Patent Publication Number: US-8971777-B2

Title: Fixing device, image forming apparatus, and fixing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-069244 filed Mar. 26, 2012. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a fixing device, an image forming apparatus, and a fixing method. 
     (ii) Related Art 
     In the past, an electrophotographic image forming apparatus including a fixing device which fixes an unfixed toner image on a recording medium has been known. 
     SUMMARY 
     According to an aspect of the invention, there is provided a fixing device including a cylinder member, a circulating member, a heat source, and a pressing member. The cylinder member, which has a shape of a cylinder and is elastically deformed in a direction of crushing the cylinder, rotates in a circumferential direction of the cylinder. The circulating member, which has a shape of a cylinder, circularly moves in a circumferential direction of the cylinder with an outer circumferential surface thereof pressed against the cylinder member. The heat source heats a recording medium which holds an unfixed toner image on a surface thereof, and which is nipped between the cylinder member and the circulating member. The pressing member, which is provided inside the circulating member, presses the circulating member against the cylinder member from inside the circulating member. The pressing member includes a nip forming portion and a thrusting portion. The nip forming portion presses the circulating member and brings the circulating member and the cylinder member into contact with each other, to thereby form a nip region therebetween. The thrusting portion thrusts, at a position upstream of the nip forming portion in a moving direction of the circulating member, and from inside the circulating member toward the cylinder member, the circulating member before being pressed against the cylinder member such that the thrust is greater in end portions of the circulating member than in a central portion of the circulating member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic configuration diagram illustrating a printer corresponding to a first exemplary embodiment of an image forming apparatus of the invention; 
         FIG. 2  is a perspective view illustrating major structural components in a fixing device; 
         FIG. 3  is an exploded perspective view illustrating a structure of a pressure structure; 
         FIG. 4  is a cross-sectional view of a central portion of the fixing device; 
         FIG. 5  is a cross-sectional view of an end portion of the fixing device; 
         FIG. 6  is a conceptual diagram illustrating a deformed state of a fixing roller and a pressure block; 
         FIGS. 7A to 7C  are diagrams illustrating states of a sheet passing the fixing device; 
         FIG. 8  is an exploded perspective view illustrating a structure of a pressure structure in a second exemplary embodiment of the invention; 
         FIG. 9  is a cross-sectional view of a central portion of a fixing device in the second exemplary embodiment of the invention; and 
         FIG. 10  is a cross-sectional view of an end portion of the fixing device in the second exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
       FIG. 1  is a schematic configuration diagram illustrating a printer corresponding to a first exemplary embodiment of an image forming apparatus of the invention. 
     A printer  100  illustrated in  FIG. 1  is a tandem-type color printer including juxtaposed image forming units  10 Y,  10 M,  10 C, and  10 K for yellow (Y), magenta (M), cyan (C), and black (K) colors, respectively. The printer  100  is capable of printing an image of a single color and a full-color image formed by toner images of four colors. Further, the printer  100  is an apparatus capable of handling not only a paper sheet, i.e., a recording medium made of paper but also a recording medium made of a resin, which is typified by an overhead projector (OHP) sheet. The following description will be made on the assumption that the recording medium is represented by the paper sheet, unless otherwise specified. 
     The printer  100  includes one exposure device  13  for the four image forming units  10 Y,  10 M,  10 C, and  10 K. 
     The four image forming units  10 Y,  10 M,  10 C, and  10 K are similar in configuration, including size and material. Thus, the image forming unit  10 Y corresponding to the yellow color will be described as a representative of the image forming units  10 Y,  10 M,  10 C, and  10 K. The image forming unit  10 Y includes a photoconductor  11 Y, a charging device  12 Y, a developing device  14 Y, a first transfer device  15 Y, and a photoconductor cleaner  16 Y. 
     The photoconductor  11 Y has the shape of a cylinder, and extends in the depth direction of  FIG. 1 . When a surface of the photoconductor  11 Y is charged, the photoconductor  11 Y holds charge. Further, when exposed to light, the photoconductor  11 Y releases the charge. Thereby, an electrostatic latent image is formed on the surface of the photoconductor  11 Y. The photoconductor  11 Y rotates around the axis of the cylinder thereof in the direction indicated by an arrow A, while holding an image (electrostatic latent image or toner image) formed on the surface thereof. The charging device  12 Y, the developing device  14 Y, the first transfer device  15 Y, and the photoconductor cleaner  16 Y are sequentially disposed around the circumference of the photoconductor  11 Y. 
