Vacuum Degassing: How It Works, What Vacuum Level You Need and How to Size the Pump

Vacuum degassing removes trapped air and dissolved gas from a liquid, such as silicone rubber, epoxy or polyurethane resin, potting compound or investment plaster, by lowering the pressure above it. As the pressure falls, every bubble swells, rises faster and bursts at the surface. At about 29 inHg on a vacuum gauge (roughly 31 mbar absolute at sea level) a bubble has grown to about 32 times its original volume.

To degas well you need four things: a vacuum level low enough for the material, a chamber with enough headspace for the foam, a pump that reaches that level well inside the material's pot life, and a way to keep foam and vapour out of the pump. This guide works through each one with the numbers shown, for people who degas in a workshop or lab and for engineers who build degassing chambers, potting machines and casting equipment.

What is vacuum degassing, and how does it work?

Mixing, stirring and pouring fold air into a liquid. Some of it stays as visible bubbles; some dissolves. Left alone, the bubbles end up in the cured part as voids, pinholes on a mould surface or weak spots in an electrical potting. Vacuum degassing deals with both kinds of gas through two physical effects.

• Bubbles grow as pressure falls. The gas in a bubble follows Boyle's law: its volume is inversely proportional to the pressure around it. Take the chamber from 1013 mbar to 93 mbar absolute and each bubble grows about 11 times in volume, or about 2.2 times in diameter.
• Bigger bubbles rise much faster. In a viscous liquid a small bubble's rising speed goes with the square of its diameter (Stokes' law). A bubble 2.2 times wider rises about 4.8 times faster, which is why a mix that would hold its bubbles for hours clears in minutes under vacuum.

Dissolved air comes out too. By Henry's law the amount of gas a liquid holds at equilibrium is proportional to the pressure of that gas above it, so at 30 mbar absolute the mix can only hold about 3% of the air it held at atmospheric pressure. The excess comes out of solution as new bubbles, which then grow and rise like the others.

In practice you see the mix rise as a foam, sometimes to several times its height, then "break" and collapse as the bubbles burst at the surface. After the break the surface calms, with only a few slow bubbles. That is the point to finish.

Vacuum degassing of molten steel in a steelworks uses the same physics on a very different scale; this guide is about liquids degassed in a chamber at room temperature.

Which materials are degassed with vacuum?

• Silicone mould rubber (tin- and platinum-cure), before pouring a mould, so the mould surface has no pinholes.
• Epoxy and polyurethane casting resins, for clear castings and for parts that must be strong or sealed.
• Potting and encapsulation compounds for electronics, where a void can mean a hot spot or a path for moisture.
• Investment and dental plaster, mixed under vacuum or degassed before casting.
• Adhesives, sealants and lab liquids that must be bubble-free before dispensing, testing or measuring.
• 3D printing resin, covered separately in vacuum pumps for 3D printer resin degassing.

What vacuum level do you need for degassing?

Think in absolute pressure, because that is what the bubbles respond to. Gauges in workshops usually read inches of mercury (inHg) or kPa below atmospheric pressure, so the same vacuum appears as three different numbers. Against a standard 1013 mbar atmosphere:

Gauge (kPa)	Gauge (inHg)	Absolute pressure	Bubble volume grows	Bubble diameter grows
−80 kPa	23.6 inHg	213 mbar	4.8×	1.7×
−90 kPa	26.6 inHg	113 mbar	8.9×	2.1×
−92 kPa	27.2 inHg	93 mbar	10.9×	2.2×
−95 kPa	28.1 inHg	63 mbar	16×	2.5×
−97 kPa	28.6 inHg	43 mbar	23×	2.9×
−98 kPa	28.9 inHg	33 mbar	30.5×	3.1×
−98.2 kPa	29 inHg	31 mbar	32.5×	3.2×

Many silicone and casting-resin datasheets ask for about 29 inHg, which is roughly 31 mbar absolute. Thinner, low-viscosity mixes release their bubbles at a weaker vacuum, only more slowly and less completely; thick mixes need the deep end of the table. Always follow the material's own datasheet, because it also tells you how long the mix stays workable.

