Short answer: PTFE parts change size after machining for two reasons: high thermal expansion (about 10× steel) moves dimensions as the part warms or cools, and stress relaxation lets the material shift for hours after cutting. Shops compensate by acclimatizing stock, finishing critical dimensions last, measuring at controlled temperature, and stating that temperature on inspection reports.
Machinists coming from metals get surprised by PTFE twice: first when the part that measured perfect at the machine reads out of tolerance the next morning, and again when the fix turns out to be process discipline rather than machine precision.
Cause 1: Thermal Expansion
PTFE’s coefficient of linear thermal expansion is roughly 120–130 × 10⁻⁶/°C — about ten times steel’s. A 100 mm dimension swings about 0.12–0.13 mm across a 10 °C temperature change: enough to consume an entire ±0.05 mm tolerance. Shop floor reality: a part cut in a cold morning workshop, then measured on a warm afternoon gauge bench, is a different size — legitimately.
Cause 2: Stress Relaxation After Cutting
Machining removes stressed material and imposes new surface stresses. PTFE responds by slowly relaxing — dimensions drift for hours after the cut, worst on thin walls and heavy stock removal. The drift is small but it lands precisely on the tight-tolerance features you care about.
Shop Practice That Works
| Practice | Effect |
|---|---|
| Acclimatize stock to shop temperature before final cuts | Removes the biggest thermal surprise |
| Rough, rest, finish: let parts relax between operations | Stabilizes stress-relaxation drift |
| Measure on a granite plate after temperature soak | Compares like with like |
| State measurement temperature on reports (23 °C convention) | Makes tolerances verifiable and disputable by nobody |
| Let the final dimension be cut last, coolest | Minimizes post-measurement drift |
What This Means for Buyers
When a supplier quotes ±0.05 mm on a PTFE part, the credible ones attach a temperature condition. Drawings for PTFE should carry realistic tolerances on non-functional features, with tight bands reserved for dimensions that matter — and inspection procedures should match the supplier’s measurement temperature. Chunhuan machines PTFE with temperature-referenced SPC on critical dimensions, so first articles and production batches verify under the same conditions they were made in.
Frequently Asked Questions
Why does a PTFE part measure differently the next day?
Two effects: thermal expansion (about 10× steel) as shop temperature changes, and stress relaxation that drifts dimensions for hours after machining.
How much does PTFE expand with temperature?
Its thermal expansion coefficient is roughly 120–130 × 10⁻⁶/°C — about ten times steel — so 100 mm shifts ~0.12 mm per 10 °C.
What shop practices control PTFE dimensional stability?
Acclimatize stock, rough-then-rest-then-finish, measure after temperature soak on a granite plate, and state the measurement temperature on reports.
What temperature should PTFE dimensions be specified at?
Conventionally 23 °C. A tolerance without a stated temperature is unverifiable — make the temperature part of the drawing note.
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