Press Fit Plastic parts 2

[RDMAN53]

Presently press fit a thin walled PTFE .1000 inside diameter part onto a PEEK part to create an air tight seal. In time I noted the PTFE to cold flow enough that I suspect low pressure air may get by. I'm looking to replace the PTFE with another material. These are screw machined parts that need to withstand mild acidic vapors The PEEK material is $$, we use 100 parts per assembly, so cost is a factor. Delran is OK but may become stiff after much exposure to the acidic vapor. So low cost, mild chemical resistance to acidic vapors, resistance to cold flow to maintain a slight PF & ability to easily screw machine thousands of parts are key. Any suggestions are most welcome, thank you.

 

[MikeHalloran]

PTFE is sintered from powder, and therefore inherently porous.

It's also a plastic, so by definition will move so as to relieve any applied stress.

It is therefore not the perfect material for this application, aside from its chemical resistance.

It might be possible to improve its effective lifetime by increasing the wall thickness a bit, and more so by providing something that will continue to apply stress even when the PTFE moves, e.g. a tight fitting rubber sleeve or even a heavy o-ring.

Most/all successful tube joint systems for PTFE store strain energy in some material other than the PTFE, typically an o-ring somewhere outside the fluid boundary.


The first alternate material that comes to mind is polypropylene, which is cheap, but very difficult to machine accurately. It can, however, be injection molded, so it may be worth exploring in that form.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Pure PTFE does cold flow due to its low inherent strength. If the cold flow problem is not significant, you might consider a filled PTFE material. There are filled PTFE materials that have a fairly high content of reinforcing fibers (like fiberglass) that significantly reduce cold flow and creep.

 

[KENAT]

There are a number of other fairly chemically resistant materialsyou could look into - though cost may be an issue.

We use KEL-F

http://en.wikipedia.org/wiki/Polychlorotrifluoroethylene>in

some solvent & acid vapor applications.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[dgallup]

Why are you screw machining thousands of plastic parts? With that kind if volume I'd be looking at injection molding.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[RDMAN53]

Thanks all for your input. I've no room for an external restraint, adding an O-ring is not preferred because that will add one mote part, GF is a good option as well as some other materials.

Dgallup - injection molding cannot hold PF tolerances of +/-.0005", also the screw machined parts a about a dollar, a tool would be around $20K

 

[3DDave]

So far it's a question for DuPont or Dow technical reps. Besides a dimension, no mechanical characteristics to the problem.

You have left out so much about your application. All that is clear are inflexible barriers for cost, size/shape, shape, and fabrication method. No information on pressure to be resisted, engagement length, install/remove cycles, tolerances on mating part interface geometry (smooth, barbed, ridged,) options for liquid adhesives.