During the casting process, small voids or holes can form in materials when changing from a liquid form to a solid state, this is called micro-porosity. Despite these small holes being hard to see, they have the ability to cause detrimental effects to the quality of the finished product.
Porosity can cause issues when materials are used in high pressure situations and environments, allowing liquids or gas to seep through. A process called ‘porosity sealing (or Impregnation)’ can help fill these small voids and reduce the permeability of the material.
This article will discuss the different ways in which porosity can form and the process of porosity sealing.~
During the casting process, there are many ways in which porosity can occur (usually unplanned); gas-related porosity, shrinkage-related porosity, pouring porosity and metallurgical porosity.
- Gas-related porosity occurs as the casting cools and small air bubbles form as dissolved gas within the liquid state is expelled as the materials turns to a solid state. These gas bubbles can either form on the surface of castings or within the metal which reduces the strength in that area.
- Shrinkage-related porosity, as the name suggests, occurs when a casting solidifies and shrinks in size. Porosity occurs when one area of the casting takes longer to solidify than its surrounding area, this can reduce the metal flow to this section and the section doesn’t completely fill. Shrinkage-related porosity has a jagged or linear appearance and occurs under the surface of the cope or drag portion. Both forms of porosity will have a negative impact on the quality of a casting.
- Pouring porosity. Essentially this is gas-related and is caused when turbulence in the molten metal as it enters the mold causes additional air entrapment in the metal.
- Metallurgical porosity. E.g. sintered components that are porous by their nature (sometimes used as filters).
To rectify the issues of porosity, manufacturers often outsource the porosity sealing process to specialists who perform the process of vacuum impregnation. During porosity sealing, casting/parts are taken through four operations:
- Impregnation Chamber – castings are put in a vacuum chamber and a vacuum is applied, this removes any air in the porosity / leak path in the casting wall (the voids are emptied of air). The casting is then covered (the vessel is flooded) in a sealant. Because the porous voids are now empty (of air) the resin fills them . Vacuum is released and a positive pressure is applied, the pressure is then released, and the chamber is drained.
- Excess Sealant Recovery – the operator will then remove any excess sealant from the surface of the casting. This can be achieved by either gravity, centrifugal force or through continuous rotation.
- Wash/Rinse Station – during this stage, the operator will wash off any residual sealant from the part’s external surface, internal passages, pockets, tapped holes and features.
- Cure Station – Most common porosity sealing resins use a thermal catalyst, so, heat is now applied to start the chemical reaction. The castings are put into a hot water bath or oven and the sealant in the leak path (porosity) will polymerize.
The process of porosity sealing is often performed on die castings and permanent mold castings as these commonly contain internal porosity. Vacuum impregnation is often integrated into manufacturing systems within many sectors including the automotive industry. Porosity sealing offers an affordable and cost-effective method for salvaging castings that would otherwise be unusable.
Impregnation Services is a market leader in vacuum impregnation and porosity sealing – based in Greater Manchester. With over 50 years’ experience within the specialist field of Impregnation we offer both in-house services from our recently refurbished facilities and on-site for clients throughout the UK and abroad.
Our skilled staff has experience working on castings from a range of industries including the automotive, marine and energy sectors.
Over the past 10 years, Impregnation Services has made major re-investments into its people, plant and machinery to improve both quality and service. We strive ourselves on rectifying porosity problems, offering services on single parts and production runs with 1000s of parts or specific applications.
Impregnation Services’ team is committed to providing our customers with the best in vacuum impregnation services.
If you would like to find out more about our porosity sealing services, speak to a member of our team on 0161 344 1004.