Patent Publication Number: US-7725053-B2

Title: Fixing unit having heater roller and pressure roller supports and image-forming device using the same

Description:
BACKGROUND CF THE INVENTION 
     1. Technical Field 
     The present invention relates to a fixing unit and an image-forming device for fixing toner images that have been transferred onto a paper or other recording medium. 
     2. Description of the Related Art 
     Conventionally, electrophotographic image-forming devices, such as laser printers, are equipped with a light-emitting unit including a laser diode for emitting a light beam, and a photosensitive member, the surface of which carries a uniform charge. When the surface of the photosensitive member is exposed to the light beam, electrostatic latent images are formed on the surface of the photosensitive member and the latent images are then developed into visible images with toner. Subsequently, the visible images are transferred onto a paper or other recording medium to form visible toner images thereon. 
     However, the images transferred onto the recording medium are merely carried to the recording medium by an electrostatic force or dispersion force at this time and can be easily removed from the recording medium. Therefore, a fixing process is required to fix and attach the visible images to the recording medium through a step of applying heat. 
     A fixing device provided in the image-forming device for fixing images using heat typically includes a heating roller and a pressure roller. As the recording medium passes between the heating roller and the pressure roller, the toner or other developing material is sintered and coalesced by the heat from the heating roller, so as to permeate and become fixed to the recording medium. The heating roller accommodates a heat source, such as a halogen lamp. Since failure if the halogen lamp is fatal to the image-forming device, resulting in a loss of the fixing function, the fixing unit must be configured in such a way as to be easily replaceable. Japanese patent application publication No. HEI 8-305205 discloses a fixing unit integrally configured of a heating roller and a pressure roller that can easily be replaced when malfunctions occur. 
     Further, in order to prevent the heating roller from rising to an abnormal temperature due to a failure of the heating source, the fixing unit is provided with a temperature fuse that interrupts the supply of power to the heating roller when the surface of the heating roller reaches an abnormal temperature. 
     Normally, the temperature fuse is fixed to contact points provided on a main fuse body by a synthetic resin disposed around the contact points in order to electrically connect these points. Upon reaching a prescribed fusing temperature, the resin melts, causing a spring provided in the fuse to break the connection between contact points and halt the supply of power. Since the temperature fuse is connected to the power source in this case, the temperature fuse is required not to electrically contact the heating roller. Normally, safety standard for electronic equipment (explosion-proof construction standards for general electronic equipment) stipulate that a distance of at least 4 mm should be maintained between the fuse and heating roller as an insulating distance. 
     Proposals have been made for technologies that interpose a fuse cover between the heating roller and the temperature fuse so as to reliably insulate the two. For example, Japanese patent application publication No. HEI 9-281842 discloses a fixing unit in which a temperature fuse is attached to the underside surface of an electrically insulated member having a heat collecting surface opposing the heating roller. 
     In Japanese patent application publication No. HEI 9-281842, the fuse cover is provided as a separate member. After the temperature fuse is attached to the fuse cover, the fuse cover is mounted in the fixing device. This assembling way may increase a cost of the image-forming device. 
     Further, the fixing unit of Japanese patent application publication No. HEI 8-305205 strictly regulates the relationship between the position of the fuse on the fuse cover and the fixing device. Accordingly, the fuse cover is not sometime appropriately mounted in the fixing device if there is any variation in the lengths of the leads on both ends of the temperature fuse. 
     SUMMARY 
     In view of the foregoing, it is an object of the present invention to provide a fixing unit and an image-forming device that are conducive to a compact size. 
     It is another object of the present invention to provide a fixing unit capable of reducing manufacturing costs. 
     It is a further object of the present invention to provide a fixing unit capable of suppressing deformation, such as warping, of a plate-shaped member used as a fuse cover. 
