PEEK Melt Density Determination in the Viscous Flow State: Testing Methods, Standards, and Applicati

PEEK melt density determination measures the density of polyetheretherketone in its viscous flow state (typically 360–420°C), a critical parameter for injection molding simulation, extrusion throughput calculation, and die design. Accurate melt density data directly improves the reliability of PVT modeling, shrinkage prediction, and part quality for this high-performance engineering thermoplastic. This article covers the technical background, sample types, test items, measurement methods, and practical applications of PEEK melt density testing.

Technical Overview

Polyetheretherketone (PEEK) is a semi-crystalline, high-performance thermoplastic with a glass transition temperature of approximately 143°C and a melting point of approximately 343°C. In industrial processing, PEEK is typically melted at 360–400°C, where it enters the viscous flow state and behaves as a compressible polymer melt. The density of PEEK in this molten condition—commonly referred to as melt density—is substantially lower than its solid-state density due to thermal expansion upon melting and the loss of crystalline packing.

Unfilled PEEK has a solid density of approximately 1.30–1.32 g/cm³ in the crystalline state and roughly 1.26 g/cm³ in the amorphous state. In the viscous flow state at processing temperatures, the density typically falls within the range of 1.05–1.12 g/cm³, depending on temperature, pressure, and filler content. Because melt density varies significantly with processing conditions, a single tabulated value is often insufficient for precise engineering calculations, and dedicated measurement is required.

Melt density determination provides the specific volume data needed to convert between mass flow rate and volumetric flow rate, to calculate shot weights, to size extrusion pumps and dies, and to build the pressure–volume–temperature (PVT) material models used in injection molding simulation software such as Moldflow and Moldex3D. For a material as thermally demanding as PEEK, careful control of temperature, atmosphere, and moisture is essential for obtaining repeatable results.

Testing Samples

Unfilled PEEK Resins

Neat PEEK granules or powders of standard viscosity grades (e.g., medium-flow, unfilled natural grades) are the baseline materials for melt density measurement. These samples establish reference values against which filled and modified compounds are compared.

Glass-Fiber and Carbon-Fiber Reinforced PEEK

Compounds containing 30% glass fiber (solid density typically 1.40–1.42 g/cm³) or 30% carbon fiber (typically 1.42–1.44 g/cm³) exhibit higher melt densities than unfilled resin. Because fibers do not expand thermally to the same degree as the polymer matrix, the temperature dependence of melt density differs from that of neat PEEK and must be characterized independently.

Bearing-Grade and Filled PEEK Compounds

PEEK modified with PTFE, graphite, molybdenum disulfide, or ceramic particles for tribological applications may show heterogeneous melt behavior. Representative sampling and consistent measurement conditions are particularly important for these multi-component systems.

PEEK Blends and Recycled Materials

PEEK/PEKK blends, PEEK-based composites, and reprocessed or recycled PEEK may contain residual crystalline nuclei or degraded chains that alter melt specific volume. Testing these materials verifies processing behavior and supports material qualification programs.

Testing Items

A comprehensive PEEK melt density characterization program typically includes the following measurable items:

• Melt density at fixed processing temperatures (commonly 360, 380, and 400°C) under atmospheric or low-shear conditions
• Melt density as a function of applied pressure for PVT curve generation
• Specific volume (cm³/g) in both the molten and solidified states across a cooling ramp
• Comparison of melt density with solid density determined by the immersion or gradient column method
• Effect of filler type and loading on melt density
• Thermal stability assessment, monitoring density drift caused by degradation during extended residence at elevated temperature
• Derived parameters for PVT modeling, including dual-domain Tait equation coefficients

Testing Methods

Sample Preparation

All PEEK samples must be dried before testing, typically at 150°C for 3 hours or 160°C for 2 hours, to reduce moisture content below 0.02%. Residual moisture causes hydrolytic degradation and bubble formation in the melt, both of which bias density results downward. Samples are then loaded into a preheated, nitrogen-purged test barrel to minimize oxidative degradation at temperatures above 360°C.

Gravimetric Piston-Displacement Method

The most widely used routine method is based on a melt flow rate instrument (ISO 1133 type). A known pressure is applied to the piston, and the melt is extruded through the die while the piston travels a precisely measured distance. Because the barrel inner diameter (9.550 mm) is known, the extruded volume equals the piston cross-sectional area multiplied by the displacement. The extrudate is cut, cooled, and weighed on an analytical balance, and melt density is calculated as the mass divided by the displaced volume.

• Applicable temperature range: 360–420°C
• Typical loads: 5 kg, 10 kg, or 21.6 kg depending on grade fluidity
• Typical accuracy: ±0.5–1.0% with calibrated instrumentation
• Sample requirement: approximately 4–8 g per determination

High-Pressure Capillary Rheometry

A capillary rheometer extends the piston-displacement principle to elevated pressures and controlled shear rates. This approach captures the pressure dependence of PEEK melt density and simultaneously yields viscosity data, making it efficient for combined rheological and densitometric characterization.

PVT Dilatometry (ISO 17744)

For simulation-grade data, a dedicated PVT dilatometer operating under the piston method described in ISO 17744 measures specific volume as a function of temperature and pressure, often up to about 200 MPa. Both isothermal and isobaric scanning modes are used to construct the full PVT diagram, from which melt density at any processing condition can be extracted. The resulting data are fitted with a dual-domain Tait equation to provide material cards for injection molding software.

Data Treatment and Reporting

Results are reported as density (g/cm³) versus temperature at specified pressures, together with the test standard, instrument, thermal history, drying conditions, and measurement uncertainty. A minimum of three replicate determinations is recommended, and inter-run variability above 1% usually indicates moisture contamination or thermal degradation.

Applications

PEEK melt density data support a wide range of engineering and manufacturing activities:

• Injection molding simulation: PVT inputs for predicting shrinkage, warpage, packing pressure, and part dimensions of PEEK components in aerospace, medical, and semiconductor tooling
• Extrusion and wire coating: conversion between gravimetric and volumetric throughput, screw and melt pump sizing, and die gap calculations for PEEK film, sheet, pipe, and insulation layers
• Shot weight and cavity filling calculations: accurate mass-to-volume conversion for mold filling and cost estimation
• Material comparison and quality control: batch-to-batch consistency verification and incoming resin qualification
• Scientific research: studies of melt compressibility, crystallization kinetics, and the thermodynamics of PEEK and its blends
• Tooling and process design: determination of holding pressure requirements to compensate for volumetric shrinkage during solidification

FAQ

What is the difference between PEEK solid density and melt density?

Solid PEEK density is approximately 1.30–1.32 g/cm³, while melt density in the viscous flow state at 360–400°C is typically 1.05–1.12 g/cm³. The difference of roughly 15–20% reflects thermal expansion and the disappearance of crystalline ordering upon melting.

Why must melt density be measured rather than estimated?

Melt density depends strongly on temperature, pressure, and filler content. Using solid density or generic values in process simulation introduces errors in predicted shrinkage, packing behavior, and volumetric flow that can lead to dimensional defects in precision PEEK parts.

At what temperature should the measurement be performed?

The test temperature should match the intended processing window—commonly 380°C for injection molding and 390–400°C for extrusion and thin-wall applications. Measuring at several temperatures across the processing range allows interpolation for process modeling.

How does moisture affect the results?

Undried PEEK can contain up to 0.5% absorbed water. Moisture volatilizes in the barrel, creating bubbles and hydrolytic chain scission, which lowers the apparent melt density and increases scatter. Strict pre-drying is therefore mandatory.

How do fillers change melt density?

Glass and carbon fibers, with densities near 2.5 and 1.8 g/cm³ respectively, raise overall melt density and reduce its temperature sensitivity because the fillers themselves undergo negligible thermal expansion within the processing window.

Which standards apply to this testing?

The gravimetric extrusion method is based on ISO 1133 apparatus and procedure, PVT determination follows ISO 17744, and solid density for comparison is measured according to ISO 1183 or ASTM D792. Internal procedures typically adapt these standards to the elevated temperatures required for PEEK.