Molding carbon fiber....Airbubble problem Please help

General Composites Discussions Composites Talk
#1

hello, Im new to this forum, but not totally new to composites. I can overlay anything in cf. Im having problems with airbubbles in my molded cf parts when i pull them out of the mold. I usually lay down a gel coat in the mold first to get a smooth shiny finish. Then lay the cf, wet out, fiberglassmat, wet out, fiberglass cloth, wet out, roll out with fiberglass rollers, and squeegee off excess resin and then vacuum bag… The parts come out looking good, except for a few small airbubbles that were trapped between the cf and the gel coat. Is there anything I can do to keep this from happening. Im a perfectionist, and I have a garage full of parts that have the airbubbles. Even vacuum bagging wont get all of them out.

Also, anything I can find at Home Depot or Lowes that I can use as sealant tape for the bagging? I always run out and I hate ordering it and having to wait a week to get it. I found the peel ply and breather fbric at wal-mart, but cant find a substitute for the sealant tape. Any ideas?

#2

Are the air bubbles every where or just in certain spots where there are sharp contours?

#3

There are small ones here and there all over the part, like between the weave pattern, but for the most part the problem is the contoured areas of the part.

#4

How much vacuum are you pulling and what type of breather did you find at Walmart exactly? I’m thinking that the breather isn’t sucking up enough to let the air bubbles pass the laminate.

#5

*air bubbles
ok try the pre preg method…
i wouldnt use more than two layers of glass for each pre preg on complicated molds. first, get two sheets of pva. lay two layers of glass on one then a layer of armalon or porforated release film on the other. pour enough resin to impregnate the glass on the two layers of glass that you have laying on the first sheet of pva. sandwich the pva layers together.
_______ <- pva
_______ <- fiberglass (laminate)
_______ <- fiberglass (laminate)
---------- <- porforated film
_______ <- pva

i hope the visual aid works.
now you have a pre preg. keep them somewhere cold like a freezer. pre pregs can stay good for months depending on the resins shelf life. the manufacturer can tell you its work life.

peel the side of pva that doesnt have the porforated release film and place the pre preg into the prepped mold with the porforated side facing out. now you can press all the air out and if your using fiberglass as your laminate you can see all the air. it helps if your molds are a bright color like orange or red.
_____________ < pre preg with pva still on
------------------ < proforated film
========== < laminate
_________________ < mold

when all the air is out you can peel the pva off and since the porforated film is in the pre preg you prepared earlier lay your absorbant material down along with your insulation and cover with the vacuum bag. place under vacuum and heat and your part is ready for cure. the porforated film can be multi layered to control the amount of resin that is pulled out of the laminate. its great to be able to control this aspect because it can control your resin to laminate ratio.

quit using the speed tape… yellow sticky tape stuff for your vacuum bags and get some dux seal. its hard to find but reuseable. it has the consistance of play dough but its oily and can be used over and over.

i hope this helps you. its a little more expensive to do this but can make less complex parts like fenders and hoods and such turn out awesome.

#6

Where do you get sheets of PVA?

#7

we use poly vinyl sheet where we work. i just use the term to generaly refer to bagging material. i think i saw vacuum bagging material on your website. but my slang might be wrong.

#8

Ok that makes sense. I just didn’t know a pva sheet existed.

#9

You need to brush the resin by itself onto the gelcoat layer after the gelcoat gets tacky. Then lay your cloth down on the resin (not dry cloth directly on the gelcoat). If you put your cloth down dry it is nearly impossible to get a good bleed-though to the opposite side between the cloth and gelcoat. That is where it sounds like most of your trouble is coming from.

Wet your part out by dabbing the resin filled brush onto the cloth which forces the resin into the cloth. Do not brush across the grain of the cloth. You do not want to move cloth that is saturated and correctly positioned where you want it. You can brush it lightly after you get the cloth fully saturated. Also I do not recommend squeegeing the cloth. You can easily force out too much of the resin and overdry it and it may also move your cloth after it is wet and set in place which will also create air pockets. Remove excess resin by dabbing the part with a semi-dry brush. It will soak up the excess and you can skim it off the brush. Be careful not to remove too much resin. If you use a dark colored mold or surface it is easier to see bubbles and imperfections. If the cloth has a sheen to it or is white looking on the surface chances are you have removed too much resin and your end product will be dry and/or full of air.

#10

^ Thanks, im going to try that today. Has anyone ever used a food saver vacuum sealer for smaller parts? A friend of mine used it yesterday to make some cf eyelids and they came out awesome. No finish work at all. Of course you cant fit a big item into those little bags, but its still a good idea for smaller parts I think.

#11

i have used the food sealer method for many projects. works great.

#12

You may also want to try degassing your resins before application. You could buy your own vacuum chamber but they can get quite expensive. I’ve built my own using 6 in pvc pipe with 1 end cap, rubber seal and a piece of glass. drill two holes in the cap for attaching the vacuum. Now you have one open end and one closed. place the resin cup on top of the glass and place the pipe over the cup. apply a vacuum for a couple seconds and voila your bubbles are gone.

#13

You can purchase PVA film in rolls. The prosthetic industry uses it all the time. We also use pre-formed bags. Although everything we do is on a positive nold.