What is Ultrasonic Welding?

Ultrasonic waves are the vibrational waves with frequencies above the hearing range of the human ear. Ultrasonic welding utilizes these waves to generate the heat required for welding. Ultrasonic welding of thermoplastics is one of the most commonly used assembly techniques. It is used in the automotive, textile, packaging, electronics and medical industries because it offers welding the advantages of speed, economy and efficiency.

Equipment and Process

Typical ultrasonic welding equipment consists of a welding press, a generator, a converter (transducer) and the welding tools (horn and anvil). Figure 1 shows a typical set-up. The welding press provides the force necessary to hold the work pieces that are being welded together. The generator converts the energy from the power supply into high frequency electrical vibrations. The converter (transducer) then converts those electrical vibrations to high-frequency mechanical vibrations. These vibrations are transferred to the work piece by the horn. Heat due to friction is then generated at the interface of the pieces to be joined, causing the plastic to melt and fusing the two surfaces together. Once the joint cools down, a strong, homogenous weld is obtained. A schematic of the ultrasonic welding process is shown in Figure 2.

Figure 1: Typical Ultrasonic Welding Equipment Setup

Figure 2: Basic Schematic of Ultrasonic Welding Operation

Welding Techniques

There are two basic techniques in ultrasonic welding. The first is near field, or direct, welding, in which the distance between the horn and the welding interface is less than 0.250” (6mm). The second technique is far field, or indirect, welding, in which the distance between the horn and the welding interface is greater than 0.250” (6mm). The reason for this distinction is that some materials are soft, and will, therefore, dampen the vibrations as they pass through the material. The longer the distance between the horn and the welding interface, the more energy that is lost. If the material is soft and if that distance is large, insufficient energy will be transferred. The welding techniques are shown in Figure 3.

Figure 3: Near Field vs. Far Field Welding

Suppliers and Manufacturers

Depending on the size of the parts and the nature of the material, the choice of the equipment will change. Ultrasonic equipment manufacturers are a good source for equipment recommendations. Below is the contact information of some ultrasonic welding equipment manufacturers.