Printed Threads in MJF Nylon: When Do They Work?

Printed threads in MJF are tempting, since they're free once they're in the model. Whether they're any good depends mostly on size.

Should You Print Threads or Tap Them in a Nylon Part?

Print them if they're bigger than about M6 and lightly loaded. For anything smaller, or anything that gets assembled over and over, use a self-tapping screw, a metal insert or a machined thread.

The r/3Dprinting thread "Printed threads vs tapped threads?" is about FDM printing in PLA and PETG, but the answers carry over. One person said neither: a straight hole and a plastite screw. Others tap the hole or melt in brass inserts, and keep printed threads for big diameters or custom threads.

What HP says about printed threads in MJF

HP's Jet Fusion design handbook recommends printing external and internal threads larger than 6 mm (M6 or 1/4") for good results in all orientations. Under that, it suggests self-tapping screws, threaded inserts, or machining the thread. It notes machining can hold up to ±0.05 mm. For printed internal threads it says to use the looser end of the standard thread tolerances.

How strong are printed threads?

A study published in PMC tested M8 threads in SLS PA12 blocks. Printed threads failed at an average of 3.41 kN. Heat-set inserts reached 3.83 kN, about 12% more. Helicoil inserts reached 4.78 kN, about 40% more. Every joint failed by pull-out. That's SLS rather than MJF, and a single thread size, so treat it as a guide.

So printed threads aren't useless, just the weakest of the three. The authors also observed that the Helicoil joint held up better to wear during repeated tightening and loosening, though they didn't put a number on it.

Tapped holes in printed nylon have their own limits too. See why they strip sooner than Delrin and why MJF press-fit bosses loosen. If you go with inserts, get the heat-set insert hole size right first.

A note on our side

We print MJF Nylon, with a listed tolerance of "±0.3 mm or ±0.3%, whichever is greater" on the 3D printing capability page. If threads matter, tell us the size and how often they'll be used. If you want inserts fitted or holes tapped after printing, ask us when you quote.