Heat-set inserts, captive nuts, screws into plastic — and the one case where printed threads are actually fine.
Metal threads in plastic parts, from best to worst: a heat-set brass insert, a captive hex nut, a thread-forming screw straight into plastic, and last, threads printed in plastic. Each step down trades durability for convenience. If the joint will be assembled more than twice, start at the top.
We fit heat-set brass inserts in-house, M3 to M5, pressed square with proper tooling — tick the option at quote time ($2.50 a part) and your parts arrive ready for machine screws.
An insert melts into a plain round hole and locks into its own knurling as the plastic cools. The hole should be slightly smaller than the insert's outside diameter, a millimetre deeper than its length (molten plastic needs somewhere to go), and surrounded by at least 2 mm of wall so the boss doesn't bulge or split.
Put insert holes on faces that print upward — inserts set into clean vertical holes hold straighter than ones fighting support-scarred surfaces.
A hex pocket sized 0.2 mm over the nut's across-flats, one layer below the surface, makes a captive nut that never spins — the classic trick for high-torque joints and for sizes above M5. Face the pocket so it prints as a clean bridge, and the bolt pulls the nut square as it bites.
Thread-forming screws work surprisingly well for light-duty joints: drive them into a plain pilot hole about 0.85× the screw's nominal diameter (M3 → Ø2.5 mm), with at least 6 mm of engagement. Good for five to ten assembly cycles; after that the plastic thread tires — which is why serviced joints get inserts.
across flats, snug not jammed
M3 → Ø2.5 mm, ≥6 mm engagement
screws into plastic are not forever
From about M10 upward, printed threads work: the thread form is large enough for the nozzle to draw faithfully, and the load spreads across enough plastic to survive. Print them vertically (threads through horizontal holes come out sagged), add 0.2–0.3 mm clearance on the profile, and chamfer the lead-in.
Custom coarse threads — jar lids, knobs, adjustment collars — are even better territory: design the profile trapezoidal, 1.5 mm pitch or coarser, and it'll outlast the part around it. Below M10 with standard pitches, don't print threads. Use an insert.
Drop an STL, STEP or 3MF and the quote applies every rule in this guide — mesh check included, GST included.