Sheet processing device and image forming system incorporating the sheet processing device

A sheet processing device comprising:

• a first conveyance passage configured to pass a sheet downstream in a sheet conveyance direction;
• a first conveyor configured to convey the sheet in the first conveyance passage;
• a second conveyor disposed downstream from the first conveyor in the sheet conveyance direction;
• a folder disposed between the first conveyor and the second conveyor,
• the folder being configured to form a fold on the sheet;
• a second conveyance passage configured to circulate, to the first conveyance passage, the sheet with the fold being formed after the sheet passes through the folder;
• and
• circuitry configured to:
• (1) cause the first conveyor to convey the sheet in the sheet conveyance direction and the second conveyor to convey the sheet in a reverse direction opposite to the sheet conveyance direction while the first conveyor and the second conveyor nip the sheet, to warp the sheet between the first conveyor and the second conveyor;
• (2) guide the sheet warped between the first conveyor and the second conveyor, to the folder, to form a first fold on the sheet by the folder, and cause the sheet to be circulated to the first conveyance passage via the second conveyance passage;
• (3) cause the first conveyor to convey the sheet in the sheet conveyance direction and the second conveyor to convey the sheet in the reverse direction opposite to the sheet conveyance direction while the first conveyor and the second conveyor nip the sheet again, to warp the sheet between the first conveyor and the second conveyor;
• (4) guide the sheet warped between the first conveyor and the second conveyor, to the folder, to form a second fold on the sheet by the folder;
• and
• (5) cause the second conveyor to convey the sheet with the first fold and the second fold, in the sheet conveyance direction.