Ultrasonic Welding
Ultrasonic welding applies high-frequency vibration through a horn to melt a thermoplastic joint locally in well under a second.
- Part
- Joining
- Revised
- 2026-08-11
At a glance
- Family
- Thermal
- Typical tolerances
- Weld collapse held to ±0.002 in (±0.05 mm) with distance-mode control; joint faces should be molded flat and parallel to the limit the tool allows
- Typical volumes
- 10,000 to 10,000,000+ per year
- Lead time
- 0.1–1.0 s weld, under 2 s total cycle; 2–4 weeks for horn and nest fixture tooling
- Materials
- Plastic, Metal
What it is
Ultrasonic welding drives a thermoplastic joint with high-frequency mechanical vibration — usually 20 kHz at 0.0008–0.005 in (20–125 µm) peak-to-peak amplitude — delivered through a tuned horn under clamping force. Friction at the joint interface generates heat exactly where the parts touch, and the weld is made in 0.1–1.0 s with a total cycle under 2 s.
Nothing else joins plastics that fast. A well-designed joint in an amorphous resin reaches 90–100% of parent material strength and can be hermetic, with no adhesive, no fastener and no added mass.
The same principle also welds metal foils and thin sheet — ultrasonic metal welding is a solid-state scrubbing process, not a melting one, and it joins copper to aluminum readily, which is why it dominates battery tab and wire-splice work. Volumes are high: the process only makes sense once horn and fixture tooling can be amortized, typically from tens of thousands of parts upward.
How it works
- Clamp. The horn descends and applies trigger force — commonly 50–500 lbf (0.2–2.2 kN) — pressing the parts together in a nest fixture that supports the joint directly beneath the weld.
- Vibrate. A converter turns 20 kHz (or 15, 30, 35, 40 kHz) electrical energy into mechanical motion; a booster scales the amplitude and the horn delivers it into the part. Motion is perpendicular to the joint face.
- Localize the heat. Cyclic surface friction and, once softening begins, viscoelastic hysteresis in the polymer concentrate heat at the interface. A molded energy director — a small triangular rib on one joint face — focuses that energy so melting starts at a defined point rather than randomly.
- Melt and collapse. The director melts and flows across the interface; the parts close by a controlled collapse distance. Welders run in time, energy, peak-power or distance mode, and distance mode holds the collapse to about ±0.002 in (±0.05 mm).
- Hold. Vibration stops and force is maintained for 0.2–1 s while the melt solidifies under pressure.
Near field, far field and resin choice
If the joint sits within 0.25 in (6 mm) of where the horn contacts the part, the weld is near field; beyond that it is far field and the energy has to travel through the part to reach the joint.
Amorphous resins — ABS, PC, PS, acrylic, PSU — soften gradually over a broad range and transmit vibration well, so they weld reliably in both near and far field. Semicrystalline resins — PA, PP, PE, POM, PBT — absorb energy melting their crystalline phase and have a sharp melting point, so they need near-field welding and usually a shear joint rather than an energy director.
Ultrasonic metal welding
The metal version applies the vibration parallel to the interface, scrubbing the oxide away and bringing clean metal into atomic contact without melting anything. It works on thin, ductile, conductive material — copper, aluminum, nickel foils, wire and tabs — and makes copper-to-aluminum joints that resistance welding and soldering both struggle with.
Design guidelines
Energy directors
For amorphous resins, mold a triangular rib on one joint face with a 90° included angle, 0.010–0.025 in (0.25–0.64 mm) high, base width roughly twice the height. One director per joint line, positioned on the part that is not in contact with the horn where possible. For hermetic seals, run the director as a continuous, unbroken ring — a single gap is a leak path.
Shear joints for semicrystalline resins
A shear joint has a small interference between a boss and a socket so the parts weld progressively down the sidewall as they telescope together, keeping the melt away from air. Use 0.008–0.012 in (0.2–0.3 mm) of interference on parts under about 0.75 in (19 mm), increasing to roughly 0.016 in (0.4 mm) on larger parts, with a 0.020–0.030 in (0.5–0.75 mm) lead-in chamfer to align the parts before the interference engages.
Support the joint and the horn contact area
The nest fixture must support the joint plane directly under the weld — unsupported walls flex and absorb energy instead of welding. The horn contact face should be flat, unbroken and perpendicular to the horn axis; textured or graphic surfaces mark under a horn. Keep sharp internal corners away from the energy path: they concentrate stress and crack under 20 kHz vibration.
Resin compatibility
| Pair | Weldable? | Notes |
|---|---|---|
| Same amorphous resin (ABS-ABS, PC-PC) | Excellent | Far-field capable; energy director joint |
| Same semicrystalline resin (PA-PA, PP-PP) | Good, near field only | Shear joint; keep the joint within 0.25 in (6 mm) of the horn |
| ABS to PC | Fair | Chemically compatible but melt temperatures differ — expect reduced strength |
| PC to acrylic | Fair | Compatible pair; validate on parts |
| PE to PP | No | Not chemically compatible despite both being polyolefins |
| Any resin to a filled grade over ~30% glass | Poor | Too little polymer at the interface to form a weld |
| Nylon after moisture pickup | Poor | Absorbed water foams at the joint — dry the parts or weld immediately after molding |
| Copper to aluminum (metal welding) | Excellent | Solid-state scrubbing; the standard battery-tab route |
Joint features are molded, so the wall, rib and radius rules that govern them are on the injection molding design guidelines chart.
