Ultrasound is a kind of elastic mechanical wave in material medium with frequency higher than 20kHz. Since 1980s, with the popularity of power ultrasonic equipment, ultrasound has been widely used in various industries.
The physical basis of ultrasonic machining and processing technology is the four basic functions of ultrasound.
1) the action of linear alternating vibration is to make use of ultrasonic wave propagation in the media, make the media particles make alternating vibration, and cause the periodic changes of stress or sound pressure in the media, thus causing a series of secondary reactions.
2) when the large amplitude vibration is propagated in the media, the periodic shock wave of the sawtooth wave surface is formed, which causes a large pressure gradient on the wave surface, thus producing a series of special effects, such as the local high temperature and high pressure.
3) the nonlinearity of the vibration causes the close proximity of the cypress, which is due to the average viscosity of the direct current caused by the periodic changes of viscosity. These direct forces can explain some mechanical effects, such as the directing action, the condensation, and so on.
4) cavitation, which is an important basic function of ultrasound in liquid media. In the sound field, the bubble of the liquid may be gradually generated and expanded and then suddenly disillusioned. In the rapid collapse of the bubble, high temperature and high pressure appears in the bubble, and a local intense shock is formed in the liquid near the bubble, thus producing a series of secondary effects.
The above four basic functions lead to five effects: mechanical effect, thermal effect, optical effect, electrical effect and chemical effect. At present, ultrasound is widely used in various industries, and it is precisely because of its five effects. With the infiltration of various disciplines, the research and application of ultrasound in polymer industry is becoming more and more extensive.
Research status of ultrasonic in the field of polymer processing
At present, scholars at home and abroad have done many exploratory researches on ultrasonic processing in polymer processing such as extrusion molding, micro hot pressing and ultrasonic plasticization.
In the application of ultrasonic to extrusion molding, ultrasonic has studied the properties of polymer melts in depth. The State Key Laboratory of materials engineering, Sichuan University, has carried out in-depth research on ultrasonic polymer degradation and polymer blending extrusion. In the process of ultrasonic degradation, ultrasonic can reduce the molecular weight of the polymer melt, increase the crystallization temperature, reduce the apparent viscosity of the melt and lead to the decrease of the tensile strength. In the process of ultrasonic polymer blending extrusion, ultrasonic can reduce the size of the mixed particles and make the distribution more uniform, and improve the machine of the blends. The mechanical properties can also influence the growth behavior of the nucleation, increase the crystallization temperature and crystallinity, and reduce the die pressure and the apparent viscosity of the blends, and improve the flow performance.
In the application of ultrasonic to micro hot pressing, the HarutakaMekaru of Japan is used to press the surface of the plastic workpiece with the ultrasonic tool head with microstructural kernel to press the surface of the plastic workpiece to press the micro structure. It is found that the ultrasonic vibration heating and embossing micro structure has excellent appearance in improving the flow performance of the polymer. The microstructure replication experiment shows that the ultrasonic vibration heating imprint lithography products have good accuracy.
In ultrasonic plasticization, the plasticizing quality of polymer melt is still in its infancy. The IKV Institute of the University of Aachen, Germany, introduced ultrasonic wave to the microplasticizing process of polymer, improved the plasticization quality of the polymer and obtained good homogeneity. The Hubei Institute of technology introduced ultrasonic, mechanical vibration and high shear micro grinding to the plasticizing process of the polymer, and found the ultrasonic wave at a certain frequency. The crystallization of polyolefin plays a positive role, and the plasticizing effect of polymer is good.
Research progress of ultrasonic application in microinjection molding
The micro injection molding technology for the weight of parts with milligrams or micrometers in micrometers was started in 1980s. In order to easily realize the advantages of low cost and large-scale commercial production, it has gradually become one of the key technologies for the popularization and application of MEMS technology. As a new technology, micro injection molding technology faces many challenges such as micro cavity filling and uneven microstructure of micro products. In order to solve the difficulties encountered in the micro injection molding technology, the Institute of mould technology of Central South University has done a lot of research on the application of ultrasonic to micro injection molding.
Study on plasticizing model and plasticizing process of polymer
The plasticizing quality of polymers is an important factor affecting microinjection molding. It is necessary to study the process of melting and plasticization of polymers under ultrasonic external field. The influence of ultrasonic vibration force field on the viscoelastic properties and apparent viscosity of the polymer and the effect of ultrasonic cavitation field on the melting and plasticizing rate of polymer are analyzed. On this basis, the ultrasonic melting plasticizing physics of polymer is established. Model。
Through the theoretical analysis and Research on the process of polymer melting and plasticization under the ultrasonic external field, the research group has developed the polymer ultrasonic plasticizing and rheological testing device, and has carried on the related ultrasonic melting plasticization experiment. The experimental results show that ultrasonic can melt the polymer and prove the melting process of polymer under ultrasonic wave. During the process of ultrasonic melting and plasticization of the polymer, the solid particles in the end of the tool head and the surrounding polymer are fused with each other under the action of ultrasonic vibration. With the continuation of the melting process, the solid particles near the end of the tool head are all fused and formed a circular columnar pool, in ultrasonic vibration and plasticizing pressure. Under the dual action of force, the molten pool extends around the tool head and extends to the periphery until the plastic particles melt.
