| PA,as one of the five engineering plastics,has excellent comprehensive properties.After decades of development since the birth of the last century,PA has become an indispensable material in automobile,transportation and military industry.However,PA has a high melting point and is extremely sensitive to the molding temperature,which limits its further application in related fields.The development of ultrasonic technology in the field of injection molding makes it possible to improve the molding performance of PA parts.However,the mechanism of ultrasonic energy on the crystallization behavior of melt condensation is still unclear,and the effect of the interaction between ultrasonic parameters and process parameters on the microstructure and macroscopic properties of the parts needs to be further studied.Aiming at the above problems,this paper mainly does the following work:Firstly,the orthogonal test of process parameter optimization was carried out on the selected PA66 material,to explore the influence of melt temperature,mold temperature,injection pressure and injection speed on the tensile strength and crystallinity of PA66 parts,and to find out the optimal combination of process parameters.The orthogonal test results were processed by various analysis methods,and the results showed that the melt temperature had the greatest influence on the crystallinity of PA66 sample,followed by the mold temperature and injection pressure,and the injection speed had the least obvious influence on the results.Melt temperature was also the biggest influence on the tensile strength of the sample,followed by injection pressure mold temperature and injection speed.By analyzing the influence of various process parameters on the crystallization behavior and mechanical performance of samples,the optimal combination of molding process parameters was determined: melt temperature 265 ?,mold temperature 60 ?,injection pressure 100 MPa,injection speed 80 mm/s.Secondly,the single-factor injection molding experiment of ultrasonic vibration power was carried out by using the ultrasonic assisted mold under the combination of the above process parameters.The direction of ultrasonic vibration in the mold was perpendicular to the direction of melt flow.The macroscopic mechanics and microstructure of the sample were analyzed by means of X-ray diffraction polarizing microscope and tensile test.The results show that: when the ultrasonic vibration power is 120 W,the low frequency vibration will promote the crystal core generation and the crystal structure perfection,the crystallinity of the parts will increase,and the mechanical properties will be improved.When the power of ultrasonic vibration exceeds 120 W,the increase of amplitude increases the impact on the melt,resulting in the inability of the molecular chain to discharge stably into the crystal lattice fromall directions,resulting in a decrease in the perfection and mechanical properties of the crystal.Finally,the single factor molding experiment with melt temperature,mold temperature and injection pressure as variables was completed under the ultrasonic vibration with fixed power to explore the influence of the interaction between ultrasonic vibration and various process parameters on the quality of the parts.The results of tensile test,X-ray diffraction and polarizing microscope showed that the ultrasonic vibration of 600 W combined with the higher melt temperature and lower injection pressure had an effect on the crystallinity of the molded sample,while the interaction with the mold temperature had no obvious effect on the mechanical properties of the sample.In addition,due to the destruction of the crystal structure in the melt caused by the high-frequency mechanical impact caused by the ultrasonic vibration of 600 W,the tensile strength of the samples of each group formed under the ultrasonic external field decreased to different degrees compared with the control group without ultrasound. |
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