I posted this as a reply to another thread, but I thought it might get a good discussion going:

When carbon fiber structures are made properly, they are extremely strong and lightweight.

THE KEY IS IN THE PROCESS.

A proper carbon fiber composite piece is processed using elevated temperature, the proper level of evenly-distributed surface pressure, and vacuum for the evacuation of volatiles and excess resin.

The vast majority of products available in the automotive aftermarket are not made this way. The reason is that it is very expensive to make a carbon fiber structure properly, and this would price a product out of the market.

Most stuff you see out there is dry carbon fiber cloth laid into a mold and then wetted out with polyester resin and allowed to cure. If you're lucky it's been vacuum bagged to remove the excess resin. This is how fiberglass has largely been processed for decades.

In addition, almost everything I've seen on the market is actually made of fiberglass with a layer of carbon fiber on the outside to give it "the look".

These crappy peices crack and fail, and lead people to say things like "I thought carbon fiber was supposed to be strong, but it looks like it cracks almost as easy as fiberglass". This could get me off on another tangent, because fiberglass can be used to make extremely strong, resilient, lightweight structures as well (think Corvette leaf springs and a gazillion other things). Again, the key is in the process.

Anyway, in most cases it's a moot point because most people just want the look of carbon fiber and don't really care about function. They want the look of function. The look of a race car.

For example, the 350Z has a very nice aluminum hood. You'd be hard pressed to make a carbon fiber replacement that was stronger and lighter at a price that would make it economically viable. However, half the posts here are from people looking for whatever carbon fiber hoods they can find under the assumption that the magic carbon fiber makes it better.

Published weights for carbon fiber pieces are as hard to come by as dual exhaust dyno sheets. The reasons are the same.

I'm sure some fine purveyors of carbon fiber products will post replies extolling the virtues of their product. Many more are afraid to. Let's see the numbers -- I'd love to be proved wrong.

 

I agree, carbon fiber is very very useful, for its strength and structural properties.

sadly only carbon fiber peices Ive seen have been in areas that serve NO structural support whatsoever. such as engine covers, the thing on the window, and all that.

there there are the overlay kits, where your taking a fiberglass kit, we'll say 10lbs and adding a lb of carbon fiber onto it. why? this is succeeding in adding a lb and giving you the cf look. yey.

this is fine, Im not one to yell at people for what I might consider rice, its there car, but if your going to do something for cosmetic purposes atleast be upfront about it, cause some people seem to think all this overlay is somehow performance oriented.

 

Reen...I noticed you work at Kennedy Space Center in Fl. (At least that's what I assume KSC is). Did you get an Aerospace Degree? Just picked mine up in May from KU. Had a couple composite courses there as well.

I agree with everything you're saying. But, I think 90 percent of the people looking for CF hoods (or for those Britich folks 'Bonnets') are getting it just for looks. At least I hope they are, cause that's all it's good for. Properly assembled CF doesn't look nearly as good as something with tons of resin still in it. (I.e. not as shiny). Oh well. Personally, I wouldn't trade the nice color of my hood for CF any day.

 

Be careful what you read...

1) In any kind of cf production elevated temperatures are necessary to speed up curing process. It almoust doubles at 10F increase in temperature. That's why you want this if you have production company. But resin will cure perfectly well at normal room temperature. It'll just take longer. Final qualities of both materials will be the same.

2) Never heard of using vacuum to get rid of excess resin... Vacuum is used to create pressure, not for evacuation of excess resin

3) While vacuum is certainly increases the final quality/strength of the cf, the usual mold/wet method can create material of sufficient strength for all kind of applications. From helicopter fuel tanks to amateur race car body. Of course F1 uses super advanced technology for their cars, but that doesn't mean small shop can't produce quality product.

Check this book for more:
http://www.amazon.com/exec/obidos/AS...444457-1888743

 

I am indeed at Kennedy Space Center, but only for another week or so.

I work at the Jet Propulsion Lab in CA, and have been out here at KSC since March as part of the Mars Exploration Rover project. We have our second launch this weekend and then we're out of here.

My degree is in Mechanical Engineering. I'm primarily a structures and mechanisms guy, but I ran the composites lab at school for three years and have a lot of hands-on experience with various structural composites.

Looking forward to the 2700 mile road trip back home in the Z!

 

1) FALSE

Read the spec sheets for structural epoxy resins. Elevated temperature is specified for full proper cure. Yes, good room temperature cure resin systems are available, but are a compromise.

2) FALSE

Vacuum is indeed used to evacuate excess resin as well as the various volatiles outgassed during the curing process. Using vacuum to apply the pressure is only used when you don't have to capability to do it a better way. The best you can do with this method is about 14.7 psi to the surface if you achieve a perfect vacuum, which you can't. This is insufficient pressure for a proper structural item.

3) FALSE

I am not saying a small shop cannot produce quality product for some applications, but certainly not all. The simple fact is that advanced applications require advanced processes.

Like I said, I am talking about efficient structural composites here, not garage parts.

 

Your original statement was:

A proper carbon fiber composite piece is processed using elevated temperature, the proper level of evenly-distributed surface pressure, and vacuum for the evacuation of volatiles and excess resin.

The vast majority of products available in the automotive aftermarket are not made this way. The reason is that it is very expensive to make a carbon fiber structure properly, and this would price a product out of the market.

As I understand you said, if one doesn't use elevated temperature, vacuum, and pressure, then final product is crap.

I'm sayin that small shop can do quality enogh products for cars.

I'm not structural engineer, but I have some experience as I did build and raced control-line speed models (F2A class) kind of like this:



Most of the aircraft, including propeller, made of composites. In quite simple ways. We did use pressure and temperature on occassion, but not always and not hi-tech of course. And those models were tested hard. It's quite often when crushed from full altitude of 16m (45ft) at speeds of over 200 km/h (130?mph). So I know what I'm talking about.

If you are talking about rockets, then probably yes, all those super technologies are necessary. But we are talking about cars...

 

I never said this at all. Read it again more carefully. The only place where I used a form of the word "crap" is where I said, "These crappy peices crack and fail..." when referring to low-grade wet layup parts made of fiberglass with a thin carbon layer on top.

In fact, I acknowledged that the "lesser" processes are indeed adequate for some applications.

 

In the field of prosthetics, we use a lot of carbon fiber/fiberglass laminations. The methods we use are very successful when used to create rigid sockects for amputees that exert hundreds of lbs. or more in force!
The key to a carbon fiber lamination is the orientation of the fibers. CF is extremely strong in tension and compression.
When I make a CF socket, (in a nutshell) I lay CF in an orientation on the socket that maximizes its tensile properties, wet it completley with polyester resin and let it set itself off over a period of a half hour or so. All of this is done under vacuum. The result is an extremely strong and light CF socket that is virtually indestructible by the wearer!!
I love CF and this thread is great!! I am always willing to learn as much as possible about materials!!
ZZ

 

One of the students who worked in the composties lab with me was way into prosthetics. She brought some pretty nice stuff around. Relative to other applications, the loads you see are pretty small -- hence, your method works great for you.

BTW, carbon (or any type) fibers have very little compressive strength. Imagine trying to push on a rope. The compressive strength of a fiber reinforced composite comes from the matrix -- in your case the polyester resin. The fibers give it the high tensile strength.

 

One of the hardest things about working with composites is getting the resin/fiber ratio right. I'll agree with you that most "homegrown" fiber parts look like crap, usually because they didn't vaccum bag the parts to get rid of the excess resin.

Pre-preg carbon is very affordable now, and you can get it with a resin that will cure at room temperature (which was hard to find in pre-preg a few years ago). If someone were to use this, and vaccum bag it, you could make a light, strong part, though I wouldn't use it for a structural piece. Using pre-preg helps take the guess work out of it.