Energy Conversion Mechanism of Plastic Ultrasonic Welding
Definition of ultrasound:
Ultrasound refers to sound waves above the frequency range that human hearing can accept. Human hearing ability is between 20Hz and 20KHz; therefore, generally speaking, all sound waves with a frequency higher than 20KHz are called ultrasonic waves. However, in industrial applications, in order to meet the special process requirements, the sound wave frequency is lower than 10KHz (such as ultrasonic dryer), 15KHz (such as plastic ultrasonic welding machine), these are also called ultrasonic waves in the habit. The upper limit of ultrasound frequency is usually within 100KHz, but in non-destructive medical examination systems, there are also up to 5000KHz. All in all, the industry usually takes the energy to be generated and used in various media to conduct, and the fluctuations of various functions and effects are generated with the characteristic differences of various media, which are collectively referred to as ultrasonic waves.
The principle of ultrasonic welding:
The sound wave is transmitted to the plastic processing object by the ultrasonic welding head (HORN), and the vibration is 15 ~ 30KHZ per second, and the amplitude is about 4 ~ 5um. High-frequency vibration of about 20-30um occurs on the joint surface of the plastic part. In this way, the maximum instantaneous speed at the end of the welding head of the metal tool can reach 4m/s. This high-frequency pounding acts on the surface of the plastic, and strong frictional heat occurs in the place where the stress is high, so that the molten plastic at the interface is bonded together. , So as to achieve welding.
The mechanism of ultrasonic welding energy conversion:
In the ultrasonic welding process, the energy conversion at the joint mainly refers to the conversion of the mechanical energy of the ultrasonic into the thermal energy to fuse the plastic. This energy change directly affects the temperature change at the joint surface and affects the quality of the joint. Due to the complexity of the ultrasonic welding process, the research on the welding process is still inadequate, and there are many explanations for its welding mechanism.
At present, there are mainly the following two views on the fusion mechanism of plastic ultrasonic welding:
1) Friction vibration mechanism. It is believed that when ultrasonic waves act on two weldments, the plastic particles will be excited by the ultrasonic waves to cause continuous alternating compression and decompression, so that the contact surface of the weldment will generate friction due to vibration. The vibration frequency is the frequency of the ultrasonic waves. At this time, the mechanical work is converted into heat, which increases the temperature of the weldment surface until it melts, and finally a welded joint is formed.
2) Stress-strain energy storage and conversion mechanism. It is considered that plastic is a viscoelastic body. When ultrasonic waves propagate in it, the stress and strain of the plastic particles are different, which causes energy loss, which is converted into heat to melt the surface of the weldment to form a joint.