Why Not Just Make the Wall Thicker

The instinct when a molded part flexes too much is to add material, and it is the wrong move almost every time. Doubling wall thickness doubles material cost, and because cooling time scales roughly with the square of thickness, it can quadruple cycle time, the dominant term in per-part price. Worse, thick sections shrink unevenly as they cool, producing sink marks, voids, and warp that no process adjustment fully removes.

A rib gets you the stiffness for a fraction of the cost. Bending stiffness scales with the cube of section depth, so a rib standing 3 times the wall height off a panel resists bending far better than any realistic thickening of the panel, while adding a few percent to shot weight. The job is placing ribs so they carry the load and proportioning them so they do not create the defects you were trying to avoid.

The Numbers That Work

These proportions are the industry baseline for unfilled thermoplastics, expressed as ratios to the nominal wall thickness T:

  • Rib base thickness: 0.5 to 0.6 T. The rule that matters most. A thicker rib creates a heavy mass at the junction that cools last and pulls a visible sink mark on the opposite, usually cosmetic, face. On a 0.080 in (2 mm) wall, that means a rib base of 0.040 to 0.048 in.
  • Rib height: 3 T maximum, ideally under 2.5 T. Taller ribs are hard to fill, hard to eject, and prone to buckling. If you need more stiffness, use two shorter ribs rather than one tall one.
  • Rib spacing: at least 2 T apart, preferably 3 T. Closely packed ribs trap heat in the steel between them and make ejection difficult.
  • Draft: 0.5 to 1.5 degrees per side minimum, more for textured surfaces. Draft narrows the rib tip, so check tip thickness after applying it; a tall rib can taper to a knife edge in the tool that is fragile and hard to fill.
  • Root radius: 0.25 to 0.5 T. Enough to relieve the stress concentration and help flow, not so much that it adds mass at the junction. A sharp rib root is a crack starter under impact.

Every one of these derives from the panel wall, so if the nominal wall is not settled, that comes first, per wall thickness for injection molded parts.

Where to Aim the Ribs

A rib only helps if it lies along the direction the part wants to bend; ribs perpendicular to the bending axis do almost nothing. For a flat panel that bows across its short dimension, run the ribs across that dimension. For a box that racks, put ribs diagonally or add corner gussets. For a cantilevered feature, run a rib down the length under the load path. When the load case is not obvious, a quick FEA simulation pays for itself, showing deflection contours and letting you test three rib patterns in an afternoon instead of three tools over six months.

Cross-hatched rib grids look reassuring and are usually inefficient. The intersections create thick junctions that sink, and much of the grid carries no load. A smaller number of ribs in the right orientation outperforms a waffle pattern and molds better.

Bosses: The Other Half of the Problem

A boss is a cylindrical projection that receives a screw, a pin, or an insert. It has the same sink problem in a more acute form, because it is inherently a mass of material attached to a thin wall. Rules that keep bosses out of trouble:

  • Outside diameter about 2 to 2.5 times the screw diameter, with a wall of 0.5 to 0.6 T like a rib, and core out the underside so the boss is a tube rather than a solid post.
  • Never attach a boss directly to a sidewall. The combined mass sinks badly. Stand it off and connect it with thin ribs, or use a keyhole-shaped cross section.
  • Add three or four support gussets from the boss to the floor, at rib proportions, and radius the base at 0.25 T. Bosses crack at the base under assembly torque more often than anywhere else.
  • Size the hole to the fastener strategy. A self-tapping screw needs a specific pilot diameter for the resin, and a heat-set insert needs a stepped hole with a lead-in. Getting this wrong strips the boss on the assembly line; the sizing tables are in threads in plastic parts.

Material Changes the Rules

The 0.5 T guideline is written for unfilled amorphous resins such as ABS and polycarbonate, which shrink modestly and forgive a slightly heavy rib. Semi-crystalline resins, including polypropylene, polyethylene, nylon, and acetal, shrink two to three times as much and show sink far more readily, so drop the rib base to 0.4 to 0.5 T. Glass-filled grades are the opposite case: they shrink less and are stiffer, so a rib at 0.6 to 0.75 T often shows no visible sink and you need fewer of them. Glass fill also makes the part notch sensitive, so root radii matter more rather than less.

Where sink is unacceptable on a cosmetic surface, three tricks help: texture the visible face, add a shallow decorative recess so the eye has no flat reference, or move the rib behind a feature line. Texture is the cheapest answer and one more reason the finish choice belongs early.

Ribs Do More Than Stiffen

Once ribs exist in the part, use them. They make good alignment features between mating halves, act as light blocks between LED zones, serve as crush ribs for press fits, retain PCBs and batteries without extra hardware, and can guide melt to a difficult corner of the cavity. Each is a component you do not have to buy, which is exactly the leverage design for manufacturing is looking for.

Quick Checklist Before Tool Release

Run this list on every ribbed part: rib base at or below 0.6 T everywhere including at intersections; no rib taller than 3 T; draft applied and tip thickness still moldable; root radii present; ribs oriented along the actual bending direction; no boss touching a sidewall; every boss cored and gusseted; cosmetic faces checked against every rib and boss for sink. Then review it with the molder before steel is cut. That review costs an hour and routinely finds two or three items that would otherwise become tool modifications.

One caveat if your first units are printed rather than molded: printed parts have none of the sink or flow constraints, so a printed prototype will happily accept ribs that are unmoldable. Design to the molding rules from the start and treat the separate constraints in design for 3D printing as prototype-stage only.

Get the Structure Right the First Time

Projects House reviews and designs molded part structure as part of mechanical engineering and DFM: rib and boss layout, stiffness analysis, sink prediction on cosmetic surfaces, and a tool-ready model. Send your CAD through our contact form and we will mark up what needs to change before you commit to tooling.