Heat set inserts are how a printed nylon part gets a thread you can actually torque against, and unscrew, and torque again. Printed threads work for a handful of assemblies. Anything that comes apart for service, or takes a real clamp load, wants metal in the hole.
The mechanics are simple enough that people skip the design work, which is why so many bosses split during installation or pull out three months later. This guide covers boss sizing, what happens during install, what fails when a joint fails, and when a different fastening method is the better answer.
What Is a Heat Set Insert?
A heat set insert is a knurled metal bushing with an internal thread that is pressed into a plastic part using a heated tip, melting the surrounding polymer so it flows around the knurl and locks the insert in place as it cools. The result is a metal thread anchored by a mechanical key rather than by adhesive or by the plastic itself being threaded.
The important consequence is that the joint strength lives in the plastic, not in the insert. Brass will out-perform sintered nylon every time, so the insert is never the weak link. What decides whether the assembly holds is how much nylon surrounds the insert and how well the melt filled the knurl.
Why Printed Nylon Parts Need Them
Sintered nylon can hold a printed or tapped thread, but not one that survives repeated assembly. Each time a steel screw turns in a nylon thread it abrades a little of the flank, and PA12 is ductile enough that a few cycles of that noticeably reduce the holding. For a part assembled once and never touched, printed threads are fine. For tooling that gets serviced, they are a warranty problem.
There is a second reason specific to laser sintering. A printed internal thread is limited by process resolution and by powder that has to be cleaned out of the thread form, so fine pitches print poorly and clean out worse. An insert sidesteps both: the printed feature becomes a plain round hole, which sinters accurately and depowders easily, and the thread precision comes from a manufactured part. Our design guidelines for SLS cover the wider set of feature limits this sits inside.
Sizing the Boss
Boss geometry decides whether the insert holds, and the numbers are not guesswork. The published design rules from insert manufacturers translate directly to sintered nylon, and the ones below come from SPIROL guidance on designing holes for heat and ultrasonic inserts.
| Feature | Design rule | Why it matters |
|---|---|---|
| Boss outer diameter | Two to three times the insert diameter, with the multiplier decreasing as insert diameter increases | Too thin and the boss splits during installation or shears out under load |
| Hole depth | At least the insert length plus two thread pitches | The extra depth gives displaced material somewhere to go and stops the screw bottoming out |
| Hole diameter | From the insert supplier table for that specific insert style, not a generic rule | Undersized bulges the boss, oversized leaves nothing for the knurl to key into |
| Straight hole taper | No more than 1 degree included angle | Straight inserts belong only in straight holes |
| Tapered hole angle | 8 degrees included angle | Tapered inserts belong only in tapered holes |
| Filled grades | Increase hole diameter by 0.08 mm at 15 percent filler and up, and by 0.15 mm at 35 percent and up | Filler resists flow, so the same hole behaves tighter than it would in unfilled nylon |
One principle sits behind the whole table: enough plastic has to be displaced to completely fill the external features of the insert. If the melt does not fill the knurl, the insert is held by friction on a smooth bore, and that joint will let go. Undersize the hole and you push material outward until the boss bulges or cracks. Oversize it and there is not enough material to fill anything.
Installation: Temperature and Technique
Installation melts a thin layer of nylon around the insert and lets it flow into the knurl, so the tip has to run above the melting range of the polymer while the part stays cool everywhere else. Laser sintered PA12 melts between 172 and 180 degrees C according to the EOS PA 2200 material data sheet, measured to EN ISO 11357-1, and the working tip temperature sits above that. The exact setting belongs to the insert supplier rather than to a universal number, because tip mass and dwell time change the result as much as the dial does.
Technique matters more than most people expect. Go in square, because an insert that starts crooked stays crooked. Let the heat do the work rather than the arm, since pushing hard on a cold insert splits the boss and pushing hard on a hot one sinks it too deep. Stop flush or slightly below the surface, and leave it alone while it cools, because the joint has no strength until the nylon solidifies around the knurl.
