Abstract:
A color image forming apparatus for forming a color image on a sheet includes a plurality of cartridge mounting portions for demountably mounting a cartridge each of which includes a developing roller for developing an electrostatic latent image formed on a photosensitive drum into a developed image and a developer accommodating portion for accommodating a developer to be used for development of the electrostatic latent image; a single intermediary transfer member which is provided opposed to the drums and onto which the developed images are transferred from the drums; an image forming unit containing the cartridge mounting portions and the intermediary transfer member and movable between a first position for transferring, onto the sheet, the developed images transferred onto the intermediary transfer member from the drums and a second position for permitting mounting and demounting of the cartridges relative to the cartridge mounting portion; an opening for permitting mounting and demounting of the cartridges relative to the cartridge mounting portions of the unit taking the a second position; and an opening-closing member movable between a closing position for closing the opening and an open position for opening the opening, wherein when the unit takes the first position, the cartridges take attitudes in which the developer is supplied to the developing roller by the weight thereof, and when the unit takes the a second position, the cartridges take attitudes in which the developer is not supplied to the developing roller by the weight thereof.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an electrophotographic color image forming apparatus which forms an image on recording medium, with the use of multiple cartridges removably mountable in its main assembly. 
         [0002]    Here, an electrophotographic color image forming apparatus (which hereafter will be referred to simply as image forming apparatus) means such a color image forming apparatus that forms a color image on recording media with the use of an electrophotographic image formation process. Examples of an electrophotographic color image forming apparatus include color copying machines, color printers (color laser beam printer, color LED printers, etc.), color facsimile machines, color word processors, etc. Recording media include any medium on which an image can be formed by an image forming apparatus. They include paper, OHP sheet, and the like, for example. 
         [0003]    A cartridge means a process cartridge, a development cartridge, or a developer (toner) cartridge, for example. It is removably mountable in the main assembly of an image forming apparatus. It contributes to a process for forming an image on recording medium by being in the main assembly of an image forming apparatus. A process cartridge is a cartridge in which a developing means (processing means) and an electrophotographic photosensitive drum (which hereafter will be referred to simply as drum) are integrally disposed. It can be removably mountable in the main assembly of an image forming apparatus. A process cartridge can be mounted into, or dismounted from, the main assembly of an image forming apparatus by a user himself or herself. Thus, it can make it easier to maintain an image forming apparatus. A processing means is a means for processing a drum. 
         [0004]    A development cartridge has a development roller. It stores developer (toner) used in combination with the development roller to develop an electrostatic latent image on the drum. It also is removably mountable in the main assembly of an image forming apparatus. In the case of an image forming apparatus which uses a development cartridge, its drum is attached to the main assembly of the image forming apparatus, a cartridge supporting member of the apparatus, or is in a process cartridge of the so-called separation type (process cartridge having no developing means). A development cartridge also can be mounted into, or removed from, the main assembly of an image forming apparatus by a user himself or herself. Thus, it also can make it easier to maintain an image forming apparatus. 
         [0005]    A developer cartridge (developer supply container) stores developer for developing an electrostatic latent image formed on a drum by a development roller. It also is removably mountable in the main assembly of an image forming apparatus. Incidentally, a drum and a development roller are attached to the main assembly of an image forming apparatus, or the cartridge-supporting member of the main assembly. The developer cartridge also can be mounted into, or removed from, the main assembly of an image forming apparatus by a user himself or herself. Therefore, it also can make it easier to maintain an image forming apparatus. 
         [0006]    There has been known a color image forming apparatus structured as follows. It is structured so that its intermediary transferring member is rotated by the driving force source of its main assembly, and its photosensitive drum is rotated by the driving force source by way of the intermediary transferring member. This structural arrangement makes it easier to position a cartridge relative to the main assembly of an image forming apparatus when mounting the cartridge into the main assembly. That is, all that is necessary for a cartridge to be precisely positioned for image formation is for the cartridge to be precisely positioned relative to the intermediary transferring member. That is, this structural arrangement makes it easier to mount a cartridge into the main assembly of an image forming apparatus, to remove the cartridge from the main assembly, and also, to precisely position a cartridge relative to the main assembly. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is a further development of the above-described structural arrangement for an image forming apparatus. Thus, the primary object of the present invention is to provide an electrophotographic color image forming apparatus which is substantially smaller in the amount by which developer (toner) leaks when a cartridge is mounted into, or removed from, the main assembly of the apparatus, and is simpler in its mechanism for supplying the developing means with developer during an image forming operation, than any of the conventional image forming apparatuses. 
         [0008]    According to an aspect of the present invention, there is provided a color electrophotographic image forming apparatus for forming a color image on a recording material, said color electrophotographic image forming apparatus comprising a plurality of cartridge mounting portions for demountably mounting a cartridge each of which includes a developing roller for developing an electrostatic latent image formed on an electrophotographic photosensitive drum into a developed image and a developer accommodating portion for accommodating a developer to be used for development of the electrostatic latent image; a single intermediary transfer member which is provided opposed to said electrophotographic photosensitive drums and onto which the developed images are transferred from said electrophotographic photosensitive drums; an image forming unit containing said cartridge mounting portions and said intermediary transfer member and movable between a transfer position for transferring, onto the recording material, the developed images transferred onto said intermediary transfer member from said electrophotographic photosensitive drums and a mounting and demounting position for permitting mounting and demounting of said cartridges relative to said cartridge mounting portion; an opening for permitting mounting and demounting of said cartridges relative to said cartridge mounting portions of said image forming unit taking the mounting and demounting position; and an opening-closing member movable between a closing position for closing said opening and an open position for opening said opening, wherein when said image forming unit takes the transfer position, said cartridges take attitudes in which the developer in said developer accommodating portions is supplied to said developing roller by the weight thereof, and when said image forming unit takes the mounting and demounting position, said cartridges take attitudes in which the developer in said developer accommodating portions is not supplied to said developing roller by the weight thereof. 
         [0009]    The present invention can provide an electrophotographic color image forming apparatus which is substantially smaller in the amount by which developer (toner) leaks when a cartridge is mounted into, or removed from, the main assembly of the apparatus, and is simpler in its mechanism for supplying the developing means with developer during an image forming operation, than any of the conventional image forming apparatuses. 
         [0010]    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 
         [0011]      FIG. 1A  is an external perspective view of an image forming apparatus of the first embodiment of the present invention, and  FIG. 1B  is a schematic vertical sectional view of the right-hand portion of the image forming apparatus shown in  FIG. 1A . 
           [0012]    Part (a) of  FIG. 2  is an enlargement of a part of  FIG. 1B . Part (b) of  FIG. 2  is a schematic drawing for describing a typical developing method of the contact type. Part (c) of  FIG. 2  is a schematic drawing for describing a typical developing method of the noncontact type. 
           [0013]    Part (a) of  FIG. 3  is an external perspective view of the image formation unit, and (b) of  FIG. 3  is a perspective view of the roughly right-hand half of the image formation unit, from which all the development cartridges have been removed. 
           [0014]    Part (a) of  FIG. 4  is a perspective view of the portion of the image formation unit, which is for precisely positioning the development cartridges relative to the photosensitive drum unit, and (b) of  FIG. 4  is an enlargement of a part of (a) of  FIG. 4 . 
           [0015]      FIG. 5A  is a perspective view of the image forming apparatus in the first embodiment, the cover of which is closed, and the image formation unit of which is in its image forming position.  FIG. 5B  is a vertical sectional view of the image forming apparatus in the first embodiment, the cover of which is closed, and the image formation unit of which is in its image forming position. 
           [0016]      FIG. 6A  is a perspective view of the image forming apparatus in the first embodiment, the cover of which is open, and the image formation unit of which is in the position for allowing cartridges to be mounted into, or removed from, the main assembly of the image forming apparatus.  FIG. 6B  is a vertical sectional view of the image forming apparatus in the first embodiment, the cover of which is open, and the image formation unit of which is in the position for allowing cartridges to be mounted into, or removed from, the main assembly of the image forming apparatus. 
           [0017]      FIG. 7A  is a schematic vertical sectional view of the image forming apparatus in the second embodiment of the present invention.  FIG. 7B  is an enlargement of a part of  FIG. 7A . 
           [0018]      FIG. 8A  is a perspective view of the image forming apparatus in the second embodiment, the cover of which is closed, and the image formation unit of which is in its image forming position.  FIG. 8B  is a vertical sectional view of the image forming apparatus in the second embodiment, the cover of which is closed, and the image formation unit of which is in its image forming position. 
           [0019]      FIG. 9A  is a perspective view of the image forming apparatus in the second embodiment, the cover of which is closed, and the image formation unit of which is being mounted into the main assembly of the image forming apparatus.  FIG. 9B  is a vertical sectional view of the image forming apparatus in the second embodiment, the cover of which is closed, and the image formation unit of which is being mounted into the main assembly of the image forming apparatus. 
           [0020]      FIG. 10A  is a perspective view of the image forming apparatus in the second embodiment, the cover of which is open, and the image formation unit of which is in the position for allowing cartridges to be mounted into, or removed from, the main assembly of the image forming apparatus.  FIG. 10B  is a vertical sectional view of the image forming apparatus in the second embodiment, the cover of which is open, and the image formation unit of which is in the position for allowing cartridges to be mounted into, or removed from, the main assembly of the image forming apparatus. 
           [0021]      FIG. 11A  is a perspective view of the image forming apparatus in the second embodiment of the present invention, the image formation unit of which is in the position for allowing cartridges to be mounted into, or removed from, the main assembly, and the three development cartridges for which have been removed.  FIG. 11B  is an enlargement of a part of  FIG. 11A . 
           [0022]      FIG. 12A , is a schematic vertical sectional view of the image forming apparatus in the third embodiment of the present invention.  FIG. 12B  is a schematic vertical sectional view of the image forming apparatus in the third embodiment, the cover of which is fully open, and the image formation unit of which is in the position for allowing cartridges to be mounted into, or removed from, the main assembly of the image forming apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the appended drawings. However, the measurements, materials, and shapes of the structural components of image forming apparatuses in the following preferred embodiments, and their positional relationships, are not intended to limit the present invention in scope unless specifically noted. 
       Embodiment 1 
     &lt;General Structure of Electrophotographic Color Image Forming Apparatus&gt; 
       [0024]      FIG. 1A  is an external perspective view of an electrophotographic color image forming apparatus  100  (which hereafter will be referred to simply as image forming apparatus  100 , or apparatus  100 ) in this embodiment of the present invention, and  FIG. 1B  is a schematic vertical sectional view of the right-hand portion of the image forming apparatus  100  shown in  FIG. 1A . The apparatus  100  is a full-color laser beam printer which uses an electrophotographic process. It uses four color toners. More specifically, the apparatus  100  forms a full-color image on a sheet S of recording medium (paper) in response to electric signals inputted into its control circuit  300  from an external host apparatus  400 , such as a personal computer, an image reader, a remote facsimile apparatus, etc. 
         [0025]    In the following descriptions of the preferred embodiments of the present invention, the front (front side) of the apparatus  100  means the side of the apparatus, from which a sheet feeder cassette  19 , in which multiple sheets S of recording medium are stored in layers, can be pulled out of the main assembly of the apparatus  100 . The rear (rear side) of the apparatus  100  means the opposite side from the front side. The top side of the apparatus  100  is where a delivery tray  110   c  is located. The frontward direction means the frontward direction relative to the rear of the apparatus  100 , whereas the rearward direction means the direction opposite to the frontward direction. The left and right of the apparatus  100  are the left and right, respectively, of the apparatus  100  as the apparatus  100  is seen from the front side of the apparatus  100 . The leftward direction of the apparatus  100  means the leftward direction as seen from the front side of the apparatus  100 , whereas the rightward direction means the opposite direction from the leftward direction. Further, the apparatus main assembly  100 A means what remains after the removal of the cartridges and image formation unit from the apparatus  100 . 
         [0026]    The apparatus  100  is to be mounted on a roughly horizontal surface F, for example, the surface of a holder dedicated to the apparatus  100 , surface of an ordinary desk, surface of a floor, etc. The apparatus main assembly  100 A employs an image formation unit  200 , which is mounted in the apparatus main assembly  100 A. Part (a) of  FIG. 2  is an enlargement of the image formation unit portion of the apparatus  100  shown in  FIG. 1A . The unit  200  has multiple (four, in this embodiment) cartridge chambers  31   c , in which four development cartridges  33  (four development cartridges, more specifically, development cartridges  33 Y,  33 M,  33 C, and  33 K, in this embodiment) are removably mountable, one for one. Further, the unit  200  has an intermediary transferring member  34 , electrophotographic photosensitive drums  32   a  (which hereafter will be referred to simply as drum  32   a ), charge rollers  32   b  (processing means), and cleaning blades  32   c  (processing means). The apparatus  100  forms a color image on a sheet S of recording medium, with the use of multiple cartridges  33  which are removably mountable in the apparatus main assembly  100 A (unit  200 ). The structure of the unit  200  will be described later in detail. 
         [0027]    The cartridges  33  in this embodiment are the same in structure, although they are different in the color of the toner they store. However, they do not need to be the same in structure. For example, the developer storage portion of a cartridge  33 K, which stores black developer, may be made greater in the capacity than those of cartridges  33 Y,  33 M, and  33 C, which store yellow, magenta, and cyan developers, one for one, other than the black one. Incidentally, this embodiment, and the following ones, will be described referring to a development cartridge as an example of a cartridge. However, the embodiments are not intended to limit the present invention in scope. For example, in this embodiment, the drum  32   a , roller  32   b , and blade  32   c , are attached to the unit  200 . However, they may be attached to the cartridge  33 . In a case where a cartridge shell, drum  32   a , roller  32   b , and blade  32   c  are structured so that the drum  32   a , roller  32   b , and blade  32   c  are attached to the cartridge shell, the cartridge will be referred to as a process cartridge instead of a development cartridge, because the roller  32   b , development roller  33   b , and blade  32   c , which are processing means, and the drum  32   a , are integrally disposed in the cartridge shell which is removably mountable in the apparatus main assembly  100 A. The unit  32  has the drum  32   a . It has also: the charge roller  32   b , which is a processing means for processing the drum  32   a;  and a cleaning blade  32   c  for removing the developer remaining on the peripheral surface of the drum  32   a . The drum  32   a , roller  32   b , and blade  32   c  are attached to the case  32   d  of the unit  32  in such a manner that preset positional relationships are maintained among them. The unit  32  is attached to the unit  200 . Each cartridge  33  has a developing device case  33   a  and a development roller  33   b . The development roller  33   b  is at one end of the case  33   a , and supplies the drum  32   a  with developer. That is, the roller  33   b  develops the electrostatic latent image on the drum  32   a , into a visible image, that is, an image formed of developer. Further, the cartridge  33  has: a developer holding portion  33   c , as developer storage, for storing the developer to be used for the development of the electrostatic latent image; and a supply roller  33   d  for supplying the roller  33   b  with the developer from the developer holding portion  33   c.    
