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Porosity sealing is a four step process:
- Air within the pores is expanded under reduced pressure (vacuum).
- Pressure reversal (adding pressure outside the part) saturates pores with monomer.
- Excess monomer is rinsed from the exterior surfaces of the part.
- The liquid monomer within the porosity is polymerized into a solid to seal the passage.
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.
Powder metal is also impregnated to enhance machinability. PM parts are generally difficult to machine and some compositions may not be machinable without ruining the cutting tool. Porosity sealing improves the life of cutting tools by ten to one-hundred times. Resin impregnation is more effective than compacting additives and can be selectively applied for near net pressed parts.