Porosity sealing is done through the process of vacuum impregnation. Vacuum impregnation seals porosity and leak paths that form during the casting or molding process. Vacuum impregnation stops casting porosity and allows manufacturers to use parts that would otherwise be scrapped.
Porosity occurs naturally and is found in most materials. In metal castings, porosity is typically considered any void found in the casting. Casting porosity can be caused by gas formation or solidification while the metal is being moved from a liquid state to a solid state. This porosity can range in size, from sub-micron to voids greater than 10 mm, depending on the casting.
Casting defects caused by porosity can affect the part’s structural integrity, creating a failure point. Porosity can also prevent the part from being pressure tight. This will impact performance if the part is designed to hold gases or fluids.
Vacuum impregnation is governed by Military Standard MIL-I-17563C and MIL-STD-276A as well as numerous proprietary and customer specifications. MIL-I-17563 tests the impregnation sealant. MIL-I-17563C demonstrates a sealant is compatible with the application and that the sealant will not degrade or fail over the life of the part. MIL-STD-276A tests the impregnation process. MIL-STD-276A provides the standards for processing to seal parts and testing process effectiveness.
In the course of sealing castings against porosity, the parts would be processed through the following four stations:
- Impregnation Chamber: The operator would seal the chamber and draw a vacuum. This would remove air in the porosity and leak path in the casting wall. Parts would then be covered with sealant, and positive pressure applied. More energy would be required to penetrate the porosity with sealant than to evacuate the air. The operator would then release the pressure and drain the chamber.
- Excess Sealant Recovery: The operator would remove excess sealant through gravity, rotation or centrifugal force.
- Wash/Rinse Station: The operator would wash residual sealant from the part's internal passages, taps, pockets and features.
- Cure Station: The operator would polymerize the impregnated sealant in the leak path.
Vacuum impregnation should be done prior to final assembly. Specifically for metal castings, vacuum impregnation should be done after final machining. Final machining may expose any porosity, creating a leak path. These paths can cause fluids and gases to leak from the casting, causing it to be non-conforming and unusable.
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.
- Shantz, Tom. "Basics of Vacuum Impregnation" (PDF). Retrieved 1 November 2012.
- "What Size of Porosity Can be Sealed?". Retrieved 2018-07-09.
- "Testing Vacuum Impregnation Sealant Compatibility vs Impregnation Process Effectiveness" (PDF). Die Casting Engineer: 8-10. September 2019.
- Marin, Andy. "Continuing Advances in Vacuum Impregnation Systems" (PDF). Foundry Management & Technology. Retrieved 16 March 2018.
- "When to Vacuum Impregnate Castings". Production Machining.