Patent ID: 8251362

Claim:
A sheet stacking apparatus which is configured to align a sheet unit, the sheet unit including one sheet or plural sheets, the apparatus comprising: a stacking section which is configured to stack sheets ejected outside; a sorting section which is configured to change a sheet ejecting position of at least one sheet on the stacking section to sort the at least one sheet as a sheet unit; an aligning section which is configured to align the at least one sheet sorted by the sorting section and ejected onto the stacking section; a shifting section which is configured to shift the aligning section based on the sheet ejecting position on the stacking section; a control section which is configured to control at least the aligning section and the shifting section, based on a time interval between successively conveyed sheets; a conveyance route which is configured to convey the sheet toward the stacking section; a detecting section which is configured to detect a time interval between a trailing edge of a preceding sheet and a leading edge of a following sheet conveyed successively through the conveyance route; and an intermediate stacking section which is provided upstream of the stacking section with respect to a sheet conveyance direction and which is configured to temporarily stack plural sheets and to convey said plural sheets to the stacking section; wherein: (i) when plural sheets are included in the sheet unit sorted by the sorting section, the control section controls the sorting section, the aligning section, the shifting section, and the intermediate stacking section such that while a foregoing sheet unit is completely ejected onto the stacking section to be aligned, a predetermined number of sheets in a following sheet unit are stacked on the intermediate stacking section and then ejected to the stacking section, and thereafter, sheets remaining in the following sheet unit are directly ejected onto the stacking section and stacked on the predetermined number of sheets on the stacking section, whereby all of the sheets in the following sheet unit are aligned on the stacking section, and (ii) when only one sheet is included in the sheet unit, the control section controls the aligning section and the shifting section not to align the one sheet.