Abstract:
An image forming apparatus includes a plurality of image carrying members and an image carrier. Each of the plurality of image carrying members is configured to form and carry a toner image. The image carrier cassette is configured to be detachably installed to the image forming apparatus and includes an image carrier holder. The image carrier holder is configured to rotatably hold the plurality of image carrying members and to allow each of the plurality of image carrying members to be independently removed therefrom.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus provided with a detachable image bearing cassette having plural image bearing members.  
         [0003]     2. Description of the Related Art  
         [0004]     Up to now, in the field of image forming apparatuses, there has been proposed a so-called process cartridge detachable to an image forming apparatus, which is manufactured by integrating a photosensitive member. Also, there has been proposed an image forming unit that integrates plural photosensitive members and another image forming mechanism and is detachable to an image forming apparatus main body.  
         [0005]     The above process cartridge and image forming unit are advantageous in that a user can easily perform replacement and maintenance without the help of a specialist. However, in such structure that the photosensitive members and the image forming mechanism are integrated, the entire process cartridge and the entire image forming unit should be replaced even if a user desires to replace a part of the photosensitive members and the image forming mechanism. Worse yet, in the case of the image forming unit having plural short-life photosensitive members, if any one of the photosensitive members needs to be replaced, the entire unit should be replaced even though the remaining members are serviceable. With regard to a color printer having four photosensitive members, the life of a black photosensitive member expires sooner than photosensitive members of the other colors due to frequent use. Thus, in order to replace the black photosensitive member, the entire image forming unit including the photosensitive members of the other colors should be replaced.  
         [0006]     Further, in the process cartridge, a container of a cleaning device for rotatably supporting a photosensitive member is integrated with the photosensitive member. The cleaning container is provided with a process mechanism for charging the photosensitive member to form an image. The process mechanism refers to a charging roller for uniformly charging the photosensitive member, and a cleaning blade and cleaning roller for scraping off residual toner that remains on the photosensitive member even after transferring a toner image onto a sheet-like medium (hereinafter referred to as “sheet”). These are provided around the photosensitive member. The process cartridge can be attached/removed to/from an image forming apparatus, and replaced by a user himself/herself at the end of its useful life or when waste toner is fully filled.  
         [0007]     In addition, the image forming unit incorporates plural photosensitive members, charging rollers and developing devices corresponding to the individual photosensitive members, and plural intermediate transfer drums as main components. The unit can be inserted/removed to/from the image forming apparatus, and replaced by the user himself/herself at the end of the useful life of the photosensitive members or intermediate transfer drums.  
         [0008]     Meanwhile, there has been known a two-station recording system. In this two-station recording system, a developing device, a writing device, and a driving mechanism are supported to an apparatus main body via a common assembly member, and are positioned with high accuracy. According to such structure, the developing device is positioned relative to the apparatus main body and thus used as a reference position (assembly reference) for all components of an image forming process. The photosensitive member (or photosensitive unit) is incorporated into the developing device, not the apparatus main body. That is, the photosensitive member (or photosensitive unit) is positioned only relative to the developing device, so a relationship between the developing device and the photosensitive member (or photosensitive unit) is a master-servant relationship. The photosensitive member (or photosensitive unit) is detachable to the developing device, and the developing device is detachable to the image forming apparatus. Further, the photosensitive drum, the charging mechanism, and the cleaning mechanism are integrated together.  
         [0009]     In recent years, stringent market requirements have been imposed on an image forming apparatus such as a printer or a copier. Accordingly, an image forming unit has become more important. Furthermore, it is known that the market requirements place importance on the photosensitive member out of the image forming mechanism. A detailed description thereof is given below from the viewpoints of three factors.  
         [0010]     A first factor is reduction in diameter in photosensitive member accompanying downsizing of image forming apparatus. In recent years, market requirements have been directed toward downsizing an office machine, and accordingly, a downsizing is required of an image forming unit. However, if a drum-like photosensitive member as an image bearing member is reduced in size, that is, in diameter, a wearing degree per sheet is increased under the same image forming conditions. For example, assuming that the photosensitive diameter is reduced from 120 mm to 40 mm, the 40-mm photosensitive member needs to rotate at a speed three times higher than that of the 120-mm photosensitive member in order to form an image of the same size. This trebles various types of wearing of a photosensitive member upon image formation, such as an electrical wearing due to discharge etc. in a charging unit or a mechanical wearing due to a blade in a cleaning unit. Hitherto, a developing device or other such devices have been downsized to some degree. However, size reduction of a photosensitive member has not been pursued as actively as other image forming units, from the viewpoint of avoiding the above wearing. Nevertheless, it is inevitable that the photosensitive member will be downsized to meet an increasing requirement for size reduction. As mentioned above, the requirement for size reduction places much weight on the photosensitive member and shortens its useful life.  
