PEEK Melt-State Molding Expansion Experiment: Die Swell and Viscoelastic Characterization Testing

The PEEK melt-state molding expansion experiment quantifies the die swell and elastic recovery behavior of polyetheretherketone in its viscous flow state, delivering essential data for extrusion die design, injection molding optimization, and dimensional accuracy prediction. Using high-pressure capillary rheometry combined with laser-based extrudate diameter measurement, our laboratory characterizes swelling ratios, melt viscosity, and pressure-volume-temperature (PVT) relationships across the full PEEK processing window of 360–400 °C. All testing is performed according to ISO 11443 and ASTM D3835 by an experienced polymer engineering team with rapid turnaround.

Technical Overview

Polyetheretherketone (PEEK) is a semi-crystalline, high-performance engineering thermoplastic with a glass transition temperature of approximately 143 °C and a melting point of approximately 343 °C. Above its melting point, typically between 360 °C and 400 °C, PEEK enters the viscous flow state in which molecular chains gain sufficient mobility for melt processing by injection molding, extrusion, and compression molding. Because PEEK is a linear, moderately entangled polymer with a relatively high melt elasticity, its behavior in the viscous flow state cannot be fully described by viscosity alone.

When a viscoelastic PEEK melt is forced through a die or gate, the extrudate emerges with a diameter significantly larger than the die opening. This phenomenon, known as die swell or the Barus effect, results from the recovery of elastically stored deformation energy and the relaxation of molecular orientation generated during converging flow and capillary shear. The molding expansion experiment measures this expansion quantitatively, typically expressed as the swelling ratio:

• Swelling ratio (B) = extrudate diameter divided by die diameter (De / D0)
• Area swelling ratio = extrudate cross-sectional area divided by die cross-sectional area
• Recoverable shear strain = elastic deformation component derived from swell and viscosity data

The magnitude of PEEK melt expansion depends on shear rate, melt temperature, die geometry (length-to-diameter ratio, entry angle), molecular weight distribution, and filler content. Because uncorrected expansion directly causes oversize extrudates, distorted profiles, dimensional scatter, and shrinkage/warpage in molded parts, systematic expansion testing is a prerequisite for reliable die design and process simulation input data. Our laboratory provides comprehensive melt-state expansion characterization services covering virgin resins, filled compounds, and specialty grades.

Testing Samples

Unfilled (Neat) PEEK Resin

Natural PEEK pellets or powder of standard, medium, and high molecular weight grades. Unfilled resin exhibits the most pronounced elastic swell response and serves as the baseline for viscoelastic characterization. Samples are dried prior to testing to prevent moisture-induced hydrolysis and bubble formation in the melt.

Glass-Fiber and Carbon-Fiber Reinforced PEEK

Reinforced compounds containing typically 30 % chopped glass fiber or 30 % chopped carbon fiber. Fiber orientation under shear suppresses die swell and alters anisotropic expansion behavior. Characterizing filled grades is essential for profile extrusion and structural injection molding applications in aerospace and industrial markets.

Bearing-Grade and Lubricated PEEK Compounds

PEEK modified with PTFE, graphite, carbon powder, or molybdenum disulfide for tribological applications. Solid lubricants modify melt elasticity and expansion, influencing the dimensional control of bushings, seals, and wear components.

Reprocessed, Blended, and Custom Formulations

Recycled or repeatedly reprocessed PEEK, polymer blends, pigmented compounds, and customer-specific formulations. Expansion testing detects degradation-induced changes in molecular weight and melt elasticity that standard melt flow index measurements may not fully reveal.

Testing Items

Our melt-state molding expansion experiment reports the following measured and derived parameters:

• Die swell ratio as a function of apparent shear rate (typically 100 to 10,000 s⁻¹)
• Die swell ratio as a function of melt temperature (360, 380, and 400 °C)
• Apparent and true shear viscosity curves after Bagley and Rabinowitsch corrections
• Shear stress and shear sensitivity (power-law index) of the PEEK melt
• Melt flow rate (MFR) determination under standardized load and temperature
• Recoverable elastic shear strain and first normal stress difference estimates
• Extrudate appearance grading: surface smoothness, sharkskin, melt fracture onset shear rate
• PVT (pressure-volume-temperature) expansion and compressibility data for molding shrinkage prediction
• Comparison of swelling behavior before and after drying, reprocessing, or additive modification

Testing Methods

Sample Conditioning

All PEEK samples are dried at 150 °C for a minimum of 3 hours, or per supplier specification, in a forced-air or vacuum oven. Residual moisture content is verified below 0.02 % before testing, because moisture causes hydrolytic chain scission at processing temperatures and distorts both viscosity and swell results.

