Analysis of morphology and performance of PP microstructures manufactured by micro injection molding

In this paper, PP (polypropylene) microstructures were manufactured by micro injection molding (MIM). The surface topography and internal defect under different process conditions were studied. An internal defect named “hollow” was observed in microstructures made without vacuum. To investigate the morphology (crystal and phase), the microstructures samples were cut to slices with 10 μm thickness along the filling direction. Results of polarized light microscopic observation reveal that these microstructures also represent “skin-core” morphology, i.e. a highly oriented non-crystalline skin layer, a shear zone with column crystal essentially parallel to the injection direction and a spherulites core. However the morphology distribution of microstructures is different from the macroscopic structure: the non-crystalline layer is much thinner, the ratio of skin layer (non-crystalline and column crystal layer) to core thickness is very big, there is no change of spherulites dimension from skin to center. So the microstructures must have a special mechanical performance differ from the macroscopic parts.