Torsional ultrasonic welding – savings potential for the automotive industry

Topic of the month - September 2014

Photo © Telsonic

The torsional ultrasonic welding process can be used to join plastic parts with fine contours and openings, such as nozzles, valves or connectors, without damaging the parts themselves. Photo © Telsonic

Savings potential for the automotive industry

Welding two plastic or metal parts securely and well together is a well-known method which can be done using conventional techniques such as longitudinal ultrasonic welding or adhesive methods, producing reliable results. Where the limits of these joining techniques really are, however, was proven in 2006 by the Swiss company, Telsonic AG, when it developed a torsional ultrasonic welding technique: SONIQTWIST®.

SONIQTWIST® has quickly established itself as a suitable joining technique for various different plastics and metals with excellent results. The torsional energy input at the joining zone is gentle and is also suitable for highly sensitive parts. However, large forces can also be exerted onto the joining zone within a short space of time, something which is important for aluminium or copper piping, for example. Even painted plastic parts can be joined securely without leaving any visible imprints.

How does this revolutionary process, which automobile manufacturers and suppliers, as well as those in the medical technology industry, cannot get enough of, actually work?

Suitable technique for many different jobs

The torsional technique is also suitable for joining plastics and metals such as copper or aluminium. However, this technique also appears to produce the best results for flanging, riveting, separating or fine recasting processes, as well as when calibrating the dimensions and shape for functional surfaces.

The minimal amount of load applied to the parts being welded together also protects sensitive electronic components. The rate of vibration in the lower section of the joining part is approximately just one tenth when compared to conventional ultrasonic welding techniques.

What is more, the membrane remains unaffected so allowing filter membranes and other thin parts, such as films, to be welded. Ridge formation or a thinning of the material is hardly visible on films, which makes this technique especially interesting for use in medical technology.

Moreover, the technique is not hindered by interference media in the weld seam. Water, grease, silicon, oil and other materials are incapable of adversely affecting the result. Consequently, the technique produces reliable results when tightly welding oil dampers for furniture parts without causing the fluid to escape.

Photo © Telsonic

The torsional ultrasonic welding process from Telsonic AG delivers the best results when joining plastic parts that are used for additional functions, such as distance sensors, cable clips or fixing pins, on painted, exterior car parts. Photo © Telsonic

Apply a high energy output within a short amount of time

Technically speaking, SONIQTWIST® applies a high-frequency friction welding technique which is somewhere between vibration welding and ultrasonic welding. The sonotrodes are subject to torsional movement around their longitudinal axis and alternate direction. Using a high frequency of 20 kHz and an amplitude of up to 80 µm in a short blast – between 0.1 and 0.4 seconds – a large amount of energy is applied to the area where the two parts meet. This interfacial friction causes the two parts to be welded securely to one another.

The torsional presses are similar to conventional ultrasonic welding and have a generator, converter and sonotrode. In contrast to conventional systems, however, the sonotrode does not oscillate longitudinally but torsionally which explains how stress on the parts is reduced. At the same time, an ultrasonic output of up to 10 kW can be passed through the sonotrode.

As a result, even tough materials can be welded securely, something which has rarely been possible using conventional ultrasonic welding techniques. A pull-off resistance of 500 N or even more can also be achieved depending on the component and specifications required. Secure welds can also be created some centimetres away in the far field.

The way this works is then basically the same. Consequently, the electrical vibrations formed by a generator in torsional processes are relayed into a PZT vibration transformer to be converted into mechanical vibrations through a piezoelectric effect. Converting linear generated vibrations into torsional movement is done by assembling the acoustic components in the SONIQTWIST® vibration head in a particular order.

High energy input possible within a short amount of time

A generator emits a frequency of 20 kHz and up to four converters can be installed into a sonotrode. Consequently, a maximum output of 10 kW is available which is most often required when welding metal parts together.

The high frequency allows the energy output to be very high for a short space of time – usually 0.3-0.4 seconds. Once the ultrasonic equipment has been turned off, the sonotrode returns to its starting position, meaning there is no need to replace any parts.

The process does not require an external supply of heat. The sonotrode makes contact with the upper part of the parts being welded together. Combined with the welding pressure exerted between parts, this movement generates friction between the molecules and where the two parts meet. Friction in the weld zone causes temperatures to rise above the melting temperature.

The result is that plastics then plasticise, or the molecules in metal are bound together. The process can be safely operated, monitored and repeated. The established monitoring procedure can be implemented in full. All parameters, such as frequency, welding time or energy input, can be set and monitored at any point.

Moreover, the control software provides a range of options for controlling the process in line with various parameters, such as energy input, weld depth or process duration. Depending on amplitude and pressure, processing speed is many times faster than in friction welding processes.

Photo © Telsonic

Difficult membranes can be embedded easily into a plastic part using the torsional ultrasonic welding process, SONIQTWIST®. Photo © Telsonic

Examples from automotive and medical technology

In the case of attaching plastic brackets to the inside of painted bumpers for holding distance sensors, conventional joining techniques only produce satisfying results at a greater cost. Above all, the low rate of adhesion is something that many consistently object to.

Besides strong adhesion, it is particularly important that no marks are left visible on exterior parts that have already been painted. The criteria are particularly strict in this regard, and are tested scrupulously using a special light test. Since SONIQTWIST® manages without an energy director and so does not press the sonotrode into the bumper, it rules out the risk of marking completely.

The torsional ultrasonic welding process also does not need to use an additional auxiliary sheet. Using SONIQTWIST®, users can avoid the areas around the bracket ("ears" or "wings") that have been sprayed. When compared to adhesive techniques where the surface has to be completely free of grease and prepared using primer, the work required for this technique is considerably less.

Membrane for drip chambers without a membrane effect

When manufacturing drip chambers for infusion procedures in medicine, an extremely fine filter made from a hydrophilic membrane has to be welded into the chamber. The welding process ensures that the joints are extremely strong and that they are able to withstand thermal stress when being sterilised. Various different mixed plastics in particular can be joined together securely using SONIQTWIST®. The new technique has shown itself to be highly efficient when joining small polypropylene parts of 8 mm in diameter since it is more reliable and has very short cycle times.

Inline-capable and reliable

SONIQTWIST® torsional presses are flexible and are inline-capable and can be integrated into almost anything. Conversions are simple and quick to carry out thanks to the way the various components are assembled. The issue of inspecting the equipment without interrupting processes is also solved by using monitoring software. Whereas samples were only possible before, a 100% inspection is now possible. It is also possible to automate the equipment without having to make a considerable investment by using custom handling settings.