New injection-moulding tool could help reduce scrappage

A tool created at the National Physical Laboratory (NPL) could help plastic injection-moulded parts manufacturers improve cycle times and reduce scrappage.

NPL’s pvT (pressure-volume-temperature) and thermal conductivity test kit measures the thermo-physical properties of polymers, which will help designers find the exact pvT and shrinkage properties of a material.

The pvT instrument operates at pressures ranging from 200 bar to 2,500 bar, and can test materials at cooling rates of up to 280°C/min and down to temperatures approaching -100°C.

NPL found that, at higher pressures, polymers can conduct heat up to 20 per cent more efficiently, leading to faster cooling rates and shorter cycle times.

Shrinkage and warpage are recurrent problems in the manufacture of plastic components, which, in turn, can increase volumes of scrap and lead to longer cycle times.

Software can help designers overcome these issues but Angela Dawson, a higher research scientist for NPL’s Materials Division, told The Engineer that such software predicts an outcome based on data obtained around ambient conditions and low pressures.

‘We’ve modified our kit to provide data that’s more real; the data is obtained in conditions approaching fast cooling rates under injection-moulding conditions. That data can then be incorporated into a database, in Moldflow for example.’

Dawson added that creating data sets for software represents one aspect of the work undertaken to create the pvT.

‘As a result of our research we now have a fantastic facility,’ said Dawson. ‘An outcome of that is that designers can use our facility to get data tailored to the exact polymer that they want to use.

‘With tighter modelling [designers] can get closer process control, reduce cycle times and therefore increase their production rate.’