What is the correct rib thickness in injection moulding?

Rib thickness should be 40-60% of the nominal wall for cosmetic parts — about 1.0-1.5 mm on a 2.5 mm wall. Thicker ribs create a local thick section and leave sink marks on the opposite face; thinner ribs fill poorly and carry little load.

How tall can a rib be in an injection-moulded part?

Keep rib height below about 2.5-3.0x the nominal wall thickness. Taller ribs are hard to fill, vent, cool and eject, and they add little stiffness beyond that ratio. Use more ribs at closer spacing instead of fewer tall ones.

How do you design a boss for a screw in plastic?

Boss outer diameter should be roughly 2-3x the screw or insert diameter, with wall thickness 40-60% of the nominal wall and a root radius of about 0.25x the boss wall. Support tall bosses with gussets, and keep the bore through to the base wall to avoid a thick section.

Why do ribs cause sink marks on the visible face?

A rib thicker than about 60% of the wall acts as extra mass behind the surface: it stays molten after the skin freezes and pulls the skin inward as it shrinks. The fix is thinner ribs, not more packing — geometry beats process for sink.

Do ribs need draft angles?

Yes — at least 0.5-1.0° per side, more for tall ribs. Draft tapers the rib thinner toward its tip, so check the tip still meets your material's minimum fillable thickness. Nordmould checks rib tip thickness in every DFM review.

How do gussets help boss design?

Gussets are small ribs connecting a boss to the adjacent wall, carrying assembly loads without making the boss wall thicker. Use 2-4 gussets at 40-60% of wall thickness with a root radius. They prevent boss cracking under screw torque far better than a thick boss wall does.

Rib and Boss Design for Injection Moulding: Ratios, Radii & Sink Control

Ribs and bosses are how you add stiffness and fastening points without adding wall thickness — and they are also the two most common sink-mark generators on cosmetic parts. Both follow the same physics: keep them thin relative to the wall they sit on, round their roots, and draft them. This guide gives the working ratios.

What are the rib design rules?

Five numbers carry most rib design:

Parameter	Recommended value
Rib thickness	40-60 % of nominal wall (lower end for glossy cosmetic faces)
Rib height	≤ 2.5-3.0 x nominal wall
Rib spacing	≥ 2x nominal wall between ribs (4x rib thickness minimum)
Root radius	~0.25-0.5 x rib thickness — never a sharp corner
Draft per side	0.5-1.0° minimum, more on tall ribs

Thickness is the sink control. A 2.5 mm wall with a 1.5 mm rib (60%) will usually show a faint witness on a glossy A-surface; the same wall with a 2.0 mm rib (80%) will show a visible dimple. On high-gloss parts, stay at 40-50%.

Height is the fill-and-eject control. Beyond 3x the wall, ribs vent poorly (burn marks at rib tips), fill inconsistently, and grip the tool at ejection. If you need more stiffness, add ribs rather than growing them — rib stiffness scales with the cube of height in theory, but mouldability collapses first.

What are the boss design rules?

A boss is a rib closed into a circle — the same ratios apply:

Parameter	Recommended value
Outer diameter	2-3x screw/insert diameter
Boss wall thickness	40-60 % of nominal wall
Bore depth	Extend to base-wall level even if the screw needs less (keeps section uniform)
Root radius	~0.25 x boss wall thickness
Support	Gussets (2-4) connecting boss to wall for tall or loaded bosses
Entry	Chamfer or lead-in for screw/insert alignment

Two failure patterns dominate. Thin-walled bosses crack under screw torque or insert installation — the gusset fix. Thick-walled bosses sink the opposite face and trap voids inside — the core-out fix. A boss designed to the ratios above avoids both.

How do ribs and bosses interact with the mould?

Ribs are machined as slots into the cavity — deep narrow slots that must be polished and vented. Very tall, thin ribs mean fragile blade-like steel in the tool, which wears and can bend under injection pressure. This is one reason a DFM review may propose two shorter ribs where you drew one tall one.

Bosses sit on cores. Deep bores need draft on both the outer wall and the inner pin, and the pin needs cooling access or it becomes a hot spot — the classic cause of a sink ring around an otherwise correct boss. Keep blind-hole depth below about 2x diameter for uncooled pins, per the 14 DFM rules.

How do you fix sink from existing ribs without retooling?

Short answer: you mostly can't, at the machine. Packing harder moves the sink marginally and trades it for stress and flash. The honest sequence: confirm rib ratio from the model; if the rib is over 60%, the real fix is a steel change thinning the rib — a relatively small, cheap tool modification. This is exactly why Nordmould's free DFM review checks every rib and boss ratio before the tool is cut: thinning a rib in CAD costs minutes, in steel it costs days.