Common ultrasonic welding problems and solutions
The basic principle of ultrasonic welding is to convert high-frequency electrical energy into high-frequency vibration mechanical energy. This reciprocating vibration is transmitted to the 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 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 small and precise electronic products (including PCB boards and microelectronic components) shell welding, use a high frequency 40Khz welding machine. 40Khz welding machine has smaller amplitude and minimum welding pressure, which can avoid damage to the internal electronic components of the product.
For small products that require the appearance of Class A surface. 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 low-frequency 15Khz or 20Khz welding machines.
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.
The 20Khz welding machine is suitable for welding most products of small to medium size, and is also the most widely used ultrasonic frequency.
2. Material
For the ultrasonic welding of plastics, it 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.
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 difficult to weld because they attenuate ultrasonic vibration. 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 needed 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 welding head 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 the welding.
3. Welded 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. There are various designs of welded joints 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 for injection molding. It is characterized by a small raised triangle on the plane, 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.
Step seam design, easy injection molding, self-positioning upper and lower parts, high welding strength, 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.
(The above detailed explanation of weld design will be introduced in a follow-up article).
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 heads 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.
Welding head materials 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 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 the best choice.
Ultrasonic welding heads can be designed and optimized with FEA (Finite Element Analysis), allowing engineers to evaluate the vibration and stress level of the welding head before actual manufacturing. The best welding head design is to have uniform output amplitude and minimum stress. In the above picture, the picture on the left is the optimized front welding head design, and the amplitude output is uneven. On the right, after optimization, the output amplitude is uniform.
In welding head design and manufacturing, symmetry must be carefully kept in mind-welding head symmetry is crucial. An asymmetric welding head causes non-axial vibration. Radial vibration will greatly increase the stress and cause the welding head to fail.
Good tooling design is also very important. The tool has two main functions: (1) align the parts under the welding head; (2) rigidly support the welding area. The rigid support helps to reflect the ultrasonic energy to the weld position, which is why the tooling is usually made of metal processing parts.
In order to increase the wear resistance of the welding head and increase the service life, the surface of the welding head can be treated with tungsten carbide or chromium plating. The tooling can be designed in sections to better fit the product.
5. Welding parameters
During the welding process, welding parameters will affect the welding result. These parameters include amplitude, welding pressure, trigger pressure, welding distance and welding energy.
Different types of plastics require different amplitudes. The amplitude can be fine-tuned by the percentage setting in the software, or can be adjusted in a wide range by changing the amplitude modulator with a different ratio. Welding pressure can be adjusted by knob or software setting. Trigger pressure means that when the welding head presses the product and the pressure reaches a certain set value, the device starts to emit ultrasound. This value can be adjusted through the knob or software settings.
There are several control methods for the ultrasonic welding process:
Time welding mode, that is, set the duration of ultrasonic welding.
Distance welding mode (position welding mode), that is, set the welding distance or position.
Energy welding mode, which sets the welding energy.
Different welding modes apply to different products. For example, sheet welding adopts energy welding mode, products with large dimensional tolerances adopt distance welding mode, and products with high tolerance requirements adopt position welding mode.
Plastic ultrasonic welding is a special process. In the early stage of product development, it is necessary to cooperate with the ultrasonic equipment manufacturer and use the equipment manufacturer’s experience in this field to evaluate the product structure and weld design, and conduct welding tests on samples. In order to improve the yield of subsequent mass production.