Two limits stop you going lower:

• Water and volatile parts boil. Water's vapour pressure is about 23 mbar at 20 °C, so a water-based mix such as plaster starts to boil below that, and the "bubbles" you see are water vapour, not air. Some hardeners and solvent-thinned products lose volatile parts under deep vacuum, which changes the mix. Stop at the level the datasheet gives, and do not hold full vacuum longer than needed.
• The atmosphere sets the scale. A gauge reading depends on local air pressure. At 1,500 m altitude the standard atmosphere is about 846 mbar, so even a perfect vacuum reads only about 25 inHg there. Specify the target in absolute mbar and the gauge confusion disappears. Our article on gauge and absolute pressure explains the two scales.

How do you degas silicone or resin in a vacuum chamber?

The steps are the same whether you use a bench chamber or build degassing into a machine:

1. Choose a container with headspace. Material makers typically ask for a container four to five times the volume of the mix, because it can rise that much before it breaks.
2. Mix thoroughly, then pump down. Close the vent valve and start the pump. Watch the mix, not the clock.
3. Control the rise. If the foam approaches the rim, open the vent valve slightly to let a little air in, then close it again. A chamber with a clear lid and a needle valve makes this easy.
4. Hold after the break. When the foam collapses, keep the vacuum for a minute or two, or as long as the datasheet says, until only the odd bubble surfaces.
5. Vent slowly. Let air back in gradually. A sudden inrush can disturb the surface and drive fine bubbles back in.
6. Pour carefully. Pour in a thin stream from a low height into the lowest corner of the mould, so you do not fold new air in.

For clear casting resins, some makers recommend a pressure pot instead of, or after, vacuum degassing. Pressure does not remove bubbles; it squeezes those left in the poured part until they are too small to see while the resin cures. Vacuum degassing works on the mix before pouring, so the two methods can complement each other.

How big should the vacuum pump be for a degassing chamber?

The pump must reach the target pressure quickly enough to leave most of the pot life for the break, the hold and the pour. The pump-down time of a chamber is estimated with:

t = (V ÷ S) × ln(p₀ ÷ p₁)

where V is the chamber volume, S the pump's average effective pumping speed over the range, p₀ the starting pressure (1013 mbar) and p₁ the target. Our guide to vacuum pump evacuation time explains the formula in detail.

Worked example 1: a 20 L bench chamber. Target 50 mbar absolute. ln(1013 ÷ 50) = ln(20.3) = 3.01. With an average effective speed of 30 L/min (1.06 CFM): t = (20 ÷ 30) × 3.01 = 2.0 minutes. Against a pot life of 20 to 40 minutes, that leaves plenty of time.

Worked example 2: a 60 L production chamber. Same target, same pump: t = (60 ÷ 30) × 3.01 = 6.0 minutes, before the foam has even risen. A pump with three times the average speed, 90 L/min (3.18 CFM), brings it back to 2.0 minutes. Use our CFM to L/min converter when pump data comes in different units.

Two corrections matter in practice:

• The average speed is well below the free-air flow. A pump's L/min rating on the datasheet is its flow at atmospheric inlet pressure. Its speed falls as the pressure approaches its ultimate vacuum, and reaches zero at the ultimate. Use the pump's speed curve, or a conservative average, not the headline figure.
• Keep the target well above the pump's ultimate. A pump whose ultimate is 33 mbar crawls for the last few millibar and never reaches 31 mbar. As a rule of thumb, choose a pump whose ultimate absolute pressure is about half the pressure you want to work at, or lower. Our article on ultimate pressure vs working vacuum explains why.

The foam itself releases gas, and every fitting and lid seal leaks a little, so a real chamber takes longer than the formula says. If the pump-down time grows over weeks, check the lid gasket and fittings; our guide to vacuum leak rate testing shows how to measure it.

Which type of vacuum pump suits degassing?

