Abstract:
There is provided a thermal fixing unit that fixes a toner image on a printing medium, the fixing unit including a heat member, a pressure pad, a movable supporting assembly, and an urging member. The heat member is in contact with the printing medium, being positionally fixed and defining an axis. The pressure pad is disposed in opposition to the heat member for nipping the printing medium in cooperation with the heat member. The supporting assembly pivotably movably supports the pressure pad. The urging member biases the supporting assembly to urge the pressure pad toward the heat member for providing pressure contact between the heat member and the pressure pad.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2008-084531 filed Mar. 27, 2008. The entire content of the priority application is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to an image forming device such as a laser printer, and also to a thermal fixing unit provided in the image forming device. 
     BACKGROUND 
     A conventional image forming device is provided with a fixing unit that fixes a toner image transferred from a photosensitive unit onto a sheet of paper. 
     One of such conventional fixing units includes a heat roller, a pressure pad disposed in opposition to the heat roller, and a fixing belt interposed between the heat roller and the pressure pad. 
     In this fixing unit, the fixing belt can press the heat roller with a large contact area by the pressure applied from the pressure pad, facilitating efficient fixation of toner images. But, at the same time, the pressure pad is required to be in close contact with the heat roller via the fixing belt. 
     To this effect, laid-open Japanese patent application publication No. 2003-5553 discloses a fixing unit including a heat roller, an endless fixing belt, and a pressure member disposed inside the endless belt. The pressure member is provided with a nip head member for pressing the belt against the surface of the heat roller. The pressure member is attached to an arm member via a belt guide member, and the arm member has one end portion serving as a pivot portion for pivotal movement of the arm member. The nip head member presses the endless belt in a direction toward the vicinity of the center of the heat roller. 
     In the above-described configuration, since the pressure member is fixed to the arm member, accurate positioning of the pressure member relative to the heat roller at the time of assembly is required so that the nip head member can be in close contact with the heat roller irrespective of the pivotal movement of the arm member. However, demand for such high positioning accuracy necessitates less product tolerance and leads to time-consuming assembly. 
     In view of the forgoing, it is an object of the present invention to provide a heat fixing unit with a simple structure capable of achieving close contact between a pressure pad and a heat roller, and to provide an image forming device including such a heat fixing unit. 
     SUMMARY 
     In order to achieve the above and other objects, the present invention provides a heat fixing unit that fixes a toner image on a printing medium. The fixing unit includes a heat member, a pressure pad, a movable supporting assembly, and an urging member. The heat member is in contact with the printing medium, being positionally fixed and defining an axis. The pressure pad is disposed in opposition to the heat member for nipping the printing medium in cooperation with the heat member. The supporting assembly pivotably movably supports the pressure pad. The urging member biases the supporting assembly to urge the pressure pad toward the heat member for providing pressure contact between the heat member and the pressure pad. 
     According to another aspect of the present invention, there is provided an image forming device having a thermal fixing unit for fixing a toner image on a printing medium. The fixing unit includes a heat member, a pressure pad, a movable supporting assembly, and an urging member. The heat member is in contact with the printing medium, being positionally fixed and defining an axis. The pressure pad is disposed in opposition to the heat member for nipping the printing medium in cooperation with the heat member. The supporting assembly pivotably movably supports the pressure pad. The urging member biases the supporting assembly to urge the pressure pad toward the heat member for providing pressure contact between the heat member and the pressure pad. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view taken along a widthwise central line of a laser printer including a fixing unit according to a first embodiment of the image forming device of the present invention; 
         FIG. 2  is a rear view of the fixing unit shown in  FIG. 1 ; 
         FIG. 3  is a left side view of the fixing unit shown in  FIG. 1 ; 
         FIG. 4  is a right side view of an essential portion of the fixing unit shown in  FIG. 1 ; 
         FIG. 5  is a view explaining how a pressure pad is pressed against a heat roller, wherein  FIG. 5A  shows a state where the pressure pad is brought into contact with the heat roller, and  FIG. 5B  shows a state where the pressure pad is tightly in contact with the heat roller; and 
         FIG. 6  is a right side view of an essential portion of the fixing unit according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming device provided with a fixing unit according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 5B . 
