Extraordinary uses for one of the most common elements on earth.
Andy Bolig - August 15, 2013 10:00 AM
Peter shared the increasing uses for carbon fiber
Carbon fiber fabric is stored on rolls for shipping and storage. The appropriate amount of fabric is then cut from the roll to be used instantly.
The fabric is comprised of strands (tow) that are woven into a pattern. This allows the fabric to be flexible, but strong.
This tow of carbon fiber is comprised of many fibers. The number of fibers for each tow can range from 3,000 to 12,000.
This mold for a lower splitter is CNC’d out of tooling material. Then they are hand-polished to provide the highest level finish possible.
Larger panels, like this fender, are formed out of epoxy molds due to their unique shape and size.
All material is hand-laid into the mold dry. This allows for longer working times and a cleaner environment.
This two-piece mold is for Callaway’s hood bulge. This allows Callaway to use the existing hood on their Corvettes but provide supercharger clearance without the issues of paint color matching.
These small blocks are the molds for the Grand Sport Corvette’s fender trim pieces. Notice how the weave of the fiber are straight and even. This is a benefit of the VARTM style of manufacturing.
VARTM uses pressure to draw epoxy evenly throughout the component being manufactured. A bag is used to seal off the component from the atmosphere and is actually used to apply pressure (due to the vacuum) to the manufactured part.
The finish of the molded part is of high quality, but final trimming is necessary. Each one of these vents must be hand-trimmed. Then the part gets clear coated to protect it from UV rays.
The 1994 Callaway-built C7R features carbon fiber monocoque construction, and as such weighs only 2,470 pounds.
The use of carbon fiber was extensive, branching into the interior and even the seats!
Layering carbon fiber allows for additional bracing contained within the composite material and also allows for the use of more or less layers to determine thickness, even within the same panel.
Another project by Callaway, these Carbon Multi-Profile components allow for customizable support brackets. Close tolerances and proven manufacturing techniques have allowed Callaway to construct complex angles and components.
The KRTM process has allowed Callaway to create great looking products that exhibit excellent structural strength. For this rear spoiler, the fasteners, foam structural core and finished Class-A surfaces are completed in one process.
OEMs are even beginning to utilize carbon fiber as seen on this hood accent of the Chevrolet ZL1 Camaro
There are nearly 10 million known carbon compounds and many are essential for life as we know it.
Even though the ancients knew about the presence of carbon, it wasn’t until recently that automotive engineers began acknowledging its existence. It began by those in racing circles wanting to exploit carbon’s strength and lightweight characteristics. Carbon fiber has 10 times the strength but half the weight of steel.
Those same properties have encouraged the use of carbon fiber in today’s production automobiles in an attempt to reduce their carbon footprint on the environment. With increasing fuel mileage standards imposed upon the automotive industry, more and more engineering types have been studying the strength, and weight-saving possibilities of carbon, striving to give this very common element more widespread uses within today’s street-going automobiles.
Callaway Cars in Old Lyme, Connecticut, and Callaway Competition in Leingarten, Germany, were some of the first to fully embrace the benefits of using carbon fiber in the construction of cars. Their C7R, a supercar built in 1994 for the GT1 class of endurance racing, was built completely of carbon fiber. Their experience, gained on the racetrack, has afforded them the ability to forge ahead in the common use of carbon fiber on both the highways and racetracks around the globe.
We visited Callaway’s Santa Ana, California, facilities recently and spoke with Peter Callaway to understand more about this exotic but common element, and observe how it can be put into more customary uses within the auto industry.
Carbon fiber is just that, individual fibers of carbon that are brought together into strands (called tow) through a complex manufacturing process performed only by fewer than a dozen companies worldwide. The process begins with a polymeric feedstock, which provides the fiber’s molecular backbone. It is through polymerization (the chemical process that creates long-chain polymers) that gives the processed material the ability to be formed into fibers. The number of fibers in each tow determines its dimensional properties and how the strands are manufactured determines its strength modulus, overall thickness of the fabric and the appearance of the weave. Each tow can hold upwards of 12,000 individual fibers of carbon. Callaway uses a 3K tow for their materials because of the formability of the thinner sheets and the ability to better control the epoxy flow and uniformity.
