Abstract:
The present invention concerns a color image forming apparatus wherein toner images formed on a plurality of image carriers are primarily transferred onto an intermediate transfer object, and then, are secondarily transferred onto a recording sheet. The color image-forming apparatus includes a plurality of image carriers; a plurality of latent image forming devices to form electrostatic latent images on the image carriers; a plurality of developing devices to develop the electrostatic latent images with toner to form visible toner images on the image carriers; an intermediate transfer member, a longitudinal direction of which is arranged at substantially a vertical direction, and onto which the toner images formed on the image carriers are sequentially transferred in such a manner that the toner images overlap relative to each other so as to form a color toner image on the intermediate transfer member; and a transferring device to transfer the color toner image formed on the intermediate transfer member onto a transfer sheet. The intermediate transfer member and the image carriers are integrated into a rack, and the rack is drawable in substantially a horizontal direction in respect to a mainframe of the color image-forming apparatus.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an image forming apparatus employing an electrophotographic system such as a copying machine, a printer and a facsimile machine, and in particular, to a color image forming apparatus wherein toner images formed on a plurality of image carriers by a plurality of image carriers, latent image forming means and developing means are primarily transferred onto an intermediate transfer object, and then, are secondarily transferred onto a recording sheet.  
           [0002]    In the case of a conventional color image forming apparatus employing an electrophotographic system, there have been proposed various types of systems wherein a plurality of image forming sections are arranged in parallel in the conveyance direction, and toner images each being different from others in terms of color formed respectively on photoreceptor drums in image forming sections are successively transferred on a recording sheet held on a transfer belt.  
           [0003]    There is also known a color image forming apparatus wherein a transfer drum around which a recording sheet fed from a sheet feeding section is wound is used in place of the transfer belt stated above. By transferring toner images visualized by developing means onto the recording sheet wound around the transfer drum successively, a full color image is formed on the recording sheet. After this, the recording sheet is separated from the transfer drum and is subjected to fixing of the toner image conducted by a fixing means to be ejected out.  
           [0004]    Further, in another color image forming apparatus, there is also proposed a system wherein a plurality of image forming sections are arranged in parallel, toner images each being different from others in terms of color formed respectively on photoreceptor drums in image forming sections are successively transferred onto an intermediate transfer object for forming superposed primary transfer images, and the primary transfer images are transferred onto a recording sheet held on a conveyance belt.  
           [0005]    A color image forming apparatus of a tandem type of this kind having a plurality of photoreceptors wherein a color image is formed through one turn of each of plural photoreceptors can output full color images at higher speed, compared with a color forming apparatus of a multi-rotation type wherein a color image is formed through plural turns of a single photoreceptor.  
           [0006]    A plurality of photoreceptors and intermediate transfer objects need to be removed from an apparatus main body because of their maintenance and life-times. Therefore, the structure which makes it easy to remove a plurality of photoreceptors and intermediate transfer objects has been required by the color image forming apparatus of a tandem type.  
           [0007]    Since a plurality of photoreceptors, intermediate transfer objects in particular, rotating means to rotate the intermediate transfer objects, and intermediate transfer units including primary transfer means have considerable mass, it has been difficult for a user to draw out plural photoreceptors from positions where the photoreceptors have been mounted in the apparatus main body, and to take out directly intermediate transfer units from positions where the intermediate transfer units have been mounted in the apparatus main body, because it has been difficult to apply the physical power. In particular, it is not easy to pull out the heavy unit horizontally toward the front side of the apparatus main body. Furthermore, a unit such as a photoreceptor and an intermediate transfer object is weak against an impact, and there is a risk that the unit becomes unusable if a part of the unit is touched while it is pulled out. A scratch caused by a shock not only affects images in image forming in a single color but also has possibility that a desired color cannot be reproduced in color image forming, and the scratch turns out to be a fatal defect for a color image forming apparatus.  
           [0008]    On the other hand, when each member is provided with its own mechanism to take out each member without interference, a large number of taking-out mechanisms (for example, slide rail mechanisms) are needed. Each taking-out mechanism requires its strength that withstands the weight. This makes an entire apparatus to be large.  
           [0009]    The more the mechanism to take out each member individually is increased, the more difficult the positional adjustment between members is. In an image forming apparatus having plural photoreceptors, in particular, it is necessary to conduct superposition of each color in extremely high accuracy. If mutual positional deviation is caused for each maintenance of a unit, it is not possible to reproduce the color desired by a user. Furthermore, an intermediate transfer object is a member on which color toner images each being of a different color are superposed, and relative positions of intermediate transfer objects for plural photoreceptor drums are required to be accurate.  
           [0010]    Further, when a color image forming unit of this kind is provided on a copying machine, there are further caused problems. Since a document image reading apparatus is arranged on the upper portion of the copying machine, there is needed a complicated opening/closing mechanism which opens the document image reading apparatus from the upper portion of the apparatus main body. Each of the document image reading apparatus and the opening/closing mechanism has considerable mass, and how to handle them is a problem. If an impact is applied on the document image reading apparatus in the course of opening/closing operations, there are caused problems of difficulty in maintaining accuracy of optical members, parts damages and injures of an operators. Drawing out from the upper portion of the apparatus main body has the aforesaid problems, although it is easy for a user to apply the physical power.  
