Investigation on the microstructures of long fiber and their influences on warpage and mechanical property in injection reinforced thermoplastics (FRT) parts

In recent years, due to its excellent properties, the fiber-reinforced thermoplastics (FRT) material has been applied into industry as one of the major lightweight technologies, especially for automotive or aerospace products. However, due to the microstructures of fiber inside plastic matrix are very complex, they are not easy to be visualized. The connection from microstructures to the final shrinkage/warpage is far from our understanding. In this study, we have proposed a benchmark with three standard specimens based on ASTM D638 where those specimens have different gate designs. Due to the geometrical effect, the local warpage behaviors are quite different for those three specimens. Specifically, it causes one specimen warped downward and bended inward, another warped upward, and the other slightly upward at the same time. The local warpage behaviors are validated by experimental study with excellent agreement. Moreover, the fiber length effect on the full warpage behavior was also conducted. When the longer fiber length is introduced, the full model warpage behavior can be reduced. The detailed of the full model warpage behavior has been analyzed side-by-side using both of numerical simulation and experiment. The trend is in a reasonable agreement for both simulation and experiment. Furthermore, the mechanical property variation of the finished parts due to the different fiber length was also investigated. Results showed that when the fiber is reinforced the tensile strength is increased linearly for all Models. However, the tensile strength of the Model I is always better than that of Model II, while Model III is much worse than others due to its double gate effect. The reason why the tensile strength of the Model I is always better than that of Model II could be due to the side-gate structure to provide strong fiber orientation and also more uniform fiber distribution at NGR.