The Principles of Ultrasonic Welding
by Steven A. Williams
Branson Ultrasonics, a business of Emerson
Ultrasonic welding is a process used to join two plastic parts together to form a strong, finished assembly. The process relies on high-frequency (ultrasonic) vibrations being generated and applied to the parts via a horn. Welding occurs as the vibrations are absorbed in the interface between the two parts, generating friction and causing the plastic to melt. The ultrasonic vibrations are generated by a series of components, including the power supply, converter, booster and horn, which ultimately delivers the mechanical vibration to the parts.
The role of the power supply is to convert the incoming line voltage (at 50 or 60Hz) into a new frequency. This electrical energy then is sent to the converter, which as the name implies, converts the electrical energy into mechanical vibrations. The converter consists of piezoelectric ceramic discs, which expand and contract at the rate of the supplied electrical energy. The magnitude of the vibrations is referred to as amplitude, a term that becomes very important when specifying an ultrasonic system for a given application. The vibrations then are transmitted through the booster, which typically increases the amplitude by a predetermined multiple, also known as gain. Finally, the booster’s output amplitude is transmitted through the horn, where it then can be delivered to the plastic parts. The combination of converter, booster and horn is commonly referred to as the ultrasonic stack.
To deliver the vibrations to the parts, the stack is placed in an actuator, a mechanical system that applies the next critical element of ultrasonic welding: force. The actuator may consist of a pneumatic cylinder or other means of actuation to drive the stack down to the part. Actuators often contain other sensors and devices, such as linear encoders or load cells, to provide feedback during the weld. The actuator drives the horn down to the part and applies force until a trigger force is met; at this point, ultrasonics is applied. The vibrations then are delivered to the parts, where they become focused on a triangular bead of material – known as an “energy director” – which can be considered sacrificial melt material in the weld joint. Once the material melts, the ultrasonic vibrations are terminated and the actuator continues pressure until the plastic is solidified and a strong bond is formed. The entire weld process, from start to finish, typically is completed in one to two seconds.
As mentioned, one of the most important elements of ultrasonic welding is the amplitude (vibrations) that the stack provides. The reason for this is that certain plastics require more amplitude to weld than others. Amorphous polymers (ABS, polycarbonate, polystyrene) tend to require low amplitude, while semi-crystallines (nylon, polypropylene) require significantly more amplitude to weld.
An experienced applications engineer always can work to determine the proper frequency and tooling selection for a given assembly and material type.
Ultrasonic welding is a widely accepted assembly method in the plastics industry for a variety of component assembly applications in all major markets: medical, B&CE, automotive, appliance, nonwovens, packaging and more. Advantages to ultrasonic welding include short cycle time, high-strength bonds and the elimination of consumables (i.e., adhesives). When considering whether ultrasonic welding is right for an application, it is best to consult an experienced applications engineer to determine the appropriate assembly method.
Ultrasonic Welding Glossary
Amplitude: The magnitude of the mechanical vibrations, measured as the peak-to-peak displacement at the face of converter, booster or horn.
Booster: The booster amplifies the magnitude of the mechanical vibrations from the converter and delivers the vibration to the horn.
Converter: Made up of piezoelectric ceramic discs, the converter takes electrical energy and converts it into mechanical vibrations.
Fixture (or nest): Supports the plastic parts during the weld.
Gain: The amount by which amplitude is increased or decreased in a booster or horn. Measured as the ratio of output amplitude to input amplitude.
Horn: The horn transmits the ultrasonic vibrations to the plastic parts themselves. The horn often is custom-tailored for a specific part and is one of the most crucial elements of an ultrasonic welding system.
Power Supply: Converts incoming line power into an ultrasonic signal.
Trigger Force: The minimum force that must be applied to the parts prior to ultrasonic energy being applied.
Ultrasonic Stack: The mechanical structure consisting of the converter, booster and horn in the ultrasonic welder.