     The present invention provides a fixing unit having a heating roller, a pressure roller, a case, and a pair of support members. The heating roller has a rotational axis having two ends, and a cylindrical surface. The pressure roller has a rotational axis having two ends, and a cylindrical surface. The pressure roller is disposed in confrontation with the heating roller to press the cylindrical surface of the pressure roller against the cylindrical surface of the heating roller. The case has two sides rotatably supporting the two ends of the heating roller. The case covers the heating roller on an opposite side of the pressure roller. The pair of support members is supported on the two sides of the case for rotatably supporting the pressure roller. The heating roller rotates to convey a recording medium in cooperation with the pressure roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
         FIG. 1  is a perspective view showing an exterior of a laser printer according to the present invention; 
         FIG. 2  is a side cross-sectional view showing the laser printer from the side surface; 
         FIG. 3  is a perspective view illustrating a fixing unit provided in the laser printer of  FIG. 1 ; 
         FIG. 4  is a partially cross-sectional view showing the fixing unit; 
         FIG. 5  is a perspective view the fixing unit viewed from the downstream side of the sheet conveying direction; 
         FIG. 6  is an enlarged perspective view of a separating pawl; 
         FIG. 7  is a perspective view showing a mounting part on the case of the fixing unit for mounting the separating pawl; 
         FIG. 8  is a perspective view showing the separating pawl being mounted in the fixing unit; 
         FIG. 9  is a perspective view showing a separating pawl mounted in a groove in the fixing unit; 
         FIG. 10  is a plan view of the case in the fixing unit from the heating roller side, with the heating roller and the like removed; 
         FIG. 11  is an enlarged perspective view showing a drive gear disposed on one end of a heating roller and exposed through an opening of the fixing unit; 
         FIG. 12  is a diagram illustrating functions of the drive gear; 
         FIG. 13  is a perspective view illustrating the inner guide members; and 
         FIGS. 14(   a ) and  14 ( b ) are perspective views showing a temperature fuse cover and illustrating a method of mounting the temperature fuse. 
     
    
    
     DESCRIPTION OF THE EMBODIMENT 
     An image-forming device according to a preferred embodiment of the present invention will be described while referring to the accompanying drawings. 
     (1) Overall Structure of a Laser Printer 
     Referring to  FIG. 1 , a laser printer  1  includes a casing  2  having a top cover  18  forming the top surface of the casing  2 , and four side surfaces  2   a ,  2   b ,  2   c , and  2   d  (side surfaces  2   c  and  2   d  are not visible in  FIG. 1 ) A portion of the top cover  18  is recessed inward to form a sheet discharge tray  52 . A paper cassette  6  is provided in the bottom section of the casing  2  and can be inserted into or removed from the casing  2  through the front side surface  2   a . The paper cassette  6  can accommodate a plurality of sheets of paper or other recording medium. The front side surface  2   a  also includes a front cover  16  that can swing open or closed on the front of the casing  2 , and a manual feed tray  11  provided on the front cover  16  for hand feeding a recording medium one sheet at a time. 
     Next, the structure of the laser printer  1  will be described in detail with reference to the  FIG. 2 . The laser printer  1  includes the casing  2  having the top cover  18  on the top surface, the front cover  16  provided on the front side surface  2   a , and a rear cover  60  provided on the side surface  2   c . Also accommodated in the casing  2  are a paper feed unit  3  for supplying a paper or other recording medium (the path of the recording medium is represented by a broken line P); a process cartridge  4  for forming visible toner images on the paper conveyed from the paper feed unit  3 ; a fixing unit  100  for fixing the toner image to the paper; and a paper discharge unit  200  for discharging the paper which has passed through the fixing unit  100 . In the preferred embodiment, the side surface near the fixing unit  100  on the left side in  FIG. 2  is the rear side surface  2   c , while the side surface opposing the rear side surface  2   c  is the front side surface  2   a.    
     The paper feed unit  3  includes the paper cassette  6 ; and feeding rollers  7  and  8 , and a separating pad  9  disposed above the leading edge of the paper stacked in the paper cassette  6  with respect to the paper conveying direction (front surface side of the laser printer  1 ). A paper feeding path  10  is formed in the paper feed unit  3  for reversing the direction of the sheets of paper fed from the paper cassette  6  and conveying the paper along the bottom of the process cartridge  4 . A pair of registration rollers  12  is provided in the paper feed unit  3  and straddle the paper feeding path  10 . Whether the paper is fed onto the paper feeding path  10  from paper stacked in the paper cassette  6  or from a sheet hand fed into the manual feed tray  11 , the registration rollers  12  initially stops progress of the sheet of paper before ejecting the sheet to image-forming units in the process cartridge  4  in synchronization with the image-forming timing in the process cartridge  4 . 
     The paper cassette  6  is disposed below the process cartridge  4  and the fixing unit  100  and can be inserted into and removed from the casing  2  through the front side surface thereof. The paper cassette  6  accommodates a paper pressing plate  13  and a spring  14  disposed on the underside of the paper pressing plate  13  for urging an end of the paper pressing plate  13  nearest the feeding roller  7  upward. The paper pressing plate  13  is capable of pivoting about an end farthest from the feeding roller  7  so that the end of the paper pressing plate  13  nearest the feeding roller  7  can move up and down to accommodate sheets of a recording medium stacked on the paper feed unit  3 . As the amount of paper stacked on the paper pressing plate  13  increases, the end of the paper pressing plate  13  nearest the feeding roller  7  opposes the urging force of the spring  14  and pivots downward about the end farthest from the feeding roller  7 . 
