Filament winding is a continuous fabrication method that can be highly automated and repeatable with relatively low material costs. A long, cylindrical tool called a mandrel is suspended horizontally between end supports, while the “head” – the fiber application instrument – moves back and forth along the length of a rotating mandrel, placing fiber onto the tool in a predetermined configuration. Computer-controlled filament-winding machines are available, equipped with from 2 to as many as 12 axes of motion.
In most applications, the filament winding apparatus passes the fiber material through a resin “bath,” just before the material touches the mandrel. This is called “wet winding.” Towpreg – continuous fiber pre-impregnated with resin – also can be wound, eliminating the need for an onsite resin bath. In a slightly different process, fiber is wound without resin (“dry winding”). The dry shape is then removed and used as a preform in another molding process, such as RTM.
Following oven or autoclave curing, the mandrel may remain in place and become part of the wound component or it may be removed. One-piece cylindrical or tapered mandrels, usually of simple shape, are pulled out of the part with mandrel extraction equipment. Some mandrels, particularly in more complex parts, are made of soluble material and may be dissolved and washed out of the part. Others are collapsible or built from several parts that allow disassembly and removal in smaller pieces. Filament-winding manufacturers often “tweak” or slightly modify off-the-shelf resin to meet specific application requirements. Some manufacturers develop their own resin formulations.
In thermoplastics winding, material is in prepregged form, so a resin bath is not needed. Material is heated as it is wound onto the mandrel – a process known as curing “on the fly” or “in situ consolidation.” The prepreg is heated, layed down, compacted, consolidated and cooled in a single, continuous operation. Thermoplastic prepregs eliminate autoclave curing (cutting costs and size limitations), reduce raw material costs and can be reprocessed to correct any flaws.
Filament winding yields parts with exceptional circumferential or “hoop” strength. The highest-volume single application of filament winding is golf club shafts. Fishing rods, pipe, pressure vessels and other cylindrical parts comprise the bulk of the remaining business.