Patent ID: 8002255

Claim:
A sheet post-processing apparatus comprising: a folding blade that presses and turns in stacked sheets of paper having images formed thereon; a middle-folding roller pair that folds in the middle the sheets of paper turned-in by the folding blade, by pinching and pressing the same at a nip; a fold-enhancing roller pair including an upper fold-enhancing roller and a lower fold-enhancing roller, which pinches and presses the sheets of paper folded in the middle at the nip; a control device configured to control the fold-enhancing roller pair to move faster in a homeward direction than in an outbound direction, both directions being orthogonal to a sheet carrying direction, and to move between one end of the sheets of paper and another end with plural switching back movements of the fold-enhancing roller pair which is in contact with the sheets of paper, the switching back movements including: a first switching point different from the another end at which the fold-enhancing roller pair switches direction of movement to move in a direction towards the one end; a second switching point nearer to the one end than the first switching point and nearer to the another end than the one end at which the fold-enhancing roller pair switches direction of movement to move in a direction towards the another end; and a third switching point different from the another end and nearer to the another end than the first switching point at which the fold-enhancing roller pair switches direction of movement to move in a direction towards the one end; and a roller guide pair which is provided at the front and rear in the direction of advancement of the fold-enhancing roller pair so as to define an opening at the nip between the upper fold-enhancing roller and the lower fold-enhancing roller, and is bent downward at free ends so as to cover part of the lower fold-enhancing roller.