Theory on Ultrasonic welding of plastics
THEORY of Ultrasonic welding of plastics
All sounds beyond the human hearing range are called ultrasound. Standardly the human hearing threshold is set at 18 kHz. An ultrasonic vibration is used as an energy carrier. The energy is converted by the converter (vibrating element) of an electrical wave into a mechanical vibration. Since air forms a high resistance to noise vibration and energy follows the path of least resistance, the energy will move in the direction of the sonotrode.
The sonotrode is a tool adapted to a maximum contact surface with the product. At this point the energy again follows the path of least resistance and continues into the product.
A weld structure is fitted on the product. This weld seam construction is basically, a tapering (barrier) of the surface for the energy flow. A barrier means resistance, resistance means heat. When the resistance is large enough, the heat generation will be large enough to achieve the melting point of the plastic. The result is a melt at the point where a pre-weld construction is made. As soon as a melt is created, the surface and the absorption of the energy will increase; the resistance to the wave reduces. The melting process stops. As a result, the temperature doesn’t rise further; burning is prevented. In order to generate the required vibrations an ultrasonic welding machine is required. This machine consists of several components (see figure).
To get high frequencies to convert into vibration, a vibrating element with a similar frequency is used. This is also known as a converter or transducer. The piëzo-electric converter changes its length as a result of an alternating voltage. In order to increase or reduce the amplitude of the vibrations produced by the converter, a mechanical amplifier or booster is used. The amplification factor of the booster is determined by the ratio between the surface area of the top and bottom. A booster which contact surface with the converter is two times as large as the lower side, will, compared to the converter, double the amplitude.
To transfer the vibration to the product that needs welding, a sonotrode or horn is indispensable. This is a highly specialized and unique part, different for every product. In addition, the sonotrode will ensure that the vibrations are given the desired amplitude.
The lower part of the work that needs welding is located in a socket for positioning. This also ensures the conduction to the upper part.
A(servo-) pneumatic - sometimes hydraulic, magnetic or fully servo controlled- system ensures that the column with the transducer, booster and sonotrode comes down, thus compressing the product. After the machine has, by means of vibration, locally melted the plastic parts, the system keeps applying pressure until the weld has cooled sufficiently. After this, the column comes up. The result is an ultrasonically welded plastic product.
The welding seam
In order to make the plastic parts weldable, a welding seam is applied to the separation seam.
Welding seam constructions vary by product and material. The requirements depend on:
- Visual aspects
- Water resistance
- Gas density
The table below shows the ultrasonic weldable plastics.