Void formation in geometry-anisotropic woven fabrics in resin transfer molding

When geometry-anisotropic fabrics, in which the thickness and width of fiber bundles differ in the warp direction and weft direction, respectively, are used for resin transfer molding (RTM), microscopic porous structures along a flow path may depend on the resin flow direction. This study investigated the influence of woven fabrics' geometric anisotropy on inter-bundle void formation due to air entrapment at the flow front during RTM. The void content-resin flow velocity relationship was measured in warp (narrow and thick bundle) and weft (wide and thin bundle) directional impregnation. In experiments, warp directional impregnation indicated higher critical resin flow velocity of void formation and void content under a given resin flow velocity than in weft directional impregnation. Void formation was also largely affected by capillary fingering, where warp directional impregnation indicated a higher critical flow velocity of fingering formation. This may be because the gap between the fiber bundle and the mold surface is smaller in the warp direction; thus, the capillary force is higher, and fingering is facilitated compared with weft directional impregnation. Additionally, this may lead to a higher critical velocity of void formation and higher void content at a given flow velocity in warp directional impregnation.