         [0028]    The developing device  33   c  of the first cartridge  33 Y contains yellow (Y) developer. It is for forming a visible image of yellow developer (which hereafter will be referred to simply as yellow developer image), on the peripheral surface of the drum  32   a . The developing device  33   c  of the second cartridge  33 M contains magenta (M) developer. It is for forming a visible image of magenta developer (which hereafter will be referred to simply as magenta developer image), on the peripheral surface of the drum  32   a . The developing device  33   c  of the third cartridge  33 C contains cyan (C) developer. It is for forming a visible image of cyan developer (which hereafter will be referred to simply as cyan developer image), on the peripheral surface of the drum  32   a . The developing device  33   c  of the fourth cartridge  33 K contains black (K) developer. It is for forming a visible image of black developer (which hereafter will be referred to simply as black developer image), on the peripheral surface of the drum  32   a . The transferring member  34  in this embodiment is a cylindrical drum, which is rotatable about its rotational axis  34   a . It is horizontally supported so that its axial line is parallel to the leftward or rightward direction. The cartridges  33 Y,  33 M,  33 C, and  33 K are on the front side of the transferring member  34 , and are stacked in the listed order so that the cartridge  33 Y is the topmost one and the cartridge  33 K is the bottommost one. They are slightly tilted relative to the apparatus placement surface F at such an angle that the developers in the cartridges  33 Y,  33 M,  33 C, and  33 K slide downward toward the supply rollers  33   d . That is, they are in such an attitude that the developers in their developer storage portion  33   c  are made to flow by their own weight toward the rollers  33   b  and  33   d  in the developer storage portions  33   c , ensuring that the developers in the storage portions  32   c  will be used up. 
         [0029]    In the case of the apparatus  100  in this embodiment, the first cartridge  33 Y is placed in the uppermost cartridge chamber, and the second cartridge  33 M is placed in the cartridge chamber which is immediately below the uppermost cartridge chamber. The third cartridge  33 C is placed in the cartridge chamber which is immediately below the cartridge chamber for the second cartridge  33 M, and the fourth cartridge  33 K is placed in the bottommost cartridge chamber. 
         [0030]    The roller  33   b  of each cartridge  33  may be placed in contact with the peripheral surface of the drum  32   a  (developing method of contact type) as shown in (b) of  FIG. 2 , or may be positioned so that a preset minute gap α (preset distance) is present between the roller  33   b  and drum  32   a  (developing method of noncontact type), as shown in (c) of  FIG. 2 . In the case where the roller  33   b  is positioned as shown in (c) of  FIG. 2 , the left and right end portions of the roller  33   b  are fitted with a pair of spacer  33   s , one for one, so that the preset amount of gap α is maintained between the roller  33   b  and drum  32   a.    
         [0031]    The image forming portion of the apparatus  100  in this embodiment is structured as shown in (c) of  FIG. 2 . The apparatus main assembly  100 A is provided with a laser scanner unit  11  as an exposing apparatus. The laser scanner unit  11  is in the top front portion of the apparatus main assembly  100 A, and is on the front side of the cartridges  33 . More specifically, this unit  11  is in the apparatus main assembly  100 A, and is between the front portion  110   a  of the main frame  110  of the apparatus main assembly  100 A, and the cartridges  33 . The unit  11  has a laser diode, a polygonal mirror, an F-θ lens, a deflection mirror, etc. The unit  11  exposes (scans) the peripheral surface of the drum  32   a  in each cartridge  33  by outputting a beam of laser light L (Y, M, C, and K) while modulating the beam of laser light L with the information of the monochromatic images (Y, M, C, and K), which is inputted into the control circuit  300  from the external host apparatus  400 . That is, the unit  11  projects a beam of light (laser light) upon each drum  32   a  while modulating the beam of light with the information of the image to be formed. As a result, an electrostatic latent image, which reflects the information of the image to be formed is effected on each drum  32   a.    
         [0032]    There is a sheet feeder unit  18  below the unit  200 . The unit  18  has: a sheet feeder cassette  19  in which sheets S of recording medium (which hereafter may be referred to simply as recording sheets S) are stored in layers; a sheet conveyance roller  20 ; a separation pad  21 ; etc. The cassette  19  is removably mountable in the apparatus main assembly  100 A from the front side of the apparatus main assembly  100 A (front loading). Further, there is a recording medium conveyance passage Z between the transferring member  34  and the rear portion  110   b  of the main frame  110 . The passage Z extends from the roller  20  to the top rear portion of the apparatus main assembly  100 A. Further, there are a pair of registration rollers  18   a , a second transfer roller  22 , a fixing apparatus  23 , and a pair of discharge rollers  24 , listing from the bottom side of the passage Z. The fixing apparatus  23  has a fixation film unit  23   a  and a pressure roller  23   b . One of the pair of discharge rollers  24  is a discharge roller  24   a  and the other is a discharge roller  24   b . A part of the top wall of the apparatus main assembly  100 A makes up the delivery tray  110   c . The apparatus main assembly  100 A is provided with a maintenance cover  10 , which makes up a part of the front wall of the apparatus main assembly  100 A. The cover  10  can be opened or closed. It exposes or covers a cartridge entrance (removal) opening with which the front side of the apparatus main assembly  100 A is provided. The cartridge entrance opening  100 B is the opening through which the cartridges  33  are mounted into, or removed from, the development cartridge chambers  31   c  of the unit  200  when the unit  200  is in the preset position into which the unit  200  is mounted, or from which the unit  200  is removed. 
         [0033]    When the apparatus  100  is in the state shown in  FIG. 1B , the driving force input portion (unshown) of the transferring member  34  of the unit  200  is in connection with the driving force output portion (unshown) of the apparatus main assembly  100 A. Further, the driving force input portion (unshown) of the unit  32 , and the driving force input portion (unshown) of each cartridge  33 , are in connection with the driving force output portion (unshown) of the apparatus main assembly  100 A. Further, the electrical contacts (unshown) of the unit  32  and the electrical contacts (unshown) of each cartridge  33 , are in connection with the electric power supply system (unshown) of the apparatus main assembly  100 A. 
         [0034]    The full-color image forming operation of the apparatus  100  is as follows. The drum  32   a  is rotated at a preset speed in the clockwise direction indicated by an arrow mark in (a) of  FIG. 2 . As the drum  32   a  is rotated, the roller  32   b  is rotated by the rotation of the drum  32   a . The transferring member  34  is rotated in the counterclockwise direction indicated by an arrow mark in (a) of  FIG. 2 , at a speed which corresponds to the speed of the drum  32   a . Further, the rollers  33   b  and  33   d  of each cartridge  33  are rotated in the counterclockwise direction indicated by an arrow mark in (a) of  FIG. 2  at a preset speed. The unit  11  also is driven. In synchronism with the driving of these components, a preset charge bias is applied to the roller  32   b  with a preset timing. As a result, the peripheral surface of the drum  32   a  is uniformly charged to preset polarity and potential level by the roller  32   b . The unit  11  exposes (scans) the charged portion of the peripheral surface of the drum  32   a  with the beam of laser light L it emits while modulating the beam with the image signals for forming a monochromatic image of color Y, M, C, or K. As a result, an electrostatic latent image corresponding to color Y, M, C, or K is formed on the peripheral surface of the drum  32   a . The electrostatic latent image on the peripheral surface of the drum  32   a  is developed by the roller  33   b  into a visible image, that is, an image formed of developer. To the roller  32   b , a preset development bias is applied with preset control timing. 
