Abstract:
An electrophotographic image forming apparatus includes a main assembly; a cartridge including process means actable on an electrophotographic photosensitive member, the process cartridge being detachably mountable relative to the main assembly; an opening, provided in the main assembly of the apparatus, for permitting mounting and demounting of the cartridge; an openable member, provided in the main assembly of the apparatus, for openably closing the opening; an exposure device, provided on the openable member, for exposing the electrophotographic photosensitive member; the cartridge including a first unit, and a second unit pivotably connected with the first unit; a first positioning portion provided in the main assembly of the apparatus, wherein the exposure device is provided with a second positioning portion for positioning between the exposure device and the second unit of the cartridge by engagement with the cartridge by closing the openable member.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an electrophotographic image forming apparatus which forms an image on recording medium while one or more removably mountable process cartridges are in the main assembly of the apparatus. 
         [0002]    In this specification, an “electrophotographic image forming apparatus” means an apparatus which forms an image on recording medium with the use of an electrophotographic image formation process. Its examples include an electrophotographic copying machine, an electrophotographic printer (laser beam printer, LED printer, etc.), a facsimile apparatus, a word processor etc. “Recording medium” means medium on which an image can be formed with the use of an electrophotographic image forming apparatus. It includes ordinary paper, OHP sheet, etc. 
         [0003]    Further, a “cartridge” means a process cartridge or a development cartridge. That is, it means a cartridge which contributes to a process for forming an image on recording medium by being removably mounted in the main assembly of an electrophotographic image forming apparatus. More specifically, it means a process cartridge which is removably mountable in the main assembly of an electrophotographic image forming apparatus. It comprises: an electrophotographic photosensitive member; at least one processing means among a charging means, a developing means, a cleaning means; and a cartridge in which the electrophotographic photosensitive member and processing means are integrally placed. In other words, a process cartridge includes a cartridge in which an electrophotographic photosensitive member is integrally placed along with at least one among a charging means, a developing means, or a cleaning means so that they can be removably mountable in the main assembly of an image forming apparatus. Incidentally, a process cartridge which has an electrophotographic photosensitive member and a developing means is referred to as a process cartridge of the integral type. Further, a process cartridge which has an electrophotographic photosensitive member and one or more processing means other than a developing means is referred to as a process cartridge of the separation type. 
         [0004]    A process cartridge is removably mountable in the main assembly of an image forming apparatus by a user himself or herself. Thus, a process cartridge makes it easier to maintain an image forming apparatus. Incidentally, processing means are means for processing an electrophotographic photosensitive member. 
         [0005]    Further, a development cartridge has a development roller. It contains developer (toner) to be used by the development roller to develop an electrostatic latent image on an electrophotographic photosensitive member. It also is removably mountable in the main assembly of an electrophotographic image forming apparatus. In the case of an electrophotographic image forming apparatus which uses a development cartridge, its electrophotographic photosensitive member is a part of the main assembly of the image forming apparatus, or a part of a process cartridge of the so-called separation type (which does not have developing means). A development cartridge also can be removably mountable in the main assembly of an electrophotographic image forming apparatus by a user himself or herself. Thus, it also makes it easier to maintain an electrophotographic image forming apparatus. 
         [0006]    That is, a “cartridge” includes both a process cartridge of the so-called integration type and a process cartridge of the so-called separation type. It also includes a process cartridge which is employed in combination with a process cartridge of the so-called separation type by an electrophotographic image forming apparatus. Further, it includes a development cartridge which is removably mountable in the main assembly of an image forming apparatus whose electrophotographic photosensitive member is an integral part of the main assembly, to process the electrophotographic photosensitive member. 
         [0007]    In the field of an electrophotographic image forming apparatus, that is, an image forming apparatus which uses an electrophotographic image formation process, it has become common practice to employ a cartridge system, that is, a system which makes it possible to removably mount a cartridge in the main assembly of an electrophotographic image forming apparatus. As a cartridge of this type, a process cartridge, which integrally comprises an electrophotographic photosensitive drum (which hereafter will be referred to simply as photosensitive drum) and processing means for processing the photosensitive drum, has been known. 
         [0008]    Further, image forming apparatuses of the so-called inline type have been known, which employ two or more process cartridges which are placed in line with each other in the main assembly of the apparatus. One of the image forming apparatuses of the so-called inline type is an electrophotographic image forming apparatus which employs four process cartridges for yellow, magenta, cyan, and black colors, one for one. It forms a full-color image by forming a monochromatic image on the photosensitive drum in each process cartridge by outputting a beam of laser light from its laser scanner unit, and layering the four monochromatic images which are different in color. 
         [0009]    It has been known that some of the above-described image forming apparatuses are structured so that process cartridges are removably mountable in the main assembly of the image forming apparatus by opening a cover (door) with which the main assembly is provided. Japanese Laid-open Patent Application 2008-216762 discloses an image forming apparatus structured so that as the cover (door) of the image forming apparatus is opened for the mounting of process cartridges into the main assembly, or removal of the process cartridges from the main assembly, the laser scanner of the image forming apparatus is moved out of the way to allow the process cartridges to be mounted into the main assembly or removed from the main assembly, through the opening exposed by the opening of the cover, and also, that as the cover is closed, the laser scanner unit is moved back into its operational position. 
       SUMMARY OF THE INVENTION 
       [0010]    However, in the case of the image forming apparatus disclosed in the abovementioned patent application, the process cartridges and laser scanner are positioned relative to the main assembly of the image forming apparatus, independently from each other. Therefore, it is necessary to provide the image forming apparatus with a space for preventing the process cartridges from interfering with the beam of laser light outputted from the laser scanner unit. Thus, the image forming apparatus is unsatisfactory from the standpoint of spatial efficiency with which its internal components are positioned in the main assembly. In other words, the employment of the above-described structural arrangement is likely to increase an image forming apparatus in size. 
         [0011]    Thus, the primary object of the present invention is to provide such an innovative design for an image forming apparatus of the separation type that can reduce an image forming apparatus of the separation type in size by minimizing the image forming apparatus in the space necessary to prevent a process cartridge from interfering with the beam of laser light outputted from the laser scanner unit of the image forming apparatus, and improving thereby the image forming apparatus in terms of the spatial efficiency with which the internal components of the apparatus can be positioned in the main assembly of the apparatus. 
         [0012]    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 THE DRAWINGS 
         [0013]      FIG. 1(   a ) is a perspective view of the electrophotographic image forming apparatus in the first preferred embodiment of the present invention, and  FIG. 1(   b ) is a sectional view of the electrophotographic image forming apparatus in  FIG. 1(   a ). 
           [0014]      FIG. 2A  is a sectional view of the electrophotographic image forming apparatus in the first preferred embodiment of the present invention when its cartridge placement door is open, and  FIG. 2B  is a sectional view of the electrophotographic image forming apparatus in the first preferred embodiment of the present invention when a couple of the process cartridges are being removed from the main assembly of the image forming apparatus.  FIG. 2C  is a perspective view of the combination of the development units, photosensitive member units, and intermediary transfer unit of the image forming apparatus shown in  FIGS. 2(   a ) and  2 ( b ). 
           [0015]      FIG. 3(   a ) is a perspective view of one of the process cartridges in the first preferred embodiment of the present invention, and  FIG. 3(   b ) is a sectional view of the process cartridge in  FIG. 3(   a ).  FIG. 3(   c ) is a side view of the process cartridge in  FIG. 3(   a ). 
           [0016]      FIG. 4(   a ) is a schematic sectional view of the development roller and photosensitive drum of the image forming apparatus in the first preferred embodiment of the present invention when the development roller is in contact with the photosensitive drum, and  FIG. 4(   b ) is a schematic sectional view of the development roller and photosensitive drum of the image forming apparatus in the first preferred embodiment of the present invention when the development roller is not in contact with the photosensitive drum. 
           [0017]      FIGS. 5(   a ) and  5 ( b ) are perspective views of the combination of the intermediary transfer unit, process cartridges, apparatus frame, laser scanner unit, and door of the image forming apparatus in the first preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus. 
           [0018]      FIG. 6  is a side view of the combination of the process cartridges and laser scanner unit, and is for describing the cartridge positioning mechanism of the apparatus. 
           [0019]      FIGS. 7(   a ) and  7 ( b ) are perspective views of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the second preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus. 
           [0020]      FIG. 8(   a ) is a perspective view of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the third preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus.  FIG. 8(   b ) is a side view of view of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the third preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus. 
           [0021]      FIG. 9(   a ) is a perspective view of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the fourth preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus, and  FIG. 9(   b ) is a side of view of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the fourth preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus. 
           [0022]      FIG. 10  is a side view of the combination of the process cartridges and laser scanner unit in the fourth preferred embodiment of the present invention, and is for describing the mechanism for keeping the development unit rotation preventing portions pressured. 
           [0023]      FIG. 11  is a side view of the combination of one of the process cartridges, and the development unit rotation preventing portion of the laser scanner unit, and is for describing the mechanism for keeping the development unit rotation portion pressured, which is in the state in which the process cartridge and development unit rotation preventing portion are in contact with each other. 
           [0024]      FIG. 12(   a ) is a perspective view of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the fifth preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus.  FIG. 12(   b ) is a side view of view of the combination of the intermediary transfer unit, process cartridges, laser scanner unit, and door of the image forming apparatus in the fifth preferred embodiment of the present invention, and is for describing the cartridge positioning mechanism of the apparatus. 
           [0025]      FIG. 13  is a side view of the combination of the process cartridges and laser scanner unit in the fifth preferred embodiment of the present invention, and is for describing the mechanism for opening or closing the laser beam outlet shutter of the laser scanner unit. 
           [0026]      FIG. 14(   a ) is a sectional view of the image forming apparatus which is in one of the miscellaneous embodiments of the present invention and is structured so that its door for mounting or removing process cartridges opens upward, and which is in the state in which the door is in its closed position.  FIG. 14(   b ) is a sectional view of the image forming apparatus in  FIG. 14(   a ) when the door is open and a couple of the process cartridges are being moved out of the main assembly. 
           [0027]      FIG. 15(   a ) is a perspective view of the process cartridge, which is in another of the miscellaneous embodiment of the present invention, and the development unit and photosensitive drum unit of which are independently replaceable from each other and are in engagement with each other.  FIG. 15(   b ) is a perspective view of the process cartridge in  FIG. 15(   a ) when the development unit and photosensitive drum unit are not in engagement with each other. 
           [0028]      FIG. 16  is a side view of the combination of the door, laser scanner unit, process cartridges, and intermediary transfer unit of the image forming apparatus which is in the abovementioned miscellaneous embodiment of the present invention and employs the process cartridges, the development unit and photosensitive drum unit of which are independently replaceable from each other, and is for describing the process cartridge positioning mechanism of the image forming apparatus. 
           [0029]      FIGS. 17(   a ) and  17 ( b ) are perspective and side views, respectively, of the combination of the process cartridges and intermediary transfer unit of the image forming apparatus which is in the abovementioned miscellaneous embodiment of the present invention and employs the process cartridges, the development unit and photosensitive drum unit of which are independently replaceable from each other. They are for describing the cartridge positioning mechanism of the apparatus and depict the combination when a couple of the process cartridges are not in engagement with the intermediary transfer unit. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Hereinafter, the preferred embodiments of the present invention are described with reference to the electrophotographic image forming apparatuses in accordance with the present invention. The measurements, materials, and shapes of the structural components of each of the electrophotographic image forming apparatuses in the following preferred embodiments of the present invention, and their positional relationship, are to be modified as necessary according to the structure of the image forming apparatus to which they belong, and conditions under which they are operated. In other words, the following preferred embodiments are not intended to limit the present invention in scope unless specifically noted. 
       Embodiment 1  
       [0031]    First, referring to  FIGS. 1-6 , the electrophotographic image forming apparatus in the first preferred embodiment of the present invention is described.  FIGS. 1 and 2  are drawings for describing the electrophotographic image forming apparatus in the first preferred embodiment. The electrophotographic image forming apparatus in this embodiment is a laser beam printer. Thus, the general structure and function of the image forming apparatus which will be described next are those of the laser beam printer. 
         [0032]    Incidentally, in the following description of the image forming apparatuses in accordance with the present invention, the front side of the main assembly of the apparatus means the side from which an operator operates the image forming apparatus. Thus, the opposite side of the main assembly of the apparatus from the front side of the main assembly is the rear side (back side) of the main assembly as seen from the operator. 
       [General Description of Image Forming Apparatus] 
       [0033]      FIG. 1(   a ) is an external perspective view of the image forming apparatus.  FIG. 1(   b ) is a sectional view of the image forming apparatus. Referring to  FIGS. 1(   a ) and  1 ( b ), the image forming apparatus is a full-color laser beam printer, which uses an electrophotographic process, and is based on four primary colors. The image forming apparatus forms images on recording medium (recording paper) in response to electrical image formation signals inputted from an external host apparatus (unshown), such as a personal computer, an image reader, a facsimile apparatus (from which images are sent). 
         [0034]    The main assembly  100  of the image forming apparatus comprises: a laser scanner unit  10 ; process cartridges P (PY, PM, PC, and PK) which are removably mountable in the main assembly  100 ; an intermediary transfer unit  40 ; a fixing apparatus  50 ; a sheet feeding-and-conveying unit  60 ; etc. 
         [0035]    The sheet feeding-and-conveying unit  60  has a sheet feeder cassette  62 , a sheet feeder roller  61 , a separation pad  63 , etc. The sheet feeder cassette  62  is removably mountable in the main assembly of the image forming apparatus, from the front side of the main assembly (front loading). The sheet feeder roller  61  is rotated in the counterclockwise direction (indicated by arrow mark a in  FIG. 1(   b )) with preset control timing. As the sheet feeder roller  61  is rotated, one of the layered recording medium sheets S in the sheet feeder cassette  62  is fed into the main assembly, while being separated from the rest, by the coordination between the sheet feeder roller  61  and separation pad  63 . Then, the recording medium sheet S is conveyed to the nip between the intermediary unit  40  and a second transfer roller  41 , by a pair of recording sheet conveyance rollers  64 . 
