Most cast iron parts belong in a sand mold: green sand for long runs of parts under a few hundred pounds, no-bake resin sand for large parts and short runs. Investment casting and permanent mold make sense only for smaller parts whose tolerance, thin walls or volume justify their metal tooling, and die casting is not a practical route for iron.
Casting Processes That Work for Cast Iron
Cast iron can be poured in every sand process, in lost foam, investment casting, permanent molds and centrifugal casting; die casting is the one common process that does not take it in practice.
- Sand casting: green sand, no-bake resin sand and shell molding all pour gray and ductile iron, and between them they make most iron castings.
- Lost foam: takes iron, but its foam patterns come off a metal tool, so it needs volume.
- Investment casting: gray and ductile iron are both investment cast, mostly as small parts that need tight tolerances or a fine finish.
- Permanent mold: gray and ductile iron can be cast in metal molds, but the process is used mainly for aluminum and copper alloys.
- Centrifugal casting: takes iron, but mainly for round, hollow parts such as pipe and cylinder liners.
How to Choose a Casting Process for Cast Iron
Start with weight and annual volume, which rule out most processes for a given part. Tolerance, wall thickness and the iron grade then narrow the choice among the processes left, and the comparison is made on the cost of the finished, machined part.
Part Size and Weight
Above a few hundred pounds, an iron part is a sand casting, and the heaviest parts go to no-bake resin sand; investment casting and permanent mold mostly stay below about 100 lb.
The chart shows the upper end of each process’s common range, from AFS weight figures that cover all alloys; the ceiling at any one foundry is set by its mold handling equipment.
Annual Volume
Sand casting pays from a single piece, while permanent mold needs about 1,000 castings before its metal tooling pays back.
The minimum economical quantities below come from a standard manufacturing-textbook table and cover all alloys. They assume conventional tooling; with rapid-prototyped patterns, any of these processes can start at one piece.
Tolerance and Surface Finish
Investment casting holds about ±0.01 in on a dimension up to 1 in, where green sand’s practical figure for small castings is ±0.0625 in; no-bake and shell molds are more rigid and hold tighter than green sand.
Green sand can hold ±0.03 in, but the scrap rate rises as the tolerance tightens, and large castings need wider tolerances.
On finish, green sand runs 250–1,000 RMS as cast, and the other processes come in finer, as the chart of AFS figures shows.
Wall Thickness and Geometry
Thin walls push a part toward investment casting, and internal passages keep it in sand. Permanent mold is the most restrictive process on draft and cores.
A standard manufacturing-textbook table puts the minimum section at 3 mm for sand and 1 mm for investment casting, for all alloys.
For ductile iron, high-volume green sand foundries generally will not quote as-cast sections under 5 mm, as an AFS research article on ductile iron notes.
In permanent mold, internal draft runs about 2–20° against 1–3° in sand, and undrafted internal walls need collapsible metal cores that add cost.
How the Iron Grade Changes the Choice
Gray iron fits any process its size and volume allow. Ductile iron is more sensitive to the mold, because it expands more while freezing and forms shrinkage cavities more easily in a mold that gives way.
For a heavy ductile iron housing or valve body that has to be sound, no-bake resin sand is the safer starting point.
Metal molds bring a different risk for ductile iron: they chill thin sections. In one study of high-silicon ductile iron poured into a preheated iron mold, cited in a 2022 paper on permanent-mold ductile iron, 20 and 30 mm sections came out carbide-free while a 10 mm section was partly chilled.
Tooling Cost and Lead Time
Sand casting has the cheapest tooling. Shell, lost foam, investment casting and permanent mold all need metal tooling, which costs more and takes longer to make.
- Green sand: automated lines run metal patterns; wood patterns serve runs of about 50 pieces a year or less.
- No-bake resin sand: wood, plastic or metal patterns all work; plastic tooling covers runs up to about 5,000 pieces a year, metal above that.
- Shell: patterns and core boxes are iron, and shell core boxes often need ejectors, which can add significantly to their cost.
- Investment casting and lost foam: a metal tool makes the wax or foam patterns, which makes both generally more expensive to tool; investment tooling and sample castings typically take 6–8 weeks.
- Permanent mold: the mold itself is machined from metal or graphite.
A part with internal passages adds a core box, or several, to any of these.
For sand, the wait is set by the pattern shop. In summer 2025, one U.S. pattern shop raised its lead time to 12–14 weeks. Where that wait does not fit, a 3D-printed sand mold or core skips the pattern; foundries use it mainly for prototypes and small batches.
Total Cost After Machining
The process with the lowest casting price is not always the cheapest finished part, because a process that casts closer to size takes less machining.
No-bake castings typically cost more than green sand unmachined, and the difference is often recovered in machining.
How much stock each surface needs depends on the molding method as well as the casting’s size and shape; typical values are in our machining allowance guide. For a part that sits between two processes, ask for both the as-cast and the machined price on each.
Green Sand, No-Bake or Shell Molding
High volume means green sand, and large size means no-bake resin sand. Shell is for small parts that need sand’s tightest tolerances in volume.
When green sand would need extra cores, coatings, machining or trial runs to make a part, no-bake is usually the better choice.
Green Sand
Green sand suits iron parts that run in volume and stay within a few hundred pounds, such as gray iron housings, flanges and pump parts.
Its limits are tolerance and, for ductile iron, soundness.
No-Bake Resin Sand
No-bake resin sand suits iron parts that are large, made in low to medium volume, or ductile with heavy sections that must be sound.
Typical runs are 1 to 5,000 castings a year.
Shell Molding
Shell molding suits small iron parts made in volume that need the tightest tolerances and best finish a sand process gives.