One of the following symptoms is sometimes observed: – The thickness suddenly increases – The sheet folds – During inflation, a sudden snap-back is observed and so on, the above list not being exhaustive. When using the KBKZ model for blow molding and thermoforming, it is good to remember that the extra-stress tensor components are evaluated by means of a decoupled and explicit scheme. This may cause possible numerical instabilities. For example, this can lead to an overestimation of the stress build-up in the material leading to a possible snap-back. Sometimes, the properties in the model are weakening. The following remedies can be considered: – Remove the contribution of the largest relaxation time as it usually has not the time to develop – Replace the contribution of the smallest relaxation time with a Newtonian counterpart since it usually develops quite fast and may actually be characterized by a time constant smaller than the time step used for the time-marching scheme.