Patent Publication Number: US-9429888-B2

Title: Fixing device and image forming apparatus which utilizes a nip supporting member to support a nip forming member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2014-142605 filed in Japan on Jul. 10, 2014 and Japanese Patent Application No. 2015-083730 filed in Japan on Apr. 15, 2015. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, such as a copy machine, a printer, or a facsimile machine. More specifically, the present invention relates to a configuration of a fixing device. 
     2. Description of the Related Art 
     A fixing device is known that includes cylindrical metal heat-conductive member and a heat source that are provided inside an endless belt that is heated via the metal heat-conductive member and further includes a pressure roller that makes a contact with the metal heat conductive member with the endless belt in between, thereby forming a nip part, in which the endless belt is caused to runs in the circumferential direction in accordance with the rotation of the pressure roller (see, for example, Japanese Laid-open Patent Publication No. 2007-334205). This configuration makes it possible to heat the whole endless belt that configures the fixing device, shorten the first print time from the heating stand-by time, and overcome the heat shortage upon high-speed rotation. 
     Another fixing device is known in which only a member that forms a nip, a member that supports the member, and a heart source that are provided inside an endless belt and thus the endless belt can be directly heated with the heat source to form a nip (see, for example, Japanese Laid-open Patent Publication No. 2007-233011). This configuration reduces the heat capacity around the heat source, thereby shortens the first print time from the heating stand-by time. 
     In the conventional configurations, a supporting member supporting a nip member has to penetrate through the inside of the belt and thus the supporting member is a double-supported beam that bears the load from the nip surface at both ends. In order to reduce the size of the fixing device and the extra heat capacity, it is preferable that the volume of the supporting member be small as possible; however, because the supporting member has to form a nip width while bearing the load applied from an elastic pressure roller, if the volume of the supporting member is reduced and accordingly the strength lowers, the center of the supporting member bends in the direction away from the nip surface and accordingly a uniform nip width in the longitudinal direction cannot be obtained. 
     As a method that solves this problem, a method can be assumed in which the center of a nip forming member or a supporting member in the longitudinal direction is formed convex against the nip surface to cancel the bend; however, it is required to form the convex part in an accuracy of 0.05 mm or smaller in order to obtain an even nip surface and thus it costs a lot due to the total inspection etc. 
     In view of the above-described conventional problems, there is a need to provide a fixing device that solves the above-described problem and increases the reliability of the parts with a configuration that allows forming of an accurate convex part relatively easily, which makes it possible to reduce the inspection costs, etc. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to the present invention, there is provided a fixing device comprising: an endless fixing belt; a fixing belt holding member that holds the fixing belt; a pressure member that makes a contact with the circumference of the fixing belt; a nip forming member that is disposed inside the fixing belt and forms a nip by making a contact with the pressure member with the fixing belt between the nip forming member and the pressure member; a nip supporting member that supports the nip forming member and that is formed of a sheet metal; a side plate that supports the fixing belt holding member and the nip supporting member; and a pressure mechanism that presses the pressure member against the nip forming member with the fixing belt between the pressure member and the nip forming member, wherein both end surfaces of the nip supporting member are shear surfaces and one of the shear surfaces makes a contact with the nip forming member and the other shear surface is supported by the side plate. 
     The present invention also provides an image forming apparatus comprising the above-described fixing device according. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exterior view of an image forming apparatus to which an embodiment of the present invention is applicable; 
         FIG. 2  is a schematic diagram of the image forming apparatus to which the embodiment of the invention is applicable; 
         FIG. 3  is a an exterior view of the image forming apparatus without an exterior cover to which the embodiment of the invention is applicable; 
         FIG. 4  is a schematic diagram illustrating an internal configuration of a fixing device that includes an exemplary paper conveyance device of the image forming apparatus to which the embodiment of the invention is applicable; 
         FIG. 5  is a schematic front cross-sectional view of a fixing device, illustrating an embodiment of the invention; 
         FIG. 6  is a schematic plane view of the fixing device, illustrating the embodiment of the invention; 
         FIG. 7  is a schematic plane view of the fixing device without an endless belt, illustrating the embodiment of the invention; 
         FIG. 8  is a schematic diagram illustrating a stay used for the embodiment of the invention; 
         FIG. 9  is a schematic diagram illustrating a pressure mechanism used for the embodiment of the invention; 
         FIG. 10  is a schematic diagram illustrating a pad supporting surface used for a modification of the embodiment of the invention; and 
         FIG. 11  is a schematic diagram illustrating a pad supporting surface used for another modification of the embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, an embodiment for carrying out the invention will be described below. 
     An exemplary paper conveyance device and an image forming apparatus  100  using a fixing device and incorporating the paper conveyance device to which the invention is applied are used for printers: however, the invention is not limited to this and it covers copy machines, facsimile machines, and multifunction peripherals of copy machine and facsimile machine. 
     The body of the image forming apparatus  100  shown in  FIG. 1  is configured of a casing that is a rectangular parallelepiped with an front-back direction serving as a first direction (direction denoted by the arrow X), a crosswise direction orthogonal to the first direction and serving as a second direction (direction denoted by the arrow Y), and a vertical direction serving as a third direction (direction denoted by the arrow Z). 
     In each of the directions, the direction indicated by the arrow X corresponds to the front-back direction of the image forming apparatus  100  and the direction of the width of paper serving as a recording medium parallel to the longitudinal direction of a fixing member and an opposing rotation member that are used for a fixing device  20  to be described below. 
     The image forming apparatus  100  mounts an original scanning device  200  at the top of an image forming unit, which will be described in detail below using  FIG. 2 , in the vertical direction (Z direction), and a paper ejection tray  17  used as an internal ejection unit is formed at the body top surface positioned under the original scanning device  200 . 
     On one side of the image forming apparatus  100  in the width direction (Y direction) that is the crosswise direction, an exterior cover  100 A that is an openable and closable member is disposed. Opening the exterior cover  100 A allows externally viewing of the fixing device  20  disposed inside as shown in  FIG. 3 . 
     The image forming apparatus  100  has the internal configuration shown in  FIG. 2 . Note that the original scanning device  200  shown in  FIG. 1  is not shown in  FIG. 2 . 
     In the image forming apparatus  100  shown in  FIG. 2 , visible images formed on photosensitive drums  120 Y,  120 C,  120 M, and  120 Bk, respectively, are sequentially transferred onto a transfer belt  11  movable in the direction denoted by the arrow A 1  while being opposed to the respective photosensitive drums. The transfer process corresponds to a primary transfer process where each image is sequentially transferred onto the transfer belt  11  so that superimposed transfer images are formed. Thereafter, by executing a secondary transfer process on paper P for which a recording sheet or the like is used, so that the superimposed transfer images are collectively transferred onto the paper P. 
     Devices for performing image forming processing according to the rotation of the photosensitive drum are disposed around each photosensitive drum  120 . A photosensitive drum  120 Bk that forms a black image is descried as follows. Along the direction of rotation of the photosensitive drum  120 Bk, a charging device  30 Bk, a developing device  40 Bk, a primary transfer roller  12 Bk, and a cleaning device  50 Bk that perform the image forming processing are arranged. For writing after charging, an optical scanning device  8  to be described below is used. 
     In the superimposition transfer onto the transfer belt  11 , during the process where the transfer belt  11  moves in the A 1  direction, the visible images formed on the respective photosensitive drums  120  are sequentially transferred as superimposed onto the transfer belt  11 . The primary transfer process is performed sequentially from the upstream to the downstream in the A 1  direction depending on the transfer bias application using the the primary transfer rollers  12 Y,  12 C,  12 M, and  12 Bk that are disposed as opposed to the photosensitive drums  120  with the transfer belt  11  in between. 
     The photosensitive drums  120 Y,  120 C,  120 M and  120 Bk are each housed in a process cartridge and are arranged in this order from the upstream in the direction denoted by the arrow A 1 . The photosensitive drums  120  are respectively provided to image stations respectively for forming yellow, cyan, magenta and black images. For a configuration for executing the primary transfer process, a transfer belt unit  10  is used that includes the transfer belt  11  and the primary transfer rollers  12  opposed respectively to the photosensitive drums  120  with the transfer belt  11  in between. The images that are transferred as superimposed onto the transfer belt  11  are transferred collectively onto the paper P by a secondary transfer roller  5  configured of a roller that rotates according to the transfer belt  11 . 
     The image forming apparatus  100  includes, in addition to the above-descried process cartridge and the transfer belt unit  10 , the optical scanning device  8  serving as an optical writing device that is disposed as opposed to the bottoms of the four image stations and a cleaning device  13  for the transfer belt  11 . 
     The optical scanning device  8  includes a semiconductor laser serving as a light source, a coupling lens, a fθ lens, a toroidal lens, a mirror, and a rotating polygon mirror. The optical scanning device  8  emits writing light Lb corresponding to each color to each photosensitive drum  120  (in  FIG. 2 , only the light from the image station for black image is denoted, but this applies to other image stations). Accordingly, a static latent image is formed on each photosensitive drum  120 . 
     The following devices are used for the image forming apparatus  100 . 
     The image forming apparatus  100  includes a sheet feeding device  61  that feeds paper P onto which images transferred as superimposed by the secondary transfer are collectively transferred and a registration roller pair  4  that sets the timing of registration on the paper P that is fed from the sheet feeding device  61  and feeds the paper P to a secondary transfer position. The image forming apparatus  100  further includes a sensor (not shown) that detects that the tip of the paper P has reached the registration roller pair  4 . 
     The paper P with a toner image T obtained by the superimposition transfer onto the transfer belt  11  by the secondary transfer process is conveyed to the fixing device  20  (see  FIG. 2 ) to be described below where the toner image is fixed. The paper P after the fixing is ejected to the paper ejection tray  17  provided outside the body of the image forming apparatus  100  via a paper ejection roller  7 . The reference numerals  9 Y,  9 C,  9 M and  9 Bk denote new toner supply tanks for the respective developing devices provided to the respective image stations for the respective colors. 
     As shown in  FIG. 4 , the fixing device  20  is used to fix the toner image T after being transferred that is carried on the paper P by heat and pressure to melt the toner image T and permeate the toner image T through the paper P. The fixing device  20  includes a fixing belt  21  that is flexible and rotatable while being heated. The fixing device  20  includes, in addition to the fixing belt  21 , a pressure roller  22  that is an opposite rotation member that causes a pressure between the pressure roller  22  and the fixing belt  21  while making a contact with the fixing belt  21  to form a nip part N. Inside the fixing belt  21 , a heater  23  with a halogen lamp that serves as a heat source and that heats the area other than the nip part N, i.e., the area of the fixing belt  21  that circles at the side opposite to the nip part N, is disposed. 
     Inside the fixing belt  21 , a nip forming member  24  that is a base member for forming a nip that is deposed inside the fixing belt  21 , a stay  25  that supports the nip forming member  24 , and a reflective member  26  that reflects light emitted from the heater  23  to the fixing belt  21  are disposed. Although the details are not shown, the nip forming member  24  that is a base member for forming a nip has a sliding sheet (low friction sheet) surrounding a base pad as a member that makes a contact with the fixing belt  21 . The nip part N of the nip forming member  24  shown in  FIG. 4  is platy, but the shape of the nip part N is not limited to this. For example, in a case where the nip part N is formed concave along the circumference of the pressure roller  22 , the tip of the paper P that passes through the nip N is closer to the pressure roller  22 , which is advantageous in that the separability from the fixing belt  21  increases. 
     The temperature of the fixing belt  21  is detected by a temperature sensor  27  disposed at a side at which the paper P enters the nip part and the temperature is used for feedback processing at the heater  23 . The arrow F shown in  FIG. 4  denotes the direction in which the paper P is conveyed. The fixing belt  21  is an endless belt that is thin, flexible, and formed sleeve-shaped. The fixing belt  21  consists of a base material and a releasable layer positioned at the top of the base material. For the base material, a metal material, such as a nickel or SUS, or a resin material, such as, polyimide is used. For the releasable layer, for example, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) or polytetrafluoroethylene (PTFE) releasable from toner is used. 
     The pressure roller  22  includes a cored bar  22   a , an elastic layer  22   b  consisting of foaming silicone rubber, silicone rubber, or fluoro-rubber and a releasable layer  22   c  provided at the surface of the elastic layer  22   b  and made of, for example, PFA or PTE. The pressure roller  22  is pressed against the fixing belt  21  by a pressing mechanism  204  to be described below and, in a state of making a contact with the fixing belt  21 , makes a contact with the nip forming member  24  serving as a base member with the fixing belt  21  in between. At the part where the pressure roller  22  and the fixing belt  21  make a contact with each other, the elastic layer  22   b  of the pressure roller  22  is crushed so that a pressure is caused between the pressure roller  22  and the fixing belt  21  so that the nip forming member  24  keeps the nip part N with a predetermined width. 
     The pressure roller  22  is driven by a drive source, such as a motor (not shown) provided to the printer body, to rotate. When the pressure roller  22  is driven to rotate, the drive force is transmitted to the fixing belt  21  at the nip part N so that the fixing belt  21  is driven to rate. In the configuration shown in  FIG. 4 , the pressure roller  22  is a solid roller, but it may be a hollow roller. In such a case, a heat source, such as a halogen heater using radiation heat may be disposed inside the pressure roller  22 . In the case where there is not the elastic layer  22   b , the heat capacity reduces and the fixability increases; however, there is a possibility that, when unfixed toner is crushed to be fixed, fine unevenness on the surface of the belt is transferred onto the image and the gloss unevenness may occur in the solid part of the image. In order to prevent this, it is preferable that an elastic layer with a thickness of 100 μm or larger be provided. As a cylindrical metal used for the hollow roller, for example, aluminum, iron, or stainless may be selected. In the case where a heat source is provided inside the pressure roller  22 , in order not to heat the supporting member by the radiant heat from the heat source, it is preferable that a heat-insulating layer be provided at the surface of the supporting member or a heat reflecting surface be provided by mirror surface processing. The heat source in this case is not limited to the halogen heater described above and it is also possible to use an IH heater, a resistance heating element or a carbon heater. 
