Die Casting

How die casting works

Die casting is the high-pressure cousin of sand casting. A two-piece steel mold — the die — clamps shut under thousands of tonnes of force. Molten metal is forced into the cavity through a sprue and runner system at 100 to 1,200 bar. The metal hits the cool steel and freezes against the wall in a millisecond, then the cycle waits a few seconds for the centre to solidify before the dies pull apart and ejector pins push the casting out. Sprue, runners, and overflows are trimmed off and recycled; the part may need a quick tumble or shot blast for cosmetics.

The defining feature is speed. Where sand casting takes minutes per pour and gravity die-casting tens of seconds, high-pressure die casting fills the cavity in under 100 milliseconds. The metal enters the gates at 30 to 60 metres per second — supersonic in an aerodynamic sense, fast enough to atomise into a spray that paints the cavity walls. That speed is why thin sections (down to 0.6 mm in zinc, 1.5 mm in aluminum) fill cleanly without freezing short.

HOT-CHAMBER (Zn, Mg)               COLD-CHAMBER (Al, Cu, brass)
  ┌─────────────┐                    ┌─────────────┐
  │   DIE       │                    │   DIE       │
  └──┬───────┬──┘                    └──┬───────┬──┘
     │ shot  │                          │ shot  │
     │       │                          │       │
  ┌──▼───────▼──┐                    ┌──▼───────▼──┐
  │  GOOSENECK  │                    │   SLEEVE    │ ← ladle pours
  │  ╔═════╗    │                    │  ▓▓▓▓▓▓▓    │   metal here
  │  ║▓▓▓▓▓║▓▓▓ │ ← molten            └──▲──────────┘
  │  ╚═════╝▓▓▓ │   metal pot          (separate pot)
  └─────────────┘

Hot-chamber vs cold-chamber

The single most important architectural choice is whether the injection plunger lives inside the molten metal (hot-chamber) or pulls metal from a separate furnace shot by shot (cold-chamber).

Hot-chamber. A submerged gooseneck sits in the melt; the plunger drops, traps a shot of metal, and pushes it up through the gooseneck into the die. Cycle time is 1 to 3 seconds per shot; small machines run 60 cycles per minute. Limited to low-melting alloys — zinc (419 °C), magnesium (650 °C), and lead — because aluminum at 660 °C dissolves the iron of a submerged plunger in hours.

Cold-chamber. A robotic ladle dips into a separate furnace and pours a measured shot into a horizontal injection sleeve outside the die. The plunger then pushes that shot into the cavity at high pressure. Slower (30 to 90 seconds per cycle) but mandatory for aluminum, copper, and brass. Every Tesla Giga Press, every aluminum engine block, every transmission case is cold-chamber.

Casting variants compared

Process	Pressure	Volume	Tolerance	Best for
Hot-chamber die casting	70 to 350 bar	100,000 to 10 M	±0.05 mm	Zinc zippers, mag housings, lead weights
Cold-chamber die casting	200 to 1,200 bar	10,000 to 1 M	±0.1 mm	Aluminum engine blocks, transmission cases, Giga-cast underbodies
Sand casting	Atmospheric (gravity)	1 to 10,000	±1 mm	Iron pump bodies, large prototypes, art castings
Investment (lost-wax)	Atmospheric or vacuum-assist	10 to 100,000	±0.1 mm	Turbine blades, surgical implants, jewelry
Gravity die casting	Atmospheric	500 to 50,000	±0.3 mm	Aluminum cylinder heads, low-volume aerospace
Squeeze casting	700 to 2,000 bar (slow)	1,000 to 100,000	±0.2 mm	Forged-quality aluminum suspension components

The difference between high-pressure die casting and squeeze casting is fill velocity: HPDC sprays the cavity at 50 m/s and traps air; squeeze casting fills at 0.5 m/s and pressurises afterwards, eliminating gas porosity. Squeeze-cast aluminum suspension links can be heat-treated to forging-grade properties, where conventional HPDC parts cannot tolerate the solution-heat treatment temperature without blistering.

