How Ultrasonic Welding Works - Science | HowStuffWorks

In the musical comedy "Victor Victoria," Julie Andrews sings a high note at the end of her character's Parisian cabaret act. She sustains the note and champagne glasses around the room shatter. This brilliantly demonstrated how high-pitched or high-frequency sounds can break materials apart.

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But did you know that high-frequency sounds can be used to bond materials together? A technology called ultrasonic welding is used to assemble products from many industries -- ranging from medical devices to athletic shoes to automobiles.

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Typically, you can bond materials by using fasteners such as nails, screws or thread. This is appropriate for metals, wood, fabrics and plastics. For many plastics, glues are used; glues form chemical bonds between the glue itself and the surfaces of the bonded plastic materials. Metals can be held together by heating other metals as a bonding agent, such as lead solder in electrical connections. Alternatively, the metals can be directly melted together (welding); once the melted metal surfaces cool, the metals bond together. Welding usually requires an open flame or torch to achieve the high temperatures necessary to melt the metal surfaces together. So, it can be an expensive process for some manufacturing jobs.

A new, more cost effective welding technique was introduced in the s. The technique, ultrasonic welding, used ultrasonic vibration and pressure to bond metals together and required less energy than conventional welding.

Ultrasonic metal welding developed during the s through the s as the electronics used in the equipment became more sophisticated and computers could control the process. Since this time, the technique has been applied to plastics, where it has really become popular.

In this article, we'll look at the equipment and physical process of ultrasonic welding, how New Balance has used it to make athletic shoes, and the advantages and disadvantages of this technique. First, let's take a closer look at how sound waves bond materials, both metal and plastic.

Special Thanks

We would like to thank Kenneth Straka, Senior Product Developer for New Balance, for his assistance with this article.

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Rub your hands together rapidly. Notice anything? They warmed up, right? If you take a hammer and pound a metal surface rapidly and repeatedly, you will find that the place where the hammer strikes the metal warms up, too. In both these examples, the heat is due to friction. Now imagine rubbing your hands or pounding that hammer thousands of times per second.

The frictional heat generated can raise the temperature significantly in a very short time. Basically, high-frequency sound (ultrasound) causes rapid vibrations within the materials to be welded. The vibrations cause the materials to rub against each other and the friction raises the temperature at the surfaces in contact. This rapid frictional heat is what sets the conditions for the materials to bind together.

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Ultrasonic welding equipment has four main parts. A power supply converts low-frequency electricity (50-60 Hz) to high-frequency electricity (20 - 40 kHz; 1 kHz = Hz). Next, a transducer or converter changes the high-frequency electricity into high-frequency sound (ultrasound). A booster makes the ultrasound vibrations bigger. Finally, a horn or sonotrode focuses the ultrasound vibrations and delivers them to the materials to be welded. Besides these pieces, there is an anvil upon which the welded materials are stacked and held. There is also some method to apply force (usually air pressure supplied by a pneumatic piston) to hold the materials together during welding.

So what materials and industries take advantage of this clever process? Ultrasonic welding of plastics is used widely in making electronics, medical devices and car parts. For example, ultrasonic plastic welding is used to make electrical connections on computer circuit boards, and assemble electrical components such as transformers, electric motors and capacitors.

Medical devices, such as catheters, valves, filters and face masks are rely on ultrasonic welding processes. The packaging industry uses this technique to make films, assemble tubes and blister packs. Even Ford Motor Company has explored using ultrasonic welding to make aluminum chassis in cars.

Now that you know the basics behind ultrasonic welding, let's look at the welding process itself.

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The basic process of ultrasonic welding can be described by the following steps:

1. The parts to be welded are placed in the anvil or fixture.
2. The horn contacts the parts to be welded.
3. Pressure is applied to keep the horn in contact with the welded materials and to hold them together.
4. The horn delivers ultrasonic vibrations to heat up the materials. The vibrations move less than a millimeter either up-and-down or side-to-side.
5. The materials are welded together.
6. The horn gets retracted and the welded materials can be removed from the anvil.

The welding times, applied pressures and temperatures are controlled by a computer or microprocessor within the welding apparatus. And what actually happens during the welding process depends on the nature of the materials. In metals, the ultrasonic vibrations are delivered parallel to the plane of the materials.

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The frictional heat increases the temperature of the metal surfaces to about one third of the melting temperature, but does not melt the metals. Instead, the heat removes metal oxides and films from the surfaces. This allows the metal atoms to move between the two surfaces and form bonds that hold the metals together.

In the case of plastics, the vibrations are perpendicular to the plane of the materials and the frictional heat increases the temperature enough to melt the plastics. The plastic molecules mix together and form bonds. Upon cooling, the plastic surfaces are welded together. Welding times can vary, but the welds can form in as little as 0.25 seconds.

The factors that vary in ultrasonic welding are the frequency of the sound waves (usually 20, 30 or 40 kHz), the pressure applied to hold the materials together, and the time over which the ultrasound is applied (fractions of a second to more than one second).

The ultrasonic welding technique described are good for materials (metals, plastics) that are similar. But what about materials that are not similar. Let's address this question by looking at how New Balance has used ultrasonic welding to assemble athletic shoes.

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Look at a pair of athletic shoes. While traditional shoes may be made of a single material such as canvas or suede leather, many athletic shoes have several materials such as lightweight plastic polymers, suede or synthetic suede, and mesh combined. These composite materials make the shoes light, flexible, durable and breathable. For example, one style of New Balance athletic shoe has an upper portion which consists of three parts.

