Abstract:
An image forming apparatus includes an image carrier that holds a toner image, a rotary developing device including a plurality of developing units to develop an electrostatic latent image on the image carrier with toner by supplying the toner from the developing unit opposing the image carrier, a foreign-substance receiving plate that receives a falling foreign substance, a transfer unit that transfers the toner image from the image carrier onto a recording medium, and a fixing unit that fixes the transferred toner image on the recording medium. Each of the developing units includes a developer container that stores developer containing magnetic carriers and toner, and a developing roller that supplies the toner in the developer for development of the electrostatic latent image. The rotary developing device further supports a foreign-substance collecting member that magnetically collects a magnetic foreign substance on the foreign-substance receiving plate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2011-274493 filed Dec. 15, 2011. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to an image forming apparatus including a rotary developing device, and the rotary developing device. 
         [0004]    (ii) Related Art 
         [0005]    There are known image forming apparatuses each including a rotary developing device in which a plurality of developing units arranged around a rotation shaft are revolved around the rotation shaft to move a developing unit used for a current developing operation to a developing position. In some of such image forming apparatuses, a foreign-substance receiving plate is provided in a lower part of the rotary developing device to receive falling foreign substances, such as developer, falling from the rotary developing device. 
         [0006]    In general, foreign metallic substances, such as spatter powder due to welding and screw chips, are often produced during production of an image forming apparatus. Although these foreign metallic substances are cleaned off during production, they are sometimes not sufficiently cleaned off, but remain in the image forming apparatus. Such remaining foreign metallic substances may fall owing to vibration occurring during transport and operation of the image forming apparatus. 
         [0007]    In the image forming apparatus in which the above-described foreign-substance receiving plate is provided in the lower part of the rotary developing device, the above-described falling foreign metallic substrates are sometimes received by the foreign-substance receiving plate. Most developing units in the rotary developing device each include a developing roller having a magnet therein to transport developer containing magnetic carriers to a developing position while attracting the developer on a surface of the developing roller. For this reason, if foreign metallic substances are present on the foreign-substance receiving plate in the lower part of the rotary developing device, they are sometimes attached to the developing roller in the developing unit passing over the foreign-substance receiving plate and are carried together with the developer to the developing position. At the developing position, a surface of an image carrier for holding an electrostatic latent image to be developed opposes the developing roller in the developing unit. Hence, if the foreign metallic substances are attached to the developing roller, they may scrape the surface of the image carrier. 
       SUMMARY 
       [0008]    According to an aspect of the invention, there is provided an image forming apparatus including: an image carrier that holds a toner image formed by developing an electrostatic latent image formed thereon with toner; a rotary developing device including a rotation shaft and a plurality of developing units supported on a revolution orbit surrounding the rotation shaft, the rotary developing device developing the electrostatic latent image on the image carrier with the toner by revolving the developing unit used for a current development operation, of the plurality of developing units, to a position opposing the image carrier and supplying the toner from the developing unit opposing the image carrier to the image carrier; a foreign-substance receiving plate that extends along the revolution orbit of the developing units in a lower part of the rotary developing device and receives a falling foreign substance; a transfer unit that transfers the toner image from the image carrier onto a recording medium; and a fixing unit that fixes the toner image transferred by the transfer unit onto the recording medium. Each of the developing units includes a developer container that stores developer containing magnetic carriers and toner electrostatically attached to the magnetic carriers, and a developing roller having a magnet therein, the developing roller supplying the toner in the developer stored in the developer container for development of the electrostatic latent image on the image carrier by transporting the developer to the position opposing the image carrier while attracting the developer on a surface of the image carrier by the magnet. The rotary developing device further supports a foreign-substance collecting member on the revolution orbit on which the plurality of developing units are provided, the foreign-substance collecting member magnetically collecting a magnetic foreign substance on the foreign-substance receiving plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0010]      FIG. 1  schematically illustrates a configuration of an image forming apparatus according to a first exemplary embodiment; 
           [0011]      FIG. 2  is an enlarged view of a rotary developing device, a catch pan, and their surroundings illustrated in  FIG. 1 ; 
           [0012]      FIGS. 3A and 3B  respectively illustrate an opposing distance between a photoconductor and a developing roller in a developing unit used for development, and an opposing distance between the photoconductor and a developing roller in a dummy developing unit; and 
           [0013]      FIG. 4  is an enlarged view of a rotary developing device, a catch pan, and their surroundings in an image forming apparatus according to a second exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Exemplary embodiments of the present invention will be described below. 
         [0015]    First, a first exemplary embodiment will be described. 
         [0016]      FIG. 1  schematically illustrates a configuration of an image forming apparatus  1  according to a first exemplary embodiment. 
         [0017]    The image forming apparatus  1  illustrated in  FIG. 1  includes a document reading section  10 , an image forming section  20 , and a paper storage section  30 . 
         [0018]    The document reading section  10  includes a document feed table  11  on which documents S are stacked. The documents S stacked on the document feed table  11  are fed out one by one, and a fed document S is transported through a transport path  13  by transport rollers  12 . Then, characters and images recorded on the transported document S are read by a document reading optical system  15  provided below a document reading plate  14  made of a transparent glass, and the document S is output onto a document output table  16 . 
         [0019]    A hinge extending in the right-left direction is provided on a back side in the document reading section  10 , and the document feed table  11  and the document output table  16  are turned upward together about the hinge. When the document feed table  11  and the document output table  16  are thus turned up, the document reading plate  14  is exposed. 
         [0020]    As a reading method adopted in the document reading section  10 , instead of placing the documents on the document feed table  11 , only one document may be placed upside down on the document reading plate  14 , and characters and images may be read from the document on the document reading plate  14  by moving the document reading optical system  15  in a direction of arrow A. 
         [0021]    Image signals obtained by the document reading optical system  15  are input to a processing control circuit  21 . The processing control circuit  21  forms an image based on the input image signals. The processing control circuit  21  also controls the operations of the units in the image forming apparatus  1 . 
         [0022]    In the paper storage section  30  provided in a lower part of the image forming apparatus  1 , three sheet feeding units  31 _ 1 ,  31 _ 2 , and  31 _ 3  are provided. For example, the sheet feeding units  31 _ 1 ,  31 _ 2 , and  31 _ 3  store stacked sheets P having different sizes. The sheet feeding units  31 _ 1 ,  31 _ 2 , and  31 _ 3  are drawable for the purpose of supply of sheets P. 
         [0023]    From a sheet feeding unit that stores sheets P of the size matching the size of the document (here, the sheet feeding unit  31 _ 3  as an example), among these three sheet feeding units  31 _ 1 ,  31 _ 2 , and  31 _ 3 , sheets P are fed out by a pickup roller  32 . Then, the sheets P are separated one by one by separation rollers  33 , and one sheet P is transported upward by transport rollers  34  in a direction of arrow B. Subsequent transport timing of the sheet P is adjusted by standby rollers  35 , and the sheet P is then transported further. Transport downstream of the standby rollers  35  will be described below. 
         [0024]    The image forming section  20  is equipped with a manual feed tray  22 . The manual feed tray  22  is of a folding type that pivots open about a lower end thereof. In the image forming section  20 , a sheet placed on the manual feed tray  22  in an open state may be fed in the direction of arrow C. 
         [0025]    In the center of the image forming section  20 , a photoconductor  51  is provided to rotate in a direction of arrow D. Around the photoconductor  51 , a charging unit  52 , a rotary developing device  60 , a charging controller  54 , a cleaner  55 , and a charge eliminator  56  are arranged. Above the photoconductor  51 , an exposure unit  53  is provided. Also, a transfer unit  57  is provided on a side of a below-described intermediate transfer belt  71  opposite the photoconductor  51 . 
         [0026]    The photoconductor  51  is cylindrical. The photoconductor  51  stores charge by charging and emits the charge by exposure so as to hold an electrostatic latent image on its surface. 
         [0027]    The charging unit  52  charges the surface of the photoconductor  51  to a certain charging potential. 
         [0028]    The exposure unit  53  receives image signals from the processing control circuit  21 , and outputs a light beam  531  modulated according to the received image signals. 
