1. Characteristics of Ultrasonic

From the perspective of ultrasonic welding, ultrasonic waves have three very important characteristics that are closely related to the quality of ultrasonic welding. Understanding these three characteristics can help us understand the process requirements of ultrasonic welding, so that product design engineers can correctly design ultrasonic welding structures to meet the process requirements of ultrasonic welding and improve the quality of ultrasonic welding.

The three characteristics of ultrasonic waves, the welding phenomena they produce, and their requirements for plastic part structural design are shown in Picture 1:

1) High energy: Ultrasonic waves can generate much more energy than sound waves, which is the basis for ultrasonic welding of plastic parts and the fundamental reason for its high strength. Due to its ability to generate huge amounts of energy, ultrasonic waves can even weld metal parts. However, on the other hand, due to the high-energy nature of ultrasonic waves, they may cause burns to the welding interface and damage to other parts of the plastic part or other assembled components on the plastic part.

2) Good directionality and almost straight-line propagation: Due to the short wavelength of ultrasonic waves and the insignificant diffraction effect, ultrasonic waves can be approximately regarded as propagating in a straight line, that is, their directionality is good and it is easy to obtain focused and directed ultrasonic beams. Therefore, this requires that the ultrasonic welding head and the welding parts have a sufficiently large contact area to ensure that ultrasonic waves can be conducted to the welding interface. Moreover, if there are holes or gaps in the propagation direction, ultrasonic waves will have difficulty in transmitting energy around them, which is also an important consideration in ultrasonic wave structure design.

3) Attenuation: Although ultrasonic waves have strong penetrating power, there is always attenuation during their propagation in objects. The further the propagation distance, the more severe the energy attenuation. In addition, the degree of energy attenuation of ultrasonic waves varies in different types of plastics. For example, in amorphous plastics such as ABS, the degree of energy attenuation is relatively small, and even remote welding of two ABS plastic parts can ensure welding quality; however, in semi-crystalline plastics such as PA66, the degree of energy attenuation of ultrasonic waves is high, the propagation distance is short, and it is difficult to ensure the quality of remote welding.

Three characteristics of ultrasound, welding phenomena, and structural design requirements for plastic parts

Three characteristics of ultrasonic :

1. High frequency vibration: Ultrasonic waves have a high frequency, generally between 20 kHz and 50 kHz. They can generate high-frequency vibrations in objects, thereby causing molecular interactions between plastics, resulting in frictional heat, and achieving plastic fusion.

2. Good directionality: Ultrasonic waves can be approximately considered to propagate in a straight line, with good directionality. They can obtain directional and focused ultrasonic beams. 3. Attenuation: Ultrasonic waves experience attenuation during their propagation in objects. The further the distance of propagation, the more severe the energy attenuation. Welding phenomenon: In ultrasonic welding, when ultrasonic waves pass through plastic parts, they generate high-frequency vibrations, causing molecular friction and producing heat, which melt the plastic. The melted plastic fills the gap between the parts, and after cooling, a permanent fusion is formed, achieving welding.

Design requirements for plastic parts:

1. The welding surface should be smooth: ensuring the concentration of ultrasonic waves and energy transmission, and allowing ultrasonic waves to melt the surface of defective parts, producing good welding effects.

2. No foreign matter between welded components: the pad or joint surface between the foreign matter and the plastic should not be too wide or too narrow.

3. Carefully select plastic materials: select plastics that are well-formed, free of contamination, and have uniform material density. The contact area is important for ultrasonic welding, so avoid surface irregularities and warping defects.

4. The thickness of the connecting surface should be uniform: to avoid excessive heating in some areas during welding, while other areas do not melt.

