Abstract:
A color image forming apparatus of the present invention comprises: an intermediate transfer member being driven to rotate by a driving roller; a plural image forming units for forming superposed toner images on the intermediate transfer member, the plural image forming units being located below the intermediate transfer belt; a transfer member for transferring the superposed toner images formed on the intermediate transfer member onto a sheet of paper in a transfer area; and a pair of rotatable conveyance rollers for conveying the paper to the transfer area in synchronization with the superposed toner images on the intermediate transfer member, wherein a first distance from a image forming position, for the image forming unit located most-downstream in the plural image forming units relative to the movement of the intermediate transfer member, to the transfer area is longer than a second distance from a nipping area formed by the pair of conveyance rollers to the transfer area.

Description:
RELATED APPLICATIONS 
     The present invention is based on Japanese Patent Application No. 2000-203661, each contents of which being incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a so-called tandem-type color image forming apparatus. 
     2. Description of the Related Art 
     As disclosed in U.S. Pat. No. 6,029,033, for example, in a conventional tandem-type color image forming apparatus, multiple image forming units that each house toner of a different color are aligned side by side above the intermediate transfer belt, toner images of different colors are formed on the image carrier, which comprises a photosensitive drum or similar component, of each image forming unit, and a multi-color combination image is formed by sequentially transferring onto the intermediate transfer belt each toner image from each image forming unit (primary transfer), and by then transferring the multi-color combination image on the intermediate transfer belt onto a sheet of recording paper (secondary transfer). Tandem-type image forming apparatuses of this type are receiving attention as color image forming apparatuses capable of high-speed printing. 
     On the other hand, Japanese Laid-Open Patent Application Hei 7-28294 discloses a tandem-type color image forming apparatus in which multiple image forming units that each house toner of a different color are aligned side by side below the intermediate transfer belt. In a tandem-type image forming apparatus in which the image forming units are located below the intermediate transfer belt in this fashion, the distance from the primary transfer position to the secondary transfer position, in regard to the image forming unit that is located furthest downstream in terms of the movement of the intermediate transfer belt, may be reduced relative to said distance in a tandem-type image forming apparatus in which the image forming units are aligned above the intermediate transfer belt. Consequently, an image forming apparatus in which the image forming units are located below the intermediate transfer belt offers the advantages that (1) the speed of first image formation may be increased. Such a construction when the image forming operation is stopped due to a problem such as a paper jam, (2) there are fewer toner images formed on the intermediate transfer belt, enabling the amount of wasted toner to be reduced, (3) it is easy to reduce the distance from the secondary transfer position to the fusing device, and (4) printing on small-size paper such as postcards may be performed. 
     In a tandem-type image forming apparatus of this type, the intermediate transfer belt is rotated using a driving roller. Single-color images are formed by each image forming unit on the intermediate transfer belt, and the combination image is then transferred onto a sheet of paper. The sheet of paper is conveyed to an area at which the combination image is transferred thereto via the rotation of the conveyance rollers. The conveyance rollers are driven to rotate together with the driving roller for the intermediate transfer belt. After the combination image is thermally fixed onto the sheet of paper, the sheet of paper is ejected onto a paper eject tray. 
     However, in a tandem-type image forming apparatus in which the image forming units are located below the intermediate transfer belt, because the load on the conveyance rollers changes from when a sheet of paper is being conveyed by the conveyance rollers rotating to when the sheet of paper has passed through them, the rate of rotation of the driving roller, which is driven by the same driving means as the conveyance rollers, fluctuates, leading to changes in the rate of rotation of the intermediate transfer belt. Where the rate of rotation of the intermediate transfer belt fluctuates, the problem arises that the single-color images formed on the intermediate transfer belt by the image forming units become disturbed, resulting in incomplete overlap of the images of the different colors. 
     SUMMARY 
     An object of the present invention is to provide an image forming apparatus that, even when the rate of rotation of the intermediate transfer belt fluctuates when the sheet of paper passes through the conveyance rollers, does not cause misalignment of the single-color images formed on the intermediate transfer belt. 
     Another object of the present invention is to provide an image forming apparatus in which (1) the speed of first image formation may be increased, (2) the amount of toner wasted in the. event of cessation of an image forming operation due to a problem such as a paper jam may be reduced, and (3) printing on small-size sheets such as postcards is enabled. 
