Process Comparison
Aluminium sand casting vs die casting.
Four ways to turn molten aluminium into a part, with very different economics. This page sets sand casting against gravity die casting, high pressure die casting and investment casting on the 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.
One clarification first, because it causes real confusion in quotations. “Die casting” on its own almost always means high pressure die casting. Gravity die casting — also called permanent mould casting — is a different process: it uses a reusable metal die, but fills it under gravity, exactly as sand casting does. That is why gravity die castings can be heat treated to T6 and high pressure die castings generally cannot. The table below covers all four routes, including investment casting.
At A Glance
All four aluminium casting processes, compared.
General process capability for aluminium, not supplier commitments. Tolerance is quoted as ISO 8062 casting tolerance grades and finish as an as-cast Ra band; the achievable tolerance on your own part is confirmed at quotation against your drawing.
| Process | Tooling | Economic volume | Part size | As-cast tolerance | As-cast finish | T6 |
|---|---|---|---|---|---|---|
| Sand castingWhat we do | Low | 1 – ~5,000 / year | 0.1 – 110 kg at Metline | CT8 – CT12 | Ra 6.3 – 25 µm | Yes |
| Gravity die casting | Medium | ~1,000 – 20,000 / year | Usually under ~20 kg | CT6 – CT9 | Ra 3.2 – 12.5 µm | Yes |
| High pressure die casting | High | Tens of thousands / year | Usually under ~30 kg | CT4 – CT7 | Ra 0.8 – 3.2 µm | No |
| Investment casting | Medium to high | Low to medium | Usually a few kg | CT4 – CT6 | Ra 1.6 – 6.3 µm | Yes |
Expendable sand
Sand casting
Best for. Large, heavy or cored parts; prototypes, spares and low to medium volume
Reusable metal die, gravity filled
Gravity die casting
Best for. Repeat medium-volume parts where a finer grain and tighter as-cast dimensions justify a metal die
Steel die, injected under pressure
High pressure die casting
Best for. Small, thin-walled, non-structural parts at high volume with a frozen design
Expendable ceramic shell around wax
Investment casting
Best for. Small intricate parts where near-net shape removes expensive machining
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 |
Gravity Die Casting
Gravity die casting vs sand casting.
The closer of the two comparisons, and the one most often confused with high pressure die casting. Gravity die casting — permanent mould casting — pours into a reusable metal die under gravity alone, with no injection pressure. Everything that follows comes from the mould material, not the filling method.
Because a cast iron or steel die pulls heat out of the casting far faster than insulating sand does, a gravity die casting solidifies with a finer grain structure. In the same alloy and the same heat-treated condition it is typically a step stronger than its sand-cast equivalent, and holds roughly one to two ISO 8062 grades tighter as-cast. Both processes take a full solution and age cycle to T6, because neither entraps the air that blisters a high pressure die casting at solution temperature.
What you pay for that is tooling. A metal die costs considerably more than a pattern and takes weeks rather than days, it constrains how large and heavy the part can be, and every design revision means recutting steel. Sand cores can be set inside a gravity die to form internal passages, but each one erodes the simplicity that made a reusable tool attractive in the first place.
| Factor | Aluminium sand casting | Gravity die casting |
|---|---|---|
| Mould | Bonded sand, broken away after every pour | Cast iron or steel die, reused for thousands of pours |
| How it fills | Gravity, no applied pressure | Gravity, no applied pressure — the same as sand |
| Tooling cost | Low — a pattern | Higher than a pattern, far below a high pressure die |
| Cooling rate | Slower — sand insulates, giving a coarser grain | Faster — the metal die chills the casting, refining the grain |
| Mechanical properties | Good; LM25 reaches a 230 N/mm² minimum in sand-cast T6 | Typically a step higher in the same alloy and condition, from the finer grain |
| As-cast tolerance | Roughly ISO 8062 CT8 – CT12 | Roughly ISO 8062 CT6 – CT9 |
| As-cast surface finish | Around Ra 6.3 – 25 µm | Around Ra 3.2 – 12.5 µm |
| Part size and weight | Effectively limited by the moulding box — 0.1 to 110 kg at Metline | Constrained by die cost and handling; usually well under 20 kg |
| Internal passages | Bonded sand cores form cavities no cutter could reach | Sand cores can be set in the die, but each one complicates an otherwise reusable tool |
| Heat treatment to T6 | Yes — gravity filling entraps no gas to blister | Yes, for the same reason |
| Design changes | Rework the pattern — days | Recut the die — weeks |
| Where it wins | Anything large, cored, low volume or still changing | A settled, repeating, medium-volume part small enough to justify the die |
In practice the two processes divide on volume and geometry rather than on quality. A settled, repeating part small enough to justify a die belongs in gravity die casting; anything large, core-intensive, low-volume or still changing belongs in sand. Metline Casting runs sand casting, and the alloys that suit it best — LM6 and LM25 — are set out in full on our aluminium casting alloy reference.
Investment Casting
Investment casting vs sand casting.
Investment casting — lost wax — builds a ceramic shell around a wax pattern, melts the wax out and pours into the shell. It reproduces detail no sand mould can, and charges for it on every single casting.
The shell picks up very fine surface detail and needs no draft angle, so investment castings come out close to net shape at roughly ISO 8062 CT4–CT6 with an as-cast surface near Ra 1.6–6.3 µm. Where a part would otherwise need extensive machining to reach its finished geometry, that can genuinely pay for itself.
The catch is that the mould is consumed and rebuilt for every part. Wax injection, shell building, dewaxing and burnout all recur per casting, which puts the piece price materially above sand. Investment casting also suits light parts — a few kilograms is typical, against the 110 kg we pour in sand. So the comparison usually turns on two questions: how big is the part, and how much machining would the near-net shape actually save?
| Factor | Aluminium sand casting | Investment casting |
|---|---|---|
| Mould | Bonded sand packed around a reusable pattern | Ceramic shell built around a wax pattern, which is melted out |
| Part size | 0.1 – 110 kg at Metline | Usually a few kilograms at most |
| As-cast tolerance | Roughly ISO 8062 CT8 – CT12 | Roughly ISO 8062 CT4 – CT6 |
| As-cast surface finish | Around Ra 6.3 – 25 µm | Around Ra 1.6 – 6.3 µm |
| Geometric complexity | Needs draft and a workable parting line; cores handle the internals | No draft required, very fine detail, undercuts reproduced directly |
| Machining allowance | Machining stock left on critical faces | Often near-net shape, so less metal to cut away |
| Piece price | Lower — one mould, one pour, one shakeout | Higher — wax, shell building, dewax and burnout on every single casting |
| Where it wins | Larger parts, cored passages, and parts whose critical features get machined anyway | Small intricate parts where near-net shape genuinely removes machining cost |
We do not run investment casting either. If your part is small and intricate enough that lost wax is the right route, we will say so — the same as we do for high pressure die casting. Where sand casting does fit, the eight-stage process and the inspection procedure behind it are both documented in full.
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.
Die cast vs sand cast — which is stronger?
Sand cast aluminum is usually the stronger choice for a structural part, which surprises people who assume the higher-pressure process must give the better metal. The reason is heat treatment: high pressure die casting traps air during its turbulent fill, so the casting blisters if it is taken to solution temperature and cannot reach T6. A gravity-poured sand casting can. LM25 rises from roughly 130 N/mm² as-cast to a 230 N/mm² minimum in sand-cast T6 condition. Die casting does win on as-cast surface, wall thinness and piece price at volume — but not on developed strength.
What is the difference between gravity die casting and sand casting?
Both pour aluminium under gravity, with no injection pressure. The difference is the mould: sand casting forms an expendable bonded-sand mould that is broken away after each pour, while gravity die casting — also called permanent mould casting — pours into a reusable cast iron or steel die. Because the metal die pulls heat out faster, gravity die castings solidify with a finer grain, so they are typically a little stronger and hold roughly one to two ISO 8062 grades tighter than sand. In exchange, the die costs considerably more than a pattern, takes longer to make, limits part size, and needs enough annual volume to pay for itself. Both processes accept full T6 heat treatment.
Is gravity die casting the same as die casting?
No, and conflating the two causes real confusion when comparing quotes. "Die casting" on its own almost always means high pressure die casting, where molten aluminium is injected into a steel die at high speed and pressure. Gravity die casting uses a reusable metal die too, but fills it under gravity alone. That single difference is why gravity die castings can be heat treated to T6 and high pressure die castings generally cannot — gravity filling does not entrap the air that later blisters the casting.
Investment casting vs sand casting — when is investment casting worth it?
Investment casting earns its cost when the part is small, geometrically intricate, and would otherwise need a great deal of machining. It builds a ceramic shell around a wax pattern, melts the wax out and pours into the shell, which reproduces very fine detail with no draft angle and holds roughly ISO 8062 CT4–CT6 with an as-cast surface near Ra 1.6–6.3 µm. The trade-offs are that it suits light parts, involves many more process steps per casting, and so carries a materially higher piece price. For a larger part, a part with big cored passages, or a part where the critical features are machined anyway, sand casting delivers the same finished component for less.
Which casting process gives the best surface finish?
In order, best to worst as-cast: high pressure die casting at roughly Ra 0.8–3.2 µm, investment casting at roughly Ra 1.6–6.3 µm, gravity die casting at roughly Ra 3.2–12.5 µm, and sand casting at roughly Ra 6.3–25 µm. Ranking the processes this way can mislead, though, because as-cast finish only decides the outcome on faces that stay as-cast. Sand blasting evens a sand casting to a clean, uniform, paint-ready surface, and any face needing better is machined — our VMC-machined faces come in at Ra 0.8–3.2 µm, the same band as a die casting.
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From core making to finished, inspected components, Metline Casting runs the full aluminium casting process in-house.