Molecular Dynamics Simulation And Experimental Study On The Transient Heat Generation Mechanism Of Polymer Ultrasonic Plasticization

With the market demand for micro-nano parts increasing year by year,ultrasonic plasticization micro injection molding technology has emerged in the field of micro-injection molding with its advantages of low power consumption and high efficiency,and is expected to become a new breakthrough in the manufacture of small parts.Thanks to the high-frequency mechanical vibration energy of the ultrasonic tool head,the polymer material can be melted from solid to liquid in a very short time.Under this process,the filling environment in the microstructure is further improved,so that the melt can complete higher-quality injection and pressure-holding actions at lower process parameter levels.The ultrasonic plasticizing stage is the starting point of the process,and the regulation of the process directly affects the subsequent injection and filling behavior and the quality of the parts.This paper mainly focuses on the heat generation process in the ultrasonic plasticizing process,using a combination of experiments and molecular dynamics simulations to clarify the heat generation mechanism of the ultrasonic plasticizing process and provide guidance for the subsequent regulation of ultrasonic plasticizing process parameters.The main research work and innovation points of this paper are as follows:1.Aiming at the problem of microscopic viscoelastic heat generation in the process of polymer ultrasonic plasticization,the united-atom model of amorphous polyethylene and the metal layer model of the sonotrode were constructed by the MOLTEMPLATE software,and the molecular dynamics simulation of the polymer by the high frequency hammering of the sonotrode was carried out using the LAMMPS software.The temperature distribution of amorphous polyethylene under high frequency vibration,the change law of molecular structure parameters and the evolution of entanglement degree in the system were calculated,and the internal viscoelastic heat generation mechanism of the polymer during ultrasonic plasticization is revealed.The results show that the temperature distribution of amorphous polyethylene in the height direction is mainly affected by the plunger pressure and ultrasonic amplitude.When the amplitude level is increased,the heat generation of the tool head end is more obvious,and the increase of the plunger pressure will inhibit the heat generation of the tool head end.The increase of amplitude and frequency can significantly improve the degree of entanglement of the system.The coefficient of entanglement decreases from 0.573 to 0.215 and 0.35,respectively,which is not positively correlated with the plunger pressure.Excessive pressure will continue to compress the molecular chain,resulting in a higher degree of tangles.2.Aiming at the problem of micro frictional heat generation in the process of polymer ultrasonic plasticization,molecular dynamics method was used to simulate two friction processes,single sliding friction and reciprocating sliding friction between polymers,corresponding to the high frequency reciprocating friction behaviors of unidirectional shearing and ultrasonic plasticization in the traditional microinjection process,respectively.The variation laws of temperature,energy,mean square displacement and molecular configuration of amorphous polyethylene system under different sliding modes are calculated,and the microscopic heat generation mechanism of interfacial friction during ultrasonic plasticization is revealed.The results show that compared with the two-way reciprocating friction behavior,the one-way sliding friction process is more likely to cause large deformation,the molecular chain has higher mobility,and the magnitude of the mean square displacement is 10~4 times.Different from the higher dihedral angle conformational transition(14%)in the single sliding process,the dihedral angle energy is stable in the high-frequency reciprocating sliding friction process,and has a smaller conversion rate(<2%),and the friction interface has a high temperature.The high frequency local molecular rearrangement is the main reason for the efficient heat generation by high frequency reciprocating friction.3.For macroscopic heat generation in the process of polymer ultrasonic plasticization,the heat generation process of high-density polyethylene rod and sphere under the hammering action of ultrasonic tool head was studied by infrared thermal imaging equipment.The temperature rise curves,temperature rise rate curves and corresponding power and energy changes of different partitions of polymer rods and particles were analyzed,revealing the heat generation mechanism of polymers under the action of ultrasound.The result shows that,when the pressure is 1MPa,the heat generation of the end face of the tool head is the most obvious.When the pressure exceeds 2MPa,the heat generation effect of the end face of the tool head will be suppressed.The transient heat generation rate of material plasticization can reach 1118.00℃/s.Different from rods,polymer spheres have multiple interfaces and smaller contact areas during ultrasonic vibration,and the spheres can be plasticized under the benchmark process parameters of 20KHz-15μm-2MPa.