Moulding for plating

Louise Smyth

Automotive designers are constantly being challenged to reflect ever changing consumer tastes, with traditional bright chrome now giving way to satin chrome. David Brereton reports

Car design has changed as much to incorporate trends in fashion as it has advances in technology and legislation changes. There is however one shining example of design consistency across all makes and models throughout the history of the car; chrome.

Since we started injection moulding plastic components for plating, most parts have been supplied in bright or noble chrome. But the balance has shifted. The majority of interior components are now required in the slightly more subdued satin chrome finish.

Bright chrome plating was traditionally produced by electroplating a thin layer of chromium directly onto a metal substrate to not only enhance the decorative appearance, but to increase corrosion resistance and hardness.

Today we are able to achieve the same results by plating plastic components, which offer the additional advantage of weight reduction. However to ensure they are tough and durable the metal must be fully integrated with the plastic surface, especially in automotive applications.

There are two basic ways of metal coating plastic. When cost is the overriding factor, the metal can be vacuum deposited, but the finish is not particularly durable and the necessary protective lacquer can leave the finish with an ‘orange peel’ effect.

The second method, typically used when manufacturing automotive components, must ensure a high degree of surface durability, which requires the metal to become integrated with the plastic surface. This is achieved by electroplating the plastic components.

The coating is chemically and physically bonded to the substrate, with metal layers applied through electrolytic deposition. An electric current is passed through the metal to be deposited and then through the workpiece, which removes atoms from the coating material and attaches them to the workpiece.

To make the plastic component electrically conductive, a layer of nickel is deposited in a chemical dipping process. This nickel is applied over a catalytic palladium layer, which must become integral with the surface of the material for the bonding to be effective. This is one of the restrictions of plating plastic which requires experience to overcome effectively.

Although Nylon and Polypropylene can be successfully plated, the vast majority of plastic plated components will be moulded from Acrylonitrile Butadiene Styrene (ABS). Surface Butadiene molecules can be chemically removed to leave spherical undercuts in the surface for the palladium/nickel to bond into.

But designers must understand that bonding hard and inflexible chromium to a plastic surface will make the parts brittle. Some designers and moulders do not take this reduction in flexural strength into account, which can lead to failure of parts in service, with a crack in the chromium layer extending into the ABS. Selectively plating components can overcome this problem and they typically out-perform parts with an all over plating.

In our experience, if you need a component to bend or flex, then do not plate it. And, if the moulding is to be handled or hit repeatedly, then only plate the areas that show, leaving the rest of the component un-plated for stress relief. This is of particular relevance for snap-fit assembly lugs, which should be un-plated as plating them would very likely cause them to become snap-off assembly lugs!

The choice of finish, whether bright chrome or satin, might be the first thought of the product designer, but it will not actually effect the way the components are moulded. Regardless of the final finish, the same consideration must be given at the outset to the moulding of parts specifically for plating.

Long and thin sections which require moulding and plating need to be over-engineered with thicker sections and strengthening webs. This will compensate for the reduced strength of plated-plastic but, to achieve a high-quality finish, the designer must allow for gently curved convex surfaces and radiused angles.

One problem common to both bright chrome and satin is caused by sharp corners on component and the orientation of parts in the plating bath. Both of these factors can create excessive plating at the extremities and chrome ‘burn’ or greying – more evident in bright chrome.

To achieve the high quality finishes required by modern automotive manufacturers, when considering moulding plastic components for plating, we would advise getting the designer, tool-maker, moulder and plater involved from the start of the process.

This is particularly true when designing assemblies and components for modern car interiors, where designers are now reflecting changes in consumer taste, by introducing components with a satin chrome finish.

Whether the introduction of more satin chrome into cabins is born of a practical desire to reduce reflections off interior surfaces, or a desire simply for change, the result is the same. Platers are having to create a range of colours and finishes, when previously bright chrome was all they had to worry about.

Satin chrome for automotive interior components has the added advantage of being far more tactile. Many switches, surrounds and fascia panels in cars are designed to be touched and for car owners, the drawback of bright chrome has been the reluctance to leave fingerprints on the highly polished surfaces.

To achieve a satin chrome finish, the changes are made to the deposited nickel layer rather than the chrome. Satin nickel is used in the electroplating process, with organics added to the dipping solution to effect the colour and finish of this nickel layer.

It is possible with precise adjustments to this organic material to vary the colour and produce a large shade range, but ensure a consistently high quality finish with no difference between batches. Designers can now create a unique look and possibly more important, feel for the interior trim and switches in their cars.

Regardless of changes made to the nickel layer, a chrome surface layer is still applied to components to increase durability. This chrome layer is relatively thin at only 0.3 micron, but it allows the satin effect nickel underneath to show through.

For many non-automotive applications, the bright or satin nickel finish will often be tough enough for embellishments added to perfume containers, high quality packaging and other low-traffic situations.

It is probably true that modern consumers are now more critical and more demanding, with fashions subject to change more quickly than in the past. This is a problem for car designers, working on new models many years in advance, who have to introduce or reflect new consumer tastes, if they want their cars to be successful.

Some chrome has recently disappeared from the exterior of lower priced cars, but the most prestigious brands remain advocates of chrome, although they are looking for new colours and finishes, particularly within the cabin.

Working closely with automotive designers, we are constantly experimenting with different styles and shades of nickel to achieve a range of finishes that are unique to different marques. But so far, colour has not featured, although it surely will not be long before it appears in requests from automotive interior designers.”

David Brereton is with chrome-plating specialists Borough Limited