Choose a chemical binder by the casting first (the metal you pour, the size of the part, and whether the sand forms the mold or a core), then by the plant: production rate, base sand, gas, shakeout, sand reclamation, emissions, shop climate and cost per finished casting. In a ferrous shop that usually works out to furan or phenolic no-bake for the molds, amine cold box for cores made in volume, and a nitrogen-free system such as alkaline phenolic ester where steel pinholes are the risk.
A chemical binder coats every sand grain with a thin film and then hardens, by a chemical reaction or by heat, to give the mold its strength. This guide assumes you have already chosen chemically bonded sand over green sand. If you buy castings rather than make them, ask a foundry about the same factors: which binder it runs for molds and which for cores, and why that suits your alloy.
Common Chemical Binder Systems
Foundries group chemical binders by how they harden: no-bake, gas-cured cold box and heat-cured.
No-Bake Binders
No-bake binders start hardening at room temperature as soon as the last component goes into the mixer, and they are the usual choice for molds and cores for medium and large castings.
The no-bake systems a ferrous foundry chooses between:
- Furan, acid-cured: the most widely used no-bake binder, for medium and large parts in small and medium batches.
- Phenolic, acid-cured: the same acid catalysts and setting mechanism as furan, with cheaper components, so it is used mainly for large parts.
- Phenolic urethane no-bake: a phenolic resin cured with an isocyanate; used less, with restrictions for steel.
- Alkaline phenolic, ester-cured: small and medium batches; it contains no nitrogen and shakes out easily, and its strength is low right after curing but rises in storage.
- Sodium silicate, ester-cured: used mostly in steel foundries for medium and large castings; weaker and harder to shake out than the resin systems.
Gas-Cured Cold Box Binders
Cold box binders cure when a catalyst or hardener gas is blown through the sand in the core box, and the cure can take only seconds, so they are the binders for cores and small molds in medium to mass production.
- Phenolic urethane, amine-cured: cores up to 100 kg and more, and small molds; a very smooth surface, high dimensional accuracy, excellent core removal, and sand that reclaims easily.
- Alkaline phenolic, methyl formate-cured: mostly cores; chosen for easy shakeout, resistance to cracking, veining and pinholes, and low odor, although its used sand is harder to recycle.
- Epoxy-acrylic, SO2-cured: long bench life and no nitrogen, phenol or formaldehyde, at a higher binder cost.
- Sodium silicate, CO2-cured: cheap and clean to work with, but weak, poorly collapsible, moisture-sensitive and hard to reclaim, so it is now used mainly in small foundries.
Heat-Cured Binders
Heat-cured binders harden inside a heated core box or against a heated pattern. They give high dimensional accuracy but need expensive metal tooling, so they are used for small and medium cores in mass production. The organic ones release ammonia and formaldehyde as they heat.
- Hot box (phenolic or furan): cures in about 5 to 60 seconds; strong, accurate small and medium cores.
- Warm box: the same process with a resin that cures at a lower tool temperature, which saves energy, but it is not widely used.
- Shell (Croning): the only process that uses pre-coated sand bought ready to use; high accuracy, a good surface finish and easy shakeout, at high equipment and energy cost.
- Core oil: linseed oil cured in a drying oven; simple patterns and good resistance to veining and cracking, still used for small specialized cores.
How to Choose a Binder for Your Castings
Start with the casting: the metal, the size, and whether the sand is a mold or a core decide which systems are possible. Then choose among those that remain by production rate, base sand and the plant’s conditions. Cores often need a different binder from the mold around them, so the choice is usually made twice.
The Metal You Pour
Gray iron is the least demanding of the ferrous metals, and the acid-cured furan and phenolic systems suit it, provided the resin’s nitrogen is kept low in medium and heavy sections. Ductile iron and steel each react with one element the binder brings: sulphur for ductile iron, nitrogen for steel.
Ductile iron foundries favor acid-cured furan and phenolic no-bake because those molds are strong and rigid enough for riserless feeding. The catalyst, p-toluene sulphonic acid, leaves sulphur in the sand, and that sulphur degrades the graphite nodules at the casting surface.
The degraded surface layer matters most on thin-walled and fatigue-loaded parts. The recommended countermeasure is a mold coating that blocks sulphur transfer. An ester-cured alkaline phenolic binder avoids the problem at the source, since it uses no sulphur-bearing catalyst.
For carbon steel, the risk is nitrogen. Phenolic urethane releases ammonia when the metal reaches it, the steel absorbs nitrogen from it, and pinholes follow. Steel foundries that run phenolic urethane add iron oxide to the sand to prevent this.
Furan resins carry nitrogen too, in amounts that vary by grade, so only certain furan grades are used for steel. Alkaline phenolic ester and sodium silicate contain none, which is why steel foundries turn to them.
