Permeability anisotropy of flax nonwoven mats in vacuum-assisted resin transfer molding

The resin flow characteristics of natural fiber mats must be taken into consideration in the design and optimization of liquid composite molding processes. This paper studied two common types of nonwoven mat structures produced by the parallel-laying process and the cross-laying process. The flax fiber orientation distribution characteristics of these mats were determined by using a fast Fourier transform (FFT) image analysis algorithm. Flax fiber mats produced by the parallel-laying method had a prominent principal direction of fiber orientation, while mats produced by the cross-laying method had a nearly random fiber orientation. The parallel-laid mats showed a higher overall compressibility than the cross-laid mats because of the tendency of nesting between layers of parallel fibers, leading to a lower porosity under the same vacuum level and thus a lower overall permeability. The cross-laid nonwoven mats demonstrated nearly isotropic in-plane permeability, while the parallel-laid mats showed a much higher permeability in the principal fiber direction than in the orthogonal direction.