Die Casting
Die casting injects molten aluminum, zinc or magnesium into a hardened steel die under high pressure, producing thin-walled net-shape metal parts.
- Part
- Forming
- Variants
- 2
- Revised
- 2026-08-11
At a glance
- Family
- Metal
- Typical tolerances
- ±0.010 in (±0.25 mm) on the first inch, then ±0.002 in per additional inch (NADCA standard linear tolerance, aluminum); dimensions crossing the parting line need added allowance
- Surface finish
- Roughly 32–63 µin Ra (0.8–1.6 µm) as cast from a new die, coarsening as the die heat-checks
- Typical volumes
- 10,000–1,000,000+ parts per die; below about 5,000/year the tooling rarely pays back
- Lead time
- Roughly 10–16 weeks for die build and sampling; 2–4 weeks per production release thereafter
- Materials
- Metal
What it is
Die casting injects molten aluminum, zinc, or magnesium into a hardened steel die under high pressure and holds it there while the part solidifies, producing net-shape metal parts with thin walls, cast-in bosses and ribs, and an as-cast skin good enough to paint. It is the highest-rate metal casting process: cycle times run 15–60 seconds, and gates fill a cavity in 20–100 milliseconds.
Typical parts are housings, gearbox cases, brackets, connector shells, and hardware from a few grams up to about 20 lb (9 kg). Aluminum A380 dominates by volume; zinc alloys (Zamak 3 and 5) allow thinner walls, finer detail, and far longer die life; magnesium AZ91D buys weight savings at higher metal cost. Standard linear tolerance is ±0.010 in (±0.25 mm) on the first inch and ±0.002 in per inch after that.
The constraint is tooling. A production die is a hardened H13 assembly that takes months to build, so die casting only makes economic sense above roughly 5,000–10,000 parts per year.
How it works
Die casting splits into two machine architectures, chosen by the melting point of the alloy.
Hot chamber machines keep the injection cylinder submerged in the melt. They run zinc, magnesium, and lead alloys, whose melting points (Zamak 3 melts near 730°F / 390°C) are low enough not to attack the steel plunger. Shot-to-shot cycles are fast because there is no ladling step.
Cold chamber machines keep the furnace separate and ladle a measured shot into an unheated sleeve each cycle. Aluminum, poured at roughly 1,150–1,250°F (620–675°C), would dissolve a submerged hot-chamber cylinder, so it is always cast cold chamber. See metal melting points for the alloy temperatures that drive this split.
The cycle itself:
- Die prep. The two die halves close under clamp force — machines are rated roughly 100–4,000 tons. The cavity has already been sprayed with a water-based die lubricant and blown dry; die surface temperature is held around 350–600°F (175–315°C) for aluminum.
- Shot. The plunger drives metal through the runner and gate at roughly 1,500–15,000 psi (10–100 MPa). Gate velocity is high — commonly 100–200 ft/s (30–60 m/s) — which is what lets a 0.050 in wall fill before the metal freezes.
- Intensification. Once the cavity is full, pressure is boosted and held while the casting solidifies, squeezing shrinkage porosity smaller.
- Solidification and open. Dwell is a few seconds for thin zinc parts and 15–20 seconds for thick aluminum sections. The die opens and ejector pins push the casting out.
- Trim. The shot — casting plus runner, biscuit, and overflows — goes into a trim die that shears off everything but the part. Runner metal is remelted.
- Secondary. Machining of critical bores, deburring, vibratory finishing, resin impregnation for pressure-tight parts, and coating.
Air trapped during the high-velocity fill is the process's defining defect. Overflows and vents at the parting line give it somewhere to go; vacuum die casting evacuates the cavity before the shot when porosity must be low enough to heat treat or weld.
Design guidelines
Keep walls thin and uniform
Aluminum die castings run a nominal wall of 0.080–0.120 in (2–3 mm), with 0.050 in (1.2 mm) practical on small parts. Zinc goes thinner: 0.040–0.060 in (1–1.5 mm) nominal, 0.030 in (0.75 mm) achievable. Thick is worse than thin here — heavy sections solidify last, so they hold the shrinkage porosity. Core out any section more than about 1.5× the nominal wall.
Draft every surface
Aluminum needs roughly 1° on outside walls and 2° on cored internal surfaces; zinc, which grips the core less aggressively, gets by with about half that. Deep cores need more, not less. Draft is not optional — without it the casting shrinks onto the die steel and galls on ejection.
Radius the corners
Minimum fillet radius is about 0.060 in (1.5 mm), and a fillet equal to the adjacent wall thickness is better. Sharp internal corners are stress risers in the casting and heat-check initiation sites in the die, which makes them a tooling cost problem as much as a part problem.
Ribs instead of thick walls
Ribs should be about 0.6× the nominal wall thickness so they do not sink, with generous root fillets and 1–2° of draft per side. Use several shallow ribs rather than a few tall ones.
Cored holes
Minimum cored diameter is around 0.10 in (2.5 mm) in aluminum and 0.060 in (1.5 mm) in zinc, with core depth kept under about 4× diameter. Core pins are cantilevers loaded by a fluid moving at highway speed — long thin ones bend, then break. Small or deep holes are cheaper drilled after casting than cored.
Plan for the parting line and ejectors
Flash at the parting line is normal and gets trimmed; dimensions that cross the parting line carry a larger tolerance than dimensions contained within one die half. Ejector pin marks land within about ±0.005 in of the surface, so keep them off cosmetic faces.
Leave machining stock, but not too much
Allow 0.010–0.030 in (0.25–0.75 mm) on machined surfaces. The chill skin is the densest metal in the casting; cutting deeply through it exposes subsurface porosity, which is the usual cause of leaks in machined sealing faces.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Nominal wall, aluminum | 0.080–0.120 in (2–3 mm) | 0.050 in (1.2 mm) | Thinner sections freeze before the cavity fills |
| Nominal wall, zinc | 0.040–0.060 in (1–1.5 mm) | 0.030 in (0.75 mm) | Lower melting point tolerates thinner sections |
| Draft, outside wall | 1–2° aluminum | 0.5° zinc | Casting shrinks onto die steel and must release |
| Fillet radius | 1× adjacent wall | 0.060 in (1.5 mm) | Sharp corners crack castings and heat-check dies |
| Rib thickness | 0.6× nominal wall | 0.8× nominal wall | Thick ribs sink and trap porosity |
| Cored hole diameter | ≥ 0.10 in (2.5 mm) aluminum | 0.060 in (1.5 mm) zinc | Slender core pins deflect and break |
| Machining allowance | 0.015 in (0.4 mm) | 0.030 in (0.75 mm) | Cutting past the chill skin exposes porosity |
Cost drivers
Cost splits into tooling, machine time, and metal. The die is the dominant fixed cost — a multi-cavity hardened steel die with slides costs substantially more than a plastic injection mold of similar size, because it runs hotter and dies of thermal fatigue rather than wear. Per-part cost is machine seconds plus alloy weight plus secondary operations, and the secondaries are routinely underestimated: trimming, machining, deburring, and impregnation frequently add more than the casting cycle itself.
Volume breakpoints: below roughly 5,000 parts per year the tooling rarely amortizes and investment casting, sand casting, or machining from billet wins. From 10,000 to 100,000, a single- or two-cavity die is normal. Above 250,000, multi-cavity dies and higher-tonnage machines cut per-part machine time proportionally.
- Cut the number of slides. Every side-action mechanism adds tooling cost, cycle time, and a maintenance point. Reorient the part or move the feature onto the parting plane if you can.
- Design for as-cast surfaces. Each machined feature adds a fixturing operation. Tolerance the casting to the ±0.010 in standard wherever function does not require better.
- Pick zinc when the part is small. Zinc dies outlast aluminum dies by roughly an order of magnitude, cycle faster, and hold finer detail — often offsetting the higher metal cost.
Variants
2 named
High Pressure Die Casting
Low Pressure Die Casting
Questions
6 questionsWhat tolerance can die casting hold?
The NADCA standard linear tolerance for aluminum is ±0.010 in (±0.25 mm) on the first inch, plus ±0.002 in for each additional inch. Precision tooling and tighter process control roughly halve that. Dimensions that span the parting line carry an extra allowance because they depend on how tightly the die halves close.
What is the minimum wall thickness for die casting?
About 0.050 in (1.2 mm) for aluminum on small parts and 0.030 in (0.75 mm) for zinc. Nominal design walls are thicker — 0.080–0.120 in (2–3 mm) in aluminum — and uniformity matters more than absolute thinness, since heavy sections are where shrinkage porosity collects.
What draft angle does a die casting need?
Roughly 1° on outside walls and 2° on cored internal surfaces for aluminum, and about half that for zinc. Deep cores need more. Zero-draft surfaces gall on ejection and shorten die life.
Can die castings be welded or heat treated?
Conventional high-pressure die castings generally cannot, because entrapped gas expands at temperature and blisters the part. Vacuum and pore-free variants reduce entrapped gas enough to allow both, at a process premium. If welding or T6 heat treatment is required, say so before the die is designed.
How many parts will a die casting die produce?
On the order of 100,000 shots in aluminum and over a million in zinc before major refurbishment. Aluminum's higher casting temperature drives thermal fatigue in the H13 die steel, which shows up as heat-check crazing transferred onto the part surface.
Die casting or investment casting?
Die casting wins above roughly 10,000 parts a year on aluminum, zinc, and magnesium geometries with uniform thin walls. Investment casting wins for steel, stainless, and superalloys, for lower volumes, and for geometries with undercuts or zero draft that a steel die cannot release.