High-pressure die casting (HPDC) forces liquid metal into a reusable steel die at high speed and pressure. Aluminium HPDC is widely used for complex, thin-walled parts produced in large quantities.

Cold-chamber process

Because aluminium alloys attack submerged pumping components at casting temperatures, aluminium HPDC commonly uses a cold-chamber machine.

A measured amount of molten metal is poured into a shot sleeve. A plunger then drives it through the runner and gate system into the die cavity.

furnace -> shot sleeve -> runner/gates -> die cavity
             plunger  --->

The die is held closed by a large clamping force while cavity pressure rises.

Why high pressure is useful

Fast filling lets metal reach thin sections before solidification blocks the flow. Continued pressure during early solidification can improve dimensional reproduction and help feed some contraction.

The reusable precision die also enables good surface finish and high production rates once tooling has been made.

Air entrapment and porosity

The same high velocities that make HPDC productive can trap air if the cavity, runner and vents are poorly designed. Entrained gas can become internal porosity.

Vacuum-assisted systems reduce cavity pressure before or during filling to limit trapped gas.

Porosity matters especially when a component must be pressure-tight, highly loaded or subsequently heat treated or welded.

Thermal control

The die repeatedly absorbs heat from each shot and then rejects it before the next cycle. Cooling channels, spray, cycle time and local wall thickness influence die temperature and solidification.

Poor thermal balance can contribute to incomplete filling, local shrinkage, dimensional variation and excessive die wear.

Part and tool design

HPDC design is constrained by the need to fill, solidify and eject the component repeatedly. Important features include

• draft for ejection;
• suitable wall-thickness transitions;
• gate and runner placement;
• vents and overflows;
• ejector-pin locations;
• parting-line selection.

Economics and trade-offs

Steel dies and casting machines require substantial investment, so HPDC is most attractive when production volume can amortize the tooling. In return it provides fast cycles, complex near-net-shape geometry and repeatability.

The process is therefore a coupled problem in fluid flow, heat transfer, materials, tooling and production economics rather than merely "injecting aluminium into a mould."