Abstract:
An image forming apparatus having a sheet support is described. Using one or more configurations, the number of parts for the sheet support may be reduced.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2006-267440, filed on Sep. 29, 2006, the entire subject matter of which is incorporated herein by reference. 
       FIELD 
       [0002]    Aspects of the invention relate to image forming apparatuses. 
       BACKGROUND 
       [0003]    An image forming apparatus including a main mechanical part covered with a housing is known. The main mechanical part includes an ejection portion configured to eject a recording sheet that undergoes a printing process in an image forming portion. The image forming apparatus is provided with an upper sheet receiving portion disposed on an upper surface of the housing and a side sheet receiving portion disposed on a side of the housing, which are configured to receive a recording sheet ejected from the ejection portion. The image forming apparatus includes a guide portion disposed facing the ejection portion and configured to guide a recording sheet ejected from the ejection portion selectively to the upper or side sheet receiving portion when the guide portion slidingly contacts the recording sheet. 
       SUMMARY 
       [0004]    Aspects of the invention provide an image forming apparatus in which the number of parts may be reduced. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Illustrative aspects of the invention will be described in detail with reference to the following figures in which like elements are labeled with like numbers and in which: 
           [0006]      FIG. 1  is a side sectional view of a general structure of a laser printer as an image forming apparatus according to a first illustrative embodiment of the invention; 
           [0007]      FIG. 2  is an enlarged side sectional view of the laser printer in which a rear cover is open according to illustrative aspects of the invention; 
           [0008]      FIG. 3  is a side view of the laser printer to which a tray extension is attached according to illustrative aspects of the invention; 
           [0009]      FIG. 4  is a perspective view of the tray extension according to illustrative aspects of the invention; 
           [0010]      FIG. 5  is an enlarged perspective view of an upper sheet discharge tray according to illustrative aspects of the invention; 
           [0011]      FIG. 6  is a perspective view of the laser printer in which the tray extension is stored in a storage recessed portion according to illustrative aspects of the invention; 
           [0012]      FIG. 7  is a perspective view of the laser printer in which the tray extension is attached to the rear cover according to illustrative aspects of the invention; 
           [0013]      FIG. 8  is an enlarged perspective view of the laser printer in which the tray extension is attached to the rear cover according to illustrative aspects of the invention; 
           [0014]      FIG. 9  is a perspective view of a tray extension of a laser printer according to a second illustrative embodiment of the invention; 
           [0015]      FIG. 10  is a perspective view of the laser printer in which the tray extension is stored in a storage recessed portion according to illustrative aspects of the invention; 
           [0016]      FIG. 11  is a perspective view of the laser printer in which the tray extension is placed in a sheet receiving position according to illustrative aspects of the invention; 
           [0017]      FIG. 12  is a perspective view of the laser printer in which the tray extension is attached to the rear cover according to illustrative aspects of the invention; 
           [0018]      FIG. 13  is a side view of a laser printer according to a third illustrative embodiment of the invention where a tray extension is attached; and 
           [0019]      FIG. 14  is a side view of a laser printer according to a fourth illustrative embodiment of the invention where a tray extension is attached to a rear cover. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A first illustrative embodiment of the invention will be described in detail with reference to  FIGS. 1 to 8 . The image forming apparatus according to illustrative aspects of the invention is applied to a laser printer in this embodiment. 
         [0021]    For purposes herein, aspects of the invention are shown in relation to an image carrier and developer carrier. In various aspects, the image carrier may include a photosensitive drum, photosensitive belt, or the combination of one of a photosensitive drum or belt and an intermediate transfer drum or belt. Further, the developer carrier may include a developer roller or other systems for conveying developer to the image carrier. 
         [0022]    It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. 
         [0023]    As shown in  FIG. 1 , a laser printer  1  is provided with a main mechanical part  1 A inside a housing  2 . The main mechanical part  1 A includes a sheet supply mechanical part  4  for supplying a recording medium (hereinafter referred to as a recording sheet  3 ), an image forming part  5  for forming an image on the supplied recording sheet  3 , and a sheet discharge mechanism  6  for discharging the recording sheet  3  on which the image has been formed. In the following description, the right side in  FIG. 1  is regarded as a front of the laser printer  1 , and the left side in  FIG. 1  is regarded as a rear of the laser printer  1 . 
         [0024]    An upper surface of the housing  2  is downwardly recessed to provide an upper output tray  100  configured to receive a recording sheet  3  on which an image has been formed. The upper output tray  100  is provided with a receiving surface  101  inclining upwardly from rear to front of the laser printer  1 . A rear end of the receiving surface  101  is provided with a receiving portion  102  standing vertically. When a recording sheet  3  is discharged to the upper output tray  100 , the recording sheet  3  is placed on the receiving surface  101  and the trailing end of the recording sheet  3  is supported by the receiving portion  102 . 
