I. Core Principles of Ultrasonic Welding

Ultrasonic welding is a technology that achieves material joining by using high-frequency mechanical vibrations (typically with a frequency of 15-40 KHz) to generate frictional heat at the joint of workpieces to be welded under pressure. The principles vary slightly depending on the welding materials, as detailed below:

II. Main Process Methods of Ultrasonic Welding

(I) Fusion Welding Method

• Operation Method: The horn vibrates at an ultra-high frequency, and under moderate pressure, frictional heat is generated at the joint surface of two plastic pieces, achieving instantaneous melting and joining.
• Advantages: The welding strength is comparable to that of the plastic itself. With appropriate workpiece and interface design, water tightness and air tightness can be achieved. No auxiliary materials are needed, making the process clean and efficient.

(II) Embedding Method

• Operation Method: With the help of vibration and pressure transmitted by the horn, metal parts (such as nuts, screws) are instantly pressed into the pre-drilled holes in the plastic and fixed at a specified depth.
• Advantages: After embedding, the tensile force and torque of the parts are comparable to those of traditional in-mold molding. It avoids damage to the injection mold and solves the problem of slow injection molding speed.

(III) Rivet Welding Method

• Operation Method: The vibrating horn presses the protrusion of the object, melting it into a rivet shape to realize mechanical riveting of two objects.
• Advantages: No additional rivets are needed, simplifying the process. It is suitable for scenarios requiring mechanical fixation, such as the connection between metal sheets and plastic parts.

(IV) Spot Welding Method

• Operation Method: For objects where it is difficult to design welding lines, point-by-point welding is adopted to achieve fusion through local high-frequency vibration and pressure.
• Advantages: High flexibility, capable of meeting the welding needs of workpieces with complex shapes while ensuring the fusion effect.

III. Complete Process of Ultrasonic Welding

The ultrasonic welding process is divided into four key stages, which are connected in sequence and jointly determine the welding quality:

1. Initial Melting Stage: The top of the energy director begins to melt. As the melting speed accelerates, the gap on both sides of the joint gradually decreases. The molten energy director fully spreads and contacts the lower part, and at this point, the melting speed of the energy director decreases.
2. Full Contact Stage: The upper and lower parts achieve full surface contact, and the melting area further expands, laying the foundation for subsequent stable melting.
3. Steady-State Melting Stage: A molten layer with a certain thickness is formed, accompanied by the generation of a constant temperature field. When the set welding energy, time, distance, or other control conditions are reached, the ultrasonic wave stops working.
4. Cooling and Forming Stage: Pressure is maintained continuously. Some excess molten material is squeezed out of the weld. Molecular bond connections are formed between the parts and cooled, finally completing the welding and forming a stable joint.

IV. Key Design Considerations for Ultrasonic Welding

(I) Material-Related Factors

1. Polymer Structure: Polymers are divided into crystalline and amorphous types. Crystalline materials (such as Nylon) need to be matched with a more suitable energy director angle (e.g., 60°), while amorphous materials (such as ABS) can use a 90° energy director.
2. Fillers
   • Influence Law: When the content is 20%, it can enhance the transmission efficiency of ultrasonic vibration in the material, especially for semi-crystalline materials. When the content reaches 35%, the weld may have reduced sealing reliability due to insufficient resin. When the content exceeds 40%, glass fibers tend to accumulate at the joint, reducing the welding strength.
   • Countermeasures: Long glass fibers are prone to accumulate on the energy director and can be replaced with short glass fibers. When the filler content exceeds 10%, a hard alloy steel horn or a titanium alloy horn with a tungsten carbide coating should be used, and a higher-power device may be selected if necessary.
3. Additives
   • Lubricants (e.g., wax, zinc stearate): Reduce the friction coefficient between polymer molecules, reducing heat generation. Due to low concentration and dispersion, their impact on welding is relatively small.
   • Plasticizers: Increase material flexibility, reduce stiffness, and interfere with the transmission of vibration energy, which is not conducive to welding.
   • Flame Retardants: Mostly inorganic oxides or halogenated organic elements, which may account for more than 50% of the material weight, greatly reducing the content of weldable materials. High-power equipment, large-amplitude horns should be selected, and the joint design should be modified to increase the proportion of weldable materials.
   • Recycled Materials: When adding recycled materials, their content and volume must be strictly controlled. In some scenarios, 100% original materials are required to avoid affecting the welding effect.
4. Release Agents (e.g., zinc stearate, silicone): Sprayed on the surface of the mold cavity to facilitate the removal of injection-molded parts. However, they may transfer to the joint surface, reducing the friction coefficient and heat generation, and also chemically contaminating the resin to affect the formation of chemical bonds. Silicone has the most serious impact. Release agents can be removed with solvents; if their use is necessary, an appropriate grade should be selected to prevent transfer.
5. Compatibility of Different Materials: When welding two materials, the melting temperature difference should be ≤ 22°C (40°F), and their molecular structures should be similar. If the temperature difference is too large, the low-melting-point material will melt and flow first, failing to provide sufficient heat for the high-melting-point material to melt. For example, ABS and PC can be well welded, while some material combinations have poor welding effects (refer to the "Compatibility of Different Plastic Welding" table for details, where black represents good welding, O represents general welding, and blank represents poor welding).

(II) Welding Joint Design

There are four common types of ultrasonic weld designs, which should be selected based on the material, part size, rigidity, and welding performance requirements (strength, appearance, sealing):

(III) Texture Treatment of Welding Contact Surfaces

• Function: Adopting the textured surface recommended by Branson can improve welding strength, increase flow resistance to control flash, and reduce burrs, but cannot guarantee sealing effect.
• Parameter Requirements: The texture depth ranges from 0.076 to 0.152 mm. Common texture models and their corresponding depths are: Branson 300/11040 (0.003"), Branson 450/11050 (0.0045"), Branson 600/11050-6 (0.006").

(IV) Welding Distance

• Near-Field Welding: The distance from the vibration source to the joint is < 6.4 mm, with small energy loss. It is suitable for crystalline materials with high energy absorption characteristics and low-rigidity materials.
• Far-Field Welding: The distance from the vibration source to the joint is > 6.4 mm, suitable for amorphous materials with low energy absorption characteristics and high-rigidity materials.

(V) Other Important Factors

1. Wall Thickness: When the thin wall is < 1 mm, it is prone to generate lateral vibration like a spring, making it impossible to effectively transmit vibration energy to the joint and affecting welding.
2. Assembly Size: When welding with a single horn, the part size generally needs to be < 250 mm, and it is also affected by the material. For example, a 250x250 mm Nylon box is difficult to weld, but a PS box of the same size may be weldable.
3. Lower Mold Support: The melted part of the fixed component needs a sufficient supporting surface to ensure the stability of the workpiece during welding.
4. Contact Surface: The horn needs to be in close contact with the part. Notches and holes should be avoided above the joint to prevent obstruction of vibration transmission.
5. Joint Design Details: Sharp corners should be avoided near the joint to prevent cracks; the joint surface of the two welded parts should be designed in a loosely fitting state, as an overly tight fit will affect the welding effect.

V. Application Examples of Ultrasonic Welding

Ultrasonic welding is widely used in various fields, and different scenarios correspond to different materials and joint designs. Specific examples are as follows: