Abstract:
An image forming apparatus has an image forming unit including an image bearing member, an intermediary transfer member onto which an image formed on the image bearing member is transferred, the intermediary transfer member being located at an upper portion of the image forming unit, a housing for supporting and accommodating the image forming unit and the intermediary transfer member, the housing being drawable from the image forming apparatus with the image forming unit and the intermediary transfer member supported thereby, a supporting portion for supporting the intermediary transfer member for rotation relative to the housing, and a mounting and demounting portion, provided in the housing, for permitting the image forming unit to be mounted and demounted relative thereto. The mounting and dismounting of the image forming unit is enabled by rotating the intermediary transfer member, after the housing is pulled out of the image forming apparatus.

Description:
This application claims priority from Japanese Patent Application No. 2005-261521, filed Sep. 9, 2005, which is hereby incorporated by reference herein. 
   FIELD OF THE INVENTION AND RELATED ART 
   The present invention relates to an image forming apparatus, such as a copying machine, a printer, etc., for forming an image with the use of an electrophotographic method. 
   In the field of image forming apparatus employing an electrophotographic image forming method, color image forming apparatuses, more specifically, color image forming apparatuses employing multiple image formation stations for forming a color image on a recording medium, have been put to practical use. In these color image forming apparatuses, each image formation station has a photosensitive drum, and processing means, which act on the photosensitive drum. For the size reduction of these apparatuses, and also, for preventing variations in recording media from resulting in the formation of unsatisfactory images, various structures have been devised. In particular, a structural arrangement, in which multiple images formed on photosensitive drums in multiple image formation stations, one for one, are transferred (primary transfer) in layers onto an intermediary transfer belt, and then, the images on the intermediary transfer belt are transferred (secondary transfer) all at once onto a recording medium, has been widely used. 
   Among the color image forming apparatuses, such as those described above, there are color image forming apparatuses in which, in order to reduce the distance by which an image formed on the intermediary transfer belt by the primary transfer moves to reach the second transfer station, an intermediary transfer belt is disposed on the top side of the juxtaposed multiple image formation stations. As examples of the technologies for replacing a photosensitive drum and the structured components disposed around the photosensitive drum in these color image forming apparatuses, there are the technologies disclosed in Japanese Laid-Open Patent Applications No. 2005-141277, and No. 2003-287939. The structure disclosed in Japanese Laid-Open Patent Application No. 2005-141277 is such that an intermediary transfer belt and multiple image bearing members are enabled to be separately pulled out of the main assembly of the image forming apparatus. According to the technology disclosed in Japanese Laid-Open Patent Application No. 2003-287939, multiple image bearing members, an intermediary transfer belt, and the structural components, disposed in the adjacencies of the image bearing members and intermediary transfer belt, are enabled to be pulled out frontward of the housing of the main assembly. There are also such structural arrangements in which the above-mentioned multiple image bearing members, intermediary transfer belt, and the structural components disposed around them, are integrally supported by a supporting plate, which can be pulled out frontward of the housing of the main assembly along with the components supported thereon. One of such structural arrangements is also discussed in Japanese Laid-Open Patent Application No. 2002-182539. 
   As described above, according to Japanese Laid-Open Patent Application No. 2003-287939, it is possible that the photosensitive member, intermediary transfer belt, and the various other members (which, hereinafter, will be referred to as members involved in image formation) are integrally supported by a supporting plate, which is enabled to be pulled out along with the components supported thereon. In the case of this structural arrangement, the intermediary transfer belt is disposed above the photosensitive drums. Therefore, this arrangement suffers from the problem that it requires a greater number of operational steps when replacing the photosensitive drums, although the severity of this problem depends on the structural arrangement for disengaging the intermediary transfer member from its support. It also suffers from the problem that the intermediary transfer member interferes with the removal of the photosensitive drums. 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to improve an image forming apparatus in terms of the operability regarding the operation carried out by a user to replace the process units, such as a photosensitive drum involved in image formation, while ensuring the accuracy in the positional relationship between the multiple photosensitive drums and the intermediary transfer member. 
   According to an aspect of the present invention, there is provided an image forming apparatus comprising an image forming unit including an image bearing member, an intermediary transfer member onto which an image formed on the image bearing member is transferred, the intermediary transfer member being disposed at an upper portion of the image forming unit, a first housing for supporting and accommodating the image forming unit and the intermediary transfer member, the first housing being drawable with the image forming unit and the intermediary transfer member supported thereby, and a mounting and demounting portion, provided in the first housing, for permitting the image forming unit to be mounted and demounted relative thereto by a change in a relative position between the intermediary transfer member and the image forming unit. 
   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 
       FIG. 1  is a sectional view of the image forming apparatus in one of the preferred embodiments of the present invention. 
       FIGS. 2(   a ) and  2 ( b ) are drawings showing the structure of the housing of the image forming apparatus. 
       FIGS. 3(   a ) and  3 ( b ) are drawings showing the positioning of the process cartridges relative to the housing for the image formation stations. 
       FIGS. 4(   a ) and  4 ( b ) are drawings showing the structure of the intermediary transfer belt unit. 
       FIGS. 5(   a ) and  5 ( b ) are drawings showing the structure of the image formation unit. 
       FIGS. 6(   a ) and  6 ( b ) are drawings showing the method for replacing the process cartridges. 
       FIG. 7  is a drawing showing the method for replacing the process cartridges. 
       FIG. 8  is a drawing showing the method for replacing the process cartridges. 
       FIG. 9  is a drawing showing the method for replacing the process cartridges. 
       FIG. 10  is a drawing showing the method for replacing the process cartridges and developing apparatus. 
       FIG. 11  is a sectional view of another example of an image forming apparatus. 
       FIGS. 12(   a ) and  12 ( b ) are drawings showing the method for replacing the process cartridges. 
       FIGS. 13(   a ) to  13 ( c ),  14 ( a ), and  14 ( b ) are drawings showing the method for replacing the replenishment toner cartridges, waste toner recovering apparatuses, and process cartridges, which can be replenished with toner, and from which waste toner can be removed. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the present invention will be described with reference to the preferred embodiments of the present invention. 
   Next, the preferable embodiments of the present invention will be concretely described in detail with reference to the appended drawings. Incidentally, if a given component in one of the drawings has the same referential symbol as the one which a given component in another drawing has, the two components are the same in structure and functions. Thus, identical components will be described only once to avoid repetition of the same description. 
   First, referring to  FIG. 1 , the basic structure of a typical image forming apparatus will be described. The image forming apparatus shown in  FIG. 1  is a color image forming apparatus of the so-called inline type. In this image forming apparatus, therefore, multiple photosensitive drums  10 , as image bearing members, are horizontally aligned in parallel, and a color image is formed by sequentially placing, in layers, the multiple toner images, different in color, formed on the photosensitive drums, onto an intermediary transfer belt  17 . 
   The image forming apparatus  1  is provided with four process cartridges  15  as image formation units, which are roughly horizontally aligned with preset intervals. The four cartridges  15  form yellow, magenta, cyan, and black toner images, one for one. 
   In each process cartridge  15 , an electrophotographic photosensitive member  10  as an image bearing member, which is in the form of a drum (which, hereinafter, will be referred to simply as a photosensitive drum), is disposed. Around the peripheral surface of the photosensitive drum  10 , a primary charger  11 , a developing apparatus  12 , a toner storage portion  13 , and a drum cleaning apparatus  14 , which act on the photosensitive drum  10 , are disposed. Below the gap between the primary charger  11  and developing apparatus  12 , an exposing apparatus  16  for exposing the photosensitive drum  10  is disposed. Further, in an area which opposes the photosensitive drum  10 , a primary transfer roller  18 , as a transferring means, is disposed, with the intermediary transfer belt  17  interposed between the photosensitive drum  10  and the primary transfer roller  18 . 
   To the four developing apparatuses  12 , yellow toner, cyan toner, magenta toner, and black toner are supplied from the corresponding toner storage portions  13 . 
   Each photosensitive drum  10  is a photosensitive member made up of a negatively chargeable organic photoconductor. More specifically, it is made up of an aluminum drum as a substrate, and an organic photoconductive layer formed on the peripheral surface of the substrate. It is rotationally driven at a preset process speed by a driving apparatus (not shown). 
   The primary charger  11 , as the primary charging means, uniformly charges the surface of the photosensitive drum  10  to a present negative potential level by the charge bias applied from a charge bias power source (not shown). 
   The developing apparatus  12  contains toner as developer, and develops an electrostatic latent image formed on the corresponding photosensitive drum  10 , into a toner image (visible image) by adhering toner to the electrostatic latent image. 
   The primary transfer roller  18 , as the primary transferring means, is disposed within an intermediary transfer belt unit  20 , being kept pressed toward the photosensitive drum  10 . 
   The drum cleaning apparatus  14  has a cleaning blade, or the like, for removing the toner remaining on the photosensitive drum  10  after the primary transfer, from the photosensitive drum  10 . The recovered waste toner is recovered into a waste toner storage portion  19 . Incidentally, there are image forming apparatuses in which the waste toner on the photosensitive drums  10  is transferred onto the intermediary transfer belt  17 , instead of being recovered into the process cartridge  15  by the drum cleaning apparatus  14 . 
   The intermediary transfer belt unit  20  is provided with a driver roller  21 , a follower roller  22 , and a tension roller  23 , around which the intermediary transfer belt  17 , as the intermediary transfer member, is stretched. The shaft of the driver roller  21  is fitted with a gear (not shown), through which a driving force is transmitted from a driver gear on the main assembly side of the apparatus, to rotationally drive the driver roller  21 . As this gear is rotationally driven, the intermediary transfer belt  17  circularly moves. The driver roller  21  is positioned so that it opposes a secondary transfer roller  5 , doubling as the roller which opposes the secondary transfer roller  5 . 
   The intermediary transfer belt unit  20  is held by a transfer frame  30 , in which a high voltage transfer power source  31  (high voltage transferring means) is disposed. The transfer frame is pivotable about point  33   e , as shown in  FIGS. 1 ,  11   12 ( b ) and  14 ( b ). The high voltage transfer power source  31  is provided for supplying a high voltage for transferring the image formed on the photosensitive drum  10  onto the intermediary transfer belt  17 . 
   On the downstream side of the secondary transfer roller  5 , in terms of the recording medium conveyance direction, a fixing apparatus  6 , having a fixation roller  6   a  and a pressure roller  6   b , is disposed. The fixing apparatus  6  is structured so that a recording medium is vertically passed through the fixing apparatus  6 . 
   The exposing apparatus  16  is made up of a laser-based light emitting means, which emits a beam of light in response to sequential picture element signals, in the form of electrical digital signals, which reflects the provided image information. The exposing apparatus  16  forms an electrostatic latent image, according to the image information, on the surface of each of the photosensitive drums  10 , having been charged by the primary chargers  11 , one for one. The four electrostatic latent images formed on the four photosensitive drums  10 , one for one, are different in color. The exposing apparatus  16  is disposed below the process cartridges  15 , and is fixedly disposed in the image forming apparatus  1 . 
   Next, the image forming operation of the above-mentioned image forming apparatus will be described. 
   An original is read by an original reading apparatus  40 . As an image formation start signal is issued, the photosensitive drum  10  of each process cartridge  15 , which is rotationally driven at a preset process speed, is uniformly charged to the negative polarity by the corresponding primary charger  11 . The exposure apparatus  16  projects, from its laser-based light emitting elements, a beam of laser light, while modulating it with the video signals inputted into the exposing apparatus  16  from outside of the exposing apparatus. The video signals reflect the color components into which the optical image of the original has been separated. As a result, four electrostatic latent images are formed on the four photosensitive drums  10 , one for one. 
   Next, the electrostatic latent image formed on each photosensitive drum  10  is developed into a visible image (toner image) by the corresponding developing apparatus  12 . More specifically, the toner of a specific color assigned to each developing apparatus  16  is adhered to the electrostatic latent image on the corresponding photosensitive drum  10 , by the developing apparatus  12  to which a development bias is being applied. The development bias is the same in polarity as the polarity (negative) to which the photosensitive drum  10  has been charged. Each toner image is transferred (primary transfer) onto the intermediary transfer belt  17  by the primary transfer roller  18 . More specifically, while the intermediary transfer belt  17  is driven, the primary transfer bias (which is opposite in polarity to photosensitive drum  10  and toner, and therefore, positive) is applied to the primary transfer roller  18 . As a result, the toner image is transferred (primary transfer) onto the intermediary transfer belt  17 . 
   The four toner images formed on the four photosensitive drums  10 , one for one, are similarly transferred onto the intermediary transfer belt  17  by the intermediary transfer belt unit  20 ; the yellow, magenta, cyan, and black toner images are sequentially transferred in layers onto the intermediary transfer belt  17 , effecting a full-color toner image on the intermediary transfer belt  17 . 
   The transfer residual toner, or the toner remaining on each photosensitive drum  10  after the primary transfer, is scraped down by a cleaner blade, or the like, with which the drum cleaning apparatus  14  is provided, and then, is recovered. 
   The leading edge of the full-color toner image on the intermediary transfer belt  17  of the intermediary transfer belt unit  20  is moved to the secondary transfer station, which is between the driver roller  21  (which opposes secondary transfer roller  5 ) and secondary transfer roller  5 . Meanwhile, each sheet of a recording medium conveyed from a feeding-and-conveying cassette  2  or a manual feeding tray  3  is conveyed through the vertical path, and then, is conveyed to the secondary transfer station by a pair of registration rollers  4 , in synchronism with the timing with which the above-mentioned leading edge of the toner image is moved to the secondary transfer station. The toner images effecting the full-color toner images are transferred (secondary transfer) all at once by the secondary transfer roller  5 , to which the second transfer bias (which is opposite in polarity to toner, and therefore, positive) is being applied, onto the recording medium conveyed to the secondary transfer station. 
   The toner remaining on the intermediary transfer belt  17  after the secondary transfer is scraped down by a transfer cleaning apparatus  24 , and then, is conveyed to a waste toner storage container  25 . 
   The recording medium, on which the full-color toner image has just been effected, is conveyed to the fixing apparatus  6  disposed downstream. In the fixing apparatus  6 , the full-color toner image is subjected to heat and pressure in the fixation nip formed between the fixation roller  6   a  and pressure roller  6   b . As a result, the full-color toner image is thermally fixed to the surface of the recording medium. Thereafter, the recording medium is discharged onto a delivery tray  8 , which constitutes the top surface of the main assembly, by a first pair of discharge rollers  7 . This concludes the image formation sequence. 
   Incidentally, the image forming apparatus is structured so that additional discharging apparatuses  9  can be placed above the first pair of discharge rollers  7 , in the main assembly. 
   Embodiment 1 
   Next, referring to  FIGS. 2(   a ) to  3 ( b ), the structure of the housing of the image forming apparatus in the first embodiment of the present invention will be described.  FIGS. 2(   a ) and  2 ( b ) are schematic perspective views of the housing of the image forming apparatus, showing the structure thereof, and  FIGS. 3(   a ) and  3 ( b ) are perspective views of the housing for the image formation stations of the image forming apparatus, and the process cartridges, showing the positioning of the process cartridges relative to the housing. 
