Abstract:
A media sheet handling system for duplex printing is disclosed. During duplex printing, the media sheet whose first side is printed is directed into a duplex print path, wherein the media sheet is reversed to a flipped over state with respect to its original position and the original leading edge is maintained to be the leading edge for subsequent printing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to media sheet handling systems, and more particularly to a printing mechanism that can produce images on both sides of a media sheet. 
     Printing to two sides of a media sheet, referred to as duplex printing, is a desirable feature in a printing system. The advantages of duplex printing include reducing the amount of paper required in one-sided (simplex) printing, and generating print sets with layouts resembling that of professionally printed books. Conventional duplex printing devices, as exemplified by what is disclosed in U.S. Pat. No. 5,392,092 for “Printing Device Duplexing Mechanism and Method Therefor,” flip the media sheet over with respect to its original position and swap the leading edge for the trailing edge in printing a second side of the media sheet. Since the page is now positioned with what was originally the trailing edge as the leading edge, as the second side of the media sheet is printed, the image has to be written in a bottom-to-top and right-to-left manner. Thus the formatter of the printer has to correspondingly reverse the order of the data supplied to the print engine. 
     In inkjet printers, drying of media sheet brings another major challenge for duplex printing, because a printed side of a media sheet tends to wrap around the drive roller surface when the second side is printed. U.S. Pat. No. 5,772,343 for “Media Handling System for Duplex Printing,” provides one solution to duplex printing for such printers. In this solution, the media sheet, after its first side is printed and before it is fed back for printing its second side, has to be held in the air space of the printer output region for a predetermined time period for the wet ink to dry. Since the printer is waiting idly for the drying, printing efficiency therefore is unnecessarily degraded. 
     Accordingly, there is a need for a convenient and efficient way to realize duplex printing. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment, the invention provides a media sheet handling system for duplex printing that adequately reorients the media sheet while drying of wet ink, if necessary, is accomplished simultaneously. 
     An embodiment of a media sheet handling system according to the invention includes a first driving roller, an output roller, a detractor, a duplex print path and a second driving roller. A media sheet is picked by a pick roller from a stack of media sheets and fed by the first driving roller towards a printing zone for imprinting images on the first side of the media sheet. The output roller then directs the media sheet whose first side has been printed towards an output path. During duplex printing, after the first side of the media sheet is printed, the detractor between the output roller and the output path is activated to direct the media sheet into the duplex print path. In the duplex print path, the media sheet is reversed to a flipped over state with respect to its original position before the first side is printed, and the original leading edge of the media sheet remains the leading edge for subsequent printing. After the reorientation, the second driving roller supplies the media sheet from the duplex print path for printing the second side of the media sheet. The detractor is inactivated and the media sheet is ejected to the output path after the second side is printed. 
     In another embodiment according to the invention, the media sheet handling system includes a pick roller, a driving roller, an output roller, a detractor and a duplex print path. Similarly, the pick roller picks a media sheet from a stack of media sheets, and the driving roller feeds the media sheet towards a printing zone for printing the first side of the media sheet. Then the output roller drives the printed media sheet forward, the detractor directs the media sheet whose first side has been printed into the duplex print path, and the duplex print path reorients the media sheet. In the duplex print path, the media sheet is reversed to a flipped over state with respect to its original position before the first side is printed; and the original leading edge of the media sheet remains the leading edge for subsequent printing. After the reorientation, the duplex print path directs the media sheet back onto the top of the stack of media sheet. Subsequently, the media sheet whose first side has been printed is picked by the pick roller and forwarded by the driving roller towards the printing zone for printing its second side. After the duplex printing, the detractor is inactivated, and the media sheet is ejected to the output path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2 ,  3 ,  4 ,  5 ,  6  and  7  are schematic side views showing different steps of duplex printing of an inkjet printer using a preferred embodiment of the invention. 
     FIG. 8 is a schematic side view showing one step of duplex printing of an inkjet printer using another preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the invention will be described in the context of an inkjet printer. Nevertheless, it is to be understood that it is equally applicable to any system that employs duplex printing processes. 
     As shown in FIGS. 1-7, a preferred embodiment of the invention includes a drive roller  20 , an output roller  26 , a duplex roller  46 , a detractor  28  and a duplex print path. The duplex print path has a first duplex intermediate drive  36 , a second duplex intermediate drive  38 , a duplex hold drive  40 , a turn table  42  and a motor  44  coupled to the turn table  42 . FIGS. 1-7 are schematic side views showing different steps of duplex printing of an inkjet printer using the preferred embodiment of the invention. 
     In FIG. 1, a pick roller  18  picks the top sheet of print media off the supply stack in a paper tray  16  and feeds the media sheet  10  towards the drive roller  20 . The drive roller  20  further draws and propels the media sheet  10  into a printing zone formed by a printing head  24  and a platen  22  (see FIG.  2 ). The printing head  24  imprints images on the first side of the media sheet  10  which is facing the printing head  24  as the media sheet  10  passes through the printing zone. When the leading edge  12  of the printed media sheet  10  reaches the output roller  26 , the output roller  26  and its mating star wheel  27  propel the media sheet  10  forward. 
