PEEK Melt State Expansion Testing: Methods, Standards, and Applications

PEEK melt state expansion testing evaluates the volumetric and dimensional expansion behavior of polyetheretherketone above its melting transition, providing critical data for molding simulation, die design, and process control. This technical article explains the underlying principles, applicable sample types, standard test methods such as TMA, pvT dilatometry, and die swell measurement, and typical industry applications. Read on to understand how melt expansion data supports reliable processing of high-performance PEEK components.

Technical Overview

PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic distinguished by a glass transition temperature of approximately 143°C and a melting point near 343°C. When PEEK is heated above its melting point, it enters the viscous flow state, in which molecular chains gain long-range mobility and the material behaves as a viscoelastic melt. Within this state, dimensional and volumetric behavior changes markedly: thermal expansion accelerates, specific volume increases discontinuously at the crystal melting transition, residual moisture or volatiles may expand rapidly, and elastic memory stored during flow is released as die swell when the melt exits a restriction.

Melt state expansion testing quantifies these behaviors under controlled temperature, time, and pressure conditions. The key outputs, including the melt-state coefficient of thermal expansion, specific volume versus temperature (pvT) curves, die swell ratios, and post-melt dimensional stability, serve as essential inputs for injection molding simulation, extrusion tooling design, shrinkage and warpage prediction, and quality control. Because PEEK processes at relatively high temperatures, typically 360 to 400°C, and is sensitive to thermal oxidation, testing must be performed under inert atmosphere with precisely controlled heating rates to generate reproducible data.

Testing Samples

Virgin PEEK Resins

Unfilled PEEK granules, pellets, and powders represent the baseline material family. Pellets are typically compression molded into plaques or compacted into cylindrical specimens before measurement to eliminate inter-particle voids that would otherwise distort expansion readings.

Filled and Reinforced PEEK Grades

Glass-fiber reinforced grades such as 30 percent GF, carbon-fiber reinforced grades such as 30 percent CF, and bearing-grade compounds containing PTFE, graphite, and carbon fillers are tested to evaluate how rigid fillers suppress melt expansion. Fiber orientation introduced during processing creates anisotropy, so specimens are usually machined and measured in both the flow and transverse directions.

PEEK Blends and Modified Compounds

PEEK blended with PEI, PTFE, or other high-temperature polymers, along with impact-modified and tribologically optimized grades, are characterized to support formulation development and comparative material selection studies.

Finished Parts, Films, and Process Materials

Injection-molded components, extruded films and sheets, additive-manufactured specimens, sprues, runners, and regrind can be evaluated to verify process consistency, diagnose sink marks or voids, or qualify recycled feedstock.

Testing Items

• Melt-state coefficient of linear and volumetric thermal expansion above the melting point
• pvT behavior: specific volume as a function of temperature and pressure in isobaric and isothermal modes
• Die swell (extrudate swell) ratio as a function of shear rate and die geometry
• Melt density at processing temperatures
• Expansion onset temperature relative to the melting transition
• Thermal expansion hysteresis between heating and cooling cycles
• Crystallization shrinkage upon cooling from the melt
• Dimensional recovery and post-expansion stability at constant temperature

Testing Methods

Thermomechanical Analysis (TMA)

A TMA measures dimensional change with a low-load probe while the sample is heated through the melting transition at a controlled rate, typically 5 to 10°C per minute, under nitrogen. Following ISO 11359-2 and ASTM E831, the melt-state expansion segment above 343°C is isolated to calculate the melt coefficient of thermal expansion. Confined or sealed sample configurations prevent probe penetration into the softened melt.

pvT Dilatometry

A high-pressure piston dilatometer tracks specific volume across the melt transition at pressures from ambient up to approximately 200 MPa in both isobaric heating and cooling modes. The resulting pvT diagram is the primary dataset for packing and holding-phase simulation in injection molding software.

Capillary Rheometry for Die Swell

Per ISO 11443 and ASTM D3835, molten PEEK is extruded through capillary dies of defined length-to-diameter ratio at controlled shear rates, and the diameter of the free extrudate is measured immediately after exit using a laser micrometer or image analysis. The swell ratio, defined as extrudate diameter divided by die diameter, quantifies the viscoelastic expansion relevant to die design.

Supplementary Melt Flow and Rheological Testing

Melt flow rate testing per ISO 1133-1 and ASTM D1238, typically at 380 to 400°C, and oscillatory rotational rheometry per ISO 6721 provide melt viscosity context so that expansion results can be interpreted alongside flow behavior during processing.

General conditions across all methods include pre-drying samples, for example 150°C for 3 hours, nitrogen purge to limit oxidative degradation above 350°C, and test temperature windows spanning approximately 25 to 450°C.

Applications

• Input datasets for pvT and thermal expansion modeling in injection molding and extrusion CAE simulation packages
• Die and tooling design compensation based on measured die swell of PEEK melts
• Charge weight and preform sizing for compression molding and overmolding processes
• Prediction of shrinkage, sink marks, and void formation in thick-walled PEEK parts
• Batch-to-batch quality control of PEEK resins and incoming compound inspection
• Comparative evaluation of virgin, regrind, and recycled PEEK feedstock
• Material selection support for aerospace structures, medical implants and instruments, semiconductor handling components, and oil and gas sealing systems
• Failure analysis of dimensional defects traced to melt expansion and packing behavior

FAQ

What temperature range is used for PEEK melt expansion testing?

Testing typically spans room temperature up to 400 or 450°C so that the full transition into the viscous flow state above 343°C is captured. A nitrogen atmosphere is maintained to prevent oxidative degradation of the melt.

How much sample material is required?

TMA requires small specimens, roughly 5 mm square and a few millimeters thick, while pvT dilatometry generally needs 5 to 10 grams of pellets or molded discs. Capillary rheometry typically consumes 20 to 50 grams depending on die geometry and the shear-rate sweep.

Which standards govern the testing?

Dimensional expansion follows ISO 11359-2 and ASTM E831, pvT behavior follows dilatometry procedures aligned with ISO 17744, die swell is measured under ISO 11443 and ASTM D3835, and melt flow rate references ISO 1133-1 and ASTM D1238.

Can filled and fiber-reinforced PEEK grades be tested?

Yes. Reinforced grades are routinely characterized, although fiber orientation can produce direction-dependent expansion. Specimens are therefore oriented relative to the flow direction and results are reported accordingly.

How do melt expansion results differ from solid-state CTE data?

Solid-state expansion below the glass transition is typically on the order of 40 to 50 ppm/K for unfilled PEEK, while the melt-state expansion above the melting point is substantially larger and strongly pressure dependent. Melt data must be gathered in confined geometries to separate true thermal expansion from melt flow under the measuring load.

What is the typical turnaround time?

Standard single-condition testing is generally completed within 5 to 7 working days after sample receipt, with multi-condition pvT mapping and full shear-rate sweeps requiring additional time.