Is it the more uniform the particle size of chromite sand the better?
Is the more uniform the particle size of chromite sand the better?
Ⅰ. Serious disadvantages of completely uniform particle size chromite sand (single mesh, full same particle size)
- Extremely high porosity (＞40%)
Uniform particles stack with fixed large gaps between them. When pouring molten steel/iron, high-temperature metal liquid easily penetrates the gaps and causes burn-on, sand inclusion, sand holes on castings; at the same time, the large porosity leads to very low sand mold strength, easy collapse/sand falling during mold lifting, mold closing and pouring.
- Excessive air permeability & poor bonding performance
Casting sand only needs moderate air permeability (to discharge gas in sand mold). Over-uniform chromite sand has overly high air permeability, and the binder (resin/water glass) cannot form a dense adhesive film between particles — the sand mold has insufficient cured strength, and sintering/cracking occurs at high temperature due to less binder adhesion.
- Reduced refractory & anti-slag performance
The core advantage of chromite sand is high refractoriness (Cr₂O₃ content) and strong anti-slag ability. Uniform single particle size forms a “single-layer protective sand layer”, the high-temperature molten metal directly erodes the sand surface, which easily causes sand particle softening and reaction with metal oxides, and the anti-burn-on effect is greatly reduced.
Ⅱ. The ideal particle size state of chromite sand for casting: continuous gradation (coarse-fine matching)
This is why chromite sand is always marked with mesh range (e.g., 30-70 mesh, 50-100 mesh) instead of a single mesh size, and this gradation has 4 core advantages (completely solve the defects of uniform particle size):
- Moderate porosity (25%~35%, the best range)
It balances air permeability and compactness perfectly: the coarse particle skeleton ensures smooth gas discharge (avoids casting blowholes), fine particles fill gaps to prevent molten metal penetration (avoids burn-on/sand inclusion) — this is the most critical performance requirement for casting chromite sand.
- Double the strength of sand mold (normal & high temperature)
The staggered stacking of coarse and fine particles creates more bonding nodes for binders. The adhesive film between particles is denser, so the sand mold has high green strength (no deformation) and high hot strength (no cracking/sintering at high temperature). This is essential for large steel castings/alloy steel castings.
- Maximize refractory & anti-erosion performance
The dense multi-layer sand structure forms a “composite protective layer”. At high temperature, Cr₂O₃ in chromite sand forms stable high-melting compounds, which can resist the scouring of molten metal and slag corrosion effectively, and reduce the sintering tendency of chromite sand.
- Cost-effective & material-saving
Dense graded sand has higher bulk density, the same volume of sand mold requires less chromite sand consumption, and the casting quality is guaranteed at the same time.
Ⅲ. Special case: Relatively uniform particle size is better (narrow mesh range, not 100% uniform)
- Precision casting / coated sand core making (thin-wall small castings, investment casting)
Choose chromite sand with 70-140 mesh (AFS 55-65), narrow mesh range, relatively concentrated particle size. The purpose is to improve the surface finish of sand core/casting, reduce post-processing grinding workload of castings. This kind of sand needs to be matched with high-purity resin to make up for the slight deficiency of strength.
- Facing sand of castings
The surface layer of casting mold (facing sand) uses relatively uniform fine chromite sand, and the back sand uses coarse sand. This “layered matching” ensures the surface smoothness of castings (facing sand) and the air permeability/strength of sand mold (back sand).
Ⅳ. Industry judgment standard for chromite sand particle size uniformity & gradation (match your AFS/mesh questions, directly applicable for procurement)
- Mesh range & AFS value correspondence (core procurement index)
- Mainstream casting (sand casting, large steel castings): wide mesh range (30-70/40-80/50-100 mesh, AFS 35-55) → standard continuous gradation (coarse + fine), the best choice.
- Precision casting/coated sand: narrow mesh range (70-140 mesh, AFS 55-65) → relatively uniform fine particles, only for precision process.
- Single mesh marking (e.g., only 70 mesh) → completely uniform particle size, not recommended for any casting process.
- Main particle grade ratio (uniformity index)
Qualified casting chromite sand requires the main particle grade (e.g., 50-70 mesh +70-100 mesh for 50-100 mesh sand) to account for ≥85%, with a small amount of coarse particles (>50 mesh) and fine powder (<100 mesh) as filling — this is the optimal gradation. If a single particle grade accounts for ≥95%, it is over-uniform and unusable.
- Fine powder control
The content of fine powder (<200 mesh) should be ≤3%: excessive fine powder blocks pores and reduces air permeability (causes blowholes); no fine powder at all will lose the micro-gap filling effect, and the compactness of sand mold will decrease.
Final Core Summary (easy to remember, direct application for procurement/use)
✅ For precision casting/coated sand/thin-wall small castings: Choose narrow mesh range (relatively uniform fine particles) → prioritize casting surface finish (matched with high-quality binder).
✅ All scenarios: Never use 100% uniform single particle size chromite sand — disadvantages far outweigh advantages (universal principle for chromite sand, quartz sand, ceramic foundry sand).