Konica Minolta PagePro 1380 MF - Manual (Page 34)

Loading media 36
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When loading envelopes, load them lengthwise with the side to be
printed facing up as shown in the illustration.
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After the envelope is fed into the output tray, immediately open its
flap (before it has cooled). Because the envelopes pass through
heated rollers, the gummed flaps may seal. Using envelopes with
emulsion-based glue avoids this problem.