Effect of infill density and re-carbon black loading on the electrical and mechanical properties of epoxy-filled 3D printed structures

Abstract

This study investigates the additive effect of recycled carbon black (rCB) loading from 0 to 3 wt% and infill densities of 50%, 70% and 90% on the multifunctional performance of hybrid polymer composites. Polylactic Acid (PLA) samples were produced using Fused Deposition Modeling (FDM) with diverse interior geometries and subsequently infilled with an insulating epoxy resin to strengthen structural integrity. Mechanical evaluation using tensile and Vickers microhardness testing indicates that a 90% infill density offers the most effective structural framework, with a 3 wt% rCB loading resulting in the highest ultimate tensile strength (UTS) of 21.9 MPa. Significantly, although mechanical strength was generally enhanced with increased infill, 100% dense structures displayed reduced efficiency attributable to thermal stress and insufficient layer adhesion. Electrical characterization via impedance spectroscopy reveals a non-monotonic behavior; while 2 wt% rCB approaches the percolation threshold for conductive pathways, a "dielectrically blocking" phenomenon arises at 3 wt% rCB, where resistance reaches a maximum of 1.0×10^11 Ω due to particle agglomeration. The study indicates that 2 wt% rCB yields a significant 104.6% enhancement in ductility compared to pure PLA at 50% infill. These findings define a method for the development of cost-effective, high-strength smart materials appropriate for sensing applications.