Metal casting into NaCl molds fabricated by material extrusion 3D printing

Abstract Aluminum die casting is a well-established industrial process for mass producing aluminum parts with complex shapes, but design restrictions exclude some features like undercuts and hollow structures from being produced with this method. Water-soluble casting molds offer a promising solution to overcome those restrains, for example by hot pressing of salt cores or 3D printing of NaCl molds. Presently, 3D printing techniques available for NaCl are limited to direct ink writing (DIW) and photopolymerization. This study presents an approach to prepare NaCl parts by thermoplastic material extrusion (MEX) 3D printing. Firstly, a 3D printable feedstock is developed consisting of an organic binder, which is usually used for ceramic injection molding, and sodium chloride (NaCl) salt crystals. Various molds are then printed on a granulate-fed MEX printer. After thermal debinding and sintering at 690 °C, the 3D printed parts consist of pure NaCl. Furthermore, the same NaCl feedstock is used for injection molding. The bending strength of 3D printed samples with and without post-treatment are measured and compared to injection molded test specimens. Finally, metal casting in 3D printed NaCl molds is shown with tin or aluminum and the metal demonstrator parts with complex geometries such as gyroid structures and turbine wheels are released by dissolving the NaCl molds in water.