The torque in the metering zone of single screw extruders

To study the energy dissipation in the different zones of an extruder, a special barrel has been designed where local torques T q can be measured at the beginning of the conveying, the transition, and the metering zone. Experiments carried out for a commercial polyethylene and poly(vinyl chloride) at different processing conditions show that in some cases negative T q in the metering zone may occur. A very basic analytical treatment of the flow situation in the metering zone shows that for a given drag flow T q is dependent on the pressure gradient and particularly on its sign. T q decreases with decreasing positive pressure gradient until there is pure drag flow. It further decreases with increasing negative pressure gradient until it becomes zero and even negative. A comparison of theoretical and experimental results for the metering zone shows good quantitative agreement though the main aim of the paper is to provide a better general understanding of the complex situations inside an extruder.