Abstract:
A color image forming apparatus of the present invention comprises an intermediate transfer member having a movable surface, a plural image forming units for forming superposed toner images on the intermediate transfer member at a predetermined image forming pitch, and a transfer member for transferring the superposed toner images formed on the intermediate transfer member onto a sheet of paper, wherein the peripheral length of the intermediate transfer member is a non-integral multiple of the image forming pitch.

Description:
RELATED APPLICATION  
         [0001]    The present invention is based on Japanese Patent Application No. 2000-200897, the content of which is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a so-called tandem-type color image forming apparatus.  
           [0004]    2. Description of the Related Art  
           [0005]    So-called tandem-type color image forming apparatuses are conventionally known to have a plurality of image forming units containing toner of different colors arranged along an intermediate transfer belt as disclosed, for example, in Japanese Laid-Open Patent No. H7-28294.  
           [0006]    In this type image forming apparatus, image forming units form an image on an intermediate transfer belt, and this formed image is printed on a sheet of a size desired by a user. The intermediate transfer belt comprises conductive carbon particles dispersed in resin so as to have semi conductivity.  
           [0007]    When images are formed in the same range of the intermediate transfer belt and transferred to paper many times, the degree of dispersion (dispersion state) of the carbon particles in this range changes such that the resistance value and surface condition of the intermediate transfer belt changes in the area between the image area of repeated electrical current flow and the image area without electrical current flow. Image drift may be generated when an image is formed in the intermediate transfer belt range including the border location of the condition-changed areas.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide an image forming apparatus which prevents image drift due to change-of-state of the intermediate transfer member.  
           [0009]    The color image forming apparatus of the present invention comprises an intermediate transfer member having a movable surface, a plural image forming units for forming superposed toner images on the intermediate transfer member at a predetermined image forming pitch, and a transfer member for transferring the superposed toner images formed on the intermediate transfer member onto a sheet of paper, wherein the peripheral length of the intermediate transfer member is a non-integral multiple of the image forming pitch.  
           [0010]    In the image forming apparatus of the present invention, since the length of the intermediate transfer member is a non-integral multiple of the image forming pitch, the image forming range on the intermediate transfer member formed at a predetermined image forming pitch by each image forming unit shifts with each single rotation of the intermediate transfer member. In this way the image forming range can be dispersed on the intermediate transfer member, the range of the state-changed part of the intermediate transfer member caused by repeated image formation in the same range of the intermediate transfer member can be dispersed, and image drift on the sheet to which the image is transferred can be prevented. The state-change of the intermediate transfer member is the change of the surface condition and the resistance vale of the intermediate transfer member. The image forming pitch is desirably a length that matches the maximum sheet length and image interval distance. Furthermore, the intermediate transfer member desirably has semi conductivity.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 shows the general construction of a printer  10  of an embodiment of the present invention;  
         [0012]    [0012]FIG. 2( a ) shows the state of an image I 1  formed on the intermediate transfer belt of FIG. 1;  
         [0013]    [0013]FIG. 2( b ) shows the state of an image I 2  formed on the intermediate transfer belt;  
         [0014]    [0014]FIG. 3( a ) shows the state of an image I 3  formed on the intermediate transfer belt; and  
         [0015]    [0015]FIG. 3( b ) shows the state of an image I 4  formed on the intermediate transfer belt. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    The embodiments of the present invention are described hereinafter with reference to the accompanying drawings. FIG. 1 shows the general construction of a tandem-type digital color printer (hereinafter referred to simply as “printer”)  10  of an embodiment of the present invention.  
         [0017]    The printer  10  is provided with an intermediate transfer belt  12  arranged in the approximate center of the interior part. The intermediate transfer belt  12  is supported by the exterior surfaces of three rollers  14 ,  16 ,  18 , and is rotated in the arrow A direction. The length of the intermediate transfer belt  12  is a non-integral multiple (the non-integral multiple n is 2&lt;n&lt;3 in the present embodiment) of a predetermined image forming pitch L described later, as shown in FIG. 2( a ). The intermediate transfer belt  12  is formed by conductive carbon particles dispersed in resin such as polyimide or the like, and has a semi-conductivity with a resistance value of approximately 10 9  to 10 11  Ω·cm.  
         [0018]    Beneath the bottom horizontal part of the intermediate transfer belt  12  are arranged four image forming unit  20 Y,  20 M,  20 C,  20 K corresponding to each color yellow (Y), magenta (M), cyan (C), black (K) along the intermediate transfer belt  12 .  
