Able to join hard and soft plastics as well as metals, the basic process for ultrasonic welding is the same as all ultrasonic welding systems are made up of the same general components:
- A press, typically with an electric or pneumatic drive, is used to apply pressure to the two parts
- An anvil, fixture or ‘nest’ is used so the parts can be placed and the high frequency vibration can be directed at the interfaces
- An ultrasonic stack (comprising a converter or piezoelectric transducer, an optional booster and a horn or sonotrode):
- Converter: Uses the piezo electric effect to convert an electrical signal into a mechanical vibration
- Booster (optional): Used to mechanically modify the amplitude of the vibration, also used to apply a clamping force to the stack in the press
- Horn: Applies the vibration to the parts to be joined while mechanically modifying the amplitude and taking the shape of the part
These three elements of the stack are tuned to resonate at the same ultrasonic frequency as each other (measured in kHz)
- A power supply or ‘electronic ultrasonic generator’ to deliver a high powered electric signal at the same resonance frequency as the stack
- A controller to manage the movement of the press and the delivery of the ultrasonic energy
Ultrasonic welding equipment can be customised to suit different part specifications, but generally-speaking, the parts are held in a nest and sandwiched in a press. A horn is then connected to a transducer and a low-amplitude acoustic vibration is emitted. These vibrations are commonly set to ranges including 15 Khz, 20 kHz, 30 kHz, 35 kHz, 40 kHz and 70 kHz.
While the basics of ultrasonic welding systems are the same, the manner in which ultrasonic welding creates a join differs between metals and plastics.
When joining plastics, the interface between the two workpieces is typically designed to help concentrate the melting process. One piece will have a spiked or rounded surface in contact at the interface with the other piece. These contact points melt under the influence of ultrasonic energy to create the joint. This melting is local when joining thermoplastics as the vibrational energy is absorbed along the join.
With metals, the weld occurs as a result of high-pressure dispersion of surface oxides and the local motion of the materials caused by the ultrasonic vibrations. While the process creates frictional heat, it is not enough to melt the base metal materials.
Research has found that ultrasonic welding processes can even be used to create highly durable bonds between carbon fibre reinforced polymer (CFRP) sheets and light metals, but there is still much to learn about ultrasonic welding with regards to the impact of process parameters on weld quality.
The ultrasonic welding method has a number of advantages over traditional welding techniques. These advantages include the low temperatures associated with ultrasonic welding, since the low thermal impact allows a greater number of materials to be welded together. This low temperature also means that less fuel or energy is required, lowering the associated costs.
Ultrasonic welding is a fast joining method that can bond materials in fractions of a second. These fast weld times provide advantages over the use of adhesives, for example, including not having to leave a fixture in place for a long period after joining to allow for drying or curing.
There is also no need for flammable fuels, open flames or consumables, meaning workers are not exposed to dangerous gases or solvents, making ultrasonic welding a safer technique that is also environmentally safe.
In addition, the process can be automated to deliver clean, repeatable and precise joints with fast cycle times and very little touch up work required.
Finally, ultrasonic welds are found to be as durable and strong as conventional welds using the same materials. The automotive industry has looked to ultrasonic welding to join lightweight aluminium faster and at lower temperatures than with traditional welding.
However, there are also a number of drawbacks associated with ultrasonic welding, depending on the application.
Ultrasonic welding is best suited to joining smaller items as the weld depths are shallow. While the technique is fine for welding thin plastics or sheets of thin, malleable metals like aluminium, copper or nickel, and wires, it would not be practical to try to weld a thick item such as a girder.
Current technologies do not allow for large joints to be welded in a single operation with ultrasonic processes, which often also require specially designed joint details.
Although it can be used for dissimilar materials, ultrasonic welding is best used with similar plastics of metals, ad tooling costs can be high, depending on the arts to be welded.
In addition, ultrasonic vibrations can damage electrical components, although the use of higher frequency equipment can reduce this problem.
Finally, the hearing of workers can be damaged by exposure to high-frequency sound, although this can be mitigated against by enclosing ultrasonic welding machines in a safety box or by using ear protection.
Ultrasonic welding has found applications in a wide range of industries. Although it is not a suitable process for joining thick parts, it is perfect for small, complex or delicate assemblies and materials such as wires, circuits, sheet metals, foils, ribbons and meshes. As a fast process that can be automated, ultrasonic welding is also used for thermoplastics and does not require ventilation to remove heat or exhaust.
Common applications for ultrasonic welding include:
1. Computing and Electrical
Ultrasonic welding is widely used in the computer and electrical industries to join wires and connect small or delicate parts including microcircuits. In addition, the technique is used for storage media such as flash drives as well as for battery components.
2. Aerospace and Automotive
Ultrasonic welding is used in the automotive industry to assemble large plastic and electrical items, ranging from instrument panels to door panels, steering wheels and engine parts. The aerospace industry uses the process to join lightweight, thin gauge sheet materials such as aluminium as well as for bonding composite materials like carbon fibre.
3. Medical
Because it doesn’t introduce contaminants or degradation into the weld, ultrasonic welding is widely used in the medical industry. It is used to make items ranging from arterial and blood filters to face masks, hospital gowns and transdermal patches.
4. Packaging
Ultrasonic welding is used to make packaging for a range of industries, including packaging for dangerous materials like fireworks, explosives or reactive chemicals. It is also used to join the parts for butane lighters, food packaging such as milk containers.
What materials can be ultrasonic welded?
Ultrasonic welding is used to join both soft and rigid thermoplastics as well as for thermoplastic composites. It can also be used to join metals such as aluminium, brass, copper and most precious metals as well as welding nickel alloys. The process can also be used to weld a range of dissimilar metals.
Is ultrasonic welding waterproof?
Ultrasonic welding does not naturally produce waterproof (or airtight) joins for products like water tanks, coffee pots, vacuum cleaner casing, etc. However, with the correct positioning of the upper and lower plastic shells to be joined, weld line structure, wall thickness, materials, and ultrasonic frequency and power, it is possible to meet requirements for water or airtightness.
Is ultrasonic welding dangerous?
Ultrasonic welding is safer than many traditional welding methods as it does not require large amounts or heat, nor does it produce fumes. Being highly targeted, this technique has fewer dangers as a result of excess electrical energy either.
Does ultrasonic welding generate heat?
Ultrasonic welding uses high frequency ultrasonic energy to produce low amplitude mechanical vibrations. These vibrations generate heat where the parts being welded meet, although this heat is enough to melt thermoplastic materials it is not enough to actually melt metals that are being joined.
What is the maximum frequency used in ultrasonic welding?
The maximum frequency used for ultrasonic welding is 70 kHz. The minimum that is used is 20 kHz, which is just audible to humans, as the audible sound for humans ranges from 16-20 kHz. Higher frequencies are perceived only as vibrations.
Ultrasonic welding is a joining technique that uses the application of high-frequency vibratory energy. This energy is applied to the workpieces as they are held securely together under pressure.
Capable of melting and joining soft and rigid thermoplastics and thermoplastic composites, ultrasonic welding can also be used to join thin and soft metals such as aluminium, brass or gold without melting the base material. In addition, ultrasonic welding can be used to weld some dissimilar metals.
Used in a variety of industries, this joining method can create fast and accurate welds with little or no contamination.