Wall Thickness & Ribs
Uniform walls, rib ratios and boss design that prevent sink, warp and failure
Wall thickness is the single most influential molding variable: it sets cooling time (cycle cost), stiffness, shrinkage and sink risk. Uniform walls (1–4 mm) are the rule; ribs stiffen without thickening the wall, sized at 0.5–0.6× wall with height ≤ 3× wall; bosses need root thickness 0.5–0.6× wall and support ribs. Getting these ratios right prevents the classic defect trio: sink marks, warpage and short shots.
1. Why walls must be uniform
Thick sections cool slower, shrink more and pull material from neighbors — creating sink marks opposite the thick area and internal voids. Cycle time scales roughly with the square of wall thickness: a 3 mm wall cycles nearly twice as long as a 2 mm wall at the same area.
Target 1.0–4.0 mm for engineering plastics (larger for large parts needing stiffness, thinner for small high-cavity parts). Transition thickness changes gradually (≥ 3:1 ratio) with a fillet to avoid stress concentration.
2. Ribs: the stiffness tool
A rib stiffens a wall without thickening it: rib thickness 0.5–0.6× the adjoining wall (thicker ribs sink opposite), height ≤ 3× rib thickness (taller ribs need draft and risk short shots), and a fillet radius at the base ≥ 0.25–0.5 mm. Rib spacing ≥ 2× rib thickness prevents sink between ribs.
For stiffness, an X or grid of low ribs beats one tall rib: tall ribs require high injection pressure and trap air. In machining/printing the rules relax (no sink), but thin tall features still risk vibration and breakage — same ratios apply for robustness.
3. Bosses and inserts
A boss (standoff for screws) should be 0.5–0.6× wall at its root with 0.5 mm minimum wall, supported by ribs tied to the nearest wall, and gusseted at the base to prevent sink and cracking. Boss outer diameter ≈ 2× the screw size is a practical start.
For threaded inserts (heat-stake, ultrasonic), boss wall ≥ 0.6–0.8× insert diameter wall; leave clearance for the insert and mold-in or post-install per the insert manufacturer tables.
4. Checking your part
Before quoting, verify: max:min wall ratio ≤ 3:1 (except small local bosses), rib thickness ≤ 0.6× wall, boss root ≤ 0.6× wall, draft 1–2° on all vertical walls, and radius at every transition. The platform DFM flags wall-ratio and rib violations with suggested fixes.
When wall thickness must vary (hinge area thin, boss area thick), place the thick section nearest the gate and keep transitions gradual.
Quick reference
| Feature | Rule |
|---|---|
| Wall (molded) | 1.0–4.0 mm, uniform |
| Max:min wall ratio | ≤ 3:1 |
| Rib thickness | 0.5–0.6× wall |
| Rib height | ≤ 3× rib thickness |
| Rib spacing | ≥ 2× rib thickness |
| Rib base fillet | ≥ 0.25–0.5 mm |
| Boss root | 0.5–0.6× wall |
| Boss support | Ribs tied to wall |
| Draft | 1–2° all vertical faces |
| Transitions | Gradual, filleted |
Sources & references
- DIN 16742 — Plastic tolerances
- ISO 294 — Molding properties
- Xometry Learning Center — Walls & Ribs (public engineering reference)