Casting metal means melting an alloy, pouring it into a cavity shaped like the part, and letting it freeze — but between those three verbs sit fourteen steps that decide whether what comes out of the mold matches the drawing you sent.
What Is Metal Casting?
Metal casting is making a part by pouring molten metal into a cavity shaped like the part and letting it solidify there. The mold carries the geometry; the metal takes it.
Casting earns its place when that geometry is expensive by any other route — internal passages, walls that change thickness, curved external form — because all of it is produced in one operation instead of several weldments or a great deal of removed metal.
What Metals Can Be Cast?
Nearly any metal that can be melted can be cast, and the practical limit is the mold rather than the alloy. Sand and ceramic mold materials hold up to roughly 3,000–3,330 °F (1,650–1,820 °C), and the common foundry alloy families all pour below that ceiling.
The families a ferrous foundry works in:
- Gray iron: flake graphite gives damping and easy machining at the lowest cost per pound of the ferrous options, which is why counterweights, machine bases and housings loaded in compression default to it.
- Ductile iron: nodular graphite buys real ductility and impact strength while keeping most of iron’s castability — the usual choice for pump and gearbox housings, flywheels and hydraulic bodies.
- Carbon and low-alloy steel: highest strength and weldability of the group, and the only sensible answer when the part is structural or has to be welded into an assembly.
- Stainless steel: corrosion and temperature service — valve bodies, pump wet ends, and anything that has to survive what it is pumping.
- Non-ferrous alloys: aluminum, zinc, magnesium and copper-base alloys all cast readily and melt hundreds of degrees lower, which puts them in different foundries running different melting and molding equipment.
Main Methods of Casting Metal
Five processes cover almost all industrial metal casting, and for a ferrous part of any real size the choice narrows to the three sand molding variants.
| Method | Metals | Practical size | As-cast tolerance | Where it fits |
|---|---|---|---|---|
| Green sand molding | Most castable metals | Ounces to many tons | CT 12–14 | Medium to high volume, lowest tooling cost |
| No-bake (resin) sand | Most castable metals | Ounces to many tons | CT 12–14; tighter than green sand above 200 lbs | Heavy castings and low-to-medium volume |
| Shell molding | Most castable metals | Limited by mold size | Tightest of the sand processes | Smaller parts needing better finish and repeatability |
| Investment casting | Most castable metals | Fraction of an ounce to 150 lbs | CT 5–7 | Small, complex, tight-tolerance parts; no draft required |
| Die casting | Aluminum, zinc, magnesium, limited brass | Not normally over 2 ft square | Not available in ferrous | Very high volume, non-ferrous only |
A CT grade is a total tolerance band, not a ± figure, and it is looser than most drawings assume. For a feature between 160 and 250 mm, CT12 allows 8 mm of total spread — about ±0.16 in. Tighter grades, down to CT 8–10, sit inside sand molding’s capability but are not available on every casting type or every feature.
Investment casting at CT 5–7 is a genuinely different class of dimensional control, which is why it gets proposed whenever a drawing looks tight. The weight ceiling of roughly 150 lbs is what takes it back off the table for medium and large castings. Die casting is in the table only to rule it out — an iron or steel part cannot be die cast at all.
How to Cast Metal Step by Step
Fourteen steps take a drawing to an inspected casting. The first two are where changes are nearly free; from the pour onward, a mistake is scrap.
Step 1: Design the Casting
Casting design starts with three things: section thickness, draft, and where the mold parts. A minimum section of 0.25 in (6 mm) is the working floor for conventional steel casting.
Draft is added on top of the design dimensions, about 3/16 in per foot — roughly 1.5° — with more on deep pockets. The parting line is a design decision too: shapes symmetric about a centerline or plane suggest their own parting line and simplify both molding and coring.
Tolerances on as-cast features are agreed between foundry and buyer, not handed down. Putting a tight tolerance on a dimension that is not critical raises the price of the casting and buys the purchaser nothing.
Step 2: Make the Pattern
The pattern is deliberately not the size of the finished part — it carries contraction allowance, draft and machining stock, all stacked on top of the design geometry.
How much contraction allowance is less predictable than the published tables suggest. The standard sand-casting shrinkage table has no row at all for ductile iron and none for plain carbon steel.
For those alloys the allowance is corrected rather than looked up: first articles are measured and the pattern equipment is adjusted. Long production runs pay for that reverse-engineering cycle, which is one reason short runs carry looser tolerances than long ones.
Step 3: Prepare the Mold
The mold is packed around the pattern in two halves, and the pattern is then drawn out to leave the cavity. Gating and risers are formed in the same operation — they are part of the mold, not something bolted on later.
The sand system decides how rigid that mold is: clay-bonded green sand is fast and reclaimable, while chemically bonded no-bake sand sets rigid and holds heavy sections better.
Step 4: Install Cores if Necessary
Cores go in wherever the part needs an opening or internal cavity the pattern cannot form. A core is a separate unit from the mold, set into it before closing.
The limit on a core is thermal. How slender it can be depends on how much metal surrounds it, how long it is, and what the foundry does to support and vent it — thick metal around a long, thin core is the worst case, because the core is heated hardest exactly where it is weakest.
