by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Creep and Stress Relaxation in High-Performance Polymer Seals

Creep and stress relaxation are types of time-dependent deformation that matter in sealing as too many engineers in the field typically see “assembled dry, passed leak test, then seeps later.” Sealing force is not a fixed number: it decays over time. And polymer seals can be affected by factors such as viscoelasticity, temperature sensitivity, and constraint effects. 

This article explores core definitions and concepts related to creep and stress relaxation, then covers how different polymer sealing materials behave and tips for the design and installation of seals to minimize these issues.

Definitions and Concepts for Creep and Stress Relaxation

Creep is defined as the increase in strain under constant applied stress. The constant stress can be, for example,  contact stress from interference, bolt load transferred through a gasket, or differential pressure loading. The results of creep are dimensional change, extrusion growth, reduced interference, and/or a contact pattern shift.

Cold flow refers to creep at moderate or ambient temperature and is controlled by a combination of stress and constraints. As a type of creep, cold flow is dominated by a combination of viscoelastic and viscoplastic deformation under a sustained compressive load.

Stress relaxation is decreasing stress under constant strain as the result of fixed gland volume, captured seal, or fixed squeeze. This can be a problem for static seals, where the gland maintains constant displacement, not constant stress. The results of stress relaxation include clamp-load loss, loss of sealing force, and an increased possibility of leakage.

Polymers can still look like they kept their shape, but they may not be pushing as hard against the metal anymore. In elastomers, “compression set” is mainly about the rubber not springing back. In polymers, the bigger issue is that the internal stress slowly bleeds off over time, so sealing force drops even if the part does not look significantly deformed.

When a polymer is compressed, part of the squeezed portion will spring back right away, but part of it returns slowly, and another part never returns because the material has permanently shifted shape. The longer a seal is under compression, the more the polymer begins to relax and flow, so even after the load is removed, it may not be able to rebound to restore the original sealing force. 

And if you compress a polymer seal and then release it, the force on the way back will usually be lower than on the way in because some energy is lost inside the material. That’s why repeated squeeze-and-release cycles will not bring the seal back to the original force level.

Material Behavior in High-Performance Seal Polymers

PTFE (unfilled): PTFE has extremely low friction and is very chemically resistant, but it gives up the sealing load over time. Virgin PTFE tends to creep and relax under sustained compression, therefore requiring a strong gland support, tight extrusion-gap control, or spring energization.

PTFE (filled): Filled PTFE holds up better because fillers increase stiffness and reduce cold-flow behavior. Filled PTFE can usually retain its sealing force longer than virgin PTFE, but the filler used can also increase friction and may affect counterface wear.

PEEK: PEEK is typically chosen when long-term load retention matters greatly. PEEK’s higher stiffness means better resistance to creep and stress relaxation, though solid gland design and surface control still matter. PEEK is also available in filled variants that can impact its properties.

UHMW-PE: UHMW-PE is excellent for abrasion and low friction, but it can still relax under long compressive dwell, especially if stresses are high or support is limited. It performs best when the design itself minimizes sustained stress and prevents extrusion.

PAI (Torlon): PAI offers the strongest resistance to time-dependent deformation in this group. It retains shape and sealing load well, making it a strong fit for high loads and elevated temperatures where other polymers may drift.

MaterialCreep ResistanceStress Relaxation ResistanceRebound After Long DwellExtrusion Risk (if poorly supported)
PTFE (unfilled)LowLowLowHigh
PTFE (filled)ModerateModerateLow–ModerateModerate
PEEKHighHighModerate–HighLow–Moderate
UHMW-PELow–ModerateLow–ModerateModerateModerate–High
PAI (Torlon)Very HighHigh–Very HighHighLow

Design Variables That Control Creep and Relaxation

Gland constraint is the first major factor. A fully confined gland gives the seal fewer places to move, which cuts down creep flow and helps prevent extrusion. If the gland is only partially confined, any clearance becomes an escape route for the seal, and support has to be both radial and axial. Radial support keeps the polymer from pushing into the extrusion gap under pressure. Axial support helps prevent shifting and uneven edge loading. The small geometry details count as well; add corner radii and lead-in chamfers, and avoid sharp edges that create stress concentrations. Also, remember tolerance stack-up: as the seal relaxes, the “effective” clearance and contact conditions can change even if the metal parts do not.

More squeeze is not going to automatically be safer. Higher initial stress can accelerate creep and stress relaxation, especially with heat. The goal is to start with enough contact stress to seal, then still have enough after the material settles. That means designing around the minimum required contact stress at end-of-life, not just at assembly.

