Abstract:
A fixing device includes first and second contact bodies that apply a pressure to a recording medium to fix an unfixed image on the recording medium; a pressing member that is movable between a first position at which the pressing member is in contact with the first contact body and a second position at which the pressing member is separated from the first contact body; a first urging member that urges the pressing member to apply a force to the first contact body; a guide member that controls a direction in which the first contact body moves; a second urging member that is interposed between the pressing member and the guide member and that urges the guide member to apply a force to the first contact body; and a changer that selectively changes a position of the pressing member between the first and second positions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2015-055506 filed Mar. 19, 2015. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a fixing device, an image forming apparatus, a pressure-applying device, and a pressing device. 
     (ii) Related Art 
     Electrophotographic image forming apparatuses, for example, generally include a fixing device that fixes an image to a recording medium, such as a paper sheet. The fixing device applies a pressure to the recording medium to fix the image to the recording medium. The fixing device is generally provided with a pressing device, which generates a pressing force by pressing a member that is in contact with the recording medium against the recording medium or another member. There has been a demand for an operational mode in which an envelope is used as the recording medium, and there has also been a demand for a function of setting the pressing force applied in the fixing process such that the pressing force is smaller in an envelope mode, in which an envelope is used as the recording medium, than in a normal mode, in which a paper sheet is used as the recording medium. 
     SUMMARY 
     According to an aspect of the invention, there is provided a fixing device including plural contact bodies including a first contact body and a second contact body and having peripheral surfaces that are in contact with each other, the contact bodies applying a pressure to a recording medium by allowing the recording medium to pass through a space between the peripheral surfaces, thereby fixing an unfixed image on the recording medium; a pressing member that applies a force to the first contact body so that the peripheral surface of the first contact body is pressed against the peripheral surface of the second contact body, the pressing member being movable between a first position at which the pressing member is in contact with the first contact body and a second position at which the pressing member is separated from the first contact body; a first urging member that urges the pressing member to apply a force to the first contact body through the pressing member; a guide member having a guide groove that receives a portion of the first contact body to control a direction in which the first contact body moves; a second urging member that is interposed between the pressing member and the guide member and that urges the guide member to apply a force to the first contact body through the guide member; and a changer that selectively changes a position of the pressing member between the first position and the second position. 
    
    
     
       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 schematic diagram of a printer, which is an example of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a sectional view of a fixing unit; 
         FIG. 3  illustrates the structure of pressing mechanisms; 
         FIG. 4  illustrating the details of a pressing plate and a guide plate; 
         FIG. 5  illustrates the state of a pressing mechanism in a normal mode; 
         FIG. 6  illustrates the state of the pressing mechanism in an envelope mode; and 
         FIG. 7  illustrates the state of the pressing mechanism in a release mode. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  is a schematic diagram of a printer  10 , which is an example of an image forming apparatus according to an exemplary embodiment of the present invention. 
     The printer  10  illustrated in  FIG. 1  is a monochrome printer. An image signal that is created outside the printer  10  and represents an image is input to the printer  10  through a signal cable (not shown) or the like. The printer  10  includes a controller  11  that controls the operation of components included in the printer  10 , and the image signal is input to the controller  11 . The printer  10  forms an image based on the image signal under the control of the controller  11 . 
     Paper trays  21  are disposed in a lower section of the printer  10 . Each paper tray  21  contains a stack of paper sheets P. The paper trays  21  are capable of being pulled out when the paper sheets P are to be supplied thereto. Instead of the paper sheets P, OHP sheets, plastic paper sheets, envelopes, etc., may be contained in the paper trays  21  as recording media according to an exemplary embodiment of the present invention. The operation of the printer  10  will be described with reference to  FIG. 1  on the assumption that the paper sheets P are contained. However, the basic operation is the same for the cases in which other recording media are contained. 
     The paper sheets P contained in the paper trays  21  are transported to standby rollers  24  by pick-up rollers  22  and separation rollers  23 . Each paper sheet P that has reached the standby rollers  24  is further transported at an adjusted time. 
     The printer  10  includes a columnar photoconductor  12  that rotates in the direction shown by arrow A at a location above the standby rollers  24 . A charging device  13 , an exposure device  14 , a developing device  15 , a transfer device  16 , and a photoconductor cleaner  17  are arranged around the photoconductor  12 . The assembly including the photoconductor  12 , the charging device  13 , the exposure device  14 , the developing device  15 , and the transfer device  16  is an example of an image-forming unit according to an exemplary embodiment of the present invention. 
     The charging device  13  charges a surface of the photoconductor  12 . The exposure device  14  irradiates the surface of the photoconductor  12  with light in accordance with the image signal transmitted from the controller  11 , thereby forming an electrostatic latent image. The electrostatic latent image is developed into a toner image by the developing device  15 . 
     The above-described standby rollers  24  feed the paper sheet P so that the paper sheet P reaches a position where the photoconductor  12  faces the transfer device  16  at a time when the toner image on the photoconductor  12  reaches that position. The transfer device  16  transfers the toner image on the photoconductor  12  onto the paper sheet P that has been fed. Thus, an unfixed toner image is formed on the paper sheet P. 
     The paper sheet P on which the unfixed toner image is formed is further transported in the direction of arrow B, and a fixing unit  18  fixes the toner image to the paper sheet P by applying heat and pressure to the paper sheet P. As a result, a fixed toner image is formed on the paper sheet P. The fixing unit  18  is an example of a fixing device or a pressure-applying device according to an exemplary embodiment of the present invention. 
     The paper sheet P that has passed through the fixing unit  18  is further transported toward an output unit  19  in the direction of arrow C, and is ejected to an output tray  20  in the direction of arrow D by the output unit  19 . 
       FIG. 2  is a sectional view of the fixing unit  18 . 
     The fixing unit  18  includes a heating roller  110  and a pressure-applying member  120 . The pressure-applying member  120  is an example of a first contact body according to an exemplary embodiment of the present invention, and the heating roller  110  is an example of a second contact body according to an exemplary embodiment of the present invention. 
     The heating roller  110  includes a heat source lamp  111  and an outer body  112  that is formed by covering a metal core with rubber and that rotates. 
     The pressure-applying member  120  includes an outer belt  121 . A guide  122 , support plates  123 , and a pressure-applying pad  124  are disposed inside the outer belt  121 . The guide  122  is made of a resin, and is configured to guide a rotational movement of the outer belt  121 . 
     The pressure-applying pad  124  presses the outer belt  121  against the heating roller  110  from the inner side of the outer belt  121 . The recording medium receives pressure and heat when the recording medium passes through a region (nip region) in which the outer belt  121  is sandwiched between the pressure-applying pad  124  and the heating roller  110  and pressed against the outer surface of the heating roller  110 . 
     The support plates  123  are made of a metal, and support the integrated inner structure of the pressure-applying member  120 . When a force is applied to the support plates  123  in a direction toward the heating roller  110 , a pressing force that is applied through the pressure-applying pad  124  and the outer belt  121  is generated. The support plates  123  extend beyond the other components of the pressure-applying member  120  at both the near and far ends of the fixing unit  18  in  FIG. 2 . A pressing mechanism  130  that presses the pressure-applying member  120  against the heating roller  110  through the support plates  123  is assembled to the fixing unit  18  at each end of the fixing unit  18 . The pressing mechanisms  130  are an example of a pressing device according to an exemplary embodiment of the present invention. 
       FIG. 3  illustrates the structure of the pressing mechanisms  130 . 
     Each pressing mechanism  130  includes a pressing plate  131 , a normal-mode spring  132 , an envelope-mode spring  133 , and a guide plate  134 . Each pressing mechanism  130  also includes a switch for switching a mode of the pressing mechanism  130 . However, the switch is not illustrated in  FIG. 3 . 
     The pressing plate  131  is an example of a pressing member according to an exemplary embodiment of the present invention, the normal-mode spring  132  is an example of a first urging member according to an exemplary embodiment of the present invention, the envelope-mode spring  133  is an example of a second urging member according to an exemplary embodiment of the present invention, the guide plate  134  is an example of a guide member according to an exemplary embodiment of the present invention, and the switch is an example of a changer according to an exemplary embodiment of the present invention. 
     Each of the pressing plate  131  and the guide plate  134  is rotatable around a fulcrum  135 . 
     The guide plate  134  has guide grooves  136  that receive and guide the support plates  123 . 
     The normal-mode spring  132  is attached to the pressing plate  131  at one end thereof so as to urge the pressing plate  131 . The envelope-mode spring  133  is retained in a central region of the pressing plate  131 , and one end of the envelope-mode spring  133  is attached to the guide plate  134  so as to urge the guide plate  134 . Since the envelope-mode spring  133  is disposed between the pressing plate  131  and the guide plate  134 , the size of the pressing mechanism  130  is reduced. 
     The normal-mode spring  132  is a tension spring, and urges the pressing plate  131  downward in  FIG. 3 . The envelope-mode spring  133  is a torsion spring having a spring constant smaller than that of the normal-mode spring  132 , and urges the guide plate  134  downward in  FIG. 3 . 
       FIG. 4  illustrates the details of the pressing plate  131  and the guide plate  134 . 
     The pressing plate  131  includes an arch portion  137  that is pressed against the support plates  123  that are received by the guide grooves  136  in the guide plate  134 , and a projection  138  that projects toward the guide plate  134 . The projection  138  is an example of a contact portion according to an exemplary embodiment of the present invention. 
     The guide plate  134  has the guide grooves  136  that guide the support plates  123  and a through hole  139  that receives the projection  138  of the pressing plate  131 . 
     The pressing plate  131  and the guide plate  134  respectively have retaining holes  141  and  142  through which the above-described common fulcrum  135  extends to support the pressing plate  131  and the guide plate  134  in a rotatable manner. 
     The pressing plate  131  and the guide plate  134  are stacked on top of each other and installed in the corresponding pressing mechanism  130 , as illustrated in  FIG. 3 . 
     In the above-described pressing mechanism  130 , a release mode, in which the pressing force is eliminated, may be set in addition to the normal mode and the envelope mode. The state of the pressing mechanism  130  in each mode will now be described. 
       FIG. 5  illustrates the pressing mechanism  130  in a normal mode. 
       FIG. 5  illustrates a switch  143 , which is not illustrated in  FIG. 3 , and also illustrates the support plates  123  that are guided by the guide plate  134 . The above-described heating roller is rotatably supported by a frame  150  of the fixing unit, and the normal-mode spring  132  is attached to the frame  150  and the pressing plate  131 . 
     The switch  143  switches the position of the pressing plate  131  by rotating a rotating cam. The rotation of the rotating cam is controlled by the controller  11  illustrated in  FIG. 1 . The position of the rotating shaft of the rotating cam is fixed with respect to the frame  150  of the fixing unit. In the normal mode, the rotating cam is separated from the pressing plate  131 . 
     When the rotating cam is separated from the pressing plate  131 , the pressing plate  131  is urged downward in  FIG. 5  by the normal-mode spring  132 , so that the arch portion  137  is pressed against the support plates  123 . As a result, a strong force generated by the urging force of the normal-mode spring  132  is applied to the support plates  123 , and the recording medium receives a strong pressing force in the above-described nip region. 
     At this time, the urging force of the envelope-mode spring  133  serves to press the guide plate  134  against the support plates  123 . However, since the distance between the pressing plate  131  and the guide plate  134 , which are at both ends of the envelope-mode spring  133 , does not change, the urging force of the envelope-mode spring  133  does not affect the pressing force applied in the nip region. The direction in which the support plates  123  are pressed by the force generated by the urging force of the normal-mode spring  132  is controlled by the guide plate  134 . 
     When the rotating cam of the switch  143  is rotated from the position in the normal mode illustrated in  FIG. 5  so as to raise the pressing plate  131 , the pressing mechanism  130  is switched to the envelope mode. 
