Study On Effects Of Processing Conditions On Microstructure And Mechanical Property Of Injection Molded I-PP

During the injection molding cycle, the polymer undergoes high shear and normal stresses, and temperature gradient influences that are introduced by the interaction between polymer properties, the geometry of melt delivery system and cavity, and molding conditions. The semicrystalline polymers also experience the changes of microstructure, such as crystallization, orientation, etc. It is well known that the ultimate physical, optical and mechanical properties of injection moldings are closely associated with their embedded microstructures. So, it is important for the study of processing variables on the microstructure and the physical properties.In this paper, the influence on the crystallization, orientation and mechanical properties of i-PP by changing processing variables, such as melt temperature, injection time (injection pressure and injection speed), holding pressure, was studied. The main works are as follows:1. First of all, based on the study of crystallization and orientation mechanism, we design the injection molding experiments, including mold designing, selecting processing variables. The process is simulated by CAE software in order to assure one dimension flow in the mold.2. Some techniques, such as polarized optical microscope (PLM), scanning electron microscope (SEM), wide X-ray diffraction (WAXD), infrared spectrum (IR), are used in analyzing the morphology of the injection molded i-PP. The results are as follows: with the melt temperature increasing, the degree of orientation is decreased, crystallinity and the content ofβ-form are decreased, in the skin-core structure, the skin layer is thinner and the spherulite size in the core region is bigger. Prolonging of the injection time result in skin layer thickness increasing, spherulit size decreasing, crystallinity decreasing and orientation increasing. There exists a small influence of the influences of holding pressure on the microstructure.3. Owing to the effects of the processing conditions on the microstructure, the mechanical properties of injection molded i-PP are changed with the processing variables. From the tensile tests, the results are as follows: performed in and perpendicular to flow direction, yield stress is found to decreased with the higher melt temperature and shorter injection time; elongation at break is increased with the higher melt temperature and shorter injection time; holding pressure has a little effects on the mechanical properties.