Abstract:
An image forming apparatus includes a belt unit, a plurality of photosensitive drums, a cartridge, a casing, a cartridge holding unit and a first guide unit. The looped belt has a first surface outside of the loop, and is stretched to form a substantially straight portion on the loop that extends in a first direction. The casing has a base surface. The cartridge holding unit holds the cartridge at a first predetermined position in the casing. Each circumferential surface of the plurality of photosensitive drums contacts the straight portion of the first surface when the cartridge is held by the cartridge holding unit at the first predetermined position. The first guide unit guides the cartridge into the first predetermined position, and allows the plurality of photosensitive drums to move apart from the straight portion of the first surface when the cartridge is detached from the first predetermined position.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/316,945, filed Dec. 27, 2005, which claims priority to Japanese Patent Application No. 2004-378083, filed Dec. 27, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming apparatus and a cartridge which can be mounted in the image forming apparatus. 
         [0004]    2. Description of Related Art 
         [0005]    A tandem type image forming apparatus has photoconductors, exposure units, developing units, and transfer units, each corresponding to the number of colors (for example, four colors of magenta, cyan, yellow and black). When the photoconductors, the developing units, and the like are replaced and maintained, or when a jam process is performed, it is necessary to remove the photoconductors, the developing units, and the like from an image forming apparatus body. 
         [0006]    An image forming apparatus disclosed in Japanese unexamined patent application publication No. 2001-272899 has processing units constituted corresponding to the number of colors and guides for sliding the process units. Each processing unit has photoconductors, a developing unit, and the like. Each processing unit is slidable horizontally along each guide to be quickly attached to and detached from the image forming apparatus body. At the time of maintenance or replacement, only a necessary processing unit which requires maintenance or replacement may be removed. 
         [0007]    However, in the image forming apparatus, in case of performing a maintenance of a member located inside the processing unit when, for example, a conveying belt for conveying a paper sheet is replaced or a jam process is performed, it is necessary to draw out the processing unit entirely from the image forming apparatus body. Accordingly, the efficiency of the maintenance is low. 
         [0008]    Further, the guide is formed to move the processing unit in parallel with the front surface of the conveying belt, etc. Accordingly, in some cases, the photoconductor or the like provided in the processing unit and the conveying belt or the like are slidably contacted with each other to be damaged. 
         [0009]    To solve such a problem, an image forming apparatus disclosed in Japanese unexamined patent application publication No. 2003-015378 has a cartridge which is configured of integrated processing units provided corresponding to the number of colors, and is attachable and detachable. According to this image forming apparatus, all of the processing units can be drawn out at a time by drawing the cartridge from the image forming apparatus body. Therefore, its maintenance efficiency is better as compared with the image forming apparatus disclosed in Japanese unexamined patent application publication No. 2001-272899. 
         [0010]    Furthermore, after the cartridge is once moved in a direction separating the photoconductor from the belt, the cartridge is drawn out from the image forming apparatus body by sliding the cartridge horizontally with the body. Accordingly, it can prevent the photoconductor and the conveying belt or the like from being brought into slidable contact with each other. 
         [0011]    However, since in the image forming apparatus disclosed in Japanese unexamined patent application publication No. 2003-015378, a guiding mechanism, or the like for guiding the cartridge is complicated, its cost is high. Further, since an operation is necessary to separate the photoconductor from the belt before the cartridge is drawn out, its operability or maintenance efficiency is not good. 
       SUMMARY OF THE INVENTION 
       [0012]    In view of the above-described drawbacks, it is an objective of the present invention to provide an image forming apparatus in which, when a cartridge, or the like is attached to and detached from the image forming apparatus, a photoconductor or the like provided in the cartridge is not damaged by slidably contacting with a belt, or the like to achieve good attaching/detaching operability of the cartridge, and a cartridge. 
         [0013]    In order to attain the above and other objects, the present invention provides an image forming apparatus including a belt unit, a plurality of photosensitive drums, a plurality of developing units, a transfer unit, a cartridge, a casing, a cartridge holding unit and a first guide unit. 
         [0014]    The belt unit includes a looped belt that circularly moves. The looped belt has a first surface outside of the loop and a second surface inside of the loop, and is stretched to form a substantially straight portion on the loop that extends in a first direction. Each of the photosensitive drums has a circumferential surface on which an electrostatic latent image is formable. Each circumferential surface is opposed to the straight portion of the first surface. The plurality of developing units is disposed in one-to-one correspondence with the plurality of photosensitive drums. Each developing unit provides the circumferential surface of the corresponding photosensitive drum with developer in order to form a visible image thereon. The transfer unit is opposed to the straight portion of the second surface. The transfer unit transfers the visible image from the photosensitive drum into either the first surface of the looped belt or a recording medium mounted on the first surface. 
