Ultrasonic Plastic Welding refers to the use of ultrasonic equipment to convert low-frequency electrical energy into high-frequency electrical energy of 20-40 kHz. , the structure of common ultrasonic welding machine is shown in Figure 1. The vibration energy is transmitted from the welding head to the plastic workpiece, and the friction between the workpieces generates a large amount of heat to melt the welding surface of the workpiece, thereby welding into a whole. The joint strength is close to that of a single piece of connected material, and the sealing performance is good. In recent years, with the large-scale application of plastics and composite materials, plastic ultrasonic welding has been widely used due to its fast welding speed, good weld quality, easy automation, and suitability for mass production. However, when using an ultrasonic welding machine for plastic welding operations, even if all preparations are sufficient, the welding effect is inconsistent, insufficient welding, excessive welding and other poor welding phenomena still often occur.
Influence of plastic materials on ultrasonic welding
Ultrasonic welding cannot weld all plastics, which is the biggest limitation of ultrasonic welding. Plastics can be divided into two categories: thermoplastics and thermosets. Thermoplastic molecules have a linear or branched structure, and can be made into a plastic part of a certain shape after heating, softening and melting, and can maintain the shape after cooling. This process can be repeated and reversible. Commonly used thermoplastics are polyethylene, polypropylene, polyamide (nylon) and acrylonitrile (A)-butadiene (B)-styrene (S) terpolymer (ABS). Thermosetting plastics, on the other hand, undergo a cross-linking reaction when heated, and the main chains of the molecules are chemically bonded, eventually becoming a substance that neither melts nor melts. From the characteristics of the two, ultrasonic welding is only suitable for welding between thermoplastics, not for thermosetting plastics. In addition, fillers (calcium carbonate, glass fiber, aluminum hydroxide, etc.) and additives (fire retardants, plasticizers, lubricants, etc.) in plastic raw materials have a great impact on ultrasonic welding.
1. Influence of ultrasonic welding machine process parameters on welding quality
For ultrasonic welding machines, welding energy is the product of power and time, and welding power is the product of pressure, falling speed, frequency and amplitude. It can be seen from the working principle of ultrasonic waves that the actual power of ultrasonic waves is not large, the working time is short, and the heat generated is limited, so it is generally only suitable for materials with low melting points (below 400 °C). According to the characteristics of the selected plastics, the process parameters of the ultrasonic welding machine can be effectively adjusted to achieve the ideal welding effect.
2. The influence of ultrasonic welding on welding quality
During ultrasonic welding, the ultrasonic energy explodes instantaneously, and there is no need for the overall vibration and heat of the weldment during operation. Although the surface contact friction of two welded parts can generate heat energy by rapid friction, the larger the contact area, the more serious the energy dispersion, and ultimately the molecular structure of the end face material cannot be destroyed and fused, so it is only necessary to selectively generate heat at the welded part. During ultrasonic welding, the ultrasonic wave is transmitted in the plastic to the joint surface with the energy-conducting angle, where it generates heat and welds. Therefore, the design of the welding surface structure of the ultrasonic welding machine affects the difficulty of welding, the appearance of the welding place, the welding strength and the waterproof and moisture-proof performance after welding.
Plastic ultrasonic welding technology has many advantages and is widely used in all walks of life in my country, and is becoming more and more mature. However, there are many parameters that need to be controlled in the welding process, such as the setting of the process parameters of the welding machine, the selection of welding materials, and the design of the structure, all of which have an important impact on the welding effect, and these effects are not linear. In daily production, it is necessary to measure the welding effect of various parameters through rigorous test analysis, so as to obtain the optimal solution, and at the same time improve the adaptability of materials, explore the influence of various process parameters on welding quality, and apply ultrasonic welding in more broad field.