Seal Failure Prevention: Gasket Design Best Practices for Polyurethane Components
An unexpected seal failure rarely announces itself in advance. A gasket that held fine in testing starts weeping oil six months into service, a hydraulic cylinder loses pressure mid-cycle, or a sealed enclosure lets in enough moisture to short a board — and now you’re troubleshooting downtime instead of running production. Most of these failures trace back to decisions made at the design stage, not defects in the finished part.
Seal failure prevention starts with correct material selection, proper compression, and gland design that resist compression set and extrusion under load. Compression set, the permanent deformation remaining after sustained compression, varies by polyurethane formulation, with TPU grades ranging from 30% to 45% at 70°C (158°F) and directly affecting how long a gasket retains sealing force.
This article walks through the root causes that most often drive seal failure — material and compression-set mismatches, extrusion under pressure, thermal and chemical degradation, and tolerance and durometer errors — along with the installation and operational practices that decide whether a properly specified gasket actually performs in the field. Two things set this guide apart from the usual seal-failure roundup: quantified compression-set data across Pepson’s TPU grades, and tear-strength-driven gland design guidance for preventing extrusion, a failure mode most competing guides mention only in passing.
1. Material Selection and Compression Set
Compression set is the permanent deformation a seal keeps after it’s been compressed and then released — the gap between how thick the gasket was before installation and how thick it stays after sustained load. It’s measured under ASTM D395 (Test Method B is the default, though it isn’t suitable for material harder than 90 IRHD) and its international counterpart, ISO 815-1. A low compression-set number means the material springs back close to its original thickness; a high one means it stays flattened, and a flattened gasket stops applying sealing force even if nothing else about it looks wrong.
Grade selection changes that number substantially. Across Pepson’s TPU series, compression set at 70°C (158°F) ranges from 30% for Special Polyester E680A, the lowest-hardness special-polyester grade, up to 45% for the harder E598A and E498A general-purpose grades. Formulation chemistry can matter even more than hardness: Pepson’s cast polyurethane grades hold permanent deformation as low as 5% for a PTMG-based formulation, against 15% for standard sheet material, even though both sit in the same 82–92 Shore A hardness band. If you’re choosing between material families for a sealing application, our comparison of polyurethane and EPDM for sealing performance breaks down where each material’s compression-set and chemical-resistance profile makes sense.
How much compression is enough without going too far? Solid elastomeric gaskets generally need a narrower installed-compression range than sponge or cellular gaskets to seal reliably, though both under- and over-compression accelerate failure — too little and the seal never fully engages, too much and you’re pushing the material past its elastic range before it’s even in service.
For engineers: Have a technical challenge? Talk to our engineering team
2. Pressure-Related Failure Prevention: Extrusion and Gland Design
Extrusion happens when system pressure pushes seal material into the clearance gap between the gland and the mating surface — the seal gets squeezed out of its housing, one micro-extrusion at a time, until enough material has migrated that the seal can no longer hold pressure. It’s the one static-seal failure mode that shows up consistently across seal-failure guidance online, but almost none of that guidance addresses it for compression-molded elastomeric gaskets specifically, where gland geometry and elastomer choice are things you actually control at the design stage.
Tear strength is the material property that resists extrusion directly — it’s what keeps the seal from shearing at the gap edge rather than deforming and stopping there. Across Pepson’s TPU grades, tear strength ranges from 80 Kg/cm² at the softest polyether grade (E580A) up to 200 Kg/cm² at the hardest grades (5470D in the polyether series, E472D in the general-polyester series), a 2.5x spread that gives you real room to specify against extrusion risk rather than accepting whatever a default grade offers.
Gland and housing design does the rest of the work. ISO 3601-2 specifies class A and class B housing tolerances for O-ring and elastomeric seal glands, along with back-up-ring (anti-extrusion ring) provisions for applications where pressure alone would push a standard seal past its extrusion limit. In practice, pairing a higher-tear-strength TPU grade with a back-up ring is the standard move whenever clearance gaps or system pressure creep toward extrusion territory — neither fix alone is as effective as the combination. For O-ring-specific profiles and extrusion behavior in dynamic applications, see our guide to specialized seal profiles and O-ring applications.
3. Thermal and Chemical Degradation
Material choice also has to account for the environment a seal lives in day to day, not just the mechanical loads it sees. Pepson’s TPU families cover different temperature bands: polyether TPU runs −40°C (−40°F — the only temperature where the two scales coincide) to 80°C (176°F), general polyester TPU covers −30°C (−22°F) to 80°C (176°F), and special polyester TPU extends down to −50°C (−58°F) while topping out at the same 80°C (176°F). Specifying the wrong family for the application’s temperature swing is one of the more avoidable ways a seal degrades — the material doesn’t fail all at once, it just loses flexibility and sealing force faster than expected.
