Heat-Set Inserts and Embedded Fasteners

What a heat-set insert does

A heat-set insert is a small threaded metal sleeve, usually brass, with knurling on the outside. You heat it and press it into a printed hole. The plastic around it melts, flows into the knurling, and locks the insert in place as it cools. LulzBot describes heating the insert past the plastic's glass transition temperature so it can seat. The result is a metal thread in a plastic part, useful where a screw will be put in and taken out more than once.

Designing the hole

• Use the insert maker's dimensions. Inserts differ in length, outside diameter and taper, so the hole size comes from the maker of the insert you bought Maker spec. Do not copy a hole size from a different brand.
• Depth. CNC Kitchen, which sells its own inserts, recommends blind holes about 1 mm deeper than the insert is long Rule of thumb, so melted plastic has somewhere to go.
• Straight walls. CNC Kitchen recommends straight holes rather than tapered ones.
• Chamfer or not. The two makers differ here. LulzBot recommends a chamfer at the hole edge to leave space for displaced plastic near the surface. CNC Kitchen says a chamfer is normally not required. Try both on a test piece if the surface finish around the insert matters.
• Enough plastic around it. LulzBot recommends two or more perimeters around the insert, and wall thicknesses that are a whole multiple of the extrusion width so they print as solid perimeters. It notes that extra perimeters also make the part stronger. See Walls and Perimeters.
• Load direction. LulzBot recommends orienting the insert so the load pushes it into the plastic, with the fastener passing through the mating part into the insert, rather than a load that pulls the insert out of its hole.

Print one test block with a few holes before committing a whole part. The hole size that works depends on your printer's accuracy too; see Designing Fits and Clearances.

Iron temperature

There is no single correct temperature, and the two makers here give different rules:

• LulzBot suggests setting the iron to the extrusion temperature you printed that filament at, if possible.
• CNC Kitchen suggests 10 to 20 °C above the print temperature Rule of thumb.

Both are relative to the print temperature, which comes from the filament maker's printed range for that filament. Start at the lower rule and go up only if the insert will not sink under light pressure Measure it. Do not chase speed with heat: CNC Kitchen warns that overheating can decompose the plastic, which is both a weak joint and a source of fumes.

Installing an insert

1. Fit the right tip with the iron cold. LulzBot recommends a tip large enough to press steadily on the center of the insert but small enough not to touch the print. Dedicated insert tips are made for this.
2. Place the insert. Put it in the hole small end down, straight. Tweezers help on the first few.
3. Heat and let it sink. Touch the tip to the insert and wait a few seconds. LulzBot says the iron's own weight should start it settling. Press slowly and evenly; pressing fast pushes molten plastic up over the top of the insert.
4. Stop short and finish flush. CNC Kitchen's method is to melt the insert about 90 percent of the way in Rule of thumb, take the iron away, and press the rest flush with a flat tool. Hold it there for a moment, because inserts tend to creep back up while the plastic is still soft.
5. Let it cool before putting any load or screw into it.

Reading the result

What you see	Likely cause	Change
Bulge or burr of plastic around or under the insert, or the screw is hard to start	Hole too small (CNC Kitchen)	Enlarge the hole on the next test
Insert tilted	Pressed unevenly or too fast	Slower, lighter pressure; use a centering tip or straightening tool
Plastic flowed over the top	Pressed too fast (LulzBot)	Slow down; stop at about 90 percent and press flush
Insert spins or pulls out	Hole too large, or too little plastic around it	Smaller hole, more perimeters
Discolored or bubbled plastic, smoke	Iron too hot or held too long	Lower temperature, shorter contact, more ventilation

Embedding nuts and magnets during a print

The other way to get a strong fastener into a part is to drop a hex nut or magnet into a pocket partway through the print, then let the printer cover it. PrusaSlicer documents this as inserting a pause at a layer: in the sliced preview, pick the layer and add a pause with a message. Prusa notes the pause is inserted before the selected layer prints, so pick the first layer that closes the pocket over the part, and check the preview. Other slicers have a similar pause-at-layer feature.

• Prusa warns that magnets must sit firmly in their slots, or they will jump to the print head as it passes. Make the pocket snug or use a small drop of glue.
• The part must sit below the top of the pocket. Anything sticking up is in the nozzle's path.
• Design the pocket with the clearance you measured for that material; see Designing Fits and Clearances.
• Prusa notes that pauses do not work with sequential (one object at a time) printing.
• Keep your hands clear of the hot nozzle and bed while placing parts.