Chromite sand For Molding & Core Sand？
Typical Applications in Molding & Cores
Chromite is often used as a facing sand or in critical sections of a mold/core due to its higher cost. It is rarely used for entire molds (except for very large, critical castings).
Mold & Core Facing: A layer (25-50 mm) of chromite sand is placed against the pattern, backed up with cheaper silica sand.
Choke Sands: In gating systems, especially for heavy steel castings, to resist erosion from high-velocity metal flow.
Cores for Hot Spots: Used in cores placed in sections of the mold that experience the highest thermal load (e.g., heavy bosses, junctions).
Casting of Active Metals: For titanium, chromium, and manganese steels where low reactivity is crucial.
Large Steel Castings: Ingot molds, slab castings, ladles, and massive valve bodies where thermal control and penetration are major concerns.
Disadvantages & Considerations
Cost: Significantly more expensive than silica sand (typically 3-8x the cost).
Health & Safety: Contains trivalent chromium [Cr(III)], which is not considered a major hazard in its natural state. However, dust must be controlled. The primary concern is the potential for the formation of hexavalent chromium [Cr(VI)] during contact with molten metal at high temperatures (>1600°C) in the presence of alkalis (e.g., from certain binders). Foundries must implement engineering controls (ventilation) and monitor air quality.
High Density: While beneficial for stability, it makes the sand heavier to handle and can lead to core/mold sagging if not properly supported.
Binder Demand: Its angular grain shape and surface chemistry can require higher binder additions (e.g., resin) compared to rounded silica sands.
Binder Systems Used with Chromite Sand
Chromite works with all standard foundry binder systems:
Resin Binders: Phenolic urethane cold box (most common), furan no-bake, alkaline phenolic no-bake, shell resin.
Inorganic Binders: Sodium silicate (CO₂ or ester hardened) is popular, especially where low fume and improved shakeout are needed.
Clay Bonding: Can be used in green sand molds as a facing sand, though less common due to cost.
Pro Tip: The binder choice is critical. Alkaline phenolic no-bake systems are often paired with chromite for large steel castings due to their high strength and thermal properties.
Direct Comparison vs. Silica Sand & Zircon Sand
| Property | Chromite Sand | Silica Sand | Zircon Sand |
|---|---|---|---|
| Cost | Medium-High | Very Low | Very High |
| Thermal Expansion | Very Low | Very High | Lowest |
| Thermal Conductivity | High | Low | Highest |
| Refractoriness | Very High (2,150°C) | Medium (1,710°C) | Extreme (2,400°C) |
| Density | High (4.5) | Low (2.65) | Very High (4.6) |
| pH | Basic | Acidic | Acidic |
| Typical Use | Facing, heavy steel | General purpose, iron | Precision castings, high-alloy steel |
Rule of Thumb: Use Silica for most general work, Chromite to solve expansion/penetration problems in steel, and Zircon for the most critical precision or high-alloy applications where budget allows.