ANALYSIS OF THE PROCESS-INDUCED MICROSTRUCTURE IN INJECTION MOLDING OF LONG GLASS FIBER-REINFORCED THERMOPLASTICS

摘要

Over all stages in the injection molding process of long fiber-reinforced thermoplastic (LFT) materials, the configuration of the fibers changes, which ultimately affects the mechanical performance of the finished part. This article presents a full microstructure analysis of an injection molded part made out of polypropylene reinforced with 40% by weight of glass fibers. The analysis takes into account local measurements of fiber orientation, fiber length, and fiber density distributions by applying sophisticated measurement techniques, such as micro- computed tomography. The results of this work show that the assumption of a uniform fiber length and fiber density throughout the entire part is not valid. The number (weight) average fiber length increases from 0.64 mm (1.63 mm) close to the gate to 1.12 mm (2.81 mm) at the end of the flow path. Similarly, the fiber density varies along the flow path from 37.7 wt% in the gate region to 44.6 wt% at the end of flow. Moreover, the fiber density measurements across the part thickness show a significant fiber agglomeration in the core of the part that consistently suggests almost 30% more fibers in the core layer than in the shell regions.