Patent Publication Number: US-6904257-B2

Title: Image forming device capable of changing pressing force between fixing members

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming device and particularly to an image forming device with a mechanism for adjusting pressing force between fixing members. 
   2. Description of the Related Art 
   A conventional image forming device, such as a laser printer, normally includes a process cartridge and a fixing unit. The process cartridge is for transferring toner images from a photosensitive drum onto a sheet at the time of when the sheet passes between the photosensitive drum and a transfer roller. The fixing unit is located downstream from the process cartridge in the direction that the sheet is transported, and includes a thermal roller and a pressing roller disposed in pressing contact with each other. While the sheet passes between the thermal roller and the pressing roller, heat from the thermal roller fuses the transferred toner image onto the sheet. 
   Sometimes a user will want to form an image on an envelope or other sheet that is thicker than a normal print sheet. Some fixing units include a switching mechanism that can reduce pressing force of the pressing roller against the thermal roller from the pressure used on a normal print sheet when such a thick sheet is to be printed on. 
   One such switching mechanism includes two springs for each end of the pressing roller, i.e., a total of four springs. When a thick sheet is to be printed on, then the switching mechanism switches urging force of these four springs to press the pressing roller against the thermal roller. 
   Another switching mechanism includes a support member that swingably supports the pressing roller and a spring that constantly urges the pressing roller to press against the thermal roller. A rod member can be freely protruded in and out to abut against the support member by an amount that depend on the thickness of the sheet to be printed on in order to switch the pressing force. 
   SUMMARY OF THE INVENTION 
   However, the fixing unit that includes the switching mechanism with four springs can show great variation in pressing force. That is, because four springs are used to urge the pressing roller against the thermal roller, any variation in the urging force of the springs greatly influences the pressing force of the pressing roller against the thermal roller. Any variation in pressing force is almost constantly a problem for the user, because the four springs are used even in the most commonly used printing mode of the image forming device, that is, when normal sheets are printed on. 
   The switching mechanism that abuts the rod member against the support member also has room for improvement. That is, the stroke of the rod member protruding in and out can vary because of assembly errors or tolerance levels allowed in production of components. For this reason, the pressing force of the pressing roller against the thermal roller when fixing an image on a thick sheet can vary among printers even to a greater extend than in the switching mechanism with four springs. 
   It is an objective of the present invention to overcome the above-described problems and to provide an image forming device capable of reducing variation in pressing force between fixing members. 
   In order to achieve the above and other objects, according to the present invention, there is provided an image forming device including a fixing unit and a pressing force adjusting mechanism. The fixing unit includes two fixing members disposed in pressable confrontation with each other, and fixes images onto a recording medium sandwiched between the fixing members. The pressing force adjusting mechanism adjusts pressing force applied between the fixing members and includes a first urging unit that urges the fixing members to press against each other and a second urging unit including a first resilient member that produces urging force that reduces the pressing force between the fixing members against urging force of the first urging unit. The second urging unit is switchable between an operating mode wherein urging force of the first resilient member operates to reduce the pressing force between the fixing members and a non-operating mode wherein the urging force of the first resilient member does not operate to reduce the pressing force between the fixing members. 
   There is also provided an image forming device including a casing, a fixing unit, a pressing force adjusting mechanism, and a first cover. The fixing unit includes two fixing members disposed in pressable confrontation with each other, and fixes images onto a recording medium sandwiched between the fixing members. The pressing force adjusting mechanism includes a pressing member that selectively applies a predetermined pressing force and a smaller pressing force between the fixing members, and a switching member that switches the pressing member between a first pressing condition for applying the predetermined pressing force between the fixing members and a second pressing condition for applying the smaller pressing force between the fixing members. The first cover selectively opens up and covers interior of the casing. The first cover in the opened-up condition enables the recording medium to be linearly transported from the fixing unit out of the casing. The first cover abuts against the switching member when closing from the opened-up condition, thereby switching member switching the pressing member from the second pressing condition to the first pressing condition. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a cross-sectional side view of a laser printer according to an embodiment of the present invention; 
       FIG. 2  is a cross-sectional side view showing essential configuration of a linking mechanism in the laser printer of  FIG. 1  with a top cover of the laser printer closed; 
       FIG. 3  is a cross-sectional side view showing essential configuration of the linking mechanism with the top cover of the laser printer opened up; 
       FIG. 4  is a magnified side view showing essential components of the linking mechanism of  FIG. 3  with the top cover of the laser printer closed; 
       FIG. 5  is a magnified side view showing essential components of the linking mechanism of  FIG. 2  with the top cover of the laser printer opened up; 
       FIG. 6  is a magnified side view showing essential components of the linking mechanism of  FIG. 2  with the top cover at a position between being opened up and closed; 
       FIG. 7  is a magnified side view showing essential components of a pressing force adjusting mechanism of the laser printer of  FIG. 1  with a rear cover in a closed condition and a second lever in a non-operating condition; 
       FIG. 8  is a magnified cross-sectional side view showing essential components of the pressing force adjusting mechanism of  FIG. 7  with the rear cover in an opened condition and the second lever in the non-operating condition; 
       FIG. 9  is a magnified cross-sectional view from the side showing essential components of the pressing force adjusting mechanism of  FIG. 7  with the rear cover in an opened condition and the second lever in position between the non-operating condition and an operating condition; 
       FIG. 10  is a magnified cross-sectional side view showing essential components of the pressing force adjusting mechanism of  FIG. 7  with the rear cover in an opened condition and the second lever in the operating condition; 
     FIG.  11 ( a ) is a cross-sectional view showing the pressing force adjusting mechanism of  FIG. 7  in the non-operating condition; 
     FIG.  11 ( b ) is a cross-sectional view showing a switching member in a position between the non-operating condition and the operating condition; and 
     FIG.  11 ( c ) is a cross-sectional view showing the switching member in the operating condition. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENT 
   Next, a laser printer  1  will be explained as an embodiment of an image forming device according to the present invention. 
