Moulding sand is the refractory granular material a foundry packs around a pattern to form a mould cavity: silica grains in almost every case, bound together tightly enough to hold the cavity’s shape while molten metal fills it, and loosely enough to break away from the casting at shakeout.
It is written molding sand in the US, and called foundry sand when it is being bought, sold or disposed of as a commodity. The word green describes sand that is still moist and uncured when the metal arrives, and says nothing about colour.
What Moulding Sand Consists Of
Moulding sand is a base sand, a binder, and — in a green sand system — water and additives, with the share of each moving according to the binder system and the metal being poured.
| Constituent | Green sand | Chemically bonded sand | What it does |
|---|---|---|---|
| Base sand (silica) | 85–95% | 93–99% | Forms the bulk of the mould and resists the heat of the pour |
| Binder | 0–12% clay, typically about 10% bentonite | 1–3% chemical binder | Sticks the grains to each other and holds the cavity shape |
| Water | 2–5% | — | Activates the clay and gives a green sand its plasticity |
| Additives | 2–10% carbonaceous | Varies; iron oxide is the common one on steel | Control what happens at the mould face when the metal arrives |
Base Sand
The base sand is silica — quartz — in almost all ferrous work, chosen because it is refractory enough for the job and cheap enough to buy by the ton.
Silica sinters somewhere between 1,450 and 1,550 °C, and alkaline oxides and other contaminants pull that figure down, so a sand that is perfectly sound under grey iron can soften at the face under a steel pour.
Finer, rounder grains give a smoother surface and need less binder to hold them; coarser grains vent gas more freely but leave a rougher face and let metal penetrate between them.
Chromite, zircon and olivine expand less and draw heat out of the metal faster than quartz.
Binder
The binder holds the grains together; sand does not stick to itself.
In a green sand mould the binder is bentonite clay, activated by water and still plastic when the metal arrives. In a no-bake mould it is a chemical resin mixed with a catalyst that cures the whole mass rigid before the pattern is drawn. Shell moulding uses the same non-clay chemistry as a coating on the grain, cured by heat against a heated pattern.
A clay-bonded mould stays slightly compliant and is re-tempered and used again, which suits high-volume iron work. A rigid cured mould holds larger dimensions closer, the usual reason a part moves from green sand to no-bake.
Additives
Which additives go into the mix depends on the metal being poured.
Carbonaceous additives, sea coal being the traditional one, burn at the mould face and stop sand fusing onto an iron casting.
On steel the same additive becomes a liability. Carbon released at the face can be picked up by the casting surface.
Starch and wood flour do a different job. They decompose at around 300 °C, before quartz expands sharply at 573 °C, so the grains have room to expand.
Iron oxide replaces coal on steel work. It softens the grain surface so the sand takes up strain that would otherwise push the mould face apart, and it releases oxygen at casting temperature, pulling some carbon back out of the surface.
Molding Sand Types
Moulding sand is classified two ways at once, and a single quote can use both: by the binder that holds the grains, and by the job a particular sand does in the flask.
| Name | Classified by | What it tells you |
|---|---|---|
| Green sand | Binder | Clay and water, moist and uncured at the pour, reclaimed and re-tempered afterwards |
| Chemically bonded sand (no-bake, resin sand) | Binder | Resin cured by a catalyst, rigid and dry |
| Shell (resin-coated) sand | Binder | Resin coating cured by heat on the pattern |
| Moulding sand, in the narrow sense | Job in the flask | The sand forming the outside shape of the cavity |
| Core sand | Job in the flask | The sand forming internal voids; chemically bonded in practice |
| Facing sand | Job in the flask | The layer packed against the pattern, sometimes a different base sand from the bulk behind it |
| System sand | Job in the flask | The circulating bulk of a green sand plant, tested and re-conditioned between pours |
A longer list of foundry sands adds a third axis, moisture state, which is where green sand and dry sand sit.
What Properties Moulding Sand Must Have
Moulding sand must be refractory, permeable, strong enough green to hold the cavity, collapsible enough to shake out, flowable into pattern detail, and at the right moisture — every one of those two-sided, with a casting defect at each end of the range.
| Property | What it controls | What goes wrong | How it is tested |
|---|---|---|---|
| Refractoriness | Whether the mould face survives contact with the metal | Too low: burn-on and metal penetration, sand welded to the casting surface | Pyrometric cone equivalent |
| Permeability | How fast mould gas escapes ahead of the metal | Too low: blows, pinholes, expansion defects. Too high: rough surface and mechanical penetration | Permeability of a standard rammed specimen |
| Green strength | Whether the mould holds shape as the pattern is drawn and the flask closed | Too low: broken moulds and poor draws. Too high: hard ramming, poor flowability, dimensions off | Green compression strength |
| Dry strength and collapsibility | How the mould behaves as the casting contracts, and how it shakes out | Too low: cuts, washes, erosion, inclusions. Too high: cracks, hot tears, hard lumps at shakeout | Dry compression strength |
| Flowability | How completely the sand packs into pattern detail | Too low: cuts and washes, unfilled pockets, penetration. Too high: oversized castings from mould wall movement, blows, poor finish | Compactability, rammer method |
| Moisture | The plasticity of the clay bond, and through it most of the others | Too low: dry friable sand that will not mould. Too high: gas evolution, lower permeability, poor flowability | Infrared or oven moisture |
How Moulding Sand Differs for Iron and Steel Castings
Moulding sand for steel has to survive a hotter pour than iron’s, and the differences in base sand, additives and mould system all follow from that.
The Steel Founders’ Society puts the refractory limit of sand and ceramic mould materials at 3,000–3,330 °F, so the top of a carbon steel pour reaches into the bottom of what the mould itself can take, while iron sits 300 to 550 °F clear of it.
On steel, foundries substitute chromite or zircon for silica at the mould face.
Size and tolerance decide which mould system the part goes into. Green sand is the most common ferrous moulding medium and handles the great majority of grey iron and ductile iron work, and the two pour in the same band and run through the same sand system. Heavy carbon steel and stainless steel sections tend to go to a chemically bonded mould, which holds dimensions at large sizes that green sand cannot.