Today virtually all castings that are required to withstand pressure and contain a gas or liquid are impregnated as an essential part of the manufacturing process. This is partly due to the increased use of aluminium and magnesium castings in the automotive industry with more complex shapes, thinner wall sections and tighter leak specifications. High volume automotive casting production often justifies 100% impregnation of all castings to reduce the risk and the high cost of an engine or other component failing in service due to the leakage of fuel, oil or water through micro porosity. Whatever the volume or application, impregnation is a widely established rectification process for castings that fail on leak test in the automotive, defence, aerospace and general engineering industries.
Machining can turn both blind and enclosed porosity into through porosity causing a leak path, which will need impregnation to render the part leak free and recover the high cost of casting and machining.
The vacuum impregnation process is governed by US MIL-STD-276A as well as numerous proprietary and customer specifications. The purpose of the impregnation process is to completely fill the porosity of a component with a sealant that when solidified is permanent, durable and suitable for the field of use.
In general, the vacuum impregnation process comprises four steps:
The methacrylate based impregnation sealants used today are very similar to those developed in the late 1970’s. There have been improvements over the years in the sealant stability (pot life), temperature and chemical resistance, flexibility to withstand shock, and washing performance. Sealants have also been developed that can be recovered from the wash water to reduce waste effluent and satisfy environmental demands.
An effective impregnation sealant is required to totally and permanently seal all leak paths through a metal casting caused by porosity (resulting from gas bubbles, shrinkage cracks or inclusions during the casting process) to render the part pressure tight but without leaving any surface residues that could interfere with subsequent finishing, assembly or product performance, and without causing corrosion or discolouration of the part.
Methacrylate sealants are therefore well tried and tested for sealing porosity in castings, with a track record of over 30 years in very demanding applications. However, there are now numerous different brands of impregnation sealants available in the market and making sure you choose one that is up to the required standard can be a risky business for the user. Fortunately, this choice is made much easier following the introduction of the US MIL Specification Accreditation, which lists all the tested and approved impregnation sealants.
Using an unqualified and unapproved sealant could result in component failure. The high cost of rectifying a leaking engine, braking system or other component assembly that has started leaking in the field could be the result of employing a poor quality sealant, ineffective process equipment or not being provided with the proper technical support for your application by the supplier. This is a risk not worth taking.
Midland Impregnations use US MIL Specification approved sealants. In the liquid state the sealants have low viscosities and low surface tensions for effective penetration of micro porosity. In the cured state the sealants can withstand temperatures from -110°C to +205°C and have excellent resistance to a wide range of acids, caustics, hydrocarbons and solvents.
Virtually all cast metals that need to retain liquids or gases or prevent the ingress of moisture through porosity can be impregnated, including:
Worldwide approvals from the major OEM’s and US Military – the internationally recognised standard for impregnation
US Department of Defense Standard MIL-STD-276A for the impregnation of porous structurally sound castings and powder metal components in aluminium, magnesium, copper, iron and zinc alloys.
US Department of Defense/US Navy MIL-I-17653C / QPL 17563 qualified list of impregnation sealants for cast and powder metal components.