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 Post subject: air bubbles
PostPosted: Tue Apr 23, 2013 5:46 am 
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So i finished my 4th board. Last one rides fine, not great but OK. Each board I have made I have struggled with air bubbles showing up in the glass after vacuum, mostly on the base, evident after I remove the board from the vac table. I am using a single sided vac setup, -25 vac. Here is my layup - from top down:
Vinyl
Breather
Peel ply
Glass x 2
Graphic on rice paper
Core (9mm cedar)
Graphic
Glass x 2
Release wax
Plexi
So my question is why am I getting these air bubbles? Everything looks good before vac but when I pull it all apart after 36hrs under vac I have various air bubbles in the glass ranging from pen tip size to 1". Am I not using enough resin? Is my vac not strong enough for a cedar core? Sounds stupid but, do I need a breather on the base? Is there a way to not have to sand so much and do flow coats after vac? Any help to limit my flow coats and sanding after vac would be appreciated!


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 Post subject: Re: air bubbles
PostPosted: Tue Apr 23, 2013 2:13 pm 
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Peelply and breather would probably help.
But if you are not in a hurry, doing a 2-shot build might be better than spending that extra day sanding.
The thing I am always looking out for is bubbles from outgassing if the wood core heats during cure.
If this is a problem put a sealer coat of epoxy on the core before the lamination, and/or warm the core before starting/watch your work room temps.


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 Post subject: Re: air bubbles
PostPosted: Tue Apr 23, 2013 2:38 pm 
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Thanks BWD. My garage is typically around 50-60 degrees, so maybe warmer? I have used breather with peelply and perfply. Is there a order that is best? Perf over peel or vice versa?
I will be constructing a new rocker table this week and will be profiling the core in hopes of laying up the new board next week.


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 Post subject: Re: air bubbles
PostPosted: Tue Apr 23, 2013 2:59 pm 
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the typical bubble issue with wood cores:
wood is porous, contains some air
start working in a.m. or mid-day
shop warms a bit into the afternoon
epoxy exotherms a little bit as it starts to cure
air in the wood expands and tries to escape, forming bubbles under glass.
Can be a big problem with light woods laid up without vaccuum.
A good vac setup should avoid this problem, but leaks happen, etc.
peelply goes on first, then film then breather.
Often good results can be had without using breather or peelply, but it's trial and error. I have experienced the "error" myself, resulting in more sanding...
For a one-time project, probably best to go by the book and make sure the air and resin has a place to go - through film and into breather.
You may find less sanding/hotcoating needed if you skip the peelply.
Or you might get a wrinkle in the film...
its a process


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 Post subject: Re: air bubbles
PostPosted: Tue Apr 23, 2013 10:22 pm 
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Location: rhode island
i'll chime in with a vote of confidence for all of BWDs comments.

odds are good that vacuum will pull air out of the wood core regardless of temperature gradient.
the wood core will suck up lots of epoxy anyway, so you have nothing to lose by skim coating a very thin layer and scraping off all excess (during falling temps) before bagging.

w/o breather on the bottom, you really dont have any path for the air between bottom and table to escape. especially bubbles that exit the core as the resin starts to kick.

worse yet, w/o a bag on the bottom, pressure distribution may not be uniformly applied from rocker table allowing resin and bubbles to pool in 'void' areas.

does the bag hold vacuum?
air from the outside is just as bad as air in the resin

there are lots of folks that believe a hand layup in this situation will yield a better result with less sanding.

everything works, but the devil is always in the details

-bill


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 Post subject: Re: air bubbles
PostPosted: Wed Apr 24, 2013 1:05 am 
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Hi, for what you are trying to do most people use a perforated core. That way the air and any excess resin in the bottom laminate has somewhere to go.

If the picture you have posted is the board bottom and you are worried about riding performance, then you might want to rethink your flip tip design.

The water needs a clean hard edge to release from. In the picture it looks like your do not have one. I am talking about the "step" right before the actual tip of the board. By hard edge I mean it needs to be a 90 degree angle or less.

The result is your board will have poor light wind performance and/or it might feel a little slow or "draggy".

If you fix this and you still have performance issues then you need to look at your rocker line and rail shape.

