Ribs and Bosses in Injection Molding: Design Guidelines for Buyers

Ribs and bosses are two of the most common features on injection-molded parts, and two of the most common sources of sink marks when they’re not proportioned correctly. Getting them right isn’t complicated—it’s mostly about keeping them thinner than the walls they join—but it’s something to check deliberately before the tool is cut, not discover on the first samples. This guide is part of the broader design for manufacturing guide.

What Ribs Do

A rib is a projecting wall used to stiffen a part without increasing wall thickness throughout. This is important: if you need a flat panel to be rigid, making the whole panel thicker adds mass, cooling time, and sink risk. Ribs let you achieve the same stiffness with far less material, at the cost of some geometry on the back of the part.

The reason ribs create sink is the same reason any thick section does: where a rib joins the wall, there’s more material than in the surrounding wall. That mass cools slower, shrinks more, and pulls the opposite surface inward—a sink mark exactly opposite the rib. The fix is keeping the rib thin enough relative to the wall that the extra mass is manageable.

What Bosses Do

A boss is a cylindrical protrusion used to accept a fastener—a self-tapping screw, a heat-set insert, a press-fit pin (when the fastener is a molded-in metal insert instead, see insert molding). Like ribs, bosses add local mass where they meet the wall. A boss that’s too thick in its outer wall creates sink on the opposite face; a boss without adequate support can crack when the fastener is driven.

Bosses also need to release from the tool. The inside of a boss is typically cored by a pin in the mold, which helps control wall thickness, but the outside taper and any surrounding support ribs need draft to eject cleanly—covered in the draft angle guide.

The Proportioning Rules

Most rib and boss sink problems come from the same place: the feature is too thick relative to the wall it’s attached to.

Feature	Common guideline	Why
Rib thickness at base	Often around 50–60% of nominal wall	Keeps added mass below the threshold that causes visible sink
Rib height	Typically limited to 2–3× the rib base thickness	Taller ribs need more draft and add fill difficulty
Boss outer wall	Often around 60% of nominal wall	Same sink logic as ribs
Boss-to-wall connection	Gradual gusset or fillet, not abrupt	Distributes load; reduces stress concentration

These are directional guidelines, not universal laws—the right ratio depends on the resin, part size, and cosmetic requirements. A textured or non-critical back surface can tolerate more than a polished show face. Confirm specific values with your supplier and the resin’s DFM guidance.

The underlying principle is consistent: keep features thinner than the wall they join. The moment a rib or boss is thicker than its nominal wall, sink on the opposite face is likely.

Support Ribs on Bosses

A boss standing alone—unsupported by the surrounding wall—is vulnerable to cracking when a fastener is driven, because the cylindrical geometry doesn’t distribute load well. Gusset ribs connecting the boss to the wall improve strength substantially. These support ribs follow the same thickness proportion as structural ribs, and they need draft just like any rib.

What to Review Before RFQ

• Find all ribs and check their thickness relative to the walls they meet. Any rib that’s thicker than the wall is a sink candidate on the opposite face.
• Find all bosses and check their outer-wall thickness by the same logic.
• Note which faces are cosmetic. A sink on a hidden face is usually acceptable; one on an A-surface is a reject. Knowing which faces are cosmetic tells the supplier where to focus.
• Check rib height and draft. Tall ribs need more draft to release cleanly.

For where thick sections can’t be avoided—structural load-bearing areas—coring out the section and using ribs is often the better design path, as covered in the wall thickness guide.

This is an independent buyer resource, not a substitute for your supplier’s DFM review. Rib and boss proportions depend on the resin and part requirements—confirm with your supplier.

Buyer FAQs

Why do ribs cause sink marks on the opposite face?

Where a rib joins the wall, there’s more material than in the surrounding wall. That extra mass cools more slowly, shrinks more, and pulls the surface opposite the rib inward—a sink mark. The fix is keeping the rib thickness at its base proportionally smaller than the nominal wall, so the added mass doesn’t exceed what the material can compensate for through packing and cooling.

How thick should ribs be relative to the wall?

A common guideline is to keep rib thickness at the base around 50–60% of the nominal wall thickness—thin enough that the added mass doesn’t drive visible sink on the opposite surface. The exact threshold depends on the resin, surface finish, and how cosmetically critical the opposite face is. Non-critical or textured surfaces can tolerate a bit more; polished A-surfaces need tighter control. Confirm with your supplier.

How do bosses create sink marks?

The same way ribs do—the outer wall of a boss where it meets the main wall adds local mass that cools slowly and contracts, pulling the opposite surface in. Keeping the boss outer-wall thickness proportionally smaller than the nominal wall, and using a gradual transition rather than an abrupt joint, reduces the risk.

Do ribs need draft angles?

Yes. The faces of a rib run along the mold’s opening direction and need draft to eject cleanly, the same as any vertical face. Taller ribs are more sensitive to insufficient draft because there’s more contact length to drag on. The inside of a rib (the root) typically gets a small radius to reduce stress concentration and ease release.

Evidence Box

This guide to ribs and bosses summarizes widely documented tooling and design practice, framed for the decisions a buyer actually makes. Any numeric ranges, cycle-life figures, or conventions mentioned are commonly published industry values, not PTA measurements or guarantees, and real values are part- and supplier-specific. It is educational synthesis, not an engineering specification—your supplier’s and moldmaker’s review of the specific part governs, and this page is preparation for that conversation.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, provide engineering services, or certify suppliers. Design and tooling decisions are part-specific—confirm them through your supplier’s and moldmaker’s engineering review.