I'll have to disagree with that opinion. If you find a carbon hood that weighs more than your steel hood, it's a steaming pile of crap. Simple.

It's not hard to save at or above 40% weight over steel and not even try, especially if you don't try to make a bonded-on copy of the stupid stamped steel substructure on the back of the hood. Stamped sheet steel parts (like the hood substructure) don't translate that well to composites, as carbon fibers don't behave the same as metals and can't be turned into a "black steel" replica of the original part.
Really good parts will save up to 65 or 70%, but that's not normally achievable by mere mortals, only professional composites companies.

Disclaimer: the "Dry carbon" I'm going to talk about is the dry looking prepreg that is/was pretty popular in Japan on time attack cars. It has different properties than standard prepreg parts, which I suppose are sometimes called "dry carbon". I really, really, hate the aftermarket industry for spreading such stupid bullshit names, but I digress..

There is no reason to use "dry carbon" (I hate that name) to be practical, any properly done carbon parts will be much lighter that the sheet metal that they replace, and with resin infusion it's not much higher of a fiber/resin ratio than prepreg. "Dry carbon" (argh) is actually on the low side of fiber to resin ratio (if you're talking about the Japanese parts that actually look dry) and there is a loss of strength there. Most hoods, fenders, etc., are not structural and it really doesn't matter how strong they are though, so that's fine. The actual weight difference between "dry carbon" (sigh) and resin infusion is not very much, especially in the context of anything but a super stripped out race car, and definitely doesn't justify the price unless it's going on a car with a lot of money invested in race parts. "Dry carbon" (rolls eyes) parts may be popular on time attack cars, but I'd rather spend half (or less) of the money and not worry about a matter of ounces of weight. Plus, a "dry carbon" hood might not survive anymore more intense than the impact of a small songbird, they're not made for strength..

Most hoods on the market are wet lay up, non-vacuum consolidated, with a heavy layer of gel coat (useless weight, but looks pretty), and are mostly fiberglass with carbon skins for looks. Given the amount of excess resin from non-vac bagged parts, and the thickness that the hood ends up being between the gel coat and the fiberglass in order to give it any kind of stiffness, you're looking at a fairly heavy hood, but one that's still lighter than stock. What do you want for a hood that's affordable though; you're not getting professional composites work for $600, that's for sure (and that doesn't include import markups).

Resin infusion yields a much lighter part and a stiffer one if a core is used (and not core mat, which is just a resin sponge basically).
Anyway, resin infusion is the perfect blend of price and performance for pretty much anything except a purpose built ($$$) racecar, you start off with the reinforcements already consolidated under vacuum so there is no excess resin to displace when it's vacuum bagged. This is just about the best fiber/resin ratio you can get from a wet resin (non-preform) composites process with readily available materials and technology. It's not worth autoclaving anything that's done with resin infusion either, you can't watch the infusion from inside an autoclave, and you're not displacing any resin if you autoclave it afterwards because you don't have anywhere for the resin to go.
Unfortunately you can't use open cell honeycomb, but core foams are cheaper and still are way above the performance required for something non-structural like a hood. If it can take the heat, that's all that matters. Just doing resin infusion with a foam core is better than most hoods on the market right now, so what more can you ask for in terms of cost and performance???

And FYI:
You don't need an autoclave to do prepreg carbon, you just need a cure oven. Autoclaves are only useful if you want more than one atmosphere on the vac bag to get a higher consolidation. All of the carbon work that we do on our racecars is prepreg, and we don't use an autoclave for most things. Our whole monocoque chassis is done in a big cure oven with a vac bag. There are some special prepreg formulations for single atmosphere cure though that work really well the way we're doing it, and we've had good luck with them.

On the hybrid composites front, ACG makes a really cool prepreg/infusion carbon fabric ($$$$$$) that is stripe impregnated so it has really good drape and good vacuum distribution. During (elevated temp) cure, the stripes of resin flow into the areas between the stripes, completing the wet out. Unfortunately last time I checked it used a black tooling resin only, so you'd have to paint the parts because you wouldn't see the carbon anyway.

Sorry, you've really made me bring out the dark composite-based recesses of my brain that don't get used here too often. I didn't mean to write a mini novella but this was a good time to do so..

 

once again "thanks" you have a great way of explaining things in detail and to the point.... if i do hoods or deck lids. i will be using aluminum honey comb as a laminated reinforcement... dont expect a hood to be under a grand unless i get more than one order at a time.

what i am going to try to do with out bankrupting is make some completely affordable body panels that will save weight and be a "radical" mod for the average tuner at a semi affordable price....

granted the cost might be a little high but honestly if this were to hit the scene in japan i dont think we would even be discussing this right now. many tuners and drifters there are already using hand made aluminum rear quarters as weight savings modifications and have been doing so.

Many high end american cars are bringing the advantages of carbon fiber to reality as we speak.

i will also be reinforcing the quarters like you said with "sponge" or "foam" maybe even balsa impregnated in the composite structure to keep the rigidity and costs down.

im not saying im by any means an engineer but i do know enough and have the experience to make functional parts a reality.

i honestly was also thinking about making composite radiator supports along with jdm fenders as the demand is high and i might get the pricing relatively close to what buying them cost.

i really dont want to make bumpers though, they are a pain the ass.

come on people i need ideas and the perfect model car to make this a reality.

also no one seems to understand that with a freezer and pva i can make my own prepreg.

dont sweat the technique

 

I'm not going to ask, and I don't want to know..

I would like to know though: Are you doing resin infusion with the Al honeycomb?

 

mayyyyy be i would use phenolic (prolly spelled it wrong) but vacuum will "pop" the cell walls.

this is how we would do it in the aircraft field.... SHHHHHHHH dont tell anyone.

heres the process.

making a honeycomb composite

1 mould 2 wax 3 prepreg (room temp) 4 honey comb 5 prepreg 6 wax 7 metal (thin "O" alluminum works best) place in bag

reinforcing

1 panel 2 hysol "MMM" (we call it triple "m") 3 honeycomb 4 hysol "MMM" 5 balsa 6 fabric 7 perforated release film 8 bleeder place in bag

the metal is used to push the fabric against the open cells, the hysol bonds to the fabric and to the cell (closing the cell), this creates a seal preventing the resin from filling the cell. in the case of reinforcing, the balsa is thin and will flex on concave surfaces sealing the cells. the panel seals from the bottom. it is recommended to use flock or microballoons around the honeycomb. ill prolly use a light weight body filler or maybe even a non-conventional fabric to do the trick.

if your not familiar with hysol products any high strength epoxy adhesive will work.

this an unconventional proccess but... it works... and fixes aircraft in a fix when neccessary.

 

How are you getting around the cell drainage, or are you using foam filled honeycomb? I'm just curious, I do pretty much everything with prepreg and don't do any street car parts anymore. I like to see cool methods of getting around problems though.

By the way, Nida-Core makes some perfect stuff for what you're doing, infusion cores and other cool things; I'm sure you've checked them out already..??

The reason that this stuff hasn't caught on is that the weight savings aren't there for the cost of the replacement parts, that and the fact that no one really wants composite parts that don't have the pretty "carbon look". If it were up to me, and the market was there, I'd make parts with a little carbon, some aramid, maybe some S-glass, and a cheap core, and make people paint them. Fuck unpainted carbon fenders, they're ugly. Hoods are kinda ugly too, but they don't break up the body lines nearly as bad as carbon fenders do from the side. I wouldn't worry about doing bumpers, they're not worthwhile unless you're using the flexible resin formulations, and they're plastic already so the weight savings aren't there as much.
I've always wanted to have a NC router table to make car specific bond-on fender flares for the rear and flared fenders for the front. Ship them with the right type of sandable adhesive, and they'd sell like hotcakes probably. Anything that isn't ricey and lets people use bigger wheels usually sells pretty well, and there isn't much of that on the market right now. Something like WRX sedan flares to put on WRX wagons would sell like mad, I've seen it done a few times and everyone wants it but nobody wants to pay for a one-off flare conversion. Enter the cheap composites guy.......

I'd love to have a long strand SMC setup in my garage, then you could whip out parts really cheap. You know, after you amortize the ridiculous cost of the equipment and the tooling..

Seriously though, if you have good tooling for resin infusion, you can pop some parts out for cheap if you keep the carbon content low or use cores to bulk them without much reinforcement cost. I just hate trimming stuff though, it's the biggest pain in the ass of the whole process.. No one has dropped a water jet or NC router in my lap though.

 

do you have any ideas on different materials instead of pva??? ive used painters plastic but it doesnt work the best. i also need to find cheaper less expensive perforated release film.... i thought about making it but dont want quality to suffer. i want something that stretches.

 

So you're not using resin infusion with the honeycomb it sounds like..?
I know how do wet layup with honeycomb, that can be done without cell drainage pretty easily.

 

Different materials for what? PVA is a mold release liquid, is that what you're talking about? Perf release film can be had in stretchy grades, and the best way to make it cheap is to buy in quantity. You can reuse is sometimes too, if everything releases properly. There are economy types of breather, release film and peel ply, so search for those. The good high elongation stuff is still more $$$ though, better materials always are.

 

first is without and second is with infusing sorry

 

Is that Hysol a film adhesive? There are like six billion different Hysol adhesive products, I don't know most of them off the top of my head..
Anything with "MMM" by Hysol is just a military qualified product though, there are a lot of "MMM" qualified Hysol adhesives.