     In  FIG. 1 , a laser printer  1  has a main casing  3  in which a feeder unit  3  and an image forming unit  4  are provided. A front cover  5  is provided in one of the sidewalls of the main casing  2  so as to be opened or closed for detachably mounting a process cartridge  10 . 
     Note that, hereinafter, the right side of the laser printer  1  in  FIG. 1  (the side of the main casing  2  in which a front cover  5  is provided) will be referred to as a “front side,” while the left side of the laser printer  1  in  FIG. 1  will be referred to as a “rear side.” Further, the left and right sides of the laser printer  1  will be defined based on a state where the laser printer  1  is seen from the front side thereof. 
     The feeder unit  3  includes a sheet tray  6  that accommodates sheets of paper P (hereinafter simply referred to as ‘sheet P’) in a stacked state. Here, the sheet P is merely used as an example of objects on which toner images are fixed. The sheet tray  6  is detachably mounted in the bottom portion of the main casing  2 . A sheet supply roller  7  is disposed above the front end portion of the sheet tray  6 , and a pair of registration rollers  8  is disposed rearward of the sheet supply roller  7 . 
     The sheet supply roller  7  rotates and conveys the sheets P accommodated in the sheet tray  6  one sheet at a time toward the registration rollers  8 . The registration rollers  8  then convey the sheet P toward the image forming unit  4 , specifically between a photosensitive drum  14  and a transfer roller  16 . 
     The image forming unit  4  includes a scanner unit  9 , a process cartridge  10  and a thermal fixing unit  11 . 
     The scanner unit  9  is disposed in the upper portion of the main casing  2 . The scanner unit  9  emits a laser beam toward the photosensitive drum  14  of the process cartridge  10  based on image data to be formed, as shown by a broken line in  FIG. 1 . 
     The process cartridge  10  is disposed below the scanner unit  9 . The process cartridge  10  includes a drum cartridge  12 , and a developing cartridge  13  detachably mounted in the drum cartridge  12 . 
     A photosensitive drum  14  is rotatably provided in the drum cartridge  12 . A Scorotron charger  15  and a transfer roller  16  are disposed around the photosensitive drum  14 . 
     The developing cartridge  13  defines therein a toner accommodating chamber  18  and a developing chamber  19  formed next to the accommodating chamber  18  in the front-to-rear direction. The toner accommodating chamber  18  and the developing chamber  19  are interconnected therebetween. 
     The toner accommodating chamber  18  accommodates nonmagnetic, single-component polymerized toner having a positive charge. An agitator  20  is provided in the toner accommodating chamber  18 . 
     The developing chamber  19  is provided with a supply roller  21 , a developing roller  22  and a thickness-regulating blade  23 . The developing roller  22  is disposed rearward of the supply roller  21  so as to be in contact with the supply roller  21  while pressing the same from rearward. The thickness-regulating blade  23  has a base end fixed to the developing chamber  19  and a free end disposed so as to be pressed against a circumferential surface of the developing roller  22  from diagonally above. 
     As the agitator  20  agitates the toner inside the toner accommodating chamber  18 , some of the toner is supplied onto the supply roller  21  in the developing chamber  19 . As the supply roller  21  rotates, the toner borne on the supply roller  21  is then supplied to the developing roller  22 . At this time, the toner is positively tribocharged between the supply roller  21  and the developing roller  22 . Subsequently, as the developing roller  22  rotates, the thickness-regulating blade  23  controls the thickness of a toner layer formed on the developing roller  22 . The toner is thus carried on the circumferential surface of the developing roller  22  as a thin layer of uniform thickness. 
     Meanwhile, as the photosensitive drum  14  rotates, the Scorotron charger  15  charges the surface of the photosensitive drum  14  with a uniform positive polarity. The laser beam emitted from the scanner unit  9  selectively irradiates the positively charged surface of the photosensitive drum  14 , thereby forming an electrostatic latent image thereon according to the image data. 
     As the photosensitive drum  14  continues to rotate, the positively charged toner borne on the surface of the developing roller  22  comes into contact with the photosensitive drum  14 , thereby supplying the toner to the electrostatic latent image formed on the surface of the photosensitive drum  14 . In this way, the latent image on the photosensitive drum  14  is developed into a visible toner image. Finally, the toner image is transferred onto the sheet P that has been conveyed to a transferring position, that is, between the photosensitive drum  14  and the transfer roller  16 . 