           [0011]    In the image forming apparatus which forms images on both sides of a sheet, a sheet fed from the a sheet feeding means is subjected to transfer and fixing of toner images formed on an image carrier on the first surface of the sheet, and then is branched from the sheet ejection path to be conveyed to a reversing means provided on the lower or upper portion. The sheet thus conveyed is reversed up side down, and then, is made to join a sheet conveyance path again from a sheet feeding section. The sheet which has joined the sheet conveyance path is conveyed to the image carrier again so that a toner image may be transferred onto the second surface of the sheet.  
           [0012]    As a method to form images on a sheet from inputted multi-color images, TOKKAIHEI No. 9-006083 discloses an image forming apparatus wherein a plurality of developing units are arranged in parallel on one side of an image carrier representing a belt arranged longitudinally, and a sheet ejection section is provided on the other side, and charging, writing of a latent image and developing are conducted in succession within one turn of one image carrier.  
           [0013]    In an image forming apparatus for a color image, when an image carrier on which a toner image is formed and a developing unit are arranged in the longitudinal direction, if joining from the reversing means is conducted in the vicinity of the sheet feeding unit positioned at the lower portion, the image forming apparatus is made to be large in the longitudinal direction, which is a problem.  
           [0014]    For example, FIG. 9 is an illustrative diagram of an image forming apparatus showing image forming flows to both sides of a sheet.  
           [0015]    In FIG. 9, image forming apparatus  101  is provided with scanner section R, image carrier  110 , scorotron charging unit  111 , image writing means  112 , developing unit  113 , transfer means  115 , fixing means H, sheet feeding section  117 , bypass sheet feeding section  109 , reversing means S and with sheet ejection tray T.  
           [0016]    The scanner section R reads an image of a document set through an optical system. The image writing means  112  exposes image carrier  110  to light to form a latent image, based on the image read. The latent image formed on the image carrier  110  is developed by developing unit  113  to turn into toner.  
           [0017]    On the other hand, the sheet feeding section  117  is provided with a plurality of sheet feeding units  117   a - 117   d  each housing therein sheets in a different size, and a sheet is fed out toward registration roller  141  through first sheet conveyance path P 0  by sheet feeding roller  118   a  of the selected sheet feeding unit, for example, of sheet feeding unit  117   a . The registration roller  141  starts rotating to convey the sheet so that the toner image formed on the image carrier  110  may be synchronized with the sheet.  
           [0018]    The sheet conveyed to the registration roller  141  is superposed on the toner image formed on the image carrier  110 , and thus, the toner image is transferred onto the sheet. The sheet on which the toner image has been transferred is sent to fixing means H to be fixed, and is ejected to sheet ejection tray T.  
           [0019]    When transferring a toner image on the second surface of the sheet on which the toner image has been transferred onto the first surface thereof, the sheet on which the toner image has been formed is fed in second sheet conveyance path G.  
           [0020]    On the second sheet conveyance path G, there is provided reversing means S that is equipped with a pair of switchback rollers  143  capable of rotating regularly and inversely and guide  148 . The sheet which has been fed in is reversed inside out by switching of the sheet conveyance direction for the switchback rollers  143  c of the reversing means S and switching of the conveyance path of guide  148 . The sheet thus reversed is sent to roller unit  118   e  by driving rollers  144  and  145  to join the first sheet conveyance path P 0  representing a sheet conveyance path from the sheet feeding section. The sheet which has joined the first sheet conveyance path P 0  is sent again to the registration roller  141 , and then, is ejected to sheet ejection tray T after the toner image formed on the image carrier is transferred onto the second surface of the sheet and fixed.  
           [0021]    Namely, in the conventional image forming apparatus, when forming toner images on both sides of a sheet, the sheet on which a toner image has been transferred on the first surface thereof is reversed inside out by the reversing means, and is made to join the sheet conveyance path coming from the sheet feeding section, and after that, a toner image is transferred onto the second surface of the sheet.  
           [0022]    From viewpoints of production cost and easiness of design for an image forming apparatus, the reversed sheet has been made to join the sheet conveyance path in the vicinity of a sheet feeding unit by a roller similar to that of the sheet feeding unit. For example, one sheet feeding unit among plural sheet feeding units has been removed to be used for the reversed sheet to join the conveyance path.  
           [0023]    For this reason, when an image carrier on which a toner image is formed and a developing unit are arranged in the longitudinal direction, if joining from the reversing means is conducted in the vicinity of the sheet feeding unit positioned at the lower portion, the image forming apparatus is further made to be large in the longitudinal direction.  
           [0024]    When the time for image forming on both sides of a sheet needs to be shortened, the conveyance time for conveying the reversed sheet to the vicinity of the sheet feeding section is a problem.  
           [0025]    The invention has been achieved to solve the aforesaid problems, and an object of the invention is, in a color image forming apparatus having a plurality of photoreceptors and intermediate transfer units, to make operations for mounting the aforesaid members and image forming means such as developing means on an apparatus main body and operations for dismounting them from an apparatus main body to be easy and to make maintenance for them to be easy, and to realize prevention of troubles of each member, high accuracy of positions and down-sizing of the total apparatus.  
           [0026]    Another object is to reduce a floor space of an apparatus and to reduce a height of an apparatus in the vertical direction, and thereby to realize down-sizing of the total apparatus.  
           [0027]    Further object is to shorten the time for image forming on both sides of a sheet and to provide an image forming apparatus requiring less floor space.  
         SUMMARY OF THE INVENTION  
         [0028]    The objects stated above are realized by either one of the following structures.  
           [0029]    (1) A color image-forming apparatus, comprising: a plurality of image carriers; a plurality of latent image forming devices to form electrostatic latent images on the image carriers; a plurality of developing devices to develop the electrostatic latent images with toner to form visible toner images on the image carriers; an intermediate transfer member, a longitudinal direction of which is arranged at substantially a vertical direction, and onto which the toner images formed on the image carriers are sequentially transferred in such a manner that the toner images overlap relative to each other so as to form a color toner image on the intermediate transfer member; and a transferring device to transfer the color toner image formed on the intermediate transfer member onto a transfer sheet; wherein at least the intermediate transfer member and the image carriers are integrated into a rack, and the rack is drawable in substantially a horizontal direction in respect to a mainframe of the color image-forming apparatus.  
           [0030]    (2) A color image-forming apparatus, comprising: a plurality of image carriers; a plurality of latent image forming devices to form electrostatic latent images on the image carriers; a plurality of developing devices to develop the electrostatic latent images with toner to form visible toner images on the image carriers; an intermediate transfer member to bear a color toner image formed by superimposing the toner images, which are sequentially transferred onto the intermediate transfer member from the image carriers; and a transferring device to transfer the color toner image formed on the intermediate transfer member onto a transfer sheet; wherein at least the intermediate transfer member and the image carriers are integrated into a rack, and the rack is drawable in substantially a horizontal direction in respect to a mainframe of the color image-forming apparatus.  
           [0031]    (3) A color image-forming apparatus, comprising: a plurality of image carriers; a plurality of latent image forming devices to form electrostatic latent images on the image carriers; a plurality of developing devices to develop the electrostatic latent images with toner to form visible toner images on the image carriers; an intermediate transfer member, a longitudinal direction of which is arranged at substantially a vertical direction, and onto which the toner images formed on the image carriers are sequentially transferred in such a manner that the toner images overlap relative to each other so as to form a color toner image on the intermediate transfer member; a sheet feeding section to store a transfer sheet and to feed the transfer sheet stored; a first sheet conveyance path, through which the transfer sheet, fed from the sheet feeding section, is conveyed to the intermediate transfer member; a transferring device to transfer the color toner image formed on the intermediate transfer member onto the transfer sheet; a reversing device to reverse a front side back of the transfer sheet, onto a first surface of which the color toner image is already transferred; and a second sheet conveyance path, through which the transfer sheet, reversed by the reversing device, is conveyed to a confluent position with the first sheet conveyance path, so that the transfer sheet re-enters the first sheet conveyance path from the confluent position; wherein the confluent position, at which the second sheet conveyance path joins the first sheet conveyance path, is located at substantially a center of the sheet feeding section and at substantially a lower position of the image carriers.  
           [0032]    (4) A color image-forming apparatus, comprising: a plurality of image carriers; a plurality of latent image forming devices to form electrostatic latent images on the image carriers; a plurality of developing devices to develop the electrostatic latent images with toner to form visible toner images on the image carriers; an intermediate transfer member, a longitudinal direction of which is arranged at substantially a vertical direction, and onto which the toner images formed on the image carriers are sequentially transferred in such a manner that the toner images overlap relative to each other so as to form a color toner image on the intermediate transfer member; a sheet feeding section to store a transfer sheet and to feed the transfer sheet stored; a first sheet conveyance path, through which the transfer sheet, fed from the sheet feeding section, is conveyed to the intermediate transfer member; and a transferring device to transfer the color toner image formed on the intermediate transfer member onto the transfer sheet; wherein the intermediate transfer member is located at substantially a upper position of the sheet feeding section, and the transferring device is shaped in a roller and contacts a lower portion of the intermediate transfer member. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0034]    [0034]FIG. 1 is a sectional structure diagram of a color image forming apparatus showing an embodiment of the invention;  
         [0035]    [0035]FIG. 2 is a perspective view showing the state wherein a process frame is drawn out of the apparatus main body;  
         [0036]    [0036]FIG. 3 is a sectional view of an image forming process frame;  
         [0037]    [0037]FIG. 4 is a perspective view of an image forming process frame;  
         [0038]    [0038]FIG. 5 is an exploded sectional view of a frame, an intermediate transfer unit and an image forming section;  
         [0039]    [0039]FIG. 6 is a perspective view of an intermediate transfer unit;  
         [0040]    [0040]FIG. 7 is a perspective view of a casing;  
         [0041]    [0041]FIG. 8 is a perspective view of four sets of photoreceptor units; and  
         [0042]    [0042]FIG. 9 is an illustrative diagram of a conventional image forming apparatus for forming images on both sides of a sheet. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]    [0043]FIG. 1 is a sectional structure diagram of a color image forming apparatus showing an embodiment of the invention.  
         [0044]    The color image forming apparatus stated above is one called a tandem type color image forming apparatus, and it is composed of a plurality of image forming sections  10 Y,  10 M,  10 C and  10 K, intermediate transfer unit  7 , a sheet conveyance means and fixing unit (fixing means)  24 . On top of image forming apparatus main body (hereinafter referred to as apparatus main body) A, there is arranged document image reading unit SC.  
         [0045]    The image forming section  10 Y that forms yellow images is provided with charging means  2 Y arranged around image carrier (photoreceptor)  1 Y, exposure means  3 Y, developing unit (developing means)  4 Y, primary transfer means  5 Y and cleaning means  6 Y. The image forming section  10 M that forms magenta images is provided with image carrier (photoreceptor)  1 M, charging means  2 M, exposure means  3 M, developing unit (developing means)  4 M, primary transfer means  5 M and cleaning means  6 M. The image forming section  10 C that forms cyanic images is provided with image carrier (photoreceptor)  1 C, charging means  2 C, exposure means  3 C, developing unit (developing means)  4 C, primary transfer means  5 C and cleaning means  6 C. The image forming section  10 K that forms black images is provided with image carrier (photoreceptor)  1 K, charging means  2 K, exposure means  3 K, developing unit (developing means)  4 K, primary transfer means  5 K and cleaning means  6 K.  
         [0046]    The intermediate transfer unit  7  has intermediate transfer object  70  in a shape of a semiconductive endless belt trained about plural rollers and supported rotatably.  
         [0047]    The intermediate transfer object  70  is stretched longitudinally with primary transfer means  5  (Y, M, C and K) inscribed in the intermediate transfer object  70 . The intermediate transfer object  70  representing a belt that is trained about upper and lower rollers is rotated to run vertically by rotation driving of a driving roller performed by an unillustrated driving source. The driven roller impresses tension on the stretched intermediate transfer object  70  to reduce speed fluctuations of the intermediate transfer object  70  caused by creases and skewing.  
         [0048]    The intermediate transfer object  70  is an endless belt having volume resistivity of 106-1015 Ω·cm, and an example thereof is a two-layered seamless belt wherein a fluorine coating having a thickness of 5-50 μm is preferably provided as a toner filming protection layer on the outer side of a (0.1-1.0)-mm-thick conductive film base in which conductive materials are dispersed in engineering plastics such as, denatured polyimde, thermosetting polyimide, ethylene-tetrafluoroethylene copolymer, polyfluorovinylidene and nylon alloy. In addition to the foregoing, a semiconductive rubber belt having a thickness of 0.5-2.0 mm in which conductive materials are dispersed in silicone rubber or urethane rubber can be used as a belt base.  
         [0049]    Images each having a different color formed by the image forming sections  10 Y,  10 M,  10 C and  10 K are successively transferred onto rotating intermediate transfer object  70  by primary transfer means  5 Y,  5 M,  5 C and  5 K, and thereby, a composite color image is formed. Sheet P housed in sheet feeding cassette  20  is fed out by sheet feeding means  21  to be conveyed to secondary transfer means  5 A through plural intermediate rollers  22 A,  22 B,  22 C,  22 D and registration roller  23 , thus, color images are transferred collectively onto the sheet P. The sheet P on which the color images have been transferred is subjected to fixing processing by fixing unit  24 , then, is nipped by sheet ejection rollers  25  and is placed on sheet ejection tray  26  located outside the apparatus.  
         [0050]    On the other hand, after color images are transferred onto sheet P by the secondary transfer means  5 A, toner remaining on the intermediate transfer object  70  from which the sheet S is curvature-separated is removed by cleaning means  6 A.  
         [0051]    In the course of image forming processing, primary transfer means  5 K is in pressure contact constantly with photoreceptor  1 K. Other primary transfer means  5 Y,  5 M and  5 C come in pressure contact with their corresponding photoreptors  1 Y,  1 M and  1 C respectively.  
         [0052]    In this case, the secondary transfer means  5 A is a roller-shaped transfer means (transfer roller).  
         [0053]    The secondary transfer means  5 A is provided below intermediate transfer object  70 , and d.c. voltage is impressed, or d.c. voltage plus a.c. voltage are impressed on the secondary transfer means  5 A so that superposed toner images on the intermediate transfer object  70  are transferred onto the surface of a sheet.  
