Patent Publication Number: US-8995874-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic image forming apparatus in which a cartridge is detachably mounted in an apparatus main body and which forms an image on a recording medium. 
     Here, the electrophotographic image forming apparatus is an apparatus that forms an image on a recording medium using an electrophotographic image forming process. Examples of the electrophotographic image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (for example, a laser beam printer and an LED printer), a facsimile machine, and a word processor. The recording medium is a medium on which an image is formed by the electrophotographic image forming apparatus, examples of which include paper and an OHT (overhead transparency) sheet. 
     2. Description of the Related Art 
     A known image forming apparatus (electrophotographic image forming apparatus) using an electrophotographic image forming process in the related art employs a process cartridge system. The process cartridge system is such that a process cartridge in which a photosensitive drum (electrophotographic photosensitive drum), a developing roller that acts on the photosensitive drum, and a developing unit that accommodates toner for use in image formation are integrated to form a single unit is detachably mounted in an apparatus main body. The process cartridge system allows a user to perform maintenance of the apparatus by himself or herself without the help of a serviceman. Therefore, the process cartridge system is widely used in electrophotographic image forming apparatuses. In addition to the foregoing process cartridge system, there are various kinds of cartridge system. For example, there are a developing cartridge system in which a developing roller and toner are accommodated and a toner cartridge system in which only toner is accommodated. An optimum system is selected from them in consideration of the apparatus configuration, cost, etc. 
     An example of known electrophotographic image forming apparatuses has a configuration in which a scanner unit serving as an exposing unit is mounted to a covering member that opens and closes an opening for mounting and demounting a plurality of cartridges (U.S. Pat. No. 7,356,283). With this configuration, when the covering member is opened to mount or demount the cartridges, the scanner unit retracts, and thus, the ease of operation for mounting and demounting the cartridges can be improved. 
     U.S. Pat. No. 7,356,283 discloses an image forming apparatus capable of color image formation. In this image forming apparatus, a plurality of cartridges are disposed next to each other diagonally above and below. 
     SUMMARY OF THE INVENTION 
     The present invention is developed from the related art described above. 
     The present invention provides an image forming apparatus capable of high-accuracy positioning of an exposing unit relative to a photosensitive member to be exposed to light so as to form high-quality images. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an external perspective view of an image forming apparatus of a first embodiment. 
         FIG. 1B  is a longitudinal left side view of the image forming apparatus of the first embodiment. 
         FIG. 2  is an enlarged view of an image forming unit in  FIG. 1B . 
         FIG. 3A  is a right rear perspective view of a cartridge. 
         FIG. 3B  is a left front perspective view of the cartridge  33 Y. 
         FIG. 4  is a longitudinal left side view of the image forming apparatus when a cover unit is at an opening position. 
         FIG. 5A  is a left perspective view of the image forming unit. 
         FIG. 5B  is a right perspective view of the image forming unit. 
         FIG. 5C  is a top view of the image forming unit. 
         FIG. 6A  is a left perspective view of an apparatus main body in a state in which a cover unit is opened. 
         FIG. 6B  is a right perspective view of the apparatus main body in a state in which the cover unit is opened. 
         FIG. 7A  is a diagram illustrating the mounting and demounting of the image forming unit. 
         FIG. 7B  is a diagram illustrating the mounting and demounting of the image forming unit. 
         FIG. 7C  is a diagram illustrating the mounting and demounting of the image forming unit. 
         FIG. 7D  is a diagram illustrating the mounting and demounting of the image forming unit. 
         FIG. 7E  is a diagram illustrating the mounting and demounting of the image forming unit. 
         FIG. 8A  is an explanatory diagram of a maintenance button. 
         FIG. 8B  is an explanatory diagram of the maintenance button 
         FIG. 9A  is a diagram illustrating the operation of an upper cover unit. 
         FIG. 9B  is a diagram illustrating the operation of the upper cover unit. 
         FIG. 10A  is a left back perspective view of a cover unit. 
         FIG. 10B  is a right front perspective view of the cover unit. 
         FIG. 11A  is a cross-sectional view of the cover unit in the lateral direction. 
         FIG. 11B  is a cross-sectional view of the cover unit taken along line XIB-XIB of  FIG. 11A . 
         FIG. 11C  is a cross-sectional view of the cover unit taken along line XIC-XIC of  FIG. 11A . 
         FIG. 12A  is a cross-sectional view of the cover unit in the front-to-back direction. 
         FIG. 12B  is a cross-sectional view of the cover unit taken along line XIIB-XIIB of  FIG. 12A . 
         FIG. 13  is a diagram illustrating the positional relationship among the scanner unit, urging members, and compressing springs. 
         FIG. 14  is a diagram illustrating the behavior of the positioning projections of the scanner unit relative to the scanner guides during mounting. 
         FIG. 15  is a diagram illustrating the behavior of the positioning projections of the scanner unit relative to the scanner guides during mounting. 
         FIG. 16A  is a diagram illustrating the positioning of the scanner unit relative to the image forming unit. 
         FIG. 16B  is a diagram illustrating the positioning of the scanner unit relative to the image forming unit. 
         FIG. 17  is a diagram illustrating the mounting of a process cartridge including a photosensitive drum and a developing unit into an image forming apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described in detail hereinbelow with reference to the drawings. It is to be understood that the scope of the present invention is not limited to the sizes, materials, shapes, the relative positions, etc. of components described in the embodiments unless otherwise stated. 
     First Embodiment 
     Overall Schematic Configuration of Example of Electrophotographic Image Forming Apparatus 
       FIG. 1A  is an external perspective view of an electrophotographic image forming apparatus (hereinafter referred to as an image forming apparatus)  100  of this embodiment.  FIG. 1B  is a longitudinal left side view of the image forming apparatus  100 . The image forming apparatus  100  is a four-full-color laser printer using an electrophotographic process. 
     Specifically, the image forming apparatus  100  forms a full color image on a recording medium (sheet) S on the basis of an electrical image signal input from an external host unit  400 , such as a personal computer, an image reader, and a sending-side facsimile machine, to a control circuit unit  300 . 
