Quality Monitoring of Micro-shrinkage Defects in Thick-walled Injection Molded Components

Abstract

Sink mark defects are commonly observed in thick-walled injection molded components as a result of under-packing in the hot-spot areas and an insufficient compensation of the polymer melt as the plastic begins to cool. These defects degrade the surface quality of the molded parts and are a reliable indicator of excessive volume shrinkage during the molding process. Consequently, effective methods for quantifying sink marks in thick-walled components, and predicting the occurrence of such defects as a function of the injection molding process parameters, are of significant interest in improving the molding quality. Accordingly, taking an injection molded plastic handle with an average thickness of 11.57 mm for illustration purposes, the present study proposes a method for measuring the micro-shrinkage (ΔΩ) of thick-walled injection molded components using a simple image-processing technique. Simulation and experimental trials are performed to investigate the effects of the injection molding parameters on the quality of the polymer melt (as evaluated using five quality indexes (PA, PB, ηA, ηB, and CFi,p) extracted from the pressure profiles within the chamber and the tie-bar elongation, respectively) and the quality of the final molded component (as quantified by two metrics, namely the weight variation of the molded parts and the micro-shrinkage). It is shown that the quality of the polymer melt and final thick-walled component is determined mainly by the magnitude of the holding pressure applied in the molding process. By contrast, the injection speed, V/P switchover point, and barrel temperature exert only minor effects. The experimental results obtained under various values of the holding pressure show that the five injection molding process quality indexes are all highly correlated with the micro-shrinkage. The correlation is particularly strong for the pressure-based quality indexes (PA, PB, ηA, and ηB). Hence, it is suggested that these indexes provide a viable means of predicting the formation of sink mark defects and excessive micro-shrinkage in thick-walled injection molded components.