Shell molding, also known as shell-mold casting, is an expendable mold
casting process that uses a resin covered sand to form the mold. As compared to
sand casting, this process has better dimensional accuracy, a higher
productivity rate, and lower labor requirements. It is used for small to medium
parts that require high precision. Shell mold casting is a metal casting
process similar to sand casting, in that molten metal is poured into an
expendable mold. However, in shell mold casting, the mold is a thin-walled shell
created from applying a sand-resin mixture around a pattern. The pattern, a
metal piece in the shape of the desired part, is reused to form multiple shell
molds. A reusable pattern allows for higher production rates, while the
disposable molds enable complex geometries to be cast. Shell mold casting
requires the use of a metal pattern, oven, sand-resin mixture, dump box, and
Shell mold casting allows the use of both ferrous and non-ferrous metals,
most commonly using cast iron, carbon steel, alloy steel, stainless steel,
aluminum alloys, and copper alloys. Typical parts are small-to-medium in size
and require high accuracy, such as gear housings, cylinder heads, connecting
rods, and lever arms.
The shell mold casting process consists of the following steps:
Pattern creation - A two-piece metal pattern is created in the shape of the
desired part, typically from iron or steel. Other materials are sometimes used,
such as aluminum for low volume production or graphite for casting reactive
Mold creation - First, each pattern half is heated to 175-370 °C (350-700 °F)
and coated with a lubricant to facilitate removal. Next, the heated pattern is
clamped to a dump box, which contains a mixture of sand and a resin binder. The
dump box is inverted, allowing this sand-resin mixture to coat the pattern. The
heated pattern partially cures the mixture, which now forms a shell around the
pattern. Each pattern half and surrounding shell is cured to completion in an
oven and then the shell is ejected from the pattern.
Mold assembly - The two shell halves are joined together and securely clamped
to form the complete shell mold. If any cores are required, they are inserted
prior to closing the mold. The shell mold is then placed into a flask and
supported by a backing material.
Pouring - The mold is securely clamped together while the molten metal is
poured from a ladle into the gating system and fills the mold cavity.
Cooling - After the mold has been filled, the molten metal is allowed to cool
and solidify into the shape of the final casting.
Casting removal - After the molten metal has cooled, the mold can be broken
and the casting removed. Trimming and cleaning processes are required to remove
any excess metal from the feed system and any sand from the mold.
Examples of shell molded items include gear housings, cylinder heads and
connecting rods. It is also used to make high-precision molding cores.