Ultrasonic welding has near and far field welding points. Near field welding means that the distance between the part of the plastic part and the end of the electrode arm is within 6mm, and the distance beyond this distance is called far field welding. Non crystalline hard plastics, such as PC, PS, SAN, ABS and PMMA, have good transmittance for ultrasonic energy. High frequency vibration can be transmitted to the interface area over a long distance, so these materials have good welding performance in both near and far regions. Semi crystalline plastics have strong noise elimination, and high frequency vibrations are transmitted to semi crystalline plastics such as PA, PP, PE and POM, and the ultrasonic energy quickly attenuates. Therefore, semi crystalline plastic is only suitable for near field welding. For elastomers and soft plastics, ultrasonic welding is not very effective because of their stronger sound absorption.
From the point of view of energy consumption, amorphous plastics have no definite melting point, and the heat required for plasticizing is less ultrasonic energy. As the temperature of the welding zone increases, the material changes from high elastic state to viscous flow state, and melts and solidifies gradually in a wide temperature range. The semi crystalline plastic has a definite melting point and requires a higher melting heat. Compared with the non crystalline polymer, higher ultrasonic energy and vibration amplitude are needed during the welding process.
In addition, in order to ensure the quality of welding products, the properties of raw materials and final properties should be fully considered. Some additives in plastics, such as lubricants and flame retardants, will also affect the welding performance of the materials. This should be considered fully in the design of components and the adjustment of welding parameters. The interface design is correctly carried out to avoid modifying the die after molding. The release agent and oil contamination in the welding area will also weaken the welding strength and clean the surface of the mold cavity in time.
Ultrasonic welding is most suitable for rigid non crystalline plastics, and the best result is when joining the same kind of plastic. However, two kinds of plastics, such as polycarbonate and acrylic acid, can be welded between polycarbonate and acrylic acid within 20 C, and the chemical properties have a certain compatibility. Table 1 shows the welding performance of the common thermoplastic itself. It is shown in the table that the welding performance of hard plastic is better than that of flexible. The welding performance of non crystalline plastic is better than that of crystallization, and it should avoid the use of far field welding in the welding of the crystalline plastic.
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