By Versatile Equipments Technical Team | April 2025

Introduction

In the world of sand casting, success often hinges on the properties of the mold itself. While much emphasis is placed on green compression strength, clay content, and compactability, there’s one property that quietly determines whether your casting will be sound—or riddled with gas-related defects.

That property is Permeability.

At Versatile Equipments, we believe that understanding and measuring permeability correctly is one of the most powerful tools a foundry engineer can use to control casting quality, especially in green sand systems.

What Is Permeability?

Permeability refers to the ability of compacted sand to allow gases to escape through the mold during pouring and solidification. In a typical AFS test setup, a standard 2” × 2” cylindrical specimen is subjected to a fixed volume of air (2000 cc) under controlled pressure (10 g/cm²), and the time taken for the air to pass through determines the Permeability Number (PN).

Why does this matter? Because every mold generates gases—through moisture evaporation, resin decomposition, and thermal reactions. If these gases can’t escape quickly enough, they will find their way into the metal, causing blows, pinholes, gas porosity, and even metal penetration.

Key Factors Influencing Permeability

  • Grain Fineness Number (AFS GFN): Coarser sands improve permeability but may reduce surface finish.
  • Clay and Additives: Excessive fines or dead clay can reduce venting capacity.
  • Moisture Content: Too much water reduces permeability; too little affects bonding.
  • Mulling Efficiency: Poor mulling leads to uneven binder distribution, affecting air flow.

This is why leading foundries monitor permeability alongside moisture, compactability, methylene blue clay, and LOI (Loss on Ignition) for a complete picture.

Best Practices for Accurate Testing

  1. Use a standard sand rammer (like our VSR model) to prepare consistent specimens with defined ramming energy.
  2. Match the test specimen’s compactability to that of your actual mold. Even a 2% variation can skew results.
  3. Use a calibrated digital tester like the VPD-II Digital Permeability Meter, which eliminates guesswork and improves repeatability.
  4. Avoid cracks or surface irregularities in test specimens, as they drastically alter airflow.
  5. Correlate results to casting outcomes. Permeability by itself isn’t the final word—compare against defect maps and casting analytics to understand what ranges work for your process.

Real-World Example

A leading ductile iron foundry using our VPD-II system identified a steady decline in permeability from 100 to 72 over two weeks. Interestingly, moisture and compactability remained in spec. Investigation revealed a rise in MB clay and LOI—signs of excessive fines and combustibles. With corrective actions in their sand preparation system, permeability was restored and gas defects reduced by 40%.

Choosing the Right Equipment

Versatile Equipments offers a full suite of tools to support permeability testing and sand quality control:

Together, these instruments enable a data-driven approach to sand control, helping foundries reduce rejections and improve yield.

Conclusion

In an era of tighter quality requirements and growing competitiveness, permeability testing is not optional—it’s essential. By applying modern tools and consistent methodology, foundries can turn permeability into a predictive quality metric, rather than just another lab number.

Looking to upgrade your lab or troubleshoot gas defects? Contact our technical team for a customized solution.