Stereolithography of alumina systems: cure depth and dimensional control
Creators: name: Alazzawi, Mustafa Kanaan Mohammed
Publisher: name: No Publisher Supplied
Year: 2021

description: This dissertation examines additive manufacturing by stereolithography carried out by using photocurable suspensions, which are typically highly viscous filled mediums. Controlling the lateral resolution, overall dimensions, and cure depth are essential aspects of the stereolithography process. High resolution, structural integrity, and dimensional control are needed within the ceramic industry. Identifying the factors that influence dimensional control and cure depth was the goal of this work. In meeting this goal, selecting commercial suspensions, identifying quantitative measurements, establishing a fundamental evaluation of tape casting based printer, establishing mixing methods, designing photocurable resins with an optimal dispersing agent, studying cure depth of developed suspensions, and finally, studying dimensional control of developed suspensions were the objectives of this dissertation. Determining the factors that influence cure depth and dimensional control will help to establish the right printing conditions. The cure depth and dimensional control depend on compositional variables and printing parameters. In this context, the compositional variables were the concentration of the suspension, the particle size of alumina, and resin compositions, and the printing parameters were energy dose and layer thickness.In the first phase of experimentations, characterization metrics and highly viscous suspensions were developed to establish a stereolithography process. The characterization measures included rheological and fineness of grind measurements. Rheological measurements were determined using the cohesive energy density model described from the interparticle interaction and degree of coalescence. The level of coarse agglomerates was determined by the fineness of grind measurement. Determining the level of agglomeration was needed to study their effect on the cure depth. The additive manufacturing printer used in this dissertation was derived from the tape casting concept, where the baseline evaluation was needed at first by using commercial suspensions. The effect of the tape casting shear rate on dimensional control was studied, and the layer thickness and cure depth effects on interlayer adhesion, lateral resolution, and surface roughness were established. The key results showed that high targeted cure depth with thicker layers needed a high energy dose, which worsened the surface finish and lateral resolution. However, they improved the interlayer adhesion. Therefore, there was a need to control the compositional variables along with printing parameters to improve the interlayer adhesion while maintaining a high lateral resolution. In the same phase, two photocurable resins and mixing methodologies were developed. The suspensions were designed with a solid content of 32.0 – 62.0 %vol. and a particle size range of ~ 0.38 – 2.76 µm. The optimal dispersant was an acidic polyether with a 0.005 weight ratio of dispersant to ceramic. All suspensions had a higher degree of dispersion, and most of them had almost Newtonian like behavior. The thermal degradation of the resins was examined, resulting in the design of conservative pyrolysis. The designed profiles were tested on samples with varying layer thickness, energy dose for printing, and compositions. In the second phase of experimentations, the cure depth and dimensional control of developed suspensions were examined. The cure depth was determined by measuring the thickness of polymerized samples as a function of energy dose. The data was fitted to Jacob's equation (C_d=S_d ln⁡(E_∘/E_d )) to determine the sensitivity (S_d) and critical energy (E_d) for curing. This included studying the effect of coarse agglomerates, particle size, and solid content on cure depth. The key results showed that the cure depth depended on coarse agglomerates and improved with coarser particle size. The sensitivity increased with particle size, while the critical energy dose was independent of particle size and solid content. The cure depth did not change with a solid content between 32.0 – 55.0 %vol. The dimensional control of printed objects was explored by examining the effect of compositional and printing variables. The compositional variables were solid content and particle size using two different resin systems. The printing variables included two levels of energy doses and two layer thicknesses of 20.0 and 50.0 µm. The key results showed that the fine particle systems required a higher energy dose to reach a similar targeted cure depth for printing coarse particle systems. The degree of scattering depended on energy dose, which impacted feature resolution and lateral dimensions. The layer thickness showed no significant impact on the lateral dimensions but allowed to produce a high targeted cure depth. The directional shrinkage was reduced with solid content, which was more pronounced in fired samples. The vertical dimensions showed no large or significant changes with particle size, energy dose, and layer thickness.