Last updated: 2026-09-05 · 5 min read
Self-Tapping Screw Boss Design — What Outer Diameter, Wall Thickness and Hole Depth Keep the Boss from Cracking When the Screw Goes In?
A self-tapping screw boss is a cylindrical boss with a cored hole that a thread-cutting or thread-forming screw drives into. The outer diameter (OD) must be at least 2× the screw major diameter to resist hoop stress. Wall thickness between hole and outer surface: 1.0–1.5mm minimum for M3–M4. Boss height sets engagement depth — typically 6–10mm for M3. Without these proportions, the boss cracks during assembly or strips under torque.
Key Facts
- Boss OD must be 2× the screw major diameter (D) or larger. For an M4 screw (D = 4mm): boss OD ≥ 8mm. The pre-drilled hole diameter depends on screw type — thread-cutting screws need a hole 0.5–1.0mm smaller than the screw major diameter; thread-forming (rolling) screws need a hole 0.2–0.5mm smaller. If the OD is too small relative to the hole, the remaining wall is too thin to resist the radial expansion force — the boss splits.
- Engagement depth determines pull-out strength. For M3 screws in ABS: 6mm engagement gives 300–400N pull-out; 10mm gives 500–600N. In glass-filled nylon (PA66 GF30): 6mm gives 500–700N because the glass fibers increase the shear strength of the engaged threads. The minimum engagement is 2× the screw diameter — anything less and the threads strip before the screw reaches its yield point. For structural joints, specify 3× diameter engagement.
- The boss must be gated at the base — not at the top. A top gate creates a weld line around the boss circumference where melt flows meet, and the weld line is the weakest point under hoop stress. A base gate (submarine or pin gate at the boss foot) fills from bottom to top, eliminating the circumferential weld line. The boss should also have a 0.5mm fillet at its base where it meets the wall — a sharp corner here concentrates stress and initiates cracks during screw insertion.
How Do Boss Dimensions Scale Across Common Screw Sizes?
| Screw Size | Boss OD (min) | Hole Dia (thread-cut) | Wall Thickness | Engagement Depth |
|---|---|---|---|---|
| M2 (2mm) | 4.5mm | 1.4–1.6mm | 1.0mm | 4–6mm |
| M2.5 (2.5mm) | 5.5mm | 1.8–2.1mm | 1.0–1.2mm | 5–7mm |
| M3 (3mm) | 6.5mm | 2.3–2.6mm | 1.2–1.5mm | 6–10mm |
| M4 (4mm) | 8.0mm | 3.2–3.6mm | 1.5–2.0mm | 8–12mm |
| M5 (5mm) | 10.0mm | 4.0–4.5mm | 2.0–2.5mm | 10–15mm |
These dimensions assume unfilled engineering resins (ABS, PC, POM). For glass-filled resins (PA66 GF30, PBT GF30), increase the boss OD by 10–15% — the glass fibers make the material stiffer but also more brittle, so the thicker wall resists cracking under the screw’s radial force. For PP and PE (soft resins), the hole diameter should be on the smaller end of the range — these resins have lower shear strength and need more thread engagement per unit depth.
Hoop stress — the circumferential tensile stress in the boss wall caused by the screw’s radial expansion force during insertion. Hoop stress = (insertion pressure × hole radius) / wall thickness. If the hoop stress exceeds the resin’s tensile strength, the boss splits longitudinally. For ABS (tensile 40 MPa) with an M4 screw: hoop stress at the recommended OD of 8mm is 15–25 MPa — within the safe range. For GF30 resins (tensile 90 MPa but brittle): the higher stiffness concentrates stress, so increase OD by 10–15%.
What Three Design Rules Prevent Boss Failure During Assembly?
- Add a gusset or rib at the boss base — a standalone tall boss snaps off under lateral load. A boss taller than 10mm without lateral support acts as a cantilever — any side load (from mating part misalignment or assembly torque) bends it until it cracks at the base. Add two or three gussets (1.0mm thick, 5mm tall) at 120° spacing around the boss. The gussets transfer lateral load into the surrounding wall. If space is tight, connect the boss to the nearest wall with a rib — the rib serves the same function as a gusset but uses less space. The snap-fit design guide covers how bosses and snaps work together in enclosure joints.
- Use a core pin to form the hole — ejector pins leave witness marks that create stress risers. The boss hole should be formed by a core pin on the mold (mounted from the A-plate or B-plate), not by a drill operation after molding. A cored hole has a smooth surface that distributes the screw’s cutting force evenly. A drilled hole may have burrs or tear-out at the entry point — these irregularities create stress concentrations that initiate cracks when the screw enters. The core pin diameter should be held to ±0.05mm to ensure consistent hole size across cavities.
