Impregnation Methods
For Sealing Metal Castings,
Powder Metal/Sintered Parts,
and Electrical Components

By Michael Ballou

Impregnating methods used to seal porosity in metal castings, powder metal parts, and electrical components are typically performed by one of four different processes.

Choosing the best process depends on a variety of factors such as:

• Number and Size of Parts to be Impregnated
• Material Used to Produce the Part
• Size and Amount of Porosity Contained in the Parts
• Desired Sealing Results
• Economics

The Four Different Processes Available Include:

• Dry Dacuum and Pressure
• Wet Vacuum and Pressure
• Wet Vacuum
• Internal Impregnation or Pressure Method

Note: Although the impregnation process typically includes a Wash Step to remove excess sealant from the part and a Cure Step to polymerize the remaining sealant in the porosity, the Actual Impregnation Process is the Act of Filling Porosity with the Sealant.

Dry Vacuum and Pressure

This method requires the highest investment in equipment yet provides the best results when sealing porosity that is very small and sealing requirements are most stringent.

The Dry Vacuum and Pressure method uses a pressure vessel or autoclave and a resin storage vessel. Part/s are placed into the autoclave and a vacuum is applied. The level of vacuum is generally between 28 to 29.5 inches of Hg. The vacuum is held until air is removed from the autoclave and from within the porosity of the part/s within. Next, the sealant which is stored in the separate vessel is transferred, usually via a pipe, from the storage vessel to the autoclave. The transfer is facilitated by differential pressure... As the sealant is being transferred, the vacuum is maintained. When the sealant reaches a level approximately 6 inches above the part/s in the autoclave, the transfer is complete. The residual vacuum is released to atmosphere and pressure is then applied. Typically pressures are brought to between 80 to 100 psig. The pressure is held for a period of time long enough to facilitate penetration into the porosity. (Large porosity will fill quicker than very fine micro porosity). After the desired time under pressure, the sealant is transferred back to the storage tank, again through the use of differential pressure. The autoclave is finally vented to atmosphere and the part/s are removed, washed and cured.

Wet Vacuum and Pressure Impregnation

This method requires a relatively high investment in equipment yet may not produce the highest quality results of a dry vacuum pressure method.

The Wet Vacuum and Pressure method uses a single pressure vessel or autoclave but no storage tank. The sealant is stored in the autoclave. Part/s are immersed in the sealant within the autoclave and a vacuum is applied. The level of vacuum is generally between 28 to 29 inches of Hg. The vacuum is held until air is removed from the vessel, sealant and porosity of the part/s (a sufficient amount of time to ensure adequate de-aeration). When complete, the residual vacuum is released to atmosphere and pressure is applied. Typical pressures are brought to between 80 to 100 psig. The pressure is held for a period of time long enough to facilitate penetration into the porosity. (Large porosity will fill quicker than very fine micro porosity). After sufficient time under pressure, the autoclave is vented to atmosphere and the part/s are removed to be washed and cured.

Wet Vacuum Impregnation

This method requires a relatively low investment in equipment, and cycle times are quicker. The Wet Vacuum method performs best for parts with large evenly distributed porosity such powdered/sintered metal.

The Wet Vacuum method requires only a single vessel that can maintain a vacuum, thus does not require a pressure rated vessel. As in the Wet Vacuum method above, the sealant is stored in the vessel. The part/s are immersed in the sealant in the vessel and a vacuum is applied. The level of vacuum is generally between 28 to 29 inches of Hg. This removes air from the vessel, sealant and from within the porosity of the part/s. (The vacuum is applied for a sufficient amount of time to ensure adequate de-aeration). When complete, the residual vacuum is released to atmosphere. Part/s are left to soak in the sealant at atmospheric pressure for a period of time to long enough facilitate penetration into the porosity. After sufficient soak time, the part/s are removed.

Internal Impregnation or Pressure Impregnation

This method requires the least investment in equipment and provides excellent sealing results in even the finest porosity, but limits the number of parts that can be processed at any one time.

The Internal Impregnation or Pressure Impregnation method utilizes the part as the vessel. This method although quite effective, allows only one part to be processed at a time. (Naturally, a requirement to process hundreds or even thousand of parts makes this method economically impractical). To internally impregnate a part, the part is first filled with sealant while venting any trapped air. The sealant within the part is then pressurized up to the test pressure of the part. Pressures of anywhere between 5 psig to 3000 psig are not uncommon. The pressure is typically held until sealant is seen weeping from the part. When this is achieved, the pressure is reduced to atmosphere and the sealant is drained from the part.

Whichever process is required, IMPCO provides the equipment, sealant or service to seal your parts to your complete satisfaction.