Robocasting and 3D concrete printing are technologies that belong under the umbrella term material extrusion additive manufacturing. These two free form fabrication methods are used to produce 3D structures/components in materials such as... more
Robocasting and 3D concrete printing are technologies that belong under the umbrella term material extrusion additive manufacturing. These two free form fabrication methods are used to produce 3D structures/components in materials such as ceramic pastes, thermosets, and concrete. Common for the materials is their viscoplastic behavior during deposition and structural buildup (i.e., increase in yield stress) after deposition. The material's complex nature makes it a nontrivial task to ensure that printed layers do not deform when depositing additional layers on top. In this paper, we numerically investigate the influence of the yield stress buildup of viscoplastic materials on the stability of the bottom layer during multilayer printing. Specifically, we have developed a computational fluid dynamics model that applies a scalar approach to alter the yield stress. The novel model provides fundamental knowledge on how to design the material's rheology, so the bottom layer can wi...
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ABSTRACT This paper presents a novel Computational Fluid Dynamics (CFD) model that simulates the non-isothermal flow of a viscoelastic fluid through the hot-end in filament-based Material Extrusion Additive Manufacturing (MEX). The... more
ABSTRACT This paper presents a novel Computational Fluid Dynamics (CFD) model that simulates the non-isothermal flow of a viscoelastic fluid through the hot-end in filament-based Material Extrusion Additive Manufacturing (MEX). The hot-end is an essential part of the printhead, as it melts and extrudes the polymeric material. Thus, robust modelling tools are necessary for optimising its operation. In this study, a viscoelastic CFD model is used to predict the filament feeding force and the results are validated against the experimental measurements at different filament feeding rates, liquefier temperatures, nozzle diameters and liquefier lengths. It is found that the viscoelastic model is more accurate than the commonly used Generalised Newtonian Fluid approximation and it captures the influence of process conditions on the feeding force. Finally, the model is used to optimise the hot-end channel. Among others, it is shown that the feeding force can be minimised by selecting an optimal liquefier diameter.
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Abstract This paper presents computational fluid dynamics simulations of the deposition flow during printing of multiple layers in material extrusion additive manufacturing. The developed model predicts the morphology of the deposited... more
Abstract This paper presents computational fluid dynamics simulations of the deposition flow during printing of multiple layers in material extrusion additive manufacturing. The developed model predicts the morphology of the deposited layers and captures the layer deformations during the printing of viscoplastic materials. The physics is governed by the continuity and momentum equations with the Bingham constitutive model, formulated as a generalized Newtonian fluid. The cross-sectional shapes of the deposited layers are predicted, and the deformation of layers is studied for different constitutive parameters of the material. It is shown that the deformation of layers is due to the hydrostatic pressure of the printed material, as well as the extrusion pressure during the extrusion. The simulations show that a higher yield stress results in prints with less deformations, while a higher plastic viscosity leads to larger deformations in the deposited layers. Moreover, the influence of the printing speed, extrusion speed, layer height, and nozzle diameter on the deformation of the printed layers is investigated. Finally, the model provides a conservative estimate of the required increase in yield stress that a viscoplastic material demands after deposition in order to support the hydrostatic and extrusion pressure of the subsequently printed layers.
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Big area additive manufacturing and 3D concrete printing are two technologies that upscale the material extrusion additive manufacturing concept to larger workpiece dimensions and higher build rates. This work presents a computation fluid... more
Big area additive manufacturing and 3D concrete printing are two technologies that upscale the material extrusion additive manufacturing concept to larger workpiece dimensions and higher build rates. This work presents a computation fluid dynamics model that simulates material extrusion and deposition using the software FLOW-3D. The numerical simulation is used to evaluate the cross-sectional shape of the printed beads. Several constitutive models have been considered to cover the wide range of material behaviours, including shear-thinning and visco-plasticity, that are expected in the flow of molten plastic and fresh concrete. The presence or absence of shear-thinning was found to have more influence on the cross-section of the bead than the actual values of the viscosity. The numerical results are also compared to the nominal bead’s dimensions used in slicer softwares. The conclusion of this study is that the actual bead’s dimensions can vary substantially from its nominal size, w...
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In this study, the force required to feed the solid filament through the nozzle in material extrusion additive manufacturing was measured and analysed. The filament is a source of the building material in this process, but also acts as a... more
In this study, the force required to feed the solid filament through the nozzle in material extrusion additive manufacturing was measured and analysed. The filament is a source of the building material in this process, but also acts as a piston to drive the flow of the molten polymer. Thus, the filament feeding force relates to the pressure inside the nozzle and the flow conditions, which are not well understood yet, due to the complex rheology of polymers. An experimental setup was designed and built to measure the filament feeding force at different extrusion temperatures and flow rates. It was observed that at high feeding rates, the force starts to oscillate, indicating pressure fluctuations in the flow, which could be due to viscoelastic phenomena of the molten polymer or oscillating length of the melt zone inside the liquefier. Moreover, the pressure fluctuations were accompanied by the fluctuations in the extrudate swelling, which might lead to a degradation of the geometrica...