The lost foam or expendable pattern-casting process is a relatively new process in commercial terms, but is gaining increased attention due to the environmental and technical benefits that are achievable for some types of casts. In the lost foam casting process, an expendable pattern is formed out of polystyrene foam. Patterns can be made manually, using automated systems or by moulding them using a permanent die. Manual pattern making typically involves carving blocks and gluing sections together to build up the desired shape. The finished pattern is a single piece (i.e. no cores) incorporating all necessary gating systems.



This carving process can be automated using computer-aided manufacturing (CAM) system sand can incorporate rapid prototyping techniques. For repetitive castings, patterns can be moulded using a permanent aluminium die. Polystyrene beads are pre-expanded using a vacuum, steam or hot air processes. Lost foam casting helps to minimised the density of the foam as much as possible, to minimise the amount of vapour that is produced during the pouring process.

The expanded material is then blown into the aluminium mould. Steam is used to cause the material to expand further, bond together and fill the mould cavity. The mould and pattern are allowed to cool, and the pattern ejected (USEPA, 1998). As for investment casting, when the casting is small, multiple castings can be joined, often to a central tree, to increase pouring efficiency. The pattern is coated with a specially formulated gaspermeable refractory slurry. When the refractory slurry has hardened, the assembly is positioned in a flask, and unbonded sand is poured around the mould and compacted into any internal cavities.

The refractory coating must be sufficiently strong to prevent the loose sand from collapsing into the cavity as the pattern vaporises, but also permeable to allow styrene vapour to escape from the mould cavity. A vacuum system can also be used to increase sand compaction. Molten metal is then poured into the polystyrene pattern, which vaporises and is replaced by the metal. This is different from the lost wax process in which the wax is removed before the pouring stage. Vents in the side of the flask allow vapour to escape. If vapour is produced more rapidly than it can be vented, the casting may become deformed. When the metal has solidified, the flask is emptied onto a steel grate for shakeout. The loose sand falls through the grate and can be reused without treatment. The refractory material is broken away from the casting in the usual manner.