A study via simulation of solidification during injection molding

Abstract

The final properties of an injection-molded part are strictly dependent on the solidification dynamics occurring during the processing. The experimental derivation of information about polymer solidification throughout the injection molding cycle is still an open challenge. To overcome the practical issues, the process simulation is proposed as a mean to derive useful data, and especially for identifying the most influencing parameters. The no-flow temperature (NFT) is a parameter used in most of injection molding simulation packages as a mean to determine whether the polymer flows or it is solid. With this simple parameter it is possible to take into account the rheological solidification of the polymer, i.e. the high viscosity of the melt hinders the flow. However, for semicrystalline polymers, a better knowledge on the crystallization kinetics, especially at high cooling rates, is needed to achieve a comprehensive picture of the process. A sensitivity analysis regarding the filling stage was carried out on two different packages (VISI Flow and Moldflow) to estimate the influences of NFT on process performance, as the frozen layer fraction and the filling pressure. The NFT resulted very influent on frozen layer, while does not affect appreciably the filling pressure. These results suggest an influence on the shrinkage and warpage estimation. A simple correlation for NFT estimation, derived from Cross-WLF equation, is proposed for both amorphous and semicrystalline polymers. The new parameter proposed considers the variation in the apparent zero shear viscosity of the polymer on temperature, thus allowing to take into account both polymer properties and cooling rate.