         [0035]    Through the above described electrophotographic image formation process, a yellow developer image, which corresponds to the yellow component of the full-color image to be formed is formed on the drum  32   a  which the cartridge  33 Y faces. Then, this developer image is transferred (first transfer) onto the intermediary transferring member  34 , in the first transfer nip, which is the interface between the drum  32   a  and transferring member  34 . On the drum  32   a  which the cartridge  33 M faces, a magenta developer image, which corresponds to the magenta component of the full-color image, is formed. This developer image is transferred (first transfer) onto the intermediary transferring member  34  in such a manner that it is layered upon the yellow developer image on the intermediary transferring member  34 , in the first transfer nip, which is the interface between the drum  32   a  and transferring member  34 . 
         [0036]    On the drum  32   a  which the cartridge  33 C faces, a cyan developer image, which corresponds to the cyan component of the full-color image, is formed. This developer image is transferred (first transfer) onto the transferring member  34  in such a manner that it is layered upon the layered combination of the yellow developer image and magenta developer image on the transferring member  34 , in the first transfer nip, which is the interface between the drum  32   a  and intermediary transferring member  34 . On the drum  32   a  which the cartridge  33 K faces, a black developer image, which corresponds to the black component of the full-color image, is formed. This developer image is transferred (first transfer) onto the transferring member  34  in such a manner that it is layered upon the layered combination of the yellow, magenta, and cyan developer images on the intermediary transferring member  34 , in the first transfer nip, which is the interface between the drum  32   a  and transferring member  34 . Consequently, an unfixed full-color developer image is synthetically formed on the transferring member  34 , of the yellow, magenta, cyan, and black developer images. Incidentally, the order in which these monochromatic developer images are transferred in layers onto the intermediary transferring member  34  does not need to be limited to the above described one. The transfer residual developer on the drum  32   a , that is, the developers remaining on the drum  32   a  after the first transfer of the developer images, is removed by the blade  32   c.    
         [0037]    Further, the roller  20  is driven with a preset control timing, whereby the sheets S of recording medium in the cassette  19  are fed into the apparatus main assembly  100 A while being separated one by one by the coordination of the roller  20  and pad  21 . Then, each recording sheet S is introduced into the second transfer nip, which is the interface between the transferring member  34  and second transfer roller  22 , with a preset timing, by a pair of rollers  18   a . To the roller  22 , the second transfer bias, which is preset in potential level and is opposite in polarity from the developer, is applied with a preset control timing, whereby the full-color developer image made up of the layered combination of the four monochromatic developer images on the transferring member  34  is transferred (second transfer) onto the recording sheet S, as if it is peeled away from the transferring member  34 , while the recording sheet S is conveyed through the second transfer nip while remaining pinched by the roller  22  and transferring member  34 . After the recording sheet S is moved through the second transfer nip, it is separated from the surface of the transferring member  34 , and then, is introduced into the fixing apparatus  23 . In the fixing apparatus, heat and pressure are applied to the recording sheet S and the unfixed developer image thereon. As a result, the four monochromatic developer images, different in color, on the recording sheet S are fixed to the recording sheet S while being mixed. Then, the recording sheet S is discharged from the fixing apparatus  23 , and then, is discharged, as a full-color print, onto the delivery tray  110   c  by the pair of rollers  24 . 
         [0038]    In this embodiment, the second transfer residual toner, that is, the toner remaining on the peripheral surface of the transferring member  34  after the separation of the recording sheet S from the transferring member  34 , electrostatically adheres to the peripheral surface of the drum  32   a  in the primary transfer nip of the unit  32  which holds the cartridge  33 Y, for example. Then, it is removed by the blade  32   c . The transferring member  34  is rotatable, and in the form of a drum. Onto the transferring member  34 , multiple monochromatic developer images, different in color, formed on multiple drums  32   a , one for one, are transferred in layers. Then, the multiple monochromatic developer images, different in color, on the transferring member  34  are transferred all at once onto the recording sheet S. Consequently, a color image is formed on the recording sheet S. Incidentally, when the apparatus  100  is used to form a monochromatic (black) image, only a monochromatic (black) developer image is formed on the drum  32   a , and then, is transferred onto the transferring member  34  from the drum  32   a . Then, the monochromatic (black) developer image is transferred from the transferring member  34  onto the recording sheet S, to form a monochromatic (black) image on the recording sheet S. The time of the completion of the formation of the finished image on the recording sheet S is when the recording sheet S comes out of the fixing apparatus  23 . 
         [0039]    Further, when the apparatus  100  is operated in the black mode, only the combination of the cartridge  33 K and unit  32 , which is for forming black images, is used. In this embodiment, the second transfer roller  22  is movable by the second transfer roller moving mechanism (unshown) so that it can be kept in the first position where it forms the second transfer nip by being placed in contact with the transferring member  34 , and the second position where it remains separated from the transferring member  34 . More specifically, when the apparatus  100  is an image forming operation, the second transfer roller  22  is kept in the first position, whereas when the apparatus  100  is not in an image forming operation, the roller  22  is kept in the second position. However, the apparatus  100  may be structured so that the roller  22  remains in contact with the transferring member  34  whether or not the apparatus  100  is in an image forming operation. 
       &lt;Image Formation Unit&gt; 
       [0040]    Next referring primarily to  FIGS. 1A , part (a) of  FIG. 2 , part (a) of  FIG. 3 , and  FIG. 4 , the structure of the unit  200  will be described. Part (a) of  FIG. 3  is an external perspective view of the unit  200 . Part (b) of  FIG. 3  is a perspective view of the roughly right-hand half of the unit  200  when the unit  200  is holding no cartridges  33 . Part (a) of  FIG. 4  is a partially broken perspective view of the unit  200 . Part (b) of  FIG. 4  is a perspective view of the portion of the unit  200 , which is for precisely positioning the cartridge  33  and unit  32  relative to each other. 
         [0041]    The unit  200  has a frame  31 , which enables the unit  200  to be mounted into, or removed from, the main frame  110  of the apparatus main assembly  100 A. The frame  31  has an intermediary transfer member supporting portions  31   a  ( 31   a -L and  31   a -R) which rotatably support the transferring member  34 . The transferring member  34  is supported by the left and right portions  31   a -L and  31   a -R, respectively, of the frame  31 . More specifically, the left and right portions  31   a -L and  31   a -R of the frame  31  are fitted with a pair of bearings, one for one, and the left and right end portions of the center shaft  34   a  (rotational axis) of the transferring member  34  are supported by the pair of bearings, one for one, so that the transferring member  34  is rotatable. The left and right portions  31   a -L and  31   a -R (which hereafter may be referred to as left and right plates) of the frame  31  have left (unshown) and right shafts  45 L and  45 R, which are integral with the left and right plates  31   a -L and  31   a -R, respectively. The left and right shafts  45 L and  45 R are coaxial with the center shaft  34   a  of the transfer member supporting portion  31   a . Further, the unit  200  has a pair of gears G 5 . The case  32   d  of each unit  32  is connected to this frame  31 . While the unit  32  is supported by the frame  31 , an elastic pawl a, with which the frame  31  is provided, remains engaged with a projection b, with which the unit  32  is provided. Therefore, the unit  32  is kept pressed toward the transferring member  34  by the resiliency of the elastic pawl a. Therefore, the drum  32   a  is kept pressed upon the transferring member  34  by a preset amount of force. Further, the frame  31  has multiple cartridge chambers  31   c  in which the cartridges  33  are removably mountable, one for one. In this embodiment, the multiple cartridge chambers  31   c  are development cartridge chambers (developing device connecting unit which holds development cartridge  33 ), and are independent from each other, making it possible for each cartridge  33  to be independently mounted into, or removed from, the corresponding cartridge chamber, from the other. 
