PEEK Tg Vs Tm: What Engineers Need To Know

Introduction

When engineers specify PEEK (Polyether Ether Ketone) for high-performance applications, two thermal properties are critical — yet frequently confused: Tg (glass transition temperature) and Tm (melting point).

Mixing up these two values during part design or process planning can lead to dimensional instability, mechanical failure, or scrapped parts.

This guide breaks down the difference, provides reference data for common Victrex PEEK grades, and offers practical guidance for machining and application selection.

What Is Tg in PEEK?

Tg, or glass transition temperature, is the point at which an amorphous polymer transitions from a rigid, glassy state to a softer, more rubbery state.

For standard Victrex PEEK, Tg is approximately 143°C (289°F).

Above this temperature, PEEK begins to lose stiffness and dimensional stability — even though it has not melted. This is a critical threshold for:

- Structural parts operating in elevated temperature environments

- Precision components where tight tolerances must be maintained

- Any application involving prolonged thermal exposure

DSC (Differential Scanning Calorimetry) test conducted in our lab on PEEK raw material, confirming Tg midpoint at 150.12°C and Tm peak at 337.77°C — consistent with published technical data.

What Is Tm in PEEK?

Tm, or melting point, is the temperature at which the crystalline regions of PEEK fully melt. For Victrex PEEK, Tm is approximately 343°C (649°F).

This is the processing temperature range used during:

- Injection molding

- Extrusion

- Compression molding

It is important to note: PEEK does not simply "melt" at 143°C. Between Tg and Tm, semi-crystalline PEEK retains significant structural integrity due to its crystalline phase — but amorphous PEEK does not.

Tg vs Tm: Side-by-Side Comparison

Victrex PEEK Grade Tg & Tm Reference Data

Different Victrex grades share similar Tg and Tm values, but vary significantly in viscosity, mechanical performance, and application suitability.

Data reference: Victrex technical datasheets. Always verify with current grade-specific TDS before final specification.

Why This Matters for Machining PEEK

If you are CNC machining PEEK from rod, plate, or tube stock, Tg is directly relevant to your process:

Heat buildup during machining can approach or exceed Tg, especially with:

- High spindle speeds without adequate cooling

- Dull tooling causing friction heat

- Deep cuts on thin-wall features

When localized temperatures exceed 143°C at the cutting zone, the material can exhibit:

- Surface smearing

- Dimensional deviation post-machining

- Residual stress leading to warping after release

Practical recommendations from our machining experience:

- Use sharp, uncoated carbide tooling

- Apply compressed air or coolant directed at the cutting zone

- Allow semi-finished parts to stabilize at room temperature before final finishing cuts

- For tight-tolerance features, take light finishing passes at reduced feed rates

Operating Temperature Limits: Designing Within the Right Range

For continuous service, most engineering applications specify PEEK components to operate below 250°C, with optimal long-term performance below Tg for precision or load-bearing parts.

PEEK vs Alternative High-Performance Polymers: Tg Comparison

This comparison helps engineers evaluate whether PEEK is the right choice or whether an alternative polymer better fits the thermal and mechanical requirements.

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