       FIG. 6  illustrates the pressing mechanism  130  in the envelope mode. 
     When the rotating cam of the switch  143  is rotated so as to raise the pressing plate  131 , the position of the pressing plate  131  with respect to the frame  150  of the fixing unit is fixed by the switch  143 . In addition, since the pressing plate  131  is raised, the arch portion  137  is separated from the support plates  123 . As a result, the urging force of the normal-mode spring  132  serves as a force that presses the pressing plate  131  against the rotating cam of the switch  143 , and does not affect the pressing force applied in the nip region. 
     The urging force of the envelope-mode spring  133  is applied to the guide plate  134 , so that the support plates  123  are guided by the guide plate  134  and receive a force from the guide plate  134  at the same time. As a result, in the above-described nip region, the recording medium receives a pressing force based on the urging force of the envelope-mode spring  133 , the pressing force being smaller than that applied in the normal mode. The pressing force generated in the nip region differs between the envelope mode and the normal mode. However, in each mode, the support plates  123  are guided by the guide plate  134  in completely the same manner. Therefore, the direction in which the pressing force is applied does not change between the envelope mode and the normal mode. When the pressing force is applied in the same direction, high transportability of the recording medium is ensured in the fixing process. 
     As described above, according to the present exemplary embodiment, the size of the pressing mechanism  130  is reduced, so that the sizes of the fixing unit  18  and the printer  10  are reduced accordingly. In addition, the pressing force is changeable between the normal mode and the envelope mode without changing the direction in which the pressing force is applied. The switching between the normal mode and the envelope mode is performed by switching between a state in which the support plates  123  are in contact with the pressing plate  131  and a state in which the support plates  123  are separated from the pressing plate  131 . Therefore, it is not necessary to achieve high accuracy and the rotation of the switch  143  may be readily controlled. 
     When the rotating cam of the switch  143  is further rotated from the position in the envelope mode illustrated in  FIG. 6  so as to further raise the pressing plate  131 , the pressing mechanism  130  is switched to the release mode. 
       FIG. 7  illustrates the pressing mechanism  130  in the release mode. 
     In the release mode, the pressing plate  131  is raised by the switch  143  to a position higher than that in the envelope mode. Accordingly, the projection  138  of the pressing plate  131  comes into contact with an edge of the through hole  139  in the guide plate  134 , so that the guide plate  134  is also raised. As a result, spaces are generated between the guide plate  134  and the support plates  123 , and the pressing force applied by the pressing mechanism  130  is eliminated. The release mode is selected when, for example, a paler jam occurs. 
     As described above, in the present exemplary embodiment, the release mode is realized by a simple structure in which the guide plate  134  is separated from the support plates  123  by bringing the pressing plate  131  and the guide plate  134  into contact with each other. Thus, switching between the three modes, which are the normal mode, the envelope mode, and the release mode, is realized by switching the position of the pressing plate  131  with the switch  143 . 
     Although the controller  11  controls the switch to perform switching between the modes in the above-described description, the switching between the modes may instead be performed manually according to an exemplary embodiment of the present invention. 
     In addition, although the three modes, that is, the normal mode, the envelope mode, and the release mode, are described above, the structure may instead be such that only the normal mode and the envelope mode are provided and the release mode is not provided according to an exemplary embodiment of the present invention. 
     In addition, although a fixing unit is described as an example of a pressure-applying device according to an exemplary embodiment of the present invention, the pressure-applying device according to an exemplary embodiment of the present invention may instead be, for example, a device for decurling a paper sheet. 
     In addition, although a monochrome printer is described in the above exemplary embodiment, the present invention may instead be applied to a color apparatus, a facsimile machine, a copier, or a multifunction machine. 
     In addition, although an apparatus that forms a toner image by an electrophotographic method is described in the above exemplary embodiment, an image forming unit according to an exemplary embodiment of the present invention may form a toner image on a recording medium by a method other than the electrophotographic method. 
     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.