Ultrasonic welding is a process that uses high-frequency mechanical vibrations to generate sufficient heat to create a bond between parts of the same or similar materials. Extensively used to join thermoplastic parts, including to some extent glass filled thermoplastics, the process can even weld a number of metals including aluminium and copper. This technique provides a permanent alternative to fixing methods such as snap fits and screws, as discussed in the previous blog post: Avoiding Fastenings with Moulded-in Features.
How it works
The parts to be welded are held under pressure between a sonotrode and a nest, also known as a horn and an anvil respectively. The shape of these are customised to fit each job as it is important that the part is held securely in the nest to ensure correct alignment. The sonotrode is comprised of a stack of piezoelectric transducers linked to a piece of aluminium, titanium or steel, specifically shaped or “tuned” to resonate at the correct frequency for the application. The Piezoelectric transducers in the sonotrode cause the metal to resonate creating a concentrated ultrasonic vibration typically between 20 kHz and 70 kHz. This is transferred through the polymer to the point at which it contacts the mating part. Here, enough localised heat energy is generated to melt both parts and once solidified create a permanent bond.
Designing parts for ultrasonic welding
A successful weld relies on a concentration of energy at a specific point, therefore contact between the two parts must be minimised. Typically, one of the components will be designed with one or more small energy directors that taper to the point where the parts interfere, focussing the energy of the vibration. With a triangular cross-section and an apex angle between 60 to 90°, the energy director quickly melts along with the surrounding material.
Even when applied to a simple butt joint between two mating parts, ultrasonic welding creates a remarkably strong bond. For cosmetically sensitive assemblies, placing the energy director in a tongue and groove joint or behind a lip joint hides the weld, creating an invisible join.
Ultrasonic welding can be applied to permanently affix specifically adapted brass inserts, as well as used to spot weld thermoplastic sheets and large assemblies. Portable ultrasonic welding equipment further increases the flexibility of spot welding.
Ultrasonic staking, a variation on the conventional welding process, enables thermoplastics to be attached to different materials. Through holes in the alternative material are mounted over pins that have been pre-moulded into the plastic part. Melting these pins via ultrasonic vibrations forms a cap over the through hole to hold the two materials together.
Although ultrasonic welding requires additional operational costs and assembly time, as a post process it is consistent, clean and accurate. It also avoids the risk of contamination with other components and can be automated. Assemblies that utilise ultrasonic welding can be found in the automotive and aerospace industries and it is particularly useful for food safe parts, medical implements and children’s toys, as it removes the need for adhesives.
At our UK based plastic injection moulding facility, Paul Norman Plastics have recently invested in a new ultrasonic welding machine from Sirius Electric, improving the accuracy and consistency of this secondary operation.