Z-Pinning approach for 3D printing mechanically isotropic materials

Conventional 3D printing approaches are restricted to building up material in a layer-by-layer format, which is more appropriately considered 2.5-D printing. The layered structure inherently results in significant mechanical anisotropy in printed parts, causing the tensile strength in the build direction (z-axis) to be only a fraction of the in-plane strength - a decrease of 50-75% is common. In this study, a novel z-pinning approach is described that allows continuous material to be deposited across multiple layers within the volume of the part. The z-pinning process is demonstrated using a Fused Filament Fabrication (FFF) printer for polylactic acid (PLA) and carbon fiber reinforced PLA. For both materials, z-pinning increased the tensile strength and toughness in the z-direction by more than a factor of 3.5. Direct comparisons to tensile strength in the x-axis showed a significant decrease in mechanical anisotropy as the volume of the pin was increased relative to the void in the rectilinear grid structure. In fact, the PLA sample with the largest pin volume demonstrated mechanically isotropic properties within the statistical uncertainty of the tests. Tensile test results were also analyzed relative to the functional area resisting deformation for each sample.