PTFE and ME (or Machining & sealing of Teflon)

[_ME_]

Sorry for the title, I liked the pun.

I would like to hear thoughts on anyone who has worked with Teflon either machining or for gasketing. I have never touched teflon aside from it being a coating but will get to machine and work with some this week. Most of the information I find online has to do with offgasing.

I am also curious to get opinions on whether thin teflon gaskets inbetween three 3/4" thick teflon blocks would sufficiently seal so as to be watertight. Since I've not worked with the material before I am not sure how 'rubbery' this semi-crystalline stuff would be.

I imagine it is probably the material that can be found here:
http://www.mcmaster.com/#8545k72/=2y1npk

Thanks!

 

[Q_Goest]

Hi ME. I've used various materials for sealing purposes. Seals can generally be broken up into 2 main types, static and dynamic. Static meaning the seal is somehow fixed between the parts it is intended to seal. Dynamic meaning it is used to seal against a surface that moves across it.

In very general terms, Teflon can make a good static seal if properly designed, though I've seen many poorly designed Teflon gaskets. The problem with sealing statically is that Teflon tends to cold flow and also requires considerable contact stress to seal properly. Filled Teflon, such as glass filled, helps reduce cold flow and has been used quite a bit for valve seats and gaskets. The main reason for using Teflon is that it has very good chemical resistance, though it's also used at times for cryogenic applications since it holds up well at low temperature.

When using any material for sealing purposes, a general rule of thumb is that the contact stress on the parts when under pressure, must be greater than the pressure it is intended to seal. How much greater depends on the sealing surface geometry, contact width, surface finish and other factors. ASME code provides some guidelines for sealing loads.

 

[Topher925]

I use PTFE on a daily basis including using it as seals. PTFE is typically only used as a seal for applications in chemical harsh environments due to its C-F bonds, and/or applications where the seal also needs to be a VERY good electrical insulator.

Teflon is not a "rubbery" or crystalline like substance. It is a polymer, and has very similar properties to other polymers such as polypropylene and polystyrene. It has a higher modulus of elasticity than rubber type materials and tends to be slippery to the touch so it doesn't really make for a very good sealing material.

I've never machined the stuff before but imagine it would require similar tooling that you would use for machining nylon or polycarbonate.

 

[Tom Kelly]

Your second question:
You mean teflon gasket between teflon blocks?
It will not seal. Instead you have to go for soft material gasket betwwen teflon blocks. It can be silicon.

Tom

 

[mohsinkhalid]

I use PTFE on a daily basis including using it as seals. PTFE is typically only used as a seal for applications in chemical harsh environments due to its C-F bonds, and/or applications where the seal also needs to be a VERY good electrical insulator.