Porosity in sand casting is caused by either trapped gas that doesn’t escape the mold before the metal solidifies, or by shrinkage as the metal cools and contracts faster than it can be fed with additional molten metal. Both show up as voids inside or near the surface of a finished casting, and both are largely preventable through mold design, sand control, and metal handling rather than fixed after the fact.
If you’re specifying or sourcing aluminum castings and want to understand what causes porosity and how a foundry prevents it, this guide covers the two main types, their root causes, and the process controls that keep them out of finished parts. It’s written for engineers and buyers evaluating casting suppliers.
What is gas porosity and what causes it?
Gas porosity forms when gas becomes trapped in the metal as it solidifies, leaving small round or teardrop-shaped voids.
- Moisture in the sand mold turns to steam when hot metal is poured, and that gas needs a path to escape
- Hydrogen can dissolve into molten aluminum and come out of solution as the metal cools, forming gas pockets if it can’t escape in time
- Poorly vented molds trap gas that would otherwise exit through the mold’s permeability or dedicated vents
- Turbulent pouring can entrain air into the metal stream, introducing gas directly into the casting
What is shrinkage porosity and what causes it?
Shrinkage porosity forms as the metal cools and contracts, creating a void where there wasn’t enough molten metal left to fill the gap.
- All metals shrink as they solidify, and if the last areas to cool aren’t fed with additional metal, a void forms in their place
- Poor riser placement or sizing means the riser runs out of molten metal before the casting finishes solidifying
- Uneven wall thickness creates hot spots that solidify last and are prone to shrinkage if not properly fed
- Uncontrolled cooling rates can create an unpredictable solidification pattern that’s harder to feed correctly
How can you tell gas porosity apart from shrinkage porosity?
The distinction matters because the fix for each is different.
- Gas porosity typically appears as small, round or teardrop-shaped voids, often clustered together
- Shrinkage porosity tends to look more irregular or jagged, and often appears near thicker sections or where risers were undersized
- X-ray or other nondestructive testing can distinguish between the two when the cause isn’t visually obvious
- A supplier addressing gas porosity with better riser design, or shrinkage porosity with better venting, won’t solve the actual problem
How is porosity prevented before a tool is even built?
Most porosity prevention happens at the design and simulation stage, before any metal is poured.
- Solidification modeling identifies areas likely to develop shrinkage porosity based on the part’s geometry and wall thickness
- Gating and riser systems are engineered to feed the last areas of the casting to solidify
- A certified simulation engineer reviewing the results can catch risk areas that automated software output alone might miss
- Design adjustments, such as more uniform wall thickness, can reduce porosity risk before tooling is even cut
How is porosity controlled during production?
Even a well-designed tool needs consistent process control to actually deliver porosity-free castings run after run.
- Automated molding machines control sand compaction and moisture consistency more reliably than manual methods
- Robotic ladling systems pour metal at controlled rates and temperatures, reducing turbulence that can entrain gas
- In-house core making allows tighter control over core permeability and venting than outsourced cores
- Degassing molten aluminum before pouring reduces dissolved hydrogen that would otherwise form gas porosity
Frequently asked questions
Can porosity be repaired after casting?
Some surface porosity can be treated with impregnation to seal minor voids for pressure-tight applications, but internal porosity generally can’t be repaired and requires the part to be scrapped or reworked at the process level.
Does porosity always weaken a casting?
It depends on the location, size, and application. Porosity in a non-critical area may not affect performance, but porosity in a load-bearing section or a pressure-sealing surface can cause failure or leaks.
Is porosity more common in aluminum casting than other metals?
Aluminum is more prone to hydrogen-related gas porosity than some other metals because hydrogen is significantly more soluble in molten aluminum than in solid aluminum, so proper degassing matters more.
How do you know if a supplier’s porosity prevention actually works?
Ask for their process for solidification modeling, degassing, and inspection, and ask whether they can show a track record of porosity-free castings on parts similar to yours in complexity.
Get a casting reviewed for porosity risk
Porosity in sand casting comes from trapped gas or uncontrolled shrinkage during solidification, and both are largely preventable through mold design and process control. LeClaire Manufacturing uses solidification modeling on every new tool, along with automated molding and ladling equipment, to deliver consistent, porosity-free castings. Request a quote to have your part’s design reviewed for porosity risk before tooling is built.