Green sand strength and durability
Moulding sand must hold its shape, resist erosion during pouring and survive repeated cycles. Weak or inconsistent bonding clay leads to scabs, erosion, swells and higher clay additions to compensate.
Industry · Foundry
Foundry Bentonite, Binders and Coating Additives
Green sand bonding clay, mould coating suspension agents and organoclay for ferrous and non-ferrous foundries, tested batch by batch.
ISO 9001 certified processes
NABL-accredited lab testing
CoA, TDS & MSDS with every shipment
Export packaging worldwide
Overview
Minerals engineered for foundry
Foundry bentonite is the bonding clay that gives green sand its strength, mouldability and resistance to the heat of molten metal. Obsidian Minerals supplies foundry-grade bentonite alongside the additives that keep refractory mould and core coatings in suspension, giving iron, steel and non-ferrous foundries one source for several mineral inputs.
In a green sand system, bentonite coats each sand grain and, once activated with water, binds the grains together. Its swelling and thermal durability decide green compression strength, wet tensile strength and how many cycles the sand survives before the clay is burned out. Sodium-rich bentonites typically give higher hot and wet strength, while calcium-rich clays give easier flowability, and many foundries blend the two. On the mould surface, refractory coatings protect against metal penetration and burn-on, and they rely on rheology modifiers to keep dense zircon or graphite fillers from settling.
Our range covers both needs: foundry-grade and sodium bentonite for moulding sand, a foundry coating rheology modifier in water-based and alcohol-based grades, attapulgite for coating suspension and organoclay for solvent-based coating systems. Material is tested in an NABL-accredited laboratory, supplied with a TDS and batch CoA, and available in bags or jumbo bags for sand plant handling. Samples are available for sand laboratory trials.
Problems we help solve
Casting surface defects
Metal penetration, burn-on and veining raise fettling costs. Refractory coatings prevent them only if the coating is applied at consistent density and thickness on every mould and core.
Settling in coating tanks
Zircon, graphite and silicate fillers are dense and settle quickly. Without effective suspension, coatings form hard sediment, density drifts during a shift and operators lose time remixing.
Batch-to-batch consistency
Variation in montmorillonite content, moisture or grain size changes sand properties from one delivery to the next. Foundries need clay and additives backed by test data so sand control stays stable.
Recommended products for Foundry
Bentonite
Foundry Grade Bentonite
Bonding clay for green sand moulding, providing green compression strength, wet tensile strength and thermal durability for ferrous and non-ferrous castings, with key properties reported on the batch CoA.
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Bentonite
Sodium Bentonite
High-swelling sodium bentonite for use alone or blended with calcium clays, where foundries need higher hot strength and wet tensile strength in demanding iron and steel moulding sands.
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Foundry Coating Rheology Modifier
Water-based Foundry Coating Rheology Modifier
Suspension agent that gels in water to hold zircon and graphite fillers in water-based mould and core coatings, with soft sediment that re-disperses easily after storage.
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Foundry Coating Rheology Modifier
Alcohol-based Foundry Coating Rheology Modifier
Builds structure in ethanol and isopropanol coatings so that ignited and fast-drying washes level evenly on cores and moulds and resist settling between batches and during long production shifts.
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Attapulgite
Attapulgite
Needle-shaped attapulgite clay used as a non-swelling thickener and anti-settling aid in refractory coatings, contributing suspension that is relatively insensitive to dissolved salts in the make-up water.
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Organoclay
Solvent-borne Organoclay
Organophilic bentonite that develops gel strength in solvent-based coating systems, supporting suspension of refractory fillers where a hydrocarbon or other organic carrier is used in place of water.
View grade specsFoundry — common questions
What does bentonite do in foundry sand?
Foundry bentonite acts as the binder in green sand. When mulled with sand and water, the clay swells and coats each grain, bonding the grains so the mould holds its shape and resists erosion during pouring. Its thermal durability determines how much of the clay survives each casting cycle, which affects clay consumption and sand stability.
Should I use sodium or calcium bentonite for green sand?
Sodium bentonite generally gives higher wet tensile and hot strength and better durability, which suits iron and steel castings. Calcium bentonite gives good flowability and easier shake-out. Many foundries blend the two to balance these properties. The right ratio depends on your metal, sand system and moulding line, and is best set through sand laboratory tests.
How do I stop foundry coatings from settling?
Settling is controlled by adding a rheology modifier that builds a weak gel structure in the coating carrier. Choose a grade matched to the carrier: a water-based grade for water-borne coatings and an alcohol-based or organoclay grade for alcohol and solvent systems. Then set the dosage through settling, density and application trials on your own coating.
Can I get foundry bentonite samples for sand laboratory testing?
Yes. Samples of foundry and sodium bentonite, and of our coating additives, are available for laboratory evaluation. Tell us your metal, moulding process, current clay and the sand properties you control, such as compactability and green compression strength, and we will send suitable samples with the Technical Data Sheet.
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Talk to a foundry specialist
Describe your process and current material. We will recommend a grade, share the datasheet and arrange samples for trial.