Die Casting

The short answer

Strength: the same headline number, two different meanings

Zinc and aluminium are quoted with tensile strengths in the same range, and that is where the confusion starts. The headline figure says nothing about how much of that strength is available per kilogram, and nothing about how the material behaves when it is overloaded. Both matter more than the tensile number itself, so the comparison has to be read column by column rather than row by row.

Two readings matter. First, Zamak 3 has a lower tensile figure than A380 in most tables and a higher yield figure, which means it reaches its working stress with less permanent deformation. Second, the elongation column is the one that predicts failure in service: zinc stretches about three times as far before it cracks, so a zinc part absorbs impact and tolerates assembly bending that would break a comparable aluminium part.

The density penalty is the real dividing line

Density is the single most decisive figure in the comparison. Zinc is 6.60 g/cm3 against 2.71 to 2.76 g/cm3 for A380, so it is 2.4 times heavier for the same geometry. A zinc part weighing 100 grams weighs about 41 grams in aluminium. Normalising strength by density makes the trade explicit: A380 lands near 116 to 120 MPa per unit of specific gravity, while Zamak 3 lands near 43 and Zamak 5 near 50.

That is why the two alloys end up in different product families rather than competing for the same one. Wherever weight drives a functional or economic outcome — vehicle fuel economy, shipping cost, a handheld product, a structural housing — aluminium wins and the density row settles the argument before any property table is consulted. Wherever the part is small and heavy use is not a concern, the density penalty is irrelevant and the zinc advantages become decisive.

Elongation and impact: where zinc wins outright

Zinc has roughly 10 percent elongation as cast and aluminium about 3.5 percent, and the gap is not a rounding difference. A material that stretches 10 percent before it breaks absorbs energy that a material stretching 3.5 percent does not, so zinc parts survive drop tests, crimping, staking, press fits and assembly torques that crack the same geometry in aluminium. Zinc also delivers higher hardness, 82 to 91 HB against 75 to 80 HB, which translates into better wear behaviour in mechanisms that rub.

The exception is temperature. Zinc creeps under sustained load above about 100 °C, and intergranular corrosion is a real risk if the alloy is made from contaminated scrap rather than from specified high-grade ingot. Where a part runs hot or has to be welded, aluminium takes over regardless of the elongation advantage.

Cost: what actually drives the per-part figure

The zinc alloy itself is cheaper per kilogram than A380 — the material index sits at roughly 0.70 to 0.85 against an aluminium baseline of 1.00 — but the density penalty reverses that on a per-part basis for anything large, because 2.4 times the density means about 2.4 times the metal in the same geometry. Per-part cost therefore comes down to four things that have nothing to do with the price of the ingot.

Put together, zinc is usually the cheaper route for small, intricate parts, where thin walls, long die life and a fast hot-chamber cycle outweigh the metal mass. Aluminium is usually the cheaper route for large parts, where its lower density means less metal, less weight and a smaller machine. The crossover is not a single quantity: it depends on part size, wall thickness, cavity count, annual volume and how much secondary machining the casting needs.

Choosing between them

• Choose zinc when the part is small, weight is not critical, walls thinner than 1 to 1.5 mm are needed, the as-cast tolerance has to hold within about plus or minus 0.05 to 0.1 mm, the surface has to plate directly, or ductility is needed for staking, crimping or assembly.
• Choose aluminium when weight drives the design, the part is large, service temperature exceeds about 100 to 120 °C, the part has to be anodised, or the programme needs pressure tightness and the option of impregnation.
• Choose zinc for the tool, aluminium for the part, when volumes are high. A zinc hot-chamber die lasts three to five times as long, so a programme that can use zinc tooling and aluminium parts is unusual but not impossible, and the two materials are not interchangeable within one die.
• Check the finishing route before the alloy is fixed. Zinc plates directly and takes a mirror polish; aluminium needs pretreatment before plating but can be anodised in colour. The finish often decides the alloy rather than the other way round.

Limits and open questions

• Published property tables disagree. Tensile strength for Zamak 3 is quoted from 215 to 283 MPa and elongation for the same alloy from 7 to 13 percent depending on source, section thickness and casting quality. Treat any single number as an indicative value and test a coupon in the real section thickness.
• Die life depends on the die, not only the alloy. The 500,000 to 1,000,000 shot figure for zinc assumes good thermal management and correct maintenance; some foundries publish a more conservative 200,000 to 500,000, especially on dies with long cores and slides.
• Zinc creeps when it is hot. Sustained loads above roughly 100 °C cause dimensional change, so zinc is the wrong choice for engine-adjacent, lighting or heat-soak components.
• Corrosion protection is not optional outdoors. Zinc develops a stable oxide layer in clean air, but outdoor and marine service still needs plating, powder coating or a conversion coating, and plated parts need the plating specified rather than assumed.
• Aluminium is not pressure tight without help. A380 can be impregnated to seal porosity, which is an extra operation; casting quality and process control decide whether it holds.
• No certification is implied here. Which quality system applies to a given programme is confirmed per programme and per factory in writing before production.

What to send for a material recommendation

Send the 3D model, the toleranced 2D drawing, the annual volume, the load path, the service temperature and the finish required. Those six items settle the alloy before price is quoted: a weight-critical or hot-running part points to aluminium, a small thin-wall or impact-loaded part points to zinc, and a part that needs a mirror-plated surface points to zinc for reasons that have nothing to do with strength. See aluminium die casting for large structural and thermal parts, surface finishing for the plating and coating routes, and send a drawing for a free design review that compares both alloys on your part.