   As shown in  FIG. 1 , the laser printer  1  includes a casing  2 , a feeder unit  4 , and an image forming unit  5 . The feeder unit  4  feeds sheets  3  one at a time to the image forming unit  5 , which forms images on the sheets  3 . 
   The feeder unit  4  includes a sheet tray  6 , a pressing plate  7 , a sheet feed roller  8 , a sheet feed pad  9 , transport rollers  10 ,  11 , and register rollers  12 . The sheet tray  6  is detachably mounted to the interior of the base of the casing  2 . The pressing plate  7  is provided within the sheet tray  6 . The sheet feed roller  8  and the sheet feed pad  9  are provided above the front end of the sheet tray  6 . The transport rollers  10 ,  11  are provided downstream from the sheet feed roller  8  with respect to a sheet transport direction in which sheets  3  are transported. The register rollers  12  are provided further downstream from the transport rollers  10 ,  11  in the sheet transport direction. 
   The pressing plate  7  is stacked with a plurality of sheets. The pressing plate  7  is swingably supported about its rear side so that its free front side can swing vertically up and down. Although not shown in the drawings, a spring is provided below the pressing plate  7  that urges the pressing plate  7  upward. As a result, the upper most sheet of the stack on the pressing plate  7  is pressed against the sheet feed roller B. The pressing plate  7  swings about the rear side as the pivot point, downward under the weight of the stacked sheets against the urging force of the spring. The sheet feed roller  8  and the sheet feed pad  9  are located in confrontation with each other. A spring  13  provided on the bottom surface of the sheet feed pad  9  presses the sheet feed pad  9  toward the sheet feed roller  8 . When the sheet feed roller  8  rotates, the uppermost sheet  3  on the pressing plate  7  is sandwiched between the sheet feed roller  8  and the sheet feed pad  9  and fed in the sheet transport direction. Sheets  3  fed one at a time in this way are transported to the register rollers  12  by the transport rollers  10 ,  11 . The register rollers  12  subject the sheet  3  to a resist operation and then transport the sheet  3  to the image forming unit  5 . 
   The feeder unit  4  further includes a multipurpose tray  14 , a multipurpose side sheet feed roller  15 , and a multipurpose side sheet feed pad  15   a . The multipurpose side sheet feed roller  15  and the multipurpose side sheet feed pad  15   a  are disposed in confrontation with each other. Although not shown in the drawings, a spring is disposed to the rear of the multipurpose side sheet feed pad  15   a  for pressing the multipurpose side sheet feed pad  15   a  toward the multipurpose side sheet feed roller  15 . Sheets stacked on the multipurpose tray  14  are sandwiched one at a time between the multipurpose side sheet feed roller  15  and the multipurpose side sheet feed pad  15   a  by rotation of the multipurpose side sheet feed roller  15  and then fed to the register rollers  12 . 
   The image forming unit  5  includes a scan unit  16 , a processes cartridge  17 , a transfer roller  24 , and a fixing unit  18 . The scan unit  16  is provided inside the upper portion of the casing  2  and includes a polygon mirror  19 , lenses  20 ,  21 , a reflection mirror  22 , and a laser emitting section (not shown). The laser emitting section emits a laser beam based on image data. As indicated by a two-dotted chain line in  FIG. 1 , the laser beam reflects off the polygon mirror  19 , passes through the lens  20 , reflects off the reflection mirror  22 , and passes through the lens  21 , in this order, and scans across the surface of a photosensitive drum  23  (described later) of the processes cartridge  17  at a high speed. 
   The processes cartridge  17  is mounted below the scan unit  16  so as to be detachable from the casing  2 . The processes cartridge  17  includes the photosensitive drum  23 , and, although not shown in the drawings, a scorotoron charge unit, a developing roller, and a toner holding section. The toner holding section is filled with positively charging, non-magnetic, single component, polymerized toner as developing agent. The toner is borne in a thin layer with a fixed thickness on the developing roller. 
   The photosensitive drum  23  is rotatably disposed in confrontation with the developing roller. The main drum of the photosensitive drum  23  is connected to ground. The surface of the photosensitive drum  23  is formed from a positively charging photosensitive layer, such as polycarbonate. 
   As the photosensitive drum  23  rotates, the scorotoron charge unit charges the surface of the photosensitive drum to a uniform positive charge. Then, the surface is selectively exposed by the high-speed scan of the laser beam from the scan unit  16 . The electric potential of the charge drops when exposed by the laser beam, thereby forming an electrostatic latent image based on the image data. When the electrostatic latent image comes into confrontation with the development roller, the positively charged toner borne on the development roller is selectively supplied onto the surface of the photosensitive drum  23 . In this way, the toner develops the electrostatic latent image into a visible toner image. 
   The transfer roller  24  is disposed below the photosensitive drum  23 . The transfer roller  24  is supported rotatable on the casing  2  in confrontation with the photosensitive drum  23 . The transfer roller  24  includes a metal roller shaft covered with a roller made from conductive rubber. A predetermined transfer bias is applied between the transfer roller  24  and the photosensitive drum  23 . For this reason, the visible toner image borne on the photosensitive drum  23  is transferred to the sheet  3  as the sheet  3  passes between the photosensitive drum  23  and the transfer roller  24 . The sheet  3  with the visible toner image is transported to the fixing unit  18  by a transport belt  25 . 
