Ultrasonic Welding of Plastics
Key Takeaways
- Viscoelastic heating at the interface — 20–40 kHz vibration melts an energy director or shear joint in under a second.
- Amorphous resins weld easily; semi-crystalline resins need a shear joint, near-field geometry and tighter control.
- Collapse-distance mode with a stated window is the robust way to run and validate the process.
- Fastest and cheapest welding method, limited by particulate, horn marking and vibration-sensitive contents.
Ultrasonic Welding of Plastics
Ultrasonic welding is a thermoplastic joining process in which mechanical vibration at 20–40 kHz and 20–120 µm peak-to-peak amplitude is transmitted through one part to the joint interface, where viscoelastic and frictional heating melt a thin layer of polymer that solidifies under pressure to form a weld. No solvent, adhesive or consumable is involved.
The stack is a piezoelectric converter giving about 20 µm at 20 kHz, a booster with a gain such as 1:1.5 or 1:2.5, and a horn tuned to the same frequency that presses on the part under pneumatic or servo force. 20 kHz suits larger parts and higher power; 30–40 kHz suits small, delicate or cosmetic parts because amplitude, stress and marking are lower.
Polymers are viscoelastic: cyclic strain is partly converted to heat by hysteresis, concentrated at the smallest section — the energy director tip or the shear-joint interference — where melting starts within milliseconds. A joint within about 6 mm of the horn face is near-field, beyond that far-field. Amorphous resins transmit with little loss and weld far-field; semi-crystalline resins and elastomers damp the vibration and weld reliably only near-field.
Material Suitability
| Material | Weldability | Notes |
|---|---|---|
| ABS | Excellent | Low damping; near- or far-field with a standard energy director. |
| PS, SAN, PMMA | Good | Low-loss; PMMA and HIPS crack at high amplitude. |
| Polycarbonate | Good | More energy than ABS; dry below 0.02% moisture; molded-in stress cracks. |
| Polypropylene, PE | Fair | High damping: near-field only, shear joint or 60° director, 60–120 µm. |
| PA, POM, PBT, PEEK | Fair | Sharp melting point, narrow window; shear joints, high amplitude and force; nylon dry. |
| Thermoplastic elastomers | Difficult | Absorb the vibration; Shore A grades rarely weld. Overmold instead. |
| Filled grades | Fair to difficult | Filler at the interface weakens the weld; unreliable above about 30%. |
| Dissimilar pairs | Case by case | Melt temperatures within about 20–40°C and chemical compatibility: ABS/PC and PC/PMMA weld; polyolefins only to polyolefins. |
Process Parameters and Joint Design
| Parameter | Typical range | What it controls |
|---|---|---|
| Frequency | 20 kHz; 30–40 kHz for small parts | Fixed by the stack; higher frequency, less amplitude and marking. |
| Amplitude at the horn face | 20–120 µm peak to peak | Amorphous 30–70, semi-crystalline 60–120 µm at 20 kHz. |
| Weld force | 100–3,000 N | Melt rate; too high squeezes melt out before chains interdiffuse. |
| Trigger force | 50–300 N | Ultrasonics start here; repeatable energy delivery. |
| Weld mode | Time 0.1–1.0 s, energy 50–2,000 J, or collapse 0.2–0.6 mm | Collapse mode compensates for part variation; preferred for validation. |
| Hold time | 0.2–1.0 s | Solidification under force; too short gives a weak joint. |
Joint design decides more than any setting. An energy director is a triangular rib on one mating face, 60–90° included angle and 0.2–0.6 mm high, base about twice the height; use 60° and the taller end for semi-crystalline resins, and locate the parts with a step or tongue-and-groove. A shear joint uses 0.2–0.4 mm radial interference between lip and wall, a 30–45° lead-in, weld depth about 1.25 times the wall, and a nest that resists hoop stress. Shear joints give the strongest, most hermetic welds in semi-crystalline resins.
