Front page / How should a screw boss be designed in a moulded part?
How should a screw boss be designed in a moulded part?
Make the boss hollow, keep its wall thinner than the part wall around it, and tie it to nearby walls with ribs rather than merging it into them. Published rules of thumb put the boss outer diameter at about 2 to 2.5 times the hole or pilot diameter, the boss wall at about 50 to 70 percent of the adjacent wall, and draft at 0.5 to 2 degrees. The pilot hole and engagement length should come from the screw maker's data for your resin.
A screw boss is the post a screw goes into. It is one of the most heavily loaded features on a part, and a common source of sink marks, voids and cracks.
What three published guides say
| Rule | Eurotec design guide | N&H Technology | Protolabs |
|---|---|---|---|
| Outer diameter | 2.0 to 2.4 × hole diameter; 2 to 2.5 × pilot hole for self-tapping screws | only as large as the loads require | follow the screw maker's guidance |
| Boss wall | above 50% of part wall gives visible sink | B ≤ 0.6 × S | no more than 60 to 70% of the adjacent wall |
| Root radius | not stated | R ≥ 0.25 × S | not stated |
| Draft | 1/2° inside and outside | 0.5° to 2.0° | not stated |
| Thread engagement | 2.5 × screw diameter | not stated | not stated |
| Near outer walls | do not merge; use ribs and gussets | avoid bosses directly behind visible walls | move inward; connect with coring ribs |
In the N&H rules, B is the boss wall and S the reference wall thickness on their drawing. The guides agree on the main point: extra material does not make a boss stronger in moulding. N&H explains that a thick junction between boss and base wall cools more slowly, which drives uneven shrinkage, sink and warpage.
Worked sizing example (our numbers)
Take a 2.5 mm housing wall and a screw whose supplier recommends a 2.2 mm pilot hole. The Eurotec rule gives an outer diameter of 2 to 2.5 × 2.2, or about 4.4 to 5.5 mm, so a boss wall of about 1.1 to 1.65 mm. The sink limits give 0.5 to 0.6 × 2.5 = 1.25 to 1.5 mm. A 5.0 mm outer diameter, giving a 1.4 mm wall, satisfies both. Engagement of 2.5 × the screw diameter sets the hole depth.
Other points from the guides
- Never solid. N&H states a well-designed boss is hollow inside.
- Support tall bosses. Freestanding bosses deflect under load; ribs and gussets add stiffness, and should themselves be slim with radii and draft.
- Add a counterbore. Protolabs notes that a thread-forming screw displaces plastic, and a counterbore at the top of the hole gives that material somewhere to go.
- Choose the screw type for the resin. Eurotec says thread-forming screws suit unfilled or lightly filled materials, while thread-cutting screws need room for chips and should not be retightened.
- Plan for re-use. Protolabs says directly threaded bosses are typically good for a single use; for parts opened repeatedly, both N&H and Eurotec point to metal threaded inserts.
These are published rules of thumb. The screw maker's pilot hole and torque data for your resin, and tests on moulded parts, decide the final design.
Questions buyers ask
Why does my boss show a sink mark on the outside face?
Usually the boss wall is too thick relative to the part wall, or the boss sits directly behind the visible wall. Thinning the boss, coring it, or moving it inward with ribs are the usual fixes.
Why did the boss crack when the screw went in?
Common causes are a pilot hole that is too small, a wall that is too thin, a sharp root, a weld line through the boss, or tightening too fast. Eurotec warns that rapid torque application should be avoided.
Should I use inserts instead?
Consider them when the part will be opened often, when loads are high, or when a machine screw thread is needed.
Where should the gate go relative to bosses?
Ask for a flow analysis to see where weld lines fall, and keep them away from bosses where possible.
Sources
- Eurotec Engineering Plastics, Part design guide (PDF)
- N&H Technology, Design guidelines for screw bosses
- Protolabs, Design better screw bosses on molded parts