Carbon Fiber 101

[PedalOnTheRight]

I know there are a lot of teams in their infancy such as ours. We've got the fiberglass process down pretty well, made our plugs, days of sanding, layup, finishing etc. However, could someone give a comprehensive procedure for making things from carbon fiber?

I'd hope that this could be a resource for others in the future. I'm familiar with making a foam plug from a CAD model, so if that is the best place to start, please take it from there.

This would be a great help in terms of teams making seats and bodywork.

EEJ

[PedalOnTheRight]

I know there are a lot of teams in their infancy such as ours. We've got the fiberglass process down pretty well, made our plugs, days of sanding, layup, finishing etc. However, could someone give a comprehensive procedure for making things from carbon fiber?

I'd hope that this could be a resource for others in the future. I'm familiar with making a foam plug from a CAD model, so if that is the best place to start, please take it from there.

This would be a great help in terms of teams making seats and bodywork.

EEJ

[James Waltman]

That's a hell of a question.
I'm always happy to attempt to answer composites questions but I don't have time to write an introductory book on the subject.
Simon McBeath had time though and I suggest that you start there:



Competition Car Composites: A Practical Guide
by: Simon McBeath
ISBN: 1859606245
http://www.amazon.com/Competition-Car-Composites-Practi...Haynes/dp/1859606245 (http://www.amazon.com/Competition-Car-Composites-Practical-Haynes/dp/1859606245)


I think you might also find the racingcomposites.net forum useful.
http://www.racingcomposites.net/
It seems to have some knowledgeable people.

Come back here with a slightly more focused question and you'll get some help. You can ask about molds/tooling, layup technique, materials, vacuum bagging, curing, trimming, secondary operations, etc. Asking about them all at once is overload.

[PedalOnTheRight]

Well stated.

My primary question deals with the layup and curing process with respect to mold design and pre-preg use. Is the layup of CF similar to fiberglass in the use of a female mold? What is the best material to make a mold out of? Is the best way to start by created a foam core out of cross sections of the part? Finally, what is the recommended curing process to ensure a quality part?

Thanks.

[Maverik]

I don't want to sound rude, so take this with a grain of salt. Composite development isn't just follow these steps and it turns out perfectly, part of the challenge for our teams is figuring out (through experience) what works and what doesn't. That being said, just get your hands on some carbon fiber and have some fun. Wetlayup, prepreg, VARTM, etc... there are a myriad of ways to do a layup and depending on the situation (and what equipment you have floating around) it will vary as far as which one is best. If you're school has a composites lab go and talk with the faculty or students there, they will give you much more insight than we could.

[nick roberts]

Competition Car Composites: A Practical Guide
by: Simon McBeath
ISBN: 1859606245
http://www.amazon.com/Competition-Car-Composites-Practi...Haynes/dp/1859606245

I found this book to be extremely basic to the point of almost useless. Most of McBeath's books are awesome but not this one.

As far as your direct questions, we have found that the better the mold, the better the part. Molds can be made out of MDF or basic low density foam, but the better molds will be aluminum, carbon fiber, or 15+ lb/ft^3 foam. For our chassis we use a 20lb/ft^3 foam for the initial bucks which are machined in 2 pieces. We then lay up a 2 piece carbon fiber mold with tooling material (160deg cure CF later post cured at 350). We then bolt these together and lay up on the inside of this mold. The benefit to this is that you can cure the chassis at 350deg F which you can't do with a straight foam or MDF mold. For smaller aesthetic parts we use multi-piece aluminum molds for good surface finish. Also, if you have access to an autoclave it will help with finish quality but it isn't required. The curing process will vary with each different resin type which you can usually look up in the datasheets which should be available from the CF supplier.

Do you have a source for CF yet? I ask because we have excess prepreg CF right now which we might be interested in getting rid of if you needed some.

-nick roberts
University of Kansas
Suspension Lead / Test Driver

[PedalOnTheRight]

If you have extra prepreg in good shape, I'll definitely be interested. Thanks for all your help, this is a great place to start.

Maverik, don't be an ass! I'm just looking for a few pointers to get started. There's no shortage of crafty work-ethic on my team, and I obviously expect to have some set backs as the learning process goes. As a team new to composites, it's nice to ask those who have experience for their advice. Nevertheless, thanks for the pointers.

[NetKev92]

For molds, any mold with the appropriate shape is adequate as long as you can remove the part without destroying the mold.

Traditional soft tooling method is to make a good plug of the part, painting it with a good hard urethane, and waxing the heck out of it with a carnuba based auto wax. Mother's is my favorite. I'm sure it's not the one and only, but I knew a guy with a 1960s sailplane with prisitine factory-original gelcoat who swore by the stuff.

After the minimum 3 coats of wax and buffing, apply PVA or poly-vinyl-alcohol either by spraying a very thin film, or using the easier method of applying it with a smooth cloth - get it wet, and wipe it gently. The surface will be just a tinge of green because the PVA forms a thin film of green plastic that hopefully doesn't stick very well to either the plug or the mold.

Once you have a well sealed plug, you shoot gelcoat over the surface and wait for it to get tacky. Once the gel is tacky, you can wet lay-up fiberglass or carbon mold structure. As little as an FSAE mold will get used, I don't see the need for carbon fiber. The textbook answer is probably that you should make the mold out of a similar material to your part so the thermal expansion rates are similar. In practice, cheap is probably good enough for most things.

Once the mold cures, you gently pry the plug out of the mold. If it's stubborn, a little water or alcohol should help melt the PVA and help you get things apart. With the mold free, you begin prepping the mold with the same 3 coats wax and one thin layer of PVA so you can make a part.

After PVA, wet layup is the old boatmaker's standard method. Epoxies, polyester, and vinyl-esters often respond well to this method. With pre-preg, you may want a different method.

Positioning the pre-preg will probably require some dragging of tacky fibers across the mold surface, and I don't know if PVA would work well or if it might tear. The wax probably also introduces some impurities into the part. With a low-cost wet layup, it probably doesn't matter as much as the skill of the layup technician. With expensive pre-pregs, a good parting agent should be relatively cheap compared to the composite materials. Ask the pre-preg supplier for an appropriate parting agent. Different parting agents will work on different mold materials, so be specific.

Good luck.

[RiNaZ]

I just wanna add some to what Kevin wrote. If you're planning on doing some painting on these carbon fiber parts, PVA is an option. But if you think that you wont be doing any painting on the parts (meaning, the CF will be the finish part), then i personally wont use PVA. PVA will leave drip marks or patterns on your finish parts.

Usually, with the amount of time and manpower that a team has, you should be able to bring the mold surfaces to an A class finish (up to 1000 grid sand paper or more). By this time, 9-10 coats of wax with 15-20min in between each coat will work just fine. So, you probably dont need PVA (but in case you do, make sure nobody from your team member use the PVA before the waxing, it will happen if you're not use to the process).

One thing you might wanna add is flanges around the mold. You dont really have to do it, but it helps alot when you're ready to pop the parts out of the molds.

Also, if you're building a plug/buck/master, i find it beneficial to paint the foam plug with a sealer mix with some red dye.

When we first started out making bodywork for our car, we would have different guys came in the morning, sand and fill bondo the plug. And the new guys who came at night will do the same. So it took us 2-3 months just to get the plug done. We would build up on the bondo and sand it all the way down, and then new guys come in, build it up again, and sand it all the way down. And you always find somebody sanding into the foam.

So if you paint the foam plug with red sealer, if someone sand the plug far enough, they will see the red color, and know exactly what to do, stop. Plug such like what we have for FSAE usually would take 2-3 w/ends. If it takes you any longer than that, you're probably doing something wrong (this is of course not counting CNC time for the foam).

Thought i'd share that with everybody http://fsae.com/groupee_common/emoticons/icon_smile.gif.

[Rachel]

something to add...

i've never had to wax or PVA a mold but last year we had a scare and had to break our female molds to get our chassis out.

this year we used a liquid mold release that was recommended to us because it's "idiot proof." it's easy as hell to put on and i'm pretty sure it's less time consuming as well. we used frekote but meguiar's also offers a liquid mold release. i think the meguiar's one is called velocity 84...unlike the frekote, it doesn't reqire a respirator but i'm not sure if it's high temp.

[RiNaZ]

some more to add ...

you can use FREKOTE release on your mold, but always come back and wipe/buff your release after you apply it. Just like PVA, it leaves marks and water stains on your surfaces. So if you're using FREKOTE release, always make sure you wipe it clean soon after. Also, new mold surfaces probably need 4-5 layer of any type of liquid release (and of course a "breathing" time in between the layers).

Unlike PVA, wax can only be applied after putting down liquid release (this is if you want to use the combination of wax and FREKOTE release).

And i would use a respirator when using FREKOTE release. Other than the health reason, the smell itself will make you nauseous.

Also, FREKOTE release or any type of release system can also be used on your gloves when you're doing pre-preg lamination. Just make sure that you wait 10-15 mins after you apply it on your gloves. This is to make sure that you dont transfer the release on to your fabrics.

[benjo]

On the topic of carbon, I was wondering what people are doing in terms of structural equivalency? How hard is it to pass the test?

For the side impact test we have been thinking to just make up a small section to impact test, then test the minium requirement by the rules and compare the data. Would this be reasonable enough?

[BStoney]

Oh man...don't even get me started on Structural Equivalency...be on the watch for a thread post-competition 2007.

[benjo]

Haha, okay I'll keep my eyes out for that. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[James Waltman]

Putting release agents on your gloves for a layup seems like a really bad idea.
I guess that I don't have any data to show it's bad but I recommend avoiding it.
The risk is pretty high that you'll contaminate your layup with a material that's designed to keep things from bonding together.

Your mold prep area should be far away from your layup area and far away from your materials.

[RiNaZ]

I take my comment back on putting release on your gloves. I never did any testing on it either, but so far, i havent seen any delamination with putting release on my gloves. But i could be wrong. And i always make sure that i blow off my gloves with air to haze it out.

I think with the bodywork that we do in FSAE, we probably dont need to use release on our gloves anyways. I do a lot of laminations on fuselage of an airplane, so it makes sense to use it since your glove will get so sticky.

So james is right, dont use release agents on your gloves. Thanks for commenting on that.