PTFE Rod, Stress Relieved

Stress-relieved PTFE rod in natural, cut to length and sold by the foot.

Molded and extruded PTFE carries internal stress from the way it is made, and the stress varies with thickness. You discover it when you machine: removing material lets the stress go, and the part moves. That is what produces hard-to-hold tolerances, poor roundness and concentricity, parts that walk during a cut, and scrap at final inspection. Stress-relieved stock has been through a controlled thermal cycle at the mill to take most of that out before you start.

What you are buying is dimensional stability during and after machining - tighter tolerances, better roundness, better surface finish, less movement under the tool, and better repeatability across a batch. It is not a strength, chemical-resistance or temperature improvement, and no source claims any property gain beyond stability.

The honest caveat comes straight from the standard: annealing relieves stress only partially, no amount of annealing guarantees complete stability, and the process can itself distort surface and flatness. Stress-relieved stock is better, not cured. For genuinely tight work the published sequence is to rough the part out, leave 0.015 to 0.030 inch of stock, stress relieve again yourself, then take the finish cut - the second anneal removes the stress your own roughing cuts put in, and no stock grade can do that part for you.

In specification terms this is ASTM D1710 Class B - the dimensional stability class. Worth knowing: Class B carries no internal-defect inspection, and D1710 only imposes the dimensional-stability requirement on rod under 1 inch diameter, so above that the claim describes the mill process rather than a tested requirement. If you need stress relief and void-free certification together, that is Class D. The aerospace equivalents are SAE AMS 3659 for sintered and stress-relieved, AMS 3658 for the same plus radiographic inspection.

PTFE - polytetrafluoroethylene - is the material people reach for when nothing else will survive the chemistry. It resists very nearly everything: strong acids, strong alkalis, solvents, oxidizers, chlorine. It has the lowest coefficient of friction of any solid, nothing sticks to it, it insulates to over 10^18 ohm-cm, and it works from cryogenic temperatures up to 500F continuously. No other thermoplastic covers that span.

Cold flow is the thing to understand before you specify it. Under about 2,000 psi for 24 hours at 73F, virgin PTFE deforms 10 to 13 percent, and when you take the load off it recovers only about half - 6 to 7.5 percent is permanent (ASTM D621). That is at room temperature; raise the temperature and it gets dramatically worse. No other common engineering plastic behaves this way at ambient. If your part has to hold a dimension under sustained load, design to the compressive strength at 1 percent deformation - about 995 psi for virgin PTFE - and not to compressive strength at yield. Anything approaching 2,000 psi sustained will cold-flow by double figures. Where that is a problem, a filled grade is the answer: 25 percent carbon-filled cuts deformation under load to 5 to 7 percent.

One quirk that causes more tolerance arguments than anything else in this material: PTFE passes through a transition near room temperature that changes its volume by 1 to 1.5 percent - roughly 0.3 to 0.5 percent on a linear dimension. A part measured in a 65F inspection room is genuinely a different size in a 75F process. For critical dimensions, measure at service temperature or compensate for it.

Temperature: 500F continuous is unanimous across every source, and PTFE stays useful down to -328F (-200C), with some sources citing -400F. Heat deflection is 250F at 66 psi but only 132F at 264 psi - read the load with the number, because PTFE softens early under load. It is UL 94 V-0.

Food contact: available in grades compliant with FDA 21 CFR 177.1550, perfluorocarbon resins. Two things worth knowing about that. The regulation controls by melt viscosity, thermal instability index and extractives - properties of a specific lot from a specific mill, not of "PTFE" generically - and paragraph (e) attaches an actual extraction test to any molded article over 100 square inches and 1.27 mm thick, which covers most plate. So compliance is the mill's dated declaration, which we will pass on to you; it is not something a distributor can originate. We do not claim NSF listing, and USDA does not approve materials at all - it requires finished articles be made of FDA-compliant ones.

It machines easily with sharp tools and good chip clearance, but it is soft and it moves, so it will not hold a tolerance the way acetal does. Heat concentrated at the cut makes it swell locally, which is how you get overcuts, undercuts and tapered holes. For close work, rough it out, leave 0.015 to 0.030 inch, stress relieve, then finish cut.

Thermal decomposition - worth reading even if you have worked with PTFE for years. Continuous service is 500F; above roughly 625F PTFE may give off hazardous fumes, and at 840F and above it can form toxic fumes including hydrogen fluoride and carbonyl fluoride, with perfluoroisobutylene above 890F. That leaves only about 125F between the top of the service range and the start of fume evolution. Inhaling decomposition products causes polymer fume fever, a temporary flu-like illness with fever, chills and cough that normally clears in 24 to 48 hours. Do not overheat it, use local exhaust when machining or grinding, and use coolant. And the one everybody leaves out: do not smoke or use tobacco where PTFE is machined or handled - PTFE residue transferred to tobacco and then burned is a documented cause of polymer fume fever. Exposure limits for the particulate are an OSHA PEL of 15 mg/m3 total and 5 respirable; ACGIH TLV-TWA 10 and 3. Ask us for the Safety Data Sheet.