In this work, we used the residence time distribution to study the polystyrene foaming during an extrusion process. The extruder associated with a gear pump is simply and quantitatively described by three continuously stirred tank reactors with recycling loops and one plug-flow reactor. The blowing agent used is CO2 and is obtained by thermal decomposition of a chemical blowing agent (CBA). This approach allows the optimization of the density of the foam in accordance with the CBA kinetic of decomposition.