Selecting seals for vacuum

Seals in vacuum place significantly higher demands than classic sealing solutions. While minor leaks are often tolerable in normal applications, in vacuum even minimal leaks have an immediate effect on the pressure and thus on the entire process. In addition, materials themselves can become a disruptive factor, for example through outgassing or unsuitable surface structures.

The selection of a suitable seal is therefore made systematically along several influencing variables. Alongside the desired vacuum level, material behaviour, temperature ranges, media contact and the design also play a central role. Each of these variables directly influences the sealing performance.

Already in the planning phase it should be clearly defined which requirements must be met and which sealing systems function stably under real conditions. Errors in this early phase frequently lead to later problems in operation, for example through unstable pressure conditions, increased maintenance effort or shortened service lives.

Requirements for seals in vacuum

In vacuum, several technical requirements are the focus:

• Leak rate: describes the actual sealing performance. As the pressure level falls, the demands on tightness rise.
• Outgassing: materials release volatile constituents that affect the vacuum and destabilise processes.
• Temperature resistance: relevant in thermal processes or when baking out systems.
• Surface quality: directly influences the sealing effect through the effective contact area.
• Media resistance: chemical influences from gases or process media must be taken into account.
• Long-term stability: material behaviour over time and under load determines maintenance intervals and process reliability.

Static and dynamic seals in vacuum

Static seals form the standard in vacuum technology. They work without movement and are typically used in flange connections, housings or vacuum chambers. Their advantage lies in the high sealing performance with comparatively simple construction, since no dynamic loads have to be taken into account.

Dynamic seals are used wherever movement is required, for example on shafts, valves or gate valves. In these applications, friction, wear and mechanical loads also act on the seal. This significantly increases the demands on material, geometry and lubrication. Selection here is considerably more restrictive, since both tightness and movability must be permanently guaranteed.

Suitable materials for vacuum seals

The choice of material determines the function of the seal.

Elastomers:

• NBR for simple applications in rough vacuum
• EPDM for media resistance and moderate temperatures
• FKM for higher temperatures and chemical exposure

PTFE and modified plastics:

• very low outgassing
• high chemical resistance
• suitable for more demanding vacuum ranges

Metallic seals:

• copper or stainless steel
• use in high and ultra-high vacuum
• extremely low leak rates

Selection by vacuum range

The requirements rise as the pressure level falls. Concrete pressure ranges are decisive:

Vacuum range	Pressure range (mbar)	Typical seal	Materials
Rough vacuum	10³ to 10⁻³	O-rings, flat gaskets	NBR, EPDM
Fine vacuum	10⁻³ to 10⁻⁷	O-rings, PTFE seals	FKM, PTFE
High vacuum	10⁻⁷ to 10⁻¹²	Metal or special seals	FKM, metal
Ultra-high vacuum	< 10⁻¹²	Metallic systems	copper, stainless steel

The lower the pressure, the greater the effect of the smallest leaks and material outgassing.

Typical seal types

O-rings

• flexible in use
• simple installation
• dependent on material and groove geometry

Flat gaskets

• large-area sealing
• suitable for flanges and housings

Profile seals

• optimised for special installation situations
• defined deformation

Metal sealing systems

• highest sealing performance
• use under extreme requirements

Influence of design and installation situation

The performance of a seal depends heavily on the design and the installation situation. Even high-quality materials only achieve their full sealing effect with correctly dimensioned contact surfaces and defined compression.

The contact pressure must be precisely matched to the material and application, since both too low and too high loads impair the sealing function. At the same time, the groove geometry plays a central role, since it determines the deformation and positioning of the seal.

The surface quality of the sealing surfaces also directly influences the function. An even, finely machined surface significantly improves the contact effect. In addition, the installation quality has an effect, since contamination or particles between the sealing surfaces lead directly to leaks.

Even small deviations in these areas lead to measurable pressure losses and shorten the service life of the entire seal.

Why many vacuum seals are chosen incorrectly

A large proportion of the problems in vacuum applications arise from incorrect selection decisions.

Typical causes:

• focus on price instead of technical requirements
• failure to take outgassing into account
• insufficient coordination between design and material
• lack of knowledge of the actual pressure ranges

These errors lead directly to unstable processes and unnecessary costs.

Common mistakes with vacuum seals

• unsuitable material for the vacuum range
• high outgassing due to wrong materials
• insufficient surface quality
• incorrect dimensioning of the seal
• installation errors

In practice, these points lead to unstable processes and increased maintenance effort.