         [0011]     A second factor is slimness of photosensitive member along with high image quality. In recent years, users have output photographic images or graphic documents more than before. Accordingly, an attempt to increase an image quality up to a film image has been made. A typical way to increase the image quality is increasing a resolution. However, a slim photosensitive member is required for increasing the resolution in an electrophotographic process. For example, in a negatively-charged photosensitive member, carriers generated in a CGL (charge carrier generation layer) pass through a CTL (charge carrier transport layer) to reach the photosensitive member surface to thereby form a latent image on the photosensitive member. If the CLT is thick in this case, the carriers should move for a long distance, with the result that the carriers are separated from one another due to electrical repulsion between the carriers. If so, a latent image corresponding to a writing signal cannot be formed, resulting in an image dot positions of which are a little incorrect. As a result, an image quality is deteriorated. This problem arises not only in the case of increasing the resolution of the electrophotography from 600 dpi to 1200 dpi but also in the case of improving an image quality with the resolution kept at 600 dpi, for example, in order to meet recent requirements for high image quality. The photosensitive member should be made slim and the movement distance of carriers should be reduced for preventing the deterioration of image quality. To that end, recent photosensitive members are made thinner. However, the photosensitive member is worn, for example, worn away by a cleaning every image formation process. Thus, the life of a thinner photosensitive member expires after fewer image formation processes. This shortens the life of the photosensitive member.  
         [0012]     A third factor is an increase in importance of photosensitive member along with rise in popularity of color image. In recent years, a popularity of a color image has risen on the market from the viewpoint of facilitating the understanding of information. Incidentally, as for a color image, a photographic image or a graphic image that would occupy a wide area of a recording sheet is printed out in many cases unlike a monochrome image which mainly includes character information. In addition, the background includes solid fills in many cases. Thus, an image formation area per image formation process increases, and accordingly, an image forming unit including the photosensitive member is worn more and more. Meanwhile, an image forming apparatus of a revolver type where plural developing devices correspond to one photosensitive member, for example, has been known. Such an image forming apparatus has been widely available on the market because the number of parts can be reduced and a color image can be formed with relatively low cost. However, latent images are developed onto the photosensitive member by plural developing devices for forming one color image, so the photosensitive member is worn at a speed several times higher than the developing devices. In the above image forming apparatus, a wearing of the photosensitive member accompanying the formation of color images is particularly conspicuous. As described above, the formation of color images is one of the factors that shorten the photosensitive member&#39;s life.  
         [0013]     Based on the foregoing three factors of the downsizing, the high-quality image formation, and the color image formation, it is supposed that the photosensitive member be worn at higher speed than the other image forming unit. Needless to say, studies about how to improve the durability of the photosensitive member and how to lengthen its usable life have been under progress, but similar studies have been made on the other image forming unit. This means that the photosensitive member&#39;s life tends to become shorter than that of the other image forming unit.  
         [0014]     This leads to an unbalanced relation in usable lifetime between the process cartridge or photosensitive member incorporated in the image forming unit and the other image forming mechanisms. That is, the conventional process cartridge or image forming unit faces a problem in that the process cartridge should be replaced with reference to an image forming mechanism having the shortest usable life. According as the photosensitive member&#39;s life is shortened, the above problem becomes noticeable. Hence, the other image forming mechanisms should be disadvantageously replaced with reference to the short-life photosensitive member. As regards the image forming unit, the unit is replaced with reference to a photosensitive member having the shortest life among the plural photosensitive members.  
         [0015]     In such a circumstance, a serviceable image forming unit should be discarded or recycled to impose an economical burden on a user, wastefully expend a manufacturer&#39;s effort to recover units, and adversely affect the environment.  
         [0016]     There has been proposed an image forming apparatus where a short-life component out of the components of image forming mechanisms is preferentially replaced to reduce the burdens on the user, the manufacturer, and the environment with a view to overcoming the drawbacks resulting from the recent market requirements. According to the image forming apparatus, the plural photosensitive members can be individually and replaced as a photosensitive cassette by a user. Hence, the apparatus can deal with even such a situation that a replacement timing varies between the photosensitive members.  
       SUMMARY OF THE INVENTION  
       [0017]     In view of the foregoing, the present invention provide a novel image forming apparatus which improves a handling of attachment and detachment of photoconductive members. In one example, a novel image forming apparatus includes a plurality of image carrying members and an image carrier. Each of the plurality of image carrying members is configured to form and carry a toner image. The image carrier cassette is configured to be detachably installed to the image forming apparatus and includes an image carrier holder. The image carrier holder is configured to rotatably hold the plurality of image carrying members and to allow each of the plurality of image carrying members to be independently removed therefrom.  
         [0018]     The above-mentioned image forming apparatus may further include a positioning mechanism configured to determine positions of driving members integrally mounted to and driving the plurality of image carrying members such that most eccentric points of the driving members are arranged at identical positions to match relative driving phases of the driving members.  
         [0019]     The positioning mechanism may includes a sensor configured to detect marks provided to the driving members to indicate positions of the most eccentric points.  
         [0020]     The image carrier cassette may allow each of the plurality of image carrying members to be independently removed therefrom and attached thereto under a condition that the image carrier cassette is installed in the image forming apparatus.  
         [0021]     The above-mentioned image forming apparatus may further include a plurality of image forming components arranged at respective predetermined fixing positions around each one of the plurality of image carrying members.  
         [0022]     The above-mentioned image carrier cassette may be removable from the image forming apparatus after at least one of the plurality of image forming components is moved away from the predetermined fixing position.  
         [0023]     The above-mentioned image carrier cassette may allow each of the plurality of image carrying members to be independently removed therefrom and attached thereto under a condition that the image carrier cassette is in a state detached from the image forming apparatus.  
         [0024]     The above-mentioned image carrier cassette may be re-installable into the image forming apparatus under a condition that the at least one of the plurality of image forming components is moved away from the predetermined fixing position.  
