Written by Tarick Walton
Global Product Manager, Ultrasonics for Assembly Technologies
Emerson (All images courtesy of Emerson)
Ultrasonic welding is a fast, energy-efficient, and permanent fastening method used to join plastic parts and materials. Different variations of this technology are used to bond rigid plastics in automotive components, medical devices, electronic products, and many types of consumer goods.
Ultrasonic welding is also necessary for bonding clear film in packaging and tamper-resistant containers used in the food, beverage, and packaging industries. The versatility of his type of welding is evident as it also provides efficient, corrosion-free bonds between the thin and exotic metals essential for the high-performance batteries used in electric vehicles, cell phones, and personal electronic products.
The COVID-19 pandemic further highlights the value of ultrasonic welding in cutting, seaming, and joining non-woven fabrics and thin plastic films essential to infection control. This includes its role in the production of many types of personal protective equipment (PPE) and infection control products used by medical personnel and first responders. Examples of PPE include surgical masks, gowns, protective curtains, absorbent pads, bandages, bedding, and more.
Interestingly, though ultrasonic welding is diverse in its applications, the core technology is similar across these many applications.
The components of an ultrasonic welding system are a power supply, an actuator, and a stack. The power supply takes the line voltage (from the wall outlet) at a nominal 120-240V and transforms it into a high-frequency signal. It also provides the preprogrammed weld parameters necessary to operate the actuator and ultrasonic tooling (the stack) that work together to complete welds.
The ultrasonic stack connects to the actuator assembly. The stack has three parts:
1. The converter, which translates signals from the power supply into mechanical vibration (amplitude).
2. The booster, which fine-tunes (increases/decreases) the amplitude as needed for the plastics being welded.
3. The horn, which precisely transmits this vibration to the parts or surfaces being joined.
The shape of the horn is extremely important. To weld efficiently, the face of this tool must match the profile of rigid parts being joined or provide a sealing profile when parts or components are being joined. For each application, the stack (converter, booster, horn) work together to deliver the optimum level of amplitude to produce a consistent ultrasonic weld.
In operation, the actuator lowers the stack onto the parts to be joined, compressing them. Vibration is then delivered to part surfaces by the tool at the bottom of the stack — or the “horn.” Vibratory energy at the interface of the plastic parts generates frictional heat that melts the plastic, which, under force from the actuator, compresses and bonds, producing a finished ultrasonic weld.
Assembling plastic parts, films, or non-wovens using ultrasonic welding requires some up-front investment, starting with the welder itself. Additionally, product-specific tooling is required to precisely hold the various plastic components in place as a part is being welded.
All of this is a one-time investment, which means that operators can manage assembly costs, amortizing a single investment over the entire production volume of the part. When the part or product design is finalized, weld tooling can be built and high-volume production can begin.
The key to amortizing assembly method costs and realizing long-term assembly savings: have a firm idea of what the annual production volume is going to be. History shows that those with production volumes ranging from tens of thousands to millions per year generally attain a significant financial benefit with an ultrasonic welding process.
Non-wovens and infection control
The low cost and utility of non-woven fabrics have made them essential in the global battle to prevent the spread of infection, including COVID-19. Disposable non-wovens provide an effective yet inexpensive barrier against micro-organisms and contamination making them ideal for surgical protective garb.
Similar to other plastics, non-woven fabrics can be joined with ultrasonic welders, using high-frequency mechanical vibration, which is transmitted through tooling to heat and join thermoplastic material. Depending on the selection of weld parameters and tooling, the frictional heat generated by ultrasonics also offers the versatility to:
- Cut or slit a large roll of non-woven fabric into narrower strips (cut edges are simultaneously seamed and sealed by the heat).
- Bond two pieces of non-woven fabric into the finished seam of a garment or a piece of protective gear, such as a surgical gown or mask.
- “Quilt” multiple layers of non-woven material into laminated absorbent products.
Compared to adhesives, ultrasonic eliminates the cost of consumables, the time required for setup and drying, and the risk of exposure to chemical contaminants. Compared to sewing, ultrasonic offers benefits because it creates high-strength, high-integrity seams without the need for thread and without creating mechanical holes in the fabric, which can hold and conceal microorganisms or contaminants.
Ultrasonic seams are ready for use as soon as they are produced.
Ultrasonic welding is widely used in the high-volume assembly of plastic parts because it:
- Integrates easily with automated processes. Ultrasonic welding equipment is available in a wide range of configurations, from compact benchtop to freestanding units that can produce multiple parts in a single cycle.
- Offers fast production cycles. Many ultrasonic welds are completed in a fraction of a second, enabling production at a greater rate than processes that rely on other joining methods (such as adhesives or mechanical fasteners).
Requires zero consumables. Ultrasonic welds are completed directly between the surfaces of the adjoining parts. This means no additional material, fasteners, or adhesives are required.
- Is extremely energy efficient. Unlike heat-sealing or joining processes that rely on a continuously heated element, ultrasonic welders consume energy only during the weld cycle itself. Since tooling is never heated, there’s no cooling time required to switch ultrasonic equipment over for production changes. Simply swap out the tooling, update the programming, and begin production.
- Enables sophisticated and secure data collection. For manufacturers that must validate product quality and ensure product traceability to meet medical, regulatory, or performance requirements, the latest ultrasonic welding systems offer extensive data collection capabilities.