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.
Internal Voids
Meaning
Vacuum bubbles form inside thick structural sections when outer plastic skins freeze while the molten core continues to shrink. Formation of internal voids occurs when packing pressure fails to deliver supplementary melt to compensate for volumetric thermal contraction during solidification. Bubble growth halts when the internal polymer core freezes completely or when internal vacuum pressure balances matrix tensile resistance.
Ultrasonic testing and X-ray computed tomography reveal these hidden cavities, which datasheet tensile values on solid test bars do not predict. Process engineers adjust hold time, boost packing pressure, or add blowing agents to eliminate interior voiding.
Volumetric Deficit
Polymer melts expand during heating and undergo significant volume reduction upon cooling inside the mold. Thick wall geometry isolates the central molten mass, preventing external packing pressure from reaching the interior. Solidification pulls melt outward toward frozen walls, creating hollow centers.
Sink Suppression
Rigid outer skins resist inward collapse, forcing volumetric contraction to manifest as interior vacuum pockets rather than exterior surface depressions. Molders increase cooling time or lower melt temperatures to shrink core dimensions before skin rigidification occurs.
Structural Integrity
Hidden internal air pockets reduce load bearing cross sectional areas and act as stress concentration sites under mechanical fatigue. Parts subjected to high hydraulic or tensile loads fail prematurely when voids reside near high stress regions.