In the die casting shop, every engineer and production manager shares the same goal: to produce castings without defects. Yet, an unseen “enemy” always lurks in the shadows—porosity.
Die casting porosity is not some theoretical concept but a real-world challenge that haunts daily production.
The truth is, porosity is quietly eating into your profits, consuming your production capacity, and testing your credibility. Behind every porosity lies wasted costs and customer doubts.
We all know there’s no magic bullet to eliminate porosity. But getting to the true root of the problem is where real improvement begins. Let’s cut through the complexity together, understand what’s really causing the porosity in die casting.
What is Die Casting Porosity?
Die casting porosity is tiny pores, holes, or voids within or on the surface of die-cast parts.
It is one of the most common defects in die casting.
Essentially, these porosities in castings are“pockets” containing gas, vacuum, or both.
They are formed when molten metal fills the mold at high speed and pressure and solidifies.
This image shows typical porosity identified in our daily production quality inspection.
Die Casting Shrinkage Porosity
The Two Types of Die Casting Porosity
Die casting porosity is primarily categorized into two main types: gas porosity and shrinkage porosity. Often, both types can be present in the same casting.
This image shows the difference between gas and shrinkage porosity. The easiest way to tell them apart is by looking at the inside of the porosity: smooth for gas, rough for shrinkage.
Two Types of Die Casting Porosity
Gas porosity occurs when gas is entrapped in the molten metal, forming voids within the casting.
Shrinkage porosity results from volumetric contraction of the molten metal during solidification, coupled with insufficient compensation (feeding).
These two types of porosity differ fundamentally in shape, location, causes, and solution.
The table below clearly illustrates the key distinctions between these two types of porosity.
| Gas Porosity | Shrinkage Porosity | |
| Shape | Round, elliptical, smooth | Irregular, dendritic, jagged, rough |
| Location | Last filling areas, poorly vented areas, thick sections | Last solidifying hot spots, thick sections, junction points (e.g., ribs & walls) |
| Internal Surface | Smooth, shiny | Rough, dendritic structure |
| Color | Often silvery bright or dark gray | Dull, oxidized |
| Cause | Air/gas entrapment in the die cavity | Metal solidification shrinkage, inadequate feeding |
| Primary Solution Direction | Reduce turbulence, improve venting, optimize the slow shot phase | Increase intensification pressure, control solidification sequence, optimize cooling |
In practical production, a very useful and quick identification method is:
Observe the internal surface of the porosity: if the hole’s inner surface is smooth, it is most likely gas porosity; if the inner surface is rough and uneven, with a visible dendritic structure, then it is shrinkage porosity.
What are the Distinct Causes of Die Casting Porosity?
The key difference lies in the root causes of the two types of porosity: gas porosity is caused by trapped gas, while shrinkage porosity results from the physical contraction of metal during solidification. Here is a breakdown of the primary causes for each type:
Main Causes of Gas Porosity
This occurs when gas is physically trapped and encapsulated by the molten metal. The gas cannot escape before the metal solidifies.
Turbulent Filling
If the molten metal is injected into the die cavity too quickly, it splashes and flows like a wave, folding air into the metal stream instead of pushing it out ahead. This is the most common cause.
Inadequate Venting
The die casting mold needs escape paths for the displaced air. If the vents are too small, clogged, or poorly placed, the air cannot exit and gets trapped.
Excessive Release Agent
If too much water-based release agent is sprayed onto the die, the water can flash into steam upon contact with the hot mold. This steam becomes trapped in the metal, creating porosity.
Overflows
Overflows are designed to trap contaminants and cold metal, but they also trap air. If they are not properly designed or vented, they can contribute to gas entrapment.
Shot End Air
In cold chamber die casting machines, air can be trapped in the shot sleeve in front of the plunger. If the slow shot phase is too fast, this air is pushed into the die instead of being allowed to escape up the shot sleeve.
Main Causes of Shrinkage Porosity
This occurs due to the natural physical property of metals contracting (shrinking) as they change from a liquid to a solid state. If this volumetric shrinkage is not compensated for, it creates voids.
Low Holding Pressure
The holding pressure is crucial. It must be high enough and held long enough to push more molten metal into the cavity to compensate for the shrinkage as the part solidifies. Low pressure or a cut-off that is too early is a major cause.
Uncontrolled Solidification
Ideal solidification should progress from the casting’s extremities toward the gate. However, thick sections (hot spots) cool slowly and solidify last. If surrounding thinner areas or the gate solidify first, they block the liquid metal supply, causing shrinkage porosity in the thick, isolated sections.
Gate Design
If the gate solidifies too early, it severs the supply of molten metal from the shot sleeve before the entire part has finished solidifying and shrinking. The part cannot be “fed,” leading to shrinkage porosity.
Alloy Selection
Some alloys have a wider freezing range than others. They are much more prone to shrinkage porosity than those with a narrow range.
This table summarizes the common causes of porosity and identifies whether they lead to gas porosity or shrinkage porosity.
| Cause Category | Specific Cause | Type of Porosity |
| Process | Turbulent Metal Flow | Gas |
| Process | Excessive Release Agent | Gas |
| Process | Low Intensification Pressure / Short Time | Shrinkage |
| Tooling | Inadequate Venting | Gas |
| Tooling | Premature Gate Solidification | Shrinkage |
| Tooling/Design | Uncontrolled Solidification (Hot Spots) | Shrinkage |
| Material | Alloy with Wide Freezing Range | Shrinkage |
How Does Porosity Affect the Castings?
The impact of porosity on castings is significant and almost entirely negative. It can be regarded as an internal “wound” that severely compromises the integrity of the casting.
Controlling porosity is not an academic exercise but a core production issue directly affecting your costs, reputation, and customer relationships.
In this blog, we will explore this topic in detail. Click here to read more.
Conclusion
Die casting porosity manifests as two distinct defect types: gas porosity, showing smooth, rounded voids from entrapped air; and shrinkage porosity, exhibiting irregular, crystalline structures caused by solidification issues.
Successful management requires not only identifying these defect types but also understanding their specific formation mechanisms to implement targeted solutions.
At CEX Casting, we employ advanced analysis techniques to diagnose porosity root causes and develop precise corrections through process optimization and tooling modifications.
Contact us with your project details to receive a comprehensive technical evaluation and customized solution proposal today!