Patent Publication Number: US-8995896-B2

Title: 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. 2013-054808 filed Mar. 18, 2013. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a fixing device and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a fixing device including: an endless belt member; a secured member disposed in a secured state to contact an inner peripheral surface of the belt member; a support roller that rotatably supports the belt member; a heating unit that heats the belt member; and a rotary pressurizing roller that includes an elastic surface layer that is elastically deformed when the belt member is pressed against the secured member to form a fixing part, in which the secured member includes a recessed part provided on a surface of the secured member that faces the pressurizing roller and configured to curve the belt member along the secured member into a recessed shape in which the belt member is recessed from the pressurizing roller toward the secured member, projecting parts provided upstream and downstream of the recessed part, respectively, along a moving direction of the belt member and configured to curve the belt member into a projected shape in which the belt member is projected toward the pressurizing roller, and a region provided between at least one of the projecting parts provided upstream and downstream of the recessed part and the recessed part, the region having a small curvature compared to the corresponding projecting part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  shows a schematic configuration of an image forming apparatus including a fixing device according to a first exemplary embodiment of the present invention; 
         FIG. 2  shows the configuration of an image preparing device of the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 3  is a perspective view of the fixing device; 
         FIG. 4  is a cross-sectional view showing the configuration of a fixing belt; 
         FIG. 5  shows the configuration of a portion of the fixing device; 
         FIG. 6  is an enlarged view showing the configuration of a portion of the fixing device; 
         FIG. 7  shows a schematic configuration of a portion of a fixing device according to a second exemplary embodiment of the present invention; 
         FIG. 8  shows a schematic configuration of a portion of a fixing device according to a third exemplary embodiment of the present invention; and 
         FIG. 9  shows a schematic configuration of a portion of a fixing device according to a fourth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
     [First Exemplary Embodiment] 
       FIGS. 1 and 2  show an image forming apparatus including a fixing device according to a first exemplary embodiment.  FIG. 1  shows the overview of the entire image forming apparatus.  FIG. 2  shows a portion of the image forming apparatus (such as an image preparing device) as enlarged. 
     &lt;Overall Configuration of Image Forming Apparatus&gt; 
     An image forming apparatus  1  according to the first exemplary embodiment is configurated as a color printer, for example. The image forming apparatus  1  includes an image forming section  200  that serves as an example of an image forming unit that forms an image on a recording material on the basis of image data. The image forming section  200  of the image forming apparatus  1  includes plural image preparing devices  10 , an intermediate transfer device  20 , a paper feed device  50 , a fixing device  40 , and so forth. The image preparing devices  10  form a toner image to be developed using a toner that serves as a developer. The intermediate transfer device  20  holds the toner images formed by the image preparing devices  10  to transport the toner images finally to a second transfer position at which the toner images are subjected to a second transfer performed onto recording paper  5  that serves as an example of the recording material. The paper feed device  50  stores and transports the prescribed recording paper  5  to be supplied to the second transfer position of the intermediate transfer device  20 . The fixing device  40  fixes the toner images on the recording paper  5  which have been subjected to the second transfer performed by the intermediate transfer device  20 . 
     In the case where the image forming apparatus  1  is additionally equipped with an image input device  60  that allows input of a document image to be formed on the recording paper  5 , for example, the image forming apparatus  1  may be configurated as a color copier. In  FIG. 1 , reference symbol  1   a  denotes a housing of the image forming apparatus. The housing  1   a  is formed from a support structure member, an outer covering, and so forth. The broken lines in  FIG. 1  indicate principal transport paths along which the recording paper  5  is transported in the housing  1   a.    
     The image preparing devices  10  are composed of six image preparing devices  10 Y,  10 M,  10 C,  10 K,  10 S 1 , and  10 S 2  that exclusively form toner images in four colors, namely yellow (Y), magenta (M), cyan (C), and black (K) and toner images in two special colors S 1  and S 2 , respectively. The six image preparing devices  10  (S 1 , S 2 , Y, M, C, K) are disposed side by side in a line in the internal space of the housing  1   a . Examples of developers  4  (S 1 , S 2 ) for the special colors (S 1 , S 2 ) include color materials etc. that are difficult or impossible to express using the four colors. Specific examples include a toner in a color different from the four colors, a toner in the same color as the four colors but with a different saturation, a transparent toner that provides an improved gloss, an expandable toner for Braille printing, and a toner in a fluorescent color. The image preparing devices  10  (S 1 , S 2 , Y, M, C, K) have substantially common configurations as described below except that the image preparing devices  10  use different types of developers. 
     As shown in  FIGS. 1 and 2 , the image preparing devices  10  (S 1 , S 2 , Y, M, C, K) each include a photosensitive drum  11  that serves as an example of a rotary image holding member. The following various devices are principally disposed around the photosensitive drum  11 . The devices include a charging device  12 , an exposure device  13 , a developing device  14  (S 1 , S 2 , Y, M, C, K), a first transfer device  15 , a pre-cleaning charging device  16 , a drum cleaning device  17 , a static eliminator  18 , and so forth. The charging device  12  charges a peripheral surface (image holding surface) of the photosensitive drum  11 , on which an image may be formed, with a prescribed potential. The exposure device  13  serves as an electrostatic latent image forming unit that radiates light LB based on information (signal) on an image to the charged peripheral surface of the photosensitive drum  11  to form an electrostatic latent image (for each color) with a potential difference. The developing device  14  (S 1 , S 2 , Y, M, C, K) serves as a developing unit that develops the electrostatic latent image using a toner of the developer  4  for the corresponding color (S 1 , S 2 , Y, M, C, K) to form a toner image. The first transfer device  15  transfers the toner image to the intermediate transfer device  20 . The pre-cleaning charging device  16  charges attached matter such as a toner remaining on and adhering to the image holding surface of the photosensitive drum  11  after the first transfer. The drum cleaning device  17  removes the recharged attached matter. The static eliminator  18  eliminates static from the image holding surface after the photosensitive drum  11  is cleaned. 
