Published: October 2026 | By BAOLAN Engineering Team
The short answer to what vacuum level is needed for bubble free potting is 5–20 mbar absolute for most two-component epoxy — not the 0.1 mbar some datasheets imply. Below about 5 mbar you stop removing air and start boiling moisture, diluents and amine accelerators out of the compound, and the cure suffers. Above about 50 mbar you shift macro bubbles only, leaving the micro-voids that fail X-ray and partial-discharge inspection. It is a window, not a number, and the window moves with filler loading, viscosity, fill depth and compound temperature.
This guide gives the 2026 numbers by application, the hydrostatic correction most vendors omit, and four reasons a machine holding 5 mbar still ships parts full of voids. Suzhou Baolan Automation Technology Co., Ltd. builds systems around this window; these are the figures we use when a customer's adhesive goes through a trial before shipment.
1. What Vacuum Level Is Needed for Bubble-Free Potting?
As low as your compound tolerates, and no lower. Working out what vacuum level is needed for bubble free potting means balancing three limits: the vapour pressure of the most volatile component sets the floor, capillary pressure sets what you must overcome, and the resin column above the deepest cavity sets a correction almost everyone forgets.
Five working tiers cover almost every production case:
• 100–500 mbar — rough vacuum, macro bubbles only; cosmetic fills only.
• 50–100 mbar — unfilled epoxy under 3,000 cP, fill depth under 15 mm, gaps above 1 mm. Potted sensors and terminal blocks.
• 10–50 mbar — the production default: most ECU housings, PCB assemblies and power modules with moderate filler loading, matching the 1–50 mbar range compound suppliers publish.
• 1–10 mbar — filled epoxy above 60% silica, viscosity above 20,000 cP, gaps below 0.5 mm, or any part that must hold partial discharge under 10 pC.
• Below 1 mbar — transformer and high-voltage castings only, and only with dried compound and heated lines. Volatile stripping becomes a real risk here.
Turning tiers into a purchase decision is harder than it looks, so vacuum potting machine specifications explained mbar Pa and Torr leave buyers unsure which tier a machine reaches. The map below is what BAOLAN uses to size chambers and pumps.
Application map — pressure and hold time:
• Potted sensor, unfilled epoxy, 10 mm fill — 30–50 mbar, hold 8–12 min.
• ECU or power module, epoxy with 60–75% silica, gaps 0.3–1 mm — 5–15 mbar, hold 15–25 min.
• Motor stator impregnation, low-viscosity epoxy at 60–80°C — 5–20 mbar, hold 10–20 min, parts preheated.
• Transformer or HV casting, filled epoxy — 1–5 mbar, staged pump-down, hold 20–30 min.
• Two-component polyurethane — 30–100 mbar, never below the water boiling point at compound temperature. Addition-cure silicone — 10–50 mbar, hold 10–15 min.
Sanity-check the minimum vacuum requirements for two component epoxy potting against the smallest gap to fill. Below 0.5 mm, single-digit mbar is not optional: a 0.1 mm-radius bubble carries about 6 mbar of capillary pressure, so a 20 mbar chamber never gives it a reason to move.
Read the number at the bottom of the cavity, not the chamber wall. A 100 mm column of filled epoxy at 1,600 kg/m³ adds 15.7 mbar, so a 5 mbar chamber is 20.7 mbar at the base of the mould. That explains a complaint we hear weekly: the same machine clears every void in a 10 mm sensor housing and leaves a sponge in a 120 mm casting. Specify pressure at the deepest point, lower the setpoint, or fill in two stages so the head never exceeds 60 mm.
✅ Bottom line: 5–20 mbar absolute covers most electronics potting; go to 1–5 mbar only for filled or high-viscosity compounds, and never below the vapour pressure of your lightest component.
2. Why a Lower Gauge Reading Can Still Leave Voids
Because chamber pressure is one of four variables, and the other three are set before the pump starts.