     In the description below, for the image forming apparatus  100 , “front” is a direction in which a feeding cassette  19  that accommodates the stacked recording medium S is drawn outward from the interior of an apparatus main body  100 A; “rear” is opposite thereto; “above” is a direction in which the recording medium S is output; “front-to-back direction” includes a direction in which the image forming apparatus  100  is viewed from the rear to the front (forward direction) and a direction opposite thereto (backward); “left and right” are the sides of the image forming apparatus  100  as viewed from the front; “lateral direction” includes a right-to-left direction (leftward) and a direction opposite thereto (rightward); and “longitudinal direction” is the axial direction of an electrophotographic photosensitive drum or a developing roller. An apparatus main body  100 A is part of the image forming apparatus  100  excluding cartridges  33  ( 33 Y,  33 M,  33 C, and  33 K) and an image forming unit  200 . The right side of the image forming apparatus  100  of this embodiment is a drive side and the left side is a non-drive side. 
     The image forming apparatus  100  is placed on a substantially horizontal installation surface F of a table, a desk, a floor, etc. At the center in the apparatus main body  100 A is disposed the image forming unit  200 . 
       FIG. 2  is an enlarged view of the image forming unit  200  in  FIG. 1B . The unit  200  has a subframe (support frame)  31  that can be mounted and demounted to/from a main frame  110  of the apparatus main body  100 A. The unit  200  is equipped with cartridge mount portions  321  ( 321 Y,  321 M,  321 C, and  321 K) in which a plurality of cartridges, that is, first to fourth four developing cartridges  33  ( 33 Y,  33 M,  33 C, and  33 K) in this embodiment, are detachably mounted. The unit  200  further includes a single intermediate transfer member  34  on the subframe  31 . In this embodiment, electrophotographic photosensitive drums  32   a  of the individual developing cartridges  33  are mounted in the unit  200  together with charging rollers  32   b  and cleaning blades  32   c . The charging rollers  32   b  and the cleaning blades  32   c  are processing units. The image forming apparatus  100  forms a color image on the recording medium S with the plurality of cartridges  33  that are detachably mounted in the apparatus main body  100 A (unit  200 ). The image forming unit  200  can be mounted and demounted to/from the apparatus main body  200 A, as described above. This can simplify the replacement of the intermediate transfer member  34 , the photosensitive drums  32   a , and the processing units, such as the charging rollers  32   b  and the cleaning blades  32   c , such as when the lifetimes have expired. More detailed configuration of the unit  200  will be described later. In this embodiment, the individual developing cartridges  33  accommodate different color developers (toners), of which the developing cartridge  33 K that accommodates a black developer has a developer accommodating portion  33   c  with a larger capacity than the developing cartridges  33  that accommodate the other color developers; however the present invention is not limited thereto. For example, they may have the same configuration except that the colors of developers (toners) accommodated in the individual developing cartridges  33  differ. In this embodiment, although developing cartridges are described as an example of the cartridges, the present invention is not limited thereto. For example, this embodiment may be configured such that the drums (photosensitive drums)  32   a , the charging rollers  32   b , and the cleaning blades  32   c  mounted in the unit  200  may be mounted in the developing cartridges  33 . With such a configuration, the cartridges are not referred to as the developing cartridges but referred to as process cartridges. This is because the drums  32   a  and the processing units, such as the charging rollers  32   b , the developing rollers  33   b , and the cleaning blades  32   c , are integrated into a cartridge and are detachably mounted in the apparatus main body  100 A. 
     Photosensitive Drum 
     The individual photosensitive drums  32   a  are fixed to the subframe  31  of the image forming unit  200 . The subframe  31  is further provided with the charging rollers  32   b  and the cleaning blades  32   c  for removing developers remaining on the surfaces of the drums  32   a , which serve as processing units for the drums  32   a . The drums  32   a , the charging rollers  32   b , and the cleaning blades  32   c  are mounted in a predetermined placement relationship. 
     Cartridge 
     As shown in  FIG. 2 , the cartridges  33  ( 33 Y,  33 M,  33 C, and  33 K) each have a developing unit. Specifically, the cartridges  33  each have a housing  33   a  and a developing roller  33   b  for supplying a developer to the drum  32   a , in other words, for developing an electrostatic latent image formed on the drum  32   a  as a developer image. The cartridges  33  each further have a developer accommodating portion  33   c  that accommodates a developer used in developing the electrostatic latent image and a supply roller  33   d  that supplies the developer in the developer accommodating portion  33   c  to the developing roller  33   b . The developer accommodating portion  33   c  has a conveying member  33   f  for conveying the internal developer to the supply roller  33   d . The first cartridge  33 Y accommodates a yellow (Y) developer in the developer accommodating portion  33   c  and forms a Y-color developer image on the surface of the corresponding drum  32   a . The second cartridge  33 M accommodates a magenta (M) developer in the developer accommodating portion  33   c  and forms an M-color developer image on the surface of the corresponding drum  32   a . The third cartridge  33 C accommodates a cyan (C) developer in the developer accommodating portion  33   c  and forms a C-color developer image on the surface of the corresponding drum  32   a . The fourth cartridge  33 K accommodates a black (K) developer in the developer accommodating portion  33   c  and forms a K developer image on the surface of the corresponding drum  32   a.    
       FIG. 3A  is a perspective view of the cartridge  33 Y viewed from the right rear.  FIG. 3B  is a perspective view of the cartridge  33 Y viewed from the left front. The cartridges  33  will be described taking the cartridge  33 Y as an example. The cartridge  33 Y is mounted to the mount portion  321 Y of the unit  200  in the direction of an arrow X 10 . The cartridge  33 Y is demounted from the mount portion  321 Y of the unit  200  in the direction of an arrow X 11  opposite to the arrow X 10 . 
     The cartridge  33 Y has the developing roller  33   b  at the distal end thereof in the mounting direction. A gear  50  is provided at the right end of the developing roller  33   b . The gear  50  receives a driving force from a drum gear  32   a   1  ( FIG. 5A ) to rotate the developing roller  33   b . The gear  50  further transmits the driving force to the supply roller  33   d  and the conveying members  33   f  via a gear train (not shown). 
