Zinc die-casting alloy
The alloy behind almost every small metal part in a consumer product — cast to shape in seconds, and rarely noticed.
Zinc with around four per cent aluminium and traces of magnesium and copper. It is not a strong or a light alloy and it is chosen for neither: it is chosen because its low melting point allows hot-chamber die casting, where the injection mechanism sits in the molten metal and cycles in seconds.
That gives thin walls, fine detail, tight tolerances, an excellent surface straight from the die, and tooling that lasts for a million shots because the metal is not hot enough to erode it. Aluminium die casting needs a cold-chamber machine and destroys dies far faster.
So zinc die castings are everywhere and almost never recognised. Zips, buckles, locks, hinges, carburettor bodies, model cars, door handles, connector shells and the weight in the base of anything that wants to feel substantial.
Processing
Hot-chamber die cast, at cycle times of a few seconds for small parts. Plated readily — chromed zinc castings are a very old and very common combination — and painted or powder coated.
The standing hazard is impurity. Lead, tin and cadmium above a few hundredths of a per cent cause intergranular corrosion that makes the casting swell, crack and crumble over years. This destroyed the reputation of early zinc castings and is why the alloy is made from high-purity zinc under tight control; collectors of pre-war die-cast models know the failure as zinc pest.
Uses
Locks, keys, hinges and door furniture; zip sliders and buckles; carburettor and fuel-system bodies historically; connector shells and electronics housings; toy and model castings; tap and valve bodies; and decorative hardware of every kind.
It competes with injection-moulded plastic more than with other metals, and wins where weight, feel, heat resistance or electrical shielding matter.
History
The alloys date from the 1920s, when the New Jersey Zinc Company established that the destructive intergranular corrosion of earlier zinc castings was caused by lead and cadmium impurities rather than being inherent. High-purity zinc solved it, and the die-casting industry that followed is essentially built on that one piece of analysis.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Zinc — element · 94–96% · the balance, and it must be high-purity — lead and cadmium above a few hundredths of a per cent cause the casting to crumble over years
- Aluminium — element · 3.5–4.3% · about four per cent, which is what makes the alloy castable and strong enough to be useful
- Copper — element · 0–3.3% · a fraction of a per cent, raising strength and hardness
- Magnesium — element · 0.02–0.06% · a trace, and it is there to suppress the intergranular corrosion that impurities cause
is an input to
- Casting — process · hot-chamber die casting at cycle times of seconds, which is the whole economic argument for the alloy
is used in
- Automotive manufacture — industry · the small castings nobody looks at — handles, latches, brackets, and carburettor bodies historically
is an alternative to
- ABS — material · the competition for a small moulded component: zinc gives weight, heat resistance and shielding, and ABS gives cost and colour
is produced by
- Alloying and melting — process · aluminium, magnesium and copper into zinc, to a composition tight enough that the name is an acronym of the four ingredients
Sources
- Material WorldOur own writing