Thermal Annealing of Injection‐Molded PLA Grades: Trade‐Offs Between Crystallinity Development, Dimensional Stability, Secondary Shrinkage, and Mechanical Performance

Injection molding is widely used for polymer components, yet producing dimensionally accurate poly(lactic acid) (PLA) parts remains challenging because shrinkage and warpage arise from thermal contraction, crystallization‐induced densification, and relaxation of frozen‐in amorphous structure. Post‐molding annealing can increase crystallinity and heat resistance, but may also induce secondary shrinkage and embrittlement. Here, two commercial injection‐molding PLA grades with different crystallization behavior were processed under identical molding and packing conditions, while only the annealing protocol was varied between 60°C and 120°C. DSC showed different crystallization tendencies after thermal‐history erasure, with crystallinity of approximately 3% for PLA_a and 20% for PLA_c, although as‐molded specimens showed similar crystallinity of 14%–17%. Isothermal DSC confirmed faster crystallization of PLA_c, with half‐times 3–5 times shorter than PLA_a at 100°C and 120°C. Annealing increased crystallinity to approximately 68% for PLA_c and 50% for PLA_a after 3 h at 120°C. However, PLA_a showed stronger secondary shrinkage, reaching 2%–2.3%, whereas PLA_c remained below 1%. Enthalpy‐recovery analysis, POM, SEM, MFI, TGA, and FTIR demonstrate that annealing produces a grade‐dependent trade‐off between crystallinity, dimensional accuracy, and mechanical performance. Impact strength peaks at 90°C, whereas tensile performance decreases at 120°C, confirming crystallinity alone cannot predict dimensional stability reliably.