The Impact Behavior of Weld-Lines in Injection Molding

Abstract

A box-like part was injection molded in a propylene copolymer with systematic variations of the processing conditions (melt and mold temperature, injection flow rate and holding pressure), based on a design of experiments approach (L9 Taguchi orthogonal array). Due to the gating options (hot runner with two gate points), the major box surface shows a central weld-line, originated by the top-to-top meeting of the two flow fronts. This work studies the influence of the processing conditions on the impact behavior of the molded plates at two locations: in and away from the weld-line. The local processing thermomechanical environments at both locations were computed from mold filling simulations (thermal and stress levels), and characterized by thermomechanical indices (aiming at interpreting locally the microstructure development). The falling weight impact tests were performed at 1 m/s with a lubricated striker. The impact behavior was characterized by the peak force and energy and type of failure (brittle and ductile). The experimental results are analyzed by analysis of variance (ANOVA) statistical tool. The dependences of the impact response upon the thermomechanical indices at the two locations are compared. All the considered processing variables are significant in determining the impact response of the moldings. The results also show that the impact response in and away from the weld-line shows distinct dependences upon the processing variables, and cannot be maximized simultaneously. Furthermore, the effect of the local thermomechanical environment on the impact properties is different according to the type of failure (brittle or ductile) occurring.