Heat Set Inserts

TL;DR

Model the pilot hole to the insert maker’s spec, give the insert a thick, well-filleted boss with enough perimeters, then install with a temperature-controlled soldering iron while the part is fully supported so the insert goes in straight and stops flush.

Insert styles: what changes in the print

Most hobby inserts are brass with knurls that bite into softened plastic. Tapered inserts self-center more easily during the first millimeters, but they can wedge the boss outward if the hole is tight or the plastic gets too soft. Straight (parallel) knurl inserts depend more on you keeping them square, but they tend to load the boss more evenly when you have enough wall thickness. Pick an insert primarily by screw size and required engagement length, then check that your part has enough boss diameter and depth to surround it with solid plastic.

Design targets (what matters and why)

Pilot hole diameter

Match the insert manufacturer spec; too small swells/splits the boss, too large reduces grip and pull-out strength.

Boss outer diameter

Leave meaningful wall thickness around the insert; thin rings crack during heating and later when tightening.

Plastic under insert

Keep solid material below so clamp load doesn’t punch through; avoid putting the insert over a thin roof or sparse infill.

Lead-in at hole

A small chamfer helps the insert start centered and reduces the chance of going in crooked.

Root fillet at boss

Generous fillets where the boss meets the part reduce stress concentration and cracking.

Final seat depth

Seat flush or slightly below the surface so washers/heads bear on plastic, not on the insert lip.

Design checklist (CAD + slicer choices)

• Place inserts in bosses tied into nearby walls or ribs (not isolated on a thin plate).
• Increase perimeters around the boss; perimeters usually matter more than infill for insert strength because the knurl grips the walls.
• Avoid internal voids directly under the insert; molten plastic needs somewhere to flow without distorting the surface.
• If you expect high tightening torque, add anti-rotation help (thicker boss, ribs/gussets, flats, or a keyed pocket).
• Keep screw load paths in mind: redesign so fasteners see mostly shear/compression instead of pure pull-out when possible.

Print orientation near inserts

Aim for uninterrupted, vertical perimeters around the boss so the insert knurl engages continuous material. Avoid orientations where the boss walls are bridges or where the screw load tends to peel layer lines apart. If you must place an insert near a seam, move the seam away from the boss or use seam painting so the boss walls are as continuous as possible.

Installation (temperature-controlled soldering iron)

1. Choose the right tip: an insert-setting tip is best; otherwise use a flat tip and keep the insert centered manually.
2. Set a starting temperature appropriate to the filament (lower for PLA, higher for PETG/ABS/nylons). If you’re unsure, start low and increase in small steps until the insert sinks with steady pressure.
3. Support the part on a flat, firm surface so the boss can’t flex; flexing is a common cause of crooked inserts and cracked bosses.
4. Place the insert in the hole, check it is square from two directions, then apply heat and light downward pressure—let the plastic soften; don’t force it.
5. As soon as the insert starts to move, keep the iron vertical and watch the top face of the insert as your reference plane for level.
6. Stop at the target depth (flush or slightly below). Hold for a moment so the plastic re-solidifies around the knurl, then lift the iron straight up without twisting.
7. Let the part cool to room temperature before installing a screw; tightening while warm can ovalize the hole and start an insert spinning later.

Troubleshooting table