A Rib in Plastic Design

A rib is not just “extra plastic for strength.” This is one of the most common mistakes in plastic part design.

A part feels flexible. The first reaction is: “Add a thicker rib.”

But in injection molding, that can create a new problem.
- Sink mark on the show surface.
- Shrinkage void at the rib base.
- Residual stress near the feature.
- Sometimes even lower real strength than before.

Why? Because the rib base becomes a local thick section. Plastic cools from the outside first. The inside stays hot longer. As the core shrinks, it pulls material inward.
- If the outer skin can still deform: sink mark.
- If the outer skin is already strong: internal void.

Both are bad.

The better way is not always a thicker rib. The better way is often a smarter rib. For bending stiffness, remember: I = b × h³ / 12

That h³ is the key. Increasing rib height is usually much more powerful than simply increasing rib thickness.

- Practical starting point: Nominal wall = T Rib thickness: 0.4T to 0.8T
- For many unfilled plastics: try to stay near or below ≈ 0.7T
- Base radius: ≈ 0.25T to 0.4T
- Draft: commonly around 1° per side, adjusted for depth, texture, and material.

Design lesson: Do not make ribs thick because the part is weak.

First ask:
Can I increase rib height?
Can I add multiple thinner ribs?
Can I improve rib direction relative to load?
Can I avoid ribs directly behind cosmetic surfaces?
Can I keep the rib base from becoming a thick mass?
Can I validate sink and stiffness together?

A thick rib may look stronger in CAD. But the molded part does not care about CAD confidence. It cares about cooling, shrinkage, packing, and stress flow.

Good plastic design is not adding material. Good plastic design is placing material where it gives strength without creating molding defects.