Abstract:
An image forming apparatus includes a main assembly; a door member which supports one of roller members constituting a feeding nip for a recording material and which is rotatable about an axis parallel with said roller members in a sidewise direction of said main assembly to open said feeding nip; a pair of urging mechanisms which are provided at opposite end portions of said door member with respect to a front-rear direction of said main assembly and which fix said door member to said main assembly so as to be capable of opening and closing said door member, said urging mechanism urging said door member toward said main assembly in a state that said door member is closed; and an operating member provided with a grip for manual opening of said door member, said grip being disposed in a position nearer to a front side than a center portion position of said door member with respect to the front-rear direction, and said operating member being rotatably supported in a position nearer to a rear side than a center position between urging mechanisms.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus whose side door can be opened to expose the recording medium conveyance nip of the image forming apparatus. More specifically, it relates to the structure of the handle with which the side door of the image forming apparatus is provided to improve the apparatus in terms of the operation for disengaging the toggle latches of the like of the side door located at the front and rear ends of the side door. 
         [0002]    An image forming apparatus designed to form a toner image, transfer the toner image onto recording medium, and fix the toner image to the recording medium by applying heat and pressure to the toner image has been widely in use. Normally, an image forming apparatus of this type is provided with a side door for removing the jammed recording medium in the nip between a pair or pairs of recording medium conveyance rollers of the apparatus, and is structured so that the side door can be rotated out of the main assembly of the apparatus to expose the nip (Japanese Laid-open Patent Application 2006-139052). 
         [0003]    Japanese Laid-open Patent Application 2006-139052 discloses an image forming apparatus provided with a side door having a pair of pressure applying mechanisms (toggle latches, for example) located at the front and rear ends of the side door to keep the side door closed. Thus, as the side door is closed, the side door is kept pulled inward of the main assembly of the image forming apparatus, by the pressure applying mechanisms themselves, generating thereby pressure in the recording medium conveyance nip between each pair of recording medium conveyance rollers. Therefore, as the side door is opened, the pair of recording medium conveyance rollers are separated from each other. In other words, the operation of this image forming apparatus is easy to understand, simple in structure, and also, it is significantly smaller in the number of mechanical components. 
         [0004]    The image forming apparatus disclosed by Japanese Laid-open Patent Application 2006-139052 is a relatively small printer. Therefore, its side door can be opened even from the front side of the image forming apparatus. That is, the side door can be opened by pulling the door outward by grasping the mid portion of the door, in terms of the front-to-rear direction of the door. In the case of a large image forming apparatus, that is, an image forming apparatus capable of handling a sheet of recording medium of a size A3, it is greater in size in terms of the front-to-rear direction, and therefore, its side door is rather difficult to open from the front side of the apparatus. 
         [0005]    In the case of an image forming apparatus which is relatively large in terms of the front-to-rear dimension, the distance from the front end of its side door to the mid portion of the side door is also relatively large. Therefore, as a user grasps the handle of the side door, which is at the middle of the door in terms of the front-to-rear direction, while standing in front of the image forming apparatus, the arm of the user becomes stretched, making it difficult for the user to pull the side door outward. 
         [0006]    As a means for dealing with the above-described problem, it was proposed to place the side door handle closer to the front end of the side door. However, as a test version of this side door was tested, this proposal also turned out to be problematic. That is, the pair of pressure applying mechanism which are at the front and rear ends of the side door, one for one, failed to synchronize in the timing with which they disengaged from the main assembly of an image forming apparatus. Consequently, it made the tester feel nervous or even uncomfortable as the tester opened the side door. 
       SUMMARY OF THE INVENTION 
       [0007]    Thus, the primary object of the present invention is to ensure that a pair of pressure applying mechanisms with which the side door of an image forming apparatus is provided always and virtually simultaneously disengage from the main assembly of the apparatus, even if the side door is relatively large, in order to make it possible to provide an image forming apparatus, the side door of which feels more solid and easier to open to a user when the user opens the side door, than the side door of any of conventional image forming apparatus. 
         [0008]    According to an aspect of the present invention, there is provided an image forming apparatus comprising a main assembly; a door member which supports one of roller members constituting a feeding nip for a recording material and which is rotatable about an axis parallel with said roller members in a sidewise direction of said main assembly to open said feeding nip; a pair of urging mechanisms which are provided at opposite end portions of said door member with respect to a front-rear direction of said main assembly and which fix said door member to said main assembly so as to be capable of opening and closing said door member, said urging mechanism urging said door member toward said main assembly in a state that said door member is closed; and an operating member provided with a grip for manual opening of said door member, said grip being disposed in a position nearer to a front side than a center portion position of said door member with respect to the front-rear direction, and said operating member being rotatably supported in a position nearer to a rear side than a center position between urging mechanisms. 
         [0009]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic sectional view of the image forming apparatus in the first preferred embodiment of the present invention, and shows the structure of the apparatus. 
           [0011]      FIG. 2  is a schematic drawing for describing the mechanical structure of the side door of the image forming apparatus in  FIG. 1 , which is for exposing the recording medium conveyance passage of the apparatus. 
