Patent Publication Number: US-9897955-B2

Title: Pressure member, fixing device, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-039312 filed Mar. 1, 2016. 
     BACKGROUND 
     Technical Field 
     The present invention relates to pressure members, fixing devices, and image forming apparatuses. 
     SUMMARY 
     According to an aspect of the invention, there is provided a fixing device including a first fixing member and a second fixing member. The first fixing member is supported in a rotatable manner. The second fixing member is pressed against the first fixing member and includes a belt-shaped component, a support component, and a pressing component. The belt-shaped component extends in a width direction of a medium that passes through a contact region where the belt-shaped component comes into contact with the first fixing member. The support component is disposed opposite the first fixing member with the belt-shaped component interposed therebetween. The pressing component has a base end portion supported by the support component and a free end portion extending toward the contact region and presses the belt-shaped component against the first fixing member. The pressing component has multiple biasing portions that are arranged apart in the width direction and that are elastically deformable in correspondence with a thickness of the medium. The biasing portions are connected in the width direction of the medium by the free end portion. The free end portion is disposed at an upstream side in a transport direction of the medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates an image forming apparatus according to a first exemplary embodiment; 
         FIG. 2  illustrates a relevant part of the image forming apparatus according to the first exemplary embodiment; 
         FIG. 3  is an overall view illustrating a fixing device according to the first exemplary embodiment; 
         FIG. 4  illustrates a pressure member of the fixing device according to the first exemplary embodiment; 
         FIG. 5  illustrates a state where a pressure belt is removed from the pressure member shown in  FIG. 4 ; 
         FIG. 6  illustrates a state where oil-impregnated felt and a sheet are removed from the state shown in  FIG. 5 ; 
         FIGS. 7A and 7B  illustrate a support component constituting the pressure member according to the first exemplary embodiment,  FIG. 7A  being a perspective view thereof,  FIG. 7B  being a side view thereof; 
         FIGS. 8A and 8B  illustrate frame bodies constituting the pressure member according to the first exemplary embodiment,  FIG. 8A  being a perspective view thereof,  FIG. 8B  being a side view thereof; 
         FIGS. 9A and 9B  illustrate a pressing component constituting the pressure member according to the first exemplary embodiment,  FIG. 9A  being a perspective view of the pressing component,  FIG. 9B  illustrating a state where a contact member is supported by a free end portion of the pressing component; 
         FIG. 10  illustrates a technology in the related art; and 
         FIG. 11  is an enlarged view of an area surrounding a bent portion according to the first exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Although a specific exemplary embodiment of the present invention will be described below with reference to the drawings, the present invention is not to be limited to the following exemplary embodiment. 
     In order to provide an easier understanding of the following description, the front-rear direction will be defined as “X-axis direction” in the drawings, the left-right direction will be defined as “Y-axis direction”, and the up-down direction will be defined as “Z-axis direction”. Moreover, the directions or the sides indicated by arrows X, −X, Y, −Y, Z, and −Z are defined as forward, rearward, rightward, leftward, upward, and downward directions, respectively, or as front, rear, right, left, upper, and lower sides, respectively. 
     Furthermore, in each of the drawings, a circle with a dot in the center indicates an arrow extending from the far side toward the near side of the plane of the drawing, and a circle with an “x” therein indicates an arrow extending from the near side toward the far side of the plane of the drawing. 
     In the drawings used for explaining the following description, components other than those for providing an easier understanding of the description are omitted where appropriate. 
     First Exemplary Embodiment 
       FIG. 1  illustrates an image forming apparatus according to a first exemplary embodiment. 
       FIG. 2  illustrates a relevant part of the image forming apparatus according to the first exemplary embodiment. 
     In  FIG. 1 , a copier U as an example of the image forming apparatus according to the first exemplary embodiment of the present invention is an example of an apparatus body and has a printer unit U 1  as an example of an image recording device. A scanner unit U 2  as an example of a reader unit as well as an example of an image reading device is supported at the upper portion of the printer unit U 1 . An auto feeder U 3  as an example of a document transport device is supported at the upper portion of the scanner unit U 2 . The scanner unit U 2  according to the first exemplary embodiment supports a user interface UI as an example of an input unit. An operator may input information to the user interface UI so as to operate the copier U. 