         [0054]    This roller-shaped secondary transfer means  5 A, when it is arranged to be combined with intermediate transfer object  70  that is arranged longitudinally, attains high transfer power and makes the layout having high degree of freedom to be possible. In the apparatus shown in FIG. 1, a sheet is housed at the lower portion of the apparatus main body, and an intermediate transfer object that carries toner images is arranged over the sheet. Further, a floor space can be reduced sharply because the intermediate transfer object is arranged longitudinally. Though fixing unit  24  is also arranged over a sheet housing section in the apparatus shown in FIG. 1, the intermediate transfer object  70  is of a longitudinal type, and a sheet can be fed out from the lower portion of the intermediate transfer object in any direction by the roller-shaped secondary transfer object  5 A, when a sheet is conveyed to the fixing unit  24 . Further, though leakage of an electric current tends to be generated at the position other than an original transfer position because the secondary transfer means is in a roller shape, leakage of an electric current from the intermediate transfer object  70  to the secondary transfer means is hardly caused because the intermediate transfer object  70  is a semiconductive object. This means that a toner image can be transferred properly only on the portion where the intermediate transfer object  70  is in contact with the secondary transfer means, and that it is possible to prevent the problem wherein the aforesaid leakage is caused at the location where the intermediate transfer object  70  is not in contact with the secondary transfer means and a toner image scatters before it is transferred. In virtue of the roller-shaped secondary transfer means  5 A combined with intermediate transfer object  70  that is arranged above the sheet housing section longitudinally as stated above, high transfer power is attained and the layout having high degree of freedom and down-sizing of the apparatus are realized.  
         [0055]    Incidentally, the secondary transfer means  5 A comes in pressure contact with the intermediate transfer object  70  only when sheet P passes through the secondary transfer means  5 A and secondary transfer is conducted.  
         [0056]    [0056]FIG. 2 is a perspective view showing the state wherein process frame  8  has been drawn out of apparatus main body A. FIG. 3 is a sectional view of process frame  8 , and FIG. 4 is a perspective view of process frame  8 .  
         [0057]    The process frame  8  is composed of image forming sections  10 Y,  10 M,  10 C and  10 K and of intermediate transfer unit  7 .  
         [0058]    The image forming sections  10 Y,  10 M,  10 C and  10 K are arranged longitudinally in the vertical direction. Intermediate transfer unit  7  is arranged on the left side of photoreceptors  1 Y,  1 M,  1 C and  1 K in the illustration. The intermediate transfer unit  7  is composed of intermediate transfer object  70  in a shape of an endless belt trained about rollers  71 ,  72 ,  73  and  74  to be rotatable, primary transfer means  5 Y,  5 M,  5 C and  5 K and cleaning means  6 A.  
         [0059]    Process frame  8  is capable of being drawn out of apparatus main body A. Namely, front door  101  of the apparatus main body A is opened, and then, the process frame  8  is held to be drawn out to the front side. The process frame  8  slides while being guided by supporting rails  82 L and  82 R positioned respectively on the left and right, and is drawn out toward the front side. When the process frame  8  is drawn out, image forming sections  10 Y,  10 M,  10 C and  10 K and intermediate transfer unit  7  are drawn out of the apparatus main body A collectively and solidly.  
         [0060]    Supporting rail  82 L on the left side in the illustration of the process  8  is arranged in the space positioned at the upper portion of fixing unit  24  and on the left side of the intermediate transfer object  70 . Supporting rail  82 R on the right side in the illustration of the process  8  is arranged in the vicinity of the lower portion of the lowermost developing unit  4 K. The supporting rail  82 R is arranged at the position where operations for mounting developing units  4 Y,  4 M,  4 C and  4 K on or dismounting them from the process frame  8  are not interfered. Accordingly, operations for mounting a developing unit and dismounting a developing unit under the condition that the process frame is drawn out and operations for maintenance are easy.  
         [0061]    The right side of photoreceptors  1 Y,  1 M,  1 C and  1 K of the process frame  8  is surrounded by developing units  4 Y,  4 M,  4 C and  4 K, while, the lower portion in the illustration is surrounded by charging means  2 Y,  2 M,  2 C and  2 K and by cleaning means  6 Y,  6 M,  6 C and  6 K and the left side in the illustration is surrounded by the intermediate transfer object  70 .  
         [0062]    A space section on the upper portion of photoreceptor  1 Y is covered by top plate  83  fixed on casing (frame)  81  of the process frame  8 . Bottom plate  84  fixed on casing  81  of the process frame  8  protects the intermediate transfer object  70  while it is mounted or dismounted, and it serves an upper guide plate of a sheet conveyance path.  
         [0063]    [0063]FIG. 5 is an exploded sectional view showing the state wherein intermediate transfer unit  7  and image forming sections  10 Y,  10 M,  10 C and  10 K are removed, FIG. 6 is a perspective view of the intermediate transfer unit  7  and FIG. 7 is a perspective view of casing  81 . Incidentally, in FIG. 6, a left half of intermediate transfer object  70  is eliminated so that an inside of the intermediate transfer unit  7  may easily be observed.  
