How do AFS values and mesh sizes correspond in chromite ore?
The AFS (American Foundry Society) Grain Fineness Number (GFN) of chromite sand is not a one-to-one fixed correspondence with a single mesh size, but a numerical representation of its continuous particle size distribution calculated based on the percentage of particles retained on a series of standard sieves. The higher the AFS value, the finer the overall particle size; the lower the AFS value, the coarser the particles. Below is a detailed, industry-standard correspondence and calculation method, which is fully applicable to casting procurement and quality control.
Core Logic of AFS and Mesh Size
- AFS GFN: It is calculated by multiplying the mass percentage of chromite sand retained on each ASTM standard sieve by the corresponding AFS factor, then summing these products. It reflects the “average fineness” of the entire particle size range, not a single particle size.
- Mesh size: Refers to the sieve aperture specification (ASTM E-11 standard), usually expressed as a range (e.g., 30-70 mesh), representing the main particle size interval of the sand.
- Correspondence: A specific AFS range corresponds to a typical mesh interval and particle size range, which is a “range-to-range” matching, not a “one-to-one” fixed value.
Industry-Standard AFS – Mesh – Particle Size Corresponding Table (Casting Grade Chromite Sand)
The following table is the mainstream standard in the casting industry, which can be directly used for procurement and process matching.
| AFS Value Range | Typical Mesh Range | Corresponding Particle Size Range | Application Scenarios |
|---|---|---|---|
| 25-30 | 20-40 mesh | 0.45-0.85 mm | Large steel castings, heavy castings (coarse sand for back sand, high permeability) |
| 30-35 | 20-50 mesh | 0.30-0.85 mm | General steel castings, sand mold back sand |
| 35-40 | 30-70 mesh | 0.21-0.60 mm | Medium-sized castings, core making with water glass |
| 40-45 | 40-70 mesh | 0.21-0.42 mm | Medium and small steel castings, sand mold facing sand |
| 45-55 | 50-100 mesh | 0.15-0.30 mm | Precision cast steel, alloy steel castings (most commonly used) |
| 55-65 | 70-140 mesh | 0.10-0.21 mm | Precision casting, coated sand core making, thin-walled small castings |
| 65-70 | 100-140 mesh | 0.10-0.15 mm | Ultra-precision casting, surface coating sand |
Calculation Example of AFS Value (Taking 50-100 Mesh Chromite Sand as an Example)
Taking a typical chromite sand sample with an AFS value of 50-55 as an example, the calculation process is as follows (data source: actual casting test report):
| Sieve Mesh (ASTM) | Aperture (μm) | Retention Percentage (%) | AFS Factor | Calculation (Percentage × Factor) |
|---|---|---|---|---|
| +40 mesh (＞425 μm) | 425 | 5.9 | 12 | 5.9 × 12 = 70.8 |
| +50 mesh (300-425 μm) | 300 | 36.7 | 20 | 36.7 × 20 = 734.0 |
| +70 mesh (212-300 μm) | 212 | 41.9 | 30 | 41.9 × 30 = 1257.0 |
| +100 mesh (150-212 μm) | 150 | 10.6 | 50 | 10.6 × 50 = 530.0 |
| +140 mesh (106-150 μm) | 106 | 2.9 | 70 | 2.9 × 70 = 203.0 |
| -140 mesh (＜106 μm) | ＜106 | 2.0 | 100 | 2.0 × 100 = 200.0 |
| Total | – | 100.0 | – | AFS = 70.8+734.0+1257.0+530.0+203.0+200.0 = 3004.8 → AFS ≈ 50.5 |
Key Notes for Practical Application
- No absolute one-to-one correspondence: The same AFS value can correspond to slightly different mesh ranges (e.g., AFS 45-50 can be 40-70 mesh or 50-100 mesh) depending on the particle size gradation (coarse-fine ratio).
- Gradation priority: The core of casting chromite sand is “continuous gradation” (coarse particles as the skeleton, fine particles filling gaps), so the mesh range is a “wide interval” (e.g., 50-100 mesh), not a single mesh size.
- Fine powder control: For all AFS ranges, the content of fine powder (-200 mesh, ＜75 μm) should be ≤3% to avoid poor permeability and increased gas holes in castings.
- AFS calculation standard: It must be tested in accordance with the AFS 1102-00 (2016) standard, using ASTM E-11 standard sieves and specified calculation factors to ensure data accuracy.
Quick Procurement and Process Matching Formula
- Coarse sand (AFS 25-35): 20-50 mesh → suitable for large castings and back sand, focusing on strength and permeability.
- Medium sand (AFS 40-50): 40-70 mesh → suitable for general castings and facing sand, balancing strength and surface finish.
- Fine sand (AFS 55-65): 70-140 mesh → suitable for precision casting and coated sand, focusing on surface finish (matching high-purity resin).