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I have a 6" x 6" x 1/4" piece of carbon filled teflon that is slightly curved (I think it must have come off an roll of enormous diameter) that I need to flatten. The carbon fill is carbon black, not carbon fiber, in order to make the material static dissipative.

Is there any good procedure to flatten it like with an iron?

Alternatively, thickness is not too important so in theory I could try stoning it to flatten. I will have to at some point to remove scratches on the surface but I imagine it will glaze the stone like nothing else and I don't want to need to stone too much of it since this material produces nasty dust.

With a chord of 6", there is a distance of about 2mm to the circumference so it would turn the 6mm plate into a 2-3mm plate if I were to stone it down to flatten it, which is fine if it were not for all the work involved and dust that would be produced.

UPDATE: Stoning does nothing. Just messes up the stone. But wet sanding with Scotch-Brite 7447 (Maroon) cleans up scratches and imperfections real nicely. A circular motion produces a very nice matte finish. Finer grades would probably produce a finer finish. Doesn't flatten it though.

UPDATE: Glass transition temperature is 120C so I think I'll try to heat it up beyond that. This is going to be used to make really thick 2" diameter washers. So I think I will cut out oversized square pieces, mount them on a big bolt with 2" steel washers separating them, maybe with a spring to continuously apply force and then put the whole thing in an oven at 150C or so...if I can get the oven to get that low.

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  • $\begingroup$ PTFE has a strong tendency to creep - I don't know how the carbon filler affects things but I would start by jigging it to be bent in the opposite direction for a few days? $\endgroup$ Sep 19, 2021 at 9:21
  • $\begingroup$ Adding to Jonathan comment, maybe also put it an oven to speed up the creep process? $\endgroup$
    – NMech
    Sep 19, 2021 at 9:40
  • $\begingroup$ reworking teflon may require 300+ deg C for an oven given its high melting point. if it were fiberglass filled, would be a no-brainer. With carbon sticking out at edges (in an air-filled oven) might want to start with an oversized sheet, jig and oven flatten followed by cutting out what you need. $\endgroup$
    – Abel
    Sep 19, 2021 at 10:13
  • $\begingroup$ @Abel Can you elaborate about the carbon? I am not sure what you mean. This is filled with carbon black, not fiber. It's carbon filled to make it static dissipative, not for strength. $\endgroup$
    – DKNguyen
    Sep 19, 2021 at 16:05
  • $\begingroup$ @Abel I looked up and the glass transition temperature is about 120C. I think that's all I need to get it to form a bit, right? $\endgroup$
    – DKNguyen
    Sep 19, 2021 at 16:09

1 Answer 1

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I have some experience dealing with off the shelf PTFE 'curves' and thought I would provide the technique we used to straighten our material as it may be helpful for your project.

For this particular case we were dealing with tubing that came off of a roll, likely much in the same as your material, and it caused it to have a natural curve. Every attempt at bending didn't last as the material had 'memory' and given enough time would go back to the original shape, regardless of how many times it was straightened.

Picture of the tube as shipped:

enter image description here

This was problematic as we were using it in the Central Thimble of a research reactor for reactivity core measurements and this bend caused clearance issues when passing through the s-bend on this particular port (as one could imagine, it is not okay to force items through a position on a nuclear reactor).

There weren't many material options due to issues with activation from radiation and a certain level of materials toughness was required under operating conditions, so this had to be made to work at the time.

Our best approach was putting together a simple jig to over-bend the tube in the opposite direction while applying heat with an off the shelf heat gun. This worked quite well once we figured out the correct temperature for our teflon polymer.

As PTFE is a flourocarbon it is quite resistant to applied heat with a melting temperature typically above 320C so can handle quite a bit of abuse, although these materials require fairly high temperatures before they will 'hold' the new shape. An infared thermometer is useful, but this could be done without if need be. In either case, be careful and wear appropriate PPE including heat resistant gloves or you could easily get burned.

It is easy to over-correct the natural bend in the opposite direction at temperature so it is best to practice and be willing to settle with 'close enough' as we did on our rig:

enter image description here

Nearly a year later, and after a number of irradiations in the core, the apparatus is generally in the same shape as when we first straightened it.

As your material is a sheet it should be easier to manipulate as you will have less challenges with potential collapse and the carbon likely will not cause much, if any, additional issues.

Hope this helps!

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  • $\begingroup$ I'm a bit lucker. If what you say is true, instead of overbending I should just be able to clamp it under high pressure in a vise two plates and heat it and rely on creep. $\endgroup$
    – DKNguyen
    Apr 14, 2023 at 22:26
  • $\begingroup$ Indeed, the 'flate' plate should be easier to deform into your desired shape. There was a decent amount of pressure put on our tube by the jig, so your proposal seems sound; although we overbent and when the part relaxed it came into a shape we were able to work with. My advice is to just be careful, if the part goes too far or gets overheated there is probably no coming back. High heat and adequate pressure are your friend for this 'controlled' creep deformation, but not so much you damage your material of course. Let me know your results, and good luck! $\endgroup$ Apr 15, 2023 at 16:50

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