二、Principle of Ultrasonic Welding

With the rapid development of industrial production, plastic with its light weight, high specific strength, corrosion resistance, easy processing and other advantages widely used in various areas of the national economy and people's lives. However, due to processing technology and other reasons, many complex structure of the product can not be processed once formed, the need to seamlessly connect multiple parts together, there are generally two methods: bonding and welding. In general, the bonding process will be low productivity and adhesive have a certain degree of toxicity, easy to cause environmental pollution and health hazards of production personnel adverse consequences. Therefore, the ultrasonic plastic welding machine plastic welding process has been more and more widely used.

The detailed steps of ultrasonic welding are shown in the figure:

(Detailed steps of ultrasonic welding)

Step 1: The two welded plastic parts are placed in the welding fixture one after another.

Step 2: The weld head is moved down and touches the upper plastic part.

Step 3: Pressure is applied through the weld head to compress the two plastic parts.

Step 4: The welding head vibrates vertically at a frequency of 15 or 20 kilohertz per second, and the mechanical vibration energy is transferred to the initial contact area of the two plastic parts, which generates heat by violent friction, and when the temperature of the weld interface reaches the melting point of the plastic, the plastic melts and flows, and the vibration stops, a period of time called melting time.

Step 5: Continue to maintain pressure for a period of time until the molten plastic cools and solidifies, this period is called the holding time.

Step 6: Once the molten plastic has cured, the pressure is removed, the weld head is returned and the two plastic parts are fused together and the ultrasonic welding process is complete.

3.The Welding Process Analysis

In the actual production process, not all plastic materials can be welded, can be laser welding of plastics are thermoplastics, only thermoplastics with the same or similar molecular structure can be welded, in the welding surface is the chemical bond between molecules, so the more similar the parent material, the better the welding effect.

Welding capacity = amplitude X pressure X oscillation time

Amplitude: -Welding head design

-Amplifiers

-Energy control

Pressing pressure: -Standard pressure

-Dynamic starter pressure

-Fusion head drop speed

-Mechanical stop

We also need to take into account the factors that affect the quality of the weld in order to get a good result. When welding, pressure, time, and heat absorption (melt volume) are the three elements that ensure the quality of the weld.

1. Ultrasonic welding pressure

Appropriate pressure is applied to the welded surface, the welded material will transition from elastic to plastic, but also promote the molecular diffusion and squeeze out the residual air in the weld, thereby increasing the sealing performance of the welded surface.

2. Ultrasonic welding time

To have the appropriate hot melt time and sufficient cooling time. When the thermal power is certain, not enough time will appear false welding, too long will cause deformation of the weld, slag overflow, and sometimes hot spots (discoloration) in non-welded parts. Must ensure that the welding surface absorbs enough heat to reach a fully molten state, in order to ensure the full diffusion of intermolecular fusion, and must also ensure sufficient cooling time so that the weld reaches sufficient strength.

3. Melting volume

Heat melting time and thermal power coordinated adjustment to the most appropriate amount of melt, to ensure sufficient intermolecular fusion, to eliminate the phenomenon of false welding. In addition to welding equipment and operator skill level, from the plastic internal or external factors, the welding quality has a certain shadow, should pay attention to.

4. The moisture absorption of plastic

If the welding of wet plastic products, the moisture contained in the heat will be turned into steam to run out and bubbles on the welding surface, so that the sealing performance of the welding surface is weakened. Moisture absorption of more serious materials such as PA, ABS, PMMA. Products made of these materials, must be dried before welding.

5. Fillers in plastics

Such as glass fiber, talc, mica, etc., they change the physical properties of the material. The content of fillers in plastics with plastic weldability and welding quality has a great relationship. Plastics with filler content below 20% can be welded normally without special treatment. If the filler content exceeds 30%, the sealing properties will be reduced due to insufficient fusion between molecules due to insufficient proportion of plastic on the surface.

6. Cleaning of the welding surface

The welding surface must be clean and free of impurities to ensure adequate welding strength and airtightness.

With the needs of industrial production, ultrasonic welding machine technology will be more widely used, there will also be more advanced technology, more convenient to use welding technology.