As the applications for additive manufacturing have continued to grow, so too has the range of available materials, with more functional or better performing materials constantly under development. This work characterizes a copper-filled polyamide 6 (PA6) thermoplastic composite designed to enhance the thermal conductivity of fused filament fabrication (FFF) parts, especially for heat transfer applications. The composite was mixed and extruded into filament using twin screw extrusion. Because the fiber orientation within the material governs the thermal conductivity of the material, the orientation was measured in the filament, through the nozzle, and in printed parts using micro-computed tomography. The thermal conductivity of the material was measured and achieved 4.95, 2.38, and 0.75 W/(m·K) at 70 °C in the inflow, crossflow, and thickness directions, respectively. The implications of this anisotropy are discussed using the example of an air-to-water crossflow heat exchanger. The lower conductivity in the crossflow direction reduces thermal performance due to the orientation in thin-walled parts.

Long fiber-reinforced thermoplastics (LFTs) are an attractive design option for many engineering applications due to their excellent mechanical properties and processability. When processing these materials, the length of the fibers inevitably decreases, which ultimately affects the mechanical performance of the finished part. Since none of the existing modeling techniques can accurately predict fiber damage of LFTs during injection molding, a new phenomenological approach for modeling fiber attrition is presented. First, multiple controlled studies employing a Couette rheometer are performed to determine correlations between processing conditions, material properties, and fiber length reduction. The results show shear stress and fiber concentration impact fiber damage. Based on these findings, a phenomenological model to predict breaking rate and unbreakable length of a fiber under giving conditions is developed. The model is based on the beam theory with distributed hydrodynamic stresses acting on a fiber. Fiber–fiber interactions are accounted for and correlated with the fiber volume fraction via a fitting parameter. The model tracks both the number-average and weight-average fiber length during processing, which can in turn be used to extract the fiber length distribution.