     The feeding roller  8  and the separating pad  9  are disposed in confrontation with each other. A spring  15  is disposed on the underside of the separating pad  9  for urging the separating pad  9  toward the feeding roller  8 . The spring  14  on the underside of the paper pressing plate  13  urges the paper pressing plate  13  so that the topmost sheet of paper stacked on the paper pressing plate  13  is pressed against the feeding roller  7 . The feeding roller  7  feeds the topmost sheet to a position between the feeding roller  8  and the separating pad  9 . The feeding roller  8  rotates to feed the topmost sheet onto the paper feeding path  10 , while the cooperative operations of the feeding roller  8  and separating pad  9  ensure that the sheets are separated and fed one sheet at a time. 
     A sheet of paper supplied from the paper cassette  6  or the manual feed tray  11  is conveyed to the registration rollers  12  disposed above the feeding roller  7 . The registration rollers  12  first register the sheet and then convey the sheet to an image-forming position beneath the process cartridge  4  (a contact position between a photosensitive drum  37  and a transfer roller  39  described later). As mentioned earlier, the front cover  16  is provided on the front side surface  2   a  of the casing  2  and is freely opened and closed or the casing  2 . When the front cover  16  is in an open state, an opening is revealed in the casing  2  through which the process cartridge  4  can be inserted or removed. 
     A scanning unit  26  is disposed above the process cartridge  4 . The scanning unit  26  includes a laser light-emitting unit (not shown), a polygon mirror  29  that is driven to rotate at a high speed, a first scanning lens (fθ lens)  30 , a second scanning lens (cylindrical lens)  31 , and reflecting mirrors  32  and  33 . The laser light-emitting unit emits a laser beam that is modulated according to image data. As indicated by the broken line in the scanning unit  26  of the drawing, the laser beam passes through or is reflected off the polygon mirror  29 , first scanning lens  30 , reflecting mirror  32 , second scanning lens  31 , and reflecting mirror  33  in the order given and is scanned over the surface of a photosensitive drum  37  in the process cartridge  4 . 
     The process cartridge  4  includes a drum cartridge  35  and a developing cartridge  36 . The drum cartridge  35  accommodates the photosensitive drum  37 , a charger  38 , and a transfer roller  39 . As described above, the process cartridge  4  is freely mounted in and removed from the main frame  2  via the opening in the casing  2  when the front cover  16  is open. The developer cartridge  36  is detachably mounted on the drum cartridge  35  and includes a developing roller  40 , a thickness regulating blade  41 , a supply roller  42 , and a toner hopper  43 . 
     The toner hopper  43  accommodates toner. The toner hopper  43  has a toner supply opening  46  formed in a side thereof and houses a rotational shaft  44  and an agitator  45  rotatably supported on the rotational shaft  44 . The agitator  45  is driven to rotate in the direction of the arrow shown in  FIG. 2  to agitate the toner accommodated in the toner hopper  43  so that some of the toner is discharged through the toner supply opening  46  formed in the side of the toner hopper  43 . The supply roller  42  is rotatably disposed at a position to the side of the toner supply opening  46 . The developing roller  40  is rotatably disposed in confrontation with the supply roller  42  and contacts the supply roller  42  with sufficient pressure so that the developing roller  40  and supply roller  42  compress to a degree. 
     The developing roller  40  is configured of a metal roller shaft covered by a roller formed of an electrically conductive rubber material. The developing roller  40  is driven to rotate in the direction indicated by the arrow (counterclockwise in  FIG. 2 ). A developing bias is applied to the developing roller  40 . The thickness regulating blade  41  is disposed near the developing roller  40  and includes a main blade member configured of a metal leaf spring member, and a pressing part provided on a distal end of the main blade member. The pressing part has a semicircular cross section and is formed of an insulating silicon rubber. The thickness regulating blade  41  is supported on the developer cartridge  36  rear the developing roller  40  so that the elastic force of the main blade member causes the pressing part to contact the developing roller  40  with pressure. 
     Toner discharged through the toner supply opening  46  is supplied to the developing roller  40  by the rotation of the supply roller  42 . At this time, the toner is positively tribocharged between the supply roller  42  and the developing roller  40 . As the developing roller  40  rotates, the toner supplied onto the surface of the developing roller  40  passes between the thickness regulating blade  41  and the developing roller  40 , thereby maintaining a uniform thickness on the surface of the developing roller  40 . 
     The photosensitive drum  37  is disposed to the side of the developing roller  40  and rotates in the drum cartridge  35  in the direction indicated by the arrow (clockwise in  FIG. 2 ) while in confrontation with the developing roller  40 . The photosensitive drum  37  is configured of a main drum body that is grounded and a surface layer formed of a positive charging photosensitive layer of polycarbonate. 
