Key takeaways
- The routes differ first in how the mould fills, and that difference sets the wall, the tolerance and the tooling cost.
- Sand casting wins on size and low volume; die casting wins on thin walls and repeatability at high volume.
- Gravity and low pressure casting sit between them and suit pressure-tight parts in a permanent mould.
- Choose on the part brief, then check the tooling payback at the real annual volume, not the peak volume.
The difference is in the fill
All three routes pour or inject liquid aluminium into a mould. The difference is the mould material, the speed of fill and the pressure held during solidification. Sand casting uses a bonded sand mould that is destroyed to remove the part. Gravity casting uses a permanent metal mould and lets the metal fill under its own weight. Die casting uses a steel die and forces the metal in at high speed and pressure.
That single difference explains most of the comparison. A sand mould is cheap and can be any size, but it is a poor conductor and a rough surface, so it cools slowly and holds a coarse tolerance. A steel die is expensive and limited in size, but it conducts heat quickly and can fill a thin wall before it freezes. A permanent mould sits between the two.
Process selection is therefore a set of trades, not a ranking. A route that is wrong for a thin housing can be right for a large frame, and a route that is right for a pressure-tight body can be wrong for a decorative cover.
Die casting: speed and repetition
Die casting fills a steel cavity in milliseconds and holds pressure while the casting solidifies. The result is a thin wall, a good as-cast surface and a repeatable part. Cycle times are short, so the process is built for high volume and for geometry that stays fixed.
The tool is the barrier. A die is a precision item with cores, slides and a cooling circuit, and its cost and lead time are much higher than a sand pattern. A change to the part after the die is cut is expensive, and a change to the volume can leave the tool underused. Die casting also has a size limit set by the machine, so a large housing may not fit.
Within its band, die casting is hard to beat for wall thickness, detail and cost per part. Aluminium die casting commonly runs 1.5 to 3 mm walls, with thinner sections possible on small parts. That is the number that decides many electrical housing designs.
Sand casting: size and flexibility
Sand casting uses a pattern to form a cavity in bonded sand. The pattern is cheap relative to a die, and the mould is used once, so the process accepts large parts, deep pockets and shapes that would need complicated slides in a steel tool. It is the usual route for a one-off, a large frame or a part that is still changing.
The trade is surface and tolerance. A sand surface is rougher, the as-cast tolerance is wider, and the minimum wall is thicker, commonly 3 to 5 mm for aluminium. Machining stock is larger because the surface is less accurate. The mould also collapses after the pour, so the process is slower per part and less repeatable than die casting.
Sand casting is often the right answer for a low volume bracket, a large enclosure base or a prototype that needs to be the real alloy and the real thickness before a die is committed. It is a poor answer for a thin, detailed part at high volume, because the machining and the scrap will absorb the tooling saving.
Gravity and low pressure: the middle route
Gravity casting pours metal into a permanent metal mould and lets it fill under its own weight. The mould is reusable, so the tooling sits between sand and die casting. The fill is slower and gentler than die casting, which reduces gas entrapment and makes the route suitable for parts that must be pressure tight.
Low pressure casting is a variant that pushes metal up into the mould from a sealed furnace. The fill is controlled and the feeding is good, which suits wheels, pump bodies and housings with a dense section. Both routes use a metal mould, so they hold a better surface and tolerance than sand, though not as tight as die casting.
The wall limit is thicker than die casting and thinner than sand, often around 3 to 4 mm. Draft is still needed because the part leaves a metal mould. The process is a good fit for a medium volume where the part needs integrity more than it needs a thin wall.
The comparison that decides the quote
The table is a starting point for a route conversation. The values are typical bands, and a specific foundry may hold tighter or looser limits on a specific part.
| Criterion | Die casting | Sand casting | Gravity or low pressure |
|---|---|---|---|
| Mould | Steel die, reused | Bonded sand, used once | Metal mould, reused |
| Typical volume | Tens of thousands and up | One-off to a few thousand | Thousands to tens of thousands |
| Tooling cost | High | Low | Medium |
| Minimum wall | About 1.5 to 3 mm | About 3 to 5 mm | About 3 to 4 mm |
| As-cast tolerance | Tightest of the three | Widest | Between the two |
| Surface finish | Good, fine detail | Rough, more stock | Good, less detail than die |
| Pressure tightness | Needs vacuum or control | Possible with a dense design | Good, especially low pressure |
| Best part size | Small to medium | Any size | Small to medium |
Bands are illustrative. Confirm wall, tolerance and integrity with the foundry for the specific alloy and geometry.
Tooling payback and the volume trap
The route with the lowest piece price is not always the route with the lowest total cost. Die casting usually wins the piece price at volume because the cycle is short and the machining allowance is small. The die cost has to be recovered, and the recovery depends on the annual volume and the programme life, not on a single large order.
A simple payback check compares the tool cost plus the piece price times the volume for each route. The crossover is usually in the thousands to tens of thousands of parts, and it moves with part size, wall thickness and the amount of machining.
The trap is designing for a peak volume that never arrives. A die that is justified on a forecast of 100,000 parts a year is a liability if the programme settles at 8,000. If the volume is uncertain, start on sand or gravity and move to die casting when the design and the demand are stable.
Tolerance, porosity and the hidden cost
As-cast tolerance follows the mould. A sand mould moves and compresses, so the tolerance is wider and the machining stock is larger. A steel die holds a sharper shape, so the tolerance is tighter and the stock is smaller. Gravity casting sits between them, with the advantage of a metal mould and the disadvantage of a slower fill.
Porosity behaves differently too. Die casting traps gas during a fast fill, so a thick section or a last-to-fill region can hold gas porosity that only appears after machining. Sand casting is slower and vents through the sand, but it can suffer shrinkage in heavy sections. Gravity and low pressure casting fill more gently, which helps integrity, though the mould and the gating still have to be right.
The hidden cost is the inspection and the scrap that follows from each route's typical defects. A die casting that must be leak tight may need a vacuum system, a pressure test and a radiographic sample. A sand casting may need more machining and more weld repair. Put those costs in the comparison, not just the piece price.
Choosing on the brief
Start with the four numbers that decide the route: annual volume, part size, minimum wall and the integrity requirement. Add the tolerance on the features that matter and the amount of machining the part needs. Those six inputs will usually point to one route before a quote is requested.
If the part is a thin, detailed housing at high volume, die casting is the default. If it is a large frame or a low volume prototype, sand casting is the default. If it is a pressure-tight body at medium volume, gravity or low pressure casting deserves a look before die casting with vacuum. Send the model, the volume and the integrity requirement, and ask for the route with the assumption behind it.
Frequently asked questions
Is die casting always cheaper at high volume?
The piece price usually is, because the cycle is short and the machining stock is small. The total cost depends on the die, the scrap rate and the volume actually achieved. Check the payback at the real annual volume.
Which route gives the best pressure tightness?
Low pressure and gravity casting fill more gently and are often preferred for pressure-tight parts. Die casting can be made tight with vacuum assistance and good process control, but the validation and the scrap risk are higher.
Can sand casting hold a thin wall?
Not as thin as die casting. Aluminium sand casting commonly runs 3 to 5 mm, and thinner sections need a special sand and a careful pattern. Below that, the mould and the metal temperature become hard to control.
When should a prototype be sand cast rather than machined?
When the alloy, the thickness and the internal shape need to be representative. Sand casting gives a real casting at low tooling cost. If only the external shape matters, machining from solid is often faster.
Sources and further reading
- ISO 8062-3, Geometrical product specifications for moulded parts, casting tolerances
- EN 1706, Aluminium and aluminium alloys, castings, chemical composition and mechanical properties
- Qumek engineering desk notes on cast and machined aluminium parts
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