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Curing epoxy under higher pressures?

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mattma
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Curing epoxy under higher pressures?

Postby mattma » Wed Aug 31, 2011 9:49 am

Has anyone tried curing epoxy under higher pressures than 1 atmosphere?

Whenever I read articles of on performance composites it seems that curing under high pressures seems to be part of story. I guess this makes sense cause it pushes the epoxy deeper into the core material for better adhesion?

I was wondering if anyone knew how high the pressure needs to be to start getting benefits? Also, wondering if higher temperatures give you more bang for your buck than high pressure?

Cheers

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Re: Curing epoxy under higher pressures?

Postby plummet » Wed Aug 31, 2011 11:10 am

I think you need the correct resin ratio. Make sure you don't squeaze too much out of the fibre's.
35% resin ratio often used, but your need to check the type of resin your using.
As for temperature its also comes down the the style of resin used. Ultimatly i think the higher temp resins (usually used in pre-preg) give a slightly higher strength.

But i doubt its going to make any huge impact on the performace of a kite board. Sure if your making parts for F1 and the greatest strength to weight ratio per part may mean the difference between victory or death. For kite boards.... overkill.

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Re: Curing epoxy under higher pressures?

Postby Bigdog » Tue Sep 06, 2011 11:12 pm

I press at 50-60psi and ramp the temp up to 70C over hours. It really makes strong boards.

Stan

mattma
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Re: Curing epoxy under higher pressures?

Postby mattma » Mon Sep 12, 2011 1:04 pm

Bigdog wrote:I press at 50-60psi and ramp the temp up to 70C over hours. It really makes strong boards.

Stan


Hi Stan

What set up are you using for pressuring it up?

Cheers
Matt

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Re: Curing epoxy under higher pressures?

Postby Bigdog » Mon Sep 12, 2011 1:09 pm

The press consists of 4 i-beams spaced apart and bolted together with another short piece of beam. and a 18in lay flat hose/bladder is the pressure device. I use mdf to form the mold surfcae which can be changed to make snowboards as well.

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Re: Curing epoxy under higher pressures?

Postby mattma » Mon Sep 12, 2011 3:39 pm

Nice! I've seen a pnuematic press on skibuilder.com and have been scratching my head about how to use one when there is concave in the board becasue the ones I've seen clamp the board between 2 rigid surfaces. Does the bladder in yours press directly on the board or do you have sheet of mdf or the like between the bladder and the board?

I'm not sure where to go looking for a bladder that is that wide and was wondering whether I might be able to use the inner tube from a tractor tyre. What do you think?

Cheers
Matt

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zob
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Re: Curing epoxy under higher pressures?

Postby zob » Mon Sep 12, 2011 4:50 pm

mattma wrote:was wondering whether I might be able to use the inner tube from a tractor tyre. What do you think?


I can answer you this one.
I build one of these. The widest tractor tube i got was 30 cm. So it was stil min. 10 cm to narrow.
I put a layer of PVC and sprinkled it with baby powder to get the tube expand easiliy. But the resaults were disastrous. It wouldnt expand at the tips (ends of the tube) because the top plate wasn't shaped to the rocker table. The tube first takes shape in the original proportions, only when is filled and the preassure starts to rise, it tends to expand. By that time the friction between the tube and other surfaces rises so much, that even with baby powder, the tube will not slide as you would want. It will find one or two places, where it is easiest for it to expand and it will do so on those few places, and there will be rincles between expanded and non-expanded tube.
You must close this press also from the sides, to get higher preassure, or the tube will expand out, or blow.
Fire hose is different, because it has that fabric around it to limit it expansion.

Overal, I made my worst board with this press! The first one I scraped, because it was not worth rebuilding.

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Re: Curing epoxy under higher pressures?

Postby Bigdog » Mon Sep 12, 2011 9:16 pm

Using a tube with structural fibers like a firehose is the easiest way to get big pressure and no need to close it in a box as mentioned. However getting the structure designed to withstand the forces is very important. There is the potential to get hurt badly with a pneumatic press. Have an engineer check your math!

Stan

mattma
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Re: Curing epoxy under higher pressures?

Postby mattma » Tue Sep 13, 2011 11:43 am

:D :D
zob wrote:
mattma wrote:was wondering whether I might be able to use the inner tube from a tractor tyre. What do you think?


I can answer you this one.
I build one of these. The widest tractor tube i got was 30 cm. So it was stil min. 10 cm to narrow.
I put a layer of PVC and sprinkled it with baby powder to get the tube expand easiliy. But the resaults were disastrous. It wouldnt expand at the tips (ends of the tube) because the top plate wasn't shaped to the rocker table. The tube first takes shape in the original proportions, only when is filled and the preassure starts to rise, it tends to expand. By that time the friction between the tube and other surfaces rises so much, that even with baby powder, the tube will not slide as you would want. It will find one or two places, where it is easiest for it to expand and it will do so on those few places, and there will be rincles between expanded and non-expanded tube.
You must close this press also from the sides, to get higher preassure, or the tube will expand out, or blow.
Fire hose is different, because it has that fabric around it to limit it expansion.

Overal, I made my worst board with this press! The first one I scraped, because it was not worth rebuilding.


Thanks Zob and Stan. You've saved me a lot of time and frustration. Nothing like real experience to go on!

A variation on the bladder idea that I have been thinking about is to replace the bladder with a bag full of sand :D The idea is to cover the entire board with the bag of sand, cover it with a thick layer(s) of MFD and then use some sort of screw clamp set up (maybe attached to then of one or more I-beam sections like Stans) to apply the pressure. the idea of the sand is that it would flow over the contours of the board surface and help to distribute the pressure evenly. This would avoid the chance of having you're head ripped off by an exploding bladder/tube and get around problem of the uneven stretching.

Do you think this could work?

Also, how do you make the rocker table surface strong enough to avoid buckling under the big pressures? I was thinking about laminating several layers of 1/4" MFD, maybe 3 or 4 of them to make a solid surface. Would this work ? Is there an easier way?

Cheers
Matt

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zob
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Re: Curing epoxy under higher pressures?

Postby zob » Tue Sep 13, 2011 3:27 pm

mattma wrote:The idea is to cover the entire board with the bag of sand, cover it with a thick layer(s) of MFD and then use some sort of screw clamp set up to apply the pressure. the idea of the sand is that it would flow over the contours of the board surface and help to distribute the pressure evenly.
Do you think this could work?
Cheers
Matt


I think the sand also wouldn't distribute as you would want it to.
With the smallest grain of sand you can find, I think this could work only by using a vibrator like the one they use in pouring concrete.

I use 5cm thick foam for seat upholstery (I don't know the density) and on top of that 1cm plyboard (thin enough to folow rockertable shape a little)and 4 wooden beams 100x60x800 mm to clamp everything together with 8 screw clamps. This works great. Think simple!


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