Casting §

Step-by-step tutorial.

1. Pour and cool. Superheated liquid metal enters the mold and immediately starts losing heat through the mold wall. The rate of heat extraction (governed by the mold material’s thermal diffusivity) sets everything that follows: steel and graphite molds cool fastest, ceramic shells slowly, sand slowest.
2. Nucleation at the mold wall — the chill zone. The cold wall produces a large undercooling, so many crystals nucleate at once and grow into each other, forming a fine-grained surface layer called the chill zone.
3. Columnar growth. Behind the chill, a few favorably oriented grains grow inward as long columns, following the heat flow. Fast cooling (metal molds) favors long columnar zones; slow cooling (sand) lets equiaxed grains take over.
4. Equiaxed center. In the last liquid, free-floating crystals grow in all directions, giving a coarser, equiaxed (equal-axed) center. Finer equiaxed grains generally mean better toughness and less segregation, which is why foundries add grain refiners.
5. Shrinkage and feeding. Most metals shrink 0.7–3 % in volume on solidification (grey iron ~0.7 %, aluminum 1.3–1.8 %, steel ~2 %, zinc 2.5 %). If the last-to-solidify region is not fed by liquid metal from a reservoir (riser or feeder), it collapses into a shrinkage cavity or dispersed porosity. Riser sizing follows Chvorinov’s rule, t = B (V/A)ⁿ with n ≈ 2: the riser must stay liquid longer than the region it feeds.
6. Segregation and hot tearing. Solute elements concentrate in the last-to-solidify liquid (segregation), and a solid skeleton that cannot accommodate the shrinkage strain tears apart while still hot (hot tearing).

Worked example — a 2 kg aluminum bracket in green sand. The sand mold extracts heat slowly, so the chill zone is thin and the interior is mostly coarse equiaxed grains. The designer places a riser on the thickest boss so it solidifies last and feeds the boss; the thin walls solidify first and are fed through the still-liquid center. Total solidification takes a few minutes; the bracket shows fine surface grains (good finish for sand) and, if the riser was undersized, dispersed shrinkage porosity in the boss that a denser riser or a chill (a metal insert that accelerates local cooling) would remove.

The microstructure below shows the three zones from mold wall (left) to center (right):

Sources: ASM Handbook, Volume 15: Casting; J. Campbell, Complete Casting Handbook (Elsevier).

Variants (cheatsheet).

• Green sand: silica sand bound with clay and water. Cheap, reusable, but moderate surface finish (Ra 6.3–25 µm) and dimensional accuracy of ±0.75–1.5 mm on a 100 mm dimension (ISO 8062-3 casting tolerance grades CT9–CT11).
• Resin-bonded sand: phenolic or urethane resin. Better surface finish, used for medium production.
• Shell molding: a thin shell of resin-bonded sand is formed around a heated metal pattern. High accuracy (±0.2 mm), surface finish reaching ~3 µm, used for precision parts.

Step-by-step tutorial.

1. Pattern. A replica of the part, slightly oversized to compensate for solidification shrinkage and machining allowance, is made of wood, metal, or resin. Loose pieces or sloped surfaces (draft) let it be withdrawn from the sand.
2. Mold. The pattern is embedded in the drag (bottom flask) filled with sand, the flask is flipped, the cope (top flask) is added, and the sprue (pour channel) and riser (feed reservoir) are formed. The two halves are separated, the pattern withdrawn, and the flasks reassembled at the parting line.
3. Core. Internal cavities (a water jacket, a bore) are made by placing pre-baked sand cores into the mold; they are destroyed during shakeout.
4. Pour. Molten metal flows down the sprue, through the gating system, and fills the cavity. Pouring temperature and rate are chosen to fill thin sections before they freeze while avoiding turbulence that entrains oxide film and gas.
5. Solidify and shakeout. The casting solidifies as in the Solidification and defects section; then the mold is broken apart (“shakeout”) and the sand is reclaimed.
6. Fettling. The sprue, risers, and flash are cut off and the part is ground and cleaned.

Worked example — 500 gray-iron pump housings. A wood pattern (with 1 % shrink allowance) is mounted on a matchplate; green sand is rammed around it in a cope-and-drag flask. Cores form the internal volute passage. Gray iron is poured at ~1400 °C; its ~0.7 % solidification shrinkage and graphite expansion make feeding easy, so a small riser suffices. Shakeout after ~20 min, fettling, and the housings are ready for machining of the sealing face. Tolerance on a 200 mm dimension: about ±1.5–2 mm (CT9–CT11), so the sealing face carries machining allowance.

Cross-section of a cope-and-drag sand mold:

Sources: ISO 8062-3:2023 (tolerance grades); ASM Handbook, Volume 15: Casting.

