How Do You Make the Same Shape Twice? Pouring Metal Into a Cavity

Casting requires a cavity the shape of the desired object, a way of getting liquid metal into it, a way of letting the air out and a way of removing the result. The mould may be expendable, destroyed to release the casting, or permanent and reusable, and that choice drives everything else. A pattern, meaning a model of the object, is used to form the cavity in expendable moulds, and it must be slightly larger than the finished piece because metal shrinks as it cools. Channels feed the metal in, and reservoirs hold extra liquid that flows in as the casting contracts, preventing voids. The process is ancient, with cast metal objects dating back thousands of years, and the underlying physical problems have not changed at all.

Several approaches trade cost, accuracy and volume against each other:

• •Sand casting, where a pattern is pressed into bonded sand, which is cheap, versatile and leaves a rough surface
• •Investment casting, where a wax model is coated in ceramic and melted out, giving fine detail
• •Die casting, forcing metal under pressure into a reusable steel mould, which is fast and suits high volumes
• •Permanent mould casting, pouring under gravity into reusable metal moulds
• •Centrifugal casting, spinning the mould so the metal is thrown against its walls
• •Continuous casting, where metal solidifies as it is drawn steadily out of a water-cooled mould

The constraints on shape are strict and are visible in every cast object. The pattern must come out of an expendable mould and the casting must come out of a permanent one, which requires draft, meaning that surfaces are tapered slightly rather than parallel so they release. Undercuts, where a feature would lock the piece in place, require the mould to split into more pieces or to include separate cores, which costs money and leaves visible lines. Wall thickness must be reasonably uniform, because thick sections cool and shrink last and form voids, which is why cast parts are ribbed rather than solid. Sharp internal corners concentrate stress and impede flow, so they are filleted. And the metal must reach every part of the cavity before it starts to solidify, which limits how thin and how long a section can be.

Choosing to cast rather than to make a part another way is an economic decision as much as a technical one. Machining from solid gives better accuracy and surface finish and wastes material, which matters enormously for expensive alloys, and it becomes uncompetitive as complexity rises. Forging gives superior strength because the internal grain structure follows the shape, which is why crankshafts and highly stressed parts are forged rather than cast. Fabrication by welding sections together suits one-off large structures where a pattern could never be justified. Additive manufacturing now produces shapes no mould could release and is slow and expensive by comparison. Casting wins where the shape is complicated, the quantity is high enough to pay for the tooling, and the strength demands are moderate, which describes an enormous share of everything manufactured.

The characteristic defects are well catalogued and each traces to a specific cause. Porosity comes from gas dissolved in the liquid metal coming out as it solidifies, or from air trapped during filling. Shrinkage cavities form where a thick section had no reservoir to draw from. Cold shuts occur where two flows of metal meet after both have begun to solidify and fail to fuse, leaving a seam that is a crack. Inclusions are particles of mould material or oxide carried in with the metal. Hot tearing happens where a casting contracts while still weak and is restrained by the mould. Each is addressed by changing the geometry, the gating, the temperature or the mould material, and the accumulated knowledge of which change fixes which defect is much of what a foundry knows.

A cavity is filled with liquid that solidifies, with the pattern made oversize because metal shrinks and with reservoirs feeding extra liquid in as it contracts. Surfaces must taper so the piece releases, walls must be reasonably uniform because thick sections form voids, and undercuts require the mould to split. The characteristic defects each trace to a specific cause in geometry or temperature.