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Vacuum impregnation is a process that seals porosity in metal castings. If left untreated, then the porosity creates a path for fluids and gasses to leak from the part. When performed properly, vacuum impregnation seals the porosity, but it is undetectable on the surface or in the machined features of the casting.
Before vacuum impregnation is applied in production, operators often request indisputable evidence that the process is capable. This is done by measuring key process characteristics of the vacuum impregnation process. Common processes that are tested are:
- Sealant gel time
- Sealant viscosity
- Vacuum level achieved and time
- Pressure level achieve and time
- Wash time
- Curing temperature and time
The sealant gel time test produces a test slug. Often the sealant slug is discarded after the sealant gel time. But before it is discarded, the operator should examine the slug’s color and clarity for other conditions. What is optimum is to have the sealant coming out of the system looking like the clean, clear sealant that originally went into the system. Below are three common reasons why the sealant may not match its original state:
- Dark
This is a result of varying amounts of contaminants that come from the impregnated parts, which consists of marker ink, carbon, dirty cutting fluids, dirty test fluids used to pressure test the parts, etc.
- Cloudy
Too much water will cause the slug to be opaque or cloudy. Sealant can absorb water, but it cannot have too much water.
- Excessive Amount of Crazing
If the slug has an excessive amount of crazing, then this is a result of being over or under catalyzed. Crazing can cause the slug to easily fracture or crumble.
If any of these occur, then the possible action plan can include:
- Filter the sealant to ensure that it is free of contamination.
- De-gas by running additional vacuum cycles
- Add new catalyzed sealant to reduce crazing.
In Summary
Vacuum impregnation is a process that seals internal pores in metal castings. Sealing the porosity allows the part to hold gas and fluid under pressure. Measuring the key process characteristics and the sealant slug provide traceable, quantifiable and actionable data to keep the vacuum impregnation process effective.
