Canon i-SENSYS LBP6670dn User Manual page 193

Before loading envelopes, align them as follows.
1. Place the stack of envelopes on a flat
surface, and then flatten them to release
any remaining air.
Then, press the edges tightly.
Printing from an application
After loading envelopes, perform printing.
"Configuring the Printer Driver and Printing"
Do not print on the reverse side (glued side) of envelopes.
The print speed drops when printing on envelopes.
When envelopes are printed, they may be creased.
If the output envelopes curl
Switch the output tray to the sub-output tray.
"Paper Loading and Output"
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2. Loosen any stiff corners of the
envelopes and flatten curls as
shown in the figure.
3. Align the edges of the
envelopes on a flat
surface.