Electroplating Process

The first and most important stage in successfully plating plastic is to make the plastic material electrically conductive by depositing a layer of nickel in the first of many chemical dipping processes. The nickel is applied over a catalytic palladium layer, (Fig 1) and for this to bond effectively, the palladium needs to become an integral part of the surface of the material, which is one of the restrictions when plating plastic.

Fig1. The surface has to be etched to provide a key for the palladium/nickel, (Fig 2) and one problem is that many materials have a surface structure which resists being keyed.

Fig 2- Most of the work done on plating has been performed 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 – It is important to understand what effect electroplating has on the properties of ABS and these effects must be allowed for in the design. The electroplating process involves bonding a skin of very hard and much less flexible chromium - with the added complication of sandwiched layers of copper and nickel, to the plastics material. This will clearly have an effect on the behaviour of the plastic and again, the changes must be understood and allowed for. In experience Borough has found that not all designers have taken account of the change in material properties and some components have failed in service as a result.

Electroplating ABS makes it brittle and it should not be 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 and cracks will occur. The theory is that crack propagation which starts in the chromium is continued on through the ABS, leading to eventual failure of the component. Selectively plated components will often perform better in service than a component that is electroplated all over.

The general rule of thumb is ‘if it has to bend, don't plate it’. Also, if it is likely to be subjected to impact, only electroplate the areas that show, leave the rest un-plated to allow for the relief of stress. This has particular importance for assembly lugs, which should be left un-plated if the component has a snap-fit.

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