Patent Publication Number: US-2009226224-A1

Title: Image forming apparatus and transfer belt turning method for image forming 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. 61/035,213, filed on 10 Mar. 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus such as a copy machine or printer, and particularly to an image forming apparatus and a transfer belt turning method for an image forming apparatus which improve the life of a transfer belt. 
     BACKGROUND 
     In an image forming apparatus such as a copy machine or printer, a developer such as toner is transferred from a photoconductor to a transfer belt, then transferred from the transfer belt to a recording medium, and heated and pressurized to carry out printing. The transfer belt is laid over plural rollers provided on the inner side of the transfer belt. 
     In a conventional image forming apparatus, a developer may be fixed to rollers over which a transfer belt is laid, and the outer diameter of the roller may change. Particularly, the developer is often fixed to a secondary transfer roller counter-driven roller facing a secondary transfer roller which transfers the developer. 
       FIG. 4  shows a conventional secondary transfer roller counter-driven roller  630 . As shown in  FIG. 4 , a transfer belt  230  is laid over the secondary transfer roller counter-driven roller  630 . The transfer belt  230  has regulating ribs  230 A which regulate the position of the transfer belt  230 , at both edges on its inner side. 
     The conventional driven roller is made of a metal such as aluminum, stainless steel or free-cutting steel, or an elastic resin such as EPDM (ethylene propylene rubber) or urethane. Therefore, the developer tends to be fixed thereto. Particularly, if the developer is fixed to an edge part P 1 , the outer diameter of the secondary transfer roller counter-driven roller  630  is increased or the belt carrying capability becomes uneven, causing the transfer belt  230  to lean in the direction of arrow X. Thus, a force is applied to a periphery P 2  of the regulating ribs  230 A, causing a problem that the transfer belt  230  may be cracked. 
     With respect to this point, for example, JP-A-2008-203729 discloses a technique of heating a transfer belt by providing a heater in order not to generate temperature inclination. 
     However, if the outer diameter of the secondary transfer roller counter-driven roller  630  is increased or the belt carrying capability become uneven, leaning of the transfer belt still occurs even if the belt is heated, and the problem of cracks cannot be solved. 
     SUMMARY 
     It is an object of the invention to provide an image forming apparatus and a transfer belt turning method for an image forming apparatus which reduce fixation of a developer to a driven roller. 
     According to an aspect of the invention, an image forming apparatus includes: an image forming unit that forms a developer image on an image carrier; a transfer belt that carries the developer image; a transfer unit that transfers the developer image carried by the transfer belt to a recording medium; and at least one driven roller that is installed on inner side of the transfer belt and has its surface covered with a material having releasability. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing an exemplary configuration of an image forming apparatus. 
         FIG. 2  is a sectional view of a secondary transfer roller counter-driven roller according to an embodiment. 
         FIG. 3  is a view showing comparison of life of a transfer belt with the secondary transfer roller counter-driven roller according to the embodiment and with a conventional secondary transfer roller counter-driven roller. 
         FIG. 4  is a view showing a conventional driven roller. 
     
    
    
     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 invention. 
     Hereinafter, an embodiment of an image forming apparatus and a recording medium carrying method for an image forming apparatus according to the invention will be described in detail with reference to the drawings. 
     (Outline of Image Forming Apparatus) 
       FIG. 1  shows an exemplary configuration of an image forming apparatus. As shown in  FIG. 1 , the image forming apparatus has a document table  602  made of a transparent material, for example, a glass plate, for setting an original on top of an apparatus body  601 . The image forming apparatus also has a cover  603  to cover the document table  602  in an openable and closable manner on the apparatus body  601 . 
