Patent Publication Number: US-6041203-A

Title: Process unit, photosensitive member cartridge, developer cartridge, and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an electrostatographic image forming apparatus employed in a copier, a facsimile, a laser printer and the like, and to a process unit, a photosensitive member cartridge and a developer cartridge that are for use in the image forming apparatus. 
     2. Description of Related Art 
     In some conventional image forming apparatuses for recording character or image data by supplying a developer to an electrostatic latent image formed on a photosensitive member (photosensitive drum) and transferring the visible image formed by the developer onto a recording medium, a cartridge-type process unit as disclosed in, for example, Japanese Patent Application Laid-open Nos. HEI 8-54786 and HEI 9-319285, (which correspond to U.S. Pat. Nos. 5,845,176 and 5,805,959, respectively) is employed in order to facilitate operations for maintenance, replacement, and the like. 
     In a conventional arrangement, after a photosensitive member cartridge having at least a photosensitive member is set into a body (housing) of an image forming apparatus, a developer cartridge having a developer containing chamber and a developing roller can be attached to or detached from the photosensitive member cartridge. In another conventional arrangement, a process unit in which a developer cartridge has been set with respect to a photosensitive member cartridge is set into the housing of an image forming apparatus. 
     The developing roller needs to be set so that the surface of the developing roller is pressed against the surface of the photosensitive member (photosensitive drum) during image forming operation. This arrangement is necessary to form a visible image on the photosensitive member (photosensitive drum) by supplying a thin layer of developer formed on the developing roller onto an electrostatic latent image that is formed on the surface of the photosensitive member. 
     However, in order to replace the photosensitive member cartridge due to deterioration thereof, or replace the developer cartridge due to exhaustion of developer (toner) or the like, the developing roller and the photosensitive member must be separated apart from each other and, after replacement, placed at adjacent positions. Therefore, during a replacement operation, there is a danger that the surface of the developing roller may impact the surface of the photosensitive member cartridge or the like. If such an impact occurs, component parts thereof may break, or the position of a developer cartridge support portion relative to the photosensitive member cartridge may deviate, so that uniform pressurization of the developing roller against the photosensitive member cannot be accomplished. 
     Further, the conventional technology requires that the handling of the developer cartridge be facilitated by simplifying the operation of setting the developer cartridge into the press contact with the photosensitive member cartridge, and the operation of removing the developer cartridge therefrom, as much as possible. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a process unit, a photosensitive member cartridge and a developer cartridge that allow smooth and simple cartridge replacement and that make it possible to precisely press the developing roller against the photosensitive member, and provide an image forming apparatus that employs the unit or cartridges. 
     According to one aspect of the invention, there is provided a process unit including a photosensitive member cartridge having a photosensitive member on which an electrostatic latent image is formed, and a developer cartridge having a developing roller that supplies a developer to the photosensitive member. The developer cartridge is detachably attachable to the photosensitive member cartridge. An urging device is provided in the photosensitive member cartridge so as to urge a developing roller relative to the photosensitive member. An action-receiving portion is provided in the developer cartridge so as to receive an action performed by the urging device. 
     In this process unit, the urging device, having a relatively complicated structure, is provided in the photosensitive member cartridge, which requires less frequent replacement. The action-receiving portion, having a relatively simple structure, is provided in the developer cartridge, which requires frequent replacement. Therefore, the production costs of the developer cartridge can be reduced, and the running cost can be reduced. 
     The process unit may be constructed so that closest points on the photosensitive member and the developing roller are located on an extension of a line of action of an urging force applied to the developing roller relative to the photosensitive member by the urging device or above the line of action. 
     This structure prevents the developer cartridge from rising relative to the photosensitive member, so that the behavior stabilizes. 
     The process unit may further include a lock device that maintains a state that the developer cartridge has been set relative to the photosensitive member cartridge. 
     When the developer cartridge is locked by the lock device, the developer cartridge is prevented from accidentally becoming detached from the photosensitive member cartridge, and from deviating in position relative to the photosensitive member cartridge. Therefore, the operation of replacing the developer cartridge becomes easy. 
     The lock device may include an action-receiving portion provided in the developer cartridge, and a lock lever member provided in the photosensitive member cartridge, the lock lever member being changeable in orientation between an acting position and a non-acting position relative to the action-receiving portion. 
     In this lock device, the action-receiving portion, having a relatively simple structure, is provided in the developer cartridge, which requires frequent replacement. The lock lever member, having a relatively complicated structure, is provided in the photosensitive member cartridge, which requires less frequent replacement. Furthermore, the action-receiving portion can be used for both the lock device and the urging device. Therefore, the production cost of the disposable developer cartridge can be reduced. 
     According to another aspect of the invention, there is provided a photosensitive member cartridge including a photosensitive member on which an electrostatic latent image is formed, and a photosensitive member cartridge case that supports the photosensitive member and is capable of receiving a developer cartridge having a developing roller that supplies a developer to the photosensitive member, in such a manner that the developer cartridge is detachably attached to the photosensitive member cartridge case. An urging device is provided in the case so as to urge, to the photosensitive member, the developing roller of the developer cartridge attached to the photosensitive member cartridge. 
     Therefore, the developer cartridge can be attached to or detached from the photosensitive member cartridge for replacement. Furthermore, since the developer cartridge fits into the case of the photosensitive member cartridge, the two cartridges can be handled as a single unit. Further, since the urging device is provided in the photosensitive member cartridge, which requires less frequent replacement, the production cost of the developer cartridge, which requires frequent replacement, can be reduced. 
     In the photosensitive member, the urging device may be disposed on an inner surface of each of opposite sides of the photosensitive member cartridge, and each urging device may be changeable in orientation between an urging direction and a non-urging direction relative to an action-receiving portion provided in the developer cartridge. 
     Therefore, even if the distance between the opposite sides of the photosensitive member cartridge is great, the opposite sides of the developer cartridge fitted in the photosensitive member cartridge can be urged simultaneously and substantially uniformly, so that the urging or pressing of the developing roller relative to the photosensitive member cartridge also becomes uniform. The urging can reliably be performed and discontinued simply by changing the orientation of the urging devices between the urging direction and the non-urging direction. 
     Each urging device may include a pivot fulcrum member, a slide support member slidable relative to the pivot fulcrum member, and an urging spring device disposed between the pivot fulcrum member and the slide support member. 
     Therefore, the degree of freedom in changing the orientation of each urging device between the urging direction and the non-urging direction increases, compared with a conventional device that employs an urging spring to directly urge an action-receiving portion and discontinue the urging. Furthermore, the operations of urging the action-receiving portions and discontinuing the urging can be reliably performed by the slide support members. 
     The pivot fulcrum member of each urging device may be turnably mounted to an inward surface of a corresponding one of opposite sides of the case of the photosensitive member cartridge. 
     Therefore, the pivot fulcrum members of the urging devices are not exposed outside the photosensitive member cartridge regardless of whether the photosensitive member cartridge stands alone or is combined with the developer cartridge into the process unit. Therefore, the danger of accidentally hitting and breaking a component part of the urging devices is considerably reduced, and the ease of handling improves. 
     The slide support member of each urging device may have, as an integral portion thereof, a motion-acting portion that guides the urging device in the urging direction and the non-urging direction, the motion-acting portion protruding outwardly from a corresponding one of opposite sides of the case of the photosensitive member cartridge. 
     Therefore, most component parts of the urging devices are not exposed outside of the photosensitive member cartridge, so that the danger of accidentally hitting and breaking any component part of the urging devices is considerably reduced, and the ease of handling improves. 
     The case of the photosensitive member cartridge may have a lock device that maintains the attachment of the developer cartridge to the case. 
     When the developer cartridge, set with respect to the photosensitive member cartridge case, is locked by the lock device, the developer cartridge is prevented from accidentally becoming detached from the photosensitive member cartridge, and from deviating in position relative to the photosensitive member cartridge. Therefore, the operation of replacing the developer cartridge becomes easy. 
     The lock device may have a lock lever member that is provided at least on an inner surface of a side of the photosensitive member cartridge case, the lock lever member being changeable in orientation between an acting position and a non-acting position relative to the action-receiving portion provided in the developer cartridge. 
     Therefore, it becomes possible to set the developer cartridge with respect to the photosensitive member cartridge in advance outside an image forming apparatus. Hence, the developer cartridge can be prevented from deviating in position relative to the photosensitive member cartridge when the photosensitive member cartridge and the developer cartridge are combined and transported as a single unit. 
     According to still another aspect of the invention, there is provided a developer cartridge including a developing roller that supplies a developer to a photosensitive member, and a case that supports the developing roller and is detachably attachable to a photosensitive member cartridge including the photosensitive member. An action-receiving portion is provided in the case so as to receive an action performed by an urging device, that is provided in the photosensitive member cartridge, so as to urge the developing roller to the photosensitive member. 
     Since the action-receiving portion is provided in the case of the developer cartridge, the action-receiving portion can be formed together with the case. Therefore, the production cost can be reduced. 
     The developer cartridge may further include a developing chamber including the developing roller, and a developer chamber that stores the developer, wherein the action-receiving portion protrudes outwardly from an outer wall of the developer chamber. 
     Therefore, the action-receiving portion can be formed together with the developer cartridge, so that the production cost can be reduced. Furthermore, it becomes easier for the action-receiving portion to receive the pressing action performed by a slide support member of the urging device provided on the photosensitive member cartridge. 
     The case of the developer cartridge may have an action-receiving portion for a lock device that maintains a state that the developer cartridge has been attached to the photosensitive member cartridge. 
     Therefore, this action-receiving portion can also be formed together with the case, so that the production cost can be reduced. 
     The action-receiving portion for the lock device and the action-receiving portion for the urging device may be the same portion. 
     Therefore, the action-receiving portion arrangement is simplified, and the production cost can be reduced. 
