PEEK Melt Volume Expansion Testing: Principles, Methods and Applications

PEEK melt volume expansion testing measures how the specific volume of polyetheretherketone changes with temperature and pressure in the molten state, producing the pvT data essential for process simulation. This article explains the technical principles, sample requirements, test methods, and practical applications of PEEK melt volume expansion experiments for injection molding and extrusion professionals.

Technical Overview

PEEK (polyetheretherketone) 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. During melt processing, typically performed between 370 °C and 400 °C, the material undergoes a substantial change in specific volume. A PEEK melt volume expansion experiment quantifies this change by measuring the specific volume of the molten polymer as a function of temperature and pressure, generating the pressure-volume-temperature (pvT) data that underpin accurate process modeling.

Several physical phenomena contribute to melt volume expansion in PEEK. First, when the crystalline phase melts, the ordered lamellar structure converts into a less densely packed amorphous liquid, producing a characteristic step increase in specific volume at the melting transition. Second, the melt itself expands thermally as temperature rises, following a volumetric expansion coefficient on the order of 0.0006 to 0.0007 per kelvin. Third, the melt is compressible; raising the hydrostatic pressure during the packing stage of injection molding reduces the specific volume noticeably, which is why pvT curves are always recorded over a pressure range, commonly from atmospheric pressure up to 200 MPa.

Understanding these effects is essential because pvT behavior directly governs volumetric shrinkage, sink marks, void formation, and part warpage. Since PEEK components are frequently used in dimensionally critical aerospace, medical, and semiconductor applications, melt volume expansion data must be measured accurately rather than estimated from generic values.

Testing Samples

Unfilled PEEK Granules

Natural, unfilled PEEK in pellet or powder form is the baseline material for melt volume expansion studies. Both medium-viscosity injection molding grades and high-viscosity extrusion grades can be characterized. Unfilled samples provide the cleanest pvT signature because the melting and crystallization transitions are not masked by reinforcement phases.

Glass-Filled PEEK Compounds

PEEK reinforced with 30 percent chopped glass fiber is widely used for structural parts. The glass phase has a much lower thermal expansion coefficient than the polymer matrix, so the compound melt expands less than neat PEEK. Dedicated pvT measurements on filled compounds are therefore required whenever simulation models target these grades.

Carbon-Filled and Bearing-Grade PEEK

Carbon fiber reinforced PEEK, as well as bearing grades containing PTFE, graphite, and carbon fiber, can also be tested. Fiber orientation that develops during flow affects dimensional change, so melt volume expansion data for these materials is typically paired with shrinkage measurements on molded plaques.

Sample Preparation Requirements

• Dry all samples at 150 °C for at least 3 hours, or as recommended by the resin manufacturer, to remove absorbed moisture.
• Keep residual moisture below 0.02 percent, since trapped water vaporizes at melt temperatures and creates voids that corrupt volume readings.
• Use clean material free of contamination, unknown pigments, and regrind of uncertain history.
• Provide sufficient quantity, typically 3 to 10 grams depending on the dilatometer cell, to completely fill the measurement chamber.

Testing Items

• Specific volume of the melt as a function of temperature at fixed pressures (isobaric pvT curves).
• Specific volume as a function of pressure at fixed temperatures (isothermal pvT curves).
• Melt density at processing temperatures between 360 °C and 420 °C.
• Volumetric thermal expansion coefficient of the melt.
• Compressibility and isothermal bulk modulus of the melt at processing temperatures.
• Melting and crystallization temperatures as shifted by pressure, typically rising by roughly 0.3 to 0.5 K per MPa.
• Solid-state specific volume after cooling at a defined rate, used to compute volumetric shrinkage.

A typical reporting package includes tabulated pvT data, plotted pvT diagrams at several pressure levels, and fitted two-domain Tait equation parameters that mold filling simulation software can import directly.

Testing Methods

High-Pressure pvT Dilatometry

The standard instrument for melt volume expansion experiments is a high-pressure dilatometer of the confined-piston type operating according to ISO 17744. The dried sample is sealed in a rigid measurement cell, a piston or bellows transmits hydraulic pressure to the melt, and a displacement sensor tracks the change in sample height, which is converted to specific volume after calibration. Pressures up to 200 MPa and temperatures up to 420 °C cover the entire PEEK processing window.

Isobaric Temperature Scan Procedure

• Load the dried sample, apply a low sealing pressure of about 5 MPa, and heat to approximately 400 °C to melt and homogenize the material while allowing trapped air to escape.
• Hold isothermally until temperature stabilizes and thermal gradients are eliminated.
• Select the measurement pressure, for example 60 MPa, and record specific volume while cooling at a controlled rate, commonly 5 K per minute, down to below the glass transition.
• Repeat the heating scan to capture the melting transition under the same pressure.
• Re-run the cycle at additional pressure levels, such as 20, 100, 150, and 200 MPa, to construct the full pvT diagram.

Isothermal Pressure Scan Procedure

In isothermal mode, the sample is held at a fixed melt temperature while pressure is stepped up and then down between set points. Specific volume is recorded after equilibration at each step. This mode delivers precise compressibility data at individual processing temperatures and serves as an independent verification of the isobaric results.

Supplementary Die Swell Measurement

For extrusion-related questions, melt volume expansion testing is often complemented by capillary rheometry with die swell measurement. The extrudate diameter is monitored with a laser scanner as the melt exits the die, and the swell ratio is reported together with shear viscosity. This captures the elastic and thermal contributions to dimensional change in a realistic extrusion geometry, complementing the equilibrium pvT data.

Instrument calibration is verified with reference materials of known specific volume, such as mercury or indium, and corrections are applied for cell expansion and sensor drift. Repeatability of specific volume measurements better than 0.3 percent is normally achievable on well-prepared PEEK samples.

Applications

• Injection molding simulation: pvT data is a mandatory input for packing and cooling analyses in Moldflow, Moldex3D, and similar CAE packages, enabling prediction of shrinkage, sink marks, and voids in PEEK parts.
• Packing pressure optimization: melt compressibility data shows how much additional material can be forced into the cavity during holding, defining hold pressure and hold time windows.
• Mold design: predicted volumetric shrinkage guides cavity steel sizing for dimensionally critical components in aerospace, medical implant, and semiconductor handling applications.
• Extrusion and die design: melt density and thermal expansion data support mass flow calculations, draw-down balance, and sizing of dies and calibration tooling for PEEK profiles, film, and monofilament.
• Material qualification: comparing pvT curves between resin lots or suppliers supports incoming inspection and consistent processing in regulated industries.
• Research and development: melt volume expansion data supports studies of crystallization kinetics under pressure, nucleated formulations, and related PAEK chemistries such as PEKK.

FAQ

Question: Why does PEEK show a large volume change at the melting point? Answer: The step in specific volume near 343 °C reflects the conversion of the dense crystalline phase into a less densely packed amorphous melt, in addition to ordinary thermal expansion.

Question: Why must the sample be dried before testing? Answer: Moisture turns to steam at PEEK melt temperatures, creating voids that inflate volume readings and potentially degrade the polymer through hydrolysis.

Question: Which standard governs the experiment? Answer: ISO 17744 describes the determination of specific volume of plastics as a function of temperature and pressure, which is the method applied to PEEK pvT and melt volume expansion measurement.

Question: What temperature and pressure ranges are typical? Answer: Temperatures from 360 °C to 420 °C and pressures from atmospheric up to 200 MPa cover the practical PEEK processing window.

Question: Can filled and modified grades be tested? Answer: Yes. Glass-filled, carbon-filled, and lubricated bearing grades are routinely characterized; their lower polymer fraction reduces expansion, so dedicated data should always replace unfilled data in simulations.

Question: How are the results delivered? Answer: Results are provided as pvT diagrams, tabulated specific volume values, melt density and expansion coefficients, and fitted Tait equation parameters ready for direct import into mold filling software.