Slush casting is a lesser known variant of the permanent mould casting used to produce hollow castings. The method was developed by William Britain in 1893 for the production of lead toy soldiers. It uses less material than solid casting, and results in a lighter and less expensive product.
The main principle of this casting process relies on the fact that when a metal casting solidifies in a mould, it will solidify from the mould wall towards the inside of the casting. Molten metal is poured into the mould and allowed to cool until a shell of metal forms in the mould. The mould should be rotated a bit. The molten metal is not allowed to completely solidify in the mould; when the desired thickness in obtained, the remaining molten metal is poured out to leave a hollow metallic shell. The longer the metal casting was allowed to solidify before pouring out the excess metal, the greater the casting’s wall thickness will be. Some mechanical means of turning over the mould is needed in order to pour out the molten metal from the cast part. If the casting seems to be too thin, the pouring in and out process is repeated until the desired thickness is built up.