Abstract:
An image forming apparatus includes: a mainframe; a process cartridge loadable in and unloadable from the mainframe; and an exposure unit that is arranged correspondingly with the process cartridge. The process cartridge includes: a photosensitive body that is exposed by the exposure unit to form an electrostatic latent image thereon, and a processing device that acts on the photosensitive body; and relative positions of the photosensitive body and the processing device are changeable at the time the process cartridge is loaded and unloaded.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus that is disposed with a photosensitive body and a processing device and forms an image on a recording medium, and to a process cartridge that is disposed with a photosensitive body and a processing device and is loadable in and unloadable from an image forming apparatus mainframe. 
   2. Background Art 
   Conventionally, as a multicolor image forming apparatus that forms a multicolor image of four colors on a recording medium such as recording paper, a so-called tandem-system device is known where photosensitive bodies, exposure means that expose the photosensitive bodies to form electrostatic latent images on the surfaces of the photosensitive bodies and developing means that supply a charged developing agent to the surfaces of the photosensitive bodies on which the electrostatic latent images have been formed are respectively disposed in line in numbers corresponding to the number of colors (e.g., the four colors of magenta, cyan, yellow and black). A 4-cycle-system device is also known where there is one exposure means and one photosensitive body, with developing means being disposed around the periphery of the photosensitive body in a number corresponding to the number of colors. 
   Here, the latter 4-cycle system is not suited for increases in the speed of image formation because steps such as exposure and development of the photosensitive body are conducted by successively changing the colors. In contrast, with the tandem system, steps such as exposure and development of the photosensitive body can be conducted substantially simultaneously for each color, and a multicolor image can be formed by successively superposing and transferring, to the recording medium, the developing agent adhering to the photosensitive bodies corresponding to the colors. For this reason, a tandem-system multicolor image forming apparatus is suited for increases in speed because the sped of image formation is not much different from that in the case of monochromatic image formation. 
   In these image forming apparatuses, it is necessary to occasionally replace the photosensitive bodies and developing means. In a tandem-system multicolor image forming apparatus, because the exposure means and photosensitive bodies are respectively disposed in numbers corresponding to the number of colors, a contrivance is necessary so that, at the time of replacing the photosensitive bodies and the developing means (there are cases where these are integrally and replaceably configured as process cartridges), these do not interfere with the exposure means. Thus, evacuating the exposure means to a non-interfering position at the time of replacing the process cartridges has been considered (e.g., see JP-A-2001-166555). 
   However, when the exposure means are evacuated at the time of replacing the process cartridges, the position of the exposure means of each color subtly relatively moves each time replacement is conducted, so that there is the potential for this to cause color shifting. 
   SUMMARY OF THE INVENTION 
   The present invention has been devised with the object of providing an image forming apparatus with which it is possible to easily replace process cartridges, without having to evacuate the exposure unit, and a process cartridge that can be used in the image forming apparatus. 
   To achieve the object, the invention provides an image forming apparatus, including: a mainframe; a process cartridge loadable in and unloadable from the mainframe; and an exposure unit that is arranged correspondingly with the process cartridge. The process cartridge includes: a photosensitive body that is exposed by the exposure unit to form an electrostatic latent image thereon, and a processing device that acts on the photosensitive body; and relative positions of the photosensitive body and the processing device are changeable at the time the process cartridge is loaded and unloaded. 
   In the invention configured in this manner, when the process cartridge disposed with the photosensitive body and the processing device acting on the photosensitive body is loaded in and unloaded from the image forming apparatus, the relative positions of the photosensitive body and the processing device can be changed so that the process cartridge does not interfere with the exposure unit. 
   In this manner, because the process cartridge is loaded and unloaded while the relation positions of the photosensitive body and the processing device are changed, interference between the process cartridge and the exposure unit can be prevented and the process cartridge can be easily replaced without having to move the exposure unit. Also, assume that the invention is a so-called tandem-system multicolor image forming apparatus disposed with the exposure unit, the photosensitive body and the processing device per color, it is suited for increases in the speed of image formation and there is the effect of preventing color shifting because, as described above, the exposure unit does not have to be moved. Moreover, because interference between the process cartridge and the exposure unit is prevented as described above, there are also the effects that the degree of design freedom increases and it becomes easy to make the device compact. 
   The invention may provide a process cartridge loadable in and unloadable from an image forming apparatus, including: a photosensitive body; and a processing device acting on the photosensitive body; wherein relative positions of the photosensitive body and the processing device are changeable when the process cartridge is loaded in and unloaded from the multicolor image forming apparatus. 
   In the invention configured in this manner, the relative positions of the photosensitive body and the processing device change when the process cartridge is loaded in and unloaded from the image forming apparatus. For this reason, because the process cartridge is loaded and unloaded in the image forming apparatus while the relation positions of the photosensitive body and the processing device are changed, it becomes easy to configure the process cartridge of this invention so that it does not interfere with members such as the exposure unit of the image forming apparatus. Thus, the degree of freedom with which the image forming apparatus can be designed increases, the image forming apparatus can be made compact, and it becomes easy to prevent color shifting by not having to move the exposure unit when the process cartridge is replaced. It should be noted that the process cartridge of this invention is suited for the multicolor image forming apparatus according to the invention and is also suited for a monochromatic image forming apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be more readily described with reference to the accompanying drawings: 
       FIG. 1  is a schematic sectional side view of a color laser printer to which the invention has been applied. 
       FIG. 2  is a schematic sectional side view showing the printer when a front cover thereof has been opened. 
       FIG. 3  is a side view showing the configuration of a process cartridge of the printer. 
       FIGS. 4A and 4B  are a side view and a perspective view showing the configuration of guide grooves of the printer. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Next, an embodiment of the invention will be described together with the drawings.  FIG. 1  is a schematic sectional side view of a color laser printer  1  functioning as an image forming apparatus to which the invention has been applied. The color laser printer  1  exemplified in  FIG. 1  is disposed with a visible image forming unit  4 , a belt-like intermediate transfer body  5 , a fixing unit  8 , a paper supply unit  9  and a paper discharge tray  10 . 
   The visible image forming unit  4  is disposed, per visible image step resulting from respective magenta (M), cyan (C), yellow (Y) and black (Bk) toners, with developing devices  51 M,  51 C,  51 Y and  51 Bk functioning as a developing unit, photosensitive drums  3 M,  3 C,  3 Y and  3 Bk functioning as photosensitive bodies, cleaning rollers  70 M,  70 C,  70 Y and  70 Bk functioning as cleaning units, chargers  71 M,  71 C,  71 Y and  71 Bk functioning as charging units, and exposure means  72 M,  72 C,  72 Y and  72 Bk functioning as exposure units. 
   These respective constituent elements will be described in detail below. First, the developing devices  51 M,  51 C,  51 Y and  51 Bk are disposed with development rollers  52 M,  52 C,  52 Y and  52 Bk. The development rollers  52 M,  52 C,  52 Y and  52 Bk are cylindrically configured with conductive silicone rubber as the base material, with a coated layer of a rubber material or resin including fluorine being formed on the surfaces. It should be noted that the base material of the development rollers  52 M,  52 C,  52 Y and  52 Bk does not invariably have to be configured by conductive silicone rubber and may also be configured by conductive urethane rubber. Additionally, the ten-point height of surface roughness (Rz) is set to 3 to 5.m and configured to be smaller than the 9.m that is the average particle diameter of the toners. 
   Supply rollers  53 M,  53 C,  53 Y and  53 Bk are disposed in the developing devices  51 M,  51 C,  51 Y and  51 Bk. The supply rollers  53 M,  53 C,  53 Y and  53 Bk are conductive sponge rollers that are disposed so as to press and contact, with the elastic force of the sponges, the development rollers  52 M,  52 C,  52 Y and  52 Bk. It should be noted that foam of an appropriate material, such as conductive silicone rubber, EPDM or urethane rubber, can be used as the supply rollers  53 M,  53 C,  53 Y and  53 Bk. 
   Layer thickness regulating blades  54 M,  54 C,  54 Y and  54 Bk are also disposed in the developing devices  51 M to  51 Bk. Base ends of the layer thickness regulating blades  54 M,  54 C,  54 Y and  54 Bk are formed of stainless steel in plate shapes and fixed to developing device cases  55 M,  55 C,  55 Y and  55 Bk, and leading ends of the layer thickness regulating blades  54 M,  54 C,  54 Y and  54 Bk are formed of insulating silicone rubber or insulating fluorine-including rubber or resin. The leading ends of the layer thickness regulating blades  54 M,  54 C,  54 Y and  54 Bk are pressed so as to contact the development rollers  52 M,  52 C,  52 Y and  52 Bk from below the development rollers  52 M,  52 C,  52 Y and  52 Bk. 
   Also, the toners accommodated in the developing device cases  55 M,  55 C,  55 Y and  55 Bk are positively-charged nonmagnetic single component developing devices that include toner mother particles of an average particle diameter of 9.m comprising a well-known colorant such as carbon black and a charge-controlling resin or charge-controlling agent such as nigrosine, triphenylmethane or quaternary ammonium salt added to styrene-acrylic resin formed in spherical shapes by suspension polymerization. Additionally, the toners are configured by adding, as an external additive, silica to the surfaces of the toner mother particles. Also, a well-known hydrophobization treatment resulting from a silane coupling agent or silicone oil is administered to the silica serving as the external additive, so that the average particle diameter of the silica is 10 nm and the added amount of the silica is 0.6% by weight of the toner mother particles. Magenta, cyan, yellow and black toners are respectively accommodated in the developing device cases  55 M,  55 C,  55 Y and  55 Bk. 
   In this manner, the toners are suspension-polymerized toners that are extremely close to spherical shapes and have excellent fluidity because hydrophobized silica whose average particle diameter is 10 nm is added at 0.6% by weight as an external additive. For this reason, a sufficient charge can be obtained by frictional charging. Moreover, because corner portions are not present as in crushed toner, it is difficult for the toners to receive mechanical force, the toners have excellent followability with respect to an electrical field and transfer efficiency is good. 
   Drums where a positively-charged photosensitive layer is formed on an aluminium base material are used as an example for the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. The photosensitive layers are formed to have a thickness of 20.m or more, and the aluminium base materials are used as grounding layers. It should be noted that, in the present embodiment, there is a slight velocity differential between the intermediate transfer body  5  and the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. 
   The cleaning rollers  70 M,  70 C,  70 Y and  70 Bk are rollers comprising elastic bodies such as conductive sponges and are configured to frictionally slide against the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk at lower portions of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. A voltage of a negative polarity, which is the opposite polarity from that of the toner, is applied by an unillustrated power source to the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk. Residual toner on the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk is removed due to the action of an electrical field resulting from this voltage and the frictional force with respect to the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. It should be noted that, because a so-called cleanerless development system is used in the present embodiment, residual toner removed by the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk is again returned to the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk, collected by the development rollers  52 M,  52 C,  52 Y and  52 Bk and returned to the developing devices  51 M,  51 C,  51 Y and  51 Bk of each color in a predetermined cycle after the development step has ended. 
   The chargers  71 M,  71 C,  71 Y and  71 Bk are scorotron chargers and are disposed so as to face, without contacting, the surfaces of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk from below the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk further at the rotational-direction downstream side of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk than the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk. 
   The exposure means  72 M,  72 C,  72 Y and  72 Bk are configured by well-known laser scanners. The exposure means  72 M,  72 C,  72 Y and  72 Bk are disposed so as to be vertically in line with the developing devices  51 M,  51 C,  51 Y and  51 Bk of the visible image forming unit  4  and horizontally in line with the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the chargers  71 M,  71 C,  71 Y and  71 Bk, and expose the surfaces of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk with laser light beams further at the rotational-direction downstream side of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk than the chargers  71 M,  71 C,  71 Y and  71 Bk. Laser light beams corresponding to image data are irradiated, by the exposure means  72 M,  72 C,  72 Y and  72 Bk, onto the surfaces of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk so that electrostatic latent images of each color are formed on the surfaces of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. 
   The toners are positively charged, supplied from the supply rollers  53 M,  53 C,  53 Y and  53 Bk to the development rollers  52 M,  52 C,  52 Y and  52 Bk, and formed in uniformly thin layers by the layer thickness regulating blades  54 M,  54 C,  54 Y and  54 Bk. At the portions of contact between the development rollers  52 M,  52 C,  52 Y and  52 Bk and the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk, the positively charged toners can excellently develop, in a reverse development system, the positive-polarity (positive charge) electrostatic latent images formed on the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and can form extremely high-quality images. 
   The belt-like intermediate transfer body  5  comprises a conductive sheet of polycarbonate or polyimide formed in a belt. As shown in  FIG. 1 , the belt-like intermediate transfer body  5  is wound around two drive rollers  60  and  62 , and intermediate transfer rollers  61 M,  61 C,  61 Y and  61 Bk are disposed near positions at which the intermediate transfer body  5  faces the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. As shown in  FIG. 1 , the direction in which the surface of the intermediate transfer body  5  facing the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk moves is set to be a direction in which it moves vertically downward from above. 
