Offset Lithography offers the highest quality of envelope printing. It produces a clear, well-defined image area and provides excellent ink coverage. It is effective on many different paper surfaces from smooth vellums through laids, embossed finishes and heavily textured felts. Offset printing is generally done after the envelope is folded, but the blank can be printed before folding to allow for closer registration if the copy wraps around the sides of the envelope
The lithographic process is based on the
principal that oil and water do not mix. The surface of the plate is chemically treated so
that the non-print areas attract water and the printing areas repel it. If the plate is
dampened before the ink is applied, the non-print areas absorb the water and the oilbase ink will only be attracted to the printing surfaces. Wet offset plates are normally made in the envelope industry
by a photographic process in which a light-sensitive
coating is activated by passing light onto it through a negative.
The dampening rollers pick up water from the reservoir and apply it to the surface of the
plate. A series of ink rollers pick up, meter, and apply paste ink to the plate. As the plate
turns against the blanket cylinder,
the ink is transformed from the plate
to the blanket.
The blanket cylinder
controls the pressure.
Wet offset can accurately apply a light coating of ink so it can be used with a very fine
screen. Screens with 133 to 150 lines are normally used with wet offset in the envelope
industry, but wet offset can be used with screens as fine as 300 lines.
