Injection-moulded features are dimensioned as ratios to the nominal wall,
not as absolute sizes. The reason is a single mechanism: a moulding cools outside-in, so any local
thickening keeps a molten core longer than its surroundings, and when that core shrinks it pulls
the visible surface in behind it. Everything below follows from that.
Ribs, bosses and gussets
Feature
Guideline
Source
Why
Rib thickness at the wall
50–60% of nominal wall
GE Plastics
Above 60% expect visible sink on the opposite face
Maximum rib height
3 × nominal wall
GE Plastics
Taller behaves as a column — add a cross-rib instead
Rib base fillet
25% of nominal wall
GE Plastics
Minimum 0.5 mm (Eurotec); enough to kill the stress riser, not enough to rebuild the thick section
Rib draft
0.5° per side
Eurotec
Remember draft thins the rib as it rises
Rib spacing
≥ 2 × nominal wall
GE Plastics
Closer and the cooling zones overlap
Boss wall at the base
< 60% of nominal wall
GE Plastics
Core it out — a solid boss is a thick section by definition
Boss base radius
25–50% of nominal wall
GE Plastics
Floor of 0.015 in (0.381 mm)
Boss spacing
≥ 2 × nominal wall
GE Plastics
Gusset thickness
50% of nominal wall
GE Plastics
Strengthen with gussets, never by thickening
Gusset height
≤ 95% of boss height, generally < 4 × nominal wall
GE Plastics
Draft and texture
The texture rule is the one most often missed — a moderately textured sidewall can need four
degrees where the drawing says one.
Item
Guideline
Source
Minimum draft in the draw
0.5°
GE Plastics
Typical draft
1°
GE Plastics
Extra draft for texture
+1° per 0.001 in (0.025 mm) of texture depth
GE Plastics
Recommended minimum (general)
1°
Eurotec
Offset cost
depth × tan(draft), per side
geometry
Self-tapping screw bosses
Pilot hole as a fraction of the screw's nominal diameter, by thread form (Covestro):
Screw type
Pilot hole
Note
Coarse thread for plastics (e.g. Plastite, Delta PT)
Di = 0.86 × D
the usual starting point
Finer thread for plastics
Di = 0.89 × D
Thread-cutting screw
Di = 0.90 × D
Machine-screw form in tough grades
Di = 0.92 × D
Item
Guideline
Note
Boss outside diameter
2.0–2.5 × screw diameter
below 2× the boss splits from thread-forming hoop stress
Thread engagement
2.0–2.5 × screw diameter
Plastite / TR Fastenings
Maximum tightening torque
0.5 × stripping torque
Covestro — and both torques must be MEASURED
Strip-to-drive ratio
≥ 5 : 1
below this there is no room for tool scatter on a line
Living hinges
Item
Guideline
Web thickness, typical
0.008–0.010 in (0.20–0.25 mm)
Web thickness, thick end
0.015 in (0.38 mm) — orientation starts to fall away
Land length
about 0.060 in (1.5 mm)
Material
Polypropylene homopolymer, unfilled
Process requirement
Flex immediately after moulding, while warm — this cold-draws and orients the web
Gate direction
Flow ACROSS the hinge, never along it
Ultrasonic weld joints
Item
Guideline
Energy director, single
90° included angle
Energy director, multiple
about 60% of single size, 60° included angle
Director peak
Must be sharp — round or flat and the weld will not initiate
Shear joint
Controlled interference melted as the parts telescope; hermetic, needs rigid well-aligned parts
Flash trap / witness gap
0.005–0.020 in (0.127–0.508 mm)
Run any of these on Rib & Boss,
Draft & Texture, Screw Boss,
Living Hinge or Ultrasonic Weld Joint, and
read the reasoning in the interactive guide.
Sources: GE Plastics Engineering Thermoplastics Design Guide, Eurotec Part Design
Guide, Covestro Self-tapping screws for thermoplastics, Plastite/TR Fastenings
Thread Forming Screws Technical Guide, Branson Ultrasonic Plastic Joining PW-3.
Every figure is a starting point a moulder will refine for your resin, tool and gate position.
A reference aid, not a substitute for engineering judgment. Verify against the governing standard.