Consistent colour

RESEARCHERS from the University of Warwick have developed a technique to paint plastics as they are moulded. The team from Warwick Manufacturing Group's Advanced Technology Centre was led by Gordon Smith, who explains that if plastic components are coloured throughout - as in washing-up bowls, they fade at different rates depending on the polymer from which they are made, thus giving an uneven effect (New Scientist, Vol 147, No 1987).

For instance, depending upon the types of plastics used for different components in a vehicle, one gets differences in shades, says Smith, adding, "You, therefore, paint components for consistent durability and colour."

Usually, good-quality plastic components are made up of 2 kinds of polymers. The first is squirted into the mould through a nozzle and clings to the surface. This, in effect, forms a coating for the second, also called the 'core polymer'- usually a coarser composite containing fibres - which is then passed through another nozzle. After the component has cooled and set, it is sent for painting.

The War-wick researchers have done away with the polymer 'skin', and instead, used the paint itself as the coating for core plastic. The process involves squirting it in first so that it forms a skin on the inside of the mould. Then the core polymer is squirted inside the paint. "When the core plastic goes in, it blows the paint up like a balloon, pro- jecting it onto the entire inner surface of the mould," says Smith.

The trick, he says, was finding a way to 'melt' the solid paint particles as they are squirted into the mould. The researchers have developed a software to control the process. The technique can even be applied on existing machines, provided they have 2 separate nozzles for injecting molten plastic into moulds.

A second technique developed by Smith's team can be used on items that only have one face visible - such as the wheel trims of a car. Here, the paint is 'preformed' into the shape that is visible, and the core polymer is fixed to the back. Rover, an international car manufacturing company, intends using this technique for its vehicles. A spokesperson for the British Plastics Federation says that there is an increasing demand for coloured plastic mouldings, particularly for bumpers that match the car colour. "Anything that reduces lead time for moulding has to be good news," the spokesperson adds.

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