     A document tray TG 1  as an example of a medium container is disposed at the upper portion of the auto feeder U 3 . The document tray TG 1  is capable of accommodating a stack of multiple documents Gi to be copied. A document output tray TG 2  as an example of a document output unit is provided below the document tray TG 1 . Document transport rollers U 3   b  are arranged along a document transport path U 3   a  between the document tray TG 1  and the document output tray TG 2 . 
     Platen glass PG as an example of a transparent document table is disposed at the upper surface of the scanner unit U 2 . In the scanner unit U 2  according to the first exemplary embodiment, a reading optical system A is disposed below the platen glass PG. The reading optical system A according to the first exemplary embodiment is supported in a movable manner in the left-right direction along the lower surface of the platen glass PG. Normally, the reading optical system A is in a stopped state at an initial position shown in  FIG. 1 . 
     An imaging element CCD as an example of an imaging member is disposed to the left of the reading optical system A. The imaging element CCD is electrically connected to an image processor IPS. 
     The image processor IPS is electrically connected to a write circuit DL of the printer unit U 1 . The write circuit DL is electrically connected to light-emitting-diode (LED) heads LHy, LHm, LHc, and LHk as an example of latent-image forming devices. 
     Photoconductor drums PRy, PRm, PRc, and PRk as an example of image carriers are respectively disposed above the LED heads LHy to LHk. 
     Charging rollers CRy, CRm, CRc, and CRk as an example of charging units are respectively disposed facing the photoconductor drums PRy to PRk. The charging rollers CRy to CRk receive charge voltage from a power supply circuit E. The power supply circuit E is controlled by a controller C as an example of a controller. The controller C performs various kinds of control by exchanging signals with, for example, the image processor IPS and the write circuit DL. 
     In write regions Q 1   y , Ql m , Q 1   c , and Q 1   k  set downstream of the charging rollers CRy to CRk in the rotational direction of the photoconductor drums PRy to PRk, the LED heads LHy to LHk radiate write light onto the surfaces of the photoconductor drums PRy to PRk. 
     In developing regions Q 2   y , Q 2   m , Q 2   c , and Q 2   y  set downstream of the write regions Q 1   y  to Q 1   k  in the rotational direction of the photoconductor drums PRy to PRk, developing devices Gy, Gm, Gc, and Gk are disposed facing the surfaces of the respective photoconductor drums PRy to PRk. 
     First-transfer regions Q 3   y , Q 3   m , Q 3   c , and Q 3   k  are set downstream of the developing regions Q 2   y  to Q 2   y  in the rotational direction of the photoconductor drums PRy to PRk. In the first-transfer regions Q 3   y  to Q 3   k , the photoconductor drums PRy to PRk are in contact with an intermediate transfer belt B as an example of an intermediate transfer member. Furthermore, in the first-transfer regions Q 3   y , Q 3   m,  Q 3   c , and Q 3   k , first-transfer rollers T 1   y , T 1   m , T 1   c , and T 1   k  as an example of first-transfer units are disposed opposite the photoconductor drums PRy to PRk with the intermediate transfer belt B interposed therebetween. 
     Drum cleaners CLy, CLm, CLc, and CLk as an example of image-carrier cleaning units are disposed downstream of the first-transfer regions Q 3   y  to Q 3   k  in the rotational direction of the photoconductor drums PRy to PRk. 
     In the first exemplary embodiment, the photoconductor drums PRy to PRk and the drum cleaners CLy to CLk are configured as photoconductor units as an example of image carrier units for the respective colors and are integrally detachable from the printer unit U 1 . Moreover, the developing devices Gy to Gk serving as developing units are also detachable from the printer unit U 1 . 
     A belt module BM as an example of an intermediate transfer device is disposed above the photoconductor drums PRy to PRk. The belt module BM has the aforementioned intermediate transfer belt B. The intermediate transfer belt B is supported in a rotatable manner by a driving roller Rd as an example of a driving member, a tension roller Rt as an example of a tension member, a working roller Rw as an example of a meander correction member, an idler roller Rf as an example of a driven member, a backup roller T 2   a  as an example of a second-transfer-region opposing member, and first-transfer rollers T 1   y , T 1   m , T 1   c,  and T 1   k.    
     A second-transfer roller T 2   b  as an example of a second-transfer member is disposed opposite the backup roller T 2   a  with the intermediate transfer belt B interposed therebetween. The backup roller T 2   a  and the second-transfer roller T 2   b  constitute a second-transfer unit T 2 . A second-transfer region Q 4  is formed by a region where the second-transfer roller T 2   b  and the intermediate transfer belt B face each other. 
