Learn me: Working with carbon fiber

[choneofakind]

As the title states. I'd like some help in getting to understand the process of building with carbon fiber. I'm on my school's ASME Human Powered Vehicle team, and we have some ideas.

This year, we're using a carbon fairing and chrome-moly frame. In past years we've made the mold for the fairing and then sourced the work out to a local company specializing in carbon fiber. But we're doing our own carbon work this year for the shell, darn it! We'll take all the help we can get.



For next year, I've already got some ideas; I'd really like to ditch the whole frame and go with a carbon monocoque tub as our chassis. However, this requires a new level of expertise (which we don't have) on working with carbon fiber. All you engineering types who've worked with laying carbon, lemme know what you've done and give me a better idea what to do. I'm thinking it would be easier to make the monocoque in pieces and use epoxy to put them all together to form our tub. It won't be as light as if we could do it all in one piece, but weight is not my #1 concern (it will still be loads lighter than the current frame, no matter how we do it). My #1 concern is making decent quality CF that will be safe to design around for use in our vehicle.

I've read about different methods of laying the stuff; dry, wet, vacuum, no vacuum, heating it, blah blah. If you guys could give me some more reading material and personal experiences on working with CF, I would appreciate it big time.

Who knows, if I get really good at it, maybe I can start making track bodywork for my ninja.

Lets learn chone some CF skills for the greater good of his HPV team!!

[adouglas]

I have some experience with moldless composite construction for homebuilt aircraft, which included a bit of carbon fiber. Lots to say so I'll just dump... and this is incomplete.

Carbon is not really much different from fiberglass as far as working it goes. It's just WAY more expensive. It's also conductive, so watch out for static electricity and galvanic corrosion.

Doing layups is fairly simple. Final finishing is VERY labor intensive if you're not using a mold.

As with any composite work, time and toxicity are issues. Be precise. Be prompt. Be neat. Wear skin and respiratory protection. You don't want to breathe this stuff or let it get into your pores.

Get hold of the Aircraft Spruce catalog... very valuable. Learn about micro, flox, the different types of epoxy and the different types of foam.

If you're using a mold, the way you get optimum strength and low weight is with vacuum bagging. Not that complex, but it does require specialized materials and equipment, along with really good mold prep.

If you think of a monococque, the load is spread across the shell. However, loads are not uniform. You can overbuild the shell so the whole thing will support every conceivable load, but that would result in something very heavy. Or, you can add strength where needed to an otherwise lightweight shell.

You do this by laying up extra layers and adding stiffeners. A stiffener is high-density foam that is laminated into the shell between the outer and inner skins.

Basically a simple layup (just fabric and resin) isn't that strong. Create a foam/fiber composite sandwich (fiber, then foam, then fiber) and you get much greater stiffness without much extra weight. The thicker the sandwich, the stiffer it gets.

Plain layup: Lowest strength.
Layup over foam core: Much higher strength.
Layup over foam core with additional ribs: Highest strength.

Basic moldless process over a foam core:

• Cut the foam core to shape. If it's blue foam (styrofoam) you do this with a hot wire. If it's urethane (white or tan) foam, you do it with a knife and sand the edges.
• Cut all your fabric ahead of time and lay it out neatly so you can pick it up with wet, messy hands... because no matter how neat you are, you're going to make a big mess. (You need a lot of clean workspace to do this right).
• Prep the core by making a slurry of epoxy and microballoons (wet micro). This is the consistency of mustard. Spread that over the foam core with a plastic squeegee.
• Lay out your first layer of fabric, pour epoxy on it and squeegee until saturated.
• Repeat with second and subsequent layers. What you're looking for is the correct resin/fiber ratio. Glistening puddles of epoxy are bad. So are resin-starved spots (lighter in color than saturated fiber). So are air bubbles (thoroughly work each layer). It's okay to go resin-rich at first because the next layer will soak up the excess. It gets critical with the final layer or two.
• Use a hair dryer to keep the epoxy warm and thin as you work it. KEEP AN EYE ON THE CLOCK. You don't want your epoxy to set up before you're done!!!
• The surface will be rough. You fill this with a slurry of microballoons and epoxy the consistency of thick cake frosting (dry micro, the drier the better because it's lighter and easier to sand). Spread it like frosting BEFORE the layup cures to save time... if you wait, you'll have to lay down fresh epoxy for it to stick to. Once fully cured, sand to shape. Fill pinholes with more dry micro, sand again. Repeat until the surface is perfect.
• Paint. Avoid dark colors... the laminate will get hot in the sun and lose strength.
• To create complex internal structures (like a T cross section, for example if you have a bulkhead inside the shell) you make a foam core sandwich for the rib (lay up fiber on both sides of a foam sheet). Once cured, this can be attached to your substrate with flox. Flox is like micro, but it's made using flocked cotton fibers. Looks like German Chocolate Cake frosting. Heavy but strong. Hard to sand. You use it as glue, and to make fillets that spread the load across interior corners. Paint edge of bulkhead with flox. Stick it where it needs to go, run a bead of flox along the junction, and lay some glass tape over it.

That's pretty much it... bottom line:

- Get the resin/fiber ratio right. Too dry and it's weak. Too wet and it's heavy.
- Neatness counts.