   First, referring to  FIGS. 2(   a ) and  2 ( b ), the image forming apparatus  1  is made up of a housing  50  (first housing) for the image formation stations (which, hereafter, will be referred to as image formation station housing  50 ), which integrally supports the process cartridges  15  and intermediary transfer belt unit  20 , and a main housing  41  (second housing), which supports the entirety of the main assembly of the image forming apparatus. The housing  50  is mounted by pins  50   c  and  50   e . The main housing  41  has a pair (left-hand and right-hand) of support rails  42  (members in the form of rails), which supports the image formation station housing  50 , when the image formation station housing  50  is pulled out of the image forming apparatus  1 . The support rails  42  can be extended or shrunk. In other words, the image formation station housing  50  can be pulled out of the image forming apparatus  1  or retracted into the image forming apparatus  1 , while remaining supported by the pair of support rails  42 , with which the main housing  41  is provided. 
   As described above, the image formation station housing  50  is structured so that it can be pulled out of the image forming apparatus  1 , or retracted into the image forming apparatus  1  while integrally supporting the process cartridge  15  and intermediary transfer belt unit  20 . Further, the image formation station housing  50  supports the intermediary transfer belt unit  20  so that the intermediary transfer belt  20  (or process cartridges) can be moved to expose the process cartridges  15 . In other words, the opening through which the process cartridges  15  can be mounted or removed can be created by moving the intermediary transfer belt unit  20 . Next, this arrangement will be described in detail. 
   The process cartridges  15  are removably mounted into the image formation station housing  50  by being inserted into the direction indicated by an arrow mark in  FIG. 3 . The image formation station housing  50  is provided with multiple sets of guiding members for mounting the process cartridges  15 , so that the process cartridges  15  are accurately positioned relative to the housing  50 , or removing the process cartridges  15  from the housing  50 . The image formation station housing  50  is structured so that after each process cartridge is inserted into the image formation station housing  50 , the process cartridge  15  is guided by the corresponding set of guiding members, to be mounted into the housing  50 , with a pair of bearings  10   a  fitted around the lengthwise ends of each photosensitive drum  10 , one for one, supported by the corresponding pair of drum supporting portions  50   a  with which the opposing surfaces of the wall of the housing  50  are provided, one for one. 
   Further, the image formation station housing  50  also integrally supports a resist-density detecting apparatus  53 , which detects the changes in the image density of the image forming apparatus and the level of positional deviation at which the four toner images are transferred onto the intermediary transfer belt  17 , by detecting the toner images on the intermediary transfer belt  17 . With the employment of the structural arrangement described above, the position of each photosensitive drum  10  is detected at an extremely high level of accuracy, and no detection error occurs. Therefore, the image forming apparatus is stabilized in terms of image density and the image position relative to the intermediary transfer belt  17 . 
   At this time, referring to  FIGS. 4(   a ) and  4 ( b ), the structure of the adjacencies of the intermediary transfer belt unit  20  will be described.  FIGS. 4(   a ) and  4 ( b ) are perspective views of the intermediary transfer belt unit  20 , showing the structure thereof. 
     FIG. 4(   a ) shows one of the intermediary transfer belt units  20  and the corresponding transfer frame  30 . Each intermediary transfer belt unit  20  and corresponding transfer frame  30  are structured so that a pair of first support shafts  20   a  (which are different in position), with which the intermediary transfer belt unit  20  is provided, are engaged, one for one, with a pair of first holding portions  30   a  (which are different in position), with which the transfer frame  30  is provided. Further, they are structured so that a pair of second support shafts  20   b  (which are different in position) with which the intermediary transfer belt unit  20  is provided, are engaged with a pair of second support portions  30   b  (which are different in position) with which the transfer frame  30  is provided. The pair of second support portions  30   b  are located so that after the joining of the intermediary transfer belt unit  20  and transfer frame  30 , they will be near the driver roller  21  used for the second transfer. Next, referring to  FIG. 4(   b ), the intermediary transfer belt unit  20  and transfer frame  30  are joined so that the pair of support shafts  20   a  are inserted into the pair of support portions  30   a , one for one, and the pair of support shafts  20   b  are inserted into the pair of support portions  30   b , one for one. 
   During this joining, a pair of first pressure application springs  30   c  located in the transfer frame  30  are attached, in the compressed state, to a pair of first support portions  20   c , one for one, with which the intermediary transfer belt unit  20  is provided, and a pair of second pressure application springs  30   d  located also in the transfer frame  30  are attached, in the compressed state, to a pair of second support portions  20   d , one for one, with which the intermediary transfer belt unit  20  is provided. The pair of second support portions  20   d  is located so that they will be located near the driver roller  21  after the joining of the intermediary transfer belt unit  20  and transfer frame  30 . 
   Next, referring to  FIGS. 5(   a ) and  5 ( b ), the relationship between the intermediary transfer belt unit  20  and the image formation station housing  50 , after the joining of the intermediary transfer belt unit  20  and transfer frame  30 , will be described. As shown in  FIG. 5(   a ), after being joined with each other by locators  12   a , the intermediary transfer belt unit  20  and transfer frame  30  are supported (at two locations, that is, at widthwise ends) by the image formation station housing  50  (in which process cartridges  15  have been mounted) so that they can be rotated about the rotational axle  30   e  (support portion) located in the transfer frame  30 . In this case, the rotational axle  30   e  is a means for moving the above-mentioned intermediary transfer belt unit  20  to expose the process cartridges  15  from behind the intermediary transfer belt unit  20 . 
   Further, the above-mentioned rotational axle  30   e  is disposed so that its rotational axis is roughly parallel to the rotational axis of the photosensitive drum  10 . 
