U N I T 1 MATERIAL RETENTION PROCESSES This Unit looks at the most common methods for shaping materials into the desired forms by displacement, conserving their mass. You will learn about casting, forging and other deformation processes. A. CASTING Casting involves pouring a molten metal into a hollow mould, where it hardens and takes the form of the cavity. Sand casting is the oldest and best-known technique, based on the three fundamental steps of any casting process: moulding, melting and casting. Firstly, the pattern of the finished product is made in order to form a mould with the desired shape. This pattern is made of wood, plastic or metal and is larger than the finished product in order to balance the casting shrinkage occurring with cooling and solidification. Secondly, sand is packed around the pattern which is then removed, leaving an empty mould. Finally, the empty cavity is filled with molten metal which is then allowed to solidify into the shape of the sand mould, breaking the mould away and removing the casting. If necessary, cores are introduced in the mould in order to form internal cavities, holes. Sand casting employs temporary, non-reusable moulds and relies on gravity to force the molten substance into them. It is relatively cheap and usually results in items with high porosity and low dimensional accuracy. Die, permanent and centrifugal casting employ permanent, reusable moulds made of metal or graphite and exploit pressure, gravity or centrifugal forces to pour the molten metal into the mould. These techniques are suitable for mass production and provide high dimensional accuracy and good surface finish. Continuous casting is largely used in the steel industry to form long bars which are then cut, processed and shaped according to the requirements. GLOSSARY to cast: fondere core: anima die casting: pressofusione hollow: vuoto to pack: pressare pattern: modello 104 to pour: versare to result in: avere come risultato shrinkage: restringimento, contrazione