     Guided portions  33   e   1  and  33   e   2  and guided portions  33   e   3  and  33   e   4  for the cartridge  33 Y to be guided to the unit  200  are provided at the right and left sides of cartridge  33 Y, respectively. The guided portions  33   e   1 ,  33   e   2 ,  33   e   3 , and  33   e   4  each have a cylindrical shape projecting outwards from the right and left sides of the cartridge  33 Y. 
     A grip  39  (see  FIG. 2 ) for gripping the cartridge  33 Y is provided at the side of the cartridge  33 Y opposite to the developing roller  33   b . The user can mount and demount the cartridge  33 Y to/from the unit  200  by gripping the grip  39 . The other cartridges  33 M,  33 C, and  33 K also have the grip  39  to allow the user to mount and demount them to/from the unit  200  by gripping the respective grips  39 . 
     As shown in  FIGS. 5A to 5   c , in this embodiment, the intermediate transfer member  34  is a cylindrical drum whose central rotation shaft  34   a  extends in the lateral direction and which is disposed horizontally so as to be rotatable about the shaft  34   a . The cartridges  33  are arranged next to each other from above to below in the vertical direction, in front of the intermediate transfer member  34 , in a substantially horizontal orientation with respect to the installation surface F for the apparatus main body  100 A. In the image forming apparatus of this embodiment, the first cartridge  33 Y is in the highest row, and the second cartridge  33 M is located therebelow. The third cartridge  33 C is located further below. The fourth cartridge  33 K is located in the lowest row. The developing rollers  33   b  of the cartridges  33  may either be in contact with the drums  32   a , as a contact developing system, or be spaced a predetermined slight gap (a fixed distance) apart from the drums  32   a , as a noncontact developing system. 
     Urging of Cartridges 
     The apparatus main body  100 A has cartridge urging members  51   a  to  51   d  ( FIG. 4  and  FIGS. 6A and 6B ) that urge the individual cartridges  33  to corresponding photosensitive member units  32  ( 32 Y,  32 M,  32 C, and  32 K) in a state in which the unit  200  is located in an image forming position. The urging members  51   a  to  51   d  are each provided at both ends in the longitudinal direction (lateral direction) of the cartridges  33 , that is, two for each of the cartridges  33 . The urging members  51   a  to  51   d  are provided at the cover unit  10  and come into contact with the rear ends of the cartridges  33  in sequence in cooperation with the closing operation of the cover unit  10 . When the unit  200  is in the image forming position, as shown in  FIG. 1B , the cartridges  33  are urged in the directions of arrows P (PY, PM, PC, and PK) by the urging members  51   a  to  51   d , respectively. 
     The developing rollers  33   b  accommodated in the cartridges  33  come into contact with the drums  32   a  with a fixed urging force by bringing control rollers (not shown) provided at both ends thereof into contact with the drums  32   a . The urging forces of the urging members  51   a  to  51   d  suitably maintain the contact state (or the spaced-apart state) between the developing rollers  33   b  and the drums  32   a.    
     Scanner Unit 
     As shown in  FIG. 1B , a scanner unit  11  serving as an exposing unit is disposed in front (ahead) of the cartridges  33 . The scanner unit  11  includes a laser diode, a polygonal mirror, an Fθ lens, and a reflecting mirror. The scanner unit  11  outputs laser light beams L (LY, LM, LC, and LK) modulated in accordance with Y-, M-, C-, K-color image information that is input to the control circuit unit  300  from the external host unit  400  to expose the drums  32   a  of the cartridges  33  of corresponding colors to the light. 
     The scanner unit  11  is mounted in the cover unit  10  which is a covering member for opening and closing an opening  100 B for mounting and demounting the cartridges  33 . 
     The details of the mounting and positioning of the scanner unit  11  of this embodiment will be described later. 
     Recording-Medium Conveying Mechanism 
     As shown in  FIG. 1B , a feeding unit  18  is disposed below the image forming unit  200 . The feeding unit  18  includes a feeding cassette  19  that accommodates the stacked recording mediums S, a feeding roller  20 , and a separation pad  21 . The feeding cassette  19  can be taken in and out (mounted and demounted to/from) from the front of the apparatus main body  100 A (front loading). In the apparatus main body  100 A, a recording-medium conveying path extending from the feeding roller  20  to the upper rear in the apparatus main body  100 A is formed between the intermediate transfer member  34  and a rear frame  110   b  of the apparatus main body  100 A. A registration roller pair  18   a , a secondary transfer roller  22 , a fixing unit  23 , and a discharge roller pair  24  are disposed from below to above in the conveying path. The fixing unit  23  includes a fixing film unit  23   a  and a pressure roller  23   b . The discharge roller pair  24  is composed of a discharge roller  24   a  and a discharge roller  24   b . An output tray  100   c  that receives the recording mediums S on which images are formed is disposed on the upper surface of the apparatus main body  100 A. 
     The cover unit  10  is an opening and closing member that can open and close the opening  100 B provided at the front of the apparatus main body  100 A. The cover unit  10  is fitted with the scanner unit  11 . As will be described below, the opening  100 B is an opening for mounting and demounting the cartridges  33  to/from the unit  200 . Thus, the cartridges  33  can be mounted and demounted by bringing the cover unit  10  that holds the scanner unit  11  to an opening position. 
       FIG. 1B  shows a state in which the image forming apparatus  100  can perform an image forming operation. 
     In this state, the cover unit  10  is in a closing position at which the opening  100 B is closed. The unit  200  is at an image forming position in the apparatus main body  100 A in which the cartridges  3  are mounted, and image formation is performed. A gear  34   b  ( FIG. 5C ), which is a drive input portion of the intermediate transfer member  34  of the unit  200 , connects to a drive output portion (not shown) provided at the apparatus main body  100 A. A power feeding system (not shown) provided at the apparatus main body  100 A is conducting to electrical contacts (not shown) between the drums  32   a  and the charging rollers  32   b  and the cartridges  33 . Although the driving system and the bias applying system described above are not shown for ease of explanation, they may have the same configuration as that of an ordinary image forming apparatus. 
