Wind Blades
The new composites application that everybody is discussing is composites wind blades. The large, three-bladed wind generators have been around for a few decades, mostly in Europe. The U.S. has been catching on in the last couple of years as a way to make cleaner electricity. These windmills are very tall, and have blades that are 100 to 400 feet long, depending upon output rating and location.
The wind blades use glass carbon fiber, resin, and coring to make a long, stiff and lightweight blade that will attach to the hub of the windmill. These blades are very long, requiring huge manufacturing facilities to make them. The transportation of these blades is important as well, as they require specialized trucks and trailers to handle such large pieces. Large cranse are required to lift them into place at the job site. They are relatively heavy, and must be lifted fairly high, requiring a significant lift capacity.
Resin infusion with epoxy resins is the normal manufacturing technique of which I am aware. They use compsite molds that have a constantly changing surface shape due to the complex geometry of the blade. The holy grail for these blades is to make longer blades at lower weight.
This application again demonstrates the advantages of composites. Complex geometry, high strength to weight ratio, and impact resistance are important aspects of wind blades.
There are several manufacturers of the wind blades in the U.S. MFG is a specialty composites molder that has been around for ages and is in the wind blade market. Vestas is another company with operations in the U.S., along with LM Glasfiber, as well as others.
Cutting Layup Reinforcement
When working with reinforcements in the form of a woven mat, cutting is necessary to allow for proper orientation, workability, and strength. The most common way to cut these mats is with industrial scissors. Other methods include rotary cutters, die cutters, and electric shears, but a good pair of oversized, resharpenable, thru-hardened shears (scissors) are the best way to get started.
Woven mats can be cut to size in the dry stage -before the resin is applied- or in the wet stage, when resin is flowing freely. There are benefits and drawbacks to both, and operators usually find their own preferred technique. Cutting mats in the dry stage requires that it gets put together correctly when it is in the wet stage. Handling dry fiberglass is typically more itchy than wet fiberglass, which is sticky. Scissors used to cut wet reinforcement must be properly cleaned in order to be used again.
Many claims are made about the difficulty of cutting kevlar reinforcement. This can be remedied by using a dedicated pair of sharpened scissors only for kevlar. Kevlar is a material that requires a different angle on the blade in order to cut it. I have demonstrated to fellow workers how a fresh pair of scissors will cut kevlar all day, then cut a bunch of fiberglass. But when going back to the kevlar, the scissors will not cut it. Carbon Fiber falls into the category side of fiberglass where it will dull the blades and not go back.
Decorative Carbon Fiber
The automotive aftermarket makes use of decorative carbon fiber as an aesthetic means to differentiate a vehicle. Carbon fiber hoods, spoilers, and interior pieces add a cool aspect to many of the “tuner” vehicles that are specialized. A sample of this is would look appear to have a black woven pattern underneath a clear topcoat. True carbon fiber panels can be much lighter and stronger than a comparative sheet metal piece.
These parts and panels can be made using a fiberglass mold that has been made in the desired shape. The mold is waxed and then sprayed with a nice layer of clear gelcoat. It is very important to have a clear layer on top of the carbon fiber to distance it from the surface finish. Then a good polyester or vinylester resin is mixed with clear catalyst before wetting out the carbon fiber and laying it in the mold. Extreme care must be taken to orient the pattern of the carbon fiber so that it has good presentation, as the topside of this first layer will be seen through the clearcoat. The laminate can be backed with additional carbon fiber, fiberglass, or coring to achieve sufficient structure for the part being made.
One of the projects I have worked with in the past was a complex carbon fiber part where orientation was tricky. This required the mold to be made to be transparent. This was done using clear gelcoat, fiberglass reinforcement, and clear catalyst. This allowed for viewing the completed surface through the mold to ensure good cosmetics for the orientation of the weave on the finished side of the part.
One of the recent developments over the last ten years is dyed and woven fiberglass that appears to be real carbon fiber. The big advantage is cost; as it is does not have the same weight savings or strength properties of carbon fiber. Offerings also include red, yellow, and combinations of these colors in the same weave to achieve interesting decorative surfaces.
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