Sheet processing apparatus and image forming system

An image forming system comprising:

• an image forming unit configured to form an image on a sheet;
• a saddle binding processing unit configured to perform a saddle binding process on a sheet bundle including the sheet on which the image has been formed by the image forming unit;
• a half-folding processing unit configured to perform a half-folding process on the sheet bundle subjected to the saddle binding process by the saddle binding processing unit;
• a conveyance unit configured to convey the sheet bundle subjected to the saddle binding process and the half-folding process such that a spine of the sheet bundle is positioned downstream of a fore edge thereof in a conveyance direction;
• a square back processing unit including a pair of clamping units configured to clamp and release a sheet bundle conveyed by the conveyance unit and a pressing roller, the square back processing unit being configured to perform a square back process in which the pair of clamping units clamp the sheet bundle such that the spine of the sheet bundle protrudes downstream from the pair of clamping units in the conveyance direction and the pressing roller presses the spine of the sheet bundle toward the pair of clamping units while moving along the spine of the sheet bundle clamped by the pair of clamping units;
• and
• a controller capable of executing a mode in which, the square back processing unit perform the square back process to the sheet bundle conveyed by the conveyance unit, after the pair of clamping units release the clamping of the sheet bundle subjected to the square back process, the conveyance unit conveys the sheet bundle downstream in the conveyance direction, and the square back processing unit performs the square back process to the sheet bundle conveyed downstream by the conveyance unit again.