   A predetermined voltage is applied to the intermediate transfer rollers  61 M,  61 C,  61 Y and  61 Bk so that the toner images formed on the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk are transferred to the intermediate transfer body  5 . Also, a secondary transfer roller  63  is disposed so as to face the position at which the toner images are transferred to paper P (corresponding to a recording medium)—i.e., facing the roller  62  in a vertically low direction with respect to the intermediate transfer body  5 —and a predetermined potential is also applied to the secondary transfer roller  63 . As a result, the toner images of the four colors retained on the belt-like intermediate transfer body  5  are transferred to the paper P. 
   As shown in  FIG. 1 , a cleaner  6  is disposed at the side of the intermediate transfer body  5  opposite from the side at which the intermediate transfer body  5  faces the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. The cleaner  6  comprises a scraping member  65  and a case  66 , uses the scraping member  65  to scrape off toner remaining on the intermediate transfer body  5  and accommodates the residual toner in the case  66 . 
   The fixing unit  8  comprises a first heating roller  81  and a second heating roller  82 , and uses the first heating roller  81  and the second heating roller  82  to nip, convey, heat and pressurize the paper P, on which the toner images of the four colors are retained, to fix the toner images to the paper P. 
   The paper supply unit  9  is disposed at the lowermost portion of the device, and comprises an accommodation tray  91  that accommodates the paper P and a pick-up roller  92  that feeds the paper P. The paper supply unit  9  is configured to supply the paper P at a predetermined timing with the image forming steps resulting from the exposure means  72 M,  72 C,  72 Y and  72 Bk, the developing devices  51 M,  51 C,  51 Y and  51 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the intermediate transfer body  5 . The paper P supplied from the paper supply unit  9  is conveyed by a pair of conveyance rollers  100  to the portion where the intermediate transfer body  5  and the secondary transfer roller  63  are pressed together. 
   The paper discharge tray  10  is disposed at the uppermost portion of the device, at the paper discharge side of the fixing unit  8 , and configured to accommodate the paper P that is discharged from the fixing unit  8  and conveyed by pairs of conveyance rollers  101 ,  102  and  103 . 
   It should be noted that, in the present embodiment, as shown in  FIG. 1 , a front cover  20  is configured so as to be pivotable around a shaft  20   a  in the direction of the white arrow in  FIG. 1 . By opening the front cover  20  as shown in  FIG. 2 , the developing devices  51 M,  51 C,  51 Y and  51 Bk can be replaced. Here, spring members  21 M,  21 C,  21 Y and  21 Bk are disposed at left-right direction center portions of the front cover  20  that are positions at which the spring members  21 M,  21 C,  21 Y and  21 Bk face the developing devices  51 M,  51 C,  51 Y and  51 Bk. When the front cover  20  is closed, the spring members  21 M,  21 C,  21 Y and  21 Bk push the developing devices  51 M,  51 C,  51 Y and  51 Bk inward (in the leftward direction of  FIG. 1 ). 
   Also, as shown in  FIG. 3 , the developing devices  51 M,  51 C,  51 Y and  51 Bk are integrally connected to the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the chargers  71 M,  71 C,  71 Y and  71 Bk via urethane elastomers  57  and  58  to configure process cartridges  50 M,  50 C,  50 Y and  50 Bk. It should be noted that, although only the process cartridge  50 M is representatively shown in  FIG. 4 , the other process cartridges  50 C,  50 Y and  50 Bk are similarly configured. 
   For this reason, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the chargers  71 M,  71 C,  71 Y and  71 Bk are simultaneously replaced when the developing devices  51 M,  51 C,  51 Y and  51 Bk are replaced. Also, as shown in  FIG. 3 , shafts  3   a M,  3   a C,  3   a Y and  3   a Bk of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk are rotatably supported by annular support plates  3   b M,  3   b C,  3   b Y and  3   b Bk. Additionally, the support plates  3   b M,  3   b C,  3   b Y and  3   b Bk are respectively connected to the chargers  71 M,  71 C,  71 Y and  71 Bk via the urethane elastomers  57  serving as first elastic bodies and to the developing device cases  55 M,  55 C,  55 Y and  55 Bk of the developing devices  51 M,  51 C,  51 Y and  51 Bk via the urethane elastomers  58  serving as second elastic bodies. 
