Heat-Set Insert Hole Size for SLS and MJF Nylon

Heat-set insert hole size in printed nylon mostly comes down to the insert maker's table.

What Hole Size Should You Model for a Heat-Set Insert in an SLS or MJF Nylon Part?

Start with the insert maker's published hole diameter and depth, which a common shortcut sizes to the smooth pilot collar rather than the knurl. Then make the hole deeper than the insert and slightly larger for filled grades.

The poster modeled a 3.5 mm hole for an insert with a 3.9 mm smooth pilot and a ~5 mm knurl. The plastic melted, flowed up inside the insert, and ruined the threads. Replies pointed to the usual causes: the iron was far too hot (one reply estimated an unregulated iron at around 480 °C), and the hole was too small or too shallow for the displaced plastic to go anywhere except into the threads.

Start with the insert maker's table

Every insert family has a published hole diameter, depth, and sometimes a taper; McMaster-Carr and the insert manufacturers list these per part number. A common community shortcut is to size the hole to the smooth pilot collar of the insert, not the knurl.

Adjusting heat-set insert hole size for printed nylon

Industrial SLS/MJF nylon behaves differently from FDM PLA:

• Filled grades need a slightly bigger hole. One SLS supplier, citing SPIROL insert guidance, adds about 0.08 mm at 15%+ filler and 0.15 mm at 35%+.
• Boss wall matters more than the insert. Failures in printed nylon tend to be the boss splitting or the plastic shearing around the knurl, so a boss outer diameter of roughly 2–3× the insert diameter is a common starting point.
• Make the hole deeper than the insert. Leave room below the insert for displaced material and for the screw tip.
• TPU is a poor candidate. At least one MJF service bureau advises against heat-set inserts in flexible TPU.

Installation habits that prevent flooding

• Use a temperature-controlled iron with an insert tip.
• Press slowly and square, and stop flush or slightly below the surface.
• Let the part cool before loading the screw or joint.

Do you even need an insert?

MJF nylon can also be tapped directly or given printed threads for larger, coarse sizes. One service bureau (Forge Labs) suggests M6 and up for printed threads. Inserts earn their place on small threads and joints that get assembled repeatedly. For why tapped nylon threads wear, see tapped nylon vs Delrin threads, and for a related failure, why MJF press-fit bosses loosen.

Source: Forerunner 3D on heat-set inserts for MJF parts.

At DigiForge

We print MJF Nylon, SLS Nylon PA12, MJF Glass-Filled Nylon, SLS PP GB Glass Filled, and Thermoplastic Polyurethane (TPU). Our listed printing tolerance is ±0.3 mm or ±0.3%, whichever is greater. Size insert holes with that tolerance in mind, and prototype a few holes before committing to a full run. If you want inserts installed, ask us when you quote.