First developed in the 1960s, ultrasonic welding is a form of friction welding that joins parts together by vibrating them against each other. Its initial growth phase came in the 1980s and 1990s, according to suppliers, who say the technology is now in an era of refinement.
Ultrasonic welding is very popular for joining amorphous and semicrystalline thermoplastics and thermoplastic composites, but not applicable to thermoset materials.
All ultrasonic welders feature a power supply (generator), computer controller, actuator, transducer, booster and a sonotrode (titanium or aluminum) horn. The actuator is driven by a pneumatic cylinder or a ballscrew attached to a stepper motor or servomotor. The transducer, booster and horn make up the ultrasonic stack.
Power is sent to the transducer, which contains piezoelectric crystals that expand and contract. This action causes mechanical vibrations that the booster increases and transfers to the horn, which, in turn, transfers ultrasonic energy to the joint interface.
Heat is generated at the joint through a combination of friction and hysteresis. The heat melts a small amount of plastic on each part. When the ultrasonic vibrations stop, the molten material solidifies and the weld is achieved. Actual weld time is usually between 200 and 400 milliseconds, and total cycle time typically less than 1 second.
Current machines allow manufacturers to control most every key parameter of the ultrasonic welding process: amplitude, weld time, hold time, energy, power, collapse distance and horn velocity. Operators can precisely program these parameters into the controller of handheld, benchtop, inline and robotic systems. The frequency range of ultrasonic welders is 15 to 70 kilohertz, with 20 kilohertz being the most common.
Higher frequencies are used for more delicate assemblies because they require smaller vibration amplitudes, which are easier on parts. These amplitudes range from 5 to 150 micrometers.
Last October, Branson unveiled its GSX-E1 Series ultrasonic welder, the first unit based on the company’s new GSX platform. Tarick Walton, global product manager for ultrasonics at Branson, says the platform took three years to develop. It is based on more than 70 years of ultrasonic welding experience, coupled with feedback from more than 400 customers.
“This first model meets manufacturers’ growing demand for damage-free welding of smaller and more intricate plastic parts, such as those with thin walls,” explains Walton. “Early users include manufacturers of medical and wearable devices with embedded electronics.”
Modular and flexible, the welder has the lowest trigger force in the industry at 5 newtons. It offers excellent repeatability with precise actuator control throughout the welding process. Also standard are various smart technologies such as intuitive graphing, quick tooling changeover and actuator setup, and an easy-to-use HMI.
Fast weld cycle time is the main benefit of ultrasonic welding, but manufacturers also like that it offers an economical way to assemble small and midsized parts, and those that are too complex to be molded as one piece. Versatility is another benefit, as the same equipment can often weld different parts and thermoplastic materials. The process also creates neither fumes nor smoke, so no ventilation is necessary.
Ultrasonic welding does produce some flash and particulate matter, but the vibrations are fine enough that it isn’t excessive. If a weld needs to be aesthetically appealing, it can be designed with a flash trap or a recessed groove to hide any excess melt.
“Servo-driven ultrasonic welders have been around for quite awhile, but their data feedback capabilities were only so-so until the last few years,” notes Brad Rogers, director of sales for North America at MS Plastic Welders LLC. “Many servo welders, including ours, are now able to get feedback and make weld adjustments in milliseconds for parameters such as time, distance and energy input.”
Rogers adds that medical-device makers tend to have a greater need for immediate feedback during ultrasonic welding than other manufacturers, such as those, for example, that swage over a thermoplastic post to join a dissimilar material onto a housing. Feedback from advanced ultrasonic welders helps manufactures ensure that each welded medical part complies with electronic records and signatures.
More than 20 years ago, MS Plastic Welders introduced modular ultrasonic systems with multiple weld heads. These systems are used for applications like automotive bumper and interior door assemblies.
A few years ago, the supplier introduced the soniTOP line of benchtop and inline servo-based ultrasonic welders. The inline systems are typically integrated into automation lines after a welding process has been validated on a benchtop system.
One of the most unique features of the soniTOP machines is a built-in tensile test area. This lets companies automatically program in when a tensile test must be performed and ensures that the operator knows which part to test. If the part passes the test, production continues. If the part fails, the end-user decides which further steps to take—including retesting the part, alerting management or stopping all line production.