion with respect to the rotation direction of the developing roller  39  that is the same direction as a direction perpendicular to the longitudinal direction of the opening  41  (or to the transverse directions). 
     The notched portions  109  are respectively located transversely inward of the side seals  116  (to be described later) and spaced a distance X 1  (e.g., 0.5 to 1 mm, see an enlarged view of  FIG. 5 ) transversely from the side seals  116  (more specifically, first seal members  117 ). 
     The notched portions  109  each have a generally open-square shape recessed rearward or downstream with respect to the rotation direction of the developing roller  39  from a front edge of the blade  53  and opening forward or upstream with respect to the rotation direction of the developing roller  39 . 
     More specifically, the notched portions  109  each have a generally rectangular shape, as seen from the bottom side, which has an outer edge located adjacent to the side seal  116  and spaced the distance X 1  transversely inward from the side seal  116 , an inner edge spaced a distance X 2  (e.g., 3 to 3.5 mm) transversely inward from the outer edge, and a depth D 1  (e.g., 1.5 to 2 mm) as measured rearward or downstream with respect to the rotation direction of the developing roller  39  from the front edge of the blade  53  over the distance between the outer edge and the inner edge. 
     2-2-2. Pressure Contact Rubber 
     The pressure contact rubber  54  is made of an insulative elastic rubber such as silicone rubber. The pressure contact rubber  54  is provided in the form of a generally rectangular elongated projection, as seen from the bottom side, which extends longitudinally of the opening  41  (or transversely) and projects downward toward the developing roller  39  from the front edge portion of the blade  53  (for example, by 1.5 to 2 mm). 
     As shown in  FIG. 5 , the pressure contact rubber  54  integrally includes first pressure contact portions  110  as an example of adjacent portions of a projection member respectively provided transversely outward of the outer edges of the notched portions  109 , second pressure contact portions  111  respectively provided rearward or downstream of the notched portions  109  with respect to the rotation direction of the developing roller  39  as continuously extending transversely inward from the first pressure contact portions  110 , and a third pressure contact portion  112  provided between the second pressure contact portions  111  as continuously extending transversely inward of the second pressure contact portions  111 . 
     The first pressure contact portions  110  respectively extend transversely outward from the outer edges of the notched portions  109  to portions of the blade  53  transversely inward of transversely opposite ends of the blade  53  on which the first seal members  117  (to be described later) are respectively attached, and each have a width corresponding to the distance X 1 . 
     Further, the first pressure contact portions  110  each extend from an upstream edge (front edge) of the blade  53  to a position downstream (rearward) beyond a downstream edge (rear edge) of the notched portion  109  with respect to the rotation direction of the developing roller  39 , and each have a length L 1  (e.g., 5 to 5.5 mm). 
     As shown in a D-D sectional view of  FIG. 10 , the first pressure contact portions  110  each have a generally rectangular sectional shape having round corners as seen in anteroposterior section (taken along the rotation direction of the developing roller  39 ). 
     The second pressure contact portions  111  are respectively located rearward or downstream of the notched portions  109  with respect to the rotation direction of the developing roller  39 , and each have a width corresponding to the distance X 2  between the outer and inner edges of the notched portion  109 . 
     Further, the second pressure contact portions  111  respectively extend from the downstream edges (rear edges) of the notched portions  109  to positions such as to align with downstream edges (rear edges) of the first pressure contact portions  110  in the rotation direction of the developing roller  39 . 
     As shown in a C-C sectional view of  FIG. 9 , the second pressure contact portions  111  each have a generally rectangular sectional shape having a round corner as seen in anteroposterior section (taken along the rotation direction of the developing roller  39 ). The second pressure contact portions  111  each have a smaller anteroposterior length in section (as measured along the rotation direction of the developing roller  39 ) than the first pressure contact portions  110 . 
     The third pressure contact portion  112  extends transversely continuously from the second pressure contact portions  111  between the transversely inner edges of the second pressure contact portions  111 . 
     Further, the third pressure contact portion  112  extends from the upstream edge (front edge) of the blade  53  to a position such as to align with downstream edges (rear edges) of the second pressure contact portions  111  in the rotation direction of the developing roller  39 . 
     As shown in a B-B sectional view of  FIG. 8 , the third pressure contact portion  112  has a generally rectangular sectional shape as seen in anteroposterior section (taken along the rotation direction of the developing roller  39 ). The third pressure contact portion  112  has substantially the same anteroposterior length in section as the first pressure contact portions  110  (as measured along the rotation direction of the developing roller  39 ). 
     That is, the first pressure contact portions  110 , the second pressure contact portions  111  and the third pressure contact portion  112  of the pressure contact rubber  54  are transversely continuously arranged. The rear edges of the first pressure contact portions  110 , the second pressure contact portions  111  and the third pressure contact portion  112  are aligned transversely with each other at the same position with respect to the rotation direction of the developing roller  39 . Further, the front edge of the third pressure contact portion  112  is located at substantially the same position with respect to the rotation direction of the developing roller  39  as the front edges of the first pressure contact portions  110 , and the front edges of the second pressure contact portions  111  are located rearward or downstream of the front edges of the first pressure contact portions  110 . 
     The pressure contact rubber  54  substantially surrounds the notched portions  109 . More specifically, the first pressure contact portions  110  are continuous from the second pressure contact portions  111  to surround the transversely outer edges of the respective notched portions  109 , and the third pressure contact portion  112  is continuous from the second pressure contact portions  111  to surround the transversely inner edges of the respective notched portions  109 . 
     2-2-3. Fixing of Layer-Thickness Regulating Blade 
     As shown in  FIGS. 5 and 7 , the layer-thickness regulating blade  40  is fixed together with a support plate  113  to the fixing wall  105  of the developer frame  36  by the fixing screws  106 . 
     The support plate  113  is made of a steel plate, and has a generally rectangular elongated shape having the same length as the blade  53  as seen from the bottom side. The support plate  113  has screw holes  115  respectively formed in transversely opposite end portions thereof as aligning with the screw holes  108  of the blade  53 . 
     The support plate  113  is attached to an upper surface of the blade  53  by a two-sided adhesive tape. 
     The layer-thickness regulating blade  40  is fixed to the fixing wall  105  by bringing the support plate  113  into contact with the fixing wall  105  in opposed relation to the fixing wall  105 , inserting the fixing screws  106  into the screw holes  108  of the blade  53  and the screw holes  115  of the support plate  113 , and threadingly engaging the fixing screws  106  with the screw thread holes  107  of the fixing wall  105 . 
     Thus, the proximal edge portion (rear edge portion) of the blade  53  is fixed to the fixing wall  105 , while the pressure contact rubber  54  provided on the free edge portion (front edge portion) of the blade  53  is opposed to the developing roller  39  in pressure contact with the developing roller  39  from above by the elastic force of the blade  53 . The transversely opposite ends of the blade  53  are respectively opposed to the seal pedestal portions  103  of the developer frame 
     2-3. Side Seals 
     As shown in  FIGS. 5 and 6 , the side seals  116  as an example of leak preventing member are provided at the longitudinally opposite ends of the opening  41  of the developer frame  36  for preventing the toner carried on the developing roller  39  from leaking out of the developer frame  36  through axially opposite ends of the developing roller  39 . 
     The side seals  116  are thick strip-like sheets (e.g., having a thickness of 3 to 4 mm) as seen from the bottom side. The side seals  116  are each made of an elastic foam material such as a urethane sponge, more specifically a high-density micro-cell urethane foam (available from Rogers Inoac Corporation under the trade name of PORON). The high-density micro-cell urethane foam has a hardness of 0.001 to 0.05 MPa, more preferably 0.005 to 0.025 MPa, under a 25% compression load. A felt member such as of Teflon (registered trade mark) felt is attached to a surface (opposed to the developing roller  39 ) of the elastic foam material for improving the slidability. 
     The side seals  116  each include a first seal member  117  and a second seal member  118  which are arranged along the rotation direction of the developing roller  39 . 
     2-3-1. First Seal Member 
     The first seal member  117  is of a generally rectangular shape as seen from the bottom side, and has a width W 1  (transverse dimension) which corresponds to a transverse distance (e.g., 5.5 to 6 mm) between the transversely outer end of the blade  53  and the transversely outer edge of the first pressure contact portion  110 . 
     The first seal member  117  has a first downstream portion (rear portion)  119  located downstream with respect to the rotation direction of the developing roller  36 . The first downstream portion  119  is attached to a lower surface (opposed to the developing roller  39 ) of the transversely outer end portion of the blade  53  (between the transversely outer edge of the blade  53  and the transversely outer edge of the first pressure portion  110 ) by a two-sided adhesive tape (see  FIG. 7 ). 
     The first downstream portion  119  is located transversely outward of the first pressure contact portion  110  in adjoining relation, and extends from the front edge of the blade  53  to the rear side of the rear edge of the first pressure contact portion  110  in the rotation direction of the developing roller  39 . 
     The first seal member  117  further has a first upstream portion (front portion)  120  located upstream with respect to the rotation direction of the developing roller  36 . The first upstream portion  120  extends forward or upstream with respect to the rotation direction of the developing roller  36  continuously from the first downward portion  119 . The first upstream portion  120  covers a lower surface of a covered portion  123  of the second seal member  118  to be described later. 
     2-3-2. Second Seal Member 
     The second seal member  118  is of a generally rectangular shape as seen from the bottom side, and has a width W 2  (transverse dimension) corresponding to a transverse distance (e.g., 9 to 10 mm) between the transversely outer edge of the blade  53  and the transversely outer edge of the notched portion  109 . 
