Polyurethanes are synthetic materials or resins covering a wide range of stiffness, hardness and densities. They are used as low density foam e.g. in upholstery fabrics and refrigerator insulation and as soft or hard plastic parts ranging from shoes to skateboard wheels to car dashboards to vibration and shock absorbers, to name just a few examples.
In most industrial production processes liquid polyurethane is injected into a closed mold where it reacts and expands to finally fill out the mold. In this process air bubbles might be entrapped or a structural failure of the hardened but still expanding plastic might occur, resulting in defects, instabilities or unattractive surfaces.
Figure 1: Foaming polyurethane
The simulation of the chemical reaction and foaming process by a viscoelastic fluid helps to formulate and verify material models for the different polyurethanes, to analyze the forming of air traps and to optimize the production process by varying the position of the injection vents or other parameters without the need of expensive prototypes.
The meshless CFD simulation software NOGRID points abilities to efficiently handle fast varying fluid surfaces in complex geometries with rapidly changing material properties make it a supreme tool for such simulations.
In figure 1 the foaming process in an open can, perforated by a small cylinder, is shown. By symmetry only a quarter of the model needs to be simulated. One can see that at the end the mold overflows and forms the typical shape before the reaction finally stops.