         [0036]    The photosensitive drum  21  is being rotated in the clockwise direction (indicated by arrow mark b in  FIG. 1(   b )). As the photosensitive drum  21  is rotated, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum  21  by the beam L of laser light outputted from the exposing portion  16  of laser scanner unit  10 . Then, the electrostatic latent image on the photosensitive drum  21  is developed by a development roller  32  into a visible image, that is, an image formed of toner (toner image). 
         [0037]    After the formation of the toner image on the photosensitive drum  21 , the toner image is transferred (first transfer) onto the intermediary transfer unit  40 . In a case where a full-color image is formed, yellow, magenta, cyan, and black monochromatic images are developed on the four photosensitive drums  21  one for one, and are sequentially transferred (first transfer) onto the intermediary transfer unit  40 . 
         [0038]    After the transfer of the toner images onto the intermediary transfer unit  40 , the toner images are transferred (second transfer) onto the recording sheet S as the recording sheet S is conveyed through the nip between the intermediary transfer unit  40  and a second transfer roller  41 . The first transfer residual toner, that is, the toner remaining on the peripheral surface of the photosensitive drum  21  after the transfer of the toner image onto the intermediary transfer unit  40 , is removed by a cleaning device  22 , and is stored in the waste toner chamber  27  ( FIG. 3(   b )) of a photosensitive drum unit  20 . 
         [0039]    After the transfer of the toner images onto the recording sheet S, the sheet S is sent to the nip between the fixation film  51  and pressure roller  52  of a fixing apparatus  50  so that the toner images on the sheet S are fixed to the sheet S by being heated and pressed in the nip. After the fixation of the toner images to the sheet S in the nip, the sheet S is discharged into a delivery tray  70  by a pair of discharge rollers  71 . 
         [0040]    The second transfer residual toner, that is, the toner remaining on the surface of the intermediary transfer unit  40  after the transfer (second transfer) of the toner images onto the recording sheet S, is electrostatically adhered to the peripheral surface of the photosensitive drum  21 , in the first transfer portion of the process cartridge PY, for example, and then, is removed by the cleaning device  22 . After being removed by the cleaning device  22 , the second transfer residual toner is stored in the waste toner chamber  27 . 
         [0041]    Each process cartridge has an integral combination of an electrophotographic photosensitive member and processing means (processing apparatus or processing apparatuses). In this embodiment, each process cartridge P has the photosensitive drum unit  20  (first unit) and development unit  30  (second unit), as shown in  FIGS. 3(   a ) and  3 ( b ). 
         [0042]    The photosensitive drum unit  20  has a photosensitive drum  21 , which is an electrophotographic photosensitive member. It has also: a charge roller  24  (charging means) which is a processing means for processing the photosensitive drum  21 ; the cleaning device  22  (cleaning means) for removing the developer T remaining on the peripheral surface of the photosensitive drum  21 ; and a photosensitive member case  23 . The image forming apparatus in this embodiment uses multiple (four) photosensitive drum units (one for each process cartridge), which are mounted into the main assembly of the image forming apparatus in such a manner that they are vertically stacked in contact with the intermediary transfer unit  40 , with the presence of preset intervals. 
         [0043]    The development unit  30  is in engagement with the photosensitive drum unit  20 , being enabled to be rotationally moved relative to the photosensitive drum unit  20  about its rotational axle. The development unit  30  has: a development device case  31  in which the developer T to be supplied to the photosensitive drum  21  is stored; a development roller  32  (developing means) which supplies the photosensitive drum  21  with the developer T; and a supply roller  33  which supplies the development roller  32  with the developer T in the developing device case  31 . 
         [0044]    The development roller  32  may be in contact with the photosensitive drum  21  as shown in  FIG. 4(   a ), or not in contact with the photosensitive drum  21  as shown in  FIG. 4(   b ). When the development roller  32  is not in contact with the photosensitive drum  21  as shown in  FIG. 4(   b ), a pair of spacers  37 , which are in engagement with the lengthwise ends of the development roller  32 , one for one, keep a preset distance between the development roller  32  and photosensitive drum  21  by being in contact with the photosensitive drum  21 . 
         [0045]    The laser scanner unit  10  (exposing apparatus) is on the front side of the group of the vertically stacked process cartridges PY, PM, PC, and PK; it is in the front portion of the apparatus main assembly. The laser scanner unit  10  scans (exposes) the peripheral surface of each photosensitive drum  21  with the beam L of laser light which it outputs while modulating the beam L according to the information regarding the images (one monochromatic image per primary color) to be formed, which is inputted from the external host apparatus. 
       [General Description of Process Cartridge] 
       [0046]    Next, the process cartridges PY, PM, PC, and PK, which are removably mountable in the main assembly  100  of the image forming apparatus, are described. 
         [0047]      FIG. 3(   a ) is an external perspective view of one of the process cartridges. The process cartridges PY, PM, PC, and PK have a mechanism for carrying out the electrophotographic image formation process. They are different only in the color of the toner they contains, and the amount of the toner therein. 
         [0048]    The left and right directions of the process cartridges PY, PM, PC, and PK is parallel to the axial line of the photosensitive drum  21  in each cartridge. They are long and narrow assemblies, the lengthwise direction of which coincides with their left and right direction. The photosensitive drum  21  is rotatably supported at its lengthwise ends by a pair of bearings (unshown) which are at the right and left end of the process cartridge frame, one for one. The position of the photosensitive drum  21  relative to the intermediary transfer unit  40  is determined by the engagement between the rotational axle  21   a  of the photosensitive drum  21  and one of the four pairs of photosensitive drum positioning portions  43  (electrophotographic photosensitive member positioning portions) of the intermediary transfer unit  40 . Incidentally, in this embodiment, the photosensitive drum positioning portions (electrophotographic photosensitive member positioning portions) belong to the intermediary transfer unit  40 . However, this setup is not intended to limit the present invention in scope. For example, the image forming apparatus may be structured so that the electrophotographic photosensitive member positioning portions are parts of the intermediary transfer unit supporting portion of the main assembly  100  of the image forming apparatus. 
         [0049]    One of the lengthwise ends of the abovementioned rotational axle  21   a  of the photosensitive drum  21  has a drum driving gear  25 , through which the photosensitive drum  21  is driven, and therefore, may be referred to as the drive side hereafter. The corresponding lengthwise end of the rotational axle  32   a  of the development roller  32  has a development roller driving gear  36 , through which the development roller  32  is driven. Hereafter, the left end of the process cartridge, that is, the opposite end from the drive side, has electrical contacts (unshown). 
         [0050]    The force for driving the photosensitive drum  21  and development roller  32  is transmitted to the photosensitive drum  21  and development roller  32  through the meshing of the photosensitive drum driving gear  25  and development roller driving gear  36  of each of the process cartridges PY, PM, PC, and PK, and the corresponding driving force output gears (unshown) of the main assembly  100  of the image forming apparatus. 
         [0051]      FIG. 3(   b ) is a sectional view of the opposite end portion of one of the process cartridges from the drive side, at a vertical plane perpendicular to the axial line of the photosensitive drum.  FIG. 3(   c ) is a side view of the opposite end of the one of the process cartridges from the drive side. The photosensitive drum  21  is rotated by the driving force which it receives from the driving force output gear (unshown) of the main assembly  100  of the image forming apparatus. The charge roller  24  is of the contact type. It is placed in contact with the photosensitive drum  21  to charge the photosensitive drum  21 , and is rotated by the rotation of the photosensitive drum  21 . The cleaning device  22  is a blade formed of a piece of elastic rubber, and is in contact with the peripheral surface of the photosensitive drum  21  in such a manner that its cleaning edge is on the upstream side of its base in terms of the rotational direction of the photosensitive drum  21 . The cleaning device  22  plays the role of removing the toner remaining on the peripheral surface of the photosensitive drum  21  after the first transfer of the toner image. After being removed by the cleaning device  22 , the transfer residual toner is stored in the waste toner chamber  27  in the photosensitive member case  23 . 
         [0052]    The development unit  30  has: the development roller  32  which is a developing means; and the development blade  35 . Referring to  FIG. 3(   b ), the functional edge of the development blade  35  is in contact with the peripheral surface of the development roller  32 . The development blade  35  plays the role of forming the toner borne on the peripheral surface of the development roller  32 , into a thin layer of toner. Referring to  FIG. 3(   c ), the photosensitive drum unit  20  has a pair of development roller positioning holes, which are at the lengthwise ends of the photosensitive drum unit  20 , one for one. The development roller  32  is supported by the photosensitive drum unit  20 . More specifically, the lengthwise ends of the rotational axle  32   a  of the development roller  32  are in the abovementioned development roller positioning holes, and therefore, the development roller  32  is properly positioned relative to the photosensitive drum  21 . 
       [General Description of Cartridge Positioning Method] 
       [0053]      FIGS. 2(   a ) and  2 ( b ) are sectional views of the main assembly  100  of the image forming apparatus in this embodiment, and are for describing the method for replacing the process cartridges PY, PM, PC, and PK.  FIG. 2C  is a perspective view of the main assembly unit, and the process cartridges held by the main assembly unit. In  FIG. 2C , the process cartridges to be replaced are being removed. It is for describing the mechanical structure for properly positioning the development cartridges as they are mounted into the main assembly unit. 
         [0054]    The door  101 , which can be rotationally opened or closed about its hinge  102 , is on the front side (user side) of the main assembly  100  of the image forming apparatus. The door  101  exposes or covers the opening  104  of the apparatus main assembly  100 . The opening  104  is for allowing the process cartridges P to be mounted into, or removed from, the apparatus main assembly  100 . In order to open the door  101 , the door  101  is to be rotated about the hinge  102  in the direction indicated by an arrow mark d in  FIG. 2A . The laser scanner unit  10  is on the door  101 , and is on the front side (user side) of the process cartridges PY, PM, PC, and PK in the apparatus main assembly  100 . Thus, as the door  101  is rotationally opened, the laser scanner unit  10  is also rotationally moved frontward, exposing thereby the opening  104  of the apparatus main assembly  100 . Therefore, it becomes possible to access the process cartridges PY, PM, PC, and PK in the apparatus main assembly  100 . That is, it becomes possible to remove the process cartridges PY, PM, PC, and PK in the direction indicated by an arrow mark e in  FIG. 2B . 
         [0055]    When it is necessary to mount the process cartridges PY, PM, PC, and PK into the apparatus main assembly  100 , the above described sequence for removing the process cartridges P is to be carried out in the reverse order. That is, the door  101  is to be opened, and then, the process cartridges PY, PM, PC, and PK are to be mounted into the apparatus main assembly  100 . 
         [0056]      FIGS. 5(   a ) and  5 ( b ) are perspective views of the combination of the intermediary transfer unit  40 , process cartridges P, main frame  15  of the apparatus main assembly  100 , laser scanner unit  10 , and door  101  of the image forming apparatus when the door  101  is open to replace one or more of the process cartridges P and when the door  101  is closed, respectively. Since the process cartridges PY, PM, PC, and PK are the same in structure, one of them is described as a process cartridge P. 
         [0057]    The laser scanner unit  10  has multiple pairs of rotation preventing portions  12  for stopping the rotation of the process cartridges P. Each of the rotation preventing portions  12  of the laser scanner unit  10  is the positioning means (second positioning portion) for properly positioning a process cartridge P relative to the laser scanner unit  10  relative to each other. As the door  101  is closed, each pair of rotation preventing portions  12  engage with the corresponding process cartridge P. More specifically, the laser scanner unit  10  has four pairs of rotation preventing portions  12 , each pair of rotation preventing portions  12  being at the ends of the laser scanner unit  40 , one for one, in terms of the direction perpendicular to the direction in which the process cartridge P is mounted into, or removed from, the apparatus main assembly  100 . 
         [0058]    The process cartridge P has a pair of development unit rotation preventing portions  34  as cartridge positioning portion, which engage with the rotation preventing portions  12  of the laser scanner unit  10 . Each pair of the development roller rotation preventing portions  34  are at the lengthwise ends of the development unit  30 , one for one, of each process cartridge P. They prevent the process cartridge P from rotationally moving, by engaging with the rotation preventing portion  12  of the laser scanner unit  10 . 
         [0059]    The main assembly  100  of the image forming apparatus has a pair of laser scanner unit positioning portions  103 , which are the positioning means for properly positioning the laser scanner unit  10  relative to the apparatus main assembly  100 . The laser scanner unit positioning means  103  are parts of the frame  15  of the apparatus main assembly  100 , and engage with the laser scanner unit  10 . The two laser scanner unit positioning portions  103  are in the end portions of the frame  15  in terms of the direction perpendicular to the direction in which the process cartridge P is mounted into, or removed from, the apparatus main assembly  100 . One of them is a round hole, and the other is a rectangular hole. 
         [0060]    The laser scanner unit  10  is held by the door  101 , and is movable within a preset range. The laser scanner unit  10  has a pair of laser scanner unit positioning portions  11 , which are the third positioning portions. As the door  101  is closed, the pair of laser scanner unit positioning portions  11  engage into the laser scanner unit positioning portions of the frame  10 , being thereby properly positioned relative to the apparatus main assembly  100 . 
         [0061]    As the door  101  is closed, the laser scanner unit positioning portions.  103  of the frame  15  of the main assembly  100  engage with the laser scanner unit positioning portions  11  of the laser scanner unit  10 , one for one, whereby the laser scanner unit  10  is properly positioned relative to the apparatus main assembly  100 . Incidentally, one of the laser scanner unit positioning portions  103  of the apparatus main assembly  100  is a round hole, and the other is a rectangular hole, as described above. Therefore, the laser scanner unit  10  is accurately positioned relative to the main assembly  100  of the image forming apparatus. 
         [0062]    As the door  101  is closed, each pair of rotation preventing portions  12 , which are at the ends of the laser scanner unit  10 , one for one, engage with the pair of development unit rotation preventing portions of the development unit  30  of the corresponding process cartridge P. Thus, the development unit  30  of the process cartridge P is properly positioned relative to the laser scanner unit  10 , ensuring that a clear path is provided between the laser scanner unit  10  and the peripheral surface of the photosensitive drum  21 , for the beam L of laser light outputted from the laser scanner unit  10 , in spite of a very small clearance between the adjacent two process cartridges P. 
