Straightening PEEK tubing

[Daniel Rocha]

Hi Guys,
I need to straighten a piece of PEEK tubing to tight tolerances. Has anyone had any experience with this? The tubing comes in a roll of about 150mm DIA from the supplier which doesn't help and I've been really struggling to succeed at this. Tube is 1/8"OD X 0.0802"ID and I need a piece 270mm long with 1.5mm maximum permissible bow. I have many quite a few attempts and I can get it pretty close. See below what I've tried so far. They all involve heat.

1. Insert PEEK tubing into a piece of brass tube(perfectly straight) then heat up to 220C (maximum my oven achieves) and leave there for various different amounts of time: 15min, 30min, 45min, 1hr, 1.5hr and 2hrs. Remove from oven (whilst inside the brass tube) and let it cool back to room temperature. Result is a 5mm bow

2. Pretty much same process as above but leave the set-up in the oven then switched oven off and let the entire system cool down for 24hrs.
Results varying between 3 and 5mm bow.

3.Same as above but apply a torsional for on both ends of the tube. Tried many variations, half a turn, 1 turn, 1+1/2 turns, 2 turns.
Results are very inconsistent. I managed a couple of good ones with 1+1/2 turns but I cannot reproduce it. Tried more than 10 and got 2 good. This will be a production process and can't afford the material and time waste of a 20% success rate.

4.Built a jig that would force the bow to go the opposite direction then heat it up and let it cool. Went 'S' shape horribly.

5.Same as 1 but I used blocks on rollers to stretch the material while it was hot. Tried various different distances. Went up to 25mm stretch (which reduced the overall DIA by 0.5mm but that is fine for what we need it for). This gave me my best results so far but at 2.5mm bow and reproducible.

6.Started to think that I may need more heat. So I used a different oven to heat it up to higher temperature. Maximum I went for was 325 but still not solving the problem. It actually got worse.

Sorry about the massive post.

Can someone please save me?

 

[Compositepro]

Keeping a piece of plastic that is 270 mm long and 3 mm in diameter straight to 1.5 mm is almost impossible. You are probably not capable of maintaining the temperature uniformity of the tube during measurement to meet that spec. Touching the tube with your fingers would cause more than that distortion. What is your measurement procedure? What is the reason that such straightness is required?

 

[Daniel Rocha]

Thanks for the reply.
The temperature is quite uniform whilst the tube is in the oven. My thermo-couple reads a change in temperature of 1 deg C max. Since I don't touch, move the tube form the oven or even open the oven door unitll the entire system has colled down to room temperature plus a give another 10 hours or so for it to relax (normally don't touch it for about 24H). By the time I do the measurement the tube should have stabilised.
I don't have any high end measuring device. I've been pushing my finger down in one end of the tube and rolling it on a flat surface. At some point the other end of the tube rises to a max distance between the end of the tube and the flat surface. That distance is the bow in the material. This part is a part of a probe for sample introduction in an Autosampler used in a Spectrometry instrument.

 

[3DDave]

I'd contact the tubing maker to see if they would supply straight lengths. It doesn't come out of the process initially rolled up. This link says they supply straight pieces:

https://www.polyflon.co.uk/products/fluoropolymer-tubing/peek-tubing/

 

[Daniel Rocha]

Hi 3DDave
I've tried to get them I straight length from our existing supplier but their minimum order quantity is 10,000 units. We need more like 100 so they refused to gives us a quote even if we wear the set-up costs. I've contacted POLYFLON UK (thanks for the tip), now lets see if they can help.
Thank you

 

[IRstuff]

> Need to determine whether bow is aligned with gravity. If so, the you might need to turn the tube as if it were on a roasting spit.
> Alternately, one might wish to consider orienting the tube with gravity, i.e., vertical.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[EdStainless]

Perhaps a slight amount of tension on them will help the process.
Going vertical would allow you to dead weight them to do this.

Of course you know that they will likely not stay straight.
This is a thermoplastic after all.

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P.E. Metallurgy, Plymouth Tube

 

[SnTMan]

OP, seriously? Also, please DO NOT mix units. Bad for the discussion, and guaranteed to bite you someday.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[EdStainless]

And I hate to say it but if you really need that kind of straightness over the long run you should be looking at metal.

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P.E. Metallurgy, Plymouth Tube