Almost every rule in plastic part design is a ratio to the nominal wall, and almost all of them come from one mechanism. Understand that mechanism and you can derive most of the rules yourself; miss it and they look like a list of arbitrary numbers to memorise.
The mechanism: why thick sections sink
A moulding cools from the outside in. The skin freezes against the tool first, and the core stays molten behind it. Anywhere the section is locally thicker — a rib, a boss, a gusset — that core stays molten longer than its surroundings. When it finally cools it shrinks, and since the frozen skin cannot follow, it pulls the visible surface in behind it. That dimple is a sink mark. Thicker still and the core cannot pull the skin at all, so it tears itself apart internally instead and you get a void.
So the rules are not about strength. They are about keeping every section close enough to the nominal wall that it freezes at roughly the same time. That is why the answer to a weak rib is never a fatter rib.
A moulded rib flexes more than you want, so you make it 30% thicker. What happens on the visible face opposite the rib?
The thicker rib holds a molten core for longer than the wall around it. When that core finally shrinks, the already-frozen skin cannot follow, so it is pulled inward — a dimple on the show face, exactly opposite the rib. Ribs buy stiffness through height, which goes as the cube, not thickness, which only buys sink. The answer to a weak rib is a taller rib or a second rib, never a fatter one.
Try it — rib thickness and sink
The shaded blob is the slow-cooling core. Push the rib past 60% of the wall and watch it swell — that is the material that will pull the opposite face in as it shrinks.
Ribs: stiffness without material
A rib buys stiffness because bending stiffness goes as the cube of depth. Doubling a rib's height is worth eight times as much as doubling its thickness — and costs nothing in sink. So the numbers fall out: keep the rib 50–60% of the nominal wall where it meets the wall, and up to 3× the wall in height. Past that height it behaves as a column and wants a cross-rib rather than more height.
Fillet the base at about 25% of the wall — enough to kill the stress riser, not so much that you have recreated the thick section you were avoiding. Space ribs at least 2× the wall apart, and give them 0.5° of draft per side, remembering that draft thins the rib as it rises.
Bosses: the same rule wearing a different shape
A boss is a rib rolled into a tube, and it obeys the same limit: keep the wall at the base under 60% of the nominal wall. A solid boss is a thick section by definition, so core it out. When it needs more strength, add gussets — 50% of the wall thick, no taller than 95% of the boss — rather than thickening it. Radius the base 25–50% of the wall, never below 0.015 in (0.381 mm), and keep bosses 2× wall apart from each other.
Draft: and the rule everybody forgets
Every surface in the direction of draw needs draft — half a degree minimum, one degree typical. Then comes the part people miss: texture needs its own draft on top, one additional degree for every 0.001 in (0.025 mm) of texture depth. A moderately textured sidewall therefore wants three or four degrees, not one. Too little and the texture drags on ejection, scuffing the surface it was supposed to decorate.
Try it — draft, texture and what it costs
Add texture and watch the required angle climb away from the 1° you probably drew. The wedge shows what the draft actually costs you in wall over the draw.
Screw bosses: a very narrow window
A self-tapping screw in plastic forms its own thread, and the boss has to survive the hoop stress of that forming. Hole too small and the boss splits; hole too large and the thread strips. The pilot hole is a fraction of the screw diameter set by the thread form — 0.86 × D for a coarse plastics thread, up to 0.92 × D for a machine-screw form in a tough grade. Boss outside diameter wants 2.0–2.5 × the screw diameter; below 2× it splits.
Then the part that decides whether it works on a line: the strip-to-drive ratio. Driving torque is what it takes to form the thread; stripping torque is what destroys it. The gap between them is your entire process window, and below about 5:1 there is not enough room for normal tool scatter. Never tighten past half the stripping torque. Both source documents insist these be measured rather than calculated, because they depend on resin, moulding conditions and the exact screw.
Try it — the boss splitting window
Section through the boss: the hole, the material left around it, and the 2× and 2.5× diameter markers. Shrink the boss below the inner marker and the ring goes red — that is where the hoop stress from thread forming splits it.
Living hinges: it is not about bendiness
A living hinge works because polypropylene is cold drawn on its first flex. That draw orients the molecules across the web and multiplies the tensile strength along the fold, giving a well-oriented hinge virtually unlimited fold life. The consequences are practical and absolute: flex it immediately after moulding, while it is still warm, or it never develops that life. An identical shape machined from stock has none of it.
Web thickness sits in a narrow window — around 0.008–0.010 in (0.20–0.25 mm), with 0.015 in the thick end where orientation starts to fall away. Land length about 0.060 in (1.5 mm). Gate so the melt flows across the hinge, never along it, and use unfilled polypropylene homopolymer — glass fibre destroys fold life, because the fibres cannot orient with the matrix.
Ultrasonic welding: the joint is the design
Two families cover nearly everything. An energy director is a raised triangular bead moulded on one face; it concentrates the ultrasonic energy so melting starts at a point and the melt then flows through the joint. Standard practice is a single director at a 90° included angle, or several smaller ones at about 60% of that size and 60°. The peak must be sharp — round or flat it and the energy spreads out and the weld never initiates.
A shear joint instead melts a controlled interference progressively as the parts telescope together. It gives a strong hermetic weld, but it needs rigid, well-aligned parts: a flexible wall just deflects instead of melting. Either way, put a flash trap of 0.005–0.020 in (0.13–0.51 mm) anywhere squeeze-out would show.
The short version
- Ribs and bosses at 50–60% of the nominal wall. If it needs to be stronger, make it taller or add a gusset — never fatter.
- One extra degree of draft per 0.001 in of texture.
- Pilot hole 0.86 × D, boss OD ≥ 2 × D, strip-to-drive ≥ 5:1.
- Flex a living hinge while it is warm.
- Sharp peak on an energy director.
Run the numbers on Rib & Boss, Draft & Texture, Screw Boss, Living Hinge and Ultrasonic Weld Joint, and keep the ratio tables to hand. For snap-fits — the other great moulded-in feature — see the snap-fit guide.