Why stiffness comes from geometry, not thickness
When a plastic or metal part feels flimsy, the instinct is to thicken the wall. Resist that urge. Adding uniform wall thickness multiplies material cost, cycle time, and weight, and in molding it invites sink marks and warping. Real stiffness is a function of geometry: a thin wall reinforced by ribs and supported by properly sized bosses is far stiffer per gram than a thick, featureless slab.
This guide treats bosses and ribs as the two workhorse features that let you hit strength targets without paying for mass. A boss is a raised cylindrical post used for mounting, fastening, or locating; a rib is a thin vertical wall that carries bending load away from a broad face. Used together and sized correctly, they let a 2 mm wall perform like a 4 mm wall on the span that matters.
What a boss does and where it fails
A boss gives you a threaded hole, a screw post, or a locating pin seat without molding or machining a solid block. In injection molding it is the cheapest way to create a fastening point; in CNC it is a turned or milled post that replaces a heavier bossed plate. The classic failure is a boss sized like the wall around it, which becomes a local thick section that cools last and sinks.
Bosses also concentrate stress at their base where they meet the main wall. Without a fillet and without gusset ribs, a tall boss acts like a cantilever and can crack at the root under assembly torque or drop impact. The design rules below exist to keep the boss strong where it must be, while keeping its cross-section thin enough to cool evenly.
Boss sizing rules that survive molding
A reliable starting point: make the boss outer diameter about twice the screw diameter, and keep the wall thickness of the boss near the nominal wall thickness of the part. If the screw is M3, the boss outside diameter lands near 6 mm. The hollow core that the screw occupies need not be solid; in molding, a solid boss that wide is exactly what causes sink.
Add a generous fillet at the base where the boss meets the wall, and provide a small relief gap or a spoke or knockout pattern in the core to reduce mass. On tall bosses, add two or three gusset ribs radiating from the base for support. Give the boss a draft angle of about 0.5 to 1.5 degrees per side so it releases cleanly from the tool or fixture.
Rib fundamentals and the thickness rule
Ribs follow one number more than any other: thickness. As a rule of thumb, a rib should be 0.5 to 0.7 times the nominal wall thickness. At 0.5x the rib adds stiffness with almost no risk of a visible sink on the opposite face; pushing toward 0.7x is acceptable for higher load but demands care with cooling and gating.
The reason is thermal. A rib that is too thick is a local hot mass that shrinks after the surrounding wall has already solidified, pulling the surface inward and leaving a sink mark. Keeping the rib thin means it solidifies at the same rate as the wall, so the face stays flat. Never make a rib thicker than the wall it reinforces; that inverts the mass balance and guarantees defects.
Rib layout, spacing, height and taper
Place ribs to carry load along the direction of bending. For a flat panel, run ribs perpendicular to the span so they act like the webs of an I-beam. Space them so that the unsupported wall between ribs does not buckle or oil-can; a spacing of roughly 3 to 5 times the wall thickness is a practical range, adjusted for the material's modulus.
Taper the rib height down toward its free edge or round the top to a small radius rather than leaving a sharp knife edge that flashes or breaks. Keep the rib top at least one wall thickness below the opposing surface so it never touches and locks the tool. Like bosses, give ribs a draft angle so they demold without dragging the surface.
Wall transitions and stress concentration
Where a boss or rib meets a wall, you are joining a thick feature to a thin one, and that junction is a stress concentrator. A sudden step change in thickness causes a sharp corner internally that becomes the starting point for a crack under cyclic load. The fix is geometric: ramp the thickness gradually over a length of several times the thickness delta.
A well-radiused, gradually ramped transition can cut peak stress concentration by roughly half compared with a hard step. This matters most for metal parts under vibration or for plastic parts that see repeated assembly and disassembly. The same principle applies at the base of gussets and at the junction between two ribs of different heights.
Common defects and how design prevents them
Sink marks are the headline defect: they appear as dimples opposite thick bosses or heavy ribs because that mass shrinks as it cools. The cure is never to add more cooling time alone; reduce the local mass with a thinner boss wall, a cored center, or shorter ribs. Warping follows uneven wall mass around the part, so balance sections so the whole part shrinks uniformly.
Short shots and voids show up when a thick section starves the flow or traps air. Air entrapment, or gas trap, burns the surface or leaves a void; tapering ribs, adding venting, and placing gates to feed thick areas first all help. In CNC metal parts the equivalent problems are chatter and tool deflection at thin ribs, solved with sharper tooling and balanced stock removal.
Plastic versus metal: the same features, different rules
In injection-molded plastics, sinks and warpage dominate the design conversation because the material shrinks several percent as it cools and is relatively soft. Boss and rib rules there are about controlling that shrinkage. In die-cast metals the shrinkage is smaller but the part is heavier, so ribs must still be modest or the casting traps gas and the tool wears unevenly.
For CNC-machined metal parts, bosses and ribs are about machinability and material removal rather than shrinkage. A thin rib on a milled aluminum bracket is easy to cut but flexible if too tall; a deep pocket between ribs can require long-reach tooling and slower feeds. The geometry logic is the same, but the failure mode shifts from sink to deflection and chatter.
Prototype, load and iterate
Do not trust the drawing alone; build a sample and load it the way the real part will be used. A 3D-printed or CNC-machined prototype reveals whether a boss cracks under insertion torque or a rib deflects more than the gap allows. Because the part is cheap to revise, this is the stage to find the weak spot.
Adjust one variable at a time: boss height, rib count, rib thickness, or fillet size. Watch for sink on the opposite face and for any new stress concentration the change introduced. A short design-of-experiments loop of three or four prints is far cheaper than discovering the defect after a steel mold is cut.
A practical decision checklist
Before you thicken any wall, run this checklist: Is the stiffness problem localized to one span or mounting point? Can a rib or boss reach that location without blocking demold or flow? Is the local wall mass kept below about 0.7x of the nominal to avoid sink? If yes, add features instead of thickness.
Thicken the wall only when stiffness is needed everywhere at once, or when a rib would interfere with function or appearance. Remember the trade: features add tooling complexity but save material and weight; uniform thickening adds material, cycle time, and defect risk with little gain in the right direction.
How FabProto supports your design
FabProto reviews your CAD for boss and rib issues before any part is cut. Our DFM feedback flags likely sink marks, warping risk, and stress concentrations, and suggests rib thickness, boss diameter, and draft angles tuned to your material and process, whether that is injection molding, die casting, or CNC machining.
We can prototype the same geometry in 3D printing or CNC to validate load behavior, then carry the proven design into molding with rapid tooling. That loop lets you prove the stiffness you designed for, not just the shape you drew, before committing to production tooling.