§1Pouring a shape
Casting forms metal by melting it, pouring it into a mould shaped like the wanted part, and letting it solidify. It makes complex shapes in one step — shapes that would be hard or wasteful to machine or forge.
The appeal is that almost any shape, however intricate — an engine block, a pump housing, a bracket with internal passages — can be made directly by filling a cavity, at any size from grams to many tonnes, and often cheaply in quantity. The challenges all trace to two physical facts. First, metal shrinks as it cools and solidifies, so a casting comes out smaller than the mould and can pull voids into itself (§3, §5). Second, thick sections solidify last, which decides where those voids form and how they must be fed (§4). The whole craft of casting — the pattern’s size, the risers, the gating — is arranged around managing shrinkage and solidification, and this page follows that thread from the mould (§2) to the metals (§6).
Contents§2The sand mould
The commonest mould is made of bonded sand, packed around a pattern in two halves, then parted to leave a cavity the metal is poured into.
In sand casting, a pattern — a model of the part — is packed in moulding sand within a two-part box (the flask): the top half is the cope, the bottom the drag. The pattern is withdrawn, leaving a cavity, and channels are cut to fill it: a sprue down which metal is poured, a gate leading into the cavity, and a riser reservoir (§5). Internal hollows are made with sand cores set into the cavity. The halves are closed and the metal poured (the hero shows the cross-section). Two features of the pattern deserve note: it carries a draft — a slight taper on vertical faces — so it can be withdrawn without tearing the sand, and it is made oversize to allow for shrinkage (§3). Sand moulds are cheap and take any size, but are destroyed to release each casting; permanent metal moulds (die casting) suit high volumes of smaller parts.
Contents§3Shrinkage allowance
Because metal contracts as it cools from pouring to room temperature, the pattern must be made larger than the finished casting by a shrinkage allowance — different for each metal.
A casting solidifying and cooling shrinks by a characteristic amount — roughly 1% for grey cast iron and about 2% for steel — so the pattern is scaled up to compensate. A steel casting meant to finish 500 mm long needs a pattern 500 × 1.02 = 510 mm long; the same part in cast iron would need about 505 mm. Patternmakers use a “shrink rule” — an oversized ruler — so every dimension is laid out already enlarged by the metal’s shrinkage. Get it wrong and the whole casting comes out undersize. This is solid shrinkage, the contraction after freezing; it is distinct from the solidification shrinkage of the liquid-to-solid change, which forms voids and is dealt with by risers (§5). Both stem from metal shrinking, but one is fixed by an oversize pattern and the other by feeding.
§4Solidification and Chvorinov's rule
How long a section takes to freeze depends on its bulk relative to its surface — captured by Chvorinov's rule, which is the key to feeding a casting soundly.
Heat leaves a casting through its surface, so a section with much volume behind little surface freezes slowly, and one with little volume and much surface freezes fast. Chvorinov’s rule makes this exact: solidification time goes with the square of the ratio V/A, called the modulus. A 100 mm cube has volume 10⁶ mm³ and surface 6 × 10⁴ mm², so its modulus is 16.7 mm; a thin plate of the same volume, with far more surface, has a much smaller modulus and freezes far sooner. The consequence is decisive: the thickest part of a casting, with the largest modulus, is the last to solidify — and that is exactly where solidification shrinkage will pull a void unless fresh liquid can be fed in as it freezes. Chvorinov’s rule tells the founder where the trouble will be and how to beat it (§5).
§5Risers and feeding
To stop the last-freezing region pulling a shrinkage void, a riser — a reservoir of molten metal — is attached to feed liquid into the casting as it solidifies. The riser must freeze after the casting.
As a section solidifies it draws in liquid to make up the volume lost in freezing; if none is available, it forms an internal cavity or surface sink. The riser supplies that liquid — a pocket of molten metal, connected to the heavy section, that stays liquid long enough to feed it. The governing condition comes straight from Chvorinov’s rule: the riser must have a larger modulus (V/A) than the casting section it feeds, so that it solidifies later and can keep supplying liquid until the casting is fully frozen. A riser that froze first would be useless. So risers are made chunky (high volume, low surface) and placed on or near the thickest sections, and the gating is arranged so those sections stay hot and fed. Sound feeding — the right riser in the right place, sized by modulus — is what separates a solid casting from one riddled with shrinkage porosity.
Contents§6The cast irons
Iron castings dominate because cast iron is cheap, pours easily and, in its several forms, spans a wide range of properties — from brittle and wear-resistant to tough and ductile.
The materials pages set out how carbon form governs cast iron; in the foundry the forms are chosen for the part. Grey iron, with its carbon as graphite flakes, is the everyday casting metal — cheap, fluid, easy to machine, excellent at damping vibration and in compression (machine bases, blocks), but brittle in tension. Ductile (nodular) iron, with the graphite formed into spheres by treatment, keeps grey iron’s castability but gains real toughness and ductility, so it takes shock and tension — crankshafts, gears, pipes. White iron, with its carbon locked as hard iron carbide, is intensely hard and wear-resistant but very brittle, used where abrasion resistance is all. Malleable iron, made by heat-treating white iron, is a tougher old alternative to ductile. Cast steel, poured where a casting must have steel’s full strength and toughness, shrinks more (§3) and is harder to cast but gives properties grey iron cannot. The founder picks the metal for the duty — grey for stiffness and economy, ductile for toughness, white for wear, steel for strength.
Contents§7Quick reference
The working core of the page on one card rack.
Sand mould
cope + drag · sprue · gate · riser
pattern with draft, oversize
Shrinkage
iron ~1% · steel ~2%
500 mm steel → 510 mm pattern
Chvorinov
t = C(V/A)² · modulus V/A
thickest freezes last
Risers
modulus > casting → freezes later
feeds shrinkage void
Cast irons
grey · ductile · white · steel