         [0015]    The cartridge holds the plurality of photosensitive drums. The casing accommodates the belt unit, the plurality of photosensitive drums, the plurality of developing units and the transfer unit. The cartridge holding unit holds the cartridge at a first predetermined position in the casing. Each circumferential surface of the plurality of photosensitive drums contacts the straight portion of the first surface when the cartridge is held by the cartridge holding unit at the first predetermined position. The first guide unit guides the cartridge into the first predetermined position, and allows the plurality of photosensitive drums to move apart from the straight portion of the first surface when the cartridge is detached from the first predetermined position. 
         [0016]    Another aspect of this invention provides a cartridge for use in an image forming device. The image forming device includes a belt unit, a plurality of photosensitive drums, a casing accommodating the belt unit and a first guide unit. The belt unit includes a looped belt that circularly moves. The looped belt has a first surface outside of the loop and a second surface inside of the loop, and is stretched to form a substantially straight portion on the loop that extends in a first direction. Each of the photosensitive drums has a circumferential surface on which an electrostatic latent image is formable. Each circumferential surface is opposed to the straight portion of the first surface. The plurality of photosensitive drums is held by the cartridge. The casing accommodates the belt unit and the plurality of photosensitive drums. The first guide unit guides the cartridge into a first predetermined position. 
         [0017]    The cartridge includes a cartridge body and a first projection member that is guided by the first guide unit in order to guide the cartridge into a first predetermined position in the casing. Each circumferential surface of the plurality of photosensitive drums contacts the straight portion of the first surface when the cartridge is held at the first predetermined position, whereas each circumferential surface of the plurality of photosensitive drums moves apart from the straight portion of the first surface when the cartridge is detached from the first predetermined position. 
         [0018]    Another aspect of this invention provides a combination of a image forming device and a cartridge. 
         [0019]    The image forming device includes a belt unit, a plurality of photosensitive drums, a plurality of developing units, a transfer unit, a casing, a cartridge holding unit, and a first guide unit. 
         [0020]    The belt unit includes a looped belt that circularly moves. The looped belt has a first surface outside of the loop and a second surface inside of the loop, and is stretched to form a substantially straight portion on the loop that extends in a first direction. Each of the plurality of photosensitive drums has a circumferential surface on which an electrostatic latent image is formable. Each circumferential surface is opposed to the straight portion of the first surface. The plurality of developing units is disposed in one-to-one correspondence with the plurality of photosensitive drums. Each developing unit provides the circumferential surface of the corresponding photosensitive drum with developer in order to form a visible image thereon. The transfer unit is opposed to the straight portion of the second surface. The transfer unit transfers the visible image from the photosensitive drum into either the first surface of the looped belt or a recording medium mounted on the first surface. 
         [0021]    The casing accommodates the belt unit, the plurality of photosensitive drums, the plurality of developing units and the transfer unit. The casing has a base surface. The cartridge holding unit holds the cartridge at a first predetermined position in the casing. Each circumferential surface of the plurality of photosensitive drums contacts the straight portion of the first surface when the cartridge is held by the cartridge holding unit at the first predetermined position. The first guide unit guides the cartridge into the first predetermined position, and allows the plurality of photosensitive drums to move apart from the straight portion of the first surface when the cartridge is detached from the first predetermined position. 
         [0022]    The cartridge includes a cartridge body and a first projection member that is guided by the first guide unit in order to guide the cartridge into a first predetermined position in the casing. Each circumferential surface of the plurality of photosensitive drums contacts the straight portion of the first surface when the cartridge is held at the first predetermined position, whereas each circumferential surface of the plurality of photosensitive drums moves apart from the straight portion of the first surface when the cartridge is detached from the first predetermined position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
           [0024]      FIG. 1  is a schematic sectional side view showing a printer in a first embodiment; 
           [0025]      FIG. 2  is a simplified view for explaining the engagement of a visible image forming unit with a cartridge in the first embodiment; 
           [0026]      FIG. 3  is a view, as seen from the front, of the state that the visible image forming unit is held in the cartridge in the first embodiment; 
           [0027]      FIG. 4  is a side view of a belt unit in the first embodiment; 
           [0028]      FIG. 5  is an explanatory view for explaining the attaching and detaching operation of the cartridge and the belt unit with respect to the printer according to the first embodiment; 
           [0029]      FIG. 6  is another explanatory view for explaining the attaching and detaching operation of the cartridge and the belt unit with respect to the printer according to the first embodiment; 
           [0030]      FIGS. 7A and 7B  are enlarged views of contact portions of the photoconductor drums with the conveying belt in the first embodiment; 
           [0031]      FIG. 8  is a schematic sectional side view showing a printer in a second embodiment; and 
           [0032]      FIG. 9  is a schematic sectional side view showing a complex machine in a third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    An image forming apparatus and a cartridge according to the first embodiment of the present invention will be described in detail with reference to the accompanying drawings.  FIG. 1  is a schematic sectional side view showing a printer  10  as an image forming apparatus of this embodiment. In the description given below, that the left side in  FIG. 1  denotes the front of the printer  10 , and the right side denotes the rear thereof. 
