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.
Cooling Time
Meaning
Injection moulding cycles include a dedicated duration during which the molten thermoplastic resides within the closed mould to solidify sufficiently for ejection. This duration, known as cooling time, directly affects both the production rate and the final dimensional stability of the moulded part. It starts after the holding phase terminates and ends when the mould opens.
The cooling period must be long enough to prevent the part from warping or collapsing under the force of the ejector pins.
Cycle Control
Thermal diffusivity of the selected polymer resin determines the speed at which heat transfers from the molten core to the cooled metal cavity. Because semi-crystalline resins like polypropylene release latent heat during crystallization, they require a longer cooling time than amorphous resins of identical thickness. Excessively long cooling periods increase the part cost by extending the cycle time.
Conversely, insufficient cooling leads to post-moulding shrinkage and structural distortions.
Moulding Defect
Thermal stress accumulation during rapid solidification causes localized structural variations in the polymer matrix. Short cooling times exacerbate these stresses, resulting in sink marks and void formation in thick sections. The polymer molecule chains lack the necessary time to relax, frozen into a non-equilibrium state that yields high internal tension.
This tension releases slowly over time, causing unexpected dimensional drift weeks after production.
Optimization Strategy
Mould designers employ conformal cooling channels to optimize heat extraction from the cavity. These channels follow the geometry of the part closely. This layout reduces cooling time.
Uniform heat removal prevents differential shrinkage and warping.