The residence time distribution (RTD) of a flowing polymer through a single screw extruder was studied. This extruder allows injecting supercritical carbon dioxide (scCO(2)) used as physical foaming agent. The tested material is Eudragit E100, a pharmaceutical polymer. RTD was measured at various operating conditions and a model describing RTD has been developed. High screw speed or high temperature implies short residence time, but these parameters do not have the same effect on polymer flow. In the flow rate range studied, scCO(2) has no significant influence. A mathematical model consisting of a plug flow reactor in series with a continuous stirred tank reactor (CSTR) cross-flowing with a dead volume fitted well the experimental data.