     The photosensitive drum  11  has an image holding surface formed by providing a photoconductive layer (photosensitive layer) made of a photosensitive material on the peripheral surface of a grounded cylindrical or columnar base material. The photosensitive drum  11  is supported so as to receive power from a rotary drive device (not shown) to rotate in the direction indicated by the arrow A. 
     The charging device  12  is configurated as a non-contact charging device, such as a corona discharger, disposed without contact with the photosensitive drum  11 . A charging voltage is supplied to the charging device  12 . In the case where the developing device  14  performs reversal development, a voltage or a current having the same polarity as the polarity for charging the toner supplied from the developing device  14  is supplied as the charging voltage. 
     The exposure device  13  radiates the light LB, formed in accordance with the information on the image input to the image forming apparatus  1 , toward the peripheral surface of the photosensitive drum  11  after being charged to form an electrostatic latent image. When a latent image is to be formed, information (signal) on the image input in any manner to the image forming apparatus  1  is transmitted to the exposure device  13 . 
     As shown in  FIG. 2 , the developing devices  14  (S 1 , S 2 , Y, M, C, K) each include a housing  140 , two developing rollers  141  and  142 , two agitation/transport members  143  and  144 , a layer thickness restricting member  145 , and so forth. The housing  140  includes an opening portion and a storing chamber for the developer  4 , and houses the other components. The developing rollers  141  and  142  hold the developer  4 , and transport the developer  4  to two development regions facing the photosensitive drum  11 . The agitation/transport members  143  and  144 , which may be screw augers, transport the developer  4  to cause the developer  4  to pass through the developing roller  142  while agitating the developer  4 . The layer thickness restricting member  145  restricts the amount (layer thickness) of the developer held by the developing roller  142 . A development voltage from a power source device (not shown) is supplied between the developing rollers  141  and  142  of the developing device  14  and the photosensitive drum  11 . Power from a rotary drive device (not shown) is transmitted to the developing rollers  141  and  142  and the agitation/transport members  143  and  144  to rotate the developing rollers  141  and  142  and the agitation/transport members  143  and  144  in a prescribed direction. Further, a two-part developer containing a non-magnetic toner and a magnetic carrier is used as the developers  4  (Y, M, C, K) for the four colors. In  FIG. 1 , reference numeral  146  (S 1 , S 2 , Y, M, C, K) denotes a developer storing container that stores a developer containing at least a toner to be supplied to the corresponding developing device  14  (S 1 , S 2 , Y, M, C, K). 
     The first transfer device  15  is a contact transfer device including a first transfer roller that rotates in contact with the peripheral surface of the photosensitive drum  11  and that is supplied with a first transfer voltage. A DC voltage having a polarity opposite to the polarity for charging the toner is supplied from a power source device (not shown) as the first transfer voltage. 
     As shown in  FIG. 2 , the drum cleaning device  17  includes a body  170 , a cleaning plate  171 , a rotary brush roller  172 , a feeding member  173 , and so forth. The body  170  has the shape of a partially open container. The cleaning plate  171  is disposed so as to contact the peripheral surface of the photosensitive drum  11 , after being subjected to the first transfer, with a prescribed pressure to clean the photosensitive drum  11  by removing attached matter such as a residual toner. The rotary brush roller  172  is disposed upstream of the cleaning plate  171  in the rotational direction of the photosensitive drum  11  to rotate in contact with the peripheral surface of the photosensitive drum  11 . The feeding member  173 , which may be a screw auger, recovers attached matter, such as a toner, removed by the cleaning plate  171  to feed the attached matter to a recovery system (not shown). A plate-like member (for example, blade) made of a material such as rubber is used as the cleaning plate  171 . 
     As shown in  FIG. 1 , the intermediate transfer device  20  is disposed at a position below the image preparing devices  10  (S 1 , S 2 , Y, M, C, K). The intermediate transfer device  20  is principally composed of an intermediate transfer belt  21 , plural belt support rollers  22  to  27 , a second transfer device  30 , and a belt cleaning device  28 . The intermediate transfer belt  21  rotates in the direction indicated by the arrow B while passing through first transfer positions between the photosensitive drum  11  and the first transfer devices  15  (first transfer rollers). The belt support rollers  22  to  27  rotatably support the intermediate transfer belt  21  by holding the intermediate transfer belt  21  in a desired state from the inner side. The second transfer device  30  is disposed on the side of the outer peripheral surface (image holding surface) of the intermediate transfer belt  21  supported by the belt support roller  26  to have the toner image on the intermediate transfer belt  21  subjected to a second transfer performed onto the recording paper  5 . The belt cleaning device  28  cleans the intermediate transfer belt  21  by removing attached matter such as a toner and paper powder remaining on and adhering to the outer peripheral surface of the intermediate transfer belt  21  after passing through the second transfer device  30 . 
     An endless belt fabricated from a material obtained by dispersing a resistance adjusting agent such as carbon black etc. in a synthetic resin such as a polyimide resin or a polyamide resin, for example, is used as the intermediate transfer belt  21 . The belt support roller  22  is configurated as a driving roller. The belt support rollers  23 ,  25 , and  27  are each configurated as a driven roller that maintains the travel position etc. of the intermediate transfer belt  21 . The belt support roller  24  is configurated as a tension applying roller. The belt support roller  26  is configurated as a second transfer back-up roller. 
     As shown in  FIG. 1 , the second transfer device  30  is a contact transfer device including a second transfer roller provided at the second transfer position, which is a portion of the outer peripheral surface of the intermediate transfer belt  21  supported by the belt support roller  26  in the intermediate transfer device  20 . The second transfer roller rotates in contact with the peripheral surface of the intermediate transfer belt  21 , and is supplied with a second transfer voltage. A DC voltage having a polarity opposite to or the same as the polarity for charging the toner is supplied as the second transfer voltage to the second transfer device  30  or the support roller  26  of the intermediate transfer device  20 . 
     The fixing device  40  includes a heating rotary member  41 , a pressurizing rotary member  42 , and so forth. The heating rotary member  41  includes a fixing belt that rotates in the direction indicated by the arrow and that is heated by a heating unit such that the surface temperature is maintained at a predefined temperature. The pressurizing rotary member  42 , which may be in a roller form, rotates in contact with the heating rotary member  41  at a prescribed pressure. In the fixing device  40 , a contact portion at which the heating rotary member  41  and the pressurizing rotary member  42  contact each other serves as a fixation processing part at which a prescribed fixation process (heating and pressurization) is performed. The fixing device  40  will be discussed in detail later. 
     The paper feed device  50  is disposed at a position below the intermediate transfer device  20  and the second transfer device  30 . The paper feed device  50  is principally composed of one or more paper storing members  51   a  and  51   b  and feeding devices  52 . The paper storing members  51   a  and  51   b  store a stack of sheets of the recording paper  5  of desired size, type, etc. The feeding devices  52  feed the recording paper  5 , one sheet at a time, from the paper storing members  51   a  and  51   b . The paper storing members  51   a  and  51   b  are attached so as to be drawn out toward the side of the front surface (a side surface that the user faces during operation) of the housing  1   a , for example. 
     A paper feed/transport path  55  is provided between the paper feed device  50  and the second transfer device  30 . The paper feed/transport path  55  is composed of plural pairs of paper transport rollers  53  and  54  and a transport guide member (not shown). The paper transport rollers  53  and  54  transport the recording paper  5  fed from the paper feed device  50  to the second transfer position. The pair of paper transport rollers  54  disposed at a position immediately before the second transfer position in the paper feed/transport path  55  are configurated as rollers (resist rollers) that adjust the transport timing for the recording paper  5 , for example. Two paper transport devices  56   a  and  56   b  are provided between the second transfer device  30  and the fixing device  40 . The paper transport devices  56   a  and  56   b , which may be in the form of a belt or the like, transport the recording paper  5  after being subjected to the second transfer fed from the second transfer device  30  to the fixing device  40 . A cooling unit  70 , a curl correcting unit  71 , and a switching member  72  are disposed downstream of the fixing device  40  along the paper transport direction. The cooling unit  70  cools the recording paper  5  which has been subjected to the fixation process. The curl correcting unit  71  corrects a curve (curl) of the recording paper  5 . The switching member  72  switches the transport direction of the recording paper  5  between a paper ejection section  73  and a double-sided-printing transport path  57 . 
     The double-sided-printing transport path  57  includes an intermediate storing container  58  that temporarily stores the recording paper  5 , on one surface (first surface) of which an image has been formed. The intermediate storing container  58  is provided with feeding rollers  59  that feed the stored recording paper  5  with the front and back sides of the recording paper  5  reversed. The feeding side of the intermediate storing container  58  is connected to the paper feed/transport path  55  via a downstream region of the double-sided-printing transport path  57 . 
     The image input device  60 , which is provided in the case where the image forming apparatus  1  is configurated as a color copier as discussed earlier, is an image reading device that reads an image in a document carrying image information to be printed, and is disposed at the upper portion of the housing  1   a  as shown in  FIG. 1 , for example. The image input device  60  is principally composed of a document placing plate (platen glass)  61 , a light source  62 , a reflective mirror  63 , a first reflective mirror  64 , a second reflective mirror  65 , an image reading element  66 , an imaging lens  67 , and so forth. The document placing plate  61  is a transparent glass plate or the like for placement of a document  6  carrying information on an image to be read. The light source  62  illuminates the document  6  placed on the document placing plate  61  while moving. The reflective mirror  63  receives light reflected from the document  6  to reflect the light in a predetermined direction while moving together with the light source  62 . The first reflective mirror  64  and the second reflective mirror  65  move with respect to the reflective mirror  63  at a predetermined speed over a predetermined distance. The image reading element  66  is a CCD or the like that receives and reads light reflected from the document  6  to convert the light into an electric signal. The imaging lens  67  forms an image on the image reading element  66  on the basis of the reflected light. In  FIG. 1 , reference numeral  68  denotes an open/close covering that covers the document placing plate  61 . 
     The image information on the document read and input through the image input device  60  is subjected to necessary image processing performed by an image processing device configurated as a part of a controller  100 . First, the image input device  60  transmits image information on a read document to the image processing device  100  as image data (for example, data with 8 bits for each color) for three colors, namely red (R), green (G), and blue (B), for example. Meanwhile, the image processing device  100  performs predefined image processing on the image data transmitted from the image input device  60 . Examples of the image processing include a shading correction, a misregistration correction, a lightness/color space conversion, a gamma correction, unframing, and color/movement editing. In addition, the image processing device  100  changes the image signals which have been subjected to the image processing into image signals for the four colors (Y, M, C, K), and thereafter transmits the resulting image signals to the exposure device  13 . The image processing device  70  also generates image signals for the two special colors (S 1 , S 2 ). 
