Numerical and Experimental Analysis of Filament-based Material Extrusion Additive Manufacturing

This thesis explores the flow phenomena during Material Extrusion Additive Manufacturing (MEX), one of the the most popular 3D printing methods. MEX has been present since the end of 80s and has undergone a tremendous development. Nowadays, relatively reliable machines are available for the price below $5000. Yet, despite the popularity, the process faces challenges in terms of geometrical accuracy, mechanical strength of parts, and production time. Moreover, there are still aspects of this manufacturing technique that are not well understood such as the material flow during extrusion and deposition.

MEX is a process in which the material is selectively deposited through an orifice to create a three-dimensional object. This work focuses on filament-based MEX, commonly known as Fused Filament Fabrication (FFF), or Fused Deposition Modeling (FDMTM), where typically a polymeric filament is melted, extruded and deposited in the form of a strand. The strand is an essential building brick of each part printed with FFF. Control over the dimensions and morphology of strands is important for reducing geometrical inaccuracies, as well as providing good conditions for creating bonds between the layers.

In the first part of the thesis, a Computational Fluid Dynamics (CFD) model for predicting the shape formation of strands during the deposition flow is presented and validated with experimental measurements. The strand shape is observed to vary from being nearly circular to a flat cuboid with rounded corners, depending on the deposition parameters.
The numerical model is then extended to account for the presence of previously deposited material on the formation of newly laid strands. The formation of mesostructure is simulated for different deposition conditions and their influence on the porosity, bond line quality and surface roughness is modelled. The developed model could be used to improve the strand representation in a 3D printer controlling software, as well as to numerically optimize the process parameters.

The second part of the thesis investigates the flow of polymer through the printhead that occurs prior to the deposition. Understanding and predicting the flow inside the printhead is imperative to increase building rates in FFF. An experimental setup to measure the filament feeding force that relates to the pressure inside the nozzle is designed and the flow is studied for different channel geometries, filament materials, and extrusion conditions. Two extrusion regimes are identified: a stable regime, where the feeding force is constant in time and increases approximately linearly as a function of the filament feeding rate; and an unstable regime, where the feeding force oscillates and increases sharply as a function of the filament feeding rate. The transition between the two regimes is termed as the maximum feeding rate during stable extrusion and is shown to increase with the length of the heating channel as well as its temperature. An analytical model for predicting the maximum feeding rate is developed and experimentally validated.

Three CFD models are implemented and tested to predict the polymer flow inside the printhead. The first model that was previously studied in the literature, simulates a nonisothermal one-phase flow of a generalized Newtonian fluid with shear- and temperaturedependent viscosity. The second, novel model, resolves the position of the polymer free surface inside the print head and gives an insight into how the material melts and fills the channel. The third, also novel model, simulates a non-isothermal flow of a viscoelastic fluid through the channel. The numerical results are compared to the experimental measurements and the models’ strengths and weaknesses are discussed. The models predict temperature and pressure distribution during the flow, and prove useful in predicting the filament feeding force for different channel geometries and flow conditions. Moreover, the numerical results provide an explanation of the origin of pressure oscillations during the unstable extrusion regime.