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 an 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 filled 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.