Patent Publication Number: US-2009232568-A1

Title: Fixing apparatus and developer fixing method for the fixing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior U.S. Patent Application No. 61/037,044, filed on 17 Mar. 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to image forming apparatuses such as a copying machine and a printer, and, more particularly to an image forming apparatus with improved durable life of a belt of a belt fixing apparatus, which fixes a developer, and a developer fixing method for the image forming apparatus. 
     BACKGROUND 
     An image forming apparatus includes a fixing apparatus that heats and presses a developer such as a toner to thereby fix the developer on a recording medium. 
     A fixing apparatus in these days includes a fixing belt for reducing heat capacity (e.g., JP-A-2004-109650). The fixing belt is wound around a fixing roller and a metal roller. A recording medium having a developer transferred thereon is nipped by the fixing roller and a pressing roller that is set in contact with the fixing roller across the fixing belt and includes a heating device. The recording medium is pressed and heated while being conveyed. 
     In such a fixing apparatus, the position of the fixing belt is regulated by regulating plates set at both ends of the fixing roller. If the fixing belt is pressed by the fixing roller and the pressing roller, it is likely that excessive pressure is applied to both ends of the fixing belt and the fixing belt is broken. 
     SUMMARY 
     It is an object of the present invention to provide a fixing apparatus with improved durable life of a fixing belt and a developer fixing method for the fixing apparatus. 
     In an aspect of the present invention, a fixing apparatus includes: 
     a pressing roller having a heating device therein; 
     a metal roller having a core bar of metal; and 
     a fixing roller that is set in contact with the pressing roller across a fixing belt wound around the fixing roller and the metal roller, has the width of the center portion formed in a cylindrical shape smaller than the width of the pressing roller, and has, at both ends, small-diameter rollers having a diameter smaller than the diameter of the center portion. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a configuration example of an image forming apparatus; 
         FIG. 2  is a side view of a fixing apparatus; 
         FIG. 3  is a top view of the fixing apparatus; 
         FIG. 4  is a perspective view of a nip section of a fixing apparatus according to a first embodiment of the present invention; 
         FIG. 5  is a diagram of a fixing roller having enlarged cutouts in the first embodiment; and 
         FIG. 6  is a perspective view of a nip section of a fixing apparatus according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and methods of the present invention. 
     Fixing apparatuses and developer fixing methods for the fixing apparatuses according to embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
     Overview of an Image Forming Apparatus 
       FIG. 1  is a diagram of a configuration example of an image forming apparatus. As shown in  FIG. 1 , a document table  602 , which is made of a transparent material such as a glass plate, for placing an original document is provided in an upper part of an apparatus main body  601 . A cover  603  is openably and closably provided in the apparatus main body  601  to cover the document table  602 . 
     On a lower surface side of the document table  602  in the apparatus main body  601 , a scan unit (not shown) that optically scans an image of an original document placed on the document table  602  is provided. This scan unit includes, for example, a carriage  604 , reflection mirrors  606 ,  607 , and  608  that reflect light of an exposure lamp  605  reflected on the original document, a lens block for magnification  609  for magnifying the reflected light, and a CCD (Charge Coupled Device)  610 . The carriage  604  includes the exposure lamp  605  that irradiates light toward the document table  602 . The carriage  604  can reciprocatingly move along a lower surface of the document table  602 . 
     The carriage  604  reciprocatingly moves while lighting the exposure lamp  605  to thereby expose the original document placed on the document table  602 . A reflected light image of the original document, which is placed on the document table  602 , formed by this exposure is projected on the CCD  610  via the reflection mirrors  606 ,  607 , and  608  and the lens block for magnification  609 . The CCD  610  outputs image data corresponding to the reflected light image of the original document projected thereon. 
     An image forming unit  220  is provided below the scan unit in the apparatus main body  601 . The image forming unit  220  includes, for example, a print engine (not shown) and a process unit (not shown). 
     The print engine includes an exposing unit  611 . The process unit includes photoconductive drums  621 ,  622 ,  623 , and  624  arranged along the exposing unit  611 , an endless transfer belt  12  arranged to be opposed to the exposing unit  611  across the photoconductive drums  621 ,  622 ,  623 , and  624 , a drive roller  626  that drives the transfer belt  12 , primary transfer rollers  641 ,  642 ,  643 , and  644  arranged to be opposed to the photoconductive drums  621 ,  622 ,  623 , and  624  across the transfer belt  12 , and a transfer-roller driving unit that drives the primary transfer rollers  641 ,  642 ,  643 , and  644 . 
     The transfer belt  12  is laid over the drive roller  626 , guide rollers  627 ,  628 , and  629 , and a driven roller  630  and receives the power from the drive roller  626  to rotationally travel in the counter clockwise direction. The guide roller  627  is provided to freely move up and down and receives the pivotal movement of a cam  631  to move to the transfer belt  12  side. Consequently, the guide roller  627  displaces the transfer belt  12  to the photoconductive drums  621 ,  622 ,  623 , and  624  side. 
     The image forming unit  220  executes an image forming process for forming an image based on image data (an image signal outputted from the CCD  610 ) and printing the image on a recording medium being conveyed. The image signal outputted from the CCD  610  is supplied to the exposing unit  611  after being appropriately processed. The exposing unit  611  emits a laser beam B 1  corresponding to an image signal of a yellow color, a laser beam B 2  corresponding to an image signal of a magenta color, a laser beam B 3  corresponding to an image signal of a cyan color, and a laser beam B 4  corresponding to an image signal of a black color to the photoconductive drum  621  for the yellow color, the photoconductive drum  622  for the magenta color, the photoconductive drum  623  for the cyan color, and the photoconductive drum  624  for the black color, respectively. 
     The primary transfer rollers  641 ,  642 ,  643 , and  644  are respectively moved (lowered) to the transfer belt  12  side to thereby bring the transfer belt  12  into contact with the photoconductive drums  621 ,  622 ,  623 , and  624  and transfer visible images on the photoconductive drums  621 ,  622 ,  623 , and  624  onto the transfer belt  12 . 
     A drum cleaner, a charge removing lamp, a charging unit, and a developing unit not shown in the figure are arranged around the photoconductive drum  621  in order. The drum cleaner has a drum cleaning blade that is set in contact with the surface of the photoconductive drum  621 . The drum cleaner scrapes off a developer remaining on the surface of the photoconductive drum  621  with the drum cleaning blade. 
     The charge removing lamp removes charges remaining on the surface of the photoconductive drum  621 . The charging unit applies high voltage to the photoconductive drum  621  to thereby charge the surface of the photoconductive drum  621  with electrostatic charges. The laser beam B 1  emitted from the exposing unit  611  is irradiated on the surface of the charged photoconductive drum  621 . An electrostatic latent image is formed on the surface of the photoconductive drum  621  by the irradiation of the laser beam B 1 . The developing unit supplies a developer (a toner) of the yellow color onto the surface of the photoconductive drum  621  to thereby visualize the electrostatic latent image on the surface of the photoconductive drum  621 . 
     The other photoconductive drums  622 ,  623 , and  624  visualize electrostatic latent images on the surfaces of the photoconductive drums  622 ,  623 , and  624  using the developers of the colors corresponding thereto, respectively, in the same manner. 
     A cleaner  636  is provided, across the transfer belt  12 , in a position of the image forming unit  220  opposed to the drive roller  626 . The cleaner  636  has a cleaning blade  673   a  that is set in contact with the transfer belt  12 . The cleaner  636  scrapes off a developer remaining on the transfer belt  12  with the cleaning blade  673   a.    
     Printing modes are changed as explained below. Hooks  671 ,  672 ,  673 , and  674  are provided near the primary transfer rollers  641 ,  642 ,  643 , and  644 , respectively. The hooks  671 ,  672 ,  673 , and  674  respectively engage with shafts of the primary transfer rollers  641 ,  642 ,  643 , and  644  while pivoting to lift the shafts and move the primary transfer rollers  641 ,  642 ,  643 , and  644  in a direction away from the photoconductive drums  621 ,  622 ,  623 , and  624 . Printing modes such as a full-color mode, a complete separation mode, and a monochrome mode are changed by not moving all of the primary transfer rollers  641 ,  642 ,  643 , and  644  or changing a combination of the primary transfer rollers to be moved. 
