PEEK Capillary Rheology Extrudate Swell Analysis: Principles, Methods, and Industrial Applications

PEEK capillary rheology extrudate swell analysis quantifies the viscoelastic behavior of polyetheretherketone melts under processing-relevant shear conditions, providing critical data on die swell ratio and shear viscosity across a wide range of shear rates. This test method supports extrusion die design, injection molding optimization, and quality control of high-performance PEEK grades used in aerospace, medical, and industrial applications. This article explains the testing principle, sample requirements, key measurement items, standard procedures, and typical applications of the analysis.

Technical Overview

PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic belonging to the polyaryletherketone (PAEK) family. It combines a glass transition temperature of approximately 143°C, a melting point of approximately 343°C, excellent chemical resistance, and outstanding mechanical strength, which makes it a preferred material for demanding applications in aerospace, medical devices, electronics, and oil and gas engineering. Because PEEK is processed at relatively high melt temperatures, typically between 370°C and 400°C, understanding its melt flow behavior is essential for stable and reproducible manufacturing.

Capillary rheology is a fundamental technique for characterizing the flow behavior of polymer melts at high shear rates that closely simulate real processing conditions such as extrusion and injection molding. A piston drives the molten polymer through a calibrated capillary die at controlled speeds while the pressure drop is recorded, allowing the apparent shear viscosity to be calculated at shear rates typically ranging from 100 to 10,000 s⁻¹. Unlike melt flow index testing, which provides only a single low-shear data point, capillary rheometry delivers a complete flow curve that reveals shear thinning behavior, thermal degradation tendencies, and critical flow instabilities.

Extrudate swell, also known as die swell or the Barus effect, refers to the phenomenon in which the diameter of the extruded polymer strand exceeds the diameter of the capillary die. This elastic recovery occurs because polymer chains stretched and oriented by the strong extensional and shear deformation in the die entrance and die land relax and recoil once the material exits the constraint of the die. The swell ratio, defined as the ratio of the extrudate diameter to the die diameter, is a direct indicator of melt elasticity and is strongly influenced by molecular weight, molecular weight distribution, branching, filler content, shear rate, melt temperature, and die geometry. For PEEK, accurate swell characterization is indispensable for dimensional control of extruded profiles, films, fibers, tubing, and wire insulation.

Testing Samples

Virgin PEEK Pellets and Granules

Standard injection molding or extrusion grade PEEK pellets are the most common sample form. Representative grades include unfined neat resin with medium and high molecular weight, which are tested to establish baseline viscosity and swell curves for process window definition.

Fine Powder Grades

PEEK powders used in compression molding, coating, or additive manufacturing can be tested after appropriate densification. Powder samples are typically pre-consolidated or loaded carefully to avoid air entrapment, which would corrupt pressure and swell measurements.

Filled and Reinforced Compounds

Glass fiber reinforced, carbon fiber reinforced, carbon filled, and PTFE lubricated PEEK compounds are frequently analyzed. Rigid fillers suppress elastic recovery and reduce swell, so filled compounds require swell data to correctly compensate die tooling dimensions.

Sample Preparation Requirements

• Dry all PEEK samples before testing, typically at 150°C for 3 hours or 120°C overnight, because moisture causes hydrolytic degradation, pressure fluctuations, and bubble formation in the extrudate.
• Ensure sample mass is sufficient to fill the rheometer barrel and allow equilibrium time, generally 30 to 100 grams depending on barrel volume.
• Avoid contamination and thermal history variation, as prior processing can alter crystallinity and measured melt elasticity.
• Store dried samples in a desiccator and load them promptly to minimize moisture reabsorption.

Testing Items

A comprehensive PEEK capillary rheology swell analysis typically reports the following measured and derived quantities:

• Apparent shear viscosity as a function of apparent shear rate, spanning the processing-relevant range from approximately 100 to 10,000 s⁻¹.
• Apparent shear stress versus shear rate, used to identify flow regimes and power-law behavior.
• Extrudate swell ratio versus shear rate, quantifying elastic recovery of the melt after die exit.
• Bagley corrected shear stress, obtained from measurements with multiple dies of different length-to-diameter ratios to eliminate entrance and exit pressure losses.
• Rabinowitsch corrected true shear rate, accounting for the non-parabolic velocity profile of shear thinning melts.
• Critical shear rate for the onset of melt fracture, sharkskin, or flow instability at the die exit.
• Temperature dependence of viscosity and swell, evaluated by testing at multiple melt temperatures such as 370°C, 385°C, and 400°C.
• Extrudate appearance documentation, including surface quality, diameter uniformity, and draw-down behavior.

Testing Methods

Instrumentation and Die Configuration

Testing is performed on a constant-rate capillary rheometer equipped with a temperature-controlled barrel, a force-rated piston drive, and a pressure transducer positioned near the die entrance. A dual-bore instrument is preferred because it simultaneously measures a long die and a zero-length orifice die, enabling direct Bagley correction in a single run. Circular capillary dies with a 1 mm diameter and length-to-diameter ratios of 10, 20, and 40 are commonly used, with a 90° flat entrance angle as the standard configuration. The extrudate diameter is measured either in-line with a laser micrometer immediately at the die exit or off-line on the cooled extrudate using an optical microscope or precision micrometer.

Test Procedure

• Dry the PEEK sample according to the resin specification, then preheat the rheometer barrel to the target melt temperature, typically between 370°C and 400°C.
• Load the sample incrementally into the barrel, compacting each portion to remove trapped air, then insert the piston and allow a melt equilibrium soak of approximately 5 to 10 minutes.
• Optionally purge with nitrogen to minimize oxidative degradation of the melt at high temperature.
• Apply a short pre-shear conditioning step to erase prior thermal and mechanical history.
• Step the piston speed through a programmed sequence of increasing shear rates, holding each rate until steady-state pressure is reached before data acquisition.
• Capture the extruded strand at each shear rate and measure its diameter to compute the swell ratio.
• Repeat the sequence for additional dies, temperatures, or replicate charges to assess repeatability.

Data Reduction and Corrections

Raw pressure and piston speed data are converted to apparent shear stress and apparent shear rate using the die geometry. The Bagley correction extrapolates pressure versus die length-to-diameter ratio data to zero length, removing the pressure contribution of entrance convergence and exit elasticity. The Rabinowitsch-Weissenberg correction then converts apparent shear rate at the wall to true shear rate for the non-Newtonian PEEK melt. The swell ratio is calculated as the extrudate diameter divided by the die diameter, and volumetric swell can additionally be derived from the square of the diameter ratio when incompressibility is assumed.

Typical Test Conditions for PEEK

• Melt temperatures from 370°C to 400°C, selected to match the intended process without inducing thermal degradation.
• Shear rates from 100 to 10,000 s⁻¹, covering extrusion, compounding, and injection molding regimes.
• Residence time in the barrel kept short to avoid viscosity drift from thermal or oxidative degradation.
• In-line laser measurement of swell performed within a few millimeters of the die exit to minimize gravitational draw-down and cooling effects.

Applications

PEEK capillary rheology swell analysis delivers actionable data across the entire product development and manufacturing cycle:

• Extrusion die design for profiles, rods, sheets, films, and tubing, where the swell ratio directly determines the required die gap compensation to achieve target final dimensions.
• Wire and cable insulation extrusion, where dimensional stability and concentricity of thin PEEK layers depend on accurate swell prediction at high line speeds.
• Monofilament and catheter manufacturing for medical devices, where precise diameter control and surface quality are regulatory requirements.
• Injection molding process optimization, including gate sizing, filling balance, and prediction of residual stress related to melt elasticity.
• Material grade selection and incoming quality control, comparing viscosity and swell fingerprints between suppliers, lots, or reprocessed content levels.
• Compound development for glass fiber, carbon fiber, and lubricant filled systems, quantifying how formulation changes alter elasticity and processability.
• Simulation input generation, supplying true viscosity curves and elastic parameters for injection molding and extrusion CAE models.
• Aerospace, oil and gas, semiconductor, and automotive qualification programs that require documented rheological characterization of high-temperature polymers.

FAQ

Why is die swell measured together with viscosity in capillary rheometry?

Viscosity describes the resistance to steady flow, whereas swell reflects the elastic energy stored in the melt during deformation. Both properties are needed for complete process simulation: two PEEK lots may show identical viscosity curves yet different swell ratios, leading to unexpected dimensional variation in extrusion. Measuring both parameters in a single test provides a more reliable processability fingerprint.

Why does PEEK generally show lower swell than many commodity polymers?

PEEK is a rigid, semi-crystalline aromatic polymer with limited chain extensibility, so it stores less recoverable elastic strain under the same deformation history compared with flexible, highly entangled commodity resins. However, swell of PEEK is still significant at high shear rates and low melt temperatures, and it increases with molecular weight and broader molecular weight distribution.

How do fillers affect PEEK extrudate swell?

Glass fibers, carbon fibers, and mineral fillers restrict molecular recoil and effectively reduce the swell ratio, often substantially. The reduction depends on filler loading, aspect ratio, and orientation. Filled PEEK compounds therefore require dedicated swell measurements rather than correlations derived from neat resin data.

What happens if the sample is not dried before testing?

Moisture in PEEK causes hydrolytic chain scission at high melt temperatures, lowering viscosity, generating volatiles that disturb pressure readings, and producing bubbles or rough surfaces in the extrudate that invalidate swell measurements. Thorough drying is therefore a mandatory precondition for accurate results.

How does melt temperature influence the results?

Increasing melt temperature lowers viscosity and reduces swell because molecular relaxation is faster relative to the residence time in the die. Testing at several temperatures reveals the sensitivity of both viscosity and swell to process temperature, helping manufacturers define robust processing windows for PEEK.

Which standards govern this test method?

Capillary rheometry of thermoplastics is standardized in ISO 11443 and ASTM D3835, which specify apparatus requirements, test procedures, and data treatment including Bagley and Rabinowitsch corrections. Swell measurement practice follows the general principles of these standards, supplemented by in-line or off-line diameter metrology appropriate to the extrudate geometry.

How is the swell ratio reported?

The swell ratio is reported as the dimensionless ratio of extrudate diameter to die diameter at each shear rate, commonly presented as a swell versus shear rate curve together with the viscosity flow curve. Replicate measurements are recommended, and swell values are typically reproducible within a few percent when drying, temperature control, and diameter metrology are carefully executed.