Foundry Grade Bentonite Specifications — Complete Technical Guide for Green Sand Molding

Introduction

Bentonite is the backbone of green sand molding in iron and steel foundries. As the primary binder that gives molding sand its strength, plasticity, and durability, the specifications of foundry-grade bentonite directly determine casting quality, scrap rate, and productivity. This technical guide covers everything a foundry engineer or procurement manager needs to know when specifying bentonite for green sand systems.

Why Bentonite is Essential in Foundry Casting

In green sand molding, a mixture of silica sand, bentonite clay, water, and additives is compacted around a pattern to form a mold cavity. The bentonite acts as the binder, coating each sand grain with a clay-water film that develops cohesive strength under compaction. When molten metal is poured into the mold, the bentonite must maintain its integrity at high temperatures, then break down cleanly after casting so the sand can be reclaimed.

Calcium bentonite and sodium bentonite behave differently in this application, and choosing the right type depends on your foundry's specific requirements.

Key Technical Specifications for Foundry Bentonite

1. Green Compression Strength (GCS)

Green compression strength measures the load-bearing capacity of moist molding sand before pouring. Typical foundry specifications require GCS in the range of 800–1400 g/cm² at standard sand-clay-water ratios. Higher GCS provides better mold rigidity and dimensional accuracy, but excessively high GCS can reduce mold permeability and cause gas defects.

For grey iron casting, GCS of 900–1100 g/cm² is typical. For ductile iron and high-integrity castings, 1100–1400 g/cm² may be specified.

2. Wet Tensile Strength

Wet tensile strength measures the sand's ability to resist cracking when the mold surface dries or when subjected to thermal expansion during pouring. This is perhaps the most critical parameter for avoiding scabbing, veining, and erosion defects. Premium foundry bentonites deliver wet tensile strength of 15–25 g/cm² at optimum moisture.

3. Compactability