     Storing mechanisms and feeding mechanisms for recording media are explained below. Plural recording media cassettes  650  for storing recording media are provided below the exposing unit  611 . In the recording media cassettes  650 , a large number of recording media P of types different from one another are stored in a stacked state. Recording-media feeding mechanisms  221  for feeding the recording media in the recording media cassettes  650  one by one from the top are respectively provided in outlet portions (on the right side in the figure) of the recording media cassettes  650 . The recording media P are taken out one by one from any one of the recording media cassettes  650  by any one of the recording-media feeding mechanisms  221 . The recording-media feeding mechanism  221  for taking out the recording media P includes a pickup roller  651 , a recording-media feeding roller  652   a , and a separating roller  652   b . The recording-media feeding mechanism  221  separates the recording media P, which are taken out from the recording media cassette  650 , one by one and feeds the recording media P to a recording-media conveying mechanism  653 . 
     A conveying path for recording media is explained below. The recording-media conveying mechanism  653  extends to a recording-media discharge port  654  in an upper part via the driven roller  630  of the image forming unit  220 . The recording-media discharge port  654  faces a recording-media discharging section  655  that continues to an outer peripheral surface of the apparatus main body  601 . Conveying rollers  656  are provided near the recording-media feeding mechanisms  221 , respectively, on a start end side of the recording-media conveying mechanism  653 . When a recording medium is fed by any one of the recording-media feeding mechanisms  221 , the recording-media conveying mechanism  653  conveys the fed recording medium to the recording-media discharging section  655 . 
     A secondary transfer roller  630   a  is provided in a position along the recording-media conveying mechanism  653  opposed to the driven roller  630  across the transfer belt  12 . Registration rollers  658  are provided in a position before the driven roller  630  and the secondary transfer roller  630   a  in a conveying direction. 
     The registration rollers  658  delivers the recording medium P to between the transfer belt  12  and the secondary transfer roller  630   a  at timing synchronizing with a transfer operation by the transfer belt  12  and the secondary transfer roller  630   a  for transferring an image formed by a developer (a toner) onto the recording medium P. The secondary transfer roller  630   a  transfers, while holding the recording medium P delivered from the registration rollers  658  in conjunction with the transfer belt  12  on the driven roller  630 , a visible image, which is formed by the developer (the toner) and transferred onto the transfer belt  12 , onto the recording medium P and prints the visible image. In this way, the registration rollers  658  convey, in synchronization with a transfer operation of the image forming unit  220 , the recording medium P to the image forming unit  220  including the transfer belt  12  and the secondary transfer roller  630   a.    
     A fixing apparatus to fix thermally is provided in a position further on a downstream side than the secondary transfer roller  630   a  of the recording-media conveying mechanism  653 . The fixing apparatus includes a metal roller, a fixing roller  10 , a fixing belt wound around the metal roller and the fixing roller  10 , an a pressing roller  40  that is set in contact with the fixing roller  10  across the fixing belt. The pressing roller  40  includes a heating device such as a heater lamp. 
     The recording medium P having the developer transferred thereon is nipped by the fixing roller  10  and the pressing roller  40  and heated and pressed while being carried. When the recording medium P is heated and pressed, the developer is fixed on the recording medium P. A recording-media discharging roller  661  is provided at a terminal end of the recording-media conveying mechanism  653 . 
     An automatic duplex unit (hereinafter referred to as ADU)  222  may be provided in the apparatus main body  601 . The ADU  222  is set to couple a sub-conveying path  662 , which is a path for conveying the recording medium P in the ADU  222 , to the terminal end of the recording-media conveying mechanism  653  and an inlet of the registration rollers  658 . The sub-conveying path  662  branches from a downstream side of the recording-medium conveying mechanism  653  with respect to the image forming unit  220  (the terminal end of the recording-media conveying mechanisms  653 ) and merges into an upstream side of the recording-media conveying mechanism  653  with respect to the image forming unit  220  (an upstream side position of the registration rollers  658 ). 
     The sub-conveying path  662  reverses the front and the back of the recording medium P to perform duplex printing. Recording media feeding rollers  663 ,  664 , and  665  are provided in the sub-conveying path  662 . The ADU  222  reverses the recording medium P conveyed from the image forming unit  220  to the recording-media discharging section  655 , conveys the recording medium P through the sub-conveying path  662 , and merges the recording medium P into the recording-media conveying mechanism  653  on the upstream side of the image forming unit  220 . If the recording medium P is conveyed in this way, the front and the back of the recording medium P is reversed. 
