Aluminum die casting injects molten aluminum at 660-700 °C into a hardened steel die at 40-100 MPa, filling the cavity in 20-100 milliseconds and solidifying in seconds. A complete cycle takes 30 to 90 seconds, and a production die lasts 80,000 to 150,000 shots. Aluminum is always cast on cold chamber machines because molten aluminum attacks the steel of a hot chamber injection system.
Aluminum die casting looks simple from the outside: metal goes in, a part comes out, roughly once a minute. What makes it work is a set of tightly controlled variables, and understanding them is what separates a design that casts cleanly from one that fights the process for its entire production life.
This is the process as it actually runs on our floor, stage by stage, with the numbers.
The short version
Aluminum ingot is melted and held at 660 to 700 °C. A measured dose is ladled into the shot sleeve of a cold chamber machine. A hydraulic plunger drives that metal through a runner and gate system into a hardened steel die cavity, filling it in 20 to 100 milliseconds at pressures of 40 to 100 MPa. Pressure is held while the casting solidifies. The die opens, ejector pins push the part out, the die is sprayed with release agent, and the cycle repeats 30 to 90 seconds later.
Everything else in this article is detail on those six sentences.
Stage 1: melting and metal preparation
Aluminum ingot, along with recycled runners and overflows from previous shots, is charged into a central melting furnace and brought to 660 to 700 °C. The exact temperature depends on the alloy and the part: thin-walled parts with long flow paths need the upper end of the range to stay fluid long enough to fill.
Two treatments happen before the metal is used:
- Degassing. Molten aluminum dissolves hydrogen readily, and that hydrogen comes out of solution as gas porosity during solidification. Inert gas is bubbled through the melt to strip dissolved hydrogen out.
- Filtration. Oxide films and inclusions are removed, because they act as crack initiation sites and cause hard spots that wreck cutting tools during machining.
Alloy chemistry is verified by optical spectrometer against the supplier certificate before the metal is released to production. Iron content matters more than most designers realise: iron between roughly 0.8 and 1.1% prevents the casting from soldering itself to the steel die, but higher iron reduces ductility. Recycled metal creeps upward in iron over time, which is why melt chemistry is checked rather than assumed.
Stage 2: dosing into the shot sleeve
This is where aluminum differs fundamentally from zinc.
Aluminum is cast on cold chamber machines. The injection system sits outside the furnace, and for each shot a ladle transfers a measured dose of metal into a horizontal shot sleeve. The reason is chemical: molten aluminum dissolves iron aggressively, so a permanently submerged steel injection system, as used in hot chamber zinc casting, would be eaten away in short order.
The cost of that necessity is cycle time. Every cold chamber shot involves a ladle transfer and a plunger stroke across a sleeve that is exposed to air, which adds seconds and gives dissolved gas a chance to be entrained. It is the main reason aluminum cycles run slower than zinc.
Stage 3: injection
Injection is not one motion. It is three phases, and the transition points between them determine casting quality more than any other setting on the machine.
| Phase | Plunger speed | Purpose |
|---|---|---|
| Slow shot | 0.1-0.5 m/s | Push metal through the sleeve without trapping air in a wave ahead of the plunger |
| Fast shot | 2-6 m/s | Fill the cavity in 20-100 ms, before the metal freezes in thin sections |
| Intensification | Static pressure | Raise pressure to 40-100 MPa and hold it while the part solidifies |
Gate velocity, the speed at which metal passes through the narrow gate into the cavity, typically runs 30 to 50 m/s. This is a genuine trade-off rather than a value to maximise. Too slow and the metal front cools and freezes before the cavity is full, producing a cold shut. Too fast and the metal atomises into a spray, entraining air that ends up as gas porosity.
The intensification phase is what die casting is named for. Aluminum shrinks roughly 6% by volume as it solidifies, and holding 40 to 100 MPa on the still-liquid interior feeds metal into that shrinking volume, pressing the skin hard against the die wall. This is why die castings reproduce surface texture so faithfully and why they achieve density that gravity casting cannot.
Stage 4: solidification and thermal management
Solidification takes a few seconds and is the stage that decides whether your part has internal porosity.
Aluminum freezes from the die wall inward. Thin sections freeze first, thick sections last. If a thick section is isolated from the gate by a thin one, the thin section freezes solid and cuts off the feed path, so the thick region shrinks with no liquid metal available to fill the void. The result is internal shrinkage porosity, often paired with a visible sink mark on the outer surface.
This is the mechanism behind the design rule everyone repeats without explaining: keep wall thickness uniform. It is not an aesthetic preference. Non-uniform walls create isolated liquid pockets, and isolated liquid pockets become voids.
Die temperature is held between 180 and 280 °C by cooling channels drilled through the die and, on demanding parts, by conformal cooling routed close to problem areas. Too cold and the metal freezes prematurely. Too hot and cycle time stretches out, the casting sticks, and the die soldiers.
Stage 5: die opening and ejection
Once the casting has enough strength to handle, the machine unlocks and the moving die half retracts. Slides that formed any undercuts pull first. Ejector pins then push the casting off the core.
Two things make or break this stage, and both are set at design time:
- Draft angle. External walls need 1 to 2° of draft, internal walls and cored holes 2 to 3°. Aluminum shrinks onto internal cores as it cools, gripping them, which is why internal features need more draft than external ones. Insufficient draft causes drag marks, distortion under ejector force, and accelerated die wear.
- Ejector pin placement. Pins must push on locations stiff enough to take the force. Push on an unsupported thin wall and you get a witness dimple, or a bent part.
The die is then sprayed with a water-based release agent, which both lubricates ejection and removes heat, and the cycle begins again.
Stage 6: trimming, deflashing and recycling
The part that comes out of the die is not just the part. It carries the solidified runner system, the overflows that were deliberately placed to catch the cold metal front and trapped air, and a thin fin of flash where metal squeezed into the die’s parting line.
All of it is removed by trim press or by hand, then the casting is deburred and shot blasted to a uniform matte surface at Ra 1.6 to 3.2 µm. The trimmed runners and overflows go straight back into the melt, which is a substantial part of why die casting material economics work.
What this means for your design
Every stage above translates into a design consequence:
- Uniform 2 to 3 mm walls, because isolated liquid pockets become porosity.
- 1 to 2° external draft and 2 to 3° internal draft, because parts must eject.
- Fillets of at least 1 mm at internal corners, because sharp corners heat-check the die at exactly that point.
- Ribs at 60 to 80% of the adjoining wall thickness, because a thick rib is just another thermal mass.
- Critical tolerances machined after casting, because as-cast features hold ±0.1 mm and no process parameter changes that.
If you are designing a part now, the aluminum die casting design guide covers each of those rules with target values. If you are still choosing between processes, die casting versus CNC machining works through the volume break-even.
And if you already have a model, send it. Our DFM review checks all of the above against your geometry and comes back in writing, before anyone commits to tooling.
