Material-Dependent Effect of Heat Treatment on the Low-Velocity Impact Response of Additively Manufactured PA6 and PA6-CF Gyroid-Core Sandwich Structures

Sandwich structures with gyroid cores produced via additive manufacturing show promise for lightweight impact-absorbing applications due to the stable plateau response provided by their continuous surface geometries; however, the effect of post-printing thermal history on the impact behavior of these structures has not been sufficiently investigated. In this study, three sandwich configurations were produced with 2 mm face sheets, a 10 mm core thickness, and a 10 mm gyroid cell size: all-PA6, all-PA6-CF, and a hybrid structure consisting of PA6-CF face sheets and a PA6 core. Non-heat-treated (Non-HT) and heat-treated (HT, 170 °C for 4 h) specimens were subjected to low-velocity impact at energy levels of 5, 10, 15, 20, and 25 J; peak force, absorbed energy, energy absorption efficiency (the ratio of absorbed energy to impact energy), and specific energy absorption (SEA) were compared. The effect of heat treatment depended on the material configuration and impact energy. DSC analyses showed that heat treatment increased the degree of crystallinity in PA6-CF from 6.4% to 11.4%, while in PA6, it increased only from 6.2% to 6.7%. In the PA6-CF configuration, heat treatment reduced the peak force by 28% and 34% at 15 and 20 J, respectively (8.41→6.04 kN; 8.20→5.42 kN), while increasing the absorbed energy from 17.5 J to 19.8 J at 20 J and giving the highest SEA of 0.243 J/g at 25 J. In the unreinforced PA6 configuration, heat treatment reduced the absorbed energy at 25 J from 15.7 J to 11.3 J and the energy absorption efficiency from 63% to 45%. The hybrid configuration exhibited the most stable response, with efficiencies of 92–101% in both cases within the 10–20 J range and limited peak forces in the 2.6–3.4 kN range. The findings indicate that post-printing heat treatment is an effective parameter for adjusting impact performance without changing the geometry or material, but that the direction of its effect depends on the presence of fiber reinforcement and the impact energy.