About Foundry Grade Bentonite
Foundry grade bentonite is the clay binder that holds silica sand together in green sand moulds. When mulled with sand and water, the bentonite coats each grain and forms bonding bridges between them. That gives the mould the green compression strength to hold its shape, the permeability to vent casting gases and the durability to be reused through the sand system many times.
Our foundry grade is selected and processed for high active clay content, steady swell and controlled fineness. Good green and wet tensile strength help reduce erosion, scabbing and drop defects at the mould face, while thermal durability helps the clay keep its bonding capacity through repeated casting cycles. The grade suits jolt-squeeze, high-pressure and automatic moulding lines producing grey and ductile iron, steel and non-ferrous castings, and it can be used on its own or in blends to suit your sand practice.
The clay is dried at controlled temperature to protect the montmorillonite structure, then ground and classified. Swell index, active clay by methylene blue, moisture and fineness are checked for every lot in our NABL-accredited laboratory. Foundries can request samples for trial mulling and sand testing in their own laboratory before changing supply, and fineness or blend ratios can be tailored for a specific sand plant on request.
Typical specifications
| Parameter | Typical Value | Test Method |
|---|---|---|
| Swell index | ≥ 22 ml/2 g | ASTM D5890 |
| Montmorillonite content (methylene blue) | ≥ 70 % | Internal method |
| Moisture | 8–12 % | Internal method |
| pH (5 % suspension) | 9.0–10.5 | Internal method |
| Passing 75 µm | ≥ 85 % | Wet sieve, internal method |
| Green compression strength | Evaluated on request in the customer's sand mix | Internal method |
Typical values for guidance. Guaranteed limits are stated on the Technical Data Sheet and batch CoA. Green sand properties depend on base sand, additives and mulling practice and should be confirmed in plant trials.
Key features & benefits
Strong green bond
High active clay and swell give the sand strong green compression and wet tensile strength, helping moulds resist erosion from metal flow and keep sharp edges and fine detail during handling.
Durability in the sand system
The clay is dried at controlled temperature to preserve its structure, so more of it survives each casting cycle and fewer new additions are needed to keep sand properties in range.
Balanced permeability
Efficient bonding at moderate addition rates lets foundries reach target strength without overloading the sand with clay, which helps preserve the permeability needed to vent gases from the mould.
Fast, even mulling
Controlled fineness, with at least 85 % typically passing 75 µm, helps the bentonite distribute quickly over the sand grains and develop strength within normal muller cycle times.
Lot-to-lot consistency
Swell, methylene blue clay, moisture and fineness are tested on each lot, so sand lab readings stay predictable and addition rates do not need constant correction.
Applications
Iron castings
Green sand moulding of grey and ductile iron castings on automatic and high-pressure lines, including automotive, pump, valve and general engineering parts that demand consistent dimensional accuracy.
Steel and non-ferrous castings
Bonding clay for green sand moulds in steel, aluminium, brass and bronze foundries, where the clay content is adjusted to the metal, pouring temperature and casting section.
Sand system premixes
Base clay for foundry premixes that combine bentonite with carbonaceous or other additives, giving foundries a single-addition product for their sand preparation plant. Blends developed on request.
Usage & handling
- Total active clay in system sand is commonly held around 6–10 %, with new bentonite additions often 0.5–2 % per cycle to replace burnt clay.
- Add bentonite with water during mulling and allow sufficient mulling time for the clay to develop full strength.
- Use methylene blue clay, compactability and green strength readings from the sand lab to set and adjust addition rates.
- Store bags or jumbo bags dry and covered near the sand plant to avoid lumps in the dosing system.
Packaging & supply
- 25 kg and 50 kg HDPE or paper bags
- 1 MT and 1.2 MT jumbo (FIBC) bags for automatic sand plants
- Custom or private-label packaging on request
Frequently asked questions
How much foundry bentonite should be added to green sand?
It depends on your sand system. Many foundries hold total active clay at roughly 6–10 % and add around 0.5–2 % new bentonite per cycle to replace clay burnt out near the casting. The right rate follows from your sand lab readings for methylene blue clay, compactability and green strength. We can support trials with samples and typical data.
Should a foundry use sodium or calcium bentonite?
Sodium-type bentonite gives higher wet tensile strength and better durability through repeated cycles, so it is widely used on modern high-pressure lines. Calcium bentonite develops strength quickly and gives easier shakeout. Many foundries use a sodium-type grade alone or blend the two to balance properties. Our foundry grade is a high-swelling type, and blends can be developed on request.
Can you supply foundry bentonite premixed with additives?
Custom grades are developed with customers, including bentonite blends for single-addition sand preparation. Share your current additive practice, metal type and the sand properties you target, and we will discuss whether a blend makes sense for your plant. Samples of any proposed blend can be supplied for trial before you commit to production volumes.
Can we trial your foundry bentonite before switching supplier?
Yes. We supply samples for trial mulling and testing in your sand laboratory, with the Technical Data Sheet and typical values. Many foundries then run a controlled plant trial, monitoring compactability, green strength and casting defects. Our NABL-accredited laboratory tests every production lot, so the material you receive later matches what you trialled.