Influence of Grain Size Specifications of Chromite Sand for Casting on Casting Quality

1. Coarse Grain Sizes (AFS25-30 AFS30-35)

Advantages

1. Large voids between sand grains deliver excellent gas permeability, effectively preventing blowholes and gas defects for heavy-section steel castings, sodium silicate sand molds and large lost foam castings.
2. Strong heat storage and chilling capacity accelerates solidification of heavy-wall castings, reducing shrinkage porosity and hot cracking tendency.
3. Small specific surface area requires less resin binder under the same sand dosage, lowering production costs.

Negative Impacts on Casting Quality

1. Wide intergranular gaps allow molten steel to penetrate easily, resulting in rough casting surfaces, pitting and mechanical metal penetration defects.
2. Distinct grain outlines leave obvious textures on casting surfaces, increasing subsequent grinding workload.
3. Porous mold surface facilitates penetration of high-temperature slag, triggering local chemical metal penetration.
4. When applied to coated sand molds and precision small castings, coarse sand creates heavy flash and poor dimensional accuracy.

Applicable Scenarios

2. Medium Grain Size (AFS40-45,mainstream universal grade)

Advantages

1. Balanced gas permeability and surface finish; avoids both gas blockage and molten steel penetration.
2. Dense grain stacking forms uniform high-temperature sintered layers, delivering stable resistance to metal penetration and veining defects.
3. Compatible with resin self-set sand, shell mold, lost foam casting, carbon steel and stainless steel production, ensuring stable casting dimensional accuracy.
4. Moderate thickness of sintered layer facilitates shakeout without over-sintering that causes difficult cleaning.

Drawbacks

Applicable Scenarios

3. Fine Grain Sizes (AFS45-50，AFS45-55 AFS50-55)

Advantages

1. Tiny sand grains stack tightly with minimal voids, achieving superior casting surface finish with almost no molten steel penetration or pitting.
2. Significantly reduces grinding and polishing procedures when used as facing sand, ideal for stainless steel and precision castings with strict appearance requirements.
3. Dense mold structure resists penetration of high-temperature slag, effectively preventing chemical metal penetration for thin-wall precision castings.

Negative Impacts on Casting Quality

1. Minimal intergranular voids lead to poor gas permeability, easily causing subsurface blowholes, gas firing and pinhole defects on heavy-wall castings.
2. Large specific surface area demands more resin or sodium silicate binder, increasing gas evolution and further aggravating blowhole risks.
3. Thin sintered layer formed by fine sand may peel off locally at hot spots of heavy sections, resulting in metal penetration.
4. Poor shakeout performance leads to difficult cleaning of thin inner sand cores.

Applicable Scenarios

4. Chromite Powder (200/325 mesh and finer, for coating only, not for molding)

1. Fine powder fills gaps between sand grains to form compact coating films, isolating molten steel from molding sand and improving casting surface finish.
2. If mixed into facing sand, fine powder will severely block gas permeability and cause mass rejection due to blowholes.

Typical Casting Defects Caused by Improper Grain Size Selection

1. Coarse sand for precision small castings: rough surface, mechanical metal penetration, heavy flash, unqualified dimensional tolerance.
2. Fine sand for heavy-wall steel castings: subsurface blowholes, gas firing and pinhole defects.
3. Unreasonable wide grain size range (e.g. mixed 10–140 mesh sand): unstable gas permeability, simultaneous occurrence of local metal penetration and blowholes.
4. Excess fine powder content: increased gas evolution, sharply reduced gas permeability and difficult shakeout.

Summary of Grade Selection

1. Heavy-section castings & high gas permeability demand: 20–40 mesh / 20–70 mesh coarse sand
2. General carbon steel & stainless steel medium castings with balanced quality requirements: 40–70 mesh (preferred grade)
3. Thin-wall precision castings with high surface finish requirements: 50–100 mesh / 70–140 mesh fine sand
4. Facing coating production: 200–400 mesh chromite powder