Trent


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 Post subject: Re: air bubbles
PostPosted: Wed Apr 24, 2013 6:10 am 
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Thanks to all for the ideas!
I agree, sounds like a skim coat prior to layup maybe a good idea (there are definitely small gaps between the lamination of each strip of the core) and there is not much to lose by doing a skim coat first. I have tried laying up each side separately, but was hoping to be more efficient than that.
My system has always held vacuum during each press, so i don't think there are leaks - as the pump rarely runs once vac is achieved. However, I have always felt that the pressure may not evenly distributed (hence the air bubbles) and have looked at constructing a pneumatic press to apply more consistent and greater pressure but I am not sure I want to get into that level of tooling.
Zfennell: Are you saying I should apply breather on the base side? I have wondered if breather on the base would allow better flow on the base since I am not sure if air is being trapped when the board is vacuumed to the table in rocker and concave.
Not Annoymous: Sorry the pic is of the top (I didn't have a base photo). The board road well in light wind and fairly flat water (SPI) a couple weeks ago. One thing was there seemed to be some spay hitting my ankles occasionally - more than my normal ride (North Jamie). Is the spray produced form too wide of a tip? Not enough shape? Too wide of rail? Or too steep of rail angle as you said? Happy to grind away at the shape of the board to learn more for the next.
Not trying to be lazy and avoid mandatory work with the sanding and skim coats, just looking forward to dialing in my system. Thanks for the feedback, it really helps.


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 Post subject: Re: air bubbles
PostPosted: Wed Apr 24, 2013 1:27 pm 
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Location: rhode island
sinker wrote:
Thanks to all for the ideas!
Zfennell: Are you saying I should apply breather on the base side? I have wondered if breather on the base would allow better flow on the base since I am not sure if air is being trapped when the board is vacuumed to the table in rocker and concave. .


yes,
BWD said it first.

for the most part, everything in the bag is at the same pressure. (core, glass, resin, air film, breather etc) there is relatively little flow associated with the vacuum pump to move excess material.

the breather provides a way to 'press' the glass up against the core while moving excess epoxy and air to the opposite side of your release film.

if the pot life of your resin is sufficient you can degass the resin with vacuum before laminating.
use a foam roller or squeegee ( no brushes) to apply resin.
move in one direction only to minimise foaming in the cloth.
its both art and science that i have yet to master, so i cling to the few things that work for me.
i'm still sure everything can work with the right recipe.

-bill


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 Post subject: Re: air bubbles
PostPosted: Wed Apr 24, 2013 6:09 pm 
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I have a few things to add.

Sealing the wood core is definitely the first step. Applying vaccuum only means the air trapped in the core is now at higher pressure relative to the air in the bag. So it's gunna bubble. 1 inch bubbles are huge

After that, In order to get rid of the air bubbles you need to be able to evacuate the air. Clamping pressure alone will not "pop" a bubble if the air has nowhere to go. On the bottom sheet this is almost impossible as the resin pretty much seals the part from any path where it could breathe so you gotta kill em before they get there.

I find temperature makes a big impact. The viscosity of the epoxy gets thicker as you go colder. making it harder to get the bubbles out. I always find if I mix epoxy in the cold, it gets tons of bubbles in it that won't come out If I mix it aggresively. So I'd increase my shop temperatures to start. you have less time but laying up at 70 degrees makes a much bigger difference to most resin viscosities

If you want to be successful with a vacuum degassing, you need a low viscosity and REALLY high vacuum. Given you're agitating the resin in a wet layup I'd be surprised if vacuum degassing helps much.....method of application would be more important...Degassing is a must if looking for perfect results in infusion where you would suck bubbles straight into the layup

In some hollow molded parts I've made, I find hitting the completed layup with a quick shot of a blow torch has been succesful in lowering the resin viscosity and increasing the air pressure to allow bubbles to form and pop. YOU HAVE to make sure your core is sealed though otherwise it will just cause air to expand out of the wood. I haven't built a board yet since I learned this trick but it should work if you shot the finished bottom sheet just before putting it on the table. If the bubbles don't pop but at least come to the surface you can hit them with a squeegee to kill em and it will likely prevent the bubbles from coming to the surface during exotherm of the epoxy. Not quite certain if the temperature differential between the hot bottom sheet and the cold table would give you some additional problems but I don't see a reason why this should cause a problem

General question for everyone, what's up with peel ply? My understanding was it's just designed to give you a rough surface for secondary bonding.
Isn't
bag,
breather
perforated release film
part
not the desired layup schedule?

Or does the peelply overall improve surface by preventing print through of the breather at high vacuum?


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 Post subject: Re: air bubbles
PostPosted: Wed Apr 24, 2013 9:46 pm 
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Joined: Tue Jan 02, 2007 11:50 pm
Posts: 271
Location: Ottawa, Canada
Is all the air between the plexi and the glass, or is there air between the core and the glass? I would think that air from the core should not be finding its way down through the glass to end up between the glass and the table. Gravity still applies inside a vacuum, or is there some other way that air would go down? Some people don't take the time to get a perfect layup onto the table, and expect or hope the vac will do the rest, but I'm not sure if that's the issue here.

If it was me I'd try doing the layup in 2 shots, with bleeder holes in the core through to the top, so the air has somewhere to go. Also is the core getting lifted up by accident while trying to place it on the table, sticking to the glass and pulling up an otherwise clean glassing job?

Finally, it is regular plain weave cloth? I have some fine weave stuff that is harder to get bubble out of, so I have to work it more carefully.

Peter


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