Explainer: Shrinkage vs Gas Porosity

What defines shrinkage porosity

Shrinkage porosity happens when the molten metal cools and contracts. As the liquid metal turns to solid, it takes up less volume. If the solidification process does not feed fresh liquid metal into the solidifying zones, voids form. These voids are often irregular in shape. They frequently appear in areas where the part is thick or where heat concentrates.

In a typical aluminum or zinc die casting, the metal flows into the die and begins to freeze from the walls inward. The last part to solidify is usually the center of the thickest section. This center cools slowly. The solidified metal around it shrinks as it cools further. If the liquid metal cannot flow into this shrinking area, a cavity opens up. The result is shrinkage.

This defect is not caused by air. It is caused by the physical properties of the metal itself. The metal wants to be smaller when it is solid than it was when it was liquid. The process must manage this change.

What defines gas porosity

Gas porosity is different. It occurs when gas gets trapped in the molten metal before the part solidifies. The gas can come from a few sources. Air can be pulled into the melt during pouring. This is called air entrapment. The gas can also come from the metal itself. Aluminum and zinc metals can dissolve hydrogen while being heated. When the metal cools, the solubility of the gas drops. The gas comes out of solution and forms bubbles.

In a die casting machine, the melt is usually heated in a pot and then fed into the die by pressure. If the melt is too hot, it can boil and release gas. If the pouring speed is too high, air can be entrained. If the die is not clean, moisture can react with the hot metal and create steam. All of these paths lead to gas bubbles.

Unlike shrinkage, gas bubbles often form on the surface of the part or just below the surface. They can be small and round. They may also be larger and clustered. The shape of the bubble is a strong clue to the cause.

Key differences between the two defects

The main difference between shrinkage and gas porosity is the origin. Shrinkage is a thermodynamic issue. It relates to how the metal changes volume as it cools. Gas porosity is a fluid dynamics and chemistry issue. It relates to how gas behaves in the melt and how it interacts with the die.

The visual appearance differs as well. Shrinkage voids often look like irregular cavities. They may be elongated or clustered in thick sections. They can have a “swirled” appearance because the solid metal pulled inward as it shrank. Gas porosity bubbles usually look like small spheres. They may appear as a string of bubbles or as a patch of small holes. They are often found in thinner sections or on the surface.

The location on the part is another clue. Shrinkage tends to form in the thickest parts of the casting. This is where the metal solidifies last. Gas porosity can appear anywhere, but it often shows up in areas where the metal surface is exposed to air or where the die was not properly prepared.

How to identify the cause

Identifying the root cause is the first step to fixing the problem. A simple visual inspection can often point to the likely cause. If the voids are in the thick center of a bracket and look irregular, shrinkage is the likely suspect. If the holes are small, round, and scattered across a thin wall, gas porosity is more likely.

However, visual inspection is not always enough. Sometimes both defects occur in the same part. A thick section may have shrinkage, while a thin section has gas bubbles. In these cases, the supplier needs to run a more detailed analysis.

A common method is sectioning. The supplier cuts the casting into thin slices. These slices are then polished and examined. This allows the inspector to see the internal structure. The shape of the voids helps determine if they are gas bubbles or shrinkage cavities. If the voids are filled with gas, they will appear as empty spaces with smooth walls. If they are shrinkage, they may show evidence of the metal pulling away from the die.

Another useful check is the mold or die condition. If the die is worn, it may not hold pressure well. This can lead to shrinkage because the melt cannot be forced into the last solidifying areas. If the die has moisture or oil on it, it can create gas porosity. Checking the die condition is a practical way to narrow down the cause.

How each defect affects sourcing decisions

When you source die castings, you need to know which defect you are dealing with. Shrinkage and gas porosity require different solutions. Buying a supplier who only addresses one issue is a risk.

If shrinkage is the main problem, you should look for suppliers who manage solidification well. This involves controlling the cooling rate and ensuring the die is properly vented. The supplier should also be able to adjust the die design if the part has thick sections that are prone to shrinkage. A good supplier will work with you on the part design to reduce hot spots.

If gas porosity is the main problem, the focus shifts to melt preparation. The supplier needs to manage the melt temperature carefully. Too high a temperature increases gas release. Too low a temperature can cause other issues like incomplete filling. The supplier should also use a degassing process to remove hydrogen from the melt. This is a standard practice in many aluminum and zinc die casting operations.

When reviewing supplier quotes, look for specific commitments. Does the quote mention degassing? Does it specify a maximum melt temperature? Does it include a sectioning plan for critical parts? These details show that the supplier understands the difference between the two defects. They also show that they have a plan to prevent them.

A supplier who treats all porosity as the same problem is likely to miss the root cause. They may apply degassing when the issue is shrinkage. Or they may try to change the die design when the issue is gas. This wastes time and money. A knowledgeable supplier will diagnose the issue first and then apply the correct fix.

Worked example: A bracket with mixed porosity

Imagine a company orders a set of aluminum brackets for an electrical enclosure. The brackets have a central boss and two thin flanges. After the first production run, quality inspection finds two types of defects.

In the central boss, there are several large, irregular voids. They are located in the middle of the boss, which is the thickest part of the part. The inspector identifies these as shrinkage. The metal in the center solidified last and shrank, creating cavities.

In the thin flanges, there are many small, round bubbles. They are scattered across the surface. The inspector identifies these as gas porosity. The bubbles are too small and round to be shrinkage. They are likely caused by air being pulled into the melt during pouring.

The supplier reviews the data. They find that the melt temperature was set a bit high for the first run. This increased gas release in the flanges. They also find that the boss is too thick for the current die design. The heat in the center takes too long to cool, leading to shrinkage.

The supplier makes two changes. They lower the melt temperature to the recommended range for the alloy. They also modify the die to reduce the thickness of the boss slightly and improve the cooling channels. They also add a degassing step to the process to remove any remaining hydrogen.

The next run shows a significant improvement. The shrinkage in the boss is reduced because the metal cools more evenly. The gas porosity in the flanges is reduced because there is less dissolved gas in the melt. The part meets the quality spec.

Common mistakes in defect management

One common mistake is assuming that all porosity is the same. If a buyer sees bubbles in a part and assumes it is gas porosity, they may request a degassing solution. If the actual problem is shrinkage, the degassing will not help. The voids will still be there.

Another mistake is ignoring the part design. A part with very thin walls and very thick bosses is difficult to cast cleanly. The design itself creates the conditions for defects. A buyer should review the part design with the supplier before production starts. This is a low-cost step that can prevent major issues later.

A third mistake is not checking the die condition. A die that is worn or dirty can cause both types of porosity. It can reduce the pressure needed to fill the part, leading to shrinkage. It can also introduce moisture or contaminants, leading to gas bubbles. Regular die maintenance is a key part of quality control.

A practical comparison table

Feature	Shrinkage Porosity	Gas Porosity
Origin	Metal contraction during cooling	Trapped air or dissolved gas
Shape	Irregular, elongated, or swirled	Small, round, or spherical
Location	Thick sections, hot spots, centers	Thin sections, surfaces, anywhere
Root Cause	Insufficient feed, slow cooling	High melt temp, air entrapment, moisture
Primary Fix	Die design, cooling control	Degassing, temp control, clean die
Inspection Clue	Internal cavity, solid walls	Empty bubble, smooth walls