Method and apparatus for image forming capable of effectively conveying paper sheets

A sheet conveying device comprising:

• a first conveying path configured to pass a recording medium (RM) sheet therethrough to a sheet processing device;
• a second conveying path branched and separated from the first conveying path and configured to temporarily store the RM sheet conveyed therein;
• a sheet conveying mechanism configured to selectably convey the RM sheet in one of forward and backward directions to the sheet processing device, the sheet conveying mechanism including a first pair of conveying rollers and a second pair of conveying rollers disposed at respective positions along the first conveying path;
• a guide member mounted at a branch point of the first and second conveying paths and configured to guide the RM sheet when the RM sheet is conveyed in the backward direction by the sheet conveying mechanism to the second conveying path;
• and
• a control unit configured to control the sheet conveying mechanism based on a conveyed distance change between the branch point and the sheet conveying mechanism, which is according to a length of the RM sheet,
• wherein when the RM sheet is smaller than a predetermined size sheet:
• the control unit causes the first pair of conveying rollers and the second pair of conveying rollers to convey a first RM sheet in the forward direction toward the sheet processing device, and after a trailing edge of the first RM sheet passes the guide member, the first RM sheet is conveyed in the backward direction to the second conveying path and is stopped while being held in a nip of the first pair of conveying rollers with a leading edge of the first RM sheet in the forward direction thereof extending towards a downstream side of the first pair of conveying rollers, and when a second RM sheet is sequentially conveyed, the first RM sheet and the second RM sheet are piggybacked and conveyed together toward the sheet processing device;
• and
• wherein when the RM sheet is equal to or greater than the predetermined size sheet:
• the control unit causes the second pair of conveying rollers to convey the first RM sheet in the forward direction toward the sheet processing device, and after the trailing edge of the first RM sheet passes the guide member, the first RM sheet is conveyed in the backward direction to the second conveying path and is stopped while being held in a nip of the second pair of conveying rollers with the leading edge of the first RM sheet in the forward direction thereof extending towards a downstream side of the second pair of conveying rollers, and when the second RM sheet is sequentially conveyed, the first RM sheet and the second RM sheet are piggybacked and conveyed together toward the sheet processing device.