Casting Methods
Casting is a manufacturing process in which a liquid material (usually molten metal) is poured into a mould containing a hollow cavity of the desired shape, and then allowed to solidify. The solidified part, known as a casting, is ejected or broken out of the mould to complete the process. Different casting methods are used depending on the material, shape, size, accuracy, and production volume required.
1. Sand Casting
Sand casting is the most widely used casting process. It uses sand as the mould material, which is inexpensive and adaptable to complex shapes.
Process Steps
- Prepare a pattern of the part to be cast.
- Pack sand around the pattern in a moulding box (cope and drag).
- Remove the pattern to leave a cavity.
- Assemble the mould, pour molten metal, allow to solidify.
- Break the mould to remove the casting.
Advantages
- Low cost for small quantities.
- Can produce very large parts.
- Suitable for almost all metals.
Limitations
- Lower dimensional accuracy.
- Rough surface finish.
2. Shell Mould Casting
Uses a thin shell of sand bonded with resin, formed over a heated metal pattern. The shell halves are joined and metal is poured in.
Advantages
- Better surface finish and accuracy than green sand.
- Good for medium to high production rates.
Limitations
- Higher tooling cost.
- Limited to smaller castings.
3. Investment Casting (Lost Wax)
A wax pattern is coated with refractory ceramic material to form a mould. Once the ceramic is hardened, the wax is melted out and molten metal is poured in.
Advantages
- Excellent surface finish and accuracy.
- Can produce complex shapes with fine details.
Limitations
- High cost per unit.
- Longer production time.
4. Plaster Mould Casting
Similar to sand casting but uses plaster of Paris as the mould material. Suitable for non-ferrous metals with low melting points.
Advantages
- Good surface finish and detail reproduction.
- Low-cost tooling for small runs.
Limitations
- Not suitable for high-temperature metals.
- Longer mould preparation time.
5. Die Casting
Molten metal is forced into a metal mould (die) under high pressure. Commonly used for non-ferrous metals like aluminium, zinc, and magnesium.
Advantages
- High production rate.
- Excellent dimensional accuracy and surface finish.
- Thin walls possible.
Limitations
- High initial tooling cost.
- Limited to low-melting-point alloys.
6. Centrifugal Casting
Molten metal is poured into a rotating mould, and centrifugal force distributes the metal to the mould walls.
Advantages
- Dense, high-quality castings with fine grain structure.
- Good for cylindrical parts like pipes and rings.
Limitations
- Limited to symmetrical shapes.
- Specialised equipment required.
7. Continuous Casting
Molten metal is continuously poured into a water-cooled mould and solidified as it exits, producing long lengths of material.
Advantages
- High productivity and yield.
- Uniform properties along the length.
Limitations
- Limited to simple cross-sections.
- High capital cost.
8. Other Special Methods
- Vacuum Casting: Reduces gas porosity by pouring under vacuum.
- Squeeze Casting: Combines casting and forging for high strength.
- Lost Foam Casting: Uses a foam pattern that vaporizes when metal is poured.
Conclusion
The choice of casting method depends on factors like material, size, complexity, required accuracy, surface finish, and production volume. Understanding the strengths and limitations of each method helps in selecting the most cost-effective and technically suitable process.