decoration | Plating on plastic
Above: Many plastics plating quality problems can be overcome by appropriate design considerations
disparities in expansion between metal and plastic to enable large and complex parts to be successfully plated – especially important for exterior automotive components. However, plating does change the properties of a
plastic part and it is essential that designers and platers consider this. The plating process effectively bonds a skin of very hard and infl exible chromium to the surface of the plastic, with the sandwiched layers of copper and nickel adding to the complication. This clearly has an effect on the behaviour of the fi nished component, with increased brittleness the most noticeable. Confi ned in a chromium jacket, ABS will not bend
Right:
Schematic showing different edge management options wthen selectively plating a part. From top: fl at surface, simple groove, and V groove
easily and fl exural strength can be reduced. If designers do not take this into account, it can lead to failures in service as cracks that start in the chromium layer continue through into the ABS substrate. A good rule of thumb is: If it has to bend, don’t plate it. And if a compo- nent is likely to be subjected to impact then only plate the areas that show, leaving the remainder un-plated to provide stress relief. This is particularly relevant to assembly lugs - where a component has a snap fi t the lugs should be left unplated to avoid the risk of breakage. Similarly, where heat-staking or ultrasonic welding is to be used, the surface needs to be free of metal. The design and moulding of long thin sections, such as handles, also requires careful consideration. In such situations, the part may need to be modifi ed with thicker sections and strengthening webs to compensate for the reduced strength of the plated ABS.
To ensure a high quality fi nish on an electroplated
plastic component, the designer must strive for gently curved convex surfaces, radiused angles and the minimum of protruberances. Sharp corners can be a big problem as they can lead to excessive plating at the extremities and chrome ‘burn’ or greying. Similar
54 INJECTION WORLD | November/December 2014
www.injectionworld.com
problems can result from poor orientation of parts in the plating bath. While an experienced plating engineer can sometimes overcome such problems using ‘robber’ bars in the plating bath to absorb excess chrome near sharp corners or at parts of components close to the electrodes, avoiding such problems at the design state is a simpler solution. Conversely, parts designed with deep recesses may be inadequately plated and surface imperfections will stand out on large plain surfaces. Moulding components with deep recesses, sharp edges or textured surfaces will often require the use of release agents to ensure good demoulding. Release agents must not be used where parts are to be plated under any circumstances. Fortunately, parts designed with the gentle curves and absence of sharp corners that make plating easier will also tend to release more easily from the mould. Selective plating represents a great advance in the use of chrome on plastic components, allowing the designer new freedom to deliver complex parts moulded in one piece. It can, for instance, be used to incorporate features such as snap fi ts that require fl exibility to be maintained. However, if the edge of a plated area lies on a fl at surface, it is likely to accept excess metal. This is caused by the higher current density that results at the edge of the plating area. The associated metal build-up can result in an unpleasant rough edge.
However, if the boundary of the plated area coincides with the natural edge of a part, or occurs at a groove or undercut provided specifi cally for this purpose, the current density is lowered and the resulting plated edge is sharp and free of irregularities. Consequently, when
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