Ultrasonic insertion: setting metal threaded inserts into plastic

Ultrasonic insertion drives a metal threaded insert into a moulded or drilled hole in a plastic part, using high-frequency vibration to melt a thin layer of plastic around it. The plastic flows into the insert’s knurls and barbs, then solidifies and locks it in place. The result is a reusable metal thread inside a plastic part, one that takes a screw, holds torque, and survives repeated assembly and disassembly. It is the answer when a moulded plastic thread would strip after a few cycles. For forming a rivet head from the plastic itself, see ultrasonic riveting; insertion is about adding a metal fastener, not shaping the plastic into one.

Why set inserts ultrasonically

Plastic threads wear out. Screw into moulded plastic a handful of times and the thread strips, which is fine for a part you assemble once and never touch, and a problem for anything a technician opens for service. A metal insert solves it: the screw bites into steel or brass, not plastic, so the joint takes real torque and survives being undone and redone.

Ultrasonic is the fast way to set that insert. Heat staking works too, with a heated tip, but it is slower and the tool has to warm up and can stick. Ultrasonic melts only a thin skin of plastic at the insert surface, in well under a second, and the surrounding material stays cool. On a production line running automotive or electronics housings by the thousand, that cycle-time gap adds up fast. The edge is real for smaller inserts, under about 6 mm; on larger ones the ultrasonic cycle stretches, and when a part needs several inserts set at once, heat staking with a multi-tip press often wins instead.

How the process works

The insert sits on the tip of the horn or is presented to a pre-formed hole slightly smaller than the insert body. The horn descends under a light pneumatic force and delivers vibration through the insert. Friction at the metal-plastic interface melts a thin layer, the insert sinks in, and molten plastic flows into the knurls and barbs on its outer diameter. Vibration stops, the plastic sets, and the insert is anchored against both pull-out and rotation.

The acoustic chain behind it is the same as any ultrasonic assembly: a generator feeding a converter, booster and horn, typically 20 kHz, sometimes 40. Start the vibration a fraction before contact and the melt initiates cleaner.

Insertion, staking, riveting: not the same job

These three ultrasonic operations get mixed up because the physics overlaps. What differs is what you are joining and why.

If the joint needs a screw later, you insert. If it just needs to hold a part in place, you stake or rivet.

Design rules for a solid insert

The insert only holds as well as the hole and the settings around it.

• Hole diameter under the insert body, with the right interference so molten plastic fills the knurls without over-stressing the boss.
• Boss wall thick enough around the hole to resist the hoop stress the insert creates. Too thin and it cracks.
• Lead-in chamfer at the hole mouth to start the insert straight and avoid cocking.

Insert geometry matters as much as the hole. Knurls resist rotation, barbs and undercuts resist pull-out, and the two together give a joint that holds torque in one direction and load in the other. Manufacturers of the inserts themselves publish hole specs per reference; match them rather than guessing.

Which plastics take inserts well

Amorphous resins, ABS, PC, PA, accept ultrasonic inserts cleanly thanks to their broad softening range. Semi-crystalline plastics such as PP work but want tighter control of amplitude and force, since they melt over a narrower band. The compatibility logic mirrors thermoplastic welding in general: the better the resin flows when it melts, the cleaner it fills the knurls.

Strength and quality control

Two numbers define a good insert: pull-out force, how hard you can yank it before it leaves the plastic, and torque-out, how much you can twist before it spins. Both are tested destructively on samples, and both depend on the melt fully filling the knurls, not just seating the insert flush. An insert that looks set but never filled the barbs will pass a glance and fail a pull test.

On a governed line, amplitude, force, depth and energy are recorded per cycle. Depth is the tell: an insert that stops short did not melt enough, one that sinks too far flooded the boss. Machines that log the curve catch both before they reach a customer.

Automation

Insertion runs well automated because it is quick and forgiving on cycle. Vibratory feeders present inserts, the horn sets them, and depth or energy monitoring flags a bad seat for rejection. Multi-head tooling sets several inserts in one stroke on parts that need them, and the whole station logs data for traceability on regulated products.

Mecasonic machines for ultrasonic insertion

For bench work, the ultrasonic gun and pencil probe set inserts by hand in awkward spots or for short runs. For repeatable production, the OMEGA 5 range (OMEGA 5 A and 5 P pneumatic, OMEGA 5 E electric) brings the descent and force control insertion depth depends on, the electric 5 E giving the finest control and full per-cycle traceability. See the ultrasonic welding range for the machines.

For integrators, compact modules (ML 40, ML 32, UBC) with the Meca-Sequence controller build multi-insert tooling and robot cells on Pulse One and Pulse Touch generators. See the integration modules. For a specific part, send us the geometry and insert reference and we will size the hole and the cell.

When should I use an insert instead of a moulded plastic thread?

Whenever the joint will be opened more than once or has to hold real torque. A moulded thread is fine for a permanent single assembly; it strips after a few cycles. A metal insert gives a reusable, load-bearing screw point.

Ultrasonic insertion or heat staking for inserts?

Ultrasonic is faster and keeps the tool cool, which suits high volumes of smaller inserts and avoids tip-sticking. Heat staking takes back the advantage on large inserts, on parts needing several inserts set in one stroke, and on semi-crystalline resins that melt and re-solidify well under a heated tip. Match the method to insert size and count, not just to volume.

What holds the insert in place?

The plastic that melts during insertion flows into the knurls and barbs on the insert’s outer diameter, then solidifies around them. The knurls stop it turning, the barbs lock it against pull-out. There is no adhesive.

Which insert and hole size should I use?

Match the insert manufacturer’s published hole spec for the reference you are using; the interference is designed per insert. If you are unsure how it maps to your resin and boss, send us the part and we will confirm the setup.

Can inserts be set in any plastic?

Most thermoplastics take them. Amorphous resins like ABS, PC and PA are the easiest; semi-crystalline ones like PP need tighter parameter control. Thermosets do not work, since they do not re-melt.