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Electroplating
The
initial advantages of metallising plastic components
are readily apparent. The coating is on a
non-corrosive base: the component is considerably
lighter in weight, which in some costing
applications can be advantageous over metals: design
is more flexible: and a high quality finish can be
achieved.
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how we
electroplate
It is
necessary to make the plastic material electrically
conductive, and this is done by first depositing a layer
of nickel in a chemical dipping process. The nickel is
applied over a catalytic palladium layer, (Fig 1) and for
this to be bonded effectively, the palladium needs to
become integral with the surface of the material, which is
one of the restrictions of plating plastic.
Fig1. The surface has to be etched to provide a key for
the palladium/nickel, (Fig 2) and many materials have a
surface structure which resists being keyed.
Fig 2- Most of the work done on plating has been on ABS.
This is because the butadiene molecules on the surface can
be removed chemically, (Fig 3) and as they are essentially
spherical, their removal leaves undercuts which act as an
excellent key for the palladium/nickel.
Fig 3 - The effect of electroplating on the properties of
ABS must be understood and allowed for. The electroplating
process involves bonding to the plastics material a skin
of very hard and much less flexible chromium - with the
added complication of sandwiched layers of copper and
nickel. This clearly has an effect on the behavior.
Borough has found that not all designers have taken
account of the change in material properties, with the
result that some components have failed in service.
Electroplating ABS makes it brittle. It is not difficult
to imagine that if a material with the surface mobility of
ABS is closely confined in a strait jacket of chromium, it
is not going to bend easily. The theory is that crack
propagation which starts in the chromium is continued
through the ABS, leading to eventual failure.
Selectively plated components will often perform better in
service than an all over electroplated component.
The
rule of thumb is that if it has to bend, don't plate it.
And if it is likely to be subjected to impact, only
electroplate the areas that show, leave the rest un-plated
to allow for stress relief. This has particular reference
to assembly lugs. Where a component has a snap-fit, the
lugs should be un-plated.
 Surface
Finishing Basics
Just
What Is Electroplating?
Electroplating
is one kind of surface finishing. There are many
other kinds. Everyone has seen and handled
electroplated objects, even if they didn't know it.
Examples include kitchen and bathroom faucets,
inexpensive jewelry and the trim on some
automobiles. There are thousands of examples. In
fact, it is certain that nearly every piece of metal
you have ever seen has been through some kind of
surface finishing process. There are three basic
reasons for surface finishing: to improve
appearance, to slow or prevent corrosion (rust) and
to increase strength and resistance to wear (in the
case of "engineering" finishes). An object
may be processed for any or all of these reasons.
The term
electroplating means the coating of an object with a
thin layer of metal by use of electricity. The
metals most often used are gold, silver, chromium,
copper, nickel, tin and zinc, but many others are
used also. The object to be plated is usually a
different metal, but can be the same metal or a
non-metal, such as a plastic grille for an
automobile.
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