Method for conveying sheet in post-processing apparatus

An image forming system comprising:

• an image forming apparatus configured as a preceding stage to form an image on a sheet;
• and
• a post-processing apparatus to perform post-processing on a sheet conveyed from the image forming apparatus,
• wherein the post-processing apparatus includes:
• a first conveyance path configured to receive the sheet that has been conveyed from the image forming apparatus in the stage, and to convey the sheet,
• a buffer unit configured to form a sheet bundle constituted by a predetermined number of sheets by stacking the predetermined number of sheets conveyed from the first conveyance path while shifting the sheets in a conveyance direction,
• a second conveyance path connected to the buffer unit and configured to convey the sheet bundle,
• a post-processing unit configured to load the sheet bundle conveyed from the second conveyance path and to perform the post-processing on the sheet bundle, and
• at least one processor configured to perform operations including:
• controlling conveyance of the sheets and the sheet bundle,
• obtaining, in a case of a delay amount, a delay amount of the sheet conveyed from the image forming apparatus in the stage or a delay amount of a preceding other sheet bundle conveyed from the buffer unit to the post-processing unit, and
• setting a shift amount between plural sheets of the sheets constituting the sheet bundle in the buffer unit based on the delay amount obtained by the at least one processor.