     In normal printing, the media sheet  10  will be fed towards an output path formed by a first output stage  30  and a second output stage  32 . The two output stages  30 ,  32  draw the media sheet  10  forward and eject it into an output tray  34 . In duplex printing, however, the detractor  28  located between the output roller and the output path is activated, and the media sheet  10  is thus directed into the duplex print path (see FIG.  3 ). 
     As shown in FIG. 4, the first duplex intermediate drive  36  and its mating star wheel  37  propel the media sheet  10  along the duplex print path. The media sheet  10  goes through the second duplex intermediate drive  38  and its mating star wheel  39 , and reaches the duplex hold drive  40 , which is positioned on the turn table  42 . As the trailing edge  14  of the media sheet  10  crosses the second duplex intermediate drive  38 , the duplex hold drive  40  is stopped. Now the media sheet  10  is on the turn table  42  with the printed side (first side) facing upwards (see FIG.  5 ). Driven by the motor  44 , the turn table horizontally rotates 180 degrees, thus bringing the leading edge  12 , which has been pointing towards an output direction, now to point towards an input direction, i.e., towards the second duplex intermediate drive  38 . During the rotation, the media sheet  10  is held by the duplex hold drive  40  to avoid being shifted. Note that the printed side of the media sheet  10  does not touch the surface of any roller involved in the duplex print path. The drying of the wet ink is simultaneous with the reorientation of the media sheet. The rotative velocity of the turn table  42  is predetermined so that the media sheet  10  is delayed enough for at least partial drying of the wet ink. 
     Moreover, in this preferred embodiment, the mid-point of the turn table  42  (in media sheet width direction) is designed to be the mid-point of letter and legal size media. Since A4 media is 6 millimeter less than the letter or legal size media in width, when an A4 media sheet is involved in the duplex printing, the A4 media sheet has to be reoriented by being moved by 6 millimeter towards the right end of the turn table  42  to positively touch the datum wall. 
     After preceding actions, during subsequent printing, the leading edge  12  of the media sheet  10  will remain the leading edge and the printed side will be facing the drive roller  20 , i.e., opposite to the printing head  24 . 
     Then the duplex hold drive  40  moves the media sheet  10  towards the second duplex intermediate drive  38 . The second duplex intermediate drive  38  starts rotating in such a way that it will pull the media sheet  10  forward. The media sheet is moved forward by the second intermediate drive  38  until it reaches the duplex roller  46 . The duplex roller  46  then advances the media sheet  10  towards the drive roller  20  for printing the second side of the media sheet  10  (see FIG.  6 ). Note that the second duplex intermediate drive  38  may be replaced by another duplex hold drive (not shown) which may be positioned on the other side of the turn table  42  opposite to the duplex hold drive  40 . The second duplex hold drive functions similarly to the second duplex intermediate drive  38 . 
     As the media sheet  10  moves towards the printing zone, the previously printed side of the media sheet  10  is facing the drive roller while the non-printed side is facing the printing head  24 . And the original leading edge  12  of the media sheet  10  remains the leading edge. Therefore, the printing head  24  imprints images on the non-printed side of the media sheet  10  without changing its manner of printing, i.e., still writing in a top-to-bottom and left-to-right fashion. 
     The detractor  28  is inactivated when the media sheet  10  reaches the printing zone. Thus, as the printing head prints the second side of the media sheet  10 , the output roller  26  and its mating star wheel  27  propel the media sheet  10  towards the output path (see FIG.  7 ). The first output stage  30  and the second output stage  32  further eject the media sheet  10  whose second side has been printed into the output tray  34 . The whole duplex printing process is now completed. 
     FIG. 8 is a schematic side view showing one step of duplex printing of an inkjet printer using another preferred embodiment of the invention. The second preferred embodiment of the invention has a pick roller  18 , a drive roller  20 , an output roller  26 , a detractor  28  and a duplex print path. The duplex print path includes a first duplex intermediate drive  36 , a second duplex intermediate drive  38 , a duplex hold drive  40 , a turn table  42  and a motor  44  coupled to the turn table  42 . The duplex printing process in the second embodiment is quite similar to the first one, except that after the duplex print path reorients the media sheet  10  whose first side has been printed, the media sheet  10  is moved back onto the top of the stack of the media sheets. As shown in FIG. 8, the second duplex intermediate drive  38  is positioned just above the paper tray  16 . Thus the second duplex intermediate drive  38  propels the reoriented media sheet  10  into the paper tray  16 . Subsequently, the sheet  10  whose first side has been printed is picked by the pick roller  18  and forwarded towards the drive roller  20  for printing the second side.