Casting is the oldest manufacturing process still in heavy use: pour molten metal into a cavity, let it solidify, break it out. The three dominant variants — sand, investment, and die casting — differ in mold material, surface finish, tolerance, and economics. Picking the wrong one can multiply unit cost by 10x or scrap your tolerances entirely.

Sand casting uses a packed sand mold formed around a wooden or metal pattern. The mold is destroyed after each pour, so cycle times are slow (minutes per part) but tooling is cheap — a wooden pattern costs hundreds, not tens of thousands. Surface finish is rough (250–500 µin Ra), tolerances loose (±0.030" typical), and minimum wall thickness is around 3 mm. Engine blocks, pump housings, and manhole covers are sand cast because nobody cares if the outside looks like sandpaper.

Investment casting (lost-wax) builds a ceramic shell around a wax pattern, melts the wax out, then pours metal in. The shell is also destroyed per part, but the wax pattern comes from an injection-molded die — so tooling costs jump to $5k–$50k. The payoff: ±0.005" tolerances, 63–125 µin Ra finish, walls down to 0.5 mm, and near-net shape that eliminates most machining. Turbine blades, surgical instruments, and firearm parts use it.

Die casting forces molten metal (usually aluminum, zinc, or magnesium) into a reusable steel die under pressure. Tooling runs $25k–$200k+ and cycle times drop to seconds. Tolerances reach ±0.002", finish is 32–63 µin Ra, and walls can go below 1 mm. But you're limited to low-melting-point alloys — steel destroys the dies. Laptop housings, transmission cases, and small-engine parts are die cast.

Rule of thumb for picking:

• Volume < 100 parts: sand cast — tooling amortization kills the alternatives.
• 100–10,000 parts, tight tolerance or exotic alloy: investment cast.
• > 10,000 parts in aluminum/zinc/magnesium: die cast.

Common failure modes to watch for: Porosity (trapped gas or shrinkage voids) plagues all three but is worst in die casting due to turbulent fill. Hot tears form when thick and thin sections cool at different rates — keep wall thicknesses uniform. Draft angles of 1–3° on vertical surfaces are mandatory; without them, parts won't release from the mold.

One subtle gotcha: cast parts shrink as they solidify, typically 1–2% for aluminum and steel. Pattern makers build oversized patterns to compensate — if you're handed a cast part drawing, the mold cavity is bigger than the nominal dimensions on the print.