Insert Installation: Heat Staking vs Ultrasonic vs Press

A threaded connection in plastic has to survive torque, pull-out, vibration, temperature cycling and sometimes repeated assembly. Metal inserts are the standard answer — but the insert is only half of the joint. The other half is the hole it goes into and the method used to install it, and that is where most failures are born.

Figure 1. Four insert installation methods with boss cross-sections and hole sizing table.

There are four practical ways to put a metal thread into a plastic part: heat staking (heat-set), ultrasonic insertion, press fit, and molding the insert in place. Each has a place. Choosing wrongly leads to cracked bosses, spinning inserts, pull-out at half the expected load, or a joint that is fine in the sample room and loose after a summer in a car.

The four methods at a glance

• Molded-in inserts — strongest and most reliable if designed well; no post-processing; but they add tooling, require the insert to be loaded in the mold (manual or automated), heat and hold the surrounding plastic, and can crack if the insert geometry or the mold temperature is wrong.
• Heat-set (heat staking) inserts — the insert is pressed into an undersized hole by a heated tip. The surrounding plastic softens and flows into the knurls or diamonds; on cooling it shrinks onto the insert. Excellent pull-out and torque resistance, low stress on the part if done correctly, and the most common choice for medium to high volume.
• Ultrasonic inserts — the insert is driven in by ultrasonic vibration that melts the plastic locally at the insert interface. Very fast (typically 1–3 seconds), no external heat, tight process control needed; ideal for high volume and for heat-sensitive assemblies, but the horn and tooling add cost.
• Press-fit inserts — an interference fit with no heat. Cheapest tooling, but the joint relies on plastic elasticity and creep; pull-out falls over time, especially at elevated temperature. Suitable only for light, non-structural, low-temperature duty.

Design rules the joint depends on

### Boss geometry

• Boss outside diameter typically 1.8–2.5 × the insert diameter, so the wall carries the hoop stress without splitting.
• Boss height enough to bury the insert fully with 0.5–1 mm of plastic below it for heat-set and ultrasonic; a blind hole with a floor is safer than a through hole.
• Boss wall thickness at least 0.8–1.0 × the nominal wall, with generous radii at the base and a fillet to the wall.
• Ribs to the wall for tall bosses — three or four gussets spread the load and stop the boss from bending away.
• Draft on the boss outside, and a flat or spigot feature for the installation tooling to locate against.

### Hole dimensions

• Molded-in inserts need a hole sized to the insert with clearance for the plastic to shrink onto it, and a locating feature — often a shoulder or a set of ribs — to stop the insert from being pushed out by melt pressure.
• Heat-set inserts need a hole slightly smaller than the insert, typically 0.1–0.4 mm undersize on diameter for the common diamond-knurl insert. The plastic displaced by the insert has to go somewhere.
• Ultrasonic inserts need a hole close to the insert diameter, typically 0.05–0.25 mm oversize, because the material melts and flows into the knurls with minimal displacement volume.
• Press fit needs a controlled interference — often 0.5–1.0% of insert diameter — and a boss that can take the hoop stress without cracking or crazing.

Exact numbers must come from the insert supplier's design guide for the specific insert and resin, and must be prototyped before tooling is cut.

### Resin behavior

• Amorphous resins (ABS, PC, PC/ABS, PS) heat-set easily and give good, consistent results; PC demands adequate pre-drying or it will craze around the boss.
• Semi-crystalline resins (PA, POM, PP, PBT) have a sharp melting range and lower melt strength; heat-set and ultrasonic both work, but the process window is narrower and POM is particularly sensitive to hot spots.
• Glass-filled grades are stiffer and more notch-sensitive — boss walls crack more easily, so add wall thickness and radii, and reduce interference.
• Hygroscopic resins (PA, PC, PBT) must be dried before installation. Moisture causes voids, bubbles and a weak interface.
• Low-surface-energy, soft grades (PP, PE, TPE) hold inserts poorly by press fit. Prefer molded-in or heat-set with a mechanical locking geometry.

Installation process control

• Temperature — heat-set tip temperature is set by resin family (typically 20–60 °C above the resin's softening range). Too cold gives incomplete flow; too hot melts the boss wall.
• Speed and dwell — inserts should go in at a controlled rate with enough dwell for the plastic to flow and then cool under load. Too fast shears the plastic; too slow allows it to chill before it reaches the knurl.
• Align the insert — a crooked insert loses most of its pull-out and makes the mating screw cross-thread. Use a locating bushing or a guided tip.
• Support the part — the boss must be backed up during installation, or the part deflects and the insert bottoms unevenly.
• For ultrasonic — tune amplitude, energy and hold time; verify with a cross-section rather than by looking at the top. A "good-looking" install can have melt only halfway down.

Verifying the joint

• Torque test — apply the assembly torque plus a safety factor to the installed screw, then check for insert rotation or boss cracking.
• Pull-out (axial) test — the definitive test for heat-set and ultrasonic joints; specify a minimum force with a defined rate.
• Push-out and side-load tests — for joints loaded in shear.
• Cross-section — cut the boss after installation and inspect the melt/flow into the knurls, the void content and the wall condition.
• Torque-cycling and re-use — test repeated assembly to the number of cycles the customer will actually perform.
• Environmental exposure — test after heat aging, thermal shock and humidity. Creep in press fits and stress relaxation in heat-set joints both show up here, not at day one.

Choosing: a short decision path

• Structural joint, high load, high volume, insert loaded in tool, material forgiving → molded-in.
• Structural joint, medium to high volume, standard insert, tolerance-friendly → heat-set.
• High volume, tight cycle, heat-sensitive or very consistent demand → ultrasonic.
• Light duty, low temperature, low volume, cost-driven → press-fit, accepting limited pull-out.
• Cannot tolerate any post-processing or a sealed part → molded-in.

Figure 2. Quick reference: the key checks and reference values from this guide.

Checklist

• Insert type and material specified with the supplier's data for the resin
• Boss OD, wall thickness, height and ribbing per supplier design guide
• Hole diameter and tolerance defined per method (molded-in, heat-set, ultrasonic, press)
• Part dried to the resin's specification before installation
• Installation equipment with controlled temperature, speed, dwell and alignment
• Boss supported during installation; part not deflected
• Torque and pull-out requirements written into the drawing and the quality plan
• Cross-section verification on the first-off and at intervals
• Environmental and cycle-test validation, not just day-one testing
• Installation parameters locked in the work instruction

A threaded insert is a system: insert geometry, hole size, boss design, resin condition and installation process all contribute to whether the joint holds. Pick the method that matches the load and the volume, size the boss from the supplier's guide, prove it with a pull-out test and a cross-section, and the joint stops being the part that fails first.