Sheet processing apparatus, image forming system, and method of additionally folding sheet bundle

A sheet processing apparatus, comprising:

• a pressing unit including a first pressing member and a second pressing member, the pressing unit being configured to sandwich and press a fold part of a folded sheet bundle between the first pressing member and the second pressing member;
• a moving unit configured to move a position pressed by the pressing unit in a direction of a fold of the sheet bundle;
• an elastic member configured to press the first pressing member in a thickness direction of the folded sheet bundle;
• and
• a guiding unit configured to determine a pressing start and release of the pressing unit, wherein
• the pressing unit is moved along a path of the guiding unit by the moving unit,
• the path includes
• a first guiding path for guiding the pressing unit in a pressing-release state during a forward movement,
• a second guiding path for guiding the pressing unit in a pressing state during the forward movement,
• a third guiding path for switching the pressing unit from the pressing-release state to the pressing state during the forward movement,
• a fourth guiding path for guiding the pressing unit in a pressing-release state during a backward movement,
• a fifth guiding path for guiding the pressing unit in a pressing state during the backward movement, and
• a sixth guiding path for switching the pressing unit from the pressing-release state to the pressing state during the backward movement.