         [0034]    As shown in  FIG. 1 , the printer  10  is a tandem type color laser printer. The printer  10  includes a visible image forming unit  11 , a cartridge  30 , a belt unit  50 , a fixing unit  60 , a sheet feeding unit  70 , a sheet discharge tray  80 , and an exposure means  35 . 
         [0035]    The visible image forming unit  11  has developing units  31 M,  31 C,  31 Y, and  31 Bk; photoconductor drums  32 M,  32 C,  32 Y, and  32 Bk; cleaning rollers  33 M,  33 C,  33 Y, and  33 Bk; and chargers  34 M,  34 C,  34 Y, and  34 Bk. The visible image forming unit  11  is held in the cartridge  30 . The configuration of the cartridge  30  will be described in detail below. 
         [0036]    The developing unit  31 M, the photoconductor drum  32 M, the cleaning roller  33 M, and the charger  34 M perform a visible image forming step with a developer of magenta (M). The developing unit  31 C, the photoconductor drum  32 C, the cleaning roller  33 C, and the charger  34 C perform a visible image forming step with a developer of cyan (C). The developing unit  31 Y, the photoconductor drum  32 Y, the cleaning roller  33 Y, and the charger  34 Y perform a visible image forming step with a developer of yellow (Y). The developing unit  31 Bk, the photoconductor drum  32 Bk, the cleaning roller  33 Bk, and the charger  34 Bk perform a visible image forming step with a developer of black (Bk). 
         [0037]    In the description given below, as an example, the configurations of the developing unit  31 M, the photoconductor drum  32 M, the cleaning roller  33 M, and the charger  34 M which perform the visible image forming step with the developer of magenta (M) will be described. However, the configurations of the developing units  31 C,  31 Y, and  31 Bk, the photoconductor drums  32 C,  32 Y, and  32 Bk, the cleaning rollers  33 C,  33 Y and  33 Bk, and the chargers  34 C,  34 Y and  34 Bk are respectively the same as the configuration of the developing unit  31 M, the photoconductor drum  32 M, the cleaning roller  33 M and the charger  34 M. 
         [0038]    The developing unit  31 M has a developing roller  36 M, a supply roller  37 M, a layer thickness control blade  38 M, and a developing unit case  39 M. The developing roller  36 M is constituted by forming a coating layer made of resin containing fluorine or made of a rubber material on the front surface of a conductive silicone rubber formed in a columnar shape. In the developing roller  36 M, the base material may not always be constituted by a conductive silicone rubber. For example, the base material may be constituted by a conductive urethane rubber. A ten-point height of irregularities (Rz) of the front surface is set to 3 to 5 μm smaller than 9 μm of the average particle diameter of toner. 
         [0039]    The supply roller  37 M is a conductive sponge roller. The supply roller  37 M is located so as to be brought into pressure contact with the developing roller  36 M by means of the elastic force of the sponge. As the supply roller  37 M, a foam of a suitable member of conductive silicone rubber, an EPDM, a urethane rubber, or the like, can be used. 
         [0040]    The base end of the layer thickness control blade  38 M is formed in a plate shape and made of a stainless steel, etc. The tip end of the layer thickness control blade  38 M is formed of an insulating silicone rubber, an insulating fluorine-containing rubber or resin. The base end of the layer thickness control blade  38 M is fixed to the developing unit case  39 M. The tip end of the layer thickness control blade  38 M is brought into pressure contact with the developing roller  36 M on the upper side of the developing roller  36 M. In the developing unit case  39 M, the toner of magenta (M) color charged positively is contained. 
         [0041]    The photoconductor drum  32 M has a configuration that a positively chargeable photoconductor layer is formed on a base material made of aluminum, for example. The thickness of the photoconductor layer is 20 μm or more. The base material made of aluminum is used as an earth layer. 
         [0042]    The cleaning roller  33 M is a roller made of an elastic material, such as a conductive sponge, etc. The cleaning roller  33 M is arranged under the photoconductor drum  32 M slidably contact with the photoconductor drum  32 M. 
         [0043]    The charger  34 M is a Scorotron type charger. The charger  34 M is arranged to face the surface of the photoconductor drum  32 M in non-contact manner at the downstream side in the rotating direction of the photoconductor drum  32 M from the cleaning roller  33 M. 
         [0044]    The exposure means  35  is constituted by a known laser scanner unit and arranged above the visible image forming unit  11 . The exposure means  35  exposes the surfaces of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk with a laser beam according to image data. 
         [0045]    With this configuration, the positively charged toner contained in the developing unit case  39 M is supplied to the developing roller  36 M by the supply roller  37 M. In this case, the toner is made to a uniform thin layer by the layer thickness control blade  38 M. On the other hand, the photoconductor layer of the photoconductor drum  32 M is charged uniformly by the charger  34 M. Then, the photoconductor layer is exposed by the exposure means  35  to form an electrostatic latent image. When the developing roller  36 M contacts with the photoconductor drum  32 M, the positively charged toner is adhered to an (positively charged) electrostatic latent image formed on the photoconductor drum  32 M to develop the electrostatic latent image by an inversion development method. 
         [0046]    The sheet feeding unit  70  is provided at the lowermost portion of the image forming apparatus. The sheet feeding unit  70  includes a container tray  71  for containing a paper sheet P, and a pickup roller  72  for feeding out the paper sheet P. The sheet feeding unit  70  is detachable from and attachable to the printer  10  from the front (left side in the drawing). The sheet feeding unit  70  supplies the paper sheet P at a predetermined timing in accordance with the image forming step by the visible image forming unit  11 . 
         [0047]    The belt unit  50  includes a conveying belt  58 , a drive roller  51  and a driven roller  52  for bridging the conveying belt  58 . The conveying belt  58  is made of an endless belt and is formed of resin, such as a conductive polycarbonate or a polyimide, in which conductive particles made of carbon, etc., is dispersed. The drive roller  51  is rotatably driven by a motor (not shown) and the driven roller  52  is rotated by following to the drive roller  51  to rotate the conveying belt  58 . Further, the driven roller  52  is brought into contact with a primary conveying roller  54 . The driven roller  52  cooperates with the primary conveying roller  54  to place the paper sheet P supplied from the sheet feeding unit  70  on the conveying belt  58 . 
         [0048]    A transfer roller  53  and a compression spring  59  (refer to  FIG. 7 ) are provided near the position opposed to the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk inside the conveying belt  58 . The transfer roller  53  is biased to the sides of the photoconductor drums  32 M,  32 C,  32 Y, and  32 Bk by the elastic force of the compression spring  59 . Thus, nips are formed between the conveying belt  58  and the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk. 
         [0049]    The paper sheet P supplied from the sheet feeding unit  70  is first conveyed to a pressure contact portion between the conveying belt  58  and the primary conveying roller  54 . Then, the paper sheet P is sequentially conveyed between the conveying belt  58  and the respective photoconductor drums  32 M,  32 C,  32 Y and  32 Bk. When the paper sheet P is passed between the conveying belt  58  and the respective photoconductor drums  32 M,  32 C,  32 Y, and  32 Bk, toner images carried by the respective photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are sequentially transferred to the paper sheet P. Thus, a color image is formed on the sheet P. 
         [0050]    The conveying belt  58  is biased to the sides of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk and is brought into contact with the photoconductor drums. Accordingly, the image is transferred without fail to the paper sheet P from the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk. 
         [0051]    Furthermore, a cleaning unit  55  is provided at the opposite side to the side where the photoconductor drums  32 M,  32 C,  32 Y and  32 B face the conveying belt  58 . The cleaning unit  55  includes a scraping member  56  and a casing  57 . The toner, dust, or the like adhered to the conveying belt  58  are scraped by the scraping member  56  and contained in the casing  57 . 
         [0052]    The fixing unit  60  includes a heating roller  61 , a pressure roller  62 , a heater  63  and a temperature sensor  64 . The heating roller  61  is brought into contact with the pressure roller  62 . The heating roller  61  and the pressure roller  62  are rotated in the state that the rollers  61  and  62  are brought into contact with each other. The heater  63  is constituted, for example, by a halogen lamp. When the heater  63  is energized, the heater  63  generates heat. The heater  63  is provided inside of the heating roller  61  and raises the temperature of the heating roller  61 . Thus, the temperature of the heating roller  61  is controlled to be a heat fixing temperature (for example, 180° C.) during a printing operation. The temperature sensor  64  is constituted, for example, by a thermistor and arranged in contact with the surface of the heating roller  61  to detect the surface temperature of the heating roller  61 . The paper sheet P on which the toner image is transferred by the visible image forming unit  11  is heated and pressurized between the heating roller  61  and the pressure roller  62 . Then, the toner image is thermally fixed. 
         [0053]    The sheet discharge tray  80  is provided at the sheet discharge side of the fixing unit  60 . The paper sheet P discharged from the fixing unit  60  is conveyed by the conveying roller pairs  91 ,  92 , and  93  to be contained in the sheet discharge tray  80 . 
