Making Dry Autoclave Carbon Fiber With K&N
By Joey Leh, Photography by the author
Carbon fiber is one of the most popular high-end composites currently used in the automotive industry. Racing series such as Formula One and the American Le Mans Series make generous use of the material for body and chassis construction. Street users, mesmerized by the weave of carbon, have made carbon fiber popular in the construction of wings, interior trim/paneling and lower grade body panels such as hoods and trunks.
Carbon fiber has a very good strength-to-weight ratio, making it an ideal replacement for metal in situations where weight is more important than cost. Versus steel, carbon fiber has more than 2.5-times the tensile strength and, due to a lower density, a much lower weight. Pure carbon fiber also has low thermal expansion, with electrical conductivity dependant on the type of material used.
But the name carbon fiber itself is a bit of a misnomer for the street-based end user because most parts readily available for sale seem to contain more fiberglass than actual carbon fiber in their construction. These cheap, often knock-off “carbon” hoods, shift knobs and trunks can be found at the local swap meet and aren’t worth much more than their weight in rocks. There are many different methods of manufacturing carbon fiber products, of which some methods can be carried out at home.
For the garage carbon fiber manufacturer, the easiest method is referred to as “wet” lay-up. A mold is manufactured of the end part’s shape (well, sometimes, some amateur users just eyeball it with varied results) and then raw carbon fiber fabric is laid on to the mold.
A fiberglass resin is then applied on top of the carbon and the part is either left to air dry in the open or put into a bag, from which all the air is vacuumed out. Vacuuming the air out seeks to more evenly distribute the resin across the surface of the part as well as remove air bubbles for strength.
The method used by most professional racing teams and high quality parts manufacturers is “dry” pre-peg carbon fiber, with the use of an autoclave. Molds are still required to produce parts in this instance, although pro teams will be more specific about the materials used for their molds. Due to the heat of an autoclave, fiberglass molds will deform and are unable to be used as many times as a metal mold.
Carbon fiber that is pre-impregnated (i.e. pre-preg) with epoxy resin is laid into the mold and then covered with releasing films and agents. The entire piece is then bagged and vacuumed clean of air. From there the piece is inserted into an autoclave, which is, basically, a large oven. The parts are heated to cure the carbon fiber and then the part is finished. Dry carbon parts can be produced with a much lower weight than wet lay-up parts since there is much less resin used (hence the “dry” name) and with a higher strength.
Formula One chassis and many common racecars parts, such as World Challenge front splitters, are made of sandwiched carbon fiber. These parts use carbon fiber sandwiched around aluminum honeycomb or foam core, increasing the strength of the part tremendously.
When you need a front splitter that can survive a curbing hop at 120mph, you need the good stuff. For now, we’re just going to cover the dry production method for non-sandwiched carbon parts.
We paid a visit to K&N Engineering’s carbon fiber department, where they produce, in-house, all the carbon used in their air filter and intake products.
Because of the temperature change throughout the day, K&N starts work at 4AM and ends by mid-morning time. This helps the composites team avoid the rise in temperature from the sun. K&N manufacturers all of their carbon fiber products in the dry pre-preg method using resin-impregnated carbon fiber sheets. From there, the carbon sheet is laid into the part mold by hand. At K&N, CNC-machined molds are made for every different mass-produced carbon product. This ensures consistency and less disposal than fiberglass molds, which deform after a few uses.
A special release film is placed over the carbon fiber and mold package and then the whole piece is bagged and placed into the vacuum bag. Using special vacuum hoses, all the remaining air is sucked out of the sealed bag.
The parts are then placed into K&N’s autoclave. Multiple molds and parts can placed into the autoclave at one time. Once out of the autoclave, the carbon parts are taken out of the molds and then trimmed. The finished carbon fiber piece is then ready for packaging and sale.
Sources
APR Performance
909-594-3796
www.aprperformance.com
K&N Engineering
www.knfilters.com
You don’t mention anywhere that the autoclave is under high pressure. It says “From there the piece is inserted into an autoclave, which is, basically, a large oven.” This is incorrect, the autoclave is a large oven under pressure. For carbon fiber composites generally 85-110psi, @ a temperature of 300-400F.
Good article Joey. Let us know when you’ll be coming by for a visit again.
Jon,
I would say that the amount of technical detail written above is sufficient for the purposes of this article, since this is more of a news article that should serve only as an brief introduction to a couple carbon fiber composite manufacturing processes. If I, as a reader, wanted to learn more about this subject, I would search elsewhere for that knowledge. There are a multitude of different manufacturing processes that can be used with pre-preg. carbon fiber material, some of which don’t necessarily require high autoclave pressures. Depending on the part, such as for a non-structural, single-layered item (i.e. engine cover), a relative pressure delta of 4 atmospheres at 140 C (284 F) is more than enough pressure and temperature to speedily cure and produce a high-quality part. For example, there may only be 2 atmospheres (approx. 29 psi) absolute pressure within the autoclave environment, plus 0.5 atmospheres absolute pressure inside the vacuum bag that encompasses the part (created via a tube that connects the vacuum bag to a vacuum generator outside the autoclave). This would produce the specified 4 atmospheres of relative pressure needed for such a part. Higher autoclave pressures and temperatures may very well be needed on a different type of part that’s used in a different application. Sometimes, sufficient relative pressure and high temperatures are produced without an autoclave at all 
Anyway, the idea here is that there isn’t a “standard” pressure or temperature range that is used during these processes, and many factors need to be considered (i.e. resin material, application of end product, dimensions of part, etc.) in order to determine the optimum pressure and temperature values needed.
Don’t be too hard on Joey