Don’t Judge PEEK by Temperature Alone!

Don’t Judge PEEK by Temperature Alone!

Confuse Tg, Tm, HDT, and RTI? Your Parts May Fail Early.

A straightforward guide to the thermal properties of PEEK, helping you avoid common material selection pitfalls.

“Can this PEEK part be used long-term at 200°C?”

If you base your decision solely on the Heat Deflection Temperature (HDT) listed in the material datasheet, you may have already fallen into a classic trap.

As a high-performance specialty engineering plastic, PEEK’s exceptional properties stem from the interplay of several precise and interrelated thermal parameters.

Glass Transition Temperature (Tg), Melting Temperature (Tm), Heat Deflection Temperature (HDT), and Relative Thermal Index (RTI) — these four key indicators are critical. Misunderstanding any one of them could lead to premature product failure in demanding environments. This article briefly introduces these four core concepts to help build a scientific framework for material selection.

01 Glass Transition Temperature (Tg)

Tg is the starting point for understanding PEEK’s thermal behavior. It is not the melting point, but the temperature range (around 143°C) where the amorphous regions of the polymer transition from a “frozen” to a “mobile” state.

Think of Tg as the material’s “turning point in character.”

Below Tg, PEEK remains in a stiff “glassy state” — rigid and dimensionally stable. Above Tg, the amorphous regions enter a softer “rubbery state,” where rigidity decreases but toughness increases, while the crystalline regions largely retain their properties.

This means we often hear questions like:

“My application temperature exceeds the Tg of PEEK. Does that mean your material won’t work?”

In fact, this is a common misunderstanding.

Thanks to PEEK’s semi-crystalline nature, it contains both crystalline and amorphous regions that interact and influence each other. This structure means that Tg does not decisively determine the material’s overall performance.

Take a precision gear made from neat PEEK: when operating above Tg, the amorphous regions do soften, but while rigidity drops slightly, toughness and impact resistance improve significantly. This is precisely the advantage of PEEK’s semi-crystalline structure — it dynamically balances stiffness and toughness across a wide temperature range.

Thus, “above Tg” does not mean “unsuitable for use.” It often signals the activation of another set of beneficial properties.

02 Melting Temperature (Tm)

Tm is the “endpoint” of PEEK as a solid — the temperature at which the crystalline structure completely collapses, typically around 343°C. This is a first-order phase transition. Above this temperature, the material begins to melt, so PEEK processing is usually carried out between 360°C and 400°C.

Tm provides two clear guidelines:

The starting point for processing: Note that the material begins to soften as it approaches, but has not yet reached, Tm. All melt-processing (e.g., injection molding) must occur above Tm.

The upper limit for use: Any service condition exceeding Tm will cause the part to melt. For components that must retain their shape at 200–300°C, Tm is more relevant than Tg. In this range, the amorphous regions have already softened, and the crystalline structure provides the necessary support. A higher Tm means a more stable backbone.

03 Heat Deflection Temperature (HDT)

HDT measures a material’s ability to resist deformation under short-term heat and load. Imagine subjecting a standard test bar to a “high-temperature weight-bearing test” and watching when it begins to bend.

HDT values are highly dependent on the test load and material formulation.

For neat PEEK, HDT (around 152°C) is close to Tg. But with fiber reinforcement, the situation changes dramatically.

Critical warning: This is one of the most dangerous traps in material selection. A high HDT does not mean the material can be used long-term at that temperature. HDT only indicates short-term, transient resistance to deformation. Prolonged exposure at such temperatures will cause creep and eventual failure due to the softened matrix.

It should be noted that neat and reinforced PEEK grades from Jutai can achieve long-term service temperatures of up to 260°C under no or light load, with instantaneous resistance up to 300°C. However, evaluating real-world application conditions based solely on a single thermal property is far from sufficient. For detailed guidance on specific products, please consult our technical team based on your actual service conditions.

04 Relative Thermal Index (RTI)

If HDT is a “100-meter sprint,” RTI is a “marathon.”

The RTI, certified by UL, defines the maximum temperature at which a material retains more than 50% of its original properties after long-term continuous thermal exposure, with testing simulating up to 100,000 hours (about 11.4 years).

RTI addresses chemical degradation caused by thermal oxidation — such as chain scission and embrittlement.

It is the gold standard for determining long-term product reliability. Choosing a material with an RTI above your continuous operating temperature provides solid assurance of long-term performance.

05 From Theory to Practice

Understanding the four parameters is key, but applying them systematically is what matters. For high-temperature applications, follow this four-step decision process:

Check RTI first — Determine the maximum continuous operating temperature of the part. The chosen material’s RTI must exceed this temperature. This is the safety baseline.

Then, review HDT — HDT essentially indicates the “thermal failure temperature for structural rigidity.” It quantifies the material’s ability to maintain shape under heat plus load. Use it for selecting structural supports, controlling processing temperatures, and validating modification effects. It’s the core metric for judging whether a material can withstand “heat + force” in engineering applications.

Cross-check Tg — Analyze the relationship between operating temperature and Tg. If the operating temperature is at or above Tg and high rigidity is required, fiber-reinforced grades are necessary to compensate for matrix softening.

Reference Tm — Finally, confirm that your processing equipment can meet the temperature requirements above the material’s Tm.

Tg, Tm, HDT, and RTI — these four parameters define the thermal behavior boundaries of PEEK from different perspectives. Successful material selection means moving beyond reliance on any single parameter and making systematic trade-offs.

Suzhou Jutai PEEK products are the result of continuous improvement across these key dimensions — from precise Tg control and ultra-high HDT through reinforcement modification, to authoritative UL RTI certification for long-term reliability. This comprehensive approach ensures robust solutions for thermal management challenges in high-end applications.