Study on the essential mechanism of ultrasonic plasticization
The theoretical analysis and simulation are used to study the essential mechanism of the polymer ultrasonic melting and plasticization. The results show that there are three thermal effects in the process of ultrasonic melting and plasticization, namely, friction generation, viscoelastic heat and ultrasonic cavitation. The heat effect of the friction is proportional to the amplitude of ultrasonic sound pressure, the frequency of ultrasonic wave and the friction coefficient of the polymer; the viscoelastic thermal effect is proportional to the phase angle of stress, strain, stress and strain, and the frequency of ultrasonic wave; the ultrasonic cavitation effect is at a lower ultrasonic frequency (20KHz) and a certain ultrasonic sound pressure amplitude (1.4 MPa), lower plasticizing pressure (1.25MPa) and suitable polymer melt kinematic viscosity (1.2685m2/s) are more intense.
Influence of ultrasonic plasticization on polymer micromorphology
The experimental material is high density polyethylene (HDPE). High density polyethylene (HDPE) tends to produce spherulites in the absence of stress or flow when the melt is cooled and crystallized. From Figure 3, it can be seen that there is no stress or flow in the natural cooling process of the two plasticized materials, so they all produce spherulites, but the size of the two Spherulites has a large gap. The spherulites formed by heating, melting and plasticizing are relatively large, and the diameter is about 500. The spherulites formed by ultrasonic plasticization are relatively small, and the diameter is only about 200. Therefore, the polymer samples obtained by ultrasonic melting plasticization are smaller and more uniform, and the microstructure of the polymer is more uniform than that of the melt plasticized polymer, and the ultrasonic plasticizing effect is better than that of the heating and melting plasticization.
Study on energy consumption of ultrasonic plasticization
Energy conservation is the development direction of modern technology, and energy consumption has become an important indicator of new technology.
The plasticizing rate, unit energy consumption of ultrasonic plasticization and the comparison with the lab imported Arburg370S in Germany are shown in Table 1. It can be seen that the energy consumption of ultrasonic melting plasticization and Arburg370S injection molding machine is basically the same, and ultrasonic plasticization as a new technology, there is more room for improvement in energy consumption. Moreover, the melt melted polymer melt has a more uniform microstructure.
From the comparison of the ultrasonic melting and plasticizing mode, the heating melting plasticizing mode and the energy consumption of the Arburg370S injection molding machine, it can be seen that the ultrasonic melting and plasticizing method not only has great potential for energy saving, but also has a great advantage in the plasticizing effect.
Development of ultrasonic plasticizing micro injection molding machine
Because of the advantages of ultrasonic plasticization to improve the crystalline form of polymer, reduce the apparent viscosity and energy saving of the melt, the development of the ultrasonic plasticizing micro injection molding machine is one of the important directions in the development of microinjection molding technology.
Based on the analysis of the plasticizing structure of the screw type screw injection molding machine, the Institute of mould technology of Central South University has designed and developed the ultrasonic plasticizing micro injection molding machine with reference to the latest micro injection molding machine at home and abroad.
(1) the plasticizing part of the ultrasonic plasticizing micro injection molding machine is transmitted to the polymer melt through the vibration of the ultrasonic tool head, and the plasticizing work is accomplished through the action of ultrasonic.
(2) the amount of single plasticization and injection volume are relatively small (<6cm3), so the control of injection accuracy, that is, the requirement of precision measurement is higher.
(3) adopt all electric drive system, that is, servo motor drives ball screws to achieve precise metering in micro injection molding.
(4) a plurality of driving motors, so that each action is relatively independent, realizing the compound action of some actions and shortening the forming cycle;
(5) due to the short injection stroke and high injection speed, the driving unit of the ultrasonic plasticizing micro injection molding machine has a very fast reaction speed to ensure that the equipment can reach the required injection pressure.
In the process of micro injection molding, the flow channel is shallow and the mold cavity is small. It is difficult to fill the plastic melt. The plasticization quality of the polymer melt has a significant influence on the molding process and the quality of the products.
The application of ultrasonic in polymer plasticizing can improve the crystalline form of polymer, reduce the apparent viscosity and energy saving of the melt. It will make the ultrasonic plastic micro injection molding technology become the important development direction of microinjection molding.
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