Sand Casting

Sand casting is one of the oldest and most widely used metal casting processes. It involves pouring molten metal into a sand mould, allowing it to solidify, and then breaking the mould to retrieve the casting. This method is valued for its versatility, low tooling cost, and ability to produce large and complex shapes.

1. Definition

Sand casting is a manufacturing process in which a mould made of sand is used to form metal castings. The sand mould is created by packing sand around a pattern of the desired shape. Once the pattern is removed, molten metal is poured into the cavity and allowed to cool and solidify.

2. Historical Background

Sand casting has been practiced for thousands of years, with evidence of its use in ancient China, Egypt, and Mesopotamia. Over time, the process has evolved with improved moulding materials, pattern-making techniques, and metal alloys, but the basic principles remain the same.

3. Basic Steps in the Sand Casting Process

Pattern Making: Creating a replica of the final product, including allowances for shrinkage, machining, and draft.
Mould Preparation: Packing sand around the pattern in a moulding box (cope and drag) to form the mould cavity.
Core Making (if needed): Producing sand cores to form internal cavities in the casting.
Mould Assembly: Placing the cope and drag together, along with cores and gating system.
Melting: Heating the metal in a furnace until it becomes molten.
Pouring: Introducing molten metal into the mould cavity through the gating system.
Cooling and Solidification: Allowing the metal to cool and solidify inside the mould.
Shakeout: Breaking the mould to remove the casting.
Cleaning and Finishing: Removing sand, gates, risers, and performing surface finishing or machining.
Inspection: Checking the casting for defects and dimensional accuracy.

4. Components of a Sand Casting Mould

Flask: The rigid frame that holds the sand mould (cope – top, drag – bottom).
Pattern: Model of the casting used to form the mould cavity.
Core: Sand insert used to create internal cavities.
Gating System: Channels (sprue, runner, gates) that guide molten metal into the mould cavity.
Riser: Reservoir that feeds molten metal to the casting during solidification to compensate for shrinkage.
Vents: Small openings to allow gases to escape.

5. Types of Sand Used

Green Sand: Moist sand with clay binder; most common type.
Dry Sand: Green sand dried before pouring; stronger and more heat resistant.
Resin-Bonded Sand: Sand bonded with synthetic resins for high strength and precision.

6. Advantages of Sand Casting

Low tooling and equipment cost.
Suitable for small to very large castings.
Can produce complex shapes, both internal and external.
Applicable to almost all metals and alloys.
Adaptable to low and high production volumes.

7. Limitations of Sand Casting

Lower dimensional accuracy compared to precision casting methods.
Rough surface finish requiring machining.
Labour-intensive for high-volume production without automation.
Possible defects such as porosity, shrinkage, and sand inclusion.

8. Applications

Automotive engine blocks, cylinder heads, and manifolds.
Machine tool beds and frames.
Pump housings and valve bodies.
Artistic sculptures and decorative items.
Large industrial components such as turbine casings.

9. Common Defects in Sand Casting

Blowholes: Gas pockets in the casting.
Misruns: Incomplete filling of the mould cavity.
Cold Shuts: Poor fusion between two metal streams.
Sand Inclusions: Loose sand particles embedded in the casting surface.
Shrinkage Cavities: Voids caused by metal contraction during solidification.

10. Conclusion

Sand casting remains a versatile and economical manufacturing process, capable of producing a wide range of metal components. By selecting the right sand type, pattern design, and process parameters, manufacturers can achieve high-quality castings suitable for diverse applications.