The Secret to Successful Metal Casting: Critical Properties of Moulding Sand
In the metal casting industry, the quality of the final product depends directly on the technical specifications of the sand used to prepare the casting mould. To obtain a flawless cast part, the moulding sand must meet specific parameters. The most important of these parameters are gas permeability and strength.
1. Vital Property: Gas Permeability
The most important and primary property expected from a casting sand is gas permeability.
Why Is It So Critical? The moment molten metal is poured into the mould, the gases within the metal and the moisture in the mould (from wet binders) rapidly vaporize. If the moulding sand cannot efficiently vent these gases (casting gases) to the outside, meaning it is not gas permeable, these high-pressure gases become trapped inside the mould.
The results of this entrapment lead to critical defects that can render the cast part unusable:
- Gas Porosity: Small air bubbles trapped inside or on the surface of the metal part.
- Mould Explosions: Sudden and high pressure physically cracking or breaking the mould.
- Incomplete Fill (Misrun): Trapped gas preventing the metal from completely filling the mould.
2. Secondary Parameter: Mould Strength (Resistance)
After gas permeability is ensured, the second most important parameter sought is strength. The moulding sand must withstand the demanding conditions of the moulding and casting process.
- Green Strength: The ability of the mould to retain its shape and not disintegrate while still moist (during preparation, transport, and assembly stages).
- Dry Strength: Its strength after drying and hardening from the high heat of the molten metal.
Result of Insufficient Strength: If the moulding sand cannot provide the required strength levels (neither green nor dry), the mould will disintegrate under the pressure of the metal during casting or while the part is cooling after casting. This situation causes the casting to fail and requires the entire process to be repeated.
Why is Silica (SiO2) Preferred as Casting Sand?
The most widely used material as moulding sand in the market is Silica (SiO2) sand. There are two main economic and technical reasons behind this preference:
- Cost Advantage: It is one of the cheapest and most readily available raw materials compared to other casting sands (zircon, chromite, etc.).
- High Refractoriness (Heat Resistance): Silica sand shows excellent resistance to the extreme temperature of molten metal. Thanks to its high refractoriness, it maintains its structural integrity up to high melting points such as 2073 K (approximately 1800°C). This allows the sand to maintain the mould's shape without melting or reacting chemically with the metal.
