Meaning
High-velocity fluid movement within the cooling channels of an injection mould maximizes the rate of heat extraction from the molten polymer by constantly mixing the coolant and preventing a stagnant insulating layer from forming against the steel. This turbulent coolant flow is essential for achieving short cycle times and uniform part cooling.
Heat Transfer
Cooling water must flow with sufficient velocity to generate random eddies that carry heat away from the hot mould surfaces. In contrast to laminar flow where the fluid moves in smooth parallel sheets with low heat transfer rates, the chaotic movement of turbulent coolant flow dramatically increases the convective heat transfer coefficient. This rapid cooling is necessary to solidify the resin quickly so the part can be ejected without distortion.
Performance Standard
Achieving this optimal flow state requires a flow rate that exceeds the critical Reynolds number for the cooling channel diameter and fluid temperature. While a datasheet for a chiller might specify a high nominal flow rate, the actual flow through the mould depends on the restriction of the cooling circuit and the viscosity of the fluid. Molders monitor pressure drops across the mould to ensure that the required velocity is maintained throughout the production run, using flow meters and pressure gauges to verify turbulent conditions in every circuit.
Processing Defect
Insufficient cooling flow rates lead to extended cycle times, uneven cooling, and severe part warpage as different areas of the part shrink at different rates. These thermal gradients can also cause internal stresses that lead to premature part failure under mechanical load. Maintaining turbulent conditions across all circuits ensures dimensional consistency and reduces scrap rates.
