Can I make this? (Carbon/Aramid, PrePreg, Inflatable Bladder, Aluminum Inserts)

General Composites Discussions Composites Talk
#1

I have years of working with fiber glass chop (unfortunately as chop is the worst substance known to man!). Recently have been working on a project where I have need for a lightweight irregular shaped tube, thought maybe carbon would be a good choice. It doesn’t need to be exceptionally strong, or exceptionally pretty as it’ll be painted. Reasonable strength and lightweight are the goals. (The picture attached is just a mockup, I haven’t looked for unmachinable features, proper tangency or areas I may need draft)

  1. See pic. Is there any reason a shape like this can not be made using a positive pressure inflatable bladder and two female mold (mould?) halves? Seems I can lay up both halves, alternate the wet-seal top and bottom, insert the bladder, and bake.

  2. I see a lot of people (airplane builders mostly) use custom shaped bladders made from garbage bags. Bike builders often use bike tubes. Some people recommend party balloons. Latex, silicon, etc. I’ve been reading that 10-20psi should be plenty… Do I need a custom shaped balloon? Benifit from one? Get by fine without?

  3. Pre-Preg sure does seem a lot less messy. Is there any reason that would not be ideal for this?

  4. I can easily cut the female moulds on a CNC, but it’s router so I’m limited to wood, ren plank, foams, plastics, etc. Anything but metal. Issue in terms of positive pressure or curing?

  5. Not shown in the picture that I have four .75" x 4" aluminum strips that need to be placed on the top/sides/bottom. I haven’t looked into how to attach these yet. Molding in seems like it would be ideal as the molds are CNC cut I can assure excellent placement. I saw a lot of warnings about combining aluminum and carbon but mostly when there is electricity evolved. This tube is part of an aluminum and steel assembly that is far removed from electricity. Is corrosion still a concern?

  6. How concerned should I be that I’ve never worked with carbon and am considering this project?

Any direction would be more than appreciated!

#2

can’t see the picture…

yah you can use a bladder. you can even use surgical tubing, cheap and easy. A trash bag is probably not a bladder but a vacuum bag. You can also make the mold in such a way that you use vacuum instead of pressure. By using a tube with vacuum you can get excellent results.

As for prepregs, they are easier in some ways but then everything has to be able to take the heat of the cure cycle.

You can also make the molds out of wood, then make a bladder or use some tubing, and then use a braided sleeve instead of fabric. THen your layup is basically sliding the sleeve over the bladder that is slightly filled, wetting out, then placing in the mold and adding more psi. Plus sleeve is strong.

I think the aluminum strips make it a little bit more complex however. It’s possible to bond them in, but not sure if it’s easier per se.

ANd yes using carbon next to metal is not good. You’ll have to put a layer of glass to avoid galvanic corrosion. It’s not an electricity thing so much as a moisture and charge differential thing.

http://en.wikipedia.org/wiki/Galvanic_corrosion

and lastly… as for whether or not to be concerned. It’s doable but if its your first composite project, you’ll have lots to learn. It can be a steep curve so things will go slow. Also the part doesn’t have to be carbon, if you were using chop strand, then woven/braided materials are huge upgrade to strength. Just remember it’s really all about the mold and your process. Think it through as much as possible. Do some tests. Spend some more money. You’ll eventually figure it out.

#3

I machined a 2 piece bicycle fork mold in UHMW polyethelyne on a CNC router.
You can not use it in an autoclave to cure prepreg, but wet layups work fine.
For a bladder I used a recycled inner tube.
Inflated to 125 PSI.

It was not a simple mold, the 2 halves were at angles to each other, to create whats called rake in the fork.

Nice thing a UHMW is epoxy does not stick to it. I still waxed, and sprayed it with release film.
Ignore advice to use high speed tooling, carbide wood bits work better to clear chips.

High molecular weight poly would probably work as well, but the UHMW machines better.

Work was done on a Anderson Stratos Pro. Roughed with a 3/8" 2 flute straight bit, upcut.
Contoured and finished with a 5/16" ball nose bit again 2 flute.