Moulding Sand in Casting
Moulding sand is a specially prepared sand used in foundry operations to create moulds and cores for metal casting. It must possess specific properties to withstand the high temperatures of molten metal and produce accurate, defect-free castings.
1. Definition
Moulding sand is a granular material, primarily composed of refractory particles such as silica, that is mixed with binders and moisture to form a cohesive mass. This mass is shaped around a pattern to form the mould cavity into which molten metal is poured.
2. Constituents of Moulding Sand
- Refractory Base Sand: Usually silica (SiO₂) 80–90%, provides heat resistance and maintains shape at high temperatures.
- Binder: Clay minerals like bentonite or fireclay (4–10%) that impart cohesiveness and strength.
- Moisture: 2–8% water to activate the binder and develop plasticity.
- Additives: Materials like coal dust, dextrin, wood flour to improve surface finish, collapsibility, and control expansion.
3. Properties of Moulding Sand
- Refractoriness
- Ability to withstand high temperatures without fusing or breaking down.
- Permeability
- Capacity to allow gases and steam to escape during pouring and solidification.
- Cohesiveness (Green Strength)
- Strength of the sand in moist condition to retain the mould shape during handling.
- Dry Strength
- Strength of the mould after moisture has evaporated; important for large castings.
- Plasticity
- Ability to flow and fill details around the pattern during ramming.
- Collapsibility
- Ease with which the mould can break away from the solidified casting, preventing hot tears.
- Adhesiveness
- Ability of the sand to stick to the moulding box surfaces.
4. Types of Moulding Sand
4.1 Based on Condition
- Green Sand: Naturally moist sand containing silica, clay, and water; used in its damp state.
- Dry Sand: Green sand dried before pouring; stronger and more heat resistant.
- Loam Sand: High clay content (up to 50%), used for large, intricate castings.
- Facing Sand: High-quality sand applied next to the pattern for a smooth surface finish.
- Backing Sand: Fills the bulk of the mould behind the facing sand; often reused sand.
- System Sand: Used in mechanical moulding systems; prepared to consistent properties.
- Core Sand: Special sand for making cores; high refractoriness and collapsibility.
4.2 Based on Origin
- Natural Sand: Contains sufficient clay as mined; requires minimal preparation.
- Synthetic Sand: Prepared by mixing clean silica sand with binders and additives to desired properties.
- Special Sands: Zircon, chromite, olivine sands for high-temperature alloys or special properties.
5. Preparation of Moulding Sand
- Selection of Base Sand: Choose silica or special sand based on casting requirements.
- Mixing: Add binder, moisture, and additives in correct proportions.
- Tempering: Allow the mixture to rest so moisture distributes evenly.
- Testing: Check properties like permeability, strength, and moisture content.
- Conditioning: Aerate and break lumps to ensure uniform texture before use.
6. Applications
- Used to form mould cavities for ferrous and non-ferrous castings.
- Core making for internal cavities in castings.
- Special sands for high-temperature alloys and precision castings.
7. Advantages
- Reusable after proper conditioning.
- Economical and widely available.
- Adaptable to various casting sizes and shapes.
8. Limitations
- Moisture control is critical; too much or too little affects quality.
- May require additives for special properties.
- Silica dust can be hazardous; requires proper handling.