Polymer foaming with the process of extrusion assisted by supercritical CO 2 is based on the injection of a supercritical fluid, in our case carbon dioxide, in a polymer, which has been previously melted in the barrel of an extruder. Upon depressurisation that happens in the die, nucleated bubbles will appear and grow. Modelling the nucleation and growth is important to better understand and master the process. A suitable and specific mathematical formulation has been implemented to calculate the number and the size of the pores. A methodology consisting in discretizing the space, to bring back to a succession of batch models was the chosen approach and a functional algorithm has been developed. The model will be applied in the foaming process of poly-lactic acid PLA and will be compared to experimental results.