         [0025]    A front sidewall (on the left side in  FIG. 1 ) of the housing  2  is formed with an opening  7  and is provided with a front cover  7 A configured to cover and uncover the opening  7 . When the front cover  7 A is opened, the opening  7  is released, and a process cartridge  20  is able to be attached to and removed from the main mechanical part  1 A through the opening  7 . 
         [0026]    A rear sidewall (on the left side in  FIG. 1 ) of the housing  2  is formed with an opening  115 , and is provided with a rear cover  116  configured to cover and uncover the opening  115 . As shown in  FIG. 2 , the rear cover  116  is mounted to the rear sidewall of the housing  2  so as to pivot on an axis extending horizontally (in a direction passing through the sheet of  FIG. 2 ) in a lower portion of the opening  115 . The rear cover  116  is pivotal between a closed position to cover the opening  115  shown in  FIG. 1  and an open position to uncover the opening  115  shown in  FIG. 2 . An inner surface of the rear cover  116  is provided with lock portions  117  ( FIG. 7 ) that protrude inwardly in a state where the rear cover  116  is in the closed position. The lock portions  117  are configured to engage a receiving portion (not shown) provided in the housing  2 , so that the rear cover  116  is maintained in the closed position against the housing  2 . When the rear cover  116  is in the open position, the inner surface of the rear cover  116  is designed so as to receive a recording sheet  3  that undergoes printing process. As shown in  FIGS. 1 and 8 , an outer surface of the rear cover  116  is provided with a finger hook portion  128  for receiving a finger of a user. The finger hook portion  128  protrudes outward at an upper end of the rear cover  116 . 
         [0027]    A sheet supply tray  9  is disposed in the lowermost part of the housing  2 , and is configured to hold a stack of recording sheets  3  on which images are to be formed. The sheet supply tray  9  can be pulled out toward the front. 
         [0028]    The sheet supply tray  9  may be in the shape of a box with an open top. A sheet pressing plate  15  is disposed in a lower portion of the sheet supply tray  9 . A stack of recording sheets can be placed on an upper surface of the sheet pressing plate  15 . The sheet pressing plate  15  is pivotally supported at its rear portion (the left end of  FIG. 1 ) so that its front portion (the right end of  FIG. 1 ) is vertically movable. 
         [0029]    A lever  17  is disposed at a front end of the sheet supply tray  9  to raise the front end of the sheet pressing plate  15 . The lever  17  is L-shaped in a cross sectional view in such a manner as to enclose the sheet pressing plate  15  from the front to the bottom. An upper end of the lever  17  is attached to a lever shaft  18  disposed at the front end of the sheet supply tray  9 . A rear end of the lever  17  contacts a lower surface of the front end of the sheet pressing plate  15 . When the lever shaft  18  rotates in a clockwise direction in  FIG. 1 , the lever  17  pivots on the lever shaft  18  such that the rear end of the lever  17  raises the front end of the sheet pressing plate  15 . When the sheet supply tray  9  is removed from the housing  2 , the sheet pressing plate is configured to move downward under its own weight. 
         [0030]    The sheet supply mechanical part  4  includes a sheet supply roller  10  disposed at the front end of and above the sheet supply tray  9 , and a pickup roller  12  disposed at the rear of the sheet supply roller  10  so as to contact an uppermost recording sheet  3  placed in the sheet supply tray  9 . A reverse guide  37  is disposed at a position facing a front half of a circumferential surface of the sheet supply roller  10 . The reverse guide  37  includes a guide surface  16  that is configured to guide a recording sheet  3  by reversing the sheet feed direction. The recording sheet  3  passes through a space defined between the circumferential surface of the sheet supply roller  10  and the guide surface  16 . A separation pad  11  is disposed in an entrance of the reverse guide  37 . The separation pad  11  is configured to be pressed into contact with the sheet supply roller  10  by an urging force of a spring. In the reverse guide  37 , a pinch roller  13  is disposed on a downstream side of the separation pad  11 . A paper dust removing roller  8  is disposed on a downstream side of the pinch roller  13 . The pinch roller  13  and the paper dust removing roller  8  face the sheet supply roller  10 . 
         [0031]    The uppermost recording sheet  3  in the sheet supply tray  9  presses against the pickup roller  12  by the sheet pressing plate  15 , fed toward the entrance of the reverse guide  37  upon rotation of the pickup roller  12 , caught between the sheet supply roller  10  and the separation pad  11 , and singly separated from a stack of recording sheets  3 . The recording sheet  3  fed from between the pickup roller  12  and the separation pad  11  is caught between the sheet supply roller  10  and the pinch roller  13 . The recording sheet  3  is also caught between the sheet supply roller  10  and the paper dust removing roller  8 , ejected from an exit of the reverse guide  37 , and fed to register rollers  14  located on a downstream side of the sheet supply roller  10 . 
         [0032]    The register rollers  14  include a pair of rollers, which are configured to correct skew of a recording sheet  3  and feed the recording sheet  3  to an image formation position of the image forming part  5 . The imaging forming part  5  is where a photosensitive drum  29  and a transfer roller  32  make contact. 
         [0033]    The image forming part  5  includes a scanner unit  19 , a process cartridge  20 , and a fixing unit  21 . 
         [0034]    The scanner unit  19  is disposed in an upper portion of the housing  2 . The scanner unit  19  includes a laser light source (not shown), a polygon mirror  22 , an fθ lens  23 , a reflecting mirror  24 , a lens  25 , and a reflecting mirror  26 . A laser beam emitted from the laser light source based on image data is deflected at the polygon mirror  22 , passes through fθ lens  23 , is deflected downward at the reflecting mirror  24 , and is directed to a surface of a drum body  34  of a photosensitive drum  29  in the process cartridge  20 , as shown by a dot-dash line in  FIG. 1 . 
         [0035]    The process cartridge  20  is disposed under the scanner unit  19 . The process cartridge  20  includes, in a frame  28 , a photosensitive drum  29 , a scorotron charger  30 , a developing cartridge  31 , a transfer roller  32 , and a cleaning brush  33 . The frame  28  is made up of an upper frame  27 A and a lower frame  27 B, which are separately formed and assembled with each other. 
         [0036]    The photosensitive drum  29  is substantially cylindrically shaped, and includes a drum body  34  and a drum shaft  35 . The drum shaft  35  is supported by the upper frame  27 A and the drum body  34  is rotatably supported by the drum shaft  35 . The photosensitive drum  29  is rotatably provided on the drum shaft  35  in the upper frame  27 A. 
         [0037]    The scorotron charger  30  is disposed diagonally above the photosensitive drum  29  to face it. The scorotron charger  30  is configured to charge the surface of the photosensitive drum  29  evenly and positively by a corona discharge generated from a charging wire (not shown) made of tungsten. 
         [0038]    The developing cartridge  31  is shaped in a box that is open at the rear and detachably attached to the lower frame  27 B. The developing cartridge  31  includes a toner chamber  39 , a supply roller  40 , a developing roller  41 , and a layer-thickness regulating blade  42 . 
         [0039]    The toner chamber  39  is formed as a front-side interior space of the developing cartridge  31  and is divided by a partition wall  43 . The toner chamber  39  is filled with a developing agent such as nonmagnetic single component toner which is to be positively charged. A coloring agent such as carbon black and wax can be added to the toner to form toner base particles. An external additive, such as silica, also can be added to the toner to improve fluidity. 
         [0040]    The toner chamber  39  is provided with an agitator  44 . Toner in the toner chamber  39  is agitated by the agitator  44  and discharged from an opening  45  defined under the partition wall  43  toward the supply roller  40 . 
         [0041]    The supply roller  40  is disposed at the rear of the opening  45  and is rotatably supported by the developing cartridge  31 . The supply roller  40  is made up of a roller made of an electrically conductive foam material and a metal roller shaft covered with the roller. The supply roller  40  is configured to rotate by an input from a motor (not shown). 
         [0042]    The developing roller  41  is rotatably supported by the developing cartridge  31  so as to contact the supply roller  40  under compression at the rear. The developing roller  41  is configured to contact the photosensitive drum  29  when the developing cartridge  31  is attached to the lower frame  27 B. The developing roller  41  is made up of a roller formed of an electrically conductive rubber material and a metal roller shaft covered with the roller. The roller shaft protrudes from both sides of the developing cartridge  31  outwardly in a width direction substantially perpendicular to the front-rear direction (in a direction passing through the sheet of  FIG. 1 ). 
         [0043]    The roller of the developing roller  41  can be made by covering a roller body (made of a conductive urethane rubber or silicone rubber including carbon fine particles) with a coat layer of urethane rubber or silicone rubber including fluorine. The developing roller  41  is configured to receive a bias during image developing. The developing roller  41  is configured to be driven by a motor (not shown) and rotated in the same direction as the supply roller  40 . 
         [0044]    The layer-thickness regulating blade  42  is provided with a blade body  46  made of a metal plate spring member and a pressing portion  47  having a generally semicircular shape in cross section, provided at a free end of the blade body  46 , and made of insulative silicon rubber. The layer-thickness regulating blade  42  is supported by the developing cartridge  31  in an upper portion of the developing roller  41 , and the pressing portion  47  presses against the developing roller  41  by an urging force of the blade body  46 . 
         [0045]    Toner discharged from the opening  45  is supplied to the developing roller  41  upon rotation of the supply roller  40 , while being positively charged between the supply roller  40  and the developing roller  41  by friction. Toner supplied to the developing roller  41  goes between the pressing portion  47  of the layer-thickness regulating blade  42  and the developing roller  41  through the rotation of the developing roller  41 , and is further charged therebetween. The toner is carried as a thin layer of a fixed thickness on the developing roller  41 . 
         [0046]    The transfer roller  32  is rotatably supported by the lower frame  27 B, and is disposed so that, when the upper frame  27 A is assembled with the lower frame  27 B, the transfer roller  32  vertically contacts the photosensitive drum  29  to form a nip between the transfer roller  32  and the photosensitive drum  29 . The transfer roller  32  is formed by covering a metal roller shaft with a roller made of a conductive rubber material. The transfer roller  32  is configured to be driven by a motor (not shown) and rotated in a direction opposite to the direction the photosensitive drum  29  rotates. 
         [0047]    The cleaning brush  33  is attached to the lower frame  27 B and is disposed to contact the photosensitive drum  29  behind where the upper frame  27 A is assembled with the lower frame  27 B. 
         [0048]    The surface of the photosensitive drum  29  is positively and uniformly charged by the scorotron charger  30  upon the rotation of the photosensitive drum  29 . The surface of the photosensitive drum  29  is exposed to the laser beam emitted from the scanner unit  19  by high speed scanning, and an electrical latent image is formed on the surface of the photosensitive drum  29  based on predetermined image data. 
         [0049]    When toner positively charged and carried on the developing roller  41  contacts the photosensitive drum  29  upon the rotation of the developing roller  41 , it is supplied to the electrical latent image formed on the surface of the photosensitive drum  29 , where the potential has become low due to exposure to the laser beam. In this manner, the electrostatic latent image formed on the photosensitive drum  29  is developed with toner, a reversal takes place, and a toner image is formed on the photosensitive drum  29 . 
         [0050]    The toner image carried on the photosensitive drum  29  is transferred onto a recording sheet  3  through the application of a bias  32  while the recording sheet  3  being fed by the register rollers  14  passes through a transfer position between the photosensitive drum  29  and the transfer roller  32 . The recording sheet  3  on which the toner image has been transferred is fed to the fixing unit  21 . Toner remaining on the photosensitive drum  29  after transferring is collected by the developing roller  41 . Foreign matter such as dust adhering to the photosensitive drum  29  after transfer is collected by the cleaning brush  33 . 
         [0051]    The fixing unit  21  is disposed at a rear of the process cartridge  20  and includes a fixing frame  48 , and an ejection portion such as a heat roller  49  and a pressure roller  50 . The heat roller  49  and the pressure roller  50  are disposed within the fixing frame  48 . 
         [0052]    The heat roller  49  includes a metal tube coated with fluorine resin and a halogen lamp configured to apply heat to the inside of the metal tube. The heat roller  49  is configured to be rotated by input from a motor not shown. 
         [0053]    The pressure roller  50  is disposed facing the heat roller  49  in such a manner as to press the heat roller  49  from underneath. The pressure roller  50  is formed by covering a metal roller shaft with a roller formed of a rubber material. The pressure roller  50  is configured to rotate following the rotation of the heat roller  49 . 
         [0054]    In the fixing unit  21 , toner transferred onto the recording sheet  3  at a transfer position is thermally fixed onto the recording sheet  3  while the recording sheet  3  passes through between the heat roller  49  and the pressure roller  50 . The recording sheet  3  on which toner has been fixed is fed to the sheet discharge mechanism  6 . 
         [0055]    The sheet discharge mechanism  6  includes a guide portion such as a sheet discharge guide  113 , and a sheet discharge roller  110  configured to eject a recording sheet to the upper output tray  100 . In the laser printer  1 , a recording sheet  3  on which image has been thermally fixed is fed to either one of two feed paths, a first feed path  118  and a second feed path  119 . When the rear cover  116  is kept in the closed position, the recording sheet  3  being fed from between the heat roller  49  and the pressure roller  50  is guided to the upper output tray  100  via the first feed path  118 . When the rear cover  116  is kept in the open position, the recording sheet  3  is guided to the rear cover  116  via the second feed path  119  as shown in  FIG. 2 . 