         [0063]    As the rotational axle  21   a  of the photosensitive drum  21  of the process cartridge P engages with the photosensitive drum positioning portions  43  (first positioning portions) of the intermediary transfer unit  40 , the photosensitive drum  21  is properly positioned relative to the intermediary transfer unit  40  ( FIG. 2C ). Then, the door  101  is to be closed. As the door  101  is closed, the rotation preventing portions  12  of the laser scanner unit  10  engage with the development unit rotation preventing portions  34  of the process cartridge P, whereby the process cartridge P is properly positioned relative to the laser scanner unit  10 . That is, the process cartridge P is properly position relative to the laser scanner unit  10  by the photosensitive drum positioning portions of the apparatus main assembly  100  (which engage with the photosensitive drum) and the rotation preventing portions  12  of the laser scanner unit  10 . 
         [0064]    Further, one of the pair of rotation preventing portions  12  which are at the ends of the laser scanner unit  10  engages with the corresponding development unit rotation preventing portion  34  of the process cartridge P with the presence of a small gap between the two rotation preventing portions  12  and  34 . The other of the pair of rotation preventing portions  12  engages with the other of the corresponding pair of development unit rotation preventing portions  34 . Shown in  FIG. 6  is the state of engagement between one of the rotation preventing portion  12  and the corresponding development unit rotation preventing portion  34 , with the presence of gap between the two rotation preventing portions  12  and  34 . As will be evident from  FIG. 6 , when one of the rotation preventing portion  12  is in engagement with the corresponding development unit rotation preventing portion  34 , the top side of the outward end portion of the rotation preventing portion  12  is in contact with the top side of the outward portion of the development unit rotation preventing portion  34 , but, the bottom side of the outward end portion of the rotation preventing portion  12  is not in contact with the bottom side of the outward portion of the development unit rotation preventing portion  34 . Designing the rotation preventing portions  12  of the laser scanner unit  10 , and the development unit rotation preventing portions  34  of the development unit  30  as described above, can prevent the problem that the warping, twisting, and/or the like deformation of the process cartridge P prevents the rotation preventing portions  12  and development rotation preventing portions  34  from properly engaging with each other. 
         [0065]    It is not mandatory that one of the pair of rotation preventing portions  12  and the corresponding development unit rotation preventing portion  34  with the presence of the gap between the two rotation preventing portions  12  and  34  as described above. That is, the state of the contact may be such that the bottom side of the outward portion of the rotation preventing portion  12  is in contact with the bottom side of the inward portion of the development unit rotation preventing portion  34 , but, the top side of the outward portion of the rotation preventing portion  12  is not in contact with the top side of the development unit rotation preventing portion  34 . That is, the state of contact between one of the pair of rotation preventing portions  12  and the corresponding development unit rotation preventing portion  34  has only to be such that top or bottom side of one of the pair of rotation preventing portions  12  contacts the top or bottom side of the corresponding development unit rotation preventing portion  34 , respectively. 
         [0066]    As for the positional relationship between the development roller  32  and photosensitive drum  21 , the development roller  32  is properly positioned relative to the photosensitive drum  21  by the development roller positioning hole  26  of the photosensitive drum unit  20  as described above. Further, the positioning of the process cartridge P (development unit  30 ) by the rotation preventing portions  12  of the laser scanner unit  10  is only in terms of the rotational direction of each process cartridge P. Therefore, it does not affect the development function of the process cartridge P. 
         [0067]    In this embodiment, the development unit  30  is properly positioned relative to the laser beam unit  10  by the direct engagement between the pair of development unit rotation preventing portions  34  of the development unit  30 , with which the lengthwise end portions of the development unit  30  are provided one for one, and the rotation preventing portions  12  with which the lengthwise end portions of the laser scanner unit  10  on the door  101  are provided one for one. Therefore, it is possible to minimize the gap between the development unit  30  and the path of the beam L of laser light outputted from the laser scanner unit  10 . 
         [0068]    Thus, the structural arrangement, in this embodiment, for positioning the development unit  30  of each of the process cartridges PY, PM, PC, and PK relative to the laser scanner unit  10  of the main assembly  100  of the image forming apparatus is different from the structural arrangement in accordance with any of the prior art, which positions the development unit  30  (process cartridges P) relative to the laser scanner unit  10 , through the contacts, different in position from those in this embodiment, between the development unit  30  (process cartridges P) and the portions of the apparatus main assembly  100 . Therefore, this embodiment has the following effects. That is, this embodiment can minimize the gap (space) between the development unit  30  and the path of the beam L of laser light outputted from the laser scanner unit  10  while ensuring that a clear path is provided for the beam L of laser light outputted from the laser scanner unit  10 . Further, in the case of a process cartridge, the development unit  30  and photosensitive drum unit  20  of which are rotationally movable relative to each other as in this embodiment, one of the two units  30  and  20  is properly positioned by the laser scanner unit  10 , and the other is properly positioned by the portion(s), other than the laser scanner unit  10 , of the apparatus main assembly  100 . Therefore, the internal components of the apparatus main assembly  100  can be efficiently positioned in terms of spatial efficiency. Thus, the image forming apparatus in this embodiment is simpler in the structure for positioning the development units  30  (process cartridges P), and smaller in the size of the apparatus main assembly  100 , than any of the image forming apparatuses in accordance with the prior arts. 
       Embodiment 2  
       [0069]    The image forming apparatus in the second preferred embodiment of the present invention is the same in general structure as the image forming apparatus in the first preferred embodiment. Thus, the members, components, etc., of the image forming apparatus in this embodiment, which are the same in function as the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, and are not going to be described. In other words, the following description of the second preferred embodiment of the present invention is focused on the portions of the image forming apparatus in this embodiment, which are different in structure from the counterparts in the first embodiment. 
         [0070]      FIG. 7(   a ) is a perspective view of the combination of the intermediary transfer unit  40 , process cartridges P (development units  30 ), laser scanner unit  10 , and door  101  when the door  101  is in its open position for the replacement of the process cartridge(s).  FIG. 7(   b ) is a perspective view of the same combination as that in the  FIG. 7(   a ), but when the door  101  is in its closed position. 
         [0071]    The laser scanner unit  10  has five pairs of rotation preventing portions  12  for preventing the process cartridges P from rotationally moving. Each pair of the rotation preventing portions  12  of the laser scanner unit  10  are portions for positioning the corresponding process cartridge P relative to the laser scanner unit  10 . Each pair of rotation prevention portions  12  of the laser scanner unit  10  are at the ends of the laser scanner unit  10 , one for one, in terms of the direction parallel to the axial line of the photosensitive drum  21 . Each rotation preventing portion  12  is in the form of a small projection. The second, third, and fourth pairs of rotation preventing portions  12 , counting from the top side of the door  101 , are for holding a preset distance between the process cartridges PY and PM, between the process cartridge PM and PC, and between process cartridges PC and PK, respectively. Thus, as the door  101  is closed, two pairs of rotation preventing portions  12  directly engage with the top and bottom edges of each process cartridge P, preventing thereby the process cartridge P (development unit  30 ) from rotationally moving. In other words, as the door  101  is closed, the process cartridges P are properly positioned relative to the laser scanner unit  10  so that a preset amount of gap is maintained between the beam L of laser light outputted from the laser scanner unit  10 , and the corresponding process cartridge P. 
         [0072]    In this embodiment, the image forming apparatus is structured so that as the door  101  is closed, each pair of rotation preventing portions  12  directly contact the frame of the corresponding process cartridge P. Therefore, it is unnecessary to provide the process cartridge P with the development unit rotation preventing portion  34  as in the first embodiment. Thus, the image forming apparatus in this embodiment is simpler in the cartridge positioning structure, being therefore lower in cost, than the image forming apparatus in the first embodiment. 
         [0073]    That is, also in this embodiment, the rotation preventing portions  12  with which the laser scanner unit  10  is provided properly position the process cartridges P by coming directly in contact with the process cartridges P as described above. Thus, this embodiment also can reduce an image forming apparatus in the gap between the development unit  30  and the corresponding beam L of laser light outputted from the laser scanner unit  10 . 
         [0074]    Therefore, this embodiment also can accurately position the development unit  30  and laser scanner unit  10  relative each other, making it therefore possible to minimizing the gap between the development unit  30  and the path of the corresponding beam L of light from the laser scanner unit  10  while ensuring that there is a clearance for the beam L of laser light as can the first embodiment described above. In other words, this embodiment also makes it possible to efficiently position the internal components of the main assembly  100  of an image forming apparatus in terms of spatial efficiency. Therefore, it can provide an image forming apparatus which is simpler in the cartridge positioning structure of the main assembly  100 , and is smaller in the size of the main assembly  100 . In addition, not only is the image forming apparatus in this embodiment simper in the cartridge positioning structure, but also, lower in cost, than the image forming apparatus in the first embodiment. 
       Embodiment 3  
       [0075]    The general structure of the image forming apparatus in the third embodiment is similar to that of the image forming apparatus in the first embodiment. Thus, the members, components, etc., of the image forming apparatus in this embodiment, which are similar in function to the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, and are not going to be described. In other words, the following description of the third preferred embodiment of the present invention is focused on the portions of the image forming apparatus in this embodiment, which are different in structure from the counterparts in the first embodiment. 
         [0076]      FIG. 8(   a ) is a perspective view of the combination of the intermediary transfer unit  40 , process cartridges PY, PM, PC, and PK, laser scanner unit  10 , and door  101  when the door  101  is in its open position for the replacement of the process cartridge(s).  FIG. 8(   b ) is a perspective view of the same combination as that in the  FIG. 8(   a ), but when the door  101  is in its closed position. 
         [0077]    The laser scanner unit  10  has four rotation preventing portions  12  for preventing the process cartridges P from rotationally moving. The rotation preventing portions  12  of the laser scanner unit  10  are portions for properly positioning the corresponding process cartridge P relative to the laser scanner unit  10 . Each rotation preventing portion  12  of the laser scanner unit  10  is at the center of the laser scanner unit  10  in terms of the direction perpendicular to the direction in which the process cartridges P are mounted into, or removed from, the apparatus main assembly  100 . Each rotation preventing portion  12  is in the form of a small projection. The rotation preventing portions  12  are for accurately positioning the development units  30  of the properly positioned adjacent two process cartridges P, with the presence of a preset gap. 
         [0078]    Further, each process cartridge P has a development unit rotation preventing portion  34  as a development unit rotation preventing portion which engages with one of the rotation preventing portions  12  of the laser scanner unit  10 . The development unit rotation preventing portion  34  is in the form of a recess. The development unit rotation preventing portion  34  is at the center of the development unit  30  of the process cartridge P in terms of the direction perpendicular to the direction in which the process cartridge P is mounted into, or removed from, the apparatus main assembly  100 . As the door  101  is closed, the development unit rotation preventing portion  34  of each process cartridge P, which is in the form of a recess, engages with the corresponding development unit rotation preventing portion  12  of the laser scanner unit  10 , which is in the form of a projection, whereby the process cartridge P (development unit  30 ) is prevented from rotationally moving. In other words, each process cartridge P is accurately positioned so that a preset amount of gap is maintained between each process cartridge P and the path of the corresponding beam L of laser light outputted from the laser scanner unit  10 . 
         [0079]    Also in this embodiment, as the door  101  is closed, each of the rotation preventing portions  12 , with which the laser scanner unit  10  on the door  101  is provided, accurately positions the corresponding process cartridge P by directly engaging with the development unit rotation preventing portion  34  of the development unit  30 , which is at the center of the development unit  30 . Thus, this embodiment also can reduce an image forming apparatus in the gap between the development unit  30  and the path of the corresponding beam L of laser light outputted from the laser scanner unit  10 . 
         [0080]    Therefore, this embodiment also can minimize the gap (space) between the development unit  30  and the path of the corresponding beam L of light from the laser scanner unit  10  while ensuring that there is a clearance for the beam L of laser light as can the embodiment described above. In other words, this embodiment also makes it possible to efficiently position the internal components of the main assembly of an image forming apparatus in terms of spatial efficiency. Therefore, it can provide an image forming apparatus which is simpler in the cartridge positioning structure of the main assembly, and is smaller in the size of the main assembly, than any of the image forming apparatuses in accordance with the prior arts. 
       Embodiment 4  
       [0081]    The general structure of the image forming apparatus in the fourth embodiment is similar to that of the image forming apparatus in the first embodiment. Thus, the members, components, etc., of the image forming apparatus in this embodiment, which are similar in function to the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, and are not going to be described. In other words, the following description of the third preferred embodiment of the present invention is focused on the portions of the image forming apparatus in this embodiment, which are different in structure from the counterparts in the first embodiment. 
         [0082]      FIG. 9(   a ) is a perspective view of the combination of the intermediary transfer unit  40 , process cartridges PY, PM, PC, and PK, laser scanner unit  10 , and door  101  when the door  101  is in its open position for the replacement of the process cartridge(s).  FIG. 9(   b ) is a side view of the same combination as that in  FIG. 9(   a ) when the door  101  is in its closed position. 
         [0083]    The laser scanner unit  10  has four pairs of rotation preventing portions  12  for preventing the process cartridges P from rotationally moving. Each pair of rotation preventing portions  12  of the laser scanner unit  10  are portions for properly positioning the corresponding process cartridge P relative to the laser scanner unit  10 . Referring to  FIG. 10 , the structure of the laser scanner unit  10  is such that each rotation prevention portion  12  of the laser scanner unit  10  is under the pressure generated by a rotation preventing spring  13  in the direction which is roughly the same as the direction in which the process cartridge P is pressed for positioning. 
         [0084]    Each rotation preventing portion  12  in this embodiment is in the form of a small projection. The rotation preventing portions  12  are for accurately positioning the development units  30  of the properly positioned adjacent two process cartridges P (PY, PM, PC, and PK), with the present of a preset interval. 
