Vibration-assisted injection molding: an efficient process for enhanced crystallinity development and mechanical characteristics for poly lactic acid

Injection molding of semi-crystalline poly lactic acid (PLA) parts requires a long cycle time and annealing as a post-fabrication technique. Crystallinity development during injection molding processing is affected by mold temperature, cooling time, and shear rates. In this study, the evolution of crystalline morphology and structure of PLA during vibration-assisted injection molding (VAIM) processing was studied. The VAIM process imparts controlled shear rates on the polymer melt during the injection stage via an oscillatory motion of the injection screw. Vibration-assisted injection molding reduced the cycle time for PLA fabrication by 40%. The overall crystallization was observed to increase from 42% for conventional injection molding to 52 ~ 60% for VAIM under identical conditions. Additionally, VAIM promoted the formation of the more stable α phase of PLA although the disordered α′ phase was the primary phase in all of the samples. Mechanical testing showed that PLA samples fabricated with VAIM had higher ultimate tensile strength which is related to the higher degree of crystallinity and the enhanced formation of α phase crystalline domains.