Vacuum Pressure Impregnation (VPI)
Vacuum pressure impregnation (VPI) fills every void in a part with resin or sealant. Air is pumped out first, so the liquid can penetrate deep into windings, pores and fibres; pressure then pushes it the rest of the way.
The process step by step
A typical VPI cycle:
- Preheat and dry the parts to drive out moisture.
- Evacuate the vessel to remove air and residual moisture from voids.
- Flood the parts with resin while still under vacuum.
- Release the vacuum and pressurise the vessel, typically with dry air or nitrogen at several bar, to force resin into the remaining gaps.
- Drain the excess resin and cure the parts in an oven.
Choosing the vacuum level
Deeper vacuum removes more air, but the resin itself has a vapour pressure: if the pressure falls too far, volatile components boil off and foam. The right level is a compromise set by the resin supplier, usually in the low mbar range for electrical resins.
Where VPI is used
Stator and rotor windings of motors and generators, transformers and coils get mechanical strength, heat conduction and insulation from VPI. Porous aluminium and iron castings are sealed against leaks, and timber is impregnated with preservatives the same way.
Typical pressures
| Process stage | Pressure |
|---|---|
| Evacuation (electrical resins, typical) | Low mbar range, set by resin datasheet |
| Casting impregnation | ≈ 1 – 10 mbar |
| Pressure phase | Several bar overpressure |
Suitable vacuum pumps
References & further reading
- K. Jousten (ed.). Handbook of Vacuum Technology, 2nd ed.. Wiley-VCH (2016). Standard reference on gas flow, conductance, pumps and gauges.
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
Frequently asked questions
What is the difference between VPI and dip impregnation?
Dip impregnation relies on capillary action at atmospheric pressure, so air pockets remain. VPI removes the air first and uses pressure afterwards, which gives far more complete filling of windings and pores.
Why does resin foam under vacuum?
Volatile components and dissolved gas boil off when the pressure drops below their vapour pressure. Lowering the pressure gradually and keeping to the resin supplier’s minimum avoids foaming.
Which vacuum pump suits a VPI plant?
Rotary vane or dry screw pumps, often with a Roots booster for large vessels. Resin vapours call for a condenser or filter, and dry pumps avoid contaminating oil.
Other applications
HVAC Evacuation: Pulling a Deep Vacuum
How to evacuate an HVAC/R system: removing air and moisture, why 500 microns, decay tests, hose and pump tips, and the physics of boiling water out.
Vacuum Distillation
Vacuum distillation lowers boiling points to protect sensitive products: choosing the pressure, the delta-20 rule for rotavaps, industrial columns and pumps.
Freeze Drying (Lyophilization) Under Vacuum
How freeze drying works: sublimation below the triple point, chamber and condenser pressures, ice vapour pressure, drying phases and the right vacuum pumps.
Vacuum Furnaces and Heat Treatment
Heat treatment in vacuum furnaces: bright surfaces, brazing, sintering, typical pressures, partial pressure, gas quenching, pumps and leak-up rate tests.
Vacuum Sewer Systems
How vacuum sewers move wastewater with air pressure instead of gravity: interface valves, the vacuum station, typical pressures, pumps and where they fit.