For plastic components requiring a high degree of surface durability, like automotive applications, the metal needs to become integrated with the plastic surface and this can be achieved by electroplating the plastic components.
The coating is chemically and physically bonded to the substrate, with metal layers applied to this surface through electrolytic deposition – passing an electric current through the metal to be deposited and then through the workpiece, removes atoms from the coating material and attaches them to the workpiece.
The plastic material must be made electrically conductive, which is done by first depositing a layer of nickel in a chemical dipping process. The nickel is applied over a catalytic palladium layer, which must become integral with the surface of the material for the bonding to be effective; one of the restrictions of plating plastic.
Although Nylon and Polypropylene can be successfully plated, about 90% of the world’s plastic plated components will use Acrylonitrile Butadiene Styrene (ABS). This is because the butadiene molecules at the surface can be removed relatively easily with chemicals to leave spherical undercuts in the surface which act as an excellent key for the palladium/nickel.
As new applications for plated plastics were developed, the size and complexity of the components to be plated also increased, requiring a higher grade ABS. This offered a lower coefficient of thermal expansion of the plastic substrate, enabling large and complex units to overcome any disparities in thermal coefficient between the plastic and the metal plating – important for exterior automotive components.
It is essential that designers and platers consider the effect plating has on the properties of ABS. The plating process effectively bonds a skin of very hard and inflexible chromium to the surface of the plastic – with the added complication of sandwiched layers of copper and nickel. All of which clearly has an effect on the behaviour of the finished component.
Plating ABS makes it brittle. It is not difficult to imagine that once a material with the surface mobility of ABS is closely confined in a chromium jacket, it will not bend easily. Some designers and moulders do not take this reduction in flexural strength into account and this can lead to failure of parts in service, with a crack which starts in the chromium layer continuing through into the ABS. This situation can be overcome by selectively plating components, which generally perform better than parts with an all over plating.