Simulation And Experiment On Crystallization And Orientation Of Isotactic Polypropylene Micro Injection Molded Parts

Micro injection molding can mold complex,size accurate thermoplastic polymer micro products at one process and it is one of the most effective processes for mass production.Due to its small size,melt undergoes a complex shear and thermal history in the filling stage,which affects the crystal structure and orientation in the thickness direction of the part.In this paper,isotactic polypropylene was used as raw material.Under different molding process,the relative crystallinity of the part,the crystal orientation factor in the thickness direction and the crystal size were simulated and analyzed.The main contents and conclusions of this paper are as follows:1.Simulation of the crystallization morphology of micro injection molded parts.Moldflow software was used to simulate the average relative crystallinity,crystal orientation factor and final average crystal size of the products under different processing conditions.The results showed that the higher mold and melt temperature,the slower crystallization rate was.The average relative crystallinity has reached maximum value before the end of the cooling.The average crystal orientation factor and final average crystal size are greatly affected by the thickness of the part,the thicker the part,the larger the crystal size was and smaller the value of the crystal orientation factor was.In addition,the mold temperature had a greater effect on the final average grain size than the melt temperature and the injection speed.2.The investigation of the influence of process parameters on crystallinity and orientation degree in thickness direction.According to the simulated process conditions,micro infrared equipment was used to detect the spectra parallel to the flow direction and perpendicular to the flow direction at different position and orientation of the molecular chain in the crystal region and amorphous region,average value of the molecular chain orientation and crystallinity were calculated by the formula.Results showed that molecular chain oriented mainly in the crystalline region,and orientation degree has a regular trend along the thickness direction,the average crystal orientation factor is mainly affected by the thickness of the parts.Crystallinity value showed a completely opposite trend in the thickness direction for samples of 1.0mm and 0.2 mm.It showed that near the surface had lower crystallinity value,and closed to the center of the parts the crystallinity was higher for 1.0mm part.However,for the 0.2mm parts,crystallinity close to the surface of the part reached relatively high value and smaller in the center.3.Effect of processing parameters on crystallization morphology and crystal type of the micro injection molded parts.The crystal type and size of the parts were studied by wide angle X-ray diffraction.For the 1.0mm and 0.5mm parts,the crystal type was mainly ? crystal,and ? crystal appeared under special process conditions.For the thickness of 0.2mm,no matter under any mold temperature and injection speed,? crystal will always appeare,but the content of ? crystals changes very little.The results indicated that the smaller the size,the more beneficial to the formation of ? crystal.Crystal size was affected by the mold temperature significantly,the higher the mold temperature,the larger the crystal size was.