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Pros and Cons of PTFE Sealing

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PTFE as been around for a while and has its own advantages and disadvantages. It was discovered by accident, not unlike Alexander Fleming’s discovery that a green fungus was eating the bacteria he was attempting to develop in a petri dish; Penicillin went on to vary the world as we know it. PTFE was discovered by Roy Plunkett, an worker of DuPont back in 1938. Apparently the old boy was attempting to make just a few million more for the already wealthy DuPont family by attempting to come up with a new chlorofluorocarbon refrigerant when the tetrafluoroethylene in his test bottle stopped flowing. Much like Penicillin, Roy’s discovery had surprisingly positive and helpful side effects; he seen a coating inside the bottle he was using, sawed it open and located a particularly slippery substance within. Abracadabra, presto change-o: polytetrafluorethylene, or PTFE for brief, was discovered. Three years later, the fabric was patented by a subsidiary of DuPont and given the trade name Teflon. The patent has since run out and there at the moment are many producers of PTFE throughout the world. For the sake of this weblog, and more importantly, to avoid patent violations (yeah, they’re supposedly expired, but it IS DuPont) we will be referring to this material as PTFE. Curiously enough, the byproduct of the production of PTFE, the stuff Roy most likely threw out, is perfluorelastomer that’s used so usually in critical o-ring applications in the semiconductor industry. That, nevertheless, is a whole different blog post. Although recently methods to mold PTFE have been developed, the fabric comes in sheet form, is a typical part in artificial joints, and can also be useful for making inside repairs in the medical area, the first focus of this submit will be on seals and backup rings machined from billets of material.

Machining PTFE Seals

Many of the PTFE seals on the market at the moment are machined from cylindrical billets. The process of making the billet is similar to the process of sintering metal. The raw material in powder form is at occasions pelletized to be able to use an automatic process to pour the beaded PTFE materials into the cylindrical, steel cast. For some very critical applications though, the powder have to be put into the mold by hand which requires more labor at usually a higher cost. The fabric is then compressed into the mold using a big press. The resulting material shouldn’t be unlike kiln-dried clay and as such could be very fragile. Within the next step the billet is careabsolutely positioned into ovens; PTFE has a really high melt point (one of many reasons it is so efficient in sizzling applications); it is heated to 342 C (648 F). The top product produces a billet that is recognizable and is now ready to be machined into the end part that will be used.

PTFE Advantages and Disadvantages

Virgin PTFE is white and has the advantages of a very high temperature ranking, extraordinarily high lubricity, and being inert to most caustic fluids. A disadvantage is that it’s additionally very soft. PTFE manufacturers, therefore, add quite a lot of elements into the mix like carbon fiber, glass, bronze, and lots of others, with the intention to strengthen weak traits of the material and improve performance like extrusion resistance and hardness. Since PTFE in its virgin form tends to cold flow or take on the form of its housing, filling the fabric with other compounds makes the end materials more immune to cold flow.

Due to its lubricity, PTFE is great in dry, oil-free environments. PTFE seals can function in static fashion and dynamic reciprocating, and make perfect high-speed rotary seals. There are a few things to consider when deciding to use a PTFE seal, though. PTFE is a ‘dead’ material, in different words, it has very, very gradual memory; when compressed it tends to stay compressed. Seal producers make up for this by loading the seal with an energizing system like an o-ring, or, more likely, a spring. Within the case of a lip seal, these energizing gadgets lend their inherent mechanical energy to the seal and allow it to proceed to do its job particularly when in a low pressure state, when the sealing media just isn’t engaging the lips.

PTFE is also fairly fragile and it’s of utmost significance to make sure that the surfaces are smooth and comparatively hard. For rotary motion for instance, hardness must be between 55C and 65C with a surface end of two and sixteen RMS. Most manufacturers will have data primarily based on the design of seal and the material that is very important to take into consideration when designing the hardware for a PTFE seal. Elastomeric seals will ride on a thin coating of fluid that inhabit the crevasses of the hardware, PTFE nevertheless is a self-lubricating material that actually coats the shaft or bore (relying on your application) with a fine film of itself; so often lubricity and sealing improves with use.

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