Thermodynamic and shear effects of ultrasonic vibration on the flow‐induced crystallization of polypropylene

Abstract

AbstractIn order to achieve optimal mechanical properties in injection‐molded parts, continuous ultrasonic vibration is applied to a mold cavity during the filling and solidification stages of injection molding. In this work, the effect of ultrasonic thermodynamics and ultrasonic shear on the condensed structure of the parts are studied. Thermocouple measurement results show that the increase in the temperature of the mold cavity surface caused by ultrasonic thermodynamics can reach up to 15°C; Molecular dynamics simulation results show that the molecular chain morphology under ultrasonic shear is more stretched during the melt flow process. Microscopic results show that with increasing ultrasonic power and ultrasonic time, the ultrasonic thermodynamic effect causes the thickness of the shear layer to increase. With increaseing ultrasonic power, the lamella thickness of the core layer increases, while the crystal size in the transition layer shows a trend of first decreasing and then increasing. The ultrasonic shear causes the overall crystallinity and β crystal content of the parts to increase with increaseing the ultrasonic power. The crystallinity starts to decrease when the ultrasonic vibration is still applied at a melt temperature below 67°C during cooling. When the ultrasonic power is increased to 1000 W and the ultrasonic vibration time is applied for 30 s, β spherulites transform to β‐transcrystalline. The yield stress results show that when the ultrasonic vibration time is 0–20 s, the yield stress of the parts increases with the ultrasonic power; while when the ultrasonic time is increased to 30 s, the yield stress decreases.