Injection molding rib and boss design: ratios, radii and sink control

Stiffness comes with a locally thick section

Bending stiffness in a plastic wall climbs with the cube of the section depth, so a rib that is half the wall thickness but several times taller than the wall adds far more rigidity than the same plastic spread into a thicker wall, which is why ribs, gussets and the occasional crossing pattern turn up on covers, housings, trays and any long part that has to resist deflection.

The polymer does not read that math.

At the base of a rib, plastic and wall merge into one locally thick region, and thick plastic cools slower than the thin wall around it, so it keeps shrinking after the surrounding surface has frozen. Melt that would have compensated that loss cannot reach the rib root, because the thin wall feeding it has already sealed over, so the shortfall shows up on the opposite face as a sink mark or stays buried in the junction as a void.

A boss makes the same problem stand up.

A screw boss is a thick-walled cylinder with a cored hole, and the wall around that hole is a rib bent into a circle, which is why the same thinning logic that keeps a rib root in check applies to the boss wall as well.

Rib thickness: read the resin, not a house rule

Two published rules that look contradictory agree once the resin is named. The Plastics Engineering analysis of rib design puts the thickness at the projected intersection with the wall at 50 to 75 percent of that wall, moving toward the thin end as shrinkage rises: grades above roughly 0.015 in/in, which covers most neat semi-crystalline materials, take the thinner ribs, while grades below about 0.008 in/in, which covers most amorphous and filled materials, carry the thicker ones.

Cosmetic faces cut those numbers again.

Where a high-gloss, dark or plated surface sits opposite the rib, PTA Plastics advises reducing the allowance by 20 percent, and the thickness to compare against the wall is measured across the root between the outer tangent points of the two fillets, because that combined section is the mass feeding the sink.

A 2.5 mm ABS cover with a textured top face puts the range to work. At 70 percent of the wall the root lands near 1.75 mm with a base radius around 0.6 mm, and any sink on the far side stays shallow enough to hide under texture; the same rib in neat polypropylene wants a root closer to 1.5 mm or less, because higher shrinkage pulls the visible surface in behind it.

Height, draft, base radius and spacing

Height is limited by filling and ejection more than by stiffness, and the long-standing Plastics Engineering rule keeps a rib below about 2.5 times the primary wall, on the reasoning that several short ribs add more rigidity than one tall rib and that a shorter rib thins less at the tip once draft is applied.

Each rib is a small, deep pocket in the steel.

Draft is what lets that pocket release, with 0.5 to 1 degree per side the common choice, and PTA Plastics notes the penalty at both ends: below half a degree the rib drags on ejection, while more than a degree fattens the top intersection, where the extra mass creates the very sink the design was avoiding, and thins the tip that still has to fill.

The base radius serves both goals at once, spreading stress where the rib meets the wall and letting melt turn the corner into the rib instead of stalling at a sharp edge. The published figure is about 0.25 times the wall thickness with a minimum of 0.010 in (0.25 mm), a floor PTA Plastics applies to every inside corner in a part.

Parallel ribs are spaced no closer than their own height, which leaves the steel at each rib base coolable, and where one row of ribs is not enough, several short ribs beat one deep one.

Sink and warp pull in opposite directions, and this is the part of rib design that most guides skip.

Plastics Engineering is blunt about the trade: a thin rib beside a thicker wall creates a step in wall thickness, the wall and the rib then cool and shrink at different rates, and the part warps away from the ribs, a problem that gets worse with high-shrinkage semi-crystalline resins and bites hardest on exactly the long flat parts that ribs were added to stiffen. The worked example there is a 44-inch automotive trim piece ribbed at roughly 40 percent of the wall, which warped away from its own ribs.

Stiffness and flatness are not the same request.

Bosses: the same math standing up

A screw boss is a thick-walled cylinder with a cored hole, so the wall around that hole follows the rib rules above, and Covestro's joining guide frames the conflict plainly: a small boss diameter sinks less because its side walls are thin, but it may not survive the hoop stress that tightening a screw puts on it.

Diameter is usually set from the screw rather than from the wall.

In Covestro's own thread-cutting screw testing the boss outside diameter ran at about twice the screw diameter, and the same guide suggests 2.0 to 2.4 times the screw diameter for the outside, an inner hole near 0.88 times the screw diameter, at least 2.3 times the screw diameter in engagement, and a counterbore lead-in that centers the screw and moves hoop stress off the top of the boss, which is where stress cracks usually start.

Tightening torque deserves a line of its own. The guide sets it at 1.2 times the driving torque or more while staying below half the stripping torque, warns that self-drilling and self-piercing screws should not be used in these thermoplastics at all because they raise hoop stress, and adds that a boss cap is the usual answer when the joint needs more torque than a bare boss tolerates.

A metal insert does not remove the mass.

A molded-in insert still sits inside a heavy wall, and the same guide warns that the plastic shrinks onto cold metal and holds residual stress as a result, which makes polycarbonate and its blends the wrong place to depend on one.

What ribs and bosses add to the tool and the cycle

Rib and boss details are paid for twice: once when the mold is cut and again in the maintenance schedule. Deep, narrow pockets are often finished by EDM or with long small-diameter cutters, each carrying its own draft and radius so the steel can be machined at all.

Venting is where thin details bite back.

Ribs, bosses and the weld lines they create are gas traps, and Plastics Technology's tooling column on venting lists deposit buildup, erosion, splay, burns, short shots and inconsistent gloss as the symptoms that follow, with bosses and ribs singled out as details where venting can be critical rather than optional.

The same column gives numbers to work with: a ring vent ground around a pin or sleeve with a 0.040 to 0.060 in land beats the older trick of grinding flats on the side of the pin, porous inserts such as Porcerax need the right machining and a cleaning routine or they plug, and ejector pins added near the problem rarely help, because they sit on nominal wall stock rather than at the end of the detail where gas collects.

Gas has to escape somewhere.

Where the detail is a rib or boss on a core pin or insert, the guidance caps the vent land at 0.030 to 0.040 in and pushes depth toward the maximum the resin allows, so the vent stays close to self-cleaning. One case reported there: a PC lens with a rib detail got a vent track cut 0.030 in from the detail and 0.002 in deep, and needed cleaning only at the 100,000-cycle maintenance interval.

Cooling and cycle time belong in the same bill. Every rib and boss is extra mass that has to freeze before the part ejects, and the steel around a deep rib is harder to cool than a flat plate, so a part covered in fine detail can hold the press longer than its nominal wall suggests.

None of that appears on a design drawing.

Model the features, then read the range

The cost estimator behind /quote reads the uploaded geometry and returns a first-pass range for tool and part cost with its assumptions listed, so ribs and bosses belong in the model the way they will be molded, not the way they were sketched. Treat that range as a budget for planning rather than a supplier's quotation, and expect a toolmaker to confirm rib thickness, radii and venting against the resin and the cooling layout before a price becomes firm.