Ultrasonic welding is a widely recognized and accepted process for joining thermoplastic materials. It offers many advantages, including process reliability and repeatability, lower energy usage than other joining techniques, material savings (because there is no need for consumables, such as glue or mechanical fasteners), and labor savings.
But as with any process, there are situations where apparent problems with this technique may interrupt the production process. The key to resolving and avoiding these problems is to understand their likely origins. Processors that are successful in using ultrasonic welding typically share two principal traits: they have a well-documented, validated welding process; and that process is supported and maintained by a resident well-trained “champion.” If one or both of these important factors are not present, you’ll likely very soon call for help. Even with both present, it is possible that you’ll need some help or technical assistance at least once in a while.
HOW THE PROCESS WORKS
Before examining common causes of ultrasonic welding problems, let’s take a moment to understand the welding cycle itself. In ultra- sonic welding, high-frequency vibrations are applied the surfaces of two parts by a vibrating tool, commonly called a “horn” or “sonotrode.” Welding occurs as the result of frictional heat generated at the interface between the parts. The ultrasonic vibrations are created by a series of components—the power supply, converter, booster, and horn—that deliver mechanical vibration to the parts.
The power supply takes a standard electrical line voltage and converts it to an operating frequency. In the following example, we will utilize a common ultrasonic welding frequency of 20 kHz, though welding can take place over a range of 15 to 60 kHz to meet specialized needs. In operation, the power supply sends electrical energy at the specified frequency through an RF cable to the converter. The converter utilizes piezoelectric ceramics to convert the electrical energy to mechanical vibrations at the operating frequency of the power supply. This mechanical vibration is either increased or decreased based on the configuration of the booster and horn. The proper mechanical vibration amplitude is determined by an applications engineer and is based on the thermoplastic materials used in the parts.
The parts to be welded are put under a mechanical load, generally with a pneumatic actuator that holds the booster and horn. Under this load, the mechanical vibrations are transmitted to the interface between the material surfaces, which focuses the vibrations to create intermolecular and surface friction. This friction creates heat and a subsequent melt, which solidifies into a welded bond.
The second section, the booster, with an attached ring in its mid-section, serves two functions: It acts as a mounting point for the stack into the actuator, and also serves to amplify or reduce the output motion created in the transducer.
The third and final component of the stack is the horn (sonotrode) that will contact the parts to be joined. The horn will be designed to match the profile of rigid parts to be joined or can have a sealing profile added to its contact face in a film/textile application. For each application, the horn is designed to combine with the other stack components to reach the optimum level of amplitude output to allow ultrasonic welding to occur as efficiently as possible.
Issues usually occur in one of four areas:
1. Equipment: The ultrasonic welding equipment or various welding components are not suited to the application.
2. Process parameters: The parameters used are not suited to the parts being joined.
3. Materials: Changes are made in the type, composition, or physical/mechanical characteristics of the materials used in the parts.
4. Part design: Certain details of the part’s geometry are not suited to repeatable or successful welding.
It should also be noted that sometimes a problem identified in one area may expose a weakness or deficiency in another area.
Let’s start with equipment. It is easy and usually logical to think the equipment and approaches that produce successful welds in one application will do so in another. But that is not universally true. Worldwide, 20-kHz ultrasonic welders are by far the most widely used; due to their versatility, these welders can deliver high-power (up to 6000 W) and high-amplitude outputs, and they can accommodate a wide range of available tooling sizes. For a contract manufacturer that produces ultrasonically welded parts, 20-kHz equipment can be a great investment since it offers the promise of future use in many applications.
However, there are some instances—especially with small and delicate parts—where the high-power, high-amplitude capabilities of 20-kHz equipment may prove too “aggressive” for certain assemblies, potentially resulting in damage. One possible solution is to reduce the input amplitude, but this won’t work if the amplitude applied is below the recommended level for the polymer being welded.