     For example, the first-transfer rollers T 1   y  to T 1   k , the intermediate transfer belt B, and the second-transfer unit T 2  constitute a transfer device T 1 +T 2 +B according to the first exemplary embodiment that transfers images formed on the photoconductor drums PRy to PRk onto a medium. 
     A belt cleaner CLb as an example of an intermediate-transfer-member cleaning unit is disposed downstream of the second-transfer region Q 4  in the rotational direction of the intermediate transfer belt B. 
     Cartridges Ky, Km, Kc, and Kk as an example of developer containers are disposed above the belt module BM. The cartridges Ky to Kk accommodate developers to be supplied to the developing devices Gy to Gk. The cartridges Ky to Kk and the developing devices Gy to Gk are respectively connected by developer supplying devices (not shown). 
     Feed trays TR 1  to TR 3  as an example of medium containers are disposed at the lower portion of the printer unit U 1 . The feed trays TR 1  to TR 3  are supported in a detachable manner in the front-rear direction by guide rails GR as an example of guide members. The feed trays TR 1  to TR 3  accommodate recording sheets S therein as an example of media. 
     A pickup roller Rp as an example of a medium pickup member is disposed at the upper right side of each of the feed trays TR 1  to TR 3 . A separation roller Rs as an example of a separation member is disposed to the right of the pickup roller Rp. 
     A medium transport path SH 1  extending upward is provided to the right of the feed trays TR 1  to TR 3 . The transport path SH 1  has multiple transport rollers Ra arranged therein as an example of medium transport members. In a downstream area of the transport path SH 1  in the transport direction of the recording sheet S, a registration roller Rr as an example of a delivery member is disposed upstream of the second-transfer region Q 4 . 
     A fixing device F is disposed above the second-transfer region Q 4 . The fixing device F has a heating roller Fh as an example of a first fixing member as well as an example of a heating member, and also has a pressure member Fp as an example of a second fixing member. A contact region between the heating roller Fh and the pressure member Fp constitutes a fixing region Q 5 . The heating roller Fh has a built-in heater (not shown) as an example of a heat source. 
     An output roller Rh as an example of a medium transport member is disposed obliquely above the fixing device F. An output tray TRh as an example of a medium output unit is provided to the left of the output roller Rh. 
     Image Forming Operation 
     The multiple documents Gi accommodated in the document tray TG 1  sequentially pass over a document read position on the platen glass PG and are output onto the document output tray TG 2 . 
     In a case where copying is to be performed by transporting the documents Gi automatically by using the auto feeder U 3 , the documents Gi sequentially passing over the read position on the platen glass PG are exposed to light with the reading optical system A maintained in the stopped state at the initial position. 
     In a case where copying is to be performed by allowing the operator to manually place a document Gi on the platen glass PG, the reading optical system A moves in the left-right direction so that the document Gi on the platen glass PG is scanned while being exposed to light. 
     The reflected light from the document Gi travels through the reading optical system A and is focused on the imaging element CCD. The imaging element CCD converts the reflected light from the document Gi focused on an imaging surface thereof into red (R), green (G), and blue (B) electric signals. 
     The image processor IPS converts the RGB electric signals input from the imaging element CCD into black (K), yellow (Y), magenta (M), and cyan (C) image information and temporarily stores the image information. The image processor IPS outputs the temporarily-stored image information as image information for latent-image formation to the write circuit DL at a preset timing. 
     If the document image is a monochromatic image, only the K image information is input to the write circuit DL. 
     The write circuit DL has Y, M, C, and K drive circuits (not shown). The write circuit DL outputs signals according to the input image information at a preset timing to the LED heads LHy to LHk arranged for the respective colors. 
     The surfaces of the photoconductor drums PRy to PRk are electrostatically charged by the charging rollers CRy to CRk. In the write regions Q 1   y  to Q 1   k , the LED heads LHy to LHk form electrostatic latent images on the surfaces of the photoconductor drums PRy to PRk. In the developing regions Q 2   y  to Q 2   y , the developing devices Gy to Gk develop the electrostatic latent images on the surfaces of the photoconductor drums PRy to PRk into toner images as an example of visible images. When the developers are consumed in the developing devices Gy to Gk, the developing devices Gy to Gk are supplied with new developers from the respective cartridges Ky to Kk in accordance with the consumed amounts. 
