Effects of Thermal Annealing on the Flexural Properties of 3D-Printed Polyetherimide (PEI) and Carbon Black (CB)-Reinforced PEI Composites

This study investigates the effect of thermal annealing on the flexural behavior of 3D-printed neat Polyetherimide (PEI) and 5 wt% Carbon Black (CB)-reinforced PEI composites manufactured via Material Extrusion (MEX). Although MEX offers substantial design flexibility, it often leads to reduced mechanical performance due to weak interlayer bonding and residual thermal stresses. To mitigate these limitations, annealing was employed as a post-processing method to promote polymer chain diffusion and structural stabilization. Neat and CB-reinforced PEI filaments were extruded and printed into ASTM D790–17 standard specimens. The samples were subjected to 18 distinct controlled annealing cycles and cooled at room temperature. Flexural properties were evaluated using a Dynamic Mechanical Analysis (DMA) instrument under strain-controlled conditions at 30 °C, 70 °C, 110 °C, and 150 °C. The results showed that annealing increased the mass-normalized flexural modulus of neat PEI by 2% to 7% at 30 °C, reaching a maximum improvement of 15.28% at 150 °C. In the CB-reinforced PEI, improvements at 30 °C were limited (0.4–2.2%) due to filler-induced chain pinning, which restricts segmental mobility. In contrast, at 150 °C, significant enhancements were achieved, with the optimized annealing protocol yielding a 23.36% increase, exceeding that of neat PEI. This superior high-temperature performance is attributed to annealing-induced densification, relaxation of residual thermal stresses, and enhanced filler–matrix interfacial load transfer, which collectively counteract matrix softening. The findings demonstrate the critical role of optimized thermal post-processing in extending the service temperature range of MEX-printed high-performance thermoplastic composites.