     On the lower side of the document table  602  within the apparatus body  601 , a scanning unit (not shown) is provided which optically scans an image of an original set on the document table  602 . For example, this scanning unit has a carriage  604 , reflection mirrors  606 ,  607  and  608  that reflect light of an exposure lamp  605  reflected by the original, a variable-power lens block  609  that varies magnification of the reflected light, and a CCD (charged coupled device)  610 . The carriage  604  has the exposure lamp  605  that casts light toward the document table  602 . The image forming apparatus has the carriage  604  in a manner that enables the carriage to reciprocate along the lower side of the document table  602 . 
     The carriage  604  reciprocates while lighting the exposure lamp  605 , thereby exposing light to the original set on the document table  602 . The CCD  610  receives a reflection light image formed by this exposure of the original set on the document table  602 , via the reflection mirrors  606 ,  607  and  608  and the variable-power lens block  609 . The CCD  610  outputs image data corresponding to the received reflection light image of the original. 
     An image forming unit  220  is provided below the scanning unit within the apparatus body  601 . The image forming unit  220  has, for example, a print engine (not shown) and a process unit (not shown). 
     The print engine includes an exposure unit  611 . The process unit includes photoconductive drums  621 ,  622 ,  623  and  624  as image carriers located along the exposure unit  611 , an endless transfer belt  230  situated at a position facing the exposure unit  611  with the photoconductive drums  621 ,  622 ,  623  and  624  provided between them, a drive roller  626  that drives the transfer belt  230 , primary transfer rollers  641 ,  642 ,  643  and  644  situated at positions facing the photoconductive drums  621 ,  622 ,  623  and  624  with the transfer belt  230  provided between them, and a transfer roller driving unit that drives the primary transfer rollers  641 ,  642 ,  643  and  644 . 
     In the image forming apparatus, the transfer belt  230  is laid across the drive roller  626 , guide rollers  627 ,  628  and  629 , and a secondary transfer roller counter-driven roller  630 . The guide rollers  627 ,  628  and  629  regulate the position of the transfer belt  230 . The transfer belt  230  turns counterclockwise by receiving power from the drive roller  626 . The image forming apparatus has the guide roller  627  provided in such a manner that the guide roller  627  can move up and down. The guide roller  627  moves toward the transfer belt  230  by the rotation of a cam  631 . The guide roller  627  displaces the transfer belt  230  toward the photoconductive drums  621 ,  622 ,  623  and  624 . 
     This image forming unit  220  executes an image forming process to form an image based on image data (an image signal outputted from the CCD  610 ) and then to print the image onto a recording medium which is being carried. That is, an image signal outputted from the CCD  610  is properly processed and then supplied to the exposure unit  611 . The exposure unit  611  emits a laser beam B 1  corresponding to a yellow image signal to the photoconductive drum  621  for yellow, a laser beam B 2  corresponding to a magenta image signal to the photoconductive drum  622  for magenta, a laser beam B 3  corresponding to a cyan image signal to the photoconductive drum  623  for cyan, and a laser beam B 4  corresponding to a black image signal to the photoconductive drum  624  for black. 
     The primary transfer rollers  641 ,  642 ,  643  and  644  move (fall) toward the transfer belt  230  and thereby bring the transfer belt  230  into contact with the photoconductive drums  621 ,  622 ,  623  and  624 . Thus, visible images on the photoconductive drums  621 ,  622 ,  623  and  624  are transferred to the transfer belt  230 . 
     The image forming apparatus has a drum cleaner, a neutralizing lamp, a charging unit and a developing unit, which are not shown, in this order in the periphery of the photoconductive drum  621 . The drum cleaner has a drum cleaning blade that contacts the surface of the photoconductive drum  621 , and scrapes off the remaining developer on the surface of the photoconductive drum  621  by the drum cleaning blade. 
     The neutralizing lamp eliminates electric charges remaining on the surface of the photoconductive drum  621 . The charging unit applies a high voltage to the photoconductive drum  621  and thereby charges the surface of the photoconductive drum  621  with electrostatic charges. The exposure unit  611  irradiates the charged surface of the photoconductive drum  621  with the laser beam B 1 . By this irradiation, an electrostatic latent image is formed on the surface of the photoconductive drum  621 . The developing unit T 1  supplies a yellow developer (toner) to the surface of the photoconductive drum  621  and thereby visualizes the electrostatic latent image on the surface of the photoconductive drum  621 . 
     As for the other photoconductive drums  622 ,  623  and  624 , electrostatic latent images on the surface of the photoconductive drums  622 ,  623  and  624  are similarly visualized using the developers of the corresponding colors. 
     The image forming apparatus has a cleaner  636  at a position facing the drive roller  626  in the image forming unit  220 , with the transfer belt  230  nipped between them. This cleaner  636  has a cleaning blade  673   a  that contacts the transfer belt  230 , and scrapes off the remaining developer on the transfer belt  230  by the cleaning blade  673   a.    
     The print mode can be changed as follows. The image forming apparatus has hooks  671 ,  672 ,  673  and  674  near the primary transfer rollers  641 ,  642 ,  643  and  644 . These hooks  671 ,  672 ,  673  and  674  become engaged with the shafts of the primary transfer rollers  641 ,  642 ,  643  and  644  while rotating, and thus lift their shafts. The hooks  671 ,  672 ,  673  and  674  thus move the primary transfer rollers  641 ,  642 ,  643  and  644  away from the photoconductive drums  621 ,  622 ,  623  and  624 . The print mode such as full-color mode, full-space mode, or monochrome mode can be changed by moving none of the primary transfer rollers  641 ,  642 ,  643  and  644 , or by changing their combination when moving the primary transfer rollers  641 ,  642 ,  643  and  644 . 
     Next, a housing mechanism and a supply mechanism for recording media will be described. The image forming apparatus has plural recording medium cassettes  650  that house recording media, below the exposure unit  611 . These recording medium cassettes  650  house multiple recording media P of different recording medium types in a stacked state. The image forming apparatus has a recording medium supply mechanism  221  which supplies recording media in the recording medium cassette  650  one by one from the top, at an exit part (the right side in the drawing) of each of these recording media cassettes  650 . This recording medium supply mechanism  221  takes out the recording media P one by one from one of the recording medium cassettes  650 . The recording medium supply mechanism  221  for taking out the medium includes a pickup roller  651 , a recording medium supply roller  652   a , and a separation roller  652   b . The recording medium supply mechanism  221  separates the recording media P taken out from the recording medium cassette  650 , one by one, and supplies each recording medium to a recording medium carrying mechanism  653 . 
     Next, the carrying path of the recording medium will be described. The recording medium carrying mechanism  653  extends to a recording medium discharge port  654  in an upper part via the secondary transfer roller counter-driven roller  630  of the image forming unit  220 . The recording medium discharge port  654  faces a recording medium discharge unit  655  connected to the outer circumferential surface of the apparatus body  601 . The image forming apparatus also has a carrying roller  656  near each recording medium supply mechanism  221 , at the starting edge of the recording medium carrying mechanism  653 . When one of the recording medium supply mechanisms  221  supplies a recording medium, the recording medium carrying mechanism  653  carries this recording medium to the recording medium discharge unit  655 . 
     The image forming apparatus also has a secondary transfer roller  630   a  at the position facing the secondary transfer roller counter-driven roller  630  with the transfer belt  230  nipped between them, in a halfway part of the recording medium carrying mechanism  653 . The image forming apparatus has a registration roller  658  at a position before the secondary transfer roller counter-driven roller  630  and the secondary transfer roller  630   a  in the carrying direction. 
     The registration roller  658  inserts the recording medium P between the transfer belt  230 , which is a transfer unit, and the secondary transfer roller  630   a  in timing synchronized with transfer operation by the transfer belt  230  and the secondary transfer roller  630   a  to transfer an image formed by a developer to the recording medium. The secondary transfer roller  630   a  nips the recording medium P inserted from the registration roller  658  together with the transfer belt  230  on the secondary transfer roller counter-driven roller  630 , transfers to the recording medium P a visible image formed by the developer and transferred to the transfer belt  230 , and then prints the image. The registration roller  658  carries the recording medium P to the image forming unit  220  having the transfer belt  230  and the secondary transfer roller  630   a  synchronously with the transfer operation in the image forming unit  220 . 
