Plastic Part Design — Cheat Sheet
Ribs, bosses, draft and texture, screw bosses, living hinges and ultrasonic joints.
The mechanism
| Why ratios, not sizes | A moulding cools outside-in |
| Thick section | Core stays molten longer than its surroundings |
| Sink mark | Shrinking core pulls the frozen skin in behind it |
| Void | Core too thick to pull the skin — it tears internally instead |
| Consequence | The fix for a weak rib is never a fatter rib |
Ribs
| Thickness at the wall | 50–60% of nominal wall |
| Maximum height | 3 × wall |
| Base fillet | 25% of wall, min 0.5 mm |
| Draft | 0.5° per side |
| Spacing | ≥ 2 × wall |
| Stiffness | Goes as depth³ — height beats thickness ~8:1 |
Bosses & gussets
| Boss wall at base | < 60% of nominal wall — core it out |
| Base radius | 25–50% of wall, floor 0.015 in (0.381 mm) |
| Boss spacing | ≥ 2 × wall |
| Gusset thickness | 50% of wall |
| Gusset height | ≤ 95% of boss height, < 4 × wall |
| Free-standing boss | Tie it back — do not fatten it |
Draft & texture
| Minimum in the draw | 0.5° |
| Typical | 1° |
| Texture penalty | +1° per 0.001 in (0.025 mm) of texture depth |
| Offset cost | depth × tan(draft) per side |
| Example | 40 mm draw at 5° = 3.5 mm per side |
| Too much draft | Moves material into one half — can create a mass |
Self-tapping screws
| Coarse plastics thread | Di = 0.86 × D |
| Finer plastics thread | Di = 0.89 × D |
| Thread-cutting | Di = 0.90 × D |
| Machine-screw form | Di = 0.92 × D |
| Boss OD | 2.0–2.5 × D (below 2× it splits) |
| Engagement | 2.0–2.5 × D |
| Tightening limit | ≤ 0.5 × stripping torque |
| Strip-to-drive | ≥ 5 : 1 for powered assembly |
| Both torques | MEASURE — do not calculate |
Living hinges
| Web thickness | 0.008–0.010 in (0.20–0.25 mm) |
| Thick end | 0.015 in (0.38 mm) |
| Land length | ≈ 0.060 in (1.5 mm) |
| Material | PP homopolymer, unfilled |
| The mechanism | Cold drawn on first flex — orients the molecules |
| Process | Flex immediately after moulding, while warm |
| Gate | Flow ACROSS the hinge, never along it |
| Filled grades | Destroy fold life — fibres cannot orient |
Ultrasonic welding
| Energy director, single | 90° included angle |
| Multiple directors | ~60% size, 60° angle |
| Peak | Must be sharp or the weld will not initiate |
| Shear joint | Interference melted as parts telescope — hermetic |
| Shear joint needs | Rigid, well-aligned parts |
| Flash trap | 0.005–0.020 in (0.13–0.51 mm) |
Quick sanity checks
| Any feature | ≤ 60% of nominal wall |
| Textured sidewall | Did you add the texture draft? |
| Screw boss | OD ≥ 2 × D? |
| Living hinge | Is it flexed warm off the tool? |
| Weld on a visible face | Is there a flash trap? |
Formula reference — verify against the governing standard. Not a substitute for engineering judgment.