Polytetrafluoroethylene (PTFE), most commonly known by the brand name Teflon, is a synthetic fluoropolymer thermoplastic that can be used for a range of applications, from non-stick frying pans to medical devices.
Resistant to chemicals and stable even at high temperatures, Teflon is one of the popular CNC machining and milling materials for many specific applications, despite its so-so mechanical properties. As is the case with non-stick frying pans, it is sometimes used as a coating rather than a base material, and it can be manufactured in several ways.
One of the best ways to fabricate Teflon parts is CNC machining. This article looks at the ins and outs of machining Teflon: its material properties, the advantages of machining it, common applications, and more.
What is Teflon?
PTFE is a thermoplastic and a fluoropolymer of tetrafluoroethylene. The name ‘Teflon’ is a brand name of American chemical company Chemours, a spinoff from chemical giant DuPont (now DowDuPont). Interestingly, the material was initially discovered by mistake.
A white solid at room temperature, PTFE consists of carbon and fluorine only. It is hydrophobic, has incredibly low friction, and is non-reactive due to the very strong bonds between carbon and fluorine, one of the strongest existing bonds in organic chemistry. Due to its low friction — it is the only surface a gecko cannot stick to! — Teflon is often used as a lubricant.
The melting point of PTFE is 327 °C, and other material properties include strength, toughness, and self-lubrication at low temperatures.
Advantages of machining Teflon
There are several advantages to Teflon machining, including inherent material advantages of PTFE and advantages derived from the process of machining.
Material advantages of machining Teflon include:
- Chemically resistant
- UV/weather resistant
- Water resistant; hydrophobic
- High impact strength
- Good electrical insulator
- Flexible even at low temperatures
- Thermal stability between -260°C and 260°C
- Very low coefficient of friction
- Low flammability
- Easy to clean
Process advantages of machining Teflon include:
- Easily machinable due to softness and density
- Excellent thermal stability prevents part deformation and tool clogging
Limitations of machining Teflon
- High coefficient of expansion
- Stress creep
- Difficult to achieve tight tolerances
- Poor mechanical properties
- Poor dimensional stability
- Risk of burrs due to material softness
Applications of Teflon machining
Teflon is not the most versatile material for CNC machining, but it has some important niche applications due to its desirable properties like thermal stability and a low friction coefficient.
About half of all PTFE in the world is used for wire insulation, but wiring and its insulation is not made using CNC machines. The other most well-known use of Teflon is for non-stick coatings on aluminum cookware; in this case, Teflon in its liquid form is sprayed or rolled onto the etched metal surface. In general, Teflon-lined cookware is not machined.
Nonetheless, CNC machining is useful for manipulating Teflon in its solid form. Teflon parts suitable for machining include industrial components like gears, bushings, fittings, and valves.
Some commonly CNC machined PTFE parts, used in industries like medicine, food processing, science, and aerospace, include:
Teflon machining tips
Teflon parts must be designed to suit the properties of PTFE; it does not behave like common machinable thermoplastics, and is not easily substituted in for more common materials.
That being said, Teflon is suitable for a variety of parts and components if proper care and precaution is taken by both designer and machinist. An achievable tolerance for Teflon parts is around 0.13 mm — tighter than this may be difficult without stress-relieving the material in advance.
When CNC machining Teflon, very sharp tools combined with water-soluble coolants such as pressurized air and spray mists work well to produce the best surface finishes and tolerances. Non-aromatic coolants are also preferable.
Another key consideration when machining Teflon is deburring. Because PTFE is so soft, even fine, sharp cutting tools can produce undesirable marks that require post-processing treatment.
Common surface finishing techniques like sanding can be used to remove burrs, although a more advanced method involves freezing the machined Teflon to make it less soft during the deburring process.
- Use very sharp cutting tools
- Apply liberal amounts of water-soluble coolant
- Aim for medium-to-loose tolerance
- Plan a deburring strategy in advance
Similar materials to Teflon may be substituted in for certain projects, either for practical or budgetary reasons.
- ECTFE: Ethylene chlorotrifluoroethylene is a semi-crystalline fluoropolymer and a copolymer of ethylene and chlorotrifluoroethylene. It is made by chemical company Solvay under the brand name Halar ECTFE and offers slightly superior chemical and corrosion resistance.
- PFA: Perfluoroalkoxy is another fluoropolymer with similar properties to PTFE. Although softer and less abrasion-resistant than Teflon, advantages of PFA include arguably better non-stick properties. PFA is less commonly machined than Teflon.
- PCTFE: Polychlorotrifluoroethylene is a thermoplastic chlorofluoropolymer similar to PTFE. Advantages include an incredibly high level of water resistance: it has the lowest water vapor transmission rate of any plastic.