Patent Publication Number: US-7899358-B2

Title: Developing cartridge, process cartridge, and image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Applications No. 2005-266586, filed on Sep. 14, 2005, and No. 2006-235097, filed on Aug. 31, 2006, the entire subject matters of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relate to a developing cartridge, a process cartridge, and an image forming apparatus. 
     BACKGROUND 
     In a photosensitive member cartridge on which a developing cartridge is mounted, as disclosed in JP-A-2000-250310, a plurality of rotatable rollers are disposed in a developing cartridge accommodating portion. When the developing cartridge is attached in the developing cartridge accommodating portion, the weight of the developing cartridge is received on an opposite side of a roller shaft of a developing roller to a photosensitive drum by the rollers. Further, with the rollers, rattling is suppressed which otherwise occurs when attaching and detaching the developing cartridge. 
     SUMMARY 
     If a power-supply terminal of the image forming apparatus is brought into contact with a conductive bearing member, which rotatably supports the developing roller shaft and applies a developing bias to the developing roller shaft, from below, pressing of the developing roller against the photosensitive drum becomes instable, and quality of a resultant image is degraded. 
     Aspects of the invention provide a developing cartridge that, even though a main body-side power-supply electrode of an image forming apparatus presses a conductive member in order to apply a developing bias, can obtain a good-quality image, without causing pressing of a developing roller against a photosensitive drum to be instable, a process cartridge, and an image forming apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side sectional view of a printer as an image forming apparatus; 
         FIG. 2  is a plan view of a photosensitive member cartridge; 
         FIG. 3  is a plan view of a developing cartridge; 
         FIG. 4  is a cross-sectional view taken along the line IV-IV of  FIG. 3 ; 
         FIG. 5A  is a left side view of the developing cartridge, and  FIG. 5B  is a right side view of the developing cartridge; 
         FIG. 6  is a plan view of a process cartridge; 
         FIG. 7  is a right side view of the process cartridge; 
         FIG. 8  is a cross-sectional view showing the structures of left and right bearings of a developing roller and a conductive member seen from an upper of  FIG. 6 ; and 
         FIG. 9  is a diagram showing a modification of this aspect. 
     
    
    
     DETAILED DESCRIPTION 
     General Overview 
     According to an aspect of the invention, there is provided a developing cartridge, which is attachable to and detachable from an image forming apparatus and is received by a developing cartridge receiving member, the developing cartridge including: a developer carrier including a rotational shaft; a case that rotatably supports the developer carrier, the case being capable of being supported by the developing cartridge receiving member; and a conductive member electrically connected to the rotational shaft of the developer carrier, at least a part of the conductive member is positioned at an opposite side of the case to the developing cartridge receiving member in a substantially vertical direction. 
     According to the aspect of the invention, when the developing cartridge is mounted on the image forming apparatus, the main body-side power-supply electrode is brought into contact with the conductive member on the vertical line of the developing cartridge receiving member from the side opposite to the developing cartridge receiving member. Accordingly, the weight of the developing cartridge is received by the developing cartridge receiving member, and thus a force that causes rattling of the developing cartridge stably accommodated is not applied. Therefore, the contact state of the electrostatic latent image carrier and the developer carrier can be prevented from being instabilized, and thus good image quality can be secured. 
     (1) Overview of Entire Image Forming Apparatus 
     Next, an aspect of the invention as a laser printer will be described.  FIG. 1  is a schematic side sectional view of a printer as an image forming apparatus. 
     As shown in  FIG. 1 , in a main body housing  1  of the printer, a process cartridge  2  having a photosensitive member cartridge  3  and a developing cartridge  4  is detachably disposed at an approximately central portion. As shown in a left portion of  FIG. 1 , a fixing device  5  is disposed close to the process cartridge  2 , and a sheet feeder  6  is disposed below the process cartridge  2 . A sheet feed cassette  8  can be mounted from a front surface (an arrow A) direction at a lower portion of the main body housing  1 . 
     A laser scanner unit  7  is attached to a lower surface of a sheet discharge tray  1   a  formed of synthetic resin through a frame. The sheet discharge tray  1   a  also serves as a cover. 
