Carbon fiber is typically produced in an inert environment at temperatures above 1,800°F. The process of producing carbon fiber accounts for its price and classification as an exotic fiber. Carbon fibers are known for their lightweight, high strength, and high stiffness.
702 and 703 Unidirectional Carbon Tapes are designed to be used with WEST SYSTEM epoxy. The hardener choice is dependent on laminate thickness (dimensionally), the time required to perform the operation, and ambient temperature when the cure will take place.
702 and 703 carbon fibers are in tow form. A tow is like a ribbon. The tow of this material consists of 12,000 fiber bundles per inch of width, so it is referred to as 12K material. The tow is continuous and unidirectional on the ‘0’ axis (along the tow length). The fibers are not twisted or woven. The white strands running on the 90 (degree) axis are polyester threads that hold the fiber bundles in place and are non-structural. This 12K material is on the order of 11 oz (per square yard) material and measures .025″ thick.
How to Apply Unidirectional Tapes
Before applying unidirectional tape, whether carbon or fiberglass, assess the anticipated load path and orient the fibers in that direction. For example, a vertical flag pole is a cantilevered structure that is subjected to bending forces. Fiber orientation would be along the pole’s length to stiffen it. Carbon fibers are effective in compression or tension, but it’s best to apply fibers equally to both sides of a structure to maintain balance. Carbon fibers are most effective placed furthest from the neutral axis.
Off-axis plies result in a rapid loss of strength and stiffness. A depression in a layup (such as a wrinkle or bump) will attract forces and jeopardize the laminate. If off-axis fibers cross one another, it is best if the laminate is epoxy-rich (on the order f 50%). An epoxy-rich laminate helps prevent the fibers from contacting each other.
Wet the substrate with epoxy. It’s best to place the first layer of unidirectional tape into the coating while that coating is still wet because it will transfer best to the reinforcing fiber. Keep the fibers straight (in columns) as best you can.
Locking sheet metal pliers with sandpaper adhered to the flange allow you to pre-tension as well as align the fibers precisely. When working with long objects, such as spars, we will cut our unidirectional tape to length and attach the grips to both ends so the tape can be pulled taut and then lowered into position.
Once they are in place, wet the fibers with epoxy using a brush and squeegee. Hold the tool at a low angle and thoroughly wet the carbon fiber. If the laminate requires more layers, continue to apply layers, wetting the fibers as you proceed. As a rule, don’t apply more than six layers at a time or you may experience excessive exothermic heat from the epoxy as it cures.
When all layers of unidirectional tape are applied, place release fabric or polyethylene plastic over the last layer and squeegee firmly to remove excess epoxy and trapped air bubbles. Leave this in place until the laminate cures. Remove the release fabric or plastic by pulling it back on itself as close to 180° as possible. Pull steadily and avoid jerking as you peel this film from the laminate.