     According to a further aspect of the invention, there is provided an image forming apparatus including an apparatus body capable of receiving a process unit including a developer cartridge and a photosensitive member cartridge, in such a manner that the process unit is detachably attached to the apparatus body, and a guide device, that changes an urging device provided in the photosensitive member cartridge, so as to urge a developing roller provided in the developer cartridge to a photosensitive member, provided in the photosensitive member cartridge, between an urging state and a non-urging state, as the process unit is moved in one of an attaching direction and a detaching direction relative to the apparatus body. 
     Therefore, due to the guide device, the urging device can be changed or switched in orientation and therefore in operation mode, through a one-action operation of setting the process unit into or removing it from the image forming apparatus. Thus, the ease of operation considerably improves. 
     In the image forming apparatus, the urging device provided in the photosensitive member cartridge may have a motion-acting portion that protrudes laterally so as to be slidable relative to the guide device. 
     In this structure, the restraint of the motion-acting portion caused by the guide device is minimum. Therefore, an appropriate degree of freedom is provided for the operations of setting and removing the process unit, and the ease of handling improves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described in detail with reference to the following figures wherein: 
     FIG. 1 is a schematic side sectional view of a printer embodying the image forming apparatus of the invention; 
     FIG. 2 is a side view of the printer, illustrating a situation where the insertion of a process unit into a body housing is started; 
     FIG. 3 is a plan view of a photosensitive member cartridge; 
     FIG. 4 is a side sectional view of the photosensitive member cartridge taken on plane IV--IV in FIG. 3; 
     FIG. 5 is a right side view of the photosensitive member cartridge; 
     FIG. 6 is a left side view of the photosensitive member cartridge; 
     FIG. 7 is a front view of the photosensitive member cartridge; 
     FIG. 8 is a partially cut-away sectional view illustrating a pressing portion of a transfer roller; 
     FIG. 9 is a sectional view taken on plane IX--IX in FIG. 8; 
     FIG. 10 is a perspective view of an urging device and an action-receiving portion; 
     FIG. 11(a) is a plan view of a lock device; 
     FIG. 11(b) is a sectional view taken on line XIb--XIb in FIG. 11(a); 
     FIG. 12(a) is a left side view of a developer cartridge; 
     FIG. 12(b) is a right side view of the developer cartridge; 
     FIG. 13 is a plan view of the developer cartridge; 
     FIG. 14 is a view of the developer cartridge taken in the direction indicated by arrows XIV in FIG. 13; 
     FIG. 15 is a sectional view of the developer cartridge, illustrating the structure of shaft bearings disposed at the right and left sides of the developer cartridge; 
     FIG. 16 is a plan view of a process unit; 
     FIG. 17 is a right side view of the process unit; 
     FIG. 18 is a left side view of the process unit; 
     FIG. 19 is a view of the process unit taken in the direction indicated by arrows XIX in FIG. 17; 
     FIG. 20(a) illustrates a situation in which the process unit is being inserted into the body housing; 
     FIG. 20(b) illustrates a situation in which the process unit is further inserted; 
     FIG. 21 illustrates a situation in which the process unit has been set in the body housing; 
     FIG. 22 is an illustration of a drive system of the printer; and 
     FIG. 23 is an illustration of a force that presses a developing roller against a photosensitive drum and other forces concerned. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment in which the invention is embodied in a laser beam-type printer will be described in detail hereinafter with reference to the accompanying drawings. FIG. 1 is a schematic side sectional view of a printer embodying the image forming apparatus of the invention. FIG. 2 is a partly cut-away side view of the printer, illustrating a situation where the insertion of a process unit 2 into a body housing 1 is started. 
     Referring to FIG. 1, the process unit 2 having a photosensitive member cartridge 3 and a developer cartridge 4 is removably disposed in a substantially central portion of the body housing 1 of the printer. As shown in a left portion of FIG. 1, a fixer 5 is disposed adjacent to the process unit 2. A sheet feeder 6 is disposed below the process unit 2. A sheet cassette 8 is attachable to a lower portion of the body housing 1 by moving the sheet cassette 8 from a front face of the body housing 1 (as indicated by arrow A). 
     A laser scanning unit 7 is mounted to a lower surface of a synthetic resin-made discharge sheet tray 1a, via a frame. The discharge sheet tray 1a also serves as a cover. 
     When print data is transmitted to the printer from an external apparatus, for example, a personal computer, upon a print instruction, a sheet P (recording medium) is separated from a stack of sheets P on a support plate 9 of the sheet cassette 8 by a separator pad 11, in a manner of one sheet at a time, as a sheet feed roller 10 of the sheet feeder 6 rotates. The separated sheet P is conveyed to a contact portion between a photosensitive drum 13 (photosensitive member) in the process unit 2 and a transfer roller 14 (transfer device) pressed against a lower face of the photosensitive drum 13, via a pair of register rollers 12a, 12b. 
     A laser beam is emitted from the laser scanning 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 scanning unit 7. The laser beam travels to an upper peripheral surface portion of the photosensitive drum 13, via a light entrance portion 31 formed in a case 30 of the photosensitive member cartridge 3, that is, a case of the process unit 2. The peripheral surface of the photosensitive drum 13 is thereby exposed corresponding to the print data, so as to form an electrostatic latent image. 
     Developer (toner) supplied from a developing roller 22 of the developer cartridge 4 becomes deposited on the electrostatic latent image on the photosensitive drum 13, thereby making the image visible. After the visible image formed by developer (toner) is transferred from the photosensitive drum 13 to the sheet P, the sheet P is conveyed between a heat roller 15 and a presser roller 16 in the fixer 5, in which the sheet P is subjected to a heat-fixing process. The sheet 2 is then discharged onto the discharge sheet tray 1a via a sheet discharge passage 17. 
     In this embodiment, the process unit 2 includes the photosensitive member cartridge 3 having at least the photosensitive drum 13, and the developer cartridge 4 having at least the developing roller 22 (developing device) that is disposed in a case 21. The developer cartridge 4 is designed so that the developer cartridge 4 is detachably attachable to the photosensitive member cartridge 3, and so that the developing roller 22 is prevented from detaching by a lock device 46 described later. 
     The structures of the photosensitive member cartridge 3 and the developer cartridge 4 will now be described in detail. As shown in FIGS. 3 through 7, in the photosensitive member cartridge 3, the photosensitive drum 13 is rotatably journaled in the synthetic resin-made case 30, near one side thereof. A lower portion of the transfer roller 14 (transfer device) disposed below the photosensitive drum 13 is covered with a bottom wall 30a of the case 30 (see FIG. 4). The transfer roller 14 is vertically movably journaled in such a manner that the transfer roller 14 can separate from a lower surface of the photosensitive drum 13 due to the eight of the transfer roller 14. 
     When the process unit 2 is set into the body housing 1, upwardly open &#34;U&#34;-shaped bearings 35 fitted to both end portions of a shaft 14a of the transfer roller 14 are raised by shaft bearing raiser 34 (see FIGS. 8 and 9) that are urged upward by springs 33 disposed at the right and left side inner faces of the body housing 1, so that the transfer roller 14 is moved upward to press a lower surface (transfer region) of the photosensitive drum 13. 
     An upper wall 30b 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 scanning unit 7, is formed in the upper wall 30b, and is elongated in the directions of an axis the photosensitive drum 13. Disposed adjacent to the light entrance portion 31 is a charger 36, such as a scorotron or the like, that charges a photosensitive surface of the photosensitive drum 13 that is formed from an organic photosensitive material or the like. 
     An electrostatic latent image is formed on the surface of the photosensitive drum 13 by scanning laser light over the surface uniformly charged by the charger 36. After the electrostatic latent image is made visible (developed) by deposition thereon of a thin layer of toner supplied via the developing roller 22 (described later), 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. 
     A portion of the case 30 other than the upper wall 30b is open upward to define an accommodating portion 32 into which the developer cartridge 4 can be detachably set. An upper end face of each of right and left side walls 30c of the case 30 defines a downwardly arched guide groove 37 (guide device) to guide a corresponding one of bearings 23a, 23b (see FIGS. 13 through 15) fitted to both end portions of the developing roller shaft 22a of the developing roller 22 while supporting the bearing slidably. The guide grooves 37 extend toward a shaft 13a of the photosensitive drum 13. Therefore, when the developer cartridge 4 is set with respect to the photosensitive member cartridge 3, the developing roller 22 can be positioned adjacent to the photosensitive drum 13 so that they face each other (see FIG. 4). 
     In particular, when the developer cartridge 4 is connected to the photosensitive member cartridge 3, the bearings 23a, 23b contact and move along a first section of the guide grooves 37 in a direction that includes a vertical component. The bearings 23a, 23b then contact and move along a second section of the guide grooves 37 in a direction that is substantially horizontal. The difference of direction of movement of the bearings 23a, 23b is due to the arcuate shape of the guide grooves 37. Also, because of this arcuate shape, the bearings 23a, 23b move faster along the first section of the guide grooves then along the second section. 
     An urging device 42 that presses the developing roller 22 against the photosensitive drum 13 via the developer cartridge 4 is pivotably and expandably mounted to an inner surface of each of the right and left side walls 30c. As shown in FIGS. 4 and 10, each urging device 42 has a pivot fulcrum member 39 provided with pivots 39a, 39b protruding integrally from the right and left sides thereof, a frame-like slide support member 40 that connects with and supports the pivot fulcrum member 39 that facilitates sliding movements therein, and an urging spring device 41, such as a coil spring or the like, that 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 30c. 
     The lock device 46 that prevents the developer cartridge 4, which is fitted into the accommodating portion 32, from moving upward out of the accommodating portion 32, is disposed at an inner side of one of the right and left side walls 30c (the right side wall in the embodiment) of the photosensitive member cartridge 3. As shown in FIGS. 4, 5, 11(a) and 11(b), the lock device 46 is designed so that a rotating shaft 48 extending through the side wall 30c axially supports a lock lever 47 in such a manner that the lock lever 47 is pivotable relative to a side surface of the lock lever 47. 