I've never done the mold/vacuum bag thing, but in theory it's easier to get the correct resin/fiber ratio and it's definitely easier to get a nice surface finish... but you have to have a perfect mold to begin with.

You prep the mold with wax and mold release, then lay in your fibers as above, working the epoxy. You add any stiffening foam pieces as needed along the way. You then lay down an absorbent blanket, put your vacuum film over it, seal and start the pump. Cure as appropriate (heat/no heat). When you're done, the absorbent blanket will have sucked up all the excess resin and the vacuum will have compacted all the fibers properly. You get a stronger, lighter layup that way. With a good mold, surface prep for painting is minimized.

If you spend a lot of money you can get prepreg fabric that already has the resin in it... it's cured with heat, which means you need an autoclave or oven.

Hope this helps.

[choneofakind]

That helps a ton! I'm searching for that catalog.

I was already thinking along the lines of strengthening the monocoque with foam ribs in the middle, good to know I'm on the right track.

When you used molds, were your molds the male 'plug' or the female 'tub'? We plan on making a mold. Only question is do we do the mold in house or do we source that? That will be the really expensive (or time consuming) part because that has to be perfect.

Any more references I can go look for? I also need to figure out how to model this stuff on the computer to figure out where to put structural bracing. (Solidworks most likely)




Is it possible to make parts of the tub separate from each other and then anchor them together? Is that where your Flox comes in? That's a new term to me.

[JohnnyBravo]

What he said... There is or was a vid on YouTube of the guys building either a Lamborghini or a Ferrari ... Showed everything you would wanna know

[choneofakind]

Link?

[Tough Noogies]

This is one of my favorite threads of all time. Tons of awesome info here.

http://www.bayarearidersforum.com/fo...hreadid=145975

[adouglas]

I was the webmaster for MARS, the NY Metro Area Recumbent Society and the guy who created its website. The website is still up and while there's very little composite stuff there, it's a wealth of HPV knowledge. See if you can find John Tetz... I think he's still around and he's a wealth of knowledge. The guy's an HPV guru, master machinist and amazing individual.

Also look up some of the luminaries in the field. Georgi Georgiev is the guy behind the Varna streamliners, which held HPV records for years. http://www.varnahandcycles.com/hpv.htm

Follow your nose to the HPV record races held every year at Battle Mountain, Nevada. They've been called the World's Fastest Bicycle Race in the past. I believe the current record is 82 mph and is held by a Dutch team.

To answer your questions... By definition the mold is female. It is formed from a male plug, which has no purpose other than to create the mold.

You make your plug using any variety of methods... moldless composite as described above, wood, whatever... This needs to be made as perfect as you can get it... mirror-smooth, no ripples, precise shapes.

Once you've got the plug you use a ton of fiberglass to make a very thick, heavy female mold. This mold is then used to produce the finished parts.

Advantages of using a mold: Utter repeatability for multiple production pieces, perfect surfaces right out of the box with little surface prep needed prior to painting, ability to use vacuum bagging for lower weight and higher strength.

Disadvantages of using a mold: Much more work once you factor in plug fabrication/finishing, much higher cost of materials (you're making the plug, plus the mold, plus the finished pieces).

Advantages of moldless construction: Very few tools needed, relatively little material needed, simpler production process overall

Disadvantages of moldless construction: Not suited to making more than one part because you have to do the whole thing every time. Labor-intensive finishing (but this is a wash, because you have to do the same thing with the plug anyway). Making hollow shells is more involved than it is when using a female mold, but it's doable.

Look up Burt Rutan, Long-EZ, VariEze, moldless composite, Nat Puffer, Cozy. Those are aviation references.

The Voyager, the plane that flew around the world non-stop, unrefueled, was a Rutan project. It was made by hand using moldless techniques. It's hanging in the Smithsonian Air and Space museum in Washington. They had an incident on takeoff and lost one of the winglets... you can still see the blue styrofoam core sticking out of the wingtip.

Look up HPRA, the Human Power journal, team Elvie, Easy Racers (the original Gold Rush is in the Smithsonian).

Sorry for the lack of organization... I'm just doing a stream-of-consciousness brain dump.

[choneofakind]

No problem here Andrew I'll check that out tomorrow sometime. We do have some different constraints than the guys doing flat out speed runs because it's a college competition; mostly time and budget. I'll still check it out though!

I've seen the Voyager and knew it was a Rutan project. The Smithsonian was cool last time I was there.

[JohnnyBravo]

Link to original page on YouTube.
Gota look harder for the more indepth one

[adouglas]

There are lots of videos on these methods from Fibre Glast on Youtube.

First of the series on plug and mold making:

Link to original page on YouTube.

First of the series on moldless construction:

Link to original page on YouTube.

These tell you pretty much everything you need to know.

[adouglas]

Link to original page on YouTube.

Link to original page on YouTube.

[choneofakind]

Bump!

Still reading about this. Thanks Andrew for the information overload

Anyhow, I stumbled onto this:
http://www.instructables.com/id/Cust...rame/?ALLSTEPS


This gives me an alternative to having to make a monocoque and gives us a way of making molds that we can handle: 3D printers.

Carbon tubing + 3D printed joints (or carbon) should bring the weight down significantly.