         [0064]    After the process frame  8  is drawn out of apparatus main body A, intermediate transfer unit  7  is moved slightly to the left side in the illustration, and then, the intermediate transfer unit  7  is held to be pulled up vertically. Since positioning pins  75  embedded on upper portions on both sides of the intermediate transfer unit  7  move upward vertically along reference grooves  85  provided on the process frame  8 , it does not happen that intermediate transfer object  70  comes into contact with a member of the process frame  8  to damage it when the intermediate transfer unit  7  is pulled up.  
         [0065]    On the internal surface of the intermediate transfer object  70 , there are arranged longitudinally primary transfer means (hereinafter referred to also as primary transfer rollers)  5 Y,  5 M,  5 C and  5 K all supported horizontally and pressurizing rollers  76 Y,  76 M,  76 C and  76 K. Lever  77 Y which supports primary transfer roller  5 Y and pressurizing roller  76 Y, lever  77 M which supports primary transfer roller  5 M and pressurizing roller  76 M, and lever  77 C which supports primary transfer roller  5 C and pressurizing roller  76 C are rotated by regulating plate  78  simultaneously, and press the intermediate transfer object  70  to release it.  
         [0066]    [0066]FIG. 8 is a perspective view of four sets of photoreceptor units  11 Y,  11 M,  11 C and  11 K.  
         [0067]    After the intermediate transfer unit  7  is removed from the process frame  8 , four sets of photoreceptor units  11 M,  11 C and  11 K can be taken out. The photoreceptor unit  11 Y on the uppermost step is composed of photoreceptor  11 Y, charging means  2 Y and cleaning means  6 Y. The photoreceptor unit  11 M on the second step is composed of photoreceptor  11 M, charging means  2 M and cleaning means  6 M. The photoreceptor unit  11 C on the third step is composed of photoreceptor  1 C, charging means  2 C and cleaning means  6 C. The photoreceptor unit  11 K on the lowermost step is composed of photoreceptor  1 K, charging means  2 K and cleaning means  6 K.  
         [0068]    After the process frame  8  is drawn out of apparatus main body A integrally through the rail, each center hole of each of photoreceptors  1 Y,  1 M,  1 C and  1 K leaves an unillustrated reference shaft of each photoreceptor fixed and supported on a cantilever basis in the apparatus main body A to become movable, and photoreceptor units  11 Y,  11 M,  11 C and  11 K are placed respectively on supporting rests  86 Y,  86 M,  86 C and  86 K of the process frame  8 . In this case, it is possible to pull out only intermediate transfer unit  7  upward easily because the process frame  8  including the intermediate transfer unit  7  has been drawn out to the front side of the apparatus. Since the intermediate transfer unit  7  is extended almost in the vertical direction, it is extremely easy to pull it out upward. It is also excellent in terms of balance in pulling out. Namely, it is easy to pull out the intermediate transfer unit  7  by avoiding the contact between the intermediate transfer object and other member because it is possible to pull out in a well-balanced way in the direction which makes it easy for a user to apply its power. Further, in the intermediate transfer unit extended almost in the vertical direction as stated above, a risk of contact with other apparatus can be reduced sharply because an area necessary for pulling out upward is small.  
         [0069]    Next, photoreceptor unit  11 Y from which the intermediate transfer unit  7  has been drawn out is made to slide on supporting rest  86 Y to be moved in the void arrow direction shown in FIG. 5, thus photoreceptor unit  11 Y can be taken out from a space section from which the intermediate transfer unit  7  is removed. Photoreceptor units  11 M,  11 C and  11 K can be taken out respectively of supporting rests  86 M,  86 C and  86 K in the same way.  
         [0070]    Because of the state where the process frame  8  has been drawn out and the state where the intermediate transfer unit  7  has been removed, each photoreceptor unit can be taken out safely without interference with the intermediate transfer unit  7 . The risk of interference with other members can further be reduced because it is possible to mount or dismount not through movement in the axial direction of the photoreceptor but through movement for the shortest distance.  
         [0071]    Developing unit  4 Y facing photoreceptor  1 Y, developing unit  4 M facing photoreceptor  1 M, developing unit  4 C facing photoreceptor  1 C and developing unit  4 K facing photoreceptor  1 K can be taken out respectively from the prescribed housing sections of the process frame  8  in the void arrow directions on the right side in FIG. 5.  
         [0072]    In virtue of this mounting and dismounting mechanism, it is not necessary to provide many guide rails for each member, a total apparatus can be made small and each member is not damaged, resulting in a color image forming apparatus wherein a burden for a user is extremely small and maintenance is easy.  
         [0073]    The process frame  8  is made to slide along supporting rails  82 L and  82 R to be mounted on apparatus main body A, and it is stopped at a prescribed position to be fixed by an unillustrated locking lever. A frame operation detection means is a means to detect that the process frame  8  has been mounted at the prescribed position on the apparatus main body A. The frame operation detection means detects by the use of a sensor that the process frame  8  is topped at the prescribed position. Or, the frame operation detection means detects that the locking lever has completed the prescribed operations.  
         [0074]    In the embodiment stated above, there has been explained intermediate transfer object  70  in a shape of an endless belt to which, however, the invention is not limited, and an intermediate transfer object in a shape of a seamed endless belt can also be used.  
         [0075]    Further, in the color image forming apparatus of the invention, it is also possible to use a light emitting element (LED) array as each of exposure means  3 Y,  3 M,  3 C and  3 K and to arrange it at the prescribed position in the process frame  8 .  
         [0076]    Furthermore, the invention is not limited to the embodiment stated above, and it can be applied also to another color image forming apparatus equipped with an image carrier (including an electrostatic recording body), an intermediate transfer object, and a plurality of developing units (including a wet developing unit, and ion current control system).  
         [0077]    In virtue of the color image forming apparatus wherein an image forming means such as an image carrier and an intermediate transfer unit is arranged on a process frame, and the image forming means such as an image carrier and an intermediate transfer unit can be taken out under the state wherein the process frame has been drawn out from the front side near an operator of the apparatus main body, operations and maintenance have been greatly improved to be easy.  
         [0078]    Further, a plurality of developing units are arranged on the process frame, and an intermediate transfer unit and the developing unit are made to be capable of being mounted and dismounted from a different side under the state wherein the process frame has been drawn out from the front side near an operator of the apparatus main body, and thereby, mounting and dismounting of the intermediate transfer unit and the developing unit can be conducted without performing mounting and dismounting operations for other members, which makes operations to be easy.  
         [0079]    Further, by virtue of the structure wherein a process frame can be drawn out of the apparatus main body after the pressure contact between an intermediate transfer object and a transfer means is released, it does not happen that the intermediate transfer object and the transfer means are damaged by operations to draw out the process frame.  
         [0080]    Furthermore, by virtue of the structure wherein a process frame is drawn out to the front side of the apparatus main body and a plurality of photoreceptors can be taken out in front of the apparatus main body accordingly, it is possible to take out the process frame without securing a large floor space, and operations of a user are made easy.  
         [0081]    Next, functions to form images on both sides of a sheet will be explained as follows.  
         [0082]    In FIG. 1, image forming apparatus  100  further has therein sheet feeding section  117 , reversing means S, bypass sheet feeding section  109 , first sheet conveyance path P 1 , second sheet conveyance path G and third sheet conveyance path Q.  
         [0083]    The sheet feeding section  117  is equipped with a plurality of sheet feeding units  117   a - 117   c , and selects the sheet feeding unit housing a sheet on which an image is to be formed from a plurality of sheet feeding units.  
         [0084]    The reversing means S is equipped with guide  48  and a pair of switchback rollers  43  capable of rotating regularly and conversely, and it reverses a sheet inside out in the second sheet conveyance path.  
         [0085]    The first sheet conveyance path P 1  is a conveyance path for a sheet provided to convey the sheet fed from a sheet feeding unit of the sheet feeding section  117  toward intermediate transfer object  70 .  
         [0086]    The second sheet conveyance path G is a conveyance path for a sheet wherein reversing means S and driving rollers  144  and  145  are provided on the conveyance path, and the sheet on which a toner image has been transferred on its first surface is reversed by the reversing means S to be made to join the first sheet conveyance path P 1 .  
         [0087]    The third sheet conveyance path Q is a conveyance path for a sheet wherein the sheet fed in from the side of image forming apparatus  100  through bypass sheet feeding section  109  is made to join the first sheet conveyance path P 1 , and it is designed so that a sheet can be fed to intermediate transfer object  70  in a straight line, and therefore, a thick sheet that is difficult to be conveyed from sheet feeding section  117  is inserted therein.  
         [0088]    The sheet fed from the sheet feeding section  117  is conveyed toward intermediate transfer object  70  through the first sheet conveyance path P 1 . The sheet thus conveyed arrives at registration roller  23  provided on the first sheet conveyance path. The registration roller  23  starts rotating in synchronization with superimposed toner images formed on the intermediate transfer object  70  to convey the sheet. The sheet conveyed by the registration roller  23  is overlapped on the superimposed toner images on the intermediate transfer object  70 , and thus, the toner images are urged by secondary transfer means  5 A to be transferred onto the sheet. After that, the sheet is conveyed to fixing means  23  where toner particles forming the toner images are fused and fixed on the sheet by fixing means H through its heating operations. The sheet on which toner is fixed is ejected to sheet ejection tray  26  through sheet ejection roller  25 .  
         [0089]    When forming images on the second surface (reverse) of the sheet on which the toner image has been transferred onto the first surface (obverse) thereof, conveyance path switching guide  149  is controlled so that the sheet may be fed in the second sheet conveyance path G.  
