Ribs, bosses and gussets: design rules for moulded parts

Why ribs beat a thicker wall

Bending stiffness in a plate rises with the cube of its thickness, which makes a thicker wall look like the simple answer to a stiffness problem. It is also the expensive one. Doubling the wall multiplies the mass and the cooling time, and it creates a section that sinks and warps. Ribs move material away from the neutral axis instead, so a thin wall with a rib field can carry a bending load that a flat wall of several times the thickness could not match.

The gain comes with conditions. The rib has to be deep enough to matter, it has to be attached without creating a thick junction, and it has to be arranged so the material can flow into it. A rib field that ignores flow produces short shots at the rib tips and weld lines across the visible face.

Think of the section as a T beam: the wall is the flange and the rib is the web. Most of the bending resistance comes from the material furthest from the neutral axis, which is why rib depth contributes far more than rib thickness.

Rib base, height and spacing

The rib base is the first number to fix. At the junction, the rib and the wall together form a local thick section. If the base is as wide as the wall, that junction cools slowly and sinks the opposite face. A base of half to two thirds of the nominal wall keeps the junction cool enough in most materials while still giving a useful web.

Height is limited by flow and by sink at the junction. A rib deeper than about three times the wall thickness is hard to fill at the tip and adds little stiffness per unit of risk. Where more depth is needed, a pair of shallower ribs often performs better than one deep one. The rib tip should be radiused rather than square, and both sides need draft.

Spacing matters for two reasons. Ribs closer than about two wall thicknesses apart leave a narrow channel between them that fills slowly and cools fast, which is where short shots appear. Tight spacing also concentrates material, so the panel between the ribs becomes a thick block in thermal terms. Two to three wall thicknesses between rib centre lines is a practical range.

Lay ribs out along the flow

A rib is a long thin channel. Material entering it from the side has to turn a corner and then fill a narrow section, which is the definition of a hesitation effect: the flow slows, cools and may stop before the tip. Ribs that run in the direction of the main flow from the gate fill far more reliably than ribs that branch off at right angles.

Where ribs must cross, the intersection is a thick node that cools last. That node is where voids and sink appear, and it is also where the part is most likely to warp. Offset the crossings if the design allows, or thin the ribs locally at the junction. A crossed rib grid on a large panel is one of the harder geometries to mould well, and it deserves a flow study before the tool is cut.

Ribs also change the appearance of the opposite face. Even a correctly proportioned rib can leave a faint read-through line on a high gloss surface, because the shrinkage pattern differs slightly over the rib. On a visible cover, move the ribs away from the show face or add a texture that hides the line.

Boss design for screws and inserts

A boss is a local cylinder that takes a screw, a threaded insert or a press-fit pin. Left solid, it is the thickest section on the part and it will sink the face behind it. Cored to a wall of about half to two thirds of the nominal wall, it cools at a rate closer to the surrounding material and the sink risk drops.

The outside diameter is set by the fixing and the load, not by a round number. For a self-tapping screw in an unfilled engineering plastic, a boss outside diameter of roughly two to two and a half times the screw diameter is a common starting point, with the pilot hole matched to the screw and confirmed by the screw supplier. For a moulded-in or heat-set metal insert, the boss must be larger: about two and a half times the insert outside diameter, with enough wall to resist the hoop stress the insert creates.

Height needs care. A boss taller than about three times its outside diameter behaves like a chimney: it fills late, cools unevenly and can crack at the root when a screw is driven. Keep the boss height down, or support it with a gusset. The bore should be drafted, and the bottom of the bore should have a radius rather than a sharp corner.

If the boss sits on a wall, thicken the wall locally under the boss rather than letting the boss stand on a thin skin. A boss that punches through the opposite face on assembly is usually a wall thickness problem at the root, not a screw problem.

Gussets tie a boss back to the structure

A gusset is a small triangular rib that connects a boss to a nearby wall. It carries the bending moment from the screw load into the panel, so the boss does not act as a free-standing column. Without gussets, a tall boss relies on the stiffness of its own root, which is the weakest part of the geometry.

Keep the gusset thickness at about 0.4 to 0.6 of the nominal wall, taper it towards the top, and give it draft on both faces. A gusset that is as thick as the wall reintroduces the sink problem it was meant to solve. Two or three gussets arranged symmetrically are better than one wide one, because they share the load and keep the flow balanced.

The gusset height is usually half to two thirds of the boss height. Running it the full height adds material at the top where it does little, and it can interfere with the mating part or the screw head. Where the gusset meets the wall, add a fillet, because that junction is a stress riser under screw torque.

Inserts, pull-out and assembly loads

A threaded insert concentrates load into a small ring of plastic. The failure modes are pull-out, where the insert and the surrounding material leave the boss, and hoop cracking, where the boss splits along its height. Both are decided by the boss wall, the insert type and the assembly method.

Heat-set inserts melt their way in and reflow around the knurl, which gives good resistance to pull-out and torque. Moulded-in inserts are placed in the tool before shot, which is the strongest option but adds cycle time and tool wear.

For any insert, ask for the insert supplier's recommended boss diameter and bore, then check that the boss wall is thick enough to take the hoop stress. A boss that is correct for a screw can be too thin for an insert of the same thread size. Pull-out and torque tests on the first article are more useful than a dimensional check alone.

The review that catches these problems early

Section the model through a rib, through a boss and through the junction where a gusset meets the wall. In each cut, compare the local section to the nominal wall. If any local section is more than about 1.5 times the nominal wall, expect sink or a void unless the flow study says otherwise.

Then check the fill. Ribs and bosses at the end of flow are the first features to short shot, and they are also the features that need the highest pressure. If the flow study shows the pressure limit is near, the fix is usually a gate change or a shallower feature, not a higher pack pressure.

Finally, look at the assembly. Screw bosses need clearance for the mating boss, space for the driver and enough depth for thread engagement. Many boss problems are found at assembly, but they were designed in at the layout stage.

1. 01

   Section the features

   Cut through the rib base, the boss wall and the gusset junction and compare to the nominal wall.

2. 02

   Check the fill

   Confirm the pressure at the rib tips and the boss tops in the flow study.

3. 03

   Size the fixing

   Match boss diameter, bore and height to the screw or insert supplier data.

4. 04

   Test the joint

   Run pull-out and torque checks on the first article, not just dimensions.