Gas porosity is the pore you don't see until you machine into it. The casting comes off the machine sound, the wall gets faced or drilled, and a clean round hole opens up where there should be solid metal. Unlike shrinkage, it isn't about how the casting froze — it's about what got trapped inside before it did.
Three ways gas gets in
- Entrained air. High-pressure die casting fills the cavity in a few milliseconds. That speed is what makes the process, but it also whips air into the metal front. Air that can't escape ahead of the metal ends up folded into it.
- Dissolved hydrogen. Aluminium holds a great deal of hydrogen when molten and almost none when solid. As the casting freezes, that hydrogen comes out of solution with nowhere to go and forms fine, round pores through the section.
- Die lubricant and steam. Water-based die lube that hasn't fully flashed off, or a cavity that's still wet, turns to steam on the next shot and is driven into the casting.
How to tell it from shrinkage
Shape is the tell. Gas pores are round and smooth — a bubble freezes with a clean spherical wall. Shrinkage voids are jagged and branching. Gas porosity tends to be distributed or to sit just under the surface; shrinkage clusters in the last-freezing mass. Confuse the two and you'll chase the wrong fix for weeks.
Why venting alone runs out of road
The classic answer to trapped air is venting — give it a path out ahead of the metal. Vents help, but they have a ceiling. At production fill speeds the metal front reaches the vents before all the air is gone, and once the gate seals, whatever is still inside is staying inside. You can only vent as fast as the air can travel, and the shot is faster.
That ceiling is why vacuum assist exists. Instead of waiting for air to find its own way out, you evacuate the cavity before and during fill so there is far less air to entrain in the first place. A well-sealed two-stage vacuum system pulls the cavity down hard and holds it through the critical part of the shot — reaching a vacuum level that simple venting never will.
What to check on a gas reject
Round pores just under the surface point at fill and vacuum; pores everywhere point at melt hydrogen. Walk the chain: melt degassing and holding practice, die-lube type and spray-then-blow timing, vent area, and above all the integrity of the vacuum circuit — a leaking seal or a tired valve quietly turns a vacuum die back into a vented one.
From defect to fix
Venting has a ceiling; evacuating the cavity does not. A sealed two-stage vacuum removes the air before the shot can trap it. Explore Vacuum technology