acdii

I just did a cowl for my P-51, looked like it would come out nice, but found tiny pockets in the finish after I popped it out of the mold. I guess there were some bubbles from mixing. What is the best way to remove the bubbles prior to applying the first coat in the mold before laying fabric?

ByLoudDesign

The stuff is to stiff usually to remove the bubbles. the trick is to mix the epoxy slowly and the other thing is squeegeeing the layup to press them out of the laminate!

acdii

Using West Systems epoxy, Stiff is something I would not call this unless I mix in too much silica. It flows like motor oil. I was thinking maybe use a shop vac to put a vacumm on it.

Gray Beard

My Feedback: (-1)

I use finishing resin/epoxy and haven't had a bubble problem, I haven't used the West System for cowls so not sure what it is like but the finishing resin is really thin when mixed. Takes over night to cure though and when I'm finished it let it cure for a couple days before I do any sanding. It's flexible too when it's finished and doesn't get stress cracks from vibration.

acdii

West systems is about the same, flexes and doesnt crack, but cures in a few hours. I should probably use a shaker table to vibrate the bubbles out, my only concern is it setting up before I have a chance to apply it. Happened to me before, I mixed too much before I had a chance to use it all and it gelled on me.

Rudeboy

Vacuum is normally employed to get bubbles out of epoxy.

speedracerntrixie

My Feedback: (29)

Maybe you can post a picture of exactly what issue you are having. Usually when epoxy resin is mixed you do introduce air. Two ways of getting it out. The first is to degas using a vacuum pot that pulls at least 20 inches. The second is to go over your surface coat with a heat gun. This will lower the viscosity and allow the air bubbles to rise to the surface and pop. This will also shorten your working time. If you are having a pin hole issue, you can greatly reduce it by laying down a surface layer of 1 or 1.5 oz cloth first then your bulk layers.

acdii

I think the vacumm trick will work. I did the heat gun trick, which I thought removed them all, then layed in .5 oz cloth, two layers, then the 1 1/2 ounce( or whatever automotive stuff is) over that. Looked promising until I pulled it from the mold and saw the pock marks. I will see if I can get some pics posted tomorrow when back in the shop.

invertmast

My Feedback: (23)

Over using resin and having your part resin rich will allow air bubbles to stick around as well

JPMacG

My Feedback: (2)

I have seen epoxy out-gassed in vacuum. It is amazing to watch. The epoxy bubbles up into a froth and then collapses into a clear liquid.

The materials engineers at work (large aerospace) say not to vacuum degas epoxy because some of the more volatile components of the epoxy will be lost. They say just to mix it gently, trying to avoid entrapping air bubbles.

speedracerntrixie

My Feedback: (29)

Lots of things in the composite world are a compromise. Yes vacuum degassing will remove some of the volatiles. Carefully folding the epoxy as to not introduce air may result in incomplete cross linking. The best solution is to lay up the part and apply your peel ply and breather materials and cure while under vacuum. Not really feasible for your average hobbiest. A good solution is to get your layup done and then apply heat allowing the resin to flow and degas some. You can also mop up excess resin when doing this too. The more I think about this thread the more I think the OP is having issues with the cloth pulling up in corners. He did put down 2 layers of .5 oz surface cloth. I just can't see this developing pin holes. I wonder if the OP went over the corners with a paste prior to the lay up.

acdii

Maybe this will clear up the issue I have.




Overall the part came out nice, just some imperfections from bubbles. I did apply heat, so thought I had it licked.

speedracerntrixie

My Feedback: (29)

It looks like you are trapping air between the mold surface and your surface cloth. Are you brushing in a surface coat of epoxy into the mold prior to the cloth? If so that could be part of the issue. Keeping in mind that when it comes to composites there are almost as many techniques as there are guys doing lay ups I will share what my technique is. Once the mold is prepped, I will apply a paste of epoxy and Cabosil around the corners where the cloth will want to bridge. I then lay in my surface cloth which is 120 cloth. This is a 3 oz 4 harness weave that has good strength properties but the weave is loose enough to allow air bubbles to escape. Then I put down my bulk layer that could be anything from 6 oz to 10 oz plain weave. There are better options then the plain weave but it is easy to work with and inexpensive. If there are areas that need more strength I will add an additional layer of either cloth or some 5.7 oz CF cloth between the surface and bulk layer. Here are a couple pics of a fusalage I just layed up today.

sensei

From the pictures it looks to be resin rich, and maybe more stippling with a shortened brush would have work out more of the trapped air in your layup. Around 20 years ago or so I was involved with the manufacturing of many glass and or carbon fiber spinners for full scale applications in various diameters including 5' diameter spinners used on the unlimited class racers, anyway back then we used an epoxy system called Hexcel 2410 resin and 2183 hardener, this system was great to use because of it's slow pot life and low viscosity. This system disappeared from the market for several years due to EPA issues but came back without the issues around 10 or maybe 12 years ago as EZ Poxy 10 resin and EZ 83, 84, or 87 hardener, it can be purchased fairly inexpensively from Aircraft Spruce & Specialty. I/we still use this system today in some of our components manufacturing and all my modeling efforts. Utilizing the EZ 84 hardener, the mixed viscosity is around 800 @ 77 degrees with a 2hr. pot life in a mass of 300 grams allowing the user plenty of time to work through the layup process before the exothermic reaction begins to thicken the resin whereas many systems like West for one have relatively short pot lives and thicker viscosity's to start with. Thicker resin systems with shorter pot lives tend to create resin rich layups that are more difficult to remove surface air from, especially for those that do this sort of thing as a casual thing from time to time. Just food for thought.

Bob

acdii

I think that was the problem. My first cowl I did was for the Chipmunk, and I didn't have trapped air, and used the same technique, a layer of resin, then cloth. Now that I think back I used a different brush for that one, a short acid brush, and this last one I used a 1" painters brush. I guess I will mix it slower next time, and use the acid brush instead. The mold itself came out really nice without a single blemish other than where I had a couple slight runs in the PVA. At least my mold making has improved!

alex5

you can also dilute the resin with alcohol,to make it thinner,and less prone to bubble buildup.use the longest potlife that you can,as well,allowing the bubbles to work out.Try the thinning technique on a landing gear cover,or something not requiring too much effort.

H5606

I learned the following from my mentor and I'm really grateful for it all:

The 1" painter's brush is fine; just cut the bristles down to about half their length - this makes a perfect brush for stippling larger layups. Cut the top off the handle so it will fit in a spaghetti jar of acetone approximately 3/8" depth for storage. Reuse brushes to avoid leaving bristles in the layup.

You should be able to achieve perfect results with West epoxies.

Just wondering if you're using the 206 Slow hardener which gives lots more working time than the 205 Fast hardener. We're using the dispensing pumps which give about a 30 gram shot of resin and a 6 gram shot of hardener - 3 oz non-waxed, paper Dixie cups are perfect. Get the mix out of the cup as quickly as you can otherwise resin will exotherm and runaway sitting in a large mass. Start at the top of the mold and let gravity be your friend... If you don't have the pumps, use a kitchen scale - divide resin wt by 5 to get hardener wt.

Cutting cloth on the bias will make for a stronger part, is less messy (no strings to deal with), and may help cloth lay in your mold better in certain instances; paste is only really necessary for sharp corners and small radiuses. For larger layups, roll the cloth up after cutting from your patterns and unroll into the mold - one layer at a time - making sure each is fully wet out. Always bring the resin up through the cloth. Hope this helps.

Thought about this some more and - assuming you're not using vacuum - would suggest omitting the two .5 oz surface layers which I've seen trap air perhaps because of the fine weave. I've had satisfactory results just using a 6 oz cloth with good drape qualities start to finish without vacuum bagging... Oh, and make sure there aren't any puddles - blot up excess epoxy with your wife's favorite TP. Final layer should have a nice, uniform sheen; trim part flush with edge or parting line of mold with a #11 or single edge blade when "green" otherwise it's a pain to trim. BTW, if you're expiditing the curing process in an oven, the heat will cause any trapped air to expand :-(

acdii

So far I have done all you said, except the part about trimming the brush. I got a bag of disposable brushes and surprisingly enough none of the bristles came out. The two items I can take from what you said is most likely WHY i got bubbles. Using the fine cloth, and using heat. I will omit the thin stuff on my next attempt and use the heavy cloth instead, trim my brush and let it cure without heat.


I use the West systems 201/205, and cure time has not been an issue, learned on my first mold, never more than 3 pumps at a time, and been sticking with just 2 pumps except when making the mold itself. First layup, the Gel coat is 2 pumps, mixed with Graphite, coat the entire part, then mix in the Collodial for a paste around all the seams and sharp edges. I let that tack up, then lay in the cloth, two layers of the woven cloth, followed by heavy mat, and thats when I use 3 pumps. let that tack, then another layer of mat, followed by a healthy layer of resin and let it cure. This last mold came out really nice.

For the part I mixed one pump, then brushed it into the mold, followed by a little of the paste at the nose of the cowl, then laid the cloth. I think you are spot on though, the fine cloth wouldnt let the air escape, and now that I look at it, I can see fine bubble trapped in that cloth. I will just use it for what I got it for and thats glassing the plane.

Thanks for the tips, I think my next one will come out perfect now.