Solid epoxy for functional 3D printing with isotropic mechanical properties by material extrusion

Material extrusion is the most commonly used additive manufacturing process. However, currently it is mainly applied for the production of prototypes or simple jigs and fixtures due to issues with anisotropic material behavior. Diffusion and entanglement of the polymer chains is limited between the layers and among the infill lines within a layer of the usually thermoplastic material as it solidifies immediately after deposition. This results in weak bonding leading to a reduced load-bearing capacity. In this study, a thermosetting polymer is developed and presented. It enables cross-linking over the interfaces between the infill lines and layers during post-curing which resolves this issue. The formulation is based on a solid epoxy resin, allowing it to be processed in filament form and a latent curing agent preventing it from curing within the hotend and ensuring a suitable shelf life. To benchmark the newly developed material it is compared to casted and milled bulk specimens and 3D printed specimens with commercial thermoplastic filaments. Tensile tests and micrographs of the fracture surfaces prove the mechanical isotropy of the solid epoxy formulation. In addition, the material formulation is modified with single-walled carbon nanotubes to add electrical conductivity and allow functional 3D printing. Due to the high aspect ratio of the nanoparticles, a significantly lower filler content is necessary compared to the commercial materials. However, an electrical anisotropy is still observed as the material remains in a solid state during post-curing to retain its shape which limits the mobility of the nanoparticles and suppresses the agglomeration needed for conductive network formation after thorough dispersion. Proof of concept studies show that the functionalized material can be used in temperature and strain sensing applications.