Numerical and experimental investigations on the mechanism of flow-induced fiber orientation in short-shot water-assisted injection-molded short-glass-fiber-reinforced polypropylene

It is a critical requirement to have an insight into the mechanism of flow-induced fiber orientation in short-shot water-assisted injection molding (SSWAIM) of fiber-reinforced polymer for improving the structural rigidity and service life of molded parts. However, this mechanism is still unclear, which stunts the development of SSWAIM. In this work, the mechanism of flow-induced fiber orientation in SSWAIM parts of short-glass-fiber-reinforced polypropylene (SGF/PP) was clarified through numerical research and experimental verification. The results showed that the difference of fiber orientation distribution at different positions both in the radial direction and along the flow direction between SSWAIM parts and conventional injection molding (CIM) parts was mainly due to the strong flow field caused by the high-pressure water penetration in SSWAIM. Moreover, fiber orientation in the SSWAIM part depended not only on its position both in the radial direction and along the flow direction, but also on the processing parameters. At the front of SSWAIM part, fiber orientation changed greatly in the radial direction and presented an obvious "shell layer-core layer-water channel layer" hierarchical structure across the part thickness, whereas this phenomenon became more inconspicuous with increasing the distance of the selected position from the water injection inlet. Short-shot size is the principal parameter affecting the fiber orientation, and within the range of investigated processing parameters, smaller short-shot size, shorter water injection delay time, higher water pressure, and lower melt temperature could significantly facilitate fiber orientation across the part thickness.