Sheet processing apparatus and image forming system

1. A sheet processing apparatus comprising:

a sheet feeder configured to feed a sheet toward a sheet conveyance passage;

a sheet processing device disposed downstream from the sheet feeder in a sheet conveyance direction in which the sheet is conveyed, the sheet processing device configured to perform a predetermined operation on the sheet conveyed in the sheet conveyance passage;

a sheet ejection device disposed downstream from the sheet processing device in the sheet conveyance direction, the sheet ejection device configured to stack the sheet when the predetermined operation has been performed on the sheet by the sheet processing device;

a purge unit disposed downstream from the sheet processing device in the sheet conveyance direction, the purge unit configured to stack the sheet when the predetermined operation has not been performed on the sheet by the sheet processing device;

a multifeed sensor disposed on the sheet conveyance passage and configured to detect whether the sheet and another sheet are multifed from the sheet feeder;

a sheet sensor disposed downstream from the multifeed sensor on the sheet conveyance passage in the sheet conveyance direction and configured to detect the sheet passing a position of the sheet sensor; and

circuitry configured to:

stop an operation of the sheet processing apparatus when the multifeed sensor has not detected multifeed of the sheet and the sheet sensor does not detect a trailing end of the sheet after a first set time set in advance in accordance with a size of the sheet in the sheet conveyance direction has elapsed from detection of a leading end of the sheet with the sheet sensor;

stop an operation of the sheet processing apparatus when the multifeed sensor has detected the multifeed of the sheet and the sheet sensor does not detect the trailing end of the sheet after a second set time longer than the first set time has elapsed from the detection of the leading end of the sheet with the sheet sensor; and

cause the sheet processing apparatus to convey the sheet toward the purge unit without stopping the operation of the sheet processing apparatus when the multifeed sensor has detected the multifeed of the sheet and the sheet sensor detects the trailing end of the sheet before the second set time elapses from the detection of the leading end of the sheet with the sheet sensor.

2. The sheet processing apparatus according to claim 1 , wherein the multifeed sensor is disposed in proximity to the sheet feeder.

3. The sheet processing apparatus according to claim 1 , wherein the multifeed sensor is an optical sensor to detect the sheet passing a position of the multifeed sensor.

4. The sheet processing apparatus according to claim 1 , wherein the circuitry is configured to:

cause the multifeed sensor to detect a leading end of the sheet passing a position of the multifeed sensor; and

determine that the sheet is multifed with said another sheet when the multifeed sensor does not detect a trailing end of the sheet after a predetermined time in accordance with the size of the sheet in the sheet conveyance direction has elapsed from detection of the leading end of the sheet with the multifeed sensor.

5. The sheet processing apparatus according to claim 1 , wherein the circuitry is configured to change the second set time based on at least one of a thickness of the sheet fed from the sheet feeder or a kind of the sheet fed from the sheet feeder.

6. The sheet processing apparatus according to claim 1 , wherein the circuitry is configured to adjust the second set time based on an input by an operator.

7. The sheet processing apparatus according to claim 1 , further comprising another sheet sensor disposed in proximity to the sheet feeder and configured to detect the sheet passing a position of said another sheet sensor,

wherein the circuitry is configured to set the second set time based on a time from when said another sheet sensor detects the leading end of the sheet to when said another sheet sensor detects the trailing end of the sheet.

8. The sheet processing apparatus according to claim 1 , wherein the sheet processing device includes a sheet separation device configured to:

perform a sheet separating operation to separate a non-bonding portion of a two-ply sheet in which two sheets are overlapped and bonded together at a bonding portion of the two-ply sheet; and

perform a sheet inserting operation to insert an inner sheet between the two sheets of the two-ply sheet separated in the sheet separating operation,

wherein the sheet feeder is configured to feed the two-ply sheet toward the sheet conveyance passage, and

wherein the multifeed sensor is configured to detect whether the two-ply sheet fed by the sheet feeder is multifed with another two-ply sheet.

9. The sheet processing apparatus according to claim 8 , wherein the sheet processing device includes a sheet lamination device configured to perform a sheet laminating operation on the two-ply sheet with the inner sheet inserted between the two sheets separated by the sheet separation device.

10. An image forming system comprising:

the sheet processing apparatus according to claim 1 ; and

an image forming apparatus to form an image on the sheet.

11. The image forming system according to claim 10 , wherein the sheet processing apparatus is detachably attached to the image forming apparatus.