Annealing PTFE sheet

[guimba]

Hello, I am having some thin parts 1.25" wide x 2.50" long machined from .125" thick PTFE sheet. Problem is the sheets come in with a curl to them so the parts don't lie too flat on the mill. Is there a way to anneal the sheets to get the curl out? Thanks!

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[patprimmer]

Roll them up in the opposite direction and leave them overnight.

Use a hair dryer to heat one side so it curls the other way. Experiment with heat and time until it ends up straight, although I don't like your chances.

Use a rectangular metal frame to hold down the edges but not cover the work area.

Regards
Pat
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[bithkits]

Get the supplier to press them flat when they come out the manufacturing plant and are still warm.

annealing them is not a good idea, you might do more damage to the material than anything. Think about bending wood - the heat can help, but you don't want to bake it in an oven...

Possibly a large set of rollers that stretch and flatten the sheet would help? Similar to those used on roll forming machines and any machines that take wire off a reel/spool, but in your case the rollers would be long and flat and bend the sheet one way then the other way, until you relieve the residual stresses that are causing it to curl...

Could you use thicker sheet (that will by nature have less curl), machine the parts and then face them to correct thickness?

Adriaan.
I am a Mechatronics Engineer/part time Master's student from South Africa.

http://www.martin-electronics.co.za

 

[guimba]

Thanks for the inputs; you guys are on the right track. I managed to come across a design handbook from DuPont that recommends heating it to just above the expected service temperature for about 10 minutes then slow cooling. I'll contact a local heat treating shop and see if they can work me in. Cheers.

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[btrueblood]

We purposely buy the PTFE sheets in thicker sections than final product size, machine parts (water jet cut), then have them double-disk ground to exact thickness, the double disk grind also gives "perfectly" parallel surfaces across the thickness.

Yes, you can anneal and/or press PTFE sheets and have them flatten, but you won't get them to stay perfectly flat without reversing the curl as Pat suggested. PTFE creeps (exhibits plastic deformation) at room temperature at stresses above about 300 to 500 psi, at least that's the rule of thumb. Raising the temperature will drop the stress required for a given strain rate.

 

[btrueblood]

Second thoughts - if you have a lot of parts to make (thousands or more), then consider talking to a compression molding shop and having blanks molded to size for you.

 

[MikeHalloran]

Slow cooling plastics takes longer than you might think, because of their poor thermal conductivity.

I worked with a product comprising 3/8" thick plates of acrylic, that had to be super flat. After flycutting, they were stacked between flat metal plates, heated to annealing temperature, then cooled. We had to buy a computer controlled oven, because the cycle we evolved through trial and error took about 40 hours, most of which was controlled cooling with a very slow ramp rate. Don't open the door until the cycle times out.





Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

On semi crystalline materials which PTFE is, there will a temperature range where they are particularly sensitive to rate of temperature change as that will be the range where crystals most readily form.

Polymers are extremely large long molecules so crystals take a long time to form. The longer the time, the more perfectly they form which means they shrink more where they have large well formed crystals.

Regards
Pat
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[guimba]

Thanks all, I have an RFQ out to a supplier of near net shapes and will also check with the shop as to how the sheets come in, rolled up and held by a string, I surmise. Maybe we can get them to ship in the flat.

Mechanical Engineer
Field Forensics, Inc.

Engineering isn't about perfect solutions; it's about doing the best you can with limited resources. - Randy Pausch

 

[4JawChuck]

We used to make similar parts and rigged a sheet stretcher on the mill so the sheet was flat and held off the platen, we used simple knurled cams to lock the sheet in place and tighten.

All operations took place before the final cutout, once completed they went through a rolling machine to flatten them and take the curl out. It was all done at room temperature with no heating, like any material once you "break" the curl the part will lie flat and can be inspected.

Good luck.