Beyond welding: staking, swaging, insertion and spot welding

Most people meet ultrasonics as a way of joining two plastic halves. The same stack does at least four other jobs, and a shop that only welds is leaving capability on the bench.

Staking

A plastic stud is moulded on one part. It passes through a hole in a second component — a PCB, a metal bracket, a label plate. The horn comes down on the exposed stud, melts it, and reforms the melt into a head. When it cools you have a rivet that was already part of the moulding.

It replaces a screw. No fastener to buy, no thread to strip, cycle time under a second, and the joint does not loosen with vibration the way a self-tapping screw in plastic often does.

The tip geometry decides how it turns out:

• Standard rosette or dome tip: for studs roughly 1.5 mm diameter and up. Gives a clean domed head. The most common choice.
• Flush tip: where the head must not stand proud of the surface.
• Hollow or knurled tip: for larger studs, spreading the melt outward into a ring rather than a dome.
• Multi-tip horn: several studs in one hit. This is where staking really beats screws — six stakes in one second instead of six screwdriver operations.

Two things go wrong. Too much energy and the melt squeezes out sideways, leaving a thin flat head that shears off. Too little and the head never forms properly. Because the process is short and shallow, low amplitude and controlled depth work far better than brute force — this is a job where a depth-mode digital generator earns its price.

Swaging and forming

Swaging folds a plastic rim over something you want to trap. A lens into a bezel, a metal mesh into a housing, a filter disc into a cap. The horn has a chamfered internal profile that pushes the wall over as it melts it.

The wall has to be the right height and thickness before you start. Too thick and it will not fold cleanly. Too thin and it tears. As a starting point, keep the wall to be folded roughly the same thickness as the section it grows from, and leave 1.5 to 2 times that as height above the component.

Forming is the same idea used to reshape a moulded feature — flattening a boss, closing a slot, reforming a tab.

Insertion

The reverse of staking. A brass threaded insert, knurled on the outside, goes into a moulded hole that is slightly smaller than the insert. The horn presses the insert in while vibrating, a thin layer of plastic at the wall melts, and it flows into the knurls and undercuts. It cools locked.

Why bother, when you can mould the insert in or press it in cold?

• Moulded-in inserts slow the moulding cycle and risk moulding stress around the metal.
• Cold pressing leaves stress in the plastic that shows up as a crack three months later.
• Ultrasonic insertion gives higher pull-out and torque-out values than cold pressing, because the plastic has genuinely flowed into the knurl rather than being wedged aside.

Hole design matters. Too tight and you get a stressed boss. Too loose and there is not enough plastic to fill the knurls. Follow the insert manufacturer's hole recommendation — they have done the work.

Spot welding

Two sheets or two mouldings with no pre-formed joint at all. A pilot-tipped horn pushes through the top sheet, displaces molten material into the lower one, and forms a small welded stud between them.

This is the answer when the parts were never designed to be joined — large sheet assemblies, panels, retrofits, repairs. No energy director, no fixture in many cases, and you can put the welds wherever they are needed rather than where the mould dictated.

Spot welding is where a hand gun welder is genuinely the right tool rather than a compromise. An operator can reach into an assembly, tack it, and move on. For rework, field repair and low-volume work it beats setting up a bench machine every time.

Which of these needs a different machine?

Mostly none. The same generator and converter drive all of it — what changes is the horn or tip, and the settings.

What does help:

• Depth or energy mode rather than time. Staking and swaging are shallow, short operations where a fixed time gives inconsistent results.
• Controlled approach speed. Slamming a horn onto a PCB stake breaks the PCB. A servo actuator or a good pneumatic with flow control matters here more than it does for welding.
• Lower amplitude. Often a lower-gain booster than you would use for welding the same material.
• A hand gun alongside the bench machine, for everything that will not sit in a fixture.

If you already own an ultrasonic welder and have been buying screws for an assembly that could be staked, the payback on a set of staking tips is usually measured in weeks. Send us the assembly and we will tell you whether it is a candidate.

Questions we get asked