     When print data is transmitted from an external apparatus (not shown), for example, a personal computer, upon a print instruction, a sheet P is separated from a stack of sheets P on a support plate  9  of the sheet feed cassette  8  by a separator pad  11  one by one according to rotation of a sheet feed roller  10  of the sheet feeder  6 . The separated sheet P is conveyed to a contact portion between a photosensitive drum  13  serving as an electrostatic latent image carrier in the process cartridge  2  and a transfer roller  14  pressed against a lower surface of the photosensitive drum  13  through a pair of register rollers  12   a  and  12   b . A laser beam is emitted from the laser scanner unit  7  having a laser light-emitting portion, a polygon mirror  18 , a lens  19 , a plurality of reflecting mirrors  20 , and the like, through a light-emitting hole formed in a lower portion of the frame supporting the laser scanner unit  7 . The laser beam travels to an upper peripheral surface portion of the photosensitive drum  13  through a light entrance portion  31  formed in a case  30  serving as the housing of the photosensitive member cartridge  3 , that is, a case in the process cartridge  2 . The peripheral surface of the photosensitive drum  13  is exposed corresponding to the print data, such that an electrostatic latent image is formed. 
     As described below, a developer (toner) supplied from a developing roller  22  serving as a developer carrier in the developing cartridge  4  is disposed on the electrostatic latent image, thereby making an image visible. After the visible image by developer (toner) on the photosensitive drum  13  is transferred to the sheet  2 , the sheet P is conveyed between a heat roller  15  and a presser roller  16  in the fixing device  5 , in which the sheet P is subjected to a heat-fixing process. The sheet P is then discharged onto the sheet discharge tray la through a sheet discharge passage  17 . 
     In this aspect, the process cartridge  2  includes the photosensitive member cartridge  3  having the photosensitive drum  13  and a developing cartridge receiving member to be described below and the developing cartridge  4  having a developing roller  22  that is rotatably disposed in a case  21  serving as the housing. The developing cartridge  4  is formed to be attachable to and detachable from the photosensitive member cartridge  3 . Then, the developing cartridge  4  and the photosensitive member cartridge  3  are detachably mounted on a printer main body together or separately. 
     Next, the configuration of the photosensitive member cartridge  3  and the developing cartridge  4  will be described in detail. 
     (2) Configuration of Photosensitive Member Cartridge 
       FIG. 2  is a plan view of the photosensitive member cartridge. In the photosensitive member cartridge  3 , the photosensitive drum  13  is rotatably journaled in the case  30  formed of synthetic resin near one side thereof. A lower portion of the transfer roller  14  serving as a transfer device disposed below the photosensitive drum  13  is covered with a bottom wall  30   a  of the case  30  (see  FIG. 7 ). 
     As shown in  FIGS. 1 and 2 , an upper wall  30   b  of the case  30  of the photosensitive member cartridge  3  covers an upper portion of the photosensitive drum  13 . The light entrance portion  31 , allowing irradiation of an upper surface of the photosensitive drum  13  with laser light emitted from the laser scanner unit  7 , is formed in the upper wall  30   b , and is elongated in an axial direction of the photosensitive drum  13 . A charger  36 , such as a scorotron charger or the like, which charges a photosensitive surface of the photosensitive drum  13  formed of an organic photosensitive material or the like, is disposed close to the light entrance portion  31 . The electrostatic latent image is formed on the surface of the photosensitive drum  13  by scanning the surface uniformly charged by the charger  36  with laser light. After the electrostatic latent image is made visible (developed) by deposition thereon of a thin layer of the toner supplied through the developing roller  22 , the toner image is transferred to the sheet P in a press-transfer region defined by the photosensitive drum  13  and the transfer roller  14 . 
     As shown in  FIG. 2 , a portion of the case  30  other than the upper wall  30   b  is opened upward to define a housing portion  32  into which the developing cartridge  4  can be detachably set from the above. An upper end surface of each of right and left side walls  30   c  of the case  30  defines a downwardly arched guide groove  37  that guides a corresponding one of bearings  23   a  and  23   b  (see  FIG. 8 ) relatively rotatably mounted on both end portions of a developing roller shaft  22   a  serving as a rotational shaft of the developing roller  22  while supporting the bearing slidably. The guide grooves  37  extend toward a shaft  13   a  of the photosensitive drum  13 . Therefore, when the developing cartridge  4  is set in the photosensitive member cartridge  3 , the developing roller  22  can be positioned to face the photosensitive drum  13  (see  FIG. 7 ). 
