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Composite Bent Shaft Case Study

Shaft curvature found in high-end lightweight carbon fiber bent or crank shaft paddles presents several manufacturing challenges. Transforming a straight boardy carbon fiber prepreg tube into a well consolidated, wrinkle free, bent shaft is the most significant challenge.

Reinforcing fibers may undergo angular changes during molding but the fibers do not stretch. The lack of fiber elongation creates molding complications when pressure pins reinforcement layers against tool surfaces on either side of a recess. Fibers bridge across the concavity and prevent complete consolidation against the tool surface in the recess. Signs of fiber bringing typically include poor resin flow and large surface voids.

Fiber bridging can also produce significant wrinkling and fiber distortion opposite the bridged recess as excess material from the partially expanded cross section is compressed against the opposite tool surface.

Wrinkles and Fiber Distortion
Opposite a Bridged Area

Bladder Inflation Molding (BIM) was an obvious choice for the non-linear, variable cross section, hollow bent shaft. CSS helped the Client select a suitable bladder, develop custom air fittings and clamshell tooling, design a forgiving shaft layup, and optimize the BIM processing parameters.

Several molding trials were required to refine the bent shaft layup, mold packing procedure, and molding cycle settings but the result was a uniform, well consolidated, exceptionally strong bent shaft with no surface porosity or fiber distortion.

BIM Carbon Fiber Bent Shaft