How would I make a tube with multiple bend from carbon fiber? 3

[garehan]

Hey guys, I'm pretty new to this forum, but I did some searching for an answer to this to no avail.

Here's what I'm looking to do...I'd like to make some intake piping for automotive applications from biaxial carbon fiber sleeves. They'll probably be about 3" ID. Dimensions asside, how would I go about making the tubes with multiple curves in one piece with repeatable results. Ultimately, I will need both a smooth interior finish and glossy exterior finish. Whether the exterior finish is that way out of the mold or needs work to bring it to show quality after the part is cast is not that important, although obviously I'd like to keep time down to a minimum.

I've had a couple of ideas but I'm not sure they're the way to go. One idea was to make a wax mold of the inside of the tube, gel coat the wax, lay my biaxial sleeve over the gel coat, then resin infuse the tube. The problem there is that the tube would be about 3' long and I'm not sure if I should be worried about being able to pull resin along a 3' length. I think I've pulled resin across about 2' before with no problems though, so I don't see this as a big problem. The issue with this method would obviously be that the wax cores could be costly, although recycling wax would cut down on that.

My second idea was to have an inflatable, tough, tube made that's a replica of the inside of the tube. This could be wrapped in the biaxial tubing and subsequently resin infused.

Both methods would yield a rough exterior that would need so me prep and clear coat to bring it to its final gloss, but that shouldn't be too big of a problem.

Does anyone have any better solutions that I've missed? Any help would be greatly appreciated.

thanks,
russ

 

[berkshire]

garehan (Mechanical)
One of the proprietary methods is to a make molded core using liquid salt. The salt is heated until it melts and is then poured into a mold and allowed to cool. The hardened salt is then dressed and cleaned up. The part is molded around this. When the part is finished the salt is removed with boiling water.
B.E.

 

[garehan]

That's an interesting method! Similar to the wax method I was toying with. I've used the wax method for parts before, but it really didn't seem practical for anything other than 1-off parts. I'll have to look into the salt. Is there a special variety I should be looking for?

thanks again.

 

[MikeHalloran]

I've used handmade wax cores to make small frp a/c ducts by wet layup.

A decent exterior finish can be had by wrapping the wet layup with Saran Wrap. It compacts the laminate, and will peel off when the resin has cured. Wrinkles at seams in the wrap leave really nasty sharp pointed spikes of fiber- reinforced resin, so some careful cleanup is required.





Mike Halloran
Pembroke Pines, FL, USA

 

[sreid]

For the ID mold, consider aquapour.

http://www.acrtucson.com/products/Aquacore/index.htm

Pretty cheap, will stand up to 380 F curing temperatue and can be removed using tap water.

 

[klaas2]

good post berkshire
where do i get more info about this method

you can use normal salt?
and at what is the temp to heat this salt so its melts?
is there any link where i can see more about this method?

 

[berkshire]

klaas2 (Mechanical)

As far as I know, Lockheed aircraft were the first to use this method to make air conditioning ducts for aircraft.
I remember reading a paper on the method in the 1980s in modern plastics magazine.
I cannot give you much more info than that.
I will mention that the same method is used to make cores for large cavities for die castings where a salt with a large percentage of soda ash is used, and is flushed out after the parts are molded.
That is about all the info I have.
B.E.

 

[Albigger]

What quantity is req'd and wall thickness/tolerances? Will this be a structural component?

One way is to make an aluminum or steel female tool, with some sort of core and resin transfer mold the part. This will produce a good finish on the outside, and, depending on the core, also on the inside. Our company does similar molds (though mostly for the aerospace industry).

Depending on the part geometry, a flexible mandrel may be able to pull out from the tube. Or a break-down metal mandrel could be used.

Finally, as stated, a wash-out core such as aquapour or aquacore could be used (but be warned, these usually do not hold up to higher pressures).

--Jay

 

[garehan]

Hey guys, thanks for the replies. I do recall having read about water soluble cores before, but it slipped my mind for some reason. I think when I read about it, the person had used a combination of baking soda and some sort of water soluble putty sold as a powder that is widely available in hobby stores. Anyhow, the quantity will be fairly low as production goes, probably about 50 at a time, maybe 100/year max. It's not structural, it's meant as intake piping for a Honda motor. I'm an intern at a company that specializes in bolt-ons for Hondas. Anyhow, wall thickness will probably be between 1/32"-1/16" and inside diameter will be near 3". I need to make some test pieces before I can determine the final specs, but mostly it just needs to be able to support its own weight from one end. The use of carbon fiber in this case is mostly for bragging rights of the end-user, as you probably know is common among aftermarket auto parts. However, it's for an application that's unique, so the intake needs to be made one way or another. I will likely incorporate an inside layer of aramid, so that in the off-chance that someone manages to destroy the intake while driving, chunks of shattered carbon fiber are not sucked into the intake.