Design the part so the core can also get out. An internal passage with no access large enough to clean the core through is a casting that cannot be finished.
Step 5: Melt the Metal
Melting is a chemistry operation as much as a heating one: the charge is melted, adjusted into the grade’s composition window, and superheated above pouring temperature to cover the heat lost in the ladle and on the way to the mold.
Cast irons pour at 2,450–2,700 °F (1,340–1,480 °C), carbon and low-alloy steels at 2,850–3,100 °F (1,565–1,700 °C). The margin above those pouring temperatures is not slack — high metal temperature is what lets thin sections fill, so pouring temperature is a geometry decision as much as a melting one.
No heat is tapped before spectrometry confirms the chemistry, and that analysis is what the material test report is built from.
Step 6: Prepare the Molten Metal
Between the furnace and the mold the metal gets its final treatment: slag off the surface, magnesium into ductile iron to make the graphite nodular, and inoculation of gray and ductile iron to control how that graphite forms as the metal freezes.
Treatment fades. Nodularizing and inoculating are done as late as possible and the molds are poured immediately after — a treated ladle left standing pours as though it had never been treated, and the microstructure shows it.
Step 7: Pour the Metal
Pouring is controlled on two variables, temperature and rate. Molten steel cools rapidly as it enters the mold, so a thin section close to the gate fills readily while the same section far from the gate may be too cold to fill at all.
Push the other way and the problems change: a fast, turbulent stream erodes the mold face, folds oxide film into the metal and drags in gas. The gating system exists to settle that conflict — fill quickly, arrive calm.
Step 8: Allow the Casting to Solidify
Solidification time rises with the square of a section’s volume-to-surface-area ratio, which is why a heavy section can take hours while a thin one is finished in seconds.
t = C (V/A)²
The constant C is determined by experiment and runs about 2 to 4 min/cm² for iron and steel.
Feeding is the whole point of that solidification time. The casting freezes from the mold wall inward, and the volume lost to contraction has to be replaced with liquid metal from an adjoining heavier section or from a riser. A heavy section that can only be fed through a thinner one is where a shrinkage cavity appears.
Step 9: Remove the Casting
Shakeout breaks the mold apart and separates the casting from the sand, which goes back for reclamation.
The timing is a real decision. Pulled too early, a casting still contracting unevenly can distort or tear, and long restrained sections are where that shows up. Left too long, the mold ties up floor space for nothing.
Step 10: Remove Gates and Risers
Gates and risers are cut off flush with the casting surface, by saw, torch or press depending on the alloy and the section size.
They are not free metal. The weight actually poured runs roughly 1.1 to 3 times the weight of the finished part, and the difference is gating, risers and returns. That metal is remelted, but the melting energy, the molding volume and the cutting labor are spent once and paid for either way — which is why a quote reflects poured weight, not just part weight.
Step 11: Clean the Casting
Cleaning takes the casting from as-shaken to shippable: residual sand and scale come off in shot blast, then flash and riser pads are ground flush.
Surface acceptance is settled against comparator plates rather than adjectives. ASTM A802 and MSS SP-55 are the two common visual standards, and naming one of them on the order is the difference between a surface you accept and an argument you have later.
Step 12: Heat Treat the Casting if Required
Heat treatment is required whenever the specification’s grade table lists a condition, because the mechanical properties in that table are only valid in that condition — not as-cast.
ASTM A216 grade WCB, for instance, is listed annealed, normalized, or normalized and tempered; quenching and tempering is permitted only when supplementary requirement S15 is invoked on the order.
In ductile iron the matrix, and therefore the grade, is set by heat treatment rather than by the pour, so the grade ordered and the treatment applied are one decision.
Step 13: Machine Critical Features
Only the features that need it are machined; everything else ships as-cast — and the stock those cuts remove has to have been put on the pattern back at step 2.
Machining allowance is superimposed on draft and pattern allowance, not shared with them. On cope surfaces — the upper half of the mold — an allowance under 0.25 in (6 mm) is generally not recommended, which is the practical argument for molding critical machined faces in the drag.
The shop rule for how much is simple: leave enough that the first cut still runs at least 1/16 in (1.5 mm) below the cast surface. A cut that grazes the skin finds sand, scale and hard spots instead of clean metal.
Step 14: Inspect the Casting
Inspection has to be ordered in two parts — the test method and the acceptance criteria — because either one alone decides nothing. A radiographic call-out names the method, ASTM E94, and separately the acceptance level, such as ASTM E186 severity level 2.
- Radiography: internal soundness — shrinkage, gas, inclusions, with the reference radiograph set chosen by wall thickness.
- Magnetic particle: surface and near-surface indications in ferromagnetic castings, run to ASTM E709, with ASTM A903 covering surface acceptance.
- Liquid penetrant: surface-breaking indications on alloys magnetic particle cannot cover, to ASTM E165.
- Ultrasonic: subsurface volume in heavier sections, to ASTM A609 for carbon, low-alloy and martensitic stainless castings.
- Visual: surface condition against the ASTM A802 comparators or MSS SP-55.
- Mechanical testing: ASTM A370, on bars cast with the heat — bars that represent the quality of the steel, not the properties of the casting itself.
Running every method on every casting is not how it works. The method follows the defect you are actually worried about and the section it would sit in.