Abstract:
An image forming apparatus has a plurality of developing devices for developing an electrostatic image formed on an image bearing member, and a rotary member holding the plurality of developing devices and rotated in a route including a developing position, the rotary member selectively positioning any one of the developing devices at the developing position. Each of the plurality of developing devices has a plurality of developer carrying members carrying a developer thereon and carrying the developer to the image bearing member.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an image forming apparatus adopting an electrophotographic process or an electrostatic recording process, and particularly to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus. 
   2. Description of Related Art 
   In an image forming portion in an image forming apparatus such as a copying machine, a photosensitive member which is an image bearing member is charged by a charging apparatus, and is exposed to the image of an original at an exposure position by an exposing optical system to thereby form an electrostatic latent image on the peripheral surface of the photosensitive member, and this electrostatic latent image is developed by a developing apparatus to thereby form a developer image (toner image), and this toner image is transferred to a transferring material by the application of a voltage by a transferring apparatus, and the photosensitive member after the transfer is cleaned by a cleaning apparatus, whereafter exposure before charging is effected to thereby remove any residual charges on the photosensitive member, and the above-described process is repeated again to thereby effect image forming. 
   There has also been proposed an image forming apparatus which is provided with a plurality of photosensitive members, charging apparatuses and developing apparatuses and repeats the above-described image forming process a plurality of times to thereby form a full-color image. 
   Further, in the above-described image forming apparatus for forming a full-color image, it has heretofore been proposed to provide a plurality of developer carrying members (hereinafter referred to as the developing sleeves) in the developing apparatuses. 
   In this case, the developing sleeves are disposed with a predetermined clearance relative to the photosensitive members, but it is possible to form images of high definition at a high speed without greatly increasing the peripheral speed of the developing sleeves. 
   Also, in the black-and-white image forming apparatus of Japanese Patent Application Laid-Open No. 2000-147900 which has a developing device provided with a plurality of developing sleeves and fixedly disposed around a photosensitive member, there is proposed a method of pressing one developing sleeve against the photosensitive member with the other developing sleeve as a fulcrum. 
   The above-described full-color image forming apparatus, however, tends to become bulky and has posed a problem in this point. That is, neither of the higher speed and higher quality of image and the downsizing of the image forming apparatus could be made compatible. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image forming apparatus which can make both of the higher speed of image forming and the downsizing of the image forming apparatus compatible. 
   It is another object of the present invention to provide an image forming apparatus which can make both of a higher quality of image in image forming and the downsizing of the image forming apparatus compatible. 
   It is another object of the present invention to provide an image forming apparatus in which a plurality of developer carrying members can be accurately positioned relative to an image bearing member by a simple construction. 
   Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic cross-sectional view of a full-color copying apparatus provided with a developing apparatus according to the present invention. 
       FIG. 2  is a cross-sectional view illustrating the developing operation of the developing apparatus according to the present invention. 
       FIG. 3  is a cross-sectional view illustrating the developing operation of the developing apparatus according to the present invention. 
       FIG. 4  is a cross-sectional view illustrating the developing operation of the developing apparatus according to the present invention. 
       FIG. 5  is a cross-sectional view illustrating the developing operation of the developing apparatus according to the present invention. 
       FIG. 6  is a cross-sectional view of the developing device positioning mechanism of the developing apparatus according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Some embodiments of the present invention will hereinafter be described with reference to the drawings. 
     FIG. 1  is a schematic cross-sectional view of a full-color printer provided with a developing apparatus according to the present invention. 
   In the full-color printer of  FIG. 1 , the image information of an original read by an image reading portion (not shown) is processed by an image processing portion (not shown). For example, a recording sheet (not shown) fed from a cassette sheet feeding portion  32  has its skew feeding corrected at a registering portion  33  and comes to a secondary transferring portion  39 . 
