Die casting is the better process for aluminum, zinc, and magnesium parts made in quantities that pay for a tool-steel die; sand casting is the better process for iron and steel at any quantity, for large or heavy-sectioned parts in any alloy, and for runs too short to amortize a die. The alloy settles it before volume or tolerance gets a vote: iron and steel are not die cast commercially, so a ferrous part is a sand casting (or an investment casting) whatever the quantity.
Where the alloy is genuinely open — the same housing could be aluminum or ductile iron — the decision turns on three things the drawing already shows: how thin the walls are, how tight the as-cast tolerance is, and how many parts the die has to be spread across.
What Is Die Casting
Die casting injects molten metal into a reusable tool-steel die under high pressure — above roughly 31 MPa (4,500 psi) is what earns the name — and holds it there until the part is rigid enough to eject. Pressure does what gravity cannot: it drives metal into millimeter-thick walls and fine detail before the melt freezes against cold steel, and the die then repeats the part shot after shot.
The alloys are the low-melting ones: zinc, magnesium, aluminum, and copper alloys. Iron and steel are not on the list. NADCA’s own product standards describe ferrous die casting as a limited-production activity that needs refractory-metal dies; no die caster will quote a production run in it.
The die is both the cost and the constraint. It is machined from heat-treated tool steel to the finished geometry, so it is expensive to cut and slow to change.
What Is Sand Casting
Sand casting pours molten metal by gravity into an expendable mold of bonded sand, formed around a reusable pattern and broken away once the casting has solidified. Because the mold is refractory and thrown away, it does not care how hot the metal is — cast irons pour at 1,200–1,480 °C and steels hotter still — or how big the part is: castings run from about 100 g to 100 tonnes.
The tooling is the pattern — wood, resin, or aluminum cut to the part shape plus shrinkage and machining allowance — and it costs a fraction of a die because it never touches liquid metal. Cores form the internal passages.
Gravity filling is the trade-off. The metal has to flow and feed under its own head, so walls stay thicker, the sand grain prints on the surface, and as-cast tolerance is coarser than a steel die holds.
Die Casting vs Sand Casting
Every row in the table follows from one difference: die casting fills a permanent steel die under pressure, and sand casting fills an expendable sand mold under gravity.
| Factor | Die Casting | Sand Casting |
|---|---|---|
| Mold | Reusable metal die | Disposable sand mold |
| Metal filling | High-pressure injection | Mainly gravity |
| Tooling cost | High | Low |
| Production rate | Very high | Low to moderate |
| Best production volume | Medium to very high | Prototype to medium volume |
| Dimensional accuracy | Excellent | Moderate |
| Surface finish | Smooth | Relatively rough |
| Minimum wall thickness | Thin walls possible | Usually thicker walls |
| Part size | Small to medium, sometimes large | Small to extremely large |
| Material selection | Mainly aluminum, zinc, magnesium and other nonferrous alloys | Aluminum, iron, steel, bronze and many other alloys |
| Machining requirements | Usually lower | Usually higher |
| Complex details | Excellent | Good, especially with sand cores |
| Cost per part at high volume | Low | Usually higher |
Dimensional accuracy is where most drawings hinge, and “excellent” against “moderate” is about three ISO 8062 grades. Light-alloy pressure die casting holds CT5–7 in long-series production; machine-molded iron or steel sand casting holds CT8–10, and hand-molded work CT11–13. The gap is closed with machining stock, not by asking a sand mold to hold a die-casting grade.
Wall thickness runs the same way. Die castings typically carry walls of about 1 to 5 mm, and small parts go to 0.5 mm with the die caster’s agreement; conventional steel sand casting starts at 6 mm.
Iron is no thinner in production. Walls of 2 to 3 mm in ductile iron were the target of a U.S. Department of Energy research project because conventionally treated iron forms brittle carbides in sections that thin. A wall in that range means aluminum or zinc in a die; a steel part means 6 mm and the weight that comes with it.
Machining stock is most of the machining row. A die casting is machined with as little as 0.25 mm of stock, because its densest, finest-grained metal is the chilled skin and cutting deeper exposes the porosity beneath it. A steel sand casting carries at least 6 mm on cope surfaces, where sand and gas float to, and enough elsewhere that the first cut stays 1.5 mm under the cast skin. Multiply that stock by the machined area and you have much of the per-part cost gap at volume.
Surface finish is specified differently on the two sides, not just achieved differently. An aluminum die casting from a new die comes off at a roughness of 63 µin (1.6 µm) or better, and that number goes on the print.
Ferrous sand castings are not bought by Ra. Steel-casting practice under SFSA accepts an as-cast surface against comparator plates — the MSS SP-55 photographs or the ASTM A802 set — because the surface is a sand print, not a machined texture. Ask for an Ra on an as-cast sand surface and the quote will include a machining operation.
Volume is a cost-structure statement, not a quantity. The die is paid once and spread over every shot; the sand mold is paid on every part, plus the machining. Die casting wins where die cost divided by quantity falls below sand casting’s per-part mold-and-machining premium. No published break-even survives a change of part, so run the arithmetic on yours.
When Should You Choose Die Casting
Choose die casting when all three hold: the alloy is aluminum, zinc, or magnesium; the quantity is high enough that die cost per part is small beside the mold and machining cost sand casting would carry; and the design uses what pressure filling gives — walls under about 5 mm, cast-in holes and bosses, and as-cast surfaces that go straight to assembly or paint.
Two conditions ride along. The design must be frozen before the die is cut, because every change afterward is re-machining hardened tool steel. And the part must be one a die can hold: the die casting standards cover parts from a fraction of an ounce to thirty pounds or more and up to several feet in size, and large, heavy parts sit outside that.
If you are reaching for die casting to get its tolerance in a ferrous part, the answer is a different ferrous process, not a ferrous die. Investment casting holds CT4–6 in steel and iron in long-series production — tighter than light-alloy die casting — and is the route to that precision in the alloys die casting cannot pour.
When Should You Choose Sand Casting
Choose sand casting when any one of these is true: the alloy is iron or steel; the part is large, heavy, or thick-sectioned; or the quantity is too small to spread a die over. One is enough.
For iron and steel it is not a choice but the process. Gray iron, ductile iron, carbon steel, and cast stainless go into sand (or investment) molds because nothing else takes their pour temperature in production, and ISO 8062 tabulates no die-casting grade for any of them.
Where the alloy is open, quantity carries it: the tooling is a pattern that is modified in wood or resin rather than in hardened steel, so a design still moving, or a short run, stays in sand even in aluminum.
Which molding method carries the job — green sand, no-bake, or shell — is the decision after this one; it moves tolerance and finish at the margin, not by a process class.
Conclusion
Neither process is better; the alloy picks first and the numbers pick second. Iron and steel are sand castings — die casting is not offered for them. Where the alloy is open, die casting wins for thin-walled, high-volume aluminum, zinc, or magnesium parts where the die cost divides down per shot; sand casting wins for large, heavy-sectioned, or short-run parts and for any design still changing. Put the CT grade and machining stock on the drawing that match the process, and the quotes will match too.