Flow velocity and flow rate of injection molding cooling water channel under stable turbulent flow
The flow velocity and flow rate in injection molding cooling channels under stable turbulent conditions are key parameters for ensuring cooling efficiency. Turbulent flow (Reynolds number > 4000) breaks down boundary layer thermal resistance, increasing heat transfer efficiency 3-5 times higher than laminar flow (Re < 2000). The superiority of turbulent flow has been demonstrated by reducing the cooling time of a mold from 35 seconds to 20 seconds. Flow velocity is crucial for achieving turbulent flow, typically requiring 1.5-3 m/s. Too low a velocity leads to laminar flow (e.g., < 1 m/s at Re = 2000). For example, a mold for an ABS part suffered uneven cooling and warpage of 0.7 mm due to a flow velocity of only 0.8 m/s. Increasing the velocity to 2 m/s (Re = 5000) reduced the warpage to 0.2 mm. There is a corresponding relationship between flow rate and flow velocity (Q=v×A, where A is the cross-sectional area of the water channel). A water channel with an 8mm diameter (A=50.27mm²) has a flow rate of approximately 3.6m³/h at a flow rate of 2m/s. A water pump with sufficient power (such as 1.5kW) is required to maintain stability.
The matching of water channel diameter and flow rate must be determined through hydraulic calculations to avoid excessive pressure loss due to undersized pipe diameters. At the same flow rate, the longitudinal resistance of