Process selection guide
Investment casting vs sand casting: which process fits your part?
Short answer: choose investment casting for complex, thin-walled or near-net-shape parts at low to mid volume, and sand casting for large, simple, heavy parts where a rougher surface and a wider tolerance are acceptable. If you need tens of thousands of small, tight parts per year, neither process is the right one — you should be looking at die casting or machining from solid.
Key takeaways
- Investment casting wins on complexity, wall thickness, as-cast surface finish and detail — economical from roughly a few hundred to a few tens of thousands of parts per year.
- Sand casting wins on large, heavy, geometrically simple parts, and on unit cost wherever surface finish and dimensional tolerance are not critical.
- Tooling cost decides at low volume. The crossover point between the two processes is usually a few hundred parts, and it shifts with part size and geometry.
- Neither is right for very high volumes of small, tight-tolerance parts — that is die casting territory, or machining from solid.
- Undecided? Send the drawing and state the annual quantity. The process choice follows from those two facts more than from anything else.
The basics
Two different ways of making a mould
The whole comparison comes down to one idea: what happens to the mould after the metal is poured.
Investment casting (lost wax)
A wax replica of the part is injection-moulded, assembled onto a tree with other wax replicas, and dipped repeatedly in ceramic slurry and stucco to build a shell. The wax is melted out in an autoclave, the hollow shell is fired, and molten metal is poured into the cavity. The shell is then broken away and the part is cut off the tree.
Because the pattern disappears, the mould can be a single piece with no parting line. That is what allows undercuts, internal channels and fine detail that would be impossible to pull from a two-part mould. Every part needs a new wax pattern, so the process is inherently flexible but individual patterns are disposable.
Sand casting
A reusable pattern — usually wood, resin or machined metal — is packed into sand in two halves. The pattern is withdrawn, leaving a cavity, and the two halves are closed together with a parting line between them. Internal cavities are formed by sand cores placed inside the mould. Metal is poured, the sand is shaken out, and the same pattern is reused for the next part.
The pattern is the capital item, not the individual part, which is why sand casting is cheap per unit at volume and for very large parts. The trade-offs are a visible parting line, a draft angle on vertical faces, coarser finish and a wider as-cast tolerance, which usually means more machining allowance.
Specification comparison
| Dimension | Investment casting | Sand casting |
|---|---|---|
| Dimensional tolerance (ISO 8062) | Our process holds ±0.1 mm; typical investment casting runs around ±0.4 mm on a 100 mm dimension | CT10–CT13 — several times wider at the same dimension |
| As-cast surface roughness | Ra 3.2–12.5 µm; fine detail reproduces cleanly | Ra 12.5–25 µm; heavier texture from the sand grain |
| Minimum wall thickness | 1.5 mm on small parts, 2.5 mm on larger ones | 3–5 mm for small parts, more for heavy sections |
| Formable complexity | High — undercuts, internal channels, no draft limit, no parting line | Moderate — needs draft angles and a parting line; internal cavities need cores |
| Tooling / pattern cost | Aluminium wax die, typically the larger single cost, but still far below a forging or die-casting die | Pattern usually cheaper for simple one-piece parts; core boxes and large patterns add cost |
| Unit cost versus volume | Relatively flat across volume; near-net shape reduces machining time | Falls quickly with volume because the pattern is reused; extra machining allowance adds back some cost |
| Suitable volume band | Prototypes and tooling trials up to roughly 50,000 pcs/year | One-offs up to tens of thousands, most economical for large or simple parts |
| Typical single-piece weight | 10 g – 50 kg per piece; 20 – 2,000 kg per piece in lost foam | About 0.5 kg up to several hundred kg |
Values are typical ranges for guidance, not guarantees. Final tolerance and finish are agreed against your drawing and the standard you specify.
Decision checklist
Which process fits your part?
Read down each list. If most of the items in one column describe your part, that is your process — and if the third column fits better than either, be honest about it before you spend money on tooling.
Choose investment casting when…
- Geometry is complex, with undercuts, ribs or internal passages.
- You want the part close to net shape to minimise machining.
- Wall sections are thin, down to 1.5–2.5 mm.
- Annual volume is low to mid, roughly 50–50,000 pieces.
- A clean as-cast surface and fine detail matter.
- The alloy is difficult to forge, weld or machine from bar.
Choose sand casting when…
- The part is large or heavy — over 100 kg, or bulky relative to its detail.
- The shape is relatively simple and a parting line is acceptable.
- You can tolerate draft angles and a rougher surface.
- Volume is high enough that a reusable pattern pays off, or as low as a single unit.
- You need a low-cost pattern for a first prototype before committing.
- Generous machining allowance is acceptable on the critical faces.
Choose neither when…
- You need tens of thousands of small, tight parts a year — die casting is usually cheaper.
- The part is a simple prismatic shape better machined from bar stock.
- Grain flow or impact strength demands a forging.
- Wall thickness is under 1 mm at high volume — consider metal injection moulding or die casting.
- You need one prototype in days — additive manufacturing or machining gets there faster.
Common misconceptions
Four beliefs that cost money
“Investment casting is always more expensive.”
Not at low and mid volume. The wax die is far cheaper than a forging die or a die-casting die, and casting close to net shape removes machining passes. Below roughly 10,000–20,000 pieces a year the total landed cost often favours investment casting even though the per-piece rate looks higher than sand. Above that range, or for very large parts, sand casting and die casting catch up and overtake it.
“Sand casting is only good for rough, low-precision parts.”
Sand casting has improved. With resin-bonded sand and a machined metal pattern it holds tolerances that would have been considered fine a generation ago. It remains the most economical route for large, heavy parts that investment casting cannot produce at a sensible price — the limitation is size and finish, not automatically precision.
“I can hold ±0.1 mm as-cast.”
Not on a cast surface. The as-cast tolerance band is defined by the ISO 8062 grade and grows with the nominal dimension. For a 100 mm dimension, even a good investment casting holds roughly ±0.4 mm as-cast. If you need ±0.1 mm you get it by machining, not by casting, so mark on the drawing which faces are machined and which can stay as-cast. Tightening the cast tolerance on the whole part just drives cost up without improving the features that matter.
“The tolerance grade is a single number for the whole part.”
ISO 8062 defines a grade, and the tolerance band that grade produces depends on the size of each dimension. A small feature and a large feature can share one grade and still have very different tolerances in millimetres. When you review a quotation or a first-article report, check the tolerance against the actual dimension, not just the grade letter.
If you are not sure
What to send so we can tell you straight
You do not need to decide the process before you ask. Send these and we can say which route fits and why — including when the answer is a process we do not run.
- 2D drawing and 3D model (DWG or PDF plus STEP) so we can read both the tolerances and the geometry.
- Annual volume and typical batch size — the single biggest factor in process choice.
- Critical dimensions and which faces are functional, so we know what must be machined.
- Material and standard, and the end-use environment if corrosion or temperature is a factor.
- Weight or size limit the part has to meet in service.
- Target market and any compliance requirement (for example CE or pressure-equipment directives).
Send one drawing and get a clear answer
We will tell you whether your part suits investment casting, sand casting or a process we do not run, and quote the route we would use.
Not sure which process your part needs?
Send the drawing even if you have not decided. You will get a written recommendation from the people who run the process — including the cases where we are not the right supplier for it.