     The image forming apparatus has a heat fixing device for heat fixation at a position downstream from the secondary transfer roller  630   a  in the recording medium carrying mechanism  653 . The heat fixing device has a metal roller, a fixing roller  645 , a fixing belt laid across the metal roller and the fixing roller, and a pressurizing roller  646  that is abutted against the fixing roller  645  with the fixing belt provided between them. The pressurizing roller  646  has a heating device such as a heater lamp provided therein. 
     The heat fixing device nips the recording medium P with the developer transferred thereto between the fixing roller  645  and the pressurizing roller  646  and heats and pressurizes the recording medium P while carrying the recording medium P. At this time, the developer becomes fixed to the recording medium P. The image forming apparatus has a recording medium discharge roller  661  at the terminal end of the recording medium carrying mechanism  653 . 
     The image forming apparatus has an entry guide  647  that guides a sheet to the nip, upstream from the fixing roller  645  and the pressurizing roller  646  in the sheet carrying direction in the heat fixing device. 
     The image forming apparatus may have an automatic double-side unit (hereinafter referred to as ADU)  222  in the apparatus body  601 . The ADU  222  is installed to connect a sub-carrying path  662 , which is a path for carrying the recording medium P in the ADU  222 , to the terminal end of the recording medium carrying mechanism  653  and the entry to the registration roller  658 . The sub-carrying path  662  is branched from the downstream side of the recording medium carrying mechanism  653  with respect to the image forming unit  220  (from the terminal end of the recording medium carrying mechanism  653 ) and merges into the upstream side of the recording medium carrying mechanism  653  with respect to the image forming unit  220  (to an upstream position from the registration roller  658 ). 
     This sub-carrying path  662  reverses the sides of the recording medium P for double-side print. The image forming apparatus has recording medium supply rollers  663 ,  664  and  665  in the sub-carrying path  662 . The ADU  222  delivers the recording medium P which the image forming unit  220  carries to the recording medium discharge unit  655 , into the opposite direction, and carries the recording medium P through the sub-carrying path  662 , thus causing the recording medium to merge into the recording medium carrying mechanism  653  on the upstream side of the image forming unit  220 . As the recording medium P is carried in this way, the sides of the recording medium P are reversed. 
     After merging the recording medium P returned to the upstream side of the image forming unit  220  by the sub-carrying path  662 , into the recording medium carrying mechanism  653 , the image forming apparatus sends the recording medium P by the registration roller  658  to the transfer position where the transfer belt  230  and the secondary transfer roller  630   a  contact each other, synchronously with the transfer operation in the image forming unit  220 . Thus, the image forming apparatus transfers and thereby prints a visible image on the transfer belt  230  onto the back side of the recording medium P as well. 
     If double-side print is designated by an operation panel  724  provided in the apparatus body  601  or by a computer or the like connected to the apparatus body  601  via a network, the sub-carrying path  662  of the ADU  222  enters the state of actuation to reverse the sides of the recording medium P. 
     Next, additional devices to be provided will be described. The image forming apparatus has two recording medium cassettes  650  as recording medium supply sources. The apparatus body  601  may also have three or more recording medium cassettes  650 . Moreover, though not shown, a recording medium supply mechanism for manual insertion (hereinafter referred to as SFB) or a large-capacity supply recording medium feeder (hereinafter referred to as LCF), which is a recording medium supply mechanism capable of housing thousands of recording media in a stacked state, can also be provided. The image forming apparatus may have these SFB and LCF provided in the apparatus body  601  in such a manner that their paths for supplying the recording medium merge into the recording medium carrying mechanism  653 . 
