by Daniel Mays Daniel Mays 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!

Electrical Conductivity in Polymers
by Daniel Mays Daniel Mays No Comments

Electrical Conductivity in Polymers: When Plastics Carry Current

Electrical conductivity in polymers is not something commonly discussed. But there are times that a polymer component must control static charge, provide a grounding path, or contribute to EMI shielding. In such cases, electrical conductivity becomes a design requirement where the goal is not to make plastics behave like copper but to engineer a stable, predictable pathway for charge or current under real operating conditions.

The demand for such designs is rapidly increasing. For example, electronics handling is sensitive to ESD, and electrified machinery needs lightweight components that also support continuity. Chemical environments push designers away from metals, even when electrical performance still matters.

The objective of this blog post is to discuss electrical conductivity in polymers, how it works, what fillers are involved, and what both processing and testing look like.

Electrical Conductivity in Polymers: What It Means

Electrical performance in polymer components is usually specified using resistivity, and two metrics are used most often. Volume resistivity refers to charge transport through the bulk of the part. It is the right metric when the component must carry current from one face to another, or when the electrical path must remain reliable after minor surface wear.

Surface resistivity, on the other hand, reflects charge movement along the surface. It can change with surface finish, machining effects, humidity, and residues from cleaning or handling. For many assemblies, surface behavior drives field performance.

Also, keep in mind that not every application requires true electrical conduction. Many applications need controlled dissipation instead. Its goal is to prevent charge accumulation without creating a rapid discharge event. 

How Electrical Conductivity in Polymers Works

Unfilled polymers do not conduct electricity because they lack mobile charge carriers and continuous pathways. Conductive behavior must be created by introducing a filler system that forms a connected network through the matrix.

This connected network is controlled by the percolation threshold. Below that threshold, particles are too isolated to provide continuity. Near the percolation threshold, however, small changes in filler loading, dispersion quality, or processing conditions can cause large changes in resistivity.

Fillers

Conductive fillers enable conductivity in polymers, but each filler has its own tradeoffs. Electrical performance, mechanical properties, and processing stability must be considered together when specifying a polymer filler for effective conduction.

Carbon black is widely used for ESD and static-dissipative grades. It is cost-effective and typically easier to source than specialty fillers. Higher loadings can increase melt viscosity and can reduce toughness, especially in applications that experience impact or cyclic strain.

Graphite supports conductivity while improving lubricity. This filler is often selected for wear components where tribology and debris control matter greatly. The conductivity achieved depends on particle morphology and dispersion, so design considerations should include processing variability.

Carbon fiber filler can improve conductivity while also improving stiffness and strength. This filler also introduces strong directionality for some of its properties. In molded parts, note that the flow orientation can result in higher conductivity along one axis. That can be beneficial when designed intentionally, but it can introduce potential failure modes when not taken into account. 

High aspect ratio fillers like carbon nanotubes and graphene nanoplatelets can achieve meaningful conductivity at relatively low filler loading, with the practical challenge being dispersion. Consistent electrical performance requires process control and robust quality verification, particularly for high-reliability applications.

Finally, metal fibers, flakes, and metal-coated particles can provide excellent conductivity while heavily contributing to EMI shielding. However, these fillers increase density and can affect wear behavior, which can be a serious issue for some applications. Depending on the environment and mating hardware, corrosion and galvanic interactions may also need to be evaluated.

Filler typeConductivity potentialMechanical impactProcessing difficulty
Carbon blackMediumMediumMedium
GraphiteMediumLow to MediumLow to Medium
Carbon fiberMedium to High (directional)High (stiffness up, toughness may drop)Medium
CNTHighLow to MediumHigh
Graphene nanoplateletsMedium to HighLow to MediumHigh
Metal or metal-coatedHighMedium to HighMedium to High

Processing for Electrical Conductivity in Polymer Components

Electrical performance in polymer components is not solely determined by filler type, but rather by how the material is compounded, shaped, and finished.

For example, poor filler dispersion creates conductive “islands” that separatedated by insulating regions. Poor dispersion can produce inconsistent resistance across the art, and unpredictable results at contact poinA And component can pass a basic resistivity check and still fail in assembly because of localized variability in how well the filler material is dispersed.

Fiber and platelet systems can be anisotropic, which means the behavior of the material is highly dependent on the direction. For injection molded parts, for example, the skin-core structure and flow direction can lead to significant differences in resistivity by axis. If the design requires isotropic conductivity, this must be addressed through filler selection, gating strategy, or alternative processing routes.

Another example is how surface resistivity can be sensitive to simple things like machining smear, polishing, and the presence of cleaning residues. If surface conduction is extremely important, validate the electrical performance after final finishing and cleaning is complete — do not rely solely on “as molded” test coupons.

Tips for Designing Polymer Components for Conductivity

Once a polymer component is required to carry current or control charge, the interface design becomes extremely critical. First, the bulk conductivity is not going to guarantee electrical continuity through the assembly, and contact resistance is often the limiting factor. Surface finish, contact pressure, oxidation on mating metals, vibration, and thermal cycling all influence the electrical performance. There, thefore electrical contact region should be treated as a functional feature and not a basic interface.

Next, keep in mind that polymers do not spread heat efficiently. Local Joule heating at contact points can cause softening, creep, and resistance drift, which can all be problematic. In addition, current level, duty cycle, and allowable temperature rise should be specified early on in the design process because they are difficult to recover later.

In addition, fillers usually raise the modulus of elasticity and reduce elongation. Furthermore, notches and sharp corners in the design become more critical issues. If the part is going to see snap-fit strain, impact, or cyclic loading, then the geometry and material selection must be closely aligned with the reduced toughness that is typical of many conductive polymers.

Uses

Electrical conductivity in polymers supports several established application areas. Dissipative polymers, for example, are used for semiconductor handling fixtures, device nests, guides, and transport components. The objective is controlled charge bleed-off and reduced risk of sudden discharge. Conductive polymer housings, covers, and internal structural elements can support grounding strategies while reducing weight and improving corrosion resistance compared to many metal solutions.

Some wear components must maintain electrical continuity during motion, which can be a demanding requirement. It should be validated with wear testing, resistance monitoring, and debris evaluation, not inferred from bulk material properties.

Static dissipation can be important in powder handling, fuel-related environments, and certain chemical processing systems. In such cases, controlled dissipation is often the priority rather for the polymer components as opposed to high current capacity.

Conclusion

Electrical conductivity in polymers is achievable, but it must be engineered and verified. Filler selection sets the baseline, processing determines whether the conductive network is consistent, and careful interface design determines whether the assembly performs. 

If you are looking for a solution to your electrically conductive polymer design, contact Advanced EMC today. Our engineers are looking forward to talking with you.