Casting processes divide by the mold and by what drives the metal into it. The mold is either expendable — sand, ceramic shell, or foam, broken away after every pour — or a permanent metal die that is reused, and the metal enters by gravity, under applied pressure, or by rotation. Those two choices set almost everything a buyer cares about: tooling cost, tolerance, wall thickness, and which alloys the process can run.
For a medium-to-large iron or steel part, four of the processes below are live options — green sand, no-bake, shell, and investment. Most of the rest are aluminum, zinc, and magnesium processes, because their dies cannot survive a ferrous melt for a production run; each section says so where it applies.
| Casting process | Mold type | Best suited for | Main advantages |
|---|---|---|---|
| Sand Casting | Expendable sand mold | Large or complex metal parts | Low tooling cost, wide alloy range |
| Investment Casting | Expendable ceramic mold | Precision components | Excellent accuracy and surface finish |
| Die Casting | Permanent metal die | High-volume non-ferrous parts | Fast production, excellent repeatability |
| Permanent Mold Casting | Reusable metal mold | Medium/high-volume aluminum parts | Better finish than sand casting |
| Gravity Die Casting | Reusable metal die | Aluminum and zinc components | Good mechanical properties |
| Low-Pressure Die Casting | Permanent metal die | Wheels and structural aluminum parts | Low porosity, controlled filling |
| High-Pressure Die Casting | Steel die | Thin-wall aluminum, zinc, magnesium parts | Very high production rate |
| Centrifugal Casting | Rotating mold | Pipes, sleeves, rings | Dense castings with few inclusions |
| Continuous Casting | Continuous mold | Billets, slabs, blooms | Extremely efficient for primary metal production |
| Lost Foam Casting | Expendable foam pattern | Complex automotive/industrial parts | Fewer parting lines and cores |
| Shell Mold Casting | Resin-coated sand shell | Small/medium precision parts | Better accuracy than conventional sand casting |
| Plaster Mold Casting | Plaster mold | Aluminum and non-ferrous precision parts | Smooth surfaces and fine details |
| Vacuum Casting | Vacuum-assisted mold | Prototype and specialty components | Reduced gas defects |
| Squeeze Casting | Metal die with pressure | High-strength aluminum parts | Low porosity and strong mechanical properties |
| Semi-Solid Casting | Permanent die | Automotive structural components | Low porosity and good dimensional control |
1. Sand Casting
Sand casting packs bonded sand around a pattern, draws the pattern, and pours into the cavity that is left; the mold is destroyed at shakeout, which is why the process has the lowest tooling cost, the widest alloy range, and the widest size range of anything on this page — 100 g to 100 tonnes. It is the default route for gray iron, ductile iron, and steel in OEM sizes: counterweights, housings, pump and valve bodies, flanges.
What separates one sand process from another is the binder. Green sand (clay and water) is the cheapest and fastest but the least rigid; no-bake (resin and catalyst, chemically bonded) holds a tighter dimension and a thinner section; shell (resin-coated sand cured on a hot pattern) tightens further at the price of metal pattern tooling. For steel castings the comparison runs:
| Green sand | No-bake (chemically bonded) | Shell | |
|---|---|---|---|
| Surface | Fair | Fair | Good |
| Minimum section | 1/4 in. (6.4 mm) | 3/16 in. (4.8 mm) | 5/32 in. (3.9 mm) |
| Base tolerance on a 1 in. cube | ±0.030 in. (0.76 mm) | ±0.020 in. (0.51 mm) | ±0.008 in. (0.20 mm) |
| Added across the parting face | ±0.030 in. (0.76 mm) | ±0.030 in. (0.76 mm) | ±0.010 in. (0.25 mm) |
| Pattern cost | Low | Low | High |
| Lead time | Shortest | Shortest | Long |
The table gives comparison values, not drawing tolerances; a drawing calls out an ISO 8062-3 grade, and which grade each molding method holds is its own subject. Plan on the heaviest machining stock of any process here — 3/16 to 3/4 in. (5 to 19 mm), depending on size and method.
2. Investment Casting
Investment casting builds a one-piece ceramic shell around a wax pattern, melts the wax out, and pours into the fired shell, so there is no parting line and the tolerance is the tightest of any expendable-mold process — about ±0.010 in. (0.25 mm) on a 1 in. dimension, widening to ±0.040 in. (1.02 mm) at 10 in. Premium tolerances of half that are available, but each one costs an extra operation.
The dipped shell that buys the accuracy also sets the limits. Parts run from a few grams to about 5 kg, with 100 kg the practical ceiling; sections go down to 1/16 in. (1.6 mm); and the as-cast surface on carbon and stainless steel is Ra 2.3–3.2 µm, good enough to leave many faces unmachined.
Pattern tooling is high-cost and the lead time is the longest of the mold methods, so it pays on small, intricate steel and stainless parts in production quantities — valve trim, impellers, manifold bodies — and not on a housing that a sand mold would make for a fraction of the tooling.
3. Die Casting
Die casting injects molten metal into a hardened steel die under pressure and holds it there until the part is solid; it is a non-ferrous process — aluminum, zinc, magnesium, and copper alloys — and the die is what rules iron and steel out. Die wear rises with pouring temperature.
Iron and steel pour far hotter. NADCA records ferrous die casting only as limited-production work on refractory-metal dies, so for a buyer it is not a production route.
Most die castings weigh under 7 kg (15 lb). If the part is an aluminum housing at volume, die casting versus sand casting is a real question; for an iron or steel housing it is already answered.
4. Gravity Die Casting
Gravity die casting pours metal into a reusable metal die with nothing but gravity behind it — no injection, no pressurized furnace — and the fast chill of the metal die gives a finer grain, a better surface (Ra 1.6–3.2 µm), and a tighter dimension (±0.1–0.2 mm) than sand. The dies are usually gray or ductile iron, and the process is used mainly for aluminum: cylinder heads, wheel hubs, pistons, typically 4–6 kg with a ceiling around 200 kg.
Iron and steel can be gravity die cast, but only into graphite dies, and die life against a ferrous melt is very short. Ferrous brake and hydraulic components do exist as gravity die castings; in OEM housing sizes, though, the die cost against that die life does not close, and the part stays in a sand mold.
5. Low-Pressure Die Casting
Low-pressure die casting stands the die on top of a sealed furnace and pushes the melt up a riser tube with 20–100 kPa of air pressure, so the cavity fills from the bottom, slowly (under 0.5 m/s) and without turbulence. That is the point of it: the metal arrives clean, the pressurized tube keeps feeding while the casting shrinks, and yield runs 80–90% because there are no risers or runners to cut off. Walls down to about 1 mm are possible.
The dies are gray iron, which settles the alloy question: it is an aluminum, magnesium, and copper-alloy process, and the textbook product is the aluminum road wheel, followed by cylinder blocks.
6. High-Pressure Die Casting
High-pressure die casting is what “die casting” usually means: a shot rammed into the die at 10–175 MPa (1,500–25,000 psi), which is what buys walls down to 0.38 mm on small aluminum parts, 2.7–4.5 mm on large ones, and the fastest cycle of any process here.
The cost is the fill itself. Metal entering that fast traps air, so HPDC carries more gas porosity than low-pressure casting; vacuum-assisted variants exist to pull that air out of the cavity before the shot.
Size caps run about 34 kg in zinc, 30 kg (about 66 lb) in aluminum, and 20 kg in magnesium. A housing heavier than that is outside what die casting normally does, and nothing ferrous goes through a die at these pressures.
7. Permanent Mold Casting
Permanent mold casting is the North American name for gravity die casting — the same reusable metal mold, the same gravity pour, the same aluminum-first alloy list; what this name usually gets attached to is the volume question. The mold is machined into mating die blocks, preheated, and clamped, and it is reused — up to 120,000 times — so the die cost spreads over the run; the usual floor is about 500 castings.
Walls have to be over 3 mm (1/8 in.) and under about 50 mm (2 in.), and a dimension that crosses the parting line picks up an extra 0.25 mm (0.010 in.) of tolerance. Against a ferrous melt the mold life that makes the volume math work is not there.
8. Centrifugal Casting
Centrifugal casting spins the mold — sand, metal, or ceramic — fast enough to hold the metal against the wall at 60–80 g, so a hollow cylinder solidifies from the outside in with no core and no gating; it is how cast iron pipe, bushings, sleeves, and cylinder liners are made.
The spin packs the wall dense and sends anything lighter than the metal — slag, oxide, sand — to the bore. “Few inclusions” is therefore true of the wall after the bore is machined: the bore is where the dirt went, and it has to come out.
Parts run to 10 m long, 173 cm in diameter, and 36,000 kg. Anything that is not a body of revolution is a sand casting; where the two overlap — flanges, rings, thick-walled bushings — sand casting is the alternative to weigh it against.
9. Continuous Casting
Continuous casting is not a way of making a part; it is the steel mill’s route from ladle to billet, bloom, or slab, and by the industry’s own count it produced 97.4% of the world’s crude steel in 2025. Liquid steel passes through a bottomless water-cooled copper mold, forms a skin, and is drawn out as a strand that is cut to length; the product is semi-finished, and the plate, bar, and sections a weldment is built from all came through it.
Continuous casting never appears on an OEM drawing: you specify a rolled product or a shaped casting, and continuous casting sits upstream of both.
10. Lost Foam Casting
Lost foam casting replaces the pattern with a polystyrene foam replica that is coated with refractory, buried in unbonded sand, and left in the mold; the metal vaporizes it on entry, so the casting has no parting line, needs no cores even for closed internal passages, and can be drawn with almost no draft.
Complex castings up to 2,000 kg have been made without cores, but the usual range is 1–50 kg, and the minimum wall is 4–5 mm. The economics turn on pattern tooling: foam patterns are molded in a machined die at roughly one a minute, and that die is the fixed cost the run has to amortize.
Iron pours hot enough to vaporize the foam and can be cast this way. Steel is the problem: the pyrolyzed polystyrene leaves carbon that diffuses into the casting surface. At the sizes and lot quantities iron and steel OEM parts usually come in, the foam die rarely pays, and the part goes into a sand mold.
11. Shell Mold Casting
Shell mold casting cures a 6–9 mm shell of resin-coated sand — fine silica with about 5% phenolic or urea resin — against a pattern plate heated to 200–250 °C, and of the three common sand molding methods it holds the tightest tolerance: the ±0.008 in. (0.20 mm) base figure in the sand casting table above.
The shell is thin, rigid, and cured hot, so it does not yield when the metal hits it, and it takes a smoother surface off the pattern. Shell also holds draft down to about 1°, against 2–3° for other sand castings.
A shell casting that skips a machining pass is often cheaper than the sand casting it replaces even though the metal pattern costs more. Output is 30–220 molds an hour. It is a small-and-medium-part process; a 6–9 mm shell is not a mold for a heavy housing.
12. Squeeze Casting
Squeeze casting closes a metal die on a measured shot of molten metal and holds 30–110 MPa on it until the part is solid, so gas porosity has nowhere to sit and shrinkage porosity is fed shut under pressure; the casting comes out fine-grained and near forging density in a shape forging could not make. Gates are large and the metal is moved slowly to avoid turbulence, which makes it the opposite of high-pressure die casting: slow, at 5–30 shots an hour, and dense.
Parts run 2–50 kg with walls of 3–50 mm, a surface of Ra 0.4–3.2 µm, and tolerances around 0.2 mm per 100 mm before heat treatment. Commercially only aluminum alloys are squeeze cast — casting and forging grades for wheels, hubs, and pistons.