The ultrasonic vibrations are generated in a generator, which converts the incoming mains voltage into high-frequency energy. This high-frequency electrical energy is converted into mechanical vibration energy of identical frequency with the help of the sound converter.

The entire resonance unit consists of generator, converter, amplitude transformation unit and sonotrode, and it works through resonance. In ultrasonic welding, the heat required to melt the moulds is created by converting ultrasonic vibrations into mechanical vibrations and guided to the work piece to be welded at a certain contact pressure through the sonotrode. The plastic components function here as the energy carriers.

The mechanical vibrations occurring on the work piece are absorbed and reflected away at the boundary. Heat that melts the plastic is generated on account the boundary- and molecular friction. The dense vapour-emitting plastic layer creates a sound blockade, which leads to additional melting of the plastic and hence, to an acceleration of the reaction.

In ultrasonic welding, one differentiates between

• Far field welding and
• Near field welding.

One speaks of far field welding when the supporting base of the sonotrode is more than 6 mm from the actual welding zone. In this process, the energy is transferred through a work-piece half. Suitable are plastic materials that transfer mechanical vibration energy up to bonding surface predominantly with very little loss. These include polystyrene and its mixed polymers, polycarbonates and others.

Design of the bonding zones

The geometry of the bonding zones has an essential influence on the welding result. The following features must therefore be considered during design.

• Welded seam load
• Leakage
• Optics
• Expansion of molten material in the inward and outward direction
• Plastic material
• Positioning and clearance between the two components
• Supporting surface of the sonotrode
• Supporting surface in the holding tool
• Position of the bonding zones

A targeted and concentrated energy guidance is achieved with the help of an energy direction encoder.

The welding time increases inordinately without the energy direction encoder. Moreover, surface welding takes place in place of seam welding, which leads to inadequate strength and lack of uniformity in the welding results. Thermal damage to the material cannot also be ruled out.

The mash seam is used preferably to weld partially crystalline thermoplasts. Mash seams however, can also be used with amorphous plastics, especially if dense or stable welding connections are achieved.

The phases of ultrasonic welding

1. Start phase: Melting of the energy direction encoder (ERG)

2. Fusion phase: Further heat supply leads to further melting and a full bonding of the top and bottom components takes place.

3. Holding phase: The work piece is held in position and cooled under pressure during which the sonotrode yields slightly.

Distance-based welding

Relative: Stopping the ultrasound is defined at the outset by the trigger point.

Absolute: Stopping the ultrasound is defined from the tool zero position.

The lowering after the stop due to the holding pressure must also be computed in both cases. Output and time are variables and they change accordingly.

Time-based welding

The welding process is stopped after the expiry of a pre-programmed time. Output and distance are variables and they change accordingly.

Energy-based welding

A specified quantity of energy (in Joules) must flow into the component to be welded. Time and distance are variables and they change depending on the circumstances.

Amplitude:
Vibration width in the μm range, which is determined through the booster sonotrode combination and the power amplitude of the generator. It depends on the material to be welded.

Working pressure:
Pneumatic pressure in the advance cylinder. The working pressure is defined with the help of power, bonding surface area and component size.

Falling speed:
Adjustable forward travel speed of the advance cylinder.

Welding time:
Ultrasonic action time on the component to be welded.

Holding period:
Timed influence of force on the bonded zone during the hardening.

Trigger pressure:
Ultrasonic triggering force, the time structure of which is controlled through the falling speed.

Advantages of ultrasonic welding

• Ultrasonic welding offers the advantage that three-dimensional bonding can be implemented in short welding cycles.
• Reworking of the welded joints is not required.
• Ultrasonic welding is environment-friendly because no adhesives are required. This makes a sorted recycling possible.
• Ultrasonic welding requires only a small fraction of the power required by other thermal welding processes.

Ultrasonic principle

Sonotrodes

The full face of the sonotrode must be in contact with the work piece so that an optimal sound transfer is guaranteed. The holding tool should prevent lateral displacement of the work piece.

In the standard welding machines sector, KLN offers the proven Omega III Series with pneumatic drive or servo-electronic drive. Please contact our experts to work out a solution for your individual welding tasks.

Picture 5: Principal structure of a pneumatic ultrasonic welding machine

We would be glad to provide our long-standing experience in helping you select the welding geometry so that you may achieve optimal results.