     An urging device  42  that presses the developing roller  22  against the photosensitive drum  13  through the developing cartridge  4  is pivotably and expandably mounted on an inner surface of each of the right and left side walls  30   c  of the photosensitive member cartridge  3 . As shown in  FIG. 7 , each urging device  42  has a pivot fulcrum member  39  having pivots  39   b  (in  FIG. 7 , only a right pivot is shown) protruding integrally from the right and left sides thereof, a frame-like slide support member  40  that slidably supports the pivot fulcrum member  39  therein, and an urging spring member  41 , such as a coil spring or the like, which is disposed in the frame of the slide support member  40  so as to urge the pivot fulcrum member  39  toward one end. The slide support member  40  has a cylindrical motion acting portion  43  extending laterally. The motion acting portion  43  of each urging device  42  is disposed so that the portion  43  protrudes outward from a guide hole  44  formed in the corresponding one of the right and left side walls  30   c . When the developing cartridge  4  is mounted on the photosensitive member cartridge  3  and then is completely mounted on the main body housing  1  of the printer, the urging device  42  presses an action-receiving portion  61  described below of the developing cartridge  4  at a front end of the slide support member  40 . Accordingly, with the urging device  42  and the action-receiving portion  61 , the developing roller  22  can be pressed against the photosensitive drum  13  through the developing cartridge  4 . 
     A lock device  46  that prevents the developing cartridge  4  fitted into the accommodating portion  32  from moving upward out of the accommodating portion  32  is disposed on an inner side of one of the right and left side walls  30   c  (the right side wall in the aspect) of the photosensitive member cartridge  3  (see  FIG. 7 ). When a lock lever  47  provided in the lock device  46  is pressed down, the developing cartridge  4  can be unlocked from the photosensitive member cartridge  3 . 
     Rollers  50  are disposed, as a developing cartridge receiving member, at a plurality of positions (two positions in the aspect, that is, at right and left end portions) in the bottom wall  30   a  of the case  30  in the accommodating portion  32  of the photosensitive member cartridge  3  (see  FIGS. 2 and 7 ). The rollers  50  protrude from the inner surface of the bottom wall  30   a . When the developing cartridge  4  is inserted down into the accommodating portion  32 , the rollers  50  receive a portion of the weight of the developing cartridge  4  on an opposite side of the developing roller shaft  22   a  of the developing roller  22  to the photosensitive drum  13 . More specifically, the rollers  50  contact lower portions of a downward-convex curved surface of a toner containing chamber  24  (see  FIG. 1 ) provided in the case  21  of the developing cartridge  4  so as to reduce rattling of the case  21  during the attaching and detaching operation. 
     The upper roller  12   a  of the pair of register rollers is mounted to the bottom wall  30   a  of the case  30  of the photosensitive member cartridge  3 . The bottom wall  30   a  has a laterally elongated introduction hole  51  close to the upper roller  12   a  so as to introduce the sheet P from the pair of register rollers  12   a  and  12   b  into a transfer portion  52  between the photosensitive drum  13  and the transfer roller  14 . The upper surface of the bottom wall  30   a  extending between the introduction hole  51  and the transfer portion  52  has many ribs  53  (see  FIG. 2 ) extending in a direction from the introduction hole  51  to the transfer portion  52 . The ribs  53  are provided so that the sheet P can be smoothly conveyed with a reduced contact resistance on the lower surface of the sheet P. 
     (3) Configuration of Developing Cartridge 
       FIG. 3  is a plan view of the developing cartridge.  FIG. 4  is a cross-sectional view taken along the line IV-IV of FIG.  3 .  FIG. 5A  is a left side view of the developing cartridge, and  FIG. 5B  is a right side view of the developing cartridge.  FIGS. 5A and 5B  are shown at an angle when the developing cartridge  4  is mounted on the photosensitive member cartridge  3  and then mounted on the printer main body. 
     Next, the configuration of the developing cartridge  4  will be described with reference to  FIGS. 1 and 3  to  5 B. In the case  21  serving as a housing of the developing cartridge  4 , an agitator  27  that agitates the toner in the toner containing chamber  24  is provided. The toner that is agitated by the agitator  27  and discharged is carried onto the outer peripheral surface of the developing roller  22 , which is rotatably disposed in the developing cartridge  4 , through a supply roller  25 . At this time, the thickness of the toner is restricted by a blade  26  (see  FIG. 1 ). 
     The action-receiving portions  61  are provided to integrally protrude from left and right outer sides of the toner containing chamber  24  in the case  21  of the developing cartridge  4 . The action-receiving portion  61  has a triangular portion directed substantially horizontally. When the developing cartridge  4  is mounted on the photosensitive member cartridge  3  and then mounted on the main body housing  1  of the printer, the action-receiving portions  61  are urged by the urging device  42  described above. Accordingly, the developing roller  22  is pressed against the photosensitive drum  13  through the developing cartridge  4 . 
     As shown in  FIG. 3 , grip portions  80  that facilitates handling, such as transportation or the like, are provided at upper and lower surfaces (in  FIG. 3 , only the upper surface is shown) of the case  21  of the developing cartridge  4 . 
       FIG. 8  is a cross-sectional view showing structures of bearings on the left and right sides of the developing roller and the conductive member. As shown in  FIG. 8 , the developing roller  22  is a rubber roller that is formed by a substrate supported on the developing roller shaft  22   a  formed of a metal, such as stainless steel or the like. The rubber roller substrate has electrical conductivity by dispersing carbon black in silicon rubber or urethane rubber. The roller surface has a coating layer containing fluorine. The bearings  23   a  and  23   b  that are rotatably fitted to the developing roller shaft  22   a  at the left and right end portions of the developing roller shaft  22   a  are formed of a material whose frictional coefficient is small, such as acetal resin or the like. Each of the bearings  23   a  and  23   b  has an engaging claw  62  that is fitted into an annular groove  63  so as not to be detached from the shaft end of the developing roller shaft  22   a.    
     Each of the bearings  23   a  and  23   b  has, at its base end side, an umbrella-shaped (conical) shaft diameter adjusting portion  64  whose diameter gradually increases. At least one of the bearings  23   a  and  23   b  (the right-side bearing  23   b  in this aspect) is slidingly urged laterally outward by a spring device  65 . Accordingly, in a state where the developing cartridge  4  is set at a predetermined position of the photosensitive member cartridge  3 , the developing roller shaft  22   a  of the developing roller  22  is supported, with no rattling, on the guide grooves  37  provided in the left and right side walls  30   c  of the photosensitive member cartridge  3 . In this case, since both end portions of the developing roller shaft  22   a  serves as protrusions on both side ends of the developing cartridge  4  relative to the guide grooves  37 , manufacturing costs can be reduced, and positioning or assembling accuracy can be improved. Further, since the developing roller shaft  22   a  that passes through the case  21  of the developing cartridge  4  is formed of a metal, strength can be increased, and thus breakage becomes unlikely. 
     The developing cartridge  4  has a conductive member  66  that is formed to be brought into contact with and electrically connected to the developing roller shaft  22   a . As shown in  FIGS. 3 and 5B , the conductive member  66  is a plate member formed of a metal. The conductive member  66  has one end portion  66   a  that is brought into contact with the developing roller shaft  22   a , the other end portion  66   c  that is brought into contact with a main body-side power-supply electrode described below when the developing cartridge  4  is mounted on the printer main body, and an intermediate portion  66   b  that is formed between the one end portion  66   a  and the other end portion  66   c . The shape of the conductive member  66  can be changed easily in accordance with an outer surface shape of the cartridge at low cost. Further, the conductive member  66  can be reliably brought into contact with the developing roller shaft  22   a  at the one end portion  66   a , and can be reliably brought into contact with the main body-side power-supply electrode at the other end portion  66   c . The conductive member  66  may be formed of conductive resin or the like, in addition to the plate member formed of a metal. 
     The conductive member  66  is formed such that the intermediate portion  66   b  follows the side wall of the case  21  of the developing cartridge  4  by a method of fixing using a two-sided tape or the like. The other end portion  66   c  is formed to have such a width that, when the developing cartridge  4  is mounted on the photosensitive member cartridge  3 , it does not protrude outward from the case side wall  30   c  of the photosensitive member cartridge  3  (see  FIG. 6 ). Therefore, when the developing cartridge  4  is mounted on the photosensitive member cartridge  3  or when the process cartridge  2  with the developing cartridge  4  mounted on the photosensitive member cartridge  3  is mounted on the printer main body, the conductive member  66  can be prevented from being deformed or broken due to interference with other members. 
     (4) Mounting of Developing Cartridge on Photosensitive Member Cartridge 
     The operation of setting the developing cartridge  4  on the photosensitive member cartridge  3  will be described with reference to  FIGS. 2 ,  6 , and  7 .  FIG. 6  is a plan view of the process cartridge  2  in a state where the developing cartridge  4  is set on the photosensitive member cartridge  3 .  FIG. 7  is a left side view when the process cartridge  2  is placed on a table  67 . 
     The developing cartridge  4  is inserted into the upwardly open accommodating portion  32  provided in a rear portion of the case  30  of the photosensitive member cartridge  3  with the developing roller  22  side being a leading side, such that the developing cartridge  3  is mounted on the photosensitive member cartridge  3 . 
     At this time, the bearings  23   a  and  23   b  disposed at the left and right side ends of the developing roller  22  slide down along the guide grooves  37  formed along the upper edges of the left and right side walls  30   c  of the case  30  of the photosensitive member cartridge  3  so as to approach the shaft  13   a  of the photosensitive drum  13 . In this case, the case  21  of the developing cartridge  4  is pivotable about the bearings  23   a  and  23   b  of the developing roller shaft  22   a  of the developing roller  22 . Therefore, when the bearings  23   a  and  23   b  come to positions on the guide grooves  37  close to the shaft  13   a  of the photosensitive drum  13  (laterally U-shaped portions), the toner containing chamber  24  of the case  21  of the developing cartridge  4  pivots about the shaft  22   a  of the developing roller  22 . Then, the developing cartridge  4  is fittingly set in the accommodating portion  32  in the photosensitive member cartridge  3 . 
     In this state, the rollers  50  in the accommodating portion  32  are slidingly brought into contact with a lower surface of the case  21  on the side of the toner containing chamber  24 , and the bearings  23   a  and  23   b  move along the guide grooves  37  to such a position that the developing roller  22  is positioned substantially closest to the photosensitive drum  13 . If mounting of the developing cartridge  4  on the photosensitive member cartridge  3  is completed, the developing cartridge  4  is locked by the lock device  46  such that the developing cartridge  4  is prevented from being removed from the photosensitive member cartridge  3 . 
     At least one of the left and right bearings  23   a  and  23   b  is urged by the spring device  65  in such a direction of an axis that the conical jaw portion of the bearing is pressed toward an inner surface of the guide groove  37 . In this manner, the bearings  23   a  and  23   b  are slid along the guide grooves  37 . Therefore, even though there is a dimensional error between the distance between the bearings  23   a  and  23   b  and the distance between the guide grooves  37 , rattling of the developing cartridge  4  laterally relative to the photosensitive member cartridge  3  (in the direction of the axis of the developing roller  22 ) does not occur. Further, since the bearings  23   a  and  23   b  are pivotable relative to the guide grooves  37 , the bearings  23   a  and  23   b  of the developing cartridge  4  can be smoothly moved along the guide grooves  37 , with no danger of being stopped on the grooves  37 , when the developing cartridge  4  is to be set on the photosensitive member cartridge  3  or removed therefrom (see  FIG. 8 ). 
     (5) Mounting of Process Cartridge on Printer Main Body 
     Referring to  FIG. 1 , in the process cartridge  2 , in a state where a lid  1   b  at a right end (a front surface) of the main body housing  1  is downwardly turned to form a large opening, the shaft  13   a  of the photosensitive drum  13  and guiding members  56  (in  FIG. 7 , only one of the guiding members  56  is shown), which protrude laterally from lower ends of the left and right sides of the process cartridge  2  (the photosensitive member cartridge  3 ), are guided to guide surfaces formed at left and right inner surfaces of the main body housing  1  and then inserted into the main body housing  1 . Then, if the process cartridge  2  is set at a predetermined position of the main body housing  1 , an upper roller  12   a  on a lower surface side of the case  30  of the photosensitive member cartridge  3  is placed on a lower roller  12   b  disposed on the side of the main body housing  1  and is pressed by a spring (not shown). Further, the action-receiving portions  61  provided in the developing cartridge  4  at a leading end of the slide support member  40  of the urging device  42  provided in the photosensitive member cartridge  3 . Accordingly, the developing roller  22  can be pressed against the photosensitive drum  13  through the developing cartridge  4 . 