• A pattern of synthetic suede called a vamp - The vamp makes up the majority of the upper shoe including the tip, tongue and eye rows for laces.
• A pattern of synthetic suede called a saddle eye row - The saddle eye row contains the uppermost two eyelets to strengthen the lacing and reduce wear.
• A layer of mesh - The mesh surrounds the heel portion of the vamp and the upper part of the opening around the ankle.

But how do you put these materials together? Most commonly, shoe companies sew the materials together. About two to three years ago, however, New Balance set out to make the upper part of a shoe without stitching. After experimenting with polymer adhesive films and irons, they came up with a way of making this portion of the shoe using ultrasonic welding.

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To assemble the upper part of the shoe, workers start with a piece of synthetic suede material. They use an iron press to bond a thin sheet of hot melt film to the back of the material. Next, an ultrasonic welding assembly presses a pattern vamp into a suede material. Likewise, a similar ultrasonic welding machine presses the saddle eye row from another piece of synthetic suede. The vamp shape gets cut out of the suede. The saddle eye row and mesh material get welded ultrasonically to the vamp. In the processes, the frictional heat from the ultrasonic welder melts the hot melt film, which bonds the saddle eye row and the mesh material to the vamp. The finished vamp then gets shaped and bonded to the sole and heel pieces using water-based solvents.

According to Kenneth Straka, Senior Product Developer for New Balance, the ultrasonic welding methods have increased productivity by saving time. Not only do the ultrasonic welders distribute heat more evenly than iron presses, they also heat up and cool down faster. So, the assembly process requires fewer steps and is faster than traditional sewing methods.

Now that we have seen how ultrasonic welding is used to bond various materials, let's look at the advantages and disadvantages of this technique.

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Ultrasonic welding has many advantages over traditional methods. For one, welding occurs at low temperatures relative to other methods. So, the manufacturer does not need to expend vast amounts of fuel or other energy to reach high temperatures. This makes the process cheaper. It's also faster and safer.

The process occurs in fractions of a second to seconds. So, it can be done more quickly than other methods. In fact, it can bond plastics better and faster than glues. For example, the new smart keys in cars have a transponder chip in them. The car can only start when it senses the chip. To make the key, one end of the metal key blank and the chip get placed into one half of the plastic top. The other half gets placed over them and bonded to the base half. This bonding would usually be done with glue, which takes time to cure. The same task can be done with ultrasonic welding in less than a second.

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Ultrasonic welding does not require flammable fuels and open flames, so compared to other welding methods, it's a safer process. Workers are not exposed to flammable gases or noxious solvents. In electronics, copper wires are usually bonded to electrical contacts on circuit boards with solder. The same task can be done using ultrasonic welding in a fraction of the time and without exposing workers to fumes from smoldering lead solder. Although workers' hearing may be damaged by exposure to high-frequency sound, this potential danger is easily reduced by enclosing the ultrasonic welding machine in a safety box or cage and/or using ear protection.

Finally, ultrasonic welds are as strong and durable as conventional welds of the same materials -- which is just one of the reasons the method is being used in car manufacturing. To make cars lighter and more fuel efficient, auto makers are turning to aluminum as the main metal in car bodies. Ultrasonic welding can be used to bond the metal in less time and at lower temperatures than traditional welding.

Ultrasonic welding does have its limitations, though. First, the depths of the welds are less than a millimeter, so the process works best on thin materials like plastics, wires or thin sheets of metal. Ultrasonically welding a steel girder for a building would not be practical. Second, it does work best when welding similar materials like similar plastics or similar metals. As you saw with New Balance shoes, ultrasonically welding dissimilar materials requires an additional material -- in the case of the New Balance shoes, it's a film that can be bonded between the synthetic suede and the mesh.

Despite these limitations, the popularity and potential of ultrasonic welding continues to grow.

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Ultrasonic welding is a precise and efficient method for joining plastic components. Various factors influence the success and quality of this process. Here are the key elements to consider:

1. Product Material

The type of plastic used significantly affects the ultrasonic welding process. Some materials weld more easily than others, depending on their melting points and molecular structures.

2. Energy Guiding Point Design

Proper design of energy directors ensures efficient energy transfer and localized heating, which is crucial for a strong weld.

3. Mold Accuracy

Accurate mold design and debugging are essential to ensure proper alignment and pressure distribution during welding.

4. Temperature Around the Workpiece

Ambient temperature can influence the welding process by affecting the material's properties and the consistency of the weld.

5. Welding Head Material

The material of the welding head must be compatible with the workpiece material to avoid wear and ensure efficient energy transfer.

6. Welding Head Design

The design of the welding head, including its shape and size, impacts the distribution of ultrasonic energy and the effectiveness of the weld.

7. Amplitude of the Welding Head

The amplitude, or the extent of vibration of the welding head, needs to be optimized for different materials and welding requirements.

8. Bottom Mold Support Design

A well-designed bottom mold provides stable support, ensuring uniform pressure distribution and preventing deformation during welding.

9. Contact and Joint Surface Design

The design of the contact and joint surfaces affects how well the parts fit together and the strength of the resulting weld.

10. Shape and Size of the Workpiece

The workpiece’s dimensions can influence the ultrasonic energy distribution and the overall welding efficiency.

11. Surface Treatment

Surface preparation, such as cleaning or roughening, can enhance the welding process by promoting better energy absorption and bonding.

12. Energy Transferred to the Workpiece

The amount of energy transferred is critical. Insufficient or excessive energy can lead to weak welds or material damage.

13. Compatibility of Plastic Materials

Different plastics have varying compatibility levels, affecting how well they can be welded together.

14. Other Factors

Additional factors include filling materials, fixtures, and other auxiliary equipment that can affect the precision and quality of the weld.

For more information, please visit Ultrasonic Plastic Welder.