Numerical Study on the Temperature-Dependent Viscosity Effect on the Strand Shape in Extrusion-Based Additive Manufacturing

Behrouz Behdani,Leah Mason,Joontaek Park,Matthew Senter,Ming Leu

doi:10.3390/jmmp4020046

Abstract

A numerical model that incorporates temperature-dependent non-Newtonian viscosity was developed to simulate the extrusion process in extrusion-based additive manufacturing. Agreement with the experimental data was achieved by simulating a polylactic acid melt flow as a non-isothermal power law fluid using experimentally fitted parameters for polylactic acid. The model was used to investigate the temperature effect on the flow behavior, the cross-sectional area, and the uniformity of the extruded strand. OpenFOAM, an open source simulation tool based on the finite volume method, was used to perform the simulations. A computational module for solving the equations of non-isothermal multiphase flows was also developed to simulate the extrusion process under a small gap condition where the gap between the nozzle and the substrate surface is smaller than the nozzle diameter. Comparison of the strand shapes obtained from our model with isothermal Newtonian simulation, and experimental data confirms that our model improves the agreement with the experimental data. The result shows that the cross-sectional area of the extruded strand is sensitive to the temperature-dependent viscosity, especially in the small gap condition which has recently increased in popularity. Our numerical investigation was able to show nozzle temperature effects on the strand shape and surface topography which previously had been investigated and observed empirically only.

Highlights

Extrusion-based additive manufacturing, called fused deposition modeling (FDM) or fused filament fabrication (FFF), is a type of additive manufacturing (AM) process which fabricates a 3D part by building up extruded material layer by layer
We used OpenFOAM to simulate the extrusion process of FDM considering the temperature and shear-dependent viscosity which was fitted from experimental data
We compared our simulated strand shape extruded under a small gap condition and straight nozzle with published experimental data under the same conditions

Summary

Introduction

Extrusion-based additive manufacturing, called fused deposition modeling (FDM) or fused filament fabrication (FFF), is a type of additive manufacturing (AM) process which fabricates a 3D part by building up extruded material layer by layer. Since accurate prediction of extruded strand shape is required for controlling the mechanical and structural properties of a final part [8,9,10], the CFD approach has garnered attention One such CFD simulation study of an extrusion-based additive manufacturing process was done by Bellini [5]. A subsequent study directly compared the strand cross-sectional areas from experimental data to the simulation results and showed that an isothermal Newtonian simulation can match the relation with the strand shape and the process conditions from the experiment [15] This agreement was only for the case where the gap between the nozzle and the substrate is similar to the nozzle diameter, and discrepancies were reported for the case of a smaller gap.

Governing Equations

Numerical Model Solver

System Description

Results and Discussion

Conclusions