Abstract:
An image forming apparatus includes: an image carrier; a developing device that supplies one of plural color developers to the image carrier and forms a developer image; a transfer unit that transfers the developer image to a recording medium; a fixing unit that fixes the developer image; a transport path for transporting the recording medium from a supply position to an exit port; a return path for transporting the recording medium subjected to the fixing process to a position upstream of the transfer unit for performing subsequent processes on a developer image of another color developer; and a plane, including a junction of the transport path and the return path and a center of rotation of the developing device, being provided at a 90°—or wider angle with respect to a plane including a transfer position and the center of rotation of the developing device, on an upstream side of the another plane in a transport direction of the recording medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-116750 filed Apr. 28, 2008, 
       BACKGROUND 
     Technical Field 
       [0002]    The present invention relates to an image forming apparatus. 
       SUMMARY 
       [0003]    An aspect of the present invention is an image forming apparatus including: an image carrier; a developing device that supplies one of plural color developers to the image carrier by rotating and visualizes an electrostatic latent image carried on the image carrier to a developer image; a transfer unit that transfers the developer image carried on the image carrier to a recording medium; a fixing unit that fixes the developer image transferred by the transfer unit to the recording medium; a transport path for transporting the recording medium from a supply position through the transfer unit and the fixing unit to an exit port; a return path for transporting the recording medium to which the developer image has been fixed by the fixing unit to a position upstream of the transfer unit on the transport path for performing subsequent transfer and fixing processes on a developer image of another one of the plural color developers formed by the developing device, the transport path and the return path being provided to circulate around the developing device; and a plane, including a junction of the transport path and the return path and a center of rotation of the developing device, being provided at a 90°—or wider angle with respect to another plane including a transfer position for transfer of the developer image to the recording medium by the transfer unit and the center of rotation of the developing device, on an upstream side of the another plane in a transport direction of the recording medium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
           [0005]      FIG. 1  is a cross-sectional view schematically showing the structure of an image forming apparatus according to an exemplary embodiment of the present invention; 
           [0006]      FIG. 2  is an explanatory view schematically showing correction of a transporting direction of a recording medium with a correction roller; 
           [0007]      FIG. 3  schematically illustrates the positional relation between a circular path and its peripheral elements in the image forming apparatus shown on  FIG. 1 ; 
           [0008]      FIG. 4  is a cross-sectional view schematically showing the structure of a modification of the image forming apparatus according to the exemplary embodiment of the present invention; and 
           [0009]      FIG. 5  schematically illustrates the positional relation between the circular path and its peripheral elements in the modification of the image forming apparatus shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Next, an exemplary embodiment of the present invention will be described based on the drawings. 
         [0011]      FIG. 1  schematically shows the structure of an image forming apparatus  10  according to an exemplary embodiment of the present invention. The image forming apparatus  10  has an image forming apparatus main body  12 , and e.g. a sheet supply unit  14  is provided on the front side of the image forming apparatus main body  12  (right side in  FIG. 1 ). Note that the sheet supply unit  14  may be integrally provided in the image forming apparatus main body  12 , or may be attached/removed to/from the image forming apparatus main body  12 . Further, a manual sheet feed part in place of the sheet supply unit  14  may be provided on the front side of the image forming apparatus main body  12 . 
         [0012]    The sheet supply unit  14  is provided with a pickup roller  16  to pick up a recording medium such as a recording sheet from the sheet supply unit  14  on the image forming apparatus main body  12  side. The pickup roller  16 , driven by a driving part (not shown), is rotated to supply the recording medium toward a transport path  18 . 
         [0013]    The transport path  18  is a recording medium path provided with guide members  20  and  22  to guide a recording medium from the pickup roller  16  to an exit port  24 . The transport path  18  has a part approximately vertically extending on the rear side of the image forming apparatus main body  12  (left side  FIG. 1 ). On the transport path  18 , an image carrier  62  and a transfer roller  78  to be described later are provided on the upstream side of a fixing device  28 , and a registration roller (timing roller)  30  is provided on the upstream side of the image carrier  62  and the transfer roller  78 . A sensor  31  which is e.g. a line sensor to detect a length, a width and passing timing of the recording medium, is provided on the upstream side of the registration roller  30 , and a correction roller  32   a  and a guide member  32   b  are provided on the upstream side of the sensor  31 . As shown in  FIG. 2 , the correction roller  32   a  transports the recording medium toward the guide member  32   b  such that a side edge of the recording medium is in contact with the guide member  32   b.  The correction roller  32   a  moves the recording medium along the guide member  32   b  thereby corrects a transporting direction of the recording medium. Further, an exit roller  33  is provided in the vicinity of the exit port  24  on the transport path  18 . 
         [0014]    Accordingly, the recording medium picked up with the pickup roller  16  from the sheet supply unit  14  is guided to the transport path  18 , transported with the correction roller  32   a  while the transporting direction is corrected, its length, width and the like are detected by the sensor  31 , and the recording medium is temporarily stopped with the registration roller  30 . Then, e.g. a black color developer image is transferred in a transfer position between the image carrier  62  and the transfer roller  78  to be described later, at predetermined timing. The transferred black developer image is fixed by the fixing device  28 , and the recording medium is output with the exit roller  33  from the exit port  24 . 
         [0015]    Note that in the case of color printing, the recording medium is returned to a return path three times. That is, the transport path  18  is branched to two paths on the downstream side of the exit roller  33 , a switching device  34  is provided in the branching portion, and a return path  36  returned from the branching portion again to the transport path  18  is formed. The return path  36  has a part approximately vertically extending on the front side of the image forming apparatus main body  12  (right side in  FIG. 1 ). The return path  36  is provided with a transport roller  42 , and a sensor  44  to detect the length of the recording medium is provided on the upstream side of the transport roller  42  on the return path  36 . In the case of color printing, the switching device  34  is switched to the side to open the return path  36 , then the recording medium is guided to the return path  36 , then the length of the recording medium is detected by the sensor  44 . The recording medium is transported with the correction roller  32   a  while the transporting direction is corrected, and the width of the recording medium is detected by the sensor  31 . The recording medium is passed through the registration roller  30 , the transfer position between the image carrier  62  and the transfer roller  78  and the fixing device  28  four times, and is output from the exit sort  24 . 
         [0016]    Note that the transport path  18  and the return path  36  form a circular path  50  to transport the recording medium passed through the fixing device  28  again to the fixing device  28 . 
         [0017]    The image forming apparatus main body  12  is provided with a rotary developing device  52  in e.g. an approximately central position. The rotary developing device  52 , having developing units  56   a,    56   b,    56   c  and  56   d  respectively forming yellow, magenta, cyan and black color toner images in a developing device main body  54 , rotates counterclockwise about a center of the rotary developing device  58  as a center of rotation. The developing units  56   a,    56   b,    56   c  and  56   d  have developing rollers  60   a,    60   b,    60   c  and  60   d.    
         [0018]    The image carrier  62  which is e.g. a photoreceptor is provided in contact with the rotary developing device  52 , and a part of the periphery of the respective developing rollers  60   a,    60   b,    60   c  and  60   d,  not in contact with the image carrier  62 , is projected from the outer periphery of the developing device main body  54  in a radial direction by e.g. 2 mm. Further, tracking rollers (not shown), having a diameter slightly larger than that of the developing rollers  60   a,    60   b,    60   c  and  60   d,  are provided on both ends of each of the developing rollers  60   a,    60   b,    60   c  and  60   d,  such that the tracking rollers are rotated coaxially with the developing rollers  60   a,    60   b,    60   c  and  60   d.  That is, the tracking rollers of the developing rollers  60   a,    60   b,    60   c  and  60   d  are in contact with flanges (not shown) provided on both ends of the image carrier  62 , thereby a predetermined gap is formed between the developing rollers  60   a,    60   b,    60   c  and  60   d  and the image carrier  62 , and a latent image on the image carrier  62  is developed with respective color developers. 
         [0019]    A charging device  64  which is e.g. a charging roller to uniformly charge the image carrier  62  is provided above the image carrier  62 . Further, an image carrier cleaner  66  is in contact with the image carrier  62  on the upstream side of the charging device  64  in a rotation direction of the image carrier  62 . The image carrier cleaner  66  scrapes off a developer remaining on the image carrier  62  after e.g. first transfer. 
         [0020]    Note that the developer contained in the developing units  56   a,    56   b,    56   c  and  56   d  is e.g. a two-component developer including non-magnetic toner and magnetic carrier in which the non-magnetic toner is attached to the periphery of the magnetic carrier. 
         [0021]    Hereinbelow, when a description is made using one of plural constituent elements such as the developing units  56   a,    56   b,    56   c  and  56   d  as a representative unit, it will be simply referred to as e.g. a “developer  56 ”. 
         [0022]    An optical writing device  76  to write a latent image with a light ay such as a laser beam on the image carrier  62  charged by the charging device  64  is provided above the rotary developing device  52 . Further, the transfer roller  78  to transfer a developer image visualized by the rotary developing device  52  to the recording medium in the transfer position is provided on the rear side of the image carrier  62 . 