Thanks again for the suggestions. They really get me thinking. I'm leaning towards a wax inside for several reasons, but I may have to abandon that idea if it becomes necessary to use epoxy that requires oven curing. Obviously I have a bit of work to do.

peace,
russ

 

[Executnr]

We built carbon fiber mountain bike handle bars in college using an aluminum mold and an inflatable tube. Worked great, sounds like your application is a larger diameter but the principle is the same. Wrapped the tube in pre-preg sheet at the appropriate angles, layed it in the mold and inflated the tube. Piece-o-cake. Your application might be easier since our ratio of wall thickness to outer diameter was fairly high. If you have a large diameter tube with a relatively thin wall this method should work fine.

 

[CompositeGeek]

Depending on your rate of production you might consider plaster. That can be molded to your shape and then "broken out" (read beat it with a hammer, er...mallet, er...vibrating whatsis). With a flexible resin, and few plies, you can make parts pretty quickly without damage to the laminate. This is a common practice in the ducting for aerospace.

The salt approach is very good as well but usually a lot more expensive than simple plaster. You use a parting film over the plaster, and there are spray on seprating films as well. Make sure your plaster is dry, dry, dry when you go to cure!

I've built thousands of ducts using the plaster technique.

Also, the bladder idea is really great for large volumes. It is commonly used now in the fabrication of graphite tennis rackets, bicycle frames and other complex geametries. There is a company called Topkey, in China that kicks world wide butt using bladders.


Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[garehan]

Thanks, composite geek. The plaster idea has crossed my mind (in fact, i remember reading about mixing plaster with baking soda so it could be dissolved with boiling water, rather than broken out). I can't really break out the plaster, because the pipe will be about 3" diameter and 3' long. Anyhow, I'm pretty sure the approach I'm going to take will involve making a two-part metal mold out of mandrel bent pipe, making male mold out of wax using the metal mold, then using biaxial sleeve on the wax. The wax I've been experimenting with melts and solidifies pretty quickly, so I think this will be a fairly quick process. Plus, the wax is reusable.

thanks,
russ

 

[bnrg]

Executnr, I am curious about the inflatable tube that you used. Was it the same shape as the finished part but slightly undersized? If the shape was much different than the mold wouldn't the sheets shift and alter the orientation of your plies as you inflated it?

 

[saksmito]

We have been making air ducts for the A380.
First we tried Aquapour and it was useless. It didn´t dry and had it to dry for weeks (works very well for small parts with high surface exposed to water). For big parts it doesn´t dry.
The inflating bag is the best idea we have found. There´s a company in France who sold us material. Aerovac. Very good system.
Anyway. Study using phenolic resins. Epoxi is toxic when burning, and if you want it for a car intake, in case of fire, a resistance burned or something that heats the intake more than calculated you could have a prety big demand. If it´s your contractor who settled the materials nothing to do, but consider this.
On the other side... if you don´t change to Phenolic... What car is it? DD

 

[flyer007]

garehan
We do lots of ducts for aerospace and motrosport industry. I perosnally belive if you are making 50-100 pcs do a female tool out of Al or steel its is very cost effective, or else if the structural properties are not that huge do use a male mold with a styrofoam and do a wet lay using (2 part 24hr cure epoxy)and do a silicon Caul plate out side if you need a good finish. ofcause you have to leave under vacuum..!

 

[nately27]

saksmito, how big were the ducts you were casting? what diameter? I am looking at the material for bicycle frames, and want to make sure it will dry if its suitably thick. Thanks, Derek

 

[IMSPwrDev]

Hello everyone. I am fairly new to the forum, but I will try to supply what information I can.

I was just talking to a co-worker about fabricating composite parts for the automotive aftermarket. I planned to test the aquacore products as they claim to be good to 125 psi and can be autoclaved.

The eutectic salt method that was mentioned earlier is also a good idea except that the salt must be heated to temperatures around 500 degF before it liquifies. Plaster is another option, but can be very difficult to seperate from the laminate in small diameter ducts.

 

[Executnr]

BNRG
Sorry for the delay in replying, I haven't been back to the site in a while.

For the handlebars, we used surgical tubing for the inner bladder. The sheets were not wrapped tight to the bladder, but 5-6 layers resulting in a tube just smaller than the mold so we could get it closed. So the bladder inflation didn't have to move the fibers much to get it to conform to the mold.

Also, we did not do any kind of study on the final orientation of the fibers in the cured state, we just wrapped it per the design and then tested it to failure to make sure it met the strength requirement.