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Alex Madsen wrote:
I have finally got around to working on my carbon fiber intake manifold design. I got some 2” carbon fiber sleeve. Not having any bulk epoxy on hand I went to the hardware store and got some EnviroTex Lite Pour-On (the only bulk epoxy they had). Pour-On is sold as a heavy duty wood finish but it is clear epoxy 2 part epoxy. Unfortunately the stuff takes a long time to dry. I used 2” foam pipe insulation as a form for the carbon fiber. The test sample was 1 ply of CF. After it was fully dry (it took 2 days) I tried to melt out foam pipe insulation with acetone. However, acetone did not have any effect on the insulation and I was forced to cut it out with a big knife. While not unexpected the CF tube was not as rigid as I had hoped. My calculations say that it would have plenty of strength to withstand a backfire but I think a more rigid manifold would be ideal. I will try making a sample with 2 layers today. A second layer will also greatly reduce the likelihood of pin whole leeks (something I have not tested for yet). Attached are some pics of the CF tube.
I will be sending out pics of my design work later today.
A few comments from a composite builder, Alex.
1. An additional layer would help stiffness and reduce or eliminate problems with pinholes. But there are some other things to consider first. I'm concerned about the epoxy you're using. There are many types of epoxies and they have a wide variety of characteristics. Some actually remain rather soft and pliable after they cure, and this is especially true of wood finishes. Wood itself is flexible, and moves with changes in temperature, humidity, etc. Wood finishes thus need to be flexible as well because a very hard finish would quickly crack.
I'd be happy to help pay for a gallon kit of "real" epoxy and have it shipped to you for your research, since this benefits everybody. E-mail me off list (crj@lucubration.com) with your address if you want this. As an alternative, let me know where you got your CF sleeve and I'll get some of the same stuff, lay it up with my MGS, and send you a sample so you can compare them and see if the difference is worthwhile.
2. The real strength of the CF, given the weave you're using, is along its long axis (compression/tension). That's good, but it also doesn't help your rigidity test. Additional layers, especially of different weaves, would help. I'm guessing that you have weight to burn, since your sample MUST be several times lighter than an aluminum or steel tube of the same length. You could probably get 2-3 layers in there while still being lighter.
3. An epoxy system designed for wetting out fiber will act very differently when you put it on the carbon fiber. It will go on almost like water and very quickly soak into the fibers. I'm betting if you examine the inside of your tube it's pretty rough. I don't know too much about the intake design aspect but I'm guessing you want a relatively smooth inside tube.
If you wrap your foam insulation core with Saran Wrap or box sealing tape and get a good "wet" you will get exactly that. It's important to stipple out air bubbles with a disposable paint brush while you do the layup. The CF makes them harder to see but they'll be there. (This will also help distribute the epoxy more evenly and remove pinholes.) Regardless of the method you use you probably want your core to be as smooth as possible. An alternative might be to give it a light coat of wax.
4. A composite epoxy system is designed to improve its physical characteristics even more if you "post cure" - throw it in an oven for several hours at a certain temperature (varies with system). Your tubes will get even harder if you do that. Do you have an oven big enough to fit the tube?
5. You could achieve a much smoother outer surface if you used something called peel ply on top of the epoxy (and then sanded and buffed the outer surface) or Saran Wrap or 4-6 mil plastic sheeting (no sanding required, will produce a glass-like finish). I would recommend the former because with the Saran Wrap trick you tend to get minor variations in the surface so you'd be sanding anyway if you want it to look really good.
Regards,
Chad
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