Casting Impregnation

Casting Impregnation Proccess

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:

1. Impregnation - where the casting is subjected to a high vacuum to remove all the air from within the porosity, the liquid sealant is introduced to fill the evacuated voids and positive pressure is applied to ensure full penetration of the part. This process is called dry vacuum pressure (DVP) and is the most complete and successful impregnation process for sealing micro porosity in pressure die-castings.
2. Sealant Recovery - using a tilt, rotation or centrifuge drain to recover excess sealant from the surface of the casting, in galleries and in blind machined holes, which is returned to the impregnation station for re-use. Effective sealant recovery at this stage is essential for an economical process.
3. Wash/Rinse - using a cold water dip tank, rolling rinse or pump over station to remove residual sealant from all casting surfaces, galleries and holes to ensure a thoroughly clean surface finish before curing.
4. Hot Cure – using a hot water dip tank, rolling cure or pump over station to heat the casting and polymerize the sealant in the porosity, converting the sealant from a liquid to a solid, whilst also maintaining a clean surface finish without corrosion or discolouration.

Impregnation Sealants

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.