Sheet creaser, sheet conveyer, sheet finisher, image forming apparatus, and sheet creasing method

摘要

A pressing unit presses a folded side of a stack of sheets folded by a folding unit (74, 81), thereby making a strong crease on the stack of sheets. The pressing unit includes a pressure roller (600) that slides on the folded side while rotating, an elastic biasing unit (609) that presses the pressure roller (600) in a thickness direction of the stack of sheets, and a driving unit (606, 607, 612) that slides the pressure roller (600) in a direction substantially perpendicular to a conveying direction of the stack of sheets. A lifting unit (602, 611), when the pressure roller (600) slides to a first position, temporarily lifts up the pressure roller (600), and when lifted-up pressure roller (600) slides to a second position, lifts the lifted-up pressure roller (600) down onto the folded side. The first position and the second position are located before a corner of the folded side, whereby the pressure roller (600) cannot slide up on the folded side.