Sand Casting
Also: sand mold casting, green sand casting, no-bake casting, resin sand casting, shell mold casting, foundry casting, iron casting, gray iron casting, aluminum sand casting, cast parts
Sand casting pours metal into a single-use sand mold made from a pattern: low tooling cost, any size, in iron, steel, aluminum or bronze, with rough surfaces.
75 shops tagged Sand casting among 514 metal casting companies in the Noramark directory
A pattern of the part, oversized for shrinkage, is packed in sand in a two-part flask (cope and drag); cores of bonded sand form the internal passages; the pattern is drawn, the halves closed, and metal poured through a gating system with risers that feed the shrinking casting as it solidifies. The mold is broken away to release the part, the gates and risers are cut off, and the casting is shot blasted, heat treated if required and machined where it must be accurate. Green sand (clay-bonded) is the cheapest and most common, especially for iron; no-bake (resin-bonded) sand gives a better surface and tighter dimensions for larger and lower-volume work; shell molding, with resin sand cured on a heated metal pattern, is the most accurate of the family. Sand molds and cores can now be 3D printed, which removes the pattern entirely for prototypes and one-offs.
The appeal is tooling that costs hundreds to a few thousand dollars for a wood or plastic pattern, a size range from a fist to a machine bed, and every castable alloy: gray and ductile iron, carbon and alloy steel, stainless, aluminum (356, A356, 319), brass, bronze and magnesium. Foundries cast one piece or ten thousand; above that, permanent-mold or die casting takes over for aluminum, and automated green-sand lines take over for iron.
The trade-offs are the surface and the tolerances. A green-sand casting is rough, ±0.8 mm (±1/32") at best, with a parting-line witness, draft on every vertical face and a possibility of porosity, inclusions and shrinkage in heavy sections. The drawing should treat the casting as a near-net blank: add machining stock to every functional surface and specify the datums the machinist will pick up.
At a glance
| Typical tolerances | Green sand ±0.8 mm (±1/32") up to 150 mm (6"), adding about ±0.25 mm (±0.010") per further 100 mm (4"); add ±0.5-1.0 mm (±0.020-0.040") across the parting line and for cored features. No-bake ±0.5 mm (±0.020"); shell mold ±0.25-0.4 mm (±0.010-0.015") with care. In ISO 8062 terms, CT10-CT12 for green sand, CT8-CT10 for shell. Machined features ±0.05 mm (±0.002") after machining. |
|---|---|
| Size limits | From about 50 g (2 oz) to castings of 50 tons and more in floor molds; job-shop foundries commonly handle 0.5-500 kg (1-1,000 lb) and envelopes to 2-3 m (6-10 ft). Minimum wall about 3 mm (0.125") in aluminum, 4-5 mm (3/16") in iron, 6 mm (1/4") in steel, more on large castings. Cored holes from about 8-10 mm (3/8") diameter. |
| Surface finish | Green sand 12-25 µm Ra (500-1,000 µin); no-bake 6-12 µm Ra (250-500 µin); shell mold 3-6 µm Ra (125-250 µin). Castings are shot or sand blasted as standard, which evens the surface but does not smooth it. Functional surfaces are machined; iron castings are primed, painted, powder coated or e-coated; aluminum castings are chem filmed or painted (anodize on 356 is gray and cosmetic-poor). |
| Lead time | Wood or plastic patterns 2-6 weeks, aluminum match plates 4-8 weeks; first castings 1-2 weeks after the pattern. Printed sand molds give castings in 1-2 weeks with no pattern. Production runs in 3-6 weeks, plus 1-2 weeks for machining and finishing. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Casting alloys are their own grades, not the wrought roster. Gray iron (ASTM A48 Class 30-40) and ductile iron (ASTM A536 65-45-12, 80-55-06) are the bulk of sand casting: cheap, easy to pour, machinable. Carbon and low-alloy steel (ASTM A27, A148, cast 4140 and 8630 equivalents) for higher strength and weldability; cast stainless CF8 and CF8M (ASTM A351, A743) are the casting equivalents of 304 and 316; Monel M-35-1 for seawater. Aluminum sand castings are 356 and A356-T6 (ASTM B26), 319 and 535, not 6061 or 7075. Bronze C932, C954 aluminum bronze and yellow brass C852/C854 (ASTM B584) cast well for bearings, marine and plumbing parts. Titanium, nickel superalloys and 17-4 PH are investment cast instead.
