Die casting is an excellent method to fabricate high volumes of aluminium, zinc, or magnesium parts. Compared with other production methods, die-cast parts have excellent surface finishes; maintain consistent characteristics and tolerances, and waste little raw material.
A variety of industrial and commercial products are made using pressure die casting, including engine blocks and motor housings, heat sinks, fittings, brackets, and a host of other components.
When specifying die casting for a product the issue of porosity must be addressed. Despite the porosity defects being unavoidable, they can nevertheless be controlled. The purpose of this article is to explain what causes porosity and how to manage it using best design practices for superior results.
What is Porosity in Die Casting?
Porosity refers to the presence of small holes, voids or pockets of air in metals. Porosity is usually caused by air trapped between the die and metal, often leaving gaps at the top of the piece. It can also be caused by filling the mold too leading to premature solidification. Additionally, porosity may occur when air injected to force the molten metal into the mold isn't forced out or is unable to escape through vents.
Causes of Die Casting Porosity
- Designs of the moulds and cast parts
- Metal purity or alloy composition
- Machine pressure and shot speed
- Shrinkage of the material wall thickness
- Too much lubricant within the die
- Moulds with sharp corners
- Low temperatures for metals
- Metal with trapped air
A common method for checking for porosity is to take an X-ray of the material, use computerised tomography, or cut and polish a sample workpiece section for analysis under a microscope.
Methods For Preventing Die Casting Porosity
Different kinds of porous materials have different degrees of severity. In some cases, it is acceptable, but it’s best practice to minimise the occurrence. In order to control porosity, work with all of your partners to devise a process for die-casting that’s both efficient and high-quality. It is important to monitor the casting process to ensure the casting has a uniform pressure throughout.
In order to prevent gas porosity, often referred to as bubbles, forming inside of castings as they cool, the material can be melted in a vacuum or in low-solubility gas atmospheres. Gasses such as argon are frequently employed. Due to the natural capacity of liquids to hold dissolved gases, these liquids are naturally porous. The melt can sometimes be removed by exposing it to another gas, which reacts and pulls the excess out of the liquid.
Degassing materials after melting and filtering them before using the metal for casting can deliver benefits if oxide formation is the cause of the porosity.
Metallurgical defects such as hot tears and hot spots occur as a result of problems with cooling. To avoid this, the local area of the casting must be properly cooled. In cases of persistent hot spots, cooling practices will need to be adjusted. Either more die spray or more localized cooling channels will be needed.
Addressing the Casting Defects
Hole, voids, and porosity are terms used to describe holes and voids which occur in casting, though the specific terms will depend on their nature, frequency, shape, and location. There may or may not be an impact on your product’s overall efficacy from the defects and causes (listed above).
Nonetheless, the goal is to reduce or eliminate porosity defects whenever possible. Defects must be analysed for their various causes. The die casting process, the design, or the material used can result in porosity – sometimes all three.
We have extensive experience in die casting, which gives us a greater understanding of materials, designs, and parts as well as factors in the casting process that lead to different types of defects. By doing so, we can prevent defects from occurring in every project. Additionally, we are able to help redesign elements to address areas prone to porosity and reposition them in a way that will not affect structural integrity.
Die casting service providers and quality manufacturers will usually spot defects in their design review process, though defects may also be discovered during machining. It's crucial to determine the best way to resolve problems based on the type, location, and frequency of defects.
It is important that you work with your partners to determine what porosity is acceptable. Whenever possible, provide your partner with all the information they need and share the casting to assist in defect resolution.
When determining tolerance limits, the American Society for Testing and Materials (ASTM) lists standards for porosity in castings. As well as on the ASTM website, some standards are posted for free on Wikipedia - but please remember that Wikipedia is seldom 100% accurate; it's best to verify these tolerances elsewhere.
How does machining affect porosity?
In die-cast parts, the skin is the most thermally stable part of the part. With the first .5 mm or so of solidification, there is little or no porosity. During machining processes like tapping and threading holes, pores may be opened in the deep sections of castings. When castings are used for hydraulic cylinders or manifolds, they must be able to handle air or liquid pressures, so pores must be sealed after machining.
Are you interested in discussing a design you’re working on? Contact us for more information. Please contact our team if you require a bracket design not listed in this guide. We can design any custom bracket design that fits within our tooling constraints.
Using vacuum impregnation to seal pores
To seal die castings effectively, vacuum impregnation is often applied to the surface. There are generally three steps to this process:
- In order to remove trapped air in micro pores, the part is placed in a chamber and vacuumed.
- With positive air pressure, a polymer resin, such as liquid polymer resin, is pumped into the micro pores and then sealed.
- A cast part is fully cured after it has been impregnated and removed from the chamber. Surface treatment for this type of material is considered one-time and permanent.
Omnidex Metal Casting Services
Our company assists our customers in dealing with technical issues, metal casting processes, metallurgy, and supply chain optimization. In many cases, we are entrusted with the responsibility of quality control, budget control, and even stock holding of cast components.
Whether your castings are produced in one of our foundries or in one of our offshore partners, we support your project from the initial design phase through testing and quality assurance at our facilities in China and/or Vietnam.
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