         [0085]    Further, each process cartridge P has a pair of development unit rotation preventing portion  34  as cartridge positioning portions which engage with the corresponding pair of rotation preventing portion  12  of the laser scanner unit  10 . The development unit rotation preventing portion  34  is in the form of a recess. Each pair of development unit rotation preventing portion  34  are at the ends of the development unit  30  of the process cartridge P, one for one, in terms of the direction perpendicular to the direction in which the process cartridge P is mounted into, or removed from, the apparatus main assembly  100 . As the door  101  is closed, each of the development unit rotation preventing portions  34  of the process cartridge P, which is in the form of a recess, engages with the corresponding development unit rotation preventing portion  12  of the laser scanner unit  10 , which is in the form of a projection, whereby the process cartridge P (development unit  30 ) is prevented from rotationally moving. In other words, the process cartridges P are positioned so that a preset amount of gap is maintained between each process cartridge P and the path of the corresponding beam L of laser light outputted from the laser scanner unit  10 . 
         [0086]    Also in this embodiment, as the door  101  is closed, each of the rotation preventing portions  12 , with which the laser scanner unit  10  on the door  101  is provided, functions to properly position the corresponding process cartridge P by directly engaging with the corresponding development unit rotation preventing portion  34 . Thus, this embodiment also can reduce an image forming apparatus in the gap between the development unit  30  and the path of the corresponding beam L of laser light outputted from the laser scanner unit  10 . Further, the laser scanner unit  10  is structured so that each of the rotation preventing portions  12  is under the pressure generated by the rotation preventing spring  13  in the direction which is roughly the same as the direction in which the process cartridge P is pressed for positioning. Therefore, the members dedicated for keeping the process cartridges P pressured are unnecessary. 
         [0087]      FIG. 11  is an enlarged sectional view of one of the development unit rotation preventing portions  34  and the corresponding rotation preventing portion  12 , in this embodiment, when the two portions  34  and  12  are in contact with each other. The laser unit scanner  10  and process cartridge P (development unit  30 ) are structured so that the areas of contacts S 1  and S 2  between each rotation preventing portion  12  and corresponding development unit rotation preventing portion  34  are slanted. Therefore, the two portions  34  and  12  engage with each other with no play between them. Further, the areas of contact of the rotation preventing portion  12 , and the areas of contact of the development unit rotation preventing portion  34  are provided with an electrical contact for supplying the processing portion of the corresponding process cartridge with electric power. Therefore, the electric power for generating the bias to be applied to the development roller, for example, can be supplied to the processing portion through the areas of contact S 1  and S 2 . Positioning the electrical contacts as described above can make the contact pressure greater than the pressure applied to the process cartridge P. Therefore, it is ensured that the electrical contacts remain in contact. Moreover, this structural arrangement makes it unnecessary to provide portions dedicated to the electrical connection between the apparatus main assembly  100  and the process cartridge P. Further, it ensures that the process cartridges P remain properly positioned. 
         [0088]    In other words, the structural arrangement, in this embodiment, for positioning the development unit  30  of each of the process cartridges PY, PM, PC, and PK relative to the laser scanner unit  10  of the main assembly  100  of the image forming apparatus is different from the structural arrangement in accordance with any of the prior art, which positions the development unit  30  (process cartridges P) relative to the laser scanner unit  10 , through the contacts, different in position from those in this embodiment, between the development unit  30  (process cartridges P) and the portions of the apparatus main assembly  100 . Therefore, this embodiment has the following effects. That is, this embodiment can minimize the gap (space) between the development unit  30  and the path of the beam L of laser light outputted from the laser scanner unit  10  while ensuring that a clear path is provided for the beam L of laser light outputted from the laser scanner unit  10 . Further, this embodiment eliminates the need for the members dedicated to the pressing of the process cartridges P. Thus, this embodiment makes it possible to efficiently position the internal components of the apparatus main assembly  100  in terms of spatial efficiency. In other words, the present invention can provide an image forming apparatus which is simpler in the cartridge positioning structure, and smaller in the main assembly, than any of the image forming apparatuses in accordance with the prior arts. 
       Embodiment 5  
       [0089]    The general structure of the image forming apparatus in the fifth embodiment is similar to that of the image forming apparatus in the fourth embodiment. Thus, the members, components, etc., of the image forming apparatus in this embodiment, which are similar in function to the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, and are not going to be described. In other words, the following description of the fifth preferred embodiment of the present invention is focused on the portions of the image forming apparatus in this embodiment, which are different in structure from the counterparts in the fourth embodiment. 
         [0090]      FIG. 12(   a ) is a perspective view of the combination of the intermediary transfer unit  40 , process cartridges PY, PM, PC, and PK, laser scanner unit  10 , and door  101  in this embodiment when the door  101  is in its open position for the replacement of the process cartridge(s).  FIG. 12(   b ) is a side view of the same combination as that in the  FIG. 12(   a ) when the door  101  is in its closed position. 
         [0091]      FIG. 13  is a side view of the combination of the process cartridges P and laser scanner unit  10  in the fifth preferred embodiment of the present invention when the door  101  for the replacement of the process cartridge(s) is in its closed position. 
         [0092]    The laser scanner unit  10  has four pairs of rotation preventing portions  12  for preventing the process cartridges P from rotationally moving. Each pair of the rotation preventing portions  12  of the laser scanner unit  10  are portions for properly positioning the corresponding process cartridge P relative to the laser scanner unit  10 . 
         [0093]    Referring to  FIG. 13 , the laser scanner unit  10  is structured so that each of the rotation preventing portions  12  is under the pressure generated by a spring  13  in the direction which is roughly the same as the direction in which the process cartridge is pressed for positioning. Further, the laser scanner unit  10  is structured so that as the door  101  is opened, each rotation preventing portion  12  engages with the corresponding shutter  14  for covering the exposure window  16  of the abovementioned exposing apparatus, and rotates the shutter  14  about its rotational axis  14   a . As the door  101  is opened, the laser scanner unit  10  moves in the direction indicated by an arrow mark c, and each rotation preventing portion  12  moves in the direction indicated by an arrow mark a. Thus, the process cartridge P is freed from the pressure under which it was. 
         [0094]    Also in this embodiment, the development unit  30  (process cartridges P) are properly positioned relative to the laser scanner unit  10  by the direct engagement between the development unit rotation preventing portions  34  of the development unit  30 , and the rotation preventing portions  12  of the laser scanner unit  10  on the door  101 , one for one. Therefore, the gap between each of the development units  30 , and the path of the corresponding beam L of laser light outputted from the laser scanner unit  10  does not need to be as wide as that in any of the image forming apparatuses in accordance with the prior arts, as descried above. Further, each of the rotation preventing portions  12  is under the pressure generated in the direction which is roughly parallel to the process cartridge positioning direction. Therefore, the apparatus main assembly  100  does not need to be provided with the members dedicated to the pressing of the process cartridges. Moreover, each rotation preventing portion  12  is in engagement with the shutter  14  with its engaging portion  12   a . Therefore, as the pressure which is being applied to the process cartridge P is removed, the rotation preventing portion  12  moves in the direction indicated by the arrow mark a, causing thereby the shielding member  14  to cover the exposure window of the exposing apparatus. 
