A rib is a thin projecting wall that stiffens a moulded part, and it works because stiffness rises with the cube of depth while material use rises only linearly. That makes ribs the most efficient tool a plastics designer has, and the single most common source of sink marks when their proportions are wrong.
The thickness ratio is the rule that matters
A rib joins the wall, and at the junction the local section is thicker than either. If the rib is as thick as the wall, that junction is substantially heavier than its surroundings, cools last, shrinks last and pulls the visible surface in. Keeping the rib to roughly half to two thirds of the wall thickness keeps the junction close enough to uniform that the sink is not visible.
Height, draft and buckling
Ribs are usually limited to a few times the wall thickness in height, because a tall thin rib is hard to fill, hard to eject and prone to buckling under load rather than stiffening. Every rib needs draft on both faces so it releases from the tool, and that draft means a tall rib is thinner at its tip, which further limits useful height.
More ribs beat thicker ribs
Because stiffness comes from depth and from the number of ribs rather than from their thickness, the correct response to a part that flexes too much is more ribs, not thicker ones. Spacing them at a few times the wall thickness apart avoids the cooling problem of many junctions close together and distributes the stiffening where it is needed.
Where to put them and where not to
Run ribs in the direction the part is loaded, and join them into corners and around bosses where the stress concentrates. Avoid ribs directly behind a cosmetic surface where any sink would show, or offset them and use a texture on the visible face. And never use a rib to fix a part that is flexing because its wall is simply too thin over a large span; that is a wall thickness problem.
Questions people ask about plastic ribs
How thick should a rib be?
Roughly half to two thirds of the wall it joins. Thicker than that creates a heavy junction that cools last and sinks on the visible face.
How tall can a rib be?
Usually a few times the wall thickness. Taller ribs are hard to fill, hard to eject and buckle rather than stiffen.
Should I use fewer thick ribs or more thin ones?
More thin ones. Stiffness comes from depth and count, and thin ribs avoid the heavy junction that causes sink marks.
How do I stop a rib showing on the outside?
Keep the thickness ratio correct, move the rib away from the cosmetic area, or texture the visible face so any residual sink is not noticeable.