Rheology scaling of spherical metal powders dispersed in thermoplastics and its correlation to the extrudability of filaments for 3D printing

3D printing metals via material extrusion utilizes a metal particle reinforced polymer matrix composite (PMC) as the filament which is typically made with gas-atomized powders as fillers. Its rheological behavior limits the maximum metal content of the printed green composite which, consequently, hinders further reductions to part porosity and shrinkage. In this paper, the scaling of the dynamic viscosity of melt-extruded PMC filaments made of PLA and Ni-Cu gas-atomized powders is studied as a function of the metal's volumetric content and feedstock pre-mixing strategies and correlated to its extrudability performance. Extrudable and uniform filaments with the highest metal content of 63.4 vol% were produced by employing solution-mixing of the PMC feedstock and compared to physical mixing which only reached 54 vol%. After sintering, the improved metal content of 3D printed parts from solution-mixing reduced linear shrinkage by 76% in comparison to physical mixing, resulting in an absolute shrinkage value of 0.49%. By characterizing the PMC feedstock via flow-sweep rheology tests, a distinct extension of the shear-thinning zone towards high shear rates (i.e. 100 s⁻¹) at high metal content was observed for the case of solution-mixed feedstock – a result that is attributed to the improved adhesion of the PMCs to the walls of the rheometer. PMCs with such characteristics correlated well to favorable extrudability and windability test outcomes (i.e. no particle jamming or metal accumulation at the die). The Krieger-Dougherty analytical model was employed to predict the zero-shear rate viscosity as a function of the metal content in the PMCs, however, the latter property is not necessarily correlated to the viscosity at high shear rates (10–1000 s⁻¹) due to a complex shear thinning response. Since PMCs experience high shear rates in the die orifice, additional theoretical models are needed to predict shear-thinning at high metal content in PMCs and, thus, improve our prediction of PMC's rheology, its design, and extrudability to maximize its metal content.