Patent Publication Number: US-8977184-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Japanese Patent Applications Nos. 2012-147199 filed on Jun. 29, 2012 and 2012-147201 filed on Jun. 29, 2012, the disclosures of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus including a re-conveyance path. 
     BACKGROUND ART 
     There is known an image forming apparatus capable of duplex printing, which comprises an image forming unit including a photoconductor and a transfer member, an exposure device for exposing the photoconductor to light, a sheet feeder unit for feeding a recording sheet to the image forming unit, sheet ejection rollers for ejecting the recording sheet having an image formed thereon by the image forming unit, a conveyance path for conveying the recording sheet, which extends from the sheet feeder unit to the sheet ejection rollers, passing through between the photoconductor and the transfer member of the image forming unit, and a re-conveyance path for conveying the recording sheet having passed through the image forming unit to a position upstream from the photoconductor in the conveyance path. 
     To be more specific, the sheet ejection rollers are configured to be rotatable in forward and reverse directions such that the recording sheet conveyed along the conveyance path is ejected outside the casing by the forward rotation of the sheet ejection rollers and that a traveling direction of the recording sheet is reversed, after the recording sheet partly protrudes outside the casing, by the reverse rotation of the sheet ejection rollers, whereby the recording sheet is retracted back into the casing. The re-conveyance path is configured such that the recording sheet of which the traveling direction has been reversed by the sheet ejection rollers is caused to pass through a space between the photoconductor and the exposure device and then conveyed to a position upstream from the photoconductor in the conveyance path. 
     SUMMARY OF THE INVENTION 
     According to the above image forming apparatus, since a recording sheet of which the traveling direction has been reversed by the sheet ejection rollers passes through the space between the photoconductor and the exposure device, it is necessary that the exposure device starts exposing the photoconductor to light after the trailing edge of the recording sheet passes through between the photoconductor and the exposure device. However, if the length of the re-conveyance path is reduced to downsize the casing of the image forming apparatus, due to the size of a recording sheet (longer in size) conveyed along the re-conveyance path, the leading edge of the recording sheet may disadvantageously reach the position between the photoconductor and the transfer member of the image forming unit before the trailing edge passes through between the photoconductor and the exposure device. 
     In view of the above, it would be desirable to provide an image forming apparatus which can guide a recording sheet conveyed along the re-conveyance path to the position between the photoconductor and the transfer member only after the recording sheet passes through the space between the photoconductor and the exposure device. 
     According to a first aspect of the present invention, there is provided an image forming apparatus comprising: a casing; a sheet placement portion configured to receive recording sheets; a photoconductor disposed in the casing and configured to carry a developer image; a transfer member disposed opposite to the photoconductor and configured to transfer the developer image carried on the photoconductor onto a recording sheet while the recording sheet passes through a nip portion between the photoconductor and the transfer member; a sheet feeder unit configured to feed a recording sheet from the sheet placement portion toward the nip portion; an exposure device disposed away from the photoconductor and configured to form an electrostatic latent image on the photoconductor based on image data; a fixing device configured to fix the transferred developer image on the recording sheet; a sheet ejection unit configured to eject the recording sheet with the transferred developer image fixed thereon by the fixing device outside the casing; a conveyance path configured to guide the recording sheet conveyed from the sheet placement portion, passing through the sheet feeder unit, the nip portion between the photoconductor and the transfer member, the fixing device, and the sheet ejection unit in this order; and a re-conveyance path configured to guide the recording sheet conveyed from the fixing device to an upstream-side path which constitutes part of the conveyance path extending upstream from the photoconductor. In this image forming apparatus, the re-conveyance path is branched off from the conveyance path at a position downstream from the fixing device and extends to pass through a space between the photoconductor and the exposure device. Further, the re-conveyance path include a switchback mechanism configured to reverse a traveling direction of a recording sheet conveyed along the re-conveyance path after passing through the space between the photoconductor and the exposure device and to convey the recording sheet toward the upstream-side path. 
     According to a second aspect of the present invention, there is provided an image forming apparatus comprising: a casing; a sheet placement portion configured to receive recording sheets; an image forming unit disposed inside the casing and configured to form an image on a recording sheet; a sheet feeder unit configured to feed a recording sheet from the sheet placement portion toward the image forming unit; a sheet ejection unit configured to eject the recording sheet with the image formed thereon by the image forming unit outside the casing through a sheet ejection opening formed in the casing; a conveyance path for conveying the recording sheet, which includes an upstream-side path connecting the sheet feeder unit and the image forming unit, and a downstream-side path connecting the image forming unit and the sheet ejection unit; and a re-conveyance mechanism configured to convey the recording sheet having passed through the image forming unit toward the upstream-side path. In this image forming apparatus, the casing has a first opening which is different from the sheet ejection opening. Further, the re-conveyance mechanism is configured to cause the recording sheet conveyed along the downstream-side path to partly protrude outside the casing through the first opening, to reverse a traveling direction of the recoding sheet, and to thereafter guide the recording sheet toward the upstream-side path. 
     This image forming apparatus may further comprise a photoconductor configured to carry a developer image, and an exposure device disposed away from the photoconductor and configured to form an electrostatic latent image on the photoconductor based on image data. The first opening may be provided between the exposure device and the sheet placement portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To better understand the claimed invention, and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: 
         FIG. 1  is a sectional view schematically showing a laser printer according to one exemplary embodiment of the present invention; 
         FIG. 2  is a sectional view illustrating a state in which the leading edge of a sheet conveyed along the re-conveyance path has passed through the conveyance rollers; 
         FIG. 3  is a sectional view illustrating a state just before reversing rotations of the conveyance rollers; 
         FIG. 4  is a sectional view illustrating a state after a sheet conveyed along the re-conveyance path has been conveyed to the sheet feeder unit; and 
         FIG. 5  is a sectional view schematically showing the laser printer according to a modified embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     A detailed description will be given of an illustrative embodiment of the present invention with reference to the accompanying drawings. In the following description, a general arrangement of a laser printer as an example of an image forming apparatus will be described, and thereafter characteristic features of the present invention will be described in detail. 
     In the following description, the direction is designated as from the viewpoint of a user who is using (operating) the laser printer. To be more specific, in  FIG. 1 , the left-hand side of the drawing sheet corresponds to the “front” side of the laser printer, the right-hand side of the drawing sheet corresponds to the “rear” side of the laser printer, the front side of the drawing sheet corresponds to the “right” side of the laser printer, and the back side of the drawing sheet corresponds to the “left” side of the laser printer. Similarly, the direction extending from top to bottom of the drawing sheet corresponds to the “vertical” or “upper-lower” (upward/downward, up/down, upper/lower or top/bottom) direction of the laser printer. 
     Schematic Arrangement of Laser Printer 
     As seen in  FIG. 1 , a laser printer  1  includes a main body  2 , a sheet feeder unit  3  for feeding a sheet of paper P (hereinafter simply referred to as a “sheet” P) as an example of a recording sheet, a scanner unit  4  as an example of an exposure device, an image forming unit  5  for forming an image on a sheet P, and a sheet ejection unit  8 . 
     The main body  2  includes a casing  21  and a front cover  22 . The casing  2  includes a sheet output tray  9  at an upper portion thereof. The sheet output tray  9  receives a sheet P ejected outside the casing  21  by the sheet ejection unit  8  to be described later. The casing  21  has a wall extending upward from the rear end portion of the upper surface of the sheet output tray  9 , and a sheet ejection opening  21 A is formed in the wall so that the sheet P is ejected by the sheet ejection unit  8  outside the casing  2  through the sheet ejection opening  21 A. 
     Further, at the front side of the casing  21 , there are provided an insertion opening  21 B for inserting a batch of sheets P and a first opening  21 C. The first opening  21 C is an opening that is different from the sheet ejection opening  21 A, and is located at a lower portion of the casing  21 . The insertion opening  21 B is located at a position lower than the first opening  21 C. 
     The front cover  22  covers the front side of the casing  21 . A lower end of the front cover  22  is pivotally supported by the casing  22 , so that the front cover  22  is swingable in the front-rear directions around the lower end thereof. Accordingly, the insertion opening  21 B and the first opening  21 C are opened and closed by the front cover  22  when the front cover  22  is swung in the front-rear directions. 
     The sheet feeder unit  3  is located in a lower portion of the main body  2 . The sheet feeder unit  3  includes a sheet feed tray  31 , which is an example of a sheet placement portion, for receiving sheets P inserted through the insertion opening  21 B, and a sheet feeder unit  32  for feeding a sheet P from the sheet feed tray  31  toward the image forming unit  5 . 
     The sheet feed tray  31  consists of a sheet receiving plate  31 A disposed at the lower portion of the casing  21 , and the front cover  22  described above. To be more specific, the sheet feed tray  31  is disposed at a position lower than the first opening  21 C, and extends from inside to outside the casing  21  beyond the first opening  21 C. The sheet receiving plate  31 A constitutes the inner portion of the sheet feed tray  31 , which is located inside the casing  21 , and the front cover  22  constitutes the outer portion of the sheet feed tray  31 , which is located outside the casing, when the front cover is opened frontward into a lying back position. 
     The sheet receiving plate  31 A extends from a position in the vicinity of the front side of the casing  21  to a position in the vicinity of the rear side of the casing, and is pivotable in the upper-lower directions around an axis of rotation at the front end portion thereof. The sheet receiving plate  31 A is configured such that the rear end portion thereof is caused to move closer to or away from a sheet feed roller  32 A to be described later by rotating a lifter member  31 B (as an example of a movement mechanism) disposed under the sheet receiving plate  31 A with the help of a known mechanism (not shown) and a controller (not shown). With this configuration, the sheet receiving plate  31 A lifts the sheets P toward the sheet feed roller  32 A each time a sheet P is to be fed, and causes the sheets P to be positioned away from the sheet feed roller  32 A while a sheet P is conveyed along a re-conveyance path  200  and guided into a conveyance path  100 . 
     To be more specific, the sheet receiving plate  31 A is in a substantially horizontal position, which is a position away from the sheet feed roller  32 A, when no print instruction is input. When a print instruction is input, the sheet receiving plate  31 A rotates upward and approaches the sheet feed roller  32 A. After the sheet feed roller  32 A feeds a sheet P, the sheet receiving plate  31 A returns to the substantially horizontal position, away from the sheet feed roller  32 A. 
     The sheet feeder unit  32  mainly includes the sheet feed roller  32 A and a separation roller  32 B, which are examples of rotating members, and a separation pad  32 C. The sheet feed roller  32 A is disposed above the rear end of the sheet receiving plate  31 A. The separation roller  32 B is disposed opposite to the separation pad  32 C at a position downstream from the sheet feed roller  32 A in a sheet conveyance direction along which a sheet P is conveyed. 
     In this sheet feeder unit  3 , the front cover  22  is swung forward into the lying back position to provide the sheet feed tray  31 , and a stack of sheets P is placed in the sheet feed tray  31 . In this state, the sheet receiving plate  31 A is in the substantially horizontal position. The sheets P placed on the sheet feed tray  31  and the sheet feed roller  32 A are brought into contact when the sheet receiving plate  31 A rotates to lift the rear ends of the sheet P. The sheet feed roller  32 A in contact with the sheets P rotates to feed sheets P from the sheet feed tray  31  to the separation roller  32 B, and thereafter the sheets P are separated one from the other between the separation roller  32 B and the separation pad  32 C and fed to the image forming unit  5 . 
     The scanner unit  4  is disposed inside the casing  21 , at the front side thereof, between the sheet feed tray  31  and the sheet output tray  9 . The scanner unit  4  is disposed away from a photoconductor drum  61  as an example of a photoconductor to be described later. The scanner unit  4  includes a laser beam emitting portion, a polygon mirror, lenses and reflecting mirrors, etc., which are not shown in the drawings. The surface of the photoconductor drum  61  is scanned at high speeds with a laser beam L produced in the scanner unit  4 . 
     The image forming unit  5  is disposed above the sheet feeder unit  32 . The image forming unit  5  includes a process cartridge  6  and a fixing device  7 . 
     The process cartridge  6  is disposed inside the casing  21  at a position rearward of the scanner unit  4 . The process cartridge  6  includes the photoconductor drum  61 , a transfer roller  62  (as an example of a transfer member) disposed adjacent to and at the rear of the photoconductor drum  61 , a charger (not shown), and a development roller, a supply roller, and a toner storage chamber, which are shown in the drawings without reference numerals. 
     In this process cartridge  6 , the surface of the rotating photoconductor drum  61  is uniformly charged by the charger, and then exposed to a rapidly sweeping laser beam L from the scanner unit  4 . Accordingly, the electric potential of the exposed area lowers, so that an electrostatic latent image associated with image date is formed on the surface of the photoconductor drum  61 . 
     Toner in the toner storage chamber is then supplied to the electrostatic latent image on the photoconductor drum  61  via the development roller, so that a toner image (developer image) is formed on the surface of the photoconductor drum  61 . Thereafter, while a sheet P is conveyed through between the photoconductor drum  61  and the transfer roller  62 , the toner image carried on the surface of the photoconductor drum  61  is transferred onto the sheet P. 
     The fixing device  7  is disposed inside the casing  21  at a position above the process cartridge  6 . The fixing device  7  mainly includes a heating roller  71  and a pressure roller  72 . 
     The heating roller  71  is configured to heat a sheet P, and a heat source such as a halogen lamp is provided inside the heating roller  71 . 
     The pressure roller  72  is pressed against the heating roller  71  and conveys a sheet P while the sheet P is nipped between the heating roller  71  and the pressure roller  72 . The pressure roller  72  is disposed opposite to the heating roller  71  in a position diagonally upward and rearward of the heating roller  71 . 
     In this fixing device  7  configured as described above, the toner image transferred onto the sheet P is thermally fixed while the sheet P passes through between the heating roller  71  and the pressure roller  72 . The sheet P with the toner image thermally fixed thereon by the fixing device  7  is conveyed to the sheet ejection unit  8  disposed downstream from the fixing device  7 . 
     The sheet ejection unit  8  includes a drive roller  81 , and a driven roller  82  disposed in contact with the drive roller  81  and driven to rotate by the driving force of the drive roller  81 . The sheet ejection unit  8  conveys a sheet P while the sheet P is nipped between the drive roller  81  and the driven roller  82 . 
     The drive roller  81  and the driven roller  82  are configured to be movable between a sheet ejection position (shown by solid lines) where the sheet P having passed through the fixing device  7  is ejected outside the casing  21  and a re-conveyance position (shown by chain double-dashed line) where the sheet P having passed through the fixing device  7  is conveyed toward the re-conveyance path  200  to be described later. 
     To be more specific, while the drive roller  81  and the driven roller  82  are in the sheet ejection position, a contacting portion (i.e., nip portion) between the drive roller  81  and the driven roller  82  are positioned to face the sheet ejection opening  21 A. When the drive roller  81  and the driven roller  82  are moved into the re-conveyance position, the driven roller  82  is swung rearward from the sheet ejection position around the drive roller  81 , so that the contacting portion (i.e., nip portion) between the drive roller  81  and the driven roller  82  are positioned to face an inlet of the re-conveyance path  200 . 
     When the image formation process ends, the drive roller  81  and the driven roller  82  are positioned in the sheet ejection position under control of the controller (not shown) provided inside the casing  21 . In contrast, during the reverse-side printing for printing the reverse side (second side) of the sheet P, the drive roller  81  and the driven roller  82  are positioned in the re-conveyance position under control of the controller. 
     In the sheet ejection unit  8 , the drive roller  81  and the driven roller  82  are positioned in the sheet ejection position when the image formation process ends, so that the sheet P having passed through the fixing device  7  is ejected outside the casing  21  through the sheet ejection opening  21 A. When the reverse-side printing is performed on the sheet P, the drive roller  81  and the driven roller  82  are positioned in the re-conveyance position, so that the sheet P having passed through the fixing device  7  is conveyed to the re-conveyance path  200  without passing through the sheet ejection opening  21 A. 
     Detailed Structure of Conveyance Path and Re-conveyance Path 
     Detailed description will be given of the conveyance path  100  and the re-conveyance path  200  as an example of a re-conveyance mechanism, which are provided inside the casing  21 . 
     As seen in  FIG. 1 , the conveyance path  100  has a generally C-shaped configuration and extends to guide a sheet P placed on the sheet feed tray  31 , passing through the sheet feeder unit  32 , a nip portion between the photoconductor drum  61  and the transfer roller  62 , the fixing device  7 , and the sheet ejection unit  8  in this order. To be more specific, the conveyance path  100  is defined by a first guide member  101 , a second guide member  102 , and a third guide member  103 . The conveyance path  100  includes a first path  110 , a second path  120 , and a third path  130 . 
     The first path  110  is an example of an upstream-side path which constitutes part of the conveyance path  100  extending upstream from the photoconductor drum  61 . The first path  110  connects the sheet feeder unit  32  and the image forming unit  5  (nip portion between the photoconductor drum  61  and the transfer roller  62 ). The first path  110  is defined mainly by a lower portion  101 A of the first guide member  101 ; the first guide member  101  extends substantially in the vertical direction and detours around the rear side of the process cartridge  6 . 
     The lower portion  101 A of the first guide member  101  extends rearward from the sheet feed tray  31 , through the sheet feeder unit  32 , and is then directed upward to a position lower than the nip portion between the photoconductor drum  61  and the transfer roller  62 . 
     The second path  120  is a path provided inside the image forming unit  5 . The second path  120  is defined mainly by an upper portion  101 B of the first guide member  101 . 
     The upper portion  101 B of the first guide member  101  extends upward from a position higher than the nip portion between the photoconductor drum  61  and the transfer roller  62  to a position lower than the nip portion between the heating roller  71  and the pressure roller  72  of the fixing device  7 . 
     The third path  130  is an example of a downstream-side path which constitutes part of the conveyance path  100  extending downstream from the fixing device  7 . The third path  130  connects the image forming unit  5  (fixing device  7 ) and the sheet ejection unit  8 . The third path  130  is defined mainly by the second guide member  102  and the third guide member  103 . 
     The second guide member  102  has a substantially arcuate guide surface which is recessed upward in section, and extends frontward from an exit of the fixing device  7  to the sheet ejection unit  8 . 
     The third guide member  103  is disposed below and opposite to the second guide member  102 . The third guide member  103  defines a path for a sheet with the second guide member  102 . The third guide member  103  extends frontward from the exit of the fixing device  7  to an inlet of a return path  210  to be described later. 
     The re-conveyance path  200  is configured to cause a sheet P having passed through the fixing device  7  (image forming unit  5 ) to partly protrude outside the casing  21  through the first opening  21 C, to reverse the traveling direction of the sheet P, and to thereafter guide the sheet P toward the first path  110 . The re-conveyance path  200  consists of a return path  210  and a switchback path  220 . 
     The return path  210  is a path for connecting the third path  130  and the switchback path  220 . The return path  210  is branched off from the third path  130  (conveyance path  100 ) at a position downstream from the fixing device  7  and extends diagonally downward and frontward to pass through a space between the photoconductor drum  61  and the scanner unit  4  and to merge with the switchback path  220  at an intermediate portion of the switchback path  220 . With this configuration, when a sheet P having passed through the return path  210  enters the switchback path  220 , the sheet P is guided to travel frontward in the switchback path  20  toward the first opening  21 C. 
     More specifically, the return path  210  is defined by a fourth guide member  201 , a fifth guide member  202 , a sixth guide member  203 , and a seventh guide member  204 . 
     The fourth guide member  201  is a wall of the casing  21 , in which the sheet ejection opening  21 A is formed. The fourth guide member  201  extends downward from a lower side of the sheet ejection unit  8  to a position higher than a path through which a laser beam L emitted from the scanner unit  4  passes (hereinafter referred to as a “light path”). 
     The fifth guide member  202  is disposed at the rear of the fourth guide member  201 , opposite to the fourth guide member  201 . The fifth guide member  202  defines a path for a sheet P with the fourth guide member  201 . The fifth guide member  202  extends diagonally downward and frontward from the lower side of the sheet ejection unit  8  to a position higher than the light path. 
     The sixth guide member  203  extends diagonally downward and frontward from a position lower than the light path on an extension of the lower end of the fourth guide member  201 , to a pair of conveyance rollers  230  to be described later. 
     The seventh guide member  204  is disposed at the rear of the sixth guide member  203 , opposite to the sixth guide member  203 . The seventh guide member  204  defines a path for a sheet P with the sixth guide member  203 . The seventh guide member  204  extends diagonally downward and frontward from a position lower than the range where the laser beam L emitted from the scanner unit  4  passes, on an extension of the lower end of the fifth guide member  202 , toward the rear side of the conveyance rollers  230  to be described later. Further, the lower end portion (end portion closer to the switchback path  220 ) of the seventh guide member  204  is bent in a diagonally upward and frontward direction, such that the front end of the seventh guide member  204  faces inward of the return path  210  toward the sixth guide member  203 . 
     To be more specific, the lower end portion of the seventh guide member  204  is tilted with respect to the tangential line passing through the nip portion of the conveyance rollers  230  such that the switchback path  220  (path for a sheet P) is enlarged as it goes upstream in the sheet conveyance direction, along which the sheet P of which the traveling direction has been reversed is conveyed. This configuration prevents the sheet P conveyed along the switchback path  220  from the front side toward the rear side from entering the return path  210 , and makes it possible to smoothly guide the sheet P toward the sheet feeder unit  32 . 
     Further, because of the return path  210  configured as described above, the laser beam L emitted from the scanner unit  4  can pass through a space between the fourth guide member  201  and the sixth guide member  203 , and a space between the fifth guide member  202  and the seventh guide member  204 , so that the return path  210  does not block the laser beam L. 
     The switchback path  220  is a path for connecting the first opening  21 C, the return path  210 , and the sheet feeder unit  32 . In the vertical direction, the switchback path  220  is located between the sheet feed tray  31  and the scanner unit  4 . In the front-rear direction, the switchback path  220  extends rearward from the front side of the casing  21  along the sheet feed tray  31 , and merges with the first path  110 . Further, the first opening  21 C is provided at a position (i.e., front end) corresponding to the opposite end of the switchback path  220  from the first path  110 . The sheet P guided along the switchback path  220  is allowed to partly protrude outside the casing  21  through the first opening  21 C. 
     More specifically, the switchback path  220  is defined by an eighth guide member  205 , a ninth guide member  206 , and a tenth guide member  207 . 
     The eighth guide member  205  extends diagonally downward and rearward from the lower edge of the first opening  21 C to a position in front of the sheet feeder unit  32 . The rear end of the eighth guide member  205  extends to a position where it does not contact the sheets P placed on the sheet receiving plate  31 A after the sheet receiving plate  31 A rotates upward to approach the sheet feed roller  32 A. In order to stably convey a sheet P from the eighth guide member  205  to the sheet feed roller  32 A, a film or the like for guiding the sheet P may be provided at the rear end portion of the eighth guide member  205 . 
     The ninth guide member  206  is disposed above the front portion of the eighth guide member  205  so as to face the eighth guide member  205 . The ninth guide member  206  defines a path for a sheet P with the eighth guide member  205 . The ninth guide member  206  extends diagonally downward and rearward from the upper edge of the first opening  21 C to a position in front of the conveyance rollers  230 . With this configuration, the sheet P conveyed along the front part of the switchback path  220  from the front side toward the rear side can be guided to the conveyance rollers  230 . 
     The tenth guide member  207  is disposed above the rear portion of the eighth guide member  205  so as to face the eighth guide member  205 . The tenth guide member  207  defines a path for a sheet P with the eighth guide member  205 . The tenth guide member  207  extends rearward from the rear side of the seventh guide member  204  in the return path  210  to a position in front of the sheet feeder unit  32 . To be more specific, the tenth guide member  207  is tilted downward toward the rearward direction such that the switchback path  220  (path for a sheet P) narrows as it goes in a direction toward the sheet feeder unit  32 . With this configuration, the sheet P can be guided toward the sheet feed roller  32 A. 
     The conveyance rollers  230  are arranged at a position shifted from a merging position where the return path  210  is merged with the switchback path  220  in a direction away from the first path  110 , namely, at a position of the switchback path  220  close to the first opening  21 C. The conveyance rollers  230  are arranged to face a sheet P guided along the switchback path  220 . The switchback path  220  and the conveyance rollers  230  constitute a switchback mechanism  240  configured to reverse the traveling direction of a sheet P having passed through the return path  210  and to convey the sheet P toward the first path  110 . 
     The conveyance rollers  230  are configured to be rotatable in forward and reverse directions. To be more specific, the conveyance rollers  230  rotate forward to guide a sheet P in the switchback path  220  in a direction away from the first path  110 , namely, toward the first opening  21 C. Meanwhile, the conveyance rollers  230  rotate reversely to guide the sheet P in the switchback path  220  in a direction toward the first path  110 . 
