| The final properties of injection molded polymeric parts are to a great extent affected by the microstructure, and the processing conditions, such as pressure, shear and temperature, determine the microstructure. The effect of processing conditions on microstructure (or named morphology) and mechanical properties of injection molded part is an extremely hot topic in both academia and industry. In this thesis, the microstructures of injection molded PP and PP/POE blend are studied by differential scanning calorimeter (DSC), scanning electron microscope (SEM), wide angle X-ray diffraction (WAXD) and polarizing optical microscope (POM) at various processing conditions. The microstructure includes the final distribution of crystallinity, crystal grain orientation, and dispersed phase morphology. The relationships among processing conditions, microstructure and mechanical properties are systematically studied. Finally, the effect of processing conditions on the shrinkage and warpage of injection molded PP was studied, and the affection mechanism was analyzed from a microscopic point of view. The main research work and conclusions are induced as follows.(1) Studies on the PP/POE blends were carry out with SEM to get the POE content distribution from skin to center of the part, with DSC to get the melt enthalpy of every layer. The final absolute crystallinity distribution were get after subtracting the POE content from the total enthalpy of sliced samples. The results indicated that the crystallinity of PP decreases with increase of holding pressure. On the other hand, hydrostatic pressure increases the equilibrium melting temperature, thereby increases the degree of supercooling and accelerates the crystallization rate. Therefore, the effects of pressure on the crystallization kinetics and final absolute crystallinity are different. The highest final crystallinity were got at slow injection speed and at low melt temperature, because the shear induced precursors can exist for a long time just above the equilibrium melting temperature.(2) The crystal form distribution and grain orientation of PP/POE blend were studied by WAXD. The peak ofβ-form PP was observed only in the skin layer. The results indicated thatα-form PP dominates in PP/POE blend except in the skin layer. The shear layer shows the highest degree of orientation because there is shish-kebab structure in shear layer.(3) The disperse phase morphology and the effect of processing conditions on it were studied by SEM. It was found that the POE phase does not exist at the skin layer of the sample (about 0.01 mm) at a variety of process conditions, and below the surface POE content increases rapidly, reaching a stabilized platform. It was also observed in brittle fracture cross-section that two-phase transition layer has been pulled with obvious lips at shear layer of the specimen, indicating that the compatibility of PP/POE increases because of the shearing of the filling process, which enhances the bonding strength between the two phases.(4) The effect of process conditions on the thickness change of the skin layer was also studied. It was observed that the thickness of the skin layer in POM decreases with increasing the melt temperature, while gets maximum when the injection rate is medium. Holding pressure had no effect on the skin layer thickness. With the combination of POM and WAXD results, we proposed that the skin layer, about 0.1mm of thickness, actually contains a lot ofβ-crystal. Because theβ-PP andα-PP have different crystal rotation angle, the skin layer in the POM shows significant difference.(5) The tensile properties, impact toughness, and dynamic mechanical properties were studied at various conditions of injection molding process. It was found that at low injection rate the tensile specimen has the best performance for both tensile strength and impact toughness; at low melt temperature, although the tensile properties of the sample is better, but its impact toughness is reduced. Combined with analysis of microstructure, it was found that this is because at low injection rate the crystallinity increases, and the residual stress of flow becomes smaller.(6) The effect of processing conditions on shrinkage and warpage of injection molded PP products were studied. Through a simple and unique designed measurement method, the shrinkage, steady warpage and unsteady warpage (buckling) of box-shaped PP products were separately measured. It was found that processing conditions have different effects on the shrinkage and warpage. The inherent mechanism of these effects has been studied from the view point of microstructure. Numerical experiments were employed to identify the reasons of unsteady warpage. Results showed that unsteady warpage may be mainly caused by buckling due to residual stress. Meantime, comparison of simulation and experimental results verifies the applicability of the linear buckling analysis method in injection molding.
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