         [0056]    The sheet discharge guide  113  is disposed on the downstream side from the heat roller  49  and the pressure roller  50 . The sheet discharge guide  113  is configured to guide the recording sheet  3  on which the image has been thermally fixed to the first feed path  118  or the second feed path  119 . The sheet discharge guide  113  is configured to rotate on an axis extending horizontally (in a direction passing through the sheet of  FIG. 1  or  2 ) in association with the rotation of the rear cover  116  between the open position and the closed position. The sheet discharge guide  113  is provided with a guide surface  114  having a gradient inclined from rear to front in a position facing the heat roller  49  and the pressure roller  50 . 
         [0057]    When the rear cover  116  is in the closed position as shown in  FIG. 1 , the recording sheet  3  on which the image has been thermally fixed is guided to the first feed path  118  in sliding contact with the guide surface  114  of the sheet discharge guide  113 . When the rear cover  116  is in the open position as shown in  FIG. 2 , the sheet discharge guide  113  is rotated counterclockwise in  FIG. 2 , and the recording sheet  3  on which the image has been thermally fixed is guided to the second feed path  119  in sliding contact with the guide surface  114  of the sheet discharge guide  113 . In this way, the recording sheet  3  is selectively guided to the first feed path  118  or the second feed path  119 . 
         [0058]    Guide ribs  120  are disposed on an inner side of the rear cover  116 , which faces inside of the laser printer  1  when the rear cover  116  is kept in the closed position in such a manner as to protrude at a downstream side of the sheet discharge guide  113 . The guide ribs  120  are arranged in an erect manner and extend along a direction where the recording sheet  3  is ejected from between the heat roller  49  and the pressure roller  50 . Each guide rib  120  is formed with an arc surface  121  having an edge substantially arcuately shaped in a side view. The recording sheet  3  guided to the first feed path  118  by the sheet discharge guide  113  is guided to the upper output tray  100  by sliding contact with the arc surface  121 . The arc surface  121  is connected to the sheet discharge roller  110  at a downstream side. 
         [0059]    The sheet discharge roller  110  is made by covering a metal roller shaft  111  with a roller made of a rubber material, and is configured to be driven by a motor (not shown) and rotated counterclockwise. An auxiliary roller  112  is disposed ahead of the sheet discharge roller  110  in the downstream direction to face the sheet discharge roller  110 . The auxiliary roller  112  is rotated upon the rotation of the sheet discharge roller  110 . The recording sheet  3  fed from a downstream end of the arc surface  121  is caught between the sheet discharge roller  110  and the auxiliary roller  112 , and fed to the receiving surface  101  of the upper output tray  100 . 
         [0060]    As shown in  FIG. 2 , when the rear cover  116  is placed in the open position, the recording sheet  3  guided to the second feed path  119  by the sheet discharge guide  113  is fed from between the heat roller  49  and the pressure roller  50  to the rear cover  116 . 
         [0061]    A tray extension  122  will be described with reference to  FIG. 3 . 
         [0062]    The tray extension  122  is attachable to the rear cover  116  as shown by a solid line in  FIG. 3  and the upper output tray  100  as shown by a double dotted line in  FIG. 3 . The tray extension  122  is configured to receive and prevent an ejected recording sheet  3  from hanging over and slipping off the rear cover  116  or the upper output tray  100 . When a recording sheet  3  is ejected to the rear cover  116  by the sheet discharge guide  113  and the guide ribs  120 , the tray extension  122  is attached to an outer end of the rear cover  116  which is on a downstream side in a direction where the recording sheet  3  is ejected (hereinafter referred to as a sheet ejection direction). Alternatively, when a recording sheet  3  is ejected to the upper output tray  100  by the sheet discharge guide  113  and the guide ribs  120 , the tray extension  122  is attached to an outer end of the upper output tray  100  which is on a downstream side in the sheet ejection direction. 
         [0063]    The tray extension  122  is made of a synthetic resin and has a plate-like structure as shown in  FIG. 4 , and is slightly curved on one surface. As shown in  FIG. 3 , the tray extension  122  is attached to the housing  2  with the curved surface facing upward. The tray extension  122  has a recessed portion  125  at its lower part in  FIG. 4  to prevent collision with the guide ribs  120  of the rear cover  116 . The tray extension  122  is provided with a pair of leg portions  129  at lower side edges to define the recessed portion  125  therebetween. The leg portions  129  are deformable in a width direction of the tray extension  122  (in the left-right direction in  FIG. 4 ). The leg portions  129  have protrusions  123  protruding outwardly at outer edges of the leg portions  129 . When the protrusions  123  are engaged in recessed portions  124  formed in the upper output tray  100  or through holes  130  formed in the rear cover  116 , the tray extension  122  is attached to the upper output tray  100  or the rear cover  116 . 
         [0064]    As shown in  FIGS. 1 and 7 , a tray extension storing portion  126  is recessed downwardly at the front portion of the upper output tray  100  (on the right side of  FIG. 1 ). The tray extension storing portion  126  is configured to store the tray extension  122  therein. The recessed portions  124  are recessed on the downstream side in the sheet ejection direction on both sidewalls of the tray extension storing portion  126  as shown in  FIG. 5 , so as to engage the protrusions  123  of the tray extension  122 . By engagement of the protrusions  123  with the recessed portions  124 , the tray extension  122  is rotatably attached to the upper output tray  100 . The tray extension  122  is rotatable between a storage position where the tray extension  122  is stored within the tray extension storing portion  126  as shown in  FIGS. 1 and 6  and a sheet receiving position shown by a double dotted line in  FIGS. 3 and 7 . When the laser printer  1  is not used, the tray extension  122  can be rotated to the storage position and stored within the tray extension storing portion  126 . As shown in  FIGS. 5 and 6 , finger recesses  127  for allowing a user to place his/her finger on the side edge of the tray extension  122  are provided at the rear side of the tray extension storing portion  126 . The user places his/her finger in the finger recess  127  to raise the tray extension  122  from the storage position to the sheet receiving position. 
         [0065]    As shown in  FIGS. 3 and 7 , when the tray extension  122  is attached to the upper output tray  100  and placed in the sheet receiving position, the tray extension  122  protrudes from the outer edge of the upper output tray  100  on the downstream side (right side in  FIG. 3 ) in the sheet ejection direction. 
         [0066]    As shown in  FIG. 8 , the through holes  130  to engage the protrusions  123  of the tray extension  122  are formed in the upper end portion of the rear cover  116  facing each other in the width direction of the laser printer  1 . As shown in  FIGS. 7 and 8 , when the rear cover  116  is in the open position, the tray extension  122  is attached to the upper end portion of the rear cover  116 . The tray extension  122  is attached to the rear cover  116 , and the guide ribs  120  fall within the recessed portion  125  of the tray extension  122 , so that the tray extension  122  is prevented from evacuating from the guide ribs  120 . 
         [0067]    As shown in  FIGS. 3 and 7 , when the tray extension  122  is attached to the rear cover  116 , the tray extension  122  protrudes from the outer edge of the rear cover  116  on the downstream side (left side in  FIG. 3 ) in the sheet ejection direction. 
         [0068]    The operation and advantages of the laser printer  1  will be described for a case when the recording sheet  3  is ejected to the upper output tray  100 . The rear cover  116  is rotated and kept in the closed position. After closing the rear cover  116 , the sheet discharge guide  113  is rotated to a position to guide the recording sheet  3  to the first feed path  118  as shown in  FIG. 1 . 
         [0069]    The tray extension  122  is attached to the upper output tray  100  by deforming the leg portions  129  inwardly in the width direction to engage the protrusions  123  of the tray extension  122  with the recessed portions  124  of the upper output tray  100 . The tray extension  122  is rotated to the sheet receiving position. When the tray extension  122  is already attached to the upper output tray  100  and stored in the tray extension storing portion  126 , the user places his/her finger in the finger recess  127 , and raises the tray extension  122  to the sheet receiving position. 
         [0070]    A recording sheet  3  undergoes a printing process in the image forming part  5  and is ejected from the heat roller  49  and the pressure roller  50 . The recording sheet  3  is guided to the first feed path  118  in sliding contact with the guide surface  114  of the sheet discharge guide  113 , and then guided to the sheet discharge roller  110  in sliding contact with the curved surface  121  of the guide rib  120 . The recording sheet  3  is ejected from between the sheet discharge roller  110  and the auxiliary roller  112  toward the upper output tray  100 . 
         [0071]    The recording sheet  3  ejected to the upper output tray  100  is loaded on the upper output tray  100  and the tray extension  122 . The tray extension  122  is attached to the upper output tray  100  so as to protrude from the outer edge of the downstream side in the sheet ejection direction of the upper output tray  100 . Thus, even when a large-sized recording sheet  3  is used, the recording sheet  3  is received by the tray extension  122  thereby preventing the recording sheet  3  from hanging over and slipping off the upper output tray  100 . 
         [0072]    When printing is completed, the tray extension  122  is rotated from the sheet receiving position to the storage position, so that the tray extension  122  is stored in the tray extension storing portion  126  of the upper output tray  100 . Thus, when the laser printer  1  is not used, the tray extension  122  can be prevented from inadvertently being detached from the laser printer  1  and getting damaged due to a careless collision with an object. 
         [0073]    The operation and advantages of the laser printer  1  will be described for a case when the recording sheet  3  is ejected to the rear cover  116 . The user places his/her finger in the finger hook portion  128 , and pulls the rear cover  116  outward to rotate it from the closed position to the open position. Following the movement of the rear cover  116 , the sheet discharge guide  113  is also rotated to a position to guide the recording sheet  3  to the second feed path  119  as shown in  FIG. 2 . 
         [0074]    When the tray extension  122  is attached to the upper output tray  100 , the tray extension  122  can be removed from the upper output tray  100  by deforming the leg portions  129  of the tray extension  122  inward in the width direction to disengage the protrusions  123  and the recessed portions  124 . When the tray extension  122  is not attached to the upper output tray  100  or if another tray extension will be used, the removal procedure of the tray extension  122  can be omitted. 
         [0075]    The leg portions  129  of the tray extension  122  are deformed inward in the width direction, and the protrusions  123  are engaged in the through holes  130  of the rear cover  116 . In this manner, the tray extension  122  is attached to the rear cover  116 . 
         [0076]    The recording sheet  3  undergoes a printing process in the image forming part  5 . The printed recording sheet  3  is ejected from the heat roller  49  and the pressure roller  50 . The recording sheet  3  is guided to the second feed path  119  in sliding contact with the guide surface  114  of the sheet discharge guide  113 , and ejected to the rear cover  116 . 
         [0077]    The recording sheet  3  ejected to the rear cover  116  is received by the rear cover  116  and the tray extension  122 . The tray extension  122  is attached to the rear cover  116  so as to protrude from the outer edge of the downstream side in the sheet ejection direction of the rear cover  116 . Thus, even when a large-sized recording sheet  3  is used, the recording sheet  3  can be received by the tray extension  122  thereby preventing the recording sheet  3  from hanging over and slipping off the rear cover  116 . 
         [0078]    When printing is completed, the tray extension  122  can be removed from the rear cover  116  by deforming the leg portions  129  of the tray extension  122  inwardly in the width direction and disengaging the protrusions  123  from the through holes  130 . Then, the rear cover  116  can be rotated to the closed position to close the opening  115  so that the rear cover  116  is kept closed. After that, the tray extension  122  can be attached to the upper output tray  100 , and rotated so that the tray extension  122  can be stored in the tray extension storing portion  126  of the upper output tray  100 . 
         [0079]    A tray extension  122  can be attached to each of the upper output tray  100  and the rear cover  116  to prevent a large-sized recording sheet from hanging over and slipping away from the upper output tray  100  or the rear cover  116 . However, this increases the number of parts and leads to the increased cost. 
         [0080]    In this illustrative embodiment, when the recording sheet  3  is ejected to the upper output tray  100 , the tray extension  122  is attached to the upper output tray  100 , and when the recording sheet  3  is ejected to the rear cover  116 , the tray extension  122  can be attached to the rear cover  116 . With this structure, when an image is formed on a large-sized recording sheet  3 , the single tray extension  122  can prevent the recording sheet  3  from hanging over and slipping off the upper output tray  100  or the rear cover  116 . Also, the number of parts in the laser printer  1  can be reduced. As the laser printer  1  is configured to eject a recording sheet  3  to the upper output tray  100  or the rear cover  116  selectively, it is only necessary to attach the tray extension  122  to either one of the upper output tray  100  and the rear cover  116  onto which the recording sheet  3  is ejected. 
         [0081]    In this illustrative embodiment, when the recording sheet  3  is not ejected from a side of the laser printer  1 , the rear cover  116  can be placed in the closed position to close the opening  115 . As such, foreign matter such as dust and dirt can be prevented from entering the housing  2  through the opening  115  when the rear cover  116  is closed. 
         [0082]    By placing the rear cover  116  in the closed position, the recording sheet  3  can be guided to the first feed path  118  by the sheet discharge guide  113  and the guide ribs  120 , and then to the upper output tray  100 . 
         [0083]    A second illustrative embodiment of the invention will be described with reference to  FIGS. 9 to 12 . In  FIGS. 9 to 12 , a tray extension  222  is a variant of the tray extension  122  of the first embodiment, parts substantially equivalent to those described above are denoted by the same reference numerals, and descriptions therefor will be omitted. 
         [0084]    As shown in  FIG. 9 , the tray extension  222  is provided with a pair of cutout portions  231  close to lower side edges of the tray extension  222  to define leg portions  229 , which are deformable in the width direction of the tray extension  222  (in the left-right direction in  FIG. 9 ). 
         [0085]    As shown in  FIGS. 10 and 11 , the tray extension  222  can be attached to the upper sheet discharge tray  100  rotatably between the storage position (in  FIG. 10 ) and the sheet receiving position (in  FIG. 11 ). 
         [0086]    As shown in  FIG. 12 , the tray extension  222  can be attached to the upper end of the rear cover  116  in the open position. Although it is not shown, the upper end of the rear cover  116  can be formed with a clearance groove for separating a lower end of the tray extension  222  from the rear cover  116  when the tray extension  222  is attached to the rear cover  116 . Thus, the recessed portion  125  can be omitted from the tray extension  222 . 
         [0087]    A third illustrative embodiment of the invention will be described with reference to  FIG. 13 . In  FIG. 13 , a tray extension  322  is a variant of the tray extension  122  of the first illustrative embodiment. Parts substantially equivalent to those described above are denoted by the same reference numerals, and descriptions therefor will be omitted. 
         [0088]    The tray extension  322  is shaped like a plate so that the front side and the back side are indistinguishable, e.g. reversibly used. Thus, the tray extension  322  is attachable to the upper output tray  100  and the rear cover  116  without paying attention to the front and back sides of the tray extension  322 . 
         [0089]    According to the third illustrative embodiment, this structure can reduce workload of the user because there is no need to confirm the front side and the back side of the tray extension  322  when the user attaches the tray extension  322  to the laser printer  1 . 
         [0090]    A fourth illustrative embodiment of the invention will be described with reference to  FIG. 14 . In  FIG. 14 , a tray extension  422  is a variant of the tray extension  122  of the first illustrative embodiment. Parts substantially equivalent to those described above are denoted by the same reference numerals, and descriptions therefor will be omitted. 
         [0091]    The tray extension  422  is rotatably attached to the rear cover  116  by engaging the protrusions  123  in the through holes  130 . The tray extension  422  is rotatable between a storage position shown by the double dotted line in  FIG. 14  and a sheet receiving position shown by the solid line in  FIG. 14 . In the storage position, the tray extension  422  folds toward a side of the rear cover  116  on which the guide ribs  120  are disposed so as not to extend off the outer edge of the rear cover  116 . In the sheet receiving position, the tray extension  422  extends outwardly from the outer edge of the rear cover  116  in the sheet ejection direction. The tray extension  422  is formed with the recessed portion  125  for separating the guide ribs  120  from the tray extension  422  when the tray extension  422  is in the storage position. 
         [0092]    A dimension L of the tray extension  422  from an end where the protrusion  123  is disposed to an end opposite of the protrusion  123  is set so that, when the rear cover  116  is placed in the open position and the tray extension  422  is rotated clockwise from the sheet receiving position to the storage position as shown in  FIG. 14 , the tip end of the tray extension  422  is maintained out of contact with an upper end of the housing  2  defining the opening  115 . 
         [0093]    According to the fourth illustrative embodiment, after the recording sheet  3  is printed and ejected to the rear cover  116 , the tray extension  422  can be rotated from the sheet receiving position to the storage position. Then, the rear cover  116  can be rotated and maintained in the closed position where the opening  115  is closed against the housing  2 . Thus, when the laser printer  1  is not used, the tray extension  422  can be prevented from inadvertently being detached from the laser printer  1  and getting damaged due to a careless collision with an object. 
         [0094]    According to the fourth illustrative embodiment, the tray extension  422  can be stored while being attached to the rear cover  116 . This structure can save the user, who often directs a recording sheet  3  via the rear cover  116 , from having to remove the tray extension  422  from the rear cover  116  after printing. 
         [0095]    Although the recording sheet  3  may be assumed to be plain paper in the above illustrative embodiments, the recording sheet  3  may be another medium such as a transparency made of a synthetic resin. 
         [0096]    In the above illustrative embodiments, the rear cover  116  also serves as a side sheet receiving portion, but is not limited to such a configuration. The rear cover  116  and the side sheet receiving portion may be formed individually, and the side sheet receiving portion may be attached to the housing  2  when a recording sheet  3  is ejected to a side of the laser printer  1 . 
         [0097]    In the above illustrative embodiments, the sheet discharge guide  113  is configured to rotate along with the rotation of the rear cover  116  to guide the recording sheet  3  to the first feed path  118  or second feed path  119 . However, the sheet discharge guide  113  may be configured to not rotate along with the rotation of the rear cover  116 . In other words, the user may select whether the recording sheet  3  is ejected from an upper portion of the laser printer  1  or a side portion thereof, and change the position of the sheet discharge guide  113  according to the selection. 
         [0098]    In the above illustrative embodiments, the guide ribs  120  are disposed on the inner surface of the rear cover  116 . However, the guide ribs  120  may be disposed in the housing  2  so as to face the heat roller  49  and the pressure roller  50  to guide the recording sheet  3  to the first feed path  118  by sliding contact therewith. 
         [0099]    Although illustrative embodiments of the invention have been described in detail herein, the scope of the invention is not limited thereto. It will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the invention. Accordingly, the illustrative embodiments disclosed herein are only exemplary. It is to be understood that the scope of the invention is not to be limited thereby, but is to be determined by the claims which follow.