         [0023]    The fixing device  28  is provided on the downstream side of the transfer position on the transport path  18 . The fixing device  28 , having a heating roller  80  and a pressure roller  82 , applies heat and pressure to the developer image transferred to the recording medium with the image carrier  62  and the transfer roller  78 , thereby fixes the developer image to the recording medium upon each transfer, and transports the recording medium. 
         [0024]    Further, the image forming apparatus  10  includes a controller  84  having a CPU and a memory (neither shown), to control the respective constituent elements of the image forming apparatus  10  and control the shape and position of a developer image transferred to a recording medium. 
         [0025]    Next, the positional relation between the circular path  50  on which a recording medium is transported and its peripheral elements will be described in detail. 
         [0026]      FIG. 3  schematically illustrates the positional relation between the circular path  50  and its peripheral elements in the image forming apparatus  10  shown in  FIG. 1 . 
         [0027]    As described above, the transport path  18  is a recording medium path on which a recoding medium is transported from the pickup roller  16  (point A) through the transfer position (point B) between the image carrier  62  and the transfer roller  78  to the exit tort  24  (point C) in one direction. The return path  36  is a recording medium path on which the recording medium is transported from a branching point in which the switching device  34  is provided and the recording medium path is branched to two paths (point D) to a position in which the branched recording medium path is combined with the transport path  18  (point E) in one direction. Further, the circular path  50  is formed such that the recording medium is transported from the junction point (point E) between the transport path  18  and the return path  36  through the transfer position (point B) and the branching point (point D), again to the junction point (point E). 
         [0028]    The junction point (point E) is provided such that a plane including the junction point (point E) and the center of the rotary developing device  58  (point F) is set at a 90°—or wider angle with respect to a plane including the transfer position (point B) and the center of the rotary developing device  58  (point F) from the transfer position (point B) on the upstream side in the transporting direction of the recording medium transported on the transport path  18 . 
         [0029]    The registration roller  30  is provided such that a plane including a rotation axis of the registration roller  30  and the center of the rotary developing device  58  (point F) is set at a 90°—or narrower angle with respect to a plane including the transfer position (point B) and the center of the rotary developing device  58  (point F) from the transfer position (point B) on the upstream side in the transporting direction of the recording medium transported on the transport path  18 . 
         [0030]    The correction roller  32   a  and the guide member  32   b  are provided between the registration roller  30  and the junction point (point E). The sensor  31  is provided on the upstream side of the registration roller  30  on the transport path  18  and on the downstream side of the correction roller  32   a  and the guide member  32   b  on the transport path  18 . 
         [0031]    Next, the operation (color printing) of the exemplary embodiment of the present invention will be described. 
         [0032]    When an image forming signal is sent, the image carrier  62  is uniformly charged by the charging device  64 , and a light ray corresponding to a yellow image is emitted based on the image signal from the optical writing device  76  to the charged image carrier  62 . The light ray from the optical writing device  76  exposes the surface of the image carrier  62 , thus an electrostatic latent image is formed. 
         [0033]    The electrostatic latent image carried on the image carrier  62  is developed with the yellow developer supplied to the developing roller  60   a  of the rotary developing device  52 , and transferred to a recording medium supplied from the sheet supply unit  14 . 
         [0034]    Note that the recording medium supplied from the sheet supply unit  14  is transported with the correction roller  32   a  toward the guide member  32   b  thereby the transporting direction is corrected. The length (initial length), width and passing timing of the recording medium are detected by the sensor  31 , and the recording medium is transported toward a position between the Image carrier  63  and the transfer roller  78  at predetermined timing. 
         [0035]    The recording medium on which the yellow developer image has been transferred is guided to the fixing device  28 . Then heat and pressure are applied to the recording medium with the heating roller  80  and the pressure roller  82 , thereby the developer image is fixed to the recording medium. The recording medium to which the yellow developer image is fixed is guided by the switching device  34  to the return path  36 , then the length contracted by the heat of the fixing device  28  is detected by the sensor  44 , and again returned to the transport path  18 . The developer remaining on the image carrier  62  is removed by the image carrier cleaner  66 . 
         [0036]    The recording medium returned to the transport path  18  is transported with the correction roller  32   a  toward the guide member  32   b,  thereby the transporting direction is corrected, then the width and the passing timing of the recording medium are detected by the sensor  31 , and the recording medium is transported toward the position between the image carrier  62  and the transfer roller  78  at predetermined timing. 
