Anisotropic shrinkage of injection molded poly vinylidene fluoride samples

A study of as-molded shrinkage of Poly Vinylidene Fluoride (PVDF) injection molded samples was carried out in this work. The polymer was injected into a simple rectangular cavity under different holding pressures and the dimensions of the resulting samples were accurately measured at room temperature 10 min after demolding. As expected, the relative difference between the sample dimensions and the corresponding cavity dimensions (namely the shrinkage) decreased on increasing holding pressure. Furthermore, the shrinkage increased on increasing distance from the injection point. A somewhat unexpected feature of experimental data was the fact that the shrinkage was much higher along the flow direction than along the transverse on-plane direction. A thorough characterization of the elastic properties of the solid samples also showed a marked anisotropy of the elastic modulus and of the coefficient of linear thermal expansion. The injection molding tests were simulated by a software developed at University of Salerno to obtain the evolution of temperature, pressure, and crystallinity inside the samples. The predicted pressure profiles were satisfactorily compared with the measured values. The simulated histories of temperature, pressure and crystallinity were used as input for a thermomechanical model for shrinkage evolution. It was found that the reason for anisotropy in shrinkage had to be ascribed mainly to the anisotropy in material properties. It was in fact shown that, adopting the correct anisotropic values for the elastic modulus and thermal expansion coefficient, a satisfactorily description of shrinkage data could be reached.