Effect of injection speed on the mechanical properties of isotactic polypropylene micro injection molded parts based on a nanoindentation test

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Abstract
Isotactic polypropylene micro parts were molded at different injection speeds by microinjection molding. The morphology and micro structure were characterized by a polarizing microscope, and the mechanical properties of differently structured layers were characterized by nanoindentation experiments. The influence of injection speed on the nanoindentation mechanical properties of each structural layer of the micro parts was analyzed. The results showed that the mechanical properties of different layers were different, the modulus and hardness of the position near the core layer were largest, and the modulus and hardness of the position near the skin were smallest. It is compelling that the modulus and hardness of each layer decreased first and then increased as the injection speed increased under a higher melt temperature (240 °C). Meanwhile, the opposite trend was observed at a lower melt temperature (220 °C). This phenomenon can be attributed to the competitive mechanism of the shear heat effect and the disorientation effect. In addition, injection speed had a greater influence on the nanoindentation mechanical properties in the perpendicular direction than in the flow direction. This work systematically explored the relationship between the microstructure and the local mechanical properties, which can provide new insights for microinjection molding design in the future. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47329.

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