Pump type	Typical ultimate	Strengths for degassing	Watch out for
Oil-free piston	About 30 to 100 mbar absolute (roughly 27 to 29 inHg), depending on the model	No oil to change or oil mist; high flow for its size; suits continuous duty and machine integration	Pick a model whose ultimate sits well below your target; protect it from foam and condensable vapour
Diaphragm	Tens of mbar single-stage, a few mbar multi-stage	Oil-free; chemically resistant versions exist for solvent vapour	Lower flow for the size, so slower on large chambers
Oil-lubricated rotary pump	Well below 1 mbar	Deep vacuum, fast pump-down on small chambers	Vapour from the mix contaminates the oil; regular oil changes; oil mist at the exhaust
Compressed-air ejector	Depends on supply pressure and design	No moving parts at the chamber	Uses a lot of compressed air for a long hold; noisy

For silicone, epoxy and polyurethane in the rough vacuum range, an oil-free piston pump is the common choice in equipment that must run every day without oil service. If your process needs a few mbar or less, or handles a lot of solvent, a piston pump is not the right tool; say so in your enquiry and we will tell you honestly. How the pump types work is explained in how vacuum pumps work.

How do you protect the pump during degassing?

• Keep foam out. Put a catch pot or trap between chamber and pump. A foam-over that reaches the pump heads can ruin the valves.
• Filter the inlet. Dust from fillers and pigments wears an oil-free pump. Our inlet filtration guide covers filter choice.
• Mind the vapour. Oil-free piston pumps are built to move air. Water or solvent vapour from the mix can condense in the heads; a cold trap or condenser in front of the pump helps, as described in our vacuum oven pump guide. Tell us the material and any solvent before you design a pump in, and we will check whether the pump suits it.
• Fit a vent valve and a gauge. A needle valve controls the rise; a gauge reading absolute pressure tells you when you have reached the target.

Which HCEM pumps fit a degassing chamber?

Our oil-free vacuum pumps are piston pumps in AC and 24 V DC versions. Each product page's parameter table gives the open-air flow and the maximum vacuum; compare both with your target, keeping the rule above in mind. The HC580A suits bench chambers, the HC680A and the twin-head HC1500A suit larger production chambers, and the HC580D runs on 24 V DC for battery-powered or mobile equipment. Check the ultimate vacuum and the flow at your working pressure in each product's parameter table and datasheet, and ask us to confirm the figures before you design a pump in.

For equipment makers we build custom voltages from 50 units, and qualified OEM customers and distributors can apply for samples, which are dispatched in 10 to 20 days. The OEM and ODM page lists the other options.

What should a degassing equipment maker put in the pump RFQ?

• Chamber: volume including lines, lid material and seal, and whether the user can see the mix.
• Target: working pressure in absolute mbar, the pump-down time you promise, and the materials your customers degas.
• Vapour and foam: any water, solvent or volatile hardener, and whether a trap or condenser is fitted.
• Duty: cycle length, cycles per hour, running hours per year, and whether the pump runs continuously or starts with each cycle.
• Installation: ambient temperature, cooling air, mounting orientation, noise target in dB(A) at a stated distance.
• Electrical and commercial: supply voltage and frequency, annual volume, sample quantity.

Common questions

How long should you degas silicone?

Until the foam has risen, broken and collapsed, then a minute or two more, within the pot life the datasheet gives. With a chamber that reaches the target in one to two minutes, the whole cycle usually takes a few minutes. If it takes much longer, the pump is too small, the vacuum too weak or the chamber leaks.

Is 27 inHg enough to degas resin?

It can be for thin, low-viscosity resins, but bubbles grow only about 10 times in volume at 27 inHg, against about 32 times at 29 inHg, so thick mixes clear more slowly and less completely. Check the material's datasheet.

Why does my mix foam over the container?

The container is too small for the rise, or the pressure falls too fast. Use a container four to five times the volume of the mix and crack the vent valve to slow the rise.

Can you degas without a vacuum chamber?

Slow mixing, warming the parts to lower viscosity and pouring in a thin stream reduce bubbles, and a pressure pot shrinks those left in a casting. None of these removes dissolved air the way vacuum does.

Does degassing remove all the bubbles?

It removes most of them from the mix. Pouring can add new ones, so pour slowly from a low height, or degas again in the mould where the material allows it.

If you are choosing a pump for a degassing chamber or a machine you build, send us the chamber volume, target pressure and time, the materials, any solvent or water in them, and your supply voltage. Our engineers will suggest a model and tell you plainly if a piston pump is not the right fit.