Carbon fiber is a combination of unlike materials (fiber and resin) created in a notoriously difficult and expensive process. The highly guarded process is proprietary to each manufacturer, and the final product which each manufacturer produces is similar, but not identical. This allows for certain characteristics of carbon strands to be tailored to an end-user’s specific needs. As the tow is produced, they are rolled onto spools, called bobbins.
The tow is then woven together to form a fabric sheet of material that is subsequently fitted onto rolls for transportation and holding until needed. The weave pattern by which the tow is woven together influences the characteristics of the fabric, as well as the mechanical properties of the finished, molded part. Callaway utilizes fabric with a 2x2 twill pattern, specifically woven and quality controlled for optical correctness.
Callaway receives rolls of sheet carbon fiber material from their supplier, cutting the necessary amount of material from the roll when needed for production of components. Of course, any fabric flexible enough to be rolled will not exhibit the structural rigidity necessary for use in automotive body panels and the like. This is where Callaway’s experience in handling and preparation of composites has allowed them to create processes that not only make use of carbon fiber’s strength and rigidity, but also ensures a high quality finish of the completed part.
There are many methods of turning a flexible sheet of carbon fiber into a high-strength component. Callaway utilizes two different techniques. The first is a process called Vacuum-Assisted Resin Transfer Molding, VARTM for short. As the name implies, vacuum is used to introduce the polymer throughout the fibers of the fabric, thus permanently fusing them together into the desired shape. The other process, Kraemer Resin Transfer Molding (KRTM) is an industry-leading process of Callaway's own development used to create structural components.
KRTM utilizes temperature-controlled aluminum tooling for longer life and mold stiffness. A more robust mold is required for the KRTM process, which injects resin into a two-sided, closed-mold cavity under pressure, instead of being pulled through the laminate under vacuum. This is a proprietary process developed over the last 10 years in Germany that uses an automated injection process and as such, parameters are recorded with each part made. This process yields a net-shape part with two class-A (finished) surfaces, as opposed to only one class A surface produced by the VARTM process (B-side shows the peel-ply texture).
Another benefit is that KRTM allows fully integrated, near net-shape moldings to be made in one fell swoop. For example, building a part like a rear spoiler used to require a few days for processing the top skin, bottom skin, foam core, bonding it all together and then trimming, with having to add hard points for fasteners after the fact. Now the entire process can all be done in about three hours rather than three days.
Concept to Composite
Peter walked us through the VARTM process during our visit and explained what it takes to go from concept to composite. He explained that once the dimensions of the desired component are known, negative molds are CNC-cut from polyurethane tooling material. Larger items like fenders and entire fascia assemblies use laminated epoxy molds pulled from CNC-generated masters, due to their lighter construction and being easier to replicate.
Since the finished component will be an EXACT replica of the mold, the surfaces of the molds must be hand-polished to a mirror finish, while still keeping the definition and detail of the original design. Any defect in the mold will replicate itself in each and every component produced. The molds are subject to wear from use but their life cycle is quite long, if protected and cared for properly.
Mold in hand, the composite material is applied to the surface of the mold dry. This allows for a much cleaner process, eliminates the fumes associated with curing resins and also allows for unlimited working times to better form the material into the mold, a benefit when trying to lay down carbon fiber without any undesirable distortion of the weave. A special adhesive is applied to retain the placement of the composite material during laminating and lastly, a layer of material called "peel-ply" is added. Then the entire mold is placed into a vacuum bag.
The bag serves several purposes. It helps compact the material closely to the surface of the mold through vacuum and, therefore, results in a perfectly uniform ratio of resin and carbon fiber. Too much resin, and the part becomes heavy and brittle, too little and the part will not have the strength and stability necessary for use. As vacuum is introduced, it compresses the finished thickness of the component and evens out the epoxy into every fiber of the item being constructed.
Under vacuum, a precise amount of epoxy is introduced through another tube. It distributes throughout the entire mold, eliminating voids and porosity. Through the VARTM process, a controlled ratio of resin-to-composite material ensures the lightest weight and stiffest component possible and also eliminates fumes from the work environment.
Once the epoxy has been infused and has had adequate time to cure, the part is then taken from the mold and placed into an oven, which final cures the epoxy and hardens it completely. Once cured, the part is then sent for final trimming and treated to a clearcoat that will protect the resin from ultraviolet rays.
For Your Information: