Fundamentals of Ultrasonics

During the ultrasonic welding process, mechanical oscillations of the sonotrode are transferred under force into the plastic piece. Heat friction develops in the plastic part, creating intermolecular and boundary friction along the joining area. This absorbed friction results in heat build-up, which melts the plastic in the joint, generating a strong molecular bond in the welded area. Ultrasonic welding is a fast, repeatable and highly economical method for joining thermoplastic materials. It generates a true molecular bond with high strength, as well as a hermetic seal.

An ultrasonic "Joint Design" is a design feature of the plastic parts themselves, engineered into the part to improve the overall ultrasonic welding process. Also, a joint design called "energy director" (ERG) must be designed into the part around the area that will be ultrasonically welded. The fast and concentrated melting of the plastic material is achieved with the integration of specific geometric contours into the joining area of the plastic part, the sonotrode or the anvil. The energy director (ERG) defines the contact areas and directs the energy to accelerate the ultrasonic welding process. The reduction of the energy required for ultrasonic welding will decrease thermal stresses to the part and reduce any potential damage from fatigue to the plastic part. A missing energy director will extend the welding time and produce an inconsistent low quality area weld. Furthermore, it may induce thermal damage to the part.

The graph shows guidelines for form and dimensions of energy directors based on material and part size. Varying thermal characteristics of the material require a specific dimensioning of the energy director. The graph also shows the progression of the shear module to the mechanical loss factor with amorphous and crystalline thermoplastic materials.