The type of sand commonly used in sand casting is silica sand, often mixed with clay and water to form a strong, heat-resistant mould.

Types of Sand Used in Sand Casting

Here’s a detailed, structured explanation of the types of sand used in sand casting, including their composition, properties, and applications.

1. Introduction

In sand casting, the mold is made from sand, which must have certain properties:

Desired Properties of Molding Sand:

Refractoriness: Can withstand molten metal without fusing or breaking
Permeability: Allows gases to escape to prevent blow holes
Plasticity: Can be compacted around the pattern without cracking
Collapsibility: Can be broken easily after casting
Cohesiveness: Holds shape when dry or moist

Depending on the composition and binder, sands are classified into natural and synthetic types.

2. Types of Sand Used

A. Silica Sand (SiO₂)

Most widely used molding sand
High refractoriness (~1700°C)
Composition: Mostly quartz
Good thermal stability and reusability

Pros:

Cheap and abundant
Reusable after reclamation
Suitable for ferrous and non-ferrous metals

Cons:

Expansion on heating → may cause casting defects (scabs, hot tears)
Not ideal for very high-temperature metals

Applications: Most general-purpose casting like iron, steel, aluminium.

B. Olivine Sand

Composition: Mg₂SiO₄
Refractoriness: 1750–1800°C
Low thermal expansion compared to silica
Good permeability and collapsibility

Pros:

Reduces sand-related defects (hot tears, cracks)
Better for high-temperature steel casting

Cons:

More expensive than silica
Limited availability

Applications: Steel casting, high-temperature alloys, large castings

C. Chromite Sand

Composition: FeCr₂O₄
Refractoriness: ~2200°C (very high)
Very dense and heavy
Excellent thermal conductivity

Pros:

Ideal for high-temperature metal casting like steel
Can be reused multiple times

Cons:

Expensive
Less permeable → requires care to prevent gas defects

Applications: High-temperature steel molds, superalloys, heavy-duty molds

D. Zircon Sand

Composition: ZrSiO₄
Refractoriness: 2300°C (highest among casting sands)
High density, low thermal expansion, high corrosion resistance

Pros:

Excellent for precision casting and superalloys
Smooth surface finish
Can withstand repeated heating

Cons:

Very expensive
Limited availability

Applications: Aerospace, investment casting, superalloys

E. Synthetic or Special Purpose Sands

Resin-bonded sands: Phenolic, furan, or sodium silicate bonded
Properties: High strength, excellent surface finish
Application: Precision casting, thin-walled castings, complex molds

3. Binder Types

Sands are often mixed with binders to improve cohesiveness:

Binder Type	Example	Application
Clay / Water	Bentonite	Green sand molds
Chemical	Sodium silicate	Cold-box molds, hard molds
Organic	Phenolic resin, furan resin	Shell molds, thin sections

4. Summary Table

Sand Type	Composition	Refractoriness	Advantages	Disadvantages	Applications
Silica	SiO₂	~1700°C	Cheap, reusable	Expands → defects	General casting (Fe, Al)
Olivine	Mg₂SiO₄	1750–1800°C	Low thermal expansion, good permeability	Expensive	Steel casting
Chromite	FeCr₂O₄	~2200°C	High refractoriness, heavy	Expensive, low permeability	High-temp steel, superalloys
Zircon	ZrSiO₄	~2300°C	Excellent finish, high temp	Very expensive	Aerospace, precision casting
Synthetic / Resin-bonded	SiO₂ + resin	1700–2300°C	High strength, thin sections	Costly	Investment casting, thin walls

5. Key Points

Silica sand: Most common, low cost, general purpose
Olivine sand: Low thermal expansion, steel casting
Chromite & Zircon: High-temperature, heavy-duty molds, aerospace
Binders: Clay or chemical binders improve mold strength
Choice depends on: Metal temperature, casting size, accuracy, surface finish

Conclusion:

Sand casting uses silica, olivine, chromite, zircon, or synthetic resin-bonded sands depending on the metal type, temperature, and desired casting quality.