What vacuum impregnation does
Vacuum impregnation seals micro-porosity that connects through the wall of a casting. The parts are placed in a vessel, air is drawn out of the pores under vacuum, a sealant fills them, and the sealant is cured inside the wall. It does not add material to the surface and does not change dimensions in a meaningful way when the parts are washed correctly.
It is a recovery step for fine, connected porosity. It does not repair cracks, cold shuts, large shrinkage or a machining breakout, and it does not raise strength.
When impregnation is acceptable
Whether impregnation is allowed is the buyer's decision, written on the drawing. Many pump, valve and enclosure buyers allow it as a standard step; others prohibit it, for example where the sealant could contact the medium, or where the customer wants the casting process itself to hold pressure.
| Drawing says | What it means for production |
|---|---|
| Impregnation permitted | Parts failing the leak test may be impregnated once and re-tested; the record shows which parts |
| Impregnation as standard | All parts impregnated after machining, then leak tested |
| Impregnation not permitted | The casting route has to achieve the leak criterion alone; squeeze casting is evaluated |
| Silent | Ask before quoting; we do not assume permission |
What to specify
- Permitted or not, and at which stage (after machining is usual)
- Sealant type compatible with the medium and temperature
- Process standard, for example MIL-STD-276 where the customer uses it
- Leak test after impregnation: medium, pressure, limit
- Number of impregnation cycles allowed per part
- Marking or record of impregnated parts
Sequence with machining and finishing
Machining opens porosity that was sealed inside the as-cast skin, so impregnation after final machining is the usual order. Coating or anodising after impregnation needs a sealant that survives the pre-treatment. The leak test that counts is the one after the last operation that can open a path.
Fix the cause or seal the symptom
Impregnation is a valid production step, but a high impregnation rate is a signal about the casting route. Where leak rejects repeat in thick sections, squeeze casting removes much of the porosity at the source, which is why we review both before tooling.