A press with a depth stop repeats better than a soldering iron by hand. If you would rather not do it in house, it is part of our post-process options for SLS.
What Actually Fails
When a heat set insert joint fails, the insert almost never breaks and the screw almost never strips. What gives way is the nylon, and it goes one of two ways: the plastic shears around the outside of the insert and the whole thing pulls out as a plug, or the boss wall splits and releases its grip radially.
Both failures point back to boss geometry rather than to the insert. Pull-out resists load across the interface between the knurl and the melted nylon, so the amount of engaged material sets the strength, which is why insert length and a properly filled knurl matter more than the thread size. Splitting is a hoop stress problem, which is what the two to three times boss diameter rule exists to prevent.
When to Use Something Else
Heat set inserts are the right default for serviceable joints in printed nylon, but they are not the only option and they are not always the best one. The table below is the comparison we run through when specifying a printed part.
| Method | Best for | Upside | Limitation |
|---|---|---|---|
| Printed thread | Assembled once, low clamp load, cost sensitive | No hardware and no secondary operation | Wears quickly under repeated assembly, and fine pitches print and depowder poorly |
| Tapped thread | Coarse threads in thicker sections, low cycle count | Cheap, uses standard taps | Same wear problem as printed threads, plus a machining step |
| Through bolt and nut | Anywhere both sides are accessible | Strongest option, load goes through the joint rather than into the plastic | Needs access to both faces, and adds a loose part to the assembly |
| Captured nut pocket | Parts where a nut can be trapped during design | No heat, no press, and the nut cannot spin out | Needs design room for the pocket and an assembly sequence that closes it |
| Press fit insert | Low load, thin walls where heat would distort | Fast, no thermal input | Relies on interference alone, so holding is lower and creep matters over time |
| Heat set insert | Serviceable joints, repeated assembly, moderate to high clamp load | Metal thread keyed into the plastic, reusable many times | Needs boss room, correct hole size, and controlled installation |
The pattern worth remembering: if the load path can bypass the plastic, let it. Inserts earn their place when only one side is accessible, which on robot end-of-arm tooling is most of the time.
How We Handle Inserts
We are Anubis 3D Industrial Solutions. We design and manufacture end-of-arm tooling and produce parts by selective laser sintering in Mississauga, Ontario, and nearly every tool we ship has inserts in it. Because we design the part as well as print it, the boss is sized for the insert from the start rather than being discovered at assembly. That means the hole is modelled to the supplier table for the specific insert, the boss carries the two to three times wall, and bosses are tied into ribs or walls where the geometry allows. The material context for all of this is on our PA12 nylon page, and if the naming of the grades is unfamiliar, our guide to polyamide vs nylon untangles that.
Heat Set Insert Questions
What size hole do I need for a heat set insert?
Take it from the insert manufacturer table for that specific insert style rather than from a general rule. The hole should be at least as deep as the insert length plus two thread pitches, and for filled grades the diameter increases by 0.08 mm at 15 percent filler and up, and 0.15 mm at 35 percent and up.
How thick does the boss need to be around a heat set insert?
A boss outer diameter of two to three times the insert diameter, with the multiplier reducing as insert diameter grows. Below that the boss tends to split during installation or shear out in service.
What temperature installs a heat set insert in nylon?
Above the melting range of the polymer, which for laser sintered PA12 is 172 to 180 degrees C per the EOS PA 2200 data sheet. The specific tip setting comes from the insert supplier, because dwell time and tip mass affect the result as much as temperature does.
Are heat set inserts strong enough for 3D printed parts?
Yes for most serviceable joints, and the limit is the plastic rather than the insert. Failure happens by the nylon shearing around the knurl or the boss splitting, so strength is set by boss wall thickness and insert length.
Need Threads That Survive the Tenth Teardown?
Tell us how the part is assembled, how often it comes apart and what torque it sees, and our team will size the bosses and pick the fastening method with you before the part is printed rather than after.
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