     A resin-made spring 49 extending downward from a lower end of the lock lever 47 is disposed so that a lower portion of the resin-made spring 49 contacts a restriction piece 30d protruding upward from the bottom wall 30a of the case 30. A lower surface of the lock lever 47 has an arched contact portion 47a that restricts upward motion of one of action-receiving portions 61 (right-side one) protruding outward from the right and left side surfaces of the case 21 of the developer cartridge 4, by contacting an upper surface of the action-receiving portion 61. The action-receiving portions 61 have a generally inverted triangular shape in side view. 
     The action-receiving portions 61 are disposed so that they communicate with the lock device 46 and, furthermore, urging devices 42 that press the developing roller 22 against the photosensitive drum 13. 
     Rollers 50 are disposed, as receiving members, at a plurality of positions (two positions in the embodiment, that is, at right and left end portions) in the bottom wall 30a of the case 30, in the accommodating portion 32 of the photosensitive member cartridge 3 (see FIGS. 3 and 4). The rollers 50 protrude from the inner surface of the bottom wall 30a. When the developer cartridge 4 is inserted down into the accommodating portion 32, the rollers 50 (receiving device) receive a portion of the weight of the developer cartridge 4, at a side across the developing roller shaft 22a of the developing roller 22 from the photosensitive drum 13, that is, a side of the developing roller shaft 22a of the developing roller 22 opposite from 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 FIGS. 1, 12(a) and 12(b)) formed in the case 21 of the developer cartridge 4, and the rollers 50 reduce the rattling of the case 21 during the setting or removing operation. 
     The upper roller 12a of the pair of register rollers is mounted to the bottom wall 30a of the case 30 of the photosensitive member cartridge 3 in such a manner that the upper roller 12a is prevented from detaching. The bottom wall 30a has a laterally elongated introduction hole 51 adjacent to the upper roller 12a, to introduce the sheet 2 from the pair of register rollers 12a, 12b into a transfer portion 52 between the photosensitive drum 13 and the transfer roller 14. The upper surface of a portion of the bottom wall 30a extending between the introduction hole 51 and the transfer portion 52 has many ribs 53 extending in a direction from the introduction hole 51 to the transfer portion 52. The ribs 53 are designed so that the sheet P can be smoothly conveyed with a reduced contact resistance on the lower surface of the sheet P. 
     The structure of the developer cartridge 4 will be described with reference to FIGS. 1, 12(a), 12(b) and 13 through 15. After toner in the downward-convex toner containing chamber 24 in the case 21 is stirred by a rotationally driven stirrer 27 and discharged therefrom, toner is carried onto the outer peripheral surface of the developing roller 22 via a supply roller 25. A blade 26 is provided for restricting the layer thickness of toner on the developing roller 22 (see FIG. 1). The generally inverted triangular-shaped action-receiving portions 61 protrude from the right and left side outer ends of the toner containing chamber 24 in the case 21. 
     The shaft bearings 23a, 23b rotatably fitted to the right and left end portions of the developing roller shaft 22a are formed from a material whose friction coefficient is small, such as an acetal resin or the like. Each of the bearings 23a, 23b has an engaging nail 62 that engages with an annular groove 63 so that the bearing will not detach from the shaft end. Each of the shaft bearings 23a, 23b has, at its base end side, an umbrella-shaped (conical) shaft diameter adjusting portion 64 whose diameter gradually increases. 
     At least one of the shaft bearings 23a, 23b (the right-side bearing 23b in the embodiment) is slidingly urged laterally outward by a spring device 65 (see FIG. 15). Therefore, when the developer cartridge 4 is set at a predetermined position with respect to the photosensitive member cartridge 3, the developing roller shaft 22a of the developing roller 22 are supported, without rattling, on the guide grooves 37 formed in the right and left side walls 30c of the photosensitive member cartridge 3. 
     As shown in FIGS. 1, 16 and 19, the case 21 of the developer cartridge 4 has, in an upper surface portion and a lower surface portion thereof, grip portions 70, 66, respectively, that facilitate the handling of the developer cartridge 4, such as transportation thereof and the like. As shown in FIGS. 5 through 7, the case 30 of the photosensitive member cartridge 3 has foot portions 69 at a plurality of positions (at least two position and, preferably, four positions) that stabilize the photosensitive member cartridge 3 when it is placed on a table 67. 
     The operation of setting the developer cartridge 4 with respect to the photosensitive member cartridge 3 will be described. When the developer cartridge 4 inserted, with the developing roller 22 side being a leading side, into the upwardly open accommodating portion 32 provided in a rearward portion of the case 30 of the photosensitive member cartridge 3 (inserting operation), one of the action-receiving portions 61 of the developer cartridge 4 contacts the lock lever 47, and turns the lock lever 47 to a position indicated by a two-dot line in FIG. 4, against the force of the resin-made spring 49. 
     When the developer cartridge 4 is thus inserted (the inserting operation is completed), the action-receiving portion 61 is lowered and, therefore, the contact between the action-receiving portion 61 and the lock lever 47 discontinues. Therefore, the lock lever 47 is returned to the position indicated by a solid line in FIG. 4, by the restoration force of the resin-made spring 49, so that the contact portion 47a of the lock lever 47 faces an upper surface of the action-receiving portion 61, thereby establishing a locked state. 
     During the insertion of the developing roller 22 into the accommodating portion 32, the shaft bearings 23a, 23b disposed at the right and left side ends of the developing roller 22 slide down along the guide grooves 37 formed along the upper edges of the right and left side walls 30c of the case 30, so as to approach the shaft 13a of the photosensitive drum 13. The case 21 of the developer cartridge 4 is pivotable about the bearings 23a, 23b of the developing roller shaft 22a of the developing roller 22 when the bearings 23a, 23b are supported by the guide grooves 37. Therefore, when the bearings 23a, 23b come to a position on the guide grooves 37 close to the shaft 13a of the photosensitive drum 13 (&#34;U&#34;-shaped portions of the guide grooves 37), a toner containing chamber 24 side portion of the case 21 of the developer cartridge 4 comes into the accommodating portion 32 of the photosensitive member cartridge 3 by a pivoting motion about the developing roller shaft 22a of the developing roller 22. Thus, the developer cartridge 4 is fittingly set in the accommodating portion 32. 
     In this state, the rollers 50 slidingly contact a lower surface portion of the case 21 on the side of the toner containing chamber 24, and the bearings 23a, 23b move along the guide grooves 37 to such a position that the developing roller 22 is located substantially closest to the photosensitive drum 13. When the lock lever 47 pivots clockwise to return from the position indicated by the two-dot line to the position indicated by the solid line in FIG. 4, the contact portion 47a of the lock lever 47 faces the upper surface (protrusion 61a) of the action-receiving portion 61, so that the developer cartridge 4 is prevented from moving out of the photosensitive member cartridge 3. 
     FIGS. 16 through 19 are a plan view, a right side view, a left side view, and a rear view (a view of the side of the sheet discharge opening) of the process unit 2 with the developer cartridge 4 set with respect to the photosensitive member cartridge 3. Since the urging devices 42 are pressed downward by the action-receiving portions 61 as the developer cartridge 4 is pushed downward into the process unit 2, each urging device 42 normally assumes such a orientation that the motion acting portion 43 is relatively lowered, as shown in FIGS. 2 and 4. 
     The process unit 2 is designed so that the process unit 2 can be set into, and removed from, the body housing 1 when a lid 1b provided in the right-side end in FIG. 1 (a front face) of the body housing 1 is downwardly turned to form a large opening (see FIG. 2). 
     That is, as shown in FIGS. 2, 20(a), 20(b) and 21, a pair of right and left guide devices 55, made of a resin or the like, are fixed to the inner surfaces of the right and left side portions of the body housing 1 (in the drawings, only the right-side guide device 55 is shown). Each guide device 55 has a rising slope surface that is open upward and extends upwardly inward from the right-side end of the body housing 1, and an upper-side guide surface 55a extending downwardly inward from a summit 55d of the rising slope surface, and a lower-side guide surface 55b that is disposed below the upper-side guide surface 55a and that extends downwardly inward from the right-side end of the body housing 1, and ends at a position near the lower roller 12(b) of the pair of register rollers. 
     FIG. 2 indicates a position at which the insertion of the process unit 2 into the body housing 1 is started. The process unit 2 is inserted into the body housing 1 so that the shaft 13a of the photosensitive drum 13 approaches an upper inward end portion 55c of the upper-side guide surface 55a. Subsequently, as shown in FIG. 20(a), the motion acting portion 43 of each urging device 42 contacts the rising slope surface of the upper-side guide surface 55a, and each of guiding members 56, protruding laterally from lower portions of the right and left sides of the process unit 2 (photosensitive member cartridge 3), fits into the lower-side guide surface 55b (only one of the guiding members 56 is shown in the drawings), so that the process unit 2 is prevented from moving upward relative to the body housing 1, and is allowed only to be moved further inward along the lower-side guide surface 55b. 
     In this state, as the process unit 2 is pushed inward, the motion acting portion 43 of each urging device 42 is raised by the corresponding upper-side guide surface 55a, so that the slide support member 40 of each urging device 42 pivots upward about the pivots 39a, 39b. Therefore, the slide support member 40 of each urging device 42 restrains the corresponding one of the action-receiving portions 61 of the developer cartridge 4 in such a direction that a distal end portion of the slide support member 40 pushes the action-receiving portion 61. When the motion acting portion 43 of each urging device 42 comes to the summit 55d of the upper-side guide surface 55a, the slide support member 40 of each urging device 42 is pivoted to a most upward orientation. In this orientation, the distal end portion of the slide support member 40 of each urging device 42 keeps restraining the corresponding action-receiving portion 61 (see FIG. 20(b)). 
     As the process unit 2 is pushed further inward, the shaft 13a of the photosensitive drum 13 is set to a predetermined position in the inward end portion 55c of each upper-side guide surface 55a. At this position, an operator releases the process unit 2 from the hands, so that the developer cartridge 4 side portion of the process unit 2 is lowered and set by the weight the process unit 2 into a state such that the upper register roller 12a disposed at a lower surface side of the case 30 of the photosensitive member cartridge 3 is placed on the lower register roller 12b disposed in the body housing 1, and is pressed by a spring 45 shown in FIG. 2 and, simultaneously, the guiding members 56 are supported at appropriate positions on the guide devices 55 (see FIG. 21). 
     When the process unit 2 is set as described above, the distal end of the slide support member 40 of each urging device 42 presses the corresponding action-receiving portion 61. Therefore, the developing roller 22 is pressed against the photosensitive drum 13 by the urging devices 42 and the action-receiving portions 61 via the developer cartridge 4. 
     As shown in FIG. 21, it is preferable that a contact portion 72a between the photosensitive drum 13 and the developing roller 22, that is, pressed portions thereof at the closest positions, (the contact portion 72a is on an inter-axis straight line 72 passing through the axis of the shaft 13a of the photosensitive drum 13 and the axis of the developing roller shaft 22a of the developing roller 22) be above or on a pressing action line 71 of the action of either one of the urging devices 42 onto the corresponding action-receiving portion 61 (that is, a straight line passing through the pivot 39a (39b) and a pressing point at which the distal end of the slide support member 40 contacts the action-receiving portion 61), or that the inter-axis straight line 72 substantially coincide with or extend substantially parallel to the pressing action line 71. 
     As shown in FIG. 22, a gear mechanism 74 that transmits force from a drive motor 73 is disposed on the inner surface of one side (the left side in the embodiment) of the body housing 1. The gear mechanism 74 rotates the sheet feed roller 10 of the sheet feeder 6, the lower register roller 12b, the developing roller 22, the photosensitive drum 13, the heat roller 15, and conveying rollers in the sheet discharge passage. The developing roller 22 and the photosensitive drum 13 are rotated in opposite directions, as indicated in FIGS. 1 and 21, that is, the developing roller 22 is rotated counterclockwise and the photosensitive drum 13 is rotated clockwise. Furthermore, the mechanism is designed so that the circumferential velocity of the developing roller 22 is greater than that of the photosensitive drum 13. 
     Therefore, as shown in FIG. 23, during image forming operation, the direction of a pressing force F1 of each urging device 42 acting on the contact portion 72a is parallel to the pressing action line 71, and the pressing force F1 is split into a component F1V in a direction of the tangent of the circumferences of the developing roller 22 and the photosensitive drum 13 and a component F1H in a direction of the inter-axis straight line 72. 
     The difference in circumferential velocity between the developing roller 22 and the photosensitive drum 13 creates a friction resistance force F2 in an upward direction in FIG. 23 with respect to the developing roller 22 and in a downward direction with respect to the photosensitive drum 13, the friction resistance force F2 having a value equal to a multiplication product of the friction coefficient and the component F1H of the pressing force in the direction of the inter-axis straight line 72 of the photosensitive drum 13 and the developing roller 22. Therefore, the rotation moment caused by the friction resistance force F2 acting on the developer cartridge 4 acts about the developing roller shaft 22a in a clockwise direction in FIG. 23. 
     If the pressing action line 71 extends above the developing roller shaft 22a, a rotation moment acts on the developer cartridge 4 about the developing roller shaft 22a counter clockwise in FIG. 23, and reduces or offsets the rotation moment caused by the friction resistance force F2. In such a case, the developer cartridge 4 tends to rise, which is unfavorable. In this invention, however, the position of the developing roller shaft 22a is above or on the pressing action line 71, or the inter-axis straight line 72 substantially coincides with, or extend substantially parallel to, the pressing action line 71. Therefore, during image formation, the developing roller 22 is substantially prevented from rising, and stable pressing action can be achieved. 
     Furthermore, the pressing structure is formed by the urging devices 42 for pressing the developing roller 22 against the photosensitive drum 13, and the action-receiving portions 61 provided on the developer cartridge 4. The action-receiving portions 61 are disposed at a side of the developing roller 22, the side being remote from the photosensitive drum 13. Therefore, the photosensitive drum 13, the developing roller 22 and the drive mechanisms will not become impediments, and the pressing action line 71 and the inter-axis straight line 72 can easily be set substantially parallel to each other and adjacent to each other. 
     If the developer cartridge 4 is supported pivotably about the developing roller shaft 22a of the developing roller 22 while image forming operation is being performed, with the developer cartridge 4 set with respect to the photosensitive member cartridge 3, that is, if the arrangement is set such that during image forming operation, the bearings 23a, 23b on both ends of the developing roller shaft 22a of the developing roller 22 are stopped at the inward sides of the guide grooves 37 of the photosensitive member cartridge 3 and therefore prevented from moving further inward, and the weight Wo of the developer cartridge 4 is dispersedly supported at two positions in a side view, that is, the position of the shaft bearing 23a (23b), and a position at a side of the developing roller shaft 22a remote from the photosensitive drum 13, for example, a position at which the bottom of the toner containing chamber 24 contacts the rollers 50, or the like, so that split loads W1, W2 of the weight Wo are supported at the two positions (see FIG. 23), then the direction of the rotation moment about the axis (the developing roller shaft 22a) of the developing roller 22 caused by the friction resistance force F2 received by the developing roller 22 from the photosensitive drum 13 becomes the same as the direction of the rotation moment about the axis (the developing roller shaft 22a) of the developing roller 22 caused by the weight (load Wo at the center of gravity G) of the developer cartridge 4, that is, the clockwise direction in FIG. 23. Therefore, during image forming operation, the aforementioned friction resistance force F2 does not act as a moment in such a direction as to raise the developer cartridge 4 against the weight (load Wo at the center of gravity G) of the developer cartridge 4, so that the behavior of the developer cartridge 4 stabilizes. 
