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Author: Subject: Air bubbles in Carbon Fibre
johncarman

posted on 1/2/10 at 02:09 PM Reply With Quote
Air bubbles in Carbon Fibre

I have recently been trying to make some parts in carbon fibre, starting out with a simple flat piece to be used on my dash, but I was hoping to do cycle wings etc. I have attempted 4 or 5 pieces now but I can't get rid of the air bubbles in the first layer of resin (the top coat once released from the mold).
I have tried using the heat gun trick on the carbon mods video and this helps, but I have to heat it for so long to remove all the bubbles it has cured before I have finished ! I have tried both West and Carbon mods Resin, I have the same issue with both.
I am wondering if the problem could be the conditions I am working in, I have a small heater in the workshop, but how cold it's been outside, it doesn't get that warm in there. I have noticed the epoxy goes runny the warmer it gets, is this the problem, do I need to do this somewhere warmer ??
Has anyone else had the same problem, or got any advise ??

[Edited on 1/2/10 by johncarman]

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Richard Quinn

posted on 1/2/10 at 02:21 PM Reply With Quote
Is it small air bubbles where the tow's cross each other or are you getting larger patches? Is this just on flat pieces and are you just doing a wet lay-up or are you applying any vacuum or pressure to consolidate the part?

[Edited on 1/2/10 by Richard Quinn]

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Steve Hignett

posted on 1/2/10 at 02:32 PM Reply With Quote
Have you used any release agents on the glass?
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Werner Van Loock

posted on 1/2/10 at 02:38 PM Reply With Quote
are you using clear gelcoat as first layer? This might do the trick.

And yes, epoxy resin likes a warm environment so start heating or wait till summer.

And what are the steps you take to make up the piece?

eg first release agent, then clear gelcoat, then epoxy resin rolled on, then epoxy resin rolled into the first layer of CF, then place the CF on the gelcoat/resin etc...





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bodger

posted on 1/2/10 at 02:46 PM Reply With Quote
Clairetoo is an expert on carbon.
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smart51

posted on 1/2/10 at 02:47 PM Reply With Quote
Whenever I've tried to use epoxy resin, I get air bubbles just by stirring the 2 parts together. I've never managed to get the bubbles out so I ended up with pin holes in the gaps between the weaves. I'd always guessed that vacuum bagging removed these.






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Richard Quinn

posted on 1/2/10 at 02:48 PM Reply With Quote
quote:
Originally posted by Werner Van Loock
are you using clear gelcoat as first layer? This might do the trick.

And yes, epoxy resin likes a warm environment so start heating or wait till summer.

And what are the steps you take to make up the piece?

eg first release agent, then clear gelcoat, then epoxy resin rolled on, then epoxy resin rolled into the first layer of CF, then place the CF on the gelcoat/resin etc...
It's unusual to have a gelcoat with an epoxy resin system. Also, if you are using a fairly light (e.g. ~ 200gsm) twill then using a roller can drag the cloth. Careful stippling is the only way. Use a slow hardener and warm the lay up with a gentle heat gun and you should really see the resin start to "wick" into the cloth. A really slow hardener should allow you to apply a reasonable bit of heat before the resin starts to go off.

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blakep82

posted on 1/2/10 at 03:00 PM Reply With Quote
using rollers is supposed to help with bubbles too.





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Steve Hignett

posted on 1/2/10 at 03:14 PM Reply With Quote
IMO the gel needs to be rollered onto the glass, you could try a different type of gel (spray gel?).

Also IMO, you need to roll the CF after you've brushed the resin on, but you need a specific roller for best results...






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Richard Quinn

posted on 1/2/10 at 04:03 PM Reply With Quote
IMHO 2x2 twill, inexperience and a roller may do away with some of the bubbles but it will end up with the twill all over the place!
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twybrow

posted on 1/2/10 at 04:21 PM Reply With Quote
Paint the resin on neat as the first coat. Not too thick, and use a paintbrush, which will help release any trapped air bubbles. Let that go tacky, or completely off before you carry on and put the fibre down...