     An operation for forming a full-color image will be described with reference to  FIGS. 1B and 2 . 
     The drums  32   a  are rotationally driven counterclockwise in the direction of the arrows at a predetermined speed. The charging rollers  32   b  are rotated as the drums  32   a  are rotationally driven. The intermediate transfer member  34  is also rotationally driven clockwise in the direction of the arrow (reverse direction to the rotation of the drums  32   a ) at a speed corresponding to the speed of the drums  32   a . The developing rollers  33   b  and the supply rollers  33   d  are individually rotationally driven clockwise in the direction of the arrows at predetermined speeds. The scanner unit  11  is also driven. In synchronization with the driving, a predetermined charging bias is applied to the individual charging rollers  32   b  at a predetermined control timing. 
     Thus, the surfaces of the drums  32   a  are uniformly charged to a predetermined polarity and potential by the charging rollers  32   b . The scanner unit  11  exposes the surfaces of the drums  32   a  to laser light beams L (LY, LM, LC, and LK) modulated in response to the Y-, M-, C-, and K-color image signals, respectively. Thus, electrostatic latent images corresponding to the corresponding color image signals are formed on the surfaces of the drums  32   a . The electrostatic latent images formed on the surfaces of the drums  32   a  are developed as developer images by the developing rollers  33   b  of the corresponding cartridges  33 . A predetermined developing bias is applied to the developing rollers  33   b  at a predetermined control timing. 
     By the foregoing electrophotographic image forming process, a Y-color developer image corresponding to the Y-color component of the full-color image is formed on the drum  32   a  that the cartridge  33 Y faces. The developer image is primarily transferred onto the intermediate transfer member  34  at a primary-transfer nip portion, which is a contact portion between the drum  32   a  and the intermediate transfer member  34 ; an M-color developer image corresponding to the M-color component of the full-color image is formed on the drum  32   a  that the cartridge  33 M faces. The developer image is superimposed, that is, primarily transferred, onto the Y-color developer image that has already been transferred onto the intermediate transfer member  34  at a primary-transfer nip portion, which is a contact portion between the drum  32   a  and the intermediate transfer member  34 . A C-color developer image corresponding to the C-color component of the full-color image is formed on the drum  32   a  that the cartridge  33 C faces. The developer image is superimposed, that is, primarily transferred, onto the Y-color+M-color developer image that has already been transferred onto the intermediate transfer member  34  at a primary-transfer nip portion, which is a contact portion between the drum  32   a  and the intermediate transfer member  34 . A K-color developer image corresponding to the K-color component of the full-color image is formed on the drum  32   a  that the cartridge  33 K faces. The developer image is superimposed, that is, primarily transferred, onto the Y-color+M-color+C-color developer image that has already been transferred onto the intermediate transfer member  34  at a primary-transfer nip portion, which is a contact portion between the drum  32   a  and the intermediate transfer member  34 . In this way, an unfixed developer image in which the four full colors, Y+M+C+K, are combined, is formed on the intermediate transfer member  34 . 
     The order of the colors of developer images transferred onto the intermediate transfer member  34  is not limited to the above. Transferred developers remaining on the surfaces of the drums  32   a  after developer images are transferred to the intermediate transfer member  34  are removed by the cleaning blades  32   c  and are pooled in individual waste-developer accommodating portions. 
     On the other hand, the feeding roller  20  is driven at a predetermined control timing. Thus, the sheet-like recording mediums S stacked in the feeding cassette  19  are separated and fed one by one in cooperation with the feeding roller  20  and the separation pad  21 . The recording mediums S are introduced to a secondary transfer nip portion, which is a contact portion between the intermediate transfer member  34  and the secondary transfer roller  22 , by the registration roller pair  18   a  at a predetermined control timing. The secondary transfer roller  22  is subjected to a secondary transfer bias with an opposite polarity to the developer charging polarity and a predetermined potential at a predetermined control timing. Thus, the four-color-superimposed developer image on the intermediate transfer member  34  is secondarily transferred onto the surface of the recording medium S in the process in which the recording medium S is conveyed through the secondary transfer nip portion. The recording medium S that has passed through the secondary transfer nip portion is separated from the surface of the intermediate transfer member  34  and is introduced to the fixing unit  23 , where it is heated and pressed in a fixing nip portion. Thus, the colors of the developer images are combined and fixed to the recording medium S. The recording medium S exits from the fixing unit  23  and is ejected as a full-color-image formed medium onto the output tray  100   c  by the discharge roller pair  24 . Secondary-transferred toner remaining on the surface of the intermediate transfer member  34  is removed after the recording medium S is separated from the intermediate transfer member  34 . In the case of this embodiment, the remaining toner electrostatically adheres to the surfaces of the drums  32   a  at the primary-transfer nip portion between the drums  32   a  and the intermediate transfer member  34  and is removed by the cleaning blades  32   c.    
     The intermediate transfer member  34  is a drum-shaped rotational body, as described above. Developer images of different colors formed on the drums  32   a  are transferred one on another onto the intermediate transfer member  34 . The intermediate transfer member  34  transfers the transferred developer images together to the recording medium S. Thus, a color image is formed on the recording medium S. 
     In the case where a monochrome image is to be formed, a K-color developer image formed on the drum  32   a  that the cartridge  33 K faces is transferred to the intermediate transfer member  34 . The intermediate transfer member  34  transfers the transferred K-color developer image to the recording medium S, so that a K-color image is formed on the recording medium S. 
     The secondary transfer roller  22  in this embodiment can be moved by a shift mechanism (not shown) to a first position at which it is in contact with the intermediate transfer member  34  to form the secondary transfer nip portion and a second position at which it is separate from, that is, out of contact with, the intermediate transfer member  34 . When the forming apparatus  100  is in an image forming operation, the secondary transfer roller  22  is moved to the first position, and not in an image forming operation, it is moved to the second position. The secondary transfer roller  22  may also be configured to be normally in contact with the intermediate transfer member  34 . 
     Image Forming Unit 
     The configuration of the image forming unit  200  will be described with reference to  FIGS. 5A to 5C . 
