Casting Lectures

1 Lecture 2. Basics of Metal-Casting
2.1. Casting methods
Metal casting process begins by creating a mold, which is the ‘reverse’ shape of the part we need. The mold is made from a refractory material, for example, sand. The metal is heated in an oven until it melts, and the molten metal is poured into the mould cavity. The liquid takes the shape of cavity, which is the shape of the part. It is cooled until it solidifies. Finally, the solidified metal part is removed from the mould.
A large number of metal components in designs we use every day are made by casting. The reasons for this include:
(a) Casting can produce very complex geometry parts with internal cavities and hollow sections.
(b) It can be used to make small (few hundred grams) to very large size parts (thousands of kilograms)
(c) It is economical, with very little wastage: the extra metal in each casting is re-melted and re-used
(d) Cast metal is isotropic – it has the same physical/mechanical properties along any direction.
Common examples: door handles, locks, the outer casing or housing for motors, pumps, etc., wheels of many cars. Casting is also heavily used in the toy industry to make parts, e.g. toy cars, planes, and so on.
Table 1 summarizes different types of castings, their advantages, disadvantages and examples. Process Advantages Disadvantages Examples
Sand Wide range of metals, sizes, shapes, low cost poor finish, wide tolerance engine blocks, cylinder heads
Shell mold better accuracy, finish, higher production rate limited part size connecting rods, gear housings
Expendable
pattern Wide range of metals, sizes, shapes patterns have low strength cylinder heads, brake components
Plaster mold complex shapes, good surface finish non-ferrous metals, low production rate prototypes of mechanical parts
Ceramic mold complex shapes, high accuracy, good finish small sizes impellers, injection mold tooling
Investment complex shapes, excellent finish small parts, expensive jewellery
Permanent mold good finish, low porosity, high production rate Costly mold, simpler shapes only gears, gear housings
Die Excellent dimensional accuracy, high production rate costly dies, small parts,
non-ferrous metals precision gears, camera bodies, car wheels
Centrifugal Large cylindrical parts, good quality Expensive, limited shapes pipes, boilers, flywheels