Pitney Bowes SendPro C User Manual page 118

For a proper seal, the envelope's minimum flap angle must meet the following criteria:
For envelopes with a flap length from 25-60 mm, the minimum flap angle is 18 degrees.
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For envelopes with a flap length greater than 60 mm to the maximum flap length for the
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particular model, the minimum flap angle is 21 degrees.
Pitney Bowes User Guide June 2022
15 • Product Information
118