What is the difference between die casting and sand casting for metal parts production?

The short answer

Same idea, opposite economics

Both processes pour liquid metal into a cavity, and almost everything else differs. Die casting uses a hardened steel die that is reused hundreds of thousands of times and pays for itself by producing parts in seconds. Sand casting uses a pattern to form a mould that is destroyed with every part, so the tooling is cheap and the cycle is slow. That single difference explains the tolerance, the surface finish, the wall thickness and the cost curve in every row below.

Two rows deserve a second look. Cycle time is the reason the unit costs diverge so sharply at volume, because a die casting machine can produce 100 to 300 parts an hour where a sand line produces 10 to 50 parts a day. Tooling cost is the reason the divergence does not help a small programme: nobody amortises a USD 50,000 die over 800 parts.

Precision, and the machining that follows it

As-cast precision is the most over-quoted difference between the two processes, because the useful number is not the tolerance itself but the machining it leaves behind. A die casting is usually close enough to use on a general feature without a cut. A sand casting is not: published figures put the as-cast surface at Ra 12.5 to 25 micrometres, and the usual practice is to leave a machining allowance on every face that has to be accurate, which adds a machining operation, a fixture and a setup to the price.

Where a part genuinely needs the tighter band without machining, the choice is made for you: a drawing that calls for plus or minus 0.2 millimetres and a surface under Ra 6 micrometres on an as-cast face cannot be satisfied by sand casting. Where machining is planned anyway, the tolerance argument weakens considerably, and the decision returns to volume and cost.

The volume arithmetic is the whole decision

Published break-even volumes for the two processes cluster between 1,000 and 5,000 units a year, and the position inside that band depends on part size and complexity. A worked comparison puts a small aluminium part at USD 1,500 of pattern cost and USD 32 per part by sand casting against USD 22,000 of die cost and USD 16.20 per part by die casting, which crosses at roughly 1,300 pieces; at 10,000 pieces the die casting route saves on the order of USD 140,000. A separate estimate puts the break-even for a 1 to 10 pound part at 2,000 to 5,000 units.

Size moves the number in the other direction, and this is the part small buyers miss. Large castings are penalised by die cost far more than by cycle time, so for parts of 50 pounds and up, sand casting stays cheaper even at 10,000 pieces a year and above. The bigger the part, the higher the volume needed before a die makes sense.

Where each process owns the answer

• Sand casting owns ferrous and copper alloys. High-pressure die casting is limited to aluminium, zinc and magnesium, so a ductile iron, steel or bronze part has one route regardless of volume.
• Sand casting owns very large parts. It handles castings from about half a kilogram to several tonnes and can exceed several metres, well past the roughly 600 millimetre ceiling of most die casting machines.
• Sand casting owns low and mid volume. It is the economic choice from one piece to about 5,000 a year, and for service and legacy parts whose volume has fallen away.
• Sand casting owns internal cavities and undercuts. Sand cores can form internal passages that a two-part steel die cannot, because the core is broken out after the pour.
• Die casting owns volume aluminium and zinc. Above roughly 5,000 to 10,000 pieces a year the tooling is repaid and the unit cost advantage is three to eight times in published comparisons.
• Die casting owns thin walls and smooth as-cast surfaces. Walls down to 1.2 millimetres and an as-cast finish near Ra 1.6 micrometres are outside what sand casting can deliver.

One honest caveat runs through both columns: neither process is porosity-free. Die casting traps gas during the fast fill, which is why pressure-tight parts often need vacuum assistance; sand casting can suffer gas, shrinkage and inclusion defects that good feeding practice reduces but does not eliminate. Either way, the defect strategy is a conversation with the foundry, not a footnote on a process name.

Four questions that settle it before volume

Work down the flow below in order. The first two can decide the process on their own, and only when they come out in favour of die casting does the volume arithmetic become the deciding factor.

In practice the two processes cooperate more often than they compete. Sand casting is a natural route for the prototype or bridge batch while a die is being cut, and it remains the route for service quantities long after the production die has been retired. If your part sits in the middle of the volume band, send both routes a drawing and compare fully-loaded delivered prices rather than tooling figures. See aluminium die casting for the high-volume route, the die casting capability overview for tooling scope and surface finishing for what happens after the pour.