3D printing (additive manufacturing) has become a popular method to create three-dimensional objects due to its high efficiency and is easy to operate. 3D printing of continuous fiber reinforced polymers has been a challenge. The fused deposition modeling (FDM) processes for this purpose were proposed and made possible only several years ago. The 3D printed continuous fiber reinforced polymers are able to improve the mechanical properties by leaps and bounds. In this paper, we aim to investigate the possibility of further improve the mechanical properties of 3D printed continuous fiber reinforced polymers by adding nano fillers to the polymer matrix. In experiment, the Kevlar fiber is chosen to be the continuous fiber material, and nylon 6 (PA 6) is chosen to be the polymer matrix material. Carbon nanotubes (CNTs) and graphene nano platelets (GNPs) nanoparticles are first mixed with nylon 6 pellets to make nanocomposites. The nanocomposites are then extruded into filaments for 3D printing. During the 3D printing process, both Kevlar filament and nanocomposite filament are fed through the printing nozzle and deposited on the platform. Tensile specimens are made from pure PA 6 and four types of nanocomposites, namely, 0.1wt% CNT/PA 6, 1wt% CNT/PA 6, 0.1wt% GNP/PA 6, 1wt% GNP/PA 6. By incorporating four layers of Kevlar fiber, which leads to the weight percentage of about 9% for Kevlar fiber in materials, fiber composite tensile specimens are made from Kevlar/PA 6 composite and four fiber reinforced nanocomposites, namely, Kevlar/0.1%CNT/PA 6, Kevlar/1%CNT/PA 6, Kevlar/0.1%GNP/PA 6, and Kevlar/1%GNP/PA 6. The tensile tests reveal that CNTs filled PA 6 nanocomposites show less significant improvements in mechanical properties as compared to the GNP filled PA 6. With only 0.1wt% of GNP, the tensile modulus improves by 101%, and with 1wt% of GNP, the modulus improves by 153%. The results also indicate that although Kevlar fibers dominate the main mechanical properties of the printed composite materials, the existence of GNP nano fillers also provide noticeable contribution to the enhancement of tensile strengths and moduli, while the effect of CNTs is much less pronounced.

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