Patent Publication Number: US-9886000-B2

Title: Image forming apparatus and fixing device controller

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-044125 filed Mar. 8, 2016. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including a fixing device that includes a pressing unit and a heating unit provided facing the pressing unit, an image-forming-apparatus body in which the fixing device is housed and to and from which the fixing device is attachable and detachable, a fixing-device-attachment-detachment-detecting member that detects attachment or detachment of the fixing device to or from the image-forming-apparatus body, and a controller that controls, if the fixing-device-attachment-detachment-detecting member detects the detachment of the fixing device from the image-forming-apparatus body, the image-forming-apparatus body to take a position where the pressing unit and the heating unit are to be pressed against each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a side sectional view of an image forming apparatus according to the exemplary embodiment; 
         FIG. 2  is a side sectional view of a fixing device according to the exemplary embodiment; 
         FIG. 3  is a schematic diagram illustrating the fixing device and a switching unit according to the exemplary embodiment; 
         FIG. 4A  is a side view of the fixing device when the switching unit is positioned such that a flat cam and a movable frame body are spaced apart from each other; 
         FIG. 4B  is a side view of the fixing device when the switching unit is positioned such that the flat cam pushes the movable frame body with a force weaker than a normal force; 
         FIG. 4C  is a side view of the fixing device when the switching unit is positioned such that the flat cam pushes the movable frame body with the normal force; 
         FIG. 5  is a side view of the fixing device that is about to be attached to an image-forming-apparatus body, with the flat cam of the switching unit being at the position for pushing the movable frame body; 
         FIG. 6  is a flow chart illustrating a printing process according to the exemplary embodiment; and 
         FIGS. 7A and 7B  are each a flow chart illustrating a part of the flow chart illustrated in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 to 7B , an image forming apparatus  10  according to an exemplary embodiment of the present invention will now be described. As illustrated in  FIG. 1 , then image forming apparatus  10  according to the present exemplary embodiment includes an image-forming-apparatus body  12 , in which at least one sheet-feeding unit  18 , an image forming unit  14 , and a fixing device  36  are provided. The image-forming-apparatus body  12  has, at the top thereof, an output port  34 . A transport path  32  along which a recording medium  130 , which is a transfer object such as recording paper, is transported extends in such a manner as to connect the sheet-feeding unit  18 , the image forming unit  14 , the fixing device  36 , and the output port  34 . The image-forming-apparatus body  12  has a covering member  13  that is opened or closed when the fixing device  36  is attached to or detached from the image-forming-apparatus body  12 . A controller  170  that controls various operations is provided in the image-forming-apparatus body  12  (see  FIG. 3 ). The image forming apparatus  10  according to the present exemplary embodiment is capable of printing on different kinds of recording media  130 , including a recording medium  130  for normal printing, and an envelope. Details of the above elements will further be described. 
     The sheet-feeding unit  18  includes a sheet-feeding-unit body  20 , a sheet cassette  22  that contains recording media  130  as transfer objects, a pickup roller  24  that picks up some of the recording media  130  stacked in the sheet cassette  22 , and a pair of a feed roller  28  and a retard roller  26  that separate one of the picked recording media  130  from the others and feed the recording medium  130  into the transport path  32 . With the rotation of the pickup roller  24 , the feed roller  28 , and the retard roller  26 , the recording media  130  stacked in the sheet cassette  22  are each transported along the transport path  32  to a pair of registration rollers  38  to be described later. The sheet cassette  22  is detachably attached to the sheet-feeding-unit body  20 . The image forming apparatus  10  includes at least one sheet-feeding unit  18 . That is, the image forming apparatus  10  may include plural sheet-feeding units  18 . In the present exemplary embodiment, the image forming apparatus  10  includes two sheet-feeding units  18  as illustrated in  FIG. 1 . 
