PATENT ABSTRACT
An image heating unit detachably mountable to an image forming apparatus, the image heating unit includes first and second rotatable members configured to and feed a recording material in an image heating process; a casing enclosing the first and second rotatable member and provided with an opening configured to permit passage of the recording material; a movable member movable relative to the opening between a first position taken when the image heating process is carried out and a second position in which a jam clearance operation is capable through the opening and in which a test finger is prevented from entering through the opening into the casing; and a locking mechanism configured to lock the movable member at the second position.

PATENT DESCRIPTION
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to a recording medium conveying unit, an image heating unit, and an image forming apparatus equipped with the preceding units. 
         [0002]    Recently, it has come to be thought that an image forming apparatus has to be reduced in downtime (length of time image forming apparatus cannot be operated because of recording medium jam). In other words, it has come to be thought that making it easier to remove jammed recording medium leads to the downtime reduction. 
         [0003]    Thus, Japanese Laid-open Patent Application 2000-157511 discloses an image forming apparatus which employs a fixation unit which is removable from the image forming apparatus. More concretely, the image forming apparatus is structured so that a casing in which a heating belt unit and a pressure roller are integrally held is removably installable. By structuring an image forming apparatus so that a fixing means, which is one of the locations in the image forming apparatus, in which recording medium jams occurs, can be moved out of the image forming apparatus as disclosed in Japanese Laid-open Patent application 2000-157511, the cause of the recording medium jam can be easily found and eliminated. 
         [0004]    By the way, a fixation unit such as the one described above is provided with an opening, which is a part of a recording medium conveyance passage through which recording medium is conveyed during image formation. Thus, it is possible that fingers of an operator of the image forming apparatus will accidentally enter the fixation unit through this opening, and damage internal components, such as the fixation roller, of the fixation unit. Therefore, it is desired that when the fixation unit is out of the main assembly of an image forming apparatus, this opening remains closed. 
         [0005]    The fixation unit disclosed in Japanese Laid-open Patent Application 2004-157511 is structured so that as an operator attempts to grasp the fixation unit, the opening is automatically closed. More concretely, the fixation unit is provided with a shutter which is pivotally attached to one of the edges of the opening, and a spring which keeps the shutter pressed in the direction to move the shutter away from the opening. 
         [0006]    With the provision of this structural arrangement, when the fixation unit is in the image forming apparatus, the shutter is in the position in which it keeps the opening exposed. On the other hand, as the operator grasps the fixation unit to remove the fixation unit from the image forming apparatus, the shutter is pressed by the operator against the resiliency of the spring, being thereby moved into the position in which it keeps the opening covered. Therefore, it is prevented for the finger of the operator from entering the fixation unit through the opening, and therefore, it is prevented that the finger of the operator come into contact with the internal components of the fixation unit. 
         [0007]    However, the fixation unit disclosed in Japanese Laid-open Patent Application 2004-157511 suffers from the following problem. That is, unless the fixation unit remains grasped by the operator, the opening remains exposed. Therefore, if the operator removes the fixation unit from the image forming apparatus, places the fixation unit on a table, and then, tries to remove the jammed recording medium from the fixation unit, the operator will end up handling the fixation unit while the opening is exposed. Therefore, it is possible that the finger of the operator will come into contact with the internal components, such as the fixation roller, of the fixation unit. 
       SUMMARY OF THE INVENTION 
       [0008]    According to an aspect of the present invention, there is provided an image heating unit detachably mountable to an image forming apparatus, said image heating unit comprising first and second rotatable members configured to and feed a recording material in an image heating process; a casing enclosing said first and second rotatable member and provided with an opening configured to permit passage of the recording material; a movable member movable relative to said opening between a first position taken when the image heating process is carried out and a second position in which a jam clearance operation is capable through said opening and in which a test finger stipulated in JIS C 0922 probe cord B is prevented from entering through said opening into said casing; and a locking mechanism configured to lock said movable member at the second position. 
         [0009]    According to another aspect of the present invention, there is provided an image heating unit detachably mountable to an image forming apparatus, said image heating unit comprising first and second rotatable members configured to and feed a recording material in an image heating process; a casing enclosing said first and second rotatable member and provided with an opening configured to permit passage of the recording material; a movable member movable relative to said opening between a first position taken when the image heating process is carried out and a second position in which a jam clearance operation is capable through said opening and in which a width of opening measured in a direction substantially perpendicular to a longitudinal direction of said casing is less than 12 mm; and a locking mechanism configured to lock said movable member at the second position. 
         [0010]    According to a further aspect of the present invention, there is provided an image forming apparatus comprising (i) an image forming station configured to form an image on a recording material; (ii) a mounting portion; (iii) an image heating unit detachably mountable to said mounting portion, said image heating unit including (iii-i) a pair of rotatable members configured to nip and feed the recording material in heating an image formed on the recording material by said image forming station; (iii-ii) a casing enclosing said rotatable members and provided with an opening configured to permit passage of the recording material; (iii-iii) a movable member movable relative to said opening between a first position taken when the image heating process is carried out and a second position in which a jam clearance operation is capable through said opening and in which a test finger stipulated in JIS C 0922 probe cord B is prevented from entering through said opening into said casing; and (iii-iv) a locking mechanism configured to lock said movable member at the second position; and (iv) an interrelating mechanism configured to interrelate a locking operation of said locking member with an operation of dismounting said image heating unit from said mounting portion and to interrelate an unlocking operation of said locking mechanism with an operation of mounting said image heating unit to said mounting portion. 
         [0011]    According to a further aspect of the present invention, there is provided an image forming apparatus comprising (i) an image forming station configured to form an image on a recording material; (ii) a mounting portion; (iii) an image heating unit detachably mountable to said mounting portion, said image heating unit including (iii-i) a pair of rotatable members configured to nip and feed the recording material in heating an image formed on the recording material by said image forming station; a casing enclosing said first and second rotatable member and provided with an opening configured to permit passage of the recording material; (iii-iii) a movable member movable relative to said opening between a first position taken when the image heating process is carried out and a second position in which a jam clearance operation is capable through said opening and in which a width of opening measured in a direction substantially perpendicular to a longitudinal direction of said casing is less than 12 mm; and (iii-iv) a locking mechanism configured to lock said movable member at the second position; and (iv) an interrelating mechanism configured to interrelate a locking operation of said locking member with an operation of dismounting said image heating unit from said mounting portion and to interrelate an unlocking operation of said locking mechanism with an operation of mounting said image heating unit to said mounting portion. 
         [0012]    According to a further aspect of the present invention, there is provided an image heating unit detachably mountable to an image forming apparatus, said image heating unit comprising first and second rotatable members configured to and feed a recording material; a casing enclosing said first and second rotatable member and provided with an opening configured to permit passage of the recording material; a movable member movable relative to said opening between a first position taken when a recording material feeding process is carried out and a second position in which a jam clearance operation is capable through said opening and in which a test finger stipulated in JIS C 0922 probe cord B is prevented from entering through said opening into said casing; and a locking mechanism configured to lock said movable member at the second position. 
         [0013]    According to a further aspect of the present invention, there is provided an image forming apparatus comprising (i) an image forming station configured to form an image on a recording material; (ii) a mounting portion; (iii) an image heating unit detachably mountable to said mounting portion, said image heating unit including (iii-i) a pair of rotatable members configured to nip and feed the recording material in heating an image formed on the recording material by said image forming station; (iii-ii) a casing enclosing said rotatable members and provided with an opening configured to permit passage of the recording material; and (iii-iii) a shutter configured to open and close said opening; (iv) a locking member configured to lock said shutter in a closing position; (v) a door opened and closed when said image heating unit is dismounted; and (vi) an interrelating mechanism configured to interrelate a locking operation of said locking member with a closing operation of said door and configured to interrelate an unlocking operation of said locking member with an opening operation of said door. 
         [0014]    According to a further aspect of the present invention, there is provided an image forming apparatus comprising (i) an image forming station configured to form an image on a recording material; (ii) a mounting portion; (iii) an image heating unit detachably mountable to said mounting portion, said image heating unit including (iii-i) a pair of rotatable members configured to nip and feed the recording material in heating an image formed on the recording material by said image forming station; (iii-ii) a casing enclosing said rotatable members and provided with an opening configured to permit passage of the recording material; and (iii-iii) a shutter configured to open and close said opening; (iv) a locking member configured to lock said shutter in a closing position; (v) a door opened and closed when said image heating unit is dismounted; and (vi) an interrelating mechanism configured to interrelate a closing operation of said shutter and a locking operation of said locking member with an opening operation of said door, and to interrelate an unlocking operation of said locking member and an opening operation of said shutter with a closing operation of said door. 
         [0015]    Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a drawing for describing the general structure of an image forming apparatus in which a fixation unit in accordance with the present invention is removably installable. 
           [0017]      FIG. 2  is a drawing for describing the internal components of the fixation unit. 
           [0018]      FIG. 3  is a drawing for describing the state in which the fixation unit in the first embodiment of the present invention is when the fixing unit is in the image forming apparatus, and the door of the image forming apparatus is closed. 
           [0019]      FIG. 4  is a drawing for describing the state in which the fixation unit in the first embodiment is when the fixation unit is in the image forming apparatus and the door of the image forming apparatus is open. 
           [0020]      FIG. 5  is a drawing for describing the state in which the fixation unit in the first embodiment is when the fixation unit is out of the image forming apparatus. 
           [0021]      FIG. 6  is a drawing for describing the states in which the sliding cover the fixing unit, and locking mechanism of the image forming apparatus are when the fixation unit is in the image forming apparatus. 
           [0022]      FIG. 7  is a drawing for describing the states in which the fixation unit in the second embodiment of the present invention is when the fixation unit is in the image forming apparatus and the door of the image forming apparatus is closed. 
           [0023]      FIG. 8  is a drawing for describing the state in which the fixation unit in the second embodiment is when the fixation unit is in the image forming apparatus and the door of the image forming apparatus is open. 
           [0024]      FIG. 9  is a drawing for describing the state in which the fixation unit in the second embodiment is when the fixation unit is out of the image forming apparatus. 
           [0025]      FIG. 10(   a ) is a sectional view of the image forming apparatus in the third embodiment of the present invention, and shows the general structure of the apparatus.  FIG. 10(   b ) is a drawing for describing the control section of the apparatus. 
           [0026]      FIG. 11  is a perspective view of the main section of the fixing device of the image forming apparatus in the third embodiment. 
           [0027]      FIG. 12  is a schematic sectional view of the fixing device of the image forming apparatus in the third embodiment, when the fixing device is in the image forming apparatus, and, and shows the general structure of the fixing device. 
           [0028]      FIG. 13  is a schematic sectional view of the fixing device of the image forming apparatus in the third embodiment, when the fixing device is out of the image forming apparatus, and shows the general structure of the fixing device. 
           [0029]      FIG. 14  is a perspective view of the fixing device of the image forming apparatus in the third embodiment, when the fixing device is in the image forming apparatus, and shows the general structure of the fixing device. 
           [0030]      FIG. 15  is a drawing for describing the sliding cover of the fixing device, and the locking lever of the image forming apparatus, in the third embodiment. 
           [0031]      FIG. 16  is a plan view of the sliding cover of the fixing device, and the locking lever of the image forming apparatus, in the third embodiment, after the complete closing of the external cover of the image forming apparatus. 
           [0032]      FIG. 17  is a plan view of the sliding cover of the fixing device, and the locking lever of the image forming apparatus, in the third embodiment, when the external cover of the image forming apparatus is slightly open. 
           [0033]      FIG. 18  is a drawing for describing the sliding cover of the fixing device and the locking lever of the image forming apparatus, in the third embodiment,  FIGS. 18(   a ) and  18 ( b ) being top and bottom views, respectively, of the sliding cover and locking lever, when the external cover is open widest. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Hereinafter, embodiments of the present invention are described in detail with reference to appended drawings. The following embodiments are described with reference to the recording medium conveying unit and image heating unit of a fixation unit which fixes an unfixed toner image to a sheet of recording medium. 
       Embodiment 1 
       [0035]    First, referring to  FIG. 1 , the image forming apparatus in this embodiment is described.  FIG. 1  is a drawing for describing the structure of the image forming apparatus in which a fixation unit which functions as both a recording medium conveying unit and an image heating unit is removably installable. This image forming apparatus is full-color laser beam printer. It uses an electrophotographic image formation process. Further, it is of the so-called tandem type. 
       [Image Forming Apparatus] 
       [0036]    Referring to  FIG. 1 , this image forming apparatus is provided with an image forming portion  10 , which forms Y (yellow), M (magenta), C (cyan) and Bk (black) toner images, through a latent image forming process, a developing process, and a transferring process. The image forming portion  10  has image formation units  1   a ,  1   b ,  1   c  and  1   d , which correspond in color to the monochromatic toner images of the aforementioned colors, one for one, which the image forming portion  10  forms. The image formation units  1  ( 1   a ,  1   b ,  1   c  and  1   d ) are provided with photosensitive drums a, b, c and d, respectively, as electrophotographic photosensitive members. These photosensitive drums a, b, c and d are charged across their peripheral surface by unshown charging devices. Then, a latent image is formed on the peripheral surface of each of the photosensitive drums a, b, c and d by a laser scanner  6 . Then, the latent images are developed by unshown developing devices into toner images. Then, the toner images on the photosensitive drums a, b, c and d are sequentially transferred onto an intermediary transfer belt  2 , as an image bearing member, by primary transfer rollers  2   a ,  2   b ,  2   c  and  2   d . As the intermediary transfer belt  2  is circularly driven, the toner images on the intermediary transfer belt  2  are conveyed to the area of contact between the intermediary transfer belt  2  and a secondary transfer roller  3   a.    
         [0037]    Meanwhile, sheets P of recording medium in a sheet feeder cassette  4  are moved out of the sheet feeder cassette  4  one by one by a sheet feeder roller  8 . Then, each sheet P of recording medium is conveyed to a pair of registration rollers  9  through a sheet conveyance passage  45 , which is on the immediately downstream side of the sheet feeder cassette  4  in terms of the recording medium conveyance direction. The pair of registration rollers  9  catch the sheet P, and temporarily hold the sheet P so that if the sheet P happens to be slanted relative to the recording medium conveyance direction, the sheet P will be corrected in attitude. Then, the pair of registration rollers  9  release the sheet P with such timing that the sheet P will reach the area of contact between the intermediary transfer belt  2  and secondary transfer roller  3   a , at the same time as the toner images on the intermediary transfer belt  2 . Then, the toner images, different in color, on the intermediary transfer belt  2  are transferred onto the sheet P by the secondary transfer roller  3   a  as a transferring member. 
         [0038]    Thereafter, the sheet P of recording medium is sent into the fixation unit  40  through a pre-fixation recording medium conveyance passage  30 . Then, the sheet P is conveyed through the fixation unit  40 , while being subjected heat and pressure by the fixation unit  40 . Consequently, the toner images on the sheet P are fixed to the sheet P. 
         [0039]    Here, the sheet P of recording medium is a medium, on the surface of which a toner image can be formed. Examples of sheet P of recording medium are a sheet of ordinary paper, a sheet of resinous substance, a sheet of cardstock, a sheet of film for an overhead projector, etc. 
         [0040]    In an image forming operation for forming a toner image on only one of the two surfaces of the sheet P, the sheet P is guided toward a pair of discharge rollers  11  by the switching in position of a flapper  46 . Then, the sheet P is discharged by the discharge rollers  11  into a delivery tray  12 . 
         [0041]    In an image forming operation for forming a toner image on both surfaces of the sheet P, after the fixation of a toner image on one (first) of the two surfaces of the sheet P by the fixation unit  40 , the sheet P is conveyed toward the discharge rollers  11 . Then, as the trailing edge of the sheet P reaches the reversal point  42 , the discharge rollers  11  begin to be rotated in reverse to convey the sheet P backward (switch-backed). Then, the sheet P is conveyed by the switching in position of the flapper  46 , through the two-side printing sheet conveyance passage  47 . Then, the sheet P is put through the same processes as those through which the sheet P was conveyed for the formation of a toner image on the first surface of the sheet P, to form a toner image on the other surface of the sheet P. Then, the sheet P is discharged into the delivery tray  12 . Incidentally, the portion of the image forming apparatus, which is made up of the flapper  46  and discharge rollers  11 , is an example of means for turning over the sheet P to form a toner image on both surfaces of the sheet P. 
         [0042]    The image forming apparatus requires various maintenance operations, for example, the operation to remove the sheet P stuck in the recording medium conveyance passage of the image forming apparatus. Therefore, the image forming apparatus is provided with a door  80 , which is attached to main assembly of the apparatus in such a manner that it can be pivotally moved about a hinge  90  of the main assembly, in the clockwise direction indicated in  FIG. 1 . 
         [0043]    By the way, in the case of the image forming apparatus in this embodiment, its pre-fixation recording medium passage  30 , secondary transfer roller  3   a , and one (right one in  FIG. 1 ) of the pair of registration rollers  9  are attached to the door  80 . Therefore, as the door  80  is opened, the recording medium conveyance passage, which is made up of the recording medium passage  45 , which is on the immediately downstream side of the sheet feeder cassette  4 , in terms of the recording medium conveyance direction, and extends to the pair of discharge rollers  11 , is exposed, except for the portion in the fixation unit  40 . Further, as the door  80  is opened, it becomes possible for an operator to install the fixation unit  40  into the unshown fixation unit chamber of the image forming apparatus, or uninstall the fixation unit  40  from the fixation unit chamber. 
         [0044]    That is, the door  80  functions as such a door that can take a closed position in which it prevents the image heating unit from being installed into, or uninstalled from, the fixation unit chamber, and an open position in which it allows the image heating unit to be installed into, or uninstalled from, the fixation unit chamber. 
       [Fixation Unit] 
       [0045]    Next, the fixation unit  40  in this embodiment is described. The fixation unit  40  in this embodiment is removably installable in the image forming apparatus. It functions as both the recording medium conveying unit and image heating unit. This fixation unit  40  is an integration of the structural components of the fixing device for the image forming apparatus. It is removably installable in the image forming apparatus. Therefore, if it becomes necessary for a given component (or components) of the fixing device to be replaced, the entirety of the fixation unit can be replaced to make it easy to replace the component(s). In addition, if a sheet P (or sheets P) of recording medium happens to jam the fixing portion, and therefore, has to be removed, the fixating portion, which is in the form of the fixation unit, can be removed from the image forming apparatus to make it easier to remove the jammed sheet P. 
         [0046]    One of the characteristic features of the fixation unit in this embodiment described above is that it is provided with a sliding cover which covers the recording medium conveyance opening of the fixation unit  40 , in such a manner that the jammed sheet(s) P can be removed, and a locking mechanism for locking or unlocking the sliding cover. This structural arrangement prevents the finger(s) of an operator from coming into contact with the internal components, such as the fixation roller, in the fixation unit, when the operator tries to remove the jammed sheet(s) P, for example. Hereafter, the structure of this fixation unit is described in detail with reference to the drawings. 
         [0047]      FIG. 2  is a drawing for describing the internal components of the fixation unit.  FIG. 3  is a drawing for describing the state of fixation unit after the proper installation of the fixation unit into the image forming apparatus, and also, after the closing of the door. Hereafter, referring to  FIGS. 2 and 3 , the fixation unit  40  is described in detail. 
         [0048]    Referring to  FIG. 2 , the fixation unit  40  is provided with a pair of rollers, more specifically, a fixation roller  100  and a contact roller  101 , which are for heating and pressing the unfixed toner image on a sheet P of recording medium to fix the toner image to the sheet P. The fixation roller  100  is a cylindrical hollow component, and has a parting layer, as a surface layer, which is low in coefficient of friction. It has a heating member  106 , in its hollow. The contact roller  101  also is a cylindrical hollow member, and has a parting layer, as a surface layer, which is low in coefficient of friction. It is positioned in parallel to the fixation roller  100  so that its peripheral surface is placed in contact with that of the fixation roller  100  and forms a nip N as shown in  FIG. 3 . The contact roller  101  is supported by a pair of supporting members  110 , with the placement of a pair of contact roller bearings  103  between the contact roller  101  and supporting members  110 , one for one. The fixation roller  100  is supported by a pair of pressure application levers  120 , with the placement of a pair of fixation roller bearings  102 , one for one. Normally, therefore, a preset amount of contact pressure is maintained between the fixation roller  100  and contact roller  101  by the pressure application levers  120  (which hereafter will be referred to simply as “pressure levers”). 
         [0049]    Next, referring to  FIG. 3 , the fixation roller  100  and contact roller  101  are rotated in the counterclockwise and clockwise directions (in  FIG. 3 ), respectively, by an unshown driving mechanism. Thus, as a sheet P of recording medium is sent to the upstream end (bottom end in  FIG. 3 ) of the nip N, in terms of the recording medium conveyance direction, it is conveyed through the nip N to the downstream end (top end in  FIG. 3 ) of the nip N, while remaining pinched between the fixation roller  100  and contact roller  101 . While the sheet P is conveyed through the nip N, the image on the sheet P is heated and pressed by the fixation roller  100  and contact roller  101 . Thus, the image on the sheet P is fixed to the sheet P. That is, the fixation roller  100  and contact roller  101  cooperate to thermally fix the image to the sheet P. 
         [0050]    Here, the fixation roller  100  and contact roller  101  function as a pair of rotational members which heat the image on the sheet P of recording medium while conveying the sheet P through the nip N which they form. 
         [0051]    Further, the fixation roller  100  and contact roller  101  function as a pair of rotational members which convey the sheet P of recording medium while keeping the sheet P pinched between them. 
         [0052]    The above-described structural components of the fixation unit  40  are internally held by a combination of a fixation unit frame  130  and a fixation unit cover  131 , which are in connection to each other. The combination of fixation unit frame  130  and fixation unit cover  131  functions as a casing for enclosing the above described structural components of the fixation unit, and prevents the structural components from coming into contact with the foreign objects to protect them. Therefore, the internal components such as the fixation roller  100  are unlikely to be damaged. Further, the possibility that an operator will come into contact with the internal components when the components are hot is minimized. 
         [0053]    The combination of the fixation unit frame  130  and fixation unit cover  131  forms an opening as an entrance through which a sheet P of recording medium can be introduced into the nip N through the upstream conveyance passage  30 . Thus, the shape of the opening is such that its lengthwise edges are parallel to the axial line of the fixation roller  100 . 
         [0054]    To describe in further detail, the fixation unit cover  131  has an entrance guide  104  for guiding a sheet P of recording medium into the fixation unit  40 . The entrance guide  104  is positioned so that it will be on the upstream side of the nip N when it is closed. That is, the opening is formed by the entrance guide  104 , and the fixation unit frame  130 , which is on the opposite side of the opening from the entrance guide  104 . The shape of the opening is such that the lengthwise direction of the opening is parallel to the axial line of the fixation roller  100 , and the widthwise direction of the opening is parallel to the direction in which the sheet P of recording medium is conveyed. Referring to  FIG. 3 , the width of the opening is equivalent to a gap V 1 . This gap V 1  is set to prevent the problem that when a sheet P of recording medium having an unfixed toner image is introduced into the fixation unit  40  to fix the unfixed toner image, the unfixed toner image is disturbed by coming into contact with the fixation roller  100 . Thus, it is set based on the predictable amount by which a sheet P of recording medium may curl. Incidentally, the value of the gap V 1  in this embodiment is roughly 20 mm. 
         [0055]    Therefore, the combination of the fixation unit frame  130  and fixation unit cover  131  has the opening which allows a sheet P of recording medium to enter the fixation unit  40 . Further, it functions as a casing which internally holds the pair of rotational members  100  and  101 . 
         [0056]    The fixation unit  40  is provided with a sliding cover  200 , which is on the upstream side of the nip N, and on the opposite side of the recording medium conveyance passage from the entrance guide  104 , extending in the lengthwise direction of the opening. The sliding cover  200  is supported by the fixation unit frame  130  in such a manner that it can be linearly moved in the direction indicated by arrow marks X and Y in the drawing. It is kept pressed in the direction Y, that is, toward the entrance guide  104 , by a pair of sliding cover springs  201  located at the lengthwise ends of the sliding cover  200 , one for one. 
         [0057]    Therefore, the sliding cover  200  functions as a moving member (shutter) which is movable relative to the opening. Further, the sliding cover  200  is slidable in the direction which is practically perpendicular to the lengthwise direction of the casing (combination of fixation unit frame  130  and fixation unit cover  131 ). Moreover, the sliding cover springs  201  functions as pressing member for pressing the sliding cover  200 . 
       [Locking Mechanism] 
       [0058]    Next, referring to  FIGS. 3 to 6 , the locking mechanism which locks or unlocks the sliding cover  200  is described about its structure. 
         [0059]      FIG. 3  is a drawing for describing the state in which the fixation unit  40  in this embodiment is when the fixation unit is in the image forming apparatus and the door of the image forming apparatus is closed.  FIG. 4  is a drawing for describing the state in which the fixation unit  40  in this embodiment is when the fixation unit is in the image forming apparatus and the door of the image forming apparatus is open.  FIG. 5  is a drawing for describing the state in which the fixation unit  40  in this embodiment is when the fixation unit  40  is out of the image forming apparatus.  FIG. 6  is a drawing for describing the state in which the sliding cover of the fixation unit, and the locking mechanism of the image forming apparatus, are when the fixation unit is in the image forming apparatus. 
         [0060]    Referring to  FIG. 3 , the main apparatus frame  300  of the image forming apparatus is provided with a fixation unit supporting member  301 , and a fixation unit holding mechanism  310 . The fixation unit supporting member  301  supports the fixation unit  40  by the bottom surface of the fixation unit  40  against the weight of the fixation unit  40 . The fixation unit holding mechanism  310  keeps the fixation unit  40  properly positioned by properly positioning the lengthwise ends of the fixation unit  40 . 
         [0061]    When the fixation unit  40  is properly positioned in the main assembly of the image forming apparatus, it remains on the fixation unit supporting member  301 , and remains fixed to the main assembly frame  300  by being pressed upon the main assembly frame  300  by the fixation unit holding mechanism  310 . In this case, the combination of the fixation unit supporting member  301  and fixation unit holding mechanism  310  functions such a section that enables the fixation unit  40  to be properly installed into, or uninstalled from, the main assembly frame  300 . 
         [0062]    Referring to  FIG. 3 , fixation unit  40  is provided with an upstream locking lever  307  and an upstream lock spring  308 . The upstream locking lever  307  locks the sliding cover  200  to prevent the sliding cover  200  from moving. The upstream lock spring  308  provides the upstream locking lever  307  with the force which keeps the upstream locking lever  307  in its locking position. The upstream locking lever  307  is held by the fixation unit frame  130  so that it can be pivotally moved about its pivot  307   a . It is under the pressure generated by the upstream locking spring  308  in the direction to pivotally move the upstream locking lever  307  in the clockwise direction of the drawing. 
         [0063]    In other words, the combination of the upstream locking lever  307  and upstream locking spring  308  functions as a locking mechanism. 
         [0064]    The frame  300  of the main assembly of the image forming apparatus is provided with a locking lever disengaging lever  306  for disengaging the upstream locking lever  308 . The locking lever disengaging lever  306  is shaped so that it protrudes in the direction in which the fixation unit  40  is inserted into, or removed from, the main assembly of the printer. It is positioned so that when the fixation unit  40  is in its image forming position in the image forming apparatus, it remains in contact with the upstream locking lever  307 . 
         [0065]    The upstream recording medium passage  30 , with which the door  80  is provided, is provided with a pair of protrusions  304 , which correspond in position to the lengthwise ends of the fixation unit  40 , one for one. The protrusion  304  are positioned so that as the door  80  is closed, the protrusions  304  are made to move the sliding cover  200  by the movement of the upstream recording medium passage  30 . As the fixation unit  40  is pulled out of the fixation unit chamber in the direction Y, the fixation unit engaging portion  302  of the main assembly separates (disengages) from the main assembly engaging portion  200   c  of the sliding cover  200 . 
         [0066]    The fixation unit engaging portion  302 , shown in  FIG. 6 , engages with the main assembly engaging portion  200   c  of the fixation unit  40  to prevent the sliding cover  200  from moving after the installation of the fixation unit  40  into the main assembly of the image forming apparatus. 
         [0067]    Next, referring to  FIG. 3 , the state in which the fixation unit  40  is when it is capable of thermally fixing an unfixed image on a sheet P of recording medium is described. 
         [0068]    Referring to  FIG. 3 , when the fixation unit  40  is in the state in which it can heat the image on a sheet P of recording medium, it is in the main assembly of the image forming apparatus, and the door  80  is shut. Further, the upstream recording medium passage  30  is in the proper position for image formation. 
         [0069]    Further, the fixation unit  40  remains held to the main assembly frame  300  by the fixation unit holding mechanism  310 . As for the upstream locking lever  307 , its end portion  307   f  which protruding from the fixation unit frame  130 , remains pressed by the locking lever disengaging member  306 . Therefore, the upstream locking lever  307  is in the position into which it has been pivotally moved in the counterclockwise direction (in drawing) about the pivot  307   a  by the end portion  307   f  against the resiliency of the upstream locking spring  308 . When the fixation unit  40  is in this state, the locking surface  307   d  of the upstream locking lever  307 , which is on the sliding cover side of the upstream locking lever  307 , is in its retreat in which it does not contact the sliding cover  200 . Therefore, it does not interfere with the movement of the sliding cover  200 . 
         [0070]    Further, the door  80  will have been closed against the main assembly of the image forming apparatus. Therefore, the upstream recording medium conveyance passage  30  will be in its preset position. At this point in the operation, the protrusion  304  of the door  80  is pressing on the surface  200   c  of the sliding cover  200 , which is the upstream surface in terms of the direction, indicated by the arrow mark X, in which the door  8  is closed. Thus, this pressure from the protrusion  304  keeps the sliding cover  200  disengaged from the locking mechanism, moves the sliding cover  200 , and keeps the sliding cover  200  open. That is, the recording medium conveyance passage  30  is kept in the state in which it is provided with the gap V 1 . Therefore, it does not occur during the thermal fixation of an unfixed image that the unfixed image comes into contact with the fixation unit frame  130 . Therefore, it does not occur during the thermal fixation of the unfixed image that the unfixed image is disturbed by coming into contact with the fixation unit frame  130 . 
         [0071]    That is, when the door  80  is in its closed position, the sliding cover  200  is kept in its first position, or the position for thermal image fixation, by the door  80 . 
         [0072]    Next, referring to  FIGS. 4 and 6 , what occurs as the door  80  is moved in the opening direction when it is in the above described position is described. 
         [0073]    As the door  80  is moved in its opening direction indicated by an arrow mark Y when it is in the position in which it enables the fixation unit  40  to thermally fix an unfixed image, the upstream recording medium passage  30  is moved by the movement of the door  80 , in the same direction, indicated by the arrow mark Y, as the door  80 . Thus, the protrusions  304  also are moved in the same direction, indicated by the arrow mark Y, as the door  80 . 
         [0074]    Next, referring to  FIG. 6 , as the door  80  is moved as described above, the sliding cover  200 , which is in contact with the protrusions  304 , are moved by the resiliency of the sliding cover springs  201  in the direction Y, which is the same direction as the direction in which the sliding cover  200  moves when it closes the recording medium conveyance direction. Thus, the lengthwise end portions of the sliding cover  200 , which are outside the recording medium conveyance passage, come into contact with the entrance guide  104 . Since the lengthwise ends of the sliding cover  200  are protruding toward the entrance guide  104  beyond the sheet conveyance portion  200   a , the sliding cover  200  is held by the protrusions  304 , without completely closing the recording medium conveyance passage, which is between the portion  200   a  and entrance guide  104 . 
         [0075]    Further, the engaging portion (surface portion)  200   c  of the sliding cover  200  becomes engaged with the sliding cover engaging portion  302   c  of the main assembly of the image forming apparatus, which is shown in  FIG. 4 . Thus, the sliding cover  200  is regulated in movement; it is locked in position. Therefore, it does not occur that when the door  80  is open, the sliding cover  200  is pushed open by an operator. 
         [0076]    Next, referring to  FIGS. 4 and 5 , what occurs when the fixation unit  40  is installed into, or uninstalled from, the main assembly of the image forming apparatus is described. 
         [0077]    In an operation for removing the fixation unit  40  from the main assembly frame  300 , the state of the fixation unit  40  changes from the state shown in  FIG. 4  to the state shown in  FIG. 5 . 
         [0078]    As the fixation unit  40  in the fixation unit chamber of the main assembly frame is moved in the direction Y, or the direction in which the fixation unit  40  is to be pulled out of the main assembly frame, the fixation unit engaging portion  302  of the apparatus main assembly disengages from the main assembly engaging portion  200   c  of the sliding cover  200 . Further, the upstream locking lever  307 , shown in  FIG. 4 , with which the fixation unit  40  is provided, separates from the locking lever disengaging member  306 . As the upstream locking lever  307  separates from the locking lever disengaging member  306 , it is pivotally moved about the pivot  307   a  by the resiliency of the upstream lock spring  308 . Consequently, the locking surface  307   d  of the upstream locking lever  307  comes into contact with the locking surface  200   d  of the sliding cover  200 , preventing thereby the sliding cover  200  from moving in the direction X. That is, the sliding cover  200  is locked in position. In other words, even though the sliding cover  200  is free from the sliding cover engaging portion  302 , it remains locked by the upstream locking lever  307 . Therefore, it does not occur that the sliding cover  200  is pushed open by an operator. 
         [0079]    It is through the above described steps that the fixation unit  40  comes out of the image forming apparatus. 
         [0080]    By the way, when the lengthwise end portions  200   b  are in contact with the entrance guide  104 , the gap V 2  between the portion  200   a  of the sliding cover  200  and entrance guide  104  is narrower than the above described gap V 1 . More specifically, the gap V 2  is small enough to prevent a finger (fingers) of an operator from entering the fixation unit  40  through the opening (gap V 2 ). Assuming here that an operator is an adult of the normal size, it is reasonable to liken a double-jointed cylindrical probe (JIS C 0922 Probe Code B, IEC 61032, test probe B), which is 12 mm in diameter, to a finger of the operator. This probe is referred to as “test finger”. Its specifications are set by JIS (Japanese Industrial Standard) IEC (International Electric Standard Conference). The two sets of specifications are identical. 
