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.

Process

Porosity sealing is a four-step process:

1. Air within the pores is expanded under reduced pressure (vacuum).
2. Pressure reversal (adding pressure outside the part) saturates pores with monomer.
3. Excess monomer is rinsed from the exterior surfaces of the part.
4. The liquid monomer within the porosity is polymerized into a solid to seal the passage.

Vacuum Impregnation Methods

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.

• Dry Vacuum/Pressure (DVP): This is the most complex and effective method of impregnation. The DVP cycle requires two separate tanks, one for processing the part and the other for storing the sealant. This process is best used for metal parts that contain extremely small, microscopic pores that may be invisible to the naked eye. The process steps are as follows:

1. Parts are loaded into a process basket and placed into a processing tank.
2. A vacuum is activated in the process tank to remove all air, including air within the pores of the parts.
3. Sealant is transferred from storage tank to the processing tank and the entire basket of parts is submerged.
4. Vacuum is released and desired pressure is added to the process tank using compressed air. This pressure will help the sealant enter the pores.
5. After desired amount of time, the pressure is released and the sealant is drained back into the sealant storage tank.
6. The processing basket full of parts is removed from the processing tank and all parts are washed and final processed according to the metal material and specifications of the part.

• Wet Vacuum/Pressure (VP): This cycle requires only one tank that contains the desired sealant. The process steps are as follows:

1. Parts are loaded into a process basket and placed into the sealant tank.
2. Vacuum is activated in the sealant tank to remove all air, including air within the pores of the parts.
3. Vacuum is released and desired pressure is added using compressed air.
4. Pressure is released from the tank.
5. The processing basket full of parts is removed from the tank and all parts are washed and final processed according to the metal material and specifications of the part.

• Wet Vacuum (WV): This cycle is the quickest impregnation method. It requires only one tank with the desired sealant. The process steps are as follows:

1. Parts are loaded into a process basket and placed into the sealant tank.
2. Vacuum is activated in the sealant tank to remove all air, including air within the pores of the parts.
3. Vacuum is released and tank is vented to atmospheric pressure.
4. Parts are soaked in the sealant for desired amount of time.
5. The processing basket full of parts is removed from the tank and all parts are washed and final processed according to the metal material and specifications of the part.^[1]

Common applications

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

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.

References