The one process most common with carbon fiber for general purpose is called dry fiber. this is where the carbon fiber is laid into a mold while the resin is poured in & brushed over each layer that is applied. This process is very similar to how people work with conventinal fiberglass & resin. The problem with this method is that air pockets can form, which adversely affect the integrity of the product. Secondly, the impregnation of the carbon fiber cloth is inconsistant as best. This can result in a heavier product (from too much resin), or a weak product (too little resin). More importantly, there will be a varying level of resin penetration throughout the product, which will result in inconsistant strength. So be forewarned
Curious as to how a manufacturer building parts in more serious numbers went about the process of building carbon fiber components, we made a visit to K&N engineering. We had the opportunity of peeking into the Advanced Composites Fabrication Facility, where K&N manufactures carbon fiber scoops & air boxes for spring cars, as well as, carbon fiber heat shields for some of it's Typhoon intake kits.
K&N uses the pre-impregnated carbon fiber material, which already has the resin impregnated within the carbon fiber mat. The pre-impregnated carbon fiber is maintained in a frozed state to prevent its curing.
By using prepreg carbon fiber, there is less mess when working with the raw material, it is easily handled, there is nothing to spill or mix, & nothing to clean up. The carbon fiber is simply cut & laid into a pre-made mold, in as many layers that are needed for the desired thickness & resulting streength.
Once the desired thickness is aquired, the mold is then placed into a vaccuum bad. Vacuum is used to create a negative pressure that extacts ambient air & holds the carbon fiber firmly against the mold during the curing process. Each mold has it's own vacuum port that is connected within the autoclave prior to the curing process. In order for the carbon fiber to become solid, it must be heated to approximately 270 degrrees F
for the resin to properly cure. By using the autoclave, it's possible to have very consistant results & a very durable product by using high temeratures & vacuum, which help form the carbon fiber into a compact package.
Once the curing process is complete, the molds are removed from the autoclave & the carbon fiber product is removed by using a tool similar to a putty knife. Some minor trimming of excess carbon fiber is removed & the product is given a quick wiper down with a cleaner/polish material before being packaged for shipping. Products like the lid that goes on the end of a filter receive an additional process to bond it to the filter material.
1. K&N engineering stores its pre-impregnated carbon fiber mat in freezers at 0 degrees to prevent the resin from curing prematurely.
2. Once the carbon fiber mat is removed from the freezer, it's sized & cut.
3. Here are the cut pieces prior to being placed into the mold.
4A,B. The carbon fiber is carefully placed into the mold.
5. Here we can see the mold inside a vacuum bag. A gauge shows -70kPa of vacuum is used for this part.
6. The molds are placed into the autoclave & connected to vacuum lines. The autoclave heats the carbon fiber to 270 degrees F
in order to cure the resin in the carbon fiber.
7. Once the piece is dried, it is removed from the mold.