Process Comparison
Aluminium sand casting vs die casting.
Two ways to turn molten aluminium into a part, with very different economics. This page sets them side by side on the twelve factors that actually decide which one your component should use.
The Short Answer
Volume decides it, most of the time.
Sand casting pours molten aluminium under gravity into an expendable mould made from bonded sand, which is broken away after every pour. High pressure die casting forces molten aluminium into a reusable hardened steel die at high speed and pressure.
That single difference — expendable sand mould against permanent steel die — sets everything else. Sand casting tooling costs a small fraction of a die, handles far larger parts, forms complex internal passages with cores, and accepts full T6 heat treatment. Die casting gives thinner walls, tighter as-cast tolerances and a smoother as-cast surface, and a lower piece price once you are making tens of thousands a year.
If you are making fewer than about a few thousand parts a year, sand casting is almost certainly the right answer — the die would never earn back its tooling cost. Above that, and if the part is also small, thin-walled and non-structural, die casting starts to win. Metline Casting is an aluminium sand casting foundry, and we will tell you plainly if your part belongs in a die instead.
Side By Side
Sand casting vs die casting, factor by factor.
Process capability ranges for aluminium. Figures describe the two processes in general — the tolerance achievable on your specific part is confirmed at quotation against your drawing.
| Factor | Aluminium sand casting | High pressure die casting |
|---|---|---|
| Mould type | Expendable bonded-sand mould, broken away after each pour | Reusable hardened steel die, good for many thousands of shots |
| Tooling cost | Low — a pattern, not a die | High — typically one to two orders of magnitude above a pattern |
| Tooling lead time | Days to a few weeks | Several weeks to months |
| Economic volume | One-offs up to a few thousand pieces a year | Tens of thousands of pieces a year and above |
| Part weight | 0.1–110 kg at Metline; the mould simply gets bigger | Limited by machine clamping force — usually well under 30 kg |
| Minimum wall thickness | Around 3–5 mm in most geometries | Around 1–2 mm, and thinner with care |
| As-cast tolerance | Roughly ISO 8062 CT8–CT12, depending on size and cores | Roughly ISO 8062 CT4–CT7 |
| As-cast surface finish | Around Ra 6.3–25 µm — a matte, granular surface | Around Ra 0.8–3.2 µm — smooth straight off the die |
| Internal passages | Bonded sand cores form cavities no cutter could reach | Limited to what a steel slide can withdraw |
| Heat treatment to T6 | Yes — LM25 reaches a 230 N/mm² minimum in sand-cast T6 | Usually not — entrapped gas blisters at solution temperature |
| Typical porosity | Shrinkage porosity, sealable by vacuum impregnation | Gas porosity from turbulent high-pressure fill |
| Cost of a design change | Rework the pattern — days | Recut or replace the die — weeks, at significant cost |
| Common aluminium alloys | LM6, LM25 (BS 1490 grades rated Excellent for sand) | LM2, LM24, ADC12 |
Decision Guide
Which process does your part need?
Read down whichever column matches your situation. Most parts fall clearly into one side.
Choose aluminium sand casting when
- Annual volume is below roughly a few thousand pieces, so a die would never pay for itself.
- The part is large or heavy — anything up to 110 kg is routine for us.
- The design is still moving and you need revisions without recutting tooling.
- The casting needs internal passages formed by cores — water jackets, oil galleries, hydraulic bores.
- The part is structural and has to reach T6 strength.
- You need prototypes, spares or a discontinued part re-created from a sample.
Choose high pressure die casting when
- Annual volume runs to tens of thousands and the tooling amortises over the run.
- Walls are thin and consistent, and the part is small and light.
- An as-cast cosmetic surface is required with minimal finishing.
- The design is frozen and will not change over the tool's life.
- The part is non-structural, so the loss of T6 heat treatment does not matter.
We do not run high pressure die casting. If that is genuinely what your part needs, we would rather say so than sell you the wrong process.
Cost Crossover
Where die casting starts to pay for itself.
Expressed as volume bands rather than a currency figure — the crossover moves with part size, alloy price and machining content, so a hard number would be out of date before you read it.
1 – 50 pieces
Prototype and pre-production
Sand casting, decisively
Die tooling cannot be justified at all. A pattern gets you real castings in the production alloy, in days.
