Skin treatment

Carmakers can save time and money by using a new form of coating for plastic parts that also provides decorative skins for car interiors. Siobhan Wagner reports.


Those in the business of car production know that coating an injection-moulded plastic part such as a car bumper is often a long and costly procedure. But a two-step, in-mould coating process that combines injection moulding with reaction injection moulding could save them time and money.

In the process developed by Bayer MaterialScience, a part is injection moulded then whirled around by a turntable to a second cavity. There, the part is reaction injected with two-component polyurethane, which rapidly cures in a closed mould.

Peter Neuwald, head of industry innovation transportation at Bayer MaterialScience, said the simultaneous injection moulding and curing ensures a shorter cycle time than in injection moulding of thermoplastics. With the traditional method, the cavity surface of the injection-moulding tool is first sprayed with a coating then the cavity is injection-filled with the melt.

The faster, simultaneous method means higher productivity, he said, adding that the process can lead to huge savings because the two-component polyurethane cures in a thermoplastic mould — making a second unnecessary.

The process also removes the need for a separate coating area, which reduces space requirements and eases logistics at a factory. Injection-moulded parts no longer need to be transported to a special coating line and stored there, which reduces the risk of the parts becoming soiled or damaged.

Another advantage to the in-mould coating process is there is no longer any overspray, which can be a problem when coating off the production line.

Other than increasing productivity and saving costs, Neuwald said the aim of the in-mould coating process is to produce more attractive finishes than the conventional coating method.

In-mould coating can be used to apply a decorative polyurethane skin at least 200µm thick to injection-moulded parts, resulting in a surface with good adhesion, a smooth feel and attractive appearance, he said.

Bayer MaterialScience uses low-emission polyurethane raw materials that cure quickly, can easily be moulded and stick well to thermoplastic polyurethanes, polycarbonate and various polycarbonate blends. They do not contain solvents, so emissions are reduced.

The company also produces lightfast polyurethanes that do not yellow. With their light stability, the reaction-injected moulded skins can be produced in pale colours, meaning they can satisfy the trend towards brighter colours for car interiors.

Neuwald said the decorative surfaces can be enhanced with a deep gloss finish when combined with Bayer’s film insert moulding process. Film-insert-moulding technology plastic parts are decorated during the injection moulding process by using a decorated film insert. The results are wear-resistant, colourful parts with symbols accurately positioned.

The Bayer team has used the decorative skins for car interior trims but other potential applications include arm rests, glove compartment flaps, door liners and trays coated with non-slip finish for the centre console. These not only look good, Neuwald said, but can reduce the troublesome grating noises that are produced when plastic parts rub against each other.

‘The potential for this innovation is considerable as it is projected that the market for coating automotive parts made of plastics will almost double by 2014,’ he said. ‘We are confident that in-mould coating will open up many more applications in the car industry.’