         [0054]    Then, the operation of the printer  10  will be described. First, the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are rotatably driven. In this state, the photoconductor layers on the surfaces of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are respectively uniformly charged by the chargers  34 M,  34 C,  34 Y and  34 Bk. The charged photoconductor layers are exposed corresponding to the respective images of magenta, cyan, yellow and black colors by the exposure means  35 . Then, the electrostatic latent images are formed. A magenta toner, cyan toner, yellow toner and black toner are respectively adhered to the electrostatic latent images formed on the photoconductor layers of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk by the magenta developing unit  31 M, the cyan developing unit  31 C, yellow developing unit  31 Y and black developing unit  32 Bk. Thus, the electrostatic latent images are respectively developed to the magenta color, the cyan color, the yellow color and the black color. 
         [0055]    The toner images of the respective colors formed on the photoconductor layers of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are transferred to the paper sheet P conveyed by the conveying belt  58 . The toner images of the respective colors are formed by providing a slight time difference according to the moving speed of the paper sheet P conveyed by the conveying belt  58  and the positions of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk. Further, the toner images of the respective colors are transferred so as to be superposed on the paper sheet P. Incidentally, the toners remained on the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk after the transfer are temporarily held by the cleaning rollers  33 M,  33 C,  33 Y, and  33 Bk. The four-color toner images transferred to the paper sheet P are thermally fixed by the fixing unit  60 . Thus, a color image is formed on the paper sheet P. 
         [0056]    As described above, the visible image forming unit  11  includes the developing units  31 M,  31 C,  31 Y and  31 Bk, the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk, the cleaning rollers  33 M,  33 C,  33 Y and  33 Bk, and the chargers  34 M,  34 C,  34 Y, and  34 Bk for the respective visible image forming steps by the respective toners. However, it takes effort if these components are respectively attached to and detached from the body of the printer  10 , contributing to low maintenance efficiency. 
         [0057]    Therefore, in this embodiment, the visible image forming unit  11  is held in the cartridge  30 . The cartridge  30  and the belt unit  50  are easily attached to and detached from the body of the printer  10 . 
         [0058]    The configuration of the cartridge  30  will be described in detail with reference to  FIG. 2 .  FIG. 2  is a simplified view for explaining the engagement of the visible image forming unit  11  with the cartridge  30 . The visible image forming unit  11  that is held by the cartridge  30  is omitted in  FIG. 2 . The cartridge  30  has a box shape which is opened at the upper surface thereof. In  FIG. 2 , only one side surface is shown. However, projection members  100   a  and  100   b , and four engaging grooves  30   a  are respectively provided on both side surfaces (front side and back side of the sheet) of the cartridge  30 . The projection members  100   a  and  100   b  are provided at both left and right ends of the cartridge  30  shown in  FIG. 2  and are projected to the front side of the sheet (the projection member  100   a  and  100   b  at the side not shown are projected to the back side of the sheet). 
         [0059]    Further, though only one side surface is shown in  FIG. 2 , the engaging projections  100   c  are respectively provided at the lower end on both side surfaces of the developing unit cases  39 M,  39 C,  39 Y and  39 Bk for constituting the visible image forming unit  11 . The respective engaging projections  100   c  are projected to the front side of the sheet (the engaging projections  100   c  of the side not shown are projected to the back side of the sheet). 
         [0060]    The engaging projections  100   c  of the respective developing unit cases  39 M,  39 C,  39 Y and  39 Bk are respectively engaged with the four engaging grooves  30   a  of the cartridge. Thus, the visible image forming unit  11  is held in the cartridge  30 .  FIG. 3  is a view showing the state that the visible image forming unit  11  is held in the cartridge  30  as seen from the front (left side of  FIG. 1 ). As shown in  FIG. 3 , in the state that the visible image forming unit  11  (the developing unit cases  39 M) is held in the cartridge  30 , the projection members  100   a  and  100   b  are projected more to the side surface side than the engaging projections  100   c.    
         [0061]      FIG. 4  is a side view of the belt unit  50 . Though only one side surface is shown in  FIG. 4 , the supporting members  104   a  and  104   b  that are respectively shafts of the drive roller  51  and the driven roller  52  are provided to the both the side surfaces (the front side and back side of the sheet) of the belt unit  50 . The supporting members  104   a  and  104   b  are provided at the left and right ends in the drawing, and projected to the front side of the sheet (the supporting members  104   a  and  104   b  of the side not shown are projected to the back side of the sheet). 
         [0062]      FIG. 5  and  FIG. 6  are views for explaining the attachment and detachment of the cartridge  30  and the belt unit  50  to/from the printer  10 . A guiding mechanism  108  is provided in the printer  10 . The guiding mechanism  108  includes a pair of guide plates  108   a  and a pair of guide plates  108   b . That is, from the front side of the sheet, the guide plate  108   a , the guide plate  108   b , the guide plate  108   b , and the guide plate  108   a  are sequentially arranged substantially in parallel with each other. Though the guide plates  108   a  and the guide plate  108   b  of the front side of the sheet are omitted in  FIG. 5  and  FIG. 6 , the cartridge  30  and the belt unit  50  are attachable to and detachable from the printer  10  same as the guide plates  108   a  and the guide plate  108   b  of the back side of the sheet are. 
         [0063]    In the guide plate  108   a , a guide groove  101   a  in which the projection member  100   a  of the cartridge  30  is inserted, and a guide groove  101   b  in which the projection member  100   b  of the cartridge  30  is inserted, are formed. In  FIG. 5 , the projection members  100   a  and  100   b  of the back side of the sheet are inserted to the guide grooves  101   a  and  101   b  respectively. The projection members  100   a  and  100   b  are guided substantially horizontally from the front end (left end in the drawing) of the guiding mechanism  108  in the inside of the printer  10  along the guide grooves  101   a  and  101   b . Further, locking members  103   a  and  103   b  are respectively formed on the guide grooves  101   a  and  101   b . The projection members  100   a  and  100   b  respectively inserted into the guiding grooves  101   a  and  101   b  are respectively elastically locked by the locking members  103   a  and  103   b.    
         [0064]    In the guide plate  108   b , four guide grooves  101   c  in which the four engaging projections  100   c  of the cartridge  30  are respectively inserted, are formed. The four engaging projections  100   c  are respectively guided from the front end (left end in the drawing) of the guiding mechanism  108  substantially horizontally into the inside of the printer  10  along the four guide grooves  101   c . Furthermore, the locking members  103   c  are formed in the respective guide grooves  101   c . The engaging projections  100   c  inserted into the guide grooves  101   c  are respectively locked elastically by the locking members  103   c.    
         [0065]    In the state that the visible image forming unit  11  is held in the cartridge  30 , the engaging projections  100   c  are not projected more to the side of the side surface than the projection members  100   a  and  100   b . Accordingly, the engaging projections  100   c  are not collided with the guide plate  108   a , but inserted into the guide grooves  101   c . Thus, the cartridge  30  is fixed to the body of the printer  10 . 
         [0066]    Further, the guide plate  108   b  is formed with a guide groove  105   a  in which the supporting member  104   a  of the belt unit  50  is inserted, and a guide groove  105   b  in which the supporting member  104   b  of the belt unit  50  is inserted. The supporting members  104   a  and  104   b  are guided from the rear front end (right end in the drawing) of the guiding mechanism  108  substantially horizontally into the printer  10 . Moreover, locking members  107   a  and  107   b  are respectively formed in the guide grooves  105   a  and  105   b . The supporting members  104   a  and  104   b  respectively inserted into the guide grooves  105   a  and  105   b  are elastically locked by the locking members  107   a  and  107   b . The supporting members  104   a  and  104   b  are projected in substantially the same length as the engaging projections  100   c  of the cartridge  30 . Accordingly, the supporting members  104   a  and  104   b  are not collided with the guide plates  108   a , but inserted into the guide grooves  105   a  and  105   b.    
         [0067]    Since the belt unit  50  is fixed to the body of the printer  10 , the cartridge  30  and the belt unit  50  can be drawn (removed) from the body of the printer  10  by applying a force against the elastic force of the locking members  103   a  and  103   b  and  107   a  and  107   b.    
         [0068]    Thus, the drawing directions of the cartridge  30  and the belt unit  50  are in a substantially horizontal direction and the force for moving the cartridge  30  and the belt unit  50  in a vertical direction is hardly required. Accordingly, the cartridge  30  and the belt unit  50  can be operated with lighter force. The cartridge  30  and the belt unit  50  are positioned at predetermined positions by the guiding mechanism  108 . Therefore, displacement (transfer deviation) when an image is transferred can be prevented. The cartridge  30  and the belt unit  50  are drawn in opposite directions. That is, the cartridge  30  and the belt unit  50  can be drawn independently from each other. Thus, after the cartridge  30  is drawn out, it takes no effort to draw out of the belt unit  50 . The maintenance efficiency is good. 
         [0069]    The guide grooves  101   a ,  101   b ,  101   c  and  105   a ,  105   b  are arranged so that the cartridge  30  and the belt unit  50  are oblique to the horizontal plane and the extending direction of the conveying belt  58  and the aligning direction of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are parallel to each other. The extending direction is a direction perpendicular to the supporting shafts of the drive roller  51  and the driven roller  52  located at both ends of the circumferential surface facing the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk of the circumferential surface of the conveying belt  58 , and directed from the one supporting shaft toward the other supporting shaft. 