     The image forming apparatus of the present invention comprises: 
     an intermediate transfer belt being supported by a plural rollers including a driving roller and being driven to rotate by the driving roller; 
     a first image forming unit being located below the intermediate transfer belt, and forming a first toner image of a first color on the intermediate transfer belt at a first image forming position; 
     a second image forming unit being located below the intermediate transfer belt and being located downstream of the first image forming unit relative to the movement of the intermediate transfer belt, the second image forming unit forming a second toner image of a second color over the first toner image formed on the intermediate transfer belt at a second image forming position; 
     a third image forming unit being located below the intermediate transfer belt and being located downstream of the second image forming unit relative to the movement of the intermediate transfer belt, the third image forming unit forming a third toner image of a third color over the first and second toner images formed on the intermediate transfer belt at a third image forming position; 
     a fourth image forming unit being located below the intermediate transfer belt and being located downstream of the third image forming unit relative to the movement of the intermediate transfer belt, the fourth image forming unit forming a fourth toner image of a fourth color over the first, second and third toner images formed on the intermediate transfer belt at a fourth image forming position; 
     a transfer roller for transferring the superposed toner images formed on the intermediate transfer belt onto a sheet of paper in a transfer area, the transfer roller being disposed so as to press against the part of the intermediate transfer belt supported by the driving roller; and 
     a pair of rotatable conveyance rollers for conveying the paper to the transfer area in synchronization with the superposed toner images on the intermediate transfer belt, 
     wherein a first distance from the fourth image forming position for the fourth image forming unit to the transfer area is longer than a second distance from a nipping area formed by the pair of conveyance rollers to the transfer area. 
     In the image forming apparatus of the present invention, the sheet of paper is conveyed by the conveyance rollers such that the tip of the sheet arrives at the transfer area when the tip of the image formed by the fourth image forming unit reaches the transfer area. The rotational torque supplied by the roller driving device to the driving roller changes when the conveyance of the sheet by the conveyance rollers is completed, and at the same time the rate of rotation of the intermediate transfer belt also changes. Because the fourth image forming unit and the conveyance rollers are respectively located such that the first distance from the fourth image forming position to the transfer area is longer than the second distance from the nipping area formed by the conveyance rollers to the transfer area, when the sheet of paper passes through the conveyance rollers, the fourth image forming unit has completed image formation. Consequently, the effect of the fluctuation in the rate of rotation of the intermediate transfer belt is not reflected in the image transferred to the sheet of paper. 
     The first distance is slightly longer than the second distance, and the difference between the first and second distances is preferably approximately 10 to 20 mm. Based on this difference, when the sheet of paper is conveyed in synchronization with the combination image formed on the intermediate transfer belt, the sheet conveyance should begin with a slight time difference. 
     The fourth image forming unit preferably comprises an image forming unit that forms monochrome images. Because misalignment in the overlap of black images is more conspicuous than in images of different colors, this preferred location helps avoid such conspicuous misalignment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing showing the construction of the entire printer comprising an embodiment of the present invention; 
     FIG. 2 is a drawing showing the positional relationships of the black-image forming unit and the timing rollers relative to the transfer area; and 
     FIG.  3 ( a ) is a drawing showing the black-image forming unit of FIG. 2 forming an image, and FIG.  3 ( b ) is a drawing showing the timing rollers of FIG. 2 finishing the conveyance of the sheet. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention are explained below with reference to the accompanying drawings. 
     FIG. 1 shows the entire construction of the tandem-type digital color printer (hereinafter simply ‘printer’)  10  comprising an embodiment of the present invention. 
     The printer  10  has an intermediate transfer belt  12  essentially in the center of its interior. The intermediate transfer belt  12  is supported by the outer circumferences of the rollers  14 ,  16  and  18 . The roller  18  comprises a driving roller that is driven to rotate by a roller driving device  19  (see FIG.  2 ), and causes the intermediate transfer belt  12  to travel in the direction of the arrow A by supplying rotational torque to the belt. 
     Fourth image forming units  20 Y,  20 M,  20 C and  20 K, which are respectively for yellow (Y), magenta (M), cyan (C) and black (K), are located at a prescribed pitch P along and below the bottom horizontal part of the intermediate transfer belt  12 . 
     The image forming units  20 Y,  20 M,  20 C and  20 K have photoreceptor drums  22 Y,  22 M,  22 C and  22 K, respectively. Around the photoreceptor drum  22 Y are located, sequentially along the rotational circumference thereof, a charger  24 Y that uniformly charges the surface of the photoreceptor drum  22 Y, a print head  26 Y that forms an electrostatic latent image by irradiating the uniformly charged surface of the photoreceptor drum with light in accordance with the image data, a developing unit  28 Y that develops the electrostatic latent image formed on the photoreceptor drum surface into a toner image using yellow toner, a primary transfer roller  30 Y that faces the photoreceptor drum  22 Y via the intermediate transfer belt  12  and that performs primary transfer onto the intermediate transfer belt  12  of the toner image formed on the photoreceptor drum surface via electrostatic attraction, and a cleaner  32 Y that collects and cleans off the toner remaining on the photoreceptor drum surface after the primary transfer. Similarly, around the photoreceptor drum  22 M are located, sequentially along the rotational circumference thereof, a charger  24 M, a print head  26 M, a developing unit  28 M that develops the electrostatic latent image formed on the photoreceptor drum surface into a toner image using magenta toner, a primary transfer roller  30 M and a cleaner  32 M. Around the photoreceptor drum  22 C are located, sequentially along the rotational circumference thereof, a charger  24 C, a print head  26 C, a developing unit  28 C that develops the electrostatic image formed on the photoreceptor drum surface into a toner image using cyan toner, a primary transfer roller  30 C and a cleaner  32 C. Around the photoreceptor drum  22 K are located, sequentially along the rotational circumference thereof, a charger  24 K, a print head  26 K, a developing unit  28 K that develops the electrostatic image formed on the photoreceptor drum surface into a toner image using black toner, a primary transfer roller  30 K and a cleaner  32 K. The print heads  26 Y,  26 M,  26 C and  26 K each comprise a number of LEDs aligned along the main scanning path parallel to the shaft of the photoreceptor drum. 
     A secondary transfer roller (transfer member)  34  is in pressure contact with the area of the intermediate transfer belt.  12  supported by the roller  18 . The nipping area formed by the secondary transfer roller  34  and the intermediate transfer belt  12  comprises the secondary transfer area  36 . A high transfer voltage is impressed to the secondary transfer roller  34 . Due to this transfer voltage, the toner image formed on the intermediate transfer belt  12  is electrostatically attracted to the sheet of paper, which comprises the recording medium and has been conveyed to the secondary transfer area  36  in the manner explained below, such that secondary transfer is performed. 
     A cleaner  38  is in pressure contact with the area of the intermediate transfer belt  12  supported by the roller  16 . This cleaner . 38  scrapes off the toner remaining on the intermediate transfer belt  12  after the secondary transfer and collects it in the waste toner box  40 . 
     A paper supply cassette  42  is detachably located under the printer  10 . The sheets of paper S, which are stacked and housed in the paper supply cassette  42 , are fed out to the conveyance path  46  one by one, beginning with the topmost sheet, via the rotation of the paper supply roller  44 . 
     The conveyance path  46  extends to the paper eject tray  11  while passing the paper supply cassette  42 , the nipping area (contact area)  49  formed by the timing rollers  48 , the secondary transfer area  36  and the fusing unit  50 . The timing rollers  48  comprise a conveyance roller  48   a  and a roller  48   b , and the conveyance roller  48   a  is driven to rotate by the roller driving device  19 . 
     In the printer  10 , as shown in FIG. 2, the black image forming unit  20 K and the conveyance roller  48   a  are located in relation to the secondary transfer roller  34  such that the distance (the first distance) c from the image forming position  23 K of the image forming unit  20 K at which the unit forms an image on the intermediate transfer belt  12  to the transfer area  36  is slightly longer by a difference e (approximately 10 to 20 mm) than the distance (the second distance) d from the nipping area (contact area)  49  at which the conveyance roller  48   a  comes into contact with the sheet S to the transfer area  36 . 
     Referring to FIG. 1 once more, a paper supply sensor  52  is located near the timing rollers  48 . The paper supply sensor  52  detects that the top edge of the sheet S fed out from the paper supply cassette  42  to the conveyance path  46  has been grasped by the timing rollers  48 . When the top edge of the sheet S is detected by the paper supply sensor  52 , the timing rollers  48  stop their rotation, and the sheet S is thereafter fed out to the secondary transfer area  36  in synchronization with the toner image on the intermediate transfer belt  12 . 
     The fusing unit  50  has a fusing belt  60  that is supported by a pair of rollers  56  and  58  and is driven to rotate in the direction of the arrow B, as well as a fusing roller  62  that is in pressure contact with the roller  56  via the fusing belt  60  and rotates in the direction of the arrow as the roller  56  rotates. The nipping area formed by the fusing belt  60  and the fusing roller  62 , through which area passes the sheet that has undergone secondary transfer of the toner image, comprises the fusing area  64 . 
     The operation of the printer  10  having the above construction will now be explained. 
     When image signals are input to the image signal processing unit (not shown in the drawings) of the printer  10  from an external device such as a personal computer, for example, the image signal processing unit prepares digital image signals comprising the original image signals converted into yellow, cyan, magenta or black digital signals, and these digital image signals are transmitted to the print head LED drive circuit. The drive circuit causes the print heads  26 Y,  26 M,  26 C and  26 K of the image forming units  20 Y,  20 M,  20 C and  20 K, respectively, to emit light for exposure based on the input digital signals. Exposure takes place sequentially in the order of print heads  26 Y,  26 M,  26 C and  26 K such that there is a time difference between each exposure session. Consequently, an electrostatic latent image of the each respective color is formed on the surfaces of the photoreceptor. drums  22 Y,  22 M,  22 C and  22 K, respectively. 
     The electrostatic latent images formed on the photoreceptor drums  22 Y,  22 M,  22 C and  22 K are developed respectively by the developing unit  28 Y,  28 M,  28 C and  28 K into a toner image of each color. The toner image of each color undergoes primary transfer onto the intermediate transfer belt  12  that is moving in the direction of the arrow A, in a sequentially superimposed fashion based on the effect of each primary transfer roller  30 Y,  30 M,  30 C and  30 K. The combination toner image formed on the intermediate transfer belt  12  in this way reaches the secondary transfer area  36  as the intermediate transfer belt  12  continues to travel. On the other hand, the timing rollers  48  supply the sheet S sent out to the conveyance path  46  from the paper supply cassette  42  to the transfer area  36  via the rotation of the conveyance roller  48   a  in synchronization with the combination toner image. When this occurs, the timing rollers  48  grasp the sheet S until the image forming unit  20 K completes the formation of an image L on the intermediate transfer belt  12 , as shown in FIG.  3 ( a ). When the tail edge of the sheet S leaves the nipping area  49 , as shown in FIG.  3 ( b ), the tail end of the image L formed on the intermediate transfer belt  12  by the image forming unit  20 K has moved in the direction of the transfer area  36  from the image forming position  23 K for the image forming unit  20 K by the difference e (approximately 10 to 20 mm) between the distances c and d. Consequently, the roller  18 , which is driven to rotate by the roller driving device  19 , which is also the drive source for the conveyance roller  48   a , experiences a change in the rotational rate when the tail edge of the sheet S leaves the nipping area  49 , but because the operation to form the image L by the image forming unit  20 K has already been completed, no image disturbance, which may result in incomplete overlap of the single-color toner images, takes place due to the above change. 
     Referring back to FIG. 1 once more, the multi-color combination toner image undergoes secondary transfer onto the sheet S at one time at the secondary transfer area  36  based on the effect of the secondary transfer roller  34 . The toner remaining on the intermediate transfer belt  12  after secondary transfer is collected by the cleaner  38 . 
     The sheet S on which the combination toner image has been transferred via secondary transfer is sent to the fusing unit  50  through the conveyance path  46 , and the combination toner image is fused onto the sheet S when the sheet S passes through the fusing area  64 . The sheet S is then ejected onto the paper eject tray  11 . 
     The intermediate transfer member comprised a belt in the above embodiment, but it may have a different configuration such as a drum. 
     Further, the explanation regarding this embodiment was given using a printer as an example, but the present invention may be applied in a different type of image forming apparatus, such as a copying machine, a facsimile machine, or a combination machine comprising a copying machine and/or a facsimile machine and/or printer. 
     Although the present invention has been fully described by way of examples, it is to be noted that various changes and modification will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.