We all know about the pores on our face or the porous nature of things like wood, which leave the material open for invaders such as water and bacteria — that’s why many cooks will recommend using plastic cutting boards instead of wood ones for cutting raw chicken.
Porosity is also a big concern in die casting because it can mean that there is some kind of defect in the material you’re working with or have just created. The good news is that porosity doesn’t always mean a casting is defective and that changes need to be made. A full inspection may show that it meets your stability and structural integrity requirements.
What is Porosity in Die Casting?
This porosity is found when there are small voids, holes or pockets of air that is found within metal.
Typically, this porosity occurs when air is trapped into the metal by the die casting machinery, often leaving gaps at the top of the die or filling a mold too slowly and having some solidification occur too soon. It can also occur when the air used to force molten metal into the mold isn’t completely forced out or able to escape via vents and overflows.
Causes of Porosity in Die Casting
- The design of the mold and cast parts
- The purity of the metal or alloy being used
- Pressure and shot speed of the machines
- Shrinkage of the material wall thickness
- Too much lubricant in the die
- Sharp corners in the mold
- Low metal temperatures
- Air trapped in the metal
The most common way to check for porosity is an X-ray of the material, using computerized tomography or by cutting and polishing a section and then analyzing it under a microscope.
Porosity varies in severity. Sometimes it is acceptable, but most often it’s best to limit it as much as possible.
The most direct way to control for porosity is to work with all of your partners to create an efficient process for die casting while ensuring the material you’re working with is of a high quality. Process monitoring should focus on equipment maintenance and stability, which can help guarantee an even and adequate amount of pressure throughout the casting.
Gas porosity, which is the formation of air bubbles inside of a casting as it cools, can be avoided by melting the material in a vacuum or in an environment of low-solubility gases, including argon. This porosity occurs because liquids can naturally hold in dissolved gas. Sometimes this can be addressed by exposing the melted material to another gas — the two gases react and pull each other out of the liquid.
If oxide formation is the cause of your porosity, then your materials will also benefit from being properly degassed after melting or filtered before using the metal for making the casting.
Hot tears and hot spots are metallurgical defects that occur because of problems during cooling. If this occurs, you’ll first need to make sure your casting is properly being cooled in that local area of the part. If hot spots persist, you will need to adjust the cooling practices by either more die spray or adding more localized cooling channels to that problem area.
Porosity essentially describes holes and voids found in a casting, though you may hear more specific terms when it comes to voids based on different shapes, locations or frequencies. The defects and causes (listed above) may or may not have an impact on the overall efficacy of your product or part.
However, we always set a goal to limit porosity defects, or remove them when possible. That means analyzing defects for various causes. Die casting can see porosity due to the design, materials being used for that design or the manufacturing process — sometimes a combination of three.
Our expertise in die casting means we’re able to better understand your materials, designs and parts as well as the casting process factors that are most likely to cause different types of defects. This allows us to prevent many defects before they occur. We can also assist with redesigning elements to address porosity-prone areas and shift them to locations that will not impact structural integrity.
The review process of a quality manufacturer and die casting service provider will typically catch defects, though you may also discover some during the machining process. It’s important to note the location, frequency and type of defect in order to determine the best path to resolving the issue.
Your partners should work with you to determine what porosity is acceptable. When you notice a problem, provide your partner with as much information as possible and share the casting whenever possible to assist in defect resolution.
The American Society for Testing and Materials (ASTM) lists standards for porosity in castings if you need assistance in developing your own tolerance limits. Standards can be found on the ASTM website, though some have also been posted for free on Wikipedia — please remember that Wikipedia is not always 100% accurate so it’s best to verify these tolerances elsewhere.
If you’re concerned about the porosity of your casting, we recommend you contact us directly to learn more.
Pair Casting with Machining
Moving the casting through multiple partners before machining can mean that the cost of the machining plus the shipping times are all gone. You can also save on testing costs because a machined die can use liquid penetrant inspection, which is cheaper than x-ray testing, to catch porosity defects on the surface of your casting.
If we’re able to test the porosity and find issues on a sample from your first castings, then we’re able to determine the cause and re-gate to address it. It also gives you more time to determine what level of porosity is acceptable, what changes you can accept for the part design, and make your choice for options to improve overall castability.