Ultrasonic Welding: A Promising Technology to Weld Both Plastics and Metals

Fastening, stitching, and gluing are some of the popular techniques that we use to join two materials. However, we cannot deny the fact that welding provides the best results.

Welds provide strong bonds that last significantly longer than most of the other forms of joints. Numerous welding methods are in use today. Each type is different in their own ways, and are used based on the requirements and the type of material involved.

One of the most modern welding methods in use is ultrasonic welding.

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Ultrasonic welding uses ultrasonic vibration to join different materials together. One of the biggest advantages of ultrasonic welding is it can be used on materials other than metals, like thermoplastics.

By applying ultrasonic vibration to the contact points, the high vibration causes the material to melt. The typical frequency range used in ultrasonic welding is in between 15 to 40 kHz.

Since ultrasonic welding equipment can concentrate the vibration on a very localized spot, the weld is very precise.

Parts of the ultrasonic welding

Ultrasonic welding machines are created to effectively convert electricity to mechanical vibrations. The machines come with a microprocessor to read the temperatures and regulate the heating accordingly. 

Some other components that make up the stack of the machine are:

Power supply: A high-frequency ultrasonic welding machine requires a high voltage power supply.

Transducer: Transducer takes the high voltage current and converts it to high-frequency vibrations.

Booster: It works like an amplifier where it takes the high frequencies and then makes them even more powerful.

Sonotrode or Horn: A sonotrode or horn as it is commonly known is the medium between the materials to be welded and the machine. It focuses the ultrasonic vibrations to a localized point.

The materials to be welded is commonly fixed on anvil or some type of fixtures that hold them together. A pneumatic press is often attached to the machine so that the pressure can be transferred to the materials using the horn.

How does ultrasonic welding melt materials?

Ultrasonic is used in welding to provide small but rapid vibrations. When the horn or sonotrode is attached to materials, the ultrasonic vibrations will cause the materials to vibrate about half a millimeter or so in to and fro motion.

Such a minute motion is enough to melt the material because the rate of vibration is quite high. These vibrations cause the materials to rub against each other, and the resulting friction between them generates heat.

You can reenact a similar environment by rubbing your hands together rapidly. You will feel the heat being generated between your palms.

And if you continue rubbing rapidly, it gets quite uncomfortable. Imagine the heat generated within the materials when they rub against each other with speeds that are hard to pick up by the human eye!

The friction raises the temperature to such a degree that the contact points between the materials melt, paving the way for molecular bonding. This is how ultrasonic welding welds plastics.

However, there is a slight difference when it comes to welding metals. Ultrasonic welding is used to heat the metal to one-third of its melting point temperature.

At this point, the molecules between the two metals will travel across each other, creating a strong molecular bond. There is no melting in metals.

Advantages of ultrasonic welding

Ultrasonic welding has many advantages when compared to traditional welding technologies. Many industries use ultrasonic welding due to these specific combinations of merits.

Does not need external heat source: The primary advantage of ultrasonic welding is that it doesn’t need an external heat source. The heat is self-generated between the materials.

Fast: Since the frequency of vibrations is very high, ultrasonic welding is one of the fastest welding methods available in the industry.

Possibility of automation: Ultrasonic welding being uncomplicated in many ways, it is easy to automate. The ultrasonic machine comes with sensors that constantly monitor the temperatures.

Clean and strong joint: The contact surfaces melt/fuse upon welding, and it produces a very clean and strong joint.

Things to remember while using an ultrasonic welder

Ultrasonic welding is very effective at sealing or welding plastics. However, you cannot weld all types of plastics.

Like metals, plastics also have different types. And it is very important to ensure that the plastics to be welded are chemically compatible.

If you try welding polyethylene and polypropylene, it may melt together, but there will be no chemical bond. A chemical bond is important for creating strong weld joints.

Another factor to keep in mind while welding any material through ultrasonic welding is maintaining a uniform contact area. When welding metals, ultrasonic vibrations are propagated parallel to the plane.

Drawbacks of ultrasonic welding

Like all joining processes, ultrasonic welding also has some drawbacks that may or may not affect you based on your use case. However, you must understand them.

Restricted to just lap joints: Ultrasonic welding can only be used to weld lap joints.It cannot be used on another type of joints as it would require a specially designed horn and fixtures, and then again, success is not guaranteed.

Restricted weld area: We have discussed how ultrasonic welding creates highly localized welds. Hence, you cannot expect the welding to provide larger joints than an area of 250x250mm².

Cannot weld thick and hard metals: Thick and hard metals do not vibrate as much as thin and soft ones. This prevents them from undergoing mechanical vibrations.

Applications of ultrasonic welding

Ultrasonic welding is used in a variety of fields. One of the best examples of its use is in athletic shoes. The bonds that you see in the upper portions are not by stitching, but by ultrasonic welding.

Ultrasonic welding creates a much better finish than stitching or gluing.

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Ultrasonic welding of thermoplastics is common in the medical industry, automotive industry, and home appliances industry. Since there are no additional materials required to secure the bond, ultrasonic welding does not increase the overall weight of the finished product.

Ultrasonic welding is a technology from the 1940s, yet it is still the go-to welding method to create clean and strong bonds between different materials. Developments in ultrasonic welding are still underway to increase its scale of use.