Vibration Welding
Vibration welding rubs two thermoplastic parts together in-plane at low frequency under pressure until the interface melts and fuses.
- Part
- Joining
- Revised
- 2026-08-11
At a glance
- Family
- Thermal
- Typical tolerances
- Meltdown controlled to about ±0.004 in (±0.1 mm); final assembly height set by a positive stop or machine collapse control rather than by molded part height
- Typical volumes
- 5,000 to 5,000,000+ per year
- Lead time
- 6–20 s cycle including 1–10 s of vibration and a 2–6 s hold; 3–8 weeks for matched nest tooling
- Materials
- Plastic
What it is
Vibration welding rubs two thermoplastic parts against each other in-plane, at 100–250 Hz with 0.03–0.16 in (0.7–4 mm) amplitude, under 100–300 psi (0.7–2 MPa) of joint pressure, until the interface melts and the parts are clamped together to fuse.
It exists to do what ultrasonic welding cannot: weld large parts, semicrystalline resins and heavily glass-filled grades, all reliably. Machines handle joints up to roughly 24 x 40 in (600 x 1,000 mm), which is why intake manifolds, instrument panels, fluid reservoirs, bumpers and door modules are welded this way.
The joint is hermetic and, in unfilled resins, approaches base-resin strength. Cycle time is 6–20 s including 1–10 s of actual vibration and a 2–6 s hold — slower than ultrasonic, but the process is far more tolerant of part variation, contamination and joint geometry.
How it works
- Clamp. The lower part is fixtured rigidly; the upper part is held in a vibrating tool head. Joint pressure of 100–300 psi (0.7–2 MPa) is applied across the joint area — total force scales with joint area, which is why large parts need large machines.
- Vibrate. Electromagnetic drivers oscillate the upper part in-plane at 100–250 Hz. Lower frequencies run larger amplitudes (up to about 0.16 in / 4 mm) and higher frequencies smaller ones (0.03 in / 0.7 mm). Friction at the interface generates the melt.
- Melt and steady-state flow. After an initial dry-friction phase the interface reaches a steady melt film that is continuously extruded outward as flash while fresh polymer melts behind it. Total meltdown (collapse) is typically 0.04–0.10 in (1–2.5 mm) and is the primary process variable.
- Stop and hold. Vibration halts, the parts align to their final position, and clamp pressure is held for 2–6 s while the joint solidifies. Solidifying under pressure is what makes the joint hermetic.
Linear versus orbital
Linear vibration welding moves the part back and forth along one axis. The joint has to lie in a single plane, or on a shallow curve, so the relative motion stays tangential to the joint face everywhere.
Orbital vibration welding moves the upper part in a small circular orbit, so every point on the joint sees the same relative velocity in every direction. That removes the single-plane constraint and suits irregular and non-planar joint lines, at the cost of more complex machinery.
Why it tolerates what ultrasonics will not
The heat comes from bulk relative motion, not from transmitting vibration through the part, so part stiffness, resin crystallinity and glass content barely matter. Glass-filled grades up to about 50% weld successfully, though the weld line itself is weaker than the base material because the fibers reorient parallel to the flow and no longer bridge the joint.
Design guidelines
The joint must lie in one plane
This is the fundamental constraint for linear welding. Design the parting line of the assembly as a single flat plane, or a curve shallow enough that motion stays tangential everywhere along it. If the joint has to be three-dimensional, specify orbital welding instead — do not try to force a linear machine onto a saddle-shaped joint.
Joint width, flash traps and clearance
Make the joint land at least as wide as the nominal wall, and 1.5–2x wall where the joint is structural or must seal. Molten polymer is extruded outward during the weld, so design flash traps on both sides of the joint — a recessed channel roughly the volume of the displaced melt — unless flash on the finished part is acceptable. Provide clearance around the joint so the moving part can travel its full amplitude without touching adjacent features: at least the peak-to-peak amplitude plus 0.04 in (1 mm).
Design in the meltdown
The parts get shorter by the meltdown distance, typically 0.04–0.10 in (1–2.5 mm). Dimension the assembly so its critical height is controlled by a positive stop or by machine collapse control, not by the nominal molded height of the two halves.
Support the joint against clamp force
Joint pressure of 100–300 psi (0.7–2 MPa) over a large joint is a large total force, and unsupported walls will bow. Design internal ribs or a supporting nest so the joint plane stays flat under load; a joint that bows open mid-weld will not seal.
Resin and fill compatibility
| Pair | Weldable? | Notes |
|---|---|---|
| Same resin, unfilled | Excellent | Joint approaches base-resin strength, hermetic |
| Same resin, up to 30% glass | Good | Weld line weaker than base material — fibers reorient and stop bridging |
| Same resin, 30–50% glass | Fair | Welds, but size the joint on the weld-line strength, not the datasheet value |
| PA (nylon), PP, PE, PBT, POM | Good | Semicrystalline resins that ultrasonic welding struggles with |
| PC, ABS, PC/ABS | Good | Also weld well ultrasonically — choose on part size |
| Dissimilar resins | Generally no | Needs chemical compatibility and close melt temperature; validate before committing |
| Nylon with absorbed moisture | Poor | Water foams at the melt line; dry the parts before welding |
Joint lands, flash traps and rib supports are molded features — wall thickness and rib rules are on the injection molding design guidelines chart.
Inspection
Meltdown distance is recorded for every cycle and is the primary process signal; a shallow meltdown means the joint did not reach steady-state melt. Beyond that: burst pressure and pressure-decay leak testing on sealed parts, destructive cross-sections through the weld line to confirm melt across the full joint width, and tensile coupons cut from welded parts for strength qualification.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Joint plane | One flat plane (linear) | Shallow curve | Motion must stay tangential to the joint face |
| Joint land width | 1.5–2x nominal wall | 1x wall | Sets weld area and seal reliability |
| Meltdown allowance | 0.06 in (1.5 mm) | 0.04–0.10 in (1–2.5 mm) | Assembly height must be dimensioned around it |
| Flash trap | Both sides of the joint | — | Displaced melt has to go somewhere |
| Motion clearance around joint | Amplitude + 0.04 in (1 mm) | — | The moving half must not strike adjacent features |
| Glass fill at the joint | Under 30% | 50% | Weld line strength falls as fiber content rises |
| Joint support under clamp | Ribbed or nested | — | 100–300 psi (0.7–2 MPa) will bow an unsupported wall |
Cost drivers
Vibration welders are substantially larger and more expensive machines than ultrasonic welders, because they must react the vibrating mass and the full clamp force across a large joint. Machine size scales with joint area, and so does capital cost — this is the dominant economic factor and it is set by part size, not by volume.
Tooling is a pair of matched nests, typically machined aluminum, which cost more than ultrasonic nests because they carry the clamp load. Cycle time of 6–20 s is longer than ultrasonic but still short next to adhesive cure, and there are no consumables at all.
The process wins where the alternative is bonding a large assembly (cure time, fixturing, adhesive cost) or fastening it (leak paths, part count, assembly labor). It loses to ultrasonic welding on small parts, where a smaller machine and a 2 s cycle do the same job.
- Keep the joint in one plane — an orbital machine costs more than a linear one and there is rarely a functional reason to need it.
- Size the joint to the load, not to the perimeter. Total clamp force scales with joint area, and machine cost scales with force.
- Design flash traps in rather than adding a deflashing operation after welding.
- Consolidate parts into the welded assembly. The economics improve every time a weld deletes a bracket, gasket or fastener stack.
- Specify unfilled resin at the joint where the part allows it; weld-line strength in a 50% glass grade is a fraction of the datasheet number.
Questions
5 questionsWhen should I use vibration welding instead of ultrasonic welding?
When the part is large, the resin is semicrystalline, or the grade is heavily glass filled. Vibration welding handles joints up to roughly 24 x 40 in (600 x 1,000 mm) and welds PP, PA, PBT and 30–50% glass grades reliably, because the heat comes from bulk relative motion rather than from transmitting vibration through the part. Ultrasonic wins on small parts with a 2 s cycle.
How much do the parts shrink during a vibration weld?
Meltdown, or collapse, is typically 0.04–0.10 in (1–2.5 mm) of combined height. That has to be designed into the assembly: dimension the critical height off a positive stop or the machine's collapse control rather than off the nominal molded heights of the two halves.
Does the joint have to be flat?
For linear vibration welding, effectively yes — the joint must lie in one plane or on a curve shallow enough that the relative motion stays tangential everywhere along it. Orbital vibration welding moves the part in a small circle so every point sees the same relative velocity, which removes the constraint and suits three-dimensional joint lines, at higher machine cost.
Can vibration welding produce a hermetic seal?
Yes — hermetic sealing is one of its main uses, in fluid reservoirs, filter housings and intake manifolds. The seal comes from holding clamp pressure for 2–6 s after vibration stops so the melt solidifies under load. Verify with burst pressure or pressure-decay leak testing rather than assuming it.
How does glass fill affect vibration weld strength?
It reduces it, and the reduction grows with fill level. During the weld the fibers reorient parallel to the melt flow and stop bridging the joint, so the weld line is weaker than the bulk material even though the process runs fine up to about 50% glass. Size structural joints on measured weld-line strength, not the resin datasheet.