Sand Casting

Safety

Molten metal and any moisture — a damp tool, a wet floor — explode. Dry everything. Face shield, leather, and no synthetics. Pour over sand, not concrete.

Steps

Set the pattern on a board in the drag; dust it; ram sand in layers, firm at the edges.

Flip, add the cope, place sprue and riser pins, ram again.

Lift the cope off; remove pattern and pins; cut a gate from sprue to cavity.

Close the flask; weight it — molten metal floats a cope.

Melt, skim the dross, and pour steadily into the sprue without stopping until the riser fills.

Let it freeze; break out; cut off sprue and riser; clean up.

Tools

Two-part flask, green sand (silica, bentonite clay, water), a pattern with draft, parting dust, a crucible and furnace, tongs, sprue and riser pins.

The Science

Molten aluminium at 700 °C or bronze at 1,100 °C flows under gravity into the mould; the sand, bonded with clay or resin, is porous enough to let the air and steam out. The metal shrinks a few percent as it freezes, so a riser feeds more in, and shrinks again as it cools, so the pattern is made oversize. Bernoulli governs the pouring basin and sprue; the metal must fill the mould before it freezes.

How It Fails

Misrun: metal froze before filling — too cold or gate too small. Porosity: gas from wet sand or a turbulent pour. Sand inclusions from a soft mould. Cracks from a hot tear where the casting could not shrink.