     In the above-described structure, the urging devices 42 are disposed at positions that are within the accommodating portion 32 in the photosensitive member cartridge 3, and that become remote from the developing roller 22 of the developer cartridge 4 when the developer cartridge 4 is set in the accommodating portion 32. The action-receiving portions 61 protrude outward from outer walls of the developer (toner) containing chamber 24, which is disposed at a side remote from the developing roller 22. Therefore, these component parts are unlikely to interfere with the operations of inserting or removing the developer cartridge 4. 
     In the embodiment of the invention, the urging devices 42, having a relatively complicated structure are disposed in the photosensitive member cartridge 3, which requires less frequent replacement. The action-receiving portions 61 having a relatively simple structure, are disposed in the developer cartridge 4, which requires frequent replacement. Therefore, the production costs of the process unit 2 and the developer cartridge 4 can be reduced, and the running cost can be reduced. Furthermore, since the protruded action-receiving portions 61 provided on the developer cartridge 4 are integral with the side surfaces of the case 21, the action-receiving portion 61 can be formed together with the case 21, thereby reducing the production cost and, further, making it easier for the action-receiving portions 61 to be pressed by the slide support members 40 of the urging devices 42 provided on the photosensitive member cartridge 3. 
     The right and left urging devices 42 are disposed on inner surfaces of the right and left sides of the photosensitive member cartridge 3, and are connected thereto in such a manner that the urging devices 42 are capable of changing the orientation between a direction of the urging force thereof and a non-urging direction. Therefore, if the developer cartridge 4 is simply placed over or adjacent to the photosensitive member cartridge 3, the developing roller 22 is not pressed against the photosensitive drum 13. Hence, the photosensitive member cartridge 3 and the developer cartridge 4 can be combined as a process unit 2 for packing and shipping, while obviating the danger of permanent deformation of the outer peripheral surface of the process unit 2, or the danger of contamination of the photosensitive drum 13 with material components from the developing roller 22. 
     Still further, since each urging device 42 is substantially made up of the pivot fulcrum member 39, the slide support member 40 slidable relative to the pivot fulcrum member 39, and the urging spring device 41 disposed between the two members, the urging devices 42 gain an increased degree of freedom in changing the orientation between the urging direction and the non-urging direction, compared with a conventional device that employs an urging spring to directly press an action-receiving portion and discontinue the pressing. Another advantage that the operations of pressing the action-receiving portions 61, and discontinuing the pressing, can be reliably performed by the slide support members 40 can also be achieved. 
     Further, since each urging device 42 is rotatably connected at its pivot fulcrum member 39 to the inner surface of the right or left side of the case 30 of the photosensitive member cartridge 3, most of the component parts of the urging devices 42 are unexposed outside the case 30, regardless of whether the photosensitive member cartridge 3 stands alone or is combined with the developer cartridge 4 into the process unit 2. Therefore, the danger of accidentally hitting and breaking any component part of the urging devices 42 is considerably reduced, and the ease of handling improves. 
     The slide support member 40 of each urging device 42 is provided integrally with the pin-like motion acting portion 43 protruding laterally to guide the slide support member 40 into the urging direction and the non-urging direction. The motion acting portions 43 of the urging devices 42 protrude outward from guide holes 44 in the right and left sides of the case 30 of the photosensitive member cartridge 3. Thus, most of the component parts of the urging devices 42 are unexposed outside the case 30. Therefore, the danger of accidentally hitting and breaking any component part of the urging devices 42 is considerably reduced, and the ease of handling improves. 
     The action-receiving portions 61 protruding outwardly from the right and left sides of the case 21 of the developer cartridge 4 perform the function to be pressed by the urging devices 42 and the function to cooperate with the lock lever 47 of the lock device 46 to prevent the developer cartridge 4 from rising relative to the photosensitive member cartridge 3. Since the action-receiving portions 61 thus perform the two functions, the predetermined cost of the developer cartridge 4 can be considerably reduced. 
     The process unit 2 is designed so as to be removably set into the body housing 1 of the image forming apparatus. The body housing 1 is provided with the guide devices 55 that guide the urging devices 42 and switch the urging devices 42 between the urging state and the non-urging state. Therefore, the orientation of the urging devices 42 and the operation thereof will be changed or switched simply by setting the process unit 2 into the body housing 1 or removing the process unit 2 from the body housing 1. Thus, the ease of operation considerably improves. 
     It is to be understood that the invention is not restricted to the particular forms shown in the foregoing embodiment. Various modifications and alternations can be made thereto without departing from the scope of the invention.