The basic principle of ultrasonic welding is to convert high-frequency electric energy into high-frequency vibration mechanical energy. This reciprocating vibration is transmitted to thermoplastic or metal, and friction and heat are generated at the interface of plastic and plastic, plastic and metal, or metal and metal. In ultrasonic welding, friction generates heat to fuse two material surfaces together. In ultrasonic riveting, the welding head controls the flow of molten plastic, forming and pressing parts. In ultrasonic nut mounting, the welding head drives the metal nut into the plastic. The ultrasonic welding system has a variety of configuration options, including different frequencies (15Khz-50Khz), different powers (600W-4800W), and various forms, such as pneumatic ultrasonic welding machines, servo ultrasonic welding machines, handheld welding machines, non-standard welding machines , Metal ultrasonic welding machine and so on.

There are also many factors that affect the success of ultrasonic welding: mold (including upper welding head and lower bottom mold), frequency, material, weld design, welding parameters, and injection molding of parts. In this article, we introduce 5 main factors.

1. Welding system frequency The frequency of typical ultrasonic welding system is 15Khz, 20Khz, 30Khz, 35Khz and 40Khz. It is necessary to select a suitable welding frequency according to the requirements of product size, internal component types, strength and appearance. Generally, you can refer to the following principles:

For the shell welding of small and precise electronic products (including PCB boards and microelectronic components), use a high frequency 40Khz welding machine for welding. The 40Khz welding machine has a smaller amplitude and minimum welding pressure, which can avoid damage to the internal electronic components of the product.

For small products with A-type surface appearance requirements. It is welded by a 40Khz welding machine, which can improve the appearance due to the small amplitude and pressure.

For the welding of medium-sized and large-sized parts, use a low-frequency 15Khz or 20Khz welding machine.

For softer materials such as PP and thin-walled products with poor rigidity, 15Khz welding machine with low frequency and large amplitude is used for welding.

For far-field welding, that is, the welding head is far away from the welding seam, for example, when it is greater than 12mm, a 15Khz welding machine with low frequency and large amplitude is used for welding.

20Khz welding machine is suitable for the welding of most products of small to medium size, and it is also the most widely used ultrasonic frequency at present.

2. Material Ultrasonic welding of plastics is only suitable for welding thermoplastics. Because they can melt in a specific temperature range. Thermosetting plastics degrade when heated and cannot be welded by ultrasonic waves.

The weldability of thermoplastics depends on material stiffness or elastic modulus, density, coefficient of friction, thermal conductivity, specific heat capacity, glass transition temperature Tg or melting temperature Tm.

Generally speaking, rigid plastics exhibit excellent far-field welding performance because they are easier to transmit vibration energy. However, soft plastics with low elastic modulus are more difficult to weld because they attenuate ultrasonic vibrations. The opposite is true for ultrasonic riveting or spot welding. The softer the plastic, the easier it is for riveting or spot welding.

Generally, plastics can be divided into two types: non-crystalline (amorphous) and crystalline. Ultrasonic energy is easily transmitted in amorphous materials, so amorphous plastics are easy to ultrasonic welding. Ultrasonic energy is not easily transmitted in crystalline materials, so greater amplitude and energy are required when welding crystalline plastics, and the weld must be carefully designed.

Factors that can further affect solderability include moisture content, mold release agents, lubricants, plasticizers, filler enhancers, pigments, flame retardants and other additives, as well as the actual resin grade. In addition, it should be noted that the degree of compatibility between different materials is different. Some materials have a certain degree of compatibility between specific grades, while others are incompatible.

Finally, consider whether the welding is near-field welding or far-field welding. When the distance from the position where the welding head touches the part to the welding rib is less than 6mm, it is called near field welding. Larger than 6mm' is called far-field welding. The greater the distance, the greater the vibration attenuation, and the more difficult it is to weld.

3. Welding joint design The most critical and important factor affecting ultrasonic welding is the joint design. When parts are in the design stage, engineers should carefully consider and evaluate. Welded joints have a variety of designs, each with their own characteristics and advantages. The choice of design depends on the type of plastic, part geometry, welding requirements, injection molding capabilities, and appearance requirements.

Typical joint design: triangular energy guiding rib design. This is the most commonly used design in ultrasonic welding and the easiest design to inject. It is characterized by a small raised triangle on the plane, and the top of the triangle is 90 or 60 degrees. Because its sharp point design is easy to guide and concentrate the vibration energy, it is called energy guiding ribs.

The step seam design is easy for injection molding, the upper and lower parts can be self-positioned, the welding strength is high, and the molten material flows into the vertical gap.

Grooved seam design, upper and lower parts can be self-positioned, with high strength, good sealing characteristics, and no flashing inside and outside. The disadvantage is that a certain wall thickness is required.

Shearing seam design is generally used for welding smaller size products that require high-strength sealing, and is especially suitable for welding crystalline plastics.

Scarf joints, which are usually used on parts with round or elliptical shapes, provide high strength and high sealing, and are especially suitable for welding crystalline plastics.

In order to determine which weld design is suitable for your product, please consult the engineer or salesperson of the ultrasonic manufacturer.

4. Tooling and welding head Generally speaking, customers will choose tooling and welding head of the same brand as the welding machine. In fact, you can freely choose the tooling and welding head provided by other brands, as long as the frequency of the welding head is the same as that of the equipment.

The welding head material can choose aluminum alloy, titanium alloy and hard alloy steel. The tooling materials can choose aluminum alloy, stainless steel and resin mold. How to choose a material generally needs to consider the type of plastic, the glass fiber content of the material, the structure and size of the joint, the welding strength and the service life. For example, in order to extend the life, the carbide steel welding head is a suitable choice.