   The urethane elastomers  57  and  58  are respectively connected to left and right side surfaces in two vertical rows. In a state where no external force is applied thereto, as shown in  FIG. 3 , the urethane elastomers  57  and  58  are disposed in a row with the chargers  71 M,  71 C,  71 Y and  71 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk. Also, in this state, the chargers  71 M,  71 C,  71 Y and  71 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk are disposed in the aforementioned row at predetermined intervals so that they do not contact each other. 
   Also, grip portions  59 M,  59 C,  59 Y and  59 Bk (only  59 M are shown in  FIG. 3 ) are disposed at both left and right ends of the front surface side of the developing device cases  55 M,  55 C,  55 Y and  55 Bk in the developing devices  51 M,  51 C,  51 Y and  51 Bk. 
   As shown in the side view of  FIG. 4A  and the perspective view of  FIG. 4B , a support member  31  for supporting the process cartridges  50 M,  50 C,  50 Y and  50 Bk is fixed to left and right side panels  30  of the color laser printer  1 . Additionally, guide grooves  32 M,  32 C,  32 Y and  32 Bk that guide the shafts  3   a M,  3   a C,  3   a Y and  3   a Bk of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk, guide grooves  33 M,  33 C,  33 Y and  33 Bk that guide shafts  52   a M,  52   a C,  52   a Y and  52   a Bk of the development rollers  52 M,  52 C,  52 Y and  52 Bk, and guide grooves  34 M,  34 C,  34 Y and  34 Bk that guide shafts  71   a M,  71   a C,  71   a Y and  71   a Bk that project from both left and right ends of the chargers  71 M,  71 C,  71 Y and  71 Bk are respectively formed in the support member  31 . 
   Of the guide grooves  32 M to  34 Bk, the guide grooves  32 M,  32 C,  32 Y and  32 Bk are the deepest, the guide grooves  33 M,  33 C,  33 Y and  33 Bk are the next deepest, and the guide grooves  34 M,  34 C,  34 Y and  34 Bk are the least deepest. Also, although the guide grooves  33 M,  33 C,  33 Y and  33 Bk are formed horizontally across the length of the support member  31 , the deep portions of the guide grooves  32 M,  32 C,  32 Y and  32 Bk curve downward near the leading ends, and the deep portions of the guide grooves  34 M,  34 C,  34 Y and  34 Bk curve further downward near the leading ends. 
   For this reason, when the cartridges are inserted deeply like the process cartridges  50 M,  50 Y and  50 Bk shown in  FIG. 4A , the photosensitive drums  3 M,  3 Y and  3 Bk are disposed below the developing devices  51 M,  51 Y and  51 Bk, and the chargers  71 M,  71 Y and  71 Bk are disposed even further therebelow. However, when the developing device  51 C is inserted or pulled out along the guide groove  33 C like the process cartridge  50 C shown in  FIG. 4A , the developing device  51 C, the photosensitive drum  3 C and the charger  71 C are disposed in a row as exemplified in  FIG. 3 . 
   Moreover, lock springs  32   a M,  32   a C,  32   a Y and  32 Bk and lock springs  34   a M,  34   a C,  34   a Y and  34   a Bk that fix the shafts  3   a M,  3   a C,  3   a Y and  3   a Bk of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the shafts  71   a M,  71   a C,  71   a Y and  71   a Bk of the chargers  71 M,  71 C,  71 Y and  71 Bk are disposed at leading ends of the guide grooves  32 M,  32 C,  32 Y and  32 Bk and the guide grooves  34 M,  34 C,  34 Y and  34 Bk (the lock springs  34   a M,  34   a Y and  34   a Bk are not shown). The lock springs  32 M to  34 Bk are configured by wires being bent in a “&lt;” shape, and position the shafts  3   a M to  71   a Bk in the leading ends of the guide grooves  32 M to  34 Bk. 
   Next, the operation of the color laser printer  1  of the present embodiment will be described. First, the photosensitive layers of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk are uniformly charged by the chargers  71 M,  71 C,  71 Y and  71 Bk. Next, the photosensitive layers are exposed in correspondence to magenta, cyan, yellow and black images by the exposure means  72 M,  72 C,  72 Y and  72 Bk. Then, magenta toner, cyan toner, yellow toner and black toner are respectively supplied onto the electrostatic latent images formed on the photosensitive layers of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk, and development of the magenta, cyan, yellow and black images is conducted. The magenta, cyan, yellow and black toner images formed in this manner are then transferred to the surface of the intermediate transfer body  5 . 
