The relevance of enhancement of crystallization kinetics by effect of shear flow and rheology during polymer solidification to the phenomena taking place during the injection moulding process has been shown by means of simulations performed on the basis of Lord and Williams [1,2] model and recent extensions [3 to 5]. A key point of the simulation was the solidification criterion based on a critical crystallization index; on the basis of simple calorimetric and rheological tests in the limit of zero shear rate, values of a few percent were given to the solidification crystallinity value. Many experimental features of pressure history both in the runner and in the cavity are recovered by model predictions, if both effects mentioned above are properly accounted for. However, predictions for gate sealing time, although improved, still have a sensitivity to gate thickness lower than that shown by the experiments. Similar conclusions, obviously, regard also the mass entering in the mould during holding. Another mechanism which sums up to shear stresses to accelerate thin gate solidification has still to be identified.