PhD defence of David Xu

This Ph.D. aims at the thermal and mechanical investigations of the additive manufacturing process of Fused Filament Fabrication (FFF). In this framework, the process is investigated at two different scales. Numerical models are developed to perform finite element simulations of the process. The numerical tools are supported by experimental observations and measurements. This is done to have a clear idea of the different phenomena occurring during the process as well as to assess the numerical models.

 

A first model focuses on the polymer fluid dynamics in the liquefier and its extrusion and deposition on a substrate. This model enables to study the influence of the nozzle geometry and printing parameters on the polymer deposition behaviour. An experimental protocol is implemented to measure the deposited strand cross section. An empirical equation is proposed to predict the strand dimensions based on the printing parameters.

 

A second model is developed to investigate the process on a larger scale. The first step aims to simulate the thermal behaviour of parts of several cm heights. Experimental measurements of the temperature via thermocouple are performed to assess the model. The thermal model is then adapted for the case of semi-crystalline polymers. The crystallisation kinetics is computed based on the cooling of the part. The results explain the degree of crystallinity measured experimentally. Solid mechanics is also implemented in the numerical model to account for part distortion upon cooling. An experimental protocol is developed to perform in-situ measurements of the strain during the printing of the part by digital image correlations. The combination of experimental and numerical investigations performed in this Ph.D. should provide the necessary information to better tailor the process depending on the needs of the user.