Torque sensor used in cold extrusion of recycled plastics

Cold extrusion is playing an ever greater part in the recycling of waste plastic, and the Department for Business, Enterprise and Regulatory Reform (BERR) 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 this 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. It is, in effect, a frequency-dependent strain gauge operating at ultrasound frequencies and consists of a transducer mounted on the mixer's rotating shaft to monitor variations in its resonance frequency as the torsional load varies. An RF (radio frequency) link is used for wireless transmission of signals to an adjacent couple so that rotation is unhindered. It is manufactured by Sensor Technology, which developed the underlying principles into a practical technology and is now recognised globally as a leader in the field.

Non-contact and robust

Dr Patel says: "With research work, there is an inordinate amount of dismantling and reassembling equipment. This can be time-consuming and, therefore, expensive; but TorqSense, being non-contact, does not need to be dismantled. It is also naturally robust, which is important in the laboratory and critical, since it is likely to be used on the full-scale plants that we will ultimately develop."

Currently the project is sending its recycled plastic to be made into acoustic baffles for use in new buildings. Dr Patel says: "This is a ready market, driven by new Building Regulations and the higher acoustic standards required for high-density building."

However, it is not the only market open to Bradford's recycled plastic. Their best results are said to be indistinguishable form virgin plastic, so can be used in demanding applications, and there are many uses for the standard-quality output.

The pilot plant is also being used to work on design and development of full-scale plant, which will eventually be used at waste processing and recycling plants across the country.

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