     The charger  38  is disposed diagonally above and to the left (in  FIG. 2 ) of the photosensitive drum  37 , confronting the photosensitive drum  37  and separated a prescribed distance therefrom. The charger  38  is a positive charging Scorotron charger having a charging wire formed of tungsten from which a corona discharge is generated. The charger  38  functions to charge the entire surface of the photosensitive drum  37  with a uniform positive polarity. 
     The transfer roller  39  is disposed below the photosensitive drum  37  and in opposition thereto, and is supported in the drum cartridge  35  so as to be capable of rotating in the direction indicated by the arrow (counterclockwise in  FIG. 2 ). The transfer roller  39  includes a metal roller shaft covered by a roller that is formed of an electrically conductive rubber material. A transfer bias is applied to the transfer roller  39  during a transfer operation. 
     As the photosensitive drum  37  rotates, the charger  33  charges the surface of the photosensitive drum  37  with a uniform positive polarity. Subsequently, the surface of the photosensitive drum  37  is exposed to a laser beam emitted from the scanning unit  26 , forming an electrostatic latent image on the surface of the photosensitive drum  37 . Next, the positively charged toner carried on the surface of the developing roller  40  is brought into contact with the photosensitive drum  37  as the developing roller  40  rotates. At this time, due to the developing bias applied to the developing roller  40 , the latent image formed on the surface of the photosensitive drum  37  is developed into a toner image when the toner is selectively attracted to portions of the photosensitive drum  37  that were exposed to the laser beam and, therefore, have a lower potential than the rest of the surface having a uniform positive charge. In this way, a reverse developing process is achieved. 
     Subsequently, the toner image carried on the surface of the photosensitive drum  37  is transferred onto the paper due to the transfer bias applied to the transfer roller  39 , as the sheet of paper passes between the photosensitive dream  37  and the transfer roller  39 . 
     The fixing unit  100  is disposed above the paper cassette  6  and to the side and downstream of the process cartridge  4  with respect to the paper conveying direction. The fixing unit  100  includes a heating roller  110 , and a pressure roller  120  disposed in confrontation with the heating roller  110  and urged to contact the heating roller  110  with pressure. The heating roller  110  accommodates an internal heater. 
     After a visible toner image is transferred onto a sheet of paper at the process cartridge  4 , the toner image is fixed to the sheet in the fixing unit  100  by the heat of the heating roller  110  as the sheet passes between the heating roller  110  and the pressure roller  120 . Subsequently, the heating roller  110  and pressure roller  120  convey the sheet onto a paper discharge path  50  formed in the paper discharge unit  200 . 
     The paper discharge unit  200  includes an inner guide member  51  and an outer guide member  62  that form the paper discharge path  50 ; a lower discharge roller  53  and an upper discharge roller  55  forming a pair of discharge rollers disposed in a discharge opening through which the paper is discharged onto the sheet discharge tray  52  provided on the top cover  18 ; and a tray member  54  constituting part of the sheet discharge tray  52 . The outer guide member  62  that functions to configure the paper discharge path  50  moves in association with the opening and the closing of the rear cover  60  provided on the rear side surface of the casing  2 . The rear cover  60  is attached to the casing  2  via a hinge  61  and is capable of swinging open and closed about the hinge  61 . When the rear cover  60  is swung to an open position, the top portion of the outer guide member  62  pivots rearward in association. In this way, the paper discharge path  50  can be exposed through the opening formed in the rear side surface  2   c  by opening the rear cover  60 . 
     The discharge tray  52  is substantially rectangular in shape in a plan view. The rear end of the discharge tray  52  is formed as a depression that recedes into the casing  2  and slopes gradually upward toward the front surface side of the casing  2 . In this embodiment, a section of the sheet discharge tray  52  that slopes gradually upward from the rear end to a point in the middle of the sheet discharge tray  52  is configured by the tray member  54 . The top surface of the tray member  54  on the front end (the downstream end with respect to the paper conveying direction) is configured to contact the bottom surface of the top cover  18 . 
     Hence, after passing through the fixing unit  100 , the sheet of paper is conveyed along the paper discharge path  50 . The paper discharge path  50 , configured by the inner guide member  51  and the outer guide member  62 , reverses the conveying direction of the sheet so that the sheet is directed toward the pair of discharge rollers  53  and  55 . The discharge rollers  53  and  55  discharge the sheet of paper onto the sheet discharge tray  52  toward the front surface side of the casing  2 . 
     (2) Detailed Structure of the Fixing Unit  100   
     Next, the structure of the fixing unit  100  will be described in detail. Referring to  FIG. 3 , the fixing unit  130  includes the heating roller  110 , the pressure roller  120  disposed beneath the heating roller  110  and pressing against the heating roller  110 , a support plate  150  for assembling the pressure roller  120  with the heating roller  110 . 