         [0090]    The sheet fed in the second sheet conveyance path G descends and arrives at switchback rollers  143  of reversing means S. The switchback rollers  143  are composed of a pair of rollers capable of rotating regularly and conversely, and it makes the sheet to be interposed to proceed to a space between plural sheet feeding units provided under the switchback rollers  143  and a side wall of the apparatus main body. In this case, the first surface transferred on the sheet is inclined to the left side. Soon, the switchback rollers  143  stop rotating while they hold the trailing edge of the sheet between them. After that, the rotation in the opposite direction is started and guide  148  is switched so that the sheet may be conveyed to driving rollers  144  and  145 . The sheet conveyed to the driving rollers  144  and  145  enters the state wherein its second surface on which no toner image has been transferred faces downward. The driving roller  145  conveys the sheet conveyed thereto to reversing path  146  where the second surface facing downward is changed to face upward by the movement along a gentle circular arc, so that the sheet may join the first sheet conveyance path P 1  from the second sheet reversing path. Namely, the sheet joins the first sheet conveyance path P 1  from the second sheet conveyance path G after it is reversed inside out by reversing means S of the second sheet conveyance path G.  
         [0091]    The point of confluence of the second sheet conveyance path G and the first sheet conveyance path P 1  is on the downstream side of point of confluence Z of the third sheet conveyance path Q and the first sheet conveyance path P 1  for a sheet coming from bypass sheet feeding section  109 , when viewed in the sheet conveyance direction, and it is provided at the upstream side of registration roller  23 . Therefore, the distance for conveying a sheet from the second sheet conveyance path G to the registration roller  23  is shorter, and sheet conveyance time is shortened. In addition, it is not necessary to provide reversing path  146  of the second sheet conveyance path G above sheet feeding section  21  of the sheet feeding section  117 .  
         [0092]    As is apparent from FIG. 1, the reversing path of the second sheet conveyance path G is arranged to be closer to the central portion of the main body than the sheet feeding means. In the apparatus shown in FIG. 1, each sheet feeding means  21  is shifted in the horizontal direction to be arranged, and the sheet feeding means positioned to be highest is arranged at the central portion of the main body. Therefore, the reversing path of the second sheet conveyance path G is arranged to be shifted from sheet feeding means  21  positioned to be highest in the horizontal direction. This makes a height of reversing section  90  for the sheet conveyed from the sheet feeding unit to be common to that of reversing path  146  of the second sheet conveyance path G. Therefore, in the image forming apparatus having therein an intermediate transfer object representing a belt that is trained about the upper and lower rollers and runs longitudinally, in particular, an effect of space saving for image forming apparatus  100  is great.  
         [0093]    The sheet which has been reversed inside out by reversing means S is guided by the second sheet conveyance path G to registration roller  23  that starts rotating from the position of the point of confluence with the first sheet conveyance path P 1 , through sheet conveyance by driving roller  145 . In this case, driving roller  145  provided on the second sheet conveyance path G feeds the sheet reversed inside out to the point of confluence, and conducts conveyance of the sheet from the point of confluence to the registration roller  23  provided on the first sheet conveyance path P 1 . Accordingly, it is not necessary to provide on the first sheet conveyance path P 1  an additional driving roller for the confluence of the sheet coming from the second sheet conveyance path G.  
         [0094]    The sheet coming from the second sheet conveyance path G which has arrived at the registration roller  23  forms a loop with its leading edge being in contact with the registration roller  23 . Skewing of the sheet generated in the course of conveyance through the second sheet conveyance path G is corrected by the loop formation. The load for conveyance for driving roller  145  is higher than that for other driving rollers, because the driving roller  145  needs to make a sheet loop in addition to conveying the sheet from reversing path  146  in a shape of a circular arc to the registration roller  23 . Therefore, a power of the driving roller  145  to convey a sheet is greater than that of other rollers.  
         [0095]    On the other hand, on intermediate transfer object  70 , there is formed a toner image, and registration roller  23  starts rotating in synchronization with the toner image formed on the intermediate transfer object  70  to convey a sheet.  
         [0096]    The registration roller  23  starts rotating in synchronization with superimposed toner images formed on the intermediate transfer object  70  to convey the sheet. The second toner image on intermediate transfer object  70  is overlapped on the second surface of the sheet conveyed in synchronization with the intermediate transfer object  70 , and thus, the toner image is urged by transfer means  5 A to be transferred on the second surface of the sheet. After that, the sheet is subjected to fixing processing and is ejected on sheet ejection tray  26  through sheet ejection roller  25 .  
         [0097]    A plurality of image carriers are arranged on the side where intermediate transfer object  70  runs from the upper portion to the lower portion to shorten a distance of the second sheet conveyance path G where a sheet is reversed inside out, and to shorten a distance to transfer means  5 A positioned at the lower portion of the intermediate transfer object  70 , and thereby to shorten the time for image forming.  
         [0098]    In the structure stated above, the time for forming images on both sides of a sheet has been shortened and space saving has been attained.  
         [0099]    Disclosed embodiment can be varied by a skilled person without departing from the spirit and scope of the invention.