The Fastener Problem Nobody Budgets For
A plastic enclosure looks finished in CAD. Then the first shot comes back and the four M3 screws strip their holes before the lid is snug. Or worse: the bosses sink on the visible face, and the whole cosmetic surface reads as wavy. Threaded features are the single most common source of late-stage rework in plastic injection work, and almost all of it is avoidable at the design stage.
What follows is the concrete geometry and material handling behind self-tapping screws, metal inserts, and bosses — the three ways to put a fastener into a molded part.
Start With the Boss, Because Everything Sits on It
A boss is a cylindrical projection that carries a thread. Its outside diameter is not a style choice; it is set by the load it carries and the material it is made from. Common starting points:
- OD: 2.0–2.5× the screw's nominal diameter. For an M3 screw that is a 6.0–7.5 mm boss OD. Going thinner invites splitting during driving; going thicker creates sink.
- Depth: 2–2.5× thread diameter of engagement for the screw, with 0.5 mm clearance below the hole bottom so the screw tip does not bottom out and pre-load the boss.
- Draft: 0.5–1° per side on the OD, and 0.25–0.5° on the core pin forming the hole. A deep 0.25° core pin at 20 mm depth will drag and can snap; keep core pins under roughly 4:1 depth-to-diameter.
- Fillet: a 0.25–0.5 mm radius at the boss root is not cosmetic. An unfilleted root concentrates stress and is where a cracked boss always starts.
The failure mode to design against is sink. A solid boss 7 mm tall standing on a 2.5 mm wall has a mass of plastic that cools far slower than the surrounding wall. The surface opposite it shrinks inward and leaves a visible dimple — typically 0.05–0.15 mm deep, which is enough to be obvious on a matte Class A surface. The fix is not to thicken the wall; it is to core the boss out or, better, to leave it attached by ribs plus a short gusset rather than a solid cone.
Self-Tapping Screws: Cheap, and Finicky
Two families dominate. Thread-forming screws (like PT/K type) displace material as they go in; thread-cutting screws (like the Hi-Lo or trilobular variants) cut a thread path. In ABS, PC, and PC/ABS, thread-forming is the usual choice because it creates a denser, stronger thread and resists vibration loosening better.
The pilot hole is the whole game. For an M3 thread-forming screw in ABS, a pilot diameter around 2.75–2.85 mm is typical; in glass-filled PA66, go slightly larger (2.85–2.95 mm) because the material is less compliant and cracks more readily. A hole that is 0.1 mm too small will split a boss on the first assembly; 0.1 mm too large and the screw pulls out at low torque. This is why specifying a pilot hole as a nominal value with no tolerance is a mistake — call out ±0.05 mm.
Other rules that matter more than they look:
- Leave at least 1.5× the boss OD of clear space between boss and any side wall, or the wall will bulge.
- Never put a self-tapping screw into a boss that is thinner than 1.2× the screw diameter at its root.
- Design for 3–5 assembly cycles, not 20. Thread-forming holes lose clamp load each time they are re-driven.
- Add a small lead-in chamfer (0.3–0.5 mm × 45°) so the screw starts square; cross-threading is usually a lead-in problem.
Metal Inserts: When the Part Gets Opened Repeatedly
If a service technician will open the housing more than a handful of times, or if the joint must hold torque against vibration, use a threaded metal insert. Two installation methods:
Heat-set (thermoplastic inserts)
A brass insert with a knurled or barbed OD is pressed in with a heated tip at roughly 230–280 °C for ABS and PC, melting the surrounding plastic so it flows into the knurl. Pull-out strength in ABS for an M3 insert is often in the 300–600 N range and torque-out in the 0.8–1.5 N·m range, depending on boss OD and wall support. Heat-set suits ABS, PC, PC/ABS, and most unfilled nylons.
Molded-in
Molded-in inserts sit on the core pin before the shot closes. Highest pull-out strength, no post-processing labor, but added mold complexity and zero tolerance for misalignment — a tilted insert scraps the part.
Design note for all inserts: the boss ID should match the insert's minor diameter plus 0.05–0.1 mm, and the boss must be taller than the insert by 0.2–0.3 mm so displaced plastic has somewhere to go. Skip that and a raised ring of flash stops the mating part from seating.
Glass-Filled Resins Change the Rules
PA66-GF30 and PBT-GF30 have roughly one-third the elongation at break of unfilled ABS and a much sharper notch sensitivity. Practical consequences:
- Boss OD must go up, often to 2.5–3× screw diameter, because the material will not stretch around the screw.
- Pilot holes need more clearance, and the hole should never have a sharp internal corner — a small radius at the bottom dramatically reduces cracking.
- Self-tapping screws in GF material are best avoided for anything structural; heat-set inserts are safer.
- Weld lines are weaker in GF material because fibers orient along the flow front and do not bond across it. Never place a boss directly on a weld line — offset it or add a flow leader.
Leading the Thread Out of the Mold
Threads molded directly into the part need a collapsing core, an unscrewing mechanism, or a split. All three add tooling cost, and at prototype volumes they are rarely worth it. If the design genuinely needs a molded thread (a bottle cap, a filter housing), expect the tool to cost noticeably more and the cycle to slow, and spec a thread relief groove at the end so the last thread does not tear.
For most prototypes the smarter sequence is: mold the part with a plain pilot hole, then cut the thread on a CNC machining center as a secondary operation. You keep the tool simple and still get a true thread. Once volumes grow past a few thousand, revisit whether a molded or inserted thread is worth the tooling change. Prototype molds are a good place to test boss geometry first — see how small features translate into tooling on our soft tooling page.
A Pre-Release Checklist
- Boss OD ≥ 2× (2.5× for GF resins) the screw diameter, with a 0.3–0.5 mm root fillet.
- Boss height ≤ 3× OD, and cored or gusseted rather than solid where sink is visible.
- Pilot hole dimensioned with an explicit tolerance of ±0.05 mm, plus a lead-in chamfer.
- 0.5 mm clearance at hole bottom; no screw tip contact.
- Insert bosses 0.2–0.3 mm taller than the insert, with a matching minor-diameter ID.
- No boss on a weld line, no boss within 1.5× OD of a side wall.
- Core pins under 4:1 depth-to-diameter, drafted 0.25° minimum.
None of these cost money to fix in CAD. All of them cost a tool revision once steel is cut.