         [0070]    A front cover  81 , a front lower cover  82 , a supporting shaft  83  and a supporting shaft  84  are provided at the front (left end of the drawing) of the printer  10 . The front cover  81  is supported by the supporting shaft  83 . The front lower cover  82  is supported by the supporting shaft  84 . The front cover  81  and the front lower cover  82  are freely opened and closed. The cartridge  30  is detachable from and attachable to the body of the printer  10  by opening and closing the front cover  81  and the front lower cover  82 . 
         [0071]    A rear cover  85  and a supporting shaft  86  are provided at the back of the printer  10 . The rear cover  85  is supported by the supporting shaft  86  and is freely opened and closed. The belt unit  50  is attachable to and detachable from the body of the printer  10  by opening and closing the rear cover  85 . The above-mentioned fixing unit  60 , and the conveying rollers  91  and  92  are provided at the rear cover  85 . When the rear cover  85  is closed, the fixing unit  60  and the conveying rollers  91  and  92  block the attaching and detaching path of the belt unit  50  as shown in  FIG. 1 . However, when the rear cover  85  is opened, as shown in  FIG. 5 , the fixing unit  60  and the conveying rollers  91  and  92  are released from the attaching and detaching path of the belt unit  50 . Therefore, when the rear cover  85  is opened, the belt unit  50  can be attached to and detached from the body of the printer  10 . 
         [0072]    When the cartridge  30  and the belt unit  50  are attached to and detached from the printer  10 , the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk loaded in the cartridge  30  and the conveying belt  58  included in the belt unit  50  are brought into contact with one another or separated from one another. The detail will be described by using  FIG. 7(   a ) and  FIG. 7(   b ).  FIG. 7(   a ) and  FIG. 7(   b ) are enlarged views of the contact portions of the conveying belt  58  with the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk. Incidentally, the configuration is as described above. 
         [0073]    When the cartridge  30  and the belt unit  50  are contained in the body of the printer  10 , as shown in  FIG. 7(   a ), the contact state of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk with the conveying belt  58  is held by the compression spring  59  and the transfer rollers  53 . 
         [0074]    When the cartridge  30  is drawn from the body of the printer  10  in a substantially horizontal direction, as shown in  FIG. 7(   b ), the contact of the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk with the conveying belt  58  is released, and the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are separated from the conveying belt  58 . On the other hand, the transfer rollers  53  are not brought into contact with the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk. Accordingly, the transfer rollers  53  are moved upward by the elastic force of the compression spring  59 . Thus, the conveying belt  58  rises upward. 
         [0075]    In this embodiment, the cartridge  30  and the belt unit  50  are arranged obliquely to separate from each other when the cartridge  30  and the belt unit  50  are drawn from the body of the printer  10 . That is, the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk and the conveying belt  58  are arranged in parallel with one another and at a predetermined angle to the drawing direction of the cartridge  30 . The angle is an angle for separating the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk and the conveying belt  58  when the cartridge  30  is drawn out. That is, when the cartridge  30  is drawn out from the body of the printer  10 , the rise height X of the conveying belt  58  is set to be smaller than the deviation Y of the lower end position of the respective photoconductor drums  32 M,  32 C,  32 Y and  32 Bk adjacent in the longitudinal direction. 
         [0076]    Therefore, even if the conveying belt  58  rises when the cartridge  30  is drawn out, an interval can be held between the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk and the conveying belt  58 . Thus, the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk can be certainly prevented from being damaged in contact with the expanded conveying belt  58 . Furthermore, when the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are separated from the conveying belt  58 , a guide for guiding the cartridge  30  in the separating direction and a complicated mechanism for moving the cartridge  30  in a separating direction are not required. Accordingly, the printer  10  can be manufactured at lower cost. 
         [0077]    As described above, in the printer  10  of this embodiment, the constituting element of the visible image forming unit  11  is held integrally in the cartridge  30 . Accordingly, the cartridge  30  becomes attachable and detachable by sliding the cartridge  30  along the guiding mechanism  108  provided in the printer  10 . Similarly, the belt unit  50  is constituted to be attachable and detachable. The cartridge  30  and the belt unit  50  can be easily attached to and detached from the body of the printer  10 . Therefore, the maintenance efficiency can be improved. 
         [0078]    Further, the sheet feeding unit  70  is constituted to be able to be drawn out in the same direction as the cartridge  30  or to be mounted. Accordingly, the usability and maintenance efficiency can be improved. 
         [0079]    Next, a printer  12  according to the second embodiment of the present invention will be described by using  FIG. 8 . As shown in  FIG. 8 , in this embodiment, a conveying belt  58 , a drive roller  51 , a driven roller  52 , and a cleaning unit  55  are arranged fixedly in the body of the printer  12 . Further, the cartridge  30  is constituted to be detachable and attachable by a guiding mechanism  109  provided in the body of the printer  10  as in the first embodiment. 
