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I have these two parts shown below, the bottom cylindrical part is stainless steel, the rectangular upper piece is PTFE (Teflon). The SS prongs from the lower part need to insert into the holes of the PTFE part.

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I'd like to create an airtight connection between the two; but I'm unsure of how to go about it.

I'd like to avoid adhesives all together as this will undergo high temps.

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  • $\begingroup$ The parts can be modified $\endgroup$
    – Sam W
    Jun 21, 2018 at 19:00
  • $\begingroup$ What about PTFE (plumbers) tape? Also, airtight to what pressure? $\endgroup$
    – Jonathan R Swift
    Jun 21, 2018 at 19:20
  • $\begingroup$ well it needs to be mechanically secure, I don't think tape alone would do it. The pressure is about 1.5 PSI $\endgroup$
    – Sam W
    Jun 21, 2018 at 20:12
  • $\begingroup$ PTFE : stands for : Pull Tight - Fu**ing Expensive $\endgroup$
    – Solar Mike
    Jun 21, 2018 at 20:14
  • $\begingroup$ The seal between the outter surface of the prongs and the innersurface of the holes in the PTFE part should be airtight, right? $\endgroup$
    – mart
    Nov 8, 2018 at 8:32

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You're going to struggle to get a reliable gas tight fit with that design.

For a start it would be better if the two connections were separate so they aren't fighting against each other in terms of fit.

Equally rather that trying to reinvent the wheel you would be better using off the shelf connectors which are known to work. It would be fairly trivial to replace the plain tubes with threaded or compression fit connectors in a suitable material.

It is also hard to offer advice without more detail of what you are ultimately trying to achieve.

If for example you wanted to be able to 'plug-in' the stainless steel section you might be ok with a taper fit if you could keep the joint compressed eg with an external spring or cam but again you could just use off the shelf quick release fittings.

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If you have to use the Teflon for some reason(s), you can bond the Teflon part to the SS by first etching the Teflon with a special etchant designed for Teflon and then you can use a high temp epoxy adhesive to get a good bond (seal) between the two assemblies.MasterBond Inc. has a large selection of epoxies for all kinds of applications. I would sandblast the SS also to get a good bonding surface. I have used this method of etching and bonding Teflon myself, the etchant turns the Teflon brownish, so if there is a problem with looks , you can't do it. The epoxy bonds nicely to the Teflon, I was amazed the first time I tried it. That's why we love engineering.

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If you can modify the stainless steel prongs with groves or indentations to accept either O-rings or Compression brass or plastic fittings you would not need any adhesive!

Plastic compression connectors can withstand high temperatures and pressures.

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A few design improvement notes:

  1. PTFE is not Nylon. I'll assume from here on out you really mean PTFE here, but it's better known as Teflon. Nylon would be done entirely differently.
  2. You can't just snap these two parts together for an airtight seal. You can get mostly air tight, but you will not make an airtight seal between stainless and PTFE with just a snapped connection.
  3. PTFE is slippery stuff. It's well known for this property. You will need some kind of mechanical way to keep it on (such as end flanges that can be screwed onto the ends of the SST parts). Otherwise most mechanical stresses will break the very fragile airtight seal of the passive method, and up the power requirements of the active method.

There are two methods for joining these two materials.

Passive Method

In the passive method, you would roughen the stainless steel's surface to a white metal blast finish (SSPC.SP5 standard or NACE #1). Then the steel would be coated with a powder coat of PFA (or PFA/FEP combination), then baked in a powder coating application. Additional layers of PFA could be added, and baked on. A minimum of three would be recommended. Finally, heat both the stainless and PTFE to 260 Celsius, and join together at that temperature. The parts will have a fairly weak bond, but so long as the stress around the joint can be mitigated mechanically as discussed above, it will be workable.

Active Method

This is more for robust applications and with thin, relatively flexible PTFE linings. In this method, you still treat the steel surface as mentioned above, but instead of all the thermal work, there is a manifold that would supply active compressed air to a thin layer of PTFE surrounding the joint. So long as the air pressure does not exceed the compressed air pressure, the mechanical push forces will maintain an active seal.

Testing

You should make samples ahead of time between the proposed materials and test them with a mechanical peel test (Such as an ASTM D1781 climbing drum test). A 50 lb/in peel is a bare minimum for pass/fail, 100 lb/in is more suitable.

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Thread (or make otherwise irregular, like machine circular indentations) in the steel part's holes to a size that fits snugly on the PTFE pipe.

Then get two cylinders of steel or other strong material that will be able to withstand whatever the pipes will carry, threaded on the outside, of outer diameter a bit larger than the inner diameter of the PTFE pipe; radius larger at least by height of the thread; they aren't just to fit snugly, they must squash the pipe a bit. Length about the width of the metal opening.

Screw them into the pipe, stretching it and squeezing the material into the threads of the steel holes. It won't be easy, because teflon is quite hard, but under sufficient pressure it starts flowing; the action of screwing the pipe in must cause it to flow into the gaps.

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  • $\begingroup$ Danger is that the PTFE will eventually slip nonetheless over time because its so slippery, atleast thats my experience. But then if you really crush it in place who knows it might work. $\endgroup$
    – joojaa
    Jun 22, 2018 at 19:31
  • $\begingroup$ @joojaa: or include periodic re-tightening as a standard maintenance procedure. It will certainly take quite a bit of time to slip out. $\endgroup$
    – SF.
    Jun 22, 2018 at 21:55
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If you dont use adhesives then you'll need some kind of pressure to keep things airtight. Problem is PTFE is soft and slowly deforms ("cold creep") over time , vibrations etc... Making it leak prone.

Try to redesign and/or use a gasket made from elastic material(PTFE isnt elastic) between the PTFE and the steel

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