REPOSITORY > RESULTS

Abstract

Thermoplastic 3D printing (T3DP) is an additive manufacturing (AM) process where micron sized droplets of thermoplastic wax-based feedstocks, with high solid content of ceramic powder, are deposited with high precision and resolution. The complete workflow of T3DP, from the properties of the suspensions to the process parameters, to deposit droplet by droplet, forming lines and layers, needs to be understood and controlled. Since T3DP is a relatively new AM technique, the studies of the suspension rheological requirements have been limited. It was reported that the suspensions should exhibit a shear thinning behavior in a given viscosity window. The details about colloidal particle interactions, whether particles should be weakly flocculated (at secondary minimum) and have a yield point, and their influence on the printability, however, have not yet been studied. Next to the properties of the used feedstocks, the role of the infill strategy for the formation of layers from lines, an important process parameter being highly sensitive to the introduced flaw type population, which ultimately determines final mechanical strength and reliability of T3DP-printed and sintered ceramic parts, is yet to be investigated.
The goal of this doctoral thesis was to study and control the complete workflow of T3DP in a systematic approach consisting of two main parts. In the first part of the study, paraffin-wax suspensions highly loaded with fine zirconia powder, stabilized by a combination of surfactants with significantly different chain lengths, were prepared. The relation between the surfactant chain length, conformation, and interparticle interactions, as well as the rheological behavior of the suspensions, such as viscosity, yield stress, storage, and loss moduli, was investigated in detail. Furthermore, the feedstocks were deposited in the form of individual droplets, lines, and single layers to correlate these rheological properties with jettability. It was found that the conformation and thickness of the steric barrier were critical parameters, affecting the rheological properties and the state of suspensions, being weakly flocculated or well dispersed, depending on the surfactant chain length (2.4 nm or 10 nm) and the ratios of the used surfactants. Thus, it was shown that T3DP jettability was dictated not only by the viscosity of the feedstocks but also by their respective yield point. In the second part of the study, wax-based zirconia feedstocks were printed in the form of rectangular bending bars with 3 different infill strategies, i.e., 0°, 90°, and alternating 0° + 90°. The homogeneity of formed bars, the type of flaw populations, final sintered density, Weibull strength distribution, and fractography were studied. It was shown that the infill strategy did not have a significant effect on the mechanical properties for T3DP, unlike other AM processes such as fused filament fabrication. However, the Weibull moduli obtained from the parts were significantly lower than the values achieved by other AM processes. Fractography revealed two types of flaws in the form of feedstock inhomogeneities and/or voids from imperfect fusion of subsequent droplets, indicating a need for improvement of both feedstock properties and process parameters.
T3DP allows for the preparation of complex-shaped ceramic parts and multi-material components that can be used in many different areas, such as dentistry, biomedicine, the semiconductor industry, and catalysis. However, understanding the interparticle interactions and resulting rheological behavior of highly-loaded paraffin-wax suspensions is essential for their successful implementation. The thesis delivers a “roadmap” for the design of the steric barrier thickness and configuration, which governs various rheological properties of the feedstocks, with defined boundaries for the successful jetting outcome, going beyond the state of the art for T3DP.