Additive manufacturing of ceramics has been developed extensively over the past years, shifting the goal from simple material proof-of-concept and prototyping to the fabrication of high-end parts. Indeed, each AM technique has been proven to be best suited for specific ceramic feedstocks, dimensional scales and component designs. Whereas such optimization has allowed setting precise requirements for successful printing processes, it also defined their limitations. A common drawback associated with most AM approaches rises from the layer-by-layer construction: printing of complex geometries characterized by overhangs or small features requires the addition of support structures, excess or sacrificial material, or longer printing times in order to avoid structural collapse. Moreover, the piling up of layers results in the presence of multiple interlayer interfaces and a stair-stepping effect on the surface of the part, leading to reduced strength and mechanical response of the printed parts. Several strategies can be adopted to limit such phenomena, including the optimization of the part orientation and/or the use of a variable layer thickness. Nonetheless, the process still remains based on a planar construction: the stair-stepping effect is reduced but not removed, and the interlayer interfaces persist in the final part. The combination of multiple manufacturing techniques into a unique hybrid system could represent a novel solution to overcome the aforementioned limitations.
Here, we present a hybrid extrusion-photopolymerization process (UV-DIW) in which a photocurable suspension is extruded through a nozzle and consequently cured by an external UV source. On one hand, light scattering and/or absorption phenomena arising from the light-particle interaction, usually impacting on vat photopolymerization processes, do not have an influence on the process resolution provided by the extrusion nozzle; on the other hand, the consolidation of the ink relies on the photo-curing process, which proceeds faster than viscosity recovery through gel reconstruction and allows for better shape retention without extensive optimization of the ink rheology. As a result, the process is fast enough to enable the production of self-supported structures which do not require a layer-by-layer construction.
We demonstrated the full potential of the UV-DIW setup by coupling it with a 6-axis robotic arm and fabricating complex, freeform shapes using transparent and dark ceramic suspensions. Specifically, silica-based and silicon nitride-based inks were prepared and used to produce different lattice structures; particles surface properties, liquid-particles interaction forces and ink reactivity were found to be the most critical parameters to control the inks particle loading, its flow behavior and freeform ability.