Freeform 3D printing includes additive manufacturing techniques that can produce 3D objects without limit of shape but also without the addition of support structure and reducing the printing time (Colly, A. et al. MRS Bulletin (2022). 10.1557/s43577-022-00348-9). Among these techniques, we find the “Dynamic Molding” which consists in printing a 3D object within a granular environment (Courtial et al. Additive Manufacturing (2022). 10.1016/j.addma.2022.102598). Unlike other freeform 3D printing techniques that use gelled matrices, dynamic molding is distinguished by the use of solid matrix based on powder that can be of different compositions.
What does dynamic molding do for ceramic materials? Existing additive manufacturing techniques allow for ceramic printing; nowadays the most used are direct ink writing, binder jetting or stereolithography. These techniques aim to obtain a 3D green part, including a chemical binder loaded with ceramic particles, which is then heat treated to obtain the sintered ceramic. Depending on the printing technique used, the user will have to make a compromise between a high resolution, a short printing time and the size of the object. Indeed, none of the three above-mentioned techniques can combine all these advantages today. Here is where dynamic molding brings an innovative solution by combining high printing resolution (up to 50 µm), printing speed (5 ml/min) and large object size (50 cm).
In dynamic molding, ceramic powder can be used as a dynamic mold where a viscoelastic paste serving as a binder is 3D-structured inside the powder to print the shape of the desired 3D object. The total immersion of the viscoelastic material during the manufacturing process prevents the object from collapsing thanks to the powder acting as a support. In the same way, this granular phase makes it possible to fight against the hydrodynamic instabilities met in the gelled matrices such as Rayleight-Plateau because the powder is a biphasic medium made up of a solid part (powder) and a gaseous one (air).
At the end of the printing process, the green part is made of the extruded viscoelastic paste and the ceramic powder constituting the ceramic mold. The success of these prints lies in the understanding of the rheological properties of the fluids and the granular phase in order to control the imbibition and drainage phenomena. Then, depending on the nature of the extruded material, it can either be retained to form a ceramic-based composite object or sintered to obtain a pure ceramic material. The ample choice in terms of final constitution of the ceramic object is another exclusive advantage of dynamic molding, which will allow us to lift the limits of additive manufacturing for technical materials by widening the range of applications such as for bone repair (e.g. Hydroxyapatite, Zirconia) in health field or fireproof in aeronautic (e.g. alumina)