Patent Publication Number: US-8543045-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. 2010-145111 filed Jun. 25, 2010. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a fixing device and an image forming apparatus. 
     (ii) Related Art 
     In image forming apparatuses, such as a printer and a copying machine, which form an image formed by developer onto a recording medium such as paper, a fixing device fixes an unfixed image on the recording medium. There are known fixing devices including a rotatable fixing roller, an endless fixing belt that rotates in contact with the fixing roller, and a pressing member that contacts the fixing belt from the back side and presses a surface of the fixing belt against the fixing roller so as to form a fixing portion (pressing portion) through which a fixing object passes. 
     SUMMARY 
     According to an aspect of the invention, there is provided a fixing device including a rotatable fixing roller; an endless fixing belt that rotates in contact with the fixing roller; a pressing unit having a first pressing member and a second pressing member, the first pressing member contacting the fixing belt from a back side of the fixing belt and pressing a surface of the fixing belt against the fixing roller to form a fixing portion through which a fixing object passes, the second pressing member contacting the fixing belt from the back side of the fixing belt and pressing the surface of the fixing belt against the fixing roller to form the fixing portion through which the fixing object passes; a fixed support member that supports the fixing roller rotatably; a movable support member that rocks closer to and away from the fixing roller, supports the first pressing member in a fixed state, and supports the second pressing member adjacent to the first pressing member in a manner such that the second pressing member is movable closer to and away from the fixing roller; a first pressure spring that applies pressure to move the movable support member closer to the fixing roller; a second pressure spring that applies pressure to move the second pressing member closer to the fixing roller; and a switch mechanism that changes a movable range of the movable support member so as to switch between a first state in which the first pressing member and the second pressing member are pressed against the fixing roller with the fixing belt being disposed therebetween and a second state in which only the second pressing member is pressed against the fixing roller with the fixing belt being disposed therebetween. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view illustrating an image forming apparatus including a fixing device according to a first exemplary embodiment; 
         FIG. 2  is a perspective view of the principal part of the fixing device used in the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3A  is a front view of the fixing device on arrow IIIA in  FIG. 2 ,  FIG. 3B  is a side view of the fixing device on arrow IIIB in  FIG. 2 , and  FIG. 3C  is a side view, as viewed in a direction opposite the direction of arrow IIIB in  FIG. 2 ; 
         FIG. 4  is a partial cross-sectional view of the fixing device, taken along line IV-IV in  FIG. 2  (in a normal mode); 
         FIG. 5  is a perspective view of a pressing rotating member and pressing rocking frames in the fixing device illustrated in  FIG. 2 ; 
         FIGS. 6A and 6B  are perspective views illustrating a structure obtained by removing an endless belt from the pressing rotating member in  FIG. 5 , as viewed in two directions; 
         FIG. 7  is an exploded perspective view of the structure illustrated in  FIG. 6 ; 
         FIG. 8  is an exploded perspective view of a part of the structure of  FIG. 6  (two support plates and separate peripheral guide plates); 
         FIGS. 9A and 9B  illustrate separate end guide plates in the fixing device in  FIG. 4 , as viewed from the front and back sides; 
         FIG. 10  illustrates the principal part of the fixing device in  FIG. 4 ; 
         FIG. 11  is a partial cross-sectional view of the principal part of the fixing device in  FIG. 10 ; 
         FIG. 12A  is a perspective view of the fixing device in an envelope mode, and  FIG. 12B  is a side view of the fixing device on arrow XIIB in  FIG. 12A ; 
         FIG. 13  illustrates the principal part of the fixing device illustrated in  FIG. 12 ; 
         FIG. 14  is a partial cross-sectional view of the fixing device, taken along line XIV-XIV in  FIG. 12  (in an envelope mode); 
         FIGS. 15A and 15B  illustrate the separate end guide plates of the fixing device in  FIG. 14 , as viewed from the front and back sides; and 
         FIG. 16  is a partial cross-sectional view of the principal part of the fixing device in  FIGS. 15A and 15B . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments for carrying out the present invention (hereinafter simply referred to as exemplary embodiments) will be described below with reference to the attached drawings. 
     First Exemplary Embodiment 
       FIG. 1  illustrates an image forming apparatus  1  (and a fixing device  4 ) according to a first exemplary embodiment of the present invention. 
     The image forming apparatus  1  includes, in a housing (not shown), an imaging device  2  that forms an unfixed toner image based on image information and finally transfers the toner image onto a recording medium  9  such as paper, a paper feed device  3  that stores the recording medium  9  and transports and supplies the recording medium  9  to the imaging device  2 , and a fixing device  4  that fixes the toner image, which is transferred in the imaging device  2 , on the recording medium  9 . In  FIG. 1 , a one-dot chain line with an arrow indicates a transport path for the recording medium  9 . 
     The imaging device  2  can form and transfer a toner image, for example, by a known electrophotographic method. More specifically, the imaging device  2  includes a photoconductor drum  12  that rotates in a direction of the arrow. Around the photoconductor drum  12 , a charging device  13 , an exposure device  14 , a developing device  15 , a transfer device  16 , and a cleaning device  17  are arranged. The charging device  13  charges a surface (image carrying surface) of the photoconductor drum  12 . The exposure device  14  irradiates the charged surface of the photoconductor drum  12  with light based on image information (signal) so as to form an electrostatic latent image having different potentials. The developing device  15  develops the electrostatic latent image on the photoconductor drum  12  with toner serving as developer so as to form a toner image. The transfer device  16  transfers the toner image onto a recording medium  9  supplied from the paper feed device  3 . The cleaning device  17  cleans the surface of the photoconductor drum  12  by removing toner remaining on the surface after transfer. 
     For example, the photoconductor drum  12  includes a cylindrical base and an image carrying surface having a photoconductive layer (photosensitive layer) made of an organic photosensitive material. The charging device  13  is of a contact charging type that charges the surface of the photoconductor drum  12  by applying a predetermined charging voltage to a charging roller rotating in contact with the surface of the photoconductor drum  12 . The exposure device  14  is formed by, for example, a light emitting diode (LED) recording head or a semiconductor laser scanning device. The exposure device  14  receives image signals obtained by subjecting, to required processing, image information, which is input from an image reading device or a storage-medium reading device provided in or connected (in a wired or wireless manner) to the image forming apparatus  1  and an external apparatus serving as a device for forming an image, such as a computer, by an image processing device (not shown). 
