Die casting and investment casting

Two processes sit at opposite ends of the casting world. One makes millions of thin walled aluminium parts a few seconds apart. The other makes shapes no other process can make at all, one shell at a time. Choosing between them is mostly a question of what you are buying.

An investment casting wax tree, a dozen red wax patterns assembled radially around a central runner, being dipped into a tank of ceramic slurry with the slurry running off it. — The wax will be melted out, which is why this shell has no parting line and needs no draft to release the pattern.

High pressure die casting

Molten metal is injected into a hardened steel die at high pressure and very high velocity, and the pressure is held while it freezes. Filling takes a fraction of a second.

That speed is what allows thin walls and fine detail to be filled before the metal solidifies. It is also the origin of the process’s defining limitation.

The process wants thin and above all uniform walls, commonly 1.5 – 4 mm. A local thick boss in a thin walled part freezes last and traps porosity at its centre, and unlike sand casting there is no riser that can reach it. It is designed out with ribs, not fed.

The die is a large block of hot work tool steel thermally cycled every few seconds, and it eventually fails by heat checking — a network of fine thermal fatigue cracks that print themselves onto every part. That, plus the cost of the die itself, is why the process needs volume.

Investment casting

A wax pattern is built, assembled onto a runner tree, dipped repeatedly in ceramic slurry and stucco to build a shell, and then the wax is melted out. What remains is a one-piece ceramic mould with no parting line through the part.

Because the pattern leaves by melting rather than by withdrawal:

No draft is needed for pattern removal from the shell
Undercuts and complex internal forms are possible
There is no parting line and no mismatch across the component
High melting point alloys — stainless steels, superalloys — are routine

That is why turbine blades, valve trim, pump components and complex brackets are made this way. The alternative is usually a great deal of machining from solid.

Choosing between them

A comparison chart of four casting processes — sand casting, gravity die casting, high pressure die casting and investment casting. For each, bars show relative tooling cost, achievable dimensional tolerance, surface roughness, minimum wall thickness and the economic annual volume. A second panel plots tooling cost against annual volume with cross-over points marked, showing sand casting economic at low volume, investment casting at low to medium volume for complex shapes, and high pressure die casting only above a large annual quantity. A third panel lists for each process whether heat treatment and welding are normally possible. — Four processes and five axes. The cross-over between them is almost always decided by annual volume and by whether the part has to be heat treated or welded afterwards.

	High pressure die casting	Investment casting
Alloys	Mainly aluminium, zinc, magnesium	Almost anything, including steels
Wall thickness	1.5 – 4 mm, uniform	Thin sections possible, less restricted
Geometry	Must eject from a rigid die	Undercuts and complex internals
Tolerance	Very good	Very good
Surface finish	Excellent	Excellent
Heat treatable	Usually not	Yes
Weldable	Usually not	Yes
Tooling cost	Very high	Moderate
Economic volume	High	Low to medium

The argument that decides it

Near net shape — producing the casting close to finished dimensions — is worth paying for because of the machining that never happens and the material that is never cut away.

That is the real comparison. Not the cost of the casting against the cost of another casting, but the cost of the casting plus its machining route against the alternative’s. A cheaper casting with four extra operations behind it is not cheaper.

What to take away

HPDC injects at high pressure and velocity — thin walls, fine detail, seconds per part.
Entrapped gas means a die casting usually cannot be heat treated or welded.
Design HPDC for uniform thin walls; use ribs, not heavy bosses, and never expect a riser.
The die fails by heat checking, so volume is needed to justify it.
Investment casting melts the wax out, so there is no parting line and no draft limit.
It handles high melting point alloys and complex internal shapes at modest volume.
Both processes still need draft and generous radii for ejection and die life.
Compare casting plus machining, not casting against casting. That is what near net shape buys.