         [0042]    The cover  10  is connected by its bottom edge portion, to the apparatus main assembly  100 A with the presence of a shaft  10   a , as a hinge, in such a manner that it can be rotationally movable relative to the apparatus main assembly  100 A. It is enabled to be placed in the position C where it keeps the front opening  100 B of the apparatus main assembly  100 A closed, and the position D where it keeps the front opening  100 B exposed. That is, the cover  10  is a part of the front wall of the apparatus main assembly  100 A, and is rotatable to expose or cover the front opening  100 B of the apparatus main assembly  100 A. The front opening  100 B is the opening through which the cartridges  33  are mounted into, or removed from, the cartridge chambers  31   c  of the unit  200  when the unit  200  is in the cartridge mounting-and-removing position B ( FIG. 6B ). Referring to  FIGS. 1A and 1B , normally, the cover  10  remains closed, keeping thereby the front opening  100 B covered. The right plate  31   a -R of the frame  31  is provided with a projection  31   f , which is on the outward surface of the right plate  31   a -R. There is a spring  35  (elastic member) between the projection  31   f  and a projection (unshown) which is on the inward surface of the right wall  110 R of the frame  110 , remaining stretched by the two projections. When the cover  10  is in the position where it keeps the opening  100 B closed (as shown in  FIGS. 5A and 5B ), the spring  35  is on the rear side of the shaft  45 R (dead center) of the unit  200 . Therefore, the unit  200  remains pressed in the direction to rotate clockwise about the shafts  45 L and  45 R, by the tension of the springs  35 . Thus, the frame  31  is kept pressed by the unit  200  so that it rotates about the shafts  45 L and  45 R in the clockwise direction in  FIG. 1B . Thus, the top surface  31   d  of the frame  31  is kept pressed upon a stay  38  (frame positioning portion of apparatus main assembly  100 A) of the apparatus main assembly  100 A. Therefore, the frame  31  remains correctly positioned relative to the apparatus main assembly  100 A. That is, the unit  200  remains placed in the transfer position A (which enable apparatus to form images) ( FIGS. 5A and 5B ) where the developer images transferred onto the transferring member  34  from the drums  32   a  are transferred onto the recording sheet S. In other words, the unit  200  is locked into, and kept locked in, the image forming position A by the coordination among the cover  10 , spring  35 , and stay  38  ( FIG. 1B ). The cover  10 , spring  35 , and stay  38  make up a locking mechanism (first locking mechanism). 
         [0043]    This embodiment makes it possible to be precisely position the unit  200  relative to the apparatus main assembly  100 A. Incidentally, the structure of the first locking mechanism does not need to be limited to the above described one. The structural arrangement is optional as long as the same effects as those described above can be realized. Further, when the unit  200  is in the transfer position A, the driving force input portion (unshown) of the transferring member  34  of the unit  200  is in connection to the driving force output portion (unshown) of the apparatus main assembly  100 A. Further, the driving force input portion of the unit  32  and the driving force input portion of the cartridge  33  are in connection with the driving force output portion of the apparatus main assembly  100 A, which entered the unit  200  from the left side. Further, the electrical contacts of the unit  32  and the electrical contacts of the cartridge  33  are in connection to the electric power supply system of the apparatus main assembly  100 A. Thus, the apparatus  100  is ready for an image forming operation (a printing operation). That is, as soon as the apparatus  100  receives an image formation start signal (printing start signal), it can start an image forming operation such as the one described above. 
         [0044]    Referring to  FIG. 1B , in this embodiment, the unit  11 , cartridge  33 , drum  32   a , transferring member  34 , and recording medium conveyance passage Z, are positioned so that they align roughly parallel to the apparatus placement surface F. Further, the top portion of the apparatus  100  has the tray  110   c , and the bottom portion of the apparatus  100  holds the cassette  19 . The beams of laser light L (Y, M, C, and K) from the unit  11  are projected upon the drums  32   a  from the rear side of the cartridges  33 , one for one. After the monochromatic developer images, different in color, are transferred from the drums  32   a  onto the transferring member  34 , they are transferred onto the recording sheet S from the opposite side of the transferring member  34  from the side where the drums  32   a  face the transferring member  34 . Also in this embodiment, when the drums  32   a  are correctly in contact with the unit  200 , and the cartridges  33  are correctly in the unit  200  (cartridge chambers  31   c ), the unit  200  can be rotationally moved to be placed in the transfer position A (image formation position) or the cartridge mounting-and-dismounting position B. As the unit  200  is rotationally moved from the transfer position A into the cartridge mounting-and-dismounting position B, it becomes possible for the cartridges  33  to be mounted into, or removed from, the unit  200 . That is, the unit  200  is made rotatable about its rotational axis so that it is allowed to take the transfer position A or cartridge mounting-and-dismounting position B. The employment of the above described structural arrangement for an image forming apparatus in this embodiment makes it possible to reduce an electrophotographic image forming apparatus ( 100 ) in size. 
       &lt;Method for Replacing Development Cartridge&gt; 
       [0045]    As each cartridge (Y, M, C, or K) is used for image formation, the developer in the developing device  33   c  (developer holding portion) is consumed. Thus, the apparatus main assembly  100 A is provided with a means (unshown) for detecting the amount of the developer remainder in the cartridge  33 . The detected amount of the developer remainder is compared by the control circuit  300  with a threshold value preset for allowing a user to predict the end of the service life of each cartridge ( 33 ), or warning a user of the nearness of the end of the service life of the cartridge ( 33 ). More specifically, as the abovementioned detected amount of the developer remainder in a given cartridge  33  falls below the threshold value, it is shown on the display  102  of the control portion  101  of the apparatus  100  ( FIG. 1A ), to inform a user of the remaining amount of the service life of the cartridge  33 , or warm the user of the nearness of the end of the service life of the cartridge  33 . In other words, it is suggested that the user is to prepare a replacement cartridge (cartridges), or replace the cartridge  33  (cartridges), in order to ensure that the apparatus  100  continues to output images of high quality. 
