TPU Compression Set, Creep, and Stress Relaxation: What Long-Term Load Tests Actually Tell You

A universal testing machine applies compressive load in a controlled test. The pictured composite specimen is not TPU; the photo illustrates equipment context. Photo: Cjp24. Source · CC BY-SA 3.0

A TPU sample can rebound quickly after a squeeze yet still lose thickness or force after hours under load. Compression set, creep, stress relaxation, and rebound describe different time-dependent behaviors. Buyers should start with the function that must be retained—dimension, force, seal, fit, cushioning, or energy return—and then choose a test that reproduces the load history.

Quick answer

Compression set measures dimensional recovery after a specimen is held at a defined compression for a defined time and temperature and then released for a stated recovery period. Creep tracks increasing deformation under sustained load. Stress relaxation tracks decreasing force under sustained deformation. Rebound describes rapid energy return. None is interchangeable with the others, and no single datasheet value predicts long-term finished-part performance unless method, geometry, temperature, time, conditioning, and acceptance criteria match the application.

Four behaviors buyers often confuse

Behavior	Controlled condition	Measured response	Typical product question
Rebound / resilience	Short impact or deformation and release	Returned energy, bounce, or rapid recovery	Will a midsole, wheel, or cushioning element feel responsive in each event?
Compression set	Held at defined compression; released after time/temperature exposure	Residual thickness loss or incomplete dimensional recovery	Will a seal, pad, or cushion recover its shape after being compressed?
Creep	Held under approximately constant load or stress	Deformation increases with time	Will a support, roller, foot, or loaded component slowly sag or flatten?
Stress relaxation	Held at approximately constant deformation or strain	Force decreases with time	Will a gasket, clamp, grip, or interference-fit part retain contact force?

The distinction is operational. A seal can recover much of its thickness after removal yet provide too little force while installed. A cushion can rebound well during a short impact but slowly lose height under a static load. A roller can appear normal after unloading but develop a flat spot during a long dwell. The required measurement follows the failure mode.

Compression-set methods are defined by their conditions

ASTM D395 and ISO 815-1 are widely referenced for compression set of rubber and thermoplastic rubber. ISO 815-1:2019 specifies methods at ambient or elevated temperatures and distinguishes procedures according to how the test piece is released at the end of an elevated-temperature test. A report that states only “compression set: 20%” is incomplete for comparison.

• Standard and method/procedure, including whether compression is applied by a specified deflection or another controlled setup.
• Specimen shape, diameter, thickness, preparation route, molding orientation, density, and conditioning.
• Compression level or spacer thickness; actual initial and final measurements; fixture alignment and surface condition.
• Exposure time and temperature, including whether release occurs while hot or after cooling.
• Recovery time and recovery temperature before the final thickness measurement.
• Whether the value comes from a plaque specimen, foam, printed lattice, molded component, gasket section, or finished assembly.

A dissected running-shoe heel illustrates that cushioning response depends on geometry, layers, density, bonding, and loading—not resin rebound alone. The pictured materials are not identified as TPU. Photo: Yottanesia. Source · CC0

Short elevated-temperature tests are useful for material classification and quality control, but they should not automatically be converted into a service-life claim. BASF’s TPU property guide also notes that finished-part properties are affected by design, processing, molecular orientation, internal stress, moisture, annealing, and environment. Compare supplier numbers only when these conditions are aligned.

Creep and stress relaxation answer different design questions

If the product is controlled by…	Test direction	What to plot	Example risk
Applied load / weight / pressure	Hold load approximately constant	Strain, displacement, thickness, or deflection versus time	Support pad sags; roller or wheel develops a flat spot; mount loses position.
Installed squeeze / interference / displacement	Hold deformation approximately constant	Force or stress versus time	Gasket loses sealing pressure; grip or clip loses retention; clamp load decays.
Repeated load with rest periods	Apply a defined cyclic waveform	Peak force, energy loss, residual set, temperature, and recovery versus cycles	Midsole or lattice loses height/rebound; belt or wheel heats and changes dimensions.

Real assemblies may combine all three. A seal begins with imposed squeeze, sees pressure pulses and thermal expansion, and may absorb fluid. A shoe midsole receives repeated impacts plus body-weight dwell. A conveyor roller sees cyclic contact while rotating, then sustained load while stopped. A useful test sequence separates mechanisms in coupons, then recombines them in the part.

Why high rebound does not guarantee long-term recovery

Rebound is associated with how much energy returns quickly after a short deformation. Compression set and relaxation develop over longer time scales and are strongly accelerated by temperature. Material morphology, hardness, density, cell structure, wall geometry, additives, processing history, and aging can influence these behaviors differently. High rebound may be desirable, but it is not evidence by itself of low compression set, low creep, or strong force retention.

