Overview
This lecture covers the various types of moulding sands, their compositions, properties, and specific uses in different casting and moulding conditions.
Types of Moulding Sands
Green Sand
- Green sand is a moist mixture of silica sand, 18-30% clay, and 6-8% water.
- It offers good plasticity and binding capacity, making it easy to shape.
- Used for casting simple, small, or medium-sized moulds without baking or drying.
- Green sand is fine, soft, light, and porous.
Dry Sand
- Dry sand is green sand that has had all moisture removed by heating in a drying oven.
- Used for preparing large and heavy casting moulds due to its increased strength and stability.
- Dry sand provides better dimensional accuracy and soundness in castings.
Facing Sand
- Facing sand is applied against the mould pattern and forms the mould surface in direct contact with molten metal.
- It must have high strength and refractoriness to withstand heat.
- Composed of silica sand and clay, often with carbon materials like graphite or molasses for protection.
- Applied as a 20-30 mm layer in the mould.
Backing (Baking) Sand
- Backing sand is used behind the facing sand to fill the rest of the mould.
- Made from repeatedly used moulding sand, giving it a black color due to coal dust and metal action.
- Must be cleaned of foreign materials before reuse.
Parting Sand
- Parting sand prevents the two halves of the mould (cope and drag) from sticking together.
- Made of clean, dry silica sand with no moulding properties.
- Spread as a fine layer between moulding boxes.
System Sand
- System sand is used in machine moulding where facing sand is not applied.
- Prepared by adding additives, binders, and water to available sand.
- Must have higher strength, permeability, and refractoriness than facing or backing sands.
- All sand in the mould is systematically prepared and reused after cleaning and reconditioning.
Core Sand
- Core sand, also called oil sand, is used specifically for making cores (projections inside moulds).
- Composed of silica sand mixed with oil, resin, molasses, charcoal, and other binders.
- Requires high refractoriness and permeability to withstand molten metal and allow gas escape.
- Provides a smooth finish to the core surface.
Key Terms & Definitions
- Green Sand — moist moulding sand mixture of silica, clay, and water for shaping moulds.
- Dry Sand — green sand with moisture removed by heating for stronger, more stable moulds.
- Facing Sand — sand layer in direct contact with molten metal in the mould.
- Backing Sand — sand used behind facing sand to fill the rest of the mould.
- Parting Sand — dry sand preventing sticking between mould halves.
- System Sand — mechanically prepared sand for entire machine-made moulds.
- Core Sand — sand with oil and binders, used to form cores inside moulds.
Action Items / Next Steps
- Review and memorize the types and uses of moulding sands.
- Prepare for questions on sand properties and correct sand selection for different casting conditions.