Working principle of plastic ultrasonic welding machine
The concept and classification of sound waves
Waves are caused by disturbances starting at a certain point, and are propagated or transmitted to other points in a predetermined manner. Acoustic wave is the propagation of vibration energy in elastic medium, a kind of elastic mechanical wave. In a fluid or solid medium, when the mass point deviates from its equilibrium position, it will cause the elastic restoring force inside the medium. This elastic restoring force is coupled with the inertia of the system, so that the vibration of the medium mass point is continuously transmitted to the adjacent mass points, thereby generating and Emit sound waves. Acoustic waves are an elastic mechanical wave. The frequency range of sound waves that the human ear can hear is usually between 20Hz and 20KHz, called audible sound. Sound waves with a frequency between 2×104Hz-2×109Hz are called supersonic waves, and sound waves with a frequency lower than 20Hz are called infrasonic waves. (Infrasonic wave). These inaudible sounds have a significant impact on human life, and they also have a wide range of applications and development prospects.
| Frequency/ Hz | Features | |
| Infrasound | 〈20 | Human ears can’t hear, the transmission attenuation is very small, and the transmission distance is very long. |
| Audible sound | 20-2×104Hz | Can be heard by the human ear |
| Ultrasound | 2×104Hz-2×109Hz | The propagation frequency is high, the propagation direction is strong, the medium vibration is strong, and the propagation in the fluid can cause cavitation. |
| Ultrasound | 2×109Hz-2×1012Hz | The propagation attenuation is large, the wavelength is short, and the frequency band roughly corresponds to the microwave. |
Ultrasound is different from ordinary sound waves in the following characteristics: high frequency, short wavelength, large energy, reflection, refraction, resonance, and loss of energy during the propagation process.
Principle and process of ultrasonic plastic welding
Ultrasonic plastic welding (ultrasonic welding) is a non-contact welding method. The principle of ultrasonic welding is to convert 50/60 Hz current into 15, 20, 30 or 40 KHz high frequency electric energy through an ultrasonic generator, and the converted electric energy It is converted into the longitudinal mechanical vibration of the same frequency by the energy conversion device, and then this change is transmitted to the welding head through the amplitude adjustment device, and the welding head transfers the received vibration energy to the joint of the welding original. The area with large acoustic resistance at the interface uses friction to convert vibration energy into heat energy to generate local high temperature. The heat is concentrated in the middle of the welding material. Due to the poor thermal conductivity of the plastic, it cannot be dissipated in time, causing the plastic contact surface to melt rapidly. Next, make the bonding surfaces fuse together. Ultrasonic welding can be used in the bonding process of most engineering plastics, becoming one of the important technical resources of the bonding process, and has a wide range of application prospects. The welding strength can be close to the strength of the raw material, and the material characteristics are not directly affected or changed. The welding method has the characteristics of maintaining the plasticity of the material, and has little effect on the elasticity and mechanical strength of the bonded parts. It meets the technical requirements of engineering bonding. .
Figure 1 Structure diagram of the working principle of the ultrasonic welding machine
Ultrasonic welding process:
The power supply activates a trigger control signal pneumatic transmission system, the cylinder pressurizes the welding head to drop and presses the weldment to trigger the ultrasonic generator to work, emit ultrasound and maintain a certain welding time, remove the ultrasonic emission, continue to maintain a certain pressure for a certain time, depressurize, and the welding head Pick-up, welding is over.
Figure 2 The 4 stages of the ultrasonic welding process
In the first stage, the welding head is in contact with the part, exerts pressure and starts to vibrate. The heat of friction melts the energy-conducting ribs, and the melt flows into the joint surface. As the distance between the two parts decreases, the welding displacement (the decrease in the distance between the two parts due to the flow of melt) begins to increase. At first, the welding displacement increases rapidly, and then it slows down when the melted energy conduction bar spreads and touches the surface of the lower part. In the solid state friction stage, heating is caused by the friction energy between the two surfaces and the internal friction in the parts. Friction heating causes the polymer material to heat up to its melting point. The calorific value depends on the frequency of action, amplitude and pressure;
The increase in the melting speed in the second stage leads to an increase in the welding displacement and the contact between the surfaces of the two parts. At this stage, a thin molten layer is formed, and the thickness of the molten layer increases due to continuous heating. The heat at this stage is generated by viscous dissipation;
In the third stage, the thickness of the solution layer in the weld remains unchanged and with constant temperature distribution, steady state melting occurs;
In the fourth stage, after the set time has passed or the specific energy, power level or distance is reached, the power supply is cut off, the ultrasonic vibration stops, and the fourth stage begins. The pressure is maintained, so that part of the extra melt is squeezed out of the bonding surface. The maximum displacement is reached when the weld is cooled and solidified, and intermolecular diffusion occurs.
Ultrasonic welding technology is widely used in various fields, and the technology is gradually mature in continuous practice. In the process of ultrasonic plastic welding, the selection of equipment model, the selection of solder and the design of welding flux will affect the welding quality. Therefore, before purchasing ultrasonic plastic welding equipment, the relevant control should be carried out to improve the quality of plastic welding.
