Everything you need to know about ultrasonic welding, that nobody ever told you..

(Basic principles)

What is “ultrasound”?
The audible frequency of mechanical waves, between 16Hz and 16.000 Hz. Frequencies not audible to the human ear below 16 Hz are called “infrasound”, those higher than 16,000 Hz, ULTRASOUND.

What is the operating principle of ultrasound welding?
From a physical point of view, ultrasound welding is a physical phenomenon and consists of repeated alternating motion around a central position or equilibrium point.  It involves the thermoplastic parts, in most cases obtained by injection moulding, and creates within them progressive intermolecular friction that results in a localized temperature increase. This increase, concentrated in an extremely short time interval (tenths of a second), causes the melting of a well-defined area (the weld area) which, under the action of a constant thrust force, causes two parts to unite, penetrating one in the other. The pieces, therefore, are welded together without the need for glue or additives.

How does the welding process happen?
The heart of the ultrasound welding process is, live and pulsating, in the generator. Made up of a number of electronic circuits, the ultrasound generator has the task of transforming low-frequency network energy (50 Hz) and elevating it to a higher frequency (20 or more kHz) with voltage and current proportional to its power. Electrical power generated this way is transported by a co-axial cable to the converter (or emitter), where the actual electrical-mechanical transformation takes place, originating the vibration. The characteristics of this vibration are transmitted to the part using a specific frequency (20, 30, 36, 40 kHz), a characteristic amplitude (from 10 up to 200 micron peak-to-peak), together with a thrust force and duration calibrated based on the characteristics of conformity required for the end piece. It is however important to emphasize that the power produced by the generator outside the welding process must be as low as possible. The converter (or emitter) is a solid formed by piezoelectric elements and, exploiting the physical principle of piezoelectricity, it produces a stationary vibratory wave which is then amplified by the two final transmission elements: the mechanical amplifier (or booster) and the sonotrode.

The weld piece, in contact with the sonotrdoe, begins to vibrate and heat locally the weld area. This is how the weld process begins. The heat is transmitted by contact to the lower piece, which begins the same process of localised welding. The weld area can be imagined as the confluence between two rivers that mix together instant after instant. The final stage of welding involves the interruption of the ultrasonic vibration followed by a controlled cooling period under pressure (or force in the most advanced systems), that leads to the resolidification and compacting of the entire area. The molecular structures of the two separate parts are therefore recombined together to create a common joint area.

Weld area for a correct ultrasound welding process

Can all plastics be ultrasound welded?
Plastics are highly molecular materials (polymers) today exclusively produced synthetically. They are divided into thermoplastics, thermosets and elastomers. Thermoplastics are materials that are sensitive to heat which in turn are divided into two major families: amorphous thermoplastics and semi-crystalline (partially crystalline) thermoplastics. Amorphous materials have a more rigid and defined structure, and because of this they are also more suitable for ultrasound welding. The high frequency vibration is generated by the converter and transmitted to the part through a dedicated tool, called the sonotrode. The characteristics of the amorphous materials offer a rigid material means that adapts perfectly to the transmission of the sonic waves.

Semi-crystalline materials are, on the other hand, suitable for welding nearer the contact surface of the sonotrode, which is called near field and is characterised by a distance between the sonotrode and the weld area of less than 6-8 mm.

Automotive is one of the segments where ultrasound welding is most commonly used

What are the main advantages of ultrasound welding?
Ultrasound welding is a repeatable and extremely rapid technology: few seconds for a complete cycle producing excellent tight seals guaranteed for high pressures, not thermally damaging the material, with consistent results due to the resistance to torsional and tensile stress. Entering into specifics, the main advantage is the extreme precision: only the specific weld area is involved in the process, and this guarantees a well-defined and accurate weld. The molten material is conveyed, collected and subsequently compressed to form a compact and homogeneous joint. Through a specific design of the part, the ultrasonic waves are conveyed and directed to limit the general stress and optimize power consumption. The result: immediate use of the welded parts.

Examples of parts manufactured with ultrasound welding technology

Are there general guidelines for the process?
Professional ultrasound welding is based on three main factors: an in-depth study of the features of the individual solution, a prototype evaluation that identifies the basic management and control parameters (for example, identifying the characteristic amplitude, the energy required and the weld measure) and guided application in production that aims to define the limits and acceptability of the final product. Overlooking design aspects and choosing tools that are not professional often results in errors being made with an impact which is hard to calculate.

If you want to know more about ultrasound welding, you might also be interested in “Saldatura a ultrasuoni: vantaggi e svantaggi.”, Ultrasonic welding - Wikipedia”  "Plastica e saldatura a ultrasuoni: unione perfetta" and for more information contact Sirius Electric Srl (www.siriuselectric.it), leader in Italy in the production and sale of plastic welders and in the design and manufacture of sonotrodes.

Keep up the good work.



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