Pressure die casting is the quickest and most cost-effective way to make high volumes of parts, normally in nonferrous metals such as aluminium, zinc or magnesium. Die cast parts are consistently dimensionally accurate, have excellent surface finishes which in turn can significantly reduce or eliminate post processing and they have very little raw material waste.

This method is used to make items such as engine blocks, heat sinks, fittings, brackets and many other common industrial and commercial products.

However, manufacturers who are developing a product and are considering using pressure die castings should be aware that the process can result in parts being produced that exhibit a problem known as ‘porosity’, which is/are small voids, holes or pockets of air within the metal .  Porosity in die casting is mostly unavoidable however, it is controllable and does not mean that the parts exhibiting this problem need to be scrapped.

Porosity in castings is formed during the casting process, which may occur as the material changes state from a liquid to a solid. While porosity is a characteristic of cast aluminium as well as many other ferrous and non-ferrous materials, it is often classified as a defect and as a consequence  replacement parts are often needed to be cast, machined, cleaned, and then tested in the hope that new die cast components do not exhibit porosity. However, thanks to vacuum impregnation, expensive but porous castings do not need to be scrapped and some products can even be enhanced through this metal impregnation process.

To mitigate the effects of porosity in castings it is useful to understand what causes it and how to manage porosity for superior results. Porosity has two main causes: solidification shrinkage and gas porosity.

Gas Porosity

Gas porosity occurs because the molten metal can hold a large amount of dissolved gas, unlike the solid form, and as a result of this, gas bubbles form as the material cools. Turbulence can also introduce gasses during the casting process. This occurs when the molten metal is poured too quickly into the mould.

In the specific case of aluminium, hydrogen may fall out of suspension and fill voids with hydrogen gas.

Solidification Shrinkage

The pores that occurs due to solidification shrinkage are normally larger and irregular in size. After molten metal is injected into a mold cavity it begins to solidify when it contacts the relatively cooler walls of the tool. This solidification causes the metal to shrink, however the rate at which this happens will vary depending on the type of mold tool and alloy being used. The semi-liquified portion of metal further away from the tool wall is called slush, and it’s this area where pores are more likely to form.

To combat any porosity resulting from the casting process, it is now common for impregnation processes to be incorporated directly into the casting production schedules to ensure quality, rather than to be used only to recover components that are tested and show signs of porosity.

Sealing Pores with Vacuum Impregnation – The Process:

  1. The part is placed in a vacuum autoclave and vacuum is used to remove any trapped air in micro pores.
  2. A methacrylate resin is then introduced into the chamber, the vacuum is released and therefore the resin is forced into the micropores with positive air pressure.
  3. Resin is drained and washed off the surface of the item so that no excess resin is left on the surface.
  4. The item is placed in a water bath at a temperature around 95 degrees C and the thermal reaction ensures that the resin cures and seals the micro porosity.
  5. This is considered as a one-time, permanent surface treatment.


If you have 1 or 1,000 pieces, new or used, large or small, for recovery or sub surface quality enhancement, ISL can help. Our purpose-built facility allows us to turn ordinary castings into high-pressure, sub-surface sealed, fit for purpose parts, for use in any number of applications.

We provide vacuum impregnation of castings to a vast array of industries including aerospace, automotive, oil and gas, and marine.

Do you suffer from leaking or porous castings? Do you want to recover your castings and seal them for life? … Call the casting impregnation experts now.