How Metal Parts Are Made — Die Casting and Precision Casting Explained

Plastic injection molding is familiar territory for most designers, but the process behind a thin, rigid metal part — a laptop hinge, a camera body — stays oddly opaque. Die casting forces molten metal into a mold under high pressure, and it’s how most non-ferrous parts in aluminum, zinc, and magnesium get made. The principle resembles injection molding, but the temperatures and pressures involved are different enough that mold design has to follow an entirely different set of rules.

1. Why Reach for Metal at All

Metal earns its place when a part needs stiffness or heat dissipation that plastic simply can’t deliver — laptop and camera frames, transmission housings, enclosures where thermal performance matters. Die casting can pull a complex shape out of a single shot, which means a design can get its rigidity and its form from one solid piece of metal instead of several parts screwed together. Where Apple famously machines its MacBook unibody from a solid block, most electronics makers instead die-cast a rough form and refine precision afterward — a faster, cheaper starting point for the same rigidity.

How it differs from injection molding

Plastic injection runs at comparatively low temperature and pressure, but die casting deals with molten aluminum above 600°C, which means the mold itself has to be built from far tougher tool steel. Metal also shrinks more than plastic as it cools, so dimensional correction for shrinkage has to be built into the design from the start — one reason die-cast tooling takes longer and costs more than an equivalent plastic mold.

2. When Precision Casting Makes More Sense

If die casting is built for volume, precision casting — investment casting — is built for prototypes, short runs, and parts with intricate internal geometry. A wax pattern gets coated in a ceramic shell, the wax is melted out, and molten metal fills the cavity left behind — no hard tooling required, which is the whole appeal. The tradeoff is that the process leans heavily on manual work, so per-unit cost runs much higher than die casting and turnaround is slower. Jewelry, medical device components, small aerospace parts, and metal prototypes are where investment casting earns its keep. A growing number of shops now 3D-print the wax pattern itself rather than molding it, which removes even the wax tooling step and makes the process especially efficient for low-volume, high-variety runs.

3. What Happens After the Part Comes Out of the Die

A freshly die-cast part typically comes out rough, often with microscopic porosity, which rules it out for exposed surfaces as-is. That’s why CNC machining follows to refine any surface that needs precision, and anodizing or painting finishes the appearance. Anodizing builds an oxide layer on aluminum that delivers corrosion resistance and color in one step, which is why it shows up so often as the final finish on die-cast parts. Magnesium and zinc alloys, which don’t anodize well, more often get powder coating or PVD instead. How much post-processing a part goes through ends up driving most of the cost and quality variation between two parts that started in the same die.

4. The Design Mistake That Comes Up Most

The most common misstep is carrying plastic injection design instincts straight over to a metal part. Die casting is far less forgiving of abrupt wall-thickness changes — uneven cooling rates in those spots invite porosity and shrink defects, so the whole part needs to be designed with wall thickness as uniform as possible. Draft angle matters just as much: without enough taper to release the part from the die cleanly, it either won’t come free or comes out scarred. None of this is optional to consider after the mold is cut — it has to be built into the 3D model from the first pass, or the design ends up getting reworked after tooling is already finished.

5. Closing Thoughts

Die casting and investment casting both start from the same question: why does this part actually need to be metal? Once stiffness, heat performance, weight, or feel gives a clear answer plastic can’t, the next decision — which process fits the volume and geometry — follows naturally. Understanding what each process actually does makes it possible to design something realistic from the prototype stage all the way through finishing, instead of discovering the constraints after tooling is already cut.