Heat-set and threaded inserts for FDM parts

Heat-set inserts are the closest thing FDM has to a permanent, reusable metal thread, and they are our default recommendation for any joint that gets assembled more than a couple of times or needs to carry real torque. A brass insert with knurled or barbed outer flanks is pressed into a plastic boss while both the insert and the surrounding plastic are heated, then cools locked into place.

Getting a reliable insert joint is less about the insert itself and more about the plastic geometry around it: boss diameter, hole diameter, wall thickness and installation depth all have to be right together, or the insert either falls out under load or cracks the boss on the way in.

How heat-set inserts work

The installation tool, typically a soldering iron fitted with a tip matched to the insert size, heats the brass insert above the plastic's glass transition temperature. The insert is then pressed straight down into a slightly undersized pilot hole. The heat softens a thin layer of plastic immediately around the insert, which flows into the insert's external knurls or barbs as it cools, creating a mechanical lock that resists both pull-out and rotation.

Because the process relies on locally melting the plastic, it works best in materials with a clear, moderate melting behaviour such as PLA, PETG, ABS and ASA. It also works in PA6 and PA12, though nylon needs slightly higher tip temperature and a steady hand because it stays soft over a wider temperature range.

Boss geometry and hole diameter

The pilot hole diameter is specified by the insert manufacturer for each size and must be followed closely: too tight and the boss splits during installation, too loose and the insert has nothing to bite into. As a general guide, the hole is roughly 0.2 to 0.4 mm smaller than the insert's nominal outer diameter, but always check the datasheet for the specific insert you are using, since barbed and knurled designs differ.

• Boss outer diameter: at least 2.5 times the insert outer diameter, so there is enough plastic to resist hoop stress during and after installation.
• Wall around the insert: minimum 1.5 to 2 mm of solid plastic on all sides after the hole is bored.
• Hole depth: 1 to 1.5 mm deeper than the insert length, to give the molten plastic somewhere to displace to.
• Lead-in chamfer: a small chamfer at the hole mouth helps the insert start straight and reduces boss stress.

Insert depth and surrounding wall

Depth is a strength-versus-space trade-off. A deeper insert has more surface area gripping the plastic and resists pull-out under axial load better, but needs a taller boss, which costs print time and can conflict with wall clearances elsewhere in the housing. For M3 inserts we typically specify 4 to 5 mm depth, for M4 around 5.5 to 6 mm, and for M5 around 7 to 8 mm, adjusted to the specific insert's datasheet.

The wall around the insert has to survive both the installation heat and the ongoing hoop stress from repeated tightening. We do not go below 1.5 mm of plastic between the insert's outer diameter and the outside of the boss, and we increase this to 2 mm or more on parts that will be handled or dropped, since a thin boss wall is the first thing to crack in a fall.

Materials that take inserts well

Pull-out and torque behaviour

A correctly installed insert in a correctly sized boss typically outperforms a printed or tapped plastic thread by a wide margin on both pull-out force and repeated torque cycles, because the load path runs through the mechanical interlock in the plastic and then through the metal thread itself, rather than through plastic thread flanks that wear with each cycle.

The weak point shifts from the thread to the boss itself. If the boss wall is undersized, the insert stays intact but pulls a cone of plastic out with it, or the boss splits under installation heat before the joint is ever loaded. Getting the wall thickness right is therefore more important than the insert brand or design.

Installation consistency in series production

For one-off prototypes, a handheld soldering iron with a good eye for depth is enough. For a production series we control installation with a fixed depth stop and a controlled dwell time, so every insert sits at the same depth and with the same amount of plastic reflow around it. Inconsistent installation is the most common reason a batch of otherwise identical parts shows a spread of pull-out strengths.

We also check boss condition after installation on a sample basis for series orders, looking for hairline cracks or insert tilt, which are early indicators that the pilot hole or boss wall needs adjustment before the rest of the batch runs.

Alternatives: self-tapping inserts

Where heat is impractical, for example on a very thin-walled part where local melting risks warping the surrounding surface, a self-tapping (cold-pressed) insert is an alternative. These are screwed or pressed into a slightly undersized pilot hole without heat, cutting their own thread into the plastic as they go. They generally hold less strongly than a heat-set insert of the same size but avoid the risk of heat distortion on delicate parts.