Basics of Metal-Casting

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