Analysis of the importance of shear-induced elastic stresses in material extrusion

Abstract

Material extrusion is an additive manufacturing process in which material is selectively dispensed through a nozzle. In this work, we are focused on fused filament fabrication (FFF) and we intend to analyze the melt flow patterns generated by a commercial version of polycarbonate, modeled by means of a viscoelastic model (Giesekus) based on rheological properties experimentally determined under viscometric flows. The axisymmetric flow through the same longitudinal section nozzle geometry used in the work of Mendes et al. (2019) has been numerically simulated in steady state at a constant temperature using OpenFOAM® (O. Ltd, 2019) and rheoTool (Pimenta and Alves, 2016). The results show that although the extensional flow type is predominant in the fluid domain, the shear-induced normal stress differences are relevant, and even dominating, not only at the exit of the die but also in the tapered region of the nozzle. Shear-induced normal stress differences are responsible for the excess pressure drop, and also for the equilibrium height ( H ∗ ) in the backflow region, where the polymer melt flows upwards, between the solid filament that enters into the print-core and the liquefier wall. • In Material Extrusion (MEX), shear-induced elastic stresses are dominant over the extensional stresses, despite the large portion of the volume being dominated by the extensional flow. • Shear-induced elastic stresses are responsible for the die swelling, as expected, but also for the equilibrium height H ∗ in the backflow region, where the polymer melt moves upwards inside the annular region. • The force needed to extrude the material through a nozzle depends on the pressure drop through the nozzle itself and the friction introduced by the polymer melt in the backflow region, the latter also being dominated by shear-induced elastic stresses.