If you want to get real technical, you could place the unmixed resin under vacuum before using it, which will remove dissoved voltatiles, but this is massive overkill to achieve a good finish.

Any hand layup type process will be prone to air bubbles, hence the need to paint the resin on first as a nice clear coat, that can be sanded/polsihed as required to produce the surface finsih you desire.

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Richard Quinn

posted on 1/2/10 at 06:41 PM Reply With Quote
If it's epoxy, don't let the first coat cure completely. Sure, let it tack and then lay up onto that but if it cures you get an amine "blush" which won't allow the 2nd coat to bond properly.
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johncarman

posted on 1/2/10 at 07:33 PM Reply With Quote
Thanks everyone for the advise, here is a bit more info to clear up some of the questions...

I am not using any Gel Coat, the first layer is epoxy straight onto the mould (covered with release agent). I let this go tacky, then lay up my first layer of fabric which I use a roller on to ensure it is well pressed into the resin. I find without the roller I was getting air where the tow's cross, between the first layer and the fabric, but now the air I have is actually in the first layer of epoxy. as Smart51 mentions, the bubbles are there as soon as you start mixing the epoxy and the hardener, and I don't mean the odd bubble, I mean it's like the head on a pint of beer !! Is this normal ??

I am suspecting this must just be down to the conditions I am working in as I seem to be following most of the advise out there short of using a vacuum and I still have a problem.

John

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Steve Hignett

posted on 1/2/10 at 07:57 PM Reply With Quote
How are you applying the first layer of epoxy to the glass/mould surface then?






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johncarman

posted on 1/2/10 at 09:39 PM Reply With Quote
I am just brushing the first layer onto the sheet of glass (my mould), which has been covered in a PVA release agent.
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Steve Hignett

posted on 1/2/10 at 09:48 PM Reply With Quote
I would strongly suggest rollering it on! (pour it out of the jug onto the surface in a cross hatch (think sauce on bread!) then roller it horiz. and vertic. until it's as flat and even as you can get it)

Also, (and I know this is going to be imposible to gauge over t'internet, but) how are you applying the PVA, and how thick have you got it?

Have you sealed the glass? How many times have you used this "mould"?

[Edited on 1/2/10 by Steve Hignett]






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johncarman

posted on 1/2/10 at 09:52 PM Reply With Quote
I have started with a fresh mould, I am not really sure how much release agent I should be using, I just pour about a cap full onto a rag and wipe it over, I am frightened of putting too much on, I didn't know if it would effect the surface finish of the part.

When you say roll it on, you mean roll the epoxy on instead of brushing it ??

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Steve Hignett

posted on 1/2/10 at 10:30 PM Reply With Quote
The performance of a mould is better after a few releases, so that could b playing a small part.

Yes I do mean to roller the epoxy on...

I'm sure the PVA is prob being done right else your part wouldn't be releasing in some areas potentially. The trick to it is, to put on as little as possible, but you can't wipe/buff it over to thin it out or it will remove what you've put down instead. Work v quickly and smoothly to get a perfectly even thin coat.

You never mentioned whether you've "sealed" the new mould yet either?

[Edited on 1/2/10 by Steve Hignett]






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Richard Quinn

posted on 1/2/10 at 11:21 PM Reply With Quote
I probably wouldn't even use pva on glass. I would just use about 10 coats of silicone free wax polish (even colron wax will do)
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Steve Hignett

posted on 1/2/10 at 11:24 PM Reply With Quote
quote:
Originally posted by Richard Quinn
I probably wouldn't even use pva on glass. I would just use about 10 coats of silicone free wax polish (even colron wax will do)


Was thinking that myself, but wasn't sure to suggest it in case that's all he had!!!






