Effects of Annealing on the Thermomechanical Properties and Crystallization Behavior of FDM ‐Printed High‐Temperature PLA and Glass Fiber PLA Composites

Polylactic acid (PLA) combines renewable sourcing and reliable printability with limited stiffness retention near its glass‐transition region. This study examines how annealing at 90°C, 110°C, 130°C, and 150°C for 30 min affects FDM‐printed high‐temperature PLA (HT‐PLA) and 12 wt% glass‐fiber‐reinforced HT‐PLA (HT‐PLA‐GF). Differential scanning calorimetry, thermogravimetric analysis, Fourier transform infrared spectroscopy, orientation‐dependent mechanical tests, tensile‐fracture SEM, and three‐point‐bending dynamic mechanical analysis were evaluated together. DSC calculations showed that the homocrystalline contribution of neat HT‐PLA increased from 26.14% as printed to 50.33% at 110°C, whereas HT‐PLA‐GF approached a plateau near 38% between 110°C and 150°C. At 150°C, longitudinal tensile strength decreased by 25.1% for HT‐PLA and 40.4% for HT‐PLA‐GF, while transverse flexural strength decreased by 37.0% and 49.5%, respectively. SEM revealed layer‐related discontinuities in transverse neat HT‐PLA and a mixed composite failure morphology involving fiber pull‐out, fiber fracture, sockets, and matrix tearing. Annealing nevertheless improved elevated‐temperature stiffness retention. The DMA‐derived apparent HDT‐1.80 MPa increased from 65.7°C to 85.7°C for neat HT‐PLA and from 66.9°C to 114.6°C for HT‐PLA‐GF. The results demonstrate that improved thermal stiffness does not imply improved failure strength. Within the investigated conditions, 90°C provided the most balanced response, whereas 150°C maximized apparent heat‐deflection resistance at the cost of substantial tensile and flexural penalties.