Extrusion gap control is about finding where pressure can escape and blocking it. The gap changes with temperature, pressure-driven hardware deflection, and assembly variation. Backup rings help by mechanically closing off that path. Their details matter, though.

Surface finish can make or break long-term performance. Roughness peaks concentrate stress and encourage localized flow, and surface lay can create leak paths. With filled polymers, counterface hardness matters because wear risk can increase with the wrong pairing. Aim for a finish that reduces stress peaks without creating new friction or lubrication issues.

Hardware stiffness also impacts load retention. Flexible joints can magnify clamp-load loss as polymers relax, so stiffer flanges, spacers, and bolt patterns will significantly assist with stability. For demanding duty cycles, spring-energized seals are an excellent option as they add an additional force to compensate for potential issues, such as relaxation, wear, thermal cycling, and small misalignment. 

Installation Tips for Mitigating Creep and Stress Relaxation

Many issues with creep start at installation, where a small nick, a cut, or a twisted seal can leak early, then get blamed on cold flow. Over-compressing the seal during assembly also makes it worse by driving high stress that speeds up relaxation and can leave permanent deformation. A simple fix is better handling and proper lubrication during installation to reduce the potential for surface damage and help the seal seat without problems due to uneven stress.

Load management matters just as much after assembly. Polymer gaskets and seats often benefit from controlled retorque protocols (when the application allows it) because the initial load can drop quickly during the first dwell. A common approach is initial torque, a short wait, then a retorque and verification check. Keep in mind that if over-torque pushes stress too high, it can accelerate creep and shorten the sealing life.

Finally, storage can quietly pre-load your failure. If a seal sits compressed on the shelf, it may relax before it sees service, starting life with serious issues related to sealing force. Temperature history matters as well, especially if parts are stored near heat sources or in hot warehouses. When possible, ship and store seals uncompressed, and for critical applications, controlled conditioning and careful packaging can protect long-term load retention.

Conclusion

Creep, cold flow, and stress relaxation are not mysterious defects, but rather predictable behaviors that appear whenever polymers sit under load for long periods. For this reason, treat them as design inputs and build a sealing system around them by choosing the right material, controlling deformation with proper gland constraint, relying on stiff hardware to maintain load, and validating the design with tests that match real pressure, temperature, and dwell-time conditions.

Advanced EMC is here to help with all your sealing needs, and our engineers are happy to help you navigate your way through potential creep and stress relaxation issues. Contact us today!

Rotary Shaft Seals for Oil and Gas Industry | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Sealing the Depths: Spring-Energized PTFE Seals for HPHT Downhole Tools

Oilfield environments often require PTFE seals for HPHT (High-Pressure, High-Temperature) downhole tools. And this should come as no surprise, because oilfield environments represent some of the harshest sealing conditions on Earth. Temperatures can exceed 200°C and pressures can easily surpass 20 ksi. There will be exposure to media, including hydrocarbons, hydrogen sulfide (H₂S), CO₂, and amines, which can quickly render traditional sealing solutions ineffective. In such unforgiving conditions, even minor seal failure can trigger catastrophic leaks, equipment damage, or non-productive time (NPT) costing thousands of dollars per hour. That’s where spring-energized PTFE seals come into play: they are engineered precisely for high-pressure, high-temperature (HPHT) downhole environments where traditional materials fall short.

In this blog post, we’ll explore the use of spring-energized PTFE seals for HPHT applications, as well as material selection and design considerations, followed by an overview of their applications in the oil and gas industry.

Why Consider Spring-Energized PTFE Seals for HPHT Oil & Gas Environments

Unlike elastomeric seals, which tend to soften, swell, or degrade when subjected to harsh fluids and heat, spring-energized PTFE seals combine the advantages of fluoropolymer chemistry with the power of metal energizers. The result is a hybrid sealing solution that can maintain a consistent sealing force in the most demanding oil & gas applications. This solution is able to compensate for thermal expansion and resist creep under sustained load. In the depths of oil wells or even in subsea installations, this hybrid spring-energized PTFE sealing solution can make the difference between failure and flawless performance.

The HPHT Challenge: Pressure, Temperature, and Chemistry

The deeper a system operates, the more intense the pressure and temperature gradients become. Under extreme differential pressure, soft polymers tend to extrude into clearance gaps between mating surfaces, especially in dynamic or reciprocating applications. At the same time, thermal cycling in HPHT environments causes differential expansion between metal housings and polymer elements. These dimensional shifts alter contact pressure and can lead to intermittent leakage or total seal failure.

Chemistry further complicates the situation. Aggressive fluids such as drilling muds, completion brines, and sour gases attack the molecular structure of conventional elastomers like NBR or FKM. When combined with vibration, mechanical shock, and pressure fluctuations, these factors cause compression set, abrasion, and fatigue cracking. In essence, a seal in a downhole environment must perform under a complex blend of mechanical, chemical, and thermal stress—often simultaneously and continuously.