     The heating roller  110  is configured of a metal cylinder accommodating a heater in the form of a halogen lamp, for example, that functions to heat the metal cylinder. The both open ends of the heating roller  120  are sealed with bearing  113 , respectively. 
     The pressure roller  120  has an elastic cylinder made of silicon rubber and a film of polytetrafluoroethylene (PTFE) wound around the elastic cylinder. The pressure roller  120  has a rotational shaft  121  extending through the center axis of the elastic cylinder. Alternatively, the pressure roller  120  may be made by inserting an elastic cylindrical member of silicon rubber into a tube of PTFE. The pressure roller  120  follows the rotation of the heating roller  110  while applying pressure to the same. 
     Referring to  FIG. 4 , the support plate  150  extends in the paper conveying direction and has a support portion  140 , a lower arc-shaped portion  150   a , and a grip  170  in the paper conveying direction. The support portion  140  has a hook-shape on the upstream end with respect to the paper conveying direction. The grip  170  is provided on the downstream end. 
     The support portion  140  is engaged with a support shaft  131  formed on the cover  130  and rotatably supported by the cover  130  so that the support plate  150  is supported and capable of rotating with respect to the cover  130 . The support plate  150  has a notch  151  formed between the lower arc-shaped portion  150   a  and the grip  170 . A spring  160  has one end engaged with the notch  151  and the other end engaged with the cover  130  to press the pressure roller  120  against the heating roller  110 . In other words, the restorative force of the spring  160  urges the pressure roller  120  toward the heating roller  110 . 
     The lower arc-shaped portion  150   a  has a bearing portion  152  to pivotably receive one end of the rotational shaft  121  of the pressure roller  120 . The lower arc-shaped portion  150   a  has a shape that extends from the bearing portion  152  beyond the peripheral surface of the pressure roller  120  supported by the bearing portion  151  when viewed from the rotational shaft  121 . 
     The upper portion of the heating roller  110  is covered with a cover  130 . The cover  130  is molded of a resin, such as polyethylene teraphthalate (PET). The cover  130  does not cover the pressure roller  120  and, hence, the pressure roller  120  is in an exposed state out of the cover  130 . With this construction, the height of the fixing unit  100  can be reduced. Therefore, the fixing unit  100  can be made more compact, contributing to a more compact image-forming device. 
     The grip  170  is provided for a user of the laser printer  1  to open the rear cover  60  and press down on the grip  170  to easily access the problem happened in the printer  1 , when a paper jam occurs in the fixing unit  100 . 
     As described above, the support plate  150  has the lower arc-shaped portion  150   a . When the fixing unit  100  is placed on a flat surface with the side of the support plates  150  facing downward, the lower arc-shaped part  150   a  contacts the flat surface so that the surface of the pressure roller  120  does not contact the flat surface. 
     If the surface of the pressure roller  120  is damaged, the fixing strength at the damaged areas differs from the other areas, resulting in irregular fixing. As described above, the bottom of the pressure roller  120  is not covered by the case  130 , thereby contributing to the manufacturing of a compact fixing unit  100 . However, during maintenance, the fixing unit  100  must be removed from the laser printer and placed on a worktable. Depending on the type of maintenance being performed, there may be some cases in which the fixing unit  100  cannot be placed on the surface with the surface of the pressure roller  120  facing upward. Accordingly, the support plate  150  is provided with the construction described above for preventing the surface of the pressure roller  120  from becoming damaged. However, the support plate may be extended from the case  130  in order to prevent the surface of the pressure roller  120  from contacting the worktable. 
     (3) Detailed Structure of the Separating Pawls 
     Next, separating pawls provided in the fixing unit  100  will be described. In order to prevent recording paper or another recording medium that passes between the heating roller  110  and the pressure roller  120  from becoming attached to the heating roller  110  and wrapping around the heating roller  110  inside the case  130 , separating pawls are provided in the fixing unit  100  for separating the recording medium from the heating roller  110 . 
     However, various problems may arise from the use of such separating pawls. For example, since the ends of the separating pawls contact the surface of the heating roller, the life of these separating pawls is short and the separating pawls should be replaced relatively frequently. Further, the conveyed recording medium can press against the ends of the separating pawls and cause the pawls to separate from the heating roller  110  accidentally. 
     Referring to  FIG. 5 , grooves  342   a ,  342   b ,  342   c , and  342   d  are formed in four locations of the case  130  along the rotational axis of the heating roller  110 . Separating pawls  340   a ,  340   b ,  340   c , and  340   d  are disposed in the respective grooves. 
     Screw holes  301   a  and  301   b  are formed in the fixing unit  100  for mounting the fixing unit  100  inside the casing  2 . An opening  302  is provided in the fixing unit  100  for engaging a drive gear  115  mounted to the heating roller  110  with a transfer gear (not shown). The transfer gear functions to transfer a drive force from a drive motor disposed outside of the fixing unit  100  to the heating roller  110 . 
