Mechanical anisotropy in polymer composites produced by material extrusion additive manufacturing

Extrusion-based additive manufacturing technologies, such as direct ink writing of filled polymer resins, have shown a great potential for the development of printed components with superior structural and functional properties. However, the associated extrusion process induces preferred orientation on high-aspect-ratio filler materials as they extrude through the deposition nozzle, causing strong mechanical anisotropy in printed components. Printing-induced anisotropy is a critical issue that complicates the straightforward design of additively manufactured components. The goal of this work is to gain a better understanding of the anisotropy in printed polymer composites by investigating the effects of filler morphology and print parameters on the mechanical properties of printed composites. Inks are formulated using fumed silica particles or nanoclay platelets as the primary viscosifying agent, and silicon carbide (SiC) whiskers as the primary mechanical reinforcement. Mechanical anisotropy is characterized via 3pt-flexural tests for epoxy ink formulations utilizing fumed silica or nanoclay, with or without SiC whiskers, and printed at three different print speeds, using three different nozzle sizes. Orientation of nanoclay is also characterized using small- and wide-angle x-ray scattering. Results show that smaller nozzle diameters and higher deposition rates lead to greater anisotropy when nanoclay or SiC fillers are utilized, while the use of fumed silica alone results in mechanical behavior that is independent of print parameters and print path. Superior flexure strength values up to 215 MPa are obtained with SiC whisker-reinforced composites when tested parallel to the print direction.