Casting Size and Section Thickness
Large, heavy castings need no-bake molds, since cold box and heat-cured systems only make molds and cores of limited size, and thin sections matter mainly in ductile iron, where the sulphur skin from acid-cured binders does the most harm.
Large upper mold surfaces on heavy castings are where furan shows surface defects: in a radiant-heat test with iron at 1,400 °C, furan sand lost strength the fastest of the binders tested.
Mold or Core
Cores usually get a different binder from the mold when they are made in volume, because cold box shoots and cures a core in seconds.
Cold box speed comes with more gas risk, so where the core is what causes porosity in the part, a lower-gas or nitrogen-free core binder is worth its slower cycle.
Production Volume and Cycle Time
High volume points to cold box, which cures a core in seconds; one-off and short-run work points to no-bake, whose cure the catalyst can stretch from minutes to hours.
No-bake covers two kinds of work: repeat small and medium molds on a loop line, such as pump and valve bodies, and large floor molds for big castings. The time the mixed sand stays workable, its bench life, is normally about one-third of its strip time, and the catalyst sets both. Long strip times suit manual shops with small crews. Short ones need high-speed mixers.
The Base Sand
Acid-cured furan and phenolic binders need a sand that does not neutralize their acid catalyst, which rules out alkaline olivine; ester-cured alkaline phenolic works with silica, olivine, chromite and zircon sand alike.
Silica sand suits acid-cured binders only when its acid demand value is low, ideally below 6 ml for no-bake resins.
Surface Finish and Veining
The binder changes surface finish and veining less than the sand does. The accepted fixes for veining are low-expansion sands such as chromite, zircon and olivine, or sand additives.
Binders still differ. In a 2002 American Foundry Society benchmark of no-bake binders, tabled in a 2021 Steel Founders’ Society paper, phenolic urethane rated better than acid-cured phenolic on veining, surface finish and metal penetration, and worse on erosion.
Gas Evolution
Judge a binder by how much gas it releases and by what that gas contains, because binder gas is behind blowholes and pinholes, the most common binder-related defects. Gas volumes measured on different test stands are not comparable, so compare binders only on figures from the same study.
Binder content matters as much as binder type: a small increase in cold box resin raises gas evolution by much more. Inorganic binders give off little gas, mostly steam, but it can build up inside a core and break out late, more so after humid storage.
Shakeout and Collapsibility
Choose a bond no stronger than the mold and core need to survive handling and pouring, because extra strength makes shakeout harder. Organic binders collapse as the metal heats them; sodium silicate first gains strength when heated, so silicate molds and cores knock out poorly and need modifiers to collapse.
Sand Reclamation
If you reclaim most of your sand, as most foundries now do, pick a binder that reclaims mechanically, which favors furan over ester-cured alkaline phenolic and both over sodium silicate.
In one published comparison, mechanical reclamation recovered 85 to 90% of furan sand and about 70% of alkaline phenolic sand; adding a thermal stage brought alkaline phenolic up to 95%. Reclaimed sand also lowers furan’s binder demand but not alkaline phenolic’s, so the reclaim rate feeds into binder cost.
Furan reclaim is limited when you pour ductile iron: the catalyst’s sulphur carries over in reused sand, so furan lines pouring ductile iron usually keep reclaimed sand below 70% of the mix.
Emissions and Odor
For the shop and its neighbors, inorganic binders are the cleanest and furan the worst of the systems measured at a working iron foundry. Alkaline phenolic ester gives a cleaner workplace overall, but it releases formaldehyde while it cures, more so in a warm shop, and US exposure is capped at 0.75 ppm averaged over 8 hours.
In the US, the binder-specific air rules cover cold box and warm box lines. An iron or steel foundry that is a major source of hazardous air pollutants, meaning 10 tons a year of any one or 25 tons of all combined, must cut triethylamine from each cold box line to 1 ppmv at the outlet or by 99%. Every US iron and steel foundry running furan warm box must use a catalyst without methanol.
Shop Temperature and Humidity
Furan no-bake needs the sand held at 15 to 25 °C to keep its hardening time under control, so a shop that cannot hold its sand temperature will see strip times drift with the season. Sodium silicate and phenolic urethane need dry storage, and humidity matters most for sodium silicate.
Warm storage also shortens resin shelf life, and alkaline phenolic resins age faster than furan.
Cost
Compare binders by cost per finished casting, not by price per kilogram. The binder is a small share of the sand mix by weight but 30 to 60% of its raw-material cost, so the addition rate, the hardener, the reclaim rate and the scrap rate all move the real figure.
Addition rates differ by system: sodium silicate, cheap per kilogram, needs a higher addition than the resins. Price each resin together with its hardener, against the casting weights you actually produce.