         [0046]    In the case of the apparatus  100  in this embodiment, in order to replace any of the cartridges  33  in the apparatus main assembly  100 A, the cover  10  has to be opened to expose the opening  100 B. That is, in order to replace any of the cartridges  33 , a user is to manually and rotationally move the cover  100  about the shaft  10   a  by placing his or her hand on the handle portion  10   d  of the cover  10 , from the position C where the cover  10  keeps the opening  100 B covered, to the position D where the cover  10  keeps the opening  100 B fully exposed.  FIGS. 6A and 6B  show the apparatus  100  when the cover  10  is in the position D. As the cover  10  is opened, the driving force outputting portion of the apparatus main assembly  100 A is disengaged from the driving force input portion of the transferring member  34  of the unit  200  by a mechanism (unshown) which is moved by the movement of the cover  10 , and also, from the driving force input portion of each unit  32  and the driving force input portion of each cartridge  33 . Further, the electric power supply system of the apparatus main assembly  100 A is disconnected from the electrical contacts of each unit  32  and the electrical contacts of the each cartridge  33 . As the cover  10  is moved into the position D, the spring  35  is moved to the front side of the shaft  45 R (dead center) of the unit  200 . Therefore, the unit  200  is under the force generated by the tension of the spring  35  in the direction to rotate the unit  200  in the counterclockwise direction ( FIGS. 6A and 6B ) about the shafts  45 L and  45 R. Thus, after the cover  10  is moved into the position D, even if the user releases the cover  10 , it does not occur that the cover  100  automatically rotates backward. The shaft  10   a  is rotatably supported by the left and right walls  110 L and  110 R of the main frame  110  of the apparatus main assembly  100 A; the left and right end portions of the shaft  10   a  are supported by a pair of bearings with which the left and right walls  110 L and  110 R of the main frame  110  of the apparatus main assembly  100 A are fitted, one for one. The shaft  10   a  is an integral part of the cover  10 . Therefore, as the cover  10  is rotationally opened or closed, the shaft  10   a  rotationally moves with the cover  10 . The left and right end portions of the shaft  10   a  have a pair of cover gears G 1 , one for one, which are in the form of a fan and are integral parts of the shaft  10   a . The pair of cover gears G 1  are symmetrically positioned relative to the center of the shaft  10   a , and are the same in rotational phase. The left gear G 1  and the aforementioned left gear G 5  (attached to unit  200 ) are indirectly in connection to each other through left idler gears G 2 , G 3 , and G 4 . The right gear G 1  and the aforementioned right gear G 5  (attached unit  200 ) are indirectly in connection to each other through right idler gears G 2 , G 3 , and G 4 . The left idler gears G 2 , G 3 , and G 4  are attached to the left wall  110 L of the main frame  110  of the apparatus main assembly  100 A, and are freely rotatable. The right idler gears G 2 , G 3 , and G 4  are attached to the right wall  110 L of the main frame  110  of the apparatus main assembly  100 A, and are freely rotatable. The provision of these gear trains causes the unit  200  to be rotationally moved by the opening or closing movement of the cover  10 . That is, the abovementioned gears G 1 -G 5  make up the mechanism for moving the unit  200  by the movement of the cover  10 . 
         [0047]    Referring to  FIGS. 5A and 5B , when the cover  10  is in the position in which it keeps the opening  100 A covered, the angle of the fan-shaped gear G 1  is such that the fan-shaped gear G 1  is not in engagement with the gear G 2 . That is, the cover  10  and unit  200  are not in engagement with each other. Thus, regardless of the position of the cover  10 , the unit  200  remains correctly positioned relative to the apparatus main assembly  100 A by the spring  35 , in a preset manner while ensuring that the unit  200  is kept in contact with the stay  38 . When the cover  10  is in the position shown in  FIGS. 5A and 5B , a user is to manually open the cover  10  by placing his or her hand on the handle portion  10   d  of the cover  10 , and to rotationally move the cover  10  to the position D shown in  FIGS. 6A and 6B . As the cover  10  is opened by an angle greater than a preset one, the gear G 1  becomes meshed with the gear G 2 . Then, the further rotational opening movement of the cover  10  causes the force applied to the cover  10  by the user to open the cover  10  to be transmitted to the gear G 5  through the gears G 2 , G 3 , and G 4 . Thus, the unit  200  is rotationally moved about the left and right shafts  45 L and  45 R in the counterclockwise direction (as seen from right-hand side of apparatus  100 ), in the apparatus main assembly  100 A. Further, as the cover  10  is opened, the front opening  100 B of the apparatus main assembly  100 A is exposed. Then, as the cover  10  is fully opened, that is, as the cover  10  is moved into the position D, it is kept in this position by the force generated by the spring  35  in the direction to open the cover  10 . Therefore, the opening  100 B is fully exposed, and remains fully exposed, as shown in  FIGS. 6B and 6B . Further, as the cover  10  is opened, the unit  200  is rotationally moved in the counterclockwise direction by roughly 40°, from the transfer position A ( FIGS. 5A and 5B ) until it becomes roughly horizontal so that the cartridges  33  become accessible by the user through the opening  100 B ( FIGS. 6A and 6B ). Thereafter, the unit  200  remains roughly parallel to the apparatus placement surface F. That is, as the cover  10  is opened, the unit  200  is moved into the cartridge mounting-and-dismounting position B by the movement of the cover  10 . Here, the cartridge mounting-and-dismounting position B is such a position that allows the user to mount any cartridge ( 33 ) into the corresponding cartridge chamber, or dismount it. 
         [0048]    In this embodiment, a user is to manually and rotationally move the closed cover  10  until the cover  10  becomes fully open. As the cover  10  is moved in the opening direction, the unit  200  is moved from the transfer position A to the cartridge mounting-and-dismounting position B. In other words, all that is necessary to move the unit  200  from the transfer position A to the cartridge mounting-and-dismounting position B is for a user to manually move the cover  10  until the opening  100 B becomes fully exposed. Further, the direction in which the cartridges  33  have to be moved to be mounted into, or dismounted from, the unit  200  (apparatus main assembly  100 A) is roughly parallel to the apparatus placement surface F. Therefore, they can be replaced without retracting the unit  11 . Further, the attitude in which the cartridges  33  have to be kept when they are mounted or dismounted is such that they remain roughly horizontal, or slightly tilted upward (in direction to prevent toner from falling). Therefore, it is possible to prevent developer (toner) from falling into and/or outside the apparatus main assembly  100 A when replacing the cartridges  33 . Therefore, a user can easily replace the cartridges  33 . Further, with the employment of the above-described structural arrangement for the apparatus  100 , when the apparatus  100  is in an image forming operation, the cartridges  33  remain tilted at such an angle that the developers (toners) therein are naturally (automatically) supplied to the development rollers. In other words, this structural arrangement makes a toner stirring mechanism unnecessary, making it therefore possible to provide cartridges ( 33 ) which are substantially lower in cost than the conventional ones, and also, to use up the developers (toners) therein. That is, when the unit  200  is in the transfer position A, the cartridges  33  (Y, M, C, and K) in the unit  200  are in such an attitude that the developer in the developer holding portion  33   c  of each cartridge  33  is supplied to the corresponding development roller  33   b  by the weight of the developer itself. Further, when the unit  200  is in the cartridge mounting-and-dismounting position B, the cartridges  33  in the unit  200  are in the abovementioned roughly horizontal attitude, that is, such an attitude that the developer in the developer holding portion  33   c  of each cartridge  33  is not supplied to the development roller  33   b  by the weight of the developer itself, or slightly upwardly tilted (such attitude that prevent toner from falling: attitude that developer in developer holding portion  33   c  is likely to be flowed in the opposite direction from development roller  33   b  by weight of developer itself). 