     The toner images on the surfaces of the photoconductor drums PRy to PRk are transported to the first-transfer regions Q 3   y , Q 3   m , Q 3   c , and Q 3   k . The first-transfer rollers T 1   y  to T 1   k  receive first-transfer voltage with a polarity opposite from the charge polarity of the toners from the power supply circuit E at a preset timing. Therefore, in the first-transfer regions Q 3   y  to Q 3   k , the toner images on the photoconductor drums PRy to PRk are sequentially superposed and transferred onto the intermediate transfer belt B in accordance with the first-transfer voltage. In the case of a K monochromatic image, the K toner image alone is transferred onto the intermediate transfer belt B from the K photoconductor drum PRk. 
     Residues and extraneous matter on the surfaces of the photoconductor drums PRy to PRk after the first-transfer process are cleaned off by the drum cleaners CLy, CLm, CLc, and CLk. The cleaned surfaces of the photoconductor drums PRy to PRk are electrostatically charged again by the charging rollers CRy to CRk. 
     A recording sheet S from one of the feed trays TR 1  to TR 3  is picked up by the corresponding pickup roller Rp at a preset feed timing. If multiple recording sheets S in a stacked state are picked up by the pickup roller Rp, the separation roller Rs separates the recording sheets S in a one-by-one fashion. The recording sheet S that has passed the separation roller Rs is transported to the registration roller Rr by the multiple transport rollers Ra. 
     The registration roller Rr delivers the recording sheet S in accordance with the timing at which the toner images on the surface of the intermediate transfer belt B move to the second-transfer region Q 4 . 
     When the recording sheet S delivered from the registration roller Rr passes through the second-transfer region Q 4 , the toner images on the surface of the intermediate transfer belt B are transferred onto the recording sheet S in accordance with second-transfer voltage applied to the second-transfer roller T 2   b.    
     After the intermediate transfer belt B passes through the second-transfer region Q 4 , the belt cleaner CLb cleans the surface of the intermediate transfer belt B by removing residual toner therefrom. 
     The recording sheet S that has passed through the second-transfer region Q 4  subsequently passes through the fixing region Q 5  where the toner images are fixed onto the recording sheet S by being heated and pressed by the fixing device F. 
     The recording sheet S having the toner images fixed thereon is output to the output tray TRh by the output roller Rh. 
     Fixing Device F According to First Exemplary Embodiment 
       FIG. 3  is an overall view illustrating the fixing device F according to the first exemplary embodiment. 
       FIG. 4  illustrates the pressure member Fp of the fixing device F according to the first exemplary embodiment. 
       FIG. 5  illustrates a state where a pressure belt is removed from the pressure member Fp shown in  FIG. 4 . 
       FIG. 6  illustrates a state where oil-impregnated felt and a sheet are removed from the state shown in  FIG. 5 . 
       FIGS. 7A and 7B  illustrate a support component constituting the pressure member Fp according to the first exemplary embodiment. Specifically,  FIG. 7A  is a perspective view, and  FIG. 7B  is a side view. 
       FIGS. 8A and 8B  illustrate frame bodies constituting the pressure member Fp according to the first exemplary embodiment. Specifically,  FIG. 8A  is a perspective view, and  FIG. 8B  is a side view. 
       FIGS. 9A and 9B  illustrate a pressing component constituting the pressure member according to the first exemplary embodiment. Specifically,  FIG. 9A  is a perspective view of the pressing component, and  FIG. 9B  illustrates a state where a contact member is supported by a free end portion of the pressing component. 
     In  FIGS. 4 and 5 , the heating roller Fh and a pressure belt Fp 1 , respectively, are not shown. 
     Referring to  FIGS. 3 to 7B , the pressure member Fp of the fixing device F has a base component  1  as an example of a support component extending in the front-rear direction. In  FIGS. 7A and 7B , the base component  1  has an upstream guide section  2  at the lower right portion thereof and a downstream guide section  3  at the upper left portion thereof. The upstream guide section  2  protrudes toward the lower left side, which is the upstream side in the transport direction of the recording sheet S. The upper left end of the downstream guide section  3  has a convex shape, that is, a so-called arc shape, so as to be capable of smoothly guiding the recording sheet S. The upper right portion of the downstream guide section  3  is provided with a first recess  3   a  as an example of a frame-body supporter. The upstream guide section  2  and the downstream guide section  3  are connected to each other by a connection section  4 . The upper right portion of the connection section  4  is provided with a second recess  4   a  as an example of a frame-body supporter. An area surrounded by the upstream guide section  2 , the downstream guide section  3 , and the connection section  4  is provided with a spring accommodation section  5  as an example of a pressing-component accommodation section. 
