Carbon Fiber Tube Layup Quesion(s)

Hey Ya'll, I'm a first timer here.

I'm a senior of Aerospace engineering at Auburn University currently working on my level 2. I'm not alone, it seems like half our class wants to make them all at the same time for some reason (-_-), but I have a few questions regarding the process of laying up carbon fiber tubes. We have a 3 and a 4 inch diameter mandrels, and have made a few of them already, but the quality of the tube varies wildly, and I can't figure out why.

First we used vacuum bags in an autoclave, that turned out really REALLY bad, it caused the carbon fiber to bunch up on itself horribly, and bound it so tightly to the aluminium mandrels that they had to be chiseled off. Now we wrap the mandrels in Teflon, then the carbon fiber rolled up like paper, then another layer of Teflon to prevent the resin from leaking out into the oven. We have them in the oven for 2 hours, rotating the tube every 30 mins to keep the resin from accumulating at the bottom. It still accumulates to an extent, we've gone through quite a lot of CF just to get where we are now quality-wise, but there's definitely still room for improvement. Do you guys know of a sure-fire way of getting perfect tubes every time? I'm considering using hose clamps and thin aluminium sheets wrapped around the outer Teflon layer next, no idea how that will turn out.

I'm sure there are many ways of doing it, but any useful tips will be MUCH appreciated. Thanks!

First of all, hello fellow Aerospace student!

I cannot give you a lot of advice, but I do know that trying to produce a carbon fiber tube with a vacuum can be really, really difficult.

You may want to look into breather material, this collects excess epoxy so you don't have to rotate the tube. Also, you should look into peel ply and perforated release film.

I cannot tell you what order or methods would work best as I have never done a tube layup. But welcome and you will get an answer soon.

Thanks for the advice! We have breather cloth available fortunately.

So would it be Teflon, CF, Breather, then Teflon?

Hey Welcome to the forum! I hope to major in aerospace as well

Honestly you don't need any of that stuff. Just roll your tube, use peel ply and you will be set. If you have resin dripping then you used too much. The peel ply will always help with that problem and give a nice smooth finish.

I would probably just roll it without the vacuum bagging, like conman says, like in this thread below. The video shows a fiberglass layup, but a carbon layup should be the same. It should still be plenty strong and extra weight from higher resin content shouldn't make a difference.

Tfish method:https://www.rocketryforum.com/showthread.php?19534 I think this is a sticky on TRF somewhere.

I've vacuum bagged 1 layer on top of an existing layup successfully. I tried two wraps and I had some small wrinkles. For a normal thickness tube you'd probably need 5 layers with 6 oz. cloth, and I can't imagine being able to vacuum bag that and not get wrinkles.

You could also try shrink tubing to get compression instead of bagging.

I have never had much luck trying to use compression on multi-wrap tubes. I just use some tension, and this produces a big improvement in the tube over a simple layup. My methods for carbon tubes are here:

https://www.raketenmodellbau.org/repository/archive/167792?view=true

https://www.raketenmodellbau.org/repository/archive/167793?view=true

I don't get perfect tubes every time, but they are usually serviceable.

Jim

If you really really want a high fiber content, what i'd do is fillament wind a tube with prepreg, then use shrink tape for compression. Vacuum bagging tubes is a disaster.

I wrote a few articles on making tubes without compression.
https://www.apogeerockets.com/Newsletter/newsletter_archive

Articles 370 and 371.
Those'll be fine for everything under mach 3

Alex

All good advice here. Here's my video on reinforcing tubes; some of the techniques may be helpful.

As far as vacuum bagging, it can certainly be done, but you need to properly prepare the mandrel before-hand so the cloth doesn't stick. I've used was to fill tiny pores and then spray-on release film. You can find out more about this in my nose cone molding video.

Finally, larger tubes are easier and fewer wraps per vacuum bag helps to avoid wrinkles. For my Nike-Asp airframe, I used Nomex honeycomb core with carbon fiber and fiberglass skins on either side. They were still a pain to remove, although it got easier with successive tubes once the mandrel became "seasoned".

Vacuum bagging a tube without wrinkles is certainly possible and not as hard as it may seem. I will try to address your problems in the general order of the original post.

1. Bunching and Leaking- It sounds like you may be using too much resin. For light weight, plain weave cloth try using 25g of resin/square foot to wet the cloth. Lay the cloth between two layers of bag material and use a squeegee to work the resin from the center out to the edges. You want to completely wet the cloth and no more. If you find that you have extra resin, move it out of the cloth with the squeegee.

2. Bunching and Leaking 2- A good vacuum schedule can help with this (especially bunching and wrinkles). After you lay the cloth, apply the vacuum schedule in this order
--120 e-glass (sacrificial layer to protect the carbon from sanding)
--Teflon peel ply
--120 e-glass
--Perforated film
--Breather cloth (non-woven glass)
--Vacuum bag
--Overlap each layer and tape with flash tape to hold it in place snugly.
A proper vacuum schedule should remove excess resin and eliminate pooling. If it is dripping, you are not getting vacuum any way. If you are not pulling 20 inHg, check the bag for leaks.

3. Sticking- Bond Teflon peel ply to the mandrel. This is how cauls are surfaced for making panels for aircraft structural repair.

Of course, cure according to resin mfr's instructions, but 160 deg for 1 hr is probably pretty close.

Many here will probably find this to be overkill but this is what I know from aircraft structural repair, I have never done a layup for a rocket. I have seen some very complex shapes and they can come out very smooth when done correctly. Check out the video at Fibreglast for more info.

War Eagle '95. Have a Toomer's lemonade for me!

Screaminhelo said:

--120 e-glass (sacrificial layer to protect the carbon from sanding)
--Teflon peel ply
--120 e-glass
--Perforated film

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What? Why glass, then peel ply, then more glass, and then perforated film? I must be missing something there.

3. Sticking- Bond Teflon peel ply to the mandrel. This is how cauls are surfaced for making panels for aircraft structural repair.

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'Teflon peel ply' as in Teflon-coated nylon fabric? How do you bond it to the mandrel so it slides off? That sounds like a recipe for a ruined mandrel and a bad day.

CarVac said:

What? Why glass, then peel ply, then more glass, and then perforated film? I must be missing something there.



'Teflon peel ply' as in Teflon-coated nylon fabric? How do you bond it to the mandrel so it slides off? That sounds like a recipe for a ruined mandrel and a bad day.

Click to expand...

Exactly! Just use mylar film taped around the mandrel for easy release of mandrel and from the tube.

CarVac said:

What? Why glass, then peel ply, then more glass, and then perforated film? I must be missing something there.



'Teflon peel ply' as in Teflon-coated nylon fabric? How do you bond it to the mandrel so it slides off? That sounds like a recipe for a ruined mandrel and a bad day.

Click to expand...

The first layer of glass is a sacrificial layer that is embedded in the surface and protects the carbon during finishing. Sorry, I was thinking in terms of repairs where sanding and smoothing is necessary to blend the repair with the surrounding surface before finishing. You can eliminate the first layer of glass and just apply a finish coat to seal and smooth the surface. The glass above the peel ply helps to control excess resin and a slightly heavier weave may be helpful if the sacrificial layer is eliminated.

I didn't thing about mylar and it would be a better choice here for a variety of reasons. You can use tape to adhere peel ply to a surface but it may have to be re taped between layups.

Looked all over for this exact post! I'm working on a sub-min diameter 4" research EX flight and this is the perfect guide I wanted to use for my fin can. You really understate how amazing your work is. Thanks for putting in the time to make this guide!