PEEK melt density variation analysis characterizes how the density of polyetheretherketone changes under the high temperatures and pressures encountered in injection molding and extrusion. Understanding this pressure-volume-temperature (PVT) behavior is critical for accurate mold-filling simulation, shot-size calculation, and shrinkage prediction. This article covers the fundamental density transitions of PEEK, testing samples and items, measurement methods, and practical engineering applications of melt density data.
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. During melt processing, which typically occurs between 360°C and 400°C, PEEK undergoes a significant reduction in density as the crystalline phase melts and the amorphous melt expands thermally. While solid, fully crystallized PEEK exhibits a density of roughly 1.30–1.32 g/cm³, the melt density at typical processing temperatures is noticeably lower, generally falling in the range of 1.10–1.22 g/cm³ depending on temperature, pressure, and grade.
Melt density variation is governed by the PVT relationship of the material. As temperature increases at constant pressure, the melt expands and density decreases in a nearly linear fashion within the processing window. Conversely, as pressure increases at constant temperature, the melt compresses and density rises measurably. During the packing phase of injection molding, cavity pressures can reach 50–150 MPa, which increases melt density by several percent compared with atmospheric conditions. This compressibility directly determines the amount of material that must be packed into the cavity to compensate for volumetric shrinkage.
Another important phenomenon captured in melt density analysis is the volume change associated with crystallization during cooling. When the melt temperature drops below the crystallization point, typically around 320–340°C depending on cooling rate and pressure, the specific volume drops sharply as ordered crystalline structures form. This step change, combined with thermal contraction, explains the total volumetric shrinkage of 1.0–1.5% typically observed in molded neat PEEK parts.
Testing Samples
Unfilled Neat PEEK Resin
Neat PEEK pellets or virgin resin serve as the baseline material for melt density characterization. Natural-colored granules with a medium to high molecular weight are dried prior to testing, typically for 3–4 hours at 150°C, to eliminate moisture that could cause degradation or bubble formation during measurement.
Glass-Fiber Reinforced PEEK
Grades containing 30% glass fiber by weight are widely tested because reinforcement raises the effective density of both solid and melt phases. Since E-glass has a density near 2.54 g/cm³, a 30% glass-filled PEEK melt shows a higher apparent melt density than the neat polymer, and the fiber fraction must be accounted for when computing shot weights and packing volumes.
Carbon-Fiber Reinforced PEEK
Carbon-fiber reinforced grades, commonly 30% by weight, are evaluated for structural and aerospace applications. Carbon fiber density of approximately 1.75–1.80 g/cm³ produces a more modest density increase than glass fiber, and the higher thermal conductivity of these compounds also influences the temperature distribution during PVT measurement.
Bearing and Tribological Grades
PEEK compounds modified with PTFE, graphite, and carbon black are tested to support tribological component manufacturing. Because these fillers have distinct densities and volume fractions, each formulation requires its own PVT dataset rather than extrapolation from neat resin values.
Testing Items
- Melt density as a function of temperature across the processing window, typically 340–420°C at atmospheric or defined reference pressure
- Melt density as a function of hydrostatic pressure, commonly evaluated from 0 up to 200 MPa
- Complete PVT diagrams plotting specific volume against temperature under isobaric conditions
- Volumetric thermal expansion coefficient of the melt, typically expressed in units of 10⁻⁴/K
- Isothermal compressibility of the melt at each test temperature
- Crystallization temperature and associated specific volume step change under elevated pressure
- Pressure-dependent density increase during simulated packing conditions
- Comparison of solid-state density, melt density, and calculated total volumetric shrinkage
Testing Methods
The primary technique for PEEK melt density determination is PVT measurement in accordance with ISO 17744. Two apparatus configurations are commonly used. The piston-die method confines a pre-weighed polymer sample in a rigid cylinder and measures the piston displacement as temperature and pressure are controlled, calculating specific volume from the known sample mass and instantaneous volume. The confining pressure method, using a pressure chamber with hydraulic medium, applies isostatic pressure to a sample in a bellows while measuring length change with a linear variable differential transformer.
For each test, a dried sample is loaded, preheated to approximately 400°C to erase thermal history, and then subjected to a programmed sequence of pressure and temperature stages. Data are collected both in heating mode and cooling mode, because cooling scans reveal the crystallization-induced specific volume transition while heating scans define the stable melt region. Typical test programs apply pressure levels of 20, 40, 60, 80, 100, 140, 180, and 200 MPa with temperature sweeps between ambient and 420°C.
A complementary approach uses a high-pressure capillary rheometer, where melt density is derived from the mass flow rate versus volumetric flow rate relationship at each test temperature. This method is convenient for processors already characterizing melt viscosity, and it provides density values under realistic shear and pressure conditions, although accuracy is somewhat lower than dedicated PVT instrumentation. All results are reported as density-versus-temperature curves at each pressure level, together with fitted dual-domain Tait or similar equations suitable for direct import into CAE software.
Applications
Melt density data are a mandatory input for injection molding simulation packages such as Moldflow, Moldex3D, and Autodesk Helius. Solver accuracy for filling, packing, and shrinkage predictions depends directly on the quality of the PVT model, so validated melt density curves ensure reliable virtual prototyping of PEEK components before tooling is cut.
In process engineering, melt density converts between mass-based and volumetric quantities. Machine shot size calculations, screw recovery settings, and metering stroke determination all require the melt density at barrel temperature to translate part weight into the volumetric demand on the machine.
For extrusion applications, melt density enables accurate throughput calculations, die swell estimation, and sizing of haul-off equipment. In melt-cast film and coating lines, density data support gauge control and mass-per-area verification.
Quality control laboratories use melt density measurement as an incoming resin verification tool, since contamination, moisture-induced degradation, or off-specification molecular weight can shift density behavior measurably. Aerospace and medical device manufacturers rely on certified PVT data packages to satisfy traceability requirements for materials used in regulated structural and implantable components.
FAQ
What is the typical melt density of PEEK at normal processing temperature?
At approximately 380°C and near-atmospheric pressure, neat PEEK melt density is typically in the vicinity of 1.12–1.18 g/cm³, decreasing further as temperature rises toward 400–420°C. Exact values depend on grade, molecular weight, and measurement method, so certified data for the specific resin lot are recommended for critical applications.
Why is melt density lower than the solid density of PEEK?
Solid PEEK contains crystalline regions with a density of roughly 1.40 g/cm³, while fully amorphous PEEK has a density near 1.26 g/cm³. Upon melting, the ordered crystalline phase disappears and thermal expansion increases intermolecular free volume, so the melt density drops below the solid density, producing the volumetric shrinkage observed in molded parts.
How much does pressure change the melt density during injection molding?
During packing at 50–150 MPa, PEEK melt density can increase by roughly 2–5% relative to atmospheric pressure at the same temperature. This compressibility must be accounted for in shot size and packing profiles to avoid short shots, sink marks, or excessive flash.
Do fiber-reinforced PEEK grades need separate melt density data?
Yes. Glass and carbon fibers alter both absolute density and compressibility behavior, and rule-of-mixtures estimates neglect interfacial and packing effects. Each reinforced formulation should be characterized individually with PVT testing.
Which standards govern melt density and PVT testing of PEEK?
ISO 17744 specifies PVT measurement methods for thermoplastics, and related ASTM procedures cover dilatometry and density determination. Test reports should state the apparatus type, pressure levels, heating and cooling rates, and thermal history so results are comparable and simulation-ready.