     &lt;Basic Operation of Image Forming Apparatus&gt; 
     Basic image forming operation performed by the image forming apparatus  1  will be described below. 
     First, image forming operation for forming a full-color image by combining toner images in four colors (Y, M, C, K) using the four image preparing devices  10  (Y, M, C, K) will be described as a representative example. 
     When the image forming apparatus  1  receives command information requesting image forming operation (printing), the four image preparing devices  10  (Y, M, C, K), the intermediate transfer device  20 , the second transfer device  30 , the fixing device  40 , and so forth are started. 
     In each of the image preparing devices  10  (Y, M, C, K), first, the photosensitive drum  11  rotates in the direction indicated by the arrow A, and the charging device  12  charges the surface of the photosensitive drum  11  with a prescribed polarity (in the first exemplary embodiment, negative polarity) and a predefined potential. Then, the exposure device  13  radiates the surface of the photosensitive drum  11  after being charged with light LB emitted on the basis of a signal for an image obtained by converting information on an image input to the image forming apparatus  1  into each color component (Y, M, C, K). Thus, an electrostatic latent image for each color component with a prescribed potential difference is formed on the surface of the photosensitive drum  11 . 
     Then, the developing device  14  (Y, M, C, K) develops the electrostatic latent image for each color component formed on the photosensitive drum  11  by supplying a toner for the corresponding color (Y, M, C, K) charged with a prescribed polarity (negative polarity) for electrostatic adhesion. As a result of the development, the electrostatic latent images for the various color components formed on the photosensitive drums  11  are rendered manifest as toner images for the four colors (Y, M, C, K) developed using toners for the corresponding colors. 
     Then, when the toner image in each color formed on the photosensitive drum  11  of the image preparing device  10  (Y, M, C, K) is transported to the first transfer position, the first transfer device  15  performs a first transfer on the toner image in each color such that the toner images in the various colors are sequentially superposed on the intermediate transfer belt  21  of the intermediate transfer device  20  which rotates in the direction indicated by the arrow B. 
     In the image preparing device  10  which has finished the first transfer, the pre-cleaning charging device  16  recharges attached matter such as a toner remaining on the surface of the photosensitive drum  11  after the first transfer. After that, the drum cleaning device  17  cleans the surface of the photosensitive drum  11  by scraping off the recharged attached matter. Lastly, the static eliminator  18  eliminates static from the surface of the photosensitive drum  11  after being cleaned. This allows the image preparing device  10  to be ready for the next image preparing operation. 
     Then, the intermediate transfer device  20  transports the toner images which have been subjected to the first transfer to the second transfer position through rotation of the intermediate transfer belt  21 . Meanwhile, the paper feed device  50  feeds the prescribed recording paper  5  to the paper feed/transport path  55  in accordance with the image preparing operation. In the paper feed/transport path  55 , the pair of paper transport rollers  54  that serve as resist rollers feed the recording paper  5  to the second transfer position in accordance with the transfer timing to supply the recording paper  5 . 
     At the second transfer position, the second transfer device  30  collectively performs a second transfer of the toner images on the intermediate transfer belt  21  onto the recording paper  5 . In the intermediate transfer device  20  which has finished the second transfer, the belt cleaning device  28  removes attached matter such as a toner remaining on the surface of the intermediate transfer belt  21  after the second transfer. 
     Then, the recording paper  5 , onto which the toner images have been transferred through the second transfer, is peeled from the intermediate transfer belt  21  and the second transfer device  30 , and thereafter transported to the fixing device  40  by the transport devices  56   a  and  56   b . The fixing device  40  performs a necessary fixation process (heating and pressurization) to fix unfixed toner images to the paper  5  as discussed later. Lastly, the cooling unit  70  cools the recording paper  5  which has been subjected to the fixation by absorbing heat from the recording paper  5  and the toner images. After that, the curl correcting unit  71  corrects curl of the recording paper  5 . In the case of image forming operation in which an image is to be formed on only one surface of the recording paper  5 , the recording paper  5  is ejected to the paper ejection section  73  provided outside the housing  1   a , for example, by a pair of paper ejection rollers (not shown) via the switching member  72 . 
     In the case of image forming operation in which an image is to be formed on both surfaces of the recording paper  5 , meanwhile, the switching member  72  switches the transport direction of the recording paper  5  for which curl has been corrected to the double-sided-printing transport path  57 , and the recording paper  5  is temporarily stored in the intermediate storing container  58  via the double-sided-printing transport path  57 . The recording paper  5  stored in the intermediate storing container  58  is fed to the paper feed/transport path  55  by the feeding rollers  59  via the double-sided-printing transport path  57  with the front and back sides of the recording paper  5  reversed. In the paper feed/transport path  55 , the pair of paper transport rollers  57  which serve as resist rollers feed the recording paper  5  to the second transfer position in accordance with the transfer timing to supply the recording paper  5 . This allows toner images to be transferred to the back surface (second surface) of the recording paper  5  through the second transfer. 
     After that, similar to the image forming operation in which an image is to be formed on only one surface of the recording paper  5 , the recording paper  5 , on the back surface of which the toner images have been transferred through the second transfer, is transported to the fixing device  40  by the transport devices  56   a  and  56   b  to be subjected to a fixation process. After that, the cooling unit  70  cools the recording paper  5  and the toner images, and the curl correcting unit  71  corrects curl of the recording paper  5 . The recording paper  5  is ejected to the paper ejection section  73  via the switching member  72 . 
