Although based on accumulated experience in the industry, many general-purpose materials have recommended design amplitudes. However, as newly developed special materials are rapidly increasing, it is hoped that there will be an experimental method to determine the minimum amplitude required for the materials. Here, we introduce a feasible method for determining the minimum required amplitude. The output amplitude of the welding head depends on the equipment frequency, the output amplitude of the transducer, the transformation ratio of the amplitude modulator (horn), and the shape design of the welding head. A typical welding head design method is: under the condition of lower than the fatigue limit stress of the material, the larger the welding head design transformation ratio, the better (that is, the larger the output amplitude, the better). If you need to reduce the amplitude, change the amplitude modulator with a smaller ratio or adjust the voltage to reduce the amplitude.
However, this design method has some disadvantages:
The transformation ratio cannot be evenly distributed, that is, the transformation ratio of the welding head and the amplitude modulator are very different. It should be noted that when the welding head and the amplitude modulator have a close transformation ratio, the performance of the ultrasonic triplet (transducer + amplitude modulator + welding head) will be improved.
By changing the voltage to reduce the welding head transformation ratio, at the same time it will reduce the output power of the electric box. For example, when the amplitude is set to 50%, an electric box with an original output power of 2400W will actually output 1200W. It is easy to cause overload.
Using large amplitudes will cause unacceptable damage to parts. For example, cracks in a small cross-sectional area may cause burns in appearance, and may cause damage to membranes, filter membranes and electronic components.
For the above reasons, it is very important to define the minimum amplitude required for ultrasonic welding of plastics in the early stage.
Amplitude calculation The magnitude of the amplitude transmitted to the plastic part is the product of the output amplitude of the transducer, the ratio of the amplitude modulator and the ratio of the welding head.
The output amplitude of the transducer and the ratio of the amplitude modulator are provided by the manufacturer of the ultrasonic equipment and are often fixed. But the welding head transformation ratio can be designed.
The welding head transformation ratio has an approximate calculation formula: welding head transformation ratio = mass above the welding head node ÷ mass below the welding head node. Since the material density and length are the same, the formula is simplified as: welding head transformation ratio = cross-sectional area above the welding head node ÷ cross-sectional area below the welding head node × 0.8. The 0.8 coefficient is to consider the influence of the fillet transition at the node.
Through this method, calculate the theoretical output amplitude of the welding head in the following test as 13.9um (peak to peak). The measured amplitude is 15um (peak to peak). The two are close.
In the test process, an amorphous material PC and a semi-crystalline material PP are selected for the experiment to determine the minimum amplitude required by the two materials.
Servo-driven ultrasonic welding equipment is very suitable for determining the minimum amplitude test. Because its welding head can hover, and generate pressure measurement data, and correlate with data such as ultrasonic vibration amplitude. In essence, the "Melt-match" function of the device allows the welding head to contact the product with a set pressure, stop descending and trigger ultrasonic vibration. The ultrasonic vibration is transmitted to the position of the plastic weld. Once the device measures the pressure drop, the surface polymer has begun to melt. Then the welding head starts to move downwards.
We generally believe that the amorphous material PC, after the temperature is higher than the glass transition temperature, will show a gradual softening characteristic as the temperature increases. The semi-crystalline material PP will suddenly liquefy when it reaches the melting point.
In this experiment, the gradually changing properties of PP may be due to the softness of this material. The rapid change of PC may be caused by the amplitude not in the correct range. Further experiments are needed, such as increasing the PC amplitude range, or measuring the temperature of the weld seam.
Conclusion This work provides an experimental method and framework for determining the minimum amplitude required for plastics. The strong correlation between the material melting time and the amplitude and welding strength also shows that this method of estimating the minimum required amplitude is feasible.