Sand casting

Metal poured into a cavity packed in sand; the sand is broken away

Sand casting makes a metal part by pouring molten metal into a hollow shaped in packed sand. Engine blocks, manhole covers and pump housings are typical examples. It is one of the oldest casting methods and still the most widely used, because the mold is cheap and almost any size is possible.

Photo: Kambai Akau, CC BY-SA 4.0, via Wikimedia Commons

Sand casting · pattern, pouring, shake-outRead the three panels left to right; circled numbers match the steps below.

Sand casting in a minute, by MecMade

How it works

Make a pattern. A pattern is a full-size copy of the part in wood, plastic or metal. It is made slightly oversize, because metal shrinks as it cools.
Pack sand around it. The pattern sits in a two-part box called a flask. Sand mixed with clay and water (green sand) or with a resin binder is rammed (pressed hard) around it until it holds its shape.
Remove the pattern, set the core, close the mold. The upper half (the cope) is lifted off the lower half (the drag) and the pattern is drawn out, leaving a cavity. A core (a hardened sand insert) goes in to form an inner hole, channels are cut, and the halves are closed again. The seam where they meet is the parting line.
Pour. Molten metal is poured from a ladle into the pouring cup. It runs down the sprue (vertical channel), along the runner (horizontal channel) and fills the cavity.
Let it solidify. Cooling takes minutes for small parts and hours, even days, for very large ones. A riser (an extra reservoir of metal) keeps feeding the part as it shrinks, so the shrinkage hole forms in the riser instead of the part.
Break out and finish. The sand is shaken off and recycled. The sprue and risers are cut off, the surface is cleaned, often by shot blasting (blasting with small steel balls), and key faces are machined.

Key facts

Materials: Almost any metal that can be melted: gray and ductile iron, steel, aluminum, bronze, brass, magnesium
Typical parts: Engine blocks, cylinder heads, pump and valve bodies, gearbox housings, manhole covers, machine bases
Part size: From a few centimeters to several meters across; from grams to more than 100 tonnes
Precision: Typically about ±0.5 to ±2 mm, more on large parts
Surface finish: Rough and grainy: Ra (average roughness) roughly 6 to 25 µm
Batch size it suits: One-offs and prototypes (hand molding) up to millions of parts (automated molding lines)
Tooling and machine cost: Patterns from a few hundred to tens of thousands of dollars; automated molding lines cost millions
Speed: Hours per mold by hand; automated lines make up to several hundred molds per hour. Cooling adds minutes to hours

Strengths and limits

Strengths

Works with almost any metal, including high-melting iron and steel
Parts from palm-size to many tonnes
Cheap tooling, so one-offs and prototypes are affordable
Cores make complex internal passages possible

Limits

Rough surface and loose tolerances, so key faces need machining
A new mold must be made for every part
Gas holes (porosity) and trapped sand grains can cause defects
Thin walls are hard to fill; about 3 to 5 mm is a practical minimum

Where it's used

AutomotiveCast-iron engine blocks, cylinder heads, brake drums, differential housings
Pumps and valvesPump casings, valve bodies, impellers
Heavy machineryMachine-tool beds, gearbox housings, mining and construction equipment
Energy and marineWind-turbine hubs, turbine casings, ship propellers
City infrastructureManhole covers, drain grates, pipe fittings
Home goodsCast-iron skillets, wood-stove parts, bells

How to recognize it

A grainy, matte surface, like fine sandpaper, on every area that was not machined.
A thin seam or small step running around the part: the parting line, where cope and drag met.
Flat, ground-down patches where the sprue, runners and risers were cut away.
Shiny machined faces (bolt flanges, bores) right next to rough cast surfaces.

Compare with

Investment casting1.2a ceramic shell built around a wax copy gives much finer detail and smoother surfaces, at a higher cost per part.Die casting1.4a reusable steel die and high pressure make fast, precise aluminum and zinc parts, but the die costs far more.Lost foam casting1.6also uses sand, but the pattern is a foam copy that stays in the mold and is vaporized by the metal.

Watch it

dharmesh barodiya · Every step, from pattern to finished casting, in a teaching workshop

mrpete222 · How a green-sand mold is rammed by hand in a cope and drag

General Foundry Service · What an industrial green-sand foundry and its pouring look like

DISA · How an automated line makes and pours molds for mass production

See real photos

Wikipedia: Sand casting — photos of molds, flasks, cores and castings, plus a process diagram
Wikipedia: DISAMATIC — the automated machine that makes vertically split sand molds
Wikimedia Commons photo search

Remember

A reusable pattern shapes the cavity; the sand mold is destroyed after each pour.
The sprue, runners and risers bring metal in and feed it, then are cut off.
Cheap and flexible for any size, but rough and only moderately precise.