   The fixing unit  28  is disposed downstream from the processes cartridge  17  in the sheet transport direction and includes a thermal roller  26  and a pressing roller  27  disposed in confrontation with the thermal roller  26 . The thermal roller  26  is made from a metal roller, and includes a halogen lamp to heat up the metal roller. The thermal roller  26  is rotatably supported on a roller shaft  34 , which is fixed inside the casing  2 . The pressing roller  27  is supported on a roller shaft  35  and presses toward the thermal roller  26 . With this configuration, the fixing unit  18  thermally fixes toner images onto sheets  3  while the sheets  3  pass between the thermal roller  26  and the pressing roller  27 . 
   Transport rollers  28 ,  29  are provided downstream from the fixing unit  18  for transporting sheets  3  that are discharged from the fixing unit  18  to sheet-discharge rollers  30 . The discharge rollers  30  discharge the sheets  3  onto a discharge tray  31 . 
   A rear cover  58  is provided freely openable at the rear side of the casing  2 . A top cover  59  is provided freely openable at the top of the casing  2 . The rear cover  58  is formed separately from the top cover  59  and is freely swingable around its lower end. When the rear cover  58  is opened up as indicated by two-dot chain line in  FIG. 1 , a “straight” path is formed that enables sheets  3  to be discharged following a substantially linear path from the fixing unit  18 . Also, a first operation lever  46  shown in  FIG. 2 and a  second operation lever  72  shown in  FIG. 7  are exposed to manipulation by an operator from outside the casing  2 . Accordingly, the first operation lever  46  and the second operation lever  72  are easier to use. 
   The top cover  59  is pivotably supported about its rear-side end above the processes cartridge  17 . The top cover  59  is pivotable from a closed portion shown in FIG.  1  and an opened position shown in FIG.  3 . In the closed position, the top cover  59  is disposed in substantially a horizontal orientation that closes the casing  2 . In the opened position, the top cover  59  extends upward and the casing  2  is opened. By opening and closing the top cover  59  in between these two positions, the processes cartridge  17  can be easily exchanged and the processes cartridge  17  can be easily removed to facilitate correction of paper jams. Also, an attachment plate  32  is formed in a downward protruding shape at the rear end of the top cover  59 . 
   The laser printer  1  further includes a linking mechanism  33  shown in  FIG. 2 and a  pressure changing mechanism  60  shown in FIG.  7 . The linking mechanism  33  operates in linked association with closing and opening movement of the top cover  59  to press the pressing roller  27  against the thermal roller  26  when the top cover  59  is closed and not press against the thermal roller  26  when the top cover  59  is opened. The pressure changing mechanism  60  changes the pressing force between the pressing roller  27  and the thermal roller  26 . 
   The configuration of the linking mechanism  33  will be explained. As shown in  FIG. 2 , the linking mechanism  33  includes a roller support member  36 , a first spring  37 , a linear movement rod  38 , the first operation lever  46 , a link member  39 , a support plate  40 , and a connection rod  41 . Although not shown in the drawings, the linking mechanism  33  including all of the components listed above is provided on both sides of the fixing unit  18 . Because both of the linking mechanisms  33  have exactly the same configuration, the linking mechanism  33  shown in  FIG. 2  will be explained as a representative example. 
   As shown in  FIG. 4 , the roller support member  36  is positioned under the thermal roller  26  and at the widthwise side of the pressing roller  27 . The front end of the roller support member  36  is swingably supported on the casing  2  about a support shaft  42 . The roller support member  36  is formed from an integral bearing portion  43  and an abutment lever  44 . The bearing portion  43  is provided to the rear of the support shaft  42 . The abutment lever  44  protrudes rearward from the bearing portion  43 . Also, the roller shaft  35  of the pressing roller  27  is rotatably supported on the bearing portion  43 . 
   The first spring  37  is attached at its lower end to a base  45  of the abutment lever  44  and at its upper end to the casing  2  at a position substantially directly above the base  45 . The first spring  37  urges the base  45  upward. With this configuration, the roller support member  36  is urged by the first spring  37  to pivot clockwise, as viewed in  FIG. 4 , around the support shaft  42  so that the pressing roller  27  presses against the thermal roller  26 . 
   The linear movement rod  38  is supported on the casing  2  at a position above the abutment lever  44  and in a substantially vertical orientation so as to be freely slidable up and down between an upper position and a lower position. The linear movement rod  38  is constantly urged upward by a suspension spring  64  so that the linear movement rod  38  is normally maintained in the upper position shown in FIG.  4 . 
   The first operation lever  46  has a plate shape and is disposed above the linear movement rod  38 . The front end of the first operation lever  46  is swingably supported on the casing  2  through a support shaft  47 , which is provided near the front side of the upper end of the linear movement rod  38 . The rear end of the first operation lever  46  serves as an operation lever  48 , which users can manually manipulate in a manner to be described later. An abutment protrusion  49  is formed on the lower surface of the first operation lever  46 , at a position between the support shaft  47  and the operation lever  48 . The abutment protrusion  49  protrudes downward and is for abutting against the upper end of the linear movement rod  38  in a manner to be described later. 
   When the linear movement rod  38  is retracted into its upper position as shown in  FIG. 4 , the lower end of the linear movement rod  38  is separated from the abutment lever  44 . Therefore, the pressing roller  27  is maintained in the pressing position in pressing abutment with the thermal roller  26  by urging force of the first spring  37 . On the other hand, when the first operation lever  46  is swung downward about the support shaft  47  as shown in  FIG. 5 , then the abutment protrusion  49  presses down on the upper end of the linear movement rod  38  so that the linear movement rod  38  moves into its downward position against the urging force of the suspension spring  64 . As a result, the lower end of the linear movement rod  38  presses the abutment lever  44  downward. This swings the roller support member  36  against the urging force of the first spring  37  counterclockwise as viewed in  FIG. 5  around the support shaft  42  so that the pressing roller  27  moves into its non-pressing position separated from the thermal roller  26 . That is, the pressing roller  27  can be selectively moved between the pressing position and the non-pressing position with respect to the thermal roller  26  by swinging movement of the roller support member  36  generated when the linear movement rod  38  is advanced and retracted. With this configuration, when the user presses the operation lever  48  of the first operation lever  46  downward, then the pressing roller  27  moves to its non-pressing position. 