         [0029]    The light beam  531  repeatedly scans a portion of the surface of the photoconductor  51  rotating in the direction of arrow D, which is charged by the charging unit  52 , in a rotation axis direction of the photoconductor  51  (a direction perpendicular to the plane of paper of  FIG. 1 ), thereby forming an electrostatic latent image on the surface of the photoconductor  51 . 
         [0030]    After the electrostatic latent image is formed on the surface of the photoconductor  51  by scanning with the light beam  531 , it is developed by the rotary developing device  60  to form a toner image on the surface of the photoconductor  51 . 
         [0031]    The photoconductor  51  corresponds to an example of an image carrier of the present invention. 
         [0032]    The rotary developing device  60  corresponds to an example of a rotary developing device in the image forming apparatus of the present invention. The rotary developing device  60  also corresponds to a rotary developing device according to a first exemplary embodiment of the present invention. 
         [0033]    In the rotary developing device  60 , four developing units and two dummy developing units, which will be described below, are supported on a revolution orbit surrounding a rotation shaft  61  of the rotary developing device  60 . The rotary developing device  60  rotates in a direction of arrow E to revolve the developing units and the dummy developing units around the rotation shaft  61  so that any one of the four developing units is moved to a position opposing the photoconductor  51 . An electrostatic latent image formed on the photoconductor  51  is developed into a toner image by the developing unit opposing the photoconductor  51 . 
         [0034]    The four developing units supported in the rotary developing device  60  contain yellow (Y) toner, magenta (M) toner, cyan (C) toner, and black (K) toner. When developing the electrostatic latent image on the photoconductor  51 , the developing unit that contains color toner to be currently used is moved to the position opposing the photoconductor  51 . The developing unit opposing the photoconductor  51  performs development with the color toner contained therein. 
         [0035]    Below the rotary developing device  60 , a catch pan  65  extends along the revolution orbit of the developing units and the dummy developing units. The catch pan  65  receives falling foreign substances such as developer from the developing units and below-described foreign metallic substances. The catch pan  65  corresponds to an example of a foreign-substance receiving plate of the present invention. 
         [0036]    Operation of the rotary developing device  60  and the catch pan  65  will be described in detail below with reference to other drawings. 
         [0037]    A toner image formed on the photoconductor  51  by development with the developing unit is transferred onto an intermediate transfer belt  71  by the action of the transfer unit  57 . 
         [0038]    The surface of the photoconductor  51  is charged to a positive polarity by transfer with the transfer unit  57 , and a history of the positive polarity is made thereon. After transfer, the history of the positive polarity is removed by the charging controller  54 . After the history is removed, toner remaining on the photoconductor  51  after transfer is removed by the cleaner  55 , and the charge is eliminated from the photoconductor  51  by the charge eliminator  56 . 
         [0039]    The intermediate transfer belt  71  is an endless belt stretched on a plurality of rollers  72 . The intermediate transfer belt  71  circulates in a direction of arrow F. On a side of a transport path of the sheet P opposite the intermediate transfer belt  71 , a transfer unit  73  is provided. On a downstream side of the transfer unit  73  in a circulation direction of the intermediate transfer belt  71 , a cleaner  74  is provided to remove toner remaining on the intermediate transfer belt  71  after transfer. The transfer unit  73  and the cleaner  74  are movable closer to and away from the intermediate transfer belt  71 . To form a multicolor image, the transfer unit  73  and the cleaner  74  are separated from the intermediate transfer belt  71 . Then, a process of forming a toner image of one certain color on the photoconductor  51  and transferring the toner image onto the intermediate transfer belt  71  is repeated corresponding to a plurality of developing units (a plurality of colors of toner) while rotating the rotary developing device  60 . Thus, a plurality of toner images of a plurality of color toners are sequentially superimposed and transferred on the intermediate transfer belt  71 . 
         [0040]    After that, the transfer unit  73  is brought into contact with the intermediate transfer belt  71 , and a sheet P is supplied from the standby rollers  35  so as to reach a transfer position, where the transfer unit  73  is located, when the superimposed color toner images reach the transfer position. At the transfer position, the color toner images on the intermediate transfer belt  71  are transferred onto the sheet P by the action of the transfer unit  73 . The sheet P on which the toner images are transferred is further transported in a direction of arrow G, and is heated and pressurized by a fixing unit  75 , so that an image is formed by the fixed toner images on the sheet P. The fixing unit  75  corresponds to an example of a fixing unit of the present invention. 