         [0080]    According to such a constitution, the conveying belt  58  is fixed to the body of the printer  12 . The belt unit  50  can suppress the displacement (transfer deviation) of an image as compared with the first embodiment in which the belt unit  50  is detachable from and attachable to the body. Thus, the image of higher quality can be formed. 
         [0081]    Next, a complex machine  13  of the third embodiment of the present invention will be described by using  FIG. 9 . In this embodiment, as shown in  FIG. 9 , a scanner unit  110  is provided integrally at an upper portion above the printer  10  so as to become parallel to the bottom of the complex machine  13 . Further, the scanner unit  110  includes an operation panel  112  having an operating unit  114 , or the like for operating the scanner unit  110  and the printer  10 . The other constitution of the complex machine  13  of this embodiment is similar to the printer  10  in the first embodiment. Thus, the description thereof will be omitted. 
         [0082]    According to such a configuration, the scanner unit  110  is provided above the printer  10 . An image can be easily read and printed. Thus, the printer device having good usability can be provided. Further, the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are arranged so that the adjacent photoconductor drums  32  overlap with one another in the horizontal direction. Accordingly, as compared with the case that the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk are arranged in the vertical direction, the height of the printer  10  can be lowered. Therefore, when the printer  10  or the complex machine  13  is provided, it is easy to constitute the printer  10  or the complex machine  13  in height easy to be handled by a user. Further, since the center of gravity is lowered, the printer  10  or the complex machine  13  can be installed stably. 
         [0083]    The scanner unit  110  is provided in parallel with the bottom of the printer device, that is, substantially horizontally. Accordingly, when the cartridge  30  is drawn from the body of the printer  10 , the cartridge  30  can be smoothly drawn out without interfering with the scanner unit  110 . 
         [0084]    Further, the sheet discharge tray  80  for containing a medium to be recorded in which an image is transferred is provided between the scanner unit  110  and the printer  10 . Accordingly, as compared with the case that the sheet discharge tray  80  is provided at the one of the longitudinal ends of the body of the image forming apparatus or one of the left and right ends, etc., of the body of the image forming apparatus, the sheet discharge tray  80  is not projected from the image forming apparatus. Thus, space saving can be achieved. Furthermore, since the sheet discharge tray  80  is not projected from the image forming apparatus, the space to be occupied by the image forming apparatus can be narrowed. Moreover, an operation panel  112  having an operating unit  114  which can be operated by a user is provided near the scanner unit  110 . Thus, as compared with the case that the operation panel  112  is arranged on the outer wall of the printer  10 , the operability is improved. 
         [0085]    While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. For example, in the above embodiments, the printer of the direct transfer type for transferring an image to the paper sheet P directly from the respective photoconductor drums  32 M,  32 C,  32 Y and  32 Bk has been described. However, if the present invention is applied to a printer of the intermediate transfer type for transferring an image to the intermediate transfer belt to transfer the image on a paper sheet, the similar effect can be obtained. 
         [0086]    Further, in the above respective embodiments, the conveying belt  58  extends between the drive roller  51  and the driven roller  52 , and the extending direction is one direction. However, for example, if the conveying belt  58  extends between three or more rollers and the extending direction is two or more directions, the present invention can be applied, and the similar effect can be obtained. The extending direction in this case is a direction vertical to the supporting shafts of the rollers located at both ends of a circumferential surface facing the photoconductor drums  32 M,  32 C,  32 Y and  32 Bk of the circumferential surface of the conveying belt  58 , and directed from the supporting shaft of one roller to the supporting shaft of the other roller. 
         [0087]    Moreover, in the above-mentioned embodiments, the belt unit  50  is arranged under the cartridge  30 . However, the belt unit  50  may be arranged above the cartridge  30 . With this configuration, the opening of the developing unit case  39  can be directed upward. Accordingly, the toner is prevented from leaking downward. 
         [0088]    In the above-mentioned embodiment, when the cartridge  30  or the belt unit  50  is drawn out to a predetermined position, the cartridge  30  or the belt unit  50  is removed from the body of the printer  10 . Alternatively, when the cartridge  30  or the belt unit  50  is drawn to a predetermined position, a stopping member for once stopping the cartridge or the belt unit at the predetermined position may be provided. 
         [0089]    In the first embodiment, the fixing unit  60  is provided to the rear cover  85  of the printer  10 . Alternatively, the fixing unit  60  may be provided to the belt unit  50 , and the fixing unit  60  and the belt unit  50  may be collectively attached to or detached from the body of the printer  10 . Further, the fixing unit  60  may be constituted so that the fixing unit  60  is individually attached to and detached from the body of the printer  10 .