   Next, toner remaining on the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk after the transfer is temporarily retained by the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk. The toner images of the colors are formed at slight time differentials to match the moving speed of the intermediate transfer body  5  and the positions of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk, and are transferred so as to be superposed on the intermediate transfer body  5 . 
   The four-color toner image formed on the intermediate transfer body  5  in this manner is transferred, at the position where the secondary transfer roller  63  and the intermediate transfer body  5  contact, onto the paper P supplied from the paper supply unit  9 . Then, the toner image is fixed on the paper P in the fixing unit  8  and discharged onto the paper discharge tray  10 . In this manner, a four-color color image is formed. 
   Also, when any of the process cartridges  50 M,  50 C,  50 Y and  50 Bk (e.g., the process cartridge  50 C) is to be replaced, the front cover  20  is opened, the grip portions  59 C are gripped and the developing device  51 C is pulled out horizontally along the guide grooves  33 C. In so doing, the development roller  52 C and the photosensitive drum  3 C are separated until the urethane elastomers  58  stretch to the length shown in  FIG. 3 . Then, due to tensile force from the urethane elastomers  58 , the shaft  3   a C of the photosensitive drum  3 C crosses over the lock spring  32   a C and moves along the guide grooves  32 C. 
   In accompaniment therewith, the shaft  71   a C of the charger  71 C also crosses over the lock spring  34   a C and moves along the guide grooves  34 C. The photosensitive drum  3 C and the charger  71 C first move diagonally upward along the guide grooves  32 C and  34 C, and then move to a position at which they are not horizontally in line with the exposure means  72 C. Then, as shown in  FIG. 4A , after the developing device  51 C, the photosensitive drum  3 C and the charger  71 C have been disposed in a row, the entire process cartridge  50 C can be horizontally pulled out. 
   When the process cartridge  50 C is to be loaded, the developing device  51 C, the photosensitive drum  3 C and the charger  71 C are first disposed in a row due to the action of the urethane elastomers  57  and  58  (see  FIG. 3 ). When the shafts  71   a C,  3   a C and  52   a C successively engage with the guide grooves  34 C,  32 C and  33 C and the entire process cartridge  50 C is horizontally pushed, the charger  71 C and the photosensitive drum  3 C are guided midway to the guide grooves  34   c  and  32 C and move downward. Then, when the developing device  51 C is pushed further, the shafts  71   a C and  3   a C are pushed via the urethane elastomers  57  and  58  and cross over the lock springs  34   a C and  32   a C. The shafts  71   a C and  3   a C of the charger  71 C and the photosensitive drum  3 C are positioned at the leading ends of the guide grooves  34 C and  32 C, and the photosensitive drum  3 C and the charger  71 C are disposed in positions horizontally in line with the exposure means  72 C. 
   From the start of insertion of the process cartridge  50 C to now, the charger  71 C, the photosensitive drum  3 C and the developing device  52 C are retained, by the action of the urethane elastomers  57  and  58 , so that they do not contact each other. When the front cover  20  is closed after the above-described operation, the spring member  21 C pushes the developing device  51 C. Thus, the shaft  52   a C of the development roller  52 C is positioned at the leading end of the guide groove  33 C, and the peripheral surface of the development roller  52 C and peripheral surface of the photosensitive drum  3 C come into contact. 
   It should be noted that the same is true in the case of replacing the process cartridges  50 M,  50 Y and  50 Bk. Also, simultaneously replacing the chargers  71 M,  71 C,  71 Y and  71 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk in this manner is extremely effective in maintaining the excellent image quality of the color laser printer  1 . 
   In the present embodiment, at the time of loading and unloading the process cartridges  50 M,  50 C,  50 Y and  50 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the chargers  71 M,  71 C,  71 Y and  71 Bk are disposed in positions in line along the loading/unloading direction and the relative positions between the chargers  71 M,  71 C,  71 Y and  71 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk are changed so that the process cartridges  50 M,  50 C,  50 Y and  50 Bk can be loaded and unloaded. For this reason, the process cartridges  50 M,  50 C,  50 Y and  50 Bk do not interfere with the exposure means  72 M,  72 C,  72 Y and  72 Bk at the time of loading and unloading. Thus, replacement of the process cartridges  50 M,  50 C,  50 Y and  50 Bk can be conducted without moving the exposure means  72 M,  72 C,  72 Y and  72 Bk, so that color shifting can be excellently prevented. 
   Also, because the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk are also positioned by the lock springs  32   a M,  32   a C,  32   a Y and  32   a Bk, color shifting can be more excellently prevented. Moreover, because the color laser printer  1  of the present embodiment is a so-called tandem-system multicolor image forming apparatus, it is suited for increases in the speed of image formation. 
   Moreover, at the time of replacing the process cartridges  50 M,  50 C,  50 Y and  50 Bk, the chargers  71 M,  71 C,  71 C and  71 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk move in a state where they do not contact one another. Additionally, the chargers  71 M,  71 C,  71 Y and  71 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk relatively move around the shafts  3   a M,  3   a C,  3   a Y and  3   a Bk with respect to the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. For this reason, in the present embodiment, the peripheral surfaces of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk can be excellently prevented from being damaged. Moreover, because the respective portions of the process cartridges  50 M,  50 C,  50 Y and  50 Bk are guided along the guide grooves  32 M to  34 Bk, the respective portions can be more excellently prevented from abutting against and damaging other members such as the exposure means  72 M,  72 C,  72 Y and  72 Bk. 
   Also, in the present embodiment, because the process cartridges  50 M,  50 C,  50 Y and  50 Bk are loaded and unloaded in directions substantially orthogonal to the shafts  3   a M,  3   a C,  3   a Y and  3   a Bk, the configuration of shaft-receiving portions can be simplified and the manufacturing costs of the color laser printer  1  can be reduced in comparison to a case where the process cartridges are loaded and unloaded along the shafts  3   a M,  3   a C,  3   a Y and  3   a Bk. 
   Moreover, in the present embodiment, the relative positions of the chargers  71 M,  71 C,  71 Y and  71 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk and the developing devices  51 M,  51 C,  51 Y and  51 Bk of the pulled-out process cartridges  50 M,  50 C,  50 Y and  50 Bk are retained in a positional relation, by the urethane elastomers  57  and  58 , immediately after being pulled out. Thus, the process cartridges  50 M,  50 C,  50 Y and  50 Bk can be loaded in the color laser printer  1  without changing the positional relation of the respective portions, so that replacement becomes easier. 
   In the tandem system, as described above, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk are positioned in the vicinity of the center of the color laser printer  1  mainframe, between the exposure means  72 M,  72 C,  72 Y and  72 Bk and the intermediate transfer body  5 , so that replacement of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk is difficult. However, by disposing the grip portions  59 M,  59 C,  59 Y and  59 Bk on the process cartridges  50 M,  50 C,  50 Y and  50 Bk, the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk can be loaded and unloaded together, whereby it becomes possible to more easily replace the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk. 
   It should be noted that, in the above-described embodiment, the developing devices  51 M,  51 C,  51 Y and  51 Bk and the chargers  71 M,  71 C,  71 Y and  71 Bk correspond to process means, and the shafts  3   a M,  3   a C,  3   a Y,  3   a Bk,  52   a M,  52   a C,  52   a Y,  52   a Bk,  71   a M,  71   a C,  71   a Y and  71   a Bk correspond to guided portions. 
   Also, the present invention is not limited to the above-described embodiment and can be variously implemented in a range that does not deviate from the gist of the invention. For example, although the developing devices  51 M,  51 C,  51 Y and  51 Bk and the chargers  71 M,  71 C,  71 Y and  71 Bk were integrally connected to the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk to configure the process cartridges  50 M,  50 C,  50 Y and  50 Bk in the above-described embodiment, the chargers  71 M,  71 C,  71 Y and  71 Bk may be separately disposed, and the cleaning rollers  70 M,  70 C,  70 Y and  70 Bk serving as process means may be integrated. 
   Moreover, a configuration that is the same as that of the process cartridges  50 M,  50 C,  50 Y and  50 Bk of the present embodiment can also be applied to a monochromatic image forming apparatus. Also, the urethane elastomers  57  and  58  may be connected to the cases of the photosensitive drums  3 M,  3 C,  3 Y and  3 Bk, and the guide members may also be rails instead of grooves. 
   While the invention has been described in conjunction with the specific embodiments described above, many equivalent alternatives, modifications and variations may become apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention as set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.