Thick Paper; Transparencies - Epson EPL-6100 Reference Manual

The printing quality on envelopes may be irregular because
different parts of an envelope have different thicknesses. Print one
or two envelopes to check the print quality. If the printing is too
light, adjust the print density as described in "Print Quality
Problems" on page 120.
Do not use window envelopes unless they are specially designed
for laser printers. The plastic on most window envelopes will melt
when it comes into contact with the hot fuser.

Thick paper

You can load 5 to 10 sheets of thick paper into the MP tray.
However, you may need to feed some types of thick paper one
sheet at a time. Use thick paper of a weight between 90 and 163
g/m² (24 to 43 lb).
To print on thick paper, be sure to change the Paper Type Setting
in the printer driver's Basic Settings menu (Windows) or Basic
Settings dialog box (Macintosh). Choose Thick Narrow (Windows)
or Thick[N] (Macintosh) for thick paper which is less than 188 mm
wide. Choose Thick Wide (Windows) or Thick[W] (Macintosh)
when printing on thick paper which is 188 mm or wider.

Transparencies

You can load up to five transparencies into the MP tray. However,
you may need to feed some types of transparencies one sheet at a
time.
You can only use overhead projector transparencies and adhesive
drafting film made for use with laser printers or plain-paper
copiers.
22 Paper Handling