7 Tips for Understanding PEEK Thermal Properties?

PEEK is often chosen for parts that face heat, friction, or repeated mechanical stress. Yet a datasheet value rarely tells the whole story. PEEK thermal properties depend on grade, processing history, crystallinity, and reinforcement. A neat polymer and a carbon-fiber-filled grade may behave quite differently in the same assembly. Details matter.

Temperature figures can also be easy to misread. PEEK’s glass transition is around 143°C, while its melting point is near 343°C. These values describe different changes, not a simple safe operating range. A component may soften or creep under load well below its melting point. That distinction deserves attention.

This guide explains seven practical ways to interpret PEEK’s thermal behavior, from heat deflection and continuous-use ratings to thermal expansion and conductivity. It also considers what happens during repeated heating, cooling, and real-world loading. Small test conditions can change the result. That is not always obvious.

The aim is not to promise one number for every application. Instead, the tips help readers compare reliable data, ask suppliers better questions, and identify when testing is needed. Published values are useful starting points, but they can leave gaps. Engineers should verify critical designs under representative conditions.

Table of Contents [Hide]

1 Define PEEK’s Thermal Baseline: Typical Tg ≈143°C and Tm ≈343°C

2 Distinguish Glass Transition from Melting in Thermal Design

3 Check Service Limits: Unfilled PEEK Is Rated for About 250°C Continuous Use

4 Compare Heat Transfer: Typical Unfilled PEEK Conductivity Is ≈0.25 W/m·K

5 Account for Grade Effects: Fillers Change PEEK’s Thermal Expansion and Conductivity

Define PEEK’s Thermal Baseline: Typical Tg ≈143°C and Tm ≈343°C

PEEK’s thermal baseline is commonly described by a glass-transition temperature (Tg) near 143°C and a melting temperature (Tm) near 343°C. These values mark different changes. Around Tg, amorphous regions gain molecular mobility, so stiffness and dimensional behavior can shift. At Tm, crystalline regions melt. The part does not simply “fail” at either temperature, but its response can change significantly.

Tips: Check whether the figures apply to the specific grade, since fillers and processing can affect measured behavior. Confirm values using the supplier’s technical data and, where needed, test parts under relevant conditions. Numbers need context. A brief heat exposure and months under load are not equivalent.

For a practical check, record the part’s actual temperature near the hottest contact point, not only the surrounding air. Also consider load, exposure time, and repeated heating cycles. I would not treat 143°C or 343°C as safe operating limits; they are reference points, not guarantees. That distinction is easy to miss. A component may deform or lose performance below its melting point, especially under sustained stress.

Distinguish Glass Transition from Melting in Thermal Design

PEEK’s glass-transition temperature is commonly near 143°C, while its melting point is around 343°C. Exact values vary by grade and test method.

At glass transition, amorphous regions become more mobile, and stiffness can decrease. The polymer has not melted.

In semi-crystalline PEEK, crystalline regions still help the part retain its shape, but load-bearing performance may change significantly.

Not the same.

For thermal design, don’t treat the melting point as a safe continuous-use limit. Check the material data and test conditions, then consider load, exposure time, part thickness, and crystallinity.

A thin clip under constant stress may creep near its glass-transition temperature. A lightly loaded component may respond differently to a brief heat exposure. Thermal cycling matters, too; repeated expansion and contraction can loosen a fit.

It is tempting to rely on one temperature number. That can mislead. Test the actual geometry under representative heat and load, and measure deformation after a realistic dwell. Leave room for manufacturing variation, especially when crystallinity or annealing is uncertain.

Check Service Limits: Unfilled PEEK Is Rated for About 250°C Continuous Use

Unfilled PEEK is commonly rated for continuous service near 250°C in industrial polymer data sheets. Treat that figure as a design limit, not a promise that every part can run there indefinitely. The rating depends on the grade, load, exposure time, and surrounding chemicals. Check the supplier’s current technical sheet for the exact material.

Heat resistance is not the same as melting resistance. Differential scanning calorimetry measurements reported using ISO 11357-3 place PEEK’s melting peak near 343°C. That gap does not make 300°C a safe operating temperature: stiffness and strength can decline well below melting. A loaded bearing near a hot oven, for example, may creep even when it keeps its shape at first. Small details matter.

For a real service check, record the hottest measured part temperature, not just the room or process setting. Include brief temperature spikes, clamp pressure, and cooling cycles. Compare those conditions with long-term data, and test a representative part if failure would be costly. I would not rely on a single headline rating; the missing load details can be decisive. The temperature figure is a starting point, not the whole answer.

Compare Heat Transfer: Typical Unfilled PEEK Conductivity Is ≈0.25 W/m·K

Typical unfilled PEEK has thermal conductivity near 0.25 W/m·K, although grade and test method can shift the measured value. That is far below most metals. In a component, heat therefore tends to move through PEEK more slowly, rather than spreading quickly across the part.

Thickness matters. A 2 mm wall usually transfers heat more readily than a 10 mm wall made from the same material, all else being equal.

Contact also matters: a small gap between a PEEK part and a metal plate can add resistance. So can surface roughness or uneven pressure. These details are easy to overlook.

For a real design, check the material data and the test conditions, then consider the part’s geometry, contact area, and temperature gradient. Filled PEEK grades may conduct heat differently from unfilled grades, so do not apply the 0.25 figure to every formulation. A simple bench test can help: warm one side of a sample and record how quickly the other side responds. I would not treat one reading as a guarantee, though. Sensor placement and clamping force can change the result.

Account for Grade Effects: Fillers Change PEEK’s Thermal Expansion and Conductivity

PEEK is not one thermal recipe. Its response changes with the grade and filler system. Unfilled PEEK typically expands more with heat than reinforced grades. Glass or carbon fibers can reduce thermal expansion, but the result depends on fiber direction and loading. A molded part may expand less along the flow direction than across it. That difference can affect a close-fitting bushing or a thin-walled housing.

Fillers can also raise thermal conductivity, helping heat move through some grades faster. But a single conductivity value may hide important differences between directions. For a part that touches a hot surface, ask for data measured in the relevant direction and temperature range. Check the exact grade, not just the word “PEEK.” Processing matters, too: fiber orientation can vary around corners and gates. I have seen specifications look reassuring while overlooking that detail. Not always a problem. Still, test a representative molded sample when tight clearances or uneven heating matter. Data sheets are useful, but they cannot fully describe every part’s behavior.