Explainer: How Gas Porosity Differs from Shrinkage Porosity

What causes gas porosity in die cast parts

Gas porosity in die casting starts when gas is present in the metal or the casting cavity. In aluminum die casting, this gas is usually hydrogen, which comes from the melt or from moisture in the die. It can also be air that gets trapped during the fill cycle.

The gas does not disappear by itself. It stays in the molten metal until the part solidifies. At that point, the gas pockets are locked in place. The result is a pore that holds no metal and no structure.

In practice, gas porosity shows up in parts that look clean on the outside but fail in service. The pores may sit just under the skin, where they are hard to see but easy to find with X-ray. They may also break the surface during machining or cleaning.

The main causes include:

• Hydrogen in the melt from flux, contamination, or poor melt handling.
• Air trapped in the die cavity from slow filling or poor venting.
• Inadequate die temperature control, which changes how gas behaves as it solidifies.
• Poor mold venting, which prevents gas from escaping before the cavity closes.

What causes shrinkage porosity in die cast parts

Shrinkage porosity is a different problem. It has nothing to do with gas in the metal. It is caused by the metal changing volume as it cools.

When molten metal solidifies, it contracts. If the cavity or the gating system does not feed it enough metal, a void forms. That void is a shrinkage pore. It is usually round and smooth, because the metal simply pulled away from the solidified structure.

Shrinkage porosity is more common in thick sections and in parts where heat moves slowly. The heat stays in the thick areas, so the metal there solidifies later. The rest of the part is already set, so the late solidifying metal has nowhere to go.

The main causes include:

• Gating that is too small or poorly placed.
• Insufficient feed metal at the end of the solidification cycle.
• Hot spots in the die that delay solidification in one area.
• Part design with uneven thickness.

How to tell gas and shrinkage porosity apart

The two defects look similar on a basic X-ray, but they have distinct physical signatures.

Gas porosity pores are usually irregular. They may be elongated, clustered, or irregularly shaped. They tend to form in the skin of the part, where the metal first touches the die. Because the gas is trapped at the surface, these pores are often shallow.

Shrinkage porosity pores are usually round and smooth. They form in the interior of thick sections. Because the metal pulls inward as it cools, the pore is a clean void with no gas residue.

The location is a strong clue. Gas porosity often appears along parting lines, in thin sections near the surface, or in areas where the metal fills quickly. Shrinkage porosity appears in thick bosses, ribs, and areas where the part design traps heat.

How each defect affects sourcing decisions

When a buyer receives a sample or a shipment, the type of porosity tells a lot about the supplier’s process control.

Gas porosity points to a melt chemistry or die management issue. If a supplier has a repeat gas porosity problem, the buyer should ask about their melt chemistry controls, hydrogen levels, and venting design. The fix is usually in the melt handling and die preparation.

Shrinkage porosity points to a gating and feeding issue. If a supplier has a repeat shrinkage porosity problem, the buyer should ask about the gating design, feed size, and hot spot control. The fix is usually in the die design or the casting parameters.

This distinction matters because the corrective actions are different. A supplier who fixes gas porosity by adding more venting will not fix shrinkage porosity. A supplier who fixes shrinkage porosity by increasing feed size will not fix gas porosity.

Buyers who can identify the defect type can direct the supplier to the right root cause. They can also avoid paying for unnecessary rework or incorrect fixes.

A worked example in plain words

Imagine a buyer orders a batch of aluminum brackets for a pump assembly. The parts have a thick boss and a thin rib. During incoming inspection, a few parts show small pores near the surface of the rib.

The buyer sends those parts to the lab. X-ray shows the pores are shallow and irregular. The lab reports them as gas porosity.

The buyer calls the supplier. The supplier checks the melt chemistry and finds hydrogen levels above their internal limit. They also check the venting and find one vent was clogged with oxide. They clean the vent and adjust the melt treatment. The next batch passes.

Now imagine the same buyer orders a different part, a heavy bracket with a thick base. During inspection, the parts show round pores in the thick base area. X-ray confirms they are internal and smooth. The lab reports them as shrinkage porosity.

The buyer calls the supplier. The supplier checks the gating and finds the feed is too small. They also find a hot spot in the die near the base. They enlarge the feed and adjust the die temperature. The next batch passes.

In both cases, the buyer used the defect type to direct the supplier to the correct fix. Without that distinction, the supplier might have spent time on the wrong problem.

A quick comparison of gas and shrinkage porosity

Feature	Gas Porosity	Shrinkage Porosity
Cause	Gas in melt or cavity	Metal contraction during solidification
Shape	Irregular, elongated, clustered	Round, smooth, isolated
Location	Surface, thin sections, parting lines	Thick sections, internal bosses
Common in	Aluminum, zinc die casting	Aluminum, magnesium, zinc die casting
Fix focus	Melt chemistry, venting, die temp	Gating, feed size, hot spot control

How to reduce both types of porosity

Controlling gas porosity requires attention to the melt and the die. Melt chemistry must be managed to keep hydrogen low. The melt should be treated with the right deoxidizer, and the handling should avoid contamination. The die must be clean and well vented. The venting system must allow gas to escape before the cavity closes.

Controlling shrinkage porosity requires attention to the part design and the gating. The gating system must be sized to feed the part until the last drop solidifies. The feed should be placed where the metal needs it most. The die must be managed to avoid hot spots that delay solidification.

Both defects can be reduced with good process control. But the controls are different. A supplier that is good at one type of porosity control may not be good at the other. Buyers should ask specifically about the supplier’s experience with the defect type they are seeing.

When to escalate a porosity problem

If a supplier cannot explain the defect type, or if the defect keeps coming back after a fix, the buyer should escalate.

Escalation may include:

• Requesting a full X-ray report of a sample lot.
• Asking for the supplier’s melt chemistry records.
• Reviewing the gating design and venting layout.
• Conducting a joint root cause analysis with the supplier.
• Considering a second source if the supplier cannot fix the issue.

The goal is to get to the root cause, not just to reject the bad parts. Rejection without understanding will not stop the problem.

How to use this explainer in your sourcing process

Use this explainer when you are evaluating a supplier or troubleshooting a quality issue. First, identify the defect type. Second, ask the supplier for their process records related to that defect. Third, verify the fix with a follow-up inspection.

If you are working with a supplier who is new to your part, ask them to show you their porosity control plan. Ask how they manage melt chemistry for gas porosity. Ask how they manage gating for shrinkage porosity. A supplier who can answer those questions clearly is more likely to deliver a part that passes inspection.