     The fixing unit  11  is disposed in the main casing  2 , rearward of the process cartridge  10 . The fixing unit  11  includes a heat roller  24 , a pressure pad  25  disposed in opposition to the heat roller  24  and an endless fixing belt  26 . The fixing belt  26  is looped around the pressure pad  25 , while being nipped between the heat roller  24  and the pressure pad  25 . The pressure pad  25  presses the fixing belt  26  against the heat roller  24  from the bottom thereof. 
     The sheet P is conveyed to the fixing unit  11  after the toner image has been transferred thereon. In the fixing unit  11 , as the sheet P passes between the heat roller  24  and the fixing belt  26 , the sheet P comes into contact with the heat roller  24  and the fixing belt  26 , and the toner image borne on the sheet P is fixed to the surface of the sheet P by the heat applied from the heat roller  24  and by the pressure applied from the pressure pad  25 . 
     After the toner image has been fixed to sheet P in the fixing unit  11 , the sheet P is then conveyed toward a pair of discharge rollers  27 . The discharge rollers  27  discharge the sheet P onto a discharge tray  28 , which is formed in the upper surface of the main casing  2 . 
     Next, details of the fixing unit  11  will be described with reference to  FIGS. 2 to 4 . Note that a belt guide (described later) is omitted in  FIG. 2 . 
     In addition to the above-mentioned heat roller  24 , the pressure pad  25 , and the fixing belt  26 , the fixing unit  11  further includes a supporting assembly  31  for supporting the pressure pad  25 , and a tension spring  32  for urging the supporting assembly  31 . 
     The heat roller  24  is configured of a substantially cylindrically-shaped metal tube that extends in a left-to-right (width) direction. The surface of the tube is coated with a layer of fluorine resin and the like. 
     More specifically, the heat roller  24  includes a fixing area  33  and side sections  34  at both axial end portions of the heat roller  24  for interposing the fixing area  33  therebetween. The width of the fixing area  33  is designed to be slightly greater than the maximum width of a sheet on which the laser printer  1  can form an image. The both side sections  34  are rotatably supported via bearing members (not shown). A gear (not shown) is fixedly coupled to the left side section  24 . 
     The heat roller  24  accommodates a rod-like heating source  30  configured of a halogen lamp. The heating source  30  extends in the left-to-right direction and is arranged along the axial direction of the heat roller  24 . 
     The pressure pad  25  includes a pad member  35  and a pad frame  36  that supports the pad member  35 . The pad member  35  is formed of an elastic material such as rubber. The pad member  35  extends in the left-to-right direction as shown in  FIG. 2 , and has a substantially rectangular cross-section in which a length in the front-to-rear direction is greater than its height, as shown in  FIG. 3 . 
     More specifically, the pad member  35  has a length is made slightly shorter than that of the fixing area  33  with respect to the left-to-right direction as shown in  FIG. 2 . Also, as shown in  FIG. 3 , the pad member  35  has an upper surface forming a substantially arcuate pressure surface  40 , which is concave downward following the circumferential surface of the heat roller  24 . 
     The pad frame  36  is formed by bending a flat metal plate, and includes a mounting portion  37  and holding portions  38 . On the mounting portion  37  the pad member  35  is fixed and the holding portions  38  are fixed to a shaft member  41  (described later) as shown in  FIG. 2 . The mounting portion  37  and the holding portions  38  are integral with each other. 
     The mounting portion  37  is flat plate-shaped, and has a length in the left-to-right direction substantially the same as the length of the fixing area  33  of the heat roller  24 , while being slightly longer than the length of the pad member  35  in the left-to-right direction. The mounting portion  37  has an upper surface on which the bottom surface of the pad member  35  is fixed. 
     Each of the holding portions  38  is formed by bending the metal plate downward at both ends thereof in the left-to-right direction. As shown in  FIG. 4 , each holding portion  38  has a substantially rectangular shape in a side view, and has a length in the front-to-rear direction slightly shorter than that of the mounting portion  37 . 
     Also, each of the holding portions  38  has a fixing hole  39  in a central region thereof in a side view. Each fixing hole  39  is provided to allow a shaft member  41  (described later) to extend therethrough and to allow the shaft member  41  to be fixed thereto. 
     The supporting assembly  31  includes the shaft member  41  for supporting the pressure pad  25 , and arm members  42  for supporting the shaft member  41 , as shown in  FIG. 2 . 
     The shaft member  41  has a cylindrical shape extending in the left-to-right direction. The shaft member  41  has a length in the left-to-right direction longer than the length of the fixing area  33  of the heat roller  24 , but shorter than the length between both axial end faces of the side sections  34  in the left-to-right direction. The shaft member  41  is disposed below the mounting portion  37  and extends in a longitudinal direction of the mounting portion  37  (i.e., the left-to-right direction). 
     The shaft member  41  penetrates the fixing holes  39  of the holding portions  38  so as to be incapable of rotating relative to the holding portions  38 , thereby enabling the pad frame  36  to be fixed to the shaft member  41 . 
     Each of the arm members  42  is disposed respectively below each side section  34  of the heat roller  24 . The arm members  42  are formed of a metal plate. As shown in  FIG. 3 , each arm member  42  has a substantially rectangular plate shape in a side view, extending from diagonally upward front to diagonally downward rear. In a middle portion of the arm members  42  with respect to the front-to-rear direction, engaging sections  43  are formed. The engaging sections  43  protrude rearward from the arm members  42  and are substantially triangular-shaped in a side view. Each arm member  42  has a front end formed with a notch  44  that are cut out from the arm member  42  in a substantially circular shape in a direction extending from the front end to rearward thereof. 
     At the base end of each engaging section  43 , i.e., at the central region of each arm member  42  with respect to the front-to-rear direction, a circular-shaped through-hole  45  is formed for allowing the shaft member  41  to extend therethrough. The shaft member  41  is rotatable with respect to the through-holes  45  of both arm members  42 . At the rear end of each engaging section  43 , a hook  46  is provided for engaging the tension spring  32 . The hook  46  protrudes downward from the engaging section  43  at the rear end thereof. 
     The right and left arm members  42  are disposed below the respective side sections  34  of the heat roller  24  as shown in  FIG. 2 . A stationary supporting shaft  48  is provided in the main casing  2  and extends in the left-to-right direction thereof. The supporting shaft  48  rotatably extends through the notches  44  as shown in  FIG. 3 . 
     Therefore, both arm members  42  are pivotably movably supported to the main casing  2  via the supporting shaft  48 . Further, both axial end portions of the pressure pad  25  are pivotably movably supported to the arm members  42 . More specifically, the pressure pad  25  is pivotably movable about an axis of the shaft member  41 . 
     As shown in  FIG. 4 , a belt guide  47  is provided on each inner side surface of the arm members  42  in the left-to-right direction. The belt guide  47  is adapted for regulating running range of the fixing belt  26  over the pressure pad  25  as described later. 
     The belt guide  47  protrudes inward from the each inner side surface of the arm members  42  in the left-to-right direction to an extent that each belt guide  47  overlaps the widthwise end areas of the fixing belt  26 . In a side view, the belt guide  47  is generally circular-shaped, surrounding the holding portion  38 . Each belt guide  47  has an upper end surface disposed in opposition to both widthwise ends of the mounting portion  37  with a space therebetween with respect to an up-down direction. That is, each upper end of the guide  47  and each end of the mounting portion  37  in the left-to-right direction are arranged so as to keep a space therebetween within which a pivotal movement of the pressure pad  25  about the axis of the shaft member  41  can be performed. 
     When the pressure pad  25  pivots about the axis of the shaft member  41 , the widthwise ends of the mounting portion  37  become in contact with the upper ends of the belt guides  47  as shown in by a dotted line in  FIG. 4 , thereby restricting further pivotal movement of the pressure pad  25 . In other words, the belt guide  47  functions to restrict the pivotally movable range of the pressure pad  25  about the axis of the shaft member  41  as well as to regulate the circularly moving range (i.e., guide the running track) of the fixing belt  26 . 
     The tension spring  32  is provided on each of the arm members  42  as shown in  FIG. 2 . The tension spring  32  includes an upper hook portion  50 , a bottom hook portion  51 , and a coil spring portion  52  provided therebetween. 
