How Does Ultrasonic Welding Work?

Last Updated: March 12, 2021

Ultrasonic Welding

Ultrasonic welding, or USW, is a welding technique that utilizes mechanical vibrations above the audible range. The process is accomplished by converting high-frequency electrical energy into high mechanical motion. The mechanical motions applied along with force generate frictional heat at the plastic joint area. This causes the plastic material to melt, forming a molecular bond between the parts. This technique is fast, economical, easy to automate, and well suited for thermoplastic materials’ mass welding.


The Basics of Ultrasonic Welding

The process uses a high frequency of ultrasonic vibrations, usually around 20 kHz to 40 kHz, to join two plastic pieces. The plastic parts to be welded are usually placed between the anvil and the sonotrode. These parts are usually made of aluminum or titanium. When welding, the sonotrode is connected to a transducer leading to the emission of acoustic vibrations. These vibrations then flow from the transducer onto the welding parts via the sonotrode with high-frequency speeds ranging between 16 kHz and 75 kHz.

The Main Parts of Ultrasonic Welding

1. Machine Press

The machine press is used to hold the welding system and apply force to the workpieces. It comprises a base-plate used to hold the cooling jig and a pneumatic cylinder to exact the pressure. The machine press also has a pressure gauge and regulator for adjusting the welding force. The functionality of pressure gauges varies with the machine’s welding force. To accurately compare the welding forces of a machine you need to calibrate its welding force using a local cell.

2. Generator

The ultrasonic machine generator converts electrical power from the single-phase main source to the required voltage and frequency. The transducer then converts the electrical energy to mechanical vibrations used for welding. The generator’s microprocessor unit drives the welding cycle and provides the welder with important welding communication through the user interface. The user interface is also instrumental in allowing an operator to key in the required welding parameters.

3. Welding Stack

This is the main section in the welding machine responsible for providing ultrasonic machine vibrations. The welding stack comprises three parts, the transducer, booster, and the welding horn. All these three parts are mounted on the welding machine press at the central point of the booster. The welding stack is normally a tuned resonator for optimal functioning. The welding stack’s frequency must closely match that of the generator’s electrical signal for quality results.

4. Transducer

A transducer, also known as a converter, is used to convert the generator’s electrical energy to mechanical vibrations used in welding. The transducer is made up of several piezoelectric ceramic discs sandwiched in between two titanium metallic blocks. Between the discs is a thin metal plate that acts as the electrode.

5. Booster

This section serves two main purposes: amplifying the mechanical vibrations produced at the transducer’s tip and transmitting them to the welding horn. The other function is that it acts as a base for mounting the stack on the welding press. Whenever the transducer’s energy reaches the booster, it causes it to expand and contract to amplify the vibrations.

6. Welding Horn

The welding horn is instrumental in transmitting energy to the workpieces being welded. The welding horn is also a tuned device providing mechanical gain to the ultrasonic machine. These components are made of aluminum or titanium. However, aluminum is only applicable in low-volume applications since it wears out quickly, leading to distortion of the welds. To control the wearing of the horns, most of them come with hardened tips.

7. Supporting Tooling

The base of the machine press is used to support the tooling that supports the workpieces when welding. The support tooling has been designed to firmly hold the lower components while the ultrasound is being applied. The supporting tooling is also designed to match the contours of the workpieces.


Applications of Ultrasonic Welding

Ultrasonic welding is extensively used in many industries due to its ability to create quality welds without directly applying heat. Here is an overview of the applications of ultrasonic welding.

1. Aerospace and Automotive Industries

In the automobile industry, ultrasonic welding is used to assemble large plastics and electrical components. These components include door panels, instrument panels, steering wheels, air ducts, upholstery, and engine components. In this industry, welders prefer ultrasonic welding due to low capital costs, flexibility, low cycle times, and automation. The method also does not damage the surface furnish. This is due to the high-frequency vibrations preventing the formation of marks on the manufactured parts.