     The recording medium P returned to the upstream side of the image forming unit  220  by the sub-conveying path  662  merges into the recording-media conveying mechanism  653 . Thereafter, while being synchronized with a transfer operation of the image forming unit  220 , the recording medium P is delivered by the registration rollers  658  to a transfer position where the transfer belt  12  and the secondary transfer roller  630   a  are in contact with each other. In this way, the visible image on the transfer belt  12  is transferred onto the back of the recording medium P and printed as well. 
     If the duplex printing is designated from an operation panel  724  provided in the apparatus main body  601 , a computer connected to the apparatus main body  601  through a network, or the like, the sub-conveying path  662  of the ADU  222  is brought into a state for performing an action for reversing the front and the back of the recording medium P. 
     Devices additionally provided in the apparatus main body  601  are explained below. In the example of the apparatus main body  601  shown in  FIG. 1 , a pair of the recording media cassettes  650  are provided as feeding sources of recording media. However, a trio or more of the recording media cassettes  650  may be provided in the apparatus main body  601 . Besides, although not shown in the figure, it is also possible to provide a manual-feed recording-media feeding mechanism (hereinafter referred to as SFB) and a large-capacity recording media feeder (hereinafter referred to as LCF), which is a recording-media feeding mechanism that can store several thousands recording media in a stack state. The SFB and the LCF are set in the apparatus main body  601  such that a path for feeding the recording media merges into the recording-media conveying mechanism  653 . 
     A recording-media type sensor  223  may be provided in the apparatus main body  601 . The recording-media type sensor  223  is arranged in a position on the upstream side of the recording-media conveying mechanism  653  with respect to the image forming unit  220  and further on the upstream side than the registration rollers  658 . The recording-media type sensor  223  detects a type of the recording medium P conveyed by the recording-media conveying mechanism  653 . As the recording-media type sensor  223 , it is possible to use, for example, a publicly-known sensor that determines a type of the recording medium P by detecting the thickness and the light transmittance of the recording medium P. 
     If the SFB and the LCF are set, the recording-media type sensor  223  is arranged further on the downstream side than a merging point of a recording-media feeding path from the SFB and the LCF and the recording-media conveying mechanism  653 . By arranging the recording-media type sensor  223  in this way, it is possible to detect, with one recording-media type sensor  223 , types of the recording media P conveyed on the recording-media conveying mechanism  653  from all the recording-media feeding sources. 
     Fixing Apparatus 
       FIG. 2  is a side view of the fixing apparatus. As shown in the figure, the fixing apparatus includes a metal roller  20 , the fixing roller  10 , and a fixing belt  30  wound around the metal roller  20  and the fixing roller  10 . 
     The fixing belt  30  includes electrocast nickel, stainless steel, or polyimide as a base material and has a heat-resistant elastic layer of silicon rubber on an outer circumferential surface thereof. An annular belt is used as the fixing belt  30 . Moreover, a belt obtained by coating an outer layer of silicon rubber with highly-releasable fluorine resin such as PFA (tetrafluoroethylene perfluoroalkyl vinyl ether copolymer) may be used. 
     As the metal roller  20 , a roller obtained by coating a hollow core bar of aluminum or iron with PFA can be used. 
     As the fixing roller  10 , a roller obtained by providing a heat-resistant elastic layer  101  of silicon sponge or the like in a hollow core bar  102  of aluminum or iron can be used. It is desirable to use a heat-resistant elastic layer having low hardness as the heat-resistant elastic layer  101  in order to secure a wide nip. 
     Moreover, the fixing apparatus includes the pressing roller  40  that is set in contact with the fixing roller  10  across the fixing belt  30 . 
     As the pressing roller  40 , it is possible to use a roller obtained by providing a heat-resistant elastic layer  402  such as silicon sponge on a hollow core bar  403  of aluminum or iron and forming a coated layer  401  on an outer circumference of the heat-resistant elastic layer  402  with PFA. A halogen heater  404  may be used as a heat source on an inner side of the pressing roller  40 . 
