Salt-Supported Thermal Annealing of FDM Printed PLA and Effects on Tensile and Impact Properties

The mechanical performance of parts manufactured using fused deposition modeling (FDM) is often limited by weak interlayer bonding, residual stresses, and the predominantly amorphous structure of rapidly cooled polymers. This study investigates a simple and low cost post processing method aimed at improving the mechanical properties of FDM printed PLA components without the use of reinforcing additives or composite materials. Based on a review of commonly used printing materials and processing parameters, a thermal post treatment procedure employing compacted table salt as a supporting medium was developed. The salt bed prevented geometric distortion during annealing while enabling controlled thermal exposure above the glass transition temperature of PLA. The proposed process was applied to specimens manufactured in both vertical and horizontal build orientations and subsequently evaluated using static tensile testing and Charpy impact testing. The experimental results showed that salt supported annealing had only a minor effect on tensile properties, while producing a clear improvement in impact resistance. The average tensile strength of vertically printed specimens remained essentially unchanged (30.05 MPa → 30.01 MPa), and horizontally printed specimens exhibited a modest increase of approximately 3% (53.31 MPa → 54.93 MPa). Elongation at break decreased slightly in both orientations. In contrast, Charpy impact strength increased by 28.9% for vertically printed specimens (5.23 → 6.74 kJ/m²) and by 224% for horizontally printed specimens (5.30 → 17.19 kJ/m²). FTIR analysis confirmed that the thermal treatment did not alter the chemical structure of the PLA. The observed improvement in impact performance is therefore attributed primarily to physical changes, most likely including residual stress relaxation, possible improvement of interlayer bonding, and reduced moisture content. An increase in crystallinity is also possible, but was not directly verified in this study.