Powder preparation for a shell mold using a new coating process

A new coating process in the powder preparation for a shell mold has been developed to increase the fracture strength of the shell mold. It is due to the homogeneous formation of a glass phase on the starting particles and the increase in the glassification efficiency by the reduction in the loss of inorganic precursors. The inorganic binder system used for the new coating process is composed of tetraethyl orthosilicate (TEOS) and sodium methoxide (NaOMe) as the silica (SiO₂) and sodium oxide (Na₂O) precursors, respectively. Three different coating processes are employed for the powder preparation with a high glassification efficiency. In process I, the starting particles are coated with NaOMe, and then TEOS are coated on the particles treated with NaOMe. Process II is the reverse sequence of process I. Process III involves coating of the particles with a mixture of TEOS and NaOMe. The particles coated with an individual or mixture precursor were fixed with an organic binder and then heated at 1000 °C for 1 h. Molds prepared through the new coating processes, especially process III, show a higher fracture strength value compared with that of the conventional convert mold process, which may be caused by the increase in the glassification efficiency of the precursors. Powder prepared by process III shows a more uniform coating of the glass phase than those by other processes, resulting from an enhancement in the phase mixing between SiO₂ and NaOH molecules.