1. Introduction to Casting
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.
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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 and Disadvantage of Casting
| Advantage | Disadvantage |
|---|---|
| 1. Can produce complex shapes 2. Suitable for large components 3. Almost any metal can be cast 4. Internal cavities possible 5. Economical for mass production 6. Minimal machining required | 1. Lower strength compared to forged parts 2. Possibility of internal defects 3. Surface finish may be poor 4. Requires further machining in some cases |
5. Classification of Casting Processes
A. Expendable Mould Casting
Expendable mould casting is a casting process in which the mould is destroyed or broken to remove the solidified casting. Since the mould cannot be reused, a new mould must be prepared for each casting.
This is the most widely used category of casting processes because it can produce components with complex shapes, internal cavities, and a wide range of sizes.
- Sand casting
- Shell moulding
- Investment casting
- Plaster mould casting
B. Permanent Mould Casting
Permanent mould casting is a metal casting process in which molten metal is poured into a reusable metallic mould (die). Unlike expendable mould casting, the mould is not destroyed after each casting and can be used repeatedly to produce many identical components.
The mould is generally made of cast iron, tool steel, or alloy steel, allowing it to withstand repeated heating and cooling cycles.
- 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
A gating system is the network of channels that guides molten metal from the pouring basin to the mould cavity in a controlled manner. It ensures that the mould is filled completely with minimum turbulence, preventing casting defects such as air entrapment, erosion, and inclusions. A well-designed gating system prevents turbulence and defects.
Components:
- Pouring basin
- Sprue
- Runner
- Gate
10. Risers (Feeders)
- Supply molten metal during solidification
- Compensate for shrinkage
- Improve soundness of casting
11. Types of Special Casting Processes
a) Die Casting
Die casting is a permanent mould casting process in which molten metal is injected into a reusable steel die under high pressure. The metal rapidly solidifies inside the die, producing components with high dimensional accuracy, excellent surface finish, and fine details.
Die casting is one of the fastest and most economical methods for producing large quantities of non-ferrous metal components.
- 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 |