Heat Set Inserts

TL;DR

Model the pilot hole to the insert maker’s spec. Give the insert a thick boss with a generous root fillet and enough perimeters. Install with a temperature-controlled soldering iron while the part is firmly supported so the insert goes in straight and seats flush.

Insert styles: what changes in your print

Most hobby inserts are brass with knurls that lock into plastic after you melt them into a printed boss. Tapered inserts self-center in the first few millimeters, but they can wedge the boss outward if the pilot hole is tight or the plastic gets over-softened. Straight (parallel) knurl inserts rely more on you to keep them square, but they tend to load the boss more evenly when you have enough wall thickness. Choose an insert by screw size and required thread engagement, then verify you can surround it with solid plastic for the full depth.

Design targets (what matters and why)

Pilot hole diameter

Use the insert maker’s spec; too small swells or splits the boss, too large cuts grip and pull-out strength.

Boss outer diameter

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

Plastic under insert

Keep solid material under the insert so clamp load can’t punch through; don’t place it over a thin roof or sparse infill.

Lead-in at hole

Add a small chamfer so the insert starts centered and is less likely to tilt as it enters.

Root fillet at boss

Use generous fillets where the boss meets the part to reduce stress concentration and splitting.

Final seat depth

Seat the insert flush or slightly below the surface so the screw head/washer bears on plastic, not the insert’s top edge.

Design checklist (CAD + slicer choices)

• Anchor bosses into nearby walls or ribs so the load spreads into the part, not a thin plate.
• Use more perimeters around the boss; the knurl grips perimeter walls more than it benefits from extra infill.
• Avoid voids directly under the insert so softened plastic can flow without sinking or doming the surface.
• For high torque, add anti-rotation features (more boss thickness, ribs/gussets, flats, or a keyed pocket).
• Design the load path so screws see shear/compression where possible, not pure pull-out on the insert.

Print orientation near inserts

Orient the part so the boss has continuous, vertical perimeters for the knurl to bite into. Avoid setups where the boss walls are bridges, or where the screw load tends to peel layer lines apart. If a seam lands near the boss, move it away or use seam painting so the boss wall stays as continuous as possible.

Installation (temperature-controlled soldering iron)

1. Pick a tip that fits the insert: use an insert-setting tip when you have one; otherwise use a flat tip and center the insert by hand.
2. Set a reasonable starting temperature for your filament (lower for PLA, higher for PETG/ABS/nylons); start low and step up until the insert sinks with steady pressure.
3. Support the part on a flat, rigid surface so the boss can’t flex while you press; flexing is a common cause of cracks and angled inserts.
4. Drop the insert into the pilot hole, square it up from two directions, then apply heat with light downward pressure so the plastic softens instead of being forced.
5. Once it starts moving, keep the iron vertical and use the insert’s top face as your level reference.
6. Stop at your target depth (flush or slightly below), pause briefly so 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 boss and encourage later spinning.

Troubleshooting table