     The developing device  15  supplies charged developer containing toner of a predetermined color (e.g., one-component developer or two-component developer) onto the surface of the photoconductor drum  12  via a developing roller  15   a  to which a developing voltage is applied. The transfer device  16  is of a contact type that performs transfer by applying a predetermined transfer voltage to a transfer roller rotating in contact with the surface of the photoconductor drum  12 . 
     The paper feed device  3  includes a storage cassette  31  that stores stacked plural recording media  9  of a predetermined size to be supplied to the imaging device  2 , and a supply unit  32  that supplies and transports the recording media  9  stored in the storage cassette  31  one by one. As required, plural storage cassettes  31  are provided. Further, the paper feed device  3  is connected to a sheet transport path for sheet feeding through which the recording media  9  are transported from the storage cassette  31  to a transfer portion in the imaging device  2  (between the photoconductor drum  12  and the transfer device  16 ). The sheet transport path includes plural pairs of transport rollers  33  and  34 , a transport guide member, etc. The pair of sheet transport rollers  34  serve as a pair of transport-timing adjusting rollers that temporarily stop a leading edge of a transported recording medium  9  and are then driven to feed the recording medium  9  at a predetermined paper feed time. The sheet transport path is also provided between the imaging device  2  and the fixing device  4 . 
     In a housing  40  of the fixing device  4 , a heating roller  41 , an endless fixing belt  42 , and a pressing unit  43  are arranged. The heating roller  41  is heated by a heating unit so that the surface temperature of the heating roller  41  is kept at a predetermined temperature, and rotates in a direction of the arrow. The endless fixing belt  42  rotates in contact with a portion of the surface of the heating roller  41  extending substantially in a rotation axis direction A. The pressing unit  43  contacts the fixing belt  42  from a back surface side, and presses a front surface of the fixing belt  42  against the heating roller  41  so as to form a pressing portion (fixing portion) NP through which a fixing object (a recording medium  9  on which a toner image is transferred). Referring to  FIG. 1 , a pair of output rollers  39  output the recording medium  9  after fixing. Details of the fixing device  4  will be described below. 
     The image forming apparatus  1  performs image formation as follows. 
     In the imaging device  2 , first, the photoconductor drum  12  starts rotation, and the surface of the rotating photoconductor drum  12  is charged with a predetermined charging potential by the charging device  13 . After that, light based on an image signal is applied onto the charged surface of the photoconductor drum  12  by the exposure device  14 , whereby an electrostatic latent image having a predetermined latent image potential is formed. Subsequently, when the electrostatic latent image moves with the rotation of the photoconductor drum  12  and passes through the developing device  15 , the toner supplied from the developing roller  15   a  of the developing device  15  electrostatically adheres to the latent image to form a developed toner image. After that, the toner image on the photoconductor drum  12  is electrostatically transferred onto a recording medium  9  supplied and transported from the paper feed device  3  at a transfer position opposing the transfer device  16 . After the toner image is transferred, the surface of the photoconductor drum  12  is cleaned by the cleaning device  17 . 
     Next, the recording medium  9  on which the unfixed toner image has been formed in the imaging device  2  is transported to the fixing device  4 , and is led into the pressing portion NP between the heating roller  41  and the fixing belt  42 . In the fixing device  4 , the recording medium  9  passes through the pressing portion NP while being nipped in the pressing portion NP, so that the unfixed toner image is heated under pressure to be fixed on the recording medium  9 . After fixing, the recording medium  9  is output from the fixing device  4 , and is transported by the pair of output rollers  39  into an output storage portion (not shown). Thus, a toner image is formed on one side of the recording medium  9 . 
     The image forming apparatus  1  can use, as a recording medium  9  serving as an object on which an image is formed, not only sheets, such as a recording sheet, thick paper, a transparent sheet, and a postcard, but also envelope-like media shaped like a bag, such as an envelope. An envelope-like recording medium  9  is stored in the storage cassette  31  of the paper feed device  3  and is transported to the transfer position in the imaging device  2  through the sheet transport path for sheet feeding during image formation. Alternatively, the envelope-like recording medium  9  is stored in a manual feeding table  35 , and is joined into the sheet transport path for sheet feeding by a supply unit  36  and transported to the transfer position in the imaging device  2  during image formation, as illustrated in  FIG. 1 . 
     Next, the fixing device  4  will be described in detail. 
     As illustrated in  FIGS. 2 to 4 , the fixing device  4  includes fixed frames  51  ( 51 A and  51 B), pressing rocking frames  81 , first pressure springs  87 , second pressure springs  91 , and switch mechanisms  45  in addition to the heating roller  41 , the fixing belt  42 , and the pressing unit  43  described above. The fixed frames  51  support the heating roller  41  rotatably. The pressing rocking frames  81  are turnably attached to ends of the fixed frames  51  with shafts  56  being disposed therebetween, rock closer to and away from the heating roller  41 , and support the pressing unit  43  in a required state. The first pressure springs  87  apply a pressure such as to move the pressing rocking frames  81  closer to the heating roller  41 . The second pressure springs  91  apply a pressure such as to move one structure that forms the pressing unit  43  closer to the heating roller  41 . The switch mechanisms  45  switch the pressing unit  43  to a required state by changing the rocking range of the pressing rocking frames  81 . 
     In the fixing device  4 , the switch mechanisms  45  change the rocking range of the pressing rocking frames  81  so as to switch a pressing state in which the pressing unit  43  is pressed against the heating roller  41  with the fixing belt  42  being disposed therebetween. That is, the switch mechanisms  45  switch between a first state (referred to as a normal mode) and a second state (referred to as an envelope mode). The first state is suitable for image formation (including a fixing step) using a sheet (different from an envelope-like medium) as a recording medium  9  (see  FIGS. 3 and 4 ). The second state is suitable for image formation using an envelope-like medium as a recording medium  9  (see  FIGS. 12 and 14 ). 
