Two common materials used for moulding in casting are sand (for sand moulds) and plaster (for plaster moulds). These materials form the cavity that shapes the molten metal.
In foundry practice, moulding materials are the substances used to create the mould cavity into which molten metal is poured. The choice of material affects casting quality, surface finish, and defect prevention. Let’s go step by step, focusing on the two main materials used in moulding.
Two Main Materials Used for Moulding
1. Sand
Description
- Sand is the most widely used moulding material in metal casting.
- Usually silica (SiO₂) sand is preferred due to its high melting point, abundance, and good thermal properties.
Types of Sand Used
- Green Sand
- Sand mixed with clay and moisture
- Used for general-purpose sand casting
- Cheap and reusable
- Dry Sand
- Baked or chemically bonded for higher strength
- Used in large or complex castings
- Special Sands
- Zircon sand, chromite sand for high-temperature alloys
Advantages of Sand
- High refractory property → can withstand molten metal
- Easy to shape and compact
- Inexpensive and widely available
- Can produce large castings
Limitations
- Surface finish may be rough
- Requires venting to prevent gas defects
- Dimensional accuracy is lower than metal or ceramic moulds
Applications
- Iron castings, steel castings, non-ferrous metal castings
- Engine blocks, pump housings, pipes
2. Metals (Permanent Mould Material)
Description
- Metallic moulds are usually made of steel, cast iron, or copper alloys.
- Used for permanent mould casting methods such as gravity die casting, pressure die casting.
Characteristics
- High thermal conductivity → faster cooling
- Reusable → suitable for mass production
- Can withstand repeated heating and pouring
Advantages
- Produces smooth surface finish
- High dimensional accuracy
- Longer mould life → economical for large production
Limitations
- High initial cost
- Limited size of casting
- Complex shapes may be difficult
Applications
- Aluminium and zinc alloy castings
- Small to medium high-precision components like automotive parts, gear blanks, and housings
Comparison Table: Sand vs Metal Mould
| Property | Sand | Metal |
|---|---|---|
| Reusability | Usually one-time (except green sand) | Reusable |
| Surface finish | Rough | Smooth |
| Dimensional accuracy | Moderate | High |
| Cost | Low | High |
| Size of casting | Large possible | Limited |
| Cooling rate | Slow | Fast |
Summary
- Sand → most common, cheap, suitable for large/complex castings, may need venting.
- Metal → used in permanent moulds, gives smooth finish and high accuracy, ideal for mass production.
Conclusion
The two main materials used for moulding are sand, which is cheap, refractory, and suitable for large castings, and metal, which is durable, reusable, and used in permanent mould casting for high-accuracy components.
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