Porosity sealing

Porosity sealing, also known as vacuum impregnating, metal impregnating, polymer impregnating, and porous metal sealing, is the process of filling a porous substrate to make it airtight.

Porosity sealing is a four-step process:

There are a variety of methods that can be used to impregnate metal parts. The method chosen depends on a part requirements, specifications and sealant that will be used. All methods used are effective and once porous metal parts are sealed and cured, they will be sealed indefinitely unless the sealant material fails in the future due to exposure to chemicals, excessive heat, wear or other damaging conditions.

Die castings and permanent mold castings commonly contain internal porosity. This porosity is generally localized to the deepest cross-sections of the part and does not extend to the outer skin. However, if the part is also machined, the internal porosity will be exposed and the part will leak if pressurized. Machined die castings that need to hold fluids (intake manifolds, coolant connectors, transmission cases, pump housings and fluid power components) are routinely sealed for life using acrylic resins. Because the sealant is internal to the part, the exterior dimensions and appearance of the part are unchanged.

Powder metallurgy (PM) components are sealed prior to plating and to reduce internal corrosion. Plating operations typically involve submerging the parts in acid solutions. After plating, residual acid internal to the part can promote corrosion and/or preclude an acceptable plating finish. The solution to this problem is to seal the internal voids prior to plating. As explained above, the porosity is saturated with monomer and is then rinsed completely clear of the surface. The resin cures to a durable polymer. Thus, the exposed surface metal is free to be plated while the interior spaces are sealed dry.

The porosity in powder metal parts becomes a liability when the part must resist a differential pressure. PM applications for compressed air, fuel handling or hydraulic housings are common and effective; however, they must be sealed first. The polymer does not add structural strength to the physical part, but it will hold high pressures without creeping. If the wall thickness of the part exceeds 1/4 inch, the leak pressure is typically on the same order of magnitude as the burst pressure of the part.

...
Wikipedia