sand casting

Imagine building a sandcastle mold, but instead of packing wet sand around your hand you pack it around a wooden or metal copy of the part you want. You lift the copy out, leaving a hollow cavity shaped like the part, then pour in molten metal and let it freeze solid. Break away the sand and there is your casting. This is the oldest and cheapest way to turn liquid metal into a shape, and it is how big rough parts like engine blocks, manhole covers, and machine bases are still made today.

In practice the sand is mixed with a binder (clay and water in a green-sand mold, or a chemical resin) so it holds its shape. The pattern is packed in two halves of a box, removed, and the halves are closed with channels cut in for the metal to run in and for air to escape. Molten metal is poured, solidifies from the mold walls inward, and the sand is knocked off. Because sand is a poor conductor of heat, the metal cools slowly, which gives coarse grains and a rough surface. Tolerances are loose, often plus or minus a millimetre or more.

The honest picture is that sand casting trades quality for cheapness and flexibility. A new pattern is cheap, so it wins for large parts, complex internal shapes, and small production runs. But slow solidification leaves a coarse grain structure, and the metal shrinks as it freezes, so castings are prone to shrinkage cavities and gas porosity unless the mold is designed with risers (extra reservoirs) to feed the shrinkage. This is the process half of the process-structure link: how you cast sets the grain size and the flaws you must live with.