Inspection
Ultrasonic welds are validated in-process and by destructive sampling. Modern welders record energy, peak power, collapse distance and weld time for every cycle and alarm on out-of-window results — that data trail is the primary quality record. Destructively, pull and burst tests on lot samples, and cross-sections to confirm the melt has spread across the full joint width. For sealed parts, pressure decay or vacuum leak testing.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Energy director height | 0.015 in (0.4 mm) | 0.010–0.025 in (0.25–0.64 mm) | Too small starves the weld, too large leaves unmelted material |
| Energy director angle | 90° included | — | Concentrates energy at a repeatable point |
| Shear joint interference | 0.010 in (0.25 mm) | 0.008–0.016 in (0.2–0.4 mm) by part size | Sets the welded sidewall area |
| Joint-to-horn distance, semicrystalline | Under 0.25 in (6 mm) | Near field required | Crystalline phase absorbs the energy en route |
| Glass fill at the joint | Under 20% | 30% | Not enough polymer at the interface above this |
| Horn contact face | Flat, unbroken, perpendicular | — | Textured surfaces mark; angled faces lose coupling |
| Collapse control | Distance mode, ±0.002 in (±0.05 mm) | — | Time mode does not compensate for part variation |
Cost drivers
Ultrasonic welding is a tooling-plus-cycle-time process. The press, converter and power supply are moderate capital; the real per-part cost is a tuned horn and a machined nest fixture, both specific to the part. Once those exist, the cycle is under 2 s and consumable cost is zero — no adhesive, no fastener, no filler — so unit cost is essentially machine time.
That structure means the process wins decisively at volume and loses at low volume, where adhesive or screws carry no tooling burden. Horns are also consumable on abrasive filled resins: titanium horns last far longer than aluminum on glass-filled material but cost more.
The hidden cost is joint development. Energy director and shear joint dimensions interact with resin, part stiffness and horn amplitude, and getting a hermetic seal usually takes an iteration on the mold — which is expensive after tooling is cut.
- Design the joint into the first mold revision. Adding or resizing an energy director after tooling is cut is a steel change, not a process tweak.
- Use one horn for multiple parts by keeping joint plane dimensions in a family across a product line.
- Specify unfilled or low-fill resin at the joint, even if the body of the part is glass filled — a locally unfilled joint region welds far better.
- Buy distance-mode (servo) control if hermeticity matters; it absorbs part-to-part molding variation that time mode cannot.
- Check whether staking or ultrasonic insertion does the job instead — both use the same machine and cheaper tooling than a full hermetic weld.
Questions
6 questionsWhat is an energy director and how big should it be?
A small triangular rib molded onto one joint face that concentrates ultrasonic energy so melting starts at a defined point. Standard practice is a 90° included angle, 0.010–0.025 in (0.25–0.64 mm) high, with a base width about twice the height. For a hermetic seal, run it as a continuous unbroken ring around the joint — a single gap is a leak path.
How strong is an ultrasonic weld?
A well-designed joint in an amorphous resin such as ABS or PC reaches 90–100% of parent material strength and can be hermetic. Strength falls off with glass content — above roughly 30% fill there is not enough polymer at the interface to form a weld — and with semicrystalline resins welded in the far field.
Why won't polypropylene weld as easily as ABS?
Polypropylene is semicrystalline: it has a sharp melting point and absorbs energy melting its crystalline phase, so vibration is attenuated on the way to the joint. It needs near-field welding, with the joint within 0.25 in (6 mm) of the horn contact, and usually a shear joint (0.008–0.016 in / 0.2–0.4 mm interference) rather than an energy director.
Can ultrasonic welding join dissimilar plastics?
Only if the resins are chemically compatible and their melt temperatures are close. ABS to PC and PC to acrylic work with reduced strength; PE to PP does not weld despite both being polyolefins. As a rule, plan on welding a resin to itself and treat any dissimilar pair as something to validate on real parts before committing tooling.
Can you ultrasonically weld metal?
Yes, but it is a different process. Ultrasonic metal welding vibrates parallel to the interface, scrubbing away oxide and bringing clean metal into atomic contact with no melting at all. It suits thin, ductile, conductive material — copper and aluminum foils, wires and battery tabs — and makes copper-to-aluminum joints that resistance welding and soldering both handle poorly.
How are ultrasonic welds inspected?
Primarily in-process: modern welders log energy, peak power, collapse distance and weld time for every cycle and alarm on out-of-window results. That trail is the quality record. Destructive pull and burst tests on lot samples, cross-sections to confirm melt across the full joint width, and pressure-decay leak testing on sealed parts back it up.