Properties of injection-molded poly (L-co-D,L-lactic acid) using different melt temperatures and stress concentrator in the specimen geometry

This paper investigates the morphology and mechanical properties of poly (L-co-D,L-lactic acid) (PLDLA) specimens injection-molded using different melt temperatures and stress concentrator in the specimen geometry. The values for the tensile strength and ultimate strain increased with a reduction in the melt injection temperature for unnotched specimens. The notched specimens molded using low and high melt injection temperatures showed similar values for the tensile strength compared with the unnotched specimens. The fracture surfaces of the notched and unnotched specimens molded at the low melt temperature showed the characteristic of ductile failure presenting fibrillation and displacement in the oriented skin layer. A restriction in the chain rotation and conformation due to the oriented skin layer explains the less viscous behavior observed in the dynamic mechanical analysis for low injection temperature specimens. The photoelastic analysis indicated a birefringence along the specimens manufactured using the low melt injection temperature, suggesting the existence of residual stress due to filling phase and rapid solidification. On the other hand, specimens injected using the high temperature showed residual stress concentration near the gate due to the packing effect of holding pressure. The enthalpic relaxation peak at the glass transition (T-g) observed in the differential scanning calorimetry analysis confirmed the existence of significant residual stress in all PLDLA specimens, especially when injected using the low temperature.