         [0037]    Then, the developing device main body  54  of the rotary developing device  52  is rotated, then the image carrier  62  is uniformly charged by the charging device  64 , and a light ray corresponding to a magenta image is emitted based on the image signal from the optical writing device  76  to he charged image carrier  62 . For example, the controller  84  controls the shape (length and width) of an electrostatic latent image and a writing position of the electrostatic latent image by the optical writing device  76 , and the like, based on the result of detection by the sensor  44  and the sensor  31 . The light ray from the optical writing device  76  exposes the surface of the image carrier  62 , thus an electrostatic latent image is formed. 
         [0038]    The electrostatic latent image carried on the image carrier  62  is developed with the magenta developer supplied to the developing roller  60   b  of the rotary developing device  52 , and overlay-transferred to the recording medium returned to the transport path  18 . The recording medium on which the magenta developer image has been transferred is guided to the fixing device  28 , and the developer image is fixed to the recording medium with the heating roller  80  and the pressure roller  82 . The recording medium on which the magenta developer image has been fixed is guided by the switching device  34  to the return path  36 , and again returned to the transport path  18 . 
         [0039]    The developer remaining on the image carrier  62  is removed by the image carrier cleaner  66 . 
         [0040]    As the recording medium on which an image is formed is returned to the return path  36  three times, as in the case of the yellow and magenta developer images, developer images developed with the cyan and black developers are fixed to the recording medium with the fixing device  28 . Then the recording medium on which a color image formed with overlaid developer images is fixed is output by she switching device  34  from the exit port  24 . 
         [0041]    Next, a modification of the image forming apparatus  10  will be described. 
         [0042]      FIG. 4  schematically shows the structure of the modification of the image forming apparatus  10 . Note that in the modification of the image forming apparatus  10 , constituent elements substantially the same as those of the image forming apparatus  10  shown in  FIG. 1  have the same reference numerals. 
         [0043]    A sheet supply unit  90  is provided in a front-side lower part (right lower side in  FIG. 4 ) of the image forming apparatus main body  12 . The sheet supply unit  90  is provided with a pickup roller  92  to pick up a recording medium from the sheet supply unit  90 . The pickup roller  92 , driven by a driving part (not shown), is rotated to supply a recording medium toward a transport roller  94 . The transport roller  94  transports the recording medium toward the registration roller  30 . 
         [0044]    A transport path  96  is a recording medium path from the pickup roller  92  to the exit port  24 . The transport path  96  has a part approximately vertically extending on the rear side (left side in  FIG. 4 ) of the image forming apparatus main body  12 . A sensor  98  is provided on the upstream side of the registration roller  30  on the transport path  96  and on the downstream side of the transport roller  94  on the transport path  96 . The sensor  98  detects the length, the width and the passing timing of the recording medium. 
         [0045]    A return path  100  is a recording medium path which returns from the branching point ion which the switching device  34  is provided to the transport path  96  again. The return path  100  has a part approximately vertically extending on the front side (right side in  FIG. 4 ) of the image forming apparatus main body  12 . Note that the transport path  96  and the return path  100  form a circular path  102  to transport the recording medium passed through the fixing device  28  again to the fixing device  28 . 
         [0046]    A sensor  104  which is e.g. a line sensor to detect the width and the passing timing of the recording medium is provided on the upstream side of the registration roller  30  on the return path  100 . The correction roller  32   a  and the guide member  32   b  are provided on the upstream side of the sensor  104  on the return path  100 . 
         [0047]    Next, the positional relation between the circular path  102  on which the recording medium is transported and its peripheral elements will be described in detail. 
         [0048]      FIG. 5  schematically illustrates the positional relation between the circular path  102  and its peripheral elements in the modification of the image forming apparatus  10  shown in  FIG. 4 . 
         [0049]    As described above, the transport path  96  is a recording medium path on which a recording medium  4 s transported from the pickup roller  92  (point A) through the transfer position (point B) between the image carrier  62  and the transfer roller  78  to the exit port  24  (point C in  FIG. 2 ) in one direction. The return path  100  is a recording medium path on which the recording medium is transported from the branching point (point D) in which the switching device  34  is provided and the recording medium path is branched to two paths to the position in which the branched path is combined with the transport path  96  (point E) in one direction. The circular path  102  is formed such that the recording medium is transported from the junction point (point E) between the transport path  96  and the return path  100  through the transfer position (point B) and the branching point (point D), again to the junction point (point E). 
         [0050]    The junction point (point E) is provided such that a plane including the junction point (point E) and the center of the rotary developing device  58  (point F) is set at a 90°—or wider angle with respect to a plane including the transfer position (point B) and the center of the rotary developing device  58  (point F) from the transfer position (point B) on the upstream side in the transporting direction of the recording medium transported on the transport path  96 . 