     The heating roller  41  includes a metal cylindrical base having a length larger than the maximum transport width of the recording medium  9  serving as a fixing object, and an elastic layer and a release layer formed on a surface of the cylindrical base in this order. A heating unit (not shown) for heating the heating roller  41  to a required temperature is provided in the cylindrical base of the heating roller  41 . The heating roller  41  is rotatably supported at both ends by the fixed frames  51 A and  51 B. 
     The heating roller  41  is attached at both ends to the fixed frames  51 A and  51 B, respectively, with bearings  54  being disposed therebetween, and is rotated at a required speed by transmitting rotational power from a rotating device (not shown) provided in a body of the image forming apparatus  1  to a gear  55  fixed to one end of the heating roller  41 . The surface temperature of the heating roller  41  is detected by a temperature detector (not shown), and a heating operation of the above-described heating unit is controlled on the basis of information about the detection so that the surface temperature of the heating roller  41  is kept at a required temperature. 
     Each of the fixed frames  51  includes a curved portion for holding the heating roller  41 , and two end portions  51   a  and  51   b  projecting on both sides between which the heating roller  41  is provided. The end portion  51   a  of the fixed frame  51  is located on an upstream side of the pressing portion NP in a passage direction B of the fixing object, and the shaft  56  to which a part of the pressing rocking frame  81  is attached is provided in the end portion  51   a.  The end portion  51   b  of the fixed frame  51  is located on a downstream side of the pressing portion NP in the passage direction B of the fixing object, and a spring support face portion  52  for supporting the corresponding first pressure spring  87  is provided in the end portion  51   b.  The spring support face portion  52  is bent toward the inner side of the fixed frame  51 . Both of the fixed frames  51  are fixed to the housing  40  of the fixing device  4 . 
     The fixing belt  42  is a cylindrical belt having a width substantially equal to the length (axial dimension) of the heating roller  41 . The fixing belt  42  is formed by coating a surface of a thin cylindrical belt base formed of synthetic resin, such as polyimide, with a release layer formed of fluorine resin or the like. 
     As illustrated in  FIG. 4 , the pressing unit  43  includes a long rigid member (first pressing member)  61  and an elastic member (second pressing member)  62  that have a length substantially equal to the width of the fixing belt  42 . In the pressing unit  43  of the first exemplary embodiment, the rigid member  61  is located on a downstream side of the pressing portion NP in the passage direction B of the fixing object, and the elastic member  62  is located on an upstream side of the rigid member  61  in the passage direction B of the fixing object. The rigid member  61  and the elastic member  62  are provided side by side. 
     The rigid member  61  is formed by an inelastic member made of synthetic resin, metal, or the like. As illustrated in  FIGS. 4 and 7 , the rigid member  61  of the first exemplary embodiment includes a contact portion  61   a  that presses the fixing belt  42  against the surface of the heating roller  41 , and a fitting portion  61   b  in which an end of a first support plate  64  for supporting the rigid member  61  on the back side of the contact portion  61   a,  which will be described below, is fitted. For example, the fitting portion  61   b  includes a side face portion  61   c  extending from the contact portion  61   a,  and holding projections  61   d  that are provided at a distance, corresponding to the thickness of the first support plate  64 , from the side face portion  61   c  and are dotted in the axial direction A. 
     As illustrated in  FIGS. 4 and 7 , the elastic member  62  includes an elastic contact element  63  that presses the fixing belt  42  against the surface of the heating roller  41 , and a holding element  60  that holds the elastic contact element  63 . The elastic contact element  63  is formed of an elastic material such as rubber. In the first exemplary embodiment, the elastic contact element  63  is shaped like a long plate (having a rectangular cross section in a no-load state) formed of silicon rubber. The holding element  60  includes a receiving body portion  60   a  that receives the elastic contact element  63 , and a fitting portion  60   b  in which an end of a second support plate  65  for supporting the elastic member  62  (elastic contact element  63 ), which will be described below, is fitted on the back side of the receiving body portion  60   a.  For example, the fitting portion  60   b  includes holding projections  60   c  and  60   d  that oppose each other in a staggered manner in the axial direction A. The holding projections  60   c  and  60   d  are provided at a distance, corresponding to the thickness of the second support plate  65 , from both edges of the back side of the receiving body portion  60   a.    
     As illustrated in  FIG. 4 , the rigid member  61  and the elastic member  62  of the pressing unit  43  are independently supported by two support plates  64  and  65 . 
     As illustrated in  FIG. 8 , the first support plate  64  is formed by a long rectangular metal plate, and supports the rigid member  61  with a body portion  64   a  serving as one end portion extending in the longitudinal direction (also in the axial direction A) being fitted in the fitting portion  61   b  of the rigid member  61 . The other end portion  64   b  of the first support plate  64  extending in the longitudinal direction is bent to increase the total mechanical strength of the first support plate  64 . As illustrated in  FIG. 8 , the second support plate  65  is formed by a long rectangular metal plate, and supports the elastic member  62  with a body portion  65   a  serving as one end portion extending in the longitudinal direction being fitted in the fitting portion  60   b  of the holding element  60  in the elastic member  62 . Longitudinal end portions  64   c,    64   d,    65   c,  and  65   d  of the end portions  64   a  and  65   a  of the two support plates  64  and  65  are attached to attachment holes or attachment cuts of the pressing rocking frames  81 , as will be described below. 
     As illustrated in  FIGS. 5 to 9 , the fixing belt  42  is supported at both ends by a pair of separate end guide plates  66  and  67 , and is supported on an inner peripheral surface by separate peripheral guide plates  68  and  69 . 
     The separate end guide plate  66  and the separate peripheral guide plate  68  serve as an upstream separate end guide plate and an upstream separate peripheral guide plate, respectively, which hold and guide a portion of the fixing belt  42  on an upstream side of almost the boundary between the rigid member  61  and the elastic member  62  of the pressing unit  43  in the passage direction B of the fixing object. The separate end guide plate  67  and the separate peripheral guide plate  69  serve as a downstream separate end guide plate and a downstream separate peripheral guide plate, respectively, which hold and guide a portion of the fixing belt  42  on a downstream side of almost the boundary between the rigid member  61  and the elastic member  62  of the pressing unit  43  in the passage direction B of the fixing object. 
