Abstract:
An image forming apparatus for forming an image on a recording material. A process cartridge, including a photosensitive drum and a process device actable on the drum, is detachably mountable to a main assembly of the image forming apparatus in a direction of an axis of the drum. The image forming apparatus includes a first positioning portion, provided above the cartridge, for positioning, with respect to a direction crossing with the axis of the drum, the drum at an upstream position with respect to a mounting direction in which the cartridge is mounted to the main assembly of the apparatus, and a second positioning portion, provided above the cartridge at a downstream position with respect to the mounting direction, for positioning the drum with respect to a direction crossing with the axis of the drum. A third positioning portion limits rotation of the cartridge relative to the first positioning portion and the second positioning portion when the main assembly coupling transmits a driving force to the drum coupling and positions the lower portion of the cartridge at an upstream position with respect to the mounting direction.

Description:
This application is a divisional of application Ser. No. 13/597,474, filed Aug. 29, 2012, which is a divisional of application Ser. No. 12,470,749, filed May 22, 2009, which issued as U.S. Pat. No. 8,280,279, on Oct. 2, 2012. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus, such as a copying machine, a printer, and a facsimile machine, etc., in which a process cartridge is removably mountable. It also relates to a process cartridge. 
     There have been available various image forming apparatuses which form a toner image (an image formed of toner). Some of them are structured so that a process cartridge or process cartridges are removably mountable in their main assembly. Further, some of them have been known to have been structured in consideration of the ease with which a process cartridge or process cartridges can be mounted into the main assembly of an image forming apparatus, or the ease with which a process cartridge or process cartridges in the main assembly of an image forming apparatus are replaceable. 
     Further, some image forming apparatuses are structured so that a process cartridge is mounted into, or removed from, their main assembly, through the opening with which one of their side walls is provided, in a direction parallel to the lengthwise direction of the process cartridge. Further, some of the image forming apparatuses structured as described above are provided with a process cartridge guide or process cartridge guides which supports or support a process cartridge from under the process cartridge (Japanese Laid-open Patent Application 2004-212986). Regarding the prior technologies related to the above described structural features of the image forming apparatuses, an image forming apparatus is provided with a door which can be opened to expose the abovementioned opening for mounting or dismounting a process cartridge, or to cover the opening. More specifically, an image forming apparatus is structured so that as the door is moved into its closed position after the mounting of a process cartridge into the main assembly of an image forming apparatus, the image bearing member of the process cartridge is precisely moved into its image forming position in the main assembly, by the closing movement of the door. Further, in order to prevent the image bearing member from being damaged while the process cartridge is mounted into, or removed from, the main assembly, the image forming apparatus is structured so that the abovementioned guides for supporting the process cartridge from under the process cartridge are movable to keep the image bearing member separated from the apparatus main assembly while the process cartridge is mounted into, or removed from, the apparatus main assembly. 
     In order for an image forming apparatus to form images of excellent quality, the image forming apparatus has to be structured so that an image bearing member is highly precisely positioned relative to the main assembly of the image forming apparatus. Further, the image forming apparatus has to be structured so that the process cartridge is precisely positioned in terms of its attitude relative to the main assembly. That is, the image forming apparatus and the process cartridge therefor have to be structured so that the process cartridge is held in its preset position in the main assembly without causing the processing means (charging means, developing means, cleaning means, etc.) in the process cartridge to be misaligned relative to the image bearing member. 
     Generally, an image forming apparatus and a process cartridge therefor are structured so that the process cartridge is precisely positioned relative to the main assembly of the image forming apparatus (which hereafter will be referred to simply as an apparatus main assembly) to precisely position the image bearing member in the process cartridge relative to the apparatus main assembly, and also, so that the process cartridge is rotated about the axial line of the image bearing member to precisely fix the process cartridge in its attitude relative to the apparatus main assembly. More concretely, for example, the apparatus main assembly is provided with a pair of image bearing member positioning portions, which are in the form of a pair of bearing catching recesses or holes for catching the bearings of the image bearing member. Thus, as the bearings of the image bearing member are caught by the bearing catching portions of the main assembly, the image bearing member becomes precisely positioned relative to the main assembly. As for the rotation stopper for precisely fixing the process cartridge in its attitude relative to the apparatus main assembly by catching the process cartridge as the process cartridge is rotated about the axial line of its image bearing member, conventionally, a process cartridge is provided with a boss which protrudes from the lengthwise ends of the external frame (shell) of the process cartridge, whereas the apparatus main assembly is provided with a pair of holes which are elongated in cross section. As the abovementioned bosses are fitted in these holes, one for one, the process cartridge becomes fixed in attitude. In the case of some image forming apparatuses and the process cartridge therefor, the process cartridge is provided with the abovementioned holes with elongated cross section, and their main assembly is provided with the bosses. 
     Also regarding the above described background technologies, as far as fixing in attitude the downstream end portion (in terms of cartridge mounting direction) of a process cartridge relative to the rear end of the apparatus main assembly is concerned, a highly precise rotation stopping means, which is relatively small in component count, can be realized by structuring an image forming apparatus and process cartridge therefor so that the boss protruding from the external frame of the process cartridge fits into a hole with elongated cross section, with which the rear panel of the apparatus main assembly is provided. However, it has been difficult to provide a rotation stopping means capable of highly precisely fixing in attitude the upstream end portion of a process cartridge relative to the apparatus main assembly, because the front end of the apparatus main assembly has the opening for mounting or dismounting the process cartridge. Besides, structuring an image forming apparatus and a process cartridge therefor so that the process cartridge is precisely fixed in attitude relative to the apparatus main assembly, only at the rear end of the apparatus main assembly, is not satisfactory to precisely position the process cartridge relative to the apparatus main assembly, and precisely fix the process cartridge in attitude relative to the apparatus main assembly. 
     Some image forming apparatuses, and the process cartridges therefor, are structured so that the front door is provided with means for precisely positioning a process cartridge relative to the apparatus main assembly, and also, in terms of attitude. This cartridge positioning structural arrangement also is affected by the accuracy with which the door is attached to the apparatus main assembly. Therefore, it was also difficult to precisely position a process cartridge relative to the main assembly of an image forming apparatus with use of this structural arrangement. 
     SUMMARY OF INVENTION 
     The present invention was made in light of the above described problems. Thus, the primary object of the present invention is to provide a combination of an image forming apparatus and a process cartridge, which is structured so that the process cartridge is mounted into, or removed from, the main assembly of the image forming apparatus in the direction parallel to the axial line of the photosensitive drum in the process cartridge; is simple in structure; and yet, is capable of highly precisely positioning the process cartridge relative to the apparatus main assembly. 
     According to an aspect of the present invention, there is provided an image forming apparatus for forming an image on a recording material. A process cartridge including a photosensitive drum and process means actable on the photosensitive drum is detachably mountable to a main assembly of the image forming apparatus in a direction of an axis of the photosensitive drum. The image forming apparatus comprises a first positioning portion, a second positioning portion, a main assembly coupling, and a supporting member. The first positioning portion is provided above the process cartridge, to position, with respect to a direction crossing the axis of the photosensitive drum, the photosensitive drum at an upstream position upstream of the second positioning portion with respect to a mounting direction in which the process cartridge is mounted to the main assembly of the apparatus. The second positioning portion is provided above the process cartridge at a downstream position downstream of the first positioning portion with respect to the mounting direction, and positions the photosensitive drum with respect to a direction crossing the axis of the photosensitive drum. The main assembly coupling is provided at a downstream position downstream of the first positioning portion with respect to the mounting direction, and engages a drum coupling provided at an end of the photosensitive drum and transmits a driving force to the photosensitive drum. The supporting member supports the process cartridge at a first position for positioning the photosensitive drum to the first positioning portion and the second positioning portion and at a second position for spacing the photosensitive drum away from the first positioning portion and the second positioning portion to permit mounting and dismounting the process cartridge relative to the main assembly of the apparatus. The supporting member includes a guide portion that engages portion-to-be-guided provided at a bottom portion of the process cartridge to guide the process cartridge along the mounting direction. The supporting member also includes a third positioning portion that limits rotation of the process cartridge relative to the first positioning portion and the second positioning portion when the main assembly coupling transmits a driving force to the drum coupling and that positions the bottom portion of the process cartridge at an upstream position upstream of the second positioning portion with respect to the mounting direction. 
     According to another aspect of the present invention, there is provided a process cartridge detachably mountable to a main assembly of an image forming apparatus. The main assembly includes a main assembly coupling, a first positioning portion, a second positioning portion, a supporting member movable between the first position and the second position and provided below the first positioning portion and the second positioning portion. The supporting member includes a guide portion and a third positioning portion. The cartridge comprises a photosensitive drum, and a process device actable on the photosensitive drum. The cartridge also includes a drum coupling, provided at an end of the photosensitive drum, that engages the main assembly coupling at a downstream position downstream of the first positioning portion with respect to a mounting direction in which the process cartridge is mounted to the main assembly and which is substantially parallel with a direction of an axis of the photosensitive drum. The cartridge further includes a first portion-to-be-positioned, provided at a first portion of the process cartridge, to be positioned with respect to a direction crossing with the axis of the photosensitive drum by contacting the first positioning portion at a position upstream of the second positioning portion with respect to the mounting direction when the supporting member supporting the process cartridge is at the first position. The cartridge also includes a second portion-to-be-positioned, provided at the first portion of the process cartridge, to be positioned with respect to a direction crossing the axis of the photosensitive drum by contacting the second positioning portion at a position downstream of the first positioning portion with respect to the mounting direction when the supporting member supporting the process cartridge is at the first position. In addition, the cartridge includes a portion-to-be-guided engageable with the guide portion to be guided in a direction of the axis of the photosensitive drum when the process cartridge is mounted to the main assembly. The cartridge also includes a third portion-to-be-positioned, provided at a second portion of the process cartridge, lower than the first portion of the process cartridge and that is limited by the third positioning portion to prevent rotation of the process cartridge relative to the first positioning portion and the second positioning portion at an upstream position upstream of the second positioning portion with respect to the mounting direction when the main assembly coupling transmits a driving force to the drum coupling. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic vertical sectional view of the image forming apparatus in the first preferred embodiment, at a plane parallel to the front panel of the image forming apparatus. 
         FIG. 2  is an external perspective view of the image forming apparatus in the first preferred embodiment, when the front door of the image forming apparatus is shut. 
         FIG. 3  is an external perspective view of the image forming apparatus in the first preferred embodiment, when the front door of the apparatus is completely open, and the first process cartridge has just been halfway pulled out of the apparatus main assembly. 
         FIG. 4  is an external perspective view of one of the cartridges for the image forming apparatus in the first preferred embodiment, as seen from the opposite lengthwise end of the cartridge, from which the cartridge is driven. 
         FIG. 5  is a perspective view of the first to fourth cartridge supporting members of the apparatus main assembly, and the front door (which is open) of the apparatus main assembly. 
         FIG. 6  is a perspective view of one of the cartridges for the image forming apparatus in the first preferred embodiment, as seen from diagonally below the lengthwise end of the cartridge, from which the cartridge is driven. 
         FIG. 7  is a phantom drawing of the image forming apparatus, and the four process cartridges having just been precisely positioned in the apparatus main assembly. 
         FIG. 8  is a partly broken-away perspective view of the image forming apparatus in the first preferred embodiment, and the process cartridges therefor, in which the image forming apparatus is partly broken away, and which is for describing the portions of the cartridges, by which the cartridges are precisely positioned relative to the apparatus main assembly, and the portions of the apparatus main assembly, which are for precisely positioning the drums relative to the apparatus main assembly. 
         FIG. 9  is a front view of the image forming apparatus, and the four process cartridges therefor, when the cartridges are in their position into which they are moved into before they are precisely positioned relative to the apparatus main assembly, and which allows them to be dismounted from the apparatus main assembly. 
         FIG. 10  is a schematic vertical sectional view of the image forming apparatus in the first preferred embodiment, at a plane parallel to the front panel of the image forming apparatus, when the four cartridges are in their intermediary positions where they are temporarily positioned before they are precisely positioned relative to the apparatus main assembly, or before they are moved out of the apparatus main assembly, and where their photosensitive drums are not in contact with the intermediary transfer belt. 
         FIG. 11  is an enlarged perspective view of one of the arms and its adjacencies. 
         FIGS. 12(   a )- 12 ( c ) are schematic drawings for describing the relationship between the movement of the cartridge supporting member and the cartridge, when the cartridge is mounted into, or removed from, the apparatus main assembly. 
         FIGS. 13(   a ) and  13 ( b ) are a combination of a top view of the cartridge supporting member and a phantom top view of the cartridge thereon, in the first preferred embodiment, in the final stage of the mounting of the process cartridge, and after the mounting of (precise positioning) of the process cartridge, respectively. 
         FIG. 14  is a perspective view of one of the cartridge supporting members in the second preferred embodiment of the present invention. 
         FIGS. 15(   a ) and  15 ( b ) are a combination of a top view of the cartridge supporting member and a phantom top view of the cartridge thereon, in the second preferred embodiment, in the final stage of the mounting of the process cartridge, and after the mounting of (precise positioning) of the process cartridge, respectively. 
         FIG. 16  is a perspective view of one of the cartridge supporting members in the third preferred embodiment of the present invention. 
         FIG. 17  is an external perspective view of one of the cartridges for the image forming apparatus in the third preferred embodiment, as seen from diagonally below the lengthwise end of the cartridge, from which the cartridge is driven. 
         FIGS. 18(   a ) and  18 ( b ) are a combination of a top view of the cartridge supporting member and a phantom top view of the cartridge thereon, in the third preferred embodiment, in the final stage of the mounting of the process cartridge, and after the mounting of (precise positioning) of the process cartridge, respectively. 
         FIG. 19  is a perspective view of the coupling in the first preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a schematic vertical sectional view of the image forming apparatus in the first preferred embodiment, at a plane parallel to the front panel of the image forming apparatus.  FIG. 2  is an external perspective view of the image forming apparatus in the first preferred embodiment, when the front door of the image forming apparatus is shut.  FIG. 3  is an external perspective view of the image forming apparatus in the first preferred embodiment, when the front door of the apparatus is completely open, and the first process cartridge has just been halfway pulled out of the apparatus main assembly. 
     This image forming apparatus  1  is a full-color laser beam printer, which uses an electrophotographic process and four monochromatic toners. It forms a color image on recording medium. It uses four process cartridges P (first to fourth process cartridge PY, PM, PC, and PK). Further, the image forming apparatus  1  and the process cartridges therefor are structured so that the cartridges are removably mountable in the main assembly of the image forming apparatus  1 . 
     In the following description of the preferred embodiments of the present invention, the front side, or user side, of the image forming apparatus is the side where the door which is for covering or exposing the process cartridge chambers of the apparatus main assembly is located. The backside, or rear side, is the opposite side from the front side. Further, the front-to-rear direction means both the direction from the rear side to the front side (frontward), and the opposite (rearward) direction from the frontward direction. The left and right sides of the image forming apparatus  1  means the left and right sides of the apparatus as seen from the front side of the apparatus. Further, the left-to-right direction means both the leftward direction, that is, the direction from the right to left, and the opposite direction (rightward direction), that is, the direction from the left side to the right side. Further, the image forming apparatus main assembly  1 A (which hereafter will be referred to simply as apparatus main assembly  1 A) is the portion of the image forming apparatus excluding the process cartridges. Further, the lengthwise direction of a process cartridge (which hereafter will be referred to simply as cartridge), structural components of the process cartridge, or structural components of the apparatus main assembly, means the direction of the axial line of the image bearing member of the process cartridge, which is in the form of an electrophotographic photosensitive drum, or the direction parallel to the axial line of the image bearing member, when the process cartridge is in the apparatus main assembly. Regarding the cartridge P, its lengthwise end to which the cartridge driving force is transmitted from the apparatus main assembly  1 A may be referred to as driven end, and the opposite end from the driven end may be referred to as non-driven end. 
     The apparatus main assembly  1 A is provided with cartridge chamber  1 B in which the first to fourth cartridges PY, PM, PC, and PK are placed. The cartridge chamber  1 B occupies roughly the center portion of the apparatus main assembly  1 A. 
     The first to fourth cartridges P (PY, P 1 , PC, and PK) are disposed in the cartridge chambers  1 B in such a manner that their lengthwise direction is perpendicular to the left-to-right direction of the apparatus main assembly  1 A, and also, that they are horizontally aligned in tandem in the left-to-right direction. Further, they are precisely position for image formation. 
     The four cartridges P are the same in structure, being designed to carry out the electrophotographic process, but are different in the color and amount of the developer (which hereafter may be referred to as toner) they contain. To each of the four cartridges P precisely positioned for image formation in the cartridge chamber  1 B, rotational driving force is transmitted from one of the couplings  51  ( FIG. 19 ) with which the apparatus main assembly  1 A is provided. Further, each of the four cartridges P is supplied with biases (charge bias, development bias, etc.) (unshown). 
     Each process cartridge P in this embodiment is made up of a drum  3 , a charging means  4 , a developing means  5 , a cleaning means  6 , and an external frame  7  (shell  7 ). The charging means,  4 , developing means  5 , and cleaning means  6  are the means for processing the drum  3 , and are integrally disposed, along with the drum  3 , in the external frame  7  (shell  7 ). As the charging means  4 , a charge roller is used. As the developing means  5 , a development roller is used. As the cleaning means  6 , a cleaning blade is used. 
       FIG. 4  is an external perspective view of one of the cartridges for the image forming apparatus in the first preferred embodiment, as seen from the non-driven end. The cartridge P is an assembly, the lengthwise direction of which is parallel to the direction A-A of the drum  3  in the cartridge P. The drum  3  is rotatably supported by the external cartridge frame  7 . More specifically, the external frame  7  is provided with first and second bearings  7 A and  7 B, which are attached to the lengthwise ends of the frame  7 , one for one. The end portion of the drum  3 , from which the drum  3  is not driven, is supported by the first bearing  7 A, and the end portion of the drum  3 , from which the drum  3  is driven, is supported by the second bearing  7 B. 
     The first cartridge PY has a developer container in which yellow (Y) toner is stored. It forms a toner image of yellow color, on the peripheral surface of the drum  3 . The second cartridge PM has a developer container in which magenta (M) toner is stored. It forms a toner image of magenta color, on the peripheral surface of the drum  3 . The third cartridge PC has a developer container in which cyan (C) toner is stored. It forms a toner image of cyan color, on the peripheral surface of the drum  3 . The fourth cartridge PK has a developer container in which black (K) toner is stored. It forms a toner image of black color, on the peripheral surface of the drum  3 . 
     The apparatus main assembly  1 A is provided with a laser scanner  8 , which is located below cartridge chamber  1 B. The laser scanner  8  is a means for exposing the peripheral surface of the drum  3  in each cartridge P, with the beam of laser light modulated with the information regarding the image to be formed. 
     The apparatus main assembly  1 A is also provided with a recording medium feeding unit  9 , which is located below the laser scanner  8 . The recording medium feeding unit  9  has a recording paper feeding cassette  10 , in which sheets S of a recording medium (transfer medium) are storable in layers, a combination of a sheet feeding roller  11  and a sheet separating portion  12 , a pair of registration rollers  13 , etc. 
     Further, the apparatus main assembly  1 A has an intermediary transfer unit  14 , which is located above the cartridge chamber  1 B. The unit  14  is a means for transferring in layers (primary transfer) the toner images formed on the drums  3  in the first to fourth cartridges P (PY, PM, PC, and PK) onto an intermediary transfer belt  15  (first recording medium, which hereafter will be referred to simply as belt  15 ). The intermediary transfer unit  14  has a pair of rollers, that is, a driver roller  16  and a tension roller  17 , which are disposed in parallel in the right and left end portions of the apparatus main assembly  1 A, and a flexible and endless belt  15 , which is suspended by the two rollers  16  and  17  in such a manner that the portion of the belt  15 , which is circulating through the bottom portion of the belt loop, remains horizontal. 
     Each cartridge P is precisely placed in its image forming position in the cartridge chamber  1 B. As the cartridge P is precisely placed in its image forming position, its drum  3  is placed in contact with the portion of the belt  15 , which corresponds in position to the bottom portion of the belt loop. The point (portion) of contact between the drum  3  and belt  15  is the primary transfer point (portion). The intermediary transfer unit  14  is also provided with four primary transfer rollers  18 , which are located in the inward side of the loop of the belt  15 , being positioned so that they oppose the four drums  3 , one for one, with the presence of the belt  15  between each primary transfer roller  18  and the corresponding drum  3 . The belt  15  is circularly driven by the driver roller  16  in the counterclockwise direction, indicated by an arrow mark, at the speed which matches the rotational speed of the drum  3 , while remaining in contact with all of the four drums  3 , by the portion which corresponds to the bottom portion of the belt loop, across the highest portion of the peripheral surface of each drum  3 . To the primary transfer roller  18 , a preset primary transfer voltage is applied with preset control timing. Further, the apparatus main assembly  1 A is provided with a secondary transfer roller  19 , which is positioned in contact with the belt  15  in such a manner that it is kept pressed against the driver roller  16 , with the presence of the belt  15  between the two rollers  19  and  16 . The point (portion) of contact between the secondary transfer roller  19  and the belt  15  is the secondary transfer point (portion). To the secondary transfer roller  19 , a preset secondary transfer voltage is applied with preset control timing. 
     Further, the apparatus main assembly  1 A is provided with a fixation unit  20  and a recording medium sheet discharging unit  21 , which are located in the top right portion of the apparatus main assembly  1 A. A part of the top surface of the external frame of the apparatus main assembly  1 A is designed as a delivery tray  22 . 
     The full-color image forming operation of the image forming apparatus  1  is as follows: First, the controller (unshown control circuit) causes the image forming apparatus  1  to start an image forming operation, in response to a print signal. As the image forming operation is started, the drums  3  in the first to fourth cartridges P (PY, PM, PC, and PK) are rotationally driven at a preset peripheral velocity in the clockwise direction, indicated by the arrow mark, and the belt  15  is circularly driven also in the clockwise direction (so that it moves in the same direction as the peripheral surface of the drum  3 , in the interface between them), indicated by another arrow mark, at the speed which matches the peripheral velocity of the drum  3 . Further, the laser scanner  8  is started. In synchronism with the driving of the abovementioned components, the peripheral surface of the drum  3  in each cartridge p is uniformly charged to preset polarity and potential level by the charge roller  4  to which a preset charge voltage is being applied. The uniformly charged portion of the peripheral surface of each drum  8  is exposed by one of the beams of laser light L (LY, LM, LC, and LK) projected by the laser scanner  8  while being modulated with signals which carry the information of an image to be formed. More specifically, as the beams of laser light LY, LM, LC, and LK outputted from the laser scanner  8  enter the cartridges PY, PM, PC, and PK, correspondingly, through the slit  7   a , with which each cartridge P is provided, they illuminate the drums  3  across the bottom portion of the peripheral surface of the drum  3 . As a result, an electrostatic latent image is formed on the peripheral surface of each drum  3 . Each electrostatic latent image reflects the signals modulated with the information regarding the image to be formed. Then, each electrostatic latent image is developed by the development roller  5  into a visible image, that is, a monochromatic image formed of toner. 
     Through the above described image forming operation, which uses the electrophotographic process, a yellow toner image is formed on the drum  3  of the first cartridge PY, which corresponds to the yellow component of the full-color image to be formed. This toner image is transferred (primary transfer) onto the belt  15 . Similarly, a magenta toner image is formed on the drum  3  of the second cartridge PM, which corresponds to the magenta component of the full-color image to be formed. This toner image is transferred (primary transfer) onto the belt  15  in such a manner that it is overlaid on the yellow toner image having just been transferred onto the belt  15 . Similarly, a cyan toner image is formed on the drum  3  of the third cartridge PC, which corresponds to the cyan component of the full-color image to be formed. This toner image is transferred (primary transfer) onto the belt  15  in such a manner that it is overlaid on the yellow and magenta toner images having just been transferred in layers onto the belt  15 . Further, a black toner image is formed on the drum  3  of the fourth cartridge PK, which corresponds to the black component of the full-color image to be formed. This toner image is transferred (primary transfer) onto the belt  15  in such a manner that it is overlaid on the yellow, magenta, and cyan toner images having just been transferred in layers onto the belt  15 . 
     As a result, an unfixed full-color toner image is synthetically formed on the belt  15 , and is composed of the yellow, magenta, cyan, and black monochromatic toner images. 
     The transfer residual toner, that is, the toner remaining on the peripheral surface of the drum  3  in each cartridge P after the primary transfer, is removed by the cleaning blade  6 . 
     Meanwhile, one of the recording sheets S in the sheet feeding cassette  10  of the sheet feeding unit  9  is separated from the rest of the recording sheets S in the cassette  10 , and is fed into the apparatus main assembly  1 A. As the recording sheet S is fed into the apparatus main assembly  1 A, it is conveyed to the pair of registration rollers  13 , and then, is introduced by the registration rollers  13  into the secondary transfer portion, which is the interface between the belt  15  and secondary transfer roller  19 . To the secondary transfer roller  19 , a preset secondary transfer voltage is applied. As a result, the four layers of an unfixed monochromatic toner image, different in color, on the belt  15  are transferred together (secondary transfer) onto the surface of the recording medium sheet S as if they are peeled away from the belt  15 . 
     Then, the recording medium sheet S is separated from the surface of the belt  15 , and is conveyed into the fixation unit  20 , in which the recording medium sheet S and the unfixed images thereon are subjected to heat and pressure. As a result, the unfixed toner images become fixed to the recording medium sheet S. Then, the recording medium sheet S is conveyed out of the fixation unit  20 , and is discharged, as a full-color copy of the image to be formed, onto the delivery tray  22  by the sheet discharging unit  21 . 
     &lt;Exchange of Process Cartridge&gt; 
     The toner in the developing means of each of the cartridges P in the apparatus main assembly  1 A is consumed as the cartridges are used for image formation. Thus, each cartridge P is provided with a detecting means (unshown) for detecting the amount of the toner remaining in the cartridge. The detected amount of the toner in each cartridge P is compared by the controller with the threshold value set up for issuing a prescribed warning message, which informs a user of the nearness to the end of the service life of the cartridge P, the actual end of the service life of the cartridge P, etc. If the controller determines that the detected amount of the toner in the cartridge P is no more than the abovementioned threshold value, it makes the display portion (unshown) of the apparatus main assembly  1 A display the abovementioned warning messages, to suggest to a user that a replacement cartridge for the cartridge in the apparatus main assembly  1 A be prepared, or that the cartridge in the apparatus main assembly  1 A be replaced to ensure that the image forming apparatus continues to output high quality images. 
     The operation for replacing any of the cartridges P in the image forming apparatus  1  is as follows: The cartridge P in the apparatus main assembly  1 A is to be replaced from the front side of the apparatus main assembly  1 A by opening the front door of the apparatus main assembly  1 A as shown in  FIG. 3 . More specifically, designated by a referential code  2   a  is a handhold portion of the front door  2 . Incidentally, the image forming apparatus  1  is structured so that the recording medium sheet feeding cassette  10  of the recording medium sheet feeding unit  9  is also to be inserted or removed from the front side of the apparatus main assembly  1 A. Designated by a referential code  10   a  is the handhold portion of the recording sheet feeding cassette  10 . 
     The front panel  23  of the apparatus main assembly  1 A is provided with an opening  24 , through which the cartridge P is inserted into, or removed from, the apparatus main assembly  1 A. The direction in which the cartridge P is mounted into, or removed from, the apparatus main assembly  1 A is parallel to the axial line of the drum  3 . 
     Further, the front panel  23  of the apparatus main assembly  1 A is provided with the door  2  (front door), which is movable between the closed position in which it keeps the opening  24  completely covered, and the open position in which it keeps the opening  24  fully exposed. 
     The front door  2  in this embodiment is rotatably supported by a door axle  25  (first rotational shaft) located at the bottom edge of the front door  2 . The door axle  25  is rotatably supported by a pair of bearings  26 , with which the front panel of the apparatus main assembly  1 A is provided. In this embodiment, in consideration of the operational convenience for a user, the door axle  25  is attached to the front panel  23  in such a manner that its axial line B-B is horizontal and parallel to the left-to-right direction of the apparatus main assembly  1 A. Thus, the front door  2  can be changed in attitude by being rotated frontward roughly 90° about the rotational axle  25 , from the one in which it is folded up against the front panel  23  of the apparatus main assembly  1 A, being therefore roughly vertical, into the one in which it is roughly horizontal. 
     The cartridge chamber  1 B of the apparatus main assembly  1 A is provided with the first to fourth cartridge supporting members  27  ( 27 Y,  27 M,  27 C, and  27 K), which are for supporting the first to fourth cartridges P (PY, PM, PC, and PK), correspondingly.  FIG. 5  is a perspective view of the cartridge supporting members  27  (which hereafter may be referred to simply as supporting members  27 ) and the adjacencies of the front door  2 . 
     Each supporting member  27  is horizontally disposed in the same direction as the first to fourth cartridges P (PY, PM, PC, and PK). 
     Further, each supporting member  27  is movable between its first and second positions by a supporting member moving mechanism, the movement of which is linked to the opening and closing movement of the front door  2 . The first position of the supporting member  27  is the position into which the supporting member  27  is moved to precisely position the cartridge P relative to the apparatus main assembly  1 A, for image formation. The second position of the supporting member  27  is the position into which the supporting member  27  is moved to make it possible for the cartridge P to be mounted into, or removed from, the apparatus main assembly  1 A. Further, the first position of the supporting member  27  is the position into which the supporting member  27  is moved by the closing movement of the front door  2  to place the drum  3  in contact with the belt  15 , and the second position of the supporting member  27  is the position into which the supporting member  27  is moved by the opening movement of the front door  2  to keep the drum  3  away from the belt  15 . 
     The direction in which each supporting member  27  is moved is vertical, that is, perpendicular to the abovementioned direction in which the cartridges P are inserted into, or removed from, the apparatus main assembly  1 A. The supporting member moving mechanism, the movement of which is linked to the opening and closing movement of the front door  2 , will be described later. 
     In this embodiment, as the front door  2  is rotationally closed, each supporting member  27  is moved up into the first position by the supporting member moving mechanism, which is moved by the closing movement of the front door  2 , whereas as the front door  2  is rotationally opened, each supporting member  27  is moved down into the second position by the supporting member moving mechanism, which is moved by the opening movement of the front door  2 . 
       FIGS. 7 and 12(   a ) depict the state of the image forming apparatus  1 , in which each supporting member  27  is in the first position into which it was moved up. When the supporting member  27  in is its first position, the corresponding cartridge P is kept in its position in which the top portion of the first bearing  7 A (which is located at one of lengthwise end of cartridge P), is kept pressed upon one of the first cartridge positioning portions  29  (cartridge catching portions), with which the front panel  23  of the apparatus main assembly  1 A is provided, and also, in which the top portion of the second bearing  7 B (which is located at other lengthwise end of cartridge P), is kept pressed upon one of the second cartridge positioning portions  30  (cartridge catching portion), with which the rear panel  28  of the apparatus main assembly  1 A is provided. The first positioning portion  29  is a recess in the form of a V-shaped valley. It is with these recess portions of the front and rear panels of the apparatus main assembly  1 A that the first and second bearings  7 A and  7 B, by which the drum  3  (cartridge P) is precisely positioned relative to the apparatus main assembly  1 A, are placed in contact. In terms of the direction perpendicular to the axial line of the drum  3 , it is by the first and second bearings  7 A and  7 B that each cartridge P is precisely positioned relative to the apparatus main assembly  1 A. Thus, as the front door  2  is closed, each cartridge P is precisely positioned relative to the apparatus main assembly  1 A, more specifically, it is moved by the movement of the front door  2  into its image forming position in the apparatus main assembly  1 A, and is held in the position until the front door  2  is reopened. While the cartridge P is in this position, the drum  3  of the cartridge P remains in contact with the belt  15 , by the top portion of its peripheral surface, as shown in  FIG. 1 . The portion of the belt  15 , with which the top portion of the peripheral surface of the drum  3  remains in contact, is the portion of the belt  15 , which is moving through the bottom portion of the belt loop. It is also while the cartridge P is in this position that the rotational driving force can be transmitted to the cartridge P from the coupling  51 , with which the apparatus main assembly  1 A is provided, as shown in  FIG. 19 . More specifically, the apparatus main assembly  1 A is provided with the coupling  51 , whereas the cartridge P is provided a coupling  3   a   1 , which is a part of the flange  3   a , with which one of the lengthwise end portions of the drum  3  is fitted. As the cartridge P is moved by the supporting member  27  into the position in which the cartridge P is kept precisely positioned relative to the apparatus main assembly  1 A by the first and second cartridge positioning portions  29  and  30  of the apparatus main assembly  1 A, the coupling  51  of the apparatus main assembly  1 A engages with the coupling  3   a   1  of the cartridge P. To describe in more detail the couplings  51  of the apparatus main assembly  1 A and the coupling  3   a   1  of the cartridge P, the coupling  51  has a hole  51   a , which is roughly triangular in cross section, whereas the coupling  3   a  has a projection, which is also roughly triangular in cross section, and which fits into the hole  51   a  of the coupling  51 . Thus, the engagement between the coupling  51  and coupling  3   a   1  means that the projection  3   a   1  fits into the hole  51   a  of the coupling  51 . Thus, as the coupling  51  rotates while the projection  3   a   1  of the coupling  3   a  is in the hole  51   a  of the coupling  51 , the rotational driving force is transmitted to the drum  3 . Further, the apparatus main assembly  1 A is provided with a coupling supporting member  52 , which supports the coupling  51 , and the movement of which is linked to the movement of the front door  2 . Thus, as the front door  2  is closed, the coupling supporting member  52  is moved in the direction indicated by an arrow mark f by the movement of the front door  2 , and therefore, the projection  31   a   1  of the coupling  3   a  fits into the coupling  51  of the apparatus main assembly  1 A, whereas as the front door  2  is opened, the coupling supporting member  52  is moved in the direction indicated by an arrow mark b by the movement of the front door  2 , and therefore, the coupling  51  of the apparatus main assembly  1 A is disengaged from the coupling  3   a  of the drum  3 . 
       FIGS. 9 and 12(   b ) depict the state of the image forming apparatus  1 , in which each supporting member  27  is in the second position into which it was moved down. When the supporting member  27  in its second position, the corresponding cartridge P is kept in the position in which the top portions of the first and second bearings  7 A and  7 B are not kept pressed upon the first and second cartridge positioning portions  29  and  30  of the apparatus main assembly  1 A; the first and second bearings  7 A and  7 B are not in contact with the first and second cartridge positioning portions  29  and  30  of the apparatus main assembly  1 A. Thus, when the supporting member  27  is in its second position, the cartridge P is in its position into which it is moved when it is mounted into the apparatus main assembly  1 A, or from which it is removed when it is removed from the apparatus main assembly  1 A. Referring to  FIG. 10 , when the cartridge P is in this position in the apparatus main assembly  1 A, into which the cartridge P is mounted, and which allows the cartridge P to be removed from the apparatus main assembly  1 A, there is a space (gap) between the top portion of the peripheral surface of the drum  3  in the cartridge P and the portion of the belt  15 , which corresponds in position to the bottom portion of the belt loop. Also when the cartridge P is in this position, the rotational driving force is not transmitted to the cartridge P from the apparatus main assembly  1 A, nor is the cartridge P supplied with the biases by the apparatus main assembly  1 A. 
     The operation for mounting the cartridge P into the apparatus main assembly  1 A, or removing the cartridge P from the apparatus main assembly  1 A is as follows: Referring to  FIG. 3 , first, the front door  2  is to be fully opened to expose the opening  24 . As the front door  2  is rotationally opened, the each supporting member  27  is moved down by the supporting member moving mechanism into its second position ( FIG. 12(   a )→ 12 ( b )). By this movement of the cartridge supporting member  27 , the cartridge P is moved into its position which allows the cartridge P to be moved out of the apparatus main assembly  1 A, and also, into which the cartridge P is temporarily moved before it is to be precisely positioned relative to the apparatus main assembly  1 A as it is mounted into the apparatus main assembly  1 A. It is when the cartridge P is in this position which corresponds to the second position of the cartridge supporting member  27 , that the cartridge P can be moved out of the apparatus main assembly  1 A through the opening  24 , by pulling the cartridge P frontward to slide the cartridge P along the supporting member  29 , which is supporting the cartridge P. As for the mounting of the cartridge P into the apparatus main assembly  1 A, if a user wants to insert the cartridge P into the apparatus main assembly  1 A, first, the user is to position the cartridge P so that the lengthwise end of the cartridge P, from which the cartridge P is driven, faces the apparatus main assembly  1 A. Then, the user is to slide the cartridge P into the apparatus main assembly  1 A along the supporting member  27  through the opening  24 . The cartridge P is to be inserted deep enough into the apparatus main assembly  1 A for the cartridge P to be caught by the rear panel  28  of the apparatus main assembly  1 A so that the cartridge P cannot be inserted any further ( FIG. 12(   c )). 
       FIG. 6  is a perspective view of one of the cartridges P, as seen from diagonally below the lengthwise end of the cartridge, from which the cartridge is driven.  FIG. 8  is a broken-away perspective view of the image forming apparatus, and the process cartridges  7  therefor, as seen from diagonally below the rear panel of the apparatus main assembly  1 A. It is for describing the state of the image forming apparatus, in which the first cartridge PY has just been pulled out halfway from the apparatus main assembly  1 A, whereas the other cartridges are in their second position into which they were moved down by the downward movement of the supporting members  27 .  FIGS. 13(   a ) and  13 ( b ) are a combination of a top view of the cartridge supporting member and a phantom top view of the cartridge thereon, in the final stage of the mounting of the process cartridge, and after the mounting of (precise positioning) of the process cartridge, respectively. 
