Ribs and Bosses in Injection Molding: Design Guidelines for Buyers

Ribs and bosses are standard features on molded parts. Proportioned wrong, they are a primary source of sink marks. Success depends on thickness ratios versus adjoining walls. Verify those proportions before tool construction. Fixes later are hard and expensive. This expands the design for manufacturing guide.

What Ribs Do

A rib is a projecting wall that adds stiffness without thickening the whole section. Thickening a flat panel for rigidity adds mass, lengthens cooling, and raises sink risk. Ribs deliver stiffness with less material.

Ribs also add local mass where they meet the main wall. That thicker junction cools slower than surrounding plastic, shrinks more, and pulls the opposite surface inward into a sink mark. The main defense is keeping the rib thin enough that added mass at the junction stays manageable.

What Bosses Do

A boss is a cylindrical protrusion that accepts a fastener, self-tapping screw, heat-set insert, or press-fit pin (for molded-in metal inserts, see insert molding). Like ribs, bosses add local mass at the wall. An outer wall that is too thick 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 is typically cored by a mold pin, which helps control wall thickness. Outside taper and surrounding support ribs need draft to eject cleanly; see the draft angle guide.

The Proportioning Rules

Most rib and boss sink problems share one cause: the feature is too thick relative to the wall it joins.

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 resin, part size, and cosmetics. A textured or non-critical back surface can tolerate more than a polished show face. Confirm values with your supplier and the resin’s DFM guidance.

The principle is consistent: keep features thinner than the wall they join. When 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. Cylindrical geometry does not distribute load well. Gusset ribs from boss to wall improve strength substantially. Those support ribs follow the same thickness proportion as structural ribs and need draft like any rib.

What to Review Before RFQ

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

Where thick sections cannot be avoided in load-bearing areas, coring out and using ribs is often the better path; see the wall thickness guide.

Buyer FAQs

Why do ribs cause sink marks on the opposite face?

Where a rib joins the wall there is more material than in the surrounding wall. That mass cools slower, shrinks more, and pulls the opposite surface inward. Keep rib base thickness proportionally smaller than nominal wall so packing and cooling can compensate.

How thick should ribs be relative to the wall?

A common guideline is about 50–60% of nominal wall at the base. Exact threshold depends on resin, surface finish, and how critical the opposite face is. Non-critical or textured surfaces tolerate a bit more; polished A-surfaces need tighter control. Confirm with your supplier before you freeze geometry.

How do bosses create sink marks?

The same way ribs do. The boss outer wall at the main wall adds local mass that cools slowly and contracts, pulling the opposite surface in. Keep boss outer-wall thickness proportionally smaller than nominal wall, and use a gradual transition rather than an abrupt joint.

Do ribs need draft angles?

Yes. Rib faces run along mold open and need draft to eject cleanly, like any vertical face. Taller ribs are more sensitive because contact length is longer. The rib root usually gets a small radius to cut stress concentration and ease release. Put draft on the drawing before steel is cut.

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.