How Aluminium Die Casting Works: A Step-by-Step Guide
A practical guide to the aluminium die casting process — die design, melting, injection, solidification, trimming, machining and inspection — written by die casters.
By Sunmax Casting India Engineering TeamReviewed by [INSERT ENGINEER NAME, ROLE]7 min read
Aluminium die casting turns molten aluminium alloy into a finished, near-net-shape part in seconds. It is used for gearbox housings, motor housings, brackets, heat sinks and hundreds of other components because it combines low weight, good strength and high productivity. This guide walks through the process the way it runs on a production floor.
1. Design for die casting
Every good casting starts with a part designed for the process. Engineers review the 3D model for:
- Draft on walls parallel to die opening, so the casting ejects cleanly.
- Uniform wall thickness, because thick sections solidify last and form shrinkage porosity.
- Fillets and ribs that help metal flow and add stiffness without mass.
- Parting line and datums, so critical features are formed in one die half and machining can locate reliably.
Small changes at this stage — coring out a boss, adding a rib, moving a hole — often save more cost than any later optimisation.
2. Die design and tooling
The die is two hardened steel halves: a fixed half on the stationary platen and a moving half on the moving platen. Inside are the cavity, the gating system (biscuit, runner and gates) that brings metal in, overflows that collect the first, coldest metal and oxides, and vents that let air escape. Cooling channels keep the die at a stable working temperature, and ejector pins push the casting out. Slides form undercuts that cannot be released by the main die opening.
3. Melting and alloy preparation
Alloy ingots (common grades include ADC12, A380, AlSi9Cu3, AlSi10Mg and LM24) are melted and held at a controlled temperature — at our plant, 685°C ± 5°C. Molten aluminium readily absorbs hydrogen and forms oxides, so the melt is degassed with an inert gas through a rotary impeller and treated with flux. Chemistry is checked by spectrometer before casting.
4. Injection
In a cold-chamber machine, a ladle transfers a measured dose of metal into a horizontal shot sleeve. The plunger then moves in three phases:
- Slow shot — pushes the metal to the gate while expelling air from the sleeve.
- Fast shot — fills the cavity in tens of milliseconds at high gate velocity.
- Intensification — raises pressure sharply to compact the metal and feed shrinkage as it solidifies.
The machine’s clamping force must resist the metal pressure acting on the projected area of the part and runners — that is how machine tonnage is selected.
5. Solidification and ejection
The metal freezes against the cooled die in seconds. After a set dwell time, the die opens and ejector pins push the casting free; on automated cells a robot extracts it. The die faces are then sprayed with release agent, which also helps control die temperature for the next shot.
6. Trimming and finishing
The casting comes out attached to its biscuit, runners and overflows. These are cut off with saws or a trim press, and parting-line flash is removed. Depending on the application, parts are then shot blasted for a uniform surface, vibratory deburred to break edges, or linished to blend visible parting lines.
7. Machining
Sealing faces, bearing bores and threads usually need machining. Doing this in the same plant lets fixtures locate on the casting datums designed into the die — which is why machining is planned at the DFM stage, not after the tool is built.
8. Inspection, packing and dispatch
Castings are checked dimensionally (gauges and CMM), visually for defects such as cold shuts and cracks, and, where required, for leak tightness or internal porosity. Each batch is labelled for traceability and packed to protect machined surfaces before dispatch.
Common defects and how the process prevents them
| Defect | Main cause | Prevention |
|---|---|---|
| Gas porosity | Trapped air or dissolved hydrogen | Degassing, slow-shot control, venting |
| Shrinkage porosity | Thick sections freezing last | Uniform walls, intensification, die thermal balance |
| Cold shuts | Metal too cold or slow | Melt and die temperature control, gate design |
| Soldering | Aluminium sticking to the die | Die spray, die temperature, die steel condition |
When is die casting the right choice?
Die casting is most economical for medium to high volumes of the same geometry, thin-walled parts with many integrated features, and components where consistent dimensions matter. For very low volumes or large, thick, heat-treated parts, gravity or sand casting may be better — a good die caster will tell you which.
Frequently asked questions
How long does one aluminium die casting cycle take?
Typically from under a minute to a few minutes depending on part size and wall thickness; filling itself takes only milliseconds, most of the cycle is solidification, die opening and spraying.
Why is aluminium cast in cold-chamber machines?
Molten aluminium attacks iron and steel components, so it cannot sit permanently in the injection system as zinc can. It is ladled into a shot sleeve for every shot instead.