How is the heat generated in the ultrasonic plastic welding machine?

How is the heat generated in the ultrasonic plastic welding machine? Unlike traditional heat sources, which generate heat in direct contact with plastic, ultrasonic welding generates heat through friction. Learn about ultrasonic welding together.

1. Amplitude, frequency and wavelength

In ultrasonic welding, longitudinal waves are transmitted in the form of high frequencies, resulting in mechanical vibrations of low amplitude. The electrical energy of the welding machine is converted into mechanical energy for reciprocating motion. In order to understand the relationship between amplitude, frequency and wavelength, and how they relate to heat generation, we need to understand the main components of an ultrasonic welding machine.

As shown in the figure above, the main components of an ultrasonic welding machine are a power generator, a transducer, an amplitude modulator (sometimes called a horn) and a welding head. The power generator converts a 50-60Hz power supply with a voltage of 120V/240V to a power supply operating at 20-40Khz with a voltage of 1300V. This energy is supplied to the transducer, which uses the disc-shaped piezoelectric ceramic to convert the electrical energy into mechanical vibration, that is, when a high-frequency current passes through the piezoelectric ceramic, the piezoelectric ceramic will generate strain displacement.

The converter transmits the vibration to the amplitude modulator. The amplitude modulator amplifies the amplitude of the ultrasonic wave and continues to transmit it to the welding head. The horn continues to amplify the amplitude of the ultrasonic waves and makes contact with the part.

Ultimately, the energy is transferred to the welded rib locations of the two parts of the assembly. Because the welding rib is designed with a sharp point, the energy is concentrated at the point of the point, and the frictional heat is generated under the pressure. This heat is generated by two kinds of friction, one is the surface friction between the upper and lower part materials, and the other is the intermolecular friction within the material. It is the heat generated by friction that causes the upper and lower parts to melt and join together at the welding location.

2. Understand the heating rate

For the same material, three factors determine the heating rate: frequency, amplitude and welding pressure. For existing equipment, such as 15Khz, 20Khz, 30Khz or 40Khz machines, the frequency is fixed. So the heating rate can usually be changed with the welding pressure. Generally, the higher the pressure, the faster the heating rate. Alternatively, you can vary the amplitude, as with pressure, the larger the amplitude, the faster the heating rate.

Of course, excessive pressure and amplitude can also adversely affect weld quality, such as causing material degradation, leaks, cracks, and flash. Therefore, ultrasonic welding requires a process of optimizing process parameters. After the parameters are determined, the welding process can achieve a stable output with fast speed and high welding strength. This is why ultrasonic welding is widely used in mass production.

3. Time, Distance, Power and Energy

The amount of heat required for welding depends on material type, weld design and equipment specifications. The traditional method of controlling heat is to weld by time mode, that is, welding for a certain time, such as 0.2-1s (generally less than 1s). However, with today's ultrasonic welding equipment, it is often possible to set and monitor welding distance, power and energy. With properly trained operators, parameters can also be adjusted according to the actual situation and different materials, resulting in consistent welding results. This also greatly improves the flexibility and reliability of welding.