Sheet atacker and finisher

Abstract

An improved sheet stacking scheme is described which is applied to a copying machine, a printer, or the like. Even when printed sheets are sorted and stacked in different stacking positions, sheet alignment is achieved by inhibiting each sheet from being displaced. When sheets are sorted into a first stacking position and a second stacking position, the finisher control unit 200 determines the common area Rc which is shared by the first stacking position and the second stacking position. Then, a finisher control unit 200 sets the positions of the pair of holding members for holding a tail edge in order to perform the tail edge holding motion in the positions F 1 and F 2 respectively which are located within the common area Rc and symmetrical about the center position Rcc of the common area Rc.

1 . A sheet stacker comprising:
 a catch tray configured to stack a sheet discharged from a discharging unit;   a holding unit including a first holding member and a second holding member which are arranged adjacent to each other in the sheet width direction that is perpendicular to the discharge direction of the sheet;   a lateral motion drive unit configured to move said first holding member and said second holding member in the sheet width direction respectively;   a holding drive unit configured to drive said first holding member and said second holding member to hold down the tail edge area of the sheet from the upper side of the sheet toward a sheet receiving surface; and   a control unit configured to control said holding drive unit to perform a tail edge holding motion in synchronization with the discharge of the sheet, and control said lateral motion drive unit,   wherein, when the sheet is stacked by sorting between a first stacking position and a second stacking position which is offset from said first stacking position in the sheet width direction, said control unit determines a common area which is shared by said first stacking position and said second stacking position, and sets the positions of said first holding member and second holding member in order that said tail edge holding motion is performed in symmetric positions which are located within said common area and symmetrical about the center position of said common area.   
     
     
         2 . The sheet stacker of  claim 1   wherein said holding unit comprises three holding members functioning as said first holding member, said second holding member and a third holding member which is arranged in said sheet width direction with said first holding member and said second holding member, and   wherein, when said stacking position is switched, said control unit performs a shift position holding motion by holding the side edge area of an uppermost sheet stacked in the stacking position before switching with said third holding member, in the side opposite the shift direction of said stacking position.   
     
     
         3 . The sheet stacker of  claim 2   wherein said control unit assigns, of said three holding members constituting said holding unit, a pair of adjacent holding members to said first holding member and said second holding member for performing said tail edge holding motion and the remaining holding member to said third holding member for performing said shift position holding motion.   
     
     
         4 . The sheet stacker of  claim 3   wherein, when said stacking position is switched, said control unit changes assignment of said three holding members constituting said holding unit in correspondence with the stacking position after switching, and   wherein said control unit sets the positions of said three holding members in accordance with said assignment thereof as changed and the stacking position after switching.   
     
     
         5 . The sheet stacker of  claim 2   wherein said control unit sets a holding pressure, with which said shift position holding motion is performed by said holding members, to be smaller than a holding pressure with which said tail edge holding motion is performed by said holding members.   
     
     
         6 . The sheet stacker of  claim 1  further comprising:
 a pair of regulating members configured to align a sheet discharged from said discharging unit in the sheet width direction; and 
 a regulating member drive unit configured to move said regulating members in the sheet width direction respectively, 
 wherein said control unit controls said regulating member drive unit to set the positions of said pair of regulating members in correspondence with a stacking position of the sheet. 
 
     
     
         7 . A finisher comprising:
 a sheet stacker;   a sheet receiving unit configured to receive a sheet on which an image is formed from an image forming apparatus;   a post-printing unit configured to perform a predetermined post-printing process with the sheet received through said sheet receiving unit; and   a discharging unit configured to discharge the sheet from said post-printing unit or from said sheet receiving unit to said sheet stacker,   wherein said sheet stacker comprises:   a catch tray configured to receive the sheet discharged from said discharging unit for stacking the sheet thereon;   a holding unit including a first holding member and a second holding member which are arranged adjacent to each other in the sheet width direction that is perpendicular to the discharge direction of the sheet;   a lateral motion drive unit configured to move said first holding member and said second holding member in the sheet width direction respectively;   a holding drive unit configured to drive said first holding member and said second holding member to hold down the tail edge area of the sheet from the upper side of the sheet toward a sheet receiving surface; and   a control unit configured to control said holding drive unit to perform a tail edge holding motion in synchronization with the discharge of the sheet, and control said lateral motion drive unit,   wherein, when the sheet is stacked by sorting between a first stacking position and a second stacking position which is offset from said first stacking position in the sheet width direction, said control unit determines a common area which is shared by said first stacking position and said second stacking position, and sets the positions of said first holding member and second holding member in order that said tail edge holding motion is performed in symmetric positions which are located within said common area and symmetrical about the center position of said common area.