     Each tension spring  32  is disposed above the arm members  42  and the belt guide  47 . The upper hook portion  50  is fixed to the main casing  2 , while the bottom hook portion  51  is engaged with the hook  46 . 
     The tension springs  32  urge the arm members  42  so that the hooks  46  can be pulled upward, and thus the rear ends of the arm members  42  are urged to move upward while the front ends thereof pivot about the supporting shaft  48 . As a result, the pressure pad  25  supported by the arm members  42  is brought into pressure contact with the heat roller  24 . 
     At this time, as shown in  FIG. 3 , the pressure pad  25  presses the heat roller  24  in a direction parallel to a line L passing through the center of the heat roller  24  and the rotational axis of the shaft member  41 . 
     The fixing belt  26  is an endless belt extending along the left-to-right direction and formed of an elastic material, such as rubber, as shown by two dotted chain lines in  FIG. 2  and  FIG. 4 . The fixing belt  26  passes between the pressure surface  40  of the pad member  35  and the heat roller  24 , and is looped around the pressure pad  25  and the belt guide  47 , as shown by the two dotted chain line in  FIG. 4 . The pressure pad  25  and the belt guide  47  are thus disposed inside the running track of the fixing belt  26 . In other words, the fixing belt  26  is circularly movable over the pressure pad  25  and the belt guide  47 . 
     The length of the fixing belt  26  with respect to the left-to-right direction is set to be shorter than the length of the pad member  35 . The circumferential length of the endless fixing belt  26  is determined so that tension can be applied to the fixing belt  26  when the latter is disposed over the pressure pad  25  and the belt guide  47 . 
     Next, operation in the fixing unit  11  will be described. First, the heating source  30  located inside the heat roller  24  generates heat by the power supplied from a power circuit board (not shown) provided in the main casing  2  for heating the heat roller  24 . Further, a motor (not shown) provided in the main casing  2  transmits a driving force to the gear of the heat roller  24 , causing the heat roller  24  to rotate. As the heat roller  24  rotates, the fixing belt  26  follows the movement of the heat roller  24  and slidingly moves over the pressure surface  40  of the pad member  35 , thereby circularly moving around the pressure pad  25  and the belt guide  47 . 
     The sheet P becomes in contact with the heat roller  24  and the fixing belt  26  when the sheet P passes therebetween. Meanwhile, the heat roller  24  applies heat while the pressure pad  25  applied pressure to the sheet P, thereby enabling the toner image to be fixed onto the sheet P. 
     Next, pressing manner of the pressure pad  25  against the heat roller  24  will be described with reference to  FIGS. 5A and 5B . As shown in  FIG. 5A , as the tension springs  32  urge the arm members  42 , the rear ends of the arm members  42  are urged to move upward while the arm members  42  pivot about the supporting shaft  48 . 
     Hence, since the pressure pad  25  is supported on the arm members  42 , the front end of the pressure pad  25  first contacts the heat roller  24 . Then, as the pressure pad  25  presses the heat roller  24 , the pressure pad  25  is urged to pivot about the axis of the shaft member  41  in clockwise direction when seen from the left side, so that the pressure surface  40  of the pressure pad  25  can come into close contact with the circumferential surface of the heat roller  24 . Thus, the pressure pad  25  is tightly pressed against the heat roller  24  so that the pressure surface  40  can be in close contact with the circumferential surface of the heat roller  24  via the fixing belt  26 . 
     Hence, close contact between the pressure pad  25  and the heat roller  24  can be provided without accurate assembly of the pressure pad  25  relative to the heat roller  24 . As a result, the fixing unit  11  can realize reduced accuracy in each component and easy assembly with a simple structure, while achieving a close contact between the pressure pad  25  and the heat roller  24 . 
     Further, the shaft member  41  of the supporting assembly  31  supports the holding portions  38  disposed at the longitudinal ends of the pressure pad  25 . Hence, in the fixing unit  11 , the supporting assembly  31  can uniformly support the pressure pad  25  in the longitudinal direction thereof. As a result, the pressure pad  25  can be reliably pressed against the heat roller  24  along the longitudinal direction of the pressure pad  25 . 
     Further, the pressure pad  25  is pressed against the heat roller  24  in a direction parallel to the line L that connects the axis of the shaft member  41  and the axis of the heat roller  24  in the radial direction of the shaft member  41 . That is, the direction in which the pressure pad  25  is pressed against the heat roller  24  by the supporting assembly  31  is approximately the same as the direction in which the pressure pad  25  applies pressure to the heat roller  24 . 
     Accordingly, the pressure pad  25  can press the heat roller  24  in the direction parallel to the line L from the time when one end of the pressure pad  25  makes a first contact with the heat roller  24 . As a result, the pressure pad  25  can be pressed against the heat roller  24  in a reliable and efficient manner without making a large pivotal movement. Also, the urging force of the tension spring  32  can be efficiently converted into the pressure that the pressure pad  25  applies to the heat roller  24 . 
     Further, the belt guide  47  regulates the circulation movement of the fixing belt  26  concurrently with restricting the pivotal movement of the pressure pad  25 . Hence, the direction in which the pressure pad  25  presses the heat roller  24  can also be regulated by the belt guide  47 . That is, restriction by the belt guide  47  enables the pressure surface  40  of the pressure pad  25  to face toward the heat roller  24 . As a result, the pressure pad  25  can stably and tightly contact the heat roller  25 . Another restricting member is thus not necessary to be prepared for restricting the pivotal movement of the pressure pad  25 , thereby contributing to reduction in the number of parts and simplification of the device configuration. 
     Moreover, the shaft member  41  rotatably penetrates the through-holes  45  of the arm members  42 . Hence, the axis of the shaft member  41  becomes a pivotal axis of the pressure pad  25 , achieving accurate positioning of the pivotal center of the pressure pad  25 . The pressure pad  25  can, therefore, be made in close contact with the heat roller  24 . 
     Instead of the tension spring  32 , a compression spring may be employed for achieving a close contact between the pressure pad  25  and the heat roller  24 . In the latter case, the compression spring is provided at a side opposite to the heat roller  24  with respect to the arm member  42 . However, controlling the expanding direction of the compression spring is generally more difficult than controlling the shrinking direction of the tension spring  32 . Therefore, in this fixing unit  11 , a tensile force of the tension spring  32  is employed for pressing the pressure pad  25  against the heat roller  24 , enabling a more stable pressure contact therebetween to be realized. 
     Since the laser printer  1  is provided with the above-described fixing unit  11 , the laser printer  1  can realize improved productivity, reduction in production costs as well as efficient fixation of a toner image. 
     As a variation of the first embodiment, the belt guide  47  may be spanned between the arm members  42  so as to extend along the full length of the fixing belt  26 . 
     Next, a fixing unit according to a second embodiment of the present invention will be described with reference to  FIG. 6 . In  FIG. 6 , like parts and components are designated by the same reference numerals as those shown in  FIG. 4  in order to avoid duplicating description. 
     In the first embodiment, the belt guide  47  provided on the inner surface of the arm members  42  with respect to the left-to-right direction serves as the restricting member that restricts pivotal movement of the pressure pad  25 . In the second embodiment, each arm member  42  is provided with ribs  53 , instead of the belt guide  47 , for restricting the pivotal movement of the pressure pad  25 . A fixing belt and a belt guide may be additionally employed, but not necessarily be provided in the second embodiment. 
     The ribs  53  protrude inward with respect to the widthwise direction from the inner side surface of the arm members  42  so that the holding portion  38  be disposed between the ribs  53  in the front-to-rear direction in a side view. More specifically, the ribs  53  are substantially columnar shaped. The ribs  53  are disposed in opposition to the widthwise ends of the mounting portion  37  with a space therebetween in the up-down (vertical) direction so that the pressure pad  25  can pivotally move within the space. 
     With this construction, when the pressure pad  25  pivots about the axis of the shaft member  41 , a front end of the mounting portion  37  gets in contact with one of the ribs  53  as shown in  FIG. 6 , thereby limiting further pivotal movement of the pressure pad  25 . The ribs  53  can thus control the direction in which the pressure pad  25  is pressed against the heat roller  24 . In other words, the pressure surface  40  of the pressure pad  25  can be directed to the heat roller  24 . As a result, the pressure pad  25  can reliably be in close contact with the heat roller  24 . 
     While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims.