Five snap-fit types for FDM and SLS. Change the gaps and the root strain moves with them; both parts download as STEP and STL.
Snap and housing as STEP and STL, oriented for the build plate, plus the settings. Kit designs use the exact CadQuery STEP; custom settings get STEP built here with round features as 96 facets.
Add designs from the studio to compare them. The list stays in your browser. The four examples solve the same job four ways in PETG on FDM.
Eighteen designs: all five types for FDM in PETG and SLS in PA12, plus a gap ladder, which is the same two-prong clip at four side gaps per process with the engagement held at 1.05 mm. Before publishing we re-imported every file and checked it: one valid STEP solid, a watertight STL, no overlap when seated, the snap holds when pulled back by its axial play, and the interference measured from the solids matches the design value to 0.001 mm. The whole-kit zip includes the CadQuery script that builds the files and the one that checks them.
Same beam, same strain; across layers the allowance halves. The studio's default PETG latch otherwise; colour is root strain against the repeated-use limit.
A sharp root concentrates the strain where the beam meets the base. The studio's default PETG latch otherwise; colour is root strain against the repeated-use limit.
Thinning the beam toward the tip spreads the strain along it instead of piling it at the root. The studio's default PETG latch otherwise; colour is root strain against the repeated-use limit.
Same geometry and the same strain; PLA is more brittle, so its allowance is lower. The studio's default PETG latch otherwise; colour is root strain against the repeated-use limit.
Two prongs, both bending inward into the gap between them; the same layer rule applies to each. The studio's default PETG latch otherwise; colour is root strain against the repeated-use limit.
| Type | Use it for | How it fails | FDM | SLS | Strain model |
|---|---|---|---|---|---|
| Cantilever latch | Lids, battery doors, one-sided catches on a box wall. | Cracks at the beam root. With no stop, someone pushes the beam past its limit and it snaps there. | Beam flat on the bed. Taper to 0.5 to 0.6 of root thickness at the tip. | Any orientation; flat is still better. Thinner beams fine. | ε = h·y / (C·L²), C 0.67 → 1.09 with taper |
| Two-prong clip | Pushing a part through a rectangular hole in a panel. | Prongs touch each other before the hooks clear if the gap between them is too small. | Print lying on its side so both prongs bend in the layer plane. | Good default. The prong gap doubles as powder escape. | same as latch, 2 beams |
| U-hook | Enclosures where there is no room for a long straight beam. Releasable by pressing the arm. | Arm cracks at the bend. The stem also flexes, so real strain is lower than shown. | Print on its side; the U lies in the layer plane. | Works well; the bend radius matters more than on FDM. | arm only from the bend (conservative) |
| Annular | Caps, round covers, knobs on a shaft. Seals if the fit is tight. | Collar splits along a layer line, or the push force is so high nobody can assemble it. | Collar axis vertical, so hoop strain runs along the extrusion lines. Keep the interference tiny. | Much more forgiving; PA12 stretches 3× further than PLA. | ε = (u − c) / (R + c) |
| Split pin | Fixing a part to a sheet or panel through a round hole. PCB standoffs. | Legs too short for a thin panel, so strain goes through the roof. Extend the slot into the head. | Add flats and print lying down; an upright pin bends across layer lines. | Print upright is fine; round pin needs no flats. | leg cantilever, ×1.15 for the half-round section |
Side gap c. Space between the snap and the housing wall when it's seated. It stops the beam being preloaded, but it comes straight off your undercut: a 1.0 mm hook in a 0.5 mm gap only holds by 0.5 mm. Size the hook as undercut = engagement + c.
Deflection room. Free space behind the beam so it can bend by the full engagement. It also works as an overtravel stop: set it too big and a heavy thumb bends the beam past its limit.
Axial play. Distance between the retention face and the wall it catches. Zero play means a slightly long print sits permanently bent, and PLA creeps under that constant load.
Entry chamfer. A 45° chamfer on the housing entry, so the lead-in ramp meets a slope instead of a square edge. On FDM keep it at the top of the print so elephant's foot can't eat it.
| Process | Fit | Side c | Room | Axial | Source |
|---|
| Feature | FDM | SLS / MJF (PA12) |
|---|---|---|
| Allowable strain, one-time | PLA 2 to 3 %, PETG 3 to 5 %. Multiply by 0.5 to 0.6 when the beam bends across layers. | PA12 5 to 8 % in XY. PA11 roughly twice PA12. Z lower on PA12, closer to XY on MJF. |
| Beam minimums | 1 mm thick at the root, 5 mm wide, three perimeters, 100 % infill for arms under 4 mm wide. | 1 mm thick, 5 mm wide. Start at 2 to 3 mm thickness for anything that cycles. |
| Mating clearance | 0.25 to 0.3 mm on a tuned printer, 0.5 mm if you don't know the printer. | 0.3 to 0.4 mm assembled; 0.2 mm for snug. |
| Print-in-place clearance | 0.3 to 0.4 mm radial with a 0.4 mm nozzle. | 0.5 to 0.7 mm. Below that the powder in the gap will not come out. |
| Living hinge | Not in PLA or PETG. PP at 0.3 to 0.5 mm with lines running across the hinge. | 0.5 mm, max 10 mm long attached on one side, 25 mm on two. Expect tens of cycles, not thousands. |
| Holes | Vertical holes print about 0.25 mm undersize. Teardrop horizontal holes over 5 mm. | 1 mm minimum, 2 mm recommended. Channels 3 mm or more for powder removal. |
| Hollow parts | No escape holes needed. | At least two escape holes of 2 mm or more. |