What is Zinc Alloy Die-Casting?

The short answer

What the process is, and why it is a hot-chamber process

Zinc alloy die casting is the permanent-mould casting of zinc-based alloys, almost always the Zamak family, by injecting molten metal into a hardened steel die at high pressure. Its defining feature is that it is run on a hot-chamber machine: the injection cylinder and gooseneck are immersed in the melt, so the metal is transferred from a bath a few centimetres away rather than ladled into a cold shot sleeve. That is only possible because zinc does not attack iron the way aluminium does, and it is the reason the process is so much faster and gentler than its aluminium cousin.

The practical consequences stack up. Melt temperature is 380 to 430 degrees Celsius against 660 to 720 for aluminium, so the die sees far less thermal shock and far less heat checking. Injection pressure is a fraction of the aluminium figure, at roughly 15 to 40 megapascals, so gates and runners erode more slowly. And because cooling is quick, published cycle times for small zinc parts run at 2 to 6 seconds per shot against 4 to 12 seconds for cold-chamber aluminium.

What the lower temperature buys: die life and tolerance

Die life is where zinc separates itself most clearly from aluminium. A well-maintained zinc die commonly exceeds 1,000,000 shots before major refurbishment, against 150,000 to 300,000 for aluminium on the same geometry. Because the die holds its dimensions for far longer, tolerances stay tight for a greater share of the tool's life, and published figures put zinc die casting at plus or minus 0.03 to 0.05 millimetres on small parts without any secondary machining. A die polished to a fine cosmetic finish reproduces an as-cast surface near Ra 0.2 to 0.4 micrometres, which is why zinc hardware often goes straight from the die to the plating line.

Wall thickness follows the same logic. Zinc is more fluid than aluminium and freezes over a narrower range, so the practical minimum falls to about 0.5 millimetres on small cosmetic parts, 1.0 millimetre as a general engineering floor, and 1.5 millimetres where the part carries load or holds a thread. Maximum uniform wall should stay under about 4 millimetres to avoid centre-line shrinkage, and draft angles can be small, around 0.5 to 1.0 degrees on outside faces and 1.0 to 1.5 degrees inside, because zinc shrinks only 0.6 to 0.8 percent.

The Zamak family, and what the copper buys

Zamak stands for zinc, aluminium, magnesium and copper, and the letter grade is essentially a statement about how much copper is in the mix. Zamak 3 is the baseline and accounts for the large majority of general zinc castings: 4 percent aluminium, no deliberate copper addition, 283 megapascals tensile and around 10 percent elongation, which makes it ductile enough for clinching, riveting and snap fits. Zamak 5 adds roughly 1 percent copper, which raises tensile strength to about 328 megapascals and hardness to around 91 Brinell at the cost of elongation, which falls to 5 to 7 percent; it is the grade chosen for latches, hinges and parts under repeated mechanical load. Zamak 2 pushes copper to 2.5 to 3 percent and reaches 360 to 400 megapascals with 3 to 5 percent elongation, for the highest-strength work a hot-chamber machine can handle.

A fourth family, the ZA grades, sits outside this table. ZA-8 contains more aluminium and can still run on hot-chamber equipment, while ZA-12 and ZA-27 pour hot enough that they destroy the gooseneck and are restricted to cold-chamber machines. They are specified for bearing and wear duty rather than for the general hardware that Zamak covers.

Where zinc die casting is the wrong choice

• Weight matters and the part is large. Zinc has a density of 6.6 to 6.8 grams per cubic centimetre, more than twice aluminium's 2.7. Below a few hundred grams the penalty is irrelevant; above it, the same part in aluminium or magnesium is lighter and cheaper to move.
• The part runs hot or carries sustained load. Zinc creeps under load at elevated temperature, which is exactly why Zamak 5 exists alongside Zamak 3. Where a part sits in a hot or continuously loaded location, the service temperature is checked against the alloy datasheet before zinc is specified, and the grade is chosen for creep rather than for strength.
• The programme is small. Zinc dies are long-lived and correspondingly expensive to cut, so the route needs volume to repay the tooling just as aluminium does.
• The surface has to be bare metal for an anodised finish. Zinc does not anodise the way aluminium does; the usual route is electroplating, typically a copper-nickel-chrome stack around 25 micrometres thick that passes salt-spray testing for 96 hours.
• The casting has to be pressure tight or structural in a high-integrity sense. Zinc keeps good properties in thin sections, but where a specification calls for a qualified structural casting the material and process are confirmed per programme in writing rather than assumed.

How to start a zinc die casting project

Send a STEP model, a toleranced drawing, the annual volume, the finish you need and the environment the part will see. Those five items decide the grade, the wall sections and whether the part goes to the plating line or comes off the die as finished. Zinc is usually the right answer for small, detailed, moderately loaded hardware that has to look good and hold a thread; aluminium is usually the right answer once the part grows or has to be light. See aluminium die casting for the comparison, the die casting capability overview for tooling scope and surface finishing for plating and coating options.