Resin steps up to bat, improves manufacturing process

Lincoln, NE--When a major sporting goods manufacturer approached Spencer Composites Corp. to develop a new filament-winding process, Spencer turned to Loctite Corp. (Rocky Hill, CT) for help.

At the time, the sporting goods manufacturer roll wrapped its softball bats with fabric prepeg, carbon fiber pretreated with a high-viscosity resin. Using prepeg is clean and efficient, but expensive. The manufacturer wanted a filament-winding process that would allow automated production of its 26-oz bat; that would be cost-effective, fast, and clean; yet still result in high-quality products.

In standard filament winding, carbon fiber is wetted with a resin and then wrapped around a mandrel or other mold. This wetting and wrapping process can be quite sloppy, and requires expensive hand labor to squeeze off excess resin and clean up floors and walls that become coated with resin spatter. During the winding process, getting the fiber to stay in place can also be challenging--if the speed of the wet winding is too fast, the fiber will slip out of place. Once the filament winding process is completed, the composite material is placed in a standard convection oven and heated until cured completely.

Loctite saw the need for a cleaner filament-winding process that would reduce the amount of resin spray in the production area and stop resin migration during the winding and curing cycles. The company developed Accuset 303, a UV-curing filament-winding resin additive that contains a photoinitiator that is sensitive to UV light. "By adding Accuset 303 to the resin and exposing the part to UV light, you can gel the fibers in place, which improves winding pattern control," explains Loctite Composite Engineer Jay Brenner.

In the new process, workers drew carbon fiber through a resin bath, wet it, and wound it onto a metal mandrel inserted into the filament-winding machine. They exposed each layer of the fiber to UV light for 10 to 15 seconds per cycle to partially cure the resin. At the end of the winding process, workers easily removed the partially cured preform from the mandrel.

A bladder was inserted in place of the mandrel and inflated to hold the shape of the preform, which was then placed into a mold. Since the preform was already partially cured, it filled the mold better and was not wet or drippy. Final cure time was two hours at 250F in a standard convection oven. The result: a smooth finish with a visible crisscross winding pattern.

The UV curing process decreased both processing and curing time. And, by reducing the amount of hand labor required to make each bat, the sporting goods company lowered manufacturing costs, speeded production, and im-proved part-to-part consistency and quality. In addition, using the filament winding process rather than prepeg reduced the cost of materials by $7 to $8 per pound.