1. Introduction
Casting is a manufacturing process in which molten metal is poured into a mould cavity of the desired shape and allowed to solidify. After solidification, the metal takes the shape of the mould and is removed to obtain the final product called a casting.
Casting is one of the oldest and most versatile manufacturing processes and is widely used to produce complex shapes that are difficult or uneconomical to manufacture by other methods.
2. Principle of Casting
The basic principle of casting involves:
- Melting the metal
- Pouring molten metal into a mould cavity
- Allowing the metal to solidify
- Removing the solidified part from the mould
The shape of the final product depends on the shape of the mould cavity.
3. Advantages of Casting
- Can produce complex shapes
- Suitable for large components
- Almost any metal can be cast
- Internal cavities possible
- Economical for mass production
- Minimal machining required
4. Limitations of Casting
- Lower strength compared to forged parts
- Possibility of internal defects
- Surface finish may be poor
- Requires further machining in some cases
5. Classification of Casting Processes
A. Expendable Mould Casting
The mould is destroyed after each casting.
- Sand casting
- Shell moulding
- Investment casting
- Plaster mould casting
B. Permanent Mould Casting
The mould is reused multiple times.
- Die casting
- Gravity mould casting
- Centrifugal casting
6. Sand Casting
Sand casting is the most widely used casting process.
Steps in Sand Casting
- Pattern making
- Mould preparation
- Core making (if required)
- Melting and pouring
- Solidification and cooling
- Shakeout and cleaning
Advantages:
- Low cost
- Suitable for large castings
- Wide range of metals
7. Pattern and Pattern Allowances
Pattern
A pattern is a replica of the final casting, used to create the mould cavity.
Pattern Allowances
- Shrinkage allowance
- Machining allowance
- Draft allowance
- Distortion allowance
- Rapping allowance
8. Moulding Sand and Its Properties
Moulding sand consists of:
- Base sand
- Binder (clay)
- Moisture
Important properties:
- Permeability
- Refractoriness
- Strength
- Plasticity
- Adhesiveness
- Collapsibility
9. Gating System
The gating system controls the flow of molten metal into the mould.
Components:
- Pouring basin
- Sprue
- Runner
- Gate
A well-designed gating system prevents turbulence and defects.
10. Risers (Feeders)
- Supply molten metal during solidification
- Compensate for shrinkage
- Improve soundness of casting
11. Types of Special Casting Processes
a) Die Casting
- Molten metal forced into mould under high pressure
- Good surface finish and accuracy
- Used for aluminium, zinc alloys
b) Investment Casting
- Uses wax pattern
- Produces very accurate and complex shapes
- Used in aerospace and medical components
c) Centrifugal Casting
- Mould rotated during pouring
- Dense and defect-free castings
- Used for pipes and cylinders
12. Casting Defects
Common defects in castings include:
- Blowholes
- Shrinkage cavities
- Cold shuts
- Misruns
- Hot tears
- Inclusions
Proper mould design and process control reduce defects.
13. Inspection and Testing of Castings
- Visual inspection
- Radiographic testing
- Ultrasonic testing
- Magnetic particle testing
- Pressure testing
14. Applications of Casting
- Engine blocks
- Machine tool beds
- Pipes and valves
- Automotive components
- Pump housings
- Turbine casings
15. Comparison of Casting with Other Processes
| Aspect | Casting | Forging |
|---|---|---|
| Shape complexity | High | Limited |
| Strength | Lower | Higher |
| Cost | Lower | Higher |
| Material waste | Less | More |