       FIG. 5A  is a perspective view of the unit  200  as viewed from the left,  FIG. 5B  is a perspective view of the unit  200  as viewed from the right, and  FIG. 5C  is a top view thereof. 
     The unit  200  includes the subframe  31  that can be mounted and demounted to/from the main frame  110  of the apparatus main body  100 A. The subframe  31  rotatably supports the intermediate transfer member  34  formed by coating the circumferential surface of a cylinder with an elastic member. The intermediate transfer member  34  is supported such that the left end and the right end of the central shaft (rotation center)  34   a  are rotatably supported between a left side plate  31 L and a right side plate  31 R of the subframe  31 . The intermediate transfer member  34  has the gear  34   b , at the right end, that transmits a driving force to the individual drums  32   a . The gear  34   b  transmits a driving force transmitted from the apparatus-main-body driving source (not shown) to the drum gears  32   a   1 . The drums  32   a  are disposed around the intermediate transfer member  34  in a state in which they are in contact with the intermediate transfer member  34 . The drums  32   a  are positioned relative to the subframe  31  with a positioning configuration (not shown) so as to be rotatable about the axes of the drums  32   a . This allows the drums  32   a  and the intermediate transfer member  34  to be positioned relative to each other with high accuracy. The drums  32   a  are in contact with the intermediate transfer member  34  under a predetermined pressure. 
     A left shaft  45 L and a right shaft  45 R are fixed integrally with the outer surfaces of the left side plate  31 L and the right side plate  31 R, respectively, coaxially with the central axis  34   a  of the intermediate transfer member  34 . The right side plate  31 R of the subframe  31  is provided with a restricted portion  311  that restricts the inclination of the unit  200  in the apparatus main body  100 A. The projection of the restricted portion  311  from the side surface of the subframe  31  is smaller than that of the right shaft  45 R, as shown in  FIG. 5C . 
     The unit  200  is located in the apparatus main body  100 A by the left shaft  45 L, the right shaft  45 R, and the restricted portion  311 , the details of which will be described later. Since the left shaft  45 L, the right shaft  45 R, and the inclination restricted portion  311 , which are positioning portions for the intermediate transfer member  34  in the apparatus main body  100 A, are provided on one subframe  31 , the intermediate transfer member  34  is located in the apparatus main body  100 A with high accuracy. As described above, the subframe  31  is provided with the cartridge mount portions  321  for detachably mounting the cartridges  33 . The function of the mount portions  321  will be described later. 
     Image-Forming-Unit Mount Portion 
     As shown in  FIGS. 6A and 6B , a left guide  80 L and a right guide  80 R are provided in an opposing state inside the left frame  110 L and the right frame  110 R of the apparatus main body  100 A, respectively. The guides  80 L and  80 R each have a positioning portion  80   a  that rotatably supports the left shaft  45 L or the right shaft  45 R of the subframe  31  and a guide portion  80   b  that guides the shaft  45 L or  45 R to the positioning portion  80   a . The right guide  80 R has an inclination restricting portion  80   c , inside in the longitudinal direction, that is recessed to a substantially horizontal surface, with which the inclination restricted portion  311  of the unit  200 , described above, is in contact to restrict the rotation of the unit  200 . 
     As shown in  FIGS. 6A and 6B , the upper part of the apparatus main body  100 A can be opened and closed as an upper cover unit  301  for the apparatus main body  100 A. 
     Mounting of Image Forming Unit 
     Next, the mounting of the unit  200  into the apparatus main body  100 A will be described with reference to  FIGS. 7A to 7E  and  FIGS. 8A and 8B .  FIGS. 7A to 7E  are longitudinal right side view taken along the right guide  80 R.  FIGS. 8A and 8B  are enlarged views of a two-dot chain line portion VIII in  FIG. 1B . 
     The lower end of the cover unit  10  is rotatably connected to the apparatus main body  100 A via a hinge shaft  10   a  and can be located at a closing position ( FIG. 1B ) at which it closes the opening  100 B in the side surface of the apparatus main body  100 A and an opening position ( FIGS. 7A to 7E ) at which it opens the opening  100 B. In other words, the cover unit  10  is an opening and closing member that is supported rotatably about the hinge shaft  10   a  at the front of the apparatus main body  100 A to open and close the opening  100 B. The closing position of the cover unit  10  is maintained by the engagement (latch engagement) of a securing claw  136   a  provided at a maintenance button  136  disposed at the front of the apparatus main body  100 A and a securing claw  10   b  provided at the cover unit  10 , shown in  FIGS. 8A and 8B . The securing claw  136   a  is a main-body-side securing portion, and the securing claw  10   b  is an opening-and-closing member-side securing portion. The closing of the cover unit  10  is released by pushing the button  136 . Pushing the button  136  backwards against a return spring (not shown) moves the securing claw  136   a  of the button  136  backwards from the securing claw  10   b  of the cover unit  10  to release the latch engagement, as shown by the two-dot chain line. This allows the cover unit  10  to rotate about the hinge shaft  10   a  to the opening position, thereby opening the opening  100 B wide. In this embodiment, the securing claw  136   a  and the securing claw  10   b  are in engagement elastically and releasably. However, this embodiment is not limited thereto. For example, a claw (securing portion) provided at one side may be elastically and releasably in engagement with a hole (securing portion) provided at the other side. 
     As shown in  FIGS. 9A and 9B , the upper part of the apparatus main body  100 A is rotatably connected to the apparatus main body  100 A, as the upper cover unit  301 , via a hinge shaft  301   a . The upper cover unit  301  can be located at a closing position ( FIG. 9B ) at which it closes an opening  100 C in the upper surface of the apparatus main body  100 A and an opening position ( FIG. 9A ) at which it opens the opening  100 C. The closing position of the upper cover unit  301  is also maintained by an engaging mechanism (not shown) similar to that of the cover unit  10 . 
     The mounting of the unit  200  into the apparatus main body  100 A will be described. 
     First, as shown in  FIG. 7A , the upper cover unit  301  and the cover unit  10  of the apparatus main body  100 A are located at opening positions. 
     Next, the image forming unit  200  is inserted into the apparatus main body  100 A through the openings  100 B and  100 C. First, the left and right shafts  45 L and  45 R of the subframe  31  are brought into engagement with the opposing guide portions  80   b  provided at the left and right guides  80 L and  80 R of the apparatus main body  100 A, respectively ( FIG. 7B ). 
     As the mounting proceeds, the right shaft  45 R reaches the rotation restricting portion  80   c  inside in the longitudinal direction ( FIG. 7C ). However, the right shaft  45 R comes into engagement with the guide portion  80   b  outside in the longitudinal direction because of the large amount of projection from the side surface of the subframe  31 , and thus, the mounting proceeds along the guide portion  80   b . The inclination restricted portion  311  starts to come into engagement with the guide portion  80   b . At that time, the image forming unit  200  can be mounted without interfering with the upper cover unit  301  and the cover unit  10 . 
     As the mounting further proceeds, the left and right shafts  45 L and  45 R reach the positioning portions  80   a  ( FIG. 7D ) provided at the extensions of the guide portions  80   b . On the other hand, the inclination restricted portion  311  comes to the rotation restricting portion  80   c  of the right guide  80 R. The protrusion of the inclination restricted portion  311  from the side surface of the subframe  31  is small, and thus, it does not come into engagement with the guide portion  80   b  outside in the longitudinal direction. Therefore, the image forming unit  200  rotates to the position in  FIG. 7E . 
     At that time, the gear  34   b  ( FIG. 5C ) provided at one end of the intermediate transfer member  34  is connected to a drive gear (not shown) provided in the apparatus main body  100 A. Finally, the upper cover unit  301  is closed, and thus, the mounting is completed. Closing the upper cover unit  301  causes a compressing spring  302  serving as an urging member urges and presses the upper part of the subframe  31 , thus allowing the image forming unit  200  to be held stably at the position in  FIG. 9B . 
     When the unit  200  is to be removed, the procedure is opposite to the foregoing mounting procedure. Specifically, the upper cover unit  301  is moved to the opening position; the unit  200  is rotated until the inclination restricted portion  311  comes to a position at which it comes into contact with the ceiling surface of the guide portion  80   b ; and the unit  200  is extracted along the guide portion  80   b.    
     After the unit  200  is mounted by the foregoing procedure, the four cartridges  33  ( 33 Y,  33 M,  33 C, and  33 K) are mounted to the mount portions  321  ( 321 Y,  321 M,  321 C, and  321 K) provided in the subframe  31 . 
     Cover Unit 
     The cover unit  10 , which is a covering member that opens and closes the opening  100 B, will be described with reference to  FIG. 10A  to  FIG. 13 .  FIG. 10A  is a left back perspective view of the cover unit  10 ;  FIG. 10B  is a right front perspective view of the cover unit  10 ;  FIG. 11A  is a cross-sectional view of the cover unit  10  in the lateral direction;  FIG. 11B  is a cross-sectional view of the cover unit  10  taken along line XIB-XIB of  FIG. 11A ;  FIG. 11C  is a cross-sectional view of the cover unit  10  taken along line XIC-XIC of  FIG. 11A ;  FIG. 12A  is a cross-sectional view of the cover unit  10  in the front-to-back direction;  FIG. 12B  is a cross-sectional view of the cover unit  10  taken along line XIIB-XIIB of  FIG. 12A ;  FIG. 13  is a diagram illustrating the positional relationship among the scanner unit  11 , urging members, and compressing springs. 
     The image forming apparatus  100  of this embodiment is provided with the cover unit  10  for opening and closing the opening  100 B. 
     Cartridge Urging Unit 
     The cover unit  10  is provided with the urging members  51   a  to  51   d  that urge the individual developing cartridges  33 , as shown in  FIG. 10A . 
     As shown in  FIG. 11C , the urging members  51   a  to  51   d  each have a cartridge contact member  511  and a spring  512  in a housing  10   c  of the cover unit  10 . When the cover unit  10  is closed, the urging members  51   a  to  51   d  come into contact with the individual cartridges  33  to urge the cartridges  33  toward the drums  32   a , as described above. The urging force brings the developing rollers  33   b  of the cartridges  33  into contact with the photosensitive drums  32   a  with a predetermined pressure. 
     Scanner Mount Configuration 
     On the other hand, the cover unit  10  is equipped with the scanner unit  11 , which is an exposing unit, as described above. 
     As shown in  FIG. 13 , the scanner unit  11  is provided with positioning projections  11   a   1  to  11   a   3 , which are engaging portions for the subframe  31  serving as a support frame. 
     Of the three positioning projection  11   a   1  to  11   a   3 , the upper two positioning projections  11   a   1  and  11   a   2  are cylindrical in shape and are located on the same axis. The two projections  11   a   1  and  11   a   2  have longitudinal-position restricting portions  11   b   1  and  11   b   2 , respectively. As shown in  FIG. 12A , the positioning projection  11   a   3  at the lower part of the scanner unit  11  has the shape of a bent cylinder. 
     The housing  10   c  of the cover unit  10  is a hollow housing, as shown in  FIG. 11B , and the scanner unit  11  is located in the hollow portion. This configuration can prevent the operator, for example, from touching the optical components etc. of the scanner unit  11  or striking the cartridges  33  etc. against the scanner unit  11 , thereby preventing troubles caused by the scanner unit  11 . 
     As shown in  FIGS. 12A and 12B , the foregoing three positioning projections  11   a   1  to  11   a   3  project outwards through openings  10   d   1  to  10   d   3  provided in the housing  10   c  of the housing  10 , respectively. 