         [0019]    Each image forming unit  20 Y,  20 M,  20 C, and  20 K has a photosensitive drum  22 Y,  22 M,  22 C,  22 K, respectively. Arranged around the photosensitive drum  22 Y sequentially in the direction of rotation of the drum are a charger  24 Y for uniformly charging the surface of the photosensitive drum  22 Y, print head  26 Y for forming an electrostatic latent image on the surface of the photosensitive drum  22 Y by optical exposure corresponding to the image data, developing device  28 Y for developing the electrostatic latent image by yellow toner as a toner image, primary transfer roller  30 Y for primary transfer of the toner image formed on the surface of the photosensitive drum onto the intermediate transfer belt  12  by electrostatic attraction, and a cleaner  32 Y for collecting the residual toner from the surface of the photosensitive drum after the primary transfer. Similarly, arranged sequentially around the periphery of the photosensitive drum  22 M in the direction of rotation are charger  24 M, print head  26 M, developing device  28 M develops the electrostatic latent image by magenta toner as a toner image, primary transfer roller  30 M, and cleaner  32 M, arranged around the periphery of the photosensitive drum  22 C are charger  24 C, print head  26 C, developing device  28 C for developing the electrostatic latent image by cyan toner as a toner image, primary transfer roller  30 C, and cleaner  32 C, and arranged around the periphery of the photosensitive drum  22 K are charger  24 K, print head  26 K, developing device  28 K for developing the electrostatic latent image by black toner as a toner image, primary transfer roller  30 K, and cleaner  32 K. The print heads  26 Y,  26 M,  26 C, and  26 K comprise a plurality of LEDs arrayed in the main scan direction parallel to the axial direction of the photosensitive drum.  
         [0020]    A secondary transfer roller  34  presses against the part of the intermediate transfer belt  12  supported by the roller  18 . The nip area formed between the secondary transfer roller  34  and the intermediate transfer belt comprises the secondary transfer region  36 . A transfer voltage is applied to the secondary transfer roller  34 . The toner image formed on the intermediate transfer belt  12  is electrostatically attracted to the sheet of the recording medium transported to the secondary transfer region via the transfer voltage as described later.  
         [0021]    A cleaner  38  presses against the part of the intermediate transfer belt  12  supported by the roller  16 . The cleaner  38  removes the residual toner remaining on the intermediate transfer belt after the secondary transfer, and collects the residual toner in a waste toner hopper  40 .  
         [0022]    A paper cassette  42  is removably installed in the bottom part of the printer  10 . Sheets S stacked in the cassette  42  are fed into the transport path  46  one sheet at a time from the uppermost sheet via the rotation of a take-up roller  44 .  
         [0023]    The transport path  46  extends from the paper cassette  42  through the nip area formed by the pair of timing rollers  48 , secondary transfer region  36 , and fixing unit  50  to a discharge tray  11 .  
         [0024]    A sheet sensor  52  is disposed near the pair of timing rollers  48 . The sheet sensor  52  detects the leading edge of a sheet S fed from the cassette  42  into the transport path  46  at the nip area formed by the pair of timing rollers  48 . When the leading edge of the sheet S is detected by the sheet sensor  52 , the pair of timing rollers  48  temporarily stop rotation, and thereafter the sheet S is transported to the secondary transfer region  36  synchronously with the toner image on the intermediate transfer belt  12 .  
         [0025]    The fixing unit  50  is provided with a fixing belt  60  supported by a pair of rollers  56  and  58  and rotated in the arrow B direction, and a fixing roller  62  pressed against the roller  56  through the fixing belt  60  and driven in rotation in the arrow direction. A nip area formed by the fixing belt  60  and fixing roller  62  through which passes a sheet bearing the toner image of the secondary transfer comprises a fixing region  64 .  
         [0026]    The printer  10  is further provided with a controller not shown in the illustrations. Image signals and the like are input to the controller, and signals are output from the controller to the print head LED drive circuit and the like, such that and image is formed at a predetermined image forming pitch L on the intermediate transfer belt  12  by the image forming units  20 Y,  20 M,  20 C,  20 K as described later. This image forming pitch is determined at a length L matching a maximum sheet size from leading edge to trailing edge, e.g., a sheet length L 1  of A3 size, and an image-interval distance L 2 , as shown in FIG. 2( a ).  
         [0027]    The operation of the printer  10  having the previously described construction is described below.  