Troubleshooting Ultrasonic Welds
| Symptom | Likely cause | Correction |
|---|---|---|
| Flash at the joint line | Excess collapse or energy; oversized director | Reduce collapse setpoint, add a flash trap, lower amplitude. |
| Marks or melt on the horn side | Amplitude too high; thin wall under horn; rough horn face | Polish the horn, fit protective film, move to 40 kHz, thicken the wall. |
| Incomplete or uneven weld | Horn not parallel; warped parts; low energy | Level the fixture, check flatness, use collapse mode, raise amplitude first. |
| Particulate | Director tip shedding before melt; shear-joint flaking | Raise trigger force, profile amplitude down after melt starts, vacuum the nest. |
| Cracking | Molded-in stress; high amplitude in PC or PMMA; sharp corners | Anneal or re-mold, lower amplitude and raise force, radius corners. |
| Diaphragming (holes in thin flats) | Thin unsupported section resonating under the horn | Thicken by 0.3–0.5 mm, add ribs or a crown, lower amplitude, change frequency. |
| Horn overload or frequency drift | Horn out of tune, cracked or loose; dirty stack joints | Re-tune or replace the horn, torque and clean stack joints, damp the fixture. |
Ultrasonic Against Laser, Vibration and Hot-Plate Welding
Ultrasonic is rarely beaten on cycle time or tooling cost, and rarely right for large, cosmetically critical, electronics-carrying or particulate-sensitive parts. Through-transmission laser welding is the usual alternative; it needs a matched resin pair (see laser transmission welding explained and resin pair and colorant selection) and joint faces within about 0.1 mm, where ultrasonic tolerates poorer fit and any color. Sabreen engineers laser welding processes, sells no equipment, and applies this comparison before recommending either.
| Criterion | Ultrasonic | Through-transmission laser | Vibration and hot plate |
|---|---|---|---|
| Cycle time | 0.1–1 s weld, 2–5 s cycle | 1–10 s | Vibration 5–15 s; hot plate 20–60 s |
| Particulate | Present; needs traps | Near zero | Heavy; hot plate strings |
| Cosmetic marking | Horn witness marks | None; faint seam | Flash unless trapped |
| Part size | Horn-limited, roughly 250 mm | Any beam reach | Vibration to about 1 m; hot plate larger |
| Clear-to-clear | Yes, any color | Needs a 1.5–2 µm source or absorber additive | Yes |
| Electronics on board | Risk to crystals and solder joints | Safe; no vibration | Vibration unsafe; hot plate adds heat |
| Medical cleanliness | Needs particulate control | Cleanest; preferred for fluid paths | Rare for small devices |
Specifying and Validating an Ultrasonic Weld
“Ultrasonically weld” on a drawing specifies nothing. State a performance requirement, an in-process window and evidence that the window delivers it.
- Strength. Pull strength on the assembly, or the AWS G1.2M/G1.2 I-beam specimen for material-pair screening; amorphous near-field welds reach 70–100% of parent strength. State minimum and failure mode.
- Burst and leak. A minimum burst pressure and a numeric pressure-decay or helium leak limit, not “no leaks”. A strong weld with a 2 mm skip still leaks.
- Collapse window. For example 0.35 ± 0.05 mm, with energy and peak power as secondary limits logged every cycle; out-of-window parts are rejected.
- IQ/OQ/PQ. Under ISO 13485 and the FDA quality system regulation: IQ confirms welder, stack, fixture and measured amplitude; OQ maps the window by designed experiment over amplitude, force and collapse; PQ runs three lots at nominal with production operators and resin. Requalify after any horn, booster or resin change; DVS 2216 covers joint design.
Related Terms and Reading
- Laser plastic welding
- Through-transmission laser welding
- Laser welding services
- Laser transmission welding explained
- Resin pair and colorant selection
- Clear-to-clear laser welding
Applying this in production
The Sabreen Group provides independent engineering support for ultrasonic and laser welding process development, joint design and weld validation. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
Should I weld at 20 kHz or 40 kHz?
Use 20 kHz for parts above roughly 50 mm, thick walls, semi-crystalline resins and far-field joints, which need more amplitude and power. Use 30–40 kHz for small, thin-walled or cosmetic parts and anything carrying electronics: amplitude and transmitted stress halve, marking drops, and the horn is half the length.
Can polypropylene and other semi-crystalline plastics be ultrasonically welded?
Yes, with a narrower window. They damp the vibration, so the joint must sit within about 6 mm of the horn at 60–120 µm amplitude, and it should be a shear joint or sharp 60° director rather than ABS’s 90° director. Their sharp melting point makes hold force matter more.
Why does a weld pass its pull test but still leak?
Because strength and continuity differ. A pull test averages the whole weld and passes if 90% of the joint is sound, while one 2 mm skip from a warped part or an unlevel horn leaks. Specify leak as a numeric limit, run collapse mode so under-welded parts are rejected, and check horn parallelism when leaks cluster.
When is laser welding the better choice?
When the part is cosmetically critical, carries electronics, needs a particulate-free fluid path, has a long or three-dimensional seam, or exceeds a horn’s reach. Laser has no vibration, horn marks or particulate, at the cost of higher capital, a matched resin pair and joint faces within about 0.1 mm. Ultrasonic still wins on cycle time and cost.
