Screws for plastics pose problems

Hot engines and corrosive chemicals are among the most demanding of environments for products made from plastics. To handle new plastics designed to cope with those environments, and composites made with abrasive glass fibres, the processing machinery makers are having to develop better machines. Rubber and plastics research body Rapra has a one-day seminar on […]

Hot engines and corrosive chemicals are among the most demanding of environments for products made from plastics. To handle new plastics designed to cope with those environments, and composites made with abrasive glass fibres, the processing machinery makers are having to develop better machines.

Rubber and plastics research body Rapra has a one-day seminar on 14 May dedicated to a single, vital element at the heart of all plastics processing machinery the screw.

The screw fulfils several roles: melting, mixing and extruding the polymer and dispersing colour pigments. Made from hardened carbon steel, the screws measure 18 200mm in diameter though some go up to 400mm. A 60mm screw weighs around 50kg.

Seminar chairman John Colbert says screw design is still viewed as a black art although computer simulation techniques are making his job more scientific. His paper on ‘The scaling up of extrusion’ will look at the transfer of a laboratory scale design to full-scale production screw which he says is ‘not a case of doubling up everything’.

The Archimedean screw design (a helix about an inclined axis) is critical to the quality of the end product.

Novel screw design includes the barrier flighted screw where a second flight path, introduced part way along the screw, acts as a barrier which improves moulding efficiency.

Colbert is technical director of Betol, a plastics extrusion machinery maker with £12m annual sales, specialising in machines for medical products such as tiny intravenous tubing.

A similar extrusion process is used to form and put a protective sheath on to wire for electric cables, and make pipes of all sizes.

Extrusion machines are used to force molten polymer through a shaping die to produce injection moulded plastic toys.

Screws for Polymer Processing, 14 May, £175 + VAT. Contact Janine Kitson at Rapra Technology, Shawbury, Shropshire on telephone 01939 250383.