         [0025]     The above-mentioned at least one of the plurality of image forming components may be reset to the predetermined fixing position when the image carrier cassette is reinstalled into the image forming apparatus. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0027]      FIG. 1  is a schematic sectional view showing the schematic structure of a two-station type image forming apparatus to which an image forming apparatus of the present invention is applied;  
         [0028]      FIG. 2  is a sectional view showing the structure of an image forming apparatus according to an embodiment of the present invention;  
         [0029]      FIG. 3  is a plan view showing the schematic structure of an image station in the image forming apparatus according to the embodiment of the present invention;  
         [0030]      FIG. 4  is a perspective view showing the schematic structure of each photosensitive member in each image station;  
         [0031]      FIG. 5  is a schematic sectional view showing the outline of a driving mechanism of a photosensitive member;  
         [0032]      FIG. 6  is a schematic sectional view showing the structure of a photosensitive cassette;  
         [0033]      FIG. 7  is a schematic sectional view showing each image station attached and positioned in a reference setting position in an image forming apparatus main body according to the embodiment of the present invention;  
         [0034]      FIG. 8  is a schematic sectional view showing a detachment step;  
         [0035]      FIG. 9  is a schematic sectional view showing a detachment step;  
         [0036]      FIG. 10  is a schematic sectional view showing a detachment step;  
         [0037]      FIG. 11  is a schematic sectional view showing how a photosensitive cassette is replaced in the image forming apparatus according to the embodiment of the present invention;  
         [0038]      FIG. 12  is a perspective view showing the schematic structure of each photosensitive member in each image station for individually attaching/detaching photosensitive members to/from a photosensitive cassette; and  
         [0039]      FIG. 13  is a perspective view showing another schematic structure of each photosensitive member in each image station. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus according to one embodiment of the present invention is explained.  
         [0041]      FIG. 1  is a schematic sectional view showing the schematic structure of a two-station type image forming apparatus to which an image forming apparatus of the present invention is applied. The image forming apparatus of  FIG. 1  typifies an intermediate transfer type color image forming apparatus in a two-station type recording system. In  FIG. 1 , an intermediate transfer belt  110  as an intermediate transfer medium is stretched over two opposing rollers  120  and  130  that are provided away from each other. The intermediate transfer belt  110  is rotated by these rollers  120  and  130 . In addition, the intermediate transfer belt  110  is arranged in a vicinity of an image formation process mechanism which is described below. Assuming that the intermediate transfer belt  110  rotates in the direction of the arrow A, a first image station  140  and a second image station  150  are provided as the image formation process mechanism in this order from an upstream side in the belt rotation direction. Further, a transfer roller  160  as an image transferring mechanism is provided to contact with and to separate from the roller  130  across the intermediate transfer belt  110 . In addition, a cleaning blade  170  is provided to contact with and to separate from the roller  120  across the intermediate transfer belt  110 .  
         [0042]     An image formation process of such an image forming apparatus is based on a general electrostatic recording system from the viewpoint of the first image station  140 . In the dark, an optical wiring unit  180  writes an electrostatic latent image of a corresponding color onto a photosensitive member  141  uniformly charged with a charging mechanism (not shown). Then, a developing device  144  develops the electrostatic latent image as a visual image to transfer a toner image onto the intermediate transfer belt  110 . Incidentally, the developing device  144  of the first image station  140  and a developing device  154  of a second image station  150  have visualizing functions using two different colors. Since four colors can be prepared by adding black to three primary colors, these four colors may be assigned to a corresponding one of a magenta developing device  142 , a cyan developing device  143 , a yellow developing device  152 , and a black developing device  153  to form a full-color image.  
         [0043]     Hence, the same image formation area of the intermediate transfer belt  110  passes sequentially through the two image stations  140  and  150  while being applied with a transfer bias voltage by transfer rolls  145  and  155  which serve as an intermediately-image-transferring mechanism and oppose the drum-like photosensitive members  141  and  151 , respectively, to thereby multiply transfer toner images onto the intermediate transfer belt  110  for each color. The image area of the intermediate transfer belt  110  that has been multiply transferred with the two toner images of different colors is passed sequentially through the two image stations  140  and  150  again while being multiply transferred with toner images of colors different from the previous ones by each image station. In this way, after the same image formation area has passed through the image stations  140  and  150  again, a full-color toner image can be formed through the multiple transfer in the same image formation area.  
         [0044]     Then, the formed full-color toner image is transferred onto a sheet P as a sheet-like medium. The transfer is carried out such that a transfer bias voltage is applied to a transfer roller  160  for a final transfer process which is brought into close contact with the roller  130  across the intermediate transfer belt  110  to pass the sheet P through a nip portion between the transfer roller  160  and the intermediate transfer belt  110 . After the final transfer process, the full-color toner image formed on the sheet P is fixed by a fixing mechanism  190  to form the final full-color image on the sheet P.  
         [0045]      FIG. 2  is a sectional view showing the structure of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus of  FIG. 2  is structured such that a transfer sheet holding unit  201   a , a sheet feeing roll  201   b , a wiring device  202 , developing devices  203  and  204 , an intermediate transfer belt  205  for intermediate transfer, a fixing mechanism  206 , and an electrical system  207  are stacked in this order from the bottom. Further, a sheet conveying path including a manual feeding roller  208 , registration rollers  209   a  and  209   b , and a transfer roller  210  is provided substantially vertically, and passes through the transfer sheet holding unit  201   a  and the sheet feeding roll  201   b  and extends upward from a sheet transfer unit as a nip portion between the transfer rollers  210  and  211  towards a sheet discharging unit  212  through a fixing unit including the fixing mechanism  206 .  
