Setting of ultrasonic welding process parameters
The process parameter setting of ultrasonic welding includes ultrasonic welding power, ultrasonic frequency, ultrasonic amplitude, ultrasonic welding pressure, ultrasonic welding time, etc.
l. Frequency of ultrasound
The working frequency of ultrasonic welding is usually 15-40kHz, and materials with poor response to low frequency, such as PvC, PE, etc., can be welded with high frequency, which can reduce the damage to the material. High-frequency ultrasonic energy transfer is concentrated, and high-frequency ultrasonic welding can be used for some delicate parts. During ultrasonic welding, the detuning phenomenon of the ultrasonic equipment will be caused due to the change of the load, which makes the welding strength stronger. In general, after the working frequency of the welding machine is determined, the acoustic system needs to be kept in resonance.
The following equation can describe the power of ultrasound:
P=µSnv=-2Aω/π=4usaf
In the formula, P ultrasonic power; F static pressure; S solder joint area; v relative velocity; A amplitude; µ a friction factor; w is the angular frequency; f is the vibration frequency.
2. Ultrasonic amplitude
Welding at a larger operating frequency and amplitude can reduce welding time and improve work efficiency. For different materials, there is an optimal welding amplitude as shown in Table 1. Ultrasonic welding has a small amplitude of 20µm. It is usually recommended to use an amplitude of 40µm. Because too large amplitude will often cause fatigue and damage to the ultrasonic power supply, the ultrasonic amplitude requirements are consistent with the ultrasonic power supply.
1212.png
3. Ultrasonic welding time
Welding time refers to the time when ultrasonic energy is emitted during the welding process. The welding time is too short and the energy is not enough to cause a reliable welding joint. As the welding time increases, the weldment can absorb more energy, the temperature of the welding surface will increase, the welding area will increase, and the welding penetration will increase, so that the welding strength will increase [22-24]. However, too long welding time will lead to excessive melting of the welding material and cause more flash. The flow of these melts in the welding area is directional, so too much melt flow will cause a decrease in strength. In addition, too long welding time will cause the temperature of the weldment to be too high, causing the weldment to burn and degrade, causing weld marks on the surface of the weldment, resulting in over-welding and lowering the strength. Too long welding time and too much energy will cause the temperature of the molten layer to be too high, discoloration, decomposition, and embrittlement of the welded plastic; and the welding edge stress is concentrated, and indentation appears on the welding surface. Therefore, in order to obtain higher welding strength, it is necessary to choose a suitable ultrasonic welding time, too short and too long will cause the decrease of welding strength.
4. Ultrasonic welding pressure
Ultrasonic welding pressure refers to the static pressure applied by the welding head to the weldment during the welding process, and the application of static pressure transmits ultrasonic energy to the weldment. In ultrasonic welding, when the welding time is fixed, the pressure is related to the welding surface to form a suitable contact, which is a very critical factor for the strength. Within a certain pressure range, as the pressure increases, the welding strength will increase. When the welding pressure is low, the contact of the weldment is not good, the friction energy can not be effectively produced, and the energy utilization rate of ultrasonic is low. Lower pressure will result in less molten material in the welded part, which makes it impossible to form an effective weld. However, when the welding pressure is too high, it will cause the melt to flow too fast, and the melt will flow out from the welding force, which reduces the solidification of the melt required for the formation of the welding head and reduces the welding strength. Excessive force will cause excessive friction, which will weaken the relative frictional movement between the weldments, cause excessive load on the welding machine, and make welding difficult. The welding pressure has a great influence on the welding strength during the ultrasonic welding of nylon 66. A slightly lower welding pressure can make the welding produce a thicker heat-affected zone, which will make more molecular chains, crystal grains, and fibers move perpendicular to the welding interface, and improve the welding strength. These welded joints are under a welding pressure of 0.66MPa. The welding strength can reach 70% of nylon 66. The welding pressure needs to be matched with the welding time in order to obtain a better welding degree. Matsuoka [27] found that for glass fiber reinforced thermoplastics, when the welding amplitude is kept constant, increasing the welding pressure can reduce the welding time.
5. Lap length and fixed position
The lap length and clamping position during ultrasonic welding will also affect the welding strength. With the increase of the lap length in the single lap test, the welding strength will be reduced. When the lap length increases, it will cause the stress concentration of the welding part and reduce the strength. Therefore, in order to obtain the best welding strength, it is necessary to design a shorter lap length, and choose an appropriate length according to the type of joint. In general, the lap length is often fixed. In order to meet the strength requirements, the lap joint is short, the welding area is small, and the strength is not enough; the lap joint is longer, and it will cause waste of materials. Design the lap length. Change the welding parameters to obtain the best welding strength. Qiu et al. found that on the anvil where the weldment is fixed, the distance between the clamping point and the welding part will affect the welding strength. A shorter distance is conducive to enhancing the heat generated by friction, which can improve the strength of welding. In actual production, the welding parts have various shapes and the fixed clamping position is not suitable. Generally, the welding parts are required to be stable in the welding process.
6. Ultrasonic welding depth
During the welding process, as the material at the welding position melts, the position of the welding head will continue to drop, and the molten material will diffuse and solidify at the end of the welding. The thickness of the final solidified material is called the penetration depth. Under normal circumstances, the welding process can be controlled. The downward displacement of the welding head controls the penetration depth. The strength of welding has a great relationship with the microstructure of the welded part, which is closely related to the thickness of the molten layer and the temperature of the welded part during the welding process. Increasing the welding pressure or welding time will increase the melting and flow of the material during the welding process, thereby increasing the penetration depth [29]. Proper penetration can increase the welding strength, but when the penetration is too large, more welding time is often required, which will cause over-welding of the material and decrease the strength. No matter how you change the welding pressure and time, you need to prove a suitable penetration depth, so as to ensure that the welding reaches a higher strength.
7. The influence of ultrasonic conductive bars
The energy guiding ribs are designed on the welded parts, which can concentrate the welding energy, reduce the welding time, reduce the stress concentration of the welding part, and improve the welding strength. Common energy guiding ribs are in the form of triangle, rectangle and semicircle. In ultrasonic welding, butt joints and lap joints are often used for welding, and the design of energy guide bars is also different. Because the energy guide tendons tend to concentrate the pressure during welding and are subject to more vibration stress, during the welding process, energy is concentrated and concentrated to the energy guide bars. Under the action of the pressure, the energy guide bars will first heat up and melt and move to both sides. Flow expansion [3o]. Liu et al. predicted that welding parts with semi-circular energy guiding ribs can have the highest welding strength when welding with suitable welding parameters [31]. Devine[32] suggested that the triangular energy-conducting ribs with a 90° apex angle are suitable for most amorphous plastics, while the triangular energy-conducting ribs with a 60° apex angle are suitable for semi-crystalline plastics, and for semi-crystalline plastics, conductive The material that can be melted by the rib may solidify as it flows around, which may cause the material to be incompletely welded, so the energy guide is not necessary for lap welding. In addition, the addition of energy guiding ribs increases the difficulty of welding and increases the cost.
Ultrasonic welding process parameter setting method needs to strictly follow the above theory and cannot be adjusted at will. Only when you understand the principle and use ultrasonic welding equipment can you become comfortable.
