Castings for Aircraft: Part 1: Wax and sand casting processes.

For castings such as cylinder heads for air-cooled engines, both the mold and the cores are usually made of baked or chemically bonded sand. These are generally referred to as "dry sand" molds and while they are more expensive, thinner sections can be cast and closer dimensional tolerances can be maintained. For the spaces between the cylinder head fins, thin cured sand segments are joined to form the mold, and cores are used for the intake and exhaust passages, spark plug holes, and other cavities.

A variation of the dry sand process sometimes used for making aircraft castings is shell molding, which requires heated aluminum tooling for forming both the cores and the mold segments. The sand used in shell molds and cores is coated with a resin that is partially melted when it comes in contact with the hot aluminum pattern or core box, causing the sand grains to adhere to each other and form the shell. The core or mold that is formed has a generally uniform wall thickness that follows the contour of the tooling and has a smooth surface. The sprue and gating system are incorporated in the tooling, along with alignment bosses and corresponding cavities on each half of the mold. When the cores are pasted in place and the mold is closed it is ready to pour. The heat of the metal degrades the binder, allowing the casting to shrink as it cools and making core removal easier. This is the most accurate of the sand molding processes and is used by both OEM and aftermarket makers of air-cooled engine cylinder heads.

George Genevro, a retired college professor at Cal State Univ. at Long Beach, CA, is an A&P mechanic, pilot, and aircraft owner.