Aug 24, 2026Company News

Impact of Grain‑Size Distribution of Ceramic Foundry Sand on Casting Surface Quality

Ceramic foundry sand grain‑size affects casting surface quality. AFS GFN & grading guide for cold‑box, coated sand, 3D‑printing, furan no‑bake and lost‑foam to reduce veining, penetration & porosity.

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Introduction

The surface finish of castings is closely related to molding materials. The grain‑size and particle‑size distribution of base sand directly govern the process performance of moulding sand, and further determine final casting quality.
Coarse sand creates large voids between grains. Molten metal can easily penetrate these gaps and cause metal penetration, resulting in rough casting surfaces. Conversely, finer sand delivers smoother casting surfaces, yet it requires higher binder dosage and lowers the permeability of moulds and cores, bringing higher risk of porosity defects. Selecting foundry sand with proper grain‑size and well‑graded particles is critical to balance surface finish, core strength and permeability.
As one high‑grade spherical special foundry sand, ceramic sand features high refractoriness, low thermal expansion, superior sphericity, moderate bulk density and excellent wear resistance. It is widely applied in cold‑box, resin‑coated sand, binder‑jet 3D printing, furan no‑bake, lost‑foam and water‑glass sand processes. Custom‑tuned wide‑range particle‑size grading can be manufactured according to client requirements to mitigate common defects such as penetration and veining from the source.
Key Parameters: AFS GFN and Particle‑Size Concentration
  • AFS Grain Fineness Number (GFN): Represents average grain fineness. Higher AFS values indicate finer sand grains.
  • Particle‑size concentration: Weight percentage of sand retained on 4‑5 main test sieves. Industry typical requirement ≥85 %.
  • Well‑graded multi‑sieve distribution: Mixed coarse and fine grains improve compaction and core strength while preserving permeability. Too scattered grading weakens core strength; overly single‑sized grains enlarge voids and raise penetration risk.
  • Note: Ceramic sand has bulk density close to silica sand, so segregation seldom occurs when blended with silica sand for cost‑effective production.

1. Cold‑box Process

Recommended AFS GFN: 40‑65, 4‑5‑sieve grading, particle‑size concentration ≥85 %
Sand within this AFS range provides good flow‑in and easy stripping during core‑shooting. Dense, uniform cores can be obtained. Its bulk density matches silica sand, avoiding segregation for blended sand systems. It is widely used for water‑jacket and air‑passage cores of engine blocks and cylinder heads, delivering stable dimension and reducing veining and metal‑penetration defects.

2. Resin‑Coated Sand Process

Recommended AFS GFN: 50‑110, 4‑5‑sieve grading, particle‑size concentration ≥85 %
Ceramic sand for coated‑sand application offers better hot‑compressive performance and lower high‑temperature expansion. It minimizes core cracking, core breakage and veining induced by thermal stress. Low tendency of shell‑separation during core curing makes it ideal for thin‑wall and intricate cores of automotive power components.

3. Binder‑Jet 3D‑Printing Process

Recommended AFS GFN: 70‑100, particle‑size concentration ≥85 %
3D printing demands outstanding sand flowability, particle uniformity and core strength. Properly graded ceramic sand enables smooth recoating and high dimensional accuracy of printed cores. It effectively alleviates veining and burn‑on defects with easy shake‑out performance, and is widely adopted for diesel cylinder heads and large machine‑tool castings.

4. Furan No‑bake Sand Process

Recommended AFS GFN: 30‑65, 4‑5‑sieve grading, particle‑size concentration ≥80 %
Coarser grades suit heavy‑duty steel and iron castings. Low acid‑demand value grants good compatibility with furan resin systems. Mould strength and permeability are improved to reduce sand‑inclusion defects for large equipment castings, pump and valve parts.

5. Lost‑Foam Casting

Common mesh grades: 10/20 mesh, 20/30 mesh coarse sand
Large‑particle ceramic sand provides high gas permeability. Low crushing rate supports high reclamation yield. Fresh sand addition rate can be kept low, cutting waste‑sand discharge for heavy iron and steel castings in lost‑foam dry‑sand filling.

Additional Factors Affecting Casting Surface Quality besides Grain‑size

  1. Al₂O₃ content & refractoriness: Higher alumina grade resists penetration of high‑temperature molten metal and prevents burn‑on at hot‑spot sections.
  1. Thermal expansion rate: Ceramic sand shows thermal expansion around 0.11‑0.15 %, far lower than silica sand, which is essential for veining suppression.
  1. Angularity coefficient: Spherical grains with angularity coefficient ≤1.15 reduce specific surface area, cutting binder consumption and gas generation to lower porosity risk.
  1. Clay content control: Fines and clay degrade core strength and increase gas evolution. Clay content should be controlled ≤0.2 %.
  1. Reclamation management: Test AFS GFN and particle‑size distribution regularly for reclaimed sand. Fines accumulate after long‑term circulation. Supplement new sand by sieving to maintain stable grading.

Selection Guidelines

  1. Select finer AFS grades for thin‑wall castings with high‑surface‑finish requirements; choose relatively coarser sand for thick‑wall hot‑spot castings to guarantee permeability and avoid metal penetration.
  1. Prioritize Ceramic sand with well‑balanced 4‑5‑sieve grading and qualified particle‑size concentration, instead of pursuing overly narrow single‑sieve distribution.
  1. Monitor bulk density for blended‑sand application to prevent particle segregation between different sand types.
  1. Conduct small‑scale process trials whenever possible, and fine‑tune grain‑size & AFS parameters according to actual casting defects.

Conclusion

Particle‑size distribution is not the sole determining factor for casting quality, yet it is a fundamental consideration for process design. Matching reasonable grain‑size grading with casting structure, pouring temperature and binder system can maximise Ceramic sand’s strengths: low thermal expansion, high refractoriness and excellent reusability. Foundries can achieve higher casting surface quality, lower reject rate, less cleaning labour and eco‑friendly production with reduced waste‑sand output.