Rib-to-wall relationship
A rib as thick as the wall is a sink and cycle risk. Size the rib from the nominal wall, then confirm on CAD.
Rib design
Injection Molding Rib Design: Stiffen Walls Without Sink Marks
Ribs are a wall-stiffening feature, not a fixed formula. Size each rib from the nominal wall, then add draft, root radius, and spacing. DFM review comments on your CAD before quoting.
Design guideline
Size ribs from the nominal wall, not as a second wall
Rib design for injection molding: stiffen a wall without sink marks. Size each rib from the nominal wall, then add draft, root radius, and spacing.
Ribs add stiffness by increasing section modulus without thickening the whole wall. Size each rib from the nominal wall , add draft , round the root, and space them so the mold can cool and vent. NetProto does not publish one rib-to-wall ratio as a spec. DFM review comments on your CAD, resin, and cosmetic class.
Ribs are a wall-stiffening feature, not a fixed formula. This page covers height, draft, sink, spacing, and boss tie-in. The rib-to-wall ratio spoke owns the ratio question. Bosses are fastening posts, not ribs. The design guide hub lists every topic.
Roles
Which page answers which rib question
Stay here for the stiffening feature. Open a sibling for the wall, the post, or the CAD check.
| Page | Use it for |
|---|---|
| Rib design (this page) | Stiffen a wall without making the skin thick |
| Wall thickness | Pick the nominal wall first |
| Rib to wall ratio | The ratio question, still not a NetProto spec |
| Bosses | Screw posts and how they tie into ribs |
| Draft angle | Draft on vertical rib faces |
| Gussets | Triangular supports that are not full-height ribs |
| Fillets and radii | Root and corner radii that change the junction mass |
| Support surfaces | Rest pads. A rib stiffens a wall; it is not the seat. |
| DFM review | CAD check of ribs before quoting |
Checks
What DFM review looks for on ribs
Qualitative checks. No ratio on this page is a NetProto specification.
Root radius and stress
A sharp root concentrates stress and can crack. A generous root helps flow, but too much root rebuilds a thick junction.
Draft and ejection
Both faces of the rib need draft. Deep or textured ribs usually need more draft per side so they do not drag.
Spacing, venting, and steel
Ribs packed too close starve cooling and leave thin steel between cavities. Space them so the tool can vent and stay stiff.
Decision
A rib as thick as the wall versus a thinner rib with draft
Wrong: treat the rib as a second wall. Right: thinner rib, root radius, draft, and spacing.
What goes wrong
- Rib as thick as the wall pulls sink onto the opposite face
- Thick junctions stay hot and stretch cycle time
- Zero-draft rib faces drag and scuff on ejection
What to try first
- Size the rib from the nominal wall, not as a matching wall
- Round the root and keep the junction from becoming a thick island
- Add draft per side and space ribs so steel can cool and vent
RFQ
Send these notes with CAD for a rib review
The quote can start from CAD alone. These notes make the DFM pass specific.
Nominal wall callout the ribs should reference
Resin family, especially if shrink or cosmetics are tight
Class-A or show-face location opposite any rib
Rib height, count, and orientation
Draft per side, if you already have a target
How ribs tie into bosses or gussets
Related
Related design guidelines
This page owns the rib as a stiffening feature. Open a sibling for the wall, the post, or the CAD check.
DFM review
How NetProto checks ribs on your CAD before quoting.
DFM reviewDesign guide hub
All injection molding design guidelines by topic.
Design guide hubFAQ
Injection molding rib design FAQ
What is the purpose of ribs in injection molding?
Ribs add stiffness by increasing section modulus without thickening the whole wall. They are a wall-stiffening feature, not a substitute for picking a nominal wall first. Use them when a thicker skin would add sink, cycle time, or resin you do not need.
How thick should a rib be compared to the wall?
Typical industry guidelines often start ribs thinner than the nominal wall, in a range commonly cited as 40 to 60 percent. Cosmetic class, resin shrink, and rib height change the answer. NetProto does not publish one rib-to-wall ratio as a specification. Open the rib-to-wall ratio spoke for that topic, then upload CAD for DFM review.
How tall can a rib be?
Height is limited by draft, cooling, and how thin the steel stays between ribs. A tall, thin rib can buckle, drag on ejection, or leave a hot spot at the root. There is no NetProto height multiplier on this page. Call out rib height and count on the RFQ.
Why do ribs cause sink marks?
The rib-to-wall junction is locally thicker than the skin. That extra mass cools slowly and can pull a sink or read-through on the opposite face, especially on a class-A surface. A rib as thick as the wall makes that worse. Root radius, a thinner rib, and spacing help.
Do ribs need draft angle?
Yes. Vertical rib faces need draft so they eject without drag. Texture and deeper ribs usually need more draft per side. Open the draft angle page for that rule. Put draft per side on the RFQ if you already have a target.
How is this page different from bosses, wall thickness, or DFM review?
This spoke owns ribs as a stiffening feature: relationship to the wall, root, draft, height, spacing, and boss tie-in. Wall thickness owns the nominal skin. Bosses own fastening posts. The rib-to-wall ratio page owns the ratio question. DFM review is how NetProto checks your CAD before quoting.
Next step
Upload CAD for DFM review of your ribs
Share geometry, resin, cosmetic faces, and rib height so review can check rib-to-wall relationship, root radius, draft, spacing, and boss tie-ins before quoting.