Heat Treatment Effects on Additively Manufactured Polymer Matrix Composites

Additive manufacturing (AM) of polymer matrix composites (PMCs) enables the fabrication of complex, structurally efficient parts, but the layer-wise thermal history intrinsic to processes such as material extrusion, vat photopolymerization, powder bed fusion, and direct ink writing leaves as-printed components in a metastable state: sub-equilibrium crystallinity or incomplete cure, interlayer porosity, weak interlaminar bonding, and locked-in residual stress. Post-fabrication heat treatments have positioned themselves as an attractive and straightforward strategy to address these deficiencies; however, the reported outcomes vary widely across material systems and treatment protocols. This review consolidates and analyzes the existing literature on heat treatment effects in AM-PMCs by organizing the different reported strategies into a taxonomy encompassing free and constrained annealing, pressure-assisted consolidation (including hot isostatic pressing and hot pressing), and post-curing processes for thermoset and photopolymers. The comprehensive literature review allowed identification of four coupled microstructural mechanisms, which govern the resulting property changes after the heat treatment: crystallization, interlayer/interface healing, porosity evolution, and residual stress relaxation. Post-treatment mechanical testing revealed that interlaminar shear strength exhibits the largest and most consistent improvement, while tensile and flexural performance respond differently depending on material systems and treatment conditions. Thermal and dimensional results are shown to be mechanistically coupled between heat deflection maximization temperature and shrinkage or warping minimization. Reinforcement type and matrix chemistry play a substantial role in modulating the treatment response, as continuous-fiber and carbon-fiber systems generally benefit most from pressure-assisted treatment. The present review concludes by identifying persistent gaps in protocol standardization, interlaminar or fatigue characterization, and reinforcement-treatment coupling studies, while outlining directions, including in situ consolidation and data-driven process-property modeling for future work.