Casting defects · gas

Blowholes in castings

A blowhole is a smooth-walled gas cavity, usually just under the skin. It is a gas and permeability problem, not metal shrinkage - and the fix is different.

By Versatile Equipments Pvt Ltd · equipment manufacturer & calibration provider · updated 2026-07-15

What a blowhole is

A blowhole is a smooth-walled gas cavity in the casting, usually subsurface and rounded. It forms when gas - from the mould, a core or the melt - cannot escape and is trapped as the metal solidifies. The smooth, often shiny wall marks it as a gas defect, with causes quite different from the rough cavities of shrinkage.

Blowhole or shrinkage?

Both are internal cavities, so they get confused. The tell is the wall: a blowhole is smooth and rounded because gas pushed the metal aside; a shrinkage cavity is rough, jagged and dendritic because metal pulled away as it fed. Blowholes sit near the cope or over cores; shrinkage sits at thermal centres under feeders. The fixes point in opposite directions - venting and dryness for gas, feeding and gating for shrinkage.

Root causes

Mould gas and low permeability. Wet or fine sand, over-ramming and poor venting hold steam and gas against the metal. The permeability and moisture numbers have to be trustworthy.
Core gas. Binders break down and release gas; without core venting to open air, it enters the casting.
Moisture on tools. Damp or rusty chills, cold ladles and wet tools flash to steam on contact with metal.
Melt gas. Hydrogen and nitrogen from damp charge, rust or turbulence come out of solution on freezing.

Prevention checklist

Hold moisture and permeability in band, with calibrated instruments so the numbers are real.
Vent the mould and every core; give gas a shorter path out than into the metal.
Dry and preheat ladles, tools and chills; keep charge and returns dry.
Avoid over-ramming; control pouring temperature and speed to limit gas pickup.
Record where blowholes land, by part and by core, so you fix the source rather than the symptom.

Frequently asked questions

How do I tell a blowhole from a shrinkage cavity?

Look at the wall. A blowhole is smooth-walled and rounded because trapped gas pushed the metal aside; a shrinkage cavity is rough, jagged and dendritic because the metal pulled away as it solidified. Blowholes sit near the cope or over cores; shrinkage sits at thermal hot spots under feeders.

Are blowholes caused by the sand or the metal?

Both are possible. Mould and core gas with low permeability is the most common source, but dissolved gas in the melt from damp charge, rust or turbulence also causes them. Whether the cavity clusters near cores or spreads through the section helps point to the source.

Log defects on the floor, free

CaRe 101 is a free shop-floor app from Versatile: your team photographs the casting, marks the defect and taps its name - no typing, works offline. The rejection register builds itself, so you can see which defects hit which parts and act on the pattern.

Open CaRe 101 - free See how it works