         [0046]     The writing device  202  includes a laser optical system using a light source such as a semiconductor laser, an LED (light-emitting diode), or the like. The following drum-like photosensitive members  213  and  214  as image bearing members are exposed to light corresponding to image information. More specifically, two semiconductor lasers (not shown) apply laser beams corresponding to the image information to stacked polygon mirrors  202   a . Then, the light reflected by the polygon mirrors  202   a  passes through scanning lenses  202   b  and  202   c , and is reflected by reflection mirrors  202   d  and focused in exposure positions of the rotating photosensitive members  213  and  214 .  
         [0047]     Incidentally, each optical part is positioned and fixed to a housing  215  serving as a base of an apparatus main body casing. In this embodiment, a two-beam laser scanning system is used, but a writing method or structure is not limited thereto. In this embodiment, the wiring device  202  is provided below the photosensitive members  213  and  214 . Thus, it is unnecessary to form an opening through which wiring light passes, in the housing  215  that supports the wiring device  202  from below the device. Thus, it is possible to improve the strength of the housing  215 .  
         [0048]     Next, a latent image forming and developing system mainly increases three cassettes or devices of a photosensitive cassette, a cleaning cassette (cleaning device), and a developing device, and the three cassettes and a sub side plate (not shown) supporting the developing device are incorporated into a unit to constitute the image station. The image station can be removably fixed to the apparatus main body. The photosensitive cassette and the cleaning cassette can be removably fixed to the sub side plate. Two image stations are prepared using the same members and provided in parallel on the right and left sides. In  FIG. 2 , a first image station  300  is provided on the left side and a second image station  400  is provided on the right side.  
         [0049]     Further, the two photosensitive members  213  and  214  constitute a photosensitive cassette  500 . In the photosensitive cassette  500 , the two photosensitive members  213  and  214  are integrated into one unit so as to be collectively attached and removed. That is, one photosensitive cassette  500  is provided for the two image stations  300  and  400 . The photosensitive cassette  500  can be attached to and removed from the two image stations  300  and  400 . The integration of the two photosensitive members into a unit facilitates insertion of the photosensitive member at the time of setting a machine and replacement thereof. Further, unlike conventional process cartridges or image formation units, the other image forming mechanisms are not incorporated, so burdens on a user, a manufacturer, and environments can be alleviated. However, further alleviation of the burdens on the user, the manufacturer, and the environments is hindered by collective replacement of the two photosensitive members different in usable lifetime (discard of a serviceable photosensitive member). As a countermeasure against this, in the present invention, the photosensitive members are individually detachable to the unitized photosensitive cassette. To be specific, the first image station  300  and the second image station  400  include the developing devices  203  and  204 , and cleaning cassettes  216  and  217 , respectively, and share the detachable photosensitive cassette  500 . Further, the photosensitive members  213  and  214  can be individually removed from and attached to the photosensitive cassette  500 .  
         [0050]      FIG. 3  is a plan view showing the schematic structure of image stations of the image forming apparatus according to this embodiment.  FIG. 4  is a perspective view showing the schematic structure of each photosensitive member of each image station. Incidentally, in  FIGS. 3 and 4 , the same components as those of  FIG. 2  are denoted by like reference numerals. The plan view of  FIG. 3  shows the schematic structure of the first image station  300  and the second image station  400  as viewed from above. Sub side plates  601  and  701  of each image station positioned in main body side plates  600  and  700  constituting the apparatus main body casing are coupled so as to keep predetermined interval and degree of parallelization with high dimensional accuracy, by a stay or a shaft (not shown).  
         [0051]     Further, the developing devices  203  and  204  are rotatably supported and positioned by rotating shafts  801  and  802  inside the sub side plates  601  and  701 . Thus, the developing devices  203  and  204  are integrated with the sub side plates  601  and  701 . The size and shape of each of the sub side plates  601  and  701  are determined so as to cover a developing driving member such as a gear or a shaft or a developing component such as a toner replenishment port provided to side portions  803 ,  804 , and  805 ,  806  of the developing devices  203  and  204 . Further, the sub side plates  601  and  701  support the cleaning cassettes  216  and  217  such that the cassettes can be individually, rotated and removed/attached. The two photosensitive embers  213  and  214  are integrated into one unit to constitute the photosensitive cassette  500 . The sub side plates  601  and  701  support the photosensitive cassette  500  such that the photosensitive members  213  and  214  can be rotated and removed/attached. Then, the sub side plates  601  and  701  are used to integrate the developing devices  203  and  204  and cleaning cassettes  216  and  217 . The first image station  300  and the second image station  400  share the photosensitive cassette  500 . The obtained first image station  300  and second image station  400  are set and fixed in positioning portions of the main body side plates  600  and  700 , and a relative relationship therebetween is thus determined. The sub side plates  601  and  701  are detachable to the main body side plates  600  and  700  together with at least the developing deices  203  and  204 .  
         [0052]     As described above, the photosensitive cassette  500  includes a holder  811  that protects and integrates, and rotatably supports the photosensitive members  213  and  214 , rotating shafts  807  and  808 , and bearings  809  and  810 . That is, unlike the conventional process cartridge or image forming unit, the photosensitive drums and other process mechanisms are not integrated. In addition, the plural photosensitive drums are not individually structured. That is, in the photosensitive cassette  500 , the bearings  809  and  810  of  FIG. 3  can be individually slid outwardly. The bearings are slid outwardly to expose the rotating shafts  807  and  808 . As shown in  FIG. 4 , cut-out portions  812  and  813  having the diameter somewhat larger than the diameters of the rotating shafts  807  and  808  are formed in both side portions of the holder  811 . The exposed rotating shafts  807  and  808  pass through the cut-out portions  812  and  813 , so the photosensitive members  213  and  214  can be individually removed/attached. Thus, the photosensitive members  213  and  214  can be attached/detached to/from the holder  811 .  
