Porosity is the most common defect in high pressure die casting, and it has no single cause. That is why "we found porosity" is not a diagnosis on its own; the first step is to separate which porosity it is. Broadly there are two: gas porosity (air, moisture and release agent vapour trapped in the cavity) and shrinkage porosity (the feed path closing on metal that is shrinking as it solidifies).
The two arise from different causes, look different and call for different countermeasures. Both can appear in the same part.
Telling the two apart
Gas porosity
Filling takes milliseconds, and any air that cannot escape in that time is trapped. Trapped gas leaves round, smooth walled voids inside the solidifying metal. Air is not the only source: release agent vapour, hydrogen dissolved in the melt and moisture in the sleeve all end the same way.
The distinguishing sign is the geometry of the void; gas voids are close to spherical and usually gather in the last regions to fill, away from the overflows.
Shrinkage porosity
Aluminium contracts as it solidifies, and that contraction is made up by liquid metal fed from behind. If the feed path closes early — because a thin section froze before a thick one — a void is left at the centre of the heavy region. These voids look angular, branched and irregular.
Shrinkage porosity is almost always the signature of a hot spot: a region that solidifies last, surrounded by material that froze before it.
Countermeasures in design
The most effective intervention is in design, because geometry decides where the hot spot forms. Abrupt jumps in wall thickness create hot spots, and that region solidifies last — unfed, it leaves a void.
- Break up heavy sections with ribs. Stiffness comes from intelligent ribbing, not from thick material. The result is both a lighter and a less porous part.
- Step the thickness transitions. Instead of jumping from a thin wall to a heavy boss, easing the transition organises the direction of solidification.
- Avoid material build-up. Screw bosses, mounting lugs and intersecting ribs can quietly form a heavy mass; rib intersections are the most commonly overlooked hot spots.
- Think about the direction of solidification. Ideally it progresses from the farthest point back towards the gate, so the feed path is the last thing to close.
Countermeasures in the die
Gate section and position decide at what speed and from which direction metal enters the cavity. The placement of vents decides where trapped air can leave. Both can be assessed with filling simulation before any die steel is cut — which, set against the cost of fixing a filling problem discovered after the die exists, makes it the cheapest step in the process.
- Overflows. Placed at the last regions to fill, they carry the first cold metal and the air ahead of it out of the part.
- Vents. With no way out, air is bound to be trapped. Vent sections must be thin enough not to let metal escape, yet sufficient to pass air.
- Cooling circuits. Cooling placed near a hot spot changes that region's solidification sequence. Sometimes the answer to shrinkage porosity is not in the design but in a single cooling channel in the die.
- Vacuum assistance. Evacuating the cavity before filling reduces gas porosity at source; it is considered on parts that must hold pressure or be heat treated.
Countermeasures in the process
Injection speed, die temperature, metal temperature and intensification pressure all act directly on porosity. Too much speed creates turbulence and folds air into the metal; too little brings cold shuts and short fills. If the die runs cold, the metal freezes early and the feed path closes prematurely.
What matters most here is that the parameters are on record. When porosity rises in a batch, only a retrospective record shows which variable drifted; without records you are left with trial and error. Release agent quantity and spray pattern belong on that record too — excess release agent vaporises straight into gas porosity.
How is it detected?
Porosity is invisible; unless it breaks the surface, the part looks perfect from outside. The methods used:
- X-ray. Non-destructive; shows the position, size and distribution of voids. A standard step on critical parts.
- Sectioning. Destructive but the clearest information; used in process validation and at first article stage.
- Leak testing. Tells you whether the part is functionally permeable. A void does not leak unless it opens to the surface — so not every void seen on an X-ray is a leak problem.
- Inspection after machining. The most awkward moment is a void appearing on a machined face. That is why the regions beneath surfaces to be machined get separate attention in design.
The acceptance criterion: "no porosity" is not realistic
Porosity is never driven to zero in high pressure die casting; the aim is to keep it in the right place and within an acceptable size. The right question is not "is there porosity" but "is it within the limit in the load bearing section, on the sealing face or in the region to be machined".
Marking the critical regions on the drawing and agreeing the acceptance criterion up front therefore removes most of the arguments that would otherwise come later. On the quality control and metrology side we define these criteria part by part.
A common mistake
Treating porosity as purely a process issue and trying to solve it with machine parameters alone. A hot spot rooted in the design never disappears through parameter tuning; past a point, the process window narrows and production becomes fragile. The order is: design first, then the die, and the process last.