   The support plate  40  has a substantially rectangular plate shape and is disposed in a substantially vertical orientation. The support plate  40  is formed on one end with an engagement protrusion  55  and at the other end is swingably supported with respect to the casing  2  by a support shaft  56 . 
   The link member  39  has a plate shape and is disposed above the roller support member  36  separated from the roller support member  36  by a predetermined distance. The link member  39  is swingably supported on the casing  2  by a support shaft  51  at an upper portion of the link member  39  near the lengthwise center of the link member  39 . A pressing protrusion  50  is provided to the lower surface of the link member  39  at its rear-side end. The pressing protrusion  50  is for abutting against the upper surface of the first operation lever  46  so as to press the linear movement rod  38  downward via the first operation lever  46 . A guide groove  52  is formed in the link member  39  from near the lengthwise center of the link member  39  to the front of the link member  39 . The guide groove  52  is elongated slot shaped substantially like the letter “L.” The guide groove  52  is for receiving the engagement protrusion  55  and allowing the engagement protrusion  55  to move following the swinging direction of the support plate  40 . 
   In more concrete terms, the guide groove  52  includes a movement groove portion  53  and a lock groove portion  54 . The lock groove portion  54  is opened to connect with the movement groove portion  53 . The movement groove portion  53  is formed in the shape of an elongated slot so as to enable movement of the engagement protrusion  55 . The lock groove portion  54  is formed to extend in a slant upward with respect to the lengthwise direction of the movement groove portion  53 . The upward slant is the direction for reducing force for swinging the link member  39  when the top cover  59  is being opened up as will be described later and is the direction that the engagement protrusion  55  moves after surpassing the fulcrum position shown in  FIG. 6  when moving in the guide groove  52  in association with swinging movement of the support plate  40 . 
   The support shaft  56  of the support plate  40  is positioned above and to the front of the guide groove  52  so that, when the top cover  59  is in the closed condition, the support plate  40  is oriented to extend in a direction substantially perpendicular to the direction in which the guide groove  52  extends. 
   A roller member  57  is freely rotatably provided on the engagement protrusion  55  for reducing resistance to swinging between the engagement protrusion  55  and the guide groove  52 . 
   As shown in  FIGS. 2 and 3 , the connection rod  41  has the shape of a rod. The front end of the connection rod  41  is pivotably attached on the attachment plate  32  of the top cover  59 . The rear end of the connection rod  41  is pivotally attached to the lower end of the support plate  40 . The connection rod  41  is disposed so as to intersect the movement direction of the guide groove  52  when the link member  39  swings while the top cover  59  is being opened up and so as to extend to outside of the swinging path of the link member  39 . 
   Next, operations of the linking mechanism  33  will be described. As shown in  FIG. 2 , the rear-side end of the connection rod  41  is in its rearmost position when the top cover  59  is in its closed up condition. Therefore, the support plate  40  is located where it is swung rearward, that is, clockwise, around the support shaft  56 . The engagement protrusion  55  is located at the rear-side end of the movement groove portion  53  of the link member  39 . As shown in  FIG. 4 , in this condition the link member  39  has swung clockwise around the support shaft  51  so that the guide groove  52  side of the link member  39  faces downward and the pressing protrusion  50  side of the link member  39  faces upward. As a result, the pressing protrusion  50  is separated from the upper surface of the first operation lever  46 . Therefore, the first operation lever  46  does not press against the linear movement rod  38 . For this reason, the linear movement rod  38  is in its retracted condition where it does not press the abutment lever  44  of the roller support member  36 . Therefore, as described above the roller support member  36  maintains the pressing roller  27  in the pressing position under the urging force of the first spring  37 . In this way, while the top cover  59  is in its closed up condition, the pressing roller  27  presses against the thermal roller  26 . 
   When the top cover  59  is moved into its opened up condition shown in  FIG. 3 , the connection rod  41  moves forward in linking association with the opening movement of the top cover  59 . In association with this, the rear-side end of the connection rod  41  moves forward as shown in FIG.  6 . As a result, the support plate  40  swings counterclockwise forward around the support shaft  56  so that the engagement protrusion  55 , which is engaged in the movement groove portion  53  of the guide groove  52 , moves forward in the lengthwise direction of the movement groove portion  53 . Therefore, the link member  39  swings counterclockwise around the support shaft  51 , the guide groove  52  side of the link member  39  moves upward, that is, in the direction toward the support shaft  56 , and the pressing protrusion  50  side of the link member  39  swings downward. 
   As a result, the pressing protrusion  50  of the link member  39  presses down on the upper surface of the first operation lever  46  so that, in the same way as when the operation lever  48  of the first operation lever  46  is pressed downward by a user, the first operation lever  46  swings in the counterclockwise direction around the support shaft  47 , thereby pressing the linear movement rod  38  downward to press the abutment lever  44 . Therefore, the pressing roller  27  moves into its non-pressing position so that pressure is no longer applied by the pressing roller  27  to the thermal roller  26 . 
   When the top cover  59  is fully opened up as shown in  FIG. 3 , then as shown in  FIG. 5  the engagement protrusion  55  of the support plate  40  is received within the lock groove portion  54  of the guide groove  52  so that movement of the engagement protrusion  55  is properly regulated. For this reason, when the top cover  59  is fully opened up, the pressing roller  27  is reliably maintained in its non-pressing position against the urging force of the first spring  37 . 
