A mathematical model for investigating the melt flow behaviour of thermoplastic natural rubber (TPNR) in a double feed system of a single-screw reciprocating injection unit using a specially written software is presented. The model enabled the prediction of processing parameters in the feed and main extruders, from which the optimum conditions for processing TPNR were obtained. The introduction of the double feed system also contributed to better mixing in the main extruder.