Hot ceramic lithography of silica-based ceramic cores: The effect of process temperature on vat-photopolymierisation

Abstract

An enhanced-temperature vat photo-polymerisation of ceramic-loaded mixtures, or so called hot ceramic lithography, has been presented in this paper, for 3D printing silica-based ceramic cores used in the investment casting of hot section parts in aero and industrial turbines. The pre and post-sintering properties of the 3D printed ceramic parts highly depend on the polymerisation degree of the base resin binders and the rheological behaviour of ceramic slurry, and hot lithography can play a significant role both. First, the paper aimed to better understand how different printing temperatures (25,35,45 and 55 °C) would affect the properties of the base binder mixtures before loading ceramic particles, by conducting tensile, DMA and FTIR analysis on binder-only prints. The impact of elevated printing temperature on the mechanical properties of ceramic loaded binder mixtures was further investigated. The highest mechanical properties for the base binder and the ceramic loaded mixtures were achieved when the process temperature was set closer to the binder mixtures' initial glass transition temperature (when the binder is cured at room-temperature). This printing temperature is yet lower than the level causing heat-induced cross-linking yet high to enhance the mobility of monomers and active species, when compared with conventional room-temperature vat photopolymerisation printing. The results indicate that printing temperature, if set as described, can significantly impact the mechanical property of parts and conversion rate of the base binders for a successful print process. The second part of this study investigates the impact of temperature on the rheological behaviour and peeling forces of highly loaded ceramic-loaded mixtures (61.2 vol% of SiO 2 -ZrSiO 4 ). The suspension showed shear thickening behaviour at 25 °C, hindering the replenishment of the slurry during printing, while a high transition from shear-thickening to shear-thinning behaviour, ideal for flawless ceramic 3D printing, was observed at an optimum process temperature of 35 °C. When print temperature increases to 45 °C and then 55 °C, the viscosity of the mixture at very low shear rate (when the material is static during printing) increases, hence affecting peeling forces. Comparing peeling force measurements and FTIR results showed the higher cross-linking density, and the reactivity of the base binder, leads to a stronger adhesion between the cured layers and the printer's transparent film, hence higher separation/peeling forces. To sum up, hot ceramic lithography at the right printing temperature can resolve many print and post print issues, such as deformation and cracking, leading to a stronger and more dimensionally-accurate ceramic components.