The NFCs have been studied by several researchers, using injection molding as a fabrication process. Several issues and challenges regarding the fabrication of NFCs by injection molding have been observed.

8.4.1 DISTRIBUTION AND ORIENTATION OF NATURAL FIBERS

The distribution and orientation of the fibers in the composite plays an important role in determining the mechanical properties of the composites. In injection molded composites, the orientation of the reinforced fibers is a critical factor to control. The fiber orientation of short fibers vary with respect to the thickness as well as the in plane direction. During the injection molding of composites, the fibers are oriented according to the complex molten polymer flow generated during the process. Con­vergent flow of the fiber matrix mixture results in fiber alignment along the flow axis while the divergent flow causes the fibers to align perpendicular to the flow direction.25The fibers near the surface are generally aligned parallel to the direction of flow and are aligned perpendicular to the direction of flow at the center. A similar orientation of fibers was also observed, in a study where distribution of flax fibers reinforced PP was studied. The fibers close to the surface and sides of the molded part were well aligned with the direction of flow, whereas, near the mid plane the fibers were randomly distributed.26This shows the dependence of fibers to align in the direction of shearing and stretching. During injection of the matrix-fiber mixture into the mold, the shear flow near the mold walls aligns the fibers in the direction of flow. This outer layer in contact with the mold surface is called as skin. Below this layer, the mixture in the molten state continues to experience shear and fibers are aligned along the flow direction. Then a core layer is formed in the center where bulk deformation of the flow occurs causing the material to stretch in and out of the paper direction aligning the fibers.25 The fiber distribution in PLA/Jute composites was studied using long fiber pellets (LFT) (fabricated via pultrusion process) and recompounded pellets (RP) (extruded LFT) and it was found that the fiber dispersion and separation in RP was better than that of LFT, resulting in a better interfacial interaction and improved mechanical properties, although severe fiber attrition was observed in case of recompounding process.27 Hence, it shows that the orientation and distribution of the fibers not only depends on the processing and flow conditions but also on the precompounding of the fibers and the matrix. A balance between the amount of compounding and the distribution of the fibers is required to keep the fiber attrition to minimum. Also, along with the processing parameters, the rheologi­cal properties of the matrix should be taken into account while designing the mold for injection molding to ensure a better flow and less fiber attrition.