         [0081]    The above-described gap V 1  is 20 mm. Therefore, if the fixation unit  40  is removed from the image forming apparatus without changing the gap V 1  in size, the test finger can be put through the gap V 1 . Thus, all that is necessary to make it impossible for the test finger to be put through the gap V 1  when the fixation unit  40  is out of the image forming apparatus is to move the sliding cover  200  to make the gap V 2  become no more than 12 mm, for example, as the fixation unit  40  is removed from the image forming apparatus. With the gap V 2  being no more than 12 mm, it is possible to prevent an operator from touching the internal components, such as the fixation roller, of the fixation unit  40 , when the fixation unit  40  is out of the image forming apparatus. 
         [0082]    Further, it is possible that when the fixation unit  40  is out of the image forming apparatus, and therefore, is completely exposed, a child will touch the fixation unit  40 . Therefore, it is desired that a double-jointed cylindrical probe (JIS C 0922 Probe Code 19, IEC 61032, test probe  19 ), which is 5.6 mm in diameter, and therefore, can be likened to a finger of an infant, cannot be put through the opening. For example, all that is necessary is to structure the fixation unit  40  so that the sliding cover  200  is movable to make the gap V 2  become no more than 5.6 mm. 
         [0083]    Incidentally, the reason why the gap V 2  is provided is as follows. That is, it is for making it possible for an operator to remove jammed object(s) when the fixation unit  40  is out of the image forming apparatus. If the gap V 2  is not provided, it is possible that a jammed sheet P will be pinched between the sliding cover  200  and entrance guide  104 , and there, it will be difficult for the jammed sheet P to be removed (pulled out). In addition, a sheet P having wrapped around the fixation roller  100  will be confined on the inward side of the sliding cover  200 , and therefore, will be difficult to remove. Therefore, it is desired that the gap V 2  is greater than the thickness of the thickest sheet P of recording medium which can be introduced into the fixation unit  40 . 
         [0084]    Therefore, the sliding cover  200  is enabled to take a second position in which it allows a sheet P of recording medium to be removed from the fixation unit  40  through the opening, and yet, prevent the test finger from being put through the opening. 
         [0085]    Further, the sliding cover  200  is slid between the first and second positions to regulate the dimension of the opening, in terms of the thickness direction of a sheet P of recording medium which is conveyed through the gap. 
         [0086]    Further, the sliding cover spring  201  keeps the sliding cover  200  continuously pressed in the direction to move the sliding cover  200  from the first position to the second position. 
         [0087]    Further, the sliding cover  200  functions as a component which is movable relative to the opening. It can take the second position in which it makes the opening no more than 12 mm in dimension in terms of the thickness direction of a sheet P of recording medium which is conveyed through the opening, and the first position which is the position for image heating, and in which it makes the opening greater in dimension than the second position. 
         [0088]    Further, the sliding cover  200  functions as a component which can be moved relative to the opening. It can take the first position which is for image heating, and the second position which is for allowing a sheet P of recording medium to be removed through the opening, and yet, preventing the test finger (JIS C 0922 Probe Code B) from entering the opening. 
         [0089]    Further, the sliding cover  200  functions as a movable component which can be moved into the second position in which it regulates the aforementioned width to no more than 5.6 mm. 
         [0090]    Further, the sliding cover  200  functions as a component movable to the second position in which it prevents the test finger (JIS C 0922 Probe Code 19) from passing through the gap. 
         [0091]    Further, the upstream locking lever  307  functions as a locking component which locks the sliding cover  200  in position when the sliding cover  200  is in the second position. 
         [0092]    During the operation for installing the fixation unit  40  into the main assembly frame  300 , the state of the fixation unit  40  changes from the state shown in  FIG. 5  to the state shown in  FIG. 4 . 
         [0093]    As the fixation unit  40  is moved in the direction X to be installed into the fixation unit chamber in the main assembly frame  300 , the upstream locking lever  307  comes into contact with the locking lever disengaging member  306 . More concretely, the locking lever contacting portion  306   e  of the locking lever disengaging member  306  comes into contact with the tapered portion  307   e  of the upstream locking lever  307 , and presses the upstream locking lever  307  against the resiliency of the upstream locking spring  308 . 
         [0094]    Thus, the upstream locking lever  307  pivots about its pivot  307   a  in the counterclockwise direction, shown in the drawing. When the fixation unit  40  is in this state, the upstream locking lever  307  is in the position in which its locking surface  307   d  does not contact the locking surface  200   d  of the sliding cover  200 . Then, as the fixation unit  40  is moved into the preset position in the main assembly frame  300 , the upstream locking lever  307  is moved into the position in which the locking disengaging surface  307   f  of the upstream locking lever  307  contacts the lever contacting surface  306   f  of the lock disengaging member  306 . Thereafter, the upstream locking lever  307  is held in the position in which it remains disengaged from the sliding cover  200 . 
         [0095]    In other words, the lock disengaging member  306 , upstream locking lever  307 , and upstream locking spring  308  function in concert as a linkage for locking or unlocking the sliding cover  200 . 
         [0096]    This linkage causes the installation of the fixation unit  40  into the fixation unit holding mechanism  310 , to unlock the sliding cover  200 . 
         [0097]    Further, the linkage causes the uninstallation of the fixation unit  40  from the fixation unit holding mechanism  310 , to lock the sliding cover  200 . 
         [0098]    According to this embodiment, there is no possibility that a finger of an operator will come into contact with the internal components of the fixation unit  40 , when the fixation unit  40  is out of the image forming apparatus. 
         [0099]    According to this embodiment, there is no possibility that a finger of an infant will come into contact with the internal components of the fixation unit  40 , when the fixation unit  40  is out of the image forming apparatus. 
         [0100]    Also according to this embodiment, there is no possibility that the sliding cover  200 , which is covering the opening, is pushed open by an operator, when the fixation unit  40  is out of the image forming apparatus. 
         [0101]    Also according to this embodiment, a sheet P of recording medium can be removed from the fixation unit  40 , when the fixation unit  40  is out of the main assembly of the image forming apparatus. 
         [0102]    Lastly, according to this embodiment, it is on the recording medium entrance side of the fixation unit  40  that the above described effects can be obtained. 
       Embodiment 2 
       [0103]    Next, the structure of the fixation unit in the second embodiment of the present invention is described. This embodiment is different from the first one in that the object to be locked in position in this embodiment is the exit guide located at the opening of the fixation unit, which is on the downstream side of the fixation nip N, instead of the sliding cover  200  located at the opening of the fixation unit, which is on the upstream side of the nip N. However, it is the same in gist as the first embodiment. By the way, the components in this embodiment, which are similar in function to the counterparts in the first embodiment, are given the same referential codes as those given to the counterparts, and are not going to be described. 
       [Fixing Device] 
       [0104]    Next, the fixing device in this embodiment is described. The fixing device in this embodiment is in the form of a fixation unit  40 , as an image heating unit, which is a combination of a recording medium conveying unit and an image heating unit. It is removably installable in an image forming apparatus. This fixation unit  40  is an integration of the structural components of the fixing device. If it becomes necessary for a component (or components) of the fixing device to be replaced, the entirety of the fixation unit can be replaced. Therefore, it makes it easier to replace the component. Further, if a sheet of recording medium becomes jammed in the fixing portion of the fixation unit  40 , and therefore, it becomes necessary to remove the jammed recording medium from the fixing portion, the fixing portion can be moved out of the image forming apparatus by removing the fixation unit  40  out of the image forming apparatus. Therefore, this fixation unit  40  makes it easier to remove the jammed recording medium. 
         [0105]    One of the characteristic features of the above described fixation unit in this embodiment is that it is provided with an exit guide and a locking mechanism. The exit guide which is disposed on the downstream side of the fixation unit  40  to guide a sheet P of recording medium as the sheet P comes out of the fixation unit  40 , so that the jammed sheet can be removed. The locking mechanism is for locking the exit guide in position. This structural arrangement prevents the problem that as an operator tries to remove a jammed sheet of recording medium, for example, from the fixation unit, a finger of the operator comes into contact with the internal components of the fixation unit  40 , when the fixation unit  40  is out of the image forming apparatus. Hereafter, the structure of this fixation unit  40  is described in detail with reference to the appended drawings. 
         [0106]    Referring to  FIG. 2 , the fixation unit  40  in this embodiment is provided with a fixation roller  100  and a contact roller  101 , which are for fixing an image on a sheet of recording medium, in the nip N. It is also provided with a fixation unit frame  130  and a fixation unit cover  131 , which integrally support and cover the fixation roller  100  and contact roller  101 . The fixation unit frame  130  and fixation unit cover  131  form an opening, through which a sheet P of recording medium is conveyed toward a downstream sheet conveyance passage  31  after being discharged from the nip N. 
         [0107]    Referring to  FIG. 7 , in this embodiment, the fixation unit cover  131  is provided with an exit guide  105  which assists the recording medium conveyance. The fixation unit cover  131  is disposed in the adjacencies of the opening of the fixation unit  40 , which is on the downstream side of the nip N. The exit guide  105  plays the role of directing a sheet P of recording medium to guide outward the sheet P as the sheet P comes out of the nip N while remaining pinched between the fixation roller  100  and contact roller  101 . More specifically, when it is in its closed position, it functions as a component for guiding the sheet P as the sheet P is discharged from the fixation unit  40  through the downstream opening of the fixation unit  40 . 
         [0108]    There is an opening between this exit guide  105 , and the fixation unit frame  130 , which is on the opposite side of the recording medium passage  31  from the exit guide  105 . The opening has preset width and depth. The width of this opening is set to prevent the problem that as a sheet of recording medium is conveyed through this opening immediately after the fixation of the image thereon, the image on the sheet P is disturbed by coming into contact with the fixation unit frame  130 . It is set based on the predictable amount by which a sheet of recording medium curls. The width of this downstream opening is desired to be such that it prevents a finger of an operator from accidentally entering the opening, as in the first embodiment. Assuming here that an operator is an adult of the normal size, it is reasonable to liken a double-jointed cylindrical probe (JIS C 0922 Probe Code B, IEC 61032, test probe B), which is 12 mm in diameter, to a finger (fingers) of the operator. This probe is referred to as “test finger”. Its specifications are set by JIS (Japanese Industrial Standard) IEC (International Electric Standard Conference). Thus, all that is necessary to make it impossible for the test finger to be put through the opening when the fixation unit  40  is out of the image forming apparatus is to move the exit guide  105  to make the opening no more than 12 mm in width, for example. With the opening being no more than 12 mm in width, it is possible to prevent an operator from touching the internal components of the fixation unit  40 , when the fixation unit  40  is out of the image forming apparatus. 
         [0109]    Further, it is possible that when the fixation unit  40  is out of the image forming apparatus, and therefore, is completely exposed, a child will touch the fixation unit  40 . Therefore, it is desired that a double-jointed cylindrical probe (JIS C 0922 Probe Code 19, IEC 61032, test probe  19 ), which is 5.6 mm in diameter, and therefore, can be likened to a finger of an infant, cannot be put through the opening. For example, all that is necessary is to structure the fixation unit  40  so that the exit guide  105  is movable to make the opening become no more than 5.6 mm in width. 
         [0110]    Incidentally, it is desired that when the fixation unit  40  is the image forming apparatus, the exit guide  105  remains in the position in which it prevents a finger of an operator from entering the downstream opening of the fixation unit  40  as the sliding cover  200  remains in the position in which it prevents a finger of an operator from entering the upstream opening of the fixation unit  40  in the first embodiment. However, as long as a finger of an operator is prevented from entering the downstream opening of the fixation unit  40 , the fixation unit  40  does not need to be structured as in this embodiment. For example, the fixation unit  40  may be structured as follows. 
         [0111]    The exit guide  105  is pivotally movable about a hinge  105   a  in the direction indicated by an arrow mark U, and is kept under the pressure generated by an exit guide spring  108  in the opposite direction from the direction indicated by the arrow mark U. 
         [0112]    Therefore, normally, the exit guide  105  remains in its closed position. However, it can be pivotally opened in the direction U by applying pressure to the exit guide  105  against the resiliency of the exit guide spring  108 . With the provision of this structural arrangement, it is possible for an operator to insert his or her finger into the unshown gap between the main assembly of the image forming apparatus and the fixation unit  40 . Therefore, the operator can easily remove a jammed sheet P of recording medium when a sheet P of recording medium has jammed the recording medium passage. More concretely, as an operator tries to pull the sheet P of recording, which is protruding downstream from the nip N, out of the fixation unit  40 , by grasping the sheet P with his or her fingers inserted through the gap, the exit guide  105  is pivotally opened in the direction U by being pushed by the sheet P, which is being moved out in the direction Y. Therefore, the exit guide  105  does not interfere with the movement of the sheet P. 
         [0113]    During this operation, the downstream opening is widened enough for a finger of an operator to enter. However, as the exit guide  105  is pivotally moved in the direction U, the gap between the image forming apparatus and fixation unit  40  narrows. 
       [Locking Mechanism] 
       [0114]    Next, referring to  FIG. 7 , the mechanism for locking the exit guide  105  in this embodiment is described.  FIG. 7  is a drawing for describing the state in which the fixation unit  40  in the second embodiment is when the fixation unit  40  in the image forming apparatus, and the door  80  of the image forming apparatus is closed. 
         [0115]    Referring to  FIG. 7 , the main assembly frame  300  of the image forming apparatus is provided with a fixation unit supporting member  301  and a fixation unit holding mechanism  310 . The fixation unit supporting member  301  supports the fixation unit  40  by the bottom surface of the fixation unit  40  against the weight of the fixation unit  40 . The fixation unit holding mechanism  310  keeps the fixation unit  40  properly positioned by properly positioning the lengthwise ends of the fixation unit  40 . 
         [0116]    Referring to  FIG. 3 , the fixation unit  40  is provided with a downstream locking lever  400  and a downstream locking spring  401 , which are on the downstream side of the nip N of the fixation unit  40 . The downstream locking lever  400  locks the exit guide  105  so that the exit guide  105  cannot be moved. The downstream lock spring  401  provides the downstream locking lever  400  with the force which keeps the locking lever  400  in the locking position. The downstream locking lever  400  and downstream lock spring  401  are outside the recording medium passage. The downstream locking lever  400  is held by the fixation unit frame  130  so that it can be pivotally moved about its pivot  400   a . It is held by the resiliency of the downstream locking spring  401 , being kept in a preset position by the stopper portion  130   a  of the fixation unit frame  130 . 
         [0117]    The main assembly frame  300  of the image forming apparatus is provided with a downstream locking lever disengaging lever  402  for disengaging the above described downstream locking lever  400 . The downstream locking lever disengaging lever  402  is in the form of a protrusion, and protrudes from the main assembly of the image forming apparatus in the direction in which the fixation unit  40  is inserted into, or extracted from, the main assembly. It is positioned so that when the fixation unit  40  is in its image forming position in the image forming apparatus, it remains in contact with the downstream locking lever  400 . 
         [0118]    Next, referring to  FIG. 7 , the state in which the fixation unit  40  is during image formation is described. 
         [0119]    During an image forming operation, the fixation unit  40  is in the main assembly of the image forming apparatus, and the door  80  is shut against the main assembly. Further, the upstream recording medium conveyance passage  30  is in the preset position. 
         [0120]    Further, the fixation unit  40  remains immovably held to the main assembly frame  300  by the fixation unit holding mechanism  310 . The end portions  400   b  of the downstream locking lever  400 , which are protruding from the fixation unit frame  130 , remain under the pressure applied by the downstream locking lever disengaging lever  402 . Thus, the downstream locking lever  400  is in the position into which it has been pivotally moved in the clockwise direction of  FIG. 7 , against the resiliency of the downstream lock spring  401 , because the lengthwise end portions  400   b  are under the pressure applied by the downstream locking lever disengaging lever  402 . When the fixation unit  40  is in this state, the locking surface  400   c , which belongs to the end portion of the downstream locking member, which is on the exit guide  105  side, is in the position into which it has been retracted, and in which it does not make contact with the exit guide  105 . Therefore, the exit guide  105  does not interfere with the movement of the exit guide  105 . 
         [0121]    Next, referring to  FIG. 8 , what occurs as the door  80 , which is in the above described position, is opened is described. 
         [0122]    When the door  80  is opened, the downstream locking lever  400  is not moved by the movement of the door  80 . Therefore, the downstream locking lever  400  remains in the position in which it does not contact the exit guide  105 , leaving thereby the exit guide  105  unlocked. Therefore, the exit guide  105  is pivotally movable about the hinge  105   a  in the direction U in the drawing. 
         [0123]    As the door  80  is opened in the direction Y when the fixation unit  40  is in its image heating position, it becomes possible for the fixation unit  40  to be moved out of the apparatus main assembly. It is assumed here that the jam has occurred to the fixation unit  40 , and a sheet P of recording medium is protruding from the fixation unit  40  through the downstream opening of the fixation unit  40 . In such a case, an operator is likely to put his or her fingers through the gap between the image forming apparatus and fixation unit  40 , in the direction indicated by an arrow mark X, and try to pull the sheet P out of the fixation unit  40  in the direction Y by grasping the portion of the sheet P, which is protruding through the downstream opening. 
         [0124]    In the above-described situation, the sheet P, which is being pulled in the direction Y, comes into contact with the exit guide  105 , and presses the exit guide  105  in the direction Y. Here, the exit guide  105  is remaining unlocked as described above. Therefore, as it is pressed by the sheet P, it pivotally moves against the resiliency of the exit guide spring  108 , in the clockwise direction indicated by an arrow mark U. 
         [0125]    Therefore, as the sheet P is pulled, the downstream opening widens. Therefore, such a situation that the sheet P is caught by the exit guide  105 , being thereby prevented from being moved out of the fixation unit  40  can be avoided. That is, it becomes easier to pull the sheet P out of the fixation unit  40  in the direction indicated by the arrow mark Y in the drawing. 
         [0126]    Next, referring to  FIGS. 8 and 9 , the operation for inserting the fixation unit  40  into the image forming apparatus, and the operation for extracting the fixation unit  40  from the main assembly of the image forming apparatus, are described.  FIG. 8  is a drawing for describing the state in which the fixation unit  40  in this embodiment is after the installation of the fixation unit  40  into the image forming apparatus and before the closing of the door  80 .  FIG. 9  is a drawing for describing the state in which the fixation unit  40  in the second embodiment is when the fixation unit  40  is out of the image forming apparatus. 
         [0127]    During an operation for removing the fixation unit  40  from the main assembly frame  300 , the state of the fixation unit  40  changes from the one shown in  FIG. 8  to the one shown in  FIG. 9 . As the fixation unit  40  in the image forming apparatus is moved in the direction Y, the downstream locking lever  400  with which the fixation unit  40  is provided separates from the downstream locking lever disengaging lever  402 . 
         [0128]    As the downstream locking lever  400  separates from the downstream locking lever disengaging lever  402 , it is pivotally moved about the pivot  400   a  in the counterclockwise direction of the drawing by the resiliency of the downstream locking spring  401 . Thus, the locking surface  400   d  of the downstream locking lever  400  comes into contact with the locking surface  105   c  of the exit guide  105 , preventing thereby the exit guide  105  from moving in the direction U. That is, the exit guide  105  is locked in position by the downstream locking lever  400 . Therefore, it does not occur that when the fixation unit  40  is out of the main assembly of the image forming apparatus, the exit guide  105  is pushed open by an operator. Therefore, it does not occur that when the fixation unit  40  is out of the main assembly of the image forming apparatus, a finger of an operator accidentally enters the fixation unit  40  through the downstream opening. 
         [0129]    During an operation for installing the fixing unit  40  into the main assembly frame  300 , the state of the fixation unit  40  changes from the one shown in  FIG. 9  to the one in  FIG. 8 . As the fixation unit  40  is inserted into the main assembly of the image forming apparatus in the direction X, the downstream locking lever  400  comes into contact with the downstream locking lever disengaging lever  402 . More concretely, the lever contacting portion  402   b  of the downstream locking lever disengaging member  402  comes into contact with the locking lever disengaging member contacting portion  40   b  of the downstream locking lever  400 , and presses the downstream locking lever  400  against the resiliency of the downstream lock spring  401 . 
         [0130]    Thus, the downstream locking lever  400  is pivotally moved about the pivot  400   a  in the clockwise direction of the drawing, into the preset position, in which the locking surface  400   c  of the downstream locking lever  400  does not make contact with the locking surface  105   c  of the exit guide  105 . Then, the downstream locking lever  400  is retained in this position, in which it does not lock the exit guide  105  in position. 
         [0131]    According to this embodiment, there is little possibility that when the fixation unit  40  is out of the main assembly of the image forming apparatus, a finger of an operator will come into contact with the internal components of the fixation unit  40 . 
         [0132]    Further, according to this embodiment, there is little concern that when the fixation unit  40  is out of the main assembly of the image forming apparatus, a finger of an infant touches the internal component of the fixation unit  40 . 
         [0133]    Further, according to this embodiment, it does not occur that when the fixation unit is out of the main assembly of an image forming apparatus, the exit guide located next to the opening is pushed open by an operator. 
         [0134]    Further, according to this embodiment, a sheet of recording medium can be removed from the fixation unit  40  when the fixation unit  40  is out of the main assembly of the image forming apparatus. 
         [0135]    Further, according to this embodiment, a sheet of recording medium can be removed from the fixation unit  40  when the fixation unit  40  is in the main assembly of the image forming apparatus. 
         [0136]    Further, according to this embodiment, it is on the recording medium exit side of the fixation unit  40  that above described effects can be obtained. 
         [0137]    Further, effects such as those obtained by the fixation units  40  in the first and second embodiments can be obtained even if the fixation unit  40  is structured as follows. 
         [0138]    That is, application of the present invention is not limited to a fixation unit which is removably installable in an image forming apparatus, and which is used as the image fixing portion of the image forming apparatus. For example, the present invention is also applicable to an image heating unit which applies heat and pressure to a sheet of recording medium, on which a fixed image, or an incompletely fixed image, is present, to modify the image in surface properties. Further, the present invention is applicable to a recording medium conveying unit made up of a pair of rotational components, such as a pair of registration rollers  9  of the secondary transferring portion  3  and a pair of recording medium conveyance rollers of the recording medium conveying portion  13 . 
         [0139]    In the first and second embodiments, the pair of rotational members of the fixation unit  40  were the heat roller (fixation roller  100 ) and contact roller  101 . However, the present invention is also applicable to a fixation unit having a heating belt and a contact belt, in place of the heat roller and contact roller, respectively. 
         [0140]    Further, in the first and second embodiments, the heat source disposed in the hollow of the heat roller was used to heat the heat roller. However, the present invention is also applicable to a fixation unit which electromagnetically heats its heat roller with the use of excitation coil or the like. 
         [0141]    Further, in the first and second embodiments, the image forming apparatus was structured so that as the fixation unit  40  is inserted into, or pulled out of, the main assembly of the image forming apparatus, the movable member is locked or unlocked. However, the present invention is also applicable to an image forming apparatus, the movable member of which is locked or unlocked by the movement (opening or closing) of the door. More concretely, the door is provided with a protrusion for disengaging the upstream locking lever, so that as the door is closed, the protrusion presses the upstream locking lever to disengage the lever. In the case of this structural arrangement, it is unnecessary to provide the main assembly of the image forming apparatus with a locking mechanism (engaging mechanism) for locking the sliding cover  200  when the fixation unit  40  is in the main assembly, but the door is open. 
         [0142]    Further, image forming apparatuses which employ an image heating unit, and to which the present invention is applicable, are not limited to image forming apparatuses for forming full-color images. That is, the present invention is also applicable to monochromatic image forming apparatuses. Further, image forming apparatuses to which the present invention is applicable are not limited to printers. That is, the present invention is also applicable to copying machines, facsimile machines, and multifunction image forming apparatuses capable of performing two or more functions of the preceding image forming apparatuses. 
       Embodiment 3 
       [0143]    Next, the third embodiment of the present invention is described. By the way, this embodiment is described with reference to a full-color printer of the so-called tandem type, which is an example of various image forming apparatuses. 
         [0144]    Referring to  FIG. 10 , the image forming apparatus  1  has a main assembly  10  (which hereafter will be referred to as apparatus main assembly). The apparatus main assembly  10  has a sheet feeding/conveying section  20 , an image forming means  30 , a sheet conveying section  40 , a sheet discharging section  50 , and a control section  60 . Incidentally, a sheet S is a sheet of recording medium across which an image is formed. For example, it may be a sheet of ordinary paper, a sheet of resinous substance (which may be used in place of ordinary paper), a sheet of cardstock, a sheet of film for an overhead projector, etc. 
         [0145]    The sheet feeding/conveying section  20  is in the bottom portion of the apparatus main assembly  10 . It has a sheet cassette  21  in which sheets S of recording paper or the like are stored in layers, and a feed roller  22 . It is structured to feed sheets of recording medium into the apparatus main assembly  10 , and convey them to the image forming means  30 . 
         [0146]    The image forming means  30  has an image forming section  31 , a laser scanner  35 , an intermediary transfer unit  33 , a secondary transferring section  34 , and a fixing device  70  as an example of unitized section. It forms images. 
         [0147]    The image forming section  31  has four image formation units  31   y ,  31   m ,  31   c  and  31   k  for forming four toner images, different in color, one for one, more specifically, yellow (y), magenta (m), cyan (c) and black (b) monochromatic toner images, respectively. The four image formation units can be individually installed into, or removed from, the apparatus main assembly  10  by a user. For example, the image formation unit  31   y  has: a photosensitive drum  32   y  which is an image bearing member on which a toner image is formed; an unshown charge roller; a development roller; a drum cleaning blade; toner; etc. By the way, the other image formation units  31   m ,  31   c , and  31   k  are the same in structure as the image formation unit  31   y , and therefore, will not be described in detail. 
         [0148]    The laser scanner  35  is an exposing means which exposes the peripheral surface of the photosensitive drums  32   y ,  32   m ,  32   c  and  32   k  to form an electrostatic latent image on the peripheral surface of the photosensitive drums  32   y ,  32   m ,  32   c  and  32   k.    
         [0149]    The intermediary transferring unit  33  is above the combination of the image formation units  31   y ,  31   m ,  31   c  and  31   k . It has: multiple rollers, more specifically, a driving roller  33   a , and four primary transfer roller  33   y ,  33   m ,  33   c  and  33   k , etc.; and an intermediary transfer belt  33   b  suspended by these rollers in a manner to envelop the rollers. The primary transfer rollers  33   y ,  33   m ,  33   c  and  33   k  are disposed so that they oppose the photosensitive drums  32   y ,  32   m ,  32   c  and  32   k , respectively. They are in contact with the intermediary transfer belt  33   b . As positive transfer bias is applied to the intermediary transfer belt  33   b  by the primary transfer rollers  33   y ,  33   m ,  33   c  and  33   k , the toner images on the photosensitive drums  32   y ,  32   m ,  32   c  and  32   k , which are negative in polarity, are sequentially transferred in layers onto the intermediary transfer belt  32   b . As a result, a full-color image is effected on the intermediary transfer belt  33   b.    
         [0150]    The secondary transferring section  34  has a secondary transfer internal roller  34   a , and a secondary transfer external roller  34   b . As positive secondary transfer bias is applied to the secondary transfer external roller  34   b , the full-color image on the intermediary transfer belt  32   b  is transferred onto a sheet S. Incidentally, the secondary transfer internal roller  34   a  supports the intermediary transfer belt  33   b  from within the loop which the intermediary transfer belt  33   b  forms. It is disposed so that it opposes the secondary transfer external roller  34   b  with the presence of the intermediary transfer belt  33   b  between the two rollers  34   a  and  34   b.    
         [0151]    The fixing device  70  has a fixation roller  71  (heating roller) and a pressure roller  72  as its primary components. It is structured so that as a sheet S is conveyed through it, remaining pinched between the fixation roller  71  and pressure roller  72 , the toner image transferred onto the sheet S is subjected to heat and pressure, whereby the toner image is fixed to the sheet S. That is, the fixation roller  71  and pressure roller  72  are in the apparatus main assembly  10 , and make up parts of the image forming means. The detailed structure of the fixing device  70  is described later. 
         [0152]    The sheet conveying section  40  has: a recording medium conveyance passage  41 , which is on the immediately upstream side of the secondary transferring section  34 ; a recording medium conveyance passage  42 , which is on the immediately upstream side of the fixing device  70 ; a recording medium conveyance passage  43  which is on the immediately downstream side of the fixing device  70 ; a recording medium discharge passage  44 ; and a recording medium re-conveyance passage  45 . The sheet conveying section  40  is structured so that as a sheet S is fed into the apparatus main assembly  10  from the sheet feeding/conveying section  20 , the sheet conveying section  40  conveys the sheet S to the image forming means  30 , and then, to the sheet discharging section  50 . 
         [0153]    The image forming apparatus is structured so that as a sheet S is fed into the apparatus main assembly  10 , the sheet S is conveyed to the secondary transferring section  34  through the recording medium conveyance passage  41 . The recording medium conveyance passage  41  is provided with a pair of registration rollers  47 , which catch the sheet S and temporarily hold the sheet S to correct the sheet S in attitude if the sheet S happens to be slanted relative to the recording medium conveyance direction. The recording medium conveyance passage  42 , which is on the immediately upstream side of the fixing device  70  catches the sheet S as the sheet S comes out of the secondary transferring section  34 , and guides the sheet S from the secondary transferring section  34  to the fixing device  70 . The recording medium conveyance passage  43  catches the sheet S as the sheet S comes out of the fixing device  70 , and then, guides the sheet S from the fixing device  70  to the flapper  46 . 
         [0154]    The sheet discharge passage  44  catches the sheet S as the sheet S passes by the flapper  46 , and then, guides the sheet S from the flapper  46  to the sheet discharging section  50 . The re-conveyance passage  45  is for reversing the sheet S in conveyance direction with the utilization of the sheet discharge passage  44  after the formation of an image on one of the two surfaces of the sheet S, and then, conveying the sheet S to the image forming means  30  to form an image on the other surface of the sheet S. 
         [0155]    The sheet discharging section  50  has: a pair of discharge rollers  51 , which are at the downstream end of the sheet discharge passage  44 ; and a face-down delivery tray  52 , which is on the immediately downstream side of the pair of discharge rollers  51 . As the sheet S is delivered to the pair of discharge rollers  51  through the sheet discharge passage  44 , the discharge rollers  55  discharge the sheet S into the face-down delivery tray  52 . 
         [0156]    Referring to  FIG. 10(   b ), the control section  60  is made of a computer, which comprises: a CPU  61 ; a ROM which stores programs for controlling various sections of the image forming apparatus; a RAM  63  which temporarily stores data; and an input/output circuit (I/F)  64  which exchanges signals with external devices. The control section  60  is in connection to the sheet feeding/conveying section  20 , image forming means  30 , sheet conveying section  40 , and sheet discharging section  50 , through the input/output circuit  64 . It exchanges signals with these sections, and also, controls their operations. 
         [0157]    Next, referring to  FIG. 10(   a ), the image forming apparatus  1  structured as described above is described about its image forming operation and recording medium conveying operation. 
         [0158]    As an image forming operation is started, first, the laser scanner  35  begins to scan the peripheral surface of each of the photosensitive drums  32   y ,  32   m ,  32   c  and  32   k , with the beam of laser light it emits while modulating the beam according to the image formation information transmitted thereto from an unshown personal computer or the like. The peripheral surface of each of the photosensitive drums  32   y ,  32   m ,  32   c  and  23   k  has been uniformly charged to preset polarity and potential level. Thus, as a given point of the uniformly charged portion of the peripheral surface of each photosensitive drum  32  is exposed to the beam of laser light, it reduces in the amount of electric charge. Consequently, an electrostatic latent image is effected on the peripheral surface of each photosensitive drum  32 . This electrostatic latent image is developed by the yellow, magenta, cyan, or black toner, supplied to the peripheral surface by the development roller, into a visible image, that is, a toner image (image formed of toner). 
         [0159]    The monochromatic toner images, different in color, are sequentially transferred in layers onto the intermediary transfer belt  33   b  by the primary transfer bias applied to each of the primary transfer rollers  33   y ,  33   m ,  33   c  and  33   k . Consequently, a full-color toner image is effected on the surface of the intermediary transfer belt  33   b.    
         [0160]    While the monochromatic toner images, different in color, are formed, the multiple sheets S stored in the sheet cassette  21  are separated one by one, and fed into the apparatus main assembly  10 . Then, the pair of registration rollers  47  are driven with such timing that each sheet S coincides in position with the full-color toner image on the intermediary transfer belt  33   b , in the secondary transferring section  34 . Thus, the sheet S is conveyed to the secondary transferring section  34 , in which the four monochromatic toner images, different in color, of which the full-color toner image is formed on the intermediary transfer belt  33   b , are transferred together onto the sheet S, by the secondary transfer bias applied to the external roller  34   b , in the secondary transferring section  34 . 
         [0161]    After the full-color toner image is transferred onto the sheet S, the sheet S is conveyed to the fixing device  70 , in which the full-color toner image is subjected to heat and pressure. Therefore, the four toners, different in color, of which the full-color image is formed, melt and mix, and then, become fixed to the sheet S. That is, the full-color toner image is fixed to the sheet S. Thereafter, the sheet S having the fixed toner image is discharged by the sheet discharging section  50  which is on the downstream side of the fixing device  70 . 
         [0162]    The apparatus main assembly  10  has an external cover  12  and a hinge  11 . The external cover  12  covers the sheet conveyance passage  40  side of the apparatus main assembly  10 , and is pivotally movable about the hinge  11  to be opened or closed. The external cover  12  is provided with a pair of walls, which oppose each other, and form the re-conveyance passage  45  between them. Further, the external cover  12  has: a pre-secondary-transfer recording medium conveyance passage  41 , pre-fixation recording medium conveyance passage  42 , post-fixation recording medium conveyance passage  43 , and recording medium discharge passage  44 , which are in one (right one in  FIG. 10 ) of the two walls. 
         [0163]    As the external cover  12  is opened, the recording medium conveying section  40  is exposed, from its pre-secondary-transfer recording medium conveyance passage  41  to its pair of discharge roller  51 , except for the recording medium passage in the fixing device  70 . Thus, opening the external cover  12  makes it possible to perform maintenance operations such as removing a sheet S stuck in the recording medium conveying section  40 . 
         [0164]    Next, referring to  FIGS. 11-18 , the fixing device  70  is described in detail. 
         [0165]    Referring to  FIG. 11 , the fixing device  70  has a pair of rollers, more specifically, the fixation roller  71  and pressure roller  72 , which are for applying heat and pressure to the toner image on the sheet S to fix the toner image to the sheet S. The fixation roller  71  is a cylindrical hollow member. Its surface layer is a parting layer which is low in coefficient of friction. There is a halogen heater in the hollow of the fixation roller  71 , to heat the fixation roller  71 . The fixation roller  71  is supported by a pair of pressure application lever  74 , with the placement of a pair of fixation roller bearings  75  between the pressure application levers  74  and fixation roller  71 . The pressure roller  72  is supported by a pair of supporting members  76  with the placement of a pair of pressure roller bearings  77  between the pressure roller  72  and supporting members  76 . The pressure application lever  74  and supporting member  76  are in connection to each other, and are pivotally movable relative to each other about their points of connection. The fixing device  70  is provided a pair of springs  78  which generate pressure in such a direction that reduces the gap between the pressure application lever  74  and pressure roller  72 . That is, the fixation roller  71  and pressure roller  72  are under the pressure generated by the springs  78 . 
         [0166]    Next, referring to  FIGS. 12 and 13 , there is a nip N (area of contact) between the peripheral surface of the fixation roller  71  and the peripheral surface of the pressure roller  72 . Further, there is provided an entrance guide  80  on the upstream side of the nip N. The upstream side of the pressure roller  72  is covered with the entrance guide  80 . There is provided an exit guide  81  on the downstream side of the nip N. The entrance guide  80  and exit guide  81  assist (support) a sheet S as the sheet S is conveyed through the fixing device  70 . The fixing device  70  is structured so that the fixation roller  71  and pressure roller  72  are rotationally driven by a driving mechanism (unshown) to convey a sheet S between the fixation roller  71  and pressure roller  72  to apply heat and pressure to the unfixed toner image on the sheet S to fix the toner image, and then, the sheet P is conveyed further downstream. 
         [0167]    Referring to  FIG. 12 , the right-hand side of the double-dot chain line shows the components attached to the external cover  12 . Thus, as the external cover  12  is opened, the components on the right-hand side of the double-dot chain line are moved away from the apparatus main assembly  10 . Consequently, the fixing device  70  is exposed as shown in  FIG. 13 . Incidentally,  FIG. 13  does not show the component of the apparatus main assembly  10 , which supports the fixing device  70 . 
         [0168]    On the upstream side of the nip N, there is a sliding cover  90 , as a fixing device entrance cover, which is on the opposite side of the exit guide  80 . Referring to  FIGS. 12-15 , the sliding cover  90  has: a main section  91 , the lengthwise direction of which coincides with the widthwise direction of the sheet S which is being conveyed through the nip N; a pair of support portions  92  which are at the lengthwise ends of the main section  91 , one for one; a pair of contact portions  93  which protrude from the support portion  92 , one for one, toward the entrance guide  80 . The support portion  92  is supported by the main section of the casing of the fixing device  70 , being enabled to linearly move in the direction which is roughly perpendicular to the recording medium conveyance direction (indicated by arrow marks Q and R in  FIGS. 12 and 13 ). That is, the sliding cover  90  is slidable in the same direction as the direction in which the external cover  12  is movable. 
         [0169]    As the support portion  92  is slid relative to the main section  70   a  of the fixing device casing, the entirety of the sliding cover  90  is slid relative to the main section  70   a , and the main section  91  of the sliding cover  90  covers the fixation roller  71 , and the upstream side of the nip N. That is, the sliding cover  90  is enabled to move between its open position in which it exposes the fixation roller  71  and the upstream side of the nip N as shown in  FIG. 12 , and its closed position in which it covers the fixation roller  71  and the upstream side of the nip N as shown in  FIG. 13 , to take one of the two positions. 
         [0170]    The support section  92  is under the pressure generated in the direction to press the support section  92  toward the entrance guide  80 , by a spring  94 , as the first pressure applying means, which is a compression spring attached to the main section  70   a  of the fixing device casing. That is, the spring  94  continuously pushes the sliding cover  90  toward the closed position. Therefore, when the fixing device  70  is out of the apparatus main assembly  10 , the sliding cover  90  is kept by the spring  94  in the state in which the end surface  92   a  of the support section  92  is in contact with the entrance guide  80 , at both of the lengthwise ends of the entrance guide  80 , as shown in  FIG. 13 . That is, when the fixing device  70  is out of the apparatus main assembly  10 , the main section  91  of the sliding cover  90  keeps the fixation roller  71  and the upstream side of the nip N covered, preventing thereby a finger, or the like, of a user, from coming into contact with the fixation roller  71  and nip N, from the upstream side of the nip N. 
         [0171]    Further, the fixing device  70  is structured so that even when the end surface  92   a  of the support section  92  is in contact with the entrance guide  80 , there remains a small gap V, as a sheet passage, between the entrance guide  80  and the main section  91  of the sliding cover  90 . Therefore, even if the sliding cover  90  is closed by the opening of the external cover  12 , there is always the small gap V. Therefore, even if a paper jam occurs and stops recording medium conveyance when a sheet S is between the fixation roller  71  and pressure roller  72 , it is possible to extract the sheet P between the fixation roller  71  and pressure roller  72  through the gap V by opening the external cover  12 . 
         [0172]    By the way, the portion of the recording medium conveying section, which is on the downstream side of the fixation roller  71  and nip N, is long and narrow. Therefore, even if there is nothing to cover the recording medium conveying section, a finger of a user, or the like, is prevented from touching the fixation roller  71  and nip N, from the downstream side of the fixing device  70 . 
         [0173]    Next, referring to  FIGS. 14 and 15 , the structure of the mechanism for locking or unlocking the sliding cover  90  is described. 
         [0174]    The main assembly frame  13  of the apparatus main assembly  10  is provided with a pair of fixing devise supporting sections  14  for supporting the fixing device  70 . The fixing device supporting sections  14  are located so that they will be in the adjacencies of the lengthwise ends of the fixing device  70  when the fixing device  70  is in the main assembly frame  13 . Thus, when the fixing device  70  is in the apparatus main assembly  10 , it is on the fixing device supporting section  14 , and remains immovably held to the main assembly frame  13  by a combination of a lever (unshown) and a spring (unshown). 
         [0175]    The fixing device supporting section  14  has: a locking lever  15  as a locking member for immovably locking the sliding cover  90 ; and a locking spring  16  as the second pressure generating means for keeping the sliding cover locking lever  15  pressured. The locking lever  15  is supported by the fixing device supporting section  14 . It is a pivotally movable lever which is pivotally movable about a pivot  15   a . It is under the pressure generated by the locking spring  16  in the direction to pivot the locking lever  15  in a preset direction. Further, it is capable of engaging with the sliding cover  90  from the direction perpendicular to the direction in which the sliding cover  90  can be opened or closed. 
         [0176]    The locking lever  15  can be switched in position between a locking position and an unlocking position. The locking position is the position in which the locking lever  15  remains engaged with the sliding cover  90  when the sliding cover  90  is in its closed position. That is, the locking position is the position in which the locking lever  15  keeps the sliding cover  90  immovably locked in the closed position ( FIG. 18 ). The unlocking position is the position in which the locking lever remains disengaged from the sliding cover  90 , and therefore, allows the sliding cover  90  to move ( FIGS. 16 and 17 ). Further, the locking spring  16  is a compression spring, and keeps the locking lever  15  continuously pressured in the direction to move the locking lever  15  from the locking position to the unlocking position. 
         [0177]    Further, the locking lever  15  has: a parallel surface (first surface of contact)  15   b  and a slant surface (second surface of contact)  15   c , which face the second protrusion  18  of the external cover  12 . The locking lever  15  is structured so that as it is moved into its unlocking position, its parallel surface  15   b  becomes roughly parallel to the closing direction, indicated by an arrow mark Q, of the external cover  12 . The slanted surface  15   c  is angled relative to the parallel surface  15   b , and extends from the parallel surface  15   b  toward the pivot  51   a.    
         [0178]    Next, referring to  FIG. 18 , the locking lever  15  has: a locking surface (surface of contact)  15   d , which engages with the locking surface (surface of contact)  92   b  of the sliding cover  90 , which is the downwardly facing surface of the support section  92  of the sliding cover  90 , as the locking lever  15  is moved into the locking position. The locking lever  15  and sliding cover  90  are shaped so that their locking surfaces  15   d  and  92   b , respectively, can contact with each other with the presence of no gap between them. Further, the angle θ of the locking surface  92   b  of the sliding cover  90 , relative to a line T which coincides with the pivot  15   a  of the locking lever  15 , and also, the inward edge of the locking surface  92   b  of the sliding cover  90 , is accurate and on the opening side (indicated by arrow mark R) of the line T. Therefore, even if a user happens to apply such force that works in the direction (indicated by arrow mark Q) to close the sliding cover  90 , with his or her finger, or the like, no moment is generated in the direction to disengage the locking lever  15 , and therefore, the sliding cover  90  is not going to open. 
         [0179]    The wall  42   a  of the external cover  12 , which is a part of the pre-fixation recording medium conveyance passage  42 , is provided with first and second protrusions  17  and  18 , as parts of linkage, which are located so that they will be in the adjacencies of the fixing device  70  when the fixing device  70  is in the apparatus main assembly  10 . The first and second protrusions  17  and  18  are positioned so that as the external cover  12  is opened or closed, the first and second protrusions  17  and  18  come into contact with, or separate from, the sliding cover  90  of the fixing device  70  in the image forming apparatus, and the locking lever  15 , to move the sliding cover  90  and locking lever  15 . 
         [0180]    When the external cover  12  is in its closed position, the first protrusion  17  keeps the sliding cover  90  in the open position against the resiliency of the spring  94 , and the second protrusion  18  keeps the locking lever  15  in the position in which the locking lever  15  remains disengaged from the sliding cover  90  against the resiliency of the locking spring  16 . On the other hand, when the external cover  12  is open, the first protrusion  17  keeps the sliding cover  90  in the closed position against the resiliency of the spring  94 , and the second protrusion  18  keeps the locking lever  15  in the locking position against the resiliency of the locking spring  16 . 
         [0181]    In this embodiment, the sliding cover  90 , spring  94 , locking lever  15 , locking spring  16 , first protrusion  17 , and second protrusion  18  make up a cover opening/closing device  2 . That is, the image forming apparatus  1  has the cover opening/closing device, and image forming means  30 . 
         [0182]    Next, referring to  FIGS. 12-18 , the movement of the sliding cover  90  and locking lever  15  of the image forming apparatus  1  in this embodiment is described in detail. 
         [0183]    Referring to  FIG. 12 , during an image forming operation, the fixing device  70  is in the apparatus main assembly  10 , and the external cover  12  remains closed, and therefore, the wall  42   a  of the pre-fixation sheet passage  42  is in its image formation position. Further, the end surface  93   a  of the contact portion  93  of the sliding cover  90  has been moved (pushed) in the closing direction Q by the first protrusion  17 , and the main section  91  of the sliding cover  90  is open, as shown in  FIG. 12 . Further, the sliding cover  90  is kept in contact with the first protrusion  17  by the resiliency of the spring  94 . 
         [0184]    Next, referring to  FIG. 16 , the locking lever  15  is in the position in which it does not contact the sliding cover  90 , that is, in its unlocking position, because the second protrusion  18  moved the locking lever  15  in the opposite direction from the direction W which is roughly perpendicular to the closing directing Q of the external cover  12 , by pressing on the parallel surface  15   b  of the locking lever  15 . Further, the locking lever  15  is kept in contact with the second protrusion  18  by being kept pressured in the direction W, that is, toward the external cover  12 . 
         [0185]    Thus, as the external cover  12  is opened when the abovementioned components of the apparatus main assembly  10  and fixing device  70  are in the state shown in  FIGS. 12 and 16 , the wall  42   a  moves with the external cover  12  in the direction R in which the external cover  12  is to be opened. Thus, the first and second protrusions  17  and  18  move in the direction R in which the external cover  12  is opened. This movement of the protrusions  17  and  18  allows the sliding cover  90 , which is in contact with the first protrusion  17 , to be moved in the direction (the same as opening direction R) to close the recording medium passage, by the resiliency of the spring  94 . 
         [0186]    The locking lever  15  is under the pressure generated by the locking spring  16  in the direction to move the locking lever  15  in the direction W, and the parallel surface  15   b  and second protrusion  18  are in contact with each other. Therefore, as long as the second protrusion  18  remains in contact with the parallel surface  15   b , the locking lever  15  does not begin to move. The parallel surface  15   b  is given a preset length to create such a time lag that the sliding cover  90  begin to move before the locking lever  15  begins to move. 
         [0187]    As the external cover  12  is opened further, that is, moved in the direction R, the first protrusion  17  is moved in the direction R (external cover opening direction), and the sliding cover  90  is moved in the direction R by the resiliency of the spring  94 . Consequently, the end surface  92   a  of the support section  92  of the sliding cover  90  comes into contact with the entrance guide  80 , that is, the sliding cover  90  is moved into its closed position in which it provides the recording medium passage with the gap V. With the provision of the gap V, the sheet S remaining pinched between the fixation roller  71  and pressure roller  72  can be easily extracted. Therefore, it is possible to prevent the problem that when a jammed sheet S is removed, the sheet S tears and/or the sliding cover  90  is soiled by the unfixed toner on the sheet S. 
         [0188]    As the external cover  12  is opened further when it is in the state shown in  FIG. 17 , the first protrusion  17  separates from the sliding cover  90 , and also, the point of contact between the second protrusion  18  and locking lever  15  begins to move from the parallel surface  15   b  to the slanted surface  15   c . Then, as the second protrusion  18  comes into contact with the slanted surface  15   c , the locking lever  15  begins to be pivotally moved toward the sliding cover  90  by the resiliency of the locking spring  16 , because the slanted surface  15   c  is slanted toward the pivot  15   a  of the locking lever  15 . Since the parallel surface  15   b  is given the preset length, there is provided such a time lag that the locking lever  15  begins to move after the sliding cover  90  stops moving. 
         [0189]    Referring to  FIG. 18 , as the external cover  12  is opened further when the fixing device  70  is in the above described state, the locking surface  15   d  of the locking lever  15  begins to engage with the locking surface  92   b  of the sliding cover  90 , and moves into the position in which it locks the sliding cover  90 . Then, the locking lever  15  is kept in this position by the resiliency of the locking spring  16 . At this point, the locking surface  15   d  of the locking lever  15  and the locking surface  92   b  of the sliding cover  90  form the accurate angle θ, relative to the straight line T which is perpendicular to the line which coincides with the outward edge of the locking surface  92   b  of the sliding cover  90  and the pivot  15   a  of the locking lever  15 . Further, the two surfaces  92   b  and  15   d  are on the closing side of the line T. Therefore, even if force happens to be applied to the sliding cover  90  in the direction (indicated by arrow mark Q) to open the sliding cover  90  by a finger, or the like, of an operator, the force does not generate moment in the direction to disengage the locking lever  15 , an therefore, the sliding cover  90  is prevented from being opened. 
         [0190]    As the fixing device  70  is removed from the apparatus main assembly  10  when it is in the state shown in  FIG. 18 , its state changes into the one shown in  FIG. 13 . The direction in which the locking lever  15  keeps the sliding cover  90  locked is opposite from the direction (indicated by arrow mark R) in which the fixing device  70  is extracted from the apparatus main assembly  10 . Therefore, it does not occur that the locking lever  15  prevents the fixing device  70  from being extracted. 
         [0191]    Further, it is the apparatus main assembly  10  that is provided with the locking lever  15 . Therefore, after the extraction of the fixing device  70  from the apparatus main assembly  10 , the sliding cover  90  can be opened by a finger or the like. Therefore, even if pieces of jammed sheet S are remaining in the fixing device  70 , they can be easily removed. Normally, the interior of the fixing device  70  remains protected by the sliding cover  90 . 
         [0192]    Further, the locking lever  15  is positioned so that when the fixing device  70  is installed into the apparatus main assembly  10 , it will be on the upstream side of the fixing device  70  in terms of the direction in which the fixing device  70  is to be inserted into the apparatus main assembly  10 . Therefore, the locking lever  15  does not interfere with the insertion of the fixing device  70  into the apparatus main assembly  10 . Further, until the fixing device  70  is inserted all the way into the apparatus main assembly  10 , the locking lever  15  remains in the locking position. Therefore, it is after the completion of the installation of the fixing device  70  into the apparatus main assembly  10  that the sliding cover  90  remains locked by the locking lever  15 . 
         [0193]    The movements of the above described components of the fixing device  70  and apparatus main assembly  10 , which occur when the external cover  12  is closed, are opposite in direction from those which occur when the external cover  12  is opened. That is, as the external cover  12  is opened, the second protrusion  18  moves from the slanted surface  15   c  of the locking lever  15  to the parallel surface  15   b  of the locking lever  15 , causing, thereby, the locking lever  15  to move from the locking position to the unlocking position. Thus, the sliding cover  90  becomes slidable, and therefore, is moved from the closed position to the open position by the spring  94 . As the external cover  12  is completely closed, the image forming apparatus  1  is ready for image formation. 
         [0194]    In the case of the image forming apparatus  1  in this embodiment, as the external cover  12  is closed, not only does the movement of the external cover  12  cause the sliding cover  90  to cover the upstream side of the fixation roller  71  and pressure roller  72 , but also, causes the locking lever  15  to lock the sliding cover  90  in position. Therefore, it is impossible for a user to open the sliding cover  90  with his or her finger or the like when the external cover  12  is open. Therefore, it is possible to prevent the problem that the finger, or the like, of a user comes into contact with the fixation roller  71  and pressure roller  72  from the upstream side of the fixing device  70 . Therefore, it is possible to prevent the problem that the fixation roller  71  and pressure roller  72  are accidentally damaged and/or soiled by the finger, or the like, of a user. 
         [0195]    Also in the case of the image forming apparatus  1  in this embodiment, as the external cover  12  is closed, not only does the movement of the external cover  12  cause the locking lever  15  to unlock the sliding cover  90 , but also, allow the sliding cover  90  to be moved to expose the upstream side of the fixation roller  71  and pressure roller  72 . Therefore, it does not occur that the sliding cover  90  interferes with the conveyance of the sheet S while the external cover  12  remains closed, that is, while the image forming apparatus is in operation. 
         [0196]    This embodiment was described with reference to the fixing device  70  which employs a pair of compression rollers. However, this embodiment is not intended to limit the present invention in scope in terms of type of fixation roller and/or pressure roller. For example, the present invention is also applicable to a fixing device which employs a fixation belt unit, instead of the pressure roller  72  and/or fixation roller  71 . 
         [0197]    Further, this embodiment was described with reference to a case where the sliding cover  90  was on the upstream side of the combination of the fixation roller  71  and pressure roller  72 . However, this embodiment is not intended to limit the present invention in scope in terms of the positioning of the sliding cover. For example, the present invention is also applicable to a fixing device, the sliding cover  90  of which is on the downstream side of the combination of the fixation roller  71  and pressure roller  72 . 
         [0198]    Also in this embodiment, it was the apparatus main assembly  10  that was provided with the locking lever  15  and locking spring  16 . However, this embodiment is not intended to limit the present invention in scope in terms of where the locking lever  15  and locking spring  16  are attached. For example, the present invention is also applicable to a case where it is the fixing device  70  that is provided with the locking lever  15  and locking spring  16 . In this case, even after the removal of the fixing device  70  from the apparatus main assembly  10 , the sliding cover  90  remains locked by the locking lever  15 . Therefore, it is possible to prevent the problem that the sliding cover  90  is accidentally opened by a finer, or the like, of a user when the fixing device  70  is out of the apparatus main assembly  10 . 
         [0199]    Further, the present invention is also applicable to a case in which the fixing device  70  is provided with the sliding cover  90  and locking lever  15 , and apparatus main assembly  10  is provided with the spring  94  and locking spring  16 . In this case, it is desired that the fixing device  70  is provided with a holding means for keeping the locking lever  15  in the locking position even after the removal of the fixing device  70  from the apparatus main assembly  10 . As for this locking lever holding means, a mechanism that makes the locking lever  15  and sliding cover  90  elastically deform to make them engage with each other, a mechanism that employs magnets to make the locking lever  15  and sliding cover  90  engage with each other, or the like, may be used. 
         [0200]    Further, this embodiment was described with reference to a case in which the fixing device  70  is in the form of a unit, the cover opening/closing device of which is removably installable in the apparatus main assembly  10 . However, this embodiment is not intended to limit the present invention in scope in terms of the cover opening/closing device. For example, the present invention is also applicable to a combination of an image forming apparatus and a fixing device, the cover opening/closing device of which is used to open or close the component fixed to the apparatus main assembly  10 . 
         [0201]    Further, this embodiment was described with reference a case in which the cover of the unit removably installable in the apparatus main assembly  10  is opened or closed. However, this embodiment is not intended to limit the present in scope in terms of which cover of the image forming apparatus (fixing device) is opened or closed. For example, the present invention is also applicable to a case in which the cover of a unit removably installable in the external cover  12  is opened or closed. In this case, the first and second protrusion  17  and  18  are to be parts of the apparatus main assembly  10 . 
         [0202]    Further, this embodiment was described with reference to a case in which the fixing device  70  is a unit of the cover opening/closing unit  2 . However, this embodiment is not intended to limit the present invention in scope. For example, the present invention is also applicable to a shutter mechanism for the photosensitive drums  32   y ,  32   m ,  32   c , and  32   k  of the image forming portion  31 , a conveying apparatus having a pair of rotationally drivable rollers, and the like. 
         [0203]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
         [0204]    This application claims priority from Japanese Patent Applications Nos. 179806/2013 and 196966/2013 filed Aug. 30, 2013 and Sep. 24, 2013, respectively, which are hereby incorporated by reference.