{"id":5363,"date":"2026-11-01T13:01:08","date_gmt":"2026-11-01T05:01:08","guid":{"rendered":"https:\/\/ptfesuppliers.com\/why-ptfe-parts-change-size-after-machining\/"},"modified":"2026-11-01T13:01:08","modified_gmt":"2026-11-01T05:01:08","slug":"why-ptfe-parts-change-size-after-machining","status":"publish","type":"post","link":"https:\/\/ptfesuppliers.com\/ru\/why-ptfe-parts-change-size-after-machining\/","title":{"rendered":"Why PTFE Parts Change Size After Machining (and How Shops Compensate)"},"content":{"rendered":"

Short answer:<\/strong> PTFE parts change size after machining for two reasons: high thermal expansion (about 10\u00d7 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.<\/p>\n

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.<\/p>\n

Cause 1: Thermal Expansion<\/h2>\n

PTFE’s coefficient of linear thermal expansion is roughly 120\u2013130 \u00d7 10\u207b\u2076\/\u00b0C \u2014 about ten times steel’s. A 100 mm dimension swings about 0.12\u20130.13 mm across a 10 \u00b0C temperature change: enough to consume an entire \u00b10.05 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 \u2014 legitimately.<\/p>\n

Cause 2: Stress Relaxation After Cutting<\/h2>\n

Machining removes stressed material and imposes new surface stresses. PTFE responds by slowly relaxing \u2014 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.<\/p>\n

Shop Practice That Works<\/h2>\n\n\n\n\n\n\n\n
Practice<\/th>\nEffect<\/th>\n<\/tr>\n
Acclimatize stock to shop temperature before final cuts<\/td>\nRemoves the biggest thermal surprise<\/td>\n<\/tr>\n
Rough, rest, finish: let parts relax between operations<\/td>\nStabilizes stress-relaxation drift<\/td>\n<\/tr>\n
Measure on a granite plate after temperature soak<\/td>\nCompares like with like<\/td>\n<\/tr>\n
State measurement temperature on reports (23 \u00b0C convention)<\/td>\nMakes tolerances verifiable and disputable by nobody<\/td>\n<\/tr>\n
Let the final dimension be cut last, coolest<\/td>\nMinimizes post-measurement drift<\/td>\n<\/tr>\n<\/table>\n

What This Means for Buyers<\/h2>\n

When a supplier quotes \u00b10.05 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 \u2014 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.<\/p>\n

Frequently Asked Questions<\/h2>\n

Why does a PTFE part measure differently the next day?<\/h3>\n

Two effects: thermal expansion (about 10\u00d7 steel) as shop temperature changes, and stress relaxation that drifts dimensions for hours after machining.<\/p>\n

How much does PTFE expand with temperature?<\/h3>\n

Its thermal expansion coefficient is roughly 120\u2013130 \u00d7 10\u207b\u2076\/\u00b0C \u2014 about ten times steel \u2014 so 100 mm shifts ~0.12 mm per 10 \u00b0C.<\/p>\n

What shop practices control PTFE dimensional stability?<\/h3>\n

Acclimatize stock, rough-then-rest-then-finish, measure after temperature soak on a granite plate, and state the measurement temperature on reports.<\/p>\n

What temperature should PTFE dimensions be specified at?<\/h3>\n

Conventionally 23 \u00b0C. A tolerance without a stated temperature is unverifiable \u2014 make the temperature part of the drawing note.<\/p>\n