Using ceramic fused sand as stuffing sand is a prime example of a composite molding system. Instead of building the entire mold from one expensive material, you strategically use different sands where their properties are most needed:
Facing Sand: The inner layer (in contact with the molten metal). This is typically a specialized sand like Chromite, Zircon, or a high-performance resin-coated sand, chosen for its direct resistance to metal penetration and chemical reaction.
Stuffing/Backing Sand: The bulk of the mold, behind the facing sand. This is where ceramic sand excels.
| Chemical Analysis[%] | |
| Al2O3 | 70-73 |
| SiO2 | 8-20 |
| Fe2O3 | ≤3 |
| TiO2 | ≤3.5 |
| CaO | ≤0.45 |
| MgO | ≤0.35 |
| K2O | ≤0.33 |
| Na2O | ≤0.08 |
| Physical Properties | |
| Grain shape | Spherical Ball |
| Roundness | 90% |
| Refractoriness: | ＞1790℃ |
| Shape Coefficient | ≤1.1 |
| Thermal Expansion Ratio | 0.13%（Heated up for 10 min at 1000℃） |
| Thermal expansion coefficient | 6×10-6 /℃(20-1000℃) |
| Thermal conductivity | 0.698W/M.K(1200℃) |
| Specific Gravity:: | 1.95-2.05g/cm3 |
| Bulk density(LPD): | 3.4 g/cm3 |
| Color: | Black |
| AVAILABLE SIZES | |
| AFS25-35,AFS35-45,AFS45-55,AFS55-65,AFS60-70,AFS70-80,AFS95-110 Customized sizes are available upon request | |
Why Ceramic Fused Sand is an Excellent Stuffing Sand
Ceramic foundry sand (spherical, sintered alumino-silicate) possesses a unique combination of properties that make it ideal for this role.
| Advantage | Explanation & Benefit |
|---|---|
| 1. High Refractoriness | With a melting point typically >1790°C, it can easily handle the heat conducted from the facing layer without sintering or melting. This ensures the mold retains its shape and the backing sand doesn’t fuse to the facing sand. |
| 2. Low Thermal Expansion | Unlike silica sand, which expands sharply around 573°C, ceramic sand expands very little. This is critical. It prevents casting defects like veining and rat tails caused by expansion-induced stress in the mold. It also reduces the risk of cracking that could allow metal to penetrate. |
| 3. High Thermal Conductivity | It draws heat away from the casting faster than silica sand. This leads to faster solidification, finer grain structure in the metal, and increased molding line productivity. |
| 4. Superior Reclamation & Reuse | Ceramic sand grains are spherical and incredibly durable. They withstand mechanical reclamation with very low breakdown rates. As stuffing sand constitutes the bulk of the mold, its high reusability (90%+) drastically reduces new sand purchases and waste sand disposal, offering massive long-term cost savings. |
| 5. Excellent Flowability | The spherical particles flow like water, allowing for rapid and uniform filling of the flask in automated molding lines. This creates a dense, consistent sand bed with no soft spots. |
Practical Implementation in the Foundry
Here’s how it typically works in a production setting:
Coating/Mulling: The ceramic sand is often coated with a suitable binder (e.g., phenolic urethane, furan) in a continuous mixer.
Molding:
A layer of facing sand (e.g., chromite) is first introduced into the pattern box.
The resin-coated ceramic stuffing sand is then blown in to fill the rest of the flask.
Curing & Pouring: The mold cures, and molten metal is poured.
Knockout & Reclamation: After casting, the mold is broken apart. The sand mixture (ceramic and facing sand) is sent through a reclamation system.
The robust ceramic sand survives reclamation and is returned to the process.
A portion of the facing sand may be recovered, but fresh facing sand is regularly added to maintain facing layer quality.