How to fasten 3D-printed parts: self-tapping screws vs heat-set inserts vs captive nuts vs through-bolts (M2–M5 hole sizes)

Four ways to fasten a printed part

Plastic is far weaker and softer than a steel screw, so the question with printed parts is not whether the screw is strong enough but whether the plastic pulls out, crushes or strips first. There are four common approaches: drive a self-tapping screw straight in, melt in a heat-set insert, trap a hex nut in a pocket, or pass a bolt all the way through and tighten a nut.

The star ratings compare the same screw size in PLA / PETG. Real pull-out forces depend on material, perimeter count, wall around the hole and print orientation and can differ several-fold — print and test a coupon with your settings when you need numbers.

TIPMany products mix methods: self-tapping screws for the housing, heat-set inserts for the service hatch that is opened repeatedly, through-bolts for the load-bearing bracket.

Self-tapping screws: hole size and boss design

The simplest option is to draw the hole smaller than the screw and let the screw form its own thread. Machine screws work, but screws designed for plastics (smaller flank angle, coarser pitch, e.g. PT-style) create less radial stress and are less likely to split the boss.

Self-tap hole ≈ 0.9 · d (typically 0.85–0.95 · d)　engagement ≥ 2 · d　boss OD ≥ 2.5 · d

• Brittle PLA wants the larger hole (about 0.95 d) to avoid cracking; tougher PETG and nylon can go to 0.85 d for more engagement.
• Surround the hole with perimeters, not infill: at least 3–4 walls, or add a local modifier around screw holes in the slicer.
• Orient the screw axis vertical (perpendicular to the bed): the hoop stress then runs within each layer instead of prying layers apart.
• Add a 0.5 mm chamfer or countersink at the entry to centre the screw and stop the rim from lifting.
• Don't reassemble the same hole too often; when you do, turn the screw backwards until it drops into the existing thread, then tighten.

TIPThe tap drill calculator's "3D print self-tap hole" row gives the 0.9·d hole directly, together with ISO 273 clearance and counterbore sizes.

Heat-set inserts: hole sizes and installation

A heat-set insert is a knurled brass nut that you press into a printed hole with a soldering iron. The surrounding plastic melts into the knurls and solidifies, giving the part a metal thread that survives repeated assembly and higher torque — the go-to method for functional parts.

The table shows vendor reference values for common standard inserts (e.g. Ruthex or CNC Kitchen sizes). Outer diameters and lengths differ between brands and between short and long versions, so always follow the data for the inserts you actually buy. Make the hole about 1 mm deeper than the insert so displaced plastic has somewhere to go and the insert doesn't bottom out and tilt.

1. Fit a flat or dedicated insert tip and set the iron near the filament's print temperature (about 200–220 °C for PLA).
2. Place the insert small end down on the hole, align the iron vertically and press lightly — let the heat sink it rather than forcing it.
3. Stop about 0.1–0.2 mm below the surface, remove the iron and press flush with a flat metal plate while the plastic is soft so the insert stays square.
4. Let it cool before driving a screw. If the insert spins, the hole was too large or heated too long — go 0.1 mm smaller next time.

TIPKeep at least 3–4 perimeters around the hole, and install the insert on the side that is pulled toward the screw head so the load pushes it into the plastic. The tap drill calculator lists reference insert holes for each size too.

Captive hex nut pockets: sizes and printing tricks

Model a hexagonal pocket, drop in a standard ISO 4032 nut and you have a steel thread at almost no cost and with high strength. The pocket stops the nut from turning, so you only need to drive the screw from the other side.

Across corners e = across flats / cos 30° ≈ 1.155 × across flats　(when drawing a hexagon by its circumscribed circle in CAD, enter e, not the across-flats size)

• Top-loaded pockets: the screw hole above the pocket is an overhang — leave a 0.2 mm sacrificial layer on top of the pocket and drill through it after printing.
• Side-loaded slots: make the slot across flats + 0.2–0.3 mm wide and nut height + 0.2 mm tall; ideal when the nut goes in after printing.
• With a horizontal hole axis, put a corner of the hexagon at the top: the two upper faces are at 60° to horizontal and need no support.
• Fully enclosed nuts: add a pause at the layer just below the pocket roof (Marlin M600, Prusa M601, Klipper PAUSE), drop the nut in and resume.

TIPDownload an ISO 4032 nut from the hex nut generator into your assembly, add 0.2–0.3 mm clearance and subtract it with a Boolean — no more mis-sized pockets.

Through-bolts and nuts: clamp load and bearing pressure

When the bolt passes through the whole part and a nut tightens on the far side, the steel carries the load and the plastic is only clamped, so this is the strongest method and can be reassembled indefinitely. The only limit is whether the plastic under the head and nut crushes or creeps.

Preload F ≈ T / (K · d) (K ≈ 0.2)　bearing pressure p = F / A　A = π/4 · (outer diameter² − hole²)

For an M3 socket head cap screw (ISO 4762, 5.5 mm head) through a ⌀3.4 mm hole: at 0.4 N·m the preload is about 667 N and the pressure under the head about 45 MPa — high for PLA or PETG, which will creep and lose preload over time. An ISO 7089 washer (7 mm OD) doubles the bearing area and cuts the pressure to about 23 MPa. That is why M3 screws in plastic should be tightened to only 0.3–0.5 N·m, always with a washer.

• Clearance holes: ISO 273 fine or medium plus 0.2 mm on FDM (3.4–3.6 mm for M3); use a teardrop for horizontal holes.
• For high clamp loads press a metal sleeve (compression limiter) into the hole so the clamp force goes through the sleeve instead of the plastic.
• For vibrating joints use nylon-insert lock nuts or a threadlocker that is safe for plastics.
• For pivots and hinges use shoulder screws or a bronze bushing so rotation happens on metal.

TIPThe socket head cap screw, hex nut and washer generators all export ISO-sized STEP files for checking clearance and tool access in your assembly; from M6 up you can also choose the 3D-print thread mode and print the threads directly.