Patent Publication Number: US-6987525-B2

Title: Image forming apparatus for duplex printing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Patent Application No. 2002-25671, filed May 9, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an image forming apparatus, such as a laser printer or a copier, and more particularly, to an image forming apparatus for duplex printing having a paper conveyance guide using both surfaces thereof as paper feeding surfaces. 
   2. Description of the Related Art 
   Generally, an image forming apparatus for printing on both sides of a paper sheet (duplex printing), for example, a laser printer  1 , includes a paper feeding cassette  10  having a stack of paper sheets  15  therein, a pickup roller  20  mounted on an upper side of the paper feeding cassette  10  to pick up the paper sheet  15 , a paper conveyance guide  22  defining a first paper conveying path that guides the paper sheet  15  picked up by the pickup roller  20 , a developing unit  30  that develops a visible toner image on a first side of the paper sheet  15  being conveyed through the paper conveyance guide  22 , a fixing unit that fixes the toner image on the first side of the paper sheet  15  passing through the developing unit  30 , a conveying roller  50  that conveys the paper sheet  15  that has passed through the fixing unit, a discharge roller  60  arranged on an upper side of the conveying roller  50  to discharge the paper sheet  15 , a reverse paper conveyance guide  62  defining a second paper conveying path that guides the paper sheet  15 , which is discharged through the discharge roller  60 , to re-enter the paper conveyance guide  22  for the duplex printing so as to have a second side of the paper sheet  15  printed, and a first and a second reverse conveying rollers  70  and  90  that reverse-convey the paper sheet  15  through the reverse paper conveyance guide  62   a  as shown in  FIG. 1 . 
   The developing unit  30  includes a photosensitive drum  32  that receives light signals, such as a laser beam, from a laser scanning unit (LSU)  36  in accordance with an image signal, thereby forming an electrostatic latent image thereon, and a developing roller (not shown) that attaches toner particles onto the electrostatic latent image of the photosensitive drum  32 , thereby forming the toner image. 
   A transfer roller  34  is disposed below the photosensitive drum  32  of the developing unit  30  to transfer the toner image from the photosensitive drum  32  to the paper sheet  15 . 
   The fixing unit includes a heating roller  41  and a fixing roller  40  fixing the toner image on the paper sheet  15  with heat and pressure. 
   Formed between the paper conveyance guide  22  and the paper reverse conveyance guide  62 , at which the discharging roller  60  is disposed, is a paper reverse guide  42  guiding the paper sheet  15  to the paper reverse conveyance guide  62 . The paper reverse guide  42  is formed as an upright arm flexibly hinged between the conveyance guide  22  and the paper reverse conveyance guide  62 . On the paper reverse guide  42  is formed a paper sensor (not shown) that senses the paper sheet  15  being conveyed. 
   The duplex printing by the laser printer  1  constructed as above will be described below. 
   First, each paper sheet  15  is picked up by the pickup roller  20  from the paper stack of the paper feeding cassette  10 , and then conveyed to the developing unit  30  along the paper conveyance guide  22 . 
   By a laser beam emitted from the LSU  36  in accordance with the image signal, the electrostatic latent image is formed on the photosensitive drum  32  of the developing unit  30 , and the electrostatic latent image formed on the photosensitive drum  32  is developed into the toner image by the developing roller that attaches the toner particles onto the electrostatic latent image. 
   The toner image formed on the photosensitive drum  32  is transferred onto the first side, e.g., an upper side, of the paper sheet  15  while the paper sheet  15  is conveyed to the developing unit  30  along the paper conveyance guide  22 , and then fixed on the upper side of the paper sheet  15  by the heating roller  41  and the fixing roller  40  of the fixing unit. 
   After passing through the heating roller  41  and the fixing roller  40 , the paper sheet  15  is advanced to the discharging roller  60  by the conveying roller  50 . By a rotational force of the conveying roller  50 , the paper sheet  15  pushes the paper reverse guide  42  from a dotted line position to a solid line position of  FIG. 1 . Accordingly, the paper sensor detects that the paper sheet  15  is passing forward the discharging roller  60 . 
   When the printing is performed on the first side of the paper sheet  15  only, i.e., when it is simplex printing, the paper sheet  15  is released onto a paper panel  59  via the discharging roller  60 . 
   In the duplex printing, however, the discharging roller  60  is reverse-driven by a separate forward/reverse rotation control unit (not shown) in accordance with a signal or signals from the paper sensor which generates the signal after a rear tip end of the paper sheet  15  passes through the paper reverse guide  42 . As a result, the paper sheet  15  is not released out, but re-entered into the reverse paper conveyance guide  62 . Since the paper reverse guide  42  is in the dotted line position, i.e., in an original position due to its recovery force, the paper sheet  15  can be easily re-entered into the reverse paper conveyance guide  62  by the paper reverse guide  42 . 
   When the paper sheet  15  reaches the reverse paper conveyance guide  62 , the paper sheet  15  is conveyed to an aligning roller  80  by the first reverse conveying roller  70 . The aligning roller  80  is in a tilted posture at an angle about 5°, and thus, a leading tip end of the paper sheet  15  is pushed by the aligning roller  80  into an alignment with the second paper convey path. 
   After being aligned by the aligning roller  80 , the paper sheet  15  is passed through the second reverse conveying roller  90  and conveyed back to the paper conveyance guide  22 . Then, after the printing is performed on the second side of the paper sheet  15  in the same way as described above, the paper sheet  15  is released out onto the external paper panel  59 . 