     Each cartridge P is provided with a rotation stopper boss  7 C, which perpendicularly protrudes from the downstream panel of the cartridge P in terms of the cartridge insertion direction. Correspondingly, the rear panel  28  of the apparatus main assembly  1 A is provided with four elongated holes  41 , which match in position to the four cartridges P, one for one, and the long axis of which is parallel to the direction in which the supporting members  27  are moved up or down. As each cartridge P is inserted into the apparatus main assembly  1 A as far as possible, the rotation stopper boss  7 C of the cartridge P fits into the corresponding elongated hole  41  of the rear panel  28  ( FIG. 13(   a )→ FIG. 13(   b )). Then, each supporting member  27  is moved up into its first position, whereby the drum  3  of the cartridge P is precisely positioned relative to the apparatus main assembly  1 A by the drum bearing positioning portions  29  and  30 . When the drum  3  remains precisely positioned by the drum bearing positioning portions  29  and  30 , the rotation stopper boss  7 C is in the corresponding elongated hole  41  of the rear panel  28 . Thus, the rotation stopper boss  7 C and elongated hole  41  prevent the lengthwise end portion of the cartridge P, which corresponds to the rear portion of the apparatus main assembly  1 A, from rotationally moving relative to the apparatus main assembly  1 A. 
     Referring to  FIG. 5 , the top surface of each supporting member  27  is provided with a groove  32  (guiding portion) which extends in the lengthwise direction of the supporting member  27 , that is, the direction parallel to the direction in which the cartridge P is mounted into, or removed from, the apparatus main assembly  1 A. Next, referring to  FIG. 6 , the bottom wall of the cartridge P is provided with a rib  33 , which extends in the lengthwise direction of the cartridge P. The rib  33  protrudes perpendicular to the bottom wall of the cartridge P, and is positioned so that when the cartridge P is mounted into, or removed from the apparatus main assembly  1 A, it can be fitted into the abovementioned groove  32  of the supporting member  27  of the apparatus main assembly  1 A to prevent the cartridge P from moving in the direction other than the direction in which the cartridge P is moved into, or removed from, the apparatus main assembly  1 A. Thus, the width of the rib  33 , that is, the dimension of the rib  33  in terms of the direction perpendicular to the lengthwise direction of the groove  32 , is made less than the width of the groove  32 , providing thereby a proper amount (1 mm-3 mm) of play (gap) between the rib  33  and the walls of the groove  32  to make it easier for the cartridge P (rib  33 ) to be mounted into, or removed from, the apparatus main assembly  1 A, across most of the moving range of the cartridge P in the rear portion of the apparatus main assembly  1 A. However, a portion  33   a  of the rib  33 , which will be at the front end of the apparatus main assembly  1 A after the mounting of the apparatus main assembly  1 A (upstream end portion in terms of cartridge mounting direction), is formed wide enough to provide virtually no play (gap) between the rib  33  and the walls of the groove  32 . Therefore, as the supporting member  27  is moved up into its first position as shown in  FIG. 12 , the drum  3  of the cartridge P is precisely positioned relative to the apparatus main assembly  1 A by the drum bearing positioning portions  29  and  30 . Further, the engagement between the portion  33   a  of the rib  33  and the front portion  32   a  of the groove  32 , which are shown in  FIG. 13 , prevents the portion of the cartridge P, which is on the front side of the apparatus main assembly  1  after the mounting of the cartridge P, from rotationally moving relative to the apparatus main assembly  1 A. To describe in more detail, as the process cartridge P receives the rotational driving force from the apparatus main assembly  1 A, that is, as the coupling  3   a  of the drum  3  receives the driving force from the coupling  51  of the apparatus main assembly  1 A, the process cartridge P is subjected to such a force that works in the direction to press the process cartridge P upon the drum bearing positioning portions  29  and  30 . This force is caught by the contact between the portion  33   a  of the rib  33 , or the third cartridge positioning portion of the cartridge P, and the portion  32   a  of the groove  32 , or the third cartridge positioning portion of the apparatus main assembly  1 A. Therefore, even when the rotational driving force is transmitted from the apparatus main assembly  1 A to the process cartridge P, the process cartridge P remains stable in attitude. That is, the third drum positioning portion  33   a  of the process cartridge P, which is the farthest portion of the process cartridge P from the drum coupling  3   a , and is in the diagonal direction from the drum coupling  3   a , is precisely positioned relative to the apparatus main assembly  1 A by the third cartridge positioning portion  32   a  of the groove  32 . Thus, the problem that as the process cartridge P is subjected to the rotational driving force transmitted to the drum  3 , it is twisted by the force, can be prevented. Therefore, the process cartridge P remains highly precisely positioned relative to the apparatus main assembly  1 A. Further, the third cartridge positioning portion  32   a  is provided as a part of the groove  32  of the supporting member  27 , which is for guiding the process cartridge P when the process cartridge P is mounted into, or removed from, the apparatus main assembly  1 A. In other words, the image forming apparatus and the cartridges therefor in this embodiment are very simple in structure, and yet, can highly precisely position the process cartridges P relative to the apparatus main assembly  1 A. 
     Further, referring again to  FIG. 13 , the process cartridge P is provided with the rotation stopper  7 C, as the fourth cartridge positioning portion of the process cartridge P, which perpendicularly projects outward from the downstream end of the external frame  7  of the cartridge P in terms of the cartridge mounting direction. Thus, as the process cartridge P is mounted into the apparatus main assembly  1 A, this rotation stopper  7 C fits into the elongated hole  41  of the apparatus main assembly  1 A, that is, the fourth cartridge positioning portion of the apparatus main assembly  1 A. Therefore, as the process cartridge P is mounted into the apparatus main assembly  1 A, not only is the front side (upstream side in terms of cartridge mounting direction) of the process cartridge P prevented from rotationally moving, by the contact between the third cartridge positioning portion  33   a  of the process cartridge P and the third cartridge positioning portion  32   a  of the groove  32  of the apparatus main assembly  1 A, but also, the rear side (downstream side in terms of cartridge mounting direction) of the process cartridge P is prevented from rotationally moving, by the contact between the rotation stopper  7 C, that is, the fourth cartridge positioning portion of the process cartridge P, and the wall of the elongated hole  41  of the apparatus main assembly  1 A. Therefore, the process cartridge P is precisely positioned in terms of its attitude relative to the apparatus main assembly  1 A. Incidentally, the elongated hole  41 , into which the rotation stopper  7 C fits, is elongated in such a direction that allows the process cartridge P to be moved by the supporting member  27 . 