For chemical exposure specifically, Special Polyester TPU (the E680A–E698A range) is formulated for oil resistance, flex resistance, and abrasion resistance, and it carries the widest operating range of the three families — which is why it tends to be the default recommendation for demanding chemical-exposure or wide-temperature-swing sealing applications. Chemical and fluid compatibility itself is evaluated under ASTM D471, which measures volume swell, hardness change, tensile-strength loss, and elongation loss after 22-, 70-, or 166-hour immersion at elevated temperature — the standard test protocol for confirming a material actually holds up against the specific oils or process chemicals it will contact in service, rather than relying on a generic resistance rating. For a deeper breakdown of oil, chemical, and temperature performance by grade, see our guide to chemical and temperature resistance in polyurethane gaskets.
4. Tolerance, Durometer, and Dimensional Design
Even the right material fails if it’s specified at the wrong hardness or fitted to the wrong tolerance — this is a dimensional-design problem, not a formulation problem, and it’s worth treating as a separate root cause rather than folding it into material selection. Pepson’s TPU spans an 80A to 98A/72D-equivalent hardness range across the polyether, general polyester, and special polyester series, which gives you enough durometer room to match seal compressibility to the application’s load and tolerance requirements rather than settling for a one-size-fits-all grade.
Matching durometer to gland dimensions and expected load is what prevents the two most common tolerance-driven failures: under-compression, where the seal never makes full contact and leaves a leak path, and over-compression, which accelerates compression set and shortens service life even on an otherwise well-chosen material. For the full tolerance and durometer selection methodology across dynamic and static seal types, see our guide to tolerance and durometer selection for seals.
5. Operational and Installation Considerations
Installation Practices That Prevent Common Seal Failure Causes
Good material and dimensional specs still get undone by bad installation. Handle seals with clean, gloved hands — skin oils and debris damage fragile elastomeric surfaces before the part ever goes into service. Follow the manufacturer’s specified installation sequence and tooling; a gasket nicked or stretched beyond its design tolerance during assembly is compromised before the equipment even starts running. And verify shaft or housing alignment before final assembly — misalignment shows up repeatedly across seal-failure analysis as one of the most common installation-stage failure drivers, and it’s one of the easiest to catch with a quick check before buttoning everything up.
Wear and Environmental Exposure in Demanding Applications
Some operating environments wear seals down faster than the material spec alone would predict. High-friction contact and abrasive process materials accelerate degradation independent of whether material selection was otherwise correct. Specialized applications compound the problem further — UAV and drone systems, for example, layer vibration and rapid environmental cycling on top of standard wear mechanisms, which is why drone gaskets get engineered against a tighter set of constraints than a typical stationary application.
For procurement: Ready to source? Request a quote from Pepson
6. FAQ
What is the most common cause of seal failure?
Improper installation and material selection account for most seal failures — either the gasket wasn’t matched to the operating temperature and chemical exposure it faces, or it was damaged, misaligned, or under- or over-compressed during assembly. Compression set and extrusion under pressure are the two failure modes most directly tied to material and gland design choices, which is why getting those right at the specification stage prevents more failures than any maintenance schedule can catch after the fact.
How does a seal fail on a pump?
On a pump, seal failure usually starts with misalignment between the shaft and housing, which creates uneven contact pressure around the seal and lets fluid or contaminants work past it over time. Add in dry running, inadequate lubrication, or temperature and chemical exposure beyond the material’s rated range, and the seal degrades faster than the maintenance interval expects, eventually leaking or losing pressure entirely.
What are the common causes of oil seal failure?
Oil seal failure typically comes down to chemical incompatibility between the elastomer and the specific oil or process fluid it contacts, combined with thermal stress that accelerates material breakdown over time. ASTM D471 testing — which measures volume swell, hardness change, and strength loss after fluid immersion — is the standard way to confirm a material will hold up against a given oil before it’s specified, rather than finding out after installation.
What is a seal failure?
A seal failure is any point at which a gasket or seal stops blocking the moisture, dust, pressure, or fluid it was installed to contain — whether that shows up as a slow weep, a sudden pressure drop, or contamination reaching a component the seal was supposed to protect. It can result from material degradation, dimensional mismatch, installation error, or operating conditions that exceed what the seal was specified for, and it’s rarely just one of those causes acting alone.
Conclusion
Seal failure prevention comes down to matching four things to the application: material chemistry, tear strength, durometer, and the dimensional tolerances and installation practices that determine whether a well-specified gasket actually performs once it’s in service. None of these can be optimized in isolation — the best compression-set-resistant material still fails if it’s installed misaligned, and the toughest tear-strength grade still extrudes if the gland design doesn’t back it up. Pepson’s TPU and cast polyurethane grades span the compression-set, tear-strength, and temperature ranges gasket designers need to specify against exactly these failure modes, from the material stage through final assembly.
