Quick Answer
Sand casting packs sand around a reusable pattern, removes the pattern, and pours metal into the cavity. It is the most flexible and lowest-tooling-cost casting process: nearly any metal, from a fist-sized bracket to a multi-ton machine base, in quantities from one piece to continuous production. The trade-offs are rougher surfaces (Ra 12.5–25 µm), looser tolerances (±0.8 mm and up) and more machining stock than precision processes.
The Process in Six Steps
- 1. Pattern and core boxes. A pattern (wood, plastic or metal, chosen by volume) replicates the part plus shrinkage allowance and machining stock. Core boxes form internal cavities.
- 2. Molding. Sand mixed with binder is compacted around each pattern half — green sand (clay-bonded) for most work, resin sand for better finish and heavy sections.
- 3. Core setting and closing. Baked or cold-box cores are placed, the two mold halves close, and weights or clamps resist metal pressure.
- 4. Pouring. Metal fills the cavity through a gating system designed to feed shrinkage as the casting solidifies.
- 5. Shakeout and cleaning. After cooling, the mold is broken up, sand reclaimed, gates cut, and surfaces shot blasted.
- 6. Heat treatment and inspection. Per grade and drawing — then machining where required.
Realistic Capabilities
| Attribute | Typical range |
|---|---|
| Weight | 0.5 kg to several tons |
| Tolerance (unmachined) | ±0.8–1.5 mm small parts; wider on large |
| Surface finish | Ra 12.5–25 µm (green sand); better with resin sand |
| Minimum wall | ~5 mm ferrous; ~4 mm aluminum |
| Metals | Gray/ductile iron, carbon and alloy steel, stainless, aluminum, copper alloys |
Where Sand Casting Is the Right Answer
- Large parts. Above ~50 kg, it is usually the only economical option
- Low volumes and prototypes. Pattern cost is a fraction of any die-based process
- Design churn. Wood patterns modify in days when the design is still moving
- Heavy machining planned anyway. If every surface gets machined, precision casting buys nothing
When surface finish or tight as-cast dimensions drive the drawing, compare against investment casting or shell molding — we compared the trade-offs in our shell molding guide.
Quality Risks and How Foundries Control Them
- Sand inclusions and veining: controlled by sand system discipline and coating practice
- Shrinkage porosity: solved in gating/riser design, verified by sectioning or radiography on critical parts
- Shift and mismatch: pattern and flask maintenance; check datum-to-parting-line callouts
- Gas porosity: moisture and venting control — ask how the foundry monitors compactability and moisture if your part is leak-tight
Our inspection sequence for sand castings — dimensional layout, material certs, and NDT where specified — is described on the quality assurance page.
FAQ
Green sand vs resin sand — which should I ask for?
Green sand for cost and speed on routine parts; resin (furan/no-bake) sand for heavy sections, better finish and tighter dimensional consistency.
How much machining stock does a sand casting need?
Commonly 2–5 mm per machined surface on small/medium parts, more on large castings — agreed per drawing at DFM stage.
Can sand casting hold a leak-tight requirement?
Yes, with gating designed for soundness plus pressure testing — but say “leak-tight” at RFQ, not after delivery.