       FIG. 9  shows the pressure mechanism  204  of the pressure roller  22 . The pressure mechanism  204  includes a substrate  204 A and a pressure roller urging plate  204 B swingably supported by the substrate  204 A. The pressure roller urging plate  204 B is supported at the lower end of the substrate  204 A serving as a swing fulcrum as shown in  FIG. 9 . A spring  205  arranged between the pressure roller urging plate  204 B and the substrate  204 A normally gives the swing end a behavior of swing toward the fixing belt  21 . 
     At the bottom end of the substrate  204 A, an input gear  206  to which the drive force is transmitted from the apparatus body is rotatably supported by a spindle  206 A. An idle gear  208  that is provided to an attachment plate  207  used to attach the input gear  206  to the apparatus body is engaged with the input gear  206  and an output gear  209  that is supported on the same shaft as that of the pressure roller  22  is engaged with the idle gear  208 . The substrate  204 A and the pressure roller urging plate  204 B can swing on a spindle  206 A serving as a swing fulcrum as the arrow in  FIG. 9  shows. The swing direction corresponds to the direction in which the pressure roller  22  is pressed against the fixing belt  21  to apply a pressure to the nip part and the direction in which the pressure is released from the nip part. 
     The operations of swinging the substrate  204 A and the pressure roller urging plate  204 B are performed by an eccentric cam  210  that is arranged on the swing end back side of the substrate  204 A and a bearing  211  that is rotatably supported on the side of the substrate  204 A so as to make a contact with the eccentric cam  210 . In other words, when the large diameter part of the rotation phase of the eccentric cam  210  pushes and moves the bearing  211 , the pressure roller  22  moves toward the fixing belt  21 . Once the opposition position of the eccentric cam  210  with respect to the bearing  211  changes from the large diameter part to the small diameter part, the pressure roller  22  moves in a direction to such that the nip pressure is released. 
     The image forming apparatus  100  using the fixing device  20  configured as described above includes a paper conveyance device for conveying paper. An exemplary paper conveyance device  300  is aimed at paper that passes through the nip part N of the fixing device  20 . As shown in  FIG. 4 , the paper conveyance device  300  configures a conveyance path for conveying the paper P that has passed through the nip part N sequentially to the outlet side of the fixing device  20  toward the paper ejection tray  17  (see  FIG. 2 ). 
     The characteristics of the present invention according to the image forming apparatus  100  configured as described above will be described. In the above-described fixing device  20 , as described in the section of “Description of the Related Art”, it is required to form the convex part of the nip forming member  24  in an accuracy of 0.05 mm or smaller in order to obtain an even nip surface and thus it costs a lot due to the total inspection etc. A configuration that solves the problem will be described below. 
       FIGS. 5 and 6  shows a fixing device  81  used instead of the fixing device  20  in the embodiment of the present invention. The fixing device  81  includes a fixing belt  82  that is an endless belt. The fixing belt  82  includes therein a pad  83  that is a nip forming member, a stay  84  that is a nip supporting member that positions the pad  83 , and a heater  85  that heats the fixing belt  82 . Both ends of the fixing belt  82  are held by holders  86  serving as a fixing belt holding member and the stay  84  and the holders  86  are supported by side plates  87 . At the front side of the fixing belt  82 , a pressure roller  88  serving as a pressure member is disposed in a position where the pressure roller  88  is opposed to the pad  83  and the pressure roller  88  is pressed against the pad  83  by springs (not shown) provided near both ends of a spindle of the pressure roller  88 , so that a nip surface  89  is formed. 
     According to the above-described configuration, in a fixing operation, the pressure roller  88  rotates clockwise as shown in  FIG. 5  by the drive force from a drive mechanism (not shown), the fixing belt  82  runs in accordance with the rotation, and the heater  85  directly heats the fixing belt  82 . Thereafter, passing through of paper is started when sufficient heat is stored. Specifically, paper  91  on which toner  90  has been transferred is conveyed from the lower part in  FIG. 5  and the paper  91  passes through the nip surface  89  so that the toner  90  is fixed onto the paper  91 . 
     In the above-descried fixing operations, when the nip width is narrow and the amount of heat applied to the toner  90  is too small, the toner  90  cannot sufficiently melt and thus the toner  90  cannot be fixed onto the paper  91 . On the contrary, when the nip width is wide and the amount of heat applied to the toner  90  is too large, the toner  90  has excessive flowability over the rubbery state and thus the toner  90  cannot be fixed onto the paper  91 . In order to prevent occurrence of such a problem, it is necessary to reduce unevenness in the nip width due to various factors. As exemplary nip width unevenness, a state can be exemplified where the stay  84  bends due to a load applied from the pressure roller  88  and the center nip width is narrowed, which causes a deviation in the nip width. 
     In order to solve the above-described problem, according to the invention, pad supporting surfaces  92  of the stay  84  serving as one of shear surfaces are formed to be convex in a stepwise fashion from both ends in the longitudinal direction toward the center as shown in  FIG. 7 . This configuration cancels the bend of the stay  84 . 
     The stay  84  includes a stay top  84   a , a stay bottom  84   b , and a stay right  84   c . The stay top  84   a  is a steel plate member and its bottom surface shown in  FIG. 8  is formed of a linear shear surface in the longitudinal direction. The top end surface of the stay top  84   a  shown in  FIG. 8  is formed of a shear surface that is convex in a stepwise fashion from both ends in the longitudinal direction toward the center. The top end surface of the stay top  84   a  penetrates through the surface of the stay right  84   c  and this top end surface forms the pad supporting surfaces  92  serving as one of the shear surfaces. The stay bottom  84   b  is also a steel plate member and its bottom surface shown in  FIG. 8  is formed of a linear shear surface in the longitudinal direction. Because the stay bottom  84   b  is bent midway, the top end surface of the stay bottom  84   b  is away from the top end surface of the stay top  84   a  by a given distance and is formed of a shear surface that is convex in a stepwise fashion from both ends in the longitudinal direction toward the center. The top end surface of the stay bottom  84   b  also penetrates through the surface of the stay right  84   c  and the top end surface forms the pad supporting surfaces  92  serving as one of the shear surfaces. The stay top  84   a  and the stay bottom  84   b  are fixed to each other by, for example, welding, bonding, calking, or screwing and the stay top  84   a  and the stay bottom  84   b  are fixed to the stay right  84   c  by, for example, welding, bonding, calking, or screwing. The bottom end surfaces of the stay top  84   a  and the stay bottom  84   b  configure the stay supporting surface  93  serving as the other shear surface and the stay supporting surface  93  is fitted to holes  87   a  formed in the side plates  87  so that the stay  84  is supported by the side plates  87 . 
     According to the above-described configuration, the pad  83  can be positioned with the shear surface that is formed in the stay  84  formed of a metal plate with which dimensional accuracy is easily obtained, which makes it possible to easily form the accurate pad supporting surfaces  92 . This makes it possible to provide a fixing device having the parts with increased reliability that can reduce the examination costs. Furthermore, by forming the pad supporting surfaces  92  to be convex in a stepwise fashion from both ends in the longitudinal direction toward the center, the nip deviation due to bending of the stay  84  can be canceled and thus an even nip can be obtained. 
     According to the above-described configuration, the pad supporting surfaces  92  are formed to be convex in a stepwise fashion from both ends in the longitudinal direction toward the center. Alternatively, instead of the pad supporting surfaces  92 , a pad supporting members  92   a  formed to form an arc as shown in  FIG. 10  may be used. This makes it possible to prevent the pad  83  from making a linear contact with the pad supporting surfaces  92   a  and obtain an even nip and thus prevent the pad  83  from being broken. According to the above-described configuration, multiple pad supporting surfaces  92  that are discontinuous and pectinate are formed. Alternatively, instead of the pad supporting surfaces  92 , a pad supporting member  92   b  that is formed to be continuous as shown in  FIG. 11  may be formed. Note that forming pad supporting surfaces separately allows independent fine adjustment of the shear surfaces, which makes it possible to easily secure the parts reliability. Furthermore, because the stay  84  consists of three metal plates of the stay top  84   a , the stay bottom  84   b , and the stay right  84   c , the stay  84  can be formed of the minimum number of parts, which allows cost reduction and easy assembly. 
     According to an aspect of the embodiment, the nip forming member can be positioned by the shear surface that is formed in the nip supporting member formed of a metal plate with which dimensional accuracy is easily obtained, which makes it possible to easily form an accurate one of the shear surfaces. This makes it possible to provide a fixing device having the parts with increased reliability that can reduce the examination costs. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.