WHAT IS COLLOIDAL SILICA?
Colloidal silica is a stable dispersion of millimicron size SiO2 particles in water. The silica particles are non-agglomerated, spherical in shape, negatively charged, and stabilized with a counter ion. Refractory flours and colloidal silica are mixed to form ceramic slurries for producing shells. These slurries are stable mixtures and have been used six months or more on instance. The slurries are destabilized by impurities and conditions leading to form an irreversible gel. Shells made from unstable slurry leads to a weak shell and causes casting quality problems. Stability of Silica Sols: The discrete silica particles in a sol begin to form agglomerates when destabilized. Such an unstable sol loses its ability to form a complete bond. Slurries of unstable sols results in weak shells and surface imperfections on the casting. Slurry with agglomerated sol cannot always be detected because the gel is continuously broken during mixing. Once the slurry has begun to gel, the rate is unpred
