PEEK melt-state expansion coefficient testing measures how the specific volume of polyetheretherketone changes with temperature and pressure above its melting point, providing essential data for precision molding and process simulation. This article explains the underlying physics, standard test methods such as ISO 17744 pvT dilatometry, sample requirements, typical measured values, and practical applications in injection molding and extrusion of PEEK components.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline, high-performance engineering thermoplastic used in aerospace, medical, semiconductor, and energy industries. It exhibits a glass transition temperature of approximately 143 °C and a melting point near 343 °C, and it is typically processed at melt temperatures between 360 °C and 400 °C. In the molten state, the specific volume of PEEK is highly sensitive to both temperature and pressure, and the melt-state expansion coefficient quantifies this volumetric expansion behavior above the melting point. Accurate knowledge of this coefficient is critical for predicting part shrinkage, warpage, sink marks, flash formation, and packing efficiency during injection molding, as well as for die design in extrusion processes.
Definition and Physical Significance
The melt-state expansion coefficient is most commonly expressed as the volumetric thermal expansion coefficient, defined as the fractional change in specific volume per unit change in temperature at constant pressure. Because a polymer melt is hydrostatically isotropic, an equivalent linear expansion coefficient can be derived by dividing the volumetric value by three. Unlike solid-state thermal expansion, melt-state expansion is strongly pressure-dependent: raising the applied pressure from atmospheric to 200 MPa can reduce the apparent expansion coefficient of a PEEK melt by a substantial margin. This is why melt-state characterization is performed along defined pressure-volume-temperature (pvT) paths rather than at a single condition.
Typical Values for PEEK
- Melting point: approximately 340–345 °C; crystallization (freezing) point on cooling: approximately 320–340 °C depending on cooling rate.
- Melt density at 380 °C under atmospheric pressure: approximately 1.10–1.18 g/cm³, compared with roughly 1.30 g/cm³ for the semi-crystalline solid.
- Volumetric expansion coefficient of the melt near atmospheric pressure: approximately 5–8 × 10⁻⁴ K⁻¹, decreasing measurably at elevated pressures.
- Equivalent linear expansion coefficient in the melt: approximately 1.7–2.7 × 10⁻⁴ K⁻¹, considerably higher than the solid-state values of roughly 45–50 × 10⁻⁶ K⁻¹ below the glass transition.
Exact values vary with molecular weight, crystallinity history, filler content, and the measurement path, which is why grade-specific testing is recommended for critical applications.
Testing Samples
Unfilled PEEK Resins
Standard natural-color granules or powders of unfilled PEEK grades are the most frequently tested materials. Pellets are loaded directly into the dilatometer chamber, or pre-consolidated slugs are used to minimize entrapped air. Unfilled results establish the baseline pvT behavior referenced in material datasheets.
Glass-Filled and Carbon-Filled Compounds
Reinforced grades containing 30 percent glass fiber or 30 percent carbon fiber, as well as bearing grades filled with PTFE, graphite, or carbon powder, are tested to capture the reduced compressibility and altered expansion behavior introduced by the fillers. Because reinforcement is partially oriented during processing, melt-state data remains isotropic and is used together with solid-state anisotropic shrinkage data for full simulation accuracy.
Specialty Grades, Colors, and Custom Compounds
Pigmented compounds, reground material blends, and custom formulations supplied by compounders can be characterized to generate input data for simulation software when supplier data is incomplete or unavailable.
Sample Preparation and Conditioning
- Drying: typically 3 hours at 150 °C or 4 hours at 120 °C in a vacuum or desiccant dryer, targeting residual moisture below 0.02 percent.
- Sample mass: commonly 5–20 grams depending on the instrument chamber volume.
- Form: as-received pellets, powder, or vacuum-compressed preforms to eliminate voids.
- Handling: dry transfer to the test cell to avoid moisture reabsorption before the chamber is sealed.
Testing Items
A comprehensive melt-state expansion characterization of PEEK typically includes the following deliverables:
- Volumetric thermal expansion coefficient of the melt as a function of temperature and pressure.
- Equivalent linear expansion coefficient derived from the volumetric data.
- Specific volume versus temperature curves at multiple constant pressures (isobaric mode).
- Specific volume versus pressure curves at multiple constant temperatures (isothermal mode).
- Melt density at defined processing temperatures and packing pressures.
- Melting point and crystallization point extracted from pvT transition knees.
- Isothermal compressibility of the melt.
- Volumetric shrinkage indicators computed from the specific volume difference between the melt at the end of packing and the solid at ejection temperature.
Testing Methods
High-Pressure Piston Dilatometry (ISO 17744)
The dominant method for melt-state expansion measurement is high-pressure piston dilatometry conforming to ISO 17744. The polymer sample is confined in a sealed cylinder and pressurized by a precision piston. As temperature or pressure is varied, the piston displacement is measured with a displacement transducer, and the change in sample volume is calculated after correcting for cell expansion and seal behavior. Hydrostatic pressure transmission is maintained either by direct contact, a flexible sealing membrane, or a mercury or silicone oil transmission fluid. This technique delivers the full two-dimensional pvT surface required by molding simulation packages.
Isobaric and Isothermal Measurement Modes
In isobaric mode, the sample is heated or cooled at a controlled rate under constant pressure, and the volumetric expansion coefficient is obtained from the slope of the specific volume versus temperature curve within the melt region. In isothermal mode, the sample is held at a set temperature while pressure is stepped through a defined profile, yielding compressibility data and isothermal pvT points. Modern instruments combine both modes to build a dual-domain (transition and no-transition) pvT model of the material.
Capillary Rheometer-Based pvT Measurement
Some laboratories perform pvT measurements using a high-pressure capillary rheometer equipped with a sealed pvT test cell, allowing specific volume data to be acquired alongside melt viscosity on the same instrument. This approach is useful when a combined rheological and thermophysical dataset is required for process modeling.
Typical Test Conditions
- Temperature range: approximately 320–420 °C, fully covering the melt region of PEEK.
- Pressure range: atmospheric to 200 MPa, with some instruments extending to 250 MPa.
- Cooling and heating rates: commonly 5–20 K/min, with faster cooling scans used to approximate molding conditions.
- Atmosphere: nitrogen purge to prevent oxidative degradation at elevated temperatures.
- Calibration: validated with certified reference materials or published pvT data for a standard polymer.
Solid-state expansion coefficients, by contrast, are measured separately by thermomechanical analysis according to ISO 11359-2 or ASTM D696, but those methods are not applicable above the melting point and must not be extrapolated into the melt region.
Applications
Melt-state expansion data for PEEK directly supports engineering decisions across the polymer processing value chain:
- Injection molding simulation: pvT curves serve as mandatory input for shrinkage and warpage prediction in software such as Moldflow, Moldex3D, SIGMASOFT, and Cadmould.
- Packing phase optimization: compressibility and pvT slopes guide holding pressure and holding time selection to minimize sink marks and voids in thick sections.
- Precision mold design: melt and solid specific volume data enable accurate steel-safe dimensioning for tight-tolerance medical implants, surgical instruments, and semiconductor wafer carriers.
- Extrusion and film processes: melt density and expansion behavior support die flow balancing, draw-down calculations, and thickness control.
- Material qualification: comparative pvT testing supports grade selection, supplier change validation, and incoming quality control for critical programs.
- Material development: compounders use melt-state data to quantify the effect of fillers, nucleating agents, and molecular weight on dimensional stability.
FAQ
What is the typical melt-state expansion coefficient of PEEK?
At pressures near atmospheric, the volumetric expansion coefficient of PEEK melt is approximately 5–8 × 10⁻⁴ K⁻¹, corresponding to a linear equivalent of roughly 1.7–2.7 × 10⁻⁴ K⁻¹. The value decreases significantly as applied pressure increases, so grade-specific pvT data should always be used for quantitative work.
Why is melt-state expansion so much larger than solid-state expansion?
Above the melting point, the crystalline phase is destroyed and the polymer chains gain substantial free volume, so specific volume rises steeply with temperature. In the solid state, the crystalline lattice constrains expansion, producing coefficients roughly an order of magnitude smaller.
Which standards govern this test?
ISO 17744 covers the determination of specific volume as a function of temperature and pressure using piston dilatometry and is the primary standard for melt-state pvT data. ISO 11359-2 and ASTM D696 apply only to solid-state linear expansion and do not cover the melt region.
How much sample is required and how should it be prepared?
Most dilatometers require 5–20 grams of pellets, powder, or pre-compressed slugs. The material must be thoroughly dried, typically 3 hours at 150 °C, and transferred to the test cell with minimal moisture exposure to prevent bubbles and hydrolytic degradation from distorting the volume measurement.
How are the results used in mold filling analysis?
The pvT dataset is fitted to a dual-domain Tait equation or equivalent model inside the simulation software. The solver then tracks specific volume through filling, packing, and cooling to predict volumetric shrinkage, dimensional change after ejection, and part warpage, enabling molders to set holding pressure, melt temperature, and cooling time scientifically rather than by trial and error.