Shrinkage compensation

Liquid aluminum shrinks in three stages: 1.4 percent volumetric on cooling from pour to liquidus, 6.6 percent volumetric during freezing, and 5.6 percent volumetric on cooling from solidus to room temperature. Of those, only the post-solidification step affects part dimensions because intensification pressure feeds the freezing shrinkage from runners back into the cavity.

For 380-series aluminum (the workhorse die-casting alloy), the practical linear shrinkage is approximately 0.6 percent. So a finished part nominally 200.0 mm long is cut into the die at:

L_die = L_part × (1 + s) = 200.0 × 1.006 = 201.2 mm

Zinc Zamak shrinks 0.4 percent linear; magnesium AZ91D, 0.5 percent; copper alloys, 1.0 to 1.5 percent. Pattern makers tabulate shrinkage as shrink rule — a steel ruler stretched by the appropriate factor that lets a designer mark out die dimensions at finished-part scale.

Real-world specs

• Tesla Giga Press (6,000-tonne and 9,000-tonne). Built by IDRA. Casts a Model Y rear underbody — a 60 kg aluminum part formerly assembled from 70 stamped pieces — in 80 to 90 seconds. The shot weighs 80 kg of molten aluminum at 670 °C, with intensification pressure of 850 bar.
• Aluminum engine blocks. Cycle time around 90 seconds on a 2,000-tonne cold-chamber machine. Each block weighs 25 kg and contains 12 cores (water jackets, oil galleries) made from sand or salt that wash out after solidification.
• Zinc Zamak fasteners and toy cars. Hot-chamber machines running 60+ shots per minute pump out zinc parts at $0.02 to $0.10 each. Hot Wheels die-cast bodies start their lives this way.
• Magnesium laptop enclosures. AZ91D magnesium thixomolded at 600 °C produces 0.8 mm walls — light, stiff, and EMI-shielding. Most ThinkPad and MacBook bodies between 2005 and 2015 used this process.
• Brass plumbing fittings. Cold-chamber casting at 1,000 °C; the fittings are then machined to thread tolerance and tested for pressure. Tooling life is short — brass is hard on dies.

Anatomy of the machine

A die-casting machine is rated by its locking force — the clamping load applied to keep the dies from blowing apart under injection pressure. The required lock equals projected cavity area times intensification pressure plus a safety factor:

F_lock = A_proj × P_inten × 1.2

  A_proj   = projected area of cavity + runners (cm²)
  P_inten  = intensification pressure (bar)
  1.2      = safety factor

Worked example. A 0.5 m × 0.4 m projected aluminum part (2,000 cm²) injected at 700 bar (= 700 daN/cm²) needs:

F_lock = 2,000 × 700 × 1.2 = 1,680,000 daN ≈ 1,700 tonnes

So a 2,000-tonne machine. Extrapolate to a Tesla underbody (5,000 cm² projected at 850 bar) and you get the 6,000-tonne Giga Press.

Common failure modes

• Gas porosity. Air trapped in the cavity, hydrogen dissolved in the melt, or steam from die lubricant. The dominant defect. Mitigated by vacuum die casting (cavity evacuated to < 50 mbar pre-shot), proper venting, and dry shot sleeves.
• Shrinkage porosity. Internal voids where intensification pressure could not feed metal into the last-to-freeze region. Caused by hot spots, undersized runners, or premature gate freeze. Visible only on cross-section or X-ray.
• Cold shuts. Two flow fronts meet at temperature too low to fuse, leaving a hairline weld crack. Caused by long flow paths, cold dies, low metal temperature, or undersized gates.
• Soldering. Aluminum dissolves into the H13 die steel and welds the part to the cavity. Mitigated by die coatings (nitriding, PVD CrN), spray lubricant, and tighter temperature control. Soldered dies need polishing or replacement.
• Heat checking. The die surface fatigue-cracks from repeated hot–cold cycles, leaving a spider web pattern transferred to every subsequent part. End of life for a die after 100,000 to 500,000 shots.
• Flash. Metal squeezes between the parting lines under injection pressure. Caused by under-tonnaged machines or worn dies. Trimmed off after each shot but indicates a real problem.