Less expensive carbon parts are usually made with polyester resin. Polyester resin is cheap, finishes well and is easy to work with. Most of your typical aftermarket performance parts for street cars are made with polyester, which is fine for many consumers. However, polyester-based carbon laminates tend to be brittle and can shatter when something is hit hard. It is also more likely to stress crack. To get the most out of carbon fiber's superior strength properties it is best to use a higher performance resin like epoxy. Epoxy is much more flexible, tougher and stronger than polyester, although it is harder to work with and has a yellowish cast that some find ugly. Epoxy also tends to turn yellow and chalky with UV light exposure so it should be clear-coated.
Carbon-fiber stuff is made in several ways. The cheapest and easiest way is called a wet layup. This is simply laying the carbon-fiber cloth in a mold and then saturating it with liquid resin. Additional layers of carbon or fiberglass are laid on top of the first layer so that each layer gets saturated with more resin. Typically inexpensive carbon parts have only the top layer that you can see made of carbon. The under layers that give it stiffness are made of cheaper and easier-to-form fiberglass. The best, strongest and lightest fiberglass to use for this is cloth, but sometimes fiberglass batting or chopped fibers are used. Batting is often preferred since it is thick and fluffy, which adds a lot of thickness in one labor step. Chopped fibers can be shot out of a gun to build up thickness and make parts quickly. Batting and chopped fiberglass is heavier and much weaker than fiberglass cloth. Although using fiberglass seems like its cheapening the part, good parts that exploit the superior strength of carbon fiber can still be made this way plus it does make large parts like a hood more affordable.
When the needed thickness of the layup for strength and stiffness is reached, the excess resin is squeegeed off and the part is set aside to cure. Sometimes a layer of gel coat is first added to the mold before the carbon cloth to assure a smooth top layer finish. There is nothing wrong with wet layups and many good parts are built this way. The advantage of wet layups is that they can have a nice glossy surface finish. The disadvantages of wet layups are that they weigh more because it is impossible to squeegee all of the excess resin out. There is less compaction and interlocking of fibers between layers of material and there are more voids within the material, making a wet layup less than optimally strong. Wet layups are the fastest, easiest and cheapest way to build a part.
Vacuum bagging is the next way to build a part. The wet layup in the mold is covered with a layer of smooth perforated plastic called a peel ply. The peel ply is covered with a fluffy batting material called a breather. Then the whole thing is placed into a strong plastic bag and the air is sucked out of it with a vacuum pump. The hard vacuum compresses the layup, compacting it thoroughly. The excess resin is squeezed out of the pores of the peel ply and soaked up by the breather. When the layup is cured, it is removed from the plastic bag and the breather and peel ply are removed. This method is called vacuum bagging or vacuum infusion. Vacuum bagged parts offer superior strength due to their better compaction and lighter weight because excess resin is completely squeezed out. Of course vacuum bagging is more labor-intensive and thus more expensive. Vacuum bagged parts can still have some fiberglass layers to them.
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