Questions: Balsa/Carbon Sandwiches

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Aquaman33
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Questions: Balsa/Carbon Sandwiches

Postby Aquaman33 » Tue Aug 28, 2018 9:38 pm

Hi all,

I am new to composites, but I am experimenting with carbon/balsa sandwich fins for a future project.

The methodology I plan to use is as follows:
1. Wet out one side of 2mm thick balsa fins with epoxy (Wests 105/206).
2. Apply a piece of oversize carbon/kevlar cloth.
3. Wet out the cloth with more epoxy until lightly saturated.
4. Add a sheet of perforated peel ply.
5. Add a sheet of breather.
6. Flip over and repeat on the other side of the balsa.
7. Place the sandwich on a sheet of flat glass in a vacuum bag to cure.

I am going to make some sample panels and test the strength. I will add additional layers of carbon if required.

I have done a lot of research online, but I still have a number of questions so I am hoping to tap into the collective wisdom of this forum:

1. Should I drill lots of tiny holes in the balsa to allow the epoxy to penetrate the balsa and bond the 2 carbon faces together? I have seen this mentioned in lots of comments, but can't find it from a reliable source.

2. I want the fins to have rounded edges. Should I sand the edge of the balsa before laminating or leave it square and then sand the rounded profile after laminating? I am concerned about the cloth not following the contour of a rounded edge if I sand the balsa before laminating.

3. Should I remove the fins from the bag at the "leather stage" and trim the excess cloth/resin, or wait until they are fully cured?

4. If I remove the fins and trim at the leather stage should I re-bag them for the remaining cure time to avoid warped fins?

5. How long does 105/206 take to reach the leather stage when used for laminating (assuming normal room temperature)?

6. Should I apply some sort of finish or epoxy to the edge of the fins where the balsa is exposed, or just leave it exposed?

Thanks in advance for your advice. Any other tips or suggestions would be most welcome.

This is an interesting article on laminate cores if anyone is interested:
https://www.compositesworld.com/articles/getting-to-the-core-of-composite-laminates

Cheers,

Derek

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Re: Questions: Balsa/Carbon Sandwiches

Postby drew » Wed Aug 29, 2018 11:39 am

Hi Derek,

I've got no experience with vacuum bagging so I'm not going to be able to address some of your questions. That said, I can answer a couple so hopefully that'll get the ball rolling.

2. I want the fins to have rounded edges. Should I sand the edge of the balsa before laminating or leave it square and then sand the rounded profile after laminating? I am concerned about the cloth not following the contour of a rounded edge if I sand the balsa before laminating.


Yes, definitely sand before laminating. because you're vac bagging the cloth should conform to the contour of the fin.

6. Should I apply some sort of finish or epoxy to the edge of the fins where the balsa is exposed, or just leave it exposed?


Ideally if you sand before laminating the cloth should "touch" at the tip of the contour on either side and a) seal the balsa on leading and trailing edges and b) eliminate the need to run a leading edge coat of epoxy to mitigate the exposed balsa.
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Re: Questions: Balsa/Carbon Sandwiches

Postby Viking » Wed Aug 29, 2018 12:25 pm

I have bagged some composite fins, using e-glass instead of Carbon though.

It is better to wet-out your cloth rather than the balsa. You will get lighter fins this way.
I'm not sure If bonding the two faces together is really needed but if you are worried about de-lam, epoxy alone will help a little but it would be easy enough to push some carbon tow through the holes. Epoxy alone will likely get sucked out during bagging anyway.
Definitely sand to shape before laminating. E-glass conforms nicely to rounded edges, I'm not sure about carbon, never used it like this.
I left the fins in the bag for a day then trimmed (easy with a sharp blade, like an x-acto) I then left them another day to harden up a bit more and then sanded the edges.

If you want to increase edge strength, you can buy carbon spread tow self adhesive tape. I've seen RC plane guys wraping wing leading and trailing edges with this stuff before adding their lams and bagging. I also saw a vid with a guy making his own using thin 3M double-sided tape, but can't find that vid now.

Sorry for the crappy photo, these aerofoiled fins are ~16mm thick balsa lite-ply (7-ply from memory) with 2 layers per side of e-glass. I forget the gsm.
They survived a decent impact onto rocky soil under tiny drogue chute (main failed).

fins.jpg
fins.jpg (19.94 KiB) Viewed 1505 times
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Re: Questions: Balsa/Carbon Sandwiches

Postby Viking » Wed Aug 29, 2018 3:22 pm

Viking wrote:...I also saw a vid with a guy making his own using thin 3M double-sided tape, but can't find that vid now....


Found the vid, though it's not exactly as I remember.
He's making edge tape with e-glass and cutting it so it's biaxial. I guess it conforms to compound curves better this way, or it's stronger, dunno, I don't understand German.
https://youtu.be/aybxBzKvd5o?list=PLVgV ... rm0c&t=172

At 4:55 he shows some pre-prepared kevlar tape and it looks like he also has some spread tow carbon tape hanging there too.
He applies the tape to a tailplane edge at around 5:15
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Re: Questions: Balsa/Carbon Sandwiches

Postby Aquaman33 » Mon Sep 17, 2018 10:02 pm

Thanks for the responses Drew and Viking.

Since my last post I have done some experimenting 25mm wide balsa strips that I have laminated with carbon, kevlar and fiber glass.

Image

Unfortunately my vacuum bag developed a leak, so I had to quickly clamp them between some boards as a last resort.....it was a bit of a scramble because I was afraid the epoxy would go off, but it worked out okay.

I plan to test them until they break and hopefully that will help me decide what to use.

I have also started making some carbon fins with a balsa core for a low powered rocket:

Image

They aren't finished yet but so far they seem strong and light....and they look good.

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Re: Questions: Balsa/Carbon Sandwiches

Postby Viking » Tue Sep 18, 2018 2:01 pm

Good Stuff!
Aquaman33 wrote:....it was a bit of a scramble because I was afraid the epoxy would go off, but it worked out okay.


Don't stress too much with 206.
I mix for a min or so and then wet out my cloth as quick as I can just so it's not sitting in the pot cooking itself.
I use a foam roller (paint trimming type) which in effect does more mixing while you're wetting out.
Once I'm done wetting the cloth I can slow down as it's easy to forget steps and f-up simple things while rushing.
The 206 datasheet states 90 - 110 min thin-film working time, so for cloth I think it's safe to say half that for working time.
So far I've never had an issue, even when the left-over in the pot is starting to go off, the cloth is fine.


Quick correction while I'm here...
Viking wrote:...these aerofoiled fins are ~16mm thick balsa lite-ply (7-ply from memory) ...

That should be 12mm. And yes, it is 7-ply but the face veneers are pretty thin, so really 5-ply plus facing.
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Re: Questions: Balsa/Carbon Sandwiches

Postby Aquaman33 » Tue Sep 18, 2018 6:43 pm

Viking wrote:Good Stuff!
Aquaman33 wrote:....it was a bit of a scramble because I was afraid the epoxy would go off, but it worked out okay.

So far I've never had an issue, even when the left-over in the pot is starting to go off, the cloth is fine.


Thanks Viking.

Yes, I have noticed that the epoxy in the pot goes off fast. For my first batch I mixed up way too much and it went off quickly in the pot.....especially around where the paint brush was sitting in the pot.

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Re: Questions: Balsa/Carbon Sandwiches

Postby Oldboy » Thu Sep 20, 2018 10:56 pm

Yes, I have noticed that the epoxy in the pot goes off fast. For my first batch I mixed up way too much and it went off quickly in the pot.....especially around where the paint brush was sitting in the pot.
Can you mix on a saucer ?
Longer time available to use it , not being cooked from the bottom up .
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Re: Questions: Balsa/Carbon Sandwiches

Postby Viking » Fri Sep 21, 2018 12:39 pm

Yep, epoxy cures faster with heat, curing epoxy creates heat(exothermic reaction) it's vicious circle. I guess the brush bristles are a bit of an insulator trapping heat inside, I wonder how close the epoxy in the brush is to thermal runaway?

When I was a grom (teen surfer) I used to surf with a guy that's now a world famous board maker. Back then he was starting to experiment with board materials and everyone thought he was a bit of an oddball, now they're copying him worldwide.
Anyway, one day he was doing a 'hot coat' (heat lets the resin spread thin and levels out nice and glossy) on some new boards and gave me a demo of thermal runaway. He demo'd both epoxy and polyester resins doing this, smoking like crazy in a pot and then bursting in flames, was cool but also scary as f to think something seemingly benign could do this.

Oldboy wrote:Can you mix on a saucer ?

Might be a bit difficult to mix, maybe mix in a pot and then transfer to a saucer?

I do my initial mix in a pot then pour it into a paint tray for the foam roller, it spreads out into a thin layer and doesn't cook as quick as the left-overs in the pot.
roller tray.PNG
roller tray.PNG (33.77 KiB) Viewed 1098 times


Edit: doh, wrong pic
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Re: Questions: Balsa/Carbon Sandwiches

Postby Aquaman33 » Fri Sep 21, 2018 8:25 pm

I mixed a small batch yesterday in a pot and then poured onto a disposable picnic plate. It definitely extended the pot life as the heat wasn't trapped.

The downside was that you waste a bit of epoxy as it tends to spread out on the plate.....but worth it for complex lay ups.

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Re: Questions: Balsa/Carbon Sandwiches

Postby OverTheTop » Sat Sep 22, 2018 9:44 am

I have been known to sit the mixing container in ice water to slow things up on hot days.
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