         [0053]     As shown in the schematic diagram of a driving mechanism of the photosensitive members in  FIG. 5 , in the photosensitive members  213  and  214 , gears  814  and  815  integrated with the rotating shafts  807  and  808  are engaged with worm gears  817  and  818  of a worm shaft  816 . The worm shaft  816  is connected with a motor  820  provided to the image forming apparatus main body, through a pulley  819 . Thus, the photosensitive members  213  and  214  are rotated in a clockwise direction that is the direction of the arrow of  FIG. 5  by a driving torque of the motor  820 . The gears  814  and  815  (or the photosensitive members  213  and  214  etc.) are formed of the same material with the same shape and quality. Identical positions on the outer side surfaces of the gears  814  and  815  are marked as marks  821  and  822 . The marks  821  and  822  are put on the most eccentric positions of the gears  814  and  815 . When the photosensitive members  213  and  214  are attached to the apparatus main body, the marks  821  and  822  are put in identical positions. For example, at the time of replacing the photosensitive member, the member is attached such that the marks  821  and  822  are put in upper positions. Hence, relative driving phases of the gears  814  and  815  are matched. Then, a speed difference between the photosensitive members  213  and  214  due to the eccentric gears  814  and  815  is eliminated to suppress color drift. Whether or not the marks  821  and  822  are put in identical positions is checked by sensors  823  and  824  for detecting the marks  821  and  822 . At the time of setting a machine just after the purchase of the image forming apparatus, a new photosensitive cassette where photosensitive members marked in identical positions are integrated is attached to the apparatus main body, so no problem arises. At the time of replacing the photosensitive members, for example, replacing one of the photosensitive members, it is necessary to precisely match mark positions. If a user needs to execute this operation, the operation should be very easy. To that end, for example, whenever printing of an image in the image forming apparatus is completed to stop the photosensitive members and other driving units, the sensors  823  and  824  detect the marks  821  and  822  as shown in  FIG. 5  to stop these components. In this way, the marks are always placed in predetermined positions (in  FIG. 5 , upper positions), so it is only necessary to attach a new photosensitive member to the photosensitive cassette while matching the mark positions. Hence, a user is free from a stress of positional alignment of the marks only by being given a message that “attach photosensitive member with mark being faced up”. At this time, the sensors  823  and  824  automatically check whether or not the positions of the marks  821  and  822  are matched. If not matched, a user is notified of the misalignment on a display or the like of the apparatus main body and required to reattach the photosensitive member.  
         [0054]     Further, in the first image station  300 , as shown in  FIGS. 2 and 3 , the photosensitive cassette  500  is positioned with high accuracy relative to the sub side plates  601  and  701  that fix and support the developing device  203  including the developing rollers  218  and  219  with high positional accuracy. The same applies to the second image station  400 . At the time of forming an image, the photosensitive members  213  and  214  of the photosensitive cassette  500  of this embodiment need to contact a developing or cleaning mechanism upon the image formation, and thus are exposed. If the removed photosensitive cassette  500  is placed outside the image forming apparatus at the time of replacing the photosensitive member, and the exposed portion contacts the floor or the like, there is a fear that the photosensitive members  213  and  214  are adversely affected. Thus, the photosensitive cassette  500  of this embodiment is structured as shown a schematic sectional view of  FIG. 6  that shows the structure of the photosensitive cassette. The holder  811  has protrusions around the photosensitive members  213  and  214 . Lines j-j, k-k, and n-n connecting tip ends of the protrusions extend outside the photosensitive members  213  and  214 . With such structure, for example, even if the photosensitive cassette  500  is provided on the lines j-j, k-k, and n-n, the holder  811  protects the members, so the exposed portion of the photosensitive members  213  and  214  never contacts the floor. Hence, it is possible to improve the operability and prevent adverse influence on the photosensitive members  213  and  214 .  
         [0055]     Further, as shown in  FIG. 2 , in the first image station  300 , the cleaning cassette  216  that supports the cleaning mechanism  220  and the charging device  221  is positioned relative to the photosensitive cassette  500  positioned with high accuracy to the sub side plates  601  and  701  of  FIG. 3  to improve a relative positional relation among the photosensitive member  213 , the cleaning mechanism including the cleaning blade  220  and a sealing roller  221 , and a roller-like charging device  222 . Further, the cleaning cassette can separately contact the photosensitive cassette such that the cleaning cassette  216  side is operated by the sub side plates  601  and  701 .  
         [0056]     In this way, the photosensitive cassette is structured as above, and the relation between the photosensitive cassette  500 , and the cleaning cassette and the sub side plates  601  and  701  is set as above, so the photosensitive cassette  500  can be removed from the apparatus main body, and the photosensitive members  213  and  214  can be removed from the photosensitive cassette  500 . Hence, the photosensitive members  213  and  214  can be solely replaced. That is, replacement timings of the photosensitive members  213  and  214  can be individually set based on each usable life. That is, the used component is only replaced, and a serviceable component is not replaced.  
         [0057]     Further, the intermediate transfer belt or developing device, the cleaning device, and the charging device provided around the photosensitive member or components thereof are extremely close to or in contact with the photosensitive member. In this state, if the photosensitive cassette  500  is removed from the apparatus main body, there is a fear that not only the photosensitive member but also the devices or components provided closer to or in contact with the photosensitive member are damaged. In addition, an operability is too low. This holds true of the attachment to the apparatus main body. For example, in  FIG. 2 , when the photosensitive cassette  500  is pulled out frontward in  FIG. 2  and thus removed from the apparatus main body, the photosensitive member is pulled out while being in contact with the intermediate transfer belt, the developing roller, the cleaning blade, and the charging roller of the first image station  300  and the second image station  400 . Thus, in the present invention, in the case of attaching/removing the photosensitive cassette  500  to/from the apparatus main body, the devices or components around the photosensitive member are temporally separated from the photosensitive member whenever the photosensitive cassette is removed/attached, after which the photosensitive cassette  500  is removed/attached. At the time of attaching the cassette, the separated devices or components are restored to the original positions after the photosensitive cassette  500  is inserted. Thus, a positional relation therebetween is returned to the original one. The individual replacement of the photosensitive members  213  and  214  alone is described above. Furthermore, if the photosensitive cassette  500  alone can be removed from the image forming apparatus main body prior to the developing device or the cleaning cassette, the replacement of the photosensitive members  213  and  214  that require frequent replacements is facilitated. A process element to be replaced most frequently is the photosensitive members  213  and  214 . During the frequent attachment/removal of the photosensitive cassette  500  and replacement of the photosensitive members  213  ad  214 , removing the serviceable developing device or cleaning cassette together with the sub side plates  601  and  701  is troublesome and low in applicability, and causes another problem that a user&#39;s hand or surroundings get dirty. To overcome such drawbacks, in this embodiment, the used component alone is replaced, and the serviceable component is not removed as long as possible. Further, a component that would be more frequently replaced is preferentially removed. Incidentally, in this embodiment, the photosensitive members  213  and  214  have the drum shape, but may be structured in a belt shape.  
         [0058]     Further, in the first image station  300 , the cleaning cassette  216  includes a cleaning mechanism including the cleaning blade  220  as a cleaning member for removing residual toner or contaminants on the photosensitive member  213 , and the sealing roller  221  for preventing the toner from being splashed at the time of cleaning, and the charging roller  222  for uniformly charging the photosensitive member  213  surface. These are integrated so as to surround the photosensitive member  213  with a waste toner recovery cleaning container  223  serving as a holder. Further, the sealing roller  221  and the charging roller  222  are rotatably supported to the cleaning container  223 , and the connected wit the photosensitive member  213  by a gear train. As mentioned above, the driving force is transmitted by the gear  814  through the worm shaft  816  to rotate the photosensitive member. At the time of removing/attaching the photosensitive cassette  500 , the gear train is disengaged. In order to minimize the wasteful replacement, the life periods of the charging roller  222  and the cleaning mechanism that would be worn and deteriorated are set almost equal in such a way that, for example, images can be printed on about 400 to 500 K sheets. The waste toner reservoir provided to the cleaning container  223  is set such that the waste toner is fully filled before the life of the charging roller  222  or the cleaning mechanism expires. The cleaning cassette  216  is housed and fixed to a cassette case  224  integrated with the sub side plates  601  and  701 , so its position relative to the photosensitive cassette  500  and the photosensitive member  213  is determined with high accuracy.  
         [0059]     In this way, the cleaning cassette  216  can be removed/housed from/to the cassette case  224  so as to be solely replaced. The cleaning cassette  216  is positioned and fixed by the sub side plates  601  and  701  similar to the photosensitive cassette  500 , and can be solely removed/attached. Incidentally, the cassette  216  can separate from/contact with the photosensitive cassette  500 , that is, the photosensitive member  213 . The same holds true of the second image station  400 .  
         [0060]     To keep up with the recent tendency to downsize the image forming apparatus, or reduce the diameter of the photosensitive member, the cleaning cassette is accordingly provided around the photosensitive member, for example, in  FIG. 2 , provided in a space extending from the right side surface to the lower portion of the photosensitive member  213 . The cleaning cassette occupies a wide area. Thus, for pulling out the cleaning cassette  216  upward, the photosensitive cassette  500  should be removed beforehand. In other words, in such a structure where the photosensitive cassette  500  is placed in a path along which the cleaning cassette  216  is to be removed, the image forming apparatus main body can be reduced, but in addition, the photosensitive member  213  to be most frequently replaced, that is, the photosensitive cassette  500  can be preferentially and easily removed. Further, the cleaning cassette  216  cannot be removed unless a correct operation procedure, for example, a process of removing the photosensitive cassette  500  first is executed. Accordingly, a user can determines which operation to execute, so an applicability is expected to improve. Further, a risk of replacing wrong parts or damaging the parts upon the replacement is eliminated.  
         [0061]     As mentioned above, in the image forming apparatus where a user replaces expendables, how to facilitate the replacement or prevent erroneous replacement is most important, so an advantage of this embodiment that a user can clearly grasp what to do next is great.  
         [0062]     Further, in the image forming apparatus of this embodiment, the photosensitive member or developing device is removable vertically to the rotational driving shaft. Thus, unlike the case of removing the device in the rotating axis direction, the photosensitive member never contacts the driving mechanism upon the attachment/removal, and image forming mechanism such as the photosensitive member can be removed/attached without being damaged. Further, the cleaning cassette is replaced by notifying a user that the waste toner is fully filled. In this embodiment, the charging roller  222  is used as the charging device, and the cleaning blade  220  and the sealing roller  221  are used as the cleaning meas. However, the present invention is not limited to the system, method, structure, material, etc. of these mechanisms. Further, the above applies to a cleaning-less cassette. Further, the sub side plates  601  and  701  are provided with the developing device  203  including the cyan developing device  225  and the magenta developing device  226  for developing the electrostatic latent image formed on the photosensitive member  213  using the developing rollers  218  and  219 , and the cassette case  224  housing the cleaning set  216  for determining a position of the charging roller  222  and the cleaning mechanism relative to the photosensitive member  213 . The side plates integrate these components Further, the photosensitive cassette  500  is positioned and fixed integrally with the sub side plates  601  and  701 , and detachably and attahcably fitted and held therein.  
         [0063]     Further, as for the cyan developing device  225  as a component of the developing device  2 - 3 , cyan toner is supplied from the replenishment port  227  to one end of a transfer screw  229 . The cyan toner supplied into the developing device by the transfer screw  229  is stirred in the direction opposite to the transfer direction by a paddle roller  220 . During this process, the toner is supplied to the developing roller  218 . The transfer screw  228  is separated from the paddle roller  229  across a partition wall  230 . Incidentally, the same applies to the remaining developing device, the magenta developing device  226  as a component of the developing device  203  in order not to mix the toners different in moving directions. Further, the yellow developing device or the black developing device that constitutes the developing device  204  of the second image station  400  has the same structure except that the processed image color is different. Further, in the case of switching the color in the firs image station  300  or the second image station  400 , while the photosensitive member is rotated, one of the cyan developing device  225  and the cyan developing device  226  switches a developing function to the other developing device to form a visual image on the photosensitive member using two colors in order. This method is classified into two: (a) a method using the electrical or magnetic force without changing the position of the image forming unit or the developing roller and (b) a method of changing the position of the device and the developing roller so as to separate from/contact with the photosensitive member. In this embodiment, at the time of removing the photosensitive cassette  500 , the developing device and the developing roller are temporally separated from the photosensitive member, so in the case of using the method (a), a mechanism for physically separating the developing device and the developing roller from the photosensitive member is added, and in the case of using the method (b), the method (a) is used and the above mechanism is added, and both of the developing rollers are separated from the photosensitive member in such a neutral position that the rollers are separated from the photosensitive member.  
         [0064]      FIG. 7  is a schematic sectional view showing each image station positioned and fixed in a reference setting position in the image forming apparatus main body of this embodiment. In  FIG. 7 , in the first image station  300 , image forming mechanism such as the charging roller  222 , the cyan developing device  225 , the magnet developing device  226 , a quenching lamp  231 , and the cleaning blade  220  are provided around the photosensitive member  213 . The unit including the sub side plates  601  and  701  having these image forming mechanism, and the cassette case  224  is also referred to as an image forming unit main body. The developing unit side plate  232  supports the cyan developing device  225  and the magenta developing device  226  on both sides to form the developing device  203 . The developing device  203  is rotatably supported by the rotating shaft  801  attached t the sub side plates  601  and  701  larger than the developing unit side plate  232 . The sub side plates  601  and  701  support the developing device  203  as well as axially support the cassette case  224  rotatably around the shaft  234 . Further, a semi-circular groove  235  extends above the sub side plates  601  and  701 , and the rotating shaft  807  is fitted into the groove  235  (bearing  814   a  of  FIG. 4 ) to position the photosensitive cassette  500 . The cassette case  224  axially supports the holder  825  rotatably about the shaft  233 . In this state, a detachment step of FIGS.  8  to  10  is carried out.  
         [0065]     First, as shown in  FIG. 8 , a lock mechanism (not shown) of the cassette case  224  is released, and then the cassette case  224  is turned to the right in the direction of the arrow about the shaft  234 . As a result, the developing roller  222 , the cleaning cassette  216  of the cleaning blade  220  of the cleaning mechanism, and the quenching lamp  231  are separated from the photosensitive member  213 . Thus, the cleaning blade  220  in close contact, and the sealing roller  221  in contact are separated from the photosensitive member  213 , so the photosensitive cassette  500  can be removed. As shown in  FIG. 7 , for example, under the normal attachment condition, the sealing roller  221  provided above the photosensitive member  213  and opposite t the developing roller  218  contacts the outer peripheral surface of the photosensitive member  213  in a position above an imaginary plane passing the centers of the developing roller  218  and the photosensitive member  213 . Under this state, the photosensitive member  213  cannot be removed upward. Thus, it is necessary to separate the sealing roller  221  from the photosensitive member  213 . In addition, the cleaning blade  220  is brought into close contact with the photosensitive member  213  from a counter direction, in the normal attachment position. Therefore, unless the blade is separated therefrom, the photosensitive member  213  cannot be attached/removed. In view of these, as shown in  FIG. 8 , the cassette case  224  is rotated about the shaft  234  to the right in the direction of the arrow beforehand. Likewise, the photosensitive member  219  should be separated from the developing rollers  218  and  210 . Since one of the developing rollers  218  and  219  is always in contact or close contact with the photosensitive member  213 , the photosensitive member  213  may be damaged or toner may spill from the developing roller if the photosensitive member  213  is removed/attached in this state. Accordingly, in this embodiment, the color is switched based on the above method (b). Further, the developing rollers  218  and  219  are moved away from the photosensitive member  213 , that is, moved to a neutral position beforehand within a color switchable range of the developing devices  225  and  226 . The way of separate the developing devices  225  and  226  from the developing rollers  218  and  219  is not limited to the above. In the case of increasing a distance between the developing devices  225  and  226  and the developing rollers  228  and  219 , the developing devices  225  and  226  may be moved to the left of  FIG. 8 , for example. In the second image station  400 , similar separating operation is executed. Unless the separating operation is carried out in both of the image stations, the photosensitive cassette  500  cannot be removed.  