     As shown in  FIG. 13 , four compressing springs  111  and  112 , which are elastic members, are provided between the scanner unit  11  and the inner wall of the cover unit housing  10   c . The compressing springs  111  and  112  urge the scanner unit  11  in the directions of arrows E 1   b  and E 2   r . For the direction of E 1   b , as shown in  FIGS. 11B and 11C , the scanner unit  11  is urged by the three compressing springs  111  to bring the three positioning projections  11   a   1  to  11   a   3  to the openings  10   d   1  to  10   d   3  of the housing  10   c  of the cover unit  10 , respectively. For the direction of E 2   r , as shown in  FIG. 12B , the scanner unit  11  is urged by the compressing spring  112 , so that the position restricting portion  11   b   1  of the positioning projection  11   a   1  is in contact with the outer wall  10   c   1  of the cover unit housing  10   c . For the remaining direction of E 3 , as shown in  FIG. 11C , the positioning projections  11   a   1  and  11   a   2  are roughly defined in the openings  10   d   1  and  10   d   2 , respectively, with a gap therebetween. 
     As described above, the scanner unit  11  is mounted in the cover unit  10  via the compressing springs  111  and  112  which are elastic members. The scanner unit  11  is mounted so as to be moved relative to the cover unit housing  10   c  in any direction if a predetermined force or more is applied thereto. 
     Positioning of Scanner Unit 
     Next, the positioning of the scanner unit  11  when the cover unit  10  is located at the closing position at which the opening  100 B is closed will be described with reference to  FIGS. 5A to 5C ,  FIGS. 14 and 15 , and  FIGS. 16A  and  16 B.  FIGS. 14 and 15  are diagrams illustrating the behavior of the positioning projections of the scanner unit  11  when being mounted and demounted to/from the scanner guides; and  FIGS. 16A and 16B  are diagrams illustrating the positioning of the scanner unit  11  to the image forming unit  200 . 
     Configuration of Scanner Unit and Subframe 
     As described above, the scanner unit  11  is mounted in the cover unit  10  via the compressing springs  111  and  112 . On the other hand, the subframe  31  of the image forming unit  200  is provided with scanner guides  312   a ,  312   b , and  312   c , which are engaged portions corresponding to the positioning projections  11   a   1  to  11   a   3  which are engaging portions of the scanner unit  11 . 
     As shown in  FIGS. 5A to 5C , there are three scanner guides  312   a ,  312   b , and  312   c , which correspond to the positioning projections  11   a   1  to  11   a   3  of the scanner unit  11 , respectively. 
     As shown in  FIG. 14 , the scanner guides  312   a  and  312   b  have a groove shape in which arc shapes  312   a   1  and  312   b   1 , straight line portions  312   a   2  and  312   b   2 , and introductory portions  312   a   3  and  312   b   3  provided at the inlet are combined, respectively. The straight line portions  312   a   2  and  312   b   2  expand outwards with an increasing distance to the inlet.  FIG. 14  shows only the scanner guide  312   a  at the left; however, the right scanner guide  312   b  has also the same shape having the arc shape  312   b   1 , the straight line portion  312   b   2 , and the introductory portion  312   b   3 . 
     The portions of the scanner guides  312   a  and  312   b  inside the subframe  31  are slopes  312   a   4  and  312   b   4  ( FIGS. 5A and 5B  and  FIG. 16A ), respectively, that expand outwards with an increasing distance to the inlet. 
     The lower scanner guide  312   c  is a square groove whose corners are R-shaped, as shown in  FIG. 5C . 
     Behavior of Scanner Unit in Cover-Unit Closing Operation 
     The scanner unit  11  is mounted in the cover unit  10  via the elastic members, and thus has a certain degree of flexibility. Therefore, when the cover unit  10  is moved close to the closing position, the positioning projections  11   a   1  and  11   a   2  are guided to the straight line portions  312   a   2  and  312   b   2  of the scanner guides  312   a  and  312   b  by the introductory portions  312   a   3  and  312   b   3  of the scanner guides  312   a  and  312   b , respectively, as shown in  FIG. 14 . Although  FIG. 14  shows only the scanner guide  312   a  and the positioning projection  11   a   1  at the left, the scanner guide  312   b  and the positioning projection  11   a   2  at the right have the same configuration. 
     The closing operation further proceeds, and the cover unit  10  reaches the closing position, the positioning projections  11   a   1  and  11   a   2  are guided to the positioning portions  312   a   1  and  312   b   1  of the scanner guides  312   a  and  312   b  by the straight line portions  312   a   2  and  312   b   2  of the scanner guides  312   a  and  312   b , respectively. At that time, the positioning projections  11   a   1  and  11   a   2  are located and held at positions coaxial with the positioning portions  312   a   1  and  312   b   1  of the scanner guides  312   a  and  312   b , shown in  FIG. 16A , by the urging force of the compressing springs  111 . 
     For the lower positioning projection  11   a   3 , when the cover unit  10  is brought close to the closing position, first a bent portion  11   a   3 A comes into contact with the scanner guide  312   c , as shown in  FIG. 15 . As the closing operation further proceeds, and the cover unit  10  reaches the closing position, a vertical cylinder  11   a   3 B comes into contact with the scanner guide  312   c  by the guide of the bent portion  11   a   3 A of the positioning projection  11   a   3 . 
     As the closing operation proceeds, the longitudinal-position restricting portion  11   b   2  of the positioning projection  11   a   2  of the scanner unit  11  is guided along the slope  312   b   4  against the urging force of the compressing spring  112 . When the cover unit  10  reaches the closing position, the longitudinal-position restricting portion  11   b   2  is urged to an inner wall  312   b   5  by the compressing spring  112  to come into the contact position, as shown in  FIG. 16A . 
     Positioning of Scanner Unit 
     As described above, when the cover unit  10  is located at the closing position at which the opening  100 B is closed, the scanner unit  11  is positioned relative to the subframe  31 , as shown in  FIG. 16A . Here,  FIG. 16A  illustrates only the scanner unit  11  and the subframe  31  for the ease of understanding the positioning configuration of the scanner unit  11  relative to the subframe  31 . 
     When the cover unit  10  reaches the closing position, the cover unit  10  is located such that the positioning projections  11   a   1  and  11   a   2  are fitted in the positioning portions  312   a   1  and  312   a   2  of the scanner guides  312   a  and  312   b , respectively, as shown in  FIG. 16A . 