     The charging device  12 Y includes a charging roller which rotates while in contact with the surface of the photoconductor  11 Y. The charging device  12 Y applies charge to the surface of the photoconductor  11 Y by using the charging roller, to thereby charge the surface. As well as the charging roller, a corona discharging device not in contact with the photoconductor  11 Y may be employed as the charging device  12 Y. 
     The exposure device  13  includes a light emitting device which emits laser light modulated in accordance with an image signal supplied from outside the printer  100 , and a rotary polygon mirror for scanning the photoconductor  11 Y with the laser light. The exposure device  13  radiates the laser light to the photoconductor  11 Y, to thereby expose the surface of the photoconductor  11 Y to the light and form an electrostatic latent image on the surface. As well as the system using the laser light, a light-emitting diode (LED) array including multiple LEDs aligned along a scanning direction may be employed as the exposure device  13 . 
     The developing device  14 Y develops the latent image on the surface of the photoconductor  11 Y by using a two-component developer formed by a toner and a magnetic carrier. The developing device  14 Y is supplied, as required, with the toner from a toner cartridge, the illustration of which is omitted. The toner is mixed and stirred with the magnetic carrier in the developing device  14 Y, and thereby is charged. The developing device  14 Y develops the latent image on the surface of the photoconductor  11 Y with the charged toner in the developer. With this development process, a toner image is formed ob the surface of the photoconductor  11 Y. 
     The first transfer device  15 Y serves as a roller facing the photoconductor  11 Y across an intermediate transfer belt  30 . The first transfer device  15 Y includes a conductive elastic layer on a surface thereof. The first transfer device  15 Y is applied with a voltage having a polarity opposite to a toner charging polarity, to thereby electrostatically attract the toner image on the photoconductor  11 Y to the intermediate transfer belt  30 . 
     The photoconductor cleaner  16 Y includes a cleaning blade in contact with the surface of the photoconductor  11 Y, and cleans the surface of the photoconductor  11 Y after the transfer. More specifically, residual toner, an external additive, and paper dust are scraped off the surface of the photoconductor  11 Y by the cleaning blade. As well as the system including the cleaning blade, a system which performs the cleaning by using a charged brush or nonwoven fabric may be employed as the photoconductor cleaner  16 Y. 
     The printer  100  further includes the intermediate transfer belt  30 , a fixing device  60 , and a sheet transport unit  80 . The intermediate transfer belt  30  is an endless belt made of a resin material containing a charge preventing agent. The intermediate transfer belt  30  is stretched over belt support rollers  31  to  33 , and circularly moves in the direction indicated by an arrow B via the image forming units  10 Y,  10 M,  10 C, and  10 K and a second transfer device  50 . The toner images of the respective colors are transferred to the intermediate transfer belt  30  from the image forming units  10 Y,  10 M,  10 C, and  10 K. The intermediate transfer belt  30  moves while holding the toner image of the respective colors. 
     The sheet transport unit  80  includes a pickup roller  81  which picks up sheets stored in a sheet container T, separation rollers  82  which separate the picked-up sheets, transport rollers  83  which transport the sheets, registration rollers  84  which transport the sheets to the second transfer device  50 , and discharge rollers  86  which discharge the sheets to the outside. The sheets stored in the sheet container T are picked up by the pickup roller  81 , and are separated from one another by the separation rollers  82 . Thereafter, each of the sheets is transported in the direction of an arrow C by the transport rollers  83 , and reaches the registration rollers  84 . The registration rollers  84  transport the sheet such that the sheet arrives a position facing the second transfer device  50  in synchronization with the arrival of the toner image on the intermediate transfer belt  30  to the position. 
     The second transfer device  50  serves as a roller which rotates while nipping the intermediate transfer belt  30  and the sheet between the second transfer device  50  and a backup roller  32  included in the belt support rollers  31  to  33 . The second transfer device  50  includes a conductive elastic layer on a surface thereof. The second transfer device  50  is applied with a voltage having a polarity opposite to the toner charging polarity, to thereby electrostatically attract the toner image on the intermediate transfer belt  30  to the sheet. 
     The combination of the image forming units  10 Y,  10 M,  100 , and  10 K, the intermediate transfer belt  30 , and the second transfer device  50  corresponds to an example of an image forming device of the invention. 
     The sheet subjected to the transfer of the toner image is further transported in the direction of an arrow D by the transport rollers  83 , and enters the fixing device  60 . The fixing device  60  includes therein a fixing roller  61  and a pressure structure  62 . The fixing roller  61  includes therein a heater  63 . In the fixing roller  61 , the sheet formed with the unfixed toner image is passed through between the fixing roller  61  and the pressure structure  62  while being nipped therebetween. Then, the sheet is heated by the heater  63 . Thereby, the toner image is fixed on the sheet. The fixing device  60  corresponds to an exemplary embodiment of a fixing device of the invention. 
     The sheet having passed the fixing device  60  proceeds in the direction of an arrow E toward the discharge rollers  86 , and is further transported and discharged onto a sheet exit tray  70  by the discharge rollers  86 . 
     Subsequently, details of the fixing device  60  will be described. 
       FIG. 2  is a perspective view illustrating major structural components in the fixing device  60 . 
     As described above, the fixing device  60  includes therein the fixing roller  61  and the pressure structure  62 , and the fixing roller  61  includes therein the heater  63 . 
     The fixing roller  61  is formed by a metal cylinder having an outer circumferential surface coated with a release material, and has opposite ends respectively rotatably supported by fixing roller support members  64 . Further, one of the ends of the fixing roller  61  is attached with a drive gear  67 . The fixing roller  61  rotates in the direction of an arrow F with rotational drive force received via the drive gear  67 . The fixing roller  61  corresponds to an example of a cylinder member of the invention, and the heater  63  included in the fixing roller  61  corresponds to an example a heat source of the invention. 
     The pressure structure  62  has opposite ends respectively supported by pressure arms  65 . Further, the pressure arms  65  are respectively biased toward the fixing roller support members  64  by pressure springs  66 . Thereby, the pressure structure  62  is firmly pressed against the fixing roller  61 . As described later, a surface of the pressure structure  62  is formed by a belt, and the belt circularly moves in the direction of an arrow G in accordance with the rotation of the fixing roller  61 . As a result, the sheet nipped between the pressure structure  62  and the fixing roller  61  is transported to the near side of the drawing. 
       FIG. 3  is an exploded perspective view illustrating a structure of the pressure structure  62 . 
     The pressure structure  62  includes a belt  621 , a belt guide member  622 , a support member  623 , a pressure block  624 , a lubricant sheet  625 , and end surface covers  626 . The belt  621  is a member forming a circumferential surface of the pressure structure  62  formed into a rod shape as a whole. As described above, the belt  621  circularly moves in accordance with the rotation of the fixing roller  61 . The belt  621  corresponds to an example of a circulating member of the invention. 
     The belt guide member  622  is a member which guides, inside the belt  621 , such circular movement of the belt  621 . The belt  621  circularly moves in an unstretched state, and thus is loosely guided by the belt guide member  622 . The support member  623  is fit in the belt guide member  622 , and supports the pressure block  624 . Supported by the support member  623 , the pressure block  624  presses the belt  621  from inside the belt  621 , to thereby press the belt  621  against the fixing roller  61  (see  FIG. 2 ). The pressure block  624  corresponds to the combination of an example of a pressing member of the invention and an example of a thrusting member of the invention. 
     The lubricant sheet  625  is a member which covers the pressure block  624  to enhance the lubricity of the belt  621  on the pressure block  624 . 
     With the belt  621  pressed against the fixing roller  61  by the pressure block  624 , a nip region is formed between the pressure structure  62  and the fixing roller  61 . 
       FIG. 4  is a cross-sectional view of a central portion of the fixing device  60 , and  FIG. 5  is a cross-sectional view of an end portion of the fixing device  60 . 
     As illustrated in  FIGS. 4 and 5 , a nip region R is formed between a nip forming portion  627  of the pressure block  624  and the fixing roller  61 . The nip forming portion  627  is a part of the pressure block  624  located on the downstream side in the rotation direction F of the fixing roller  61  and the circular movement direction G of the belt  621 , and serves as an example of the pressing member of the invention. Further, at a position upstream of the nip forming portion  627  in the rotation direction F of the fixing roller  61  and the circular movement direction G of the belt  621 , a belt thrusting portion  628  is provided. The belt thrusting portion  628  thrusts the belt  621  toward the fixing roller  61  at a position upstream of the nip region R, and serves as an example of the thrusting member of the invention. Further, the thrust exerted on the belt  621  by the belt thrusting portion  628  is greater in the end portion illustrated in  FIG. 5  than in the central portion illustrated in  FIG. 4 . 
     With the belt  621  pressed against the fixing roller  61  by the thus configured pressure block  624 , the fixing roller  61  is elastically deformed in a direction of crushing the cylinder thereof, and is also elastically deformed in a direction of flexing a central portion of the cylinder relative to the opposite ends of the cylinder. Meanwhile, the pressure block  624  is pressed against the fixing roller  61 , and thereby is also elastically deformed in a direction of flexing a central portion of the pressure block  624  relative to the opposite ends of the pressure block  624 . 
       FIG. 6  is a conceptual diagram illustrating a deformed state of the fixing roller  61  and the pressure block  624 . In  FIG. 6 , the upper and lower sides are opposite to those of  FIGS. 1 and 2 . 
     The fixing roller  61  receives the force of the pressure block  624 , while being supported at the opposite ends thereof. Thus, the fixing roller  61  is elastically deformed in the direction of flexing the central portion thereof. The nip forming portion  627  of the pressure block  624  is formed to be thicker in a central portion thereof than in end portions thereof such that the pressure force applied to the circumferential surface of the fixing roller  61  is substantially equal from a central portion of the circumferential surface to each of end portions of the circumferential surface (also see  FIGS. 4 and 5 ). Further, together with the support member  623  supporting the pressure block  624 , the pressure block  624  is elastically deformed in the direction of flexing the central portion thereof. Such flexural deformation of the fixing roller  61  and the pressure block  624  causes bending deformation in the belt  621  (not illustrated in  FIG. 6 ; see  FIGS. 4 and 5 ) and a sheet passing through between the fixing roller  61  and the nip forming portion  627 , making the belt  621  and the sheet project toward the upper side of  FIG. 6  (i.e., toward the fixing roller  61 ). 
     At a position upstream of the nip region R where such bending deformation occurs, the belt thrusting portion  628  of the pressure block  624  projects toward the fixing roller  61 , and thrusts end portions of the belt  621  and the sheet in a direction of suppressing the bending deformation before the end portions enter the nip region R. Therefore, the degree of the bending deformation occurring in the belt  621  and the sheet is small immediately before the belt  621  and the sheet enter the nip region R. In the example illustrated in  FIG. 6 , the thrust by the belt thrusting portion  628  is adjusted to an extent that offsets the bending deformation occurring in the nip region R. Therefore, the belt  621  and the sheet are substantially flat when passing over the belt thrusting portion  628 . 
       FIGS. 7A to 7C  are diagrams illustrating states of the sheet passing the fixing device  60 . In  FIGS. 7A to 7C , the upper and lower sides are the same as those of  FIG. 6 , and are opposite to those of  FIGS. 1 and 2 . 
       FIG. 7A  illustrates the bending deformation occurring in the sheet when the sheet is nipped in the nip region R. The bending deformation occurs in the sheet, making the sheet project toward the upper side of the drawing. Such bending deformation generates, in the sheet, tension acting in the directions indicates by an arrow in the drawing. 
       FIG. 7B  illustrates a state in which a front portion of the sheet is nipped in the nip region R of the fixing device  60  and a rear portion of the sheet is nipped between the transport rollers  83 . As illustrated in  FIG. 1 , a path of the sheet passing the fixing device  60  is bent inward toward the fixing roller  61 . While being transported in the direction of the arrow D by the transport rollers  83  provided before the fixing device  60 , a portion of the sheet arrives the nip region R of the fixing device  60 . This positional relationship is illustrated in  FIG. 7B , in which, when the leading end of the sheet located on the far side of the drawing has reached the nip region R, the rear end of the sheet located on the near side of the drawing is pulled toward the upper side of the drawing. If the rear end side of the sheet is oriented in such a direction, the sheet is not easily bent owing to the above-described tension. It is therefore desired to provide a configuration which suppresses the formation of a crease in a central portion of the sheet. 