In the aerospace industry, ultrasonic welding is used to join gauge metals and other light aircraft parts such as the lights. Ultrasonic welding is also useful in welding aluminum, widely used in manufacturing different parts of an aircraft.

2. Computer and Electrical Industries

Ultrasonic welding is used in the computer industries to join wired connections to create small and delicate circuits. The method is also used to fuse junctions of wire harnesses used in distributing electrical power and signals. Ultrasonic welding is also instrumental in assembling transformers, electric motors, capacitors, and field coils.

Ultrasonic welding is used in the assembly of storage media such as flash drives and computer disks due to the high volumes required. The cycle time of an ultrasonically welded computer disk has been estimated to be less than 300 milliseconds.

The method is employed when conducting research and experiments focused on microcircuits. The bonds created in this process are reliable and prevent impurities or thermal distortion on the components. The method is also applicable in bonding wires, ribbons, and chips when producing microcircuits.

3. Medical Industry

Ultrasonic welding is used in the medical industry since it prevents degradation of the welds. For this reason, the method is used in the production of specialized machines used for cleaning rooms. Automation of ultrasonic welding provides strict control over dimensional tolerances preventing biocompatibility of the welding parts. This lowers production costs without compromising on the quality of the features.

Ultrasonic welding is used to manufacture medical items such as intravenous spike or filters, face masks, blood and gas filters, dialysis tubes, cardiometry reservoirs, arterial and anesthesia filters. The method is also instrumental in manufacturing medical textiles such as transdermal patches, fabrics for cleaning, sterile garments, and hospital gowns. Ultrasonic welding is efficient in medical textile sewing due to its ability to prevent contamination and reduce the risk of infections.

4. Packaging Industry

In the packaging industry, ultrasonic welding is often used in creating, packing, and sealing blister packs, tubes, and containers. The method is also crucial in packaging dangerous materials that require hermetic sealing and cannot be subjected to high temperatures. These items include fireworks, explosive and reactive chemicals such as potassium. Ultrasonic welding is also used in packaging ammunition and propellants. These packages are known to withstand pressure and stress protecting the consumer from unnecessary accidents during application.

The food industry also uses ultrasonic welding for packing since it is fast and can be used to produce hermetic seals. Paper seals used for packaging are usually coated with plastic materials such as polypropylene before being fused using ultrasonic welding to create an airtight seal. Ultrasonic welding is also used to seal food items such as frozen food packages, beverage containers, and candy bar wrappers.


The Advantages of Ultrasonic Welding

Ultrasonic welding has found its relevance in most industries and applications due to its ability to blend with thermoplastics and other welding materials like polypropylene. Here are some advantages of ultrasonic welding:

1. The Speed

Ultrasonic welding uses the transmission of high-frequency ultrasonic acoustic vibrations to heat, weld, and cool. All these processes occur very fast and allow you to weld more products within minutes. You will experience a high throughput and short turnaround times when you use ultrasonic welding.

2. Unmatched Safety

It is very safe to weld using the ultrasonic welding technique. The procedure uses highly targeted energy, and this reduces injuries to the welders. The heat produced during welding is localized, minimal, and quickly dissipated to the required welding joints and materials. This process does not cause any damage to the welds and the surrounding environment.

3. Reliability

Ultrasonic machines and equipment are known to be durable. Once they are assembled and tested, you can do mass welding with minimal breakdowns and faults. Depending on your welding project, you may automate the process to reduce human intervention. This will reduce the operation costs and increase the quality of welds.

4. Flexibility

You can use ultrasonic welding techniques to weld different materials. Whether you intend to weld thermoplastics or metals, you can use the method without fear of uneven joints. The technique is widely preferred for welding dissimilar materials.

5. Minimal Material Costs

Since the process is speedy, it takes less time, money, and resources to complete a welding project. The welding process does not require connective bolts, solders, and adhesive materials. As such, you realize a lot of savings in material cost. With high precisions, ultrasonic welding gives you a smooth finish with minimal waste.