- Specify the insertion torque — not just the screw size — in the assembly drawing. The assembly torque determines the clamp force and the hoop stress on the boss. For an M3 self-tapping screw in ABS: recommended torque is 0.3–0.5 N·m. Above 0.7 N·m, the boss starts to crack. For M4 in PC: 0.6–0.8 N·m. The torque must be controlled on the assembly line with a clutch-controlled driver — not a standard drill. If the drawing does not specify torque, the assembler defaults to “tight as possible,” which almost always exceeds the boss’s capacity.
Weld line (in screw bosses) — a linear weakness where two flow fronts meet and bond incompletely. In a top-gated boss, melt flows around the circumference and meets on the opposite side — the weld line runs vertically. Under hoop stress from screw insertion, the weld line splits because polymer chains are not entangled across the joint. A base gate eliminates the circumferential weld line by filling bottom to top. See the injection molding weld lines guide for how gate location affects weld line strength.
Screw boss: OD ≥ 2× screw diameter. Wall 1.0–2.0mm. Engagement 2–3× diameter. Base gate, not top gate. Fillet 0.5mm at base. Gussets for bosses >10mm tall. Cored hole ±0.05mm. Specify insertion torque on the drawing.
- Boss OD ≥ 2× screw major diameter — smaller OD means wall too thin to resist hoop stress.
- Base gate, not top gate — top gate creates circumferential weld line that splits under insertion.
- Fillet 0.5mm at boss base — sharp corner concentrates stress and initiates cracks.
- Gussets for bosses >10mm tall — standalone tall bosses snap off under lateral load.
- Insertion torque specified on drawing — uncontrolled torque is the #1 cause of boss cracking.
Frequently Asked Questions
Why does my boss crack during screw insertion even though the dimensions look correct?
Three causes: wrong hole diameter, a weld line in the boss wall or excessive insertion torque. If the hole is 0.2mm too small for the screw type, the screw cuts deeper threads than designed — the radial expansion force exceeds the boss wall’s tensile strength. Check the hole diameter against the screw manufacturer’s recommendation for that specific screw series.
The second cause is a weld line — if the boss was gated at the top, the melt flows around the boss circumference and meets on the opposite side, creating a weld line that runs vertically through the wall. Under hoop stress, this weld line splits open. Switch to a base gate. The third cause is uncontrolled torque — if the assembly operator uses a drill without a clutch, peak torque can reach 2–3× the recommended value.
Can I use a heat-set insert instead of a self-tapping screw boss?
Yes — and for high-volume production or high-torque joints, a heat-set insert is the better choice. A brass heat-set insert (M3) costs $0.02–0.05 per piece and requires a 5.0mm boss OD with a 4.2mm hole (for M3). The insert is pressed into the molded hole with a heated tip (250–350°C), and the knurled outer surface bonds into the plastic. Pull-out strength is 2–3× higher than a self-tapping screw in the same boss size.
The trade-off is the insert cost and the secondary operation (pressing it in). For 10,000 units with 4 screws each: insert cost is $800–2,000 plus labor. For lower volumes, self-tapping screws avoid the insert cost entirely. For higher volumes or safety-critical joints, the insert’s higher pull-out strength justifies the expense.
How does boss design change for ultrasonic welding energy directors?
An ultrasonic energy director is a small triangular bead (0.3–0.5mm tall, 90° included angle) on the boss top face — it concentrates the ultrasonic vibration to melt a controlled joint surface. The boss itself must be stiffer than a screw boss: OD at least 2.5× the joint diameter, wall thickness 1.5mm minimum. The energy director collapses during welding (0.2–0.3mm melt), so the boss height must account for this collapse to maintain the correct stack-up dimension after welding.
The gate location rule is the same — base gate, not top gate — because the weld line would weaken the joint under both screw insertion and ultrasonic vibration. The boss must also have a 0.2mm interference fit with the mating part’s hole to ensure the energy director contacts the mating surface before the horn finishes.
Key Takeaways
- Boss OD ≥ 2× screw dia. Wall 1.0–2.0mm. Engagement 2–3× dia.
- Base gate — top gate creates a circumferential weld line that splits.
- Fillet 0.5mm at boss base. Gussets for bosses >10mm tall.
- Specify insertion torque on the drawing — not just screw size.
Send the assembly drawing with the screw size, resin and required pull-out strength — the quotation specifies the boss dimensions, hole tolerance and gate location needed to survive the target assembly torque.
Sources: BASF — Boss Design Guidelines for Engineering Resins · DuPont — Delrin POM Design Guide for Screw Bosses and Self-Tapping Joints · Plastics Technology — Screw Boss Design Rules and Failure Analysis · Society of Plastics Engineers (SPE) — Fastener Boss Design Technical Papers · MatWeb — Material Property Database with Hoop Stress and Tensile Strength Data
— Arthur, Tooling Engineer at aCatMould