       FIG. 7C  illustrates a state in which the end portions of the sheet are thrust by the belt thrusting portion  628  of the pressure block  624 . When a preceding portion P 1  of the sheet is nipped in the nip region R, a subsequent portion P 2  subsequent to the preceding portion P 1  is thrust toward the upper side of the drawing (i.e., toward the fixing roller  61 ) at left and right end portions of the subsequent portion P 2  in the drawing by the belt thrusting portion  628  of the pressure block  624 . As a result, the bending deformation illustrated in  FIG. 7A  is limited to an area near the nip region R, and the sheet is substantially flat in the subsequent portion P 2 , as described above. Even if the rear end side of the sheet is pulled toward the upper side of the drawing, therefore, the sheet is easily bent, and a crease is not formed. 
     The sheet is not necessarily required to be flat, as long as the thrust by the belt thrusting portion  628  is greater in the end portions of the sheet than in the central portion of the sheet. This is because, if the thrust reduces the deformation of the sheet to be less than the bending deformation occurring in the nip region R, the tension of the sheet is reduced accordingly, and the formation of a crease is suppressed. Further, the belt thrusting portion  628  may thrust the end portions of the sheet to an extent that makes the sheet project toward the lower side of the drawing. This is because the above-described tension of the sheet does not obstruct the bending of the sheet toward the lower side of the drawing. 
     A second exemplary embodiment of the invention will be described below. An image forming apparatus and a fixing device of the second exemplary embodiment are similar to the image forming apparatus and the fixing device of the above-described first exemplary embodiment, except for a difference in structure of the pressure structure. In the following, therefore, the structure of the pressure structure will be particularly described. Further, constituent components similar to those of the first exemplary embodiment will be designated by the same reference numerals, and repetitive description thereof will be omitted. 
       FIG. 8  is an exploded perspective view illustrating a structure of a pressure structure in the second exemplary embodiment of the invention. 
     In a pressure structure  69  in the second exemplary embodiment, the support member  623  supports a base  691 , and a plate spring  692  is mounted on the base  691 . The plate spring  692  presses the belt  621  from inside the belt  621 , and thereby the belt  621  is pressed against the fixing roller  61  (see  FIG. 2 ). The plate spring  692  corresponds to an example of the pressing member of the invention. 
     With the belt  621  pressed against the fixing roller  61  by the plate spring  692 , the nip region R is formed between the pressure structure  69  and the fixing roller  61 . The pressing by the plate spring  692  also causes, in the fixing roller  61  and the nip region R, flexure similar to the flexure illustrated in  FIG. 6 . Further, the bending deformation as illustrated in  FIG. 7A  occurs in the belt  621  and the sheet passing the nip region R, and the above-described tension is also generated. 
       FIG. 9  is a cross-sectional view of a central portion of the fixing device in the second exemplary embodiment of the invention.  FIG. 10  is a cross-sectional view of an end portion of the fixing device in the second exemplary embodiment of the invention. 
     As illustrated in  FIGS. 9 and 10 , the nip region R is formed between the plate spring  692  and the fixing roller  61 . At a position upstream of the plate spring  692  in the rotation direction F of the fixing roller  61  and the circular movement direction G of the belt  621 , a part of the base  691  projects toward the fixing roller  61 . Further, the base  691  thrusts the belt  621  and the sheet toward the fixing roller  61  by using the projecting portion. The thrust exerted on the belt  621  by the base  691  is greater in the end portion illustrated in  FIG. 10  than in the central portion illustrated in  FIG. 9 . That is, the base  691  corresponds to an example of the thrusting member of the invention. With the thrust exerted on the belt  621  and the sheet by the base  691 , the sheet in the state illustrated in  FIG. 7C  enters the fixing device. Accordingly, the formation of a crease in the sheet is suppressed. 
     In the above-described exemplary embodiments, the heater  63  is provided inside the fixing roller  61 . The heat source of the invention, however, may be provided inside the circulating member of the invention, or may be provided independently, not provided inside the cylinder member or the circulating member of the invention. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.