   On the other hand, the image data processed by the image processing portion (not shown) is recorded as a latent image on a photosensitive drum  35  by a laser scanner portion  34 . When a full-color image is to be formed, the latent image is developed by a rotary type developing apparatus  30 . Here, the rotary type developing apparatus  30  develops the latent image on the photosensitive drum  35  as toner images of four colors while changing over a plurality of (four) developing devices 37K (black), 37C (cyan), 37M (magenta) and 37Y (yellow) to a rotary containing portion  36 . The toner images of four colors are superimposed and primary-transferred onto an intermediate transferring belt  38 , and the toner images on the intermediate transferring belt  38  are collectively transferred to the recording sheet at the secondary transferring portion  39 . The toner image on the recording sheet is fixed on the recording sheet by a fixing portion  40 , and the recording sheet having the toner image thereon fixed is discharged to a sheet discharging portion  41 . 
   Description will now be made of the action of the rotary type developing apparatus according to the present invention. 
   First, as a premise, when the rotary type developing apparatus  30  provided with two developing sleeves is used, both of the higher speed (or higher quality of image) of image forming and downsizing can be made compatible. 
     FIGS. 2 to 5  show the developing operation of the rotary type developing apparatus  30  provided with two developing sleeves. While for simplicity, a developing device  37  alone is taken as an example and the action thereof will hereinafter be described, the other developing devices are also similar in construction. 
   Also, the electrostatic image formed on the photosensitive drum  35  is adapted to be developed by a developing sleeve S 1  and a developing sleeve S 2  which will be described later, in the named order. 
   Referring to  FIG. 2 , a regulating member  42  for ensuring a gap (hereinafter referred to as the SD gap) with respect to the photosensitive drum  35  is mounted on the developing sleeve S 2  outside a developing area, often on the opposite end portions thereof. 
     FIG. 2  shows the retracted positions of the developing sleeves S 1  and S 2  in the developing device  37  being rotated, and the developing sleeve S 2  on the downstream side with respect to the direction of rotation (the direction indicated by the arrow R) of the developing device  37  is retracted to a position in which it does not interfere with the photosensitive drum  37  when the rotary type developing apparatus  30  is being rotated. When the developing sleeve S 1  on the upstream side comes to a developing position, the rotation of the rotary type developing apparatus  30  is stopped. At this time, as shown in  FIG. 3 , the photosensitive drum  35  and the developing sleeve S 1  are adjusted so that the gap between the two may be a, and this also holds true of the other developing devices. 
   When in the above-described state, predetermined developing is effected by the developing sleeve S 1 , the developing device  37  is rotated about the center of rotation A of the developing sleeve S 1  by driving means (not shown) to a position in which the regulating member  42  for ensuring the gap between the photosensitive drum  35  and the developing sleeve S 2  contacts with the photosensitive drum  35 , as shown in  FIG. 4 . Thereupon, the gap between the photosensitive drum  35  and the developing sleeve S 2  is kept at a predetermined value “b”, and predetermined developing is effected by the developing sleeve S 2 . 
   After the termination of the developing, the developing device  37  or the developing sleeve is rotated (retracted) to a position in which the developing sleeve S 2  does not interfere with the photosensitive drum  35  even if the rotary type developing apparatus  30  is rotated, whereafter the rotary type developing apparatus  30  is rotated (see  FIG. 5 ). 
   The timing at which the movement of the developing sleeve S 2  to the developing position is started may be before the rotation of the rotary type developing apparatus  30  is stopped, and by doing so, the changeover time can be made shorter than that by rotating the developing device  37  after the rotary type developing apparatus  30  is stopped. 
   Thus, according to the present embodiment, design is made such that after the predetermined developing has been effected by the use of the developing sleeve S 2 , the developing device  37  or the developing sleeve S 2  is rotated (retracted) to the position in which the developing sleeve S 2  does not interfere with the photosensitive drum  35  and therefore, the plurality of developing sleeves S 1  and S 2  of each developing device  37  can be disposed in proximity to the photosensitive drum  35 . 
   Also, design is made such that the developing sleeve S 1  is positioned by the developing device  37  so as to form the predetermined gap (SD gap) “a” between it and the photosensitive drum  35 , and the developing device  37  is rotated to thereby bring the position-regulatable regulating member  42  provided on the developing sleeve S 2  into contact with the photosensitive drum  35  and form the predetermined gap (SD gap) “b” between the developing sleeve S 2  and the photosensitive drum  35  and therefore, the gaps (SD gaps) “a” and “b” between the photosensitive drum  35  and the developing sleeves S 1 , S 2  can be positioned highly accurately. 
   While in the foregoing, description has been made of a case where developing is effected by the developing sleeve S 1 , whereafter developing is effected by the developing sleeve S 2 , the developing by the developing sleeve S 1  and the developing by the developing sleeve S 2  may be started at a time after the developing sleeves S 1  and S 2  have been stopped at the developing position. Also, design may be made such that a mode in which the rotation of the developing sleeve S 2  is selectively controlled and developing is effected by the developing sleeve S 1  alone, and a mode in which developing is effected by the use of both of the developing sleeves S 1  and S 2  are set to thereby set a high image quality mode, a standard image quality mode, a high speed output mode, etc. so as to enable a user to select the modes with the quality of image, the output time, etc. taken into account. 
   Description will now be made of a modification according to the present invention, i.e., not the construction in which the regulating member of the developing sleeve as described above is directly abutted against the photosensitive drum, but an example in which the regulating member is indirectly abutted against the photosensitive drum to thereby ensure the distance between the photosensitive drum surface (image forming area) and the developing sleeve surface (image forming area). 
     FIG. 6  is a cross-sectional view of a positioning mechanism for the developing device (the developing sleeve S 2  on the downstream side)  37 . 
   The positioning of the developing device  37  is done by pressing the developing device  37  against a developing device rotating rail  61  and a developing position determining rail  62  by means of a pressure spring  64 , and abutting a runner  63  against the rail  62 . 
   The developing device rotating rail  61  is for determining the position of the developing device  37  when not used, and holds the runner  63  of the developing device  37  in its retracted position so that during the rotation of the rotary type developing apparatus  30 , the developing sleeve S 2  may not interfere with the photosensitive drum  35 . 
   Also, the developing position determining rail  62  is a member for guiding the developing sleeve S 2  on the downstream side to the developing position as the developing device  37  approaches the developing position, and when the developing sleeve S 1  on the upstream side leaves the developing position, the rail  62  contains the developing device  37  in the rotary type developing apparatus  30  so that the developing sleeve S 2  on the downstream side may not interfere with the photosensitive drum  35 . The developing position determining rail  62  is mounted on the rotary shaft of the photosensitive drum  35 , and the gap (hereinafter referred to as the SD gap) between the developing sleeve S 2  on the downstream side and the photosensitive drum  35  is accurately positioned by a portion. The developing position determining rail  62 , if its accuracy can be ensured, need not be mounted on the rotary shaft of the photosensitive drum  35 , and the developing position determining rail  62  may be adjusted so that a desired SD gap may be obtained. 
   The developing position determining rail  62  has a portion having a curvature of a radius P about the center of the photosensitive drum  35  within a range C which determines the SD gap of the developing sleeve S 2  on the downstream side, whereby even if the stopped position of the rotary type developing apparatus  30  deviates, the SD gap of the developing sleeve S 2  on the downstream side can be ensured. 
   Also, as previously described, the developing device rotating rail  61  retracts the developing device  37  by a minimum amount for which the developing sleeve S 2  does not interfere with the photosensitive drum  35 , but it may be designed to further retract the developing device  37  and suppress the moment of inertia of the rotary type developing apparatus  30  to a small value to thereby reduce the load of the driving motor of the rotary type developing apparatus  30 . 
   As is apparent from the foregoing description, according to the present embodiment, both of the higher speed (higher quality of image) of image forming and the downsizing of the image forming apparatus can be made compatible. In addition, the developer carrying member can be positioned highly accurately with a predetermined gap with respect to the image bearing member.