Reaching injection molded strengths in high performance plastics via reinforced multi-material 3D printing and annealing

Material extrusion additive manufacturing is often only viewed as a sophisticated prototyping technique due to its poor mechanical performance, especially in the vertical “z”-direction. This deficiency limits the use of high-performance plastics in 3D printing. While recent attempts have been made to increase the tensile strength of printed polyetherimide (PEI) via thermal annealing or radiative heating, the mechanical strength ceases to reach injection molded strengths, likely due to the short durations of annealing possible without severe deformation. The present work aims to enhance the mechanical performance of 3D printed parts, printed in the upright z-direction, up to injection molded strength via dual material filament printing and annealing. We fabricated a core/shell filament composed of a high T_g PEI (T_g: 217 °C) asterisk-shaped core surrounded by low T_g PEI (T_g: 181 °C) shell. The parts fabricated thus consist of a high T_g material scaffolding, which allows structural stability during thermal annealing at high temperatures for longer durations. With optimized thermal annealing at 215 °C for 24 hours, the vertically printed parts showed an increase in tensile strength from 40.58 MPa to 81.9 MPa or 98.6% of the bulk tensile strength of the shell ULTEM material, without noticeable deformation. This phenomenon opens the possibility of fabricating high-performance functional parts via 3D printing with exceptional freedom of design and low start-up costs, without sacrificing the strength of the material.