         [0041]    The above-described catch pan  65  is provided between the rotary developing device  60  and the fixing unit  75 , and also serves to prevent heat generated by the fixing unit  75  during fixing from being transmitted to the rotary developing device  60 . This prevents the developer in the developing units of the rotary developing device  60  from sticking owing to heat generated during fixing. 
         [0042]    The sheet P that has passed through the fixing unit  75  is further transported in a direction of arrow H, and is output onto a sheet output table  23 . 
         [0043]    The cleaner  74  is also moved into contact with the intermediate transfer belt  71 . The cleaner  74  removes residual toner from the intermediate transfer belt  71  after transfer using the transfer unit  73 . 
         [0044]    The image forming apparatus  1  form images on both surfaces of a sheet P. To form images on both surfaces of the sheet P, after an image is formed on only one surface of the sheet P, the transport direction of the sheet P is switched by a guide member  36 , and the sheet P is transported by transport rollers  37  in a direction of arrow I, instead of being output onto the sheet output table  23 , as described above. After that, the transport direction is reversed to a direction of arrow J, and the sheet P is transported in a direction of arrow K by being guided by another guide member  38 . Then, the sheet P is further transported by transport rollers  39 , and reaches the standby rollers  35 . 
         [0045]    Subsequently, an image is formed on a second surface of the sheet P in a manner similar to the above. The sheet P having images on both surfaces is output onto the sheet output table  23 . 
         [0046]    Next, the operation of the rotary developing device  60  and the catch pan  65  will be described in detail. 
         [0047]      FIG. 2  is an enlarged view of the rotary developing device  60 , the catch pan  65 , and their surroundings illustrated in  FIG. 1 . 
         [0048]    Besides the rotary developing device  60  and the catch pan  65 ,  FIG. 2  also illustrates the photoconductor  51 , the charging unit  52 , the exposure unit  53 , the charging controller  54 , the cleaner  55  for the photoconductor  51 , the charge eliminator  56 , the intermediate transfer belt  71 , and the cleaner  74  for the intermediate transfer belt  71  that are also illustrated in  FIG. 1 . 
         [0049]    In the rotary developing device  60 , developing units  62 Y,  62 M,  62 C, and  62 K corresponding to four colors Y, M, C, and K are supported on the revolution orbit surrounding the rotation shaft  61 . These four developing units  62 Y,  62 M,  62 C, and  62 K have the same structure except in the toner to be used. Here, the structure of the developing unit  62 Y, of the four developing units  62 Y,  62 M,  62 C, and  62 K, will be described as an example. 
         [0050]    The developing unit  62 Y includes a case  621  that contains developer formed by toner and magnetic carriers. The developing unit  62 Y also includes, in the case  621 , two augers  622  and a developing roller  623  for carrying the developer to an opposing position opposing the photoconductor  51 . In the case  621 , the toner is charged by friction against the magnetic carriers while being agitated by the two augers  622 . Thus, the toner in the developer is electrostatically attached onto the magnetic carriers. 
         [0051]    The developing roller  623  has an unillustrated magnet therein. The developer is attracted toward the developing roller  623  by the magnet and is attached onto a surface of the developing roller  623  in a state in which the toner remains attached to the magnetic carriers. When the rotary developing device  60  rotates in the direction of arrow E to revolve the developing unit  62 Y to the opposing position opposing the photoconductor  51 , the developing roller  623  of the developing unit  62 Y meets the photoconductor  51  at that position. As a result, the developer is carried to the opposing position. The electrostatic latent image on the surface of the photoconductor  51  is developed with the toner in the developer carried to the opposing position. 
         [0052]    The four developing units  62 Y,  62 M,  62 C, and  62 K supported in the rotary developing device  60  and having the above-described structure correspond to examples of a plurality of developing units of the present invention. Further, the case  621  of each developing unit corresponds to an example of a developer container of the present invention. The developing roller  623  in each developing unit corresponds to an example of a developing roller of the present invention. 
         [0053]    Two dummy developing units  63  are further supported in the rotary developing device  60 . The dummy developing units  63  are supported on the revolution orbit, where the four developing units  62 Y,  62 M,  62 C, and  62 K are located, and between the Y-color developing unit  62 Y and the K-color developing unit  62 K. 
         [0054]    While the dummy developing units  63  are not used for development, they have a structure equivalent to the structure of the developing units  62 Y,  62 M,  62 C, and  62 K. That is, each of the dummy developing units  63  also includes a case  631 , two augers  632 , and a developing roller  633  having a magnet therein. However, the case  631  of the dummy developing unit  63  does not contain developer, and the two augers  632  and the developing roller  633  move idly. Further, the position of the developing roller  633  in the dummy developing unit  63  is slightly closer to the inner side of the case  631  than the position of the developing roller in the developing unit used for development. The position of the developing roller  633  in the dummy developing unit  63  will be described in detail below. 
         [0055]    In general, foreign metallic substances, such as spatter powder due to welding and screw chips, are frequently produced during production of an image forming apparatus. Although these foreign metallic substances are cleaned off during production, they are sometimes not sufficiently cleaned off, but remain in the image forming apparatus. Such remaining foreign metallic substances may fall owing to vibration occurring during transport and operation of the image forming apparatus. 
         [0056]    In the image forming apparatus  1  of the first exemplary embodiment, the catch pan  65  is provided below the rotary developing device  60 . The catch pan  65  receives developer from the rotary developing device  60  and the above-described falling foreign metallic substances. 
         [0057]    Supposing foreign metallic substances received by the catch pan  65  are magnetically attracted by the developing roller of the developing unit in the rotary developing device  60  and are attached to the surface of the developing roller, the developing roller may scrape the surface of the photoconductor  51  when opposing the photoconductor  51  during development. 
         [0058]    In the first exemplary embodiment, the developing rollers  633  with the magnets in the two dummy developing units  63  provided in the rotary developing device  60  serve to magnetically collect foreign metallic substances on the catch pan  65 . The dummy developing units  63  correspond to an example of a foreign-substance collecting member of the present invention. The dummy developing units  63  also correspond to an example of a dummy developing unit of the present invention. 
         [0059]    Since each dummy developing unit  63  in the rotary developing device  60  has a structure equivalent to the structure of the developing units used for development, a support structure for the dummy developing unit  63  is also equivalent to a support structure for the developing units used for development. For this reason, if image formation is performed using some special color in addition to the four colors Y, M, C, and K in future, the dummy developing unit  63  may be replaced with a developing unit for the special color. 
         [0060]    In addition, in the first exemplary embodiment, a home position where the rotary developing device  60  stays from when an operation is completed and to when the next operation starts is set as follows. That is, in the first exemplary embodiment, the home position of the rotary developing device  60  is set at a position where the two dummy developing units  63  oppose the catch pan  65 . For this reason, in the first exemplary embodiment, operation of the rotary developing device  60  is started in a state in which foreign metallic substances on the catch pan  65  are collected by the dummy developing units  63 . 
         [0061]      FIG. 2  illustrates the rotary developing device  60  located at the home position. 
         [0062]    The home position is not limited to the position where the dummy developing units  63  oppose the catch pan  65  as in the first exemplary embodiment. The home position may be a position where the dummy developing units  63  are located on an upstream side of the position opposing the catch pan  65  in the rotating direction of the rotary developing device  60 . In this case, however, the dummy developing units  63  are positioned on the above-described upstream side so that the K-color developing unit  62 K downstream of the dummy developing units  63  does not oppose the catch pan  65 . That is, in this case, the home position is a position where the dummy developing units  63  meets the catch pan  65  earlier than any of the developing units used for development after operation of the rotary developing device  60  starts. 
         [0063]    The rotary developing device  60  starts rotation from the above-described home position, and temporarily stops every time any of the developing units  62 Y,  62 M,  62 C, and  62 K and the dummy developing units  63  meets the photoconductor  51 . 
         [0064]    When the rotary developing device  60  stops at the position where any of the developing units  62 Y,  62 M,  62 C, and  62 K opposes the photoconductor  51 , toner is supplied from the opposing developing unit to the photoconductor  51  to develop an electrostatic latent image. After stopping for a predetermined development time, the rotary developing device  60  restarts rotation. 
