Ultrasonic welding joins thermoplastic parts in under one second by vibrating them together at 15-40 kHz — the friction at the interface melts the plastic locally and forms a molecular bond upon cooling. For nylon, ABS, and acetal parts in automotive, medical, and consumer electronics, it is the fastest and cleanest plastic joining method available.
How Ultrasonic Welding Works
A titanium or aluminum horn (sonotrode) presses against the top part and vibrates vertically at ultrasonic frequency. The vibration transmits through the top part to the joint interface, where an energy director — a small triangular ridge — concentrates the mechanical energy into a tiny area. This spot melts almost instantly, and the vibration continues for a fraction of a second to allow the molten material to flow across the joint. The horn then holds pressure for 0.5-2 seconds while the weld solidifies.
Energy Director Design for Nylon
For nylon 6 and 66, the energy director should be a 90° triangular ridge 0.5-0.8mm tall. Nylon’s higher melt temperature (220-260°C vs 180°C for ABS) requires more ultrasonic energy — about 30-50% more amplitude than welding ABS of the same joint area. Nylon also absorbs moisture, which can boil during welding and create porosity. Parts should be welded immediately after molding while still dry, or dried at 80°C for 4 hours before welding.
Frequently Asked Questions
What materials can be ultrasonically welded?
Amorphous thermoplastics (ABS, polycarbonate, acrylic) weld most easily. Semi-crystalline materials (nylon, acetal, PEEK) require more energy. Thermosets and elastomers cannot be ultrasonically welded because they don’t melt.
How strong is an ultrasonic weld in nylon?
With proper energy director design and moisture-free material, nylon ultrasonic welds achieve 80-90% of the base material strength. The limit is that the semi-crystalline structure doesn’t fully recrystallize at the weld interface as quickly as the amorphous melt zone cools.
What joint designs work best?
Butt joints with energy directors are standard for small parts. Shear joints (interference-fit walls) work better for larger nylon parts because the melting surface area is larger and more forgiving of material variation. Tongue-and-groove joints provide the most consistent results for production.
What causes ultrasonic welding failures?
Moisture in the material (nylon is the worst offender), insufficient amplitude, wrong energy director geometry, or parts that don’t fit together with intimate contact before welding. The number one cause we see: wet nylon — it boils during welding and leaves a porous, weak joint.
Need ultrasonic welding for your plastic parts?
We provide ultrasonic welding as a secondary operation for injection molded and CNC machined nylon, acetal, and ABS components.
