Question 1

Why must a rib be thinner than the wall it sits on?

Accepted Answer

Because a moulding cools from the outside in. Where the section is locally thicker the core stays molten longer, and when it finally shrinks the frozen skin cannot follow — so it pulls the visible surface in behind it. That dimple is a sink mark. Keeping the rib to 50–60% of the nominal wall keeps everything freezing at roughly the same time. Thicker still and the core cannot pull the skin at all, so it tears internally instead and you get a void.

Question 2

My rib is not stiff enough — should I make it thicker?

Accepted Answer

No. Bending stiffness goes as the cube of depth, so doubling a rib's height is worth about eight times as much as doubling its thickness — and costs nothing in sink. Make it taller (up to about 3× the wall), add a second rib, or cross-rib it. A fatter rib buys very little stiffness and guarantees a cosmetic defect.

Question 3

What about bosses?

Accepted Answer

A boss is a rib rolled into a tube and it obeys the same limit: keep the wall at the base under 60% of the nominal wall, which means coring it out rather than leaving it solid. When it needs strength, add gussets at 50% of the wall thickness rather than thickening the boss. Radius the base 25–50% of the wall with a 0.015 in (0.381 mm) floor, and keep bosses at least 2× wall apart.

Rib & Boss Design