Cool gas and plasma assist plastic processing

Plasma and pressure have both played a part in two advances in the world of plastics processing. Mark Fletcher examines the technologies

Researchers at the University of Warwick have hit upon a novel approach to gas-assisted injection moulding (GAIM), speeding up the process by as much as 40%. They have also developed an improved method for preparing plastic surfaces prior to bonding, to achieve engineering-strength joints. Both technologies will have significant implications for industry, as both speed of processing and engineering integrity are essential for many modern-day plastic products.

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GAIM is already used by many companies, so any improvements to the process, especially with regard to speed of processing, will be welcomed with open arms. The plasma technology, on the other hand, is initially targeted at the automotive industry, due to the increased use of plastic in both internal and body-in-white structures. However, as with most automotive-lead ideas, it will have many applications in general industry.

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Ordinarily, GAIM is used for two reasons, either to reduce the weight of plastic products or to increase the rate at which components can be made by using the gas to displace volumes of unnecessary hot plastic from the components' core. As a cooling agent it is used to speed the cooling of injection-moulded components so they can be demoulded quicker. However, according to the researchers, few people have considered using gas that has been cooled prior to use.

With this in mind, they have created what has been dubbed 'Kool Gas'. A cryogenic heat exchanger is used to cool high-pressure nitrogen to temperatures as low as -150 deg C prior to injection into the plastic component. The results are dramatic. The Kool Gas allows the components to cool 40% faster than normal methods or would allow a moulder to produce 40% more product in the same time. There are no detrimental effects and it was observed that components produced using this method exhibit a more controllable wall thickness.

The plasma treatment developed by the university has been born out of necessity as plastics start to play a larger role in automotive construction. Getting plastics to join using adhesives can be a major headache (Eureka cover feature, April 2000), so many methods have been created to 'key' the surfaces. Many processes can be difficult to control or do not suit the particular product, so the team investigated many ideas before arriving at plasma treatment, which was already being used to great success in the US to apply paint and labels.

The technology - forced air plasma surface treatment - is the result of a successful collaboration between the university and industry (University of Warwick, University of Bristol, Dow Automotive, Solvay Engineered Polymers and Hamilton Machinery Sales). It uses a plasma spraying tool, attached to a robot arm, to rapidly treat/oxidise the mating surface of the plastic components. It deposits oxygen onto the surface, to create "hand holds" which then aid the adhesive. Bonds created using this method have passed all the standard automotive impact tests. An additional feature of the treatment, of benefit to the stock-holding nature of the industry, is that the parts remain treated for up to one month.

Design pointers

The 'Kool Gas' process cools parts 40% faster or enables 40% more product to be made in the same time period

Hollow components have a more controllable wall thickness

The plasma treatment keys the plastic prior to adhesive application and can stay treated for up to one month

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