How to Design Functional Features: Ribs and Bosses

Strengthening with Ribs

Ribs are thin, wall-like features designed to add stiffness and support to a part. A well-designed grid of ribs can make a 2mm thick part feel as rigid as a 4mm solid slab, but with a fraction of the material and cooling time.

The single most important rule for designing ribs is: A rib's thickness should be approximately 40% to 60% of the nominal wall thickness of the part.

Why? If a rib is too thick at its base, it creates a mass of plastic that will cool slowly and cause a sink mark on the cosmetic surface directly opposite it. By keeping the ribs thin, you add the structure you need without creating a cosmetic problem.

Creating Mount Points with Bosses

A boss is typically a cylindrical feature designed to receive a screw, a threaded insert, or an alignment pin. They are the go-to feature for assembling your product.

The same rules for ribs apply to bosses: they are essentially thick walls, so they must be designed to avoid sink.

• Core them out: Never design a solid boss. It should be hollowed out to maintain a wall thickness consistent with the rest of your part.
• Use Gussets: Freestanding bosses can be weak and get knocked over by the flow of plastic. It's best practice to connect them to nearby walls with thin support ribs, often called gussets, for stability.

By correctly designing ribs and bosses, you create a part that is both lightweight and strong—the hallmark of an efficient and well-engineered plastic product.

What's Next?

Now that you can strengthen your part and screw it together, let's look at a more elegant assembly method. Our next article dives into a classic feature: A Guide to Designing Snap-Fits for Injection Molding.