Influence of the injection molding thermal boundary conditions on the filling flow of PET

Abstract

During the filling phase of the injection molding process, the polymer in contact with the wall solidifies, developing the so-called skin layer. Although the no-flow temperature is often exploited in numerical simulations to model the skin layer formation mechanism, there is still an incomplete understanding of what happens for semi-crystalline PET. In fact, for this polymer, the high shear stresses near the cavity wall combined with the thermal gradient at the polymer-mold interface can cause the so-called flow-induced crystallization. This work aims to experimentally investigate the skin layer formation at different injection molding processing conditions. A full-factorial experimental plan was performed using a commercial PET injected into an open-cavity mold. The thermal and mechanical boundary conditions at the polymer-wall interface were varied using insulating coatings and different flow rates, respectively. The mold coatings were applied to different steel substrates to consider the substrate thermal conductivity contribution. Different flow rates were imposed, varying the cavity thickness and the injection speed. The in-line acquisition of the cavity pressure allows for assessing the contributions of the modified boundary conditions to the solidified skin layer formation. The results show that the insulating mold coating changes the thermal boundary conditions, leading to a maximum pressure decrease of 14 %. The cavity pressure vs. filling time curves for the uncoated inserts show an evident derivative discontinuity, suggesting the onset of flow-induced crystallization.