Hot chamber vs cold chamber die casting: which process will your part run on?

How does a hot chamber die casting machine work?

In a hot chamber machine the injection cylinder and plunger are permanently immersed in the holding furnace, so each shot is drawn straight from the melt and pushed through a gooseneck and nozzle into the die. There is no ladling step between the furnace and the die.

NADCA describes the cycle this way: as the plunger rises, a port in the cylinder opens and metal flows in; as it descends, it closes the port and drives the metal through the gooseneck into the cavity; once the casting has frozen, the plunger withdraws, the die opens and the part is ejected (NADCA, hot chamber process). The International Zinc Association glossary defines the same process in one line, as one where the metal injection system stays permanently in liquid metal, and names the conical runner it forms the sprue (IZA, glossary).

The catch is that the pot, cylinder and plunger live in the molten alloy all day. That only works for alloys that do not readily attack and erode those steel parts, which is why NADCA says the route is used primarily for zinc, magnesium, lead and other low-melting alloys. The U.S. Department of Energy’s industry profile puts hot chamber cycles anywhere from below one second on small parts up to roughly half a minute on multi-pound castings, with typical operating pressures of 1,500 to 4,500 psi (DOE metalcasting profile, 1999).

Which alloys are cast in each process?

Low-melting zinc alloys such as Zamak 3 run hot chamber, aluminum alloys such as A380 and ADC12 run cold chamber, and magnesium can go either way. The high-aluminum zinc grades ZA-12 and ZA-27 are the exception inside the zinc family and are cast cold chamber.

The DOE profile groups die casting alloys by melting point and states the rule plainly: low-melting alloys generally go to hot chamber machines, moderate- and high-melting alloys to cold chamber (DOE metalcasting profile, 1999). The table below maps the common alloy families against that rule.

Two rows deserve a closer look. ZA-12 and ZA-27 carry more aluminum than Zamak, and the IZA lists both as cold chamber die casting alloys, so a “zinc” callout does not automatically mean hot chamber economics (IZA, zinc alloys). Magnesium is the other: it can run hot chamber, but the IZA notes lower productivity than zinc because the die needs regular stops to clear oxide, and the DOE profile quotes cold chamber injection pressures for magnesium alongside aluminum, so both machine types are in use. If your drawing says ADC12, the ADC12 vs A380 guide covers how that grade relates to the ASTM B85 alloys.

Does the process affect porosity and pressure tightness?

Both processes inject metal at high speed and can trap gas, so neither is porosity-free. In practice, hot chamber zinc parts are more often pressure tight than cold chamber aluminum parts, and aluminum parts that must hold fluid are commonly impregnated.

The IZA states that zinc die castings are much more often pressure tight than aluminum ones, and gives the mechanism: aluminum die castings more often need finish machining, which opens subsurface porosity, so impregnation is normal for aluminum fluid-handling parts (IZA, comparison with alternative materials). Within aluminum, alloy choice matters too: NADCA’s Table 4.3 rates 380 at 2 and 413 at 1 for pressure tightness on its 1–5 scale (NADCA, aluminum alloys).

Entrapped gas is one reason conventional die castings are rarely heat treated to full tempers. NADCA notes that even small amounts of porosity can blister at heat-treating temperatures, which is why temper designations are not used for magnesium die casting alloys, and it describes squeeze casting as a separate route developed to fill without turbulence or gas entrapment. If your design assumes a T6 temper, a structural weld or a leak rate, that requirement has to be on the drawing and discussed before the die is built; the process family alone will not deliver it.

For leak-critical housings, write the test on the drawing: medium, pressure, hold time and allowed leak rate. Then ask whether the quote includes impregnation, so a zinc part that passes without it and an aluminum part that needs it are compared on the same basis.

What should an RFQ to an overseas die caster say about the process?

Specify the alloy and its standard, not the machine, and ask the supplier to confirm the process it will use and what the piece price assumes about die life. The machine type then becomes a check on the quote rather than a line on the drawing.

Suppliers in Vietnam and elsewhere in Asia often quote Japanese grade names, so “ADC12” or “ZDC2” may come back where the drawing says A380 or Zamak 3. The IZA lists ZDC2 as an equivalent of Zamak 3, while ADC12 is a close relative of the 380 family rather than an identical alloy. Ask for the grade and its governing standard in writing. Beyond that, a short list covers most of what matters:

• The process the supplier will use, and whether it matches the alloy family in the table above.
• The die-life assumption behind the piece price, and who pays for replacement dies.
• Which features are held as cast and which are machined afterwards.
• Whether impregnation, leak testing and finishing are included, and at what stage.
• The annual volume and program life the quote is built on.

The RFQ preparation guide covers the rest of the package. Die ownership, storage and transfer are contract terms rather than process questions; agree them in writing before the die is cut.