Alloy and annual volume decide this, and for a ferrous part they decide it immediately: permanent mold casting is in practice an aluminum, magnesium and copper-alloy process, so an iron or steel casting goes to sand. Where both processes really can pour the part — light alloys, under a few hundred pounds — the choice comes down to whether the annual quantity pays for a metal mold.
What Is Permanent Mold Casting?
Permanent mold casting pours molten metal under gravity into a reusable metal mold, which opens to eject the casting and is then re-coated and closed for the next pour. Pressure is zero to minimal, which is what separates it from die casting, where metal is injected at over 15,000 psi.
The metals actually run this way are aluminum and copper-base alloys, with magnesium, zinc, and — marginally — gray and ductile iron also possible. In high-volume production the castings weigh from 1 oz. to more than 100 lbs., and parts up to 400 lbs. are produced commercially.
What Is Sand Casting?
Sand casting packs a bonded sand mold around a pattern, pours into it, and breaks the mold away after solidification: one mold per casting. The binder, not the process, sets the envelope. Green sand handles parts from a few ounces to 500 lbs. as routine and has been taken to 7,000 lbs.; chemically bonded no-bake sand covers 1 to 50,000 lbs.
Because the mold is consumed, nothing about it has to survive the pour. That is why sand takes any alloy that can be melted, including the iron and steel castings whose pouring temperatures no reusable mold tolerates for long.
Permanent Mold Casting vs. Sand Casting
The two processes separate cleanly on mold economics, part size and alloy. On the quality rows they sit far closer together than the usual comparison chart shows.
| Factor | Permanent Mold Casting | Sand Casting |
|---|---|---|
| Mold | Reusable metal mold | Disposable sand mold |
| Typical materials | Aluminum, magnesium, copper alloys | Any pourable alloy — iron, steel, bronze, aluminum |
| Tooling cost | High | Low |
| Part cost at volume | Lower | Higher |
| Production volume | Economical from about 3,000/year | 1–5,000/year no-bake; 10,000–50,000+ automated green sand |
| Surface finish | 125–300 µin. RMS | 250–1,000 µin. RMS (green sand) |
| Dimensional accuracy | About ±0.01 in. | ±0.005–0.015 in. (no-bake); looser in green sand |
| Wall thickness | 0.1 in. minimum | 0.09 in. minimum (green sand) |
| Part size | 1 oz. to 100 lbs. typical; 400 lbs. commercially | To 7,000 lbs. green sand; 1–50,000 lbs. no-bake |
| Design flexibility | Moderate — every feature must clear a rigid mold | Very high — cores and undercuts are routine |
| Cooling rate | Faster; the metal mold chills the casting | Slower; sand insulates |
| Mechanical properties | About 10% higher spec minima, same alloy (aluminum) | Spec baseline for the alloy |
| Mold life | 10,000–120,000 castings in aluminum; about 500 in iron | One mold per casting |
| Best suited for | Repeat non-ferrous production | Prototypes, large parts, iron and steel, any volume |
Wall thickness is the row usually printed backwards. Published AFS figures put the minimum permanent mold wall at 0.1 in. and green sand at 0.09 in. — sand reaches marginally thinner, not thicker. The mechanism is plain once stated: permanent mold fills under gravity into a mold that is chilling the metal hard, so a thin section freezes before it fills. The process that genuinely casts thin is high-pressure die casting, at 0.04 in.
Dimensional accuracy is directional rather than absolute. Permanent mold’s basic linear tolerance is about ±0.01 in.; no-bake sand holds ±0.005 to ±0.015 in. A well-controlled chemically bonded sand casting matches or beats permanent mold on a given linear dimension. What permanent mold reliably wins is repeatability across a long run, because the cavity is machined once instead of rebuilt every cycle.
Sand is not the low-volume process in general — no-bake is. No-bake molding runs roughly 1 to 5,000 castings a year, while automated vertically-parted green sand lines are aimed at 10,000 to 50,000 and above at up to 500 molds an hour. Permanent mold turns economical around 3,000 castings a year, which puts it below automated green sand, not above it.
When Should You Choose Permanent Mold Casting?
Choose permanent mold when the alloy is aluminum, magnesium or copper-base, the casting is under about 100 lbs., and the annual quantity runs to a few thousand pieces. The economic threshold sits near 3,000 castings a year, though jobs with runs as few as 100 a year are done.
The payback is mold life against tooling cost. A mold that survives 10,000 to 120,000 aluminum castings spreads a high tooling bill across enough parts to beat sand on unit cost. Below that, the mold is the dominant line item and never amortizes.
The chill also buys real properties: about 10% higher spec minima in tensile and yield for the same alloy and temper, from finer grain structure and less porosity. Those are minima on separately cast test bars, not measured values from a production part.
When Should You Choose Sand Casting?
Choose sand whenever the part is iron or steel, whenever it is heavier than a few hundred pounds, or whenever the annual quantity will not amortize a metal mold. For a ferrous part the first of those is decisive on its own.
A permanent mold’s service life falls as the pouring temperature rises. Each pour thermal-shocks the mold face, the coating has to be resprayed every cycle, and the mold eventually cracks from thermal fatigue. Published estimates for iron molds run about 500 cycles pouring iron against 100,000 pouring aluminum.
Ferrous permanent molding does exist, but it is not a route a North American OEM can realistically source through.
Size closes the rest of the question. No-bake sand covers 1 to 50,000 lbs. while permanent mold is capped near 400 lbs. commercially, so anything in the counterweight, housing or large valve-body class has one process available to it. And if a ferrous part needs tolerances tighter than sand will hold, the alternative is investment casting, not permanent mold.