         [0065]    When the rotary developing device  60  stops at the position where any of the two dummy developing units  63  opposes the photoconductor  51 , the developing rollers  633  in the dummy developing units  63  move idly. In this case, the rotary developing device  60  also restarts rotation after stopping for the above-described development time. 
         [0066]    In this way, the rotary developing device  60  repeats the operations of temporarily stopping every time any developing unit meets the photoconductor  51  and restarting rotation, and then stops at the home position again. 
         [0067]    In the first exemplary embodiment, when the rotary developing device  60  performs the first operation after power-on of the image forming apparatus  1 , it rotates while switching the rotation speed as follows. 
         [0068]    For example, the first operation after power-on refers to the first operation after power-on of the image forming apparatus  1  that has been subjected to handling such as transport associated with vibration which easily causes fall of foreign metallic substances in the image forming apparatus  1 . In the first operation, a lot of foreign metallic substances may exist on the catch pan  65 . The below-described switch of the rotation speed is performed on the assumption of such possibility. 
         [0069]    In the first operation, the rotary developing device  60  rotates at a first rotation speed from when it starts rotation from the home position to when any of the four developing units  62 Y,  62 M,  62 C, and  62 K first meets the photoconductor  51 . The first rotation speed is lower than a second rotation speed at which the rotary developing device  60  rotates until it stops at the home position again after the developing unit first meets the photoconductor  51 . 
         [0070]    In the first exemplary embodiment, while the rotary developing device  60  starts rotation from the home position and rotates at the lower first rotation speed, the two dummy developing units  63  slowly pass over the catch pan  65 . Thus, even if there are a lot of foreign metallic substances on the catch pan  65 , the foreign metallic substances are collected over time by the slowly moving dummy developing units  63 . 
         [0071]    After any of the developing units first meets the photoconductor  51 , the rotation speed is switched to the higher second rotation speed, and the rotary developing device  60  rotates at the second rotation speed for development. 
         [0072]    In the second and subsequent operations, the rotary developing device  60  starts rotation from the home position, and consistently rotates at the second rotation speed until it returns to the home position again. 
         [0073]    The above-described rotating operation of the rotary developing device  60  is performed according to instructions from the processing control circuit  21  illustrated in  FIG. 1 . 
         [0074]    Instead of being performed in the first operation after power-on as in the first exemplary embodiment, for example, the above-described switching of the rotation speed of the rotary developing device  60  may be performed every time rotation starts from the home position after power-on. Alternatively, such switching of the rotation speed may be performed in the first operation after an event different from power-on, such as replacement of the development unit, occurs, or may be performed every time rotation starts from the home position after the event. 
         [0075]    As described above, the developing roller  633  in each dummy developing unit  63  is slightly closer to the inner side of the case  631  than the developing roller in the developing unit used for development. As a result, an opposing distance between the developing roller  633  in the dummy developing unit  63  and the photoconductor  51  when the developing roller  633  opposes the photoconductor  51  is longer than an opposing distance between the developing roller in the developing unit used for development and the photoconductor  51  when the developing roller opposes the photoconductor  51 . 
         [0076]      FIGS. 3A and 3B  respectively illustrate the opposing distance between the photoconductor and the developing roller in the developing unit used for development and the opposing distance between the photoconductor and the developing roller in the dummy developing unit. 
         [0077]      FIG. 3A  illustrates the opposing distance between the photoconductor  51  and the developing roller  623  in the developing unit used for development. As illustrated in  FIG. 3A , the developing roller  623  is located at a position in the case such that the opposing distance is 290 μm. The opposing distance of 290 μm allows the toner to be smoothly transferred from the developing unit onto the photoconductor  51 . 
         [0078]      FIG. 3B  illustrates the opposing distance between the photoconductor  51  and the developing roller  633  in the dummy developing unit  63 . As illustrated in  FIG. 3B , in the dummy developing unit  63 , the developing roller  633  is located at a position in the case  631  such that the opposing distance is 1000 μm. The position of the developing roller  633  in the case  631  of the dummy developing unit  63  is closer by 710 μm to the inner side of the case  631  than the position of the developing roller in the developing unit used for development. 
