During extrusion of starchy products, the molten material is forced through a die so that the instantaneous pressure drop causes part of the water to vaporize, giving an expanded, cellular structure. The objective of this work was to elaborate a phenomenological model of expansion and couple it with Ludovic ® mechanistic model of twin screw extrusion process. From experimental results that cover a wide range of thermomechanical conditions, a concept map of influence (relationships between input and output variables) was built. It took into account the phenomena of bubbles nucleation, growth, coalescence, shrinkage and setting, in a viscoelastic medium. The input variables were the moisture content MC; melt temperature T p , specific mechanical energy SME, shear viscosity at the die exit, computed by Ludovic ® , and the melt storage moduli E'( at T p > T g). The outputs of the model were the macrostructure (volumetric expansion index VEI, anisotropy) and cellular structure (fineness) of solid foams. Then a general model for VEI was suggested and the link between anisotropy and fineness was established.