     The conveyance rollers  230  are controlled by the controller such that they are driven to rotate forward while the sheet P is being conveyed along the return path  210  and to rotate reversely after the tailing end of the sheet P having conveyed along the return path  210  reaches the position between the return path  210  and the switchback path  220  (i.e., after the sheet P passes through the lower end of the seventh guide member  204 ). 
     Accordingly, the switchback mechanism  240  can guide the sheet P from the return path  210  to the switchback path  220 , using the conveyance rollers  230 , so as to cause the sheet to partly protrude outside the casing  21  through the first opening  21 C. Thereafter, after the trailing edge of the sheet P reaches the merging position where the return path  210  is merged with the switchback path  220 , the switchback mechanism  240  reverses the traveling direction of the sheet P, so that the sheet P can be conveyed toward the sheet feeder unit  32  with the trailing edge positioned ahead. 
     Further, the switchback path  220  is configured to allow sheets P to be inserted through the first opening  21 C and to feed a sheet P from the inserted sheets P toward the conveyance path  100  (first path  110 ). In other words, the switchback path  220  also works as a manual sheet feeding path, and the first opening  21 C also works as an opening for manual sheet feeding (second opening). During printing a sheet P by the manual sheet feeding, the controller controls the conveyance rollers  230  to rotate reversely. 
     Operation of the laser printer  1  configured as described above will be described below. 
     To perform duplex printing, as seen in  FIG. 1 , the lifter member  31 B is raised first, so that sheets P placed on the sheet receiving plate  31 A approach the sheet feeder unit  32 . This causes the sheets P placed on the sheet receiving plate  31 A (sheet feed tray  31 ) to be fed out by the sheet feeder unit  32 , toward the image forming unit  5  via the first path  110 . During this time, the laser beam L emitted from the scanner unit  4  sweeps the surface of the photoconductor drum  61  for high speed scanning. 
     The sheet P fed to the image forming unit  5  then passes through the nip portion between the photoconductor drum  61  and the transfer roller  62 , whereby an image is formed on the surface of the sheet P. Thereafter, the sheet P is conveyed to the fixing device  7  via the second path  120 . The sheet P having passed through the fixing device  7  is then conveyed to the third path  130 . In the sheet ejection unit  8 , as seen in  FIG. 2 , the drive roller  81  and the driven roller  82  are moved into the re-conveyance position, so that the sheet P conveyed along the third path  130  is guided toward the return path  210  without passing through the sheet ejection opening  21 A. 
     The sheet P is conveyed along the return path  210 , and after passing through the space between the scanner unit  4  and the photoconductor drum  61 , the sheet P is guided to the conveyance rollers  230  provided in the switchback path  220 . It is noted that the image formation on the surface of the sheet P is completed by the time when the leading edge of the sheet P reaches the space (light path) between the scanner unit  4  and the photoconductor drum  61 , and so the irradiation with the laser beam L is not blocked by the sheet P. 
     Thereafter, as seen in  FIG. 3 , the sheet P conveyed from the return path  210  to the switchback path  220  is guided by the conveyance rollers  230  toward the first opening  21 C until the trailing edge of the sheet P passes through the return path  210 . During this time, the leading edge of the sheet P protrudes outside the casing  21  through the first opening  21 C; an protruding part of the sheet P is supported by the front cover  22  which constitutes the sheet feed tray  31 . 
     Once the trailing edge of the sheet P passes through the return path  210  and enters the switchback path  220 , the conveyance rollers  230  rotate reversely. Accordingly, as seen in  FIG. 4 , the sheet P conveyed along the switchback path  220  is guided toward the sheet feeder unit  32  (first path  110 ) with the trailing edge positioned ahead. While the sheet P is guided along the switch back path  220 , the lifter member  31 B is lowered and the sheets P placed on the sheet receiving plate  31 A are positioned away from the sheet feeder unit  32 . 
     In this way, the sheet P having passed through the fixing device  7  and conveyed along the re-conveyance path  200  is conveyed upside down toward the conveyance path  100 , and an image is formed on the reverse surface of the sheet P. In order to form an image on the reverse surface of the sheet P, the irradiation with the laser beam L starts after the trailing edge of the sheet P passes through the space (light path) between the scanner unit  4  and the photoconductor drum  61 . Thereafter, as seen in  FIG. 1 , the sheet P having the image formed thereon is ejected outside the casing  21  by the driving roller  81  and the driven roller  82  of the sheet ejection unit  8 , which have been moved into the sheet ejection position. 
     With the configuration of the laser printer  1  according to this embodiment, the following advantageous effects can be achieved. 
     Since the trailing edge of the sheet P having passed through the space (light path) between the photoconductor drum  61  and the scanner unit  4  in the re-conveyance path  200  (return path  210 ) is reversed by the switchback mechanism  240  and positioned ahead, and then guided toward the nip position between the photoconductor drum  61  and the transfer roller  62 , even if the size of the sheet P is large, the laser printer  1  can reliably guide the sheet P conveyed along the re-conveyance path  200  to the nip portion between photoconductor drum  61  and the transfer roller  62  after the sheet P passes through the space between the photoconductor drum  61  and the scanner unit  4 . 