         [0049]    Next, the process of moving the unit  200  back into the transfer position A after the cartridge  33  or cartridges  33  are replaced while the unit  200  is kept in the cartridge mounting-and-dismounting position B, will be described. Referring to  FIGS. 6A and 6B , each cartridge  33  is to be insert into the corresponding cartridge chamber  31   c  of the unit  200  when the unit  200  is in the cartridge mounting-and-dismounting position B. As the cartridge  33  is mounted into the corresponding cartridge chamber  31   c , the elastic member c ((a) of  FIG. 2 ) of the cartridge chamber  31   c  engages with the projection d of the cartridge  33 , whereby the cartridge  33  is retained in the cartridge chamber  31   c . After the replacement of the cartridge  33 , a user is to rotationally move the cover  10 , which is in the opening exposing position D, in the direction to close (cover) the opening  100 B. As the cover  100  is rotationally moved in the closing direction, the unit  200  is rotationally moved about the shafts  45 L and  45 R in the clockwise direction ( FIG. 6A ) by the movement of the cover  10 . At the same time, the force applied to the cover  10  to rotationally move the cover  10  by the user is transmitted to the unit  200  through the gears G 1 -G 5 . As the cover  100  is moved in the closing direction by an angle greater than a preset one, the gear G 1  becomes disengaged from the gear G 2 . That is, the cover  10  and unit  200  become disengaged from each other. Thus, the unit  200  is rotationally moved in the clockwise direction about the shafts  45 L and  45 R by the rotational force applied to the unit  200  by the spring  35 , being thereby placed, and kept, in contact with the stay  38 . Therefore, the unit  200  remains in the transfer position A thereafter ( FIGS. 1A and 1B ). Further, while the cover  10  is rotationally moved in the closing direction, from the abovementioned angle beyond which the cover  10  becomes disengaged from the unit  200 , to the position in which it completely covers the opening  100 B, the driving force outputting portion (unshown) of the apparatus main assembly  100 A is engaged with the driving force input portion (unshown) of the transferring member  34 , and also, with the driving force input portion (unshown) of the unit  32  and the driving force input portion (unshown) of the cartridge  33 , by the abovementioned mechanism (unshown) which is moved by the movement of the cover  10 . Further, the electrical contacts of the apparatus main assembly  100 A are connected to the electrical contacts of the unit  32  and cartridge  33 . Thereafter, the cover  10  remains fully closed. Through the above-described process, the apparatus  100  is put back into the state shown in  FIG. 5B , being readied for image formation. Incidentally, the gears G 1  and G 5  may be integral with, or independent from, the cover  10  and frame  31 , respectively. Further, the apparatus main assembly  100 A may be structured so that the unit  200  is directly moved by the movement of the cover  10  with the employment of a combination of a rack and a pinion gear, instead of the above-described mechanism. Although this embodiment was described with reference to the development cartridge  33  as a cartridge to be replaced, this embodiment is also applicable to an image forming apparatus, whose cartridge or cartridges to be replaced are a process cartridge or process cartridges, that is, a cartridge or cartridges made up of the photosensitive unit  32  and development cartridge  33 . 
       Embodiment 2  
       [0050]      FIGS. 7A-FIG .  11 B are drawings for describing the apparatus  100  in the second embodiment of the present invention. In this embodiment, a toggle mechanism is used as the mechanism for moving the unit  200  by utilizing the movement of the cover  10 . The apparatus  100  in this embodiment is structured so that the cartridge positioning member is moved into the cartridge positioning position or moved out of the cartridge positioning position, by the rotational movement of the unit  200 . The structural members, portions, etc., of the apparatus  100  in this embodiment, are the same as the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, and will not be described here. In this embodiment, in order to keep the cartridges  33  pressed upon the units  32 , one for one, so that a preset amount of pressure is maintained between the cartridges  33  and units  32 , the frame  31  of the unit  300  is structured as follows. This structural arrangement will be described referring primarily to  FIGS. 7A and 7B . In this embodiment, the frame  31  has a pressing portion  60   a  and pressing levers  60  (Y, M, C, and K). The pressing levers  60  have a gear-shaped portion  60   b  (pressing lever gear) for eliminating pressure. The frame  31  has also compression springs  61  (Y, M, C, and K) which keep the pressing levers  60  always pressed in the clockwise direction. It has also gears  63   a - 63   c  which are between adjacent two lever gears portions  60   b  and transmit driving force. The apparatus main assembly  100 A has a separation gear  62 , which is stationary. When the unit  200  is in the image forming position, the relationship between the gear portion  60   b  and gear  62  is such that the driving force is not transmitted. Therefore, the contacting portion  33   e  of the cartridge  33  is kept pressed by the lever  60  which is kept in the positioning position by the resiliency of the pressing spring  61  (elastic member). Thus, the shaft  33   b - 1  of the roller  33   b  is kept in contact with the positioning portion  39  ( FIG. 5B ). Therefore, it is ensured that the roller  33   b  is correctly positioned relative to the drum  32   a  ((b) of  FIG. 2 ). Incidentally, the gear portion  60   b  may be an integral part of the pressing portion  60   a , or a component independent from the pressing portion  60   a.    
         [0051]    The cover  10  is provided with a toggle bar  51 , which is rotatable about the lever supporting point  51   c . The lever  51  is provided with a boss  51   b , which is engaged with a guide rail of the apparatus main assembly  100 A. Thus, the movement of the lever  51  is regulated by this rail  52 . Further, the lever  51  has a spring ( 35 ) anchoring portion  51   a , which is at the opposite end from the supporting point  51   c . Both  FIGS. 7 and 8  show the apparatus  100  when the cover  10  of the apparatus main assembly  100 A is in the closed position. As a user opens the cover  10 , the lever  51  is rotationally moved by the movement of the cover  10 . Thus, the anchoring portion  51   a  (projection) of the lever  51  is moved from the rear side of the dead center (shaft  45 R) to the front side of the dead center (shaft  45 R), as shown in  FIG. 9 . As a result, the unit  200  is pulled by the resiliency of the spring  35  in such a manner that it is rotated in the counterclockwise direction about the shaft  45 L and  45 R. Thus, the unit  200  rotates in the counterclockwise direction until it is caught by a stopper  37 , which is the unit positioning portion of the apparatus main assembly  100 A. More specifically, the unit  200  is rotationally moved roughly 40° in the counterclockwise direction from the transfer position A ( FIGS. 8A and 8B ). Next, referring to  FIGS. 10A and 10B , as the unit  200  is moved as described above, the cartridges  33  are moved into the positions where they face the opening  100 B, and are kept in the positions. In other words, as the cover  10  is opened, the unit  200  is moved into the position B in which the cartridges  33  can be mounted into, or dismounted from, the corresponding cartridge chambers  31   c.    
         [0052]      FIG. 11A  is a partially broken perspective view of the apparatus  100 , and  FIG. 11B  is a sectional view of the unit  200 . They show the pressing levers  60  (Y, M, C, and K) when the unit  200  has just been rotationally moved from the transfer position A to the cartridge mounting-and-dismounting position B. As the unit  200  is rotated in the counterclockwise direction by a certain angle, the gear  62  is engaged with the gear  60   b . Then, as the unit  200  is rotated further, the gear  60   b  is driven by the gear  62 , whereby the pressing lever  60  is rotated in the counterclockwise direction about the gear  60   b  against the resiliency of the pressing lever spring  61  (elastic member). Thus, the pressing portions  60   a  simultaneously separate from the corresponding cartridges  33 . That is, the levers (Y, M, C, and K), which are cartridge positioning members, are retracted from their cartridge positioning positions into their home positions, whereby the cartridges  33  are freed. As described above, when the unit  200  is in the topmost position (transfer position A), each lever  60  is in the cartridge positioning position Q 1 . However, while the unit  200  is moved to the cartridge mounting-and-dismounting position B, each lever  60  is retracted into the home position Q 2 . In other words, all that is necessary for a user to do to move the unit  200  from the transfer position A to the cartridge mounting-and-dismounting position B is to manually move the cover  10  from the closed position to the fully open position. Further, as the cover  10  is fully opened, the cartridges  33  are freed from the corresponding cartridge positioning members  60 . In other words, this embodiment can realize the above-described structural arrangement for an image forming apparatus, which makes it possible to replace a cartridge or cartridges without retracting the unit  11 . Therefore, it makes easier for a user to replace the cartridge  33 . Further, when replacing cartridges, the cartridges can be kept roughly horizontal, or slightly tilted (in such direction that prevents toner from falling). Therefore, it is possible to prevent developer from falling into, and/or outside, the apparatus main assembly  100 A when a user is replacing cartridges. Further, when the apparatus  100  is in an image forming operation, the cartridges  33  remain tilted in such a manner that the developers (toners) therein can be supplied to the corresponding development rollers with the utilization of their own weight. That is, this embodiment makes a toner stirring mechanism unnecessary, makes it possible to provide an image forming apparatus which is substantially lower in cost than conventional image forming apparatus, and also, can use up the developers in the cartridges. 
         [0053]    Next, the process of moving the cartridges  33  into the transfer position A after the cartridge  33  or cartridges  33  are replaced, will be described. Referring to  FIGS. 11A and 11B , each cartridge  33  is to be inserted into the corresponding cartridge chamber  31   c  of the unit  200  when the unit  200  is in the cartridge mounting-and-dismounting position B. As the cartridge  33  is mounted into the corresponding cartridge chamber  31   c , the projection of the cartridge  33  engages with the elastic member c ( FIG. 7B ) of the cartridge chamber  31   c , whereby the cartridge  33  is temporarily held in the cartridge chamber  31   c . After the replacement of the cartridges to be replaced, a user is to rotationally move the cover  10 , which is in the open position D, in the closing direction. As the cover  10  is rotationally moved, the lever  51  is rotationally moved, while being guided by the rail  52 , by the rotational movement of the cover  10 . Thus, the spring anchoring projection  51   a  of the lever  51  is moved from the rear side of the dead center (shaft  45 R) to the front side of the dead center, by the movement of the lever  51 . Thus, the unit  200  is rotationally moved in the clockwise direction about the shafts  45 L and  45 R by the rotational force applied to the unit  200  by the tension of the spring  35 . Therefore, the unit  200  rotates in the clockwise direction until it is caught by the stay  38 . That is, the unit  200  is rotated in the clockwise direction by roughly 45° from the cartridge mounting-and-dismounting position B ( FIGS. 10A and 10B ). Further, as the unit  200  is rotated in the clockwise direction by a certain angle from the cartridge mounting-and-dismounting position B, the positional relationship between the gear  62  and gear portion  60   b  becomes such that driving force is not transmitted. Thus, each lever  60  is moved into cartridge positioning position ( FIG. 7B ) from the home position ( FIG. 11B ) by the resiliency of the compression spring  61  (elastic member), coming into contact with the corresponding cartridge  33 . Thus, the roller  33   b  is pressed upon the unit  32  by a preset amount of pressure, being thereby correctly positioned relative to the unit  32 . That is, as the unit  200  is moved from the cartridge mounting-and-dismounting position B to the transfer position A, the roller  33   b  is correctly positioned relative to the drum  32   a . Incidentally, each lever  60  may be made to double as an electrical contact of the cartridge  33 . Not only does the above described structural arrangement for the image forming apparatus make it easier for a user to replace cartridges  33 , but also, precisely position the cartridges  33  relative to the apparatus main assembly  100 A. 
       Embodiment 3  
       [0054]      FIGS. 12A and 12B  are a drawing for describing the apparatus  100  in the third embodiment. This embodiment is basically the same as the second embodiment. Thus, this embodiment will be described utilizing the drawings used for describing the preceding embodiments. The apparatus  100  in this embodiment is the same as the apparatus  100  in the second embodiment, except for the following feature. In this embodiment, as a user manually opens the cover  10  when the cover  10  is in the closed state, the user is allowed to access a handle portion of the unit  200 , which is for moving the unit  200 . Therefore, the user is allowed to manually pull down the unit  200  into the cartridge mounting-and-dismounting position B. That is, as the user manually pulls down the unit  200  after opening the cover  10 , it becomes possible for the cartridges  33  to be replaced. The frame  31  has: a spring  35  (elastic member) for keeping the unit  200  pressed in the clockwise direction; and a handle for a user to manually pull down the unit  200 . The pressing of the cartridges  33 , structural arrangement for correctly positioning the cartridges, and how each cartridge  33  is made to come into contact with, or separate from, the corresponding cartridge by the rotational movement of the frame  31 , are the same as those in the second embodiment. Therefore, they will not be described here. 
         [0055]    In this embodiment, it is not by the rotational movement of the cover  100  that the unit  200  is rotationally moved. More specifically, a user is to rotationally move the closed cover  10  into the preset open position for the cover  10 , and then, to pull the unit  200  by grasping the handle  38 . As the unit  200  is pulled downward, the unit  200  is rotationally moved into the cartridges mounting-and-dismounting position B, and is kept in the position B by the spring  35  of the toggle mechanism. That is, it becomes possible for the cartridges in the unit  200  to be replaced. The cartridge positioning mechanisms in this embodiment are the same as those in the second embodiment; they are the mechanisms  60 - 63  made up of levers  60 . As the unit  200  is moved from the transfer position A into the cartridge mounting-and-dismounting position B, each lever  60  retracts from its cartridge positioning position into its home position. Thus, when the unit  200  is in the cartridge mounting-and-dismounting position B, the cartridges  33  are not restricted in movement by the levers  60 . With the employment of the above described structural arrangement in this embodiment, the cartridges  33  in the unit  200  can be replaced without retracting the unit  11 ; a user can easily replace the cartridges  33  in the unit  200 . Further, it is possible to prevent the problem that when a user is replacing the cartridges  33 , the developers in the cartridges  33  fall into, and/or outside, the apparatus main assembly  100 A. Further, it is possible to precisely position the cartridges  33  relative to the apparatus main assembly  100 A. Moreover, the employment of the structural arrangement in this embodiment makes it possible to realize image forming apparatuses which are much simpler in structure, and substantially lower in cost, than any of the conventional image forming apparatuses. 
         [0056]    After the cartridge replacement, the user is to move the unit  200  upward by grasping the handle  38 . As the unit  200  is moved upward, the unit  200  is rotationally moved into the transfer position A, and is kept therein by the spring  35  of the toggle mechanism. Then, the user is to close the cover  10 . Further, as the unit  200  is moved from the cartridge mounting-and-dismounting position B into the transfer position A, the retracted levers  60  are moved from their home positions into the cartridge positioning positions, whereby the cartridges  33  are correctly positioned relative to the unit  32 . 
         [0057]    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. 
         [0058]    This application claims priority from Japanese Patent Application No. 280308/2009 filed Dec. 10, 2009 which is hereby incorporated by reference.