   On the other hand, when the top cover  59  is closed from its opened condition, the top cover  59  presses down on the connection rod  41  so that the rear-side end of the connection rod  41  moves rearward. As a result, the support plate  40  swings clockwise to the rear around the support shaft  56 . This pulls the engagement protrusion  55  out from the lock groove portion  54  and moves the engagement protrusion  55  following the swinging direction of the support plate  40  in the movement groove portion  53  to the rear-side end. For this reason, the link member  39  swings in the clockwise direction around the support shaft  51 . As a result, the pressing protrusion  50  of the link member  39  stops pressing down on the upper surface of the first operation lever  46 . Therefore, the pressing roller  27  presses against the thermal roller  26  while the top cover  59  is closed up. 
   Because pressing force of the pressing roller  27  against the thermal roller  26  can be released by opening the top cover  59 , the user can easily remove a sheet  3  that is jammed between the pressing roller  27  and the thermal roller  26  by merely opening the top cover  59 . Also, after removing the jammed sheet  3 , the user needs to merely close the top cover  59  to press the pressing roller  27  against the thermal roller  26  so that fixing operations can be simply and reliably performed. 
   Although the opening movement of the top cover  59  presses the link member  39  down on the linear movement rod  38  through the first operation lever  46  against the urging force of the first spring  37 , the load that is transmitted to the link member  39  by the urging force of the first spring  37  is transmitted in the swinging direction of the link member  39 , that is, toward the support shaft  56 . However, as described previously, the support plate  40  extends in a direction that is substantially perpendicular with the direction in which the guide groove  52  extends. Therefore, the load can be reliably received by the support plate  40 . Further, because the connection rod  41  is disposed in an orientation so as to extend in a direction that intersects with the support plate  40  and to the outside of the swinging path of the link member  39 , only an extremely small load will be transmitted to the top cover  59  through the connection rod  41 . Moreover, because closing movement of the top cover  59  stops the link member  39  from pressing the linear movement rod  38 , the load is not transmitted through the link member  39  to the top cover  59 . 
   For this reason, the pressing roller  27  can be pressed against and separated from the thermal roller  26  in association with closing and opening movement of the top cover  59 , while also reducing the amount of the resultant load that is transmitted to the top cover  59 . Therefore, the top cover  59  does not need to be made particularly strong. The top cover  59  can be made with a simple and light configuration so that costs can be reduced. Also, there is no need to produce a force equivalent to the load when opening and closing the top cover  59 . The top cover  59  can be opened easily with a simple force. 
   Moreover, when the top cover  59  is being opened up and closed, the support plate  40  swings in association with the opening or closing movement so that the engagement protrusion  55  slides within the guide groove  52 . This insures that the link member  39  can be more reliably swung so that the associated load can be reliably prevented from being transmitted to the top cover  59 . 
   The lock groove portion  54  is formed at an angle with respect to the movement groove portion  53  to reduce force for swinging the link member  39  when the top cover  59  is being opened up. Therefore, when the top cover  59  is being closed shut, the engagement protrusion  55  can be moved into the movement groove portion  53  of the guide groove  52  by merely applying force for moving the engagement protrusion  55  from the lock groove portion  54  to the movement groove portion  53 , that is, force sufficient to move the engagement protrusion  55  over the fulcrum point shown in FIG.  6 . Accordingly, opening and closing operations of the top cover  59  can be performed using even less force. 
   Moreover, because the roller member  57  is rotatably provided on the engagement protrusion  55  in order to reduce resistance to the sliding between the guide groove  52  and the engagement protrusion  55 , the engagement protrusion  55  can be moved within the guide groove  52  with resistance to sliding reduced by the roller member  57 . Accordingly, opening and closing operations of the top cover  59  can be performed using even less force. 
   Because the first operation lever  46  is provided as a member separated and independent from the link member  39 , if a sheet  3  become jammed between the pressing roller  27  and the thermal roller  26 , the sheet  3  can be easily removed without opening up the top cover  59 , but by opening the rear cover  58  as indicated in FIG.  1  and pulling the first operation lever  46  by hand to release pressure between the pressing roller  27  and the thermal roller  26 . That is, when a sheet  3  jams between the thermal roller  26  and the pressing roller  27 , the user can reach the paper Jam by either opening up the top cover  59  or by operating the first operation lever  46 . Because the user has a choice on how to remove the sheet  3 , the device is more easily to repair. 
   Because the first operation lever  46  is swingably provided between the pressing protrusion  50  of the link member  39  and the linear movement rod  38 , the first operation lever  46  can be provided in a simple and inexpensive configuration. 
   It should be noted that although the embodiment describes the first operation lever  46  as being swingably provided between the linear movement rod  38  and the pressing protrusion  50  of the link member  39 , the first operation lever  46  can be dispensed with so that the pressing protrusion  50  of the link member  39  presses directly against the linear movement rod  38 . 
   In this case, for example, the link member  39  and the linear movement rod  38  could be formed integrally together so that the link member  39  and the linear movement rod  38  configure a single pressing member. Alternatively, the pressing member configured from the integral linear movement rod  38  and the link member  39  could be formed integrally with the roller support member  36  to configure a single movement member from the link member  39 , the linear movement rod  38 , and the roller support member  36 . 
   Next, the pressure changing mechanism  60  for changing pressing force of the pressing roller  27  onto the thermal roller  26  will be described in detail. It should be note that the same pressure changing mechanism  60  shown in  FIGS. 7  to  11  is provided on both sides of the fixing unit  18 . However, only the pressure changing mechanism  60  shown in  FIGS. 7  to  11  will be described as a representative example to simplify explanations. 
   As shown in  FIG. 7 , the pressure changing mechanism  60  includes the above described roller support member  36 , the first spring  37  and the linear movement rod  38 . The linear movement rod  38  includes a linear movement rod  61 , a switch  62 , a second spring  63 , and the suspension spring  64 . 
   The linear movement rod  61  includes a linear motion member  65  and a protrusion portion  66 . The linear motion member  65  has a long thin plate shape. An abutment portion  65 A is formed on the lower end of the linear motion member  65 . A hook-shaped holding protrusion  65 B is formed on the front side of the abutment portion  65 A. The protrusion portion  66  is formed integrally with the linear motion member  65  and protrudes rearward from the substantial lengthwise center of the linear motion member  65 . The protrusion portion  66  is formed in a substantial rectangular shape with a downward slant at the lower section. The protrusion portion  66  is formed form a pair of plate members disposed in confrontation with each other and separated by a predetermined space. Also, a slot  66 A is formed through the rear-end section of the protrusion portion  66  to extend downward following the lengthwise direction of the linear motion member  65 . 
   The switch  62  includes a switching member  70  and a slide member  80 . The switching member  70  is formed in an L-shape as viewed from the side and includes an integrally formed abutment plate  71  and a second operation lever  72 . A swing shaft  76  of the abutment plate  71  is pivotably supported in the slot  66 A of the linear movement rod  61 . By this, the switching member  70  is swingably supported on the linear movement rod  61 . 
   As shown in FIG.  11 ( a ), the abutment plate  71  has a substantially rectangular shape and includes a first portion  71   a , a first angled portion  71   b , and a second portion  71   c  connected together in this order. The first portion  71   a  is a flat section formed at the upper end edge of the abutment plate  71 . The second portion  71   c  is the front side edge of the abutment plate  71 . The first angled portion  71   b  forms a rounded corner of the abutment plate  71  and connects the first portion  71   a  and the second portion  71   c . The first portion  71   a  extends in the direction that is substantially perpendicular with the direction in which the second operation lever  72  extends. The second portion  71   c  is formed in an arched shape that follows the swing path followed by the abutment plate  71  when the abutment plate  71  swings about the swing shaft  76 . 
   The swing shaft  76  is formed to protrude in an inward direction that intersects the abutment plate  71 . The swing shaft  76  is formed in a substantial teardrop shape in cross section. The outer peripheral surface of the swing shaft  76  includes a third portion  76   c , a fourth portion  76   a , a second angle portion  76   b , and a circular arched portion  76   d . The third portion  76   c  and the fourth portion  76   a  are formed in a substantially flat shape. The second angle portion  76   b  is formed in an acute angle that connects one side of the third portion  76   c  and the fourth portion  76   a . The circular arched portion  76   d  is formed in a circular arched shape that connects the other side of the third portion  76   c  and the fourth portion  76   a . The third portion  76   c  is formed substantially parallel with the first portion  71   a  of the abutment plate  71 . 
   The second operation lever  72  has a plate shape that can be manually manipulated. The second operation lever  72  is formed integrally with the abutment plate  71  so as to extend from the rear-side end of the abutment plate  71  in a direction that is substantially perpendicular to the abutment plate  71 . 
   The slide member  80  has an integrally formed thick plate portion  81 , a thin plate portion  82 , and a protrusion portion  84 . The thick plate portion  81  has a substantially triangular shape. The thick plate portion  81  is interposed between the pair of plate members of the protrusion portion  66  so as to be slidable vertically. An upper end  83  of the thick plate portion  81  is formed in a flat shape. The thin plate portion  82  is mounted within a through-hole type groove that is opened in the linear motion member  65  following the lengthwise direction of the linear motion member  65  and is also engaged, so as to be slidable vertically, in a slide groove formed in the rear-side end of the linear motion member  65 . The protrusion portion  84  is formed at the lower end of the thin plate portion  82 . The thick plate portion  81  of the slide member  80  is interposed between the plate members of the protrusion portion  66 , and the thin plate portion  82  of the slide member  80  is guided in the through-hole type groove. Also, while the slide member  80  is interposed in the slide groove, the slide member  80  is slidable vertically up and down with respect to the linear movement rod  61 . 
   As shown in  FIG. 7 , the second spring  63  is mounted in the through-hole type groove of the linear motion member  65  with the upper end of the second spring  63  engaged with the protrusion portion  84  of the slide member  80  and the lower end engaged with the abutment portion  65 A of the linear motion member  65 . The second spring  63  urges the slide member  80  and the abutment portion  65 A to separate from each other. The elastic modulus of the second spring  63  is set smaller than the elastic modulus of the first spring  37 . 
   The suspension spring  64  is attached at its lower end to the holding protrusion  65 B and fixed at its upper end to the casing  2 . The suspension spring  64  urges to pull the holding protrusion  65 B upward. The elastic modulus of the suspension spring  64  is set smaller than the elastic modulus of the second spring  63 . 
   For this reason, as shown in  FIG. 7 , the linear movement rod  38  is constantly urged by the suspension spring  64  so that the abutment plate  71  of the switch  62  abuts against a reference abutment portion  90  that is formed on the casing  2 . 
   Next, operations of the pressure changing mechanism  60  will be explained. While the rear cover  58  is closed shut, then as shown in FIGS.  7  and  11 ( a ), the second operation lever  72  is oriented in a substantially vertical posture. Because of this, the first portion  71   a  of the abutment plate  71  and the third portion  76   c  of the swing shaft  76  are oriented in substantially horizontal postures. In this condition, the linear movement rod  38  is urged upward by urging force of the suspension spring  64  so that the first portion  71   a  of the abutment plate  71  abuts against the lower surface of the reference abutment portion  90  and the abutment portion  65 A is positioned in a non-operative position separated from the abutment lever  44  of the roller support member  36 . This condition will be referred to as the non-operating condition, hereinafter. Accordingly, the roller support member  36  presses the pressing roller  27  against the thermal roller  26  under only the urging force of the first spring  37 . Therefore, a predetermined pressing force is generated between the pressing roller  27  and the thermal roller  26 . This will be referred to as a first pressing condition, hereinafter. In the non-operating condition, the thin plate portion  82  of the slide member  80  is urged by urging force of the second spring  63  to the upper portion of the slide groove. In this condition, the third portion  76   c  of the swing shaft  76  abuts the upper surface  63  of the thick plate portion  81 . Also, at this time, as shown in FIG.  11 ( a ) the first portion  71   a  of the abutment plate  71  abuts the lower surface of the reference abutment portion  90  and is maintained in that position. In this condition, printing is performed on thin sheets  3  made from normal paper, for example. 
   Next, when printing on a sheet  3  of relatively thick paper, such as a thick envelope, it is desirable to transport the sheet  3  substantially linearly from the fixing unit  18  in order to prevent the sheet  3  from being bent. For this reason, the rear cover  58  is opened up as shown in  FIG. 8  from its closed condition. As a result, as shown in  FIG. 1  a straight path is formed for discharging a sheet  3 , onto which a toner image was thermally fixed by the fixing unit  18 , onto the rear cover  58  in a substantially linear manner from the transport rollers  28 , and also the second operation lever  72  is exposed. 
   If the user presses the second operation lever  72  downward and rearward in this condition, then the pressure between the thermal roller  26  and the pressing roller  27  is reduced from the pressure applied for printing on sheets  3  made from normal paper. That is, when the user presses the second operation lever  72  downward and rearward using his or her finger, then as shown in FIGS.  11 ( b ) and  11 ( c ) the second operation lever  72  swings downward around the swing shaft  76 , and the abutment plate  71  swings upward around the swing shaft  76 . At this time, first the first portion  71   a  of the abutment plate  71  abuts against the reference abutment portion  90  as shown in  FIG. 11  ( a ). Then as swinging movement proceeds, then the second operation lever  72  enters temporarily into a slanted posture as shown in FIG.  11 ( b ), whereby the first angled portion  71   b  of the second operation lever  72  abuts the reference abutment portion  90 . When, the second operation lever  72  becomes oriented substantially horizontal as shown in FIG.  11 ( c ). The second portion  71   c  abuts the reference abutment portion  90 . 
   Also, at first as shown in FIG.  11 ( a ) the third portion  76   c  of the swing shaft  76  abuts the upper surface  83  of the slide member  80 . However, in the condition shown in FIG.  11 ( b ) when the first angled portion  71   b  abuts the reference abutment portion  90 , the second angle portion  76   b  abuts the upper surface  83 . Afterward, in the condition shown in FIG.  11 ( c ), when the second portion  71   c  abuts the reference abutment portion  90 , the fourth portion  76   a  abuts the upper surface  83 . 
   As a result, the swing shaft  76  moves downward only by the difference between the distance separating the second portion  71   c  from the swing shaft  76  and the distance separating the first portion  71   a  from the swing shaft  76 . As a result, the slide member  80 , which is abutted by the swing shaft  76 , is also pressed down. Because the elastic modulus of the second spring  63  is larger than the elastic modulus of the suspension spring  64 , as shown in  FIG. 9  the linear movement rod  61  moves downward under the urging force of the second spring  63  so that the abutment portion  65 A abuts the abutment lever  44  of the roller support member  36 . 
   At this time, the abutment portion  65 A receives upward moving repulsive force from the roller support member  36 , which is urged by the first spring  37 . However, because the elastic modulus of the first spring  37  is set larger than the elastic modulus of the second spring  63 , the second spring  63  contracts under the repulsive force as shown in FIG.  10 . Therefore, the linear movement rod  61  slidingly moves upward relative to the slide member  80 , which is maintained in its position by the reference abutment portion  90 . Therefore, substantially only the urging force of the second spring  63  operates to the roller support member  36  through the abutment portion  65 A, without the abutment portion  65 A pressing the roller support member  36  further downward. In the following explanation, the position where the abutment portion  65 A abuts the abutment lever  44  will be referred to as the operation position, and the condition wherein only the urging force of the second spring  63  operates on the roller support member  36  in the direction against urging force of the first spring  37  will be referred to as the operating condition. 
   In the operation condition, the second spring  63  urges the roller support member  36  against the urging force of the first spring  37 . Therefore, the roller support member  36  moves by a corresponding amount in the direction for releasing pressure between the pressing roller  27  and the thermal roller  26  so that pressure of the pressing roller  27  on the thermal roller  26  is reduced. As a result, a second pressure condition, wherein pressure applied to the thermal roller  26  is less than the predetermined pressing force, can be realized so that fixing operations can be properly performed on sheets  3  made from thick sheets, such as envelops. 
   In the pressure changing mechanism  60 , the elastic modulus of the second spring  63  is set to a smaller value than the elastic modulus of the first spring  37 . Therefore, in the operating condition the pressing force between the pressing roller  27  and the thermal roller  26  can be reliably reduced without completely releasing the pressing force between the thermal roller  26  and the pressing roller  27 . 
   Once the user completes printing on thick sheets  3  and wants to print on normal sheets  3  of thinner print paper, then the user presses the second operation lever  72  up manually with his or her finger. As a result, the second operation lever  72  swings upward around the swing shaft  76  and the abutment plate  71  swings downward. As a result, the compression against the second spring  63  is released so that the urging force of the first spring  37  raises the linear movement rod  61  upward to separate from the abutment lever  44 . Afterwards, the linear movement rod  61  is pulled upward by the urging force of the suspension spring  64  until the first portion  71   a  of the abutment plate  71  comes into abutment with the reference abutment portion  90 . In this manner, components revert to the non-operating condition, that is, to the first pressing condition where the predetermined pressing force is generated between the pressing roller  27  and the thermal roller  26 . 
   It should be noted that as the user presses the second operation lever  72  up, the abutment plate  71  changes from abutting the reference abutment portion  90  with the second portion  71   c  as shown in FIG.  11 ( c ), to abutting the reference abutment portion  90  with the first angled portion  71   b  as shown in FIG.  11 ( b ), and then to abutting the reference abutment portion  90  with the first portion  71   a . Also, simultaneously with this, the swing shaft  76  changes from abutting the upper surface  83  of the slide member  80  with the fourth portion  76   a  as shown in FIG.  11 ( c ), to abutting the upper surface  83  with the second angle portion  76   b  as shown in FIG.  11 ( b ), and then to abutting the upper surface  83  with the third portion  76   c  as shown in FIG.  11 ( a ). 
   Next, the user closes the rear cover  58  and prints using sheets  3  of normal paper. During printing, toner images are fixed on the sheets  3  by the predetermined pressure between the thermal roller  26  and the pressing roller  27  and discharged onto the discharge tray  31 . 
   Here, if the laser printer  1  will not properly fix images onto normal sheets  3  if the user forgets to switch into the non-operating condition after printing on thick sheets  3  and before printing on normal sheets  3 . However, in the embodiment, even if the user forgets to press the second operation lever  72  upward, the laser printer  1  will switch to the non-operating condition automatically when the rear cover  58  is closed shut. That is, in the operating condition shown in  FIG. 10  the second operation lever  72  abuts against the inner surface of the rear cover  58 . Therefore, when the rear cover  58  is closed shut, the second operation lever  72  presses against the second operation lever  72  and returns the second operation lever  72  into the non-operating condition. Accordingly, images can be properly fixed onto normal sheets  3 . 
   In this way, by switching the second operation lever  72  into the non-operating condition, the pressing roller  27  presses the thermal roller  26  with the predetermined pressure in the first pressing condition. Therefore, images can be properly fixed onto normal sheets. On the other hand, by switching the second operation lever  72  into the operating condition, the pressing roller  27  will press against the thermal roller  26  in the second pressing condition with a pressing force that is smaller than the predetermined pressing force. Therefore, images can be properly formed on thick sheets, such as envelops. 
   Also, while the pressure changing mechanism  60  is in the operating condition, the linear movement rod  61  urges the roller support member  36  downward by only the urging force of the second spring  63 . Therefore, the urging force applied by the first spring  37  to the roller support member  36  can be reduced by a fixed rate using a simple configuration so that the stable pressing force can be reliably reduced. 
   That is, when fixing images onto normal sheets  3  of paper, the pressing force between the pressing roller  27  and the thermal roller  26  can be designated by the urging force of only the first spring  37 . Therefore, variation among laser printers in pressing force used to fix images on normal paper can be reduced compared to conventional printers that normally use two springs on each side of the pressing roller to generating pressing force during image fixation. Also, when fixing images onto thick sheets, the pressing force can be designated by the urging force of the first spring  37  and the urging force of the second spring  63 , which operates against the urging force of the first spring  37 . Therefore, variation among laser printers in the pressing force generated while fixing images onto thick sheets can be less than when the pressing force is switched by the stroke of a rod member as in the conventional technology. 
   Also, because the user can manually operate the second operation lever  72  to switch between the operating condition and the non-operating condition, this switching operation can be performed reliably at any desired time. 
   While the abutment portion  65 A is in the operating position, the position of the slide member  80  is regulated by the abutment plate  71 , whose position is constantly restricted by the reference abutment portion  90 . Therefore, the position of the slide member  80  can be properly regulated using a simple configuration. By attaching the second spring  63  to the slide member  80 , the urging force of the second spring  63  against the roller support member  36  can be reliably stable. 
   The first angled portion  71   b  is formed between the first portion  71   a  and the second portion  71   c . Therefore, the abutment plate  71  will always swing into abutment with the reference abutment portion  90  by either the second portion  71   c  or the first portion  71   a . That is, even though the first angled portion  71   b  abuts against the reference abutment portion  90  while the second operation lever  72  is being switched between the operating condition and the non-operating condition, the condition is not stable so the abutment plate  71  will swing one way or the other. For this reason, the second operation lever  72  stably supports the abutment portion  65 A in the operating condition so that the switching operation is more reliably and stably performed. 
   Furthermore, the second portion  71   c  is formed with an arched shape that follows the swinging path of the abutment plate  71 . Therefore, the second operation lever  72  will swing smoothly when the linear movement rod  61  is switched from the operating position to the non-operating position by the second operation lever  72 . For this reason, the switching operation can be even more reliably and stably performed. 
   Further, the second angle portion  76   b  is formed between the third portion  76   c  and the fourth portion  76   a  of the swing shaft  76  of the abutment plate  71 . Therefore, even if the second angle portion  76   b  abuts against the slide member  80 , the condition is unstable so the swing shaft  76  will rotate to abut the slide member  80  with either the third portion  76   c  or the fourth portion  76   a . For this reason, the linear movement rod  61  is stably held in either the non-operating position or the operating position. Therefore, the switching operation can be even more reliably and stably performed. 
   Also, the second operation lever  72  swings centered around the swing shaft  76 . Therefore, by forming the swing shaft  76  to such a shape, the second operation lever  72  can be switched smoothly with only a light force. Therefore, the switching operation can be even more stably performed. 
   Although the embodiment describes a pressure changing mechanism  60  as being provided to both sides of the fixing unit  18 , even if one is in the operating condition and the other is in the non-operating condition, pressing force that prevents paper jams can be set without obstructing the passage of sheets. 
   While some exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention. 
   For example, only a single pressure changing mechanism  60  could be provided for moving the entire pressing roller  27  with respect to the thermal roller  26 .