Sheet folding device, image forming apparatus, and sheet folding method

1. A sheet folding device for performing single-sheet folding and multi-sheet folding with a single feed path, the device comprising:

a conveying unit that conveys a sheet;

a restraining unit that stops a leading edge of the sheet;

a pair of folding rollers that fold the sheet;

a guiding member that guides a bend of the sheet to a nip between the pair of folding rollers, wherein the guiding member is positioned at different sheet-guiding positions for single-sheet folding and multi-sheet folding;

a driving unit that makes the guiding member advance and retreat with respect to the pair of folding rollers; and

a control unit that controls the driving unit,

wherein the control unit is configured to,

acquire information indicating whether the single-sheet folding or the multi-sheet folding is performed,

control the position of the guiding member so that a gap between the pair of folding rollers and the guiding member during the multi-sheet folding becomes larger than the gap during the single-sheet folding, prior to the sheet passing through the gap, and

pass the sheet through the gap, after controlling the position of the guiding member.

2. The sheet folding device according to claim 1 , wherein

the guiding member moves in a direction to increase a gap between the guiding member and outer peripheries of the folding rollers during multi-sheet folding.

3. The sheet folding device according to claim 2 , wherein

the guiding member makes a shifting movement when moving from a single-sheet folding position to a multi-sheet folding position, the shifting movement being in a direction that is about perpendicular to a conveying direction of the sheet.

4. The sheet folding device according to claim 2 , wherein

the gap varies according to a number of the sheet to be folded or a folding thickness of the sheet.

5. The sheet folding device according to claim 4 , wherein

the gap increases as the number of the sheet to be folded increases, or as the folding thickness of the sheet increases.

6. The sheet folding device of claim 4 , wherein the control unit is further configured to,

acquire additional information that indicates the number of sheets to be folded, and

control the position of the guiding member so that the gap becomes larger after the sheet passes through the gap, if the number of sheets to be folded is two or more.

7. The sheet folding device according to claim 1 , wherein

the guiding member includes

a protruded member for positioning that secures a sheet feeding gap, and

an elastic body that holds the protruded member in contact with an opposed guide plate; and

the guiding member is held by the elastic body during single-sheet folding, and moved by the driving unit against an elastic force of the elastic body during multi-sheet folding.

8. The sheet folding device according to claim 7 , wherein

the guiding member operates in conjunction with an advancing operation of a pusher member that pushes the sheet into between the pair of folding rollers for sheet folding; and

the pusher member is driven by a single driving source.

9. An image forming apparatus comprising the sheet folding device according to claim 1 .

10. A sheet folding method for performing single-sheet folding and multi-sheet folding with a single feed path, the method comprising:

conveying a sheet by a conveying unit;

restraining a leading edge of the conveyed sheet by a restraining unit;

guiding by a guiding member a bend of the sheet in the restrained state into a nip between a pair of folding rollers for sheet folding, thereby folding the sheet, wherein in the guiding, the guiding member is positioned at different sheet-guiding positions for single-sheet folding and multi-sheet folding;

driving, by a driving unit, the guiding member to advance and retreat with respect to the pair of folding rollers; and

acquiring, by a control unit, information indicating whether the single-sheet folding or the multi-sheet folding is performed,

controlling, by the control unit, the position of the guiding member so that a gap between the pair of folding rollers and the guiding member during the multi-sheet folding becomes larger than the gap during the single-sheet folding, prior to the sheet passing through the gap, and

passing, by the control unit, the sheet through the gap, after controlling the position of the guiding member.