Canon C5051 User Manual page 24

Hold down the four corners of the envelopes firmly, so that they and the sealed or glued portion stay flat.
1
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If you use envelopes that have glue attached to their flaps, the glue may melt due to the heat and pressure of the fixing unit.
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Take particular care to spread the envelopes out in the direction that they will be fed.
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Do not print on the back side of the envelopes (the side with the flap).
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If the envelopes become filled with air, flatten them by hand before loading them into the envelope feeder attachment.
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Envelope Feeder Attachment-D1 (Optional)
Flap
Flap