Shell molding, also known as shell-mold casting, is the process of creating and using resin molds made up of thin-walled sections. The process is typically used to create small to medium metal parts. It is well-suited to items requiring precision and consistent accuracy. Shell molding first came into use in Germany during World War II, where it was patented by Johannes Croning, and so it may also be known as the "Croning process."
To create a shell molding, silica sand is coated with a resin that will harden into a solid when subjected to heat. This mixture is placed into a “dumpbox” — a box made of heat-resistant material and open at the top. A metal pattern is then heated to approximately 450°F (approximately 230°C), coated with a lubricant, and secured over the opening of the box. The whole is inverted so that the resin sand comes into contact with the pattern. After 15 to 30 seconds, the whole is returned upright, excess sand falls back into the box, and the pattern is removed.
The shell molding created by the process may be only 1/8 to 3/16 inch (approximately 3 to 5 mm) thick, but still possess reasonable tensile strength, depending on the resin used. The mold may be heated further in an oven in order to cure and harden it before removing it from the pattern. Once all the pieces of the shell mold are created, they may either be clamped together or held together with adhesives. The finished mold is then typically placed into a container that is filled with shot, sand, or another similar material to add strength.
Once the mold is fully assembled, molten metal is poured into it and allowed to cool. After the metal has cooled, the shell is “broken,” though the shell sections will not actually be damaged under normal conditions. The modular nature of the mold allows the finished metal piece to be removed without damaging the mold, thus making the mold reusable.
The materials used in the shell molding process may be more expensive than those used in other molding processes, but the finished mold requires less material due to its thinness, which may offset the difference in cost. Having a reusable mold is is one of the key advantages of shell molding over sand casting. Though the materials needed for sand casting are much cheaper, the mold is destroyed, while the mold in shell molding is reusable.