6. Joint Quality

The precision in ultrasonic welding guarantees high-quality joints. These joints will exhibit no plastic flashes, deformations, or faults. The result is a clean and almost invisible seam that needs zero retouches.


The Disadvantages of Ultrasonic Welding

While Ultrasonic welding has its advantages, it also has drawbacks that may hinder a welding project’s effectiveness. Below are some of the downsides of the method:

1. Limits Material

While ultrasonic welding is flexible and effective for most materials, it does not work well with particular thermoplastics. If you intend to use high moisture content thermoplastics, ultrasound welding becomes ineffective. The technique is also not applicable in welding hard and strong thermoplastics such as polypropylene.

2. Limits the Sizes

Though the technique is useful in a wide scope of metals, it cannot handle products with joints larger than 150 mm since the transducer’s output ranges between 100-150 mm. Welding thicker materials such as polypropylene also pose a challenge as they require a lot of energy to break the molecular bonds.

3. A High Initial Investment

Basic ultrasonic welding equipment and tools are costly. Prices further increase for the automated ultrasonic welding machines.

4. Limits the Joint Types

Ultrasonic welding can only be used for lap joints. These joints overlap one another using a flat surface. The technique will be unsuitable for welding joints such as a corner, butt, tee, and edge joint.

5. Lead Time

Ultrasonic welding requires a series of equipment, tools, and processes to weld. Adding another part or equipment into the process will translate to an overall increase in the lead time. Additional tools or equipment require a custom-built tooling setup.


FAQs

1. What is the Minimum Frequency That I Can Use in Ultrasonic Welding?

The minimum frequency required in ultrasonic welding is 20,000 Hz.

2. How Do You Break an Ultrasonic Weld?

You can break by squeezing the weld from the opposite corners to crack and open it along the welded lines. The process is usually tricky due to the molecular bonds formed through welding. You can also crack the weld by whacking it using a hammer.

3. What Electronic Components are Most Susceptible to Damage from Ultrasonic Welding, and Why?

When welding using the ultrasonic method, the upper sections connected by the sonotrode usually vibrate. Having any other electronic components mechanically fastened to the vibrating parts will alter the vibration pattern damaging the welds. The damage is likely to occur in high-stress concentration areas such as small cross-sectional areas or the ones with sharp corners. To solve this problem easily, isolate the electrical components from the upper part of the assembly’s vibrating parts.

4. What is the Proper Gap for Ultrasonic Welding of Polycarbonate?

A welding gap refers to the difference in the height between the welding materials and the final welds. Welders usually provide an estimated welding gap of around 0.025 mm to accommodate the melt layer. However, you can check this gap by designing an energy director on the parts to be ultrasonically welded. The energy director’s function is to control the power needed to produce and shape the welds.

Whenever you set up the welding process using energy directors, ensure they are not fully melted when welding. A complete melt of these energy directors will cause the molten to flow out of the joint, leading to a flash that weakens the weld’s bonds.

For you to achieve perfect precision, you should select an ultrasonic machine that can weld to the exact distance while defining the final weld’s length automatically. Using a small energy director is also a perfect alternative since it can be enlarged when the need arises.


Conclusion

Ultrasonic welding requires precision and accuracy. For this reason, the personnel handling the machines and the processes must be well-trained and highly skilled. Safety is also paramount in this process. As such, ultrasonic welders should gear up appropriately to mitigate bodily injuries and intake of hazardous fumes and gases emitted when welding. To enhance the durability of the ultrasonic welding machines, ensure you conduct regular maintenance and servicing. During the purchase of an ultrasonic machine, ensure you outsource from a licensed vendor and check for the machines’ quality mark.


Featured Image Credit: NearEMPTiness, Wikimedia Commons

Cameron Dekker
 

Cameron grew up in Allentown, Pennsylvania, a once-proud steel town on the Lehigh River, where he got a taste of TIG welding in his high school shop class. He holds certificates for Certified WeldingEducator (CWE) and Certified Resistance Welding Technician (CRWT) from the American Welding Institute. His interests include scuba diving, sculpture, and kayaking.

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