Ultrasonic welding process parameters
The main process parameters of ultrasonic welding are: amplitude, welding time, holding pressure time, welding pressure, frequency, etc. The best welding specification depends on the components to be welded and the welding equipment used. The adjustment of welding parameters depends on the size and stiffness of the part, especially the distance between the contact point of the welding head and the welding joint. The welding ability is limited by the plastic's ability to transmit ultrasonic vibration (and the parts are not damaged).
1 frequency
Commonly used frequencies for ultrasound are 20, 30, and 40 kHz, and 15 kHz is often used for semi-crystalline plastics. 20 kHz is the most commonly used ultrasonic frequency, because the amplitude and power necessary for melting thermoplastics at this frequency are easy to reach, but it may generate a lot of mechanical vibrations that are difficult to control, and the tool becomes very large. A higher frequency (40 kHz) that produces less vibration is feasible, and is generally used for welding engineering plastics and reinforced polymers. The advantages of high-frequency welding equipment include: low noise, small parts size, enhanced part protection (due to reducing cyclic stress and unselective heating of the outer area of the joint interface), improved mechanical energy control, lower welding pressure, and faster processing speed. The disadvantage is that it is difficult to perform far-field welding due to the small size of the parts, the reduced power capacity and the reduced amplitude. Higher frequency ultrasonic welding machines are usually used to weld small, precision parts (such as electrical switches) and parts that require less material degradation. The 15 kHz welder can quickly weld most thermoplastics, in most cases, less material degradation than the 20 kHz welder. Parts that can barely be welded with 20 kHz (especially those made of high-performance rubber and plastic technology and equipment) can be welded effectively with 15 kHz. At lower frequencies, the welding head has a longer resonance length and can be made larger in all dimensions. Another important advantage of using 15 kHz is that compared with using higher frequencies, it greatly reduces the attenuation of ultrasonic waves in plastics, allowing the welding of softer plastics and greater far-field distances.
2 amplitude
Successful welding depends on the proper amplitude of the end of the welding head. For all horn/welding head combinations, the amplitude is fixed. Select the amplitude according to the material to be welded to obtain an appropriate degree of melting. Generally speaking, semi-crystalline plastics require more energy than non-crystalline plastics, and therefore require greater amplitude of the tip end. Process control on modern ultrasonic welding machines allows for gradation. The high amplitude is used to start melting, and the low amplitude is used to control the viscosity of the molten material. Increasing the amplitude will improve the weld quality of the shear joint design part. For butt joints, as the amplitude increases, the weld quality improves and the welding time decreases. In ultrasonic welding with energy guide bars, the average heat loss rate (Qavg) depends on the composite loss modulus (Eʺ), frequency (ω) and acting strain (ε 0) of the material: Qavg=ωε 02 Eʺ/2
The composite loss modulus of thermoplastics is closely related to temperature. When the melting point or glass transition temperature is reached, the loss modulus increases and more energy is converted into heat. After the heating starts, the temperature at the welding interface rises sharply (up to 1 000 ℃/s). The acting strain is proportional to the amplitude of the welding head, so the heating of the welding interface can be controlled by changing the amplitude. Amplitude is an important parameter that controls the flow rate of thermoplastic extrusion. When the amplitude is high, the welding interface heating speed is higher, the temperature rises, and the molten material flows faster, which leads to an increase in molecular orientation, a large number of flashes and a lower weld strength. High amplitude is necessary to start melting. Too low an amplitude produces uneven melting and premature melt solidification. When the amplitude is increased, a greater amount of vibration energy is consumed in the thermoplastic, and the parts to be welded are subjected to greater stress. When the amplitude is constant throughout the welding cycle, the highest amplitude that will not cause excessive damage to the parts to be welded is usually used. For crystalline plastics such as polyethylene and polypropylene, the impact of amplitude is much greater than for non-crystalline plastics such as ABS and polystyrene. This may be due to the need for more energy for melting and welding of crystalline plastics. The amplitude can be adjusted mechanically (by changing the horn or welding head) or electrically (by changing the voltage supplied to the transducer). In practice, the larger amplitude adjustment adopts a mechanical method and the finer uses an electrical method. High melting point materials, far-field welds, and semi-crystalline plastics generally require greater amplitude than non-crystalline plastics and near-field welds. The typical total amplitude range of amorphous plastics is 30-100 μm, while that of crystalline plastics is 60-125 μm. Amplitude profiling can achieve good melt flow and consistent high weld strength. For combined amplitude and force levels, high amplitude and force are used to start melting, and then the amplitude and force decrease to reduce the molecular orientation along the weld line.
