Pre preg carbon

I have access to unlimited free pre preg uni carbon (3.7 -4.7oz) .
I want to build some fins,but I dont really know how to use the stuff.
I know I need a heat source to cure it 250 -350 F( heat blanket / autoclave) .
Do I have to vacuum bag it?
Is the stuff only good to use in a mold?
If so ,will a non metal mold hold up to those temps?
Any advice is appreciated.

Do you have to bag it?

No, but I highly recommend it.

Is it only to be used in a mold…?

well… not exactly where you are going… no bag, no mold… were you planning on just wrapping it around something and going like that?

How many plies? Etc… Unlimited supply… is astonishing… I hate to even look up the price on pre-preg… it is down right ugly… frightfully expensive.

I use an autoclave (at a friends shop) and you can reduce multiple plys down to dimensions that are crazy thin and incredibly strong… but that DOES require being bagged.

If you have access to a shop that is capable of this type of work, I think you would be best off telling them what you want, and paying them to do the work for you. This is not the sort of thing that you pick up in a few hours time. Sure, there are lots of guys that will do good work I am sure with no experience… but you would be beating the odds. If you already get the material for free… pony up the dough to have one of the fabricators there get you set the rest of the way.

Do the molds HAVE to be out of metal? No and few are, the are quite expensive to machine/mill molds from a metal such as aluminum. They may be made from a tooling resin that will withstand the heat of the ovens, I do that all the time. They may well have a CNC router on site as well that could make the parts you need in half molds from such material, and then be created from them.

Good Luck, I sure wish I had your good fortune.


The correct temperature needs to be known, and how long to keep it there, as well as the speed to get it up to temp, and the speed of cooling (ramp rates).

It does need to be squished (de-bulking). What ever form of you can do
(vacuum, compression, jammed in the autoclave, etc.), so that it reaches the desired thickness, has the resin flow to where it needs to be, when the right temperature is reached, for the rght amount of time.

It can be vac’d against a male core form, but again, you need the right amount of resin to go where it should (to bond it to the core). The stuff usually has enough resin in it for the reinforcment alone, so an adhesive film is also required to bond it propperly to the core (say a carved foam keel or something).

Non-metal molds are comonly used, so long as the tooling surface layer, and the sauce in the reinforcing layers can withstand the heat.
Personally, I like Hetron vinylester (good to 600 degrees), with a high graphite or cobalt powder mixed in for the tooling coat, and then in the reinforcement to construct the mold. Hi-temp epoxies are awsome too.

Using prepeg opens up a whole new kettle of fish, and to use it, other corresponding methods must be used aswell. Always consider the practicallity of your methods, and remember that the final results are what matters, not the methods, proceedures, or mindless arguing of the best way to get there!


Thanks for the great information, I always pay attention to both your posts.

I would like to build a small autoclave and the molds too if they dont have to be machined.

I want to learn the whole process, its the journey I’m interested in not just the destination( the fins)

I’m in A and P school working toward my ticket.
We have a hot bonder and vacuum bagging set up here and I am going to start working with it soon.

It was through the school that I found the supply of prepreg. They do supply the specs for proper curing.

The school just bought a new freezer for thier prepreg and I will have to too.

Nigelpheron,thanks for information on the molds, do you know of any books or sites on building high temp molds?

         Thanks again

I don’t know what exists out in web-land, without doing a search (learned from doing it, and from more experienced dudes) There will be a lot of amature info out there, but the good stuff you must pay for. The more gigs ya do, the accumulated engineering you learn, in the form of what is required to provide the strengths needed, for the particular job. That’s the gold. :graduate: The key is to keep experimenting. The more ya F–k up, the more ya learn. Expensive yes, but so is buying the processes. Nature of the game. :wink:

There usually isn’t much, because the processes are what the engineering companies are selling. That info aint free. in fact, that’s the valuable part of the business…Selling the processes. Ya get the plans, get the composite engineers, buy the processes, and hire the competant builders. Big money, big competition, big secrets (F1 and AC for eg.)

Never label yourself master, always be a student, and continue to exeriment.


Thanks nigelpheron.