     As a result of the operation described above, the recording paper  5  is output with a full-color image formed thereon by combining the toner images in the four colors. 
     Next, operation of the image forming apparatus  1  for a case where special-color toner images are formed using developers for the special colors S 1  and S 2  together with a normal image formed as described above, for example, will be described. 
     In this case, first, the image preparing devices  10 S 1  and  10 S 2  perform image preparing operation similar to that performed by the image preparing devices  10  (Y, M, C, K) discussed earlier. This allows the special-color toner images (S 1 , S 2 ) to be formed on the respective photosensitive drums  11  of the image preparing devices  10 S 1  and  10 S 2 . Then, as in the image forming operation for the toner images in the four colors discussed earlier, the special-color toner images formed by the image preparing devices  10 S 1  and  10 S 2  are transferred to the intermediate transfer belt  21  of the intermediate transfer device  20  through the first transfer, and thereafter transferred from the intermediate transfer belt  21  to the recording paper  5  through the second transfer performed by the second transfer device  30  (together with the toner images in the other colors). Lastly, the recording paper  5 , on which the special-color toner images and the toner images in the other colors have been transferred through the second transfer, is subjected to a fixation process performed by the fixing device  40 , and thereafter ejected to the outside of the housing  1   a.    
     As a result of the operation described above, the recording paper  5  is output with two special-color toner images superposed on the entirety or a part of the full-color image formed on the recording paper  5  by combining the toner images in the four colors discussed earlier. 
     Besides, in the case where the image forming apparatus  1  is a color copier equipped with the image input device  60 , the basic image forming operation is performed as follows. 
     In this case, a document  6  is set on the image input device  60 . When command information requesting image forming operation (copying) is received, the image input device  60  reads a document image from the document  6 . After that, the image processing device  100  performs image processing on information on the read document image as discussed earlier to generate an image signal. After that, the image signal is transmitted to the exposure device  13  in each image preparing device  10  (S 1 , S 2 , Y, M, C, K). This causes each image preparing device  10  to form an electrostatic latent image and a toner image on the basis of the information on the image from the document  6 . After that, operation similar to that in the case of the image forming operation (printing) discussed earlier is performed. Finally, an image formed from the toner images is formed on the recording paper  5  to be output. 
     In  FIG. 1 , reference numeral  101  denotes a user interface that allows a user to input image formation conditions etc. 
     The image forming apparatus  1  may be configurated to transfer the toner images formed by the image preparing devices  10  (S 1 , S 2 , Y, M, C, K) directly onto the recording paper  5 , not via the intermediate transfer belt  21  of the intermediate transfer device  20 . 
     &lt;Configuration of Fixing Device&gt; 
       FIG. 3  shows the configuration of the fixing device according to the first exemplary embodiment. 
     The fixing device  40  is roughly composed of a fixing belt module  41  and a pressurizing roller  42 . The fixing belt module  41  serves as a heating rotary member that heats the recording paper  5 . The pressurizing roller  42  serves as a pressurizing rotary member disposed selectively in contact with or away from the fixing belt module  41 . A nip part N is formed between the fixing belt module  41  and the pressurizing roller  42 . The nip part N serves as a fixation processing part at which the recording paper  5  holding unfixed toner images is heated and pressurized to fix the unfixed toner images to the recording paper  5 . 
     The fixing belt module  41  includes a fixing belt  43 , a fixing pad  44 , and plural support rollers  45  to  49 . The fixing belt  43  serves as an example of a belt member formed as an endless belt. The secured pad  44  serves as an example of a secured member disposed in a secured state in contact with the inner peripheral surface of the fixing belt  43  to bring the fixing belt  43  into press contact with the pressurizing roller  42  from the inner side. The support rollers  45  to  49  rotatably support the fixing belt  43  in a tensioned state. In the exemplary embodiment, some of the plural support rollers  45  to  49  also serve as heating rollers that serve as a heating unit that heats the fixing belt  43 . 
     The plural support rollers  45  to  49  include an internal heating roller  45 , an external heating roller  46 , a first driven roller (pre-nip roller)  47 , a second driven roller  48 , and a third driven roller  48 . The internal heating roller  45  heats the fixing belt  43  from the inner side with the fixing belt  43  in a tensioned state. The external heating roller  46  heats the fixing belt  43  from the outer side with the fixing belt  43  in a tensioned state. The first driven roller  47  is disposed upstream of the nip part of the secured pad  44  to hold the fixing belt  43  in a desired state. The second driven roller  48  is disposed between the first driven roller  47  and the internal heating roller  45  to hold the fixing belt  43  in a desired state. The third driven roller  49  is disposed between the secured pad  44  and the external heating roller  46  to hold the fixing belt  43  having passed through the nip part N in a desired state. 
     As shown in  FIG. 4 , the fixing belt  43  is a flexible endless belt, and includes a base layer  431 , an elastic member layer  432 , and a release layer  433 , for example. The base layer  431  is formed from a polyimide resin. The elastic member layer  432  is formed from a silicone rubber laminated on a surface (outer peripheral surface) of the base layer  431 . The release layer  433  is formed from a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) applied to a surface of the elastic member layer  432 . The configuration of the fixing belt  43 , such as material, thickness, and hardness, may be selected appropriately in accordance with conditions required for the fixing device  40  such as purpose of use and conditions of use. In the exemplary embodiment, the elastic member layer  432  is provided on the surface of the base layer  431  for the purpose of improving the image quality of the color image. The recording paper  5  on which powder toners in various colors are superposed passes through the nip part N which serves as a press contact region in which the fixing belt module  41  and the pressurizing roller  42  are in press contact with each other. Therefore, the elastic member layer  432  of the fixing belt  43  is deformed in accordance with the toner images on the recording paper  5 , which makes it possible to supply heat to the entire toner images. 