     In the base component  1  according to the first exemplary embodiment, the upstream guide section  2 , the downstream guide section  3 , and the connection section  4  are formed as a single unit by using resin. 
     Referring to  FIGS. 3 to 6  and  FIGS. 8A and 8B , a first plate  11  and a second plate  12  as an example of frame bodies are supported at the upper portion of the base component  1 . The first plate  11  has a first section  11   a  supported by the first recess  3   a  and a first bent section  11   b  bent toward the lower right side from the outer edge of the first section  11   a . The first section  11   a  is disposed for reinforcement at the downstream side of the fixing region Q 5  in accordance with a position where the force acting on the base component  1  locally increases. The position where the force acting on the base component  1  locally increases is measured and set in advance based on, for example, tests or simulation. 
     The outer surface of the first bent section  11   b  supports oil-impregnated felt  13  as an example of a lubricant supplying member. The oil-impregnated felt  13  is supported in contact with the inner surface of the pressure belt Fp 1  as an example of a belt-shaped component. The oil-impregnated felt  13  is impregnated with silicone oil as an example of a lubricant. The silicone oil may be supplied by being applied to the inner surface of the pressure belt Fp 1  from the oil-impregnated felt  13 . 
     The second plate  12  has a second section  12   a  supported by the second recess  4   a  and a second bent section  12   b  bent toward the upper left side from the outer edge of the second section  12   a . The second section  12   a  is disposed at the upstream side of the fixing region Q 5  in accordance with a position where the force acting on the base component  1  locally increases. 
     Each of the first plate  11  and the second plate  12  has a plate shape extending in the front-rear direction, and the front and rear ends thereof are supported by a frame plate (not shown) of the fixing device F. 
     Referring to  FIGS. 3 to 6  and  FIGS. 9A and 9B , the spring accommodation section  5  supports a plate spring  21  as an example of a pressing component. With regard to the plate spring  21  according to the first exemplary embodiment, the front and rear widthwise ends thereof are fixed to the base component  1 . The plate spring  21  is electrically connected to ground. The plate spring  21  has a base end portion  22  that extends toward the downstream guide section  3  from the upstream guide section  2  in the transport direction of the recording sheet S. With regard to the plate spring  21  according to the first exemplary embodiment, an end of the base end portion  22  closer toward the upstream guide section  2  is in contact with the inner surface of the upstream guide section  2  so as to be fixed thereto in a positioned state. 
     An end of the base end portion  22  closer toward the downstream guide section  3  is provided with a folded portion  23  extending continuously from the base end portion  22  and folded in a substantially U-shape toward the fixing region Q 5 . An end of the folded portion  23  closer toward the fixing region Q 5  is provided with a pressing portion  24  extending continuously from the folded portion  23  toward the upstream guide section  2 . 
     Furthermore, in the plate spring  21  according to the first exemplary embodiment, multiple slits  26  as an example of gaps extend from the end of the base end portion  22 , that is, the fixed base end, to an intermediate point of the pressing portion  24  via the folded portion  23 . The slits  26  are arranged at predetermined pitch in the front-rear direction. Thus, the areas interposed between the slits  26  constitute reed-shaped biasing portions  27 . The slits  26  do not extend to the end of the pressing portion  24 , that is, the free end, and the free ends of the biasing portions  27  are connected together by a free end portion  28  extending in the front-rear direction, that is, the width direction of the recording sheet S. 
     Referring to  FIGS. 9A and 9B , the free end of the pressing portion  24  according to the first exemplary embodiment is provided with a bent portion  24   a  that is bent toward the heating roller Fh. As shown in  FIG. 9A , in the first exemplary embodiment, the slits  26  extend toward the free end beyond the bent portion  24   a . Thus, the plate spring  21  may be fabricated by forming the slits  26  in a single metallic plate and then bending the metallic plate along the folded portion  23  and the bent portion  24   a.    
