WELDING EQUIPMENT

Ultrasonic plastic welding: stacks, joint design and process choice

How the ultrasonic stack works, what decides whether a part can be welded, and when hot-plate, spin or vibration welding is the better route.

By WeldOrigin Editorial · Published · Updated

What sits inside an ultrasonic welder

Ultrasonic welding joins thermoplastic parts by pressing them together while a tool vibrates at high frequency and small amplitude. TWI describes the equipment as a press, a nest or anvil that holds the parts, and an ultrasonic stack: a piezoelectric converter that turns the electrical signal into vibration, an optional booster that changes the amplitude, and a horn, also called a sonotrode, that carries the vibration into the part. A generator drives the stack at its resonant frequency, and a controller coordinates press travel with the energy delivered.

TWI's job-knowledge article gives 20 kHz and 40 kHz as typical frequencies and weld times usually below one second, one reason it is common for high-volume moulded assemblies.

Chinese suppliers often sell parts of this chain separately. Chenggong counts transducers and drive supplies among its main products, RPS-SONIC catalogues transducers, boosters and tools next to finished machines, and Guobiao frames its business around drive and automatic-control technology. Before comparing prices, decide whether you need a component, a bench welder or a station built around your part.

Frequency and power are matching data, not a specification

Every element of the stack is tuned to resonate at one frequency, so a 20 kHz generator cannot drive a 35 kHz horn, and a replacement converter has to match the mounting, connections and controls of the existing machine as well as its frequency. Read the frequency and generator output on a datasheet the way you would read the supply and output data on a welding power source nameplate: they identify the equipment; a trial on your part shows whether it welds.

TWI adds that higher frequencies reduce the risk of vibration damaging electronics inside an assembly. Otherwise, ask which frequency the supplier proposes for your joint and why, and see it proven on your parts rather than judged by wattage.

When offers quote different frequencies and ratings, list each package's contents (generator, converter, booster, horn, press, nest, controls) and align them using the method in comparing welding equipment quotations.

Joint design and polymer decide whether it works

Ultrasonic energy has to be concentrated at the interface. TWI explains that plastic joints use small pointed or rounded contact features that melt first and form the weld, and that the process often needs specially designed joint details. So the moulded part, not the machine, is the starting point; an unsuitable joint may need a mould change or another process.

TWI lists soft and rigid thermoplastics and thermoplastic composites as weldable, and is clear that the process suits small items because weld depths are shallow; large joints usually cannot be made in one operation. For sealed joints it stresses housing location, weld-line form, wall thickness, material, frequency and power.

Send the polymer grade, fillers, both mating-part drawings and the function of the joint: seal, load, insert retention or appearance. Automotive interiors show why. Taisonic describes robotic cells combining interior-part joining with laser cutting and clip assembly, and Dongho offers ultrasonic welding beside heat staking for door interiors. Staking reforms a plastic post around another component, whereas welding fuses two compatible plastic surfaces, so ask which one each attachment point needs.

When hot-plate, spin or vibration welding fits better

Several Yuyao and Ningbo suppliers sell more than one plastic-joining process, so a buyer can request alternatives for one part. Mingfeng lists ultrasonic, hot-melt and hot-plate, spin and high-frequency families, and Dongho adds vibration-friction systems.

Hot-plate welding presses the joint faces against an electrically heated tool, withdraws the tool, then forces the melted faces together. It handles large faces and is used for battery cases, pallets, automotive trim and pipes; TWI's job-knowledge article spans small automotive reservoirs up to pipelines above 1000 mm in diameter. The same heated-tool principle underlies thermoplastic pipe butt-fusion machines such as those Xuansheng supplies, although pipe jointing follows its own material procedures.

Vibration welding rubs one part linearly against the other under load. TWI says it suits joints too long for ultrasonic welding and runs about four times faster than hot-plate on large joint areas, but three-dimensional joints are difficult and capital cost is high. Spin welding needs a circular joint: one part rotates against the other on a direct-drive or inertia machine, and modern servo machines can control final orientation.

If a supplier proposes laser transmission welding, which TWI also lists, the questions in our laser welding cells guide apply.

Metal, nonwoven and other ultrasonic work

Ultrasonic metal welding works differently. TWI describes it as a solid-state process in which pressure and vibration disperse surface oxides while the base metal stays unmelted. It suits thin, malleable metals such as aluminium, copper and nickel, along with wires, foils, meshes and some dissimilar pairs. HCSONIC's welding material concerns cable-to-terminal connections, where conductor construction, cross-section and plating define the job more than generator output.

The same physics sets a ceiling. TWI calls thick items impractical, so heavier wire structures belong to other processes. ZGTEK, for example, builds filter-cage lines within its resistance welding equipment range rather than around ultrasonic heads, and a buyer joining steel wire should start from that process.

Chenggong also lists nonwoven and geogrid joining, and Guobiao names nonwoven and paper-cup work. For continuous material, feeding, tension and seam pattern matter as much as the stack, and liquid-treatment products from the same firms are not welders.

Tooling, trials and the delivered package

The horn and the nest are made for one part, and TWI notes that tooling costs can be high. Ask who designs them and how they are reordered; welding fixtures and tooling covers ownership and spares. TWI also flags hearing protection or an acoustic enclosure for operators, so include noise control in the installation scope.

Judge the process on production mouldings that include normal variation, and agree the strength, leak or appearance checks beforehand. The guide to sample trials explains how to record the configuration behind each accepted part. When the welder sits inside a robotic or multi-station line, treat it as a welding automation project with its own interfaces and acceptance. Match any CE or other conformity paperwork to the quoted model; verifying welding equipment certificates explains how.

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