What is ultrasonic welding? Functional principle and applications

Ultrasonic welding uses the mechanical vibrations in the material that are achieved with ultrasound for welding. The process is completed in a very short time and is characterized by high efficiency. It is also suitable for joining different materials together.

What is ultrasound?

Sound is much more than what we can perceive with our ears. It can also cause much more than just the sound of certain tones. It can cause mechanical vibrations that cause glass to shatter, for example.

When a note is struck, for example on the side of a guitar, this leads to vibrations. These are caused by the pressure and density of the air. They extend from the sound source in all directions in the form of waves. This is not only possible in air. Any elastic medium can transmit sound. This includes solids, fluids and gases.

Ultrasound is a sound that humans cannot perceive. However, this does not mean that it does not exist. It cannot be heard because it vibrates in a very high frequency range. This makes it very suitable for use in various areas of medicine and industry.

Sound waves are differentiated according to their frequency. The frequency in turn indicates how many waves per second are generated by the sound. For ultrasound, the frequency is between 20 kHz and 1 GHz. The frequencies from 20 to 70 kHz are used for ultrasonic welding.

The human ear, on the other hand, is only able to perceive vibrations in the range of 16 Hz to 20 kHz. This means that ultrasound is located exactly above the audible range and, at best, can still be detected as a vibration.

How does ultrasonic welding work?

As soon as the ultrasound hits a material, the molecular chains it contains are set into vibration. This causes the molecules to move in relation to each other, which leads to friction and thus to the release of heat. This frictional heat causes the material to begin to melt. When pressure is added, the workpieces are joined together at the level of the molecules within the joining zone.

What is ultrasonic welding used for?

Ultrasonic welding creates joints in fractions of a second. No auxiliary materials are used. The process is therefore very suitable for toys, car parts and packaging. This allows these tasks to be completed:

• It is possible to combine dissimilar materials, which is what happens when installing airbags, for example.
• Membranes can be embedded, which is used for medical components with filter membranes.
• Injection molded parts can be joined together, which is particularly important for toys.
• Form-fit connections can be created, as is the case with magnetic contacts in chargers, for example.
• Magnets and sockets can also be countersunk, for example to produce encapsulated magnets that can be used to control sensors.

What are ultrasonic welding systems?

Welding systems are used for ultrasonic welding. These differ depending on the work and the objective of the work, but they always consist of the same components. These include active components, which generate or conduct the sound, and passive components, which absorb the forces used in the process and thus support the weld seam.

Active parts are:

• The ultrasound generator that produces the sound
• A converter
• An amplitude transformation piece
• The sonotrode that is used as a welding tool

This is often referred to as the oscillating structure. This consists of the sonotrode, the converter and the amplitude transformation piece.

The passive components are:

• The holder for the workpiece
• The anvil

How is ultrasonic welding used?

The principle of ultrasonic welding is very simple. The vibrational energy of the ultrasound is focused on a single point. This is known as energy focusing. This ensures the development of heat and the melt. Focusing allows the welding process to be precisely defined and energy consumption to be kept to a minimum.

The energy can be focused in different ways. However, the aim is always to apply the vibration energy in exactly the right place. The geometry of the workpieces must also be designed for this. The energy direction generator, often abbreviated to ERG, focuses the energy on the material. This is how it works:

Focusing energy through the anvil

The anvil has its own structure. Its shape is now used to focus energy. To do this, the energy is focused on the raised parts of the anvil. This is where the contact points are located and where the melting begins. This is mainly used for web materials such as cardboard, film and nonwovens.

Energy focusing with the help of the sonotrode design

The energy can also be focused by the welding tool. The sonotrode or its contour is used as an aid for melting. The energy is concentrated at the tip so that it is at its hottest there. This is used in ultrasonic riveting, for example.

Focusing energy through the design of the seam

Focusing the energy through the seam design bundles the energy with the shape of the components. For this purpose, the area where the joint is to be created is shaped accordingly. This can take the form of an edge or a point, for example.

What are the parameters for ultrasonic welding?

A number of process parameters are important for the quality of the weld seam during ultrasonic welding. They must be adjusted to the process itself and the material. Once set correctly, not only is a good seam created, but it can also be created again and again.

The amplitude

The welding tool itself oscillates with an amplitude of between 5 and 50 micrometers. This cannot be seen with the naked eye. The corresponding power is achieved by adjusting the mechanical oscillation generated in the converter in the amplitude transformation piece accordingly.

The welding force

To ensure that the weld seams are even and tight, it is necessary to apply force to the material. The strength of the force depends on factors such as the joining surface, the size of the parts and the power.

The trigger point

The trigger point determines when and where welding starts. The sonotrode presses against the workpiece. This is done with the so-called trigger force. If the material now yields only minimally, the ultrasound is triggered. The force acting on the sonotrode now changes from the trigger force to the welding force. This ensures that the starting point for welding remains the same. This in turn ensures that the result is of consistent quality.

The welding time

The welding time is the time spent on the actual welding. It starts when the ultrasound is triggered and switched off. The ultrasound starts with the trigger point and switches off as soon as the corresponding predefined criterion is reached.

The holding time

Once the welding time is complete, the workpieces should be kept under pressure for a while. This allows the welding seam to cool down and solidify evenly over its entire length. The welding tool, which does not heat up while working, supports the cooling process.

What materials are used for ultrasonic welding?

The materials that can be joined using ultrasonic welding range from thermoplastics, i.e. plastics, to hard materials. These include non-ferrous metals such as nickel, aluminum, copper and brass.

Plastics

We can no longer imagine a world as we know it without plastics. Whether it's a printer cartridge, a toy for children or the remote control for the TV, plastic is used everywhere.

Plastics have to be joined together to form the various devices that we know and use. This is done with ultrasonic welding. It can also be used to seal them.

Metals

Metals are joined using ultrasonic welding, particularly in the consumer goods and automotive industries. Copper and aluminum can be joined using this process. This produces high-quality weld seams thanks to the process stability and verifiability of the processes. The result can also be repeated as often as required.

Non-wovens

Ultrasound can also be used very efficiently in the field of non-wovens. Permanent and secure bonds can be produced here without a heating phase and without the use of adhesives. This also eliminates the need for typical cleaning intervals in the machines.

Ultrasonic welding can be used to produce diapers, cotton pads, hygiene articles and surgical masks. Ultrasound is not only used for joining. It is also used for precise cutting, perforating and embossing.

Packaging materials

Packaging is also super important in our society. It keeps the coffee powder dry and retains its aroma and the cheese stays fresh for a long time. The packaging is sealed using ultrasound. This is done with very thin materials so that packaging materials can be saved and the environment protected.

A large number of welding processes are available in different categories. This means that the right process can be selected depending on the economic requirements, the demands of the material and the requirements for the weld seam.