     In this state, since the scanner unit  11  is urged in the direction of E 1   b  by the compressing springs  111 , the positioning projections  11   a   1  and  11   a   2  are pushed against the positioning portions  312   a   1  and  312   a   2  of the scanner guides  312   a  and  312   b , respectively. 
     Therefore, the positioning projections  11   a   1  and  11   a   2  in the directions of E 1  and E 3  are positioned and held at a position in the directions of E 1  and E 3  at which the positioning projections  11   a  and  11   a   2  and the positioning portions  312   a   1  and  312   a   2  of the scanner guides  312   a  and  312   b  are coaxial. Even if the positions of the positioning projections  11   a  and  11   a   2  are determined in the directions of E 1  and E 3 , the scanner unit  11  has flexibility in the direction in which it rotates about the positioning projections  11   a   1  and  11   a   2 , and thus, it is not completely positioned. Thus, as shown in  FIG. 16B , the lower positioning projection  11   a   3  is configured to come into contact with the scanner guide  312   c  so that the angle of the rotation about the positioning projections  11   a  and  11   a   2  can be determined. 
     With the above positioning configuration, the position of the scanner unit  11  relative to the subframe  31  in the directions of E 1  and E 3  can be determined. 
     For the position in the direction of E 2 , the scanner unit  11  is positioned because the position restricting portion  11   b   2  of the positioning projection  11   a   2  is pushed against the inner wall  312   b   5  of the subframe  31  by the compressing spring  112 . 
     With such a configuration, the scanner unit  11  can be positioned relative to the subframe  31  that supports the intermediate transfer member  34 , the drums  32   a , and the developing cartridges  33  simply by closing the cover unit  10 . As described above, since the positioning of the scanner unit  11  relative to the subframe  31  is performed using the urging forces of the compressing springs  111  and  112  interposed between the scanner unit  11  and the cover unit  10 , there is no need to provide a separate urging member only for positioning. 
     Summary 
     Thus, the image forming apparatus  100  can perform an image forming operation (printing operation), and thus, the image forming operation as described above is performed on the basis of an image-formation start signal (printing start signal). In other words, a drive output portion (not shown) at the apparatus main body  100 A side is connected to the gear  34   b  serving as a driving input unit for the intermediate transfer member  34  of the unit  200  at the image forming position. The drum gears  32   a   1  of the individual photosensitive drums  32   a  are connected to the gear  34   b . The gears  50  of the developing rollers  33   b  of the cartridges  33  are connected to the drum gears  32   a   1 . 
     Contacts at the apparatus main body  100 A side are electrically connected to the electrical contacts between the photosensitive drums  32   a  and the cartridges  33 . This allows the image forming apparatus  100  to perform an image forming operation. 
     As shown in  FIG. 1B , the image forming apparatus  100  of this embodiment has the scanner unit  11 , the cartridges  33 , the drums  32   a , the intermediate transfer member  34 , and the conveying path for the recording medium S substantially side by side horizontally on the installation surface F. The output tray  100   c  is disposed at the upper part of the apparatus  100 , and the cassette  19  is disposed at the lower part of the apparatus  100 . The laser light beams L (LY, LM, LC, and LK) from the scanner unit  11  are emitted to the drums  32   a  from the front of the apparatus  100 . Color developer images transferred from the drums  32   a  to the intermediate transfer member  34  are then transferred to the recording medium S at the opposite side to the drums  32   a , with the intermediate transfer member  34  interposed therebetween. 
     This embodiment has been described taking the intermediate transfer member  34  for indirectly transferring images from the drums  32   a  to the recording medium S as an example of a transfer member; instead, a system of directly transferring images from the drums  32   a  to the recording medium S is also possible. In this case, the transfer member  34  plays the roll of directly transferring developer images on the drums  32   a  onto the recording medium S. This embodiment shows the configuration in which the replaceable cartridges are the developing cartridges  33 ; instead, it is also possible to have a configuration in which the replaceable cartridges are integrated process cartridges  133  in each of which the drum  32   a , the charging roller  32   b , the cleaning blade  32   c , the developing roller  33   b  serving as a developing unit, etc. are integrated.  FIG. 17  is a diagram illustrating the mounting of the integrated process cartridges  133  ( 133 Y,  133 M,  133 C, and  133 K) to the mount portions in the direction of arrow X 10  and the demounting thereof from the mount portions in the direction of X 11 . In other words, cartridges that can be mounted and demounted to/from the apparatus main body may each have an electrophotographic photosensitive drum and a developing unit for developing an electrostatic latent image formed on the electrophotographic photosensitive drum. 
     As described above, with the configuration of this embodiment, the urging of the cartridges  33  is performed by the urging members  51  attached to the housing  10   c  of the cover unit  10 . On the other hand, the scanner unit  11  is mounted in the housing  10   c  of the cover unit  10  via the compressing springs  111  and  112  which are elastic members, and the positioned is performed relative to the subframe  31 , which is a supporting member, of the image forming unit  200 . Thus, this embodiment is configured such that the scanner unit  11  is positioned directly to the subframe  31 . 
     Therefore, if the housing  10   c  of the cover unit  10  is slightly deformed by urging the cartridges  33 , the positions of the compressing springs  111  and  112  may change correspondingly to exert an influence on the force of urging the scanner unit  11 . However, such a change in urging force due to the slight positional change has no influence on the position of the scanner unit  11 . 
     Accordingly, this configuration can significantly reduce the adverse influence on an exposing process due to the urging of the cartridges  33  as compared with a configuration in which the scanner unit  11  is fixed in the housing  10   c  of the cover unit  10 . 
     Furthermore, the scanner unit  11  can be positioned directly to the subframe  31  of the image forming unit  200 , which supports and positions the drums  32   a  to be exposed to light, the cartridges  33 , or cartridges each including the drum  32   a . Therefore, this can achieve high-accuracy images and can reliably prevent the laser light beams L of the scanner unit  11  from being blocked by the cases  33   a  of the cartridges  33  or the like to cause underexposure. Here, the direct positioning of the scanner unit to the subframe is that the scanner unit is positioned by part thereof coming into contact with part of the subframe. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-273911 filed on Dec. 8, 2010, which is hereby incorporated by reference herein in its entirety.