     As illustrated in  FIGS. 6 ,  7 , and  9 , the upstream separate end guide plates  66  and the downstream separate end guide plates  67  include semicircular peripheral-surface guide portions  66   a  and  67   a  having semicylindrical guide faces that hold and guide the ends of the fixing belt  42  from the inner peripheral side, and flange guide portions  66   b  and  67   b  having vertical guide faces that project in a substantially vertical direction at outer ends of the peripheral-surface guide portions  66   a  and  67   a  and that guide the ends of the fixing belt  42  by contact therewith. Portions of the peripheral-surface guide portions  66   a  and  67   a  and the flange guide portions  66   b  and  67   b  of the separate end guide plates  66  and  67  where the rigid member  61  and the elastic member  62  of the pressing unit  43  are provided (hereinafter these portions will be referred to as pressing-portion opposing portions  66   c  and  67   c ) are cut in a linear form (or in a planar form) ( FIGS. 6A ,  6 B, and  9 B). 
     As illustrated in  FIGS. 7 and 9 , the peripheral-surface guide portions  66   a  of the upstream separate end guide plates  66  have rectangular fitting holes  71  in which end portions  65   c  and  65   d  of the body portion  65   a  of the second support plate  65  are fixedly fitted, and substantially rectangular insertion holes  72  in which attachment end portions  68   b  of the upstream separate peripheral guide plate  68  are fixedly fitted. In contrast, the peripheral-surface guide portions  67   a  of the downstream separate end guide plates  67  have rectangular fitting hole  73  in which end portions  64   c  and  64   d  of the body portion  64   a  of the first support plate  64  are fixedly fitted, and fitting holes  74  of substantially cylindrical cross section in which attachment end portions  69   b  of arc-shaped cross section of the downstream separate peripheral guide plate  69  are fixedly fitted. 
     In each pair of separate end guide plates  66  and  67 , an end portion along the fitting hole  71  and an end portion along the fitting hole  73  are formed as linear end faces. The separate end guide plates  66  and  67  can be relatively displaced in the longitudinal direction of the fitting holes  73  and  74  while the end faces oppose each other. 
     As illustrated in  FIG. 8 , the upstream separate peripheral guide plate  68  has a curved body portion  68   a  extending long in the axial direction A and having a semicylindrical cross section. At either end of the curved body portion  68   a,  attachment end portions  68   b  each shaped like a flat plate project in the axial direction A. Further, plural ribs  68   c  are arranged on an outer peripheral surface of the curved body portion  68   a  at intervals in the axial direction A. The ribs  68   c  are linearly raised in the rotating direction of the fixing belt  42 . The separate peripheral guide plate  68  also has a portion where a felt  49  impregnated with lubricant oil is provided to apply the lubricant oil onto the inner peripheral surface of the fixing belt  42  (see  FIGS. 4 and 11 ). 
     The downstream separate peripheral guide plate  69  has a curved body portion  69   a  extending long in the axial direction A and having a semicylindrical cross section. At either end of the curved body portion  69   a,  arc-shaped attachment end portions  69   b  project in the axial direction A. Further, plural ribs  69   c  are arranged on an outer peripheral surface of the curved body portion  69   a  at intervals in the axial direction A. The ribs  69   c  are linearly raised in the rotating direction of the fixing belt  42 . 
     For example, the pressing unit  43  (rigid member  61  and elastic member  62 ) is assembled as follows: 
     The rigid member  61  is assembled by passing the end portions  64   c  and  64   d  of the body portion  64  of the first support plate  64 , which extends between the two downstream separate end guide plates  67 A and  67 B, through the fitting holes  73  of the separate end guide plates  67 A and  67 B and then inserting the body portion  64   a  of the first support plate  64  into the fitting portion  61   b.  The elastic member  62  is assembled by passing the end portions  65   c  and  65   d  of the body portion  65   a  of the second support plate  65 , which extends between the two upstream separate end guide plates  66 A and  66 B, through the fitting holes  71  of the separate end guide plates  66 A and  66 B and then inserting the body portion  65   a  of the second support plate  65  into the fitting portion  60   b.    
     The downstream separate peripheral guide plate  69  is combined with and fixed to the rigid member  61  by fitting the attachment end portions  69   b  into the fitting holes  74  of the downstream separate end guide plates  67 A and  67 B. The upstream separate peripheral guide plate  68  is combined with and fixed to the elastic member  62  by fitting the attachment end portions  68   b  into the fitting holes  72  of the upstream separate end guide plates  66 A and  66 B. 
     The rigid member  61  and the elastic member  62  of the pressing unit  43  are assembled and the separate peripheral guide plates  68  and  69  are attached thereto, thereby forming a structure having an outer appearance illustrated in  FIGS. 6A and 6B . When the fixing belt  42  is attached to the outer peripheral portions of the guide plates  66  in the structure in  FIGS. 6A and 6B , a structure having an outer appearance illustrated in  FIG. 5  is formed (hereinafter this structure will also be referred to as a pressing rotating body  44 ). The fixing belt  42  illustrated in  FIG. 5  can rotate while being held and guided in a substantially cylindrical form by the separate end guide plates  66  and  67  and the separate peripheral guide plates  68  and  69 . The fixing belt  42  can also rotate while passing outside the rigid member  61  and the elastic member  62  of the pressing unit  43 . 
     A pair of pressing rocking frames  81 A and  81 B are bent once away from the heating roller  41  between the end portions  51   a  of the fixed frames  51  upstream of the pressing portion NP in the passage direction B of the fixing object and the end portions  51   b  of the fixed frames  51  downstream of the pressing portion NP in the passage direction B. 
