Sheet stacking apparatus, sheet processing apparatus, and image forming apparatus

An image forming apparatus comprising:

• an image forming portion which forms an image in a sheet;
• and
• a sheet processing apparatus which processes the sheet, wherein the sheet processing apparatus includes:
• a sheet stack tray which provides an inclined sheet stack surface on which the image formed sheet is stacked;
• a regulating member, having a regulating surface disposed opposite the inclined sheet stack surface, which is movable toward the inclined sheet stack surface so as to decrease a clearance between the regulating surface and the inclined sheet stack surface, and thereby regulate the sheet stacked on the inclined sheet stack surface;
• a clamping member which clamps an upper portion of the sheet stacked on the inclined sheet stack surface;
• a pair of folding rollers which performs a folding process to the sheet stacked on the sheet stack tray, the pair of folding rollers being disposed on a side to which the pair of folding rollers faces the inclined sheet stack surface at a predetermined distance from the inclined sheet stack surface;
• and
• a controller which controls movement of the clamping member and movement of the regulating member so that,
• the clamping member clamps the upper portion of the sheet after the regulating member regulates the sheet, and
• the regulating member moves to increase the clearance between the regulating surface and the inclined sheet stack surface while the upper portion of the sheet is clamped by the clamping member.