           [0012]      FIG. 3  is a drawing for showing the steps for exposing the recording medium conveyance passage of the image forming apparatus in  FIG. 1 . 
           [0013]      FIG. 4  is a side view of the toggle latch mechanism of the apparatus main assembly when the side door is closed. 
           [0014]      FIG. 5  is a side view of the toggle latch mechanism of the apparatus main assembly when the side door is open. 
           [0015]      FIG. 6  is a side view of the auxiliary locking mechanism when the side door is closed. 
           [0016]      FIG. 7  is a side view of the auxiliary locking mechanism when the side door is open. 
           [0017]      FIG. 8  is a side view of the image forming apparatus, and shows the positioning of the hand lever of the side door. 
           [0018]      FIG. 9  is a perspective view of the hand lever of the side door. 
           [0019]      FIG. 10  is a schematic sectional view of the hand lever of the side door, and its adjacencies, and is for describing the structure of the essential portions of the hand lever. 
           [0020]      FIG. 11  is a drawing for describing the force to which the side door is subjected as the hand lever is pulled. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Hereinafter, one of the preferred embodiments of the present invention is described in detail with reference to the appended drawings. The present invention can be embodied in the form an image forming apparatus other than the image forming apparatus which is described hereafter, even if the image forming apparatus to which the present invention is to be applied is partially or entirely different in structure from the image forming apparatus in the following embodiment of the present invention, provided that the image forming apparatus to which the present invention is to be applied is structured so that the side door of the apparatus can be rotationally opened, and also, so that the grip portion of the hand lever of the side door is positioned substantially frontward from the center of the side door. 
         [0022]    That is, the side door of the image forming apparatus in this embodiment is provided with a pair of pressure applying means, and one of the pair of pressure applying means is at one end of the side door of the image forming apparatus, in terms of the front-to-rear direction, whereas the other is at the other end of the side door. However, the preferred embodiment is not intended to limit the present invention in scope. That is, each end of the side door may be provided with two or more pressure applying means, or one end of the side door may be provided with one pressure applying means, whereas the other end is provided with a pair of pressure applying means. In other words, the present invention is applicable to any image forming apparatuses regardless of developer type, image formation method (system). That is, the present invention is applicable to any image forming apparatus regardless of whether the apparatus is of the tandem type or single drum type, and/or of the intermediary transfer type or direct transfer type, as long as the apparatus is structured so that one of the pair of recording medium conveyance rollers which form the recording medium conveyance nip, can be moved away from the other recording medium conveyance roller to expose the nip portion of the recording medium conveyance passage. 
         [0023]    Hereafter, this embodiment is described with reference to only the portions of an image forming apparatus, which are essential to the formation of a toner image. However, the present invention is also applicable to image forming apparatuses other than the one in this embodiment. That is, the present invention is applicable to a printer, a copying machine, a facsimile machine, a multifunction image forming apparatus, etc., which are examples of a combination of an image forming apparatus such as the image forming apparatus, and additional device or devices, equipment, and external structure (shell). The measurements and materials of the structural components of the image forming apparatus, their positional relationship, etc., in this embodiment of the present invention are not intended to limit the present invention in scope, unless specifically noted. 
         [0024]    The features of the image forming apparatus in this embodiment, which are similar to the general features of the image forming apparatus disclosed in Japanese Laid-open Patent Application 2006-139052 are neither illustrated nor described. 
       &lt;Image Forming Apparatus&gt; 
       [0025]      FIG. 1  is a schematic sectional view of an image forming apparatus  100 , to which the present invention is applicable. It describes the structure of the apparatus. As shown in  FIG. 1 , the image forming apparatus  100  is a full-color printer of the so-called tandem type, and also, is of the so-called intermediary transfer type. That is, the apparatus  100  has an intermediary transfer belt  7 , and yellow, magenta, cyan, and black image forming stations Sy, Sm, Sc, and Sb which are sequentially arranged in the listed order along the intermediary transfer belt  7 . 
         [0026]    In the image forming portion Sy, a yellow toner image is formed on a photosensitive drum  1   y , and is transferred onto the intermediary transfer belt  7 . In the image forming portion Sm, a magenta toner image is formed on a photosensitive drum  1   m , and is transferred onto the intermediary transfer belt  7 . In the image forming portions Sc and Sb, a cyan toner image and a black toner image are formed on a photosensitive drum  1   c  and a photosensitive drum  1   b , respectively, and are transferred onto the intermediary transfer belt  7 . 
         [0027]    The intermediary transfer belt  7  is suspended, and remains stretched, by a driver roller  72 , a backup roller  71 , and a tension roller  73 . It is rotated by the driver roller  72  in the direction indicated by an arrow mark R 2 . After the transfer of the four monochromatic toner images, different in color, onto the intermediary transfer belt  7 , the toner images are conveyed to the second transfer station T 2 , and are transferred (second transfer) onto a sheet P of recording medium. 
         [0028]    A recording medium cassette  10  contains a substantial number of sheets P of recording medium. As the sheets P are moved out of the cassette  10 , each sheet P is separated from the rest, and is conveyed to a pair of registration rollers  15 , which release the sheet P with such timing that the sheet P arrives at the second transfer station T 2  at the same time as the toner images on the intermediary transfer belt  7 . 
         [0029]    When the image forming apparatus  100  is in the mode for forming an image on only one surface of the sheet P of recording medium, the sheet P is conveyed to a fixing device  17  after the formation of an image on one of the two surfaces of the sheet P. In the fixing device  17 , the toner images on the sheet P are fixed to the surface of the sheet P by the heat and pressure applied to the toner images by the fixing device  17 . Then, the sheet P is discharged from the image forming apparatus  100  through a sheet outlet  19   m  by a pair of discharge rollers  19 . On the other hand, when the image forming apparatus  100  is in the mode for forming an image on both surfaces of the sheet P, the sheet P is moved back into the image forming apparatus  100  by the pair of discharge rollers  19  after being discharged halfway from the apparatus. Then, the sheet P is conveyed to pairs of recording medium conveyance rollers  18 , which are on the outward side of the apparatus relative to the fixing device  17 . 
         [0030]    Then, the sheet P of recording medium is conveyed for the second time to the pair of registration rollers  15  by the pairs of recording medium conveyance rollers  18 . Then, toner images are transferred onto the back side of the sheet P from the intermediary transfer belt  7 . Then, the toner images on the back surface of the sheet P are fixed to the back surface of the sheet P by the fixing device  17 . Then, the sheet P is discharged from the apparatus  100  by the discharge rollers  19 . The fixing apparatus  17  comprises: a fixation roller  17   a  having an internal heater; and a pressure roller  17   b . It fixes the unfixed toner images on the sheet P to the sheet P by applying heat and pressure to the sheet P and unfixed toner images thereon as the sheet P is conveyed through the fixing apparatus  17 . 
         [0031]    The image forming stations Sy, Sm, Sc, and Sb are roughly the same in structure although they are different in the color of the toner which their developing devices  4   y ,  4   m ,  4   c , and  4   b , respectively, use. Thus, in the following description of this embodiment of the present invention, only the image forming station Sy is described, since the description of the image forming stations Sm, Sc, and Sb are the same as that of the image forming station Sy except for the suffixes m, c, and b of their referential codes. 
         [0032]    The image forming station Sy has: a photosensitive drum  1   y ; and a charge roller  2   y , an exposing device  3 , a developing device  4   y , and a first transfer roller  5   y , which are in the adjacencies of the peripheral surface of the photosensitive drum  1   y . The photosensitive drum  1   y  comprises: a cylindrical substrate; and a photosensitive layer which covers virtually the entirety of the peripheral surface of the photosensitive drum  1   y  and is negative in intrinsic polarity. It is rotated in the direction indicated by the arrow mark R 1  at a preset process speed. The charge roller  2   y  uniformly charges the peripheral surface of the photosensitive drum  1  to the negative polarity. The exposing device  3  writes an electrostatic latent image on the uniformly charged portion of the peripheral surface of the photosensitive drum  1   y , by scanning the uniformly charged portion with a beam of laser light with the use of its rotational polygonal mirror. 
         [0033]    The developing device  4   y  forms a visible image, that is, an image formed of toner (which hereafter will be referred to simply as toner image) by developing the electrostatic latent image on the peripheral surface of the photosensitive drum  1   y , with the use of developer. The first transfer roller  5   y  transfers (first transfer) the toner image on the photosensitive drum  1   y , onto the intermediary transfer belt  7  by being supplied with a positive DC voltage. 
         [0034]    The recording medium cassette  10  stores in layers a substantial number of sheets P of recording medium of a preset size. It comprises a feed roller  11  and an internal plate  10 A. The feed roller  11  takes the sheets P out of the cassette  10 . The internal plate  10 A is rotatably held to the cassette  10  by its upstream end, and is kept pressured toward the feed roller  11  by unshown coil springs. The image forming apparatus  100  has also a pair of separation rollers  12 , which are on the downstream side of the feed roller  11  in terms of the direction in which the sheet P is conveyed by the feed roller  11 . As the sheets P are taken out of the recording medium cassette  10  by the feed roller  11 , they are sent to the compression nip of the pair of separation rollers  12 . As the sheets P are fed into the compression nip, one of the sheets P is separated from the rest by the difference between the top and bottom separation rollers  12  in terms of their friction relative to the sheet P. Then, the separated sheet P is conveyed to the pair of registration rollers  15  by two pairs of recording medium conveyance rollers  13  and two pairs of recording medium conveyance rollers  14 . 
       &lt;Door Unit&gt; 
       [0035]      FIG. 2  is a schematic side view of the side door, and its adjacencies, of the main assembly of the image forming apparatus  100 . The side door is for allowing the recording medium conveyance passage of the apparatus main assembly to be exposed.  FIG. 3  is a drawing for describing the operational steps for opening the side door to expose the recording medium conveyance passage. 
         [0036]    In order to allow a user to take care of the recording medium jam, the image forming apparatus  100  is structured so that a part of its external shell can be opened. That is, the external shell is provided with a door unit  100 B, which is opened or closed relative to the external shell of the apparatus main assembly. The door unit  100 B makes up a part of the recording medium conveyance passage, and can be rotationally opened or closed relative to the external shell, about its bottom edge by which it is attached to the external shell. When it is necessary to take care of the recording medium jam, the door unit  100 B can be opened to expose the recording medium conveyance passage, wide enough for the recording medium jam to be easily taken care of. 
         [0037]    Referring to  FIG. 2 , the recording medium conveyance passage can be exposed by rotationally moving the door unit  100 B out of the apparatus main assembly  100 A about a door unit supporting shaft  27  in the clockwise direction. That is, the door unit  100 B can be moved out of the apparatus main assembly  100 A in such a manner that the door unit  100 B rotationally moves out of the apparatus main assembly  100 A. As the door unit  100 B is rotationally moved out of the apparatus main assembly  100 A, the recording medium conveyance passage is widely exposed, allowing thereby the jammed sheet P of recording medium to be easily taken out of the apparatus main assembly  100 A. As described above, the apparatus main assembly  100 A has the fixing apparatus  17 , intermediary transfer belt  7 , backup roller  71 , recording medium conveyance roller  14 A, and registration roller  15 A, which are within the apparatus main assembly  100 A. 
         [0038]    The door unit  100 B, which is an example of a side door, supports one of each of pairs of rollers which form the nips, one for one, through which the sheet P of recording medium is conveyed. The apparatus main assembly  100 A and door unit  100 B are structured so that the door unit  100 B can be rotationally moved out (or back into) the apparatus main assembly  100 A, about the door unit supporting shaft  27 , which is parallel to the axial line of each of the recording medium conveyance rollers in order to move one of each of pairs of nip forming rollers away from the corresponding roller. The door unit  100 B comprises: a one-piece frame  28  molded of resinous substance; a second transfer roller  16 , a conveyance roller  14 B, a registration roller  15 B, etc., which are attached to the frame  28 . 
         [0039]    Next, referring to  FIG. 3(   a ), the apparatus main assembly  100 A is provided with a pair of toggle latch mechanisms  37 , which are parts of the aforementioned pressure applying mechanism. The toggle latch mechanisms  37  are positioned so that they correspond to the ends of the door unit  100 B in terms of the lengthwise direction of the recording medium conveyance rollers. They have a toggle arm  39  (lever) which can be switched in position to keep the door unit  100 B closed relative to the external shell of the apparatus main assembly  100 A, or to allow the door unit  100 B to be rotationally moved out of the external shell. Further, the door unit  100 B is provided with a pair of auxiliary locking mechanism  40 . The auxiliary locking mechanism  40  can be set to prevent the door unit  100 B from being opened. 
         [0040]    If a sheet P of recording medium becomes jammed in the recording medium conveyance passage of the image forming apparatus  100  for some reason during an image forming operation, and therefore, the image forming apparatus  100  stops, the jammed sheet P of recording medium has to be removed. The procedure for removing the jammed sheet P of recording medium is as follows: 
         [0041]    As a sheet P of recording medium becomes jammed in the recording medium conveyance passage of the image forming apparatus  100 , a message “Remove jammed paper by opening side door” is shown on the display of the unshown control panel of the apparatus main assembly  100 A. After the confirmation of the message, a user is to carry out the operation for opening the door unit  100 B. 
         [0042]    Next, referring to  FIG. 3(   b ), the user is to pull the hand lever  29  of the door unit  100 B in the outward direction of the apparatus main assembly  100 A. The hand lever  29  is one of the members to be operated to open the door unit  100 B. As the hand lever  29  is pulled, the auxiliary locking mechanism  40  is unlocked. When the door unit  100 B is remaining closed, each toggle latch mechanism  37  solidly attached to the apparatus main assembly  100 A remains engaged with a hook  35  solidly attached to door unit  100 B, and therefore, the pressure applying mechanism ( 37 ) keeps the door unit  100 B pressed upon the external shell of the apparatus main assembly  100 A by the front and rear end portions of the door unit  100 B. 
         [0043]    Therefore, while the door unit  100 B remains closed as shown in  FIG. 1 , a certain amount of pressure is present in the recording medium conveyance nip between each pair of the recording medium conveyance rollers. 
         [0044]    Next, referring to  FIG. 3(   c ), the user is to pull the hand lever  29 , which is an example of the members to be operated to open the door unit  100 B, further in the direction to open the door unit  100 B, the toggle arm  39  of the toggle latch mechanism  37  is flipped by the hook  35  of the door unit  100 B, allowing thereby the toggle latch mechanism  37  of the pressure applying mechanism to be disengaged from the hook  37 . Consequently, the pressure in the nip of each of the aforementioned pairs of recording medium conveyance rollers vanishes. Then, the door unit  100 B is to be opened, as far as it can be, by the pulling of the hand lever  29 , as shown in  FIG. 3(   d ), so that the recording medium conveyance passage is fully exposed to allow the jammed sheet P of recording medium to be removed. 
         [0045]    Then, the user is to remove the jammed sheet P of recording medium from the exposed recording medium conveyance passage. Then, the user is to close the door unit  100 B to enable the image forming apparatus  100  to restart the interrupted image forming operation. 
         [0046]    In the case of the image forming apparatus  100 , the second transfer roller  16  for transferring toner images onto a sheet P of recording medium, and the recording medium conveyance passage for two-sided image formation mode, are on the door unit  100 B as shown in  FIG. 2 . In other words, the door unit  100 B of the image forming apparatus  100  is greater in the number of components attached thereto than the door unit ( 100 B) of any of conventional image forming apparatuses. In other words, the door unit  100 B is greater in size and/or weight than the door unit ( 100 B) of any of conventional image forming apparatuses. Therefore, in order to compensate for the extra amount of the force, which has to be applied to the door unit  100 B to close the door unit  100 B, the apparatus main assembly  100 A is provided with the aforementioned pair of toggle latch mechanisms  37 , which are attached to the side plates of the apparatus main assembly  100 A. As for the position of the toggle latch mechanisms  37 , they correspond to the front and rear ends of the door unit  100 B, one for one. The details of the door unit  100 B are shown in  FIG. 8 . 
         [0047]    Usually, when a user operates an image forming apparatus, the user stands in front of the apparatus. Thus, if the hand lever for opening the side door of the apparatus is closer to the rear end of the apparatus, it is difficult to operate. Therefore, in the case of the image forming apparatus  100 , the hand lever  29  for opening or closing its door unit  100 B is located near the front end of the apparatus main assembly  100 A, in order to make it easier for the user to operate the hand lever  29  to open or close the door unit  100 B. 
       &lt;Toggle Latch Mechanism&gt; 
       [0048]      FIG. 4  is a drawing for describing the state of the toggle latch mechanism when the door unit  100 B is remaining closed, whereas  FIG. 5  is a drawing for describing the state of the toggle latch mechanism when the door unit  100 B is open. 
         [0049]    Referring to  FIG. 4  along with  FIG. 2 , the door unit frame  28  is provided with the aforementioned pair of hooks  35 , which engage with the corresponding toggle latch mechanisms  37 . On the other hand, the side plate  36  of the apparatus main assembly  100 A is provided with the aforementioned pair of toggle latch mechanisms  37 , which are precisely position relative to the side plate  36  so that they catch the hooks  35  at preset positions to precisely hold the door unit  100 B closed relative to the side plate  36 . That is, the door unit  100 B is held closed to the apparatus main assembly  100 A by being pulled inward of the apparatus main assembly  100 A by the toggle latch mechanisms  37  of the apparatus main assembly  100 A. 
         [0050]    As described above, the apparatus main assembly  100 A is provided with the pair of toggle latch mechanisms  37 , which correspond in position to the front and rear ends of the door unit  100 B, one for one. 
         [0051]    The amount of force F 1  which the front toggle latch mechanism  37  generates to hold the door unit  100 B closed is the same as the amount of force F 1  ( FIG. 1 ) which the rear toggle latch mechanism  37  generate to hold the door  100   b  closed. Each toggle latch mechanism  37  comprises: a toggle latch supporting shaft  38  attached to the side plate  36 ; a toggle arm  39  rotationally supported by the shaft  38 ; and a tension spring  39   a  which keeps the hook  35  pulled toward the toggle lever mechanism  37 . 
         [0052]    Next, referring to  FIG. 5  along with  FIG. 2 , as the user rotationally moves the door unit frame  28  outward by pulling the hand lever  29  outward, the hook  35  rotationally moves the toggle arm  39  outward against the resiliency of the tension spring  39   a . As the door unit frame  28  is rotationally moved outward beyond the angle at which the tension springs  39   a  is largest in tension, the toggle arm  39  is sprung in the opposite direction from the direction in which it was kept pressed. Thus, the hook  35  is pushed outward by the toggle arm  39 , being thereby disengaged from the toggle arm  39 . 
         [0053]    As for the procedure for moving the door unit  100 B back into the apparatus main assembly  100 A after the removal of the jammed sheet P of recording medium, first, the user is to rotationally move the door unit  100 B toward the side plate  36 . As the door unit  100 B is rotationally moved toward the side plate  36 , the toggle arm engaging portion  35   b  of the hook  35 , which is lower in position than the toggle arm engaging portion  35   a  of the hook  35 , pushes the toggle arm  39 , rotationally moving thereby the toggle arm  39 . As the angle by which the toggle arm  39  is rotated exceeds the angle at which the tension spring  39   a  is largest in tensional, the toggle arm  39  snappingly falls inward of the apparatus main assembly  100 A, engaging thereby with the toggle arm engaging portion  35   a  of the hook  35 . Thus, the hook  35  is pulled inward of the apparatus main assembly  100 A by the resiliency of the tension spring  39   a , creating thereby a nip with a proper amount of internal pressure, between the top and bottom rollers of each of the pairs of recording medium conveyance rollers. 
         [0054]    Providing the apparatus main assembly  100 A with the toggle latch mechanisms  37  can assist a user in terms of the amount of force required of the user to close the door unit  100 B, and therefore, can improve the door unit  100 B (image forming apparatus  100 ) in operability. 
       &lt;Auxiliary Locking Mechanism&gt; 
       [0055]      FIG. 6  is a side view of the auxiliary locking mechanism when the door unit  100 B is kept closed.  FIG. 7  is a side view of the door unit locking auxiliary mechanism when the door unit  100 B is open. 
         [0056]    Referring to  FIG. 6  along with  FIG. 2 , the door unit frame  28  is provided with a pair of auxiliary locking mechanisms  40 , each of which has a latch  41 , a levers  42  (as part of mechanical linkage), a lever  44  (part of mechanical linkage), and a compression spring  43 . The auxiliary latch  41  is for preventing the door unit  100 B from opening. The lever  42  is for disengaging the auxiliary latch  41 , and is in contact with the lever  44  (part of mechanical linkage). The door unit frame  28  rotationally supports a pair of auxiliary latch shafts  42   a , to which the linkage lever  42  is solidly attached. Therefore, the auxiliary latch  41  and linkage lever  42  rotationally move with the auxiliary latch shaft  42   a.    
         [0057]    The auxiliary latch shaft  42   a  is kept in a preset position by being under an auxiliary latch spring  43  (compression spring) which is between one end of the linkage lever  42  and the door unit frame  28 . The door unit frame  28  rotationally supports a handle linkage  44 , which is positioned so that its upper end can be pressed by a part of the hand lever  29 , and also, so that its bottom end is kept pressed by the top portion of the linkage lever  42 . 
         [0058]    Each of the front and rear plates  36  of the apparatus main assembly  100 A is provided with a cylindrical projection  45  (auxiliary lock catch), which is precisely positioned so that it catches the corresponding auxiliary latch  41  at a preset position. 
         [0059]    When the door unit  100 B is remaining closed, it is kept pulled inward of the apparatus main assembly  100 A by the toggle latch mechanisms  37  as shown in  FIG. 4 . As the door unit  100 B is pulled into the apparatus main assembly  100 A, the auxiliary latch  41  is guided by the cylindrical projection  45 , and becomes locked with the cylindrical projection  45 , as shown in  FIG. 6 . In other words, as the door unit  100 B is closed, not only is it locked by the toggle latch mechanisms  37 , but also, by the door unit locking auxiliary mechanisms. On the other hand, when the door unit  100 B is opened, the auxiliary locking mechanisms  40  are disengaged by the rotational movement of the hand lever  29  before the toggle latch mechanisms  37  are disengaged by the rotational movement of the hand lever  29 . 
         [0060]    As the door unit frame  28  is subjected to force, it is likely to deform. Thus, if a user pulls the door unit frame  28  by the portion of the door unit frame  28 , which is very close to the front end of the door unit frame  28  to open the door unit frame  28 , the door unit frame  28  is deformed by the torsional force generated as the door unit frame  28  is pulled. Thus, the front toggle latch mechanism  37  is disengaged from the door unit frame  28  before the rear toggle latch mechanism  37  is disengaged. That is, the front end portion of the door unit frame  28  separates from the apparatus main assembly  100 A while the rear end portion of the door unit frame  28  is still kept in contact with the apparatus main assembly  100 A by the rear toggle latch mechanism  37 . Therefore, the door unit frame  28  is deformed by the torsional force generated by the combination of the force generated by the rear toggle latch mechanism in the direction to pull the door unit frame  28 , and the force applied to the front end portion of the door unit frame  28  to open the door unit  100 B. 
         [0061]    In order for a user to open the door unit  100 B, the user has to pull the door unit  100 B with a force which is greater than the force applied to the door unit  100 B by the toggle latch mechanisms  37  in the direction to keep the door unit  100 B pulled inward of the apparatus main assembly  100 A. Therefore, if the frame  28  of the door unit  100 B deforms because the door unit  100 B is insufficient in rigidity, and/or because of the like reason, the door unit  100 B (image forming apparatus  100 ) feels flimsy and uncomfortable to operate. 
         [0062]    In particular, in the case where the hand lever  29 , which is to be pulled by a user to open the door unit  100 B, is positioned very close to the front side of the door unit  100 B, the force to which the front side of the door unit  100 B is subjected is significantly greater than the rear side of the door unit  100 B. In other words, the amount of the force to which the front toggle latch mechanism  37  is subjected becomes different from the amount of the force to which the rear toggle latch mechanism  37  is subjected. Consequently, the front end portion of the door unit  100 B separates from the apparatus main assembly  100 A before the rear end portion of the door unit  100 B separates from the apparatus main assembly  100 A, making the door unit  100 B (image forming apparatus  100 ) feel even more uncomfortable to the user. 
         [0063]    One of the solutions for preventing the door unit  100 B from feeling uncomfortable to operate is to insert reinforcing members such as metallic plates in the door unit frame  28  in order to minimize the door unit frame  28  in deformation. Such a solution, however, makes the door unit  100 B heavier, and also, increases the door unit  100 B in component count, increasing therefor the door unit  100 B in cost. 
         [0064]    In this embodiment, therefore, the door unit  100 B is kept locked also by the aforementioned pair of auxiliary locking mechanisms  40  on the front and rear sides, respectively, of the door unit  100 B so that it is only by operating (pulling) the hand lever  29  that the door unit  100 B can be opened. Further, the hand lever  29  is mounted in a specific way so that the force applied to the hand lever  29  by a user is transmitted to both toggle latch mechanisms  37  neither excessively nor insufficiently to disengage the toggle latch mechanisms  37  so that both toggle lever mechanisms  37  can be repeatedly disengaged virtually simultaneously in a satisfactory manner. Because the image forming apparatus  100  and its door unit  100 B are structured as described above, both toggle latch mechanisms  37  are disengaged at the same time, and therefore, the door unit  100 B is unlikely to be torsionally deformed. 
         [0065]    The hand lever  29  is provided with a latch  29   a  which is to be pulled by a user when the user wants to open the door unit  100 B. The latch  29   a  is near the front end of the hand lever  29 . The hand lever  29  is also provided with a mechanism, through which the hand lever  29  is in connection to an opposite portion of the door unit  100 B from the portion to which the aforementioned portion  29   a  is attached, in terms of the direction parallel to the lengthwise direction of each of the recording medium conveyance rollers. The linkage between the hand lever  29  and door unit frame  28  is structured so that as the hand lever  29  of the door unit  100 B is pulled by a user by the latch  29   a  in the direction to open the door unit  100 B, the two toggle latch mechanisms  37  virtually simultaneously disengage. 
       Embodiment 1 
       [0066]      FIG. 8  is a drawing for describing the positioning of the hand lever  29  relative to the door unit frame  28 .  FIG. 9  is a perspective view of the hand lever  29 .  FIG. 10  is a schematic sectional view of the essential portions of the hand lever  29 , which are on the inward side of the door unit  100 B.  FIG. 11  is a drawing for describing the force to which the door unit  100 B is subjected as the hand lever  29  is pulled by a user, by the latch  29   a.    
         [0067]    Referring to  FIG. 8 , the latch  29   a  of the hand lever  29 , which is to be pulled by a user when the user wants to open the door unit  100 B, is positioned very close to the front end of the door unit frame  28 . Thus, the user can open the door unit  100 B while remaining in the same position and attitude as those in which the user was when the user confirmed the message on the display of the control panel while standing upright in front of the apparatus main assembly  100 A. The latch  29   a  of the hand lever  29 , which the user is to pull to operate the hand lever  29 , is partially visible through an opening  28   n  of the door unit frame  28 . 
         [0068]    Next, referring to  FIG. 9 , it is the front end portion of the hand lever  29  that has the latch  29   a  and spring seat portion  29   c . The rear end portion of the hand lever  29  has the hole for the hand lever  29  to be rotationally supported by the door unit frame  28 . 
         [0069]    Next, referring to  FIG. 10 , the hand lever  29  is rotatably supported by its rear end portion, by a hand lever supporting shaft  29   b , which is on the rear side of the mid point between the pair of toggle latch mechanisms  37 . It makes up a part of a hand lever mechanism. The hand lever  29  is kept pressed toward the recording medium conveyance guide  33  the door unit  100 B by a compression spring  32 , which is between the spring seats  29   c  of the front end portion of the hand lever  29 . 
         [0070]    When the door unit  100 B remains closed, the front end portion of the hand lever  29  is kept in contact with a part of the recording medium conveyance guide  33 , whereby the hand lever  29  is kept within the door unit  100 B by the compression spring  32  in such a manner that the outward surface of the latch  29   a  of the hand lever  29  is level with the outward surface of the side plate  36  of the apparatus main assembly  100 A; the door unit  100 B is on the inward side of the side plate  36 . 
         [0071]    The door unit frame  28  is provided with a hand lever catch  34 , which catches the force applied to the hand lever  29  by a user to open the door unit  100 B. Incidentally, in the first embodiment, the roughly mid point of the door unit frame  28  means the range of the door unit frame  28 , which corresponds to the range of the apparatus main assembly  100 A, which falls within 50 mm from the mid point of the apparatus main assembly  100 A in terms of the front-to-rear direction of the apparatus main assembly  100 A. Incidentally, the overall length of the apparatus main assembly  100 A in its front-to-rear direction is 470 mm. 
         [0072]    As the latch  29   a  of the door unit  100 B is pulled by a user in the direction to open the door unit  100 B, the hand lever  29  rotates about the hand lever supporting shaft  29   b  of the door unit frame  28 , until the hand lever  29  comes into contact with the door unit frame  28 . 
         [0073]    As the latch  29   a  is pulled, the force applied to the latch  29   a  by the user to open the door unit  100 B is caught by the center portion of the door unit frame  28 , because the hand lever catch  34  of the door unit frame  28  is at roughly the mid point between the pair of toggle latch mechanisms  37 . Therefore, the problem that because the latch  29   a  (hand lever  29 ) is positioned close to the front end of the door unit frame  28 , the front side of the door unit  100 B separates from the apparatus main assembly  100 A before the rear side of the door unit  100 B separates from the apparatus main assembly  100 A, does not occur. Therefore, the door unit  100 B does not feel flimsy (uncomfortable) to the user when the user opens the door unit  100 B. 
         [0074]    Further, the door unit  100 B in this embodiment is structured so that the hand lever  29  is rotationally movable. Therefore, the deformation of the hand lever  29  in this embodiment is more difficult for a user to recognize than the deformation of a hand lever ( 29 ) immovably attached to the door unit  100 B. Therefore, the door unit  100 B of the image forming apparatus  100  in this embodiment is easier to open, and also, is significantly better in terms of how the door unit  100 B feels to a user when the user opens the door unit  100 B. 
         [0075]    Next, referring to  FIG. 11 , the hand lever catch  34  of the door unit frame  28  is at roughly the mid point of the door unit  100 B in terms of the front-to-rear direction. Thus, as the door unit  100 B pulled by a user by the latch  29   a  comes into contact with the hand lever catch  34 , the force applied to the latch  29   a  of the hand lever  29  to open the door unit  100 B begins to be directed as indicated by arrow marks. 
         [0076]    As the door unit frame  28  and hand lever  29  are subjected to the force applied by the user to open the door unit  100 B, they are deformed by the force. Therefore, as the hand lever  29  comes into contact with the hand lever catch  34  of the door unit frame  28 , a moment M, which works in the direction to rotationally move the hand lever  29  about the center of the hand lever catch  34  of the door unit  100 B, is generated in the joint between the hand lever  29  and the door unit frame  28 , that is, the hand lever supporting shaft  29   b.    
         [0077]    This moment M generates forces F 2  and F 3  in the front (left in  FIG. 11 ) and rear (right in  FIG. 11 ) toggle latch mechanisms  37 , respectively. The force F 2  works in the direction to pull the door unit  100 B away from the apparatus main assembly  100 A, whereas the force F 3  works in the direction to pull the door unit  100 B into the apparatus main assembly  100 A. 
         [0078]    The generation of these forces destroys the balance between the front and rear toggle latch mechanisms  37  in terms of the amount of the force which keeps the door unit  100 B held to the apparatus main assembly  100 A, making it likely for the front end of the door unit  100 B to separate from the apparatus main assembly  100 A ahead of the rear end of the door unit  100 B. In this embodiment, therefore, the hand lever catch  34  is positioned on the rear side of the center of the door unit frame  28  in terms of the front-to-rear direction, in order to cancel the above-described imbalance. Therefore, the door unit  100 B (image forming apparatus  100 ) in this embodiment is significantly better, in terms of how the door unit feels when a user opens the door unit, than a door unit ( 100 B), the hand lever catch ( 34 ) of which is at the center of the door unit frame  28 . 
         [0079]    In the first embodiment, the front and rear ends of the door unit  100 B are the same in terms of the amount of a force F 1  by which the hook  35  is held (pulled). Thus, the hand lever catch  34  is positioned at roughly center of the door unit  100 B. This setup, however, is not mandatory. For example, even if the present invention is applied to an image forming apparatus, the front and rear ends of the door unit  100 B of which are made different in the amount of the force with which they are kept pressed upon the side plate  36 , in order to make the front end of the nip which the second transfer roller ( 16 ) forms, different in nip pressure from the rear end of the nip, the same effects as those described above can be obtained by positioning the hand lever catch  34  at a point where the moment generated by the front toggle latch mechanism  37  and the moment generated by the rear toggle mechanism  37  cancel each other. 
         [0080]    Further, it is not mandatory that the hand lever  29  is rotationally movable. That is, it may be connected to the point of the door unit frame  28 , at which the moment generated by the front toggle latch mechanism  37  and the moment generated by the rear toggle mechanism  37  cancel each other. As for the positioning of the toggle latch mechanisms  37 , all that is necessary is that their positions corresponds to the front and rear ends of the door unit in terms of the direction parallel to the axial line of each recording medium conveyance rollers. In other words, the image forming apparatus  100  may be structured so that the hooks  35  are solidly attached to the apparatus main assembly  100 A, and the toggle lever mechanisms  37  are attached to the door unit  100 B. 
         [0081]    As will be evident from the description of the image forming apparatus  100  and its door unit  100 B in this embodiment, the present invention can provide a door unit for an image forming apparatus, which is significantly smaller in component count, simpler in structure, better in operability, and also, has significantly better operational feel than any of conventional door units. In other words, the present invention can provide an image forming apparatus which is significantly superior in customer satisfaction than any of conventional image forming apparatuses. 
         [0082]    As will be evident also from the description of the image forming apparatus  100  and its door unit  100 B in this embodiment, according to the structural design of the image forming apparatus in the first embodiment, the force applied to the door unit  100 B is caught by the center portion of the door unit  100 B. Thus, the door unit  100 B does not feel as flimsy and uncomfortable as any of conventional door units. Further, it does not occur that one end of the door unit  100 B separates from the apparatus main assembly  100 A ahead of the other end. That is, the present invention can realize a door unit (side door unit) for an image forming apparatus, which is smaller in component count and simpler in structure than any of conventional door units. 
         [0083]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0084]    This application claims priority from Japanese Patent Application No. 124705/2010 filed May 31, 2010 which is hereby incorporated by reference.