     The internal heating roller  45  which serves as an example of the heating unit is a cylindrical roller formed from aluminum or stainless steel, for example. One or more halogen heaters  451  that serve as an example of a heating source are disposed inside the internal heating roller  45  to heat the surface of the internal heating roller  45  to a predefined temperature (for example, 190° C.). The internal heating roller  45  is provided with a meandering controller (not shown) that serves as a meandering control unit that controls meandering of the fixing belt  43 . The meandering controller includes a detection unit (end portion sensor) (not shown) that detects the position of an end portion of the fixing belt  43  along the width direction. One end portion of the internal heating roller  45  along the axial direction is moved in the direction perpendicular to the axial direction on the basis of information on the position of the end portion of the fixing belt  43  detected by the detection unit to control meandering of the fixing belt  43 . 
     The external heating roller  46  which serves as an example of the heating unit is a cylindrical roller formed from aluminum or stainless steel, for example. A release layer made of a fluorine resin is formed on the surface of the external heating roller  46 . One or more halogen heaters  461  are disposed inside the external heating roller  46  as an example of a heating source to heat the surface of the external heating roller  46  to a predefined temperature (for example, 190° C.). Spring members (not shown) are disposed at both end portions of the external heating roller  46  along the axial direction to press the fixing belt  43  inward to apply a tension of 15 kgf, for example, to the entire fixing belt  43 . 
     In the exemplary embodiment, the fixing belt  43  is heated by the internal heating roller  45  and the external heating roller  46 . However, the present invention is not limited thereto, and a heating source may be disposed inside the secured pad  44  so that the secured pad  44  heats the fixing belt  43  in addition to the internal heating roller  45  and the external heating roller  46 . 
     Thus, the fixing belt  43  is a member formed as an endless belt, and has a low heat capacity compared to a fixing member formed as a roll. Thus, the surface of the fixing belt  43  is heated to a predefined temperature while the fixing belt  43  passes through the internal heating roller  45  and the external heating roller  46 . In addition, the internal heating roller  45  and the external heating roller  46  are disposed to contact the fixing belt  43  over a large area. 
     The pressurizing roller  42  is composed of a substrate  421 , an elastic member layer  422 , and a release layer  423 . The substrate  421  is a cylindrical or columnar roller made of aluminum or stainless steel, for example. The elastic member layer  422  is made of a silicone rubber, and is applied to the outer peripheral surface of the substrate  421 . The release layer  423  is made of a PFA tube, and is applied to the surface of the elastic member layer  422 . The pressurizing roller  42  is disposed to be movable into contact with and away from the fixing belt module  41  through a movement unit (not shown), and rotationally driven along the direction of the arrow by a drive unit (not shown). The fixing belt  43  is driven for rotation in the direction of the arrow along with rotation of the pressurizing roller  42  with the pressurizing roller  42  in press contact with the fixing belt  43  at the nip part N. 
     The secured pad  44 , which serves as an example of a secured member, is a member made of a rigid material such as aluminum, stainless steel, or a synthetic resin, for example, and formed to have a generally rectangular cylindrical or generally rectangular columnar cross-sectional shape. The secured pad  44  is disposed on the inner peripheral side of the fixing belt  43  with both end portions secured to a frame (not shown) of the fixing device  40  so that the secured pad  44  extends over the entire length of the pressurizing roller  42  along the axial direction, for example. The secured pad  44  is disposed to uniformly press the pressurizing roller  42  via the fixing belt  43  over a predefined width region (for example, 10 to 50 mm) with a predefined load (for example, 3.0 to 6.0 kgf/cm 2 ). The secured pad  44  thus forms the nip part N in press contact with the pressurizing roller  42  via the fixing belt  43 . 
     As shown in  FIG. 5 , the secured pad  44  includes a pressing part  441 , which serves as an example of a recessed part, disposed in a surface of the secured pad  44  that faces the pressurizing roller  42  to press the fixing belt  43  against the pressurizing roller  42 . The pressing part  441  is configured to curve the fixing belt  43  on the pressurizing roller  42  side into a recessed shape (arcuate shape) in which the fixing belt  43  is recessed from the pressurizing roller  42  toward the secured pad  44 . For further description, the pressing part  441  is formed by recessing the surface of the secured pad  44  into a recessed (arcuate) shape with the center O 1  of the circle disposed on the side of the pressurizing roller  42  facing the secured pad  44 . The radius of curvature R 1  of the pressing part  441  is set to be larger than the radius R 0  of the pressurizing roller  42 , for example. Bringing the pressurizing roller  42  into press contact with the secured pad  44  elastically deforms the elastic member layer  422  of the pressurizing roller  42  in accordance with the cross-sectional shape of the pressing part  441 . This makes it possible to form the nip part N that is wide along the circumferential direction on the outer peripheral surface of the pressurizing roller  42  compared to a case where a fixing member formed as a roll is brought into press contact with the secured pad  44 . 
