Sound is the propagation of smallest pressure and density variations in an elastic medium (gas, liquid, solid-state body). For example, a noise is generated when the air in a specific spot is compressed more than in the surrounding area. Subsequently, the layer with changed pressure propagates remarkably fast in all directions at speed of sound of 343 m/s.
Acoustic frequencies between 16 kHz and 1 GHz are referred to as ultrasound; in industrial settings we call it “ultrasonics”. To clarify: people are able to hear frequencies between 16 Hz and 20 kHz; i.e. the lower frequencies of industrial ultrasonics are audible, especially if secondary frequencies are generated. And what is more, ultrasonics is palpable when touching the weld tool. For ultrasonic welding, the frequency range is between 20 kHz and 70 kHz.
Additional fields of application: Imaging ultrasound in the field of medical diagnostics ranges between 1 and 40 MHz. It is not audible or palpable. In the field of industrial material testing, ultrasonics is used at frequencies from 0.25 to 10 MHz.
Ultrasonic welding of thermoplastic materials is a weld technology utilizing mechanical vibrations to generate heat due to molecular friction. These vibrations excite the molecules in the plastic so that they start moving. The plastic becomes soft and starts melting. The components are bonded by cohesive or form-fit joints After a short hold time under pressure, they are firmly joined molecularly.
Ultrasonic weld technology
Ultrasonic joining technology has been established as joining method for technical thermoplastic materials in a large variety of applications throughout the plastic-processing industry.
- high process speeds
- repeatable weld results
the technology is preferred for high-volume production in the automotive, electrical, medical, packaging, hygiene, and filter industry.
Good bonding quality in terms of strength, tightness and visual appearance are particularly achieved if the part material and design is suited for the ultrasonic process. This means that from the beginning they must be designed such that ultrasonic waves are focused in the weld zone.
The secret of ultrasonic welding is focusing the ultrasound with an energy director. In this way, it is possible to generate heat and subsequently melt, restricted to a locally defined area, while using only little energy. Large-area contact surfaces are counterproductive; they require high power and only achieve undefined joining areas with poor strength.
Energy focusing is achieved by:
- the energy director (ED)
- sonotrode design
- contouring of the anvil profile
Possibilities of energy focusing due to variations in the joint design
Focusing by weld geometry of the plastic part. Thus, injection molded, thermoformed, or blow-molded parts often have specially shaped zones within the joining area. This is referred to as joint design, e.g. in the form of a tip or an edge. These special geometric features are the energy directors (ED). With the help of these defined, small contact surfaces, it is possible to apply the ultrasonic energy into the joining area in a targeted and locally defined manner.
Possibilities of energy focusing due to variations in sonotrode design
Focusing by the sonotrode shape (weld tool); e.g. during the ultrasonic staking process, the sonotrode assumes the task of energy concentration.
The centering tip serves as melting initiation aid.
Possibilities of energy focusing due to variations in anvil structures
Focusing by means of contouring of the anvil structure for web materials such as film, cardboard, and nonwovens. Local deformation is achieved by anvil or sonotrode
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