High-Pressure Capillary Rheometry with Online Swell Measurement

Testing is conducted on a computer-controlled high-pressure capillary rheometer in accordance with ISO 11443 and ASTM D3835. The barrel is preheated to the set temperature, charged with dried PEEK, and packed to eliminate air. After an appropriate melt time (typically 5–7 minutes), a servo-driven piston forces the melt through a calibrated capillary die (commonly 1 mm diameter, L/D of 20 or 30, 180° or flat entry) at programmed shear rates.

A laser micrometer positioned directly beneath the die exit continuously measures the extrudate diameter in the still-molten state, while a synchronized take-off unit controls draw-down. Simultaneous acquisition of piston force, speed, and extrudate diameter generates paired viscosity and swell data at each shear rate. Correction dies of multiple L/D ratios allow Bagley entrance-pressure correction for true viscosity calculation.

Offline Dimensional and Morphological Analysis

Quenched and air-cooled extrudates are additionally measured with an optical micrometer, and diameters are corrected for thermal contraction and density change between the melt and solid state. Extrudate surfaces are examined optically to identify gross melt fracture and sharkskin onset. Optional microscopy of cross-sections reveals internal voids and fiber orientation in reinforced grades.

PVT Dilatometry for Molding Expansion

Where mold shrinkage prediction is required, PVT measurements are performed per ISO 17744 in a confining-fluid or piston-die dilatometer. Specific volume is recorded along cooling and heating paths at several hydrostatic pressures, providing the thermal expansion and compressibility coefficients used in injection molding simulation packages.

Data Reduction and Reporting

Swelling ratio, viscosity, shear stress, and recoverable strain are plotted against shear rate at each temperature. Results are delivered in a formal test report including raw data tables, correction details, instrument configuration, drying history, and engineering interpretation of processing implications.

Applications

• Extrusion die design: die land, entry angle, and exit geometry compensation for rod, sheet, film, pipe, and profile extrusion of PEEK
• Wire and cable insulation: controlling coating thickness and concentricity in PEEK-coated wires for aerospace, automotive, and downhole systems
• Injection molding process optimization: gate sizing, packing pressure profiles, and hold-time settings to offset elastic expansion and shrinkage
• Molded part dimensional accuracy: shrinkage and warpage prediction for tight-tolerance components in semiconductor handling and medical devices
• Additive manufacturing: strand diameter control and die swell compensation in fused filament fabrication and pellet-based 3D printing of PEEK
• Material qualification and QC: batch-to-batch consistency verification, regrind content assessment, and supplier comparison
• Simulation input generation: Cross-WLF and power-law viscosity model parameters, PVT coefficients, and elastic data for Moldflow, Moldex3D, and Sigmasoft
• Failure analysis: diagnosing dimension-related part rejections linked to melt elasticity variation or thermal degradation

FAQ

What is the viscous flow state of PEEK?

The viscous flow state is the temperature region above the melting point (about 343 °C for PEEK) where the polymer behaves as a viscoelastic melt capable of sustained flow. For PEEK processing, this window is typically 360–400 °C. Above approximately 420 °C, thermal degradation accelerates and invalidates expansion measurements, so test temperatures are carefully bounded.

Why does PEEK expand after exiting a die?

During flow through the die, converging streamlines and capillary shear stretch and orient the polymer chains, storing elastic energy. When the constraining die wall is removed at the exit, the chains recoil and recover their random coil conformation, causing the extrudate to swell perpendicular to the flow direction. Typical swelling ratios for unfilled PEEK range from about 1.2 to 1.8, increasing with shear rate and decreasing with temperature and die L/D ratio.

Which standards govern the test?

Capillary rheometry and die swell measurement follow ISO 11443 and ASTM D3835. Melt flow rate is determined per ISO 1133-1 / ASTM D1238. PVT expansion data follow ISO 17744. Our reports reference the applicable standard for every reported parameter.

What sample form and quantity are required?

Pellets, granules, or pulverized PEEK are acceptable; a minimum of approximately 100–200 g is recommended for a full multi-temperature, multi-shear-rate sweep. For molded or finished parts, representative chips milled from the article can be tested to evaluate lot condition, provided the thermal history is documented.

How do fillers affect the measured expansion?

Glass and carbon fibers generally reduce die swell because rigid fillers constrain molecular recoil and absorb part of the elastic energy, though they introduce anisotropic swelling and directional shrinkage. Solid lubricants such as PTFE and graphite also lower swell. Testing each specific compound is necessary because supplier formulations differ significantly.

What is the typical turnaround time?

Standard single-temperature expansion sweeps with report delivery are typically completed within 5–7 working days after sample receipt. Full multi-temperature characterization with PVT analysis and simulation model fitting generally requires 10–15 working days, depending on sample availability and scope.

Can the experiment detect PEEK degradation?

Yes. Thermal or hydrolytic degradation lowers molecular weight, reduces melt elasticity, and shifts both viscosity and swell curves downward. Comparing expansion data against a control or retained reference lot is a sensitive method for detecting excessive reprocessing, improper drying, or overheating during fabrication.