Residence-time distribution model for twin-screw extruders

A model for the residence-time distribution (RTD) is validated by the analysis of data collected from the extrusion of polyethylene on a 30-mm Krupp Werner and Pfleiderer (W&P) corotating twin-screw extruder. Transformation of the RTD to give both the residence-volume distribution (RVD) and the residence-revolution distribution (RRD) yields new physical insights into the extrusion process. It is observed that operating conditions with equivalent specific throughput result in a equivalent RVD and RRD, and for a given screw configuration the axial mixing of extrusion material as measured by a tracer is essentially the same for all operating conditions. This allows the experimental RVD curves to be superimposed to form a single master curve for a given screw geometry. These new tools motivate the development of a simple residence model that characterizes the partially filled and fully filed screw sections and is capable of distinguishing between screw configurations and operating conditions. A least-square error-fit method used to identify the parameters of the RTD model indicated that the model function is appropriate to describe the RTD experimental data.