   Further, the above-mentioned rotational axle  30   e  is located at one of the ends of the transfer frame  30  in terms of the direction in which the process cartridges  15  are aligned in the image formation station housing  50 . In this embodiment, the image forming apparatus is structured so that the rotational axle  30   e  is located at the end of the transfer frame  30 , which is located next to the process cartridge slot located farthest from the secondary transfer station. However, the image forming apparatus may be structured so that the rotational axle  30   e  is located at the end of the transfer frame  30 , which is located next to the closest process cartridge slot to the secondary transfer station. 
   The intermediary transfer belt unit  20 , which is rotatable about the rotational axle  30   e , is provided with a pair of bearings  20   e , by which the driver roller  21  is supported at its lengthwise ends. The bearings  20   e  constituting the pair are located at the free end of the intermediary transfer belt unit  20 , and are inserted into the support holes  50   b  (different in position), one for one, with which the image formation station housing  50  is provided. Further, the transfer frame  30  is provided with a pair of positioning members  30   f  ( FIGS. 4(   a ) and  4 ( b )), which are engaged, one for one, with a pair of support shafts  50   e  ( FIG. 3(   b )) with which the image formation station housing  50  is provided. With the employment of this structural arrangement, it is possible to form an image formation unit  60  ( FIGS. 5(   a ) and  5 ( b )) in which the intermediary transfer belt unit  20  is accurately positioned relative to the image formation station housing  50 , relative to which the process cartridges  15  are accurately positioned. 
   With the employment of the structural arrangement, which supports the process cartridges  15  and intermediary transfer belt unit  20  by the image formation station housing  50 , the intermediary transfer belt unit  20  can be positioned relative to the photosensitive drums  10  at an extremely high level of accuracy, making it possible to produce excellent images, more specifically, images which do not suffer from positional deviation. 
   Further, the above-mentioned image formation station housing  50  supports so that the intermediary transfer belt unit  20  can be rotationally moved to expose the process cartridges  15  mounted in the housing  50 . With the employment of this structural feature, the image forming apparatus can be improved in terms of the operability regarding the replacement of the process cartridges  15  by a user, while ensuring that the intermediary transfer belt unit  20  is positioned relative to each of the photosensitive drums  10  at a high level of accuracy. 
   The process cartridges  15  are to be replaced in the following manner. That is, first, the above-mentioned image formation station housing  50  is pulled out of the image forming apparatus, and then, the intermediary transfer belt unit  20  is rotationally moved away from the process cartridges  15 . Then, the process cartridges  15  are replaced. Therefore, it does not occur that toner scatters in the image forming apparatus when a process cartridge is or process cartridges are replaced. Therefore, the contamination of the interior of the image forming apparatus, and the formation of unsatisfactory images, which are attributable to the scattering of toner in the image forming apparatus, which occurs when a process cartridge or process cartridges are replaced, do not occur. In particular, the exposing apparatus  16  is positioned below the process cartridges  15  and is fixedly disposed in the main assembly of the image forming apparatus. Therefore, the formation of unsatisfactory images, the defects of which are attributable to the falling of toner, which occurs when cartridges are replaced, does not occur. 
   The image formation station housing  50  (image formation unit  60 ), by which the process cartridges  15  and intermediary transfer belt unit  20  are integrally supported, is retractable into the image forming apparatus  1  by being supported and guided by the pair of support rails  42 . Support rails  42  are supported by angle bracket  40   a . As the housing  50  is retracted, the driving force transmitting portion for transmitting driving force to the process cartridges  15  and intermediary transfer belt unit  20 , which the housing  50  supports, engages with the driving portion, which is disposed in the main housing  41 , to transmit the driving force to the above-mentioned driving force transmitting portion for transmitting driving force to the process cartridges  15  and intermediary transfer belt unit  20 . More specifically, a first drum coupling  10   b , shown in  FIG. 5(   a ), for driving the photosensitive drum  10  of the process cartridge  15 , engages with a second drum coupling  43   a , shown in  FIG. 5(   b ), disposed in the main housing  41  of the main assembly of the image forming apparatus  1  to drive the photosensitive drum  10 . Further, a first development coupling  12   a  for driving the developing apparatus  12  of the process cartridge  15  engages with a second development coupling  43   b , disposed in the main housing  41 , to drive the developing apparatus  12 . Further, a first driver roller coupling  21   a  of the intermediary transfer belt unit  20  engages with a second driver roller coupling  43   c , disposed in the main housing  41 , to drive the driving roller  21 . Thus, as the driving portion  43  of the main assembly operates, a driving force can be transmitted to the process cartridges  15  and intermediary transfer belt unit  20 , which are supported by the image formation station housing  50 , through these couplings. 
   Incidentally, the image forming apparatus is structured so that four positioning pins  50   e  (different in position), shown in  FIG. 5(   a ), with which the image formation station housing  50  is provided, fit into the positioning holes  41   a , with which the main housing  41  is provided. 
   Next, referring to  FIGS. 6(   a ),  6 ( b ) and  7 , the method for replacing the process cartridges  15  will be described.  FIGS. 6(   a ),  6 ( b ) and  7  are perspective views showing the flow of the operation for replacing the process cartridges  15 . 
