Aero65

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Hello Everyone This is an Awesome Forum, I have learned so much from everyone here. Thank you!

I made a 3 piece mold for a cowl, and I followed the example on the www.cstcomposites.com website.

I'm wondering on the initial gel coat using west systems epoxy (105 resin) (206 hardener) and, graphite powder, what techniques could be used to reduce or eliminate small bubbles from forming very small voids on the inside surface of the mold.

I did not get extensive voids just a few small ones.

I was thinking of these possible methods.

Vacuum degassing the mix before painting on gel coat (Degas would occur prior to adding graphite.

Heating mix for lower viscosity

use different product like duratec for base coat.

Aloha in advance to all that help

CurtD

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Try to keep "entrained" air (the technical term for air bubbles in the mix) to a minimum by stirring less vigorously and taking a little more time. After you mix your epoxy well add a little isopropyl alcohol (the "anhydrous" kind - no water) or denatured alcohol and gently mix it in. The alcohol will lower the viscosity of the mix and allow the bubbles to rise out, giving you a nice, smooth first layer in your mold with no pinholes. It does lower the viscosity a bit but I haven't found this to be a problem - make it fairly thick with Cabosil to begin with.

Heat will also work by lowering the viscosity (to some extent) but it also accelerates the curing - something I don't want. Haven't tried the "vacuum" method of removing the bubbles but I don't see why it wouldn't work. Just don't suck epoxy up the vacuum hose!

Aero65

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Thank you CurtD

Sounds good ill try those things on my next mold.

Troy-RCU

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De-gassing is the preferred method of eliminating the bubbles from surface coat. It's a simple as using a coffe can, or strong bucket of some sort, with a clear top (I've used thick plexi glass), stick your mixing cup inside, and apply a few inches of mercury to it. You have to be patient and watch for bubbles to stop surfacing. Mixing slowly is definately a good idea. I really don't recommend adding alcohol to the mix because it can interfere with the epoxy matrix and it also makes the surface coat slide off vertical surfaces. Surface coat is thick for a reason
Another way to make sure you don't get surface imperfections is to do a thin intial coat, let it gel up, and come back with a thicker coat over top. Once that is gelled up enough to where you don't leave finger prints when you touch it, you're ready to start your lay up over top.

scole

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For anyone wondering why the link wont work, its cstsales.com

Aero65

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Thanks Scole

Got the website goofed up. Sounds easy on the vacc setup Troy, I have a vaccum pump that will pull 29. inches at 10 cfm so my only worry I guess is collapsing the vacc chamber. Lol that would be a mess in more places then one.

ptxman

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A lot of these issues were discussed on the rcu thread

http://www.rcuniverse.com/showthread...82&forumid=206


- if you are going to de-gas the resin mixture, that should occur at the final stage prior to application ie add your carbon to the resin mixture, then de-gass
- adding thinner to resin is a no-no, it adversley affects strength & cure time. Tooling coats are generally thicker anyway.
- gently heating the resin will lower the viscosity & help with bubbles but also decrease the pot life
- Ive had good results applying the tooling coat on with a foam brush vr bristle brushes. Pros use spray equipment but generally thats overkill for our application
- A lot of people think the tooling coat has to be applied in one shot, even to the extent that the reinforcemnt layers have to be applied when tacky etc. I dont support that. You can in fact build up your tooling coat in a few layers/sessions with no adverse effects. Take your time, it will give you more control, lower exotherm, thinner layers will also help control entrained air

Good luckk

CurtD

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Guys go back and read my post. I said add a LITTLE thinner (isopropyl or denatured alcohol) to the mix. Not everyone has the degassing equipment that you mentioned. I certainly don't and it isn't necessary. A SMALL AMOUNT of the correct alcohol WILL do the job and it won't have ANY noticeable affect on the strength on the first tooling coat on a mold for a cowling.

We ain't building full size airplanes here, just models. Try to keep that in mind.

Aero65

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Hello everyone

One of my friends mentioned that I should try a Polyester Gelcoat, Since you can mold epoxy in it and it seems like an easier way to go ill try that next time.

Troy-RCU

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Unless you're going to make the whole mold out of polyester resin, don't do it. The epoxy won't stick to the polyester gel coat and you'll end up with surface coat peeling out of the mold.
And about de-gassing: I don't normally do it at home but we did it at my work often enough. Like I said before and ptxman backed me up, you can always put the gelcoat on in thin layers so you don't end up with bubbles. The thing with composites is that there are often countless ways to do the same job. Some ideas just work more consistently than others.

Aero65

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For the most part im going to try polyester mold's for awhile due to lower cost. I agree with the thinner coats that sounds like that will work well.

When I go to back to epoxy molds Ill try that.

Just awesome advice from everone, I have learned sooo much from this forum, Thanks all.

Flyingmike-inactive

Hi

What do you use to apply the resin, a sponge, brush?????

Mike

Aero65

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In the case of Epoxy i have used a brush.

For the polyester Gel Coat I'm going to use a spray gun with a 2 mm or greater needle.

ptxman

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If you can access less expensive polyester resins & related tools that’s worth considering for small projects & initial learning I suppose. I’ve seen & made some very nice polyester resin based molds, but like anything, those results were achieved with experience & often repetitious trials to get it right. You will probably not see the quality difference on a cowl for a sport plane, but you will when the mold requirements get more stringent like a wing or fuselage.

In reality, you might not be as ‘money-ahead’ as the $/gal comparison would indicate either. There are different grades of polyester resin - general purpose, isopthalic, vinylester. The qualities of these vary among formulators within the same class. Better grades cost more. Better means stronger & less distortion as their chemistries start take on properties more like epoxy. All things being equal, you need a proportionately thicker polyester based mold than you would with epoxy mostly to control shrinkage distortion, requiring additional reinforcement materials & consumables. Most good polyesters have a tough time approaching ordinary grades of epoxy. Polyesters are more sensitive to catalyst variations, ambient conditions, layup thickness & smell badly (if that’s an issue). Viscosity of epoxies can be just as low a as polyesters for layup.

In terms of gels, both systems from what Ive heard contain similar additives anway. The qualities you want in a gel are are viscosity, strength, cure time control, finish & durability. There are limits to how thin you can make successive polyester gel coats, at least that was my experience. I don’t know if it’s the high styrene content, exotherm temp, gel time curve or what, but you can give your mold surface a case of the uglies with thin coats of polyester gels. Epoxy is much more forgiving, controllable & durable particularly on the edges like on mold flanges etc.

The real money is the effort value put into the plug. If you have something ‘go wrong’ on a mold, the cost difference of the resin becomes a very small issue by comparison. When it comes to laying up the composite parts, all these same parameters apply. I cant think of many commercial models these days that are even made from polyester. Certainly not many in the competition world. Just my opinions, hope this helps, show us some pics!