Investigation of Crystal Structure Change and Mechanical Properties of 3D Printed PLA After Annealing

ABSTRACT Fused Deposition Modeling (FDM) is a widely used 3D printing technology, yet the inherent mechanical anisotropy and low crystallinity of printed poly(lactic acid) (PLA) parts limit their application in demanding engineering fields. This study systematically investigates the effects of thermal annealing on the thermal, microstructural, and mechanical properties of FDM‐printed PLA across three specific temperature–time conditions (70°C for 4 h, 80°C for 1 h, and 90°C for 30 min). The structural evolution from a predominantly amorphous state (0.5% crystallinity) to a semi‐crystalline structure was comprehensively evaluated using Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), and X‐ray Diffraction (XRD). These combined techniques confirmed the formation of the metastable α′ crystal polymorph, with DSC and FTIR showing a peak crystallinity of approximately 46.6% at 90°C, while XRD highlighted surface‐specific crystallization behaviors and skin‐core effects. Mechanical testing (tensile, compressive, flexural, and impact) revealed highly direction‐dependent responses. In the longitudinal print direction, the increased crystallinity significantly enhanced all mechanical properties, yielding a maximum tensile strength of 49.5 MPa and flexural strength of 100.4 MPa under the 90°C/30 min condition. Conversely, transverse properties were mostly adversely affected or unchanged, indicating that interlayer bond strength remains the primary limiting factor regardless of bulk crystallization. Scanning Electron Microscopy (SEM) further supported these findings by showing a transition from ductile to brittle fracture modes in highly crystalline samples. Ultimately, while thermal annealing at 90°C for 30 min provides an optimal balance for improving the overall strength and thermal stability of FDM‐printed PLA, the persistent mechanical anisotropy and weak transverse interlayer adhesion remain critical design considerations.