Comparison Of Structure And Properties Between Micro-injection Molded And Micro-injection Compression Molded Parts

In recent years, with the rapid development of3C (computer, communication, consumerelectronics) industries and microfluidics technology, the trend of miniaturization for plasticparts is prevailing. Micro-injection molding (MIM) and micro-injection compression molding(MICM) become the main micro molding technologies due to their high molding efficiencyand low molding cost. In this dissertation, the microparts were molded via MIM and MICM,which included the rectangular polystyrene (PS) plate with micro-features, polypropylene (PP)and PP/halloysite nanotubes (HNTs) nanocomposite parts with a0.7mm thickness. Theinfluences of processing and geometric parameters on the replication quality of themicro-features were investigated. The microstuctures and properties of the MIM, MICM andconventional injecteion molded (CIM) PP parts were comparatively studied.The processing and geometric parameters had more obvious effects on the replication ofthe micro-features. Under the same processing parameters, the replicated capability of themicro-features in MICM was better than that in the MIM. In the MIM, the first to the thirdmicro-features along the flow path were mainly filled in the injection stage, their aspect ratios(ARs) were approximately linearly decreased, whereas the last micro-feature was mainlyfilled in the packing stage and its AR was the highest. The micro-features in a thinner moldcavity could be filled more completely.Compared with the CIM PP parts with a3.5mm thickness, the MIM and MICM PPparts exhibited increases of67%and38%in the tensile strengths, and increases of48%and27%in the storage modulus (at40℃), respectively. This was attributed to highercrystallinities (76.7%and70.8%) and shear layer thickness percentages (57.8%and44.1%)and the presence of high contents of the shish-kebab structures in shear layer in the middleregion (position P2) of the MIM and MICM PP parts.The HNTs had the β-nucleating ability for the PP. The MIM and MICM PP/HNTscomposite parts exhibited the highest contents of β-crystal (26.3%and23.5%) upon theaddition of2phr HNTs. The tensile strengths of the MIM PP/HNTs parts were obviouslyhigher than those of the MICM PP/HNTs parts, which was due to higher degrees of themolecular orientation in the former parts. Despite higher content of β-crystal in the MIMPP/HNTs parts, their elongations at break were evidently lower than those of the MICMPP/HNTs parts, because the effect of toughness decrease caused by molecular orientation wasmore significant than the toughening effect of the β-crystal. Adding a5phr HNTs yielded thehighest tensile strengths of both MIM and MICM PP/HNTs parts, owing to the fact that the shear layer thickness percentages in position P2of the molded parts were the highest.Furthermore, the introduction of the HNTs improved the thermal stability of both MIM andMICM PP/HNTs parts. The content of the HNTs showed an obvious influence on the thermalstability of the molded parts.