Foundry sands are named on three different axes, and the ten types below mix all three: the binder that holds the mold together, the base grain that takes the heat, and the position the sand occupies in the flask. Green sand, dry sand, resin-bonded, sodium silicate and shell are binder systems. Silica, chromite, zircon, olivine and ceramic are base grains. Facing, backing, system, core and parting sand are positions.
Knowing which axis a name sits on tells you what it controls. The binder sets tolerance, cycle time and mold cost; the base grain sets refractoriness and how much the mold moves while the metal is still liquid; the position decides how much of the expensive sand you actually pay for. A quote reading “furan no-bake on silica, chromite facing” is answering all three at once.
1. Green Sand
Green sand is a moist, clay-bonded mix rammed around the pattern and poured wet — nothing is baked and nothing is chemically cured, which is why the mold can be broken down and the sand used again the same shift. A mold mix runs roughly 85 to 95% silica sand, 5 to 10% bentonite clay, 3 to 9% carbonaceous additive and 2 to 5% water.
Green sand covers the whole practical size range up to about a tonne, and holds close tolerances where compaction and sand properties are held steady. Surface finish is set mostly by grain fineness: AFS 50 to 60 is standard, AFS 90 to 110 buys a smoother surface.
It is the default for volume ferrous work: virtually every ferrous sand mold is green sand, and gray iron castings are poured this way almost without exception. It is also the cheapest sand per mold.
2. Dry Sand
Dry sand is a clay-bonded mold with the water dried out of it before pouring, bought for rigidity and low gas evolution at the price of a drying cycle. As a distinct process it has largely disappeared.
What survived is the oil-bonded baked core. Everything the baked mold was used for — heavy sections, long pouring times, cores that had to hold their shape under a head of iron — moved to chemically bonded sand, which delivers the same rigidity at shop temperature with no drying step at all.
Treat the words “dry sand” on a quote as a flag. Today they usually describe a no-bake mold, and the two are not priced or scheduled the same way.
3. Resin-Bonded Sand
Resin-bonded sand is dry silica mixed with 1 to 3% liquid organic binder and a catalyst that cures it rigid in place — no clay, no water, no pattern heating. Furan, phenolic and phenolic-urethane are the common binder families.
Cold-setting systems cure at shop temperature, and the catalyst level sets the clock. That timing control is what makes the process fit medium and large parts in small to medium batches: the mold is hard and dimensionally stable, and the pattern is wood or resin rather than heated metal.
For ductile iron castings and steel parts above green sand’s comfortable size, this is the standard route. One caveat on steel: not every furan grade is suitable, and the wrong one produces cracks, fins or pinholes, so the binder matters as much as the process name on the quote.
The sand costs more than green sand and the quote shows it. Mechanical reclamation returns 75 to 80% of a cold-setting monosand, and the binder is bought by the ton and burned once.
4. Sodium Silicate Sand
Sodium silicate sand is an inorganic system: 2 to 4% water glass mixed into the sand and hardened in place by gassing with CO₂ for 10 to 60 seconds, with no purging afterward. It is cheap, easy to handle, and clean to work around.
Its problem is getting it back out. The binder does not burn away — as the mold heats, residual strength drops and then peaks a second time between 900 and 1000 °C. Cores that spend their time in that window come out of deep passages hard. Reclamation is correspondingly poor, at 45 to 85%.
For iron molds and open cores that shake out easily, it is a sound, low-cost choice. For steel cores in blind passages, settle the knockout method before specifying it.
5. Shell Molding Sand
Shell molding sand is silica pre-coated with a solid phenolic novolac resin, which a heated metal pattern cures at 250 to 270 °C in two to three minutes into a thin, rigid shell. Whatever has not cured is tipped out of the pattern and used again.
The payoff is dimensional accuracy and as-cast finish at the top of the sand-casting range, with clean shakeout and easy de-coring. The cost sits in two places: pre-coated sand is dearer per ton than plain silica, and the heated metal pattern is a real tooling investment. Shell molding therefore earns its place on small to medium parts in long production runs and almost never on a short one.
6. Core Sand
Core sand is chemically bonded sand selected for a contradictory duty: strong enough to hold its shape while metal fills around it, weak enough afterward to be broken out of a passage. Green sand does not do this job — cores are formed from silica, and occasionally zircon, chromite or olivine, with strong chemical binders.
Amine-gassed cold-box urethane is the dominant system. It makes cores of 100 kg and more, holds very high dimensional accuracy, releases cleanly from the casting and reclaims easily, which is why it became the default for anything with internal passages.
7. Facing Sand
Facing sand is the layer packed directly against the pattern — the only sand that touches the metal — so it is where a plant spends its sand budget and nowhere else. A facing layer is how a foundry gets chromite or zircon performance on the surfaces that matter without filling a whole flask with it.
Chromite is the usual choice on heavy carbon steel castings and solid iron sections: it gives a better surface on large castings and chills the sections where chilling is wanted. On small steel parts the same sand is often used throughout the mold instead, since there is not enough volume behind the surface to be worth splitting.
8. Backing Sand
Backing sand is the cheaper, usually reclaimed sand filling the rest of the flask behind the facing layer. It never contacts the metal, so its job is bulk, rigidity and a path for gas to leave the mold — which makes its permeability, not its refractoriness, the property that matters.
9. System Sand
System sand is a single sand doing both jobs — no facing and backing split — continuously reconditioned and returned to the molding line. This is what a mechanized green sand line runs on, and the reconditioning loop is what makes green sand cheap.
10. Parting Sand
Parting sand is dry, binder-free silica dusted onto the parting plane so the cope lifts off the drag without tearing the mold. It works precisely because it carries no clay and no moisture — there is nothing at the joint face for the two halves to bond through.
Used heavily, or dusted where it can fall into the cavity, it becomes a straightforward source of sand inclusions in the finished casting.
Types of Base Sand Used in Foundries
Five base sands cover essentially all foundry work: silica, chromite, zircon, olivine and manufactured ceramic sand. Silica is the default, and each of the other four is bought to fix one specific thing silica does badly.
That thing is usually expansion. Quartz inverts at 573 °C and grows as it does, and on a large steel casting that growth decides whether the mold holds its dimensions while the metal is still liquid.
| Base sand | Important properties | Typical applications |
|---|---|---|
| Silica sand | Density 2.65; quartz inversion at 573 °C drives 2.2–2.5% peak expansion; sinters at 1450–1550 °C; compatible with all binders; cheapest and most widely available | General iron, steel and aluminum casting |
| Chromite sand | Density 4.3–4.6; melts at 1700–1900 °C depending on impurities, and needs under 2% SiO₂ to avoid low-temperature sintering; thermal diffusivity over 25% above silica; expansion near 1% with no inversion point; basic pH of 7 to 10 | Steel and heavy-section castings; facing sand where chilling is wanted |
| Zircon sand | Density 4.4–4.7; melts above 2000 °C; thermal diffusivity over 30% above silica; fine rounded grain gives a better finish but very low permeability; highest price of the four minerals | Precision steel and alloy castings; cores and facings, usually blended with silica |
| Olivine sand | Density 3.2–3.6; forsterite melts at 1890 °C; peak expansion near 1.2%; pH around 9, so it cannot be used with acid-catalysed binders | Steel and manganese-steel casting, where silica is ruled out |
| Ceramic sand | Spherical fused grain of synthetic bauxite or mullite; hot distortion of 0.04 mm against 0.76 mm for fine silica in the same test; absorbs more binder for equal strength | High-precision castings and cores where silica moves too much |
How to Choose Sand for Sand Casting
Choose on whichever constraint is tightest — annual volume, casting size, tolerance, finish, or the alloy’s chemistry — because the sand that satisfies one of those usually costs you another.
| Requirement | Common choice |
|---|---|
| Low-cost mass production | Green sand |
| Large industrial casting | Resin sand / dry sand |
| High dimensional accuracy | Resin sand / shell molding |
| Excellent surface finish | Shell sand / fine resin sand |
| Complex internal passages | Resin-bonded core sand |
| Heavy steel casting | Chromite or zircon-containing sand |
| High-temperature resistance | Zircon / chromite / ceramic sand |
| Highly automated molding | Green system sand |
Three constraints are vetoes rather than preferences, and they are the ones worth checking first. Coal dust leaves the green sand mix for steel because the casting picks up carbon from it. Olivine and acid-catalysed no-bake binders do not go together. And silica is kept away from manganese steel entirely, since the two react to form a very fusible compound at the mold face.
Conclusion
The ten names in this article collapse into three questions about your part: which binder system, on which base grain, and whether the expensive sand goes everywhere or only against the pattern.
In practice you do not need to specify the sand. Give the foundry the alloy and grade, the casting weight, the tolerances and finish you actually need, and the annual quantity — the sand follows. Where it does not follow, ask which of the three the foundry was solving for, and the quote becomes readable.