Rheological expansion (die swell) determination of PEEK in the viscous flow state is a high-temperature rheological test that quantifies the viscoelastic behavior of polyetheretherketone melts during extrusion and injection molding. Using capillary rheometry at temperatures between 360 °C and 420 °C, this measurement provides swell ratio data, melt viscosity curves, and critical processing parameters essential for die design, molding simulation, and quality control of PEEK components in aerospace, medical, automotive, and semiconductor applications.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic built on a rigid aromatic backbone, with a glass transition temperature of approximately 143 °C and a melting point of approximately 343 °C. Above its melting point, typically between 370 °C and 400 °C, PEEK enters the viscous flow state in which it can be shaped by injection molding, extrusion, and fiber coating. In this state the material behaves as a viscoelastic melt: it flows under shear while simultaneously storing recoverable elastic energy.
Rheological expansion, commonly referred to as die swell or extrudate swell (the Barus effect), describes the phenomenon in which a polymer melt extrudate emerges from a capillary die with a diameter significantly larger than the die orifice. The expansion originates from the elastic recovery of molecular chains that were stretched and oriented during convergent entrance flow and capillary shear flow. The swelling is quantified by the swell ratio B, defined as the extrudate diameter D divided by the die diameter D0.
- PEEK typically exhibits swell ratios in the range of 1.1 to 1.5, depending on shear rate, temperature, residence time, and molecular weight.
- The rigid aromatic chain structure of PEEK produces lower elastic recovery than flexible-chain polymers such as polyethylene, yet swell remains significant at injection-molding shear rates.
- Determination of rheological expansion is essential for die design compensation, shrinkage and warpage prediction, melt-fracture avoidance, and accurate injection-molding simulation inputs.
- Filled PEEK grades generally show reduced swell because fibers and mineral particles suppress molecular recoil and orientation recovery.
Testing Samples
Unfilled PEEK Resin Granules
Natural or virgin PEEK pellets in standard flow and high-flow molecular weight grades are the primary test material. Because hydrolysis and degradation at melt temperatures are strongly accelerated by moisture, samples are dried before testing, for example 3 hours at 150 °C, and handled in a dry atmosphere to ensure reproducible rheological data.
Filled and Reinforced PEEK Compounds
Glass-fiber reinforced (typically 30 % GF), carbon-fiber reinforced (typically 30 % CF), and mineral- or particulate-filled PEEK grades are tested to characterize how reinforcement affects swell ratio, viscosity, and extrudate surface quality. These compounds dominate structural applications, and their anisotropic fiber orientation makes accurate swell data critical for dimensional prediction.
Modified PEEK Blends and Specialty Grades
PEEK-PEI blends, wear-resistant grades containing PTFE, graphite, or molybdenum disulfide, and medical implant grades are evaluated to compare elastic memory behavior across formulations. Blend components and solid lubricants measurably alter the elastic recovery and therefore the swelling tendency of the melt.
PEEK for Additive Manufacturing
PEEK filament for fused filament fabrication and powder for laser sintering are tested to control filament diameter tolerances during melt extrusion and to model bead spreading and die-landing behavior in powder-bed processes.
Regrind, Recycled Material, and Finished Parts
Regranulated material cut from molded parts, runner systems, or recycled feedstock can be tested for quality assurance, batch-to-batch consistency, and assessment of thermomechanical degradation during reprocessing.
Testing Items
The determination of rheological expansion in the viscous flow state comprises the following measurable quantities, typically reported as functions of shear rate, temperature, or residence time:
- Apparent and true (Rabinowitsch-corrected) shear viscosity as a function of shear rate at multiple melt temperatures.
- Die swell ratio B = D/D0 versus apparent shear rate, the core deliverable of the expansion determination.
- Temperature dependence of the swell ratio and the viscous flow activation energy from Arrhenius analysis.
- Entrance pressure loss and die length dependence of pressure (Bagley correction) to separate elastic entrance effects from viscous capillary pressure.
- Critical shear rate and shear stress at the onset of extrudate surface defects (sharkskin and melt fracture).
- Melt thermal stability, monitored as viscosity drift versus residence time at the test temperature.
- Extrudate geometry quality: diameter uniformity, ovality, and post-extrusion relaxation behavior.
- Optional supporting data: melt flow rate per ISO 1133 and linear viscoelastic functions from oscillatory rheometry.
Testing Methods
The standard method is high-temperature capillary rheometry in accordance with ISO 11443 and ASTM D3835. A capillary rheometer with a heated barrel, precision force-driven piston, and interchangeable dies is used. Round-hole dies with a common diameter (for example 1 mm) and length-to-diameter ratios of 10, 20, and 30 are employed, together with a short (zero-length) orifice die for the Bagley entrance correction.
The typical test procedure is as follows: the dried PEEK granules are loaded into the barrel preheated to the set melt temperature, commonly between 360 °C and 420 °C; after a compaction and equilibration period of about 5 minutes, the piston drives the melt through the die at programmed speeds, generating apparent shear rates from roughly 50 to 10,000 s⁻¹, which spans extrusion through injection-molding conditions. Pressure at the piston is recorded continuously and converted into wall shear stress.
For the expansion measurement, the emerging extrudate diameter is determined either online with a non-contact laser diameter gauge positioned immediately at the die exit, or offline by measuring the cooled, freely collected extrudate with an optical microscope or precision micrometer at multiple axial positions. Free collection without drawing or gravity-induced necking is essential, and the average of at least five diameter readings is taken.
The swell ratio is calculated as B = D/D0, where D is the mean extrudate diameter and D0 the die capillary diameter; an area swell ratio may additionally be reported as B squared. Results are presented as swell ratio versus shear rate curves at each temperature, together with corrected viscosity flow curves. Complementary techniques include plate-plate oscillatory rheometry for storage and loss modulus in the linear viscoelastic regime, and DSC confirmation of melting behavior and crystallinity of the tested grade.
Applications
- Extrusion die design: compensation of profile dies, film dies, pipe and sheet tooling, and monofilament spinnerets so that the final PEEK extrudate meets dimensional specification after swell and thermal contraction.
- Wire and cable insulation: control of coating thickness in high-temperature PEEK wire coating, where swell directly governs insulation diameter and concentricity.
- Injection molding engineering: prediction of part shrinkage, warpage, flash tendency, and gate sizing, and provision of authentic viscosity and elasticity data for CAE filling and packing simulation.
- Additive manufacturing: diameter control of PEEK 3D-printing filament during melt spinning and extrusion, and prediction of bead geometry in large-format extrusion printing.
- Material development: comparison of molecular weight, branching, blending, and filler loading on melt elasticity for grade selection and formulation optimization.
- Quality control: incoming resin certification, batch consistency verification, and evaluation of regrind or recycled content on processing behavior.
- Industry sectors: aerospace structures, automotive powertrain and transmission components, medical implants and instruments, oil and gas seals, and semiconductor handling parts, where dimensional precision of PEEK parts is mission-critical.
FAQ
Why does PEEK show die swell if its chains are rigid?
Even though the aromatic backbone of PEEK is relatively rigid, the long polymer chains still become oriented by the strong extensional and shear deformation at the die entrance and inside the capillary. When the constraining die wall is removed at the exit, the stored elastic energy relaxes and the chains recoil, producing measurable expansion of the extrudate.
At which temperatures is the viscous flow state determination performed?
Testing is normally conducted between 360 °C and 420 °C, comfortably above the 343 °C melting point but below the onset of pronounced thermal degradation. Common set points are 380 °C, 400 °C, and 420 °C to capture the temperature dependence of swelling and viscosity.
What shear rate range is covered?
Capillary rheometry typically covers apparent shear rates from about 50 to 10,000 s⁻¹. This range spans slow extrusion processes at the low end and the high shear rates experienced by the melt during injection molding at the high end.
How do glass or carbon fibers affect the swell ratio?
Fibrous and particulate fillers restrict molecular recoil and add a rigid phase, so 30 % glass-filled or carbon-filled PEEK grades generally show lower swell ratios than unfilled resin, along with higher viscosity and an earlier onset of surface defects at extreme shear rates.
Which standards govern the measurement?
ISO 11443 and ASTM D3835 govern capillary rheometry of plastics, including pressure, viscosity, and extrudate swell determination. Melt flow rate measurements that may accompany the study follow ISO 1133 or ASTM D1238.
How is die swell distinguished from thermal shrinkage?
Die swell is an instantaneous viscoelastic expansion occurring at the die exit while the material is molten, whereas thermal shrinkage is the later contraction during cooling and crystallization governed by the coefficient of thermal expansion and crystallization behavior. Both effects must be superimposed when predicting the final dimensions of a PEEK part.
How much sample is required and how long does a test take?
A full multi-temperature, multi-shear-rate determination with swell measurement typically requires roughly 100 to 200 grams of dried granules and can be completed within several hours, including drying, barrel loading, equilibration, and replicate runs.