It is known that if one casts the molds made out of synthetic sand (silica, zircon for example connected by organic resins) without heating them first, one obtains elements with substantial risks of faults (blisters, micropores).
The heating of the core is generally carried out in an oven and the heating of the molds is generally carried out with a blowtorch. The mold is then assembled and closed before the casting.
However heating with the blowtorch is irregular and may not touch all of the surfaces, and in the interval of time necessary for the remolding before casting, the volatile products can return towards the impression.
In low pressure casting of sand molds, the metal is injected from bottom to top through the lower surface of the mold. In gravity casting, on the other hand, the mold thus has an orifice on the other side of the mold. Futhermore, the mold, having no deadhead, is without an orifice at the upper portion.