     The secured pad  44  also includes an input-side guiding part  442  and an output-side guiding part  443 . The input-side guiding part  442  serves as an example of a projecting part provided upstream (on the input side) of the pressing part  441  along the moving direction of the fixing belt  43  (travel direction of the recording material). The output-side guiding part  443  serves as an example of a projecting part provided downstream (on the output side) of the pressing part  441  along the moving direction of the fixing belt  43 . The input-side guiding part  442  and the output-side guiding part  443  are formed to curve the fixing belt  43  on the pressurizing roller  42  side into a projected (arcuate) shape in which the fixing belt  43  is projected from the secured pad  44  toward the pressurizing roller  42 . For further description, the input-side guiding part  442  projects toward the pressurizing roller  42  in an arcuate shape with the center O 2  of the circle disposed on the side of the secured pad  44  facing the pressurizing roller  42 . Meanwhile, the output-side guiding part  443  projects toward the pressurizing roller  42  in an arcuate shape with the center O 3  of the circle disposed on the side of the secured pad  44  facing the pressurizing roller  42 . The respective radii of curvature R 2  and R 3  of the input-side guiding part  442  and the output-side guiding part  443  are set to be greatly smaller than the radius of curvature R 1  of the pressing part  441 . The respective radii of curvature of the input-side guiding part  442  and the output-side guiding part  443  may be set to be equal to each other, for example. In the exemplary embodiment, however, the radius of curvature R 3  of the downstream guiding part  443  is set to be smaller than the radius of curvature R 2  of the upstream guiding part  442  (R 3 &lt;R 2 ) so that the fixing belt  42  is curved with a larger curvature on the downstream side in consideration of the detachability of the recording paper  5 . 
     The secured pad  44  further includes a second guiding part  444  provided on the input side between the input-side guiding part  442  and the pressing part  441 . The second guiding part  444  is a region having a small curvature (large radius of curvature) compared to the input-side guiding part  442 . For further description, the input-side guiding part  442  includes the second guiding part  444  provided on the pressing part  441  side. The radius of curvature R 4  of the second guiding part  444  is set to be larger than that of the input-side guiding part  442 . The second guiding part  444  is shaped to project toward the pressurizing roller  42  in an arcuate shape with the center O 4  of the circle disposed on the side of the secured pad  44  facing the pressurizing roller  42 . 
     The input-side guiding part  442  and the second guiding part  444  are connected to be smoothly continuous. That is, the input-side guiding part  442  and the second guiding part  444  are formed such that the respective tangents to the input-side guiding part  442  and the second guiding part  444  at a connection portion at which the input-side guiding part  442  and the second guiding part  444  are connected extend in the same direction, for example. For further description, the input-side guiding part  442  and the second guiding part  444  are connected on a straight line that connects between the center O 2  of the arc forming the input-side guiding part  442  and the center O 4  of the arc forming the second guiding part  444 . 
     Similarly, the second guiding part  444  and the pressing part  441 , and the pressing part  441  and the output-side guiding part  443 , are connected to be smoothly continuous. The second guiding part  444  and the pressing part  441  are connected on a straight line that connects between the center O 4  of the arc forming the second guiding part  443  and the center O 1  of the arc forming the pressing part  441 . Meanwhile, the pressing part  441  and the output-side guiding part  443  are connected on a straight line that connects between the center O 1  of the arc forming the pressing part  441  and the center O 3  of the arc forming the output-side guiding part  443 . The nip part N is formed from the pressing part  441  of the secured pad  44 . However, the nip part N may be formed to include a part of the second guiding part  444  and a part of the output-side guiding part  443 . 
     The fixing device  40  may include a low-sliding-resistance member  440  in the form of a sheet interposed between the secured pad  44  and the fixing belt  43  to reduce the sliding resistance therebetween. 
     &lt;Operation of Fixing Device&gt; 
     In the image forming apparatus  1 , as shown in  FIG. 1 , a toner image is electrostatically transferred from the intermediate transfer belt  21  onto the recording paper  5  through the second transfer performed at the second transfer position, and the recording paper  5  to which an unfixed toner image has been transferred is transported to the fixing device  40 . In the fixing device  40 , as shown in  FIG. 3 , the unfixed toner image is fixed onto the recording paper  5  with the recording paper  5  heated and pressurized while the recording paper  5  passes through the nip part N formed between the fixing belt module  41  and the pressurizing roller  42 . 
     In the fixing device  40  according to the exemplary embodiment, the recording paper  5  which passes through the nip part N is heated by the fixing belt  43 . The fixing belt  43  is heated by the internal heating roller  45  which contacts the inner peripheral surface of the fixing belt  43  and the external heating roller  46  which contacts the outer peripheral surface of the fixing belt  43 . 
     In the fixing device  40 , the secured pad  44  is a rigid member formed from aluminum, stainless steel, or the like, and the pressurizing roller  42  is a soft roller coated with the elastic member layer  423 . Therefore, the nip part N which has some width in the moving direction of the fixing belt  43  is formed with the secured pad  44  hardly warped but with the elastic member layer  422  positioned on the surface of the pressurizing roller  42  warped. 
     On the output side of the nip part N of the secured pad  44 , as shown in  FIG. 5 , the fixing belt  43  is transported along the output-side guiding part  443  disposed on the output side of the secured pad  44 . Thus, the transport direction of the fixing belt  43  is abruptly changed there. That is, the fixing belt  43  moves along the output-side guiding part  443  of the secured pad  44 , and therefore the curvature of the fixing belt  43  becomes large enough to match the radius of curvature R 3  of the output-side guiding part  443 . This allows the recording paper  5 , the adhesion of which to the fixing belt  43  has been reduced in the nip part N, to be separated from the fixing belt  43  by the firmness (rigidity) of the paper itself. 
     The recording paper  5  separated from the fixing belt  43  is transported toward the cooling unit  70  by a paper ejection guiding plate  75  and a paper ejecting belt  76  disposed downstream of the secured pad  44 . The fixing device  40  thus finishes the fixation process. 
