Injection Molding Boss Design - Interactive Reference

Designing Bosses for Injection Molding

Interactive reference: boss anatomy, a live boss designer, boss types, screw details, defects and material guidelines. Hover or tap any number on the drawings.

1. Boss anatomy

Key dimensional rules

Boss wall t	0.5–0.6 × T
Outer diameter	≈ 2 × screw d (2.5 × glass-filled)
Pilot hole	≈ 0.8 × d (screw maker's chart)
Height H	≤ 3 × OD; gussets above 2 × OD
Draft	0.5–1° per side, inside and out
Base fillet R	0.25–0.5 × T
Thread engagement	2–2.5 × d

Why 0.5–0.6 × T? A boss wall as thick as the part wall cools much later than the wall around it; the late-shrinking plastic pulls the show face in (sink) and can leave voids inside.

2. Live boss designer – drag the sliders

Screw diameter d 3 mm Nominal wall T 2 mm Boss OD 2.6 × d Boss height 2.6 × OD Base fillet R 0.1 × T Draft 0.25 ° Material Add gussets

OD–

Pilot hole–

Boss wall–

Height–

3. Common boss types

Standalone boss

Tied only to the base wall. Fine for light loads and short bosses (height up to about 2 x OD). Core it and keep the wall at 0.5-0.6 x T so the show face does not sink.

4. Screw boss details

Thread-forming screw bosses

→Pilot hole from the screw maker's chart (about 0.8 × d)
→Engagement 2–2.5 × d; hole deeper than the screw tip
→0.5 mm chamfer or short counterbore at the entry
→OD about 2 × d (2.5 × for glass-filled)

Heat-set / ultrasonic insert bosses

→Hole size and taper from the insert maker's spec
→Boss OD about 2 × insert OD
→Hole deeper than the insert so displaced plastic has room

Molded-in insert bosses

→Uniform wall around the insert to limit hoop stress
→Knurls or grooves on the insert for pull-out and torque
→Pre-heat inserts to cut residual stress

5. Design do’s and don’ts

✓ Do

✓Keep the boss wall at 50–60% of the nominal wall
✓Use gussets or ribs on tall bosses
✓Core the boss so it is a tube, not a solid post
✓Draft 0.5–1° inside and outside
✓Keep bosses away from outside corners
✓Radius the base 0.25–0.5 × T
✓Tie bosses near a side wall to it with a thin rib

✗ Don’t

✗Make the boss wall as thick as the part wall
✗Merge a boss straight into a side wall
✗Go above about 3 × OD unsupported
✗Leave bosses solid – they always sink
✗Space bosses closer than 2 × OD
✗Leave a sharp corner at the base
✗Skip draft – zero-draft bosses stick and scuff

6. Defects caused by poor boss design

High riskSink marks

Thick boss walls or a heavy root shrink late and pull the show face in. Fix: boss wall 0.5–0.6 × T, core the boss, moderate fillet.

High riskVoids

The skin freezes before the thick core of a solid boss, leaving bubbles inside. Fix: core the boss, keep walls uniform.

Medium riskWeld lines

Melt splits around the core pin and meets on the far side - a weak spot when the screw is driven. Fix: gate so fronts meet hot and at a shallow angle.

Medium riskWarpage

The boss area shrinks differently from the wall around it. Fix: uniform walls and cooling close to the bosses.

Lower riskCracking at the base or rim

Sharp roots, too-small OD or the wrong screw split the boss. Fix: base fillet, OD about 2 × d, screws made for plastics.

Lower riskSticking on ejection

No draft on the core pin or outer wall. Fix: 0.5–1° draft, polished core pins, ejector sleeve on deep bosses.

7. Quick reference – dimension ratios

Parameter	Recommended	Notes
Boss wall thickness	0.5–0.6 × T	T = nominal part wall
Boss OD	≈ 2 × screw d	2.5 × for glass-filled
Pilot hole	≈ 0.8 × d	Follow the screw maker’s chart
Height	≤ 3 × OD	Gussets above 2 × OD
Draft	0.5–1° per side	Inside and outside
Base fillet	0.25–0.5 × T	Larger radii add mass at the root
Thread engagement	2–2.5 × d	Hole deeper than the screw tip
Rib / gusset thickness	0.5 × T	Thicker ribs sink
Boss to side wall	Rib-connected	Do not merge into the wall
Boss to boss	≥ 2 × OD centre to centre	Avoids merged thick sections

8. Material-specific considerations

Standard

Unfilled thermoplastics (ABS, PC, PP)

Standard rules apply. Good ductility makes them forgiving with thread-forming screws.

Caution

Glass-filled (PA-GF, PBT-GF)

Raise OD to about 2.5 × d. Fibres add stiffness but cut ductility, and weld lines are weaker – place gates carefully.

Critical

Semi-crystalline (POM, PA, PBT)

Higher shrinkage makes uniform walls even more important. Core every boss; consider heat-set inserts for repeated assembly.

Special

Soft / flexible (TPE, TPU)

Screw bosses rarely work. Use metal inserts or snap features instead.

Not sure your bosses will sink or crack? Send the CAD – we check every boss in a free DFM review.Get a free DFM review

Source: Topworks Plastic Mold