Ribs are the most efficient way to stiffen a plastic part, and the most common cause of sink. The proportions, spacing and layouts that deliver stiffness without cosmetic or moulding problems.
A product team needed a flat ABS panel to stop flexing under a user's thumb. The first fix was to thicken it from 2.5 to 4 mm. It became stiffer, but it also gained 60% more material, an extra 12 seconds of cooling time and enough warpage to rock on a table. The second fix used the original 2.5 mm wall with a grid of 1.4 mm ribs, 8 mm tall. It was more than twice as stiff as the 4 mm panel, lighter than the original and flat.
That is why ribs are the plastic designer's most powerful tool, and why getting their proportions wrong is so costly.
Why ribs work
Stiffness depends on the second moment of area, which rises with the cube of depth. Adding height to a section through a rib increases stiffness far faster than adding the same material as wall thickness. Ribs let you keep walls thin, which keeps cooling times short and shrinkage even, while still making the part rigid.
The proportions that prevent sink
The trouble is that every rib creates a local thick spot where it meets the wall. That intersection shrinks more and later than the wall around it, and shows on the opposite face as a sink mark, the defect we described in sink marks: causes and cures. The design rules exist to keep that thick spot small:
- Rib thickness at the base: 50–60% of the nominal wall for most amorphous materials such as ABS and PC; 40–50% for high-gloss cosmetic surfaces and for high-shrink materials such as PP and PA.
- Rib height: up to about three times the wall thickness. Taller ribs are hard to fill and vent, and thin at the tip once drafted.
- Draft: 0.5–1° per side, which further reduces thickness towards the tip.
- Base radius: 0.25–0.4 times the wall thickness. Enough to reduce stress concentration and help flow, not so much that it recreates the thick spot you just removed.
- Spacing: at least two to three times the wall thickness between parallel ribs, so the steel between them can be cooled and the ribs can be filled.
Several small ribs beat one big one
If one rib at the correct thickness is not stiff enough, the answer is usually more ribs, not a thicker one. Two ribs at 50% of wall thickness are stiffer than one at 100%, and they do not cause sink. Where height is limited, a rib grid or honeycomb pattern gives stiffness in two directions.
Rib layout and orientation
- Align ribs with the direction of bending load. A rib placed across the load path adds little stiffness.
- Avoid ribs meeting in a cross (+) where possible. The intersection is a large thick spot. Offset T-junctions, or core out the intersection.
- Run ribs in the direction of flow where you can, so they fill easily and help, rather than block, the flow front.
- Tie ribs into walls and bosses to share load, but watch the combined thickness at each junction.
When cosmetic surfaces are critical
On high-gloss show faces, even a correctly proportioned rib can leave a faint shadow. Options include:
- Going to the lower end of the thickness range, around 40%.
- Adding a small step, groove or feature line on the show face to hide the rib line.
- Texturing the surface.
- Moving structural ribs to internal parts or frames, rather than the cosmetic housing.
Check before you commit
A wall thickness colour map in CAD will show every rib junction that exceeds your rule. A mould flow analysis will predict where volumetric shrinkage is likely to show. Both cost far less than adding steel back into a tool, which is what thinning a rib after the tool is cut requires. It is one more reason rib design sits at the heart of every Design for Manufacture review.
