Vacuum and Non-Vacuum Hydrothermal Annealing as a Post-Processing of MEX PLA on Tensile Response and Fracture Characteristics

Material extrusion (MEX) components remain limited by incomplete inter-road bonding and process-induced voids, which govern both load transfer and final fracture. This study investigated whether vacuum pretreatment can intensify the effect of short-term hydrothermal annealing on the tensile response and fracture characteristics of MEX-manufactured PLA. Ninety treated specimens were distributed among 30 treatment cells, comprising 15 temperature–time combinations for each pretreatment condition, with three specimens processed simultaneously within each treatment cell. Hydrothermal annealing was performed at 50–90 °C for 1–3 min. Fracture morphology was examined by digital optical microscopy and scanning electron microscopy (SEM). Tensile properties were evaluated according to ISO 527 and analyzed using general linear models (GLMs) with Tukey-adjusted comparisons. Vacuum pretreatment increased the estimated marginal mean maximum tensile stress from 61.8 MPa to 67.3 MPa (+5.53 MPa; 95% CI: 4.19–6.86 MPa; p < 0.001) and elongation at maximum load from 3.95 mm to 4.38 mm (+0.426 mm; 95% CI: 0.332–0.519 mm; p < 0.001). Temperature significantly affected maximum tensile stress (p = 0.002) and elongation (p = 0.001), with significant processing-route-by-temperature interactions for both responses (p = 0.014 and p = 0.027, respectively). Annealing time affected neither response significantly, while stress at break was not significantly affected by processing route (p = 0.194). Fractography revealed persistent inter-road voids and mixed interfacial–cohesive failure. Vacuum-pretreated specimens exhibited more compact central fracture regions and altered crack localization, although process-induced voids remained preferential sites of final failure. The results demonstrate that vacuum pretreatment was primarily associated with load transfer and deformation before maximum loading, whereas resistance to final fracture remains controlled by defects.