     The image forming apparatus may also have a recording medium type sensor  223  in the apparatus body  601 . The image forming apparatus has the recording medium type sensor  223  at a position that is on the upstream side of the recording medium carrying mechanism  653  with respect to the image forming unit  220  and that is upstream from the registration roller  658 . The recording medium type sensor  223  detects the recording medium type of the recording medium P carried by the recording medium carrying mechanism  653 . For the recording medium type sensor  223 , a known sensor can be used which determines the type of the recording medium P, for example, by detecting the thickness or light transmittance of the recording medium P. 
     If an SFB or LCF is installed, the recording medium type sensor  223  is arranged downstream from the merging point of the recording medium supply path from the SFB or LCF and the recording medium carrying mechanism  653 . With such arrangement, the type of the recording medium P carried on the recording medium carrying mechanism  653  from any of the recording medium supply sources can be detected by the single recording medium type sensor  223 . 
     (Driven Roller) 
       FIG. 2  is a sectional view of the secondary transfer roller counter-driven roller  630  according to this embodiment. As shown in  FIG. 2 , the secondary transfer roller counter-driven roller  630  has a base member  12  made of a metal such as aluminum, stainless steel or free-cutting steel, or an elastic resin such as EPDM (ethylene propylene rubber) or urethane, an adhesive layer  11  that covers the surface of the base member and attaches a material having releasability to the base member, and a material having releasability layer  10  that covers the surface of the adhesive layer  11 . 
     For the material having releasability, at least one base material selected from silicone, that is, organosilicon compound polymers, and fluorocarbon resins including PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy polymer resin) and PVDF (poly(vinylidene fluoride)) can be used. 
     In the case of silicone and PTFE, a thin film is formed on an adhesive layer. In the case of PFA and PVDF, a tube is formed and this tube covers an adhesive layer. A film thickness of 10 to 30 μm is desirable and 20 μm is the most desirable. 
     The material having releasability is formed by dispersing a conductive material into this base material. As the conductive material, powder of a conductive substance such as carbon or copper is used. The material having releasability contains the conductive material so that its volume resistance value is 1×10 5 Ω when the base member is covered with the material having releasability. 
       FIG. 3  shows a comparison of life of the transfer belt  230  in the secondary transfer roller counter-driven roller  630  according to this embodiment and in a conventional secondary transfer roller counter-driven roller. 
     The vertical axis represents the number of sheets printed by the image forming apparatus. The letter “k” indicates×1000. Graphs S 1  to S 3  show the results of using the conventional secondary transfer roller counter-driven roller. As shown in  FIG. 3 , the transfer belt  230  breaks when approximately 73,000 to 227,000 sheets are printed. 
     Graphs E 1  and E 2  show the results of using the secondary transfer roller counter-driven roller  630  according to this embodiment. If the secondary transfer roller counter-driven roller  630  is used, the transfer belt  230  does not break until approximately 1,322,000 to 1,516,000 sheets are printed. Thus, according to this embodiment, the life of the transfer belt  230  can be extended approximately seven times. 
     Moreover, in this embodiment, it is possible to further extend the life of the transfer belt  230  by covering other driven rollers with a material having releasability. Here, the driven rollers covered with the material having releasability are preferably driven rollers which are situated on the inner side of the transfer belt  230 , contact the transfer belt  230  and rotate by following the turn of the transfer belt  230 . The driven rollers include, for example, the guide rollers  627 ,  628  and  629  but are not limited to these. 
     As described above, the driven roller according to this embodiment is covered with a material having releasability. Therefore, there is an advantage that the developer can be prevented from being fixed to the driven roller, enabling extension of the life of the transfer belt  230 . 
     Although exemplary embodiments of the invention have been shown and described, it will be apparent to those having ordinary skills in the art that a number of changes, modifications, or alterations to the invention as described herein may be made, none of which departs from the spirit of the invention. All such changes, modifications, and alterations should therefore be seen as within the scope of the invention.