   Referring to  FIG. 6(   a ), first, a front cover  9   a  of the image forming apparatus  1  is to be opened in the direction indicated by an arrow mark Q 1 . Next, the pressure being applied to the intermediary transfer belt  17  by the secondary transfer roller  5  is to be removed. For this pressure removal, the second transfer roller  5  is electrically or mechanically moved in the direction indicated by an arrow mark V 1  in  FIG. 1 . As an example of the structural arrangement for electrically removing the pressure being applied by the secondary transfer roller  5 , there is a structural arrangement that applies or removes the pressure from the secondary transfer roller  5  by turning on or off a solenoid switch. As a structural arrangement for mechanically removing the pressure being applied by the secondary transfer roller  5 , there is a structural arrangement in which the pressure being applied by the second transfer roller  5  can be removed by rotating a lever, which supports the secondary transfer roller  5 , so that the secondary transfer roller  5  can be pressed against, or moved away from, the driver roller  21  (intermediary transfer belt  17 ). 
   Incidentally, the structural arrangement for removing the pressure being applied to the intermediary transfer belt  17  by the secondary transfer roller  5  does not need to be limited to those described above. For example, the secondary transfer roller  5  may be supported by a conveyance door  9   b , which can be opened or closed relative to the main assembly of the image forming apparatus  1 , as shown in  FIG. 8 , so that the pressure being applied by the secondary transfer roller  5  can be removed by opening this conveyance door  9   b  in the direction indicated by an arrow mark Q 3 . 
   Next, the image formation unit  60  (integral assembly of transfer frame  30 , intermediary transfer belt unit  20 , process cartridges  15 , and image formation station housing  50 , which supports preceding components), is to be pulled out frontward of the main assembly, that is, in the direction indicated by an arrow marked P 2 , as shown in  FIG. 6(   b ). 
   Next, referring to  FIG. 7 , the integral combination of the intermediary transfer belt unit  20  and transfer frame  30  is to be rotated about the rotational shaft  30   e  in the direction indicated by an arrow mark Q 2 , to cause the combination to be held upright by a holding means, such as a damper. By this operation, the process cartridges  15 , mounted in the image formation station housing  50 , are exposed from behind the intermediary transfer belt unit  20 , making it possible to replace the process cartridges  15 . In other words, the space necessary to mount or to remove the process cartridge  15  is provided. Thus, the process cartridges  15  can be pulled out upward (indicated by arrow mark T 1 ) to be replaced. 
   In other words, the intermediary transfer belt unit  20  is rotated away from, or rotated to, the image formation station housing  50 , after the image formation unit  60  is pulled out of the image forming apparatus. Therefore, it does not occur that the toner, which scatters as the intermediary transfer belt unit  20  is rotated, falls into the image forming apparatus. Further, the process cartridges  15  and intermediary transfer belt unit  20  are integrally held by the image formation station housing  50 . Therefore, the level of accuracy, at which the photosensitive drums  10  are positioned relative to the intermediary transfer belt unit  20 , is extremely high, and the amount by which toner scatters is extremely small. 
   In the case of the structural arrangement shown in  FIGS. 6(   a ) to  8 , the rotational shaft  30   e  of the transfer frame  30  is parallel to the axial lines of the photosensitive drums  10  in the process cartridges  15 , and is located at the farthest end of the transfer frame  30  from the secondary transfer station. The structural arrangement for allowing the above-mentioned rotation of the intermediary transfer belt member  20  does not need to be limited to the above-mentioned one. For example, the above-mentioned rotational shaft  30   e  may be located at the other end of the transfer frame  30 , that is, the end which is next to the secondary transfer station (position  30   f  indicated by dotted line in  FIG. 7) , so that the intermediary transfer belt unit  20  can be rotated about the thus positioned rotational shaft  30   e  in the direction indicated by an arrow mark Q 4 . 
   Further, in this embodiment, the rotational shaft  30   e  of the transfer frame  30  is parallel to the axial lines of the photosensitive drums  10  of the process cartridges  15 . However, the rotational shaft  30   e  does not need to be parallel to the axial lines of the photosensitive drums  10 . For example, a rotational shaft  30   e , as a supporting portion, may be positioned so that its rotational axis is roughly perpendicular to the rotational axes of the photosensitive drums  10 . More specifically, referring to  FIG. 9 , the rotational shaft  30   e  may be positioned at the back end of the transfer frame  30  in terms of the direction in which the image formation station housing  50  is moved relative to the main assembly, so that the intermediary transfer belt unit  20  can be rotated about the rotational shaft  30   e  in the direction indicated by an arrow mark Q 5 , or the rotational shaft  30   e  may be positioned at the front end of the transfer frame  301 , in terms of the direction in which the image formation station housing  50  is moved relative to the main assembly, so that the intermediary transfer belt unit  20  can be rotated frontward about the rotational shaft  30   e . In these cases, however, the amount by which the image formation unit  60  is pulled out of the image forming apparatus must be increased by S 1 , as compared to the setup shown in  FIGS. 6(   a ) to  8 . Also, as shown in  FIG. 9 , the image formation station housing  50  is to be pulled out frontward of the main assembly in the direction indicated by an arrow marked P 3 . 