         [0095]    That is, the structural arrangement, in this embodiment, for positioning the development unit  30  of each of the process cartridges PY, PM, PC, and PK relative to the laser scanner unit  30  of the main assembly  100  of the image forming apparatus is different from the structural arrangement in accordance with any of the prior art, which positions the development unit  30  (process cartridges P) relative to the laser scanner unit  10 , through the contacts different in position from those in this embodiment, between the development unit  30  (process cartridges P) and the portions of the apparatus main assembly  100 . Therefore, this embodiment has the following effects. That is, this embodiment can minimize the gap (space) between the development unit  30  and the path of the beam L of laser light outputted from the laser scanner unit  10  while ensuring that a clearance is provided for the beam L of laser light outputted from the laser scanner unit  10 . Further, this embodiment eliminates the need for the members dedicated to the pressing of the process cartridges P. Moreover, this embodiment can simplify the mechanism for covering the exposure window of the exposing apparatus. Thus, this embodiment makes it possible to efficiently position the internal components of the apparatus main assembly  100  in terms of spatial efficiency. In other words, the present invention can provide an image forming apparatus which is simpler in the cartridge positioning structure, and smaller in the main assembly, than any of the image forming apparatuses in accordance with the prior arts. 
       [Miscellanies] 
       [0096]    In the preceding embodiments of the present invention, the laser scanner unit  10  is attached to the door  101 , being allowed to move within a preset range, and as the door  101  is closed, the laser scanner unit  10  is accurately positioned relative to the frame  15  of the main assembly  100  of the image forming apparatus. However, these embodiments are not intended to limit the present invention in scope. That is, the present invention is also effectively applicable to an image forming apparatus, the laser scanner unit  10  of which is immovably attached to the door  101 , but, is not accurately positioned relative to the frame  15  of the apparatus main assembly  100 . That is, according to the present invention, the development unit  30  of the process cartridge P is accurately positioned by the rotation preventing portions  12  of the laser scanner unit  10 . Thus, even if the laser scanner unit  10  on the door  101  is not movable relative to the door  101 , it is ensured that a clear path is provided for the beam L of laser light outputted from the laser scanner unit  10 , between the laser scanner unit  10  and the peripheral surface of the photosensitive drum  21 . 
         [0097]    Also in the preceding preferred embodiments described above, the door  101  was rotationally moved frontward of the main assembly  100  of the image forming apparatus to replace the process cartridge(s) in the apparatus main assembly  100 . However, this structural arrangement is not intended to limit the present invention in scope. That is, referring to  FIGS. 14(   a ) and  14 ( b ), the present invention is also compatible with an image forming apparatus structured so that the door  101  is to be rotationally moved upward about the hinge  102  in the direction indicated by an arrow mark in  FIG. 14(   b ). In such a case, it is in the upward direction indicated by an arrow mark g that the process cartridges PY, PM, PC, and PK are to be removed. The application of the present invention to the above described structural design for an image forming apparatus can provide an image forming apparatus which is significantly smaller in the gap between the development unit  30  of each of the process cartridges PY, PM, PC, and PK in the main assembly of the image forming apparatus, and the path of the beam L of laser light outputted from the laser scanner unit  10 , and therefore, in the front-to-rear dimension, than any of the image forming apparatuses in accordance with the prior arts. 
         [0098]    Further, in the preceding embodiments, the development unit  30  belonged to the process cartridge (PY, PM, PC, and PK), the development unit  30  and photosensitive drum unit  20  of which were not separable from each other. However, the present invention is also compatible with a process cartridge, the development unit and photosensitive drum unit of which are independently replaceable from each other. That is, the application of the present invention to an image forming apparatus which employs such process cartridges also provide the same effects as those described above. For example, referring to  FIG. 15 , the present invention is compatible with a process cartridge structured so that as the lengthwise end portions of the rotational axle  32   a  of the development roller  32  of the development unit  30  engage into the pair of development unit positioning grooves  29  of the photosensitive drum unit  20 , the development unit  30  and photosensitive drum unit  20  become accurately positioned to each other. Further, referring to  FIGS. 16 ,  17 ( a ) and  17 ( b ), the present invention is applicable to such an image forming apparatus design that the photosensitive drum unit  21 , with which the development unit  30  engages, is accurately positioned relative to the intermediary transfer unit  40  by the engagement of the rotational axle  21   a  of the photosensitive drum  21  of the photosensitive drum unit  20 , and the pair of photosensitive drum rotation preventing portions  28  of the photosensitive drum unit  20 , into the pair of photosensitive drum unit positioning grooves  43  of the intermediary transfer unit  40 . 
         [0099]    Further, the application of the present invention is not limited to the image forming apparatus design which places the means for positioning the photosensitive drum unit  20 , on the intermediary transfer unit  40 . That is, the present invention is compatible with also such an image forming apparatus design that the means for accurately positioning the photosensitive drum unit  20  is placed on the side walls of the apparatus main assembly  100 . 
         [0100]    Further, in the preceding embodiments, each of the image forming apparatuses was such an image forming apparatus that employed four process cartridges which are removably mountable in the main assembly of the image forming apparatus. However, the present invention does not require that the process cartridge count is limited to four. That is, the present invention is compatible with any image forming apparatus design of the above described type regardless of the process cartridge count of the design. 
         [0101]    Further, each of the exposing apparatuses in the preceding embodiments was a laser scanner unit. However, the present invention does not require that the exposing apparatus is a laser scanner unit. That is, the present invention is also compatible with exposing apparatuses made up of an LED array, or the like. 
         [0102]    Further, each of the process cartridges in the preceding embodiments was such a process cartridge that integrally comprises a photosensitive drum, and a combination of processing means, more specifically, charging means, a developing means, and a cleaning means. However, the preceding embodiments are not intended to limit the present invention in scope. That is, the present invention is also compatible with a process cartridge that integrally comprises a photosensitive drum and only one processing means among a charging means, a developing, and a cleaning means. 
         [0103]    Also in the preceding embodiments, each image forming apparatus was a printer. However, these embodiments are not intended to limit the present invention in scope. That is, the present invention is compatible with an image forming apparatus other than a printer. For example, the present invention is compatible with a copying machine, a facsimile machine, etc., and a multifunction apparatus capable of performing two or more functions of the preceding image forming apparatuses. Further, each image forming apparatus was such an image forming apparatus that uses an intermediary transfer member; sequentially transfers in layers multiple monochromatic toner image, different in color, onto the intermediary transfer member; and transfers all at once the multiple toner images onto recording medium. However, these embodiments are not intended to limit the present invention in scope. That is, the present invention is also compatible with such an image forming apparatus that uses a recording medium bearing member; and sequentially transfers in layer multiple monochromatic toner images, different in color, directly onto the recording medium on the recording medium bearing member. The effects of the application of the present invention to these image forming, apparatuses (other than those in preceding embodiments) are the same as those described above. 
         [0104]    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. 
         [0105]    This application claims priority from Japanese Patent Applications Nos. 281858/2009 and 258559/2010 filed Dec. 11, 2009 and Nov. 19, 2010 which are hereby incorporated by reference.