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When welding thermoplastics using ultrasonic energy, the welder creates an inseparable bond between two or several parts. Each connection requires a weld joint that is designed in a way that perfectly suits the material, the geometry and the position of the two components to be joined. The process follows a basic rule: the weld joint design needs to guarantee a high level of repeatability at minimum risk when the joint is reproduced in a welding machine.
The high-frequency alternating voltage produced by the generator is converted into mechanical vibration energy of the same frequency by the converter. The molecular and interfacial friction generated by this process will then produce heat that causes the plastic to plasticise. The components are afterwards joined by applying the right amount of pressure. The welding stack required for this process is made up of a converter, a booster (amplitude amplifier) and a sonotrode operating at resonant frequency.
In theory, all components made of thermoplastic materials as well as of amorphous and of semi-crystalline polymers are suitable for welding, although the welding characteristics of these plastics may differ significantly. For instance, amorphous and semi-crystalline polymers cannot be fused together. On the other hand, hard, amorphous plastics such as PC, PS and PMMA possess a coefficient of transmission that is favourable for ultrasonic energy. Finally, semi-crystalline plastics such as PA, PP and POM possess a high acoustic attenuation coefficient.
Plane welding is used wherever materials need to be joined plane to plane or fused tightly with a circumferential weld joint. The planar welding of two components requires a weld joint design created with the help of energy directors.
This welding method is used primarily in applications that require, for instance, the joining of two planar parts without any joint preparation. Both parts to be joined must, however, lie on top of each other with parallel planes and without a gap. The wall thickness of the lower part should not be too small. The sonotrode should be positioned as perpendicular as possible in relation to the weld contact area in order to prevent the parts from shifting while being welded.
Pin welding is a type of mash joint connection that is used in cases where, for instance, thin bars do not allow for the use of a different type of weld joint. The pins ensure that the components centre themselves while being welded. During the welding process the melt penetrates the cavity of the body, thereby preventing any visible weld expulsion. The advantage of this technique lies in the lower welding time afforded to the welder and the associated reduction in energy consumption.
Ultrasonic staking makes it possible to join weldable plastics with metal or non-weldable plastics. Overcoming the rebound elasticity, staking creates a rigid joint between the materials. The benefit of this technique is that it subjects only a small area to heat, protecting the component against thermal overload. What is more, the heat generated by this welding technique is dissipated quickly by the mass of the sonotrode.
Swaging is often used in cases where it is not possible mould a staking pin to the plastic component. The processing times correspond to the times required for staking. It is important to note that the plasticised material will cool off under pressure below the sonotrode when welding is complete. This reliably prevents the component from rebounding.
Compared to other types of joining technologies, ultrasonic welding is not only faster and more efficient, but also safer for the environment. For these reasons and due to the fact that it offers shorter cycle times and lower energy consumption, this technology is well established in the marketplace.