A key role in ultrasound welding is played by the sonotrode. But what kind of “tool” is this exactly, and what is it used for? Let's find out, step by step.
Sonotrode is the name given to the component that transmits vibration to the weld parts (ultrasound welding of plastic materials). It fulfills the following functions:
- transmission of vibration energy
- transmission of pressure
- transmission of amplitude
- shaping of the workpiece while the material is malleable
The sonotrode must be designed and produced with extreme care, because only a design that perfectly meets the needs of the workpiece can guarantee good results. If the sonotrode is poorly constructed (unsuitable frequency, transformation and amplitude) this may cause the transducer to break.
(Examples of sonotrodes of different sizes)
Three different materials are used to make sonotrodes, and each has its own specific characteristics. They are chosen according to the intended use.
- Aluminium: this is used for sonotrodes that are not particularly subject to mechanical stresses, or for large sonotrodes, where weight and cost are important factors. The material most commonly used is an aluminum alloy with good mechanical properties (hardness and sound propagation speed), usually Ergal or Avional.
- Titanium: this is the material chosen when the working conditions are the main criterion. Its metallurgical characteristics give it the best vibration properties, as well as an excellent lifespan that can be further increased through specific surface treatments. Sonotrodes for processing plastics containing glass fillers are usually made from titanium.
- Sintered steel: this is used for welding parts that are particularly harsh for metals like aluminum and titanium. It has high hardness and always undergoes a hardening process before being used. The sonotrodes in this material are used for assembling metal inserts.
For optimal welding, it is recommended not to use large sonotrodes. The vibration surface must in fact be as uniform as possible. Therefore, when welding large pieces, several resonator stacks must be used. The following, in particular, should be noted:
- the maximum amplitude that can be applied at the sonotrode input ξ1=40 µm
- the maximum sonotrode output amplitude for simple, round shapes ξ2=90-120 µm
- the maximum output amplitude for large sonotrodes with grooves ξ2=60 µm
- the need to avoid sharp edges. All edges must be rounded to avoid breakages
- sonotrodes must be assembled using proper tools.
The use of advanced design systems has made it possible to study sonotrode behaviour during the design stage itself. Sonotrodes can be designed in a 3D environment and simulated using the finite element method (FEM). It is possible to study the vibration behaviour of the sonotrode, and thus to achieve an extremely uniform distribution of its amplitude. This results in high efficiency during the welding process while also avoiding potential critical issues that could lead to breakage. The FEM makes it possible to define the resonance frequency with extreme precision.
(examples of FEM simulation in sonotrode construction)
Sonotrodes are available in different types, suitable for different needs. The most common are:
- stepped sonotrodes: these have circular shape in which the two diameters are connected with a small half round edge. They produce a strong amplitude. Structurally, they are subjected to strong mechanical stress.
- exponential sonotrodes: these sonotrodes have the ideal fatigue curve. Their amplitude is relatively low. They are widely used for the insertion of metal parts into thermoplastics.
- catenoid sonotrodes: these are the most commonly used type. They are characterised by good amplitude and low fatigue.
- rectangular sonotrodes: these, too, are widely used. To avoid horizontal or reflection oscillations that could break the sonotrode, it is necessary to make grooves (slots) in it. Rectangular sonotrodes are often used as adapters for other sonotrodes when performing multiple welds.
- bell sonotrodes: these are used for large circular welds. Bell sonotrodes, too, need to be grooved.
For those interested to learn more about ultrasound welding and the components it requires, see “Ultrasound welding: the leading players, that is the elements involved”. More information can also be obtained from Sirius Electric Srl (www.siriuselectric.it), a leading Italian manufacturer and supplier of welding machines for plastic materials and a leading designer and producer of sonotrodes.