If you have read anything about carbon fiber you probably have run across the term “pre-preg” but you may not know what it does or even what it is. Typically carbon fiber fabric is shipped as a dry loosely woven cloth. To use it, you use a variety of methods to apply wet epoxy resin to the cloth and then let it set at room temperature to cure. Pre-preg refers to carbon fiber fabric that is pre-impregnated with epoxy resin from the manufacturer. It is a thick material that you apply in layers to your mold. It is slightly sticky as well so it holds together and to the mold fairly well. Once it is applied, a special clear plastic sheet is applied over the pre-preg and affixed to the edges of the mold with a type of foam tape. This process creates an air tight seal between the inside of the mold and the outside. Next, an air hose is attached to the plastic sheet with a special connector and the other end of the air hose is connected to a vacuum pump. Once the pump is started it removes all the air from between the plastic and the mold “sucking” the plastic against the pre-preg and against the inside of the mold. What is really happening is that normal atmospheric air pressure is forcing the plastic and the pre-preg against the inside of the mold very firmly. Normal air pressure is about 15 lbs per square inch. Imagine that you had a 15 pound weight on each and every square inch of pre-preg, forcing it against the mold. As you can imagine, the pre-preg will have a very good fit against the mold. Because the air seal of the foam tape is not perfect the vacuum pump has to run continuously to ensure that pressure is always applied.

Now that the mold and pre-preg are ready, its time for the epoxy in the pre-preg to cure. Normally you mix resin and hardener together when you are ready to use it and it starts to setup immediately. Since the pre-preg came with the epoxy and hardener already mixed in, you need a different system to cure it. The answer is heat. All epoxies cure with heat, the question is how much is needed. Normal epoxies make their own heat when you mix the resin and hardener together. So long as they are used at about 70F degrees or so, the heat they create will be enough to cure them. The epoxy that is part of a pre-preg fabric requires much more heat, the heat from an oven. Once you have vacuumed the air out of your mold you put the whole thing in the oven to cure. Times and temperatures vary but its a couple of hours at 300 degrees or so.

And that’s how you use pre-preg. It makes the highest quality part because the mix of fabric to epoxy is always exactly right. Also, the fabric is more stable and you are less likely to have the weave disturbed as you use it. As you can see its more complicated and it costs a lot more too. But when you look at expensive cars like a Lambo or Zonda or even race cars, they are always using pre-preg.