         [0066]     Next, as shown in  FIG. 9 , the photosensitive cassette  500  is pulled out upward from the sub side plates  601  and  701  at the cut-out angle of the groove  235  formed above the sub side plates. If necessary, the holder  825  is rotated to the left about the shaft  233  to retract the quenching lamp  231  from the removal area of the cleaning cassette  216 , after which as shown in  FIG. 10 , the cleaning cassette  216  is removed upward from the cassette case  224 . In  FIG. 10 , a container of the cleaning cassette  218  is a box, and the cleaning blade  220  and the lower portion of the sealing roller  221  define a sealed space. This space serves as a waste developer container  236  that contains waste toner. A waste developer scraped off by the cleaning blade  220 , in this embodiment, the waste toner is recovered to the waste developer container  236 . The cleaning cassette  216  is integrated with the waste developer container  236  and can be removed/attached to the cassette case  224 , that is, can be removed/attached to the sub side plates  601  and  701 . Accordingly, when the waste developer container  236  is filled up with waste toner, the entire cleaning cassette  216  ire replaced or only the waste toner is disposed of to recycle the cleaning cassette  216 .  
         [0067]     As shown in  FIG. 9 , the removed photosensitive cassette  500  is place on the table as shown in  FIG. 4  or  6 . At this time, phase-matching marks  821  and  822  of the gears  814  and  815  of the photosensitive members  213  and  214  as shown in  FIG. 5  are positioned in identical upper positions. Then, a used photosensitive member out of the photosensitive members is pulled out from the holder  811  (by sliding bearing  814   a  or  815   a ) to replace the used one with a new photosensitive member. At the time of attaching the new photosensitive member, the new photosensitive member is attached to match a phase-matching mark position with that of the unreplaced one. If both of the photosensitive members should be replaced at a time, both of them may be replaced, but in this case, the positional alignment of the marks is troublesome and requires much time. Thus, in this case, it is efficient to replace the photosensitive cassette with a new cassette including the mark-adjusted photosensitive members, and thus a user never gets his/her hands dirty.  
         [0068]      FIG. 11  is a schematic sectional view showing how the photosensitive cassette is replaced in the image forming apparatus of this embodiment. In  FIG. 11 , an upper casing  900  is rotated about a shaft  901  to the left to separate the intermediate transfer belt  205  from the photosensitive members  213  and  214  to remove/attach the photosensitive cassette  500 .  
         [0069]     Further, in this embodiment, in order to improve the operability in individual attachment/removal of the photosensitive members to/from the photosensitive cassette  500 , even if the photosensitive cassette  500  is attached to the apparatus main body, the photosensitive members can be individually removed/attached. In this case, however, after the photosensitive members are removed, the photosensitive cassette  500  needs to be positioned in the first image station  300  and the second image station  400 . The photosensitive cassette  500  of  FIG. 4  is structured such that the sub side plates  601  and  701  are supported to the bearing of the photosensitive member, so if the photosensitive member alone is removed as shown in  FIG. 12 , the holder  811  not supported cannot keep its posture. To that end, in order to enable the individual attachment/removal of the photosensitive member in the state where the photosensitive cassette  500  is attached, the protrusions  814   a  and  815   a  concentric with the bearings  814   a  and  815   a  of the photosensitive members  213  and  214  are formed on the side surfaces of the holder  811  as shown in  FIG. 13 . The inner diameter thereof is set to receive the bearings  814   a  and  815   a  of the photosensitive member, and the outer diameter thereof is set within a range of the sub side plates  601  and  701 . Accordingly, the holder  811  is supported to the sub side plates  601  and  701 , and the photosensitive members  213  and  214  are supported to the holder  811 , so even if the photosensitive member is removed from the photosensitive cassette  500 , the holder  811  can keep its posture inside the apparatus main body.  
         [0070]     In this case as well, the process for matching phases of the gears of the photosensitive member is the same as above. Further, the way to retract and separate the intermediate transfer belt, the developing mechanism, the cleaning mechanism, and the charging mechanism are the same as above at the time of removing/attaching the photosensitive member.  
         [0071]     Incidentally, in this embodiment, mechanisms for fixing or unfixing the sub side plates to/from the apparatus main body, mechanisms for fixing or unfixing the photosensitive cassette to/from the sub side plates, or the like have no particular function and structure, and thus are omitted. A general lock mechanism may be used. As regards the attachment/removal of the photosensitive member to/from the holder of the photosensitive cassette is described using an example of sliding the bearing of the photosensitive member, but the present invention is not particularly limited thereto and an appropriate mechanism may be adopted. Further, the embodiment of the present invention describes the two photosensitive members, but the present invention is not limited thereto, and is applicable to any image forming apparatus using plural photosensitive members.  
         [0072]     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.  
         [0073]     This patent specification is based on Japanese patent application, No. JPAP2005-050807 filed on Feb. 25, 2005 in the Japanese Patent Office, the entire contents of which are incorporated by reference herein.