---
type: process
name: "Sand Casting"
category: "Forming"
subcategory: "Metal"
materials: ["Metal"]
tolerances: "About ±0.030 in (±0.8 mm) on the first inch plus roughly ±0.003 in per additional inch in green sand; no-bake and shell molds hold tighter. Allow up to 0.030 in (0.8 mm) mismatch across the parting line"
volumes: "1–100,000 parts per year; economical from a single piece"
lead_time: "1–4 weeks for a pattern, then 1–3 weeks per casting run; prototype castings from printed patterns in 1–2 weeks"
url: https://manufacturingprocesses.org/processes/forming/sand-casting
---

# Sand Casting

Sand casting pours molten metal into a cavity formed in bonded sand around a pattern, then breaks the mold away to release the part.

- **Category**: [Forming](https://manufacturingprocesses.org/processes/forming.md)
- **Family**: Metal
- **Materials**: Metal
- **Typical tolerances**: About ±0.030 in (±0.8 mm) on the first inch plus roughly ±0.003 in per additional inch in green sand; no-bake and shell molds hold tighter. Allow up to 0.030 in (0.8 mm) mismatch across the parting line
- **Surface finish**: Roughly 250–900 µin Ra (6–23 µm) as cast; no-bake and shell molds reach the low end of that range
- **Typical volumes**: 1–100,000 parts per year; economical from a single piece
- **Lead time**: 1–4 weeks for a pattern, then 1–3 weeks per casting run; prototype castings from printed patterns in 1–2 weeks

## Overview

Sand casting forms a cavity by packing bonded sand around a pattern, removing the pattern, and pouring molten metal into the void; the mold is destroyed to release the part. It remains the default process for large, heavy, or low-volume metal parts — engine blocks, pump housings, machine bases, manifolds, valve bodies.

Nothing else casts a multi-ton part economically, and nothing else reaches first article as cheaply: a wooden or 3D-printed pattern costs a fraction of a die and can be ready in days. Almost any pourable alloy works, including gray and ductile iron, aluminum, bronze, and carbon and stainless steels.

The price is precision and finish. Standard linear tolerance is about ±0.030 in (±0.8 mm) on the first inch, as-cast surfaces run 250–900 µin Ra, and every functional face needs machining stock. Economical from a single part up to roughly 100,000 per year.

## How it works

1. **Pattern.** A pattern of the part is made in wood, plastic, aluminum, or printed polymer, built deliberately oversize to compensate for solidification shrinkage — the shrink allowance is roughly 1% for aluminum and gray iron and closer to 2% for steel. Draft and machining stock are added to the pattern, not to the part model.
2. **Molding.** The pattern is placed in a flask and sand is packed around it. Green sand — silica sand with roughly 6–10% bentonite clay and 2–4% water — is the highest-volume medium. Chemically bonded no-bake sand (furan or phenolic urethane) gives better dimensional stability and finish for lower volumes and larger parts.
3. **Cope and drag.** The mold is made in two halves so the pattern can be withdrawn. The parting line between them is a permanent feature of the casting, visible as a fin.
4. **Cores.** Internal passages are formed by separate sand cores, usually shell or cold-box bonded, set into the mold on core prints. Cores are what make a water jacket or a hollow valve body possible.
5. **Gating and risers.** Sprue, runners, and gates are cut to fill the mold without turbulence; risers are reservoirs of liquid metal that feed the casting as it shrinks. Directional solidification — the casting freezing toward the riser — is what keeps shrinkage cavities out of the part.
6. **Pour.** Metal is poured with superheat above its melting point: gray iron at roughly 2,500–2,600°F (1,370–1,425°C), aluminum at 1,300–1,400°F (700–760°C). See [metal melting points](/charts/metal-melting-points).
7. **Cool and shake out.** Cooling runs from minutes for a small aluminum part to a day or more for a heavy steel casting. The sand mold is then broken up and most of the sand reclaimed.
8. **Cleaning.** Gates and risers are sawn or torched off, the parting fin is ground, and the casting is shot blasted. Heat treatment and machining follow.

Cooling rate governs properties: a heavy section cools slowly, giving coarser grain and lower strength than a thin section in the same casting. Chills — iron inserts placed in the mold — locally speed up sections that would otherwise be the weak spot.

## Design guidelines

### Uniform sections, and feed the heavy ones

Aim for a uniform wall. Minimum practical section is about 0.125 in (3 mm) in aluminum, 0.25 in (6 mm) in gray iron, and 0.25–0.375 in (6–10 mm) in steel; below that, metal freezes before the section fills. Where a heavy boss or junction is unavoidable it must be reachable by a riser, or it will contain a shrinkage cavity.

### Draft on every vertical face

Green sand molding needs 1.5–3° of draft to withdraw the pattern without tearing the mold; deep pockets need more, and rigid no-bake molds can approach 1°. Machined surfaces get draft too — it comes off later with the machining stock.

### Radius everything

Internal fillets should be at least 0.25 in (6 mm), or roughly half the adjoining wall thickness, whichever is larger. Sharp inside corners create hot spots and sand erosion; sharp outside corners break off in the mold and end up as inclusions in the metal.

### Machining allowance

Add 0.060–0.125 in (1.5–3 mm) of stock per surface on small and medium castings, and more on large ones or faces far from the parting line. Under-allowing is the most common cause of scrap: a casting that moves 0.050 in within tolerance leaves an uncleaned face.

### Design for a flat parting line

A parting line that follows a single plane is cheap. A stepped or contoured parting line costs pattern work and adds mismatch — expect up to about 0.030 in (0.8 mm) of offset across it, and never let a critical dimension depend on cope-to-drag alignment.

### Cores need prints and vents

Every core must be supported by core prints and must vent, or trapped gas will blow into the metal. Cores under about 0.5 in (12 mm) diameter, or slender cores longer than roughly 6× diameter, will shift or float; drill those features instead.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Minimum wall, aluminum | 0.19–0.25 in (5–6 mm) | 0.125 in (3 mm) | Thin sections freeze before the cavity fills |
| Minimum wall, gray iron | 0.25–0.375 in (6–10 mm) | 0.25 in (6 mm) | Iron loses fluidity fast in thin sections |
| Draft | 1.5–3° | 1° in no-bake sand | Pattern must withdraw without tearing sand |
| Internal fillet | 0.5× adjoining wall | 0.25 in (6 mm) | Sharp corners create hot spots and shrinkage |
| Machining stock | 0.060–0.125 in (1.5–3 mm) | More on large castings | Casting movement must not leave a black face |
| Cored hole diameter | ≥ 0.5 in (12 mm) | Drill anything smaller | Slender cores shift, float, and break |
| Parting-line mismatch | Design to tolerate 0.030 in (0.8 mm) | — | Cope and drag never align perfectly |

## Cost drivers

Pattern cost is low and per-part cost is high — the reverse of die casting. A wooden or printed pattern is inexpensive and can be modified with hand tools, which is why sand casting owns prototype and low-volume metal work. But every part consumes a new mold, so molding labor, sand handling, and cleaning dominate at volume. Cleaning-room labor — cutting gates, grinding parting lines, blasting — is routinely a third of the cost of a small casting, and it is driven directly by how many gates and risers the part needs.

Volume breakpoints: 1–100 parts on a hand-molded no-bake pattern; 100–5,000 on a matchplate pattern in an automated green sand line; above roughly 25,000–50,000 per year, permanent mold or die casting starts to win on aluminum, and shell molding or investment casting on steel.

1. **Simplify the cores.** Cores carry their own tooling, labor, and scrap rate. A part redesigned to need two cores instead of five drops sharply in cost.
2. **Keep the parting line flat.** It cuts pattern cost, reduces mismatch, and makes automated matchplate molding possible.
3. **Consolidate machined faces.** Group faces that must be machined onto the same setup axis; the machining bill often exceeds the casting bill.
4. **Specify the loosest tolerance that works.** Asking for ±0.010 in as cast forces a different process; ±0.030 in on the first inch is free.
5. **Choose an alloy for castability, not just strength.** Gray iron and aluminum-silicon alloys fill thin sections and feed well; high-strength steels and pure copper are much harder to cast sound.

## FAQ

### What tolerance can sand casting hold?

Plan on about ±0.030 in (±0.8 mm) on the first inch of any dimension, plus roughly ±0.003 in per additional inch, in green sand. No-bake and shell molds do better. Any dimension crossing the parting line must also absorb up to about 0.030 in of cope-to-drag mismatch.

### What is the minimum wall thickness for a sand casting?

Roughly 0.125 in (3 mm) in aluminum, 0.25 in (6 mm) in gray iron, and 0.25–0.375 in (6–10 mm) in steel. These are fluidity limits: below them the metal solidifies before the section fills, producing misruns and cold shuts.

### How much machining stock should I add to a sand casting?

0.060–0.125 in (1.5–3 mm) per surface on small and medium parts, and more on large castings or faces far from the parting line. Too little stock is the most common cause of scrap, since a casting that shifts within tolerance can leave an unmachined low spot.

### Why do sand castings need draft?

The pattern has to come out of the packed sand without tearing the mold wall. Green sand molding needs 1.5–3°; rigid chemically bonded no-bake sand can approach 1°. Draft is added to the pattern on top of the finished part geometry, then machined off where it matters.

### Sand casting or die casting?

Sand casting for large parts, low volumes, iron and steel, and anywhere the pattern still needs to change. Die casting for aluminum, zinc, and magnesium above roughly 10,000 parts a year, where its ±0.002 in/in tolerance and 32–63 µin Ra finish eliminate most machining.

### What metals can be sand cast?

Effectively any pourable metal: gray and ductile iron, aluminum-silicon alloys such as A356, bronzes and brasses, carbon and stainless steels, and nickel alloys. That alloy range is sand casting's main advantage over die casting, which is limited to low-melting-point metals.

## Alternative processes

- [Die Casting](https://manufacturingprocesses.org/processes/forming/die-casting.md): Die casting injects molten aluminum, zinc or magnesium into a hardened steel die under high pressure, producing thin-walled net-shape metal parts.
- [Investment Casting](https://manufacturingprocesses.org/processes/forming/investment-casting.md): Investment casting builds a ceramic shell around a wax pattern, melts the wax out and pours metal into the cavity, giving fine detail in almost any alloy.
- [CNC Machining](https://manufacturingprocesses.org/processes/cutting/cnc-machining.md): CNC machining removes material from solid stock with a programmed cutting tool, producing accurate parts directly from a CAD model with no tooling.
- [Forging](https://manufacturingprocesses.org/processes/forming/forging.md): Forging deforms solid metal under compressive load in a hammer, press or dies, so the grain flow follows the part contour and fatigue strength rises.

## Related processes

- [Investment Casting](https://manufacturingprocesses.org/processes/forming/investment-casting.md): Investment casting builds a ceramic shell around a wax pattern, melts the wax out and pours metal into the cavity, giving fine detail in almost any alloy.
- [Die Casting](https://manufacturingprocesses.org/processes/forming/die-casting.md): Die casting injects molten aluminum, zinc or magnesium into a hardened steel die under high pressure, producing thin-walled net-shape metal parts.
- [Centrifugal Casting](https://manufacturingprocesses.org/processes/forming/centrifugal-casting.md): Centrifugal casting pours molten metal into a spinning mold so centrifugal force packs it against the wall and drives inclusions toward the bore.
- [Heat Treatment](https://manufacturingprocesses.org/processes/forming/heat-treatment.md): Heat treatment cycles metal through controlled heating and cooling to change its hardness, strength, ductility or internal stress state.

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/forming/sand-casting)*

*Last updated: August 11, 2026*