Cheatsheet. Surface finish Ra 1.6–6 µm (specialized processes reach ~0.8 µm); dimensional accuracy ±0.1 mm. Used for turbine blades, jewelry, surgical instruments, and complex aerospace parts. The ceramic shell must withstand the pour temperature and thermal shock.

Step-by-step tutorial.

1. Wax pattern. Molten wax is injection-molded in an aluminum die — the same injection principle as die casting, but with wax. Each pattern is an exact replica of the part plus shrink and machining allowances.
2. Treeing. Patterns are welded onto a central wax sprue so many parts share one shell (“tree”).
3. Shell build. The tree is dipped in a fine ceramic slurry (the faced coat, which sets the surface finish), stuccoed with coarse grit, and dried; this dip–stucco–dry cycle is repeated 6–10 times to build a shell thick enough to survive handling and the pour.
4. Dewax. The shell is flash-heated (steam autoclave or furnace) so the wax melts and drains out, leaving a hollow ceramic mold.
5. Fire and pour. The shell is fired at ~1000 °C for strength, then poured while hot (to avoid thermal shock and premature freezing of thin sections). For high-integrity parts the shell is evacuated or a small overpressure is applied to minimize gas porosity and ensure complete fill of thin sections.
6. Knockout. The ceramic is broken away mechanically or with water jets, and parts are cut from the tree.

Worked example — a nickel-superalloy turbine blade. A wax pattern with a ceramic-wire core inside it forms the internal cooling channels. Eight zirconia slurry layers build a shell that survives a 1500 °C vacuum pour. Because the shell is fired hot and the metal is superalloy, the as-cast blade needs only grinding of the root and polishing of the airfoil — the complex airfoil and channels come out of the mold finished.

Shell cross-section (wax pattern inside layered ceramic):

Sources: ASM Handbook, Volume 15: Casting; Investment Casting Institute publications.

Variants (cheatsheet).

• High-pressure die casting (HPDC): 50–150 MPa injection pressure. Very fast cycle (seconds), surface finish Ra 0.8–3.2 µm as-cast (zinc Zamak at the lower end, aluminum A380 at the upper; worn dies or very complex features can degrade this to ~6.3 µm), but porosity is common due to gas entrapment.
• Low-pressure die casting (LPDC): 0.02–0.2 MPa (most commonly 0.06–0.15 MPa for aluminum). Slower but lower porosity; used for engine blocks and transmission housings.
• Squeeze casting: pressure is maintained during solidification to minimize porosity.
• Permanent mold (gravity die) casting: a reusable metal mold is filled by gravity or low-pressure head, with no high-pressure injection. It sits between sand casting and HPDC in the process spectrum: better surface finish and dimensional accuracy than sand, lower tooling cost and porosity than HPDC. Typical for aluminum and magnesium parts in medium-to-high volume.

The steel die is expensive but lasts 100,000+ shots, making die casting economical above ~10,000 parts.

Step-by-step tutorial.

1. Die preparation. Two steel die halves are clamped and sprayed with a release agent that also cools the die surface.
2. Shot. In a cold-chamber machine (aluminum, magnesium) a measured dose of molten metal is ladled into a shot sleeve and a hydraulic plunger drives it into the die at 50–150 MPa; in a hot-chamber machine (zinc, whose alloys do not attack the gooseneck) the injection mechanism sits in the melt and cycles continuously.
3. Fill and solidify. The metal fills the cavity in tens of milliseconds; venting lets air escape, but some is entrained — the source of HPDC porosity. The metal solidifies against the steel die in seconds (fast cooling → fine surface grains, per Solidification and defects).
4. Eject. The die opens and ejector pins push the part out; the cycle repeats every ~30–60 s.
5. Trim. The overflow, gates, and flash are trimmed in a punch die and remelted.

Worked example — an aluminum A380 gearbox housing, 200,000 units/year. A cold-chamber HPDC machine injects at ~70 MPa; cycle time 45 s, so one machine produces the annual volume. As-cast finish Ra ~1.6–3.2 µm and ±0.1 mm on a 100 mm dimension mean only the bearing bores need machining. The entrapped-gas porosity is acceptable because the housing is not pressure-tight and will not be welded (welding would expand the gas and blister the surface).

Cold-chamber HPDC schematic:

Sources: NADCA (North American Die Casting Association) standards; ASM Handbook, Volume 15: Casting.

Step-by-step tutorial.

1. EPS pattern fabrication. Polystyrene beads are pre-expanded with steam, then steam-molded in an aluminum tool into the full pattern shape (or segments glued together). The pattern is an exact positive of the part — it is the pattern and the core at once, which is why cores disappear in this process.
2. Coating. The pattern is dipped in a thin refractory slurry and dried. The coating is the actual mold surface: it sets the finish, and it must be permeable enough to let the vaporized foam gas escape.
3. Flasking. The coated pattern is placed in a flask and loose, unbonded dry sand is poured around it and vibrated to compact. No binder, no hardening: the coating holds the sand off the metal.
4. Pouring dynamics. This is the heart of the process. As metal advances, the foam ahead of it must vaporize and its gas must escape through the coating. The pour rate must match the foam’s vaporization rate: pour too slowly and the mold can collapse or the front stalls (misrun); pour too fast and liquid overtakes the foam, trapping foam decomposition products in the metal (carbon films, porosity).
5. Solidify and shakeout. Solidification follows the same physics as every other process (Solidification and defects); the loose sand simply falls away — no raking, no core knocking.

Typical defects. Surface roughness and “orange peel” laps where foam residue was trapped against the coating; dimensional inaccuracy from pattern expansion during steam molding (EPS patterns can distort after molding, so they are aged before use); misruns in thin sections where the vaporization front outran the metal.

Worked example — an aluminum cylinder head. The water jacket and ports, which would each need a separate core in sand casting, are simply part of the glued EPS pattern. The pattern cluster is coated, flasked in dry sand, and poured at ~740 °C with the pour rate tuned to the vaporization front. The result is a near-net head with no core-assembly step and far less machining of the port surfaces — the reason foundries adopted it for complex blocks and heads.

Sources: ASM Handbook, Volume 15: Casting; American Foundry Society, lost-foam process publications.

Cheatsheet. The effective acceleration is $G = \omega^2 r / g$; foundries run 60–100 g so the metal forms a uniform annulus regardless of gravity. Dimensional accuracy ±0.2–0.5 mm on the outside diameter (the bore is set by the poured metal volume); surface finish Ra 3–12 µm; cycle minutes; metal or graphite mold, reusable for thousands of pours.

Step-by-step tutorial.

1. Spin up. A cylindrical metal (or graphite) mold is rotated about its axis — horizontal for long pipes and cylinder liners, vertical for rings and short tubes.
2. Pour. Metal is poured into the spinning mold; the centrifugal field flings it to the wall, where it spreads into an even annulus. The axis of rotation is the bore of the finished part — no core, no core prints, no core assembly.
3. Solidify. Solidification proceeds from the mold wall inward, i.e. directionally toward the bore. This is the process’s key advantage: the wall solidifies first and is always fed by liquid metal closer to the free surface, so the casting is dense and sound.
4. Segregation to the bore. The same field that presses the metal outward also floats less-dense inclusions and traps shrinkage at the free surface — the bore. The bore carries the defects, and it is machined off (or is the surface that gets bored to size anyway).
5. Extract. Rotation stops, the tube or ring is pulled from the mold, and the bore is machined.

Variants. True centrifugal casting (a hollow part whose bore is the rotation axis: pipes, cylinder liners, bushings); semi-centrifugal (solid-of-revolution parts like wheels, spun about a central riser); centrifuging (several small molds on a spinning table, using the field to feed thin sections).

Worked example — a 6 m ductile-iron water pipe. A horizontal water-cooled steel mold spins at ~1000 rpm; a measured ladle of ductile iron runs in along the axis and spreads into a 6 mm wall. The pipe solidifies from the mold wall inward in about a minute, dense and free of shrinkage; any inclusions sit in the bore skin, which is machined or spun out. One mold pours thousands of pipes — this is how most of the world’s ductile-iron water mains are made.

Sources: ASM Handbook, Volume 15: Casting; ISO 2531 (ductile-iron pipes).

• What is simulated: transient mold filling (velocity fields, air entrapment, oxide-film folding), heat transfer and solidification (temperature fields, solidification-time maps, feeding criteria such as the Niyama criterion for shrinkage porosity), and thermo-mechanical behavior (distortion, hot-tear risk, residual stress).
• Commercial tools: MAGMASOFT, ProCAST (ESI Group), AnyCasting, FLOW-3D CAST.
• How it is used: the foundry meshes mold + casting, runs fill and solidification, and iterates gating, riser, and chill design until the predicted porosity and hot-tear indices are acceptable — cutting physical trial pours from several to one or none.
• Limitations: results depend on accurate thermophysical data; the metal–mold interface heat-transfer coefficient is the largest uncertainty, so simulation predicts trends and risk locations more reliably than exact defect sizes.

A turbine blade with a complex airfoil cross-section, internal cooling channels, and a 1.5 mm minimum wall thickness must be produced from a nickel superalloy (Inconel 718). Which casting process is appropriate, and what does the full route look like?