Ultrasonic welding of plastic: joining technology for adhesive-free applications

Anyone who has ever had to wear a plaster for a long time knows that this can sometimes irritate the skin. This is often triggered by the adhesives that hold the individual layers of a plaster together. Medical wearables, such as CGM sensors for diabetics, are also often glued together. As these have to be worn permanently, skin irritation is more than just annoying for sensitive patients. One option is therefore to dispense with the adhesive altogether and join the plastic components using ultrasonic welding. We spoke to Michael Boerner, Head of Global Business Development Plastics at Herrmann Ultraschalltechnik, about this process for K-Mag.

Mr. Boerner, chemicals such as isobornyl acrylate (IBOA) in adhesives can cause skin irritation in wearers of medical wearables. You therefore rely on ultrasonic welding. How can skin reactions be avoided?

Michael Boerner: Ultrasonic welding makes it possible to join medical plastics without the use of chemical joining processes such as adhesives or solvents. Instead, mechanical vibrations are used to briefly heat the materials at the contact points and securely join them together. As no potentially irritating substances such as isobornyl acrylate are used, there is no risk of skin irritation or allergic reactions. This is a great advantage for patients, especially in the case of medical wearables such as CGM sensors, which are often worn directly on the skin for long periods of time.

Can you explain the ultrasonic welding process in more detail and how mechanical vibrations are used to securely join plastics without adhesives?

Boerner: During the ultrasonic welding process, electrical voltage is converted into mechanical vibrations via a converter. These vibrations are transferred to the two joining partners via the welding tool - the sonotrode. The friction between the two components generates heat at previously defined contact points, which melts the plastic in a targeted manner. This creates high-strength joints at a molecular level.

In the laboratory, the parameters for each plastic and for each application are checked and the results analysed in detail. This means that customers can be sure of receiving an optimised product; Copyright: Herrmann Ultrachalltechnik

What specific advantages does ultrasonic welding offer compared to traditional bonding or welding methods, particularly in terms of process time and energy efficiency?

Boerner: Depending on the application, ultrasound often only needs a few hundredths of a second to create tight connections. The components can also be processed immediately. This is particularly advantageous for the manufacture of medical products, which are produced automatically due to the high demand.

In addition, ultrasonic joining requires very little energy and no other composite materials. Adhesives have to be cured or, in the case of thermal processes, the temperature on the tool has to be maintained permanently, which requires a lot of energy.

Thermal joining processes are slower and generate stress on the component due to the direct external temperature effect, which can lead to deformation or damage, for example. Energy, and here we are talking about very low energy requirements, for the ultrasonic process is available worldwide. Chemical joining agents often have to be transported over long distances. This environmental factor is also eliminated by using ultrasound as a joining technology.

Thermal joining processes require considerably more energy, as heat has to be constantly supplied. The targeted use of energy only in the joining zone, as with ultrasonic welding, is not possible.

Ultrasonic welding is suitable for sensitive components such as skin patches and sensors. How is it ensured that the thermal and mechanical stress on the component is kept to a minimum?

Boerner: The decisive factor here is that the entire welding process is developed individually for the customer's application. We use our ultrasonic laboratories for this, where the application developers determine the correct welding parameters.

The most important parameters include force, time and amplitude. Welding tests can be used to determine exactly how these parameters need to be set in order to create a safe and gentle joint. The values are stored in the welding system's control system and can be precisely reproduced in the customer's production process. Another crucial component is the energy direction generator - this is the name given to the points where the melt is created during the welding process. It is important that this energy direction generator is designed correctly so that the two plastics are only joined at this specific point. As a result, the surrounding areas of the component are only subjected to minimal thermal and mechanical stress.

How does the choice of plastic material influence the effectiveness of ultrasonic welding?

Boerner: In principle, all common thermoplastics can be joined with ultrasound. Which plastic is ultimately chosen has an immense influence on the welding process, as each plastic has unique properties. We have access to databases and many years of experience. As a trusted advisor, we also solve challenging applications.

What measures do you use to ensure consistent welding quality and how are the optimum welding parameters determined for different applications?

Boerner: The reproducible results are one of the biggest advantages of ultrasonic welding technology. This is made possible by intelligent process control. The ideal parameters for the process are stored there. It also monitors the entire joining process and collects up to 150 values per weld. These can be called up and evaluated at any time. This gives manufacturers the option of seamless, digital quality control.

In order to determine the correct parameters, extensive series of tests are carried out in the ultrasonic laboratory for each application. Thanks to the experience of our application engineers, they often know which parameters are suitable for which plastics after just a few tests, which speeds up process development.

To what extent is ultrasonic welding suitable for components with complex geometries, and what design considerations need to be taken into account?

Boerner: Ultrasonics can also be used to reliably join components with complex geometries. However, there are some factors that need to be taken into account, such as the wall thickness of the material or the use of different materials within the same application.

As already mentioned, the seam design also plays an important role in the welding process. This is why close cooperation between customer and ultrasonic partner is so important - especially when it comes to more complex components.

You will also be exhibiting at K 2025. What can visitors look forward to when they visit your stand?

Boerner: Ultrasound offers many advantages as a sustainable and fast joining technology, especially in the medical industry. We want to demonstrate this in particular at our stand at K 2025. To this end, we have specially developed a new test specimen on which we can demonstrate the many different possible applications of ultrasound live.

As most medical products are manufactured automatically, we will also be showing how our modules and components can be integrated into various production systems to save space - thanks to a retrofit of this kind, manufacturers with existing systems can also benefit from our technology.

To what extent is K the perfect platform for you to present your company's innovations?

Boerner: With its wide reach and international audience, K is always a highlight for us and an excellent stage for presenting new trends and topics. As we are also active in numerous other sectors besides medical, such as automotive, packaging and nonwovens, we also benefit from the wide range of visitors. We are therefore looking forward to an exciting trade fair and a lot of interest in our stand F28 in hall 11.