Investigation of thermal evolution and fluid flow in the hot-end of a material extrusion 3D Printer using melting model

Abstract

This work presents Computational Fluid Dynamics (CFD) simulations of the thermal evolution and fluid flow in the hot-end during material extrusion in fuse filament fabrication (FFF) additive manufacturing. The hot-end is the most fundamental and complex part of a FFF 3D printer. The fluid dynamics during material extrusion depend on the hot-end design while the printing speed depends on the speed at which the hot-end can melt the filament. In this work, a new melting model developed specifically for the FFF process is introduced. The CFD melting model incorporated the liquid fraction evolution to account for the presence of an air gap between the liquefier wall and the solid filament. For the first time, the thermal contact resistance was implemented using a melting model. The governing transport equations for the melting model were presented both in the dimensional primitive and non-dimensional primitive representations to understand the relevance of each term in a physical sense. The model equations were implemented with the aid of written user-defined function (UDF) codes coupled in the ANSYS Fluent. The grids were refined systematically to ensure that the influence of the mesh size on the computational results is minimized. The Richardson extrapolation technique was used to estimate the grid convergence index (GCI) and to quantify discretization error. The maximum discretization error is found to be 1.68%, which is an indication that the solution is in the asymptotic range. The thermal boundary conditions were implemented using simple liquid fraction-heat transfer coefficient relation and the results were validated using previously measured feeding force by Serdeczny et al. [1]. The predicted feeding force agrees reasonably with the measured feeding force. The model predicted a strong counter-clockwise recirculating vortex close to the barrel inlet. The further simulation revealed that the filament did not melt immediately at entry into the heated barrel, and variation in the barrel length indicates that the solid penetration depth ratio increases with an increase in the P é clet-number ( Pe ). Since high-speed 3D printing depends on the flow rate, an optimized thermal dissipation is desired in the hot-end. Our approach in presenting the model equations and subsequent results obtained should advance the understanding of the flow characteristics in the liquefier hot-end and thermal history within the barrel and the nozzle exit. Hence, the presented melt flow dynamics model would help in developing hot-end designs for 3D printers capable of melting filaments at an improved rate, by reducing the barrel length and printing at a higher liquefier temperature, while preheating the solid filament. • A new CFD melting model developed specifically for the FFF process is introduced. • Thermal contact resistance was implemented using the CFD melting model. • Effect of the air gap between the filament and the liquefier wall was implemented. • The true position of the melting front at the liquefier center was identified. • CFD Grid Sensitivity Analysis reveals solution is within the asymptotic limits.