     Next, the construction of the separating pawls  340   a - 340   d  will be described. Since the four separating pawls  340   a - 340   d  have the same construction, an arbitrary separating pawl will be referred to as a “separating pawl  340 ”; similarly, the grooves  342   a - 342   d  will be referred to collectively as a “groove  342 ”. 
     Referring to  FIG. 6 , the separating pawl  340  has a substantially triangle pawl member  366  and a rotational shaft  361  extending in a crossing direction of the pawl member  366 . The pawl member  366  has a tip  365  on the acutest angle corner to contact the surface of the heating roller  110 . The rotational shaft  361  has two engaged portions  362  on both ends to be engaged with the case  130 . The engaged portion  362  has a semicylindrical portion  362   a  and a flat portion  362   b , and has a semicircular cross-section in a direction crossing the axis of the shaft  361 . The flat portion  362   b  faces the surface of the case  130  when the separating pawl  340  is mounted to the case  130  to be in an operable condition. A spring  363  is wound around the rotational shaft  361 . The spring  363  generates an elastic force for pressing the tip  365  against the surface of the heating roller  110  when the separating pawl  340  is mounted to the case  130 . 
     Referring to  FIG. 7 , the case  130  has semicircular holding parts  351  (only one semicircular holding part  351  is shown in  FIG. 7 ) formed in the mounting portion  350  for engaging and holding the engaged portion  362  of the rotational shaft  361 . The semicircular holding part  351  rises from the surface of the mounting portion  350  and curves with the tip of the semicircular holding part  351  facing toward the mounting portion  350 . Spaces  352  are between the semicircular holding part  351  and the case  130  for receiving the engaged portion  362 . When the separating pawl  340  is mounted, the engaged portions  362  on both ends of the rotational shaft  361  are inserted in the spaces  352 . 
     Referring to  FIG. 8 , when mounting the separating pawl  340 , the engaged portion  362  are inserted into the spaces  352  under the semicircular holding parts  351  while the flat portions  362   b  are facing the semicircular holding parts  351 . Subsequently, the separating pawl  340  is rotated about the rotational shaft  361  in a direction indicated by the arrow X until the pawl member  366  of the separating pawl  340  comes into the groove  342 . At this time, the semicylindrical portion  362   a  of the separating pawl  340  fits into the semicircular holding parts  351 . 
     By engaging one end of the spring  363  at a prescribed position on the case  130 , for example, the tip  365  of the separating pawl  340  can be made to contact the surface of the heating roller  110  with pressure by the elastic force of the spring  363 . According to the semicylindrical shape of the engaged portion  362 , the separating pawl  340  can be easily and readily mounted to the case  130 . 
     Referring to  FIG. 9 , after the separating pawl  340  is mounted, the engaged portions  362  are maintained in the space  352  by the semicircular holding parts  351 . Unless the engaged portions  362  are almost completely inverted, the rotational shafts  361  do not remove from the semicircular holding parts  351 , even if the leading edge of the recording medium catches on the tips  365 . 
     Alternatively, a covering member for covering the groove  342  can be attached on the inner guide member  51  made integrally with the tray member  54  (referred to as “inner guide members  51 ” hereinafter). This structure can prevent the separating pawl  340  from removing from the case  130  if the leading edge of the recording medium catches on the separating pawl  340 . 
     (4) Structure of the Separating Ribs 
     In this embodiment, referring to  FIG. 3 , separating ribs  371   a ,  371   b ,  371   c , and  371   d  (the separating rib  371   a  is not shown in  FIG. 3 ) are provided at four locations on the case  130  in addition to the separating pawls  340  described above. Next, the structure of the separating ribs  371   a - 371   d  will be described. All separating ribs  371   a - 271   d  have the same structure and hereinafter will be collectively referred to as a “separating rib  371 ”. 
     Referring to  FIG. 10 , the case  130  includes a bottom surface  135  substantially rectangular in shape from a plan view and positioned above the fixing unit  100  when the case  130  is mounted in the casing  2 ; and four side walls  136   a ,  136   b ,  136   c , and  136   d  erected from each of the four sides of the bottom surface  135  toward the heating roller  110 , thereby forming an open box shape for accommodating the heating roller  110 . Holding parts  360   a  and  360   b  are formed in both ends of the case  130  for holding the bearings  113  on both ends of the heating roller  110 . In  FIG. 10 , the separating pawls  340  have been removed from the grooves  342 . Next, the structure of the case  130  will be described in detail with reference to  FIG. 10 . 
     A wind-screening member  373  is integrally formed with the case  130  by injection molding so as to be substantially parallel to the side wall  136   c . The wind-screening member  373  is plate-shaped and functions to prevent wind from blowing on a thermistor (not shown) provided on the case  130  for measuring the surface temperature of the heating roller  110 . A temperature fuse cover  375  is also integrally formed with the case  130  by injection molding. The temperature fuse cover  375  is substantially plate shaped and extends from the side wall  136   c  to provide insulation between the heating roller  110  and a temperature fuse (not shown) provided for preventing the heating roller  110  from rising to an abnormal temperature. 
     The temperature fuse is mounted on the bottom surface  135  by the temperature fuse cover  375 . The temperature fuse has lead wires provided one on each end that are fastened to a power supply line by screws  378   a  and  378   b . Support members  379   a  and  379   b  are integrally formed with the case  130  by injection molding. The support members  379   a  and  379   b  are positioned between the bottom surface  135  and the both ends of the temperature fuse cover  375 , extending in the direction crossing the side wall  136   c . The support members  379   a  and  379   b  help prevent the plate-shaped temperature fuse cover  375  from warping or otherwise deforming due to heat from the heating roller  110 . A tape formed of a thermosetting resin such as polyimide is fixed to the surface of the temperature fuse cover  375 . Since the tape does not melt even if the heating roller  110  reaches an abnormal temperature, the tape can prevent the temperature fuse from contacting the heating roller  110 . The bearings  113  described above (see  FIG. 3 ) are molded of a resin that melts when the heating roller  110  reaches the abnormal temperature so as to ensure that a thermistat  380  described below and the temperature fuse operate reliably during abnormal heating. 
     The thermostat  380  is disposed on the power supply line. The thermostat  380  functions to prevent abnormal rises in temperature in the heating roller  110 . In this embodiment, the thermostat  380  is set to interrupt the power supply when the surface temperature of the heating roller  110  reaches about 400° C. The temperature fuse is adjusted so as to interrupt the power supply at about 500-600° C. Hence, the temperature fuse is particularly effective when an abnormality occurs in the thermostat  380 . 
     The separating ribs  371  protrude from the side wall  136   a  toward the heating roller  110  at four locations on the downstream side of the case  130  with respect to the sheet conveying direction. While there is no particular restriction in the number and positions of the separating ribs  371 , consideration should be given for the size of the recording medium being used (A4, letter). Holes  372   a ,  372   b ,  372   c , and  372   d  corresponding to the separating ribs  371  age provided for removing the mold after integrally forming the case  130  and separating ribs  371  by injection molding. When providing the holes  372 , an appropriate number of separating ribs is set in order to maintain the strength of the case  130 . 
     The separating ribs  371  provided in this way can separate the recording medium from the heating roller  110  even when an aberration occurs in the separating pawls  340 . If the material of the recording medium is restricted, the separating palls  340  can be eliminated in order to reduce manufacturing costs. 
     The opening  302  is formed in the end of the case  130  on the side surface  2   d  side so that a drive gear of the heating roller  110  can engage with a gear for transferring a drive force from a drive motor (not shown). Next, the opening  302  will be described. 
     As shown in  FIG. 12 , the drive gear  115  exposed in the opening  302  engages with a gear  400  through the opening  302 . The gear  400  functions to transfer a driving force from a drive motor (not shown). Here, the opening  302  is provided on the opposite side of the heating roller  110  from the separating ribs  371  (only the rib  371   d  is shown in  FIG. 12 ) when viewed along a rotational axis  112  of the heating roller  110 . By providing the opening  302  at this position, pressure from the gear  400  when mounting the fixing unit  100  in the casing  2  causes the surface of the heating roller  110  to move closer to the separating ribs  371 , contributing to an improvement in the separating performance of the separating ribs  371 . 
     Next, the structure of the inner guide members  51  described above will be described. As is also illustrated in  FIG. 2 , the tray member  54  configures a portion of the depressed part formed in the sheet discharge tray  52  that receive into the casing  2 . In this embodiment, the tray member  54  is integrally configured with the inner guide member  51  and the discharge rollers  53  that reversed the direction of the recording paper or other recording medium passing through the fixing unit  100  to proceed in a conveying direction towards the sheet discharge tray  52 . 
     In addition to the inner guide members  51 , separation preventing members  343   a ,  343   b ,  343   c , and  343   d  are formed for preventing the separating pawls  340  from removing from the fixing unit  100 . When fastening the inner guide members  51  inside the casing  2  with screws  56   a  and  56   b , portions of the separation preventing members  343  are positioned in the grooves  342 . With this construction, even when the separating pawls  340  move excessively from pressure received from the leading edge of the recording medium, the separating pawls  340  are stopped when contacting the separation preventing members  343 . Hence, the separation preventing members  343  prevent separation of the separating pawls  340 . The separation preventing members  343  may be formed separately from the inner guide members  51  and provided separately therebetween. Alternatively, the inner guide members  51  may be positioned to correspond with the grooves  342  and to extend downward themselves. 
     With this construction, the inner guide members  51  can be removed from the casing  2  in order to replace the separating pawls  340  without removing the fixing unit  100  from the casing  2 . In this embodiment, the tray member  54  constituting part of the sheet discharge tray  52  is formed integrally with the inner guide members  51 . Alternatively, the tray member  52  and the inner guide member  51  may also be configured separately. 
     (5) Method of Mounting the Temperature Fuse 
     A method of mounting a temperature fuse  389  in the case  130  will be described. The temperature fuse  389  includes a main fuse body  390  having an internal resinous part that melts at a high temperature, and lead wires  391   a  and  391   b  extending from both ends of the main fuse body  390 . The other ends of the lead wires  391   a  and  391   b  have loops  392   a  and  392   b  to be fastened to screw holes  393   a  and  393   b , respectively, by screws (not shown). Alternatively, it is also possible to attach the temperature fuse  389  by solder rather than screws. 
     The case  130  is also provided with plate-shaped rail members  376   a  and  376   b  for guiding the temperature fuse  389  into a fixed position on the bottom surface  135  when mounting the temperature fuse. The rail members  376   a  and  376   b  are provided substantially parallel to each other and protrude substantially perpendicularly from the bottom surface  135  toward the temperature fuse cover  375 . The rail members  376   a  and  376   b  also span from the bottom surface  135  side of a gap  382  (see  FIG. 10 ) to the bottom surface  135  side of the temperature fuse cover  375 . The gap  382  allows the temperature fuse  389  to be mounted on top of the rail members  376   a  and  376   b.    
     When mounting the temperature fuse  389 , the temperature fuse  389  is placed with the lead wires  391   a  and  391   b  resting on top of the rail members  376   a  and  376   b  ( FIG. 14(   a )). Subsequently, the temperature fuse  389  is slid on the rail members  376   a  and  376   b  in the direction indicated by an arrow Y in  FIG. 14(   a ) to a position at which a stopper  394   a  is provided ( FIG. 14(   b )). In this way, the temperature fuse  389  can be easily guided to a fixed position beneath the temperature fuse cover  375 . Here, the guide parts are not limited to the rail members  376   a  and  376   b . For example, a block member having a depression in which the main fuse body  390  can be mounted may be used. Alternatively, a downward slope may be provided from the bottom of the gap  382  toward the bottom of the temperature fuse cover  375 , so that the temperature fuse slides down by its own weight. 
     While the invention has been described in detail with reference to preferred 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. For example, the following variations may also be implemented. 
     1) In the above embodiment, the rear cover  60  and the outer guide member  62  are configured as separate components, wherein the outer guide member  62  moves rearward in association with the opening of the rear cover  60 . However, the outer guide member  62  may also be molded integrally with the rear cover  60 . 
     2) In the above embodiment, the heating roller  110  is disposed above the pressure roller  120 , and the pressure roller  120  is urged toward the heating roller  110  by the restorative force of the spring  160 . However, the fixing unit  100  is not limited to this construction. It is also conceivable to dispose the pressure roller  120  above the heating roller  110  and to provide a mechanism for urging the pressure roller  120  toward the heating roller  110 . 
     3) In the above embodiment, the tips  365  of the separating pawls  340  are pressed into contact with the surface of the heating roller  110  by the elastic force of the spring  363 . However, another construction may be used to press the tips  365  against the surface of the heating roller  110 . For example, the tips  365  may be pressed against the surface of the heating roller  110  by the weight of the separating pawls  340 . 
     4) In the above embodiment, the separation preventing members  343  are disposed between the plurality of inner guide members  51 . However, other members for preventing the separating pawls  340  from separating from the heating roller  110  may be used and there are no particular restrictions in the positions of these members. For example, depending on the shape of the sheet conveying path, separation preventing members may be provided integrally with the rear cover, or may be disposed so as to extend from the sheet discharge tray. 
     5) In the above embodiment, the support members  379   a  and  379   b  are integrally molded with the case  130 . However, these components may be formed separately with the purpose of preventing warping or other deformation in the temperature fuse cover  375  and may be fixed by adhesion or bonding. Alternatively, the support members  379   a  and  379   b  may be fixed by screws formed of an insulating plastic or by another method. 
     6) above embodiment, the temperature fuse cover  375  is integrally molded with the case  130 , but need not be configured this way. For example, an end of the temperature fuse cover  375  may be fixed to the side wall  136   c  by adhesive. Further, it is not necessary that the temperature fuse cover  375  be a substantially plate-shaped member.