     Further, since the sheet P conveyed along the switchback path  220  toward the first opening  21 C is allowed to partly protrude outside the casing  21  through the first opening  21 C, the size of the casing  21  can be reduced as compared to the configuration in which the traveling direction of the sheet P is reversed within the casing  21 . 
     Since the first opening  21 C for causing the sheet P to partly protrude outside the casing  21  during the switchback is provided separately from the sheet ejection opening  21 A, even if the sheet P partly protrudes outside the casing  21  and is reversed during the duplex printing, the user is less likely to consider the protruding sheet P as a printed sheet P and to pull out the sheet P by mistake. 
     Further, since the re-conveyance path  200  is configured to cause the sheet P conveyed along the third path  130  to partly protrude outside the casing  21  through the first opening  21 C, to reverse the traveling direction of the sheet P, and to thereafter guide the sheet P toward the first path  110 , and further the first opening  21 C is provided between the scanner unit  4  and the sheet feed tray  31 , the trailing edge of the sheet P having passed through the space (light path) between the photoconductor drum  61  and the scanner unit  4  in the re-conveyance path  200  (return path  210 ) is reversed and positioned ahead and then guided toward the nip portion between the photoconductor drum  61  and the transfer roller  62 . With this configuration, even if the size of the sheet P is large, the laser printer  1  can reliably guide the sheet P conveyed along the re-conveyance path  200  to the nip portion between photoconductor drum  61  and the transfer roller  62  after the sheet P passes through the space between the photoconductor drum  61  and the scanner unit  4 . 
     Since the first opening  21 C is located at the lower portion of the casing  21 , the under surface of the sheet P protruding outside the casing  21  through the first opening  21 C can be supported by the sheet feed tray  31  (part of the sheet feed tray  31  which extends outside beyond the first opening  21 C) located just under the first opening  21 C. Even by the configuration in which the sheet feed tray  31  does not extend outside from the casing  21 C, if the laser printer  1  is placed on the installation surface and the installation surface extends frontward of the first opening  21 C, the installation surface can support the under surface of the sheet P protruding outside the casing  21  through the first opening  21 C. This is advantageous, as compared to the configuration in which the first opening  21 C is located at the upper portion of the casing  21  and the sheet is hung down from the first opening, because the sheet P partly protruding outside through the first opening  21 C can be held in a substantially horizontal posture and easily retracted back into the casing  21  during reversing the sheet P. 
     Since the sheet feed tray  31  extends from inside to outside the casing  21  beyond the first opening  21 C, the sheet feed tray  31  can reliably support the sheet P partly protruding outside through the first opening  21 C, even if the laser printer  1  is installed on a stand and no installation surface is present in front of the first opening  21 C. With this configuration, the sheet P partly protruding outside through the first opening  21 C can be held in a substantially horizontal posture and easily retracted back into the casing during reversing the sheet P. 
     Further, since part of the sheet feed tray  31  which extends outside the casing  21  is formed by the front cover  22  for opening and closing the first opening  21 C, the laser printer  1  becomes compact when not in use. Closing the front cover  22  can prevent dust from entering the casing  21  through the first opening  21 C. 
     Since the downstream end of the re-conveyance path  200  (switchback path  220 ) is connected to the sheet feeder unit  32 , it is not necessary to provide, other than the sheet feeder unit  32 , a feed-out member for feeding a sheet P conveyed from the re-conveyance path  200  toward the nip portion between the photoconductor drum  61  and the transfer roller  62  (image forming unit  5 ). This can reduce the number of the required parts. Further, since only the sheet feeder unit  32  is provided as a member for feeding a sheet P toward the nip portion between the photoconductor drum  61  and the transfer roller  62 , the control for determining timings of the image formation and the like can be simplified. 
     Since the lifter member  31 B lowers to position the sheet receiving plate  31 A away from the sheet feeder unit  32  while a sheet P is being guided along the re-conveyance path  200  toward the conveyance path  100 , the risk of feeding a sheet P placed on the sheet receiving plate  31 A (sheet feed tray  31 ) by mistake can be reduced. 
     Further, since the switchback mechanism  240  is provided between the sheet feed tray  31  and the scanner unit  4 , the casing can be downsized as compared to the configuration in which the switchback mechanism  240  is provided on the opposite side of (above) the scanner unit  4  from the sheet feed tray  31 . 
     Further, since the drive roller  81  and the driven roller  82  of the sheet ejection unit  8  are configured such that the sheet P is ejected outside the casing  21  when they are in the sheet ejection position and that the sheet P is conveyed toward the return path  210  when they are in the re-conveyance position, and further the rollers for discharging the sheet P outside the casing  21  and the rollers for guiding the sheet P along the re-conveyance path  200  are the same rollers, the size of the laser printer  1  can be reduced as compared to the configuration in which these rollers are provided individually. 
     Further, since the switchback path  220  also works as a manual sheet feeding path, the casing  21  can be downsized as compared to the configuration in which the manual sheet feeding path is provided separately from the switchback path  220 . 
     Since the first opening  21 C for causing the sheet P conveyed along the re-conveyance path  200  to partly protrude outside the casing  21  and the opening for manual sheet feeding (second opening) through which sheets P are inserted are the same opening, the structure of the laser printer  1  can be simplified. 
     Further, since the scanner unit  4  is located between the sheet feed tray  31  and the sheet output tray  9 , the casing  21  can be downsized as compared to the configuration in which the scanner unit  4  is provided to a position other than that between the sheet feed tray  31  and the sheet output tray  9 . 
     Although an illustrative embodiment of the present invention have been described in detail, the present invention is not limited to this specific embodiment. It is to be understood that various changes and modifications, such as those described below, may be made without departing from the scope of the appended claims. In the following description, parts similar to those previously described in the above embodiment are denoted by the same reference numerals and detailed description thereof will be omitted. 
     In the above exemplary embodiment, the drive roller  81  and the driven roller  82  of the sheet ejection unit  8  are configured to be movable between the sheet ejection position and the re-conveyance position, so that the traveling direction of the sheet P conveyed along the third path  130  can be switched. However, the present invention is not limited to this specific configuration. For example, as seen in  FIG. 5 , a flapper  300  as an example of a switching member is swingably provided in the third path  130 , between the fixing device  7  and the sheet ejection unit  8 ; the traveling direction of a sheet P conveyed along the third path  130  can be switched by the flapper  300 . 
     To be more specific, the flapper  300  extends downward from above the third path  130  so as to intersect the third path  130  and configured to be switchable between a sheet ejection posture (shown by solid line) in which a distal end of the flapper is directed toward the sheet ejection unit  8  and a re-conveying posture (shown by broken line) in which the distal end of the flapper is directed toward the inlet of the re-conveyance path  200 ; the sheet P passed through the fixing device  7  is guided to the sheet ejection unit  8  by the flapper  300  in the sheet ejection posture, whereas guided to the return path  210  (re-conveyance path  200 ) by the flapper  300  in the re-conveying posture. 
     When the image formation process ends, the flapper  300  is switched into the sheet ejection posture under control of the controller. When the reverse side of the sheet P is printed, the flapper  300  is switched into the re-conveying posture under control of the controller. 
     The traveling direction of a sheet P conveyed along the third path  130  can be reversed by a mechanism using the flapper  300 , which is less complicated in structure than that described in the above embodiment. 
     In the above exemplary embodiment, the first opening  21 C for causing a sheet P to partly protrude outside the casing  21  during the switchback and the opening for manual sheet feeding (second opening) are the same opening. However, the present invention is not limited to this specific configuration. For example, the casing  21  may have a second opening which is not the same as the first opening  21 C, and a sheet P inserted from the second opening can be fed to the switchback path  220 . 
     Further, in the above exemplary embodiment, two rollers including the sheet feed roller  32 A and the separation roller  32 B are provided in the sheet feeder unit  32 . However, only one roller may be provided in the sheet feeder unit or alternatively, three or more rollers may be provided in the sheet feeder unit. 
     In the above exemplary embodiment, the lifter member  31 B configured to cause the sheet receiving plate  31 A to be moved upward or downward is exemplified as a movement mechanism, and the sheet receiving plate  31 A is moved closer to or away from the sheet feeder unit  32  by the lifter member  31 B. However, the present invention is not limited to this specific configuration. For example, the sheet feed roller  32 A may be movable upward or downward relative to the sheet receiving plate  31 A, and a movement mechanism for moving the sheet feed roller  32 A may be provided. In this embodiment, when a sheet P is conveyed from the sheet feed tray  31  to the conveyance path  100 , the movement mechanism causes the sheet feed roller  32 A to approach the sheet receiving plate  31 A, whereas when a sheet P is conveyed from the re-conveyance path  200  to the conveyance path  100 , the movement mechanism causes the sheet feed roller  32 A to be moved away from the sheet receiving plate  31 A. 
     In the above exemplary embodiment, the conveyance rollers  230  are located at a position shifted from the merging position where the return path  210  and the switchback path  220  are merged together in a direction toward the first opening  21 C. However, the conveyance rollers  230  may be provided at the merging position between the return path  210  and the switchback path  220 . 
     Further, in the above exemplary embodiment, the sheet receiving plate  31 A is moved away from the sheet feed roller  32 A (see  FIG. 4 ) when a sheet P is guided into the switchback path  220 . However, the present invention is not limited to this specific configuration. For example, the sheet receiving plate  31 A may be moved away from the sheet feed roller  32 A after the trailing edge of a sheet P placed on the sheet receiving plate  31 A reaches the separation roller  32 B. 
     In the above exemplary embodiment, the photoconductor drum  61  is employed as an example of a photoconductor. However, a belt-type photoconductor may be employed instead. 
     In the above exemplary embodiment, the photoconductor drum  61  is exposed to light by the scanner unit  4 . However, the present invention is not limited to this specific configuration. For example, an LED unit may be provided adjacent to the photoconductor drum  61  as an exposure device for exposing the photoconductor drum  61  to light. In this embodiment, the return path  210  of the re-conveyance path  200  may extend to pass along the front side of the photoconductor drum  61  and the LED unit. 
     Further, in the above exemplary embodiment, a sheet P such as a cardboard, a postcard, and a thin paper, etc. is used as an example of a recording sheet. However, an OHP sheet may be used as the recording sheet.