What green sand is and the role of bentonite
Bentonite for foundry green sand is the clay binder that holds silica sand grains together in a mould. Green sand is a mixture of sand, bentonite and water, and in many iron foundries a carbonaceous additive such as seacoal. The word green refers to the moist, unbaked state in which the mould is used, not to its colour.
When bentonite is mulled with sand and water, the clay hydrates and forms a thin, plastic coating on each grain. Contact points between coated grains bond the mass together. This bond provides green strength to hold the mould shape, dry and hot strength to resist the molten metal, and enough flowability to compact evenly around a pattern.
System sands commonly contain around 6–10 % active clay and 2.5–4.0 % moisture, although the right figures depend on the moulding line, casting size and metal poured.
Sodium, calcium and blended bentonite
The exchangeable cation in the clay changes how a foundry bentonite performs. The table shows the general tendencies.
Many foundries use blends of sodium and calcium bentonite, or soda-activated bentonite in which calcium clay is converted towards the sodium form with soda ash. Blending lets a foundry balance hot strength and durability against flowability and shakeout for its own castings.
| Property | Sodium bentonite | Calcium bentonite |
|---|---|---|
| Green compression strength | Develops more slowly; needs more mulling | Develops quickly at lower moisture |
| Dry and hot strength | Higher | Lower |
| Wet tensile strength | Higher; helps resist expansion defects | Lower |
| Durability through repeated pours | Higher; clay survives more cycles | Lower; more make-up clay needed |
| Flowability and shakeout | Firmer sand; shakeout can be harder | Better flowability; easier shakeout |
Key quality tests for foundry bentonite
Foundry bentonite is usually evaluated with test procedures published by the American Foundry Society (AFS), or with equivalent national or internal methods. The parameters below are those most often found on a Technical Data Sheet.
- Methylene blue value: measures montmorillonite, or active clay, content. A higher value usually means more bonding power per kilogram.
- Swelling index: the settled volume of a fixed mass of clay in water. It indicates hydration capacity and helps distinguish sodium from calcium character.
- Green compression strength: measured on a standard rammed specimen of sand, clay and water. It shows how well the clay holds a mould before pouring.
- Wet tensile strength: the strength of the moist condensation zone that forms just behind the mould face during pouring. It is closely linked to resistance to scabs and other expansion defects.
- Moisture: free moisture as supplied, which affects addition accuracy and storage behaviour.
- Fineness: residue on a 75 µm sieve. Coarse grit adds no bond and can contribute to casting defects.
- Thermal durability: the share of active clay that remains after heating, which shows how well the clay survives repeated pours.
Controlling the sand system
In a recirculating green sand system, part of the bentonite near the metal is burnt out with each pour and becomes dead clay. New bentonite added at the muller replaces this loss. The methylene blue test on system sand is the usual way to track active clay and set addition rates.
Carbonaceous additives, core sand and cooling water all interact with the clay, so a change in any one input can appear as a change in clay behaviour.
- Measure active clay, moisture, compactability and green compression strength at a fixed frequency.
- Keep compactability within the band suited to your moulding line; around 35–45 % is a common target for many high-pressure lines.
- Add bentonite at a controlled, continuous rate or as a premix rather than in large intermittent doses.
- Allow enough mulling time for the clay to coat the grains; sodium bentonite needs more mulling energy than calcium bentonite.
- Monitor fines and dead clay build-up, which raise water demand and reduce permeability.
- Track new sand and core sand inputs, which dilute the system and change clay demand.
Common defects linked to bentonite
Most casting defects have several possible causes, but bentonite selection and clay control are often part of the picture. Use the table as a starting point for investigation, alongside sand test records.
| Defect | Possible clay-related cause | Typical response |
|---|---|---|
| Scabs, buckles and rat tails | Low wet tensile strength; silica expansion not accommodated | Raise the sodium bentonite share or active clay; review carbonaceous additive |
| Erosion and cuts | Low hot strength at the mould face | Raise active clay; consider more sodium bentonite |
| Blows and pinholes | Excess moisture, or high fines raising water demand | Reduce moisture; control dead clay and fines |
| Friable mould edges | Moisture too low for the clay level; short mulling | Adjust water addition and mulling time |
| Difficult shakeout | Very high hot strength or clay content | Adjust the sodium to calcium ratio; review clay level |
Specifying and buying foundry bentonite
A clear purchase specification makes deliveries consistent and easier to approve. Before switching supplier, run trials in your own sand system rather than relying on data sheets alone.
Consistency matters as much as the headline numbers. A bentonite that varies from lot to lot forces repeated changes to water and clay additions and makes defects harder to diagnose. Ask how the supplier controls raw clay selection and blending, how often finished product is tested, and whether retained samples are kept for each lot. Reviewing several consecutive CoAs from the same supplier is a simple way to judge batch-to-batch variation.
- State the bentonite type: sodium, calcium, activated or blend.
- Specify limits for methylene blue value, swelling index, green compression strength, wet tensile strength, moisture and fineness, with the test methods to be used.
- Request a batch Certificate of Analysis for every lot and a Technical Data Sheet with guaranteed limits.
- Ask for samples large enough for a muller trial.
- Agree packaging that suits your handling: bags for manual addition, or FIBC jumbo bags for silo or hopper filling.
- Store bags dry and off the floor, because bentonite readily picks up moisture.
How Obsidian Minerals can help
Obsidian Minerals manufactures foundry grade bentonite for green sand moulding at sites in Gujarat and Rajasthan, alongside sodium and calcium bentonite grades. For core and mould coatings, the company also supplies a foundry coating rheology modifier in water-based and alcohol-based grades.
Testing is carried out in an NABL-accredited laboratory under ISO 9001 certified processes. Each shipment is supplied with a batch CoA, Technical Data Sheet and MSDS/SDS, in 25 kg or 50 kg HDPE or paper bags or 1 MT and 1.2 MT jumbo (FIBC) bags. Samples are available on request, and custom grades or particle sizes can be developed with foundries to suit their sand systems.
Frequently asked questions
How much bentonite is used in foundry green sand?
System sands commonly contain around 6–10 % active clay, with moisture of roughly 2.5–4.0 %, but the right level depends on the moulding line, casting weight and metal. New bentonite additions replace clay burnt out during pouring and are usually set using the methylene blue test on system sand. Foundries should set targets from their own sand testing rather than a fixed recipe.
Is sodium or calcium bentonite better for foundry sand?
Neither is better in every case. Sodium bentonite offers higher hot strength, wet tensile strength and durability, which helps with heavy castings and expansion defects. Calcium bentonite develops green strength quickly and gives better flowability and shakeout. Many foundries use a blend or an activated bentonite to balance these properties for their own castings and moulding equipment.
What does the methylene blue test measure in foundry bentonite?
The methylene blue test measures how much dye the clay adsorbs, which reflects its montmorillonite or active clay content. On new bentonite, a higher value generally indicates more bonding capacity. On system sand, the test shows how much active clay remains after repeated pouring, which lets foundries calculate make-up additions and keep sand properties consistent.
How should foundry bentonite be stored?
Store bags or FIBCs in a dry, covered area, off the floor and away from water. Bentonite absorbs moisture readily, and damp material can cake, dose inaccurately and behave inconsistently in the muller. Use stock in order of receipt and record lot numbers, so that any change in sand behaviour can be traced back to a specific batch.