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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.   The most significant challenge is reshaping and molding a straight, round, boardy carbon fiber prepreg tube into a well consolidated, wrinkle free, bent shaft.

Straight and Boardy Bent Shaft Carbon Fiber Prepreg Tubular Preform

Reinforcements such as carbon fiber 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 concavity.   Fibers bridge across the recess and prevent expansion and consolidation of the preform in the tool concavity.   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 concavity as excess material resulting in part from the partially expanded cross section is compressed against the opposite tool surface.

Wrinkles and Local Fiber Distortion Opposite a Bridged Concavity

Bladder Inflation Molding (BIM) was an obvious choice for the non-linear, variable cross section, hollow bent shaft geometry.   CSS worked closely with the Client to select a suitable bladder, develop custom air fittings and clamshell tooling, design a forgiving shaft layup, and develop a BIM procedure which collectively addressed the anticipated manufacturing challenges.

Several molding trials were conducted to refine the bent shaft layup, mold packing procedure, BIM parameters.   The result was a uniform, well consolidated, exceptionally strong bent shaft with no surface porosity or fiber distortion.

BIM Carbon Fiber Bent Shaft

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