Vacuum Impregnation for Porosity Sealing of Metal Castings, Powder Metal/Sintered Parts

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 Vacuum 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.

Since "Dry Vacuum & Pressure" and "Wet Vacuum & Pressure" processes require very high investment and yet may not provide very high quality results, the other two processes are being discussed further.

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 psi to 3000 psi 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.

STRUCTURAL ADHESIVES CHALLENGE MECHANICAL FASTENERS

Adhesives are becoming a formidable competitor to mechanical fasteners in structural applications. The reason: recent improvements in peel strength, flexibility, and resistance to moisture, temperature, and chemicals.

The potential advantages of adhesives over mechanical fasteners have long been recognized; however, industrial acceptance of adhesives for structural applications has been slow. The main reason for this lack of use is that adequate information about the characteristics of adhesives was not available until recently. Development studies by the aerospace industry greatly improved the technical database on structural adhesives, and many industrial designers are now convinced that the adhesives can provide bonds that are both cost effective and durable.
Structural adhesive bonds have replaced both mechanical fasteners and assembly techniques such as soldering and welding in many industrial applications. The advantages of structural adhesive bonding over other joining techniques include:

• Cost savings, including lower labor costs.
• Weight reduction.
• Elimination of stress point concentrations by even distribution of stress over the entire bonded surface, plus improved load bearing capacity.
• Protective sealing against contamination by liquids or gases.
• Bonding of dissimilar materials. Often the adhesive bond line acts as an insulator against galvanic corrosion in metal assemblies.
• Improved fatigue resistance, and resistance to shock, vibration, and thermal cycling.
• Enhanced structural appearance because protrusions, punctures, and attachments are eliminated.
• Increased tolerances on machined parts because some structural adhesives can fill gaps left when parts mate poorly.

Few available adhesive materials meet the stringent requirements of structural bonding, such as durability, high strength, and dimensional stability over a wide range of environmental conditions. The main classes of adhesives meeting structural requirements are epoxies, cyanoacrylates, reactive acrylics, and polyurethanes. Also, recently developed materials that appear to be especially suited to high temperature use include polyimides and some silicones.