Carbon Fiber Techniques

[mark zhai]

Thanks to anybody who can help. I've used carbon fiber for about 6 years, all kinds of lay-up systems, and prepreg. However when every web site, every team member, and every "expert" tells you something different, it's hard to sift between the hype and the real deal. Please give me your opinions, ideally if you could support them with an example or personal experience it would be helpful.

1) What pressures are you guys vacuum bagging at? Even among my own team, I've heard a range of 2-20psi.

2) What is the best way to avoid inclusions, voids, excess epoxy buildup, using a vacuum bag system? I've heard to first vacuum at 2psi and remove all wrinkles near the part, then turn to 20psi. I've also heard that high pressures pull too much epoxy away, which results in voids. Prepreg is not an option for this application.

3) If there are voids, what is the best way to fill them for cosmetics? I have used more epoxy, but this is tedious and not the best looking.

4) What is the best way to gloss up carbon fiber? I already use vacuum bag, non-PVA release, and then high gloss enamel clear spray-paint. Would sanding with very fine paper help?

5) What is the best mold release wax in your opinions? What is the optimal number of layers of mold release wax?

1) What pressures are u guys vacuum bagging at? Even among my own team, I've heard a range of 2-20psi.

2) What is the best way to avoid inclusions, voids, excess epoxy buildup, using a vacuum bag system? I've heard to first vacuum at 2psi and remove all wrinkles near the part, then turn to 20psi. I've also heard that high pressures pull too much epoxy away, which results in voids. Prepreg is not a option for this application.

3) If there are voids, what is the best way to fill them for cosmetics? I have used more epoxy, but this is tedious and not the best looking.

4) What is the best way to gloss up carbon fiber? I already use vacuum bag, non-PVA release, and then high gloss enamel clear spraypaint. Would sanding with very fine paper help?

5) What is the best mold release wax in your opinions? What is the optimal number of layers of mold release wax?

[James Waltman]

I'm not an expert but I'll tell you my thoughts about your first 5 questions. Maybe someone else can answer the second 5.

If you only use a vacuum bag you can't get pressure differentials greater than 1 atmosphere. So a 20psi pressure differential is not likely. I suppose it might be possible on the right day in Death Valley or on the shore of the Dead Sea because of the altitude at those locations. I suspect that 20 is not psi but inches of mercury. Then it makes more sense. With a good pump and a good bag it is possible to get very close to atmospheric pressure differential. The vacuum supply lines in our building usually give us a pressure differential of about 25 inches of mercury (12.28psi).

Are you talking about wrinkles in the bag? Those don't contribute much to wrinkles in the part. If you are having trouble with voids and excess resin you may just need practice with that particular mold. It may help to make a pressure intensifier for those areas too. I don't think that you'll be able to pull out too much resin with just a vacuum bag.

Are you just making cosmetic parts (body work)? If you have serious voids it may just be best to start over. It probably isn't realistic to expect them to ever look good. If it's just a cosmetic part you can fill in the voids with body filler and figure out a clever paint scheme to hide the problem areas.

If you are after a cosmetic finish it works well to wet out the mold with a healthy coat of epoxy before you add fibers. It is possible to get a decent finish if you are careful when you spray on the PVA. The better the mold surface is the easier it is to get a nice looking part. A small amount of wet sanding will probably help a little. Try it in a hidden area first.

We use Partall wax and PVA from Rexco. I don't know if it's the best but it works well enough for us. We usually wax the mold with 3 to 5 coats and then follow the direction for the PVA (using a paint gun). On our aluminum molds we try to use Frekote.

[Ashley Denmead]

I'm no expert either but i'll have a crack at the second 5 questions http://fsae.com/groupee_common/emoticons/icon_wink.gif heeheehe

"If you only use a vacuum bag you can't get pressure differentials greater than 1 atmosphere. So a 20psi pressure differential is not likely. I suppose it might be possible on the right day in Death Valley or on the shore of the Dead Sea because of the altitude at those locations. I suspect that 20 is not psi but inches of mercury. Then it makes more sense. With a good pump and a good bag it is possible to get very close to atmospheric pressure differential. The vacuum supply lines in our building usually give us a pressure differential of about 25 inches of mercury (12.28psi).

Are you talking about wrinkles in the bag? Those don't contribute much to wrinkles in the part. If you are having trouble with voids and excess resin you may just need practice with that particular mold. It may help to make a pressure intensifier for those areas too. I don't think that you'll be able to pull out too much resin with just a vacuum bag.

Are you just making cosmetic parts (body work)? If you have serious voids it may just be best to start over. It probably isn't realistic to expect them to ever look good. If it's just a cosmetic part you can fill in the voids with body filler and figure out a clever paint scheme to hide the problem areas.

If you are after a cosmetic finish it works well to wet out the mold with a healthy coat of epoxy before you add fibers. It is possible to get a decent finish if you are careful when you spray on the PVA. The better the mold surface is the easier it is to get a nice looking part. A small amount of wet sanding will probably help a little. Try it in a hidden area first.

We use Partall wax and PVA from Rexco. I don't know if it's the best but it works well enough for us. We usually wax the mold with 3 to 5 coats and then follow the direction for the PVA (using a paint gun). On our aluminum molds we try to use Frekote." (Waltman,2005)

couldnt resist.... http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

[mark zhai]

Yeah I don't know why it posted the questions 2x in a row. Yes, when I say psi, I meant inches of Hg, sorry, I was really, really tired. I just did 2 more lay-ups this weekend and still voids appeared, not at the same places. One lay-up was vacuum-bagged at 5 inches of Hg and the other at 2 inches of Hg. I plan on doing another layup at 10, or maybe 15 inches of Hg tonight to see results. Previous attempts at high pressures resulted in an abnormal amount of voids, and pressure has always been kept relatively low. But I give it another shot. Thanks for all your info.

[Dr Claw]

what kind of parts are you trying to make? i've also found that everyone is an expert and everyone tells things differently.


1: i dont vacume bag, i do vacume infusion. try http://www.fibreglast.com/contentpages-Vacuum%20Infusion-316.html I generaly get 26-27 inches of mercury

2: vacume infusion is my best bet http://fsae.com/groupee_common/emoticons/icon_smile.gif

3:we've had dry spots....but we just soak it in resin. i use a very thin vinyl ester resin

4: i use in-mould automotoive clear coat and the resin grabs it from the back side when you infuse with resin. when it cures, and you pop it, the clear comes with the part

5: i've used wax + PVA , and then i've used Zyvax....havnt had too much luck with zyvax, but PVA is garanteed to pop IMHO.


look into vacume infusion...its an entirely hands off process, and its really easy to get started. the benifit of doing this way over vacum baging, is you can have the vacume on all day trying to find your leaks without worrying that your resin is going to cure before you get to apply vacume (being that your resin is the last thing to go into your part). its kinda fun too, and less toxic because you dont really end up touching any resin or breathing fumes for very long.