Ultrasonic riveting: forming plastic rivet heads in seconds

Ultrasonic riveting uses high-frequency vibration to melt and reshape a moulded plastic stud into a rivet head, locking two or more parts together with no screw, no adhesive and no metal fastener. The cycle takes a few seconds and forms a permanent, single-material joint. It is one of the assembly methods available for thermoplastic parts, alongside welding and insert placement, and it is the go-to when you need to trap a dissimilar component, a metal bracket or a PCB, inside a plastic housing.

How ultrasonic riveting works

A thermoplastic stud is moulded onto the base part and passed through a hole in the mating component. The horn, shaped with a cavity, presses onto the stud tip and delivers vibration. Friction melts the tip, the softened plastic flows into the cavity and takes its shape, then solidifies as the vibration stops. What is left is a rivet head that clamps the parts, formed from the part’s own material.

The cycle in order: the stud aligns with the hole; the horn descends under force; vibration melts the tip; the plastic fills the cavity; the material sets and the head is formed. Under a second of actual weld time on most parts. It is one branch of the broader ultrasonic welding family, applied to head-forming rather than surface fusion.

The equipment behind the cycle

Four elements decide accuracy and repeatability. The ultrasonic generator delivers stable energy at a controlled frequency, usually 20 to 40 kHz. The converter and booster carry that energy to the horn with minimal loss. The riveting horn (sonotrode), cavity-shaped, forms the head. The nest holds the assembly aligned and stops energy leaking into the fixture.

Riveting vs screwing vs gluing

Where it is used

In automotive, riveting fixes dashboards, trims, lighting modules and under-hood parts, swapping metal fasteners for lighter plastic heads. In electronics, it fixes covers and internal supports without heating the circuits, and leaves no visible joint on the outside. In medical, the absence of adhesive and solvent keeps assembly clean for filter housings and diagnostic tools that need validated, repeatable joints. Consumer goods use it wherever a smooth surface and a hidden joint matter.

Joint design rules that decide success

A rivet is only as good as the stud and horn behind it. Get these right before tuning the machine.

• Stud dimensions match the horn cavity. Diameter and height set how the head forms; a mismatch gives a weak or malformed head.
• Pilot hole with clearance. The hole takes the stud with room to flow, while keeping the parts aligned.
• Base thickness. Enough material at the stud base to stop it cracking under pressure.

Horn cavity shape drives the head profile. A flat cavity gives a wide shallow head for thin material; a spherical cavity a domed head for better load spread; a conical cavity a high pull-through head for structural joints. Multi-cavity horns form several rivets in one shot.

Which plastics rivet well

Amorphous thermoplastics with good melt flow, ABS, PC, PA, form clean heads reliably. Semi-crystalline plastics like PP demand tighter parameter control and a stud shaped for their sharper melt behaviour. The material-compatibility logic is the same one that governs ultrasonic insertion, so if you already run inserts on a given resin, riveting on the same part is usually a safe bet.

Process control and quality checks

Four variables shape the result: amplitude, time, force and energy. Too little amplitude and the head never forms; too much and the part is stressed or damaged. Controlled force lets the stud flow without cracking, and energy monitoring flags a drifting cycle before it makes scrap.

On the check side: visual inspection confirms head shape and seating, destructive pull-through and shear tests measure strength, and statistical process control tracks batch-to-batch drift. Machines that log weld data feed the traceability that automotive and medical production demand.

Automation and Industry 4.0

Riveting suits robotic cells because it is fast and predictable. Robot arms position parts, indexing tables feed stations, and in-line sensors verify each head and reject bad ones. Weld curves and cycle data log for traceability, horn wear is tracked for predictive maintenance, and the whole thing ties into an MES for real-time quality tracking.

Mecasonic machines for ultrasonic riveting

For bench work and tight spots, the ultrasonic gun and the lightweight pencil probe give on-tool adjustment and easy handling for spot rivets and inserts in confined areas. When head forming needs press consistency, the OMEGA 5 range (OMEGA 5 A and 5 P pneumatic, OMEGA 5 E electric) brings recipe control and scalable automation, with the electric 5 E adding fine descent control and full weld traceability. See the ultrasonic welding range for the machines.

For integrators, compact linear modules (ML 40, ML 32, UBC) pair with acoustic assemblies and the Meca-Sequence controller to build multi-rivet tooling, rotary tables or robot-mounted cells without redesigning the acoustic path. See the integration modules. They run on Pulse Touch and Pulse One generators for stable power and simple recipe management. For a specific part, send us the geometry and we will size the stud, the horn cavity and the cell.

Ultrasonic riveting vs ultrasonic staking: what is the difference?

Same physics, different goal. Staking flattens a stud to trap a component flush against the surface, often a PCB or a metal plate. Riveting forms a defined head, usually domed or conical, when the joint has to carry a real load or take repeated stress. If the part just needs to stay put, you stake it; if the head has to hold, you rivet it.

What is the difference between ultrasonic riveting and ultrasonic welding?

Welding fuses two plastic surfaces into one. Riveting deforms a single stud into a head that mechanically traps another part, which can be a different material such as a metal bracket or a PCB. Riveting is the choice when the parts are not weldable to each other.

Is ultrasonic riveting the same as heat staking?

The result looks similar but the energy source differs. Heat riveting uses a heated tool to soften the stud; ultrasonic riveting uses vibration, so the tool stays cool and the cycle is faster. Ultrasonic suits high volumes and avoids the warm-up and tip-sticking of a thermal tool. Heat riveting can be gentler on materials sensitive to vibration.

Can you rivet a metal part to plastic?

Yes, and it is a common reason to choose riveting over welding. The plastic stud passes through a hole in the metal component and the formed head clamps it in place. No fastener, no adhesive, and the metal never needs to be weldable.

What causes a weak or cracked rivet head?

Usually one of three things: a stud that does not match the horn cavity, too much amplitude stressing the material, or too little base thickness so the stud fractures under pressure. Fix the stud and horn geometry before touching the machine settings.