50 – 1,000 / year
Low volume production
Sand casting
Tooling amortisation still dominates the piece price. This is the band most of our automotive, tractor and hydraulic work sits in.
1,000 – 5,000 / year
Medium volume
Usually sand casting
The crossover band. Machining content and part size decide it — heavy or core-intensive parts stay with sand well past this point.
5,000+ / year
High volume
Die casting starts to win
If the part is also small, thin-walled and non-structural, a die will beat sand on piece price. If it is large or needs T6, sand can still be the right answer.
One caveat worth stating: volume is the usual deciding factor, but it is not the only one. A large or heavy part, a part with cored internal passages, or a structural part that has to reach T6 can stay firmly in sand casting territory at any volume — because die casting simply cannot make it. See the full casting process, tolerances and cost drivers.
Frequently Asked Questions
Sand casting vs die casting — frequently asked questions.
The questions engineers ask us most when choosing between the two processes.
What is the difference between sand casting and die casting?
Sand casting pours molten aluminium under gravity into an expendable mould made from bonded sand, which is broken away after each pour. Die casting forces molten aluminium into a reusable hardened steel die under high pressure. The practical consequences: sand casting has far cheaper tooling, handles much larger parts, allows complex internal passages formed by cores, and accepts full T6 heat treatment. Die casting gives a smoother as-cast surface, tighter as-cast tolerances and thinner walls, and a lower piece price once volumes reach tens of thousands a year.
Is sand casting cheaper than die casting?
It depends entirely on volume. Sand casting tooling costs a small fraction of a die, so at prototype, spare-part and low-volume quantities sand casting is dramatically cheaper per part. Die casting has a lower piece price but has to amortise expensive tooling, so it only overtakes sand casting somewhere in the low thousands of pieces per year. Below roughly a few thousand parts a year, sand casting almost always wins on total cost.
Which process gives better tolerances, sand casting or die casting?
Die casting holds tighter as-cast tolerances — broadly ISO 8062 grades CT4–CT7, against CT8–CT12 for sand casting. That difference matters less than it appears, because critical features on either process are normally machined anyway. At Metline Casting we machine bores, sealing faces, mounting datums and threads on our 3-axis and 4-axis VMC line after casting, so the finished part meets drawing tolerance regardless of the as-cast grade.
Can sand castings be heat treated to T6 when die castings cannot?
Yes, and it is one of the strongest arguments for sand casting a structural part. High pressure die casting traps air during the turbulent high-speed fill; heating that casting to solution temperature makes the entrapped gas expand and blister the surface. Sand castings are gravity poured and do not carry that entrapped gas, so they take a full solution and age cycle. LM25 rises from roughly 130 N/mm² as-cast to a 230 N/mm² minimum in sand-cast T6 condition.
How large a part can each process make?
Die casting is limited by the clamping force of the machine, which in practice caps aluminium parts well under 30 kg. Sand casting has no equivalent constraint — the mould simply gets larger. Metline Casting pours single pieces from 0.1 kg up to 110 kg on the same moulding line.
What surface finish should I expect from a sand casting?
An as-cast sand casting surface typically falls around Ra 6.3–25 µm — matte and slightly granular, reflecting the sand grain. Sand blasting cleans and evens that surface for painting or powder coating, and any face requiring a finer finish is machined. Die casting produces a smoother as-cast surface, roughly Ra 0.8–3.2 µm, straight from the die.
Can sand cast parts be made pressure tight like die castings?
Yes. Both processes can leave fine porosity that weeps under pressure, and both are treated the same way. Metline Casting runs vacuum impregnation in-house, drawing sealant into the porosity so the part holds pressure — the standard route for hydraulic manifolds, pump bodies, valve housings and fire fighting fittings.
I need 500 parts a year. Which process should I use?
Sand casting, without much doubt. At 500 pieces a year the cost of a die casting tool would be spread across too few parts to ever recover, and the tooling lead time alone would delay your programme by months. A pattern can be ready in days, and design revisions stay cheap. Send us the drawing and we will quote it.
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Where to go next.
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From core making to finished, inspected components, Metline Casting runs the full aluminium casting process in-house.