3 Welding time
The welding time is the time when vibration is applied. The appropriate welding time for each application is determined by experiment. Increasing the welding time will increase the strength of the weld until the optimal time is reached. A further increase in welding time will result in a decrease in weld strength or only a slight increase in strength, while at the same time it will increase weld flash and increase the possibility of part indentation. It is important to avoid over-welding, as it will produce excessive flash that needs to be trimmed, which may reduce the quality of the weld and create leaks in the parts that need to be sealed. The welding head may scratch the surface. For longer welding times, melting and fracture may also occur in parts far away from the joint area, especially at the holes, weld lines and sharp corners in the molded part.
4 Holding time
Holding pressure time refers to the nominal time for the parts to be combined and solidified under vibration-free pressure after welding. In most cases, it is not a critical parameter, 0.3~0.5 s is generally sufficient, unless the internal load is easy to disassemble the welded part (such as a coil spring compressed before welding).
5 pressure
The welding pressure provides the static force required for coupling between the welding head and the part so that vibration can be transmitted into the part. When the molten material at the joint solidifies during the pressure holding phase of the welding cycle, the same static load ensures that the parts are integrated. The determination of the optimal pressure is essential for good welding. If the pressure is too low, it will cause poor or insufficient melt flow in energy transfer, leading to unnecessary long welding cycles. Increasing the welding pressure will reduce the welding time required to achieve the same displacement. If the pressure is too high, it will cause molecular orientation along the flow direction and reduce the strength of the weld, which may cause part indentation. In extreme cases, if the pressure is too high relative to the amplitude of the end of the welding head, it may overload and stop the welding head. In ultrasonic welding, high amplitude requires low pressure, and low amplitude requires high pressure. As the amplitude increases, the acceptable pressure range narrows. Therefore, the most important thing for high amplitude is to find the best pressure. Most ultrasonic welding is performed under constant pressure or constant force. For some devices, the force can be changed during the cycle, that is, force profiling is performed, and the welding force is reduced during the application of ultrasonic energy to the part. The welding pressure or force that drops at the end of the welding cycle reduces the amount of material extruded from the joint, prolongs the diffusion time between molecules, reduces the molecular orientation and increases the strength of the weld. For materials with lower melt viscosity similar to polyamide, this may greatly increase the weld strength.
6 Welding mode
Welding by time is called an open loop process. The parts to be welded are assembled in the fixture before the welding head drops and touches. Then the ultrasonic wave acts on the component for a fixed period of time, usually 0.2 to 1 s. Successful welding did not occur during this process. Successful welding is an ideal situation under the assumption that a fixed welding time causes a fixed amount of energy to act on the joint, resulting in a controlled amount of melting. In fact, the power absorbed by maintaining the amplitude from one cycle to the next is not the same. This is due to multiple factors (such as the fit between two parts). Because energy changes with power and time, and time is fixed, the applied energy will change from one part to the next. For mass production where consistency is important, this is clearly undesirable. Energy welding is a closed loop process with feedback control. Ultrasonic machine software measures the absorbed power and adjusts the processing time to deliver the required energy input to the joint. The assumption of this process is that if the energy consumed by each weld is the same, the amount of molten material at the joint is the same every time. However, the actual situation is that there is energy loss in the welding kit and especially at the interface between the welding head and the part. As a result, some parts may receive more energy than others, which may cause inconsistent weld strength. Welding by distance allows parts to be joined at a specific welding depth. This mode of operation does not depend on time, absorbed energy or power, and compensates for any dimensional deviations in the molded part, thus best ensuring that the same amount of plastic is melted in the joint every time. In order to control the quality, a limit can be set on the energy or time used to form the weld