     The image forming unit  14  employs an electrophotographic method and includes an image carrier  44  as a photosensitive member, a charging device  56  provided in the form of a charging roller or the like and that uniformly charges the image carrier  44 , an optical drawing device  58  that draws a latent image with light on the image carrier  44  charged by the charging device  56 , a developing device  60  including a developing roller for visualizing the latent image formed on the image carrier  44  by the optical drawing device  58  into a toner image by using toner, a transfer device  42  provided in the form of a transfer roller or the like and transfers the toner image developed by the developing device  60  to the recording medium  130 , a cleaning device  62  that removes residual toner particles from the image carrier  44 , and the fixing device  36  that fixes the toner image transferred to the recording medium  130  by the transfer device  42  on the recording medium  130 . 
     The optical drawing device  58  is, for example, a scanning laser exposure device and forms a latent image on the image carrier  44  while moving along a process cartridge  64  to be described later. The optical drawing device  58  may alternatively be a light-emitting diode (LED), a surface-emission laser, or the like. 
     The process cartridge  64  is a unit including the image carrier  44 , the charging device  56 , the developing device  60 , and the cleaning device  62  and is interchangeable as the unit. 
     The transport path  32  is a passageway for the recording medium  130  and extends from the pickup roller  24  of the sheet-feeding unit  18  at the bottom of the image-forming-apparatus body  12  to the output port  34  at the top of the image-forming-apparatus body  12 . A portion of the transport path  32  in the image-forming-apparatus body  12  extends substantially vertically from the pickup roller  24  of the sheet-feeding unit  18  at the bottom to the fixing device  36 . 
     The transfer device  42  and the image carrier  44  are provided on the upstream side of the fixing device  36  on the transport path  32 , and the pair of registration rollers  38  are provided on the upstream side of the transfer device  42  and the image carrier  44  on the transport path  32 . Furthermore, a pair of discharge rollers  40  are provided on the transport path  32  near the output port  34 . 
     Hence, one of the recording media  130  picked up from the sheet cassette  22  of the sheet-feeding unit  18  by the pickup roller  24  is separated from the other recording media  130  by the retard roller  26  and the feed roller  28 , is guided into the transport path  32 , is stopped temporarily by the pair of registration rollers  38 , and passes through the nip between the transfer device  42  and the image carrier  44  at an appropriate timing, whereby a toner image is transferred to the recording medium  130 . Then, the toner image is fixed by the fixing device  36 . Eventually, the recording medium  130  having the fixed toner image is discharged from the output port  34  by the pair of discharge rollers  40  and is output to an output portion  16 . 
     Referring now to  FIGS. 2 and 3 , the fixing device  36  includes a heating roller  66  as a heating unit, and a pressing unit  70 . The pressing unit  70  includes an endless belt  72  as an endless belt-like member, and a guide member  74  that guides the endless belt  72  such that the endless belt  72  is rotatable. The pressing unit  70  is pressed against the heating roller  66 . The fixing device  36  further includes a switching unit  140  and is capable of changing the pressing force, which is applied to the heating roller  66 , by moving the pressing unit  70  relative to the heating roller  66 . In the present exemplary embodiment, the pressing force exerted by the pressing unit  70  of the fixing device  36  is changeable among a normal pressing force applied in a normal case, a weak pressing force weaker than the normal pressing force, and a zero pressing force in a case where the pressing force is removed. The changing of the pressing force of the fixing device  36  by the switching unit  140  is controlled by the controller  170  provided in the image-forming-apparatus body  12 . 
     The heating roller  66  includes a cylindrical roller portion  84 , and a heater  86  provided in the roller portion  84 . The roller portion  84  is rotatably supported by heating-roller bearings (not illustrated) and is rotatable in a direction of arrow A (see  FIG. 2 ). The roller portion  84  includes a core  88  made of metal such as iron, stainless steel, or aluminum, and a release layer  90  provided over the core  88 . The roller portion  84  is a so-called hard roller having no elastic layer. The release layer  90  is made of an insulating material having high releasability, such as perfluoroalkoxy alkane (PFA). A releasing member  92  is provided in contact with the roller portion  84 . 
     The heater  86  includes, for example, two lamp heaters. A thermostat  94  is provided across the heating roller  66  from the endless belt  72  in such a manner as to face the heating roller  66 . 
     The endless belt  72  is present between the heating roller  66  and the pressing unit  70  and rotates in a direction of arrow C (see  FIG. 2 ) by following the rotation of the heating roller  66 . Since the pressing unit  70  is configured such that the endless belt  72  is pressed against the heating roller  66 , a nip area  102  is provided. The toner image formed on the recording medium  130  is fixed in the nip area  102 . The endless belt  72  includes a belt member made of synthetic resin such as polyimide, and a release layer made of PFA or the like provided over the belt member. 
     An inlet chute  96  is provided on the upstream side of the nip area  102 . The recording medium  130  transported in a direction of arrow B is guided by the inlet chute  96  such that the leading end thereof is introduced into the nip area  102  (see  FIG. 2 ). An outlet chute  98  is provided on the downstream side of the nip area  102 . 
     As illustrated in  FIG. 2 , the pressing unit  70  further includes a holder  100  provided on the inner side of the endless belt  72 , a pressing member  110  attached to the holder  100 , and frames  78  and  79  that support the holder  100 . Furthermore, an impregnated pad  80  that supplies lubricant to the endless belt  72  is provided on the inner side of the endless belt  72 . 
     The nip area  102  provided by the pressing unit  70  pressed against the heating roller  66  is defined by the pressing member  110  attached to the holder  100 . The holder  100  and the frames  78  and  79  of the pressing unit  70  cooperate to cause the pressing member  110  and the holder  100  to be pressed against the heating roller  66 . 
     The holder  100  is made of a resin material such as liquid-crystal polymer (LCP) that is highly heat resistant. As illustrated in  FIG. 2 , the holder  100  has, on a side thereof facing the heating roller  66 , a seating surface  106  on which the pressing member  110  that defines the nip area  102  is seated. 
     The pressing member  110  has substantially the same length as the holder  100  in the longitudinal direction of the holder  100 . The pressing member  110  is a rectangular parallelepiped member made of a heat-resistant resin material such as silicon rubber. 
     The guide member  74 , illustrated in  FIG. 2 , is provided at each of two ends of the endless belt  72  (the pressing unit  70 ). The pressing unit  70  is supported by the guide members  74 . 
     As illustrated in  FIG. 2 , the frames  78  and  79  support the holder  100  and are each supported at two ends thereof by the respective guide members  74 , whereby the holder  100  is pressed toward the heating roller  66 . The ends of the frames  78  and  79  extend through the guide members  74  and are connected to a movable frame body  142  included in the switching unit  140 . The frames  78  and  79  are urged toward the heating roller  66  by urging members (not illustrated) such as springs. 
     Referring now to  FIG. 3 , the switching unit  140  includes a fixed frame body  148  that supports the heating roller  66  of the fixing device  36 , and the movable frame body  142  that is rotatably attached to a shaft portion  150  provided on the fixed frame body  148 . The frames  78  and  79  of the pressing unit  70  are fixed to the movable frame body  142 . 
     The movable frame body  142  and the fixed frame body  148  are connected to each other with a spring member  152  as an urging member that generates an urging force pulling the movable frame body  142  and the fixed frame body  148  toward each other. 
     The movable frame body  142  includes a long, narrow, stick-like arm portion  144  extending on a side thereof opposite the shaft portion  150 . A flat cam  160  and a rotating shaft  162  are provided near the arm portion  144 . The flat cam  160  serves as a pushing member that pushes and thus moves the arm portion  144  of the movable frame body  142 . The flat cam  160  is rotated about the rotating shaft  162 . The flat cam  160  has a shape defined by different distances from the center thereof to the outer periphery thereof. When the flat cam  160  is rotated about the rotating shaft  162 , the arm portion  144  of the movable frame body  142  is pushed by the flat cam  160 . Thus, the movable frame body  142  is moved by a length corresponding to the distance thereto from the center of the flat cam  160 . That is, the length of movement of the movable frame body  142  is variable. In accordance with such variations, the pressing force of the fixing device  36  is changed. The rotation of the rotating shaft  162  is controlled by the controller  170  provided in the image-forming-apparatus body  12 . 
     In the image forming apparatus  10  according to the present exemplary embodiment, the pressing force of the fixing device  36  is adjusted, i.e., changed, by the switching unit  140  as described above. Specifically, when a point of the flat cam  160  where the distance from the center of the flat cam  160  to the outer periphery of the flat cam  160  is shortest faces the arm portion  144  of the movable frame body  142  as illustrated in  FIG. 4A , the outer periphery of the flat cam  160  is out of contact with the movable frame body  142 . Hence, the arm portion  144  of the movable frame body  142  is not pushed by the flat cam  160 . Therefore, the movable frame body  142  is pulled by the spring member  152  toward the fixed frame body  148  in such a manner as to rotate about the shaft portion  150 . Thus, the pressing unit  70  connected to the movable frame body  142  and to the frames  78  and  79  is pressed against the heating roller  66 . The pressing force generated in this state is regarded as, for example, a pressing force for normal printing. Note that the point of the flat cam  160  that faces the arm portion  144  of the movable frame body  142  is not limited to the point of the flat cam  160  where the distance from the center to the outer periphery is shortest and only needs to be a point where the flat cam  160  does not push the arm portion  144  of the movable frame body  142  or where a gap is produced between the flat cam  160  and the arm portion  144  of the movable frame body  142 . 
     When a point of the flat cam  160  where the distance from the center to the outer periphery is longest faces the movable frame body  142  as illustrated in  FIG. 4C , the arm portion  144  of the movable frame body  142  is pushed by the outer periphery of the flat cam  160 . Hence, the movable frame body  142  is moved such that the pressing unit  70  and the heating roller  66  are spaced apart from each other against the urging force exerted by the spring member  152 . That is, no force of pressing the pressing unit  70  against the heating roller  66  is generated. Such a state where no pressing force is generated is established if a predetermined gap needs to be provided between the pressing unit  70  and the heating roller  66  so that a malfunction, if any occurs, is easily removed. Exemplary malfunctions include a situation where a recording medium  130  jammed in the image-forming-apparatus body  12  needs to be removed, or a situation where the covering member  13  of the image-forming-apparatus body  12  is opened. 
     When a point of the flat cam  160  between the two points where the distance from the center to the outer periphery is shortest and longest faces the arm portion  144  of the movable frame body  142  as illustrated in  FIG. 4B , the movable frame body  142  is pushed and moved by the outer periphery of the flat cam  160 . However, the length of movement of the movable frame body  142  is shorter than that in the case where the force for normal printing is applied from the pressing unit  70  to the heating roller  66 . Therefore, the pressing force of the pressing unit  70  applied to the heating roller  66  is weaker than the pressing force for normal printing. Such a pressing force weaker than the normal pressing force is used in, for example, performing printing on an envelope. 
     The flat cam  160  and the rotating shaft  162  are provided on the image-forming-apparatus body  12 . Therefore, even if the fixing device  36  is detached from the image-forming-apparatus body  12 , the flat cam  160  and the rotating shaft  162  remain in the image-forming-apparatus body  12 . The flat cam  160  and the rotating shaft  162  are rotated by a driving device (not illustrated), such as a motor, controlled by the controller  170  of the image forming apparatus  10 . 
     In the image forming apparatus  10  according to the present exemplary embodiment, the controller  170  changes the pressing force of the fixing device  36  by rotating the rotating shaft  162  and thus the flat cam  160  in accordance with signals transmitted thereto from a malfunction detector  172  and a fixing-device-attachment/detachment detector  174  to be described later. 
     Referring basically to  FIGS. 4A to 4C, 6, and 7A and 7B , a printing process performed by the image forming apparatus  10  according to the present exemplary embodiment will now be described along with an operation of controlling the switching unit  140 . Note that the image forming apparatus  10  according to the present exemplary embodiment is capable of performing normal printing and envelope printing. 
     When a printing process of the image forming apparatus  10  according to the present exemplary embodiment is started (step S 1 ), the image forming apparatus  10  checks whether the current printing job is for normal printing or for envelope printing (step S 2 ). This step may be performed by, for example, the user before the printing process is started or by using a sensor or the like that detects the type of the recording medium  130 . 
     A process of normal printing will first be described. If it is determined that normal printing is to be performed (if NORMAL in step S 2 ), the flat cam  160  of the switching unit  140  is rotated such that the pressing unit  70  of the fixing device  36  applies the pressing force for normal printing to the heating roller  66  (step S 3 ). In this step, the flat cam  160  is positioned such that the point where the distance from the center to the outer periphery is shortest faces the arm portion  144  of the movable frame body  142 , that is, the movable frame body  142  is not pushed by the flat cam  160  (see  FIG. 4A ). 
     After the pressing force of the fixing device  36  is adjusted for normal printing by the switching unit  140 , a normal printing job is started (step S 4 ). 
     While the normal printing job is being performed, the malfunction detector  172  keeps detecting any malfunction of the fixing device  36  and checks whether or not there is any malfunction occurring in the fixing device  36  (step S 5 ). Exemplary malfunctions detected during the printing process include jamming of the recording medium  130 , or the opening of the covering member  13  of the image-forming-apparatus body  12 . That is, the malfunction is regarded as a situation where the fixing device  36  has become incapable of performing or difficult to perform the normal operation. The detection of any malfunction is performed by, for example, receiving a signal from a sensor or the like provided to the fixing device  36 , the covering member  13 , or any other associated element. 
     If no malfunction of the fixing device  36  is recognized while the normal printing job is performed (NO in step S 5 ), it is checked whether or not the current normal printing job is finished (step S 6 ). If the current normal printing job is not finished (NO in step S 6 ), the detection of any malfunction is continued (step S 5 ) until the normal printing job ends. 
     On the other hand, if any malfunction of the fixing device  36  is recognized while the normal printing job is performed (YES in step S 5 ), the printing job is suspended (step S 7 ). In this step, the occurrence of a malfunction in the fixing device  36  is notified by means of a sound, a screen display (not illustrated), or the like. 
     After the normal printing job is suspended, the flat cam  160  of the switching unit  140  is rotated such that the point where the distance from the center to the outer periphery is longest pushes the arm portion  144  of the movable frame body  142 . Thus, the pressing force applied to the heating roller  66  by the pressing unit  70  of the fixing device  36  is removed (step S 8 ). That is, since the above rotation of the flat cam  160  pushes the movable frame body  142 , a gap is produced between the pressing unit  70  and the heating roller  66 , whereby a state where no pressing force is generated is established (see  FIG. 4C ). 
     Subsequently, the detection of any malfunction of the fixing device  36  is performed by the malfunction detector  172  (step S 9 ). If no malfunction of the fixing device  36  is recognized (NO in step S 9 ), the normal printing job is restarted. In that case, as described above, the flat cam  160  of the switching unit  140  is rotated again such that the pressing unit  70  of the fixing device  36  is pressed against the heating roller  66  with the pressing force for normal printing (step S 3 ). 
     On the other hand, if any malfunction of the fixing device  36  is recognized and it is determined that the malfunction of the fixing device  36  is yet to be removed (YES in step S 9 ), the fixing-device-attachment/detachment detector  174  detects the presence of the fixing device  36  (step S 10 ) so as to check whether or not the fixing device  36  has been detached from the image-forming-apparatus body  12 . If the presence of the fixing device  36  is recognized (YES in step S 10 ) and it is determined that the fixing device  36  remains in the image-forming-apparatus body  12 , the detection of any malfunction of the fixing device  36  is continued (step S 9 ) until the malfunction is removed. 
     If the presence of the fixing device  36  is not recognized (NO instep S 10 ) and it is determined that the fixing device  36  has been detached from the image-forming-apparatus body  12 , the pressing unit  70  of the fixing device  36  is pressed against the heating roller  66 . That is, the flat cam  160  of the switching unit  140  is rotated such that the point where the distance from the center to the outer periphery is shortest faces the arm portion  144  of the movable frame body  142 . Thus, the flat cam  160  and the movable frame body  142  are spaced apart from each other, whereby the pressing force for normal printing is generated (step S 11 , see  FIG. 4A ). 