         [0051]    The registration roller  30  is provided such that a plane including the rotation axis of the registration roller  30  and the center of the rotary developing device  58  (point F) is set at a 90°—or narrower angle with respect to a plane including the transfer position (point B) and the center of the rotary developing device  58  (point F) from the transfer position (point B) on the upstream side in the transporting direction of the recording medium transported on the transport path  96 . 
         [0052]    The correction roller  32   a  and the guide member  32   b  are provided between the registration roller  30  and the junction point (point E) on the return path  100 . The sensor  98  is provided on the upstream side of the registration roller  30  on the transport path  96 . The sensor  104  is provided on the upstream side of the registration roller  30  on the return path  100  and on the downstream side of the correction roller  32   a  and the guide member  32   b  on the return path  100 . 
         [0053]    Next, the operation (color printing) of the modification of the image forming apparatus  10  will be described. 
         [0054]    When an image forming signal is sent, the image carrier  62  is uniformly charged by the charging device  64 , and a light ray corresponding to a yellow image is emitted based on the image signal from the optical writing device  76  to the charged image carrier  62 . The light ray from the optical writing device  76  exposes the surface of the image carrier  62 , thus an electrostatic latent image is formed. 
         [0055]    The electrostatic latent image carried on the image carrier  62  is developed with the yellow developer supplied to the developing roller  60   a  of the rotary developing device  52 , and transferred to a recording medium supplied from the sheet supply unit  90 . 
         [0056]    Note that the length (initial length), width and passing timing of the recording medium supplied from the sheet supply unit  90  are detected by the sensor  98  and the recording medium is transported toward the position between the image carrier  62  and the transfer roller  78  at predetermined timing. 
         [0057]    The recording medium on which the yellow developer image has been transferred is guided to the fixing device  28 . Then heat and pressure are applied to the recording medium with the heating roller  80  and the pressure roller  82 , thereby the developer image is fixed to the recording medium. The recording medium to which the yellow developer image is fixed is guided by the switching device  34  to the return path  100 , and then the length contracted by the heat of the fixing device  28  is detected by the sensor  44 . The recording medium is transported with the correction roller  32   a  toward the guide member  32   b,  thereby the transporting direction is corrected Then the width and the passing timing of the recording medium are detected by the sensor  104 , and the recording medium is again returned to the transport path  96 . Then the developer remaining on the image carrier  62  is removed by the image carrier cleaner  66 . 
         [0058]    The recording medium returned to the transport path  96  is transported to the position between the image carrier  62  and the transfer roller  78  at predetermined timing with the registration roller  30 . 
         [0059]    Then, the developing device main body  54  of the rotary developing device  52  is rotated, then the image carrier  62  is uniformly charged by the charging device  64 , and a light ray corresponding to a magenta image is emitted based on the image signal from the optical writing device  76  to the charged image carrier  62 . For example, the controller  84  controls the shape (length and width) of an electrostatic latent image and a writing position of the electrostatic latent image by the optical writing device  76 , and the like, based on the result of detection by the sensor  98 , the sensor  44  and the sensor  104  The light ray from the optical writing device  76  exposes the surface of the image carrier  62 , thereby an electrostatic latent image is formed. 
         [0060]    The electrostatic latent image carried on the image carrier  62  is developed with the magenta developer supplied to the developing roller  6 Db of the rotary developing device  52 , and overlay-transferred to the recording medium returned to the transport path  96 . The recording medium on which the magenta developer image has been transferred is guided to the fixing device  28 , and the developer image is fixed to the recording medium with the heating roller  80  and the pressure roller  82 . The recording medium on which the magenta developer image has been fixed is guided by the switching device  34  to the return path  100 , and again returned to the transport path  96 . 
         [0061]    The developer remaining on the image carrier  62  is removed by the image carrier cleaner  66 . 
         [0062]    As the recording medium on which an image is formed is returned to the return path  36  three times, as in the case of the yellow and magenta developer images, developer images developed with the cyan and black developers are fixed to the recording medium with the fixing device  28 . Then the recording medium on which a color image formed with overlaid developer images is fixed is discharged by the switching device  34  from the exit port  24 . 
         [0063]    In this manner, in the image forming apparatus in which a developer image is fixed plural times to a recording medium circulating around the rotary developing device  52 , since the size of the recording medium differs upon each circulation, it is necessary to accurately detect the size of the recording medium on which a developer image is not transferred, and the positional relation between the supply position of the recording medium and the developer image transfer position is important in image formation precision (prevention of color shift). 
         [0064]    The foregoing description of the exemplary embodiment 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 exemplary embodiment was 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.