     As shown in  FIG. 7 , the second seal member  118  is attached to the front portion and the upper portion of the seal pedestal portion  103  of the developer frame  36  by two-sided adhesive tapes. 
     More specifically, the second seal member  118  has a second downstream portion (upper edge portion)  121  located downstream with respect to the rotation direction of the developer roller  36 . The second downstream portion  121  is disposed on the transversely outer end portion of the blade  53  (between the transversely outer end of the blade  53  and the transversely outer edge of the notched portion  109 ). The second downstream portion  121  is held between the upper portion of the seal pedestal portion  103  and an upper surface of the blade  53  (opposite from the opposed surface of the developing roller  39 ), and attached to the upper portion of the seal pedestal portion  103  and the upper surface of the blade  53  by two-sided adhesive tapes. 
     As shown in  FIGS. 5 and 7 , the second downstream portion  121  is opposed to the first downstream portion  119  of the first seal member  117  with the intervention of the blade  53 . As shown in  FIGS. 5 and 10 , the second downstream portion  121  is also opposed to the first pressure contact portion  110  with the intervention of the blade  53 . 
     As shown in  FIGS. 5 and 7 , the second seal member  118  further has a second upstream portion  122  (which corresponds to an upper end portion thereof except for the upper edge portion (second downstream portion  121 ) and a front portion thereof) located upstream with respect to the rotation direction of the developing roller  36 . The second upstream portion  122  extends forward or upstream with respect to the rotation direction of the developing roller  36  continuously from the second downstream portion  121 . The second upstream portion  122  is attached to the front portion of the seal pedestal portion  103  of the developer frame  36  by a two-sided adhesive tape. 
     The second upstream portion  122  includes the covered portion  123 , an opposed portion  124  and an exposed portion  125  as shown in  FIG. 5 . 
     The covered portion  123  is defined as a downstream portion (rear portion) of the second upstream portion  122  located downstream with respect to the rotation direction of the developing roller  36  and transversely outward. As described above, the covered portion  123  is covered with the first upstream portion  120 . 
     The covered portion  123  has a dimension L 2  of 0.5 to 1 mm as measured in the rotation direction of the developing roller  39 . 
     The opposed portion  124  is defined as a downstream portion (rear portion) of the second upstream portion  122  located downstream with respect to the rotation direction of the developing roller  39  and transversely inward of the covered portion  123  in transversely adjoining relation. 
     The opposed portion  124  is located adjacent the first upstream portion  120  covering the covered portion  123  on the transversely outer side, and located adjacent the first pressure contact portion  110  on the upstream side (front side) with respect to the rotation direction of the developing roller  39  in opposed relation. 
     That is, the opposed portion  124  is surrounded by the first upstream portion  120  on the transversely outer side, and by the first pressure contact portion  110  on the rear side. Level differences are present between the opposed portion  124  and the first upstream portion  120  and between the opposed portion  124  and the first pressure contact portion  110 . That is, the opposed portion  124 , which is surrounded by the first upstream portion  120  on the transversely outer side and by the first pressure contact portion  110  on the rear side, is recessed from the first upstream portion  120  and the first pressure contact portion  110 . 
     The exposed portion  125  is defined as an upstream portion (front portion) of the second upstream portion  122  located upstream of the covered portion  123  and the opposed portion  124  with respect to the rotation direction of the developing roller  36 . As shown in  FIG. 7 , the exposed portion  125  is attached to the front portion of the seal pedestal portion  103  of the developer frame  36  by a two-sided adhesive tape. 
     2-4. Developing Roller 
     As shown in  FIGS. 3 and 6 , the axially opposite ends of the developing roller shaft  51  of the developing roller  39  are received in the shaft support grooves  102  formed in the opposite side walls  101  of the developer frame  36 , whereby the developing roller  39  is rotatably supported by the opposite side walls  101  of the developer frame  36 . 
     With the developing roller  39  supported by the opposite side walls  101 , the exposed portions  125  of the second seal members  118  are pressed to a thickness of not greater than two thirds of the original thickness thereof (which is measured in an unpressed state) by the transversely opposite end portions of the developing roller  39  (see  FIG. 7 ). 
     Further, the rubber roller  52  of the developing roller  39  is pressed across its width by the pressure contact rubber  54  as shown in  FIG. 8 . 
     As shown in  FIG. 10 , there is a small gap S between each of the opposed portions  124  of the second seal members  118  and the rubber roller  52 . The gap S is defined by the opposed portion  124 , the rubber roller  52  and the first pressure contact portion  110  in a section perpendicular to the transverse directions (or the longitudinal axis of the opening  41 ), i.e., perpendicular to the axis of the developing roller  39 . More specifically, the gap S has an area of 0.2 to 0.7 mm 2  in the section perpendicular to the axis of the developing roller  39 . 
     3. Effects of First Embodiment 
     
         
         (1) As described above, the side seals  116  of the developer cartridge  27  each include the first seal member  117  and the second seal member  118 . 
       
    
     The first downstream portion  119  of the first seal member  117  is located adjacent the first pressure contact portion  110  on the lower surface of the blade  53 , and the second downstream portion  121  of the second seal member  118  is located on the upper surface of the blade  53  in opposed relation to the first downstream portion  119  and the first pressure contact portion  110  with the intervention of the blade  53 . 
     Further, the second upstream portion  122  continuous from the second downstream portion  121  integrally includes the covered portion  123  which is covered by the first upstream portion  120 , the opposed portion  124  which is disposed adjacent the covered portion  123  on the transversely inward side and opposed to the first pressure contact portion  110  on the upstream side with respect to the developing roller  39 , and the exposed portion  125  which extends upstream with respect to the rotation direction of the developing roller  39  from the covered portion  123  and the opposed portion  124 . 
     The opposed portion  124  is disposed adjacent the first upstream portion  120  covering the covered portion  123  on the transversely outer side, and disposed adjacent the first pressure contact portion  110  on the upstream side (front side) with respect to the rotation direction of the developing roller  39 . Therefore, the level differences are present between the opposed portion  124  and the first upstream portion  120  and between the opposed portion  124  and the first pressure contact portion  110 . 
     That is, the opposed portion  124  is surrounded by the first upstream portion  120  on the transversely outer side and by the first pressure contact portion  110  on the rear side with the level differences and, therefore, recessed from the first upstream portion  120  and the first pressure contact portion  110 . 
     Therefore, the gaps S are formed between the opposed portions  124  and the rubber roller  52  of the developing roller  39  even with the developing roller  39  kept in pressure contact with the surfaces of the side seals  116 . 
     When the developing roller  39  is rotated for the development, toner intruding into the gaps S of the opposed portions  124  would be liable to further intrude into boundary portions between the first seal members  117  and the first pressure contact portions  110  along the rotation direction of the developing roller  39 . However, the level differences between the opposed portions  124  and the first pressure contact portions  110  blocked the intrusion. Since the toner sequentially enters the gaps S of the opposed portions  124 , the toner blocked from intruding into the boundary portions is released transversely inward from the gaps S due to the further entering toner. That is, the toner entering the gaps S of the opposed portions  124  on the transversely opposite end portions of the developing roller  39  is circulated in the gaps S, and released inward of the transversely opposite end portions of the developing roller  39  from the gaps S. 
     As a result, the leak of the toner from the boundary portions between the side seals  116  and the pressure contact rubber  54  is effectively prevented with the simple construction.
     (2) As described above, the gaps S of the opposed portions  124  each have an area of 0.2 to 0.7 mm 2  in the section perpendicular to the transverse directions. Therefore, the toner is assuredly circulated in the gaps S of the opposed portions  124 . As a result, the leak of the toner from the boundary portions between the side seals  116  and the pressure contact rubber  54  is more reliably prevented.   

     Table 1 shows the results of evaluation of developer cartridges which vary in the areas of the gaps S of the opposed portions  124 . The evaluation is based on three evaluation tests, i.e., a toner intrusion test for checking for the intrusion of the toner into the boundary portions between the first seal members  117  and the first pressure contact portions  110 , an impact test and a drop test. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Gaps S (mm 2 ) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 0.2 
                 0.3 
                 0.5 
                 0.7 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Intrusion into boundary portions between first 
                 ∘ 
                 ∘ 
                 ∘ 
                 Δ 
               
               
                 seal members and first pressure contact members* 1   
               
               
                 Impact test* 2   
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 Drop test* 3   
                 Δ 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
               
               
                 * 1 A symbol ∘ indicates that the toner did not intrude. 
               
               
                 A symbol Δ indicates that the toner intruded to some extent. 
               
               
                 * 2 A symbol ∘ indicates that the toner did not intrude into the boundary portions between the first seal members and the first pressure contact members. 
               
               
                 A symbol x indicates that the toner passed through the boundary portions between the first seal members and the first pressure contact members. 
               
               
                 * 3 A symbol ∘ indicates that the toner did not intrude into the boundary portions between the first seal members and the first pressure contact members. 
               
               
                 A symbol Δ indicates that the toner intruded into the boundary portions between the first seal members and the first pressure contact members to some extent. 
               
               
                 A symbol x indicates that the toner passed through the boundary portions between the first seal members and the first pressure contact members. 
               
            
           
         
       
     
     In the toner intrusion test for checking for the intrusion of the toner into the boundary portions between the first seal members  117  and the first pressure contact portions  110 , the developer roller  39  of a new developer cartridge  27  was rotated at a rotation speed of 20 ppm for 10 hours in a warm-up mode, and then a testing operator visually checked the developer cartridge for the intrusion of the toner for the evaluation. Here, “Warm-up mode” means that the developer roller  39  is rotated with no sheet being fed thereto. 
     More specifically, the testing operator pulled the first seal member  117  attached to the blade  53  away from the first pressure contact portion  110  to expand the boundary portion between the first seal member  117  and the first pressure contact portion  110 , and then visually checked whether or not the toner intruded into the boundary portion between the first seal member  117  and the first pressure contact portion  110 . 
     The evaluation was based on the following criteria which are expressed by the symbols o, Δ and x. The symbol o indicates that the toner did not intrude into the boundary portion at all, and the symbol Δ indicates that the toner intruded into the boundary portion to some extent. The symbol x indicates that the toner passed through the boundary portion. 
     In the impact test, the developer cartridge  27  was tapped down on a table six times so as to bring the developing roller  39  thereof into contact with the table, and then the toner intrusion test was performed in the aforesaid manner to check for the leak of the toner from the developer cartridge  27 . More specifically, the expression “the developer cartridge  27  was tapped down on a table” means that the developer cartridge  27  which has a weight of 620 g was freely dropped onto a surface of a hard iron table. 
     In the drop test, the 620 g developer cartridge  27  was freely dropped from a height of 30 cm above the hard iron table so as to cause the developing roller  39  to collide with the surface of the table, and then the toner intrusion test was performed in the aforesaid manner to check for the leak of the toner from the developer cartridge  27 . 
     As can be understood from Table 1, the leak of the toner is effectively prevented with the provision of gaps S each having an area of 0.2 to 0.7 mm 2 . Here, the area of each of the gaps S was varied by changing the attaching position of the first seal member  117  with respect to the blade  53 .
     (3) The bland  53  has the notched portions  109  respectively provided transversely inward of the first pressure contact portions  110 . Therefore, the toner circulated in the gaps S of the opposed portions  124  and released inward of the transversely opposite end portions of the developing roller  39  is received in the notched portions  109 , so that the flow of the toner is facilitated. As a result, the leak of the toner is more reliably prevented.   (4) The notched portions  109  are respectively spaced the distance X 1  transversely from the first seal members  117 . Therefore, the toner circulated in the gaps S of the opposed portions  124  and released inward of the transversely opposite end portions of the developing roller  39  flows further inward, and is received in the notched portions  109 . As a result, the toner is more reliably prevented from stagnating on the transversely opposite end portions of the developing roller  39 .   (5) In the developer cartridge  27 , the exposed portions  125  of the respective second seal members  118  are pressed to a thickness of not greater than two thirds of the original thickness (which is measured in an unpressed state) by the transversely opposite end portions of the developing roller  39 . This prevents formation of gaps between the exposed portions  125  of the second seal members  118  and the transversely opposite end portions of the developing roller  39 . As a result, the leak of the toner from the boundary portions between the exposed portions  125  of the second seal members  118  and the transversely opposite end portions of the developing roller  39  is prevented.   (6) The toner contained in the toner accommodating chamber  43  of the developer cartridge  27  has an average particle diameter of not greater than 10 μm, so that the toner would be liable to leak through the boundary portions between the side seals  116  and the pressure contact rubber  54 . However, the leak of the toner is effectively prevented with the aforesaid simple construction.   (7) The toner contained in the toner accommodating chamber  43  of the developer cartridge  27  has a sphericity of not smaller than 0.95 and hence excellent fluidity. This facilitates the circulation of the toner in the gaps S of the opposed portions  124 . Therefore, the leak of the toner is more effectively prevented.   (8) In the drum unit  25  and the laser printer  1  each including such a developer cartridge  27 , the leak of the toner is effectively prevented.   

     Second Embodiment 
     In the foregoing description, the tandem color laser printer is employed as the image forming apparatus by way of example. However, the image forming apparatus in which the inventive developing device (developer cartridge) is mounted is not limited to the aforementioned one. Other examples of the image forming apparatus include a color laser printer of an intermediate transfer type in which color developing agent images are once transferred onto an intermediate transfer member from respective image carriers and then transferred onto a recording medium, and a monochrome laser printer. 
     Examples of the monochrome laser printer include a monochrome laser printer (image forming apparatus) including a fixing unit (fixing section) and an image carrier device (drum unit) which is detachably mounted therein and includes an image carrier (photosensitive drum) and the inventive developing device (developer cartridge) detachably mounted in the image carrier device, and a monochrome laser printer (image forming apparatus) including an image carrier (photosensitive drum), a fixing unit (fixing section) and the inventive developing device (developer cartridge) detachably mounted therein. 
     The embodiments described above are illustrative and explanatory of the invention. The foregoing disclosure is not intended to be precisely followed to limit the present invention. In light of the foregoing description, various modifications and alterations may be made by embodying the invention. The embodiments are selected and described for explaining the essentials and practical application schemes of the present invention which allow those skilled in the art to utilize the present invention in various embodiments and various alterations suitable for anticipated specific use. The scope of the present invention is to be defined by the appended claims and their equivalents.