A high-voltage electric field is created between the fixture of the plastic
part and a negative electrode, the metal target (e.g. aluminum, or an alloy)
that serves as a donor of metal atoms. The positively charged gas ions are
attracted by the negative metal electrode and are accelerated in its
direction. They transfer their kinetic energy to the metal atoms when they hit
the negative electrode, thus enabling metal atoms to escape from the solid
metal target. The plastic part is bombarded by these metal atoms and is coated
with a thin metal layer.
More refined, high rate sputtering processes make use of an additional
electromagnetic field (magnetron) to deposit the metal atoms at higher rates.
Sputter coatings have a better adhesion and are more resistant to abrasion
than vapor deposition coatings, due to the higher kinetic energy of the
deposited metal atoms. Also, sputtered coatings can be easily applied over
large surface areas with a uniform layer thickness.
Sputter coating can be done batchwise or in line with the injection molding
process. A well-known application is compact discs, which are sputter coated
in line, with a cycle time of less than 2 seconds. Also reflectors for car
lights are often sputter coated one by one, in line with the molding machine.
Reactive sputtering
In reactive sputtering, also called ‘Plasma Enhanced Chemical Vapor
Deposition’ (PECVD), a chemical reaction is incorporated in the vacuum
metallization process.
A gas can be used to react with a metal, like nitrogen with titanium to yield
a titanium nitride coating with a golden appearance and high hardness, used
for jewellery for instance. Another example reacts oxygen with aluminum to
yield an aluminum oxide coating.
Lacquering steps can be replaced by plasma polymerization of a topcoat, eg
siloxane to prevent corrosion, or depositing an aluminium adhesion promoting
layer in the same coating chamber. A monomer is let into the vacuum chamber
and is precipitated as a polymer coating under the influence of the ionized
gas particles.
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