Molecular orientation in injection molding: experiments and analysis

Abstract

Modeling and simulation of the injection molding process of thermoplastic polymers has remarkably improved over the last decade. One of the most challenging scientific objectives is currently the reliable prediction of molecular orientation simulations of the molding process. Indeed, although pressure and velocity distribution can be satisfactorily described by viscous models, the viscoelastic nature of the polymer needs to be accounted for in the description of molecular orientation evolution. In this work, an amorphous PS was injection molded into a line gate rectangular cavity. Molding tests are carefully characterized and all information needed for further analysis is provided. The molds contained special dies that could accept various sized gates. In particular two gates were used whose thicknesses were 1.5 mm and 0.5 mm, respectively. Birefringence distribution (which for PS is essentially the orientation distribution) along the thickness direction was measured by using the wedge method at different positions in the moldings, and inside the gates. Data regarding the amount of frozen-in molecular strain were gathered by measuring the thermal shrinkage at different positions along the flowpath. Molding tests were simulated by means of a software developed at the University of Salerno, and a simple viscoelastic model was used to describe the evolution of molecular orientation due to the effect of kinematics obtained using a viscous approach. Simulation results describe the main features of experimental data collected from the molded samples; in particular, the effect of the packing flow is clear in both the data and simulations. In addition, the importance of the effect of pressure on relaxation time is discussed.