For ferrous parts, sand casting is the default and investment casting is the exception you buy for a reason. Sand carries the size and keeps the tooling proportional to the order; investment casting buys tolerance, thin walls and geometry a pattern cannot be drawn out of.
What Is Investment Casting
Investment casting builds a ceramic shell around a wax replica of the part, melts the wax out, and pours metal into the one-piece mold left behind. Because the pattern is melted rather than drawn, there is no parting line to open and draft can often be eliminated altogether — the shell reproduces undercuts, cored passages and vertical walls that no sand pattern could be pulled out of.
For steel, that geometry buys ISO 8062 tolerance grades CT 5 to 7 under the Steel Founders’ Society tolerance guidelines. Wall sections of 0.060 in. (1.5 mm) are ordinary, and sections tapering to 0.030 in. (0.76 mm) are achievable.
What you pay for it is a new wax pattern for every casting and a die to produce the wax, plus a size ceiling set by what a shell can support through dewax and pour.
What Is Sand Casting
Sand casting packs bonded sand around a reusable pattern, splits the mold, draws the pattern, and pours metal into the cavity; the mold is broken away to free the casting. It is the dominant process for ferrous work.
Its advantage is that mold cost scales gently with part size. Production steel castings span 6.5 to 12,000 lbs in the same sand molding processes.
What you give up is repeatability, and sand tolerance is not one grade. Long-run work holds CT 8 to 10 on features a foundry sets out to hold tight, CT 10 to 12 across most casting types and sand processes, and CT 12 to 14 on large castings — with a short production run landing one full grade looser than a long one.
Design minimums follow from the same physics: about 1.5° of draft on most features, and 0.25 in. (6 mm) of section thickness on conventional steel castings.
Investment Casting vs Sand Casting at a Glance
Investment casting trades size and tooling cost for precision, surface and thin walls; sand casting trades precision for size, low tooling spend and a cheap change of mind.
| Factor | Investment Casting | Sand Casting |
|---|---|---|
| Mold type | Ceramic shell around wax pattern | Sand mold |
| Dimensional accuracy | High | Moderate |
| Surface finish | Excellent | Rougher |
| Part complexity | Very high | Moderate to high |
| Wall thickness | Can produce relatively thin walls | Generally requires thicker walls |
| Part size | Small to medium parts are most common | Small to extremely large parts |
| Tooling cost | Higher | Lower |
| Machining required | Usually less | Usually more |
| Production cost | Higher per part | Generally lower |
| Production volume | Low to high, depending on economics | Prototype to medium-volume production |
| Alloy flexibility | Excellent | Excellent |
| Typical applications | Aerospace, valves, medical, precision machinery | Pumps, housings, engine components, industrial equipment |
Two rows in that table carry the money, and both are quantifiable. On a 4 to 6 in. feature, CT 5 to 7 means a total tolerance of 0.02 to 0.05 in. (0.62 to 1.2 mm); the CT 10 to 12 a sand casting typically holds means 0.14 to 0.27 in. (3.6 to 7 mm), and CT 14 on a large casting opens it to 0.47 in. (12 mm).
The machining row follows directly. On that same feature, an investment casting is quoted at required machining allowance grade E — 1.1 mm of stock — against 1.5 to 3 mm for machine-molded sand and 2.2 to 6 mm for hand molding.
When Should You Choose Sand Casting?
Choose sand casting when the part is heavier than a shell will carry, when the tight features are surfaces you were going to machine anyway, and when tooling spend has to stay proportional to the order. That covers most medium-to-large ferrous parts: a ductile iron gearbox housing with machined bores and mounting faces is a sand casting at every foundry that quotes it.
Sand casting is not one process, and the choice among its three main forms is mostly a weight question.
| Sand process | Practical casting weight | What it gives you |
|---|---|---|
| Green sand | Up to about 500 lbs | Lowest tooling cost; dimensionally equal to the other sand processes below 50 lbs |
| No-bake | Up to about 2,000 lbs | Tighter tolerances than green sand above 200 lbs |
| Shell molding | Under about 100 lbs | Tightest tolerances of any sand process, but size-limited |
Treat those weight bands as a starting point for the conversation rather than a specification. SFSA’s own data shows the differences between the three molding processes are smaller than the variation between two foundries running the same process — one shop may need shell molding to hold a dimension another holds routinely in green sand.
When Should You Choose Investment Casting?
Choose investment casting when a feature on the drawing is outside what sand will repeat and the part is small enough to shell. Three triggers account for nearly every conversion:
- As-cast tolerance: the feature has to hold tighter than the CT 8 to 10 band sand reaches at its most capable, and you are not machining it afterwards.
- Wall thickness: sections run below the 0.25 in. (6 mm) minimum that conventional steel casting is designed around.
- Undrawable geometry: internal passages, undercuts or zero-draft walls that no split mold can release.
The machining argument often decides it on cost. Roughly 1.1 mm of stock instead of up to 6 mm, no draft to cut away, and features that arrive finished rather than located, clamped and cycled through a machine — on a part with many machined faces, that can outweigh a higher piece price.
The honest counter is that you can tighten a sand casting without changing process. Rough machining to gage, coining, straightening and press-to-gage all upgrade dimensions after pouring, at a cost per part. Compare the finished, inspected part against the investment quote, not the casting price against the casting price.
Conclusion
Sand casting should be the working assumption for a ferrous part, and investment casting the answer to a specific, nameable problem with it: a wall too thin to fill, a surface that has to hold size as-cast, a passage no pattern can be pulled out of. If you cannot point at the feature that forces the change, sand casting and a machining allowance will almost always land cheaper.
Whichever way the drawing points, the process sets the range and the foundry sets where you land inside it. Send the print and ask what the shop actually holds on features like yours.