Practical examples

Mechanical engineering: sealing of vacuum clamping systems or conveyor systems

Semiconductor technology: extremely high demands on purity and tightness

Laboratory and medical technology: stable processes and reproducible conditions

Decision logic for selection

Selection can be managed in a practical way via typical scenarios:

Application	Problem	Solution
Vacuum chamber	Pressure loss	FKM O-ring or metal
Flange sealing	Leakage at the connection	Flat gasket or metal
Moving shaft	Wear + leakage	Special seal PTFE/FKM
High-temperature process	Material failure	FKM or metal

In addition:

• high temperature → FKM or metal
• aggressive media → PTFE or FKM
• highest tightness → prefer metal

Material comparison for vacuum seals

Material	Temperature range (°C)	Outgassing	Media resistance	Area of use
NBR	-30 to +100	medium	limited	Rough vacuum
EPDM	-40 to +150	medium	good against water	Rough vacuum
FKM	-20 to +200	low	very high	Fine to high vacuum
PTFE	-200 to +260	very low	extremely high	demanding applications
Metal	> 300	minimal	extremely high	High and ultra-high vacuum

These values serve as a guide and vary depending on quality and manufacturer.

Using O-rings correctly in vacuum

O-rings are the most frequently used seal, but only achieve their performance with correct dimensioning.

The groove geometry determines the deformation of the O-ring. Too little squeeze leads to leakage, too high a load to premature wear. At the same time, surface roughness directly influences the contact area.

Typical mistakes in practice are the wrong choice of material, unsuitable lubrication or contamination during installation. Especially in vacuum, these factors have an immediate effect on the sealing performance.

Leak rates and technical classification

As the demands on the vacuum rise, the permissible leak rate falls significantly.

Typical classification:

• Rough vacuum: up to approx. 10⁻³ mbar·l/s
• Fine vacuum: up to approx. 10⁻⁶ mbar·l/s
• High vacuum: up to approx. 10⁻⁹ mbar·l/s
• Ultra-high vacuum: < 10⁻¹² mbar·l/s

These values show how greatly the requirements differ depending on the application. Material selection, design and installation interact directly here.

Installation and practice

Installation directly influences the sealing performance. Even small contamination or unclean surfaces lead to leaks.

Important points in practice:

• clean, grease-free surfaces
• controlled installation without damage
• suitable lubrication depending on the material
• even contact pressure

These factors determine the long-term stability of the seal.

Have a seal selected for you

The selection of the right vacuum seal always depends on several factors at the same time. Even small deviations lead to leaks, unstable processes or increased maintenance effort.

Describe your application with pressure range, temperature and operating environment and receive a concrete recommendation for material, design and dimensioning.

Frequently asked questions about seals for vacuum applications

Which seal is suitable for vacuum applications?

That depends on the pressure range, the temperature, the media used and the design. For simpler applications in rough vacuum, NBR or EPDM are often used. For higher requirements, FKM, PTFE or metallic seals are the appropriate choice.

When are O-rings suitable for vacuum?

O-rings are suitable above all for rough and fine vacuum, provided that material, groove geometry and surface quality are cleanly matched to one another. With higher demands on tightness and purity, standard solutions reach their limits more quickly.

Which material makes sense for high vacuum?

For high vacuum, FKM in a suitable quality, PTFE solutions or metallic sealing systems are often used. Which variant makes sense depends on temperature, process conditions and the desired leak rate.

Why is outgassing so important with vacuum seals?

Materials can release volatile constituents that worsen the vacuum and make processes unstable. The more demanding the application, the more important a material with low outgassing becomes.

What role does the surface of the sealing surfaces play?

The surface quality directly influences the contact area between seal and component. Unevenness, scratches or contamination increase the risk of leaks and significantly worsen the sealing effect.

What is the difference between static and dynamic seals in vacuum?

Static seals work without movement, for example on flanges or housings. Dynamic seals must additionally cope with movements, friction and wear, which significantly increases the demands on material and dimensioning.

Which seal is suitable for ultra-high vacuum?

In ultra-high vacuum, metallic sealing systems are generally used, for example made of copper or stainless steel. They offer very low leak rates and particularly high purity in the system.

Which mistakes occur particularly frequently with vacuum seals?

Typical ones are the wrong choice of material, excessive outgassing, unsuitable groove geometry, insufficient surface quality and installation errors. Even small deviations quickly lead to pressure losses and unstable processes here.

How does temperature influence the choice of vacuum seal?

As temperature rises, the elasticity, ageing behaviour and resistance of the material change. For thermally loaded applications, FKM, PTFE or metallic seals are therefore often the better solution.

How do I find the right seal for my application?

It makes sense to select via pressure range, temperature, media contact, movement and desired tightness. Only when these points are assessed together can a material and a seal type be clearly determined.

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