Die castings are porous

Trapped gas, and why you cannot machine a casting wherever you like.

Die castings are porous because molten metal is injected into the mould at high speed to fill it before it can cool, and that speed traps small pockets of air and gas inside the metal as it solidifies, leaving the finished casting with tiny internal voids scattered through it that no amount of care afterwards can fully remove.

Petroski's The Evolution of Useful Things argues objects are shaped by their failures rather than their purposes, and a die casting's internal porosity is a quiet, permanent example of exactly that idea, a defect nobody wanted that nonetheless shapes what a designer is and is not allowed to ask a casting to do.

Speed fills the mould and traps the air

Die casting forces molten metal into a mould cavity at considerable speed and pressure, because filling the mould quickly is essential before the metal begins cooling and losing the fluidity it needs to reach every corner of the shape. That same speed is exactly what traps air. As the metal front rushes into the cavity, it can fold over on itself or enclose pockets of the air that was already sitting inside the mould before the shot began, and once the surrounding metal solidifies, those pockets have nowhere left to go, remaining as small voids frozen permanently inside what looks, from the outside, like a completely solid piece of metal.

Dissolved gas carried in the molten metal adds to the same problem, coming out of solution as tiny bubbles as the metal cools, in much the same way the gas in a fizzy drink appears as bubbles once the pressure holding it in is released. Slowing the fill would reduce the trapped air, but it would also let the metal begin solidifying before it had reached every corner of the mould, so die casting accepts some internal porosity as the price of filling a complex thin-walled shape fast enough to fill it completely at all.

Bubbles in a quickly filled ice tray

Filling an ice cube tray quickly from a fast-running tap shows the same trade in a completely different material. The water settles and looks solid and unremarkable, but small bubbles of air get carried in with the rush and, once the tray goes into the freezer, those bubbles freeze in place exactly where the rushing water happened to leave them, invisible from outside until the ice is cracked open or held up to a light. Pouring gently from a jug traps far fewer bubbles but takes longer, and a die casting foundry makes the same choice deliberately every time it picks fill speed over cleanliness in order to get metal into every corner of a mould before it starts to set.

Machining only where it is needed

Because porosity is scattered somewhat randomly through a die casting rather than concentrated in one predictable spot, machining into its surface carries a real risk of opening up a hidden void right at the machined face, exposing a small pit or hole that was previously sealed safely beneath an unbroken skin of solid metal. That skin is the densest part of the casting, since the metal touching the cold die freezes first and fastest, before gas has much chance to gather there.

So die castings are usually designed with as-cast surfaces left alone wherever function permits, with machining reserved for the specific features that need it, a bearing bore or a precise mounting face, and even then kept as shallow as the design allows. Every extra millimetre of cut goes further past the dense skin and into the more porous interior, raising the odds of breaking into a pocket the original shot happened to trap in exactly that spot.

Leak paths through solid-looking metal

Porosity also rules die castings out of many jobs that need to hold pressure or stay watertight. A chain of trapped voids that happens to connect, however faintly, from one surface of the part to another creates a leak path straight through metal that looks, and mostly is, completely solid. A die-cast housing expected to seal against water or air is therefore often designed with a separate gasket or O-ring doing the actual sealing at a joint, and some castings are impregnated afterwards, soaked under vacuum in a resin that fills the connected pores and is then cured, to close any leak paths that the casting brought with it.

Why critical castings are inspected one at a time

A well-run die casting process keeps porosity low and concentrated mostly in the thickest sections and near the centre of the part, where the metal finishes filling and cooling last. Even so, the voids are unevenly distributed enough that no specification sheet can tell a designer exactly where they will sit on any individual part. Two castings pulled from the same tool one shot apart can carry porosity in quite different places, which is why die-cast parts destined for structural or pressure-critical duty are often X-rayed or otherwise inspected individually rather than trusted on the strength of the process's average performance.

An average that looks perfectly acceptable across a thousand shots says very little about whether the one part in a designer's hand is carrying a void in exactly the wrong place. That gap between an average and an individual part is the kind of failure Petroski's book keeps returning to, the specific instance a purely statistical view of a process would never have flagged.