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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 2 8 3D PRINTING OF CERAMICS: PROCESSES AND CONSTRAINTS Techniques for the 3D printing of ceramics are evolving beyond product development and prototypes to enable full production-scale operations. Joe Cesarano, John Stuecker, and Paul Calvert, Robocasting Enterprises, Albuquerque, New Mexico Fig. 1 — Schematic of a CNC robocasting platform showing an optional mixing head and the capability of multi-material deposition through a common nozzle. [3] M ost 3D printing methods involve converting a fluid into a solid point-by-point by building up lines, sheets, and then solids. Different approaches [1,2] offer their own advan- tages and disadvantages. Similarly, var- ious processes are available for printing on paper—from pen and ink to offset li- thography—and each has found its spe- cific area of application. Two major approaches to 3D print- ing include plastic extrusion (fused deposition modeling or just 3D print- ing) and photopolymerization of liq- uid monomers (stereolithography). The polymer extrusion approach came from kitchen table experiments on building with a glue gun by Scott Crump, who then founded Stratasys to produce en- gineering versions of this system. A sec- ond phase of development was driven by the RepRap open-source project and led to the inexpensive and high-per- formance 3D printers of today. All 3D printing methods involve complex in- teractions between many aspects of technology, and we should recognize that the open-source approach is very powerful under these circumstances. Stereolithography was originally developed by 3D Systems and involved an ultraviolet (UV) laser scanning across the surface of a bath of liquid mono- mer to form a part. This approach has the advantage that the liquid bath sup- ports the part and so allows overhang- ing shapes, while extrusion systems (in air) require separable supports for overhangs. Photocurable resins tend to be heavily crosslinked and brittle, but toughness has improved with specially formulated resins. Newer systems can replace the laser with some form of projection so that a whole layer can be cured at once, shortening the printing time. Photopolymeriza- tions tend to slow down with the degree of reac- tion, so some kind of post- cure is often used. These systems look promising for faster production and are leading a move from prototyping uses to plas- tic part manufacturing. Many other types of 3D printers, as well as multiple variations on the two approaches describ- ed here, are commercial- ly available. While a full overview is beyond the scope of this article, these examples provide an idea of the kinds of tradeoffs that must be addressed in developing a 3D printing system for ceramics. CERAMIC METHODS An obvious approach to the 3D printing of ceramics is to modify one of the previously mentioned methods to include particles in the resin and print a green body that can be burnt out and sintered, similar to familiar meth- ods like tape casting. Efforts have been made to produce a ceramic-loaded fil- ament that could be extruded through a nozzle using a system like a 3D plas- tic printer [4] . An intrinsic problem is that the high volume fraction of particles needed to produce a green body that will sinter well causes the filament to be brittle as a solid and very viscous as a liquid. The viscosity means that extru- sion pressures will be very high, making the feed system problematic. At Advanced Ceramics Research Inc. in Tucson, Arizona, a system was developed that used amolded bar of ce- ramic-filled resin that was melted at the