   Further, in the case of the embodiment described above, each of the process units was made up of a photosensitive drum and a developing apparatus, which are integrally combined. However, the makeup of the process unit does not need to be limited to that in this embodiment. For example, a developing apparatus  12 , as a processing means which acts on the photosensitive drum  10 , may be made to be a process unit, which is independent from the process cartridge  15  (process unit) inclusive of the photosensitive drum  10 , and which is removably mountable in the image formation station housing  50 , as shown in  FIG. 10 . In this case, when removing any of the process units (process cartridges  15  and developing apparatus  12 ) from the image formation station housing  50 , first, the pressure applied by the pressure application springs  51  and  52 , which are pressing each of the units  15  and  12 , are to be removed, so that formation station housing  50 , as shown in  FIG. 10 . In this case, when removing any of the process units (process cartridges  15  and developing apparatus  12 ) from the image formation station housing  50 , first, the pressure applied by the pressure application springs  51  and  52 , which are pressing each of the units  15  and  12 , are to be removed, so that these process units may be removed from each of the units  15  and  12  can be individually replaced. This structural arrangement makes it possible to individually replace the process cartridges  15  and developing apparatus  12  according to their lives, this being extremely effective to reduce the operational cost of the image forming apparatus. 
   Further, the above-described embodiment exemplifies a structured arrangement in which the intermediary transfer belt unit  20  is supported by the transfer frame  30 . However, the application of the present invention does not need to be limited to the above-described one. For example, even if no transfer frame is provided, effects similar to the above-described ones can be obtained by the employment of a structural arrangement in which the intermediary transfer belt unit  20  is provided with a rotational shaft (supporting portion) and the intermediary transfer belt unit  20  is rotatably attached to the image formation station housing  50 . 
   In addition, the above-described embodiment exemplifies a structural arrangement in which the intermediary transfer belt unit  20  is rotationally moved away to allow the process cartridges to be replaced. However, the application of the present invention does not need to be limited to the above-described one. For example, in order to make it possible to replace process units, an image forming apparatus may be provided with a means for sliding the intermediary transfer belt unit  20  away from the image formation station housing  50 . Further, an image forming apparatus may be structured so that process units can be moved, instead of the intermediary transfer belt unit, to allow the process units to be exposed from behind the intermediary transfer belt unit, in order to allow the process units to be replaced. In other words, all that is necessary is that an image forming apparatus is structured so that, after the image formation station housing is pulled out, the positional relationship between the intermediary transfer belt unit and process units can be changed, to create the space through which the process units can be mounted or removed. 
   Embodiment 2 
   Next, referring to  FIGS. 11 ,  12 ( a ) and  12 ( b ), the image forming apparatus in the second embodiment of the present invention will be described. This embodiment exemplifies a structural arrangement in which the photosensitive drums  10  are not horizontally aligned, more specifically, they are aligned in such an inclined straight line L that the closer a photosensitive drum  10  is to the secondary transfer station, the lower in position is the photosensitive drum  10 . 
   First, referring to  FIG. 11 , which is a sectional view of the image forming apparatus in the second embodiment of the present invention, the multiple process cartridges  15  are arranged in parallel or juxtaposed, and also, are aligned so that the farther a process cartridge  15  is from the secondary transfer station J, the higher in position the process cartridge than a process cartridge  15  located closer to the secondary transfer station J. Therefore, the process cartridge  15  having the photosensitive drum  10 , which is closest to the secondary transfer station J, is positioned lowest, and the process cartridge  15  having the photosensitive drum  10 , which is farthest from the secondary transfer station J, is highest, in position. Further, the intermediary transfer belt  17  is tilted downward toward the secondary transfer station, in parallel to the theoretical line connecting the center of the photosensitive drum  10  positioned highest and that of the photosensitive drum  10  positioned lowest. 
   Next, referring to  FIGS. 12(   a ) and  12 ( b ), the method for replacing the process cartridges  15  will be described.  FIGS. 12(   a ) and  12 ( b ) show the method for replacing the process cartridges  15 . 
   Referring to  FIG. 12(   a ), similar to the first embodiment, after the image formation unit  60  is pulled out frontward of the main assembly (in the direction indicated by arrow mark P 2 ), the integrally combined intermediary transfer belt unit  20  and transfer frame  30  are rotationally moved about the rotational shaft  30   e , in the direction indicated by an arrow mark Q 6 . With this operation, the process cartridges  15  are exposed from behind the intermediary transfer belt unit  20 , being rendered replaceable. Thereafter, the process cartridges  15  can be taken out of the image formation station housing  50  in the direction indicated by an arrow mark T 2  to be replaced. 
   Incidentally, referring to  FIG. 12(   b ), in order reduce the size of the main assembly of the image forming apparatus in terms of the direction indicated by an arrow mark X, it is necessary to place two adjacent process cartridges  15  closer to each other. In this case, if an attempt is made to take a process cartridge  15  out of the image formation station housing  50  in the vertical direction (direction indicated by arrow mark T 1 ), the process cartridge  15  collides with the adjacent process cartridge on the upstream side (portion K in  FIGS. 12(   a ) and  12 ( b )). Therefore, such a structural arrangement is adopted in order for the process cartridges  15  to be moved out in the direction indicated by the arrow mark T 2 , which is roughly perpendicular to the theoretical line which connects the center lines of four photosensitive drums  10 . 
   In this case, it is desired that the rotational shaft  30   e  be positioned at the end of the transfer frame  30 , which is next to the farthest cartridge  15  from the secondary transfer station J, as shown in  FIG. 12(   b ). The employment of this structural arrangement prevents the intermediary transfer belt unit  20  from interfering when taking the process cartridges  15  out of the image formation station housing  50  in the direction indicated by the arrow mark T 2 , drastically improving the image forming apparatus in terms of the replaceability of the process cartridges  15 . 
   Incidentally, regarding the structural arrangement in which multiple process cartridges are aligned in parallel in the above-described inclined straight line, the position of the rotational shaft  30   e  does not need to be limited to the end of the transfer frame  30 , which is next to the farthest process cartridge  15  from the secondary transfer station J, shown in  FIGS. 12(   a ) and  12 ( b ). All that is necessary is to position the rotational shaft  30   e  at the end of the transfer frame  30 , which is next to the process cartridge  15 , which is positioned highest among the process cartridges aligned in the inclined straight line. 
   For example, an image forming apparatus, in which multiple process cartridges are aligned in parallel in such an inclined straight line that the farthest process cartridge form the secondary transfer station is positioned lower than the closest process cartridge to the secondary transfer station, is feasible. In this case, it is desired that the above-mentioned rotational shaft be positioned at the end of the transfer frame, which is next to the closest process cartridge to the above-mentioned secondary transfer station. 
   Miscellaneous Embodiments 
   The above-described embodiments exemplify a structural arrangement in which each process cartridge  15  integrally comprises the toner storage portion (developer supplying means) for supplying toner, and the waste toner storage portion (developer recording means). However, the application of the present invention is not limited to this arrangement. For example, the developer supplying means for supplying developer or the developer recovering means for recovering developer may be rendered independent from the process cartridges and be removably mountable in the image formation station housing, as shown in  FIGS. 13(   a ) to  4 ( b ). Next, this structural arrangement will be described. 
   First, referring to  FIG. 13(   a ), which is a sectional view of the right-hand side of the main assembly, the image formation unit  60  is present. This image formation unit  60  includes a process cartridge  15  removably mountable in the image formation unit  60 , a developer supplying means independent from the process cartridge  15 , and a developer recovering means independent from the process cartridge  15 . The transfer frame  30  integrally combined with the intermediary transfer belt unit  20  is supported so that it can be rotated, much like a hinged door. Further, the image formation unit  60  includes a replenishment toner cartridge  61  as the developer supplying means, which is removably mountable. The toner in the replenishment toner cartridge  61  is supplied to the developing apparatus in the process cartridge by a toner supplying apparatus  62 . Further, the image formation unit  60  includes a waste toner recovering apparatus  63  as the developer recovering means, which is removably mountable. Referring to  FIGS. 13(   a ) to  13 ( c ), designated by a referential symbol  64  is a high voltage apparatus for providing the process cartridges with a high voltage for a development process and a charging process. The high voltage apparatus  64  is contained in the image formation unit  60 . The replenishment toner cartridge  61  and waste toner recovering apparatus  63  are removably mounted on the front portion of the image formation unit  60  (image formation station housing), which can be pulled out in the direction indicated by the arrow mark P 2 . Therefore, the replenishment toner cartridge  61  and waste toner recovering apparatus  63  can be individually replaced according to their service life, while the image formation unit  60  remains retracted in the main assembly of the image forming apparatus. 
   Next, referring to  FIGS. 13(   b ) and  13 ( c ), when it is necessary to replace any of the process cartridges  15 , first, the image formation unit  60  is to be pulled out frontward of the main assembly (direction indicated by arrow mark P 2 ). Then, the joined intermediary transfer belt unit  20  and transfer frame  30  are to be rotated in the direction indicated by an arrow mark Q 7  to expose the process cartridges  15 . Then, the process cartridges  15  can be pulled out in the direction indicated by the arrow mark T 2  to be replaced. 
   In the case of a structural arrangement, such as the above-described one in which the process cartridges  15 , replenishment toner cartridges  61 , and waste toner recovering apparatuses  63  are individually replaceable, toner is transferred among the units. 
   In the embodiments described above, the replenishment toner cartridges  61 , process cartridges  15 , and waste toner recovering apparatuses  63  are removably supported by the image formation station housing  50 , being thereby ensured in terms of the accuracy in their positional relationship among the units. Therefore, the scattering of toner rarely occurs when any of the units is replaced or toner is conveyed. Further, the process cartridges  15  are replaced after the replenishment toner cartridges  61  and waste toner recovering apparatus  63  are moved out of the main assembly of the image forming apparatus  1 , along with the image formation unit  60 . Therefore, the scattering of toner, in the image forming apparatus, which is attributable to the replacement of the process cartridges  15 , and the formation of unsatisfactory images attributable to the scattering of toner, do not occur. 
   Incidentally, the embodiments described above exemplify a structural arrangement in which both the developer supplying means and developer recovering means are rendered removably mountable in the image formation station housing. However, the application of the present invention is not limited to this structural arrangement. For example, an image forming apparatus may be structured so that either the developing supplying means or developer recovering means is removably mountable in the image formation station housing. 
   Further, the measurements, materials, and shapes of the structural components, and their positional relationships, of the image forming apparatus described above, are not intended to limit the scope of the present invention, unless specifically noted. 
   While the invention has been described with reference to the preferred embodiments of the present invention disclosed herein, it is not confined to the embodiments described above, 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.