Introduction
Casting investment materials have three main components: a refractory component, a binder and modifiers. The types are named after their binder: gypsum, phosphate and silica-bonded.
Components
Cristobalite expands more on heating than quartz (1.6 to 1.8% against 1.2 to 1.4% at about 1000 °C), so modern investments contain mainly cristobalite.
The three types
Gypsum investments
Mixed with water, the alpha-hemihydrate turns into dihydrate and binds the refractory particles. Above 1000 °C, calcium sulfate breaks down and the investment cracks. The sulfur dioxide released contaminates the casting with sulfides, and silver and palladium alloys become brittle. This is why phosphate investments are preferred even for noble alloys.
Phosphate investments
Monoammonium phosphate and magnesium oxide form magnesium-ammonium phosphate, a crystal hydrate with six water molecules. Above 300 °C, it turns into highly refractory magnesium pyrophosphate. After drying, the structure is porous.
Silica-bonded investments
Tetraethyl orthosilicate is hydrolyzed in a liquid of hydrochloric acid, alcohol and catalysts to form silicic acid, which binds the particles. At about 1000 °C it becomes silicon dioxide, so the binder itself turns refractory. Their use is limited by the complicated technique, and because they are not porous, they need gas collection channels.