Sand moulded casting is the process of creating a metal object by pouring molten metal into a mould made of sand. Castings are made at factories specifically known as foundries, where the process is considered something of an art.
The preparation process begins with making a mould of the casting using greensand – sand with a small percentage of clay and water. No water should be visible. The sand is tested in a fist to see if its moisture level is enough to retain shape and not break apart. If the sand is too moist, the mould may explode during the pour; if it is too dry, the mould may collapse.
Castings are first designed and formed as replica objects normally made of wood or plastic split into two halves. Each half will form part of the mould to be put together. The bottom half is placed into a container that is then covered with the sand. Sand is continually added and compacted until the entire container volume is filled with a solid surface. Turning the container around, the replica object, referred to as a pattern in casting, is removed using releasing agents and the first half of the mould is neatened up.
The top half of the replica object is then placed over the hollow shape of the bottom half. A top container is connected to the bottom container and sand is once again packed on top. Cavities are created to allow metal to pour through into the shape of the mould. The boxes are then separated and the top half of the object is removed. When the mould is put back together, the metal is poured into the cavities, filling up the space, and then left to cool and solidify. The greensand mould is broken off to reveal a perfect metal casting. The sand is then recycled and reused for the next mould.
Sand-moulded casting is ideal for fairly small production rates, complex casting shapes and large castings. For mass production fully automated moulding machines increasingly replace the labour intensive techniques.