       FIG. 3  is a top view of the fixing apparatus. As shown in  FIG. 3 , the fixing roller  10  includes, at both ends thereof, regulating plates  11  that regulate a position of the fixing belt. The diameter of the regulating plates  11  is larger than the diameter of the fixing roller  10 . 
     The recording medium P is nipped between the fixing roller  10  and the pressing roller  40  together with the fixing belt  30  and heated and pressed while being conveyed. 
     First Embodiment 
       FIG. 4  is a perspective view of a nip section of a fixing apparatus according to a first embodiment of the present invention. In  FIG. 4 , the fixing belt  30  is not shown. As shown in  FIG. 4 , in the fixing apparatus, the fixing roller  10  has cutouts at both ends thereof. 
     The width L 1  of the center portion formed in a cylindrical shape of the fixing roller  10  is smaller than the width L 2  of the pressing roller  40 . The fixing roller  10  includes small-diameter rollers  10 A at both ends thereof. The diameter φ 2  of the small-diameter rollers  10 A is smaller than the diameter φ 1  of the center portion of the fixing roller  10 . 
     It is desirable that a half of a difference between φ 1  and φ 2  is larger than the thickness of the fixing belt  30 . If the half of the difference is smaller than the thickness of the fixing belt  30 , since excessive pressure is applied to both the ends of the fixing belt  30 , the fixing belt  30  is easily broken. 
     It is desirable that φ 2  is larger than φ 1 /2. If φ 2  is smaller than φ 1 /2, the small-diameter rollers  10 A cannot support the regulating plates  11 . 
       FIG. 5  is a diagram of the fixing roller  10  having enlarged cutouts in the first embodiment. In  FIG. 5 , the fixing belt  30  is not shown. As shown in  FIG. 5 , if the regulating plates  11  are firmly fixed to a shaft  12 , the diameter φ 3  of small-diameter rollers  10 B can be set the same as the diameter of the shaft  12 . 
     The fixing belt  30  is bent to the small-diameter rollers  10 A or  10 B side in the nip section between the fixing roller  10  and the pressing roller  40 . Therefore, excess pressure is not applied to both the ends of the fixing belt  30 . 
     As explained above, the fixing apparatus according to this embodiment has the fixing roller  10  with both the ends cut out. Therefore, there is an effect that excessive pressure is not applied to both the ends of the fixing belt  30  and it is possible to extend the durable life of the fixing belt  30 . 
     Second Embodiment 
       FIG. 6  is a perspective view of a nip section of a fixing apparatus according to a second embodiment of the present invention. In  FIG. 6 , the fixing belt  30  is not shown. As shown in  FIG. 6 , in the fixing apparatus, the fixing roller  10  has tapered portions  10 C at both the ends thereof. 
     In the fixing roller  10 , the width L 1  of the center portion formed in a cylindrical shape is smaller than the width L 2  of the pressing roller  40 . The diameter φ 4  of the distal ends of the tapered portions  10 C is smaller than the diameter φ 1  of the center portion of the fixing roller  10 . 
     It is desirable that a half of a difference between φ 1  and φ 4  is larger than the thickness of the fixing belt  30 . If the half of the difference is smaller than the thickness of the fixing belt  30 , since excessive pressure is applied to both the ends of the fixing belt  30 , the fixing belt  30  is easily broken. 
     It is desirable that φ 4  is larger than φ 1 /2. If φ 4  is smaller than φ 1 /2, the tapered portion  10 C cannot support the regulating plates  11 . 
     If the regulating plates  11  are firmly fixed to the shaft  12 , the diameter φ 4  of the distal ends of the tapered portions  10 C can be set the same as the diameter of the shaft  12 . 
     The fixing belt  30  is bent to the tapered portions  10 C side in the nip section between the fixing roller  10  and the pressing roller  40 . Therefore, excessive pressure is not applied to both the ends of the fixing belt  30 . Since the pressure applied to the fixing roller  10  is dispersed to the tapered portions  10 C, there is an effect that it is possible to use a thinner core bar and reduce the weight of an image forming apparatus. 
     Although exemplary embodiments of the present invention have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which depart from the spirit of the present invention. All such changes, modifications, and alterations should therefore be seen as within the scope of the present invention.