Modeling and evaluation of freeform extruded filament based on numerical simulation method for direct ink writing

Abstract

Direct ink writing (DIW) is an extrusion-based additive manufacturing (AM) technique. One of the required factors for printing quality in DIW is to extrude continuous and tubular freeform extruded filaments (FEF) with diameters close to the nozzle inner diameter. This paper established a simulation model of FEF with volume of fluid (VOF) method and then evaluated the FEF based on the simulation results. First, ink properties including density, surface tension coefficient and rheological properties were determined by properties tests. Then, the model of FEF was established using VOF simulation method with the software OpenFOAM and two evaluation indicators, mean diameter and stability, were proposed to evaluate the FEF. Two inks, a cellulose ink and a Nivea Crème, were used under three levels of piston velocity to verify the method universality for different inks and variable process parameters. The prediction accuracy of the established simulation model was verified as the relative error between simulation and experimental results of diameter was less than 10.22%. In this work, it was found that: (1) piston velocity needed to be set higher than a minimum value to overcome yield stress and surface tension to successfully extrude filaments; (2) mean diameter of FEF decreased while stability of FEF decreased firstly and then increased as piston velocity increased. The proposed model and evaluation indicators gave an accurate and effective framework to analyze effect of the piston velocity on FEF, characterize the ink printability and find suitable printable windows for process parameters in DIW.

Numerical simulation and experimental validation of deposited corners of any angle in direct ink writing

“…In recent years, to fully understand the DIW process and improve the geometrical quality of parts in DIW, numerical simulation methods have been widely used to model the MEAM process [20][21][22][23]. Comminal et al simulated the strand deposition [20] and the material deposition along a toolpath with a sharp corner [21] with computational fluid dynamics (CFD) models to study the effect of process parameters on deposited filaments and corners, respectively.…”

“…Göhl et al [22] conducted computer based simulations for DIW to evaluate newly developed inks. Tu et al [23] established a model of freeform extruded filaments with numerical simulation and then evaluate the filaments based on the model for DIW. However, the previous work [21] for numerical simulation of deposited corners could only achieve the corner simulation under the condition of small angle which was less than 90° but failed to realize corner simulation of any angel.…”