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Heat-set threaded inserts in 3D printed parts

Workshop · Assemble

A printed thread strips. A brass heat-set insert melts into the plastic and gives you a metal thread that survives being undone repeatedly, which is the difference between a prototype and a part. The hole diameter is the whole job, and it comes from your insert manufacturer rather than from a universal number, because insert geometry varies enough that a figure copied from elsewhere will either fall out or split the wall.

  1. 01Print the holeTo the diameter your insert manufacturer publishes for its size.
  2. 02Sit it squareThe insert should rest in the mouth of the hole without being pushed.
  3. 03Heat the ironHot enough to soften the plastic, not to burn it. Start low.
  4. 04Press straight downLet the heat do the work. Force is what tips it out of square.
  5. 05Stop at the surfaceFlush, or a hair proud. Below the surface and the threads sit in a crater.
  6. 06Let it coolBefore touching it. The plastic re-forms around the knurl as it sets.
Installing a heat-set insert. Six steps, in order. The hole diameter is not shown because it is set by your insert, not by this drawing.

The hole diameter comes from your insert

Heat-set inserts are knurled, tapered and dimensioned differently between manufacturers, and the recommended hole is a property of that specific insert rather than of the thread size. A number that works perfectly for one brand of M3 insert will be loose or splitting in another.

Find the figure in your insert supplier datasheet and use it. If none is published, test on a scrap coupon printed in the same material and orientation before committing to the real part. This site does not publish a universal hole table, because there is no honest one to publish.

Temperature, and going slowly

Set the iron well above the material glass transition but well below its decomposition point. For PLA that generally means somewhere around 200 to 230C, and for PETG a little higher; the exact figure matters less than going slowly enough for the heat to conduct into the plastic instead of pooling at the tip.

Let the insert sink under the weight of the iron plus light pressure. Pushing hard forces molten plastic sideways rather than letting it flow around the knurling, which is where the holding strength comes from.

Why the wall bulges, and how to stop it

Displaced plastic has to go somewhere. If the boss wall is thin, it goes outward and you get a visible bulge or a split. Give the boss at least about 1.5 to 2mm of wall around the insert, and more in a brittle material.

Setting the insert flush or very slightly below the surface, rather than proud, also helps: the last fraction of a millimetre is where most of the displaced material ends up.

PLA against PETG

PLA is stiffer and more brittle, so it holds a crisp knurl impression but splits more readily if the hole is undersized. PETG is more ductile and tolerates a slightly tight hole, but it is stringier when molten and tends to web around the insert.

Neither is a problem if the hole is right. Both become a problem if you use a number you found for the other one.

What this needs

Each item is listed with the specification that decides whether a given product will actually do the job, because that is the part product titles tend to leave out.

Brass heat-set inserts

Knurled, with a published recommended hole diameter

The published hole figure is the reason to prefer a documented brand. An unlabelled bag of inserts leaves you testing coupons to discover a number the manufacturer already knows.

M3 brass heat-set inserts, 100 piecesruthex · M3 brass heat-set inserts, 100 piecesPublishes a recommended hole diameter, which is the number the whole job depends on

Soldering iron with insert tips

Temperature controlled, with tips sized to your inserts

A flat soldering tip fights you: it slips off the insert and heats unevenly. A matching insert tip sits in the bore and drives it in square, which is most of what stops a crooked insert.

Digital soldering iron with insert tipsMintion · Digital soldering iron with insert tipsTemperature controlled with M2 to M8 insert tips, so the insert goes in square

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Common questions

What size hole for a heat-set insert?
Use the diameter your insert manufacturer publishes for that specific insert. Heat-set inserts differ in knurl and taper between brands, so a figure taken from another manufacturer will either be loose or split the boss. If no figure is published, test on a scrap coupon in the same material and orientation.
What temperature for heat-set inserts in PLA?
Generally around 200 to 230C, above the glass transition and below decomposition. Going slowly matters more than the exact number, because the heat needs time to conduct into the plastic rather than pooling at the tip.
Why did the wall bulge when I installed an insert?
Displaced plastic had nowhere to go. Give the boss at least 1.5 to 2mm of wall around the insert, set it flush rather than proud, and let it sink under light pressure instead of forcing it.

Materials this matters most for

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