     As described above, each supporting member  27  plays the role of a guiding member for preventing the problem that when the cartridge P is mounted into, or removed from, the apparatus main assembly  1 A, it moves in the direction other than the direction in which it is mounted or removed. Further, as the cartridge P is inserted into the apparatus main assembly  1 A, each supporting member  27  supports the cartridge P from underneath, and pushes the cartridge P upward to place the cartridge positioning portions  7 A and  7 B, that is, the drum supporting bearings  7 A and  7 B, in contact with the cartridge positioning (drum positioning) portions  29  and  30  of the apparatus main assembly  1 A. Further, the groove  32  of the supporting member  27  and the rib  33  of the cartridge P are structured so that when the cartridge P is completely in the apparatus main assembly  1 A, there is virtually no gap between the portion  33   a  of the rib  33  of the cartridge P, which will be at the front of the apparatus main assembly  1 A after the proper mounting of the cartridge P into the apparatus main assembly  1 A, and the wall of the front portion  32   a  of the groove  32  of the supporting member  27 . Thus, when the cartridge P is completely in the apparatus main assembly  1 A, the engagement between the groove  32  of the supporting member  27 , and the upstream end portion  33   a  of the rib  33  of the cartridge P and groove  32  prevent the lengthwise end portion (in terms of cartridge mounting direction) of the cartridge P, makes up the means for preventing the upstream end portion of the cartridge P, from rotationally moving relative to the apparatus main assembly  1 A; they play the role of keeping the cartridge P precisely positioned in its image forming position in the apparatus main assembly  1 A. 
     &lt;Supporting Member Moving Mechanism&gt; 
     Next, the supporting member moving mechanism, that is, the mechanism for moving the supporting members  27  upward or downward, and the linkage between the supporting member moving mechanism and front door  2 , will be described. 
     Referring to  FIG. 3 , the first to fourth supporting members  27  ( 27 Y,  27 M,  27 C, and  27 K) are connected to each other by their front end to a single (common) supporting member linking shaft  34  (second rotational shaft, which hereafter may be referred to simply as linking shaft); the front end of each supporting member  27  is connected to the linking shaft  34 ). The linking shaft  34  is disposed so that its axial line C-C is horizontal, that is, perpendicular to the direction in which the cartridges P are moved upward or downward. The linking shaft  34  is rotatably supported by the pair of bearings  26 , with which the front panel  23  of the apparatus main assembly  1 A is provided. Next, referring to  FIG. 12 , the front end portion of each supporting member  27  and the linking shaft  34  are connected to each other by a first arm  35 , which rotates with the linking shaft  34 . More specifically, the front end portion of the supporting member  27  and arm  35  are connected to each other with a connective shaft  36  in such a manner that the supporting member  27  is allowed to rotationally move about the shaft  36 .  FIG. 11  is an enlarged view of one of the arms  35 , and its adjacencies. Referring to  FIGS. 5 ,  11 , and  12 , the rear end portion of the supporting member  27  is connected to the apparatus main assembly  1  with a link  37 . Designated by a reference number  38  is a connective shaft which connects the link  37  to the apparatus main assembly  1 A. Designated by a reference number  39  is a connective shaft which connects link  37  to the rear end portion of the supporting member  27 . The distance between the axial line of the shaft  34  of the arm  35  and the axial line of the shaft  36  of the arm  35  is the same as the distance between the axial line of the shaft  38  of the link  37  and the axial line of the shaft  39  of the link  37 . 
     That is, the supporting member  27 , the connective the shaft  34 , the arm  35 , the link  37 , the shafts  38 , and the shaft  39  form a parallelepipedic linkage. Thus, as the linking shaft  34  is rotated, each supporting member  27  diagonally moves while remaining horizontal. More concretely, as the linking shaft  34  is rotated in the direction to cause the arm  35  to stand upright, the supporting member  27  is moved up into its first position where it holds the cartridge P in the image forming position in the apparatus main assembly  1 A, as shown in  FIG. 12(   a ). Further, referring to  FIG. 12(   b ), as the linking shaft  34  is rotated in the direction to cause the arm  35  to fall frontward of the apparatus main assembly  1 A, the supporting member  27  is moved down into its second position where it holds the cartridge P in the position which allows the cartridge P to be removed from the apparatus main assembly  1 A. Incidentally, the linking shaft  34  moves all the supporting members  27  together. 
     The above-described downward and upward movement of the first to fourth supporting members  27 , which is caused by the supporting member moving mechanism, is linked to (and caused by) the opening and closing of the front door  2 , respectively. 
     That is, referring to  FIG. 5 , the right end of the linking shaft  34  is provided with a linking arm  40  (second arm), which extends roughly perpendicular to the shaft  34 . The linking arm  40  is solidly attached to the shaft  34  so that it rotates with the shaft  34 . Further, the opposite end of the linking arm  40  from the linking shaft  34  is provided with a pin  40   a , which protrudes parallel to the linking shaft  34 . The pin  40   a  is engaged with the front door  2  in such a manner that it is guided by the groove of a linking arm pin guiding member  42 , which is attached to the inward surface of the front door  2 . 
     Thus, as the front door  2 , which is open, is closed, it rotates about the shaft  25 , causing thereby the pin  40   a  protruding from the opposite end of the linking arm  40  from the shaft  26 , to move along the groove of the linking arm pin guiding member  42 . As a result, the linking shaft  34 , to which the linking arm  40  is solidly attached, is rotated in the direction to move the supporting member  27  upward. On the other hand, as the front door, which is closed, is opened, the pin  40   a  is moved along the groove of the linking arm pin guiding member  42 , causing the linking shaft  34 , to which the linking arm  40  is solidly attached, to rotate in the direction to cause the supporting member  27  to move downward. 
     As described above, in this embodiment, the problem that when the process cartridge P is in its image forming position in the apparatus main assembly  1 A, the portion of the process cartridge P, which is on the front side of the apparatus main assembly  1 A, rotationally moves, is prevented by structuring the main assembly of the image forming apparatus and the process cartridges therefor so that the portion  33   a  of the rib  33  of each process cartridge tightly fits in the portion  32   a  groove  32  of the corresponding vertically movable cartridge supporting member  27 . Thus, not only is the cartridge positioning structural arrangement in this embodiment superior to conventional cartridge positioning structural arrangement, for example, the one that uses a large component, such as the front door, to precisely position multiple process cartridges relative to the apparatus main assembly, but also, it makes it possible to make the front door of resinous components, being therefore advantageous in terms of cost to the conventional cartridge positioning structural arrangement. Further, the cartridge positioning structural arrangement in this embodiment is such that the cartridge rotation preventing means moves with the process cartridge(s) as the process cartridge(s) are moved upward or downward. Therefore, the cartridge rotation preventing means does not interfere with the operation (mechanism) for moving the process cartridge(s) upward or downward, making the image forming apparatus in this embodiment more reliable than the image forming apparatuses employing the conventional cartridge positioning structural arrangement. Further, the cartridge rotation preventing means in this embodiment is engaged at the last moment of the push applied to the process cartridge to mount the process cartridge into the main assembly of the image forming apparatus. Thus, the cartridge positioning structural arrangement in this embodiment does not reduce the image forming apparatus in operability. 
     Generally speaking, in consideration of the possibility that as the driving force is transmitted to the drum in a process cartridge, the process cartridge may be twisted by the driving force, a means for preventing a process cartridge from rotationally moving is designed so that only one of the lengthwise end portion of the process cartridge is used to precisely position the process cartridge relative to the main assembly of an image forming apparatus, whereas a small play (gap) is provided between the lengthwise end of the process cartridge and the apparatus main assembly. In comparison, in the case of the cartridge positioning structural arrangement in this embodiment, the tight engagement between the groove  32  of the supporting member  27 , and the front end portion  33   a  (trailing end portion in terms of cartridge insertion direction) of the rib  33  of the process cartridge, is utilized as the cartridge positioning means, whereas a slight play (gap) is provided between the rotation control boss  7 C of the process cartridge, and the elongated hole  41  of the rear panel of the apparatus main assembly, in order to prevent the cartridge rotation preventing means on the rear side from interfering with the upward or downward movement of the supporting member  27  (cartridge). 
     Therefore, it is ensured that while the image forming apparatus is in an image forming operation, the external frame  7  (shell) of the cartridge P, and the processing means, that is, the charging means  4 , the developing means  5 , and the cleaning means  6 , which are in the external frame  7 , remain precisely aligned relative to the drum  3 , making it possible for the image forming apparatus to continuously output images of excellent quality. 
     Embodiment 2 
     The image forming apparatus in the second preferred embodiment of the present invention is similarly structured to the image forming apparatus in the first preferred embodiment, except that its cartridge rotation preventing means on the downstream end of each cartridge P is the same as that on the upstream end. That is, it is made up of the groove with which the cartridge supporting member is provided, and the rib which is protruding from the cartridge P. 
       FIG. 14  is a perspective view of one of the cartridge supporting members  43  in the second preferred embodiment of the present invention.  FIGS. 15(   a ) and  15 ( b ) are a combination of a top view of the cartridge supporting member and a phantom top view of the cartridge thereon, in the second preferred embodiment, in the final stage of the mounting of the process cartridge, and after the mounting of (precise positioning) of the process cartridge, respectively.  FIGS. 15(   a ) and  15 ( b ) show what occur toward the end of the mounting of the cartridge P into the apparatus main assembly. 
     Each cartridge supporting member  43  in this embodiment is equivalent to each cartridge supporting member  27  in the first preferred embodiment. It is provided with a groove  44 , which matches in position to the rib  33  which protrudes from the corresponding cartridge P, as is the supporting member  27 . The rear end portion  44   a  of the groove  44  is narrower than rest of the groove  44 , and is just wide enough for the rib  33  to fit with virtually no play (gap). The cartridge positioning (rotational movement preventing) means on the front end of the apparatus main assembly is the same as that in the first embodiment; the portion  33   a  of the rib  33  fits into the groove  44  with virtually no play (gap). It should be noted here, regarding the cartridge positioning means in this embodiment, that unlike the process cartridge in the first preferred embodiment, the process cartridge in this embodiment is not provided with the rotational movement preventing boss  7 C which protrudes from the downstream (in terms of cartridge mounting direction) wall of the process cartridge, and also, that unlike the rear panel  28  of the apparatus main assembly in the first embodiment, the rear panel  28  of the apparatus main assembly in this embodiment is not provided with the elongated holes  41 , which match in position the bosses  7 C, one for one. 
     In this embodiment, both the means for preventing the downstream end portion of the cartridge P (external frame  7 ) from rotationally moving, and the means for preventing the upstream end portion of the cartridge P (external frame  7 ) from rotationally moving are made up of the rib  33  of the cartridge P and the groove  44  of the supporting member  43 . Thus, there are no portions of the cartridge P which rub against the apparatus main assembly  1 A when the supporting member  43  is moved upward or downward, and the engagement between the rib  33  and groove  44  does not interfere at all with the upward or downward movement of the supporting member  43  (and cartridge P thereon). Further, since the rear panel  28  of the apparatus main assembly does not need to be provided with the elongated holes  41 , more spatial latitude is afforded. That is, in this embodiment, the fourth cartridge (drum) positioning portion of the apparatus main assembly is a part of the supporting member  43 , and the fourth cartridge (drum) positioning portion of the cartridge P, which engages with the fourth cartridge (drum) positioning portion of the apparatus main assembly, is the end portion (downstream end portion in terms of cartridge mounting direction) of the rib  33  which is on the bottom surface of the cartridge P. Further, the play (gap) between the third cartridge positioning portion  33   a  of the cartridge P and the third cartridge positioning portion of the  32   a  of the apparatus main assembly  1 A (supporting member  43 ) is smaller than that between the fourth cartridge positioning portion of the apparatus main assembly  1 A, and the fourth cartridge positioning portion of the cartridge P, which fits into the fourth cartridge positioning portion of the apparatus main assembly  1 A (supporting member  43 ). Otherwise, the cartridge positioning structural arrangement (cartridge rotation preventing structural arrangement) in this embodiment is the same as that in the first preferred embodiment, and further, its effects are the same as those in the first preferred embodiment. 
     Embodiment 3 
     The image forming apparatus in the third preferred embodiment of the present invention is similar to the image forming apparatus in the first preferred embodiment of the present invention described above, except that the image forming apparatus in this embodiment is opposite from that in the first embodiment in the positional relationship between the rib and groove, which controls the mounting and removal of the cartridge P. That is, in this embodiment, each cartridge P is provided with the cartridge guiding groove, and the corresponding cartridge supporting member is provided with a cartridge guiding rib, which is positioned so that it protrudes toward the cartridge supporting member when the cartridge is in the apparatus main assembly. 
       FIG. 16  is a perspective view of one of the cartridge supporting members in the third preferred embodiment of the present invention.  FIG. 17  is an external perspective view of one of the cartridges for the image forming apparatus in this embodiment, as seen from diagonally below the lengthwise end of the cartridge, from which the cartridge is driven.  FIG. 18  is a schematic sectional view of one of the cartridge supporting members in this embodiment, at the plane coinciding with the top surface of the supporting member, when the supporting member is in the second position into which it was lowered. It shows what occurs to the relationship between the cartridge positioning means of the cartridge and the cartridge positioning means of the apparatus main assembly  1 A, toward the end of the mounting of the cartridge into the apparatus main assembly  1 A. 
     Each of the cartridge supporting members  45  in this embodiment is equivalent to the cartridge supporting members  27  in the first embodiment. Unlike the cartridge supporting member  27  in the first embodiment, the cartridge supporting member  45  in this embodiment is provided with a cartridge movement controlling rib  46  instead of the cartridge movement controlling groove. The rib  46  extends in the lengthwise direction of the cartridge supporting member, and is positioned on the surface of the supporting member, which faces upward when the cartridge is properly positioned in the apparatus main assembly  1 A. Further, each of the cartridges P in this embodiment is provided with a cartridge guiding groove  47  instead of the cartridge guiding rib  33  in the first embodiment. The cartridge guiding groove  47  is positioned so that when the cartridge P is properly positioned in the apparatus main assembly  1 A, the cartridge guiding groove  47  opposes the cartridge movement controlling rib  46 . The guiding groove  47 , except for the downstream end portion  47   a  (in terms of cartridge mounting direction), is given such a width that is wide enough to provide a proper amount of play (gap) (roughly 1-3 mm) for allowing the cartridge P to be smoothly mounted or removed. The downstream end  47   a  of the groove  47  is made narrower than the rest so that when the downstream end portion of the rib  46  is in the downstream end  47   a  of the guiding groove  47 , there is virtually no play (gap) between the rib  46  and the wall of the downstream end  47   a  of the guiding groove  47 , so that the engagement between the rib  46  and the downstream end  47   a  of the guiding groove  47  makes up the means for preventing the downstream end portion of the cartridge P from rotationally moving. 
     In the case of the image forming apparatuses in the preferred embodiments of the present invention described above, the image bearing member was an electrophotographic photosensitive member. However, the present invention is also compatible with an image forming apparatus, the image bearing member of which is an electrostatically recordable dielectric member or a magnetically recordable magnetic member. 
     Further, the present invention is also compatible with an image forming apparatus which employs a recording medium conveying belt (instead of the intermediary transfer belt) for conveying a sheet S of recording medium, so that the toner images formed on the image bearing members in the cartridges, one for one, are sequentially transferred in layers onto the sheet S of recording medium. 
     As described above, according to the present invention, it is possible to provide an image forming apparatus which is structured so that a process cartridge is removably mountable in the main assembly of the image forming apparatus in the direction parallel to the axial line of the photosensitive drum in the cartridge, and which is characterized in that it is simple in structure, and yet, can highly precisely position, and keep highly precisely positioned, the process cartridge relative to the main assembly of the image forming apparatus. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 135684/2008 filed May 23, 2008 which is hereby incorporated by reference.