Preparation must be deeper than potting. Degas in the tank at 5 mbar, then pot at 50 mbar. Run both at 5 mbar and dissolved gas comes out of solution inside the cavity — the machine generates the bubbles it was bought to remove. Keep a 5–10× ratio. Preparation depth is a separate parameter from the minimum vacuum requirements for two component epoxy potting, which apply to the chamber during fill; one number for both steps loses a week of yield.
Pump-down must be staged. Pulling 1013 to 5 mbar in one step on 3,000 cP filled material makes it froth, and froth traps more air than it releases. Two or three stages, with a 5–10 minute dwell at 200 and again at 50 mbar, let the foam collapse. A 100 L chamber on a 40 m³/h pump reaches 5 mbar in 48 seconds on paper; with outgassing and leak-up the real figure is 70–95 seconds, so never set the hold timer from the curve.
The gauge cannot see the difference. A machine specified at "-0.1 MPa" is quoted at 13 mbar absolute, not a deep vacuum; "-0.098 MPa" is 33 mbar. At 20 mbar the needle already sits at 99% of full vacuum, so a compound gauge cannot separate 20 from 5. It is why vacuum potting machine specifications explained mbar Pa or Torr rarely line up between two quotations, and why anything below 20 mbar needs a 0–100 mbar absolute sensor, not a dial.
Unit cheat sheet — absolute, gauge, water boiling point:
• 1013 mbar = 0 MPa = 0% vacuum — water boils at 100°C
• 100 mbar = −0.091 MPa = 90.1% — 46°C
• 50 mbar = −0.096 MPa = 95.1% — 33°C
• 20 mbar = −0.099 MPa = 98.0% — 18°C
• 10 mbar = −0.100 MPa = 99.0% — 7°C
• 5 mbar and 1 mbar = beyond gauge range = 99.5% and 99.9% — about 1°C and −20°C
Going too deep damages the compound. Below the vapour pressure of the lightest fraction, low-molecular-weight amine accelerators and reactive diluents leave the resin: mix ratio drifts, the surface stays tacky, the interface cures incompletely. Polyurethane is worse — moisture plus isocyanate releases CO₂ and the part foams in the oven two days later. For moisture-cure and PU systems the floor is the water boiling point at compound temperature; at 40°C that is already around 55 mbar.
✅ Bottom line: Degas deeper than you pot, stage the pump-down, and fit an absolute sensor below 20 mbar — a compound gauge cannot tell 5 mbar from 20.
FAQ
Q: Is a lower number always better when deciding what vacuum level is needed for bubble free potting?
A: No. Single-digit mbar pays off only with gaps below 0.5 mm, filler above 60%, or a partial-discharge target. On unfilled 800 cP epoxy it buys a longer cycle, more volatile loss and a tackier surface. The minimum vacuum requirements for two component epoxy potting are driven by the hardest feature you must fill, not the lowest pressure a pump can reach.
Q: How long should the hold last after the chamber reaches setpoint?
A: 10–30 minutes at 15–40°C for two-component epoxy, longer for heavy filler loadings because bubbles travel further. The hold ends when the surface foam collapses, not when a timer expires. Run coupons at 5, 10, 15 and 20 minutes and X-ray them: the curve flattens early, and most plants hold twice as long as needed.
Q: Can a machine be configured for one compound's vacuum window?
A: Yes. BAOLAN sizes pump, chamber volume, sensor range and pump-down recipe around the compound and the part, then runs the customer's adhesive in a trial to verify mix ratio and void level before shipment. Standard and OEM/ODM builds ship from the factory in Suzhou with a documented pump-down curve.
Get your window in writing
Send BAOLAN three things: the compound datasheet, the deepest fill section in millimetres, and the smallest gap to fill. Suzhou Baolan Automation Technology Co., Ltd. returns a written vacuum window, a staged pump-down recipe and a hold time within two working days, free of charge. As a China manufacturer running custom and ODM programmes, we quote factory-direct and trial your own adhesive before shipment.
Start with the vacuum potting machine range, or send your drawing through the contact page.
www.metermix.cn
Suzhou Baolan Automation Technology Co., Ltd.
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