   In the conventional laser printer  1  constructed as above, the paper conveyance guide  22  and the reverse paper conveyance guide  62  are arranged separately from each other. Further, the conventional laser printer  1  requires a number of parts for defining a space for the first and the second paper conveying paths, such as three upper guide plates and four lower guide plates. As a result, a manufacturing cost increases. Further, since an assembling process requires fasteners, such as screws, which are rather complex to assemble, the assembling process takes a lengthy time which deteriorates productivity. 
   Further, according to the conventional laser printer  1 , the first and second paper conveying paths are formed all across the printer  1  in a lengthwise direction. Accordingly, it also takes time in the duplex printing, deteriorating a printing efficiency (both a duplex printing speed and a simplex printing speed). Also, it has a high possibility of having problems, such as a paper jam, a paper skew, or the like. 
   As the paper conveying path is long, it needs a considerable number of rollers to convey the paper sheet  15  to the paper conveyance guide  22 . For example, the reverse paper conveyance guide  62  needs at least three types of rollers, i.e., the first reverse conveying roller  70 , the aligning roller  80  and the second reverse conveying roller  90 . 
   Not only are these rollers  70 ,  80 ,  90  quite complex to assemble, but the assembling process also requires a number of parts, such as a shaft fixing ring, a power transmitting belt, a pulley for belt, bushing, etc. Accordingly, the assembling process becomes complex, requiring a longer assembling time and a higher cost. Additionally, there is a high possibility of having defects. 
   More specifically, driving power is transmitted to the first reverse conveying roller  70  and the aligning roller  80  by an idle gear assembly  95  which is employed as a power transmitting unit shown in  FIG. 2 . The idle gear assembly  95  includes a first idle gear  97   a  rotatably secured to a first boss  96   a  by a first shaft fixing ring  99   a  to be meshed with a gear portion (not shown) formed at an end of a shaft of the first reverse conveying roller  70  while the first boss  96   a  is secured at the reverse paper conveyance guide  62 , a second idle gear  97   b  rotatably secured to a second boss  96   b  by a second shaft fixing ring  99   b  to be meshed with the first idle gear  97   a  while the second boss  96   b  is secured at the reverse paper conveyance guide  62 , and a third idle gear  97   c  rotatably secured to a third boss  96   c  by a third shaft fixing ring  99   c  to be meshed with the gear portion (not shown) of the aligning roller  80  and the second idle gear  97   b  while the third boss  96   c  is secured at the reverse paper conveyance guide  62 . 
   Since the aligning roller  80  and the second reverse conveying roller  90  are spaced apart from each other by a predetermined distance, as shown in  FIG. 3 , the driving power needs to be transmitted through a belt pulley  93  secured to a side of the shaft  91  of the second reverse conveying roller  90  by an E-shaped ring  94 , a belt pulley (not shown) formed corresponding to the shaft of the aligning roller  80  and a power transmitting belt  93   a  connecting such belt pulleys. 
   Further, each of the rollers  70 ,  80 ,  90  is fabricated through a rather complex process. For example, the second reverse conveying roller  90  has the shaft  91  formed separately from the gear portion and the belt pulley  93 , while the roller portion  92  is formed on the shaft  91  by molding. 
   Further, since the rollers  70 ,  80 ,  90  are secured to the reverse paper conveyance guide  62  using the bushings  98 ,  98 ′, the assembling process requires complicated additional work that takes much longer time. 
   As a result, the conventional laser printer  1  has the first and second long paper conveying paths for duplex printing, requiring many parts and processes, and thus has the problems of a manufacturing cost increase, printing efficiency deterioration, a frequent paper jam, and productivity deterioration. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an aspect of the present invention to provide an image forming apparatus for duplex printing that uses both opposite faces of a paper conveyance guide as paper conveyance guiding surfaces, thereby shortening a paper conveying path and achieving a high printing process during the duplex printing and a reduction of the number of rollers for the paper conveyance path. 
   It is another aspect of the present invention to provide an image forming apparatus for duplex printing that provides a plurality of ribs formed on opposite surfaces of the paper conveyance guide, thereby reducing problems, such as a paper jam, a skew, or the like. 
   It is yet another aspect of the present invention to provide an image forming apparatus for duplex printing that extends a diameter of a conveying roller to feed a paper sheet picked up by a pickup roller to a developing unit, and uses the conveying roller having an extended diameter not only to feed the paper to the developing unit but also to feed the paper to re-enter the developing unit during the duplex printing, thereby achieving a reduction of a paper jam and an improvement in a conveyance efficiency. 
   It is yet another aspect of the present invention to provide an image forming apparatus for duplex printing that uses components of improved shapes and assembling methods, thereby achieving a reduction of the number of parts and a manufacturing cost, and an improvement of productivity. 
   Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
   The above and/or other aspects are accomplished by an image forming apparatus for duplex printing according to the present invention, including a paper storing unit mounted in a frame of a body, a pickup unit picking up a paper sheet from a paper stack of the paper storing unit, a developing unit forming a toner image on the paper sheet, a fixing unit fixing the toner image into a visible image on the paper sheet, a paper conveyance guide having a first surface (side) defining a first paper conveyance path that guides the paper sheet to the fixing unit through the developing unit in cooperation with the frame of the body, a paper discharging unit discharging the paper sheet after the visible image is fixed on the paper sheet, and a duplex printing unit conveying the paper sheet conveyed to the paper discharge unit back to the developing unit and the fixing unit to form another image on a non-printed side of the paper sheet. The duplex print unit includes an auxiliary conveyance guide disposed on a second surface (side) of the paper conveyance guide defining a second paper conveyance path in cooperation with the second surface of the paper conveyance guide, the second paper conveyance path formed to guide the paper sheet having a first side printed and conveyed to the paper discharging unit back to the developing unit in a reversed posture, a reverse conveying roller assembly disposed between the second surface of the paper conveyance guide and the auxiliary conveyance guide to convey the paper sheet through the second paper conveyance path, and a paper conveying portion formed at a point where the first paper conveyance path and the second paper conveyance path meet, to convey the paper sheet picked up by the pickup unit and the paper sheet conveyed through the reverse roller assembly to the first paper conveyance path. 
   According to another aspect of the present invention, the paper conveying guide comprises a plurality of protruding ribs formed on the first and second surfaces at predetermined intervals in a paper advancing direction to enable a paper conveyance to be performed with a reduced friction. Further, the paper conveyance guide includes one or more ground members secured on the first surface of the paper conveyance guide to ground the paper sheet by a fixing portion having one or more protrusions formed either on the corresponding ground members or on the paper conveyance guide, and one or more protrusion receiving holes formed either on the corresponding ground members or on the paper conveyance guide. 
   The paper conveyance guide further includes one or more main grounds being secured on a side of the paper conveyance guide by a main ground fixing portion to connect the ground members and a transfer member with a printed circuit board of the body, the main ground fixing portion including one or more protrusions formed either on the corresponding main grounds or on the paper conveyance guide, and one or more protrusion receiving holes formed either on the corresponding main grounds or on the paper conveyance guide. 
   The auxiliary conveyance guide includes a first guide portion disposed on the paper conveyance guide to be extended over the second surface of the paper conveyance guide to define a part of the second paper conveyance path with the frame of the body to selectively restrict the paper sheet conveyed to the paper discharge unit, thereby guiding the paper sheet to enter the second paper conveyance path, a second guide portion disposed on the second surface of the paper conveyance guide, constituting a part of the second paper conveyance path in cooperation with the second surface of the paper conveyance guide, and a third guide portion disposed on the frame of the body to guide the paper sheet to be conveyed through the second guide portion along the second paper conveyance path to the first paper conveyance path defined along the developing unit and the fixing unit. 
   The first and the second guide portions are secured on a cover which is pivotally mounted on the frame of the body to pivotally open and close the paper conveyance guide. 
   The reverse conveying roller assembly includes one or more reverse conveying rollers disposed on the second surface of the paper conveyance guide by a fixing member, one or more tension rollers disposed on the second guide portion of the auxiliary conveyance guide to be in contact with the reverse conveying roller with a predetermined pressure, and gear parts disposed on one end of a shaft of the corresponding reverse conveying rollers, to transmit a driving force from a driving motor to the reverse conveying rollers. 
   Alternatively, the reverse conveying roller assembly includes two reverse conveying rollers formed on the second surface of the paper conveyance guide by the fixing member at a predetermined distance from each other. In this case, an idle gear assembly is formed between the gear parts of the shafts of the two reverse conveying rollers to transmit the driving force in an uniform direction from one reverse conveying roller to the other reverse conveying roller. The idle gear assembly includes a first idle gear rotatably supported on a first boss secured on the paper conveyance guide in an engagement with the gear part of the shaft of one reverse conveying roller, and having a first washer integrally formed either on upper or on lower side thereof, a second idle gear rotatably supported on a second boss secured on the paper conveyance guide in an engagement with the first idle gear, and having a second washer integrally formed either on an upper or on a lower side thereof, and a third idle gear rotatably supported on a third boss secured on the paper conveyance guide in an engagement with the second idle gear and the gear part of the shaft of the other reverse conveying roller, and having a third washer integrally formed either on an upper or on a lower side thereof. The first, the second and the third idle gears are arranged on the first, the second and the third bosses in a manner such that the washers thereof are positioned in an alternate arrangement, enabling a fixing thereof with one idle gear secured on the corresponding boss without having to secure all the idle gears on the corresponding bosses. 
   The reverse conveying roller includes a plastic shaft having a gear part integrally formed thereon and a roller part of a tubular shape being fit with the shaft. 
   The fixing member fixing the reverse conveying roller to the paper conveyance guide includes one or more holes formed on the paper conveyance guide, a shaft bushing having a hook formed at one end to be securely inserted in the corresponding holes, and a C-shape opening formed in the other end to securely receive the shaft of each reverse conveying roller. 
   The duplex printing unit includes a paper side guiding portion disposed on the second surface of the paper conveyance guide to guide an edge of the paper sheet during a paper conveyance by the reverse conveying roller assembly, thereby preventing a paper skew. The side guiding portion includes a rail portion formed on one end having a flattened U-shape to guide the edge of the paper sheet, and a fixing portion formed on the other end to fix the rail portion to the second surface of the paper conveyance guide. 
   The paper conveying portion includes a conveying roller of a large diameter disposed on the paper conveyance guide in a point where the first paper conveyance path and the second paper conveyance path join, one or more first backup rollers disposed on the frame of the body to define the first paper conveyance path to be in contact with the conveying roller with a predetermined pressure to convey the paper sheet picked up by the pickup unit along the first paper conveyance path, and one or more second backup rollers disposed on the third guide portion of the auxiliary conveyance guide to be in contact with the conveying roller with a predetermined pressure to convey the paper sheet being conveyed through the reverse conveying roller to the first paper conveyance path. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a sectional view of a conventional laser printer for duplex printing; 
       FIG. 2  is a side view of an idle gear assembly transmitting a driving power among reverse conveying rollers of the laser printer of  FIG. 1 ; 
       FIG. 3  is a side view of a reverse conveying roller of the laser printer of  FIG. 1 ; 
       FIG. 4  is a schematic view of a laser printer for duplex printing according to an embodiment of the present invention; 
       FIG. 5  is a partial perspective view showing a cover of the laser printer of  FIG. 4 ; 
       FIG. 6  is an exploded perspective view of a paper conveyance guide and a duplex printing unit of the laser printer of  FIG. 4 ; 
       FIG. 7  is a partial perspective view showing a structure at a rear portion of the paper conveyance guide of the duplex printing unit of the laser printer of  FIGS. 4 and 6 ; 
       FIG. 8  is a partial perspective view of the paper conveyance guide and the duplex printing unit of the laser printer of  FIGS. 4–7 ; 
       FIG. 9  is a cross-sectional view of a front ground portion being engaged with the paper conveyance guide of the laser printer of  FIG. 4 ; 
       FIG. 10  is a cross-sectional view of a transfer roller and a transfer roller holder being engaged with the paper conveyance guide of the laser printer of  FIG. 4 ; 
       FIG. 11  is a cross-sectional view of a main ground being engaged with the paper conveyance guide of the laser printer of  FIG. 4 ; 
       FIGS. 12A and 12B  are side views showing a reverse conveying roller and fixing members of the duplex printing unit of the laser printer of  FIGS. 4 and 6 , respectively; and 
       FIG. 13  is a side view of an idle gear assembly transmitting a driving power among reverse conveying rollers of the duplex printing unit of the laser printer of  FIGS. 4 and 6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described in order to explain the present invention by referring to the figures. 
