Wall Thickness Principles for Injection Moulding Tool Design
Uniform wall thickness design balances polymer flow, prevents shrink defects, cuts cycle times, and reduces landed part cost in injection moulding tools.
Rib Thickness Ratio
Meaning
Design proportions defining internal stiffening feature wall thickness relative to nominal part wall thickness dictate structural stiffness and cosmetic sink resistance. Maintaining a rib thickness ratio between fifty and sixty percent for unreinforced resins prevents mass concentrations that cause surface depressions on opposing cosmetic faces. The geometric rule ceases to apply when structural foams or gas-assisted injection moulding methods are used to hollow out rib intersections.
Polymer datasheets list mechanical flexural modulus but omit feature design guidelines, which depend on part geometry and resin volumetric shrinkage characteristics. Proper ratio selection avoids sink marks while adding required moment of inertia to flat surfaces.
Sink Minimization
Thick rib bases contain localized material mass that cools slower than adjacent nominal walls. Shrinkage forces pull frozen surface skins inward, creating cosmetic depressions unless rib thickness stays below recommended ratios.
Structural Stiffness
Taller, thinner ribs increase section moment of inertia more effectively than increasing nominal part wall thickness. Adding multiple thin ribs preserves low cycle times while providing high structural bending resistance.
Tooling Geometry
Draft angles added to rib walls reduce thickness further toward the top tip while widening the rib root base. Tool designers balance draft requirements against sink depth limits when cut core slots into mold cavities.