Ansys Polyflow is an implicit code that uses a predictor-corrector scheme to solve time-dependent problems. The implicit nature of the time-marching scheme has tremendous advantages for process simulations, because most of the process is in a nearly steady-state regime. Blow molding is not an exception to this rule: pure dynamic effects (like inertia) are usually small and the dominant event that occurs over time is the contact.

In the predictor-corrector scheme, Ansys Polyflow first evaluates all variables (position, velocity, temperature, and so on) at the next time step with an explicit extrapolation scheme. In view of the explicit nature of this prediction step, contact is not detected; this would require a large computational effort. Ansys Polyflow then evaluates corrections to the predicted values, using an implicit iterative scheme (that is, solving a nonlinear system). The implicit step builds all finite-element matrices and hence takes contact into account.

The deviation between predicted values and corrected values is used as an error estimate, and determines the next time step. Occurrence of contact cannot be predicted during the prediction step, since Ansys Polyflow does not anticipate the contact. This means that it is not possible to use the difference between the prediction and correction steps to control the time step for a contact detection problem. More precisely, the error evaluation cannot be performed on all variables, and in particular cannot be performed on the velocity field.

Whenever contact occurs, the position of the parison remains unchanged regardless of the inflation pressure. There is a discontinuity in the time derivative of the position, but not in the position of the parison itself. The discontinuity for position variables is rather mild. For the velocity field, however, the discontinuity is much stronger because the value of the velocity suddenly jumps from a nonzero value before contact to a zero value after contact (for a zero wall velocity).

The discontinuity in the velocity field at the occurrence of contact makes use of a predictor-corrector scheme inappropriate. Since the Reynolds number for blow molding simulations is usually small, inertia terms can almost always be neglected with respect to diffusion (viscosity) terms, and the velocity field after contact is influenced very little by the history of the momentum.

Therefore, adjustment of the next value of the time step based on a predictor-corrector scheme should be avoided for all simulations that include contact. When you set up a contact detection problem, Ansys Polydata will automatically disable the prediction for the velocity field, as described in Inputs for 2D and 3D Contact Detection. The option for disabling the prediction is in the Numerical parameters menu.