     End portions  81 Aa and  81 Ba of the pressing rocking frames  81 A and  81 B upstream of the pressing portion NP in the passage direction B of the fixing object are hook-shaped. The pressing rocking frames  81 A and  81 B are attached so as to rock in the directions of arrows C and D closer to and away from the heating roller  41  while the end portions  81 Aa and  81 Ba are caught by the first shafts  56  of the fixed frames  51 A and  51 B ( FIG. 4 ). 
     Pressure for rocking the pressing rocking frames  81 A and  81 B in the direction of arrow C closer to the heating roller  41  is applied from the first pressure spring  87  to the pressing rocking frames  81 A and  81 B. End portions  81 Ab and  81 Bb of the pressing rocking frames  81 A and  81 B downstream of the pressing portion NP in the passage direction B of the fixing object are provided with spring pressing face portions  82  with which the first pressure springs  87  contact at one end to apply the spring force. The spring pressing face portions  82  are bent inward, and oppose the spring support face portions  52  of the fixed frames  51 . The first pressure springs  87  are formed by coil springs having a required free length, spring constant, etc. 
     As illustrated in  FIG. 4 , each of the first pressure springs  87  is attached by a column  88  that is inserted in the space in a coil portion from one end and that has a length such as to protrude from the other end (length larger than the free length of the pressure springs). The column  88  includes a column body portion  88   a,  a flange portion  88   b  provided at the top of the column body portion  88   a  and having a diameter larger than the outer diameter of the coil portion of the coil spring, and a screw portion  88   c  provided at the bottom of the column body portion  88   a  and having a diameter smaller than the diameter of the column body portions  88   a.  Each spring pressing face portion  82  has a column passing hole  82   a  (hole having a diameter smaller than the outer diameter of the coil spring), as illustrated in  FIG. 5 . 
     The first pressure springs  87  are held between the flange portions  88   b  of the columns  88  fixed to the spring support face portions  52  of the fixed frames  51  by screws or nuts and the spring pressing face portions  82  of the pressing rocking frames  81 , and are compressed by a predetermined compression amount. The first pressure springs  87  press the spring pressing face portions  82  of the pressing rocking frames  81 A and  81 B toward the spring support face portion  52  of the fixed frames  51  by a spring force F 1  acting in accordance with the compression amount and spring constant, whereby the pressing rocking frames  81 A and  81 B are entirely rocked closer to the heating roller  41  (rocked in the direction of arrow C). Therefore, a pressure (pressing force) produced by thus rocking the pressing rocking frames  81 A and  81 B in the direction of arrow C is finally transmitted to the pressing unit  43 . 
     As illustrated in  FIG. 5 , the bent portions of the pressing rocking frames  81 A and  81 B to which the pressing unit  43  is attached include rectangular attachment holes  83  in which the end portions  64   c  and  64   d  of the body portion  64   a  of the first support plate  64  are inserted and fixedly supported, and linear attachment cuts  84  in which the end portions  65   c  and  65   d  of the body portion  65   a  of the second support plate  65  are inserted so that the second support plate  65  is movable in the directions of arrows E and H closer to and away from the heating roller  41 . 
     The end portions  64   c  and  64   d  of the first support plate  64  and the end portions  65   c  and  65   d  of the second support plate  65  that protrude from the separate end guide plates  66  and  67  of the pressing rotating body  44  illustrated in  FIG. 5  are fitted in the attachment holes  83  and the attachment cuts  84  of the pressing rocking frames  81 A and  81 B, respectively. 
     Thus, the first support plate  64  is fixed to the pressing rocking frames  81 A and  81 B via the attachment holes  83 . The rigid member  61  fixed to the first support plate  64  is also fixed to the pressing rocking frames  81 A and  81 B. In contrast, the second support plate  65  is attached to the pressing rocking frames  81 A and  81 B via the attachment cuts  84  in a manner such as to be movable in the directions of arrows E and H closer to and away from the heating roller  41 . Further, the elastic member  62  fixed to the second support plate  65  is also attached to the pressing rocking frames  81 A and  81 B in a manner such as to be movable in the directions of arrows E and H closer to and away from the heating roller ( FIG. 10 ). 
     The second pressure springs  91  apply a pressure P 1  such as to move the elastic member  62  serving as one component of the pressing unit  43  in the direction of arrow E toward the heating roller  41 . The second pressure springs  91  are formed by coil springs having a predetermined free length, spring constant, etc. As illustrated in  FIGS. 2 ,  3 , and  10 , the second pressure springs  91  are supported at one end by the spring support face portions  85  provided in the bent portions of the pressing rocking frames  81 A and  81 B, and are pressed at the other end against spring pressing portions  65   e  provided in the end portions  65   b  and  65   c  of the second support plate  65  that protrude from the separate end guide plates  66  and  67 . The second pressure springs  91  are attached while being compressed to a predetermined length as a whole. 
     Thus, the second pressure springs  91  apply, to the spring pressing portions  65   e  of the second support plate  65 , the pressure P 1  that moves the elastic member  62  in the direction of arrow E toward the heating roller  41 , and further apply the pressure P 1  to the elastic member  62  via the second support plate  65 . The elastic member  62  to which the pressure P 1  is applied is allowed to move in the direction of arrow E toward the heating roller  41  along the attachment cuts  84  of the pressing rocking frames  81 , and is finally pressed against the heating roller  41  with the fixing belt  42  being disposed therebetween. 
     The spring support portions  85  of the pressing rocking frames  81  are face-shaped portions bent outward at the bent portions of the pressing rocking frames  81 A and  81 B, and are each provided with plural bent portions  85   a  for holding the end portions of the second pressure springs  91 . The spring pressing portions  65   e  of the second support plate  65  form projections  65   f  to be inserted in the coil portions of the second pressure springs  91 . The pressure P 1  applied by the second pressure springs  91  may be any pressure that can reduce wrinkles in a special bag-shaped recording medium, such as an envelope, during fixing. In the first exemplary embodiment, the pressure P 1  is set at a value smaller than the above-described pressure F 1  applied by the first pressure springs  87  (e.g., a value about 30 to 40% smaller). Further, since the first exemplary embodiment adopts this setting of the relationship in volume of the pressure and the above-described configuration, a pressure Pb at the pressing portion NP in the second state finally becomes smaller than a pressure Pa of the pressing portion NP in the first state (Pb&lt;Pa). For this reason, by adopting a structure such that the pressures at the pressing portion NP have the relationship Pb&lt;Pa (for example, by combining a structure using the principle of leverage), the relationship in volume between the pressure F 1  applied by the first pressure springs  87  and the pressure P 1  applied by the second pressure springs  91  may be different from the above relationship (P 1 &lt;F 1 ), for example, may be (P 1 ≧F 1 ). 
