The viscosity of the PA66 reinforced with glass fiber at 30% by weight was measured at 280 C and at 300 C by means of a capillary rheometer RH7 Flowmaster Bohlin Instruments (Malvern Panalytical Ltd., Malvern, UK, Diameter 1 mm, length 10 mm), in the range of shear rates from 5/s to 5000/s. A regression analysis was then conducted in order to find the parameters of the Cross-WLF model [26]. In particular, the dependence of shear viscosity upon shear rate was assumed to agree with the Cross model: