While in the wet vacuum process the batch to be impregnated is inserted directly into the impregnating agent, in the dry vacuum process there is first an evacuation before the workpieces come into contact with the impregnating agent.

Compared to other workpieces, castings have a rather fine and closed porosity and therefore they require an effective impregnation method, which is realized by the dry vacuum processes.

In these processes, a deep vacuum is first drawn before the workpieces are wetted with the impregnation agent - so the impregnation chamber is still dry at the time of evacuation.

This has the advantage that all workpieces in the impregnation basket experience the same vacuum value and the air does not have to be drawn through the impregnation agent.
As a result of the vacuum now prevailing in the workpiece structure, the impregnation agent can more easily penetrate into the porosities and leakage paths after the impregnation container has been ventilated above the media level.

When using the dry vacuum process, a maximum pressure difference of one atmosphere (vacuum in the porosity - ambient pressure above the impregnating agent) can be built up. This pressure difference then forces the impregnating agent into the porosity, where it then remains. This means that expensive pressure vessels are not necessary.

The impregnation with the DV process is an effective method to seal simple porosities and leaks and is currently the most frequently used method in Europe. If you want to significantly improve the penetration of the impregnating agent, pressure vessels are necessary to build up an overpressure (overpressure process).

Additional references:
Vacuumimpregnation
Impregnation chamber
Impregnation