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twybrow

posted on 1/2/10 at 11:52 PM Reply With Quote
PVA put on too thick will make a dodgy surface finish, but not air bubbles so much as a crazing type effect (although I guess it would depend on just how much PVA you did apply!?).

Your resin shouldn't froth like a pint of beer. Some bubbles sure, but not super frothy! Is the froth only there when you mix the components? Can you see signs within either individual component of the air? Are you being careful with your mix ratio - epoxy is not tolerant of poorly measured mixes?!

Is the problem uniformly distributed all over your part, or just in very localised points?

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Benonymous

posted on 4/2/10 at 05:26 AM Reply With Quote
I have just finished making some big parts in moulds and I'd actually avoid putting epoxy onto the mould to begin with. When you see racing car (F 1) parts you're looking at stuff that's made from pre-impregnated fabric, put in moulds and then autoclaved at fairly high temperatures in a near vacuum. No wet layup will ever approach this level of finish. The problem you're describing is practically unavoidable.

I have been doing my layups in a female mould using 450GSM twill with very wide tows. My method is to lay the cloth in the mould then wet out by pouring the epoxy in and using a squeegee to move it around. Once I have wet the cloth out, I use a metal roller to consolidate and ensure the cloth is wet through. I alternate between a roller with a sort of really coarse knurl and a paddle roller with corrugations that go across the roller.

The trick to the "glassy" carbon look is actually to use a high fill clear coat and buff it. I vacuum bag my parts at around 20 to 30 inches of mercury and not even that will give you a top surface free of pits in the crosses of the tows.

With any composite, you're trying to get the correct resin to reinforcement ratio for maximum strength. If you drown the carbon in resin to get the top surface so thick that the weave isn't showing through you've got way too much resin and the part will be brittle.

If you want a good surface, vacuum bagging is the way to go.

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phelpsa

posted on 4/2/10 at 09:04 AM Reply With Quote
quote:
Originally posted by Benonymous
I have just finished making some big parts in moulds and I'd actually avoid putting epoxy onto the mould to begin with. When you see racing car (F 1) parts you're looking at stuff that's made from pre-impregnated fabric, put in moulds and then autoclaved at fairly high temperatures in a near vacuum. No wet layup will ever approach this level of finish. The problem you're describing is practically unavoidable.

I have been doing my layups in a female mould using 450GSM twill with very wide tows. My method is to lay the cloth in the mould then wet out by pouring the epoxy in and using a squeegee to move it around. Once I have wet the cloth out, I use a metal roller to consolidate and ensure the cloth is wet through. I alternate between a roller with a sort of really coarse knurl and a paddle roller with corrugations that go across the roller.

The trick to the "glassy" carbon look is actually to use a high fill clear coat and buff it. I vacuum bag my parts at around 20 to 30 inches of mercury and not even that will give you a top surface free of pits in the crosses of the tows.

With any composite, you're trying to get the correct resin to reinforcement ratio for maximum strength. If you drown the carbon in resin to get the top surface so thick that the weave isn't showing through you've got way too much resin and the part will be brittle.

If you want a good surface, vacuum bagging is the way to go.


The autoclave applies very high pressure, not vacuum

I was going to suggest a thick coat of lacquer and buff it back. Its not cheating, its a trick used by many carbon fibre manufactures, even on prepreg if you have a dry weave or a dull finish.






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johncarman

posted on 14/3/10 at 09:58 PM Reply With Quote
Thanks everyone, some great advice here. Given up on CF at the moment, was taking up all my time and was getting no other winter mods done, I will revisit over summer.






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RK

posted on 15/3/10 at 01:14 PM Reply With Quote
As a mortal who's tried it many times, i gave up too. It is VERY Temperature dependant, and I never got rid of the small holes. They wouldn't fill afterwards either, no matter what I did, because the release agent stops the new stuff from sticking to the epoxy, even when dried. Obviously, it is possible to do, I am convinced, but I want an expert to show me first hand the next time!
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