Development of a co-dispersion binder for the three-dimensional printing process to facilitate handling of ceramic casting shells in the green state

Abstract

Three Dimensional Printing, a rapid prototyping technology, fabricates ceramic casting shells directly from 3-D CAD models. To create a shell, loose alumina powder in thin layers is joined by selectively depositing a colloidal silica binder. Firing fuses the silica to the alumina, and subsequent processing removes the unbound powder from the casting cavity. In some instances, it may be advantageous to perform powder removal before firing. This requires higher green strength than what is obtainable with the current binder. An alternative binder consisting of 18 vol.% colloidal silica co-dispersed with 10 vol.% acrylic emulsion in an aqueous medium was developed. The polymeric component imparts higher green strength to the part (3.6 vs. 0.4 MPa), yet decomposes during firing to minimize interference with the fusion of silica to alumina. The reduction of fired strength in samples printed with co-dispersions was less than 10%. Through designed experiments, performance of the co-dispersion was quantified by additional criteria such as shrinkage, warping, and binder stability. Average linear shrinkage in parts printed with co-dispersions was 0.05% after firing at 900°C. A simple model for warping, based on elastic response of a plate to free thermal strain, predicts that flat plates tend to warp into a paraboloid after firing. The experimental results correlated well with this prediction.