Why Spring-Energized PTFE Seals Excel in HPHT Conditions

Spring-energized PTFE seals are capable of solving some of the weaknesses that are inherent in traditional designs. The PTFE jacket provides a low-friction, chemically inert sealing interface, while the internal metallic energizer (often a canted coil, V-spring, or helical design) applies a consistent load across the sealing surface. This ensures reliable contact pressure even when temperature or system pressure fluctuates dramatically.

As well pressure increases, the system itself reinforces the seal: fluid pressure energizes the PTFE jacket, pushing it against the mating hardware for a tighter, more reliable seal even in the presence of wide-ranging pressure changes and across wide temperature ranges. Filled PTFE jackets, such as those reinforced with glass, graphite, or carbon, can also minimize cold flow and enhance wear resistance, further extending service life in dynamic applications.

Because PTFE’s coefficient of friction is among the lowest of any solid material, it minimizes heat generation and stick-slip behavior in reciprocating or rotating motion. This self-lubricating, no stick quality is particularly useful when used with directional drilling motors and measurement-while-drilling (MWD) systems, where frictional heating can distort readings or damage sensitive components.

Material Selection for Downhole Sealing

Material selection defines the performance window of a seal. Virgin PTFE offers excellent chemical resistance and thermal stability, but has its limitations. PTFE’s natural creep resistance under prolonged load can be limited; this issue can be addressed through the use of fillers such as carbon, bronze, or glass are incorporated to increase the elastic modulus and wear resistance. These filled PTFE formulations combine durability with the chemical inertness essential for downhole fluids.

The choice of spring material is also critical. Inconel 718 and Elgiloy are common due to their superior strength, fatigue life, and corrosion resistance in sour environments. These two alloys in particular maintain a highly stable spring force even after extensive compression cycles. They aid the PTFE spring-energized seal in ensuring consistent sealing load over long service intervals.

Design Considerations for Spring-Energized PTFE Seals for HPHT Downhole Applications

Precision in design directly impacts seal life. Hardware surface finish must be carefully controlled (Ra values below 8 µin are typical for dynamic sealing surfaces) to prevent wear and micro-leakage. Extrusion gaps must be minimized, particularly when pressure exceeds 15 ksi. Where necessary, designers incorporate anti-extrusion rings or step-cut backup rings to maintain stability.

Thermal expansion is another major design consideration for downhole applications. PTFE expands significantly more than steel when heated, which can affect gland squeeze and frictional characteristics. Engineers must calculate clearances and tolerances to account for this mismatch.

Seal geometry also matters. Single-lip designs are suitable for rotary applications, while double-lip or pressure-balanced configurations are preferred in static or reciprocating systems to prevent fluid entrapment. Venting pathways behind seals may also be required to prevent trapped pressure differentials, which can cause extrusion or blowout during depressurization cycles.

Applications Across Oil and Gas Systems

Spring-energized PTFE seals have proven themselves across a wide range of oil and gas applications. In MWD (Measurement While Drilling) and LWD (Logging While Drilling)  systems, they protect internal electronics from drilling mud and hydrocarbon ingress while enduring continuous vibration and pressure pulsing. In completion tools, packers, and valves, these seals maintain long-term reliability under chemical attack and mechanical load.

Subsea valves, connectors, and actuators rely on spring-energized PRFE seals for high-pressure, high-temperature applications in oil & gas to maintain zero leakage across temperature extremes and under deep-water hydrostatic pressure. High-pressure pumps and reciprocating actuators use these seals to minimize downtime, extend maintenance intervals, and maintain consistent performance across thousands of operational cycles.

Performance that Endures Where Elastomers Fail

In HPHT downhole service, traditional elastomer seals simply can’t cope with the combined assault of heat, pressure, and chemistry. Spring-energized PTFE seals for high-pressure, high-temperature applicants bridge the gap between flexibility and endurance, offering predictable performance where failure is not an option. Their combination of low friction, corrosion resistance, and mechanical adaptability makes them indispensable in modern oilfield equipment.

As operators push deeper into the earth’s crust and toward higher-pressure, higher-temperature reserves, the demand for advanced sealing materials will continue to rise. Spring-energized PTFE seals—especially those engineered with filled PTFE and high-performance alloys—represent the next generation of reliability in extreme sealing applications.

If you are looking for a reliable sealing solution for HPHT download tools, contact Advanced EMC today. Our engineering team is ready to work with you to extend equipment life, enhance safety, and maintain uptime in some of the world’s most demanding operating environments.