HPDC vs gravity die casting: which process suits your part?

Both processes pour the same aluminium into a steel die. What differs is how the metal gets there, and that single difference drives wall thickness, mechanical properties, tooling cost and the economics of your part.

High pressure die casting (HPDC) injects molten aluminium into a hardened steel die at high velocity and pressure, filling the cavity in milliseconds. Gravity die casting (GDC) pours metal into a reusable metal mould and lets gravity do the work, filling in seconds rather than milliseconds.

That difference in fill speed is the root of every other trade-off below.

The short answer

• Thin walls, complex geometry, high volume → high pressure die casting
• Thicker sections, structural or pressure-tight parts, lower volume → gravity die casting

Side-by-side comparison

Why thin walls need high pressure

Aluminium loses heat rapidly. In a thin section, metal begins freezing almost as soon as it touches the die wall. If you fill slowly, the flow front solidifies before it reaches the far side of the cavity, producing a misrun or cold shut.

High pressure solves this by filling the cavity faster than the metal can freeze. That is why a 1.5 mm wall is routine in HPDC and effectively impossible in gravity casting.

Why thick sections favour gravity

The reverse problem applies to heavy sections. Injecting metal at high velocity entrains air, and in a thick wall that trapped gas has nowhere to escape before solidification, it becomes porosity. Because porosity is distributed through the section, heat treatment can cause it to expand and blister.

Gravity casting fills calmly, giving gas time to escape and allowing directional solidification from the far end back toward the feeder. The result is a denser casting with better mechanical properties in thick sections, and one that can be heat treated to T6 if required.

Tooling cost and break-even

HPDC dies cost more. They are built from hardened hot-work tool steel and must survive thermal shock, high injection pressure and tens of thousands of cycles. Gravity moulds are simpler and cheaper.

But HPDC cycles much faster, so the piece cost is lower. The two curves cross at a volume that depends on your part, and that crossover point is the real question:

If you expect a few hundred parts a year, gravity tooling usually wins even though each part costs more to produce. If you expect tens of thousands, the HPDC die pays for itself quickly.

A decision sequence that works

1. Is any wall below 3 mm? If yes, HPDC is likely the only viable route.
2. Must the part be pressure-tight or heat treated? If yes, lean gravity.
3. What is the annual volume? Below a few thousand, gravity tooling is usually more economical.
4. How complex is the geometry? Thin ribs, bosses and undercuts push toward HPDC.
5. What surface finish do you need as-cast? HPDC gives a better starting surface for cosmetic or coated parts.

When the answer is "both"

Occasionally a family of parts splits across processes, a thin-walled cover in HPDC, a thick manifold in gravity. Having both under one roof means one supplier, one quality system and one delivery schedule rather than coordinating two vendors.

You can read more about each process on our high pressure die casting and gravity die casting pages.