Why Ribs Exist
Plastic parts need stiffness, but solid thick walls are the enemy. They waste material, extend cooling time, and cause sink marks on the opposite surface. Ribs solve this. They are thin reinforcing walls molded into the part that add rigidity without the penalty of thick sections.
A well-ribbed part can be several times stiffer than a flat part of the same weight. A badly ribbed part warps, sinks, traps gas, and breaks at the rib base. The difference is in the dimensions.
The Core Dimensional Rules
Rib thickness: 50–60% of the adjacent wall thickness. This is the single most important rib rule. A rib thicker than 60% of the wall creates a thick section where it meets the wall. That thick section cools slower than the surrounding plastic. It shrinks more, and pulls a sink mark on the cosmetic surface opposite the rib.
For a 3 mm wall, the rib should be 1.5–1.8 mm thick. For glass-filled resins, go thinner — 40–50% — because filled materials sink more aggressively.
Rib height: maximum 3× the wall thickness. Taller ribs are hard to fill, hard to cool, and hard to eject. A rib taller than 3× wall thickness tends to flex during ejection and drag against the mold steel. If the design needs more stiffness than a 3× rib provides, use two shorter ribs instead of one tall one.
Rib spacing: at least 2× the wall thickness apart. Ribs packed too closely create thin steel walls between them in the mold — fragile, hard to cool, and prone to breaking. Give the mold steel room to do its job.
Base radius: 0.25–0.4× the wall thickness. A sharp inside corner where the rib meets the wall is a stress concentrator and a filling problem. The fillet radius distributes stress and helps plastic flow into the rib. Too large a radius, though, and you recreate the thick-section problem — keep it under 0.4× wall.
Draft: 1–2° per side minimum. Ribs are tall relative to their thickness, which makes them prone to drag during ejection. Draft the rib walls so the part releases cleanly. Deep ribs in textured molds need more.
What Goes Wrong
Sink marks opposite the rib. The classic rib failure. The rib-to-wall junction is thicker than the nominal wall, cools last, and pulls the cosmetic surface inward. Prevention: keep rib thickness at 50–60% of wall, add the base fillet, and make sure cooling reaches the rib area. If sink persists, the rib is too thick — redesign it, don’t just raise packing pressure.
Rib breakage during ejection. Tall thin ribs with no draft grip the mold steel. Ejector pins push the part, the rib flexes, and either the rib breaks or the part deforms. Prevention: adequate draft, generous base radius, and ejector pins positioned to push near ribs rather than on unsupported flat areas.
Gas traps at the rib base. Air caught where the rib meets the wall has nowhere to go. It compresses, heats, and burns — leaving a brown streak or a weak weld at the highest-stress point of the rib. Prevention: vent the rib area, typically with a shallow vent at the parting line near the rib base or an ejector-pin vent.
Weld lines in the rib. If plastic flows around a core feature and meets inside the rib, the weld line lands in the rib. That is exactly where stress concentrates. Prevention: gate the part so flow fronts meet away from ribs, or redesign so the rib doesn’t sit on a weld path.
Warping from asymmetric ribs. Ribs on only one side of a flat part pull unevenly as they cool, bowing the part. Prevention: balance ribs symmetrically where possible, or add counter-ribs on the opposite face.
Ribs vs. Thicker Walls: The Trade-off
Sometimes a designer asks: why not just make the wall thicker instead of adding ribs? The math answers it. Stiffness increases with the cube of thickness. Doubling wall thickness gives 8× the stiffness — but also 2× the material, 2× the cooling time, and severe sink risk. A ribbed design achieves similar stiffness with far less material and much shorter cycle time.
The exception: very small parts where ribs become too thin to mold reliably. Below about 0.8 mm rib thickness, filling gets difficult and the mold steel between ribs gets fragile. For tiny parts, a slightly thicker wall with no ribs can be the better choice.
Ribs in Specific Applications
Crates and containers: perimeter ribs plus a cross-hatch pattern on the base. Rib height up to 4× wall is sometimes accepted here. The parts are large and the ribs are visible structural elements. But sink on the outside wall must still be controlled with good cooling.
Housings and enclosures: ribs around bosses and along edges where the cover meets the base. Keep ribs away from cosmetic outer surfaces or accept minor sink — communicate this to the customer before tooling.
Automotive: structural ribs in PP with glass fill. Thinner rib ratios (40–50%) because filled PP sinks aggressively. Rib-to-wall junctions are the first place we check for sink in DFM.
What We Flag in DFM
Rib review is standard in our DFM reports. We check every rib against the dimensional rules. We flag ribs that will sink on cosmetic surfaces. We identify gas-trap risks at rib bases. We verify that ejector layout supports the ribbed areas. The most common finding: ribs drawn at 80–100% of wall thickness by designers who didn’t know the 50–60% rule. Catching it on paper saves a mold correction later.