   Hereinbelow, an image forming apparatus for duplex printing according to an embodiment of the present invention will be described in greater detail with reference to the accompanying drawings. 
   Referring to  FIG. 4 , a laser printer  100  is schematically illustrated, as one example of the image forming apparatus according to the present invention. 
   The laser printer  100  includes a paper feeding cassette  110  removably mounted in a frame (not shown) of a body  101 , a pickup unit  106  picking up paper sheets from a paper stack in the paper feeding cassette  110 , a developing unit  130  developing a toner image on a paper sheet, a fixing unit  140  fixing the toner image on the paper sheet into a visible image by thermal-pressing, a paper conveyance guide  103  having a first surface (side) that defines a first printing paper conveyance path (A) in cooperation with a part of the frame of the body  101  to guide the paper sheet to the fixing unit  140  via the developing unit  130 , a paper discharging unit  150  discharging the paper sheet on which the visible toner image is completely fixed by the fixing unit  140 , and a duplex printing unit  160  feeding the paper sheet to re-enter the developing unit  130  after the paper sheet is conveyed to the paper discharging unit  150  via the fixing unit  140 . 
   The paper feeding cassette  110  is formed at a lower portion of the body  101 , and has a spring  111  that elastically lifts and lowers a paper pressing plate  112  together with the paper sheet on the paper pressing plate  112 . 
   The pickup unit  106  is formed above the paper feeding cassette  110  to sequentially pick up the paper sheets from the paper stack of the paper feeding cassette  110  one sheet each time. The pickup unit  106  includes a pickup roller  107  picking up the paper sheet and a supporting portion  108  rotatably supporting the pickup roller  107 . The pickup roller  107  is connected to an output side of a driving motor (not shown) via a belt or gear (not shown). 
   The developing unit  130  is removably mounted in the frame of the body  101  to be disposed above the pickup unit  106 . The developing unit  130  includes a photosensitive drum  132  forming an electrostatic latent image thereon by a laser beam that is emitted from a laser scanning unit (LSU)  136  in accordance the image signal as received, a charging roller (not shown) charging the photosensitive drum  132  to a predetermined voltage, and a developing roller (not sown) developing the electrostatic latent image on the photosensitive drum  132  into the toner image by attaching toner particles onto the electrostatic latent image. 
   As shown in  FIGS. 5 and 6 , disposed in turn at the first surface of the paper conveyance guide  103  defining the first paper conveyance path (A) are a front ground portion  116  removing static elasticity from the paper sheet, a transfer roller  134  transferring the toner image of the photosensitive drum  132  onto the paper sheet, and a rear ground portion  105  grounding the paper sheet passing through the transfer roller  134 . 
   The front ground portion  116  is formed as an elongated bar of a flattened U-shape having a plane surface thereon to increase a contact area with the paper sheet, and connected to a printed circuit board (not shown) of the body  101  via a front main ground (not shown). 
   Further, as shown in  FIG. 9 , the front ground portion  116  is snap-fit in a first accommodating recess (portion)  103 ′ formed on the first surface of the paper conveyance guide  103  by front ground portion fixing members  103   b ,  103   b ′,  116   a ,  116   b.    
   The front ground portion fixing members  103   b ,  103   b ′,  116   a ,  116   b  include a plurality of protrusions  103   b ,  103   b ′ formed on a sidewall of the first accommodating portion  103 ′ at predetermined intervals, and protrusion accommodating holes  116   a ,  116   b ,  116   a ′,  116   b ′ ( FIG. 6 ) formed in both sides  116 ′ of the front ground portion  116  at predetermined intervals to correspond to the protrusions  103   b ,  103   b′.    
   As shown in  FIG. 10 , the transfer roller  134  connected to the printed circuit board of the body  101  via a transfer roller main ground  114  is fixed by inserting protrusions  137  formed on an outer surface of a sidewall of a general holder  136  that supports both sides of shaft  135  by a spring, into accommodating holes  103   d  formed on a second accommodating portion  103 ″ formed on the the first surface of the paper conveyance guide  103 . 
   The rear ground portion  105  includes a plurality of ribs  105   a  ( FIG. 5 ) formed on a side in an advancing direction at predetermined intervals, a serrated bracket  102  ( FIG. 6 ) connecting the ribs  105   a , a holder bracket  104  holding the serrated bracket  102 , and a rear main ground  115  connecting the holder bracket  104  to the printed circuit board. 
   The front main ground connected with one end of the front ground portion  116 , the transfer roller main ground  114  connected with one end of the shaft  135  of the transfer roller  134 , and the rear main ground  115  connected with one end of the rear ground portion  105  are secured at a side of the paper conveyance guide  103  by main ground fixing members. For example, as shown in  FIG. 11 , the rear main ground  115  is secured to the side adjacent to the first surface of the paper conveyance guide  103  by main ground fixing members  103   c ′,  103   c ,  115   a ,  115   b . The main ground fixing members  103   c ′,  103   c ,  115   a ,  115   b  include protrusions  103   c ′ protruding from the first surface of the paper conveyance guide  103 , an accommodating hole  103   c  formed on a side of the paper conveyance guide  103 , and another accommodating hole  115   a  and another protrusion  115   b  formed on one end of the two sidewalls  115 ′ of the rear main ground  115 . 
   As described above, the front and rear ground portions  116 ,  105 , the transfer roller and rear main grounds  114 ,  115  are secured to the paper conveyance guide  103  by snap-fitting instead of using fasteners, such as screws, which are complex to assemble and take a longer time. As a result, an assembling process becomes easier, an assembling time is reduced, and productivity increases. 
   On a remaining portion of the first surface of the paper conveyance guide  103 , i.e., on a place where the front and rear ground portions  116 ,  105  and the transfer roller  134  are not arranged, a plurality of protruding ribs  103   a  ( FIG. 5 ) are formed in the paper advancing direction at uniform intervals in order to prevent friction with the paper sheet. Due to the presence of the protruding ribs  103   a , an area subject to the friction with the paper sheet decreases, and as a result, a possibility of having a paper jam is reduced. 
   Referring back to  FIG. 4 , the fixing unit  140  pressing and thereby fixing the toner image from the photosensitive drum  132  on the paper sheet includes a pressing roller  142  and a heating roller  141 . As will be described in greater detail below, a paper sensor  143  is disposed downstream of the first paper conveying path (A) of the fixing unit  140  to detect a passage of the paper sheet and subsequently outputs a paper detection signal to the control unit (not shown) that controls the paper discharging and duplex printing. 
   The paper sensor  143  includes a pivot lever supported on a pivot axis and connected to, namely, a limit switch or a solenoid, so as to pivot by a contact with the paper sheet. Upon sensing a position of the conveyed paper sheet by the contact of the pivot lever with the paper sheet, the paper sensor  143  outputs the paper detection signal to the control unit that will accordingly control either the paper discharging or a reverse paper conveyance to the second paper conveyance path (B). 
   During a printing process of the laser printer  100  according to the present invention, the paper sensor  143  can be disposed at proper locations in addition to the downstream of the first paper conveyance path (A), such as a certain position between the pickup roller  107  of the pickup unit  106  and a conveying roller  121  of a paper conveying unit  120  which will be described later, another certain position between one of paper discharging rollers  151  of the paper discharging unit  150  and a first reverse conveying roller  181  of a reverse conveying roller assembly  180  which will be described later, and another certain position between the conveying roller  121  and a second reverse conveying roller  183  of a second reverse conveying roller assembly  180 . Therefore, according to the paper detection signal indicating that the paper sensor  143  contacts the paper sheet, the control unit can sequentially control the paper sheet along the first and the second paper conveyance paths A, B. 
   In order to discharge the paper sheet to an external paper panel  159  after fixing the toner image on the paper sheet by the fixing unit  140 , the paper discharging unit  150  includes the paper discharging rollers  151  and backup rollers  152  formed correspondingly with each other. The paper discharging unit  150  not only discharges the paper sheets to the external paper panel  159 , but also reverses the paper sheet by a driving force of the paper discharging rollers  151  being rotated in forward and reverse directions by the control unit in accordance with the paper signals detection signal from the paper sensor  143 . 
   The duplex printing unit  160  includes a forward/reverse rotation controlling member  155  rotating another one of the paper discharging rollers  151  in alternate directions in accordance with a printing mode and in association with the paper discharging unit  150 . The forward/reverse rotation controlling member  155  generally includes a driving power transmitting gear portion  158  having a forward driving gear train (not shown) driving the paper discharging rollers  151  in a forward direction, and a reverse driving gear train (not shown) driving the paper discharging rollers  151  in a reverse direction, a swing gear train  157  shifting engagement between the forward driving gear train and the reverse driving gear train of the driving force transmitting gear portion  158 , and a solenoid portion (motor)  156  operating the swing gear train  157 . 
   A general paper reversing guide (not shown) can be movably arranged on the first paper conveyance path A in proximity of the fixing unit  140  to block the paper from being conveyed back into the paper conveyance path A, thereby pivoting to selectively guide the discharging direction of the paper sheet in the forward and reverse directions. The paper reversing guide can include a plurality of protrusions formed at uniform intervals on a single rotary shaft that is disposed along a width of the single rotary shaft, guiding a leading end of the paper sheet to a direction as desired. Alternatively, the paper reversing guide can be formed to be moved in the same way as the conventional paper reversing guide, i.e., it can be formed to be moved in linkage with the paper sensor  143  disposed in the proximity of the fixing unit  140 . 
   The duplex printing unit  160  further includes an auxiliary conveyance guide  170  formed to face a second surface (side) of the paper conveyance guide  103  to define the second paper conveyance path B with the second surface of the paper conveyance guide  103 . The paper sheet is fed from the paper discharging unit  150  back to the developing unit  130  second surface of the paper conveyance guide  103  through the second paper conveyance path B. The reverse conveying roller assembly  180  is disposed between the second surface of the paper conveyance guide  103  and the auxiliary conveyance guide  170  to convey the paper sheet along the second paper conveyance path B. The paper conveying unit  120  is formed at a location where the first and the second paper conveyance paths A, B meet, to convey the paper sheet being conveyed through the reverse conveying roller assembly  180  as well as the paper sheet picked up by the pickup unit  106 , to the first paper conveyance path A. 
