Analyzing Annealing Effects on Residual Stresses in Injection Molded Microstructures

Abstract

Annealing, a critical heat treatment process, is known for its effectiveness in reducing residual stresses in parts affected by temperature and pressure fluctuations during injection molding. This study explores the influence of annealing on residual stress distribution inmicrostructured parts made of three materials—polycarbonate (PC), polymethyl methacrylate (PMMA), and polystyrene (PS)—and featuring two depth-to-width ratios of microprismatic structures. This investigation combines molecular dynamics simulation with annealing experiments. Findings demonstrate that annealing significantly reduces residual stresses across all three materials, albeit with varying degrees of effectiveness. This variation in annealing efficacy is linked to the materials’ properties, particularly the presence of a carbonate base in PC, which notably enhances its annealing response. In detail, for PC parts with a 1:1 depth-to-width ratio in micropillar structures, there was a 15.45% reduction in peak residual stress, while parts with a 2:1 ratio experienced a more substantial reduction of 41.56%. The study attributes these differences in annealing outcomes to the depth-to-width ratios of the microstructures, with larger cavity sizes allowing for more effective molecular chain stretching.