     The switch mechanisms  45  change the rocking range of the pressing rocking frames  81 A and  81 B so as to switch between a first state in which the rigid member  61  and the elastic member  62  of the pressing unit  43  are pressed against the heating roller  41  with the fixing belt  42  being disposed therebetween ( FIGS. 4 and 11 ) and a second state in which only the elastic member  62  is pressed against the heating roller  41  with the fixing belt  42  being disposed therebetween ( FIGS. 14 and 16 ). Here, the first state corresponds to the above-described normal mode, and the second state corresponds to the above-described envelope mode. 
     As illustrated in  FIGS. 2 ,  3 , and  10 , the switch mechanisms  45  of the first exemplary embodiment are formed by disc cams  95  that can contact cam receiving face portions  89  provided at free ends (end portions  81   b  opposite the end portions  81   a  supported by the shafts  56 ) of the pressing rocking frames  81 A and  81 B. The disc cams  95  are fixed to a rotation shaft  96  rotatably provided in the housing  40  of the fixing device or parts of the fixed frames  51 . 
     Further, as illustrated in  FIGS. 10 and 13 , the disc cams  95  have first cam faces  95   a  that do not contact the cam receiving face portions  89  in the first state and second cam faces  95   b  that contact the cam receiving face portions  89  in the second state so as to rock the pressing rocking frames  81  in the direction D away from the heating roller  41  and to stop and hold the rigid member  61  fixed to the pressing rocking frames  81  at a position separate from the heating roller  41 . The second cam faces  95   b  are at a greater distance from the rotation shaft  96  than the first cam faces  95   a.    
     The rotation shaft  96  of the disc cams  95  can be manually rotated by the user of the image forming apparatus or automatically rotated by the driving force of a rotation device. When the rotation shaft  96  is manually rotated by the user, for example, an operation lever capable of rotating the rotation shaft  96  in a predetermined direction is attached to the rotation shaft  96  directly or indirectly. When the first cam faces  95   a  of the disc cams  95  are at positions opposing the cam receiving face portions  89  of the pressing rocking frames  81 , the rotation shaft  96  is located at a position such that the first cam faces  95   a  are out of contact with the cam receiving face portions  89 . In this case, the pressing rocking frames  81  bring the rigid member  61  of the pressing unit  43  into contact with the heating roller  41  with the fixing belt  42  being disposed therebetween, and the pressing rocking frames  81  are pressed in the direction of arrow C toward the heating roller  41  by receiving the pressure F 1  from the first pressure springs  87 . 
     Next, the operation of the fixing device  4  will be described. 
     First, a description will be given of an operation performed when image formation is performed using a sheet  9 A different from an envelope-like medium as a recording medium  9  (normal mode). 
     When a normal mode is selected, as illustrated in  FIGS. 4 ,  10 , and  11 , the disc cams  95  of the switch mechanisms  45  in the fixing device  4  are held in a state in which the first cam faces  95   a  oppose the cam receiving face portions  89  of the pressing rocking frames  81 A and  81 B (strictly, in the above-described non-contact state). 
     Thus, the cam receiving face portions  89  of the pressing rocking frames  81 A and  81 B are out of contact with the disc cams  95 . For this reason, the rocking range of the pressing rocking frames  81 A and  81 B is not particularly restricted by the disc cams  95 , and therefore, the pressing rocking frames  81 A and  81 B are kept in a state such as to be able to rock on the shafts  56  in the direction of arrow C toward the heating roller  41  while receiving the spring force F 1  from the first pressure springs  87 . 
     As a result, the pressing rocking frames  81  rock in the direction of arrow C toward the heating roller  41 , and the rigid member  61  of the pressing unit  43  is placed closer to the heating roller  41  via the first support plate  64  fixed to the pressing rocking frames  81 , as illustrated in  FIGS. 4 ,  10 , and  11 . Finally, the rigid member  61  is kept in contact with the heating roller  41  with the fixing belt  42  being disposed therebetween. In this case, the second support plate  65  remains pressed in the direction of arrow E toward the heating roller  41  along the attachment cuts  84  of the pressing rocking frames  81  by receiving the pressure P 1  from the second pressure springs  91 . Hence, the elastic member  62  attached to the second support plate  65  is finally kept in contact with the heating roller  41  with the fixing belt  42  being disposed therebetween. 
     Therefore, in this case, the pressing unit  43  is kept in a state in which both the rigid member  61  and the elastic member  62  press the fixing belt  42  against the surface of the heating roller  41  (see  FIGS. 4 and 11 ). That is, this state corresponds to the above-described first state (normal mode). 
     In this first state, the separation end guide plates  66  and  67  that hold and guide the fixing belt  42  are in a state in which the pressing-portion opposing portions  66   c  and  67   c  are aligned (coincide) to form one straight line or surface ( FIG. 9 ). The separate peripheral guide plates  68  and  69  are also in a state in which the peripheral-surface guide portions  68   a  and  69   a  are aligned (coincide) to form a common cylindrical surface ( FIGS. 4 and 6 ). Thus, the fixing belt  42  is allowed to rotate while being entirely kept in a substantially cylindrical form by the separate end guide plates  66  and  67  and the separate peripheral guide plates  68  and  69  ( FIGS. 4 and 5 ). 
