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.
Classification of Ultrasonic frequency ranges.
Basic Principals of Ultrasonic Welding
The two thermoplastic parts to be assembled are placed together, one on top of the other, in a supportive nest called a fixture.
A titanium or aluminum component called a horn is brought into contact with the upper plastic part.
A controlled pressure is applied to the parts, clamping them together against the fixture.
The horn is vibrated vertically 20,000 (20 kHz) or 40,000 (40 kHz) times per second, at distances measured in thousandths of an inch (microns), for a predetermined amount of time called weld time.
The clamping force is maintained for a predetermined amount of time to allow the parts to fuse as the melted plastic cools and solidifies. This is known as hold time.
Once the melted plastic has solidified, the clamping force is removed and the horn is retracted. The two plastic parts are now joined as if molded together and are removed from the fixture as one part.