Envelope processing apparatus and image forming system

An image forming system comprising:

• an image forming apparatus configured to form an image on a medium;
• and
• an envelope processing apparatus configured to enclose the medium sent from the image forming apparatus into an envelope conveyed to an enclosing position,
• the envelope processing apparatus comprising,
• a conveyance roller configured to convey the envelope to the enclosing position,
• a first envelope length calculator configured to calculate a first envelope length of the envelope in an envelope conveyance direction in which the envelope is conveyed in a close state in which a flap of the envelope closes,
• a flap opener configured to open the flap while the conveyance roller conveys the envelope,
• a second envelope length calculator configured to calculate a second envelope length of the envelope in the envelope conveyance direction in an open state in which the flap opens,
• a first envelope detector configured to detect a leading end of the envelope in the envelope conveyance direction at a first detection time and detect a trailing end of the envelope in the envelope conveyance direction in the close state at a second detection time,
• a second envelope detector configured to detect the leading end of the envelope in the envelope conveyance direction at a third detection time and detect the trailing end of the envelope in the envelope conveyance direction in the open state at a fourth detection time,
• the first envelope length calculator is further configured to calculate the first envelope length based on a time difference between the first detection time and the second detection time and a conveyance speed of the envelope at which the envelope is conveyed,
• the second envelope length calculator is further configured to calculate the second envelope length based on a time difference between the third detection time and the fourth detection time and the conveyance speed of the envelope, and
• a controller configured to,
• calculate a flap length of the flap in the envelope conveyance direction based on the first envelope length and the second envelope length, and
• control enclosing of the medium into the envelope based on the flap length.