Can PTFE (Teflon) be machined?
Yes, with care. PTFE cuts easily but creeps, cold-flows and expands a lot with temperature, so ±0.13 mm is a realistic tolerance on most features.
PTFE cannot be injection molded in the usual way, so most PTFE parts are machined from compression-molded or ram-extruded stock. It is soft (about 50-60 Shore D), cuts with very little force and leaves a good surface with sharp tools. It is chosen for chemical inertness, one of the lowest friction coefficients of any solid (about 0.05-0.10) and service from cryogenic temperatures up to 260 °C (500 °F): seals, valve seats, gaskets, bearings, insulators and lab ware. Teflon is the Chemours trade name; shops use the two words interchangeably.
Holding size is the hard part. PTFE creeps and cold-flows under steady load, so a part clamped hard in a vise deforms and springs back out of round when released, and press fits and bolted seals relax over time. Its thermal expansion is well over 100 µm/m·°C, and it has a crystal phase transition near 19 °C (66 °F) that changes its volume by about 1%. A part machined in a warm shop and inspected in a cool one can be out of tolerance with nothing wrong in the process. A realistic general tolerance is ±0.13 mm (0.005"); tighter is possible on small features with stress-relieved stock, temperature control and a stated inspection temperature.
Machine it with very sharp, polished tools with high positive rake (HSS or sharp uncoated carbide), light clamping in soft jaws, collets or vacuum fixtures, and air or light coolant to keep heat out. Chips come off long and stringy; plan chip control so they do not wrap the tool and mark the part. Burrs are soft and fuzzy and need hand or cryogenic deburring on seal lips. Filled grades (glass, carbon, graphite or bronze, typically 15-60% by weight) cut creep and wear sharply and are the usual choice for seals and bearings under load; they are more abrasive to tools. PTFE is safe to machine, but overheated PTFE gives off toxic fume, so keep chips and dust away from open flames, heaters and cigarettes.
CNC Machining for PTFE parts: where to send them
What to specify
- Grade: virgin PTFE or filled (for example 25% glass, 25% carbon or 60% bronze), and the stock specification (ASTM D3294 for molded shapes, ASTM D1710 for extruded rod)
- General tolerance of ±0.13 mm (0.005"), tighter only on small, critical features
- Inspection temperature, normally 23 °C (73 °F), with parts stabilized before measurement
- Stress-relieved stock, and an anneal after roughing for thin-walled or close-tolerance parts
- Edge and burr requirements on seal lips and sealing faces, and the surface finish there
- Cleaning and packaging for oxygen, semiconductor or food service where it applies
Pitfalls
- Measuring at a different temperature than the part was machined at; the 19 °C transition alone moves dimensions
- Clamping hard and getting parts that spring out of round when released
- Press fits and bolted seals relaxing through cold flow; use filled grades, spring energizers or retained designs
- Long stringy chips wrapping the tool and scoring the part
- Fuzzy burrs on seal lips that leak; specify sharp tools and deburring
- Overheating PTFE well past its 260 °C service limit, which releases toxic fume
Frequently asked questions
- Why is it hard to hold tolerance on PTFE?
- It expands about ten times as much as steel with temperature, changes volume by about 1% through a phase transition near 19 °C, and creeps under clamping load. ±0.13 mm (0.005") is a realistic general tolerance.
- Is PTFE the same as Teflon?
- Teflon is the Chemours trade name for PTFE and some related fluoropolymers. On drawings, specify PTFE and the grade rather than the trade name.
- What is filled PTFE used for?
- Seals, bearings, piston rings and valve seats under load. Glass, carbon, graphite or bronze fillers cut creep and wear sharply compared with virgin PTFE, at some cost in chemical resistance and electrical properties.