         [0079]    In the first embodiment, it is assumed that foreign metallic substances remaining without being completely cleaned off during production of the image forming apparatus  1  have a size less than 1000 μm. 
         [0080]    In the first exemplary embodiment, the opposing distance between the photoconductor  51  and the developing roller  633  in the dummy developing unit  63  is set to be 1000 μm so that foreign metallic substances of the assumed size sufficiently separate from the surface of the photoconductor  51 . 
         [0081]    In the first exemplary embodiment, since the developing roller  633  in the dummy developing unit  63  is located closer to the inner side of the case  631 , the opposing distance between the photoconductor  51  and the developing roller  633  is longer than the opposing distance between the photoconductor  51  and the developing roller in the developing unit used for development. However, the opposing distance between the photoconductor and the developing roller in the dummy developing unit is not increased only by shifting the developing roller to the inner side of the case as in the first exemplary embodiment. For example, the opposing distance may be increased by setting the diameter of the developing roller in the dummy developing unit to be less than the diameter of the developing roller in the developing unit used for development. 
         [0082]    Next, a second exemplary embodiment will be described. 
         [0083]    The second exemplary embodiment is equivalent to the first exemplary embodiment except that a member for collecting foreign metallic substances is different from the above-described dummy developing units  63 . The second exemplary embodiment will be described below with attention to differences from the first exemplary embodiment. Since an overall configuration of an image forming apparatus according to the second exemplary embodiment is equivalent to that of the image forming apparatus  1  of the first exemplary embodiment, illustration and description thereof are skipped. 
         [0084]      FIG. 4  is an enlarged view of a rotary developing device  80 , a catch pan  65 , and their surroundings in the image forming apparatus of the second exemplary embodiment. 
         [0085]    In  FIG. 4 , constituents equivalent to the constituents of the image forming apparatus  1  of the first exemplary embodiment illustrated in  FIG. 2  are denoted by the same reference numerals as those in  FIG. 2 . Hereinafter, redundant descriptions of the equivalent constituents are skipped. 
         [0086]    In the rotary developing device  80  of the second exemplary embodiment, a permanent magnet  81  is supported on a revolution orbit on which four developing units  62 Y,  62 M,  62 C, and  62 K are provided, and between the Y-color developing unit  62 Y and the K-color developing unit  62 K. In the second exemplary embodiment, the permanent magnet  81  has magnetism that is strong to an extent such as not to hinder the operation of the rotary developing device  80  by magnetic interference with peripheral metal structures. In the second exemplary embodiment, the permanent magnet  81  thus having strong magnetism collects foreign metallic substances on the catch pan  65 . 
         [0087]    The permanent magnet  81  is supported at a position such that an opposing distance between the permanent magnet  81  and a photoconductor  51  is longer than an opposing distance between developing rollers in the developing units and the photoconductor  51 . 
         [0088]    The permanent magnet  81  also corresponds to an example of a foreign-substance collecting member of the present invention. The permanent magnet  81  further corresponds to an example of a permanent magnet of the present invention. 
         [0089]    In the second exemplary embodiment, a home position of the rotary developing device  80  is set at a position where the permanent magnet  81  opposes the catch pan  65 . 
         [0090]    The home position is not limited to the position where the permanent magnet  81  opposes the catch pan  65  as in the second exemplary embodiment. The home position may be a position where the permanent magnet  81  meets the catch pan  65  earlier than any of the developing units after the rotary developing device  80  starts operation. 
         [0091]    In the second exemplary embodiment, the operation of the rotary developing device  80  starts in a state in which foreign metallic substances on the catch pan  65  are collected by the permanent magnet  81 . Further, in the second exemplary embodiment, the rotary developing device  80  also rotates while switching the rotation speed in a first operation after power-on, similarly to the rotary developing device  60  of the first exemplary embodiment. 
         [0092]    While the color copying machine  1  is given as an example of an image forming apparatus in the above-described first and second exemplary embodiments, the image forming apparatus may be, for example, a color printer or a color facsimile machine. 
         [0093]    The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.