     In this case, the spring pressing face portions  82  of the pressing rocking frames  81  are kept at a predetermined distance (S 1 ) from the spring support face portions  52  of the fixed frames  51 . The distance from the spring pressing face portions  82  of the pressing rocking frames  81  to the flange portions  88   b  of the columns  88  serves as a distance for the normal mode. This distance for the normal mode is set to be shorter than a free length L 1  of the first pressure springs  87 . Therefore, the first pressure springs  87  are kept in a compressed state. As a result, the first pressure springs  87  provide the pressure F 1  (spring force=compression amount×spring constant) in accordance with the compression amount and the spring constant, thereby continuously pressing the spring pressing face portions  82  of the pressing rocking frames  81  toward the spring support face portions  52  of the fixed frames  51 . In this case, the principle of leverage using the shafts  56  as the pivots, the spring pressing face portions  82  as the points of effort, and the insertion holes  83  as the points of load acts on the pressing rocking frames  81 . Hence, a strong force obtained by increasing the spring force F 1  by the principle of leverage is transmitted from the pressing rocking frames  81  to the first support plate  54  of the attachment holes  83  serving as the points of load. 
     With this, the rigid member  61  is pressed against the heating roller  41  by the pressing rocking frames  81  and the first pressure springs  87  while receiving, via the first support plate  64 , the pressure increased by the principle of leverage. 
     In contrast, the elastic member  62  is pressed against the heating roller  41  while receiving, via the second support plate  65 , the pressure (spring force) P 1  provided by the second pressure springs  91  compressed in the pressing rocking frames  81 . This pressure P 1  is lower than the pressure F 1  applied from the first pressure springs  87 , as described above. 
     From the above, in the normal mode, the high pressure F 1  is applied to the pressing portion NP in the fixing device  4  via the rigid member  61 , and the pressure P 1  lower than the pressure F 1  is also applied to the pressing portion NP via the elastic member  62 . Moreover, the pressing portion NP is formed by the presses of the rigid member  61  and the elastic member  62  in the pressing unit  43 . Further, the pressure (distribution) applied to the pressing portion NP of the heating roller  41  when the normal mode is selected is higher at the rigid member  61  provided downstream in the passage direction B of the fixing object than the elastic member  62  provided upstream in the passage direction B of the fixing object. 
     As illustrated in  FIG. 11 , in the normal mode, when the sheet-like recording medium  9 A serving as a fixing object having an unfixed toner image enters the pressing portion NP in the fixing device  4 , the elastic member  62  provided on the upstream side in the passage direction B of the fixing object in the pressing portion NP first presses the recording medium  9 A against the rotating heating roller  41  (with the fixing belt  42  being disposed therebetween). Subsequently, the rigid member  61  provided on the downstream side in the passage direction B of the fixing object presses the recording medium  9 A against the heating roller  41  (with the fixing belt  42  being disposed therebetween). In the normal mode, the fixing operation is thus performed so that the sheet-like recording medium  9 A passes through the pressing portion NP having a nonuniform pressure distribution in the passage direction B. 
     Next, a description will be given of a case in which image formation is performed using an envelope-like medium  9 B, such as an envelope, as a recording medium  9  (envelope mode). 
     When an envelope mode is selected, in the fixing device  4 , the disc cams  95  of the switch mechanisms  45  are held in a state in which the second cam faces  95   b  oppose the cam receiving face portions  89  of the pressing rocking frames  81 A and  81 B, as illustrated in  FIGS. 12 and 13 . 
     Thus, the pressing rocking frames  81 A and  81 B are stopped after the cam receiving face portions  89  are pressed and displaced by a predetermined amount by the second cam faces  95   b  of the disc cams  95 . That is, the pressing rocking frames  81 A and  81 B are rocked on the shafts  56  in the direction D away from the heating roller  41 , and are stopped at a position where the rigid member  61  of the pressing unit  43  fixed to the pressing rocking frames  81  is separate from the heating roller  41 . In the first exemplary embodiment, the cam receiving face portions  89  of the pressing rocking frames  81  are displaced and held at a distance K ( FIG. 13 ) from the rotation shaft  96  by the second cam faces  95   b  of the disc cams  95 . 
     As a result, when the pressing rocking frames  81  rock in the direction D away from the heating roller  41 , the rigid member  61  is moved away from the heating roller  41  via the first support plate  64  fixed to the pressing rocking frames  81 , as illustrated in  FIGS. 14 and 16 . Therefore, the rigid member  61  is finally kept separate from the heating roller  41 . In this case, the second support plate  65  receives the pressure P 1  from the second pressure springs  91 , and is brought into a state such as to be able to be pressed and moved in the direction E toward the heating roller  41  along the attachment cuts  84  in the pressing rocking frames  81 . Therefore, the elastic member  62  attached to the second support plate  65  moves in the direction E toward the heating roller  41  from the position in the normal mode, and is finally kept in contact with the heating roller  41  with the fixing belt  42  being disposed therebetween. The elastic member  62  moves relative to the rigid member  61  in the opposite direction on the pressing rocking frames  81 . 
     Therefore, the pressing unit  43  is kept in a state in which only the elastic member  62  presses the fixing belt  42  against the surface of the heating roller  41  (see  FIGS. 14 and 16 ). That is, this state corresponds to the above-described second state (envelope mode). Switching to the second state is made by one operation of the disc cams  95  of the switching mechanisms  45 . 
     Since the second support member  65  moves in the direction E toward the heating roller  41  in the second state, as described above, the upstream separate end guide plates  66  attached integrally with the second support plate  65 , of the separate end guide plates  66  and  67  that hold and guide the heating roller  41 , move together with the second support plate  65  in the direction E toward the heating roller  41 , and come out of alignment with the downstream separate end guide plates  67  ( FIGS. 15A and 15B ). In  FIG. 15B , α represents the relative displacement amount of the upstream separate end guide plates  66  from the downstream separate end guide plates  67 . 
     The upstream separate peripheral guide plate  68  attached integrally with the second support plate  65  also moves in the direction E toward the heating roller  41  by the same amount as that for the upstream end guide plates  66 , and comes out of alignment with the downstream separate peripheral guide plate  69  ( FIGS. 14 and 16 ). 
     This brings the fixing belt  42  into a state in which the fixing belt  42  can be rotated in a substantially elliptic shape as a whole by the separate end guide plates  66  and  67  and the separate peripheral guide plates  68  and  69  that are out of alignment ( FIGS. 14 and 16 ). In  FIGS. 12 and 16 , reference numeral  42   a  denotes an inflected portion (deformed portion) formed in the fixing belt  42  by the misalignment of the upstream guide plates  66  and  68 . 
     In this case, the spring pressing face portions  82  of the pressing rocking frames  81  are kept in a state in which a distance S 2  thereof from the spring support face portions  52  of the fixed frames  51  is longer than the distance S 1  in the normal mode (S 2 &gt;S 1 ). This keeps the first pressure springs  87  more compressed than in the normal mode. However, the rigid member  61  fixed to the pressing rocking frames  81  via the first support plate  64  is separate from the heating roller  41 , and the rocking movement of the pressing rocking frames  81  in the direction C toward the heating roller  41  is restricted (stopped) by the disc cams  95 . For this reason, the spring force F 2  (&gt;F 1 ) of the compressed first pressure springs  87  is not transmitted to the rigid member  61  via the first support plate  64 . 
     In contrast, in a manner similar to that adopted in the normal mode, the elastic member  62  is pressed against the heating roller  41  while receiving the pressure (spring force) P 1  from the compressed second pressure springs  91  in the pressing rocking frames  81  via the second support plate  65 . 
     From the above, in the envelope mode, only the pressure P 1  lower than the pressure F 1  is applied to the pressing portion NP of the fixing device  4  via the elastic member  62 . Moreover, the pressing portion NP is formed by only the press of the elastic member  62  of the pressing unit  43 . For this reason, in the envelope mode, the pressure Pb at the pressing portion NP in the second state becomes lower than the pressure Pa at the pressing portion NP in the first state of the normal mode (Pb&lt;Pa). 
     In this case, the pressure P 1  is also lower than the pressure F 1  applied from the first pressure springs  87 , as described above. Hence, the separate end guide plates  66  and the separate peripheral guide plate  68  on the upstream side relatively move and are displaced, and the fixing belt  42  is kept in a substantially elliptic shape as a whole. To the fixing belt  42  of this shape, the relatively low pressure P 1  is applied from the second pressure springs  91 . For this reason, an excessive pressure is not applied to the fixing belt  42  entirely deformed in a substantially elliptic shape, and therefore, it is possible to prevent a situation in which the total tension of the fixing belt  42  increases and the frictional resistance (torque) to the elastic member  62 , and the guide plates  66  to  69  increases. As a result, it is possible to avoid a possibility that the transport of the envelope-like recording medium  9 B passing through the pressing portion NP will be delayed because of the increase in total tension of the fixing belt  42  and increase in frictional resistance to the elastic member  62  and so on, and that this will cause trouble such as disturbance of an image to be fixed and decrease in durability of the fixing belt  42 . 
     In the envelope mode, when the envelope-like recording medium  9 B serving as the fixing object having an unfixed toner image enters the pressing portion NP in the fixing device  4 , as illustrated in  FIG. 16 , only the elastic member  62  to which the low pressure P 1  is applied presses the recording medium  9 B against the heating roller  41  (with the fixing belt  42  being disposed therebetween). In this way, in the envelope mode, the fixing operation is conducted on the envelope-like recording medium  9 B in a (dynamically) well-balanced environment in which the applied pressure is lower than in the normal mode, the pressing portion NP formed by the elastic member  62  is elastically deformed in accordance with the passage state of the recording medium  9 B, and the second pressure springs  91  are also compressed by the required amount in accordance with the passage state. 
     Also, in the envelope mode, the pressure of the first pressure springs  87  is not applied via the rigid member  61 , and the pressure at the pressing portion NP becomes lower than in the normal mode (first state), as described above. Hence, the recording medium  9 B passing through the pressing portion NP is not compressed more than in the normal mode. As a result, in the envelope mode, the envelope-like recording medium  9 B (particularly an end area on the upstream side in the passage direction B) is not wrinkled, and this makes proper fixing possible. 
     Further, in the envelope mode, with the relative movement of the elastic member  62  of the pressing unit  43 , the separate end guide plates  66  and the separate peripheral guide plate  68  on the upstream side move similarly. Hence, the fixing belt  42  is stably held and guided to an entrance side of the pressing portion NP by the guide plates  66  and  68 . With this, the recording medium  9 B is also stable led to the entrance side of the pressing portion NP, in a manner substantially similar to that adopted for the recording medium  9 A in the normal mode. This prevents wrinkling of the recording medium  9 B and disturbance of the image on the fixing object. 
     Further, in the envelope mode, when the rigid member  61  of the pressing unit  43  is displaced with rocking of the pressing rocking frames  81 , the separate end guide plates  67  and the separate peripheral guide plate  69  on the downstream side are displaced similarly. Hence, a portion of the fixing belt  42  coming out of the exit side of the pressing portion NP is stably held and guided by the downstream guide plates  67  and  69 . With this, the recording medium  9 B is also stably output from the exit side of the pressing portion NP, in a manner substantially similar to that for the recording medium  9 A in the normal mode, and the running ability of the recording medium  9 B is prevented from being reduced during the output. 
     Other Exemplary Embodiments 
     While the disc cams  95  are used as the switch mechanisms  45  in the first exemplary embodiment, other structures may be adopted as long as they allow an operation in which the pressing rocking frames  81  rock away from the heating roller  41  and in which one of the structures that form the pressing unit  43  fixed to the pressing rocking frames  81  is stopped and held at a position separate from the heating roller  41 . 
     As the first pressure springs  87  and the second pressure springs  91 , for example, plate-shaped springs can be used instead of the coil springs. 
     Further, in the fixing device  4 , a fixing roller having no heating element can be used instead of the heating roller  41 , and a pressing and heating member having a heating element can be used as the pressing unit  43 . For example, when the heating element of the pressing and heating member is an electromagnetic induction heating element, an endless belt having a conductive layer can be used as the fixing belt  42 . 
     As the imaging device  2  in the image forming apparatus  1 , an imaging device that forms a multicolor image by forming toner images of plural colors and transferring the toner images onto the recording medium  9  can be adopted. As the transfer method adopted in the imaging device  2 , for example, a known intermediate transfer method can be used instead of the direct transfer method. 
     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.