   The auxiliary conveyance guide  170  includes a first guide portion  172  disposed on the body  101  to be extended above the second surface of the paper conveyance guide  103  and to face a curved rib guide  144  that is fixed on the frame of the body  101  in the proximity of the fixing unit  140 , to selectively restrict the paper sheet reaching the paper discharging unit  150 , thereby guiding the paper sheet to enter into the second paper conveyance path B, a second guide portion  174  screw-engaged with fixing bosses  174   a ′,  174   b ′,  174   c ′,  174   d ′ ( FIG. 7 ) through fixing brackets  174   a ,  174   b ,  174   c ,  174   d  ( FIG. 6 ) that are formed on the second surface of the paper conveyance guide  103  to define a part of the second paper conveyance path B in cooperation with the second surface of the paper conveyance guide  103 , and a third guide portion  176  formed on the frame of the body  101  to guide the paper sheet, which is conveyed onto the second paper conveyance path B through the second guide portion  174 , to enter the first paper conveyance path A. Together with the paper conveyance guide  103 , the first and the second guide portions  172 ,  174  are securely supported on a cover  190  that is movably mounted on the frame of the body  101  to be pivotally opened and closed about a hinge  192 . 
   As shown in  FIGS. 6 ,  12 A and  12 B, the reverse conveying roller assembly  180  includes the first and second reverse conveying rollers  181 ,  183  rotatably secured on the second surface of the paper conveyance guide  103  by fixing parts  103   f ,  103   f ′,  185 ,  186 ,  185 ′,  186 ′, two tension rollers  182 ,  184  disposed on the second guide portion  174  of the auxiliary conveyance guide  170  to be meshed with the reverse conveying rollers  181 ,  183  with a predetermined pressure, and gear parts  181   a ,  183   a  disposed at certain ends of shafts  181   b ,  183   b  of the first and second reverse conveying rollers  181 ,  183  to transmit a driving force of a driving motor (not shown) to the first and second reverse conveying rollers  181 ,  183 . 
   As shown in  FIGS. 5 ,  6 , and  13 , an idle gear assembly  195  is disposed between the gear parts  181   a ,  183   a  of the shafts  181   b ,  183   b  of the first and second reverse conveying rollers  181 ,  183  to transmit the driving force in the uniform direction from the second reverse conveying roller  183  to the first reverse conveying roller  181 . The idle gear assembly  195  includes a first idle gear  197   a  rotatably supported on a first boss  196   a  that is secured at a fixing bracket  117  of the paper conveyance guide  103 , to be in mesh with the gear part  183   a  of the shaft  183   b  of the second reverse conveying roller  183 , and having a first washer  198   a  integrally formed at an upper or lower side of the first idle gear, a second idle gear  197   b  rotatably supported on a second boss  196   b  that is secured at the fixing bracket  117  of the paper conveyance guide  103 , to be in mesh with the first idle gear  197   a , and having a second washer  198   b  integrally formed at an upper or lower side of the second idle gear, and a third idle gear  197   c  rotatably supported on a third boss  196   a  that is secured at the fixing bracket  117  of the paper conveyance guide  103 , to be in mesh with the second idle gear  197   b  and the gear part  181   a  of the shaft  181   b  of the first reverse conveying roller  181 , and having a third washer  198   c  integrally formed at an upper or lower side. The first, second and third idle gears  197   a ,  197   b ,  197   c  having washers  198   a ,  198   b ,  198   c  integrally formed on an upper or a lower side thereof, are arranged on the fixing bosses  196   a ,  196   b ,  196   c , respectively, in a manner such that the washers  198   a ,  198   b ,  198   c  are in an alternate arrangement, and the second idle gear  197   b  alone is secured at the second fixing boss  196   b  by a shaft fixing ring  199 . 
   Further, as shown in  FIG. 12A , each of the reverse conveying rollers  181 ,  183  includes the shaft  181   b  or  183   b  made of plastic and having the gear part  181   a  or  183   a  integrally formed thereon, and a roller part  181   f  or  183   f  of a tubular shape which is fit in fixing groove  181   d  or  183   d . Unlike a conventional structure that requires a shaft  91  separately from the gear part and a belt pulley  93 , and a rubber roller part  92  being formed on the shaft  91  by molding as shown in  FIG. 3 , a structure of each of the reverse conveying rollers  181 ,  183  as described above requires no additional assembling work. 
   As shown in  FIG. 12B , the fixing members  103   f ,  103   f ′,  185 ,  186 ,  185 ′,  186 ′ fixing the reverse conveyance rollers  181 ,  183  to the paper conveyance guide  103  include fixing holes  103   f ,  103   f ′ formed in the paper conveyance guide  103 , and a plurality of shaft bushings  185 ,  186 ,  185 ′,  186 ′ having hooks  186   b ,  186   b ′ formed at corresponding ends thereof to be inserted in the corresponding fixing holes  103   f ,  103   f ′ and C-shape openings  186   a ,  186   a ′ formed in another ends thereof to receive and secure accommodating parts  181   e ,  183   e  of the shafts  181   b ,  183   b  of the reverse conveying rollers  181 ,  183 . A linking part  186   c  or  186   c ′ is disposed between each hook  186   b  or  186   b ′ of the shaft bushings  185 ,  186 ,  185 ′,  186 ′ and corresponding C-shape opening  186   a  or  186   a ′ to connect the hook  186   b  or  186   b ′ and the C-shape opening  186   a  or  186   a ′, and supports the corresponding fixing members with respect to the paper conveyance guide  103 . As the reverse conveying rollers  181 ,  183  are fixed on the paper conveyance guide  103  by snap-fitting, which does not require any complicated process and time like conventional fastening members as shown in  FIGS. 1–3 , the assembling efficiency and the productivity are improved. 
   Two tension rollers  182 ,  184  are arranged on rotary shafts  182   a ,  184   a  on the inner side of a second guide portion  174  and at a predetermined distance from each other. The tension rollers  182 ,  184  elastically bias the reverse conveying rollers  181 ,  183 . 
   As shown in  FIGS. 6 and 7 , a side guide portion  175  is arranged on the second surface of the paper conveyance guide  103  to guide a corner of a certain end of the paper sheet to prevent skewing of the paper sheet during the conveyance of the paper sheet by the reverse conveying roller assembly  180 . The side guide portion  175  is formed as a bracket that includes a flattened U-shape rail  176  formed at one end to guide the corner of one end of the paper sheet, and a fixing (coupling) part  177  formed at the other end to fix the side guide portion  175  to the second surface of the paper conveyance guide  103 . 
   Referring back to  FIG. 4 , the paper conveying portion  120  includes the conveying roller  121  of a large diameter disposed at the paper conveyance guide  103  at a point where the first paper conveyance path A and the second paper conveyance path B meet, a pair of first backup rollers  122 ,  123  disposed on the frame of the body  101 , which defines the first paper conveyance path A, to be in contact with the conveying roller  121  with a predetermined pressure through the paper sheet picked up by the pickup unit  106  to be conveyed along the first paper conveyance path A, a pair of second backup rollers  124 ,  125  disposed on the third guide portion  176  of the auxiliary conveyance guide  170  to be in contact with the conveying roller  121  with a predetermined pressure to convey the paper sheet conveyed by the reverse conveying roller assembly  180  along the second paper conveyance path B to the first paper conveyance path A. 
   The conveying roller  121  is in mesh with the first backup rollers  122 ,  123  to convey the paper sheet picked up by the pickup unit  107  to the developing unit  130 , while it is in mesh with the second backup rollers  124 ,  125  to feed the paper sheet conveyed along the second paper conveyance path B back to the developing unit  130 . Accordingly, since a driving power transmitting portion (not shown) is required to be mounted inside of the body  101  to drive only one conveying roller  121 , the structure becomes simplified. 
   Further, since the conveying roller  121  of the large diameter compared to other rollers is disposed at a point where the paper sheet is returned to the first paper conveyance path A from the second paper conveyance path B, a contact area between the conveying roller  121  and the paper sheet increases. As a result, a paper jam is prevented, and paper conveying efficiency increases. 
   Further, as shown in  FIG. 7 , the protruding ribs  103   a  are formed at predetermined intervals on the second surface of the paper conveyance guide  103  that defines the second paper conveyance path B, to enable the paper sheet to be conveyed as easily as in the first surface of the paper conveyance guide  103  without having the paper jam. 
   As described above, both first and second surfaces of the paper conveyance guide  103  are used to define the paper conveyance paths, i.e., the first and the second paper conveyance paths A, B. Accordingly, the paper conveyance path is shortened, which is quite efficient for duplex printing, while the number of the rollers is also reduced. Further, due to the presence of the protruding ribs  103   a  formed on both surfaces of the paper conveyance guide  103 , problems like the paper jam and paper skew are prevented. 
   An operation of the laser printer  100  for the duplex printing constructed as above according to the embodiment of the present invention will be described below with reference to  FIGS. 4 through 13 . 
   First, as shown in  FIG. 4 , with the paper feeding cassette  110  having a paper stack therein and mounted in the body  101 , the pickup roller  107  of the pickup unit  106  is rotated to be in tight contact with the paper sheet in accordance with the control of the control unit by the commands from a computer. Accordingly, one sheet is picked up from the paper stack by the pickup roller  107 , and conveyed between the conveying roller  121  and the first backup rollers  122 ,  123 . 
   The paper sheet reaching the conveying roller  121  and the first backup rollers  122 ,  123  is guided along the first surface of the paper conveyance guide  103  that defines the first paper conveyance path A toward the developing unit  130  by a rotational force of the conveying roller  121  and the first backup rollers  122 ,  123 . In such a situation, as a surface of the front ground portion  116  positioned in front of the developing unit  130  comes in contact with the paper sheet, the static electricity that can influence a toner image transfer process is removed from the paper sheet. 
   Meanwhile, in accordance with the image signal, the LSU  136  emits the laser beam onto a surface of the photosensitive drum  132  charged by general charging methods, such as a corona discharge, thereby forming the electrostatic latent image on the photosensitive drum  132 . After being formed on the photosensitive drum  132 , the electrostatic latent image is developed into the visible toner image as the toner particles are attached thereon by the developing roller (not shown) being rotated opposite to the photosensitive drum  132 . 
   Next, as the paper sheet having the static electricity removed by the front ground portion  116  is kept being conveyed by the conveying roller  121  and the first backup rollers  122 ,  123  and thus passed along the photosensitive drum  132 , the toner image on the photosensitive drum  132  is transferred onto the paper sheet by being squeezed between the photosensitive drum  132  with a transfer roller  134  and the high voltage of opposite polarity applied on the transfer roller  134 . 
   Then as the paper sheet reaches at the fixing unit  140 , the toner image on the first side of the paper sheet is fixed into a printed image by the heat of the heating roller  142  and the pressure of fixing roller  141 . 
   After the desired image is printed, the paper sheet is moved by the heating roller  142 , the fixing roller  141 , and so on, and then comes to push the paper sensor  143 . Accordingly, the paper sensor  143 , which is fixed on the rotary shaft connected to the limit switch or to the solenoid, pivots by the contact with the paper sheet. The paper sensor  143  outputs the paper detection signal to the control unit, and accordingly, the control unit drives the paper discharging rollers  151  of the paper discharging unit  150  to convey the paper sheet toward the external paper panel  159 . 
   In simplex printing, the control unit of the laser printer  100  keeps driving the paper discharging rollers  151  of the paper discharging unit  150  until the paper sheet is released out onto the external paper panel  159 . 
   In the duplex printing, however, the control unit controls so that the paper discharge unit  150  is reverse-rotated after a predetermined time that a rear end of the paper sheet is further moved from the paper sensor  143  by a predetermined distance, thereby shifting an advancing direction of the paper sheet and feeding the paper sheet to re-enter the second paper conveyance path B. 
   Controlling a reverse-rotation of the paper discharge unit  150  is performed by the forward/reverse rotation control member  155  which is operated in linkage with the discharge unit  150 . More specifically, the solenoid portion  156  of the forward/reverse rotation control member  155  is operated to switch the swing gear train  157  to an engagement with the reverse driving gear train of the power transmitting gear portion  150  that rotates the paper discharge rollers  151  in the reverse direction, thereby causing the driving power of the motor (solenoid portion)  156  to be transmitted to the paper discharge rollers  151  in the reverse direction. 
   During the reverse conveyance of the paper sheet, the paper reversing guide disposed on the first paper conveyance path A in the vicinity of the fixing unit  140  selectively pivots to make the paper sheet that is conveyed in the reverse direction not move into the first paper conveyance path A. As a result, the rear end of the paper sheet is guided to the second paper conveyance path B. 
   Being guided along the second paper conveyance path B, the paper sheet enters between the first guide portion  172  and the curved-rib guide  144  formed in the proximity of the fixing unit  140  by the rotational force of the another one of paper discharge rollers  151 . 
   As the paper sheet continues to be conveyed and thus reaches a space defined between the protruding ribs  103   a  on the second surface of the paper conveyance guide  103  that defines a part of the second paper conveyance path B with the second guide portion  174 , the control unit drives the driving motor (not shown) to drive the reverse conveying rollers  181 ,  183  in mesh with the tension rollers  182 ,  184  with a predetermined pressure according to signals from the paper sensor (not shown) disposed in the proximity of the second guide portion  174 . 
   The driving force being transmitted to the second reverse conveying roller  183  via the power transmitting portion (not shown) and a gear portion  187  ( FIG. 5 ), is transmitted by the idle gears  197   a ,  197   b ,  197   c  to the first reverse conveying roller  181  in the form of a rotational force in the same direction, such as, the rotational force in a clockwise direction. 
   During the conveyance of the paper sheet by the rotational force of the reverse conveying rollers  183 ,  184 , one tip end of the paper sheet is aligned by a side guide portion  175  disposed at one side of the second guide portion  174 , and guided by the protruding ribs  103   a  formed on the second surface of the paper conveyance guide  103 . Thus, problems, such as the paper skew and the paper jam, do not occur. 
   Even when the paper jam occurs in the second paper conveyance path B, the cover  190 , which is pivotably disposed on the hinge  92  of the frame of the body  101 , can pivot to be open, the first and the second guide portions  172 ,  174  and the paper conveyance guide  103  are easily exposed, and accordingly, the paper jam can be easily dealt with. 
   As the paper conveyance continues, and accordingly when the paper sheet reaches the paper conveying portion  120 , the conveying roller  121  of the paper conveying portion  120  being driven by the power transmitting portion (not shown) of the body  101  comes into contact with the second backup rollers  124 ,  125  of the third guide portion  176  on the second paper conveyance path B, thereby being driven clockwise. As a result, the paper sheet is reversed along the third guide portion  176 , and conveyed toward the first backup rollers  122 ,  123  that are located on the first paper conveyance path A. 
   As the paper sheet is conveyed between the first backup rollers  122 ,  123 , the conveying roller  121  feeds the paper sheet to re-enter the developing unit  132  in an engagement with the first backup rollers  122 ,  123  with a predetermined pressure. After that, the toner image is transferred and fixed on a second side of the paper sheet in the same manner as described above, and then the paper sheet is released out to the external paper panel  159 , and the duplex printing is completed. 
   As described above, the laser printer for duplex printing according to the present invention uses both surfaces (sides) of the paper conveyance guide as the paper conveyance paths, and also are provided with the ribs, thereby shortening the paper conveyance path for duplex printing, increasing duplex printing efficiency, preventing problems, such as the paper jam and paper skew, and reducing the number of the rollers for conveying the paper sheet. 
   Further, the laser printer according to the present invention is provided with the conveying roller of an enlarged diameter for feeding the paper sheet picked up by the pickup roller to the developing unit, and uses the conveying roller of the enlarged diameter not only for feeding the paper sheet to the developing unit, but also for conveying the paper sheet to be fed back to the developing unit for duplex printing. Accordingly, the paper jam is reduced while the conveying efficiency increases. 
   Further, the laser printer according to the present invention uses snap-fitting for securing the ground portion, the reverse conveying roller, or the like on the paper conveyance guide instead of fasteners, such as screw or bushing, that require complicated assembling process and time. As a result, the assembling process becomes simpler, the assembling time is shortened, and the productivity increases. 
   Finally, since the laser printer according to the present invention has a plastic shaft having a gear part integrally formed thereon as a reverse conveying roller, an additional assembling process, such as assembling the gear part to the shaft, is omitted. 
   Although the preferred embodiment of the present invention has been described, it will be understood by those skilled in the art that the present invention should not be limited to the described preferred embodiment, but various changes and modifications can be made within the spirit and scope of the present invention as defined by the appended claims and their equivalents.