PTFE CNC Machining
PTFE is the material you reach for when nothing else survives — it resists essentially every chemical, works from cryogenic temperatures to 260 °C, and has the lowest friction of any solid. It is also the least mechanically useful plastic on this site: its stiffness is a fraction of acetal's, it cold-flows under any sustained load, its thermal expansion is enormous, and almost nothing will bond to it. The honest summary is that PTFE is a seal, a liner and an insulator. It is not a structural part, and the failures come from forgetting that.
Specification at a glance
- Material family
- Semi-crystalline fluoropolymer — polytetrafluoroethylene
- Density
- ~2.14–2.20 g/cm³ — heavy for a plastic
- Tensile strength
- ~20–35 MPa — low
- Modulus
- ~0.5–0.7 GPa — very low; the material is flexible rather than stiff
- Service temperature
- About -200 °C to +260 °C continuous — the widest range of any common plastic
- Chemical resistance
- Essentially universal — attacked only by molten alkali metals, elemental fluorine and a few fluorinating agents
- Friction
- The lowest of any solid — roughly 0.04–0.10
- Cold flow
- Significant — it deforms under sustained load even at room temperature
- Thermal expansion
- Among the highest of any plastic
- Bonding
- Essentially impossible without sodium etching or plasma treatment
- Machinability
- Difficult — very soft and waxy; it compresses rather than cuts, so tolerances and measurement both need care
- Precision tolerance
- ±0.01 mm on precision features, with the caveats set out below
- General tolerance
- ISO 2768-m unless stated on the drawing
- MOQ
- From 50 pieces per style and material
The extreme material, and its one big weakness
PTFE’s properties are genuinely extreme, and each one is the best in its class.
| Property | Roughly | What it means |
|---|---|---|
| Chemical resistance | Essentially universal | Nothing else comes close |
| Service temperature | -200 °C to +260 °C | The widest range of any common plastic |
| Friction | 0.04–0.10 | The lowest of any solid |
| Dielectric properties | Excellent, across the temperature range | The standard high-frequency insulator |
| Density | ~2.14–2.20 g/cm³ | Heavy for a plastic |
| Modulus | ~0.5–0.7 GPa | Very low — and this is the problem |
| Tensile strength | ~20–35 MPa | Low |
And then the weakness that governs how it can be used: PTFE cold-flows.
Cold flow — creep — is the slow, continuous deformation of a material under a sustained load. In PTFE it happens at room temperature, not only when hot, and the material does not fully recover when the load is removed. A PTFE part under a permanent load will change shape over time, and eventually fail its function.
That single property explains why PTFE is used the way it is. Where it works in a load-bearing position, it is always one of two things:
Fully confined. A gasket in a groove, a seat in a pocket, a seal in a gland — the surrounding metal prevents the material from flowing anywhere. This is why PTFE is so successful as a seal and so unsuccessful as a bracket.
A filled grade. Glass, carbon or bronze fillers resist the deformation and give the material enough stiffness and wear resistance to work as a bearing or a wear part.
Neither is a workaround; both are how the material is meant to be used. The failures come from treating PTFE as if it were a general engineering plastic and giving it an unsupported span, a bolted joint tightened directly onto it, or a cantilever.
Filled grades: what the filler buys
Virgin PTFE is the right choice for chemical sealing. For anything mechanical, a filled grade is usually necessary.
| Filler | What it improves | What it costs |
|---|---|---|
| Glass fibre | Creep resistance, wear resistance, stiffness | Some reduction in chemical resistance; more abrasive to mating surfaces |
| Carbon / graphite | Wear resistance, stiffness, thermal conductivity | Dark colour; lower chemical resistance than virgin |
| Bronze | Load capacity, thermal conductivity, wear resistance | Significantly lower chemical resistance; not a universal barrier |
The trade runs in one direction: the more mechanical performance the filler provides, the less universal the chemical resistance becomes. A bronze-filled PTFE bearing is a good bearing and is not a chemical seal. Where a part has to do both, that is worth discussing at the enquiry rather than assuming.
Where a filled grade is specified, the filler and the percentage belong on the drawing. “PTFE” and “25% glass-filled PTFE” are different materials with different behaviour, and a quote that treats them as one will be wrong.
Machining PTFE
PTFE is one of the more difficult plastics to machine to a reliable tolerance, and the reason is not hardness — it is softness.
It deforms rather than cutting. The tool pushes material ahead of the edge, the part moves away from the cutter, and it springs back afterwards. A cut that looks correct can leave a dimension that is not.
It compresses under clamping. PTFE marks and deforms under clamp pressure, so soft jaws and distributed pressure are necessary — and even then the material is being squeezed.
Measurement is genuinely difficult. A part that compresses under the tool also compresses under a micrometer. A dimension measured with a heavy gauge can read smaller than the same dimension measured with a light-touch gauge, and both can differ from the free-state dimension. On a tight-tolerance PTFE part this is a real constraint, and it is why the achievable tolerance depends as much on how the part is measured as on how it is cut.
Its thermal expansion is enormous. A warm part is not a cold part, and PTFE’s expansion is among the highest of any plastic. Critical dimensions belong on a cooled part.
What works:
- Extreme tool sharpness. More than any other material on this site, PTFE demands a genuinely sharp, high-rake, polished edge. A dull tool does not cut PTFE; it pushes it.
- High spindle speeds with very light cuts, so material is removed before heat builds and before the part deflects.
- Support the work wherever possible. A PTFE part with unsupported sections will deflect under the cutter. Backing, a close-fitting fixture, or a design with generous wall thickness all help.
- Sharp support and gentle clamping. Soft jaws, low clamp pressure, and no point loading.
- Air blast. PTFE does not absorb water and does not need heavy cooling; air clears the swarf.
- Measuring with light gauge pressure, on a cooled part. This is part of the process rather than an afterthought.
Temperature, and the decomposition warning
PTFE’s service range — roughly -200 °C to +260 °C — is the widest of any common plastic, and it is the reason the material appears in cryogenic seals and in high-temperature chemical service alike.
The limit at the top end exists because above roughly 400 °C PTFE begins to decompose, and the decomposition products are toxic — hydrogen fluoride and carbonyl fluoride, which cause a condition known as polymer fume fever. Two practical rules follow:
Do not let a cut scorch the material. Normal machining does not approach the decomposition temperature, but a badly run cut can scorch and discolour PTFE, and a scorched cut means the parameters are wrong. The material should never be overheated.
Never burn PTFE swarf. Dispose of it as waste rather than through any thermal process. This is standard industrial practice and it is worth stating because the swarf does not look hazardous.
The classic warning from the trade is also worth repeating: do not smoke where PTFE has been handled. Tobacco contaminated with PTFE residue produces the same toxic fumes when it burns, and polymer fume fever from this route is a well-documented occupational illness rather than a theoretical concern.
None of this is a reason to avoid the material. It is the standard handling discipline for a fluoropolymer, and it is the reason PTFE is treated with a little more care than a polyolefin.
Where PTFE is the right choice
- Chemical seals, gaskets and O-rings — the core application
- Valve seats, liners and diaphragms
- Pump components in aggressive chemical service
- Laboratory ware, tubing and fittings
- High-frequency electrical insulation and coaxial dielectric
- Cryogenic seals, where almost no other plastic remains usable
- High-temperature seals up to about 260 °C
- Low-friction slide surfaces where the load is light or the material is confined
- Non-stick components where nothing else releases
- Filled grades for bearings and wear parts in chemical or high-temperature service
Where PTFE is the wrong choice
The part carries sustained load without being confined. Cold flow will deform it. This is the defining failure mode and it is worth checking specifically — a PTFE part that is bolted, cantilevered or spanning a gap will move. A filled grade or a different material is the answer.
The part needs to be bonded or painted. Nothing adheres without sodium etching or plasma treatment. Design a mechanical joint — which suits a seal anyway — or choose a different material.
A precision fit is required. PTFE’s softness, compression and thermal expansion all work against a tight tolerance. It can be done, but the measurement and the part’s duty both have to be reviewed rather than assumed.
Abrasion resistance is the requirement. Virgin PTFE wears quickly. A filled grade or acetal is the answer.
Stiffness is required. PTFE’s modulus is a fraction of acetal’s; it is a flexible material. A structural part belongs in a different family.
Weight matters. At around 2.15 g/cm³, PTFE is one of the heaviest plastics — more than twice polypropylene’s density.
Cost matters and the extreme properties are not needed. PTFE is among the most expensive materials on this site. Where the duty is within what polypropylene or PVC handles, those materials are far cheaper.
Finishing PTFE
PTFE is usually used as-machined, and finishing is limited because so little adheres to it.
- Deburring — done very gently, since PTFE is soft and deforms easily
- Sodium etching or plasma treatment — the only routes to a bondable surface, arranged through our vetted partner network
- Sand blasting — a uniform matte surface, though the effect is shallow on such a soft material
- Annealing — a controlled heat cycle to relieve machining stress, worth considering on a thin or complex part
- No painting, plating or coating in the ordinary sense — nothing will adhere without treatment, and a coating on a non-stick surface is self-defeating
All finishing runs through our vetted partner network, managed under one quality process. The grades we run across every process are listed on the materials page.
What to put on the drawing
| Instead of | Write |
|---|---|
| PTFE | PTFE, virgin or a named filled grade — they behave differently |
| PTFE, filled | Name the filler and the percentage |
| PTFE, carries load | Say how the material is confined — cold flow is the defining limit |
| PTFE, tight tolerance | Say which dimensions matter, how they are measured, and at what temperature |
| PTFE, will be bonded | Say so — sodium etching or plasma treatment is required |
| PTFE, unsupported span or cantilever | Flag it — PTFE will deform under sustained load |
| PTFE, abrasive or wear duty | Specify a filled grade, or choose a different material |
| PTFE, service temperature | State it — up to about 260 °C continuous, with no overheating allowed |
The three lines that decide most PTFE specifications are how the material is confined, which chemical and at what temperature, and whether a filled grade is needed. The first is the difference between a part that works and one that creeps out of position. The second is why PTFE is on the drawing at all. The third decides whether the part can also do a mechanical job.
Material is bought against your drawing for your job, and every batch ships with a material certificate (MTR) tracing the grade and filler to its mill lot plus pre-shipment photos of the actual parts. Orders start at 50 pieces per style and material, samples from a single piece in 5–10 days, and production runs in 25–35 days after sample approval. Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on the filler grade, the confinement and the measurement method where any of them need settling.
Frequently asked
- What is PTFE used for?
- Chemical seals, gaskets and O-rings, valve seats and liners, pump diaphragms and components, laboratory ware and tubing, electrical insulators and coaxial cable dielectric, high-temperature and cryogenic seals, low-friction slide surfaces, non-stick components, and — in filled grades — bearings. Its role is almost always sealing, insulating or low-friction sliding, not carrying load.
- Why is PTFE described as a seal rather than a structural material?
- Because of cold flow. PTFE deforms slowly and continuously under a sustained load, even at room temperature, and it does not recover fully when the load is removed. That means a PTFE part with an unsupported span, a bolted joint tightened onto it, or a cantilever will change shape over time and eventually fail its function. Where PTFE is used successfully in a load-bearing position — a valve seat, a gasket, a bearing — it is either fully confined in a groove or pocket so it cannot flow away, or it is a filled grade that resists the deformation. The material is superb at sealing and terrible at holding a shape under load.
- What are filled PTFE grades?
- PTFE compounded with a filler to improve its mechanical behaviour. Glass fibre improves creep resistance and wear; carbon and graphite improve wear and add some stiffness and thermal conductivity; bronze gives the best combination of load capacity and heat dissipation. The trade is that most fillers reduce chemical resistance to some degree — bronze-filled PTFE is not the universal chemical barrier that virgin PTFE is — and friction rises. Where the part's job is chemical sealing, virgin PTFE is usually right; where it is a bearing or a wear part, a filled grade is the answer, and the filler has to be named on the drawing.
- Does PTFE really resist everything?
- Almost. It is attacked by molten alkali metals, by elemental fluorine, and by a small number of aggressive fluorinating agents — none of which are common in normal service. For practical purposes, PTFE resists every acid, base, solvent and oxidiser you are likely to encounter, at concentrations and temperatures that would destroy any other plastic. That is why it holds the position it does in chemical sealing. If the chemical is genuinely unusual, name it.
- Why is PTFE difficult to machine?
- Because it is very soft and waxy, and it deforms rather than cuts. The tool pushes the material ahead of the edge rather than shearing it, so the part moves away from the cutter and springs back afterwards. It compresses under clamping pressure and under the tool, which means a dimension taken with a micrometer can read differently from the same dimension taken with a light-touch gauge. And its thermal expansion is enormous, so a warm part measures differently from a cold one. The answers are extreme tool sharpness, high spindle speeds, very light finishing cuts, gentle clamping, sharp support where the geometry allows, and measuring at a controlled temperature with light gauge pressure.
- What is the safe working temperature for PTFE?
- Around 260 °C continuously, and it remains usable down to about -200 °C — the widest range of any common plastic. The reason for the upper limit is that above roughly 400 °C PTFE begins to decompose, and the decomposition products are toxic: hydrogen fluoride and carbonyl fluoride, which cause a condition known in the trade as polymer fume fever. Normal machining does not reach those temperatures, but two rules follow. Do not let a cut scorch the material — a scorched, discoloured cut means the parameters are wrong. And **never burn PTFE swarf**; dispose of it as waste rather than in any thermal process. The classic industrial warning is also worth repeating: do not smoke where PTFE has been handled, because contaminated tobacco produces the same fumes.
- Can PTFE be glued?
- Not with ordinary adhesives. PTFE has the lowest surface energy of any plastic, so nothing wets it and no adhesive will form a reliable bond. The treatments that work are sodium etching — a chemical process that attacks the surface to make it bondable, usually applied by the material supplier as an etchable grade — and plasma treatment. Both are specialist processes arranged through our vetted partner network. For most assemblies the practical route is mechanical: a groove, a pocket, a clamped flange or a compression fitting, all of which suit PTFE's role as a seal anyway.
- Does PTFE change size with temperature?
- A great deal — its thermal expansion is among the highest of any plastic, and it is also unusual in being non-linear, with a transition region where the rate changes. Two consequences. In service, a PTFE seal in a metal housing has to be free to move or it will bind or extrude. In the shop, a dimension measured on a warm part is not the dimension when cold, so critical measurements belong on a cooled part.
- Is PTFE electrically insulating?
- Yes — it is one of the best electrical insulators available, combining very high resistivity and low dielectric loss with the ability to work across a wide temperature range. That combination is why it is the standard dielectric in high-frequency coaxial cable and in demanding electrical and RF applications, where alternatives lose performance or break down at temperature.
Related pages
Other Engineering Plastics materials
Have a part in PTFE?
Send the drawing, the grade and the quantity. You get DFM feedback and a price within 24 hours, and orders start at 50 pieces per style and material.