Sheet post-processing apparatus and image forming system

A sheet post-processing apparatus comprising:

• a processing tray on which a sheet is stacked and a predetermined processing is performed on the sheet;
• a sheet discharge port disposed on a downstream side of the processing tray in a sheet discharge direction;
• a conveyance rollers pair disposed on an upstream side of the sheet discharge port in the sheet discharge direction;
• a lower discharge roller and an upper discharge roller which are disposed in the sheet discharge port, rotate independently from each other, and have circumferential surfaces bringing into contact with each other to form a nip area;
• a discharge tray disposed below a downstream side of the sheet discharge port in the sheet discharge direction;
• and
• a controller controlling an operation of the conveyance rollers pair, the lower discharge roller and the upper discharge roller, wherein
• when the sheet having a folded portion formed by folding a front half portion of the sheet in the sheet discharge direction downward in a Z-shape is stacked on the processing tray with the folded portion downward,
• the controller rotates the lower discharge roller and the upper discharge roller in a normal direction to advance the folded portion of the sheet conveyed by the conveyance rollers pair to the sheet discharge port,
• stops rotation of the lower discharge roller before an upstream end of the folded portion in the sheet discharge direction passes through the sheet discharge port,
• rotates the conveyance rollers pair and the upper discharge roller forward in a state where the upstream end of the folded portion in the sheet discharge direction is nipped between the lower discharge roller and the upper discharge roller so as to slide a rear half portion of the sheet on the folded portion in the nip area, to convey the rear half portion to the downstream side in the sheet discharge direction until an upstream end of the sheet in the sheet conveyance direction passes through the conveyance rollers pair and to form a deflection on the sheet, and then
• rotates the lower conveyance roller and the upper conveyance roller reversely to convey the sheet to the upstream side in the sheet conveyance direction and then to stack the sheet on the processing tray.