         [0028]    When an image signal from an external device (e.g., a personal computer) is input to the image signal processor (not shown) in the printer  10 , a digital image signal is generated by color conversion of the input image signal to yellow, magenta, cyan, and black by the image signal processor, and this digital image signal is transmitted to the print head LED drive circuit. The drive circuit accomplishes exposure by driving the print heads  26 Y,  26 M,  26 C,  26 K to emit light based on the input digital signal. The exposure is accomplished in the sequence of print head  26 Y,  26 M,  26 C,  26 K with respective time differential. In this way electrostatic latent images for each color are formed on the surface of the photosensitive drums  22 Y,  22 M,  22 C,  22 K, respectively.  
         [0029]    The electrostatic latent images formed on the photosensitive drums  22 Y,  22 M,  22 C,  22 K are developed by the developing devices  28 Y,  28 M,  28 C,  28 K, respectively, to form a toner image of each color. Then, is sequentially overlaid from the leading edge position “a” of the image forming region T 1  on the intermediate transfer belt  12  moving in the arrow A direction, and transferred in a primary transfer so as to form a toner image I 1  via the operation of the primary transfer rollers  30 Y,  30 M,  30 C,  30 K.  
         [0030]    In this way the overlaid toner image I 1  formed on the intermediate transfer belt  12  reaches the secondary transfer region  36  in conjunction with the movement of the intermediate transfer belt  12 . In the secondary transfer belt  12 , the overlaid toner image I 1  is batch transferred in a secondary transfer onto a sheet S fed from the cassette  42  into the transport path  46  and transported by the pair of timing rollers  48 . Residual toner remaining on the intermediate transfer belt  12  after the secondary transfer is collected by the cleaner  38 .  
         [0031]    The sheet S bearing the secondary transferred-toner image I 1  passes through the transport path  46  to the fixing unit  50 , where the toner image I 1  is fixed on the sheet S as the sheet S passes through the fixing region  64 . Then the sheet S is ejected to the discharge tray  11 .  
         [0032]    As shown in FIG. 2( b ), after the toner image I 1  is formed, a toner image I 2  is formed from the leading edge position “b” of an image forming region T 2  positioned on the downstream side of the leading edge position “a” at an image forming pitch L in the direction of movement of the intermediate transfer belt  12 . Then, as shown in FIG. 3( a ), a toner image I 3  is formed from the leading edge position “c” of an image forming region T 3  positioned on the downstream side of the leading edge position “b” at an image forming pitch L.  
         [0033]    Since the length of the intermediate transfer belt  12  is a non-integral multiple of the image forming pitch L, the leading edge position “d” of the image forming region T 4  for forming a toner image T 4  is positioned on the downstream side from the leading edge position ”a” and does not match the leading edge position “a” for forming the toner image T 1 . The image forming region T 4  for forming the toner image T 4  is shifted to the downstream side in the direction of movement of the intermediate transfer belt  12  and does not match the image forming region T 1 . In this way the image forming regions are gradually shifted to the downstream side each completed rotation of the intermediate transfer belt  12 . Therefore, the image forming regions can be dispersed on the surface of the intermediate transfer belt  12 , and the parts of the intermediate transfer belt  12  subject to a change-of-state due to the multiple and repeated formation of images in the same region on the intermediate transfer belt  12 , e.g., parts of the intermediate transfer belt  12  subject to changed resistance value caused by changes in the state of dispersion of carbon particles contained in the intermediate transfer belt  12 , and parts of the intermediate transfer belt  12  subject to changed surface state, can be dispersed to as to prevent image drift and degrading of image quality, and produce excellent images.  
         [0034]    Furthermore, the distance from the primary transfer position of the image forming unit K to the secondary transfer position can be reduced by positioning the image forming units  20 Y,  20 M,  20 C, and  20 K below the intermediate transfer belt. Therefore, the speed of the first image formation can be increased, toner waste is reduced because few toner images are formed on the intermediate transfer belt when the image forming operation is interrupted due to paper jam and the like, and the distance from the secondary transfer position to the fixing device can be easily reduced, which is advantageous relative to small size sheets such as postcards and the like.  
         [0035]    Although the intermediate transfer belt has been described in terms of a belt in the present embodiment, various other forms may be used, such as, for example, a drum.  
         [0036]    In the present embodiment, the image forming units are arranged below the intermediate transfer belt, but the image forming units also may be arranged, for example, above the intermediate transfer belt.  
         [0037]    A printer has been described as an example in the present embodiment, but the present invention is also applicable to other types of image forming apparatuses, including copiers, facsimiles, and multipurpose machines combining a printer and these other devices.  
         [0038]    Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art.  
         [0039]    Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.