Boss Design for Injection Molding — How Do You Design Screw Bosses to Avoid Sink Marks, Hold Structural Strength and Prevent Breakage?
Last updated: 2026-09-03 · Reading time: about 6 min
A boss is a cylindrical protrusion that takes a screw or a press-fit pin. Get four numbers right — wall at 50–60% of nominal, outer diameter at 2× the screw hole, height under 3× the diameter, and a root fillet — and you prevent sink marks, root cracks and assembly breakage.
See also related article.
Key facts
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Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.
What are the boss design rules — and why does each one matter?
Four rules cover almost every screw boss. Each one trades appearance against strength.
- Wall at 50–60% of nominal. The boss is a hollow cylinder, not a solid post — on a 2.5 mm wall that is 1.25–1.5 mm. Thicker than 60% makes a local thick section that sinks the opposite face; thinner than 50% cracks when the screw is tightened. Set the OD at 2× the screw hole (M4 → 8–10 mm).
- Height no more than 3× the OD. Above that the long, thin core pin bends under injection pressure and the boss comes out off-center — the thick side sinks, the thin side cracks. For a taller boss, step the diameter or add gussets.
- Fillet the root at 25–50% of the wall. A sharp corner concentrates stress 2.5–3.0×; a 0.5 mm fillet drops it to 1.2–1.5× and roughly doubles boss strength. The fillet also lets a ball-end mill cut it instead of burning a sharp corner with EDM.
- Add gussets for tall or high-load bosses. Three or four triangular gussets (120° or 90° apart) raise lateral strength 2–3× and spread the root load over more wall. Size them 50–80% of boss height, 50% of nominal wall thick and 1–2× wall wide at the base.
How do you prevent sink marks on the boss?
Five levers reduce or hide the sink. One eliminates it outright.
| Solution | How it works | Effectiveness |
|---|---|---|
| Reduce boss wall to 50% of nominal | Less material shrinks less | Sink depth down 40–60% |
| Core-out (hollow boss) | Removes the thick section | Eliminates sink on the opposite face |
| Cosmetic recess on the back | Texture or groove hides it | Hides sink up to 0.15 mm deep |
| Chemical foaming agent | Micro-bubbles compensate shrinkage | Sink depth down 60–80% |
| Optimize packing pressure/time | More packing feeds the shrinkage | Sink depth down 20–40% |
The core-out is the one that eliminates sink. A hollow boss with a uniform wall at 50% of nominal has no thick section, so there is no differential shrinkage — and the core pin conducts heat from the inside, so it cools faster and weighs less. Its only limit is pin length: above 3× the OD the pin bends, so fall back to a cosmetic recess or a foaming agent for tall bosses.
“Boss defects are inexpensive to fix on the screen and expensive to fix in steel. In the DFM we check every boss for wall ratio, height, root fillet and gussets, and we run the sink prediction before we cut — so the show surface stays clean and the screw does not strip on the line.”
— Project Engineer, aCatMould
Frequently asked questions
What is the pull-off force — and how do you size the boss for it?
Pull-off force = root area × material tensile strength × a 0.5 safety factor for the stress concentration.
The root area is π × OD × wall — an 8 mm OD boss with a 1.25 mm wall gives 31.4 mm².
For PP at 30 MPa that is 31.4 × 30 × 0.5 ≈ 471 N (about 48 kgf). An M4 screw at 1.5 Nm pulls roughly 200 N, so the safety factor is 2.4×. Below 1.5×, redesign — larger OD, thicker wall or gussets.
What screw hole diameter prevents boss breakage on assembly?
Size the pilot hole at 75–85% of the screw major diameter for amorphous resins (ABS, PC, PMMA) and 65–75% for semi-crystalline (PP, PA, POM), which flex and grip tighter. An M4 screw in ABS takes a 3.0–3.4 mm hole.
Breakage comes from a hole too small (hoop stress as the screw cuts in), a wall too thin, or a screw that bottoms out. Hold the wall at 50–60% of nominal and make the hole 1–2 mm deeper than the screw.
When should you use a metal insert instead of a plastic boss?
When the joint is taken apart repeatedly — a plastic boss wears after 10–20 cycles, a brass or steel insert lasts 100+. Install it by heat staking, ultrasonic insertion or molding it in.
The insert runs $0.10–0.50 plus $0.02–0.10 to install. Justify it for high-volume assembly (above 10,000/year), high torque, or a safety-critical joint that must not loosen under vibration — and it raises pull-off and push-out strength too.
Key takeaways
- Boss wall 50–60% of nominal, OD 2× the screw hole, height max 3× the OD — gussets for taller or high-load bosses.
- A root fillet at 25–50% of the wall halves the stress concentration; kill sink with a core-out, a cosmetic recess or a foaming agent.
- Pilot hole 75–85% of screw diameter (amorphous) or 65–75% (semi-crystalline); step up to a metal insert for repeated disassembly.
The boss touches all three things a customer notices — the show surface, the structural strength and the assembly line. Get the four rules right (50–60% wall, 3× height limit, root fillet, gussets) and the sink, crack and strip risks fall away. If a boss already shows defects, a DFM review catches the cause before the mold is cut.
Sources
DuPont — injection molding part design guidelines (boss and rib)
Autodesk Moldflow — sink mark and shrinkage prediction
Plastics Technology — boss design and part geometry practice
BASF — plastic part design and material guidelines
MoldMaking Technology — designing plastic parts for moldability
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet