There are a couple of possibilities that would require a double cycle impregnation.
- The porosity is so large that the sealant washes out during the washing/curing cycle.
- Additional machining content that create new leak paths not previously addressed.
In the above example, we will assume that the porosity is large enough to allow the sealant to wash out during the washing and or curing process.
Why does this occur?
Surface tension of the water during the wash process could potentially “pull” the sealant off and the sealant will become slightly less viscous as temperature rises. This typically occurs within the takt time of the curing process but still allows the sealant to migrate to the surface of the part and wash off.
In the second example, additional machining content has been added to the part to create a new leak path. As per MIL-STD-276A paragraph 4.11 Re-impregnation. Components may be re-impregnated once either before or after pressure testing unless there is a reason to doubt the quality of the impregnation process or the component. Parts which leak after one re-impregnation shall be rejected. An exception to this requirement shall be permitted if machining after impregnation is required. In which case, one additional impregnation shall be permitted before final rejection.
For more information about vacuum impregnation, contact Godfrey & Wing.