• Mollah, Md. Tusher

Abstract

This PhD dissertation concerns the Computational Fluid Dynamics (CFD) modelling and experimental analysis of Material Extrusion Additive Manufacturing (MEX-AM). MEX-AM is an umbrella term that includes Fused Deposition Modelling (FDM), Robocasting (RC), and 3D Concrete Printing (3DCP). The technology offers fabrication of parts/structures on various scales using a wide range of materials. Despite it being a popular fabrication method, the technology faces several challenges that are yet to be solved. Some of the challenges are addressed in this dissertation. Specifically, numerical and experimental work is presented on FDM printing of dimensional accurate corners. Furthermore, simulations are carried out to study the geometrical stability (i.e., uniform size) of layers produced by RC and 3DCP. Finally, simulations and experiments are exploited to investigate the possibility of integrating reinforcement bars with 3DCP.<br/><br/>The dissertation outlines the governing equations for the deposition of materials in MEX-AM. It discusses constitutive models such as Newtonian, generalized Newtonian, and elasto-viscoplastic fluid, and how they can be used to simulate MEX-AM of different materials. Subsequently, experimental details and the post-processing of results are presented. Finally, the results are presented in seven appended publications.<br/><br/>The study on the deposition of corners is carried out and compared for different corner angles and for two different extruders, Bowden and direct drive. The Bowden extruder cannot control the amount of material extruded during the nozzle movement at the corner, whereas the direct drive can. It is found that the direct drive extruder produces a more rounded edge at the outer side of the corner. Furthermore, the study enabled novel insight into the accuracy of the CFD model and the state of the material at the corners, as well as the discrepancy between experimental, analytical, and simulation results.<br/><br/>Three different printing strategies are considered when analysing the geometrical stability of layers: wet-on-wet, wet-on-semisolid, and wet-on-solid printing. In wet-on-wet printing, a wet layer is printed on top of a wet layer (i.e., the material properties do not change over time). In the other two cases, the printed layer is semi-solidified or solidified. The cross-section of deposited layers, deformation of the bottom layer, and extrusion pressure are studied when printing wet-on-wet for different material properties (i.e., yield stress and plastic viscosity) and processing parameters (i.e., extrusion speed, printing speed, nozzle diameter, and layer height). The results illustrate that the deformation is highly dependent on these parameters. In addition, the simulations show that the deformation can be reduced but not eliminated, i.e., a stable print without deformation cannot be obtained by wet-on-wet printing. The wet-on-solid printing simulations illustrate stable printing and give a conservative estimate of the yield stress required by the already printed layer in order not to deform. The wet-on-semisolid printing is simulated using a yield stress buildup of the already printed layer. The yield stress buildup is modelled by applying a scalar approach that changes the materials’ property between the layers. Based on the results, it is demonstrated that altering the process parameters can reduce the requirement for the yield stress buildup, while still print stable layers.<br/><br/>The bonding between rebars and deposited concrete is analysed in terms of the formation of air voids inside the structure. The simulated cross-sectional shapes of the printed structure are compared with experiments and are found to capture the formation of air voids accurately. The CFD models are applied to analyse three scenarios: no rebar, a horizontal rebar, and cross-shaped rebars. The models illustrate that by changing the process parameters one can eliminate the air voids around the horizontal rebar, while alterations in the toolpath and rebar geometry are necessary to fully mitigate air voids when printing around cross-shaped rebars.

Topics

• Deposition
• impedance spectroscopy
• polymer
• experiment
• simulation
• extrusion
• viscosity
• void
• material extrusion