Cold extrusion is playing an ever greater part in recycling of waste plastic, and the UK Department for Business and Enterprise is sponsoring research into development of the machinery necessary for such work.

Engineers at Bradford University have built a pilot plant and are running a series of trials with different processes, feed plastics, additives, and final products.

Leader of the project, Dr Raj Patel explains: “We started out comparing cold extrusion and warm extrusion, in which the material is heated prior to processing, and quickly realised that cold extrusion was more suited to recycled plastic. Our process now is to granulate the recycled materials to a consistent size, mix in binders as appropriate to basically, glue the particulates back together, and cold extrude it into sheets or blocks.

“The extrusion process is the critical stage, so we are monitoring many parameters relating to this.”

The single most important parameter is torque in the extruder drive. This indicates the force required to process the material and hence both the power requirements of the drive and the viscosity of the plastic. It is measured using a digital TorqSense torque sensor, which uses Surface Acoustic Wave techniques to provide a non-contact method of monitoring continuous rotary torque, allowing accurate modelling of the instantaneous load changes.

Sensor Technology Ltd is based in Bicester, Oxfordshire. www.sensors.co.uk