     When the fixing belt  43  moves to the nip part N through the first driven roller  47 , as shown in  FIG. 5 , the fixing belt  43  contacts the input-side guiding part  442  provided upstream of the nip part N of the secured pad  44  to be curved into a shape in which the fixing belt  43  is projected downward from the secured pad  44  toward the pressurizing roller  42  at the curvature of the input-side guiding part  442  of the secured pad  44 . After that, at the pressing part  441  of the secured pad  44  forming the nip part N, the travel direction of the fixing belt  43  is changed by the pressurizing roller  42  such that the fixing belt  43  is recessed toward the secured pad  44  (projected upward) at the curvature in the opposite direction. Further, the fixing belt  43 , which has been curved to be projected upward at the curvature of the pressurizing roller  42  at the pressing part  441  of the secured pad  44 , contacts the output-side guiding part  443  provided downstream of the pressing part  441  of the secured pad  44  to be curved again into a shape in which the fixing belt  43  is projected toward the pressurizing roller  42  (downward) at the curvature of the output-side guiding part  443  of the secured pad  43 . 
     Thus, when the fixing belt  43  passes through the nip part N, the fixing belt  43  is curved into a downwardly projected shape, then deformed into a shape in which the fixing belt  43  is recessed toward the secured pad  44  (upwardly projected shape), and thereafter curved into a downwardly projected shape again in accordance with the shape of the input-side guiding part  442 , the pressing part  441 , and the output-side guiding part  443 , respectively, formed on the surface of the secured pad  44  on the pressurizing roller  42  side. That is, the travel direction of the fixing belt  43  is changed with curvatures in opposite directions from a projected shape to a recessed shape and then from a recessed shape to a projected shape again. 
     Moreover, the guiding parts  442  and  443  positioned on the input side and the output side, respectively, of the nip part N change the moving direction of the fixing belt  43  from the first driven roller  47  to the third driven roller  49 . In particular, the curvature of the output-side guiding part  443  is set to be large compared to the nip part N (the radii of curvature R 2  and R 3  are set to be small) in order to separate the recording paper  5  from the fixing belt  43 . 
     In the exemplary embodiment, as shown in  FIG. 5 , the second guiding part  444  is provided on the input side in a region between the input-side guiding part  442  and the pressing part  441 . The second guiding part  444  has a small curvature (large radius of curvature) compared to the input-side guiding part  442 . In this way, when the fixing belt  43  moves from the input-side guiding part  442  to the pressing part  441 , the fixing belt  43  moves from the input-side guiding part  442  to the pressing part  441  via the second guiding part  444 . As a result, the fixing belt  43  reaches the pressing part  441  from the input-side guiding part  442  with its curvature reduced (with its radius of curvature increased) via the second guiding part  444 . 
     For further description, when the fixing belt  43  moves from the input-side guiding part  442  to the pressing part  441 , the curvature of the fixing belt  43  is not immediately changed from the curvature of the input-side guiding part  441  to the curvature of the pressing part  441 , but changed to the curvature of the pressing part  441  via the curvature of the second guiding part  444 , which is provided on the pressing part  441  side with respect to the input-side guiding part  442 . The curvature of the second guiding part  444  is set to be smaller than the curvature of the input-side guiding part  441 . Therefore, after the curvature of the fixing belt  43  is reduced as the fixing belt  43  moves from the input-side guiding part  441  toward the pressing part  441 , the curvature of the fixing belt  43  is inverted to reach the curvature of the pressing part  441 . 
     Therefore, the curvature of the fixing belt  43  is not abruptly inverted from the curvature of the input-side guiding part  442  to the curvature of the pressing part  441  when the fixing belt  43  moves from the input-side guiding part  442  to the pressing part  441 . 
     Second Exemplary Embodiment 
       FIG. 7  is a cross-sectional view showing the configuration of a fixing device according to a second exemplary embodiment of the present invention. 
     In the fixing device  40  according to the second exemplary embodiment, as shown in  FIG. 7 , a straight portion  445  formed to be straight is provided between the input-side guiding part  442 , which is provided upstream of the pressing part  441 , and the pressing part  441 . The input-side guiding part  442  and the pressing part  441  are connected to be smoothly continuous by the straight portion. 
     Providing the straight portion  445  between the input-side guiding part  442  and the pressing part  441  makes the fixing belt  43  planar along the straight portion  445  when the fixing belt  43  moves to the nip part N. 
     The configuration and the operation are otherwise similar to those of the first exemplary embodiment described earlier. Thus, such similarities are not described. 
     Third Exemplary Embodiment 
     In the fixing device  40  according to a third exemplary embodiment, as shown in  FIG. 8 , a straight portion  447  formed to be straight is provided between the output-side guiding part  443 , which is provided downstream of the pressing part  441 , and the pressing part  441 . The output-side guiding part  443  and the pressing part  441  are connected to be smoothly continuous by the straight portion. 
     The configuration and the operation are otherwise similar to those of the first exemplary embodiment described earlier. Thus, such similarities are not described. 
     Fourth Exemplary Embodiment 
     In the fixing device  40  according to a fourth exemplary embodiment, as shown in  FIG. 9 , two rollers, namely the internal heating roller  45  and a support roller  49  for meandering control, are used as support rollers that apply a tension to the fixing belt  43 . 
     In the case of the fixing device  40 , the entire device may be reduced in size, contributing to a reduction in size of the image forming apparatus  1 . In order to supply a sufficient amount of heat to the fixing belt  43 , the secured pad  44  may be provided with a heating source. 
     The configuration and the operation are otherwise similar to those of the first exemplary embodiment described earlier. Thus, such similarities are not described. 
     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.