Research On The Key Technologies Of On-line Ultrasonic Measurement Of Polymer Melt Temperature And Orientation During The Injection Molding Process

Injection molding process is regarded as the most important process used to manufacture highquality polymer products,however,the injection molding process is a complex and dynamic batch process,and the polymer melts undergo drastic variations of temperature,pressure and shear-rate in the cavity.Parameters such as melt temperature and orientation during injection molding process dramatically affect the morphology quality and mechanical properties of the molded products.It is of great significance to measure the macro / mesoscopic information of polymer melts during injection molding process to improve product quality and performance,and improve yield.Ultrasonic detection technology has the advantages of non-destruction,low cost and convenient installation.This paper will carry out the research on the key technologies of on-line ultrasonic measurement of melt temperature and orientation during the injection molding process.The main research contents of this paper are as follows:(1)An on-line ultrasonic measurement experimental device for non-isothermal and high-shear rate injection molding process was established.The cavity structure of the non-isothermal and highshear rate mold is designed as a slit with a thickness of 1 mm or 2 mm,and the bottom is un-sealed to maintain the continuous flow of the melt.The pressure and temperature sensors as well as the ultrasonic probe are amounted on the center line along the flow direction to detect the real-time evolution information of the polymer melt in the mold cavity.A mold temperature machine was ulilized to control the mold temperature.PXIe acquisition system was ultilized to collect,display and record the experimental data.The experimental results show that the established experimental device can accurately obtain the melt information of LDPE in injection molding process,and measure the apparent viscosity of melts with errors less than 18% at different shear rates and tempertures.This device can meet the requirements for on-line ultrasonic measurement of polymer melt information during actual injection molding process.(2)A new on-line ultrasonic measurement method for polymer melt temperatures in the injection molding process was proposed.The correlation model of ultrasonic signals and different process conditions such as injection time,pressure,speed,and temperature was investigated.The results show that the ultrasonic velocity can real-time reflect the changes of process parameters in the injection molding process,and can effectively characterize the evolution of physical quantities such as melt pressure and temperature during the process.The ultrasonic velocity equation and polymer PVT equation was Derived to construct the correlation model of melt temperature,ultrasonic velocity and pressure,and realized the on-line measurement of melt temperature during the injection molding process.The measurement results of melt temperature from different methods were compared.The results showed that the methods proposed in this paper are closer to the measurement results of infrared fiber optic temperature sensor than thermocouples and other methods,and the errors are less than 10%.Moreover,the cost of this method is only 1/5 of the cost of infrared fiber optic temperature sensor,and the method has the advantage of non-destruction,and has an enormous potential in the field of on-line measurement during the injection molding process.(3)Established an on-line polymer orientation measurement method in injection molding process via ultrasonic longitudinal wave.It was found that the degree of polymer orientation affects the propagation velocity of ultrasonic longitude waves.Based on the ultrasonic wave equation and Boltzmann superposition principle,a mathematical model was established between the orientation degree of polymer materials and the velocity of longitudinal wave,and a method for measuring the orientation degree of polymer molecular chains by ultrasound was proposed.The dichromic FTIR spectroscopy method was used to measure the molecular chain orientation degree of the polymer,and the model was verified with the online measurement results of the longitudinal wave velocity,and the fitting degree reached 0.956.The method was successfully applied to the online characterization of fiber orientation in the injection molding process of fiber composite polymer.The research results obtained in this paper provide a new method for the on-line detection of the injection molding process,and realize the online measurement of the macroscopic temperature evolution and mesoscopic orientation of the polymer melt,which is of great significance to the optimization and on-line diagnosis in the injection molding process.With the advantages of low cost and non-destruction,the method proposed in this paper shows a great prospect in the injection molding industry.