Injection Molding Rib Design Checker

Screen rib base thickness, height, clear spacing and base radius against the adjacent nominal wall.

Formula and method

Thickness ratio = rib base thickness / wall × 100. Height and spacing ratios = dimension / wall. Radius ratio = base radius / wall × 100.

Inputs and units

Measure at the rib base and adjacent wall in CAD. Enter clear spacing between parallel ribs, not center-to-center pitch. The screen uses 40–60% thickness, a conservative 3× height review point, 2.5× minimum spacing and 25–40% base radius.

Worked example

A 3 mm wall, 1.5 mm rib, 9 mm height, 7.5 mm clear spacing and 0.9 mm radius gives a 50% rib-to-wall ratio, 3× height, 2.5× spacing and 30% radius; 4 of 4 reference checks pass.

When to use it

Use while a rib can still be changed without cutting steel, especially below cosmetic surfaces or near other thick features.

How to interpret the result

A flag identifies a feature for engineering review; it does not predict sink, warp, short shot or structural failure. Passing all four checks is not tooling approval.

Assumptions and limitations

The height screen is deliberately conservative within broader published guidance. Loads, material, draft, gate direction, rib intersections, tool access and appearance requirements may justify different values.

Validation guidance

Have the product engineer confirm stiffness, then ask the moldmaker to review fill direction, venting, steel condition, ejection and visible-surface risk.

FAQ

Does a passing screen approve the part for tooling?

No. It creates a traceable CAD review record. Formal DFM, material review, mold design, simulation where appropriate, and molded-part validation remain required.

Sources

Protolabs design essentials · Protolabs injection-moulding design guide · Covestro strategic-radii guidance