Most rubber seals fail for one of six reasons: compression set (the seal stays flattened), ozone or UV cracking, swelling from incompatible fluids, hardening from heat, wear or tearing, or a wrong size or fit.
Each failure has visible signs and a specific fix, usually a different compound, hardness or profile.
Walker Rubber’s technical team in Norwich can identify the cause from a failed sample and make a replacement that lasts.
How do I tell why my seal failed?
What you see | Likely cause | Fix |
Seal is flat and doesn’t spring back; leaks | Compression set | Lower-compression-set compound (EPDM, silicone); less squeeze; sponge profile |
Fine cracks across the surface, worse where stretched | Ozone or UV cracking | EPDM, silicone or neoprene instead of nitrile or natural rubber |
Seal is soft, swollen or sticky | Fluid incompatibility | Match the compound to the fluid: nitrile or Viton for oils and fuels |
Seal is hard, brittle or cracks when bent | Heat ageing | Higher-temperature compound: peroxide-cured EPDM, silicone or Viton |
Seal is torn, nicked or worn away | Abrasion, sharp edges or extrusion into a gap | Harder or tougher compound; chamfer edges; change the profile |
New seal leaks or door won’t shut | Wrong size or hardness | Re-measure the gap; adjust the section or Shore hardness |
What is compression set?
Compression set is the permanent deformation left in rubber after it has been held compressed. A seal with high compression set no longer pushes back against the surface, so it stops sealing.
Lower compression set is generally better. Heat speeds it up, so a seal near its temperature limit sets much faster.
How to prevent it:
- Choose a compound with low compression set at your working temperature. Silicone and peroxide-cured EPDM perform well.
- Don’t over-compress. Solid seals typically work at 15-30% squeeze, and sponge at 30-50%.
- Use a hollow or sponge profile, so the seal bends rather than crushing.
What causes ozone cracking?
Ozone in the air attacks the double bonds in some rubbers, causing cracks at right angles to the stretch. It is worst outdoors, near electric motors and in stretched seals. Natural rubber, SBR and nitrile are most at risk. EPDM, silicone and Viton are highly resistant, and neoprene is good.
If a seal works outdoors, specify an ozone-resistant compound.
Why does a seal swell or go soft?
A seal swells when it absorbs a fluid it isn’t compatible with. EPDM in mineral oil and silicone in fuel are common examples. The seal gets larger, softer and weaker, and may extrude out of its groove.
Always list every fluid the seal touches, including cleaning chemicals and lubricants used during assembly.
Why does a seal harden and crack?
Long exposure to heat continues the curing reaction and drives off plasticisers. The rubber hardens, loses elasticity and cracks when flexed. This is heat ageing. See The Ageing of Rubber.
Choose a compound rated comfortably above your maximum working temperature, not just at it.
How long should a rubber seal last?
There’s no single figure. Life depends on material, temperature, compression, movement and exposure. As a guide, a well-specified EPDM seal outdoors can last many years, while the wrong compound in the same place may crack much sooner.
Frequently asked questions
Why do rubber seals fail?
The most common causes are compression set, ozone or UV cracking, swelling from incompatible fluids, heat ageing, wear and incorrect sizing. Each leaves different visible signs.
What is compression set in rubber?
It is the permanent flattening left after rubber has been compressed for a long time, especially when hot. It’s measured to ISO 815 as a percentage, and lower is better.
Which rubber has the lowest compression set?
Silicone and peroxide-cured EPDM generally have low compression set. Viton performs well at high temperatures.
What causes cracks in rubber seals?
Ozone and UV usually cause surface cracks, especially in nitrile and natural rubber outdoors. Heat ageing causes hardening and cracking when the seal is bent.
Why has my rubber seal swollen?
It has absorbed a fluid it isn’t compatible with, such as oil in an EPDM seal. Switch to a compound rated for that fluid, such as nitrile or Viton.
How do I make a rubber seal last longer?
Choose a compound for the actual temperature, fluids and exposure. Size it for the right compression, and avoid sharp edges and over-stretching during fitting.
Stop replacing the same seal
Send us your failed seal and its operating conditions, and we’ll recommend a replacement that lasts.Send us your drawing