PTFE is the easiest plastic in the shop to cut and one of the hardest to hold to size. The cutter goes through it like cold butter. Then the part is measured in the afternoon, measured again the next morning, and the two numbers disagree — and both are right.
Here is what PTFE does before, during and after the cut, and how to write a drawing it can meet.
What PTFE is
PTFE is polytetrafluoroethylene, the fluoropolymer most people know by the Chemours trade name Teflon®. It is specified for its environment, not its strength: it resists almost every chemical a process line will throw at it, it runs from cryogenic temperatures up to 500 °F continuous, and it has one of the lowest coefficients of friction of any solid. Virgin grades are available that comply with FDA 21 CFR 177.1550 for food contact.
The same structure that makes it inert makes it soft. Four properties drive almost every machining decision, and they are all on the PTFE material page:
| Property | Virgin PTFE stock | What it means at the machine |
|---|---|---|
| Thermal expansion (ASTM D696, 23–100 °C) | 7.2 × 10⁻⁵ in/in/°F (72 µin/in·°F) | About eleven times steel |
| Tensile modulus (ASTM D638) | 80,000 psi | Deflects under a clamp, springs back when released |
| Hardness (ASTM D2240) | 55 Shore D | Marks easily; burrs fold over instead of breaking off |
| Tensile strength (ASTM D4894) | 3,200 psi, at break | PTFE has no yield point; this is not a design stress |
Treat these as typical published values for virgin, unfilled stock, not a specification. Filled grades (glass, carbon, graphite, bronze) change the numbers, and the fillers are abrasive to tooling. Tell us the grade on the print, not just "PTFE".
The number that matters: thermal expansion
Steel expands about 6.5 µin/in·°F. Virgin PTFE: about 72.
A 12" PTFE part machined at 68 °F and measured at 78 °F has grown at least:
12 in × 72 µin/in·°F × 10 °F = 0.0086"
Ten degrees of shop drift is worth nearly nine thousandths on a foot of material. A ±0.005" length tolerance on that part is a temperature question before it is a machining question.
The second number: a transition at room temperature
PTFE has a crystal transition close to 19 °C (66 °F) — right in the middle of the temperature range most shops, inspection rooms and warehouses sit in. Through that transition the material changes volume by about 1% over a few degrees, on top of its ordinary expansion. A second, smaller transition near 30 °C (86 °F) means expansion runs higher than the published average across the whole 66–86 °F band — which is why the 0.0086" above is a floor, not an estimate.
In practice that means:
- A part measured at 64 °F and again at 70 °F can differ by much more than the expansion coefficient predicts. The step happens across the transition, not evenly.
- Machine and inspect on the same side of it. A part cut in a warm shop and inspected in a cold receiving dock has crossed it.
- Put the reference temperature on the print. Drawing tolerances are defined at 68 °F (20 °C) under ASME Y14.5 and ISO 1 — which, for PTFE, is just above the transition. On a critical dimension, agree how the part is conditioned and at what temperature it is measured before the part is quoted.
The third problem: it creeps
PTFE flows slowly under sustained load, even at room temperature. Engineers call it cold flow. It matters twice:
- Under the clamp. Anything that squeezes the part distorts it, and a part machined while distorted springs back when it is released. Light, spread-out clamping, soft jaws, and fixtures that support the part rather than pinch it are what keep a dimension honest.
- In service. A PTFE seal or gasket bolted to a fixed torque will relax over time. That is a design issue, not a machining one, but it is the reason PTFE is a poor choice for parts that carry a structural load. If the part has to hold a preload, tell us; it changes the answer.
PTFE stock also carries internal stress from how the shape was made. Removing a lot of material from one side lets that stress rebalance and the part move. Roughing, letting the part rest, and then finishing is the lever when flatness or roundness is the thing that matters.
Tooling, chips and burrs
- Sharp, polished, positive rake. PTFE cuts easily but it smears and tears if the edge is dull. A sharp edge that shears the chip leaves a clean surface; light rubbing passes generate heat and push the material instead of cutting it.
- Keep the heat down. PTFE conducts heat poorly, so the heat the cutter puts in stays at the surface. Air blast or a water-based coolant keeps the part near shop temperature, which matters more than usual because of everything above. Overheated PTFE also gives off harmful fumes, so use extraction and keep swarf away from smoking areas.
- Expect long, stringy chips. Clear them so they do not wrap the tool and re-cut the surface.
- Plan for burrs. PTFE throws soft burrs that fold over rather than snapping off. They have to be trimmed, and edge condition should be called out on the drawing.
- Filled grades wear tools. Glass- and carbon-filled PTFE are abrasive; they machine more like a composite than like virgin PTFE.
Setting tolerances a PTFE part can hold
We have deliberately not published a PTFE tolerance table. What a PTFE part holds depends on its size, how much material comes off one side, the grade, and above all the temperature it is measured at — and a plausible-looking table would hide exactly the variables that decide whether a part passes.
What helps, on every PTFE drawing:
- Say which dimensions are critical. Tight tolerances on the features that matter, a sensible general tolerance on the rest.
- State the measurement temperature. It costs nothing on the print and it settles the argument before it starts.
- Tolerance for the service range, not just the bench. A seal that runs from 40 °F to 300 °F changes size far more than any machining tolerance; design the fit around that.
- If a fit has to hold, consider another material for that feature. Acetal holds a fit and machines to a good finish; PEEK carries load at high temperature in aggressive chemistry. PTFE is the right call when the chemistry or the friction rules out the alternatives.
When PTFE is the right call
- Seals, gaskets, diaphragms and valve seats in aggressive chemistry.
- Chemical-processing components and linings where very little else survives the medium.
- Low-friction and non-stick surfaces: slide bearings, wear pads, release surfaces.
- Semiconductor and food-processing hardware, where purity and washdown matter.
- Electrical insulators, where its dielectric properties are the reason it is specified.
When it is not
- Structural or load-bearing parts. It creeps. Its tensile strength is a break value, not a design stress.
- Tight fits that must hold over time. See creep, expansion and the room-temperature transition.
- Bonding. Like most fluoropolymers, PTFE does not bond without special surface treatment; design for mechanical fastening or a captured fit.
How we quote PTFE parts
Send the drawing and tell us what the part does, the chemistry and temperature it sees, and which dimensions are critical. We carry PTFE in sheet, rod and tube, cut to size or machined to print in Linden, NJ, from one-off prototypes through production runs. If a tolerance on the print is going to fight the material, we would rather say so on the quote than after your inspection.