Expendable molds: sand, plaster, ceramic (broken to remove the part).
Permanent molds: metals that hold strength at high T.
Composite molds: 2+ materials (sand, graphite, metal) combining advantages.

Sand Casting

Steps: place a pattern in sand to make an imprint → add a gating system → fill with molten metal → cool/solidify → break away the sand → remove the casting.

Sands: mostly silica (SiO₂), cheap, high-T resistant. Fine grains → ↑mold strength but ↓permeability (gas escape). Collapsibility lets the casting shrink without cracking.

Sand-mold types: green sand (sand + clay + water, cheap); cold-box (organic/inorganic binders); no-bake (synthetic liquid resin).

Components: mold supported by a flask; two-piece = cope (top) + drag (bottom); seam = parting line; pouring basin → sprue → runner → cavity; cores (sand inserts) form internal cavities; vents carry off gases.

Patterns (wood/plastic/metal): one-piece (simple), split (complex), match-plate (halves on a plate).

Other Expendable-Mold Processes

Shell-mold casting: pattern heated 175–370°C, coated with parting agent; sand + 2.5–4% thermoset resin falls on it; oven-cured; smooth walls (high fluidity) but low permeability; uses ~1/20 the sand.
Expendable-pattern (lost foam): polystyrene pattern evaporates on contact with molten metal; no parting line, cores, or risers.
Ceramic-mold casting: refractory slurry (zircon, alumina, silica, binders) — high-T capable.
Investment casting: wax/plastic pattern invested with refractory; wax recovered & reused; precision casting of steels and high-T alloys.

Permanent-Mold & Die Casting

Permanent-mold (hard-mold): two metal halves (cast iron, steel, bronze, graphite); cavity coated with refractory/graphite; heated 150–200°C; not economical for small runs; intricate shapes hard.
Die casting (a permanent-mold process): metal forced in at 0.7–700 MPa → thin sections possible.
- Hot-chamber: piston forces trapped metal through a nozzle; ~15 MPa typical (up to ~35 MPa).
- Cold-chamber: metal poured into an unheated shot chamber; ~20–70 MPa.
- Machines rated by clamping force; dies single/multiple/combination/unit-cavity.

Centrifugal Casting

Rotation-induced inertia distributes the metal:

True centrifugal: hollow cylinders (pipes, gun barrels) in a rotating mold.
Semi-centrifugal: rotationally symmetric parts (spoked wheels).
Centrifuging: metal poured at center, forced outward into mold cavities.

Risers

Reservoirs feeding shrinkage; must be the last to solidify (so made large enough). A blind riser (enclosed by mold) can be smaller than an open riser (exposed to air). Heating the mold → better fluidity & detail but risks weak binder and turbulence.