For foamed TPU and printed lattices, density and architecture become part of the material system. Cell collapse, skin thickness, strut buckling, anisotropy, bonding, and local stress concentration may dominate the response. Report both resin/compound identity and structure; a bulk plaque result cannot represent every foam density or lattice geometry.

Match the test to the application failure

Different gasket shapes show why section geometry, squeeze, and assembly determine whether material data transfers to a seal. The pictured compounds are not identified as TPU. Photo: Vis M. Source · CC BY-SA 4.0

Application	What must be retained	Recommended evidence	Common misread
Seal or gasket	Contact pressure, dimensions, and leakage control	Compression set plus stress relaxation; fluid/heat aging; representative groove and leakage cycles	Low plaque set is treated as proof of sealing life.
Midsole / cushioning / lattice	Height, feel, rebound, impact attenuation, and stability	Static creep/set; cyclic compression; recovery after rest; density/cell or geometry inspection	High rebound is treated as proof of no long-term thickness loss.
Roller / wheel / belt	Roundness, traction, dimensions, force transfer, and heat control	Static dwell/flat-spot test; cyclic load; temperature rise; wear and runout after recovery	Hardness is treated as the only predictor of flat spotting.
Overmolded grip / soft-touch part	Fit, interference, clamp/contact force, and surface integrity	Relaxation in the assembly; thermal cycles; substrate adhesion; chemical/sweat aging	Immediate pull-off or hand feel is treated as long-term retention.
Vibration pad / bumper	Thickness, preload, isolation, and impact recovery	Creep under load; dynamic stiffness/damping; compression set; temperature sweep	One compression-set value is treated as a vibration-performance specification.

Temperature, aging, chemicals, and processing history matter

• Temperature: viscoelastic deformation and relaxation generally accelerate as temperature rises. Test continuous, peak, and thermal-cycle conditions rather than choosing an arbitrary convenient temperature.
• Water and humidity: hydrolysis-sensitive systems can change molecular weight and mechanical recovery. Include the actual wet/hot environment and measure retained properties.
• Oils, fuels, cleaners, sweat, and plasticizer contact: swelling or extraction changes dimensions, hardness, and force. Age in the exact media, concentration, temperature, and contact mode.
• Processing: moisture, residence time, melt temperature, shear, orientation, annealing, weld/knit lines, regrind, and pigments can change morphology and variation. Use production-representative specimens.
• Geometry and tolerances: compression ratio, section thickness, groove fill, load area, radius, venting, cell/lattice architecture, and surface friction alter stress distribution.
• Recovery protocol: a result measured immediately after release can differ from one measured after a longer recovery. Record the complete timing sequence.

An industrial roller conveyor illustrates sustained loads, repeated contact, and potential flat-spot or force-retention concerns. The roller materials are not identified as TPU. Photo: Kay-Uwe Rosseburg. Source · CC BY 3.0

Common failures and the first evidence to collect

Observed failure	Mechanisms to separate	First evidence
Seal leaks after hot dwell	Stress relaxation, compression set, fluid swell, low squeeze, tolerance stack	Retained force and leakage versus time; section/groove dimensions; aged material measurements.
Midsole stays compressed	Creep/set, cell collapse, geometry buckling, insufficient recovery time	Height and force over cycles/rest; density/cell or lattice inspection; temperature history.
Roller flat spot after shutdown	Sustained load creep, temperature, hardness/morphology, bearing or support geometry	Dwell load/time, runout profile, roller temperature, recovery curve and construction.
Grip becomes loose	Stress relaxation, substrate creep, adhesive/bond failure, chemical exposure	Assembly force versus time, interface failure mode, thermal/chemical history and dimensions.
Results vary by lot/cavity	Moisture/process history, dimensions, density, orientation, conditioning	Lot/cavity map, process record, specimen dimensions, mass/density and conditioning log.

A practical long-term deformation test plan

1. Define the retained function and failure consequence: thickness, position, contact force, sealing pressure, leakage, rebound, damping, fit, or roundness.
2. Translate service into load or deformation, time, temperature, cycles, rest/recovery, fluid exposure, and geometry. Identify whether load or displacement is controlled.
3. Run comparable material specimens using a named compression-set, creep, relaxation, or cyclic method. Preserve raw thickness/force/displacement-versus-time data where possible.
4. Process representative parts using intended resin lot, drying, color/additives, regrind, mold/extrusion/printing conditions, density, annealing, and assembly.
5. Add relevant aging before and during load: heat, humidity, water, oils, cleaners, fuels, sweat, UV, or thermal cycling.
6. Test the real assembly. Measure leakage, retained clamp/seal force, part dimensions, runout, cushioning, bond integrity, or other functional outputs.
7. Repeat across lots, cavities or line conditions and define production controls. Do not derive a lifetime guarantee from one short laboratory point without a validated correlation.

