In vacuum evaporation the metallic coating is usually aluminum, but other
metals can also be deposited by evaporation. The metal is heated to the point
at which it evaporates. The vapor then migrates through the chamber and
condenses on the cold plastic part. The process takes place in a vacuum in
order to allow the metal vapors to reach the plastic surface without being
oxidized by oxygen in the air. The parts to be metallized are held in an
appropriate fixture that can rotate in order to expose all surfaces to be
metallized.
Several power sources can be used for metal evaporation, such as resistance
heating, induction evaporation, electron beam guns, or a vacuum arc.
Resistance heated tungsten filaments are most often used. The filaments are
placed in the required position to obtain uniform coverage, and aluminum chips
or staples are placed on the filaments. The metal to be evaporated can also be
placed in a thermally heated boat or crucible.
Evaporation coating is normally done batchwise in a cylindrical coating
chamber. The coating chamber may have a diameter of up to several meters,
depending on the size and number of parts to be coated. The parts may make a
planetary movement around the vapour source in order to equally cover all
sides of the parts with a metal layer. If desired, the areas not to be coated
can be masked, usually by metal sheets.
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