Fiber coiling during bladder molding of thermoplastic composites

Abstract An investigation is presented into fiber movement during the consolidation of cylindrical, filament‐wound preforms when pressure is applied internally (bladder molding). It is necessary to characterize the fiber movement to predict the final fiber orientation of successful parts as well as to predict which parts will not be successful. Unsuccessful parts have fibers that clump together in thick coils at the interior surface of a part, resulting in an uneven wall thickness and poor mechanical properties. Cylindrical preforms (∼7.5 cm in diameter) made from glass fiber (20 μm in diameter, ∼50 vol%) coated with polypropylene powder (powder‐coated towpregs) were filament‐wound in a range of angles, interweaving patterns, and diameters. Trellising, a mode of fiber movement in which the fibers rotate as if they are pinned at the interweaving points, occurs first as the preforms expand. If the preforms “trellis” as much as possible without reaching the mold wall, shearing between adjacent fibers must occur to complete the expansion. Coiling occurs when a large amount of shearing is necessary to provide the final expansion needed to bring the preform to the mold diameter. A dimensionless “coiling” number is presented that predicts coiling.