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T. Rath, K. Seidler, R. Addison, E. Holzhausen, J. Stampfl:
"An ivory-like material for stereolithography based additive manufacturing";
Talk: EUROMAT 2021, Online; 09-13-2021 - 09-17-2021.

Ivory is one of the most controversial materials in history. On the one hand, it is desired for its aesthetic appearance and its convenient workability, but, on the other hand, its unethical harvesting is not justifiable in modern days. In 1989 an ivory trading ban was passed to restore elephant populations that were diminished due to excessive poaching. Since then, the available resources are shrinking and many natural and synthetic materials were introduced as replacement. These substitute materials are typically only available in bulk and some are deficient in the most important characteristics, such as color, translucency and the characteristic Schreger lines. A newly developed substitute material called "Digory" is presented that mimics these aesthetic criteria and is processible with additive manufacturing. This allows for an efficient, high quality restoration of valuable art objects and is also applicable for modern use. Layer by layer, a photosensitive slurry, which consists mainly of a dimethacrylic resin filled with calcium phosphate particles, is polymerized into the desired shape with a UV laser. The solids loading, which was adjusted to fit the translucency of ivory, was around 30 vol.%. At this content, also the density of about 1.79 g/cm3 is equivalent to the values for ivory found in the literature. The mechanical properties were compared to those of natural ivory as well. Small quantities of yellow and red color pigments were used to modify the basic color, which is thereby adjustable to match each piece of natural ivory. With additional post-processing using traditional handcraft techniques the resemblance to natural ivory was further enhanced.