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This freeform fabrication method produces dense ceramics in less than a day without requiring organic binders.
complex shapes that are completely solid or thin-walled.
The process uses highly loaded ceramic slurries that are typically 50 to 65 volume percent ceramic powder, less than 1 volume percent organic dispersants, and 35 to 50 volume percent volatile solvent (usually water). The ceramic slurries are deposited in sequential layers. Any conceivable two-dimensional pattern may be "written" layer by layer into a three-dimensional shape (Figure 1). Orifice openings can range from several millimeters to tenths of millimeters.
In some regards, robocasting is analogous to the ceramic near-net-shape processing techniques, slip casting and gel casting;6 however, robocasting is moldless and binderless, and fabrication times can be quicker. To maintain structural integrity while building a component, robocasting relies on a rheological transition of the deposited slurry induced by partial drying of the individual layers. The slurry is deposited upon a heated plate and undergoes a pseudoplastic to dilatant transition during the build process. In contrast to gel casting and other freeform fabrication techniques, robocasting does not require organic polymerization reactions or solidification of a polymeric melt to maintain the shape of components.
OPTIMIZING THE PROCESS
Robocasting is no more complicated than caulking a bathtub-except that during robocasting, the substrate actually moves underneath the point where material is dispensed from a stationary orifice. While conceptually simple, transforming this concept into reality for the manufacture of ceramics requires a synergistic control of the viscosity and rheology of the slurry, percent solids in the ceramic powder slurry, dispensing rate of the slurry through the orifice, drying kinetics of the dispensed bead of slurry, and computer code for optimal machine instructions.
The slurry viscosity must be tailored during processing for optimal performance. Additionally, the rheological dependence of viscosity with shear rate must be controlled. During dispensing, the slurry will experience high shear conditions while flowing through the orifice and as the moving substrate interacts with the dispensing slurry. However, immediately following this process, the slurry experiences a shear rate near zero.
Therefore, to control the shape of dispensed beads, the slurry rheology must be extremely pseudoplastic with a yield point (shear-thinning) so that the material can flow smoothly during dispensing but then solidify in...