What drives the cost
- Pattern and core-box tooling, a one-time cost that depends on size and complexity
- Weight: metal cost per kg plus melting, with iron cheapest and stainless and bronze dearest
- Cores: every internal passage is a core to make, set and clean out
- Quantity, which spreads the pattern cost and the setup
- Machining and heat treatment after casting
- Inspection: pressure testing, radiography, chemistry and mechanical test bars
- Surface requirements: shell molding, blasting and painting
When to use it
- Large and heavy parts: machine bases, pump and valve bodies, housings, gear blanks, manifolds
- Low to medium volumes, from one prototype to a few thousand a year
- Iron, steel, bronze and other alloys that die casting cannot pour
- Thick-walled and complex parts where machining from solid would waste most of the block
- Prototypes from printed sand molds when a pattern is not yet justified
When not to
- Walls under 3 mm (0.125") or fine detail: investment or die casting
- Tolerances under ±0.25 mm (±0.010") without machining
- Tens of thousands of small aluminum or zinc parts a year: die casting
- Pressure-tight thin-wall aluminum: porosity is a risk; use permanent mold or impregnation
- Alloys that are not cast, such as 6061, 7075 or 17-4 PH, in their wrought properties
Design tips
- Give every vertical face 1-2° of draft, 3° on cores and deep pockets, and put the parting line where it is easiest to machine off.
- Keep walls uniform and taper heavy sections toward the risers; isolated heavy bosses become shrinkage cavities.
- Use fillets of at least 3 mm (0.125") at every wall junction; sharp inside corners are hot spots and stress raisers.
- Add machining stock of 3 mm (0.125") on aluminum and iron, 5-6 mm (0.2-0.25") on steel, to every functional surface.
- Cast in locating bosses or pads the machinist can use as datums for the first setup.
- Reduce cores: an open pocket is cheaper than a cored cavity, and a through hole cheaper than a blind one.
- Specify the alloy by spec and grade (A48 Class 30, A536 65-45-12, B26 A356-T6, B584 C93200), the heat treatment, and the inspection class.
- Call out the tolerance standard (ISO 8062 grade or a general as-cast ±0.8 mm note) and identify which surfaces are cosmetic.
Frequently asked questions
- How much does sand casting cost?
- Patterns run $500-5,000 for wood or plastic and $3,000-15,000 for aluminum match plates with core boxes. Castings are priced by weight and alloy: gray iron about $3-6 per kg ($1.50-3 per lb) finished and blasted in moderate quantities, aluminum and bronze several times that, plus machining. One-offs from printed sand molds cost more per piece but skip the pattern.
- What tolerances can sand casting hold?
- About ±0.8 mm (±1/32") on the first 150 mm (6") in green sand, plus ±0.25 mm per further 100 mm, and looser across the parting line and on cored features; no-bake and shell molding roughly halve that. Anything tighter is machined, which is why casting drawings carry machining stock.
- Sand casting vs die casting?
- Sand casting has cheap tooling, pours any alloy including iron and steel, and makes very large parts, but the surface is rough and the tolerances loose. Die casting needs a steel die costing tens of thousands, pours only aluminum, zinc and magnesium, and makes thin-walled, accurate, smooth parts in seconds; it wins above a few thousand pieces a year.
- Sand casting vs investment casting?
- Investment casting gives fine detail, thin walls, a smooth surface and ±0.13 mm (±0.005") tolerances in steel, stainless and superalloys, on parts under about 50 kg, at a higher cost per part and longer lead time. Sand casting is cheaper, faster and larger, at the price of surface and accuracy; it is the choice for iron, for heavy parts and for anything that will be machined all over anyway.
- What is the minimum wall thickness for sand casting?
- About 3 mm (0.125") in aluminum, 4-5 mm (3/16") in gray and ductile iron and 6 mm (1/4") in steel for small castings, increasing with the size of the casting and the distance the metal must flow. Thinner walls misrun or cold shut; investment or die casting handles them.