     In a state where the process cartridge  2  with the developing cartridge  4  mounted thereon is mounted at a predetermined position of the main body housing  1  of the printer, the main body-side power supply electrode  70  (indicated by the virtual line in  FIGS. 5 and 7 ) that is provided in the main body housing  1  is elastically brought into contact with the upper surface of the other end portion  66   c  of the conductive member  66  toward to the rollers  50 . 
     The main body-side power-supply electrode  70  is connected to a high-pressure substrate (not shown) that is provided in the main body housing  1 , and the developing bias is applied from the high-pressure substrate. For this reason, if the main body-side power supply electrode  70  is elastically brought into contact with the upper surface of the other end portion  66   c  of the conductive member  66 , the developing bias that is applied from the high-pressure substrate is applied from the main body-side power-supply electrode  70  to the developing shaft roller  22   a  through the conductive member  66  and then applied from the developing roller shaft  22   a  to the conductive rubber roller. 
     As shown in  FIG. 5 , the other end portion  66   c  of the conductive member  66  is brought into contact with the main body-side power-supply electrode  70  on a vertical line of the rollers  50  serving as a developing cartridge receiving portion provided at the bottom wall  30   a  of the case  30  of the photosensitive member cartridge  3 . When the developing cartridge  4  is inserted down into the accommodating portion  32  of the photosensitive member cartridge  3 , the rollers  50  are designed to receive a portion of the weight of the developing cartridge  4  on a side across the developing roller shaft  22   a  of the developing roller  22  opposite to the photosensitive drum  13  and to reduce rattling of the case  21  of the developing cartridge  4  during the setting or removing operation. Accordingly, the developing cartridge  4  is stably mounted on the photosensitive member cartridge  3  in a state where the center of gravity thereof is applied to the rollers  50 . Since the main body-side power-supply electrode  70  is brought into contact with the other end portion  66   c  of the conductive member  66  on the vertical line of the rubbers  50 , rattling of the developing cartridge  4  relative to the photosensitive member cartridge  3  by the contact of the main body-side power-supply electrode  70  does not occur. Therefore, pressing of the developing roller  22  against the photosensitive drum  13  can be prevented from being instable, and thus good image quality can be secured. 
     The urging device  42  presses the action-receiving portion  61  substantially horizontally. Thus, the pressing direction is substantially perpendicular to a direction in which the conductive member  66  is brought into contact with the main body-side power-supply electrode  70 . Accordingly, even when the developing cartridge  4  moves due to the pressing force of the urging device  42 , a distance between the conductive member  66  and the main body-side power-supply electrode  70  is kept substantially constant. Thus, the contact condition between the conductive member  66  and the main body-side power-supply electrode  70  is not degraded. 
     (6) Modification 
       FIG. 9  is a diagram showing a modification of this aspect. A difference between this modification and the above-described aspect is that a separate main body-side power-supply electrode  70  is not formed, and the rollers  50  serving as the developing cartridge receiving portion also serve as a power-supply electrode for applying the developing bias to the developing roller  22 . That is, the rollers  50  are formed as a conductive electrode and are connected to a high-pressure substrate (not shown) that is provided in the main body housing  1 , such that the developing bias is applied from the high-pressure substrate. 
     The conductive member  66  that is formed to be electrically connected to the developing roller shaft  22   a  extends onto the vertical line of the rollers  50  at the lower surface of the toner containing chamber  24  along the case  21  of the developing cartridge  4  and is brought into contact with the rollers  50  at that position. Accordingly, the developing bias is applied to the developing roller  22 . The rollers  50  may be provided in the photosensitive member cartridge  3  or may be provided in the printer main body. 
     With the configuration according to the modification, since the rollers  50  are brought into contact with the conductive member  66  at a position where rattling relative to the developing cartridge  4  does not occur, the contact state of the photosensitive drum  13  and the developing roller  22  can be prevented from being instable, and thus good image quality can be secured. 
     In the above description, the developing cartridge  4  is mounted on the photosensitive member cartridge  3  in advance, and then the process cartridge  2  is attached to and detached from the main body housing  1 . Alternatively, the photosensitive member cartridge  3  and the developing cartridge  4  may be separately attachable to and detachable from the main body housing  1 . In this case, the guide grooves  37  that slidably guide and support the bearings  23   a  and  23   b , protruding from both outer surfaces of the developing cartridge  4 , formed at both ends of the developing roller shaft  22   a  are formed at the left and right sides of the main body housing  1 .