Graphite Filled PTFE Compressibility ?

[cnuk]

This may be a simple question, but it PTFE incompressible? I have a graphite filled teflon material used for a lip seal and want to do some preliminary analysis. I need to know the poisson's ratio of the material. I have some experience dealing with elastomers but little experience with PTFE and other polymers that may be used for seals/bearings like Torlon, PEEK, etc. Some good introductory book recommendations on polymers would also be helpful. I am a mechanical engineer that knows metals but feel a little intimidated by polymers and I'd like to educate myself.

Thank You

 

[MikeHalloran]

Here are some typical properties, not including the exact stuff you want:

http://www.goodfellow.com/csp/active/gfMaterialTables.csp?type=30&prop=MEC

You may be able to get better properties from the manufacturer of the material you're trying to model, if you can identify exactly what it is.




Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

My first advice to Mechanical Engineers that have extensive knowledge of the properties of steel, is disregard all you know about physical properties of steel when dealing with plastics. They are an entirely different animal.

All thermoplastics flow under load.

They all compress to some extent under load. By compression, I mean increase in SG by decreasing in volume.

The actual compression is usually so small as to be negligible, but creep or distortion can be significant.

Plastics appear to behave like extremely viscous liquids with very small amounts of gas entrapped.

PTFE of one of the most "liquid" of plastics.

Peek is on of the least "liquid" plastics.

Fillers like bronze and glass fibre reinforce some physical properties.

Graphite mainly acts as a surface lubricant of friction modifier.

Regards

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[Demon3]

Good summary from Pat. I did a Google search for "creep of PTFE" and got some nice hits.

Wikipedia says "Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore it has no "memory," and is subject to creep (also known as "cold flow" and "compression set"). This can be both good and bad. A little bit of creep allows PTFE seals to conform to mating surfaces better than most other plastic seals. Too much creep, however, and the seal is compromised. Compounding fillers are used to control unwanted creep, as well as to improve wear, friction, and other properties. Sometimes metal springs are used to apply continuous force to PTFE seals to give good contact, while permitting some creep."

Plus there were links to reducing the creep of PTFE:

http://www.bayplastics.co.uk/Product Materials/prod-ptfe.htm

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[cnuk]

Thank you all for the info. I've looked at the links and am compiling info to start reading. Looks like I have a lot to learn but that's not such a bad thing.