     The surface of the pressing portion  24  at the fixing region Q 5  side supports felt  31  as an example of a contact member. If the felt  31  is not provided, there is a risk of the free end portion  28  of the plate spring  21  locally coming into contact with the heating roller Fh. In the first exemplary embodiment, the pressing portion  24  is provided with the felt  31  so that contact pressure may be ensured over a large surface area, as compared with a case where the felt  31  is not provided. 
     Referring to  FIGS. 3 to 5 , the fixing region Q 5  side of the base component  1  and the plate spring  21  supports a sheet  32 . In  FIG. 5 , the sheet  32  extends around the lower left end of the upstream guide section  2  from the upper right portion thereof, comes into contact with the felt  31  on the surface of the plate spring  21 , and extends around the upper left end of the downstream guide section  3  to the upper right portion thereof. Thus, as shown in  FIGS. 3 and 4 , the sheet  32  covers the outer side so as to prevent the base component  1 , the plate spring  21 , and the felt  31  from directly coming into contact with the pressure belt Fp 1 . In other words, if the sheet  32  is not provided, there would be areas that may possibly come into direct contact with the rotating pressure belt Fp 1 . The surface of the sheet  32  according to the first exemplary embodiment is composed of fluorocarbon resin as an example of a low friction material. 
     Therefore, the pressure belt Fp 1  according to the first exemplary embodiment is supported in a stretched state at the positions of the lower left end and the lower right portion of the upstream guide section  2 , the plate spring  21 , the upper left portion of the downstream guide section  3 , and the oil-impregnated felt  13 . As described above, the pressure belt Fp 1  is not directly in contact with, for example, the upstream guide section  2  or the plate spring  21 , but is supported indirectly via the sheet  32 . 
     Operation of First Exemplary Embodiment 
     When image forming operation, that is, a job, commences in the copier U according to the first exemplary embodiment having the above-described configuration, electrostatic latent images on the surfaces of the photoconductor drums PRy to PRk are developed, and the images are transferred onto a recording sheet S passing through the second-transfer region Q 4  via, for example, the belt module BM. The unfixed images on the surface of the recording sheet S are fixed thereon by being heated and pressed by the fixing device F. In this case, the pressure belt Fp 1  is pressed against the heating roller Fh by the plate spring  21  of the pressure member Fp so that the fixing region Q 5  according to the first exemplary embodiment is formed. 
     Thus, in the copier U according to the first exemplary embodiment, the plate spring  21  has multiple biasing portions  27  formed in the width direction of the recording sheet S. Therefore, even in the case of a single recording sheet S having various thicknesses in the width direction, such as an envelope having a bonding area or a flap, the biasing portions  27  elastically deform in correspondence with the thicknesses when the recording sheet S passes through the fixing region Q 5 . Supposing that a single plate spring not having slits arranged in the width direction is used, when the plate spring elastically deforms in correspondence with a thick area of the recording sheet S, the fixation pressure may be locally insufficient for a thin area of the recording sheet S, possibly resulting in a fixation defect. In contrast, the occurrence of such a fixation defect may be reduced in the first exemplary embodiment. 
       FIG. 10  illustrates a technology in the related art. 
     Referring to  FIG. 10 , if the free end portion is disposed at the downstream side, as in the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308, a folded portion  04  would be disposed upstream of a fixing region  03  where a heating roller  01  and a pressure belt  02  come into contact with each other. Thus, in the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308, when a leading edge  06   a  of a recording sheet  06  having various thicknesses, such as an envelope, enters the fixing region  03 , the leading edge  06   a  of the recording sheet  06  passes the folded portion  04  disposed at the upstream side. Therefore, due to slits formed in the folded portion  04 , the biasing portions elastically deform in correspondence with the differences in thicknesses of the recording sheet  06  in the width direction. Thus, in the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308, when the leading edge  06   a  of the recording sheet  06  enters the fixing region  03 , specifically, in a state prior to the recording sheet  06  being nipped by the heating roller  01  and the pressure belt  02  with a preset fixation pressure, the biasing portions deform in correspondence with the thicknesses, so that the elastic force received by the recording sheet  06  varies in the width direction. When the force received by the recording sheet  06  varies in the width direction, the leading edge  06   a  of the recording sheet  06  may sometimes cockle in the width direction. If the cockled leading edge  06   a  of the recording sheet  06  is transported downstream and is nipped by the heating roller  01  and the pressure belt  02 , the recording sheet  06  may possibly become wrinkled. Therefore, in the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308, image formation defects may possibly occur, such as a fixation defect in which an image is not fixed at a wrinkle or near a wrinkled area. 
     In contrast, the fixing device F according to the first exemplary embodiment differs from the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308 in that the free end portion  28  not having the slits  26  formed therein is disposed upstream of the fixing region Q 5 . Thus, when the recording sheet S enters the fixing region Q 5 , the free end portion  28  extending continuously in the width direction suppresses independent movements of the biasing portions  27 , thereby reducing cockling of the recording sheet S in the width direction. Consequently, the occurrence of wrinkles in the recording sheet S may be reduced, as compared with the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308. Therefore, the occurrence of image formation defects may be reduced, as compared with the configuration of Japanese Unexamined Patent Application Publication No. 2012-208308. 
     Furthermore, in the first exemplary embodiment, the plate spring  21  is provided with the slits  26  in the fixing region Q 5  so that when the recording sheet S passes through the fixing region Q 5 , each biasing portion  27  elastically deforms in correspondence with the thickness of the recording sheet S, thereby reducing the occurrence of a fixation defect. 
       FIG. 11  is an enlarged view of an area surrounding the bent portion  24   a  according to the first exemplary embodiment. 
     In the first exemplary embodiment, the pressing portion  24  is provided with the bent portion  24   a . Therefore, as shown in  FIG. 11 , the pressure belt Fp 1  is pressed by the plate spring  21  at the front and rear sides of the bent portion  24   a  in the transport direction of the recording sheet S. Thus, the range in which the pressure belt Fp 1  is pressed against the heating roller Fh, that is, the fixing region Q 5 , is increased, as compared with a case where the bent portion  24   a  is not provided. The larger fixing region Q 5  may allow a sufficient amount of heat to be applied to recording sheets S successively passing therethrough, as compared with a case where the fixing region Q 5  is small, thereby allowing for a stable fixing process. 
     Furthermore, the slits  26  extend toward the free end beyond the bent portion  24   a  in the first exemplary embodiment. Supposing that the slits  26  only extend to the folded portion  23  side of the bent portion  24   a , the biasing portions  27  would not be provided upstream of the bent portion  24   a  in the large fixing region Q 5 , possibly causing a fixation defect to occur due to a local shortage of fixation pressure. In contrast, in the first exemplary embodiment, the slits  26  extend toward the free end beyond the bent portion  24   a  so that the occurrence of a fixation defect may be reduced. 
     Modifications 
     Although the exemplary embodiment of the present invention has been described in detail above, the present invention is not to be limited to the above exemplary embodiment and permits various modifications within the technical scope of the invention defined in the claims. Modifications H 01  to H 05  will be described below. 
     In a first modification H 01 , the image forming apparatus according to the above exemplary embodiment is not limited to the copier U, and may be, for example, a facsimile apparatus or a multifunction apparatus having multiple functions of such apparatuses. Furthermore, the above exemplary embodiment is not limited to a multicolor image forming apparatus and may be applied to a so-called monochrome image forming apparatus. 
     In the above exemplary embodiment, the pressure belt Fp 1  is an endless belt. Alternatively, for example, in a second modification H 02 , a belt wound around a feeding roller may be fed therefrom and be wound around a winding roller after passing through the fixing region Q 5 . 
     In the above exemplary embodiment, the number, the pitch, and the width of the slits  26  are not limited to those described in the exemplary embodiment. Alternatively, for example, in a third modification H 03 , the slits  26  may be configured similarly to those in Japanese Unexamined Patent Application Publication No. 2012-208308. Moreover, although the slits  26  desirably extend toward the free end beyond the bent portion  24   a , the slits  26  may alternatively extend toward the base end relative to the bent portion  24   a.    
     In the above exemplary embodiment, it is desirable that the bent portion  24   a  be provided. Alternatively, in a fourth modification H 04 , a configuration in which the bent portion  24   a  is not provided is also permissible. 
     In the above exemplary embodiment, the plate spring  21  has a substantially U-shape with a single folded portion  23 . Alternatively, for example, in a fifth modification H 05 , the plate spring  21  may have a freely-chosen shape, such as a lateral L-shape or S-shape as viewed from the front. 
     The foregoing description of the exemplary embodiment 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 embodiment was 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.