What to send GOAFLEX for material review

• Application drawing and assembly: section thickness, loaded area, groove/interference, tolerances, support, surface finish, foam density or lattice geometry.
• Whether the part is held under constant load, constant deformation, repeated load, or a combined duty cycle; include magnitude, frequency, dwell, rest, and life target.
• Continuous and peak temperatures, thermal cycles, recovery temperature/time, and any heat generated during use.
• Exact fluids, humidity, water, oils, cleaners, fuels, sweat, UV, and pre-aging sequence.
• Current grade/compound, hardness, color/additives, drying and processing conditions, annealing, regrind level, lot/cavity/line history, and failed/control parts.
• Named standards/methods and complete conditions, raw force/displacement/thickness data, acceptance limits, and the functional failure being prevented.

Need help selecting a long-term load test for TPU? Send the part geometry, load history, temperature, exposure media, current data, and failure mode to hello@goaflex.com. For related context, see High-Rebound TPU: How Molecular Design Improves Elastic Recovery and TPU Seals and Gaskets.

Sources and further reading

• ISO — ISO 815-1:2019: https://www.iso.org/standard/74943.html — Compression-set methods for vulcanized and thermoplastic rubbers at ambient or elevated temperatures; procedures differ in release handling.
• ASTM International — Rubber standards catalog / ASTM D395-18(2025): https://store.astm.org/products-services/standards-and-publications/standards/rubber-standards.html — Current catalog context for ASTM D395 compression-set methods; confirm the purchased active standard before testing.
• BASF — Elastollan TPU Material Properties: https://download.basf.com/p1/8a8082587fd4b608017ff4e900830157/en/Elastollan — TPU-specific background on creep, compression set, temperature dependence, processing history, and finished-part validation.
• PREPOL — Compression Set: What Do I Need to Know?: https://www.prepol.com/downloads/compression-set/ — Practical interpretation of ASTM D395 and ISO 815 values and why method, time, and temperature must accompany a number.
• Stockwell Elastomerics — Compression Set, Stress Relaxation and Creep: https://www.stockwell.com/blog/rubber-compression-set-stress-relaxation-and-creep/ — Application-level distinctions among dimensional set, increasing deformation, and loss of force.
• University of Birmingham — Compression set of TPU under different thermal-mechanical conditions: https://research.birmingham.ac.uk/en/publications/compression-set-of-thermoplastic-polyurethane-under-different-the/ — Peer-reviewed research record reinforcing that TPU compression set depends on thermal-mechanical conditions; do not generalize one formulation’s result.

Image credits and licensing

• Image 1: A universal testing machine applies compressive load in a controlled test. The pictured composite specimen is not TPU; the photo illustrates equipment context. Creator/source: Cjp24. https://commons.wikimedia.org/wiki/File:Compression_test-Universal_testing_machine.jpg. License: CC BY-SA 3.0. Alt-text guidance: “Universal testing machine applying a compressive load to a cylindrical specimen”
• Image 2: A dissected running-shoe heel illustrates that cushioning response depends on geometry, layers, density, bonding, and loading—not resin rebound alone. The pictured materials are not identified as TPU. Creator/source: Yottanesia. https://commons.wikimedia.org/wiki/File:Nike_Running_Shoe_Dissected_Rear.jpg. License: CC0. Alt-text guidance: “Dissected running-shoe heel showing layered cushioning construction”
• Image 3: Different gasket shapes show why section geometry, squeeze, and assembly determine whether material data transfers to a seal. The pictured compounds are not identified as TPU. Creator/source: Vis M. https://commons.wikimedia.org/wiki/File:Rubber_gaskets_02.jpg. License: CC BY-SA 4.0. Alt-text guidance: “Assorted gasket shapes illustrating sealing geometries and compression requirements”
• Image 4: An industrial roller conveyor illustrates sustained loads, repeated contact, and potential flat-spot or force-retention concerns. The roller materials are not identified as TPU. Creator/source: Kay-Uwe Rosseburg. https://commons.wikimedia.org/wiki/File:Accumulation_Roller_Conveyor_for_Cartons_Totes.jpg. License: CC BY 3.0. Alt-text guidance: “Industrial accumulation roller conveyor carrying cartons and totes”

Context note: These search-sourced photographs are used to illustrate application, equipment, or testing context. They do not prove GOAFLEX ownership of the pictured products, creator endorsement, exact TPU chemistry or grade identity, or the performance of any GOAFLEX material.

Intellectual property notice: If any information or material in this article is believed to violate intellectual property rights, please contact GOAFLEX so it can be reviewed and addressed.