     In this step, even if any malfunction of the fixing device  36  is recognized and the flat cam  160  stays at the position for pushing the movable frame body  142 , the fixing device  36  is detachable from the image-forming-apparatus body  12 . However, when the fixing device  36  is reattached to the image-forming-apparatus body  12 , the movable frame body  142  that has returned to the initial position under the urging force exerted by the spring member  152  interferes with the flat cam  160  provided in the image-forming-apparatus body  12  (see  FIG. 5 ). 
     Hence, the flat cam  160  is rotated in advance to the position where the pressing force for normal printing is generated. Thus, a gap is produced between the flat cam  160  and the movable frame body  142 . 
     Subsequently, the presence of the fixing device  36  is detected by the fixing-device-attachment/detachment detector  174  (step S 12 ), and it is checked whether or not the fixing device  36  has been attached to the image-forming-apparatus body  12 . If the presence of the fixing device  36  is not recognized (NO in step S 12 ) and it is determined that the fixing device  36  is still out of the image-forming-apparatus body  12 , the presence of the fixing device  36  continues to be detected until it is determined that the fixing device  36  has been attached to the image-forming-apparatus body  12 . 
     On the other hand, if the presence of the fixing device  36  is recognized (YES in step S 12 ) and it is determined that the fixing device  36  has been attached to the image-forming-apparatus body  12 , the flat cam  160  of the switching unit  140  is rotated again from the position for normal printing to the position taken when the malfunction has been recognized, that is, the flat cam  160  is rotated such that the point where the distance from the center to the outer periphery is longest pushes the movable frame body  142  (step S 8 , see  FIG. 4C ) so that the pressing unit  70  and the heating roller  66  are spaced apart from each other. Then, the detection of any malfunction of the fixing device  36  is performed by the malfunction detector  172  again (step S 9 ). 
     Subsequently, the detection of any malfunction of the fixing device  36  by the malfunction detector  172  is continued until the malfunction of the fixing device  36  is removed. Then, if no more malfunction of the fixing device  36  is recognized (NO in step S 9 ), the normal printing job is restarted, as described above (step S 3  and thereafter, see  FIG. 4A ). 
     If the current normal printing job is finished (YES in step S 6 ), it is checked whether or not all printing jobs are finished (step S 13 ). If all printing jobs are not finished (NO in step S 13 ), the printing process is continued. In that case, it is checked whether the subsequent printing job is for normal printing or for envelope printing (step S 2 ). 
     If all printing jobs are finished (YES in step S 13 ), the whole printing process ends (step S 14 ). 
     Now, a process of envelope printing will be described. Redundant description of the steps that are common to those for the process of normal printing is omitted. 
     If it is determined that envelope printing is to be performed (if ENVELOPE in step S 2 ), the flat cam  160  of the switching unit  140  is rotated such that the pressing unit  70  of the fixing device  36  is pressed against the heating roller  66  with the pressing force weaker than the pressing force for normal printing (step S 15 ). In this step, the flat cam  160  is positioned such that the point between the two points where the distance from the center to the outer periphery is shortest and longest pushes the arm portion  144  of the movable frame body  142  (see  FIG. 4B ). 
     After the pressing force of the fixing device  36  is adjusted for envelope printing by the switching unit  140 , an envelope printing job is started (step S 16 ). 
     While the envelope printing job is being performed, the malfunction detector  172  keeps detecting any malfunction of the fixing device  36  and checks whether or not there is any malfunction occurring in the fixing device  36  (step S 17 ), as with the case of normal printing. 
     If no malfunction of the fixing device  36  is recognized while the envelope printing job is performed (NO in step S 17 ), it is checked whether or not the current envelope printing job is finished (step S 18 ). If the current envelope printing job is not finished (NO in step S 18 ), the detection of any malfunction of the fixing device  36  is continued (step S 17 ) until the envelope printing job is finished. 
     On the other hand, if any malfunction of the fixing device  36  is recognized while the envelope printing job is performed (YES in step S 17 ), the printing job is suspended (step S 19 ). 
     After the envelope printing job is suspended, the pressing force applied to the heating roller  66  by the pressing unit  70  of the fixing device  36  is removed, as with the case of normal printing. That is, the flat cam  160  of the switching unit  140  is rotated such that the point where the distance from the center to the outer periphery is longest pushes the arm portion  144  of the movable frame body  142  (step S 20 , see  FIG. 4C ). 
     If no malfunction of the fixing device  36  is recognized (NO in step S 21 ) as a result of detection by the malfunction detector  172  (step S 21 ) and if it is determined that the malfunction of the fixing device  36  has been removed, the envelope printing job is restarted. In that case, as described above, the flat cam  160  of the switching unit  140  is rotated again such that the pressing unit  70  of the fixing device  36  is pressed against the heating roller  66  with the pressing force for envelope printing (step S 15 ). 
     On the other hand, if any malfunction of the fixing device  36  is recognized (YES in step S 21 ) and it is determined that the malfunction of the fixing device  36  is yet to be removed, the fixing-device-attachment/detachment detector  174  detects the presence of the fixing device  36  (step S 22 ), as with the case of normal printing, so as to determine whether or not the fixing device  36  has been detached from the image-forming-apparatus body  12 . If the presence of the fixing device  36  is recognized (YES in step S 22 ) and it is determined that the fixing device  36  remains in the image-forming-apparatus body  12 , the checking of whether or not the malfunction of the fixing device  36  has been removed is continued (step S 21 ). 
     If the presence of the fixing device  36  is not recognized (NO in step S 22 ) and it is determined that the fixing device  36  has been detached from the image-forming-apparatus body  12 , the flat cam  160  of the switching unit  140  is rotated such that the point where the distance from the center to the outer periphery is shortest faces the arm portion  144  of the movable frame body  142 , whereby the pressing force for normal printing is generated (step S 23 , see  FIG. 4A ). 
     Positioning the flat cam  160  such that the pressing force is adjusted for normal printing produces a gap between the flat cam  160  and the movable frame body  142 . 
     Subsequently, the presence of the fixing device  36  is detected by the fixing-device-attachment/detachment detector  174  (step S 24 ), and it is checked whether or not the fixing device  36  has been attached to the image-forming-apparatus body  12 . If the presence of the fixing device  36  is not recognized (NO in step S 24 ) and it is determined that the fixing device  36  is still out of the image-forming-apparatus body  12 , the presence of the fixing device  36  continues to be detected until it is determined that the fixing device  36  has been attached to the image-forming-apparatus body  12 . 
     On the other hand, if the presence of the fixing device  36  is recognized (YES in step S 24 ) and it is determined that the fixing device  36  has been attached to the image-forming-apparatus body  12 , the flat cam  160  of the switching unit  140  is rotated again from the position for normal printing to the position for removing the malfunction, that is, the flat cam  160  is rotated such that the point where the distance from the center to the outer periphery is longest pushes the movable frame body  142  (step S 20 , see  FIG. 4C ). Then, the detection of any malfunction of the fixing device  36  is performed by the malfunction detector  172  again (step S 21 ). 
     Subsequently, the detection of any malfunction of the fixing device  36  by the malfunction detector  172  is continued until the malfunction is removed. Then, if no more malfunction of the fixing device  36  is recognized (NO in step S 21 ), the envelope printing job is restarted, as described above (step S 15  and thereafter, see  FIG. 4B ). 
     If the current envelope printing job is finished (YES in step S 18 ), it is checked whether or not all printing jobs are finished (step S 13 ). If all printing jobs are not finished, the printing process is continued (NO in step S 13 ). In that case, it is checked whether the subsequent printing job is for normal printing or for envelope printing (step S 2 ). 
     Subsequently, if all printing jobs are finished (YES in step S 13 ), the whole printing process ends (step S 14 ). 
     This is the end of the whole printing process performed by the image forming apparatus  10  according to the present exemplary embodiment. 
     The foregoing description of the exemplary embodiment of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiment was chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.