Pros and Cons of PTFE Sealing

PTFE as been round for a while and has its own advantages and disadvantages. It was discovered accidentally, not unlike Alexander Fleming’s discovery that a green fungus was eating the micro organism he was trying to develop in a petri dish; Penicillin went on to change the world as we know it. PTFE was discovered by Roy Plunkett, an worker of DuPont back in 1938. Apparently the old boy was trying to make a few million more for the already rich DuPont family by trying to come back 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 useful side effects; he observed a coating inside the bottle he was utilizing, sawed it open and found a particularly slippery substance within. Abracadabra, presto change-o: polytetrafluorethylene, or PTFE for brief, was discovered. Three years later, the material was patented by a subsidiary of DuPont and given the trade name Teflon. The patent has since run out and there are actually many manufacturers of PTFE throughout the world. For the sake of this blog, and more importantly, to keep away from patent violations (yeah, they’re supposedly expired, however it IS DuPont) we will be referring to this material as PTFE. Apparently sufficient, the byproduct of the production of PTFE, the stuff Roy most likely threw out, is perfluorelastomer that is used so typically in critical o-ring applications within the semiconductor industry. That, nevertheless, is an entire other blog post. Although just lately methods to mold PTFE have been developed, the fabric comes in sheet form, is a common component in artificial joints, and can also be useful for making inside repairs within the medical area, the primary focus of this put up will be on seals and backup rings machined from billets of material.

Machining PTFE Seals

A lot of the PTFE seals on the market today 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 instances pelletized with the intention to use an automated process to pour the beaded PTFE material into the cylindrical, steel cast. For some very critical applications though, the powder should be put into the mold by hand which requires more labor at normally a higher cost. The fabric is then compressed into the mold utilizing a large press. The resulting materials is not unlike kiln-dried clay and as such may be very fragile. Within the next step the billet is careabsolutely positioned into ovens; PTFE has a very high soften level (one of many reasons it is so effective 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 rating, extremely high lubricity, and being inert to most caustic fluids. A disadvantage is that it’s also very soft. PTFE producers, therefore, add quite a lot of components into the blend like carbon fiber, glass, bronze, and lots of others, as a way 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 shape of its housing, filling the material with different compounds makes the tip materials more immune to cold flow.

Because of its lubricity, PTFE is superb in dry, oil-free environments. PTFE seals can perform in static fashion and dynamic reciprocating, and make best 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 other words, it has very, very sluggish memory; when compressed it tends to remain compressed. Seal producers make up for this by loading the seal with an energizing device like an o-ring, or, more likely, a spring. In the case of a lip seal, these energizing units 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 is just not engaging the lips.

PTFE is also pretty fragile and it’s of utmost significance to make certain that the surfaces are smooth and relatively hard. For rotary motion for example, hardness must be between 55C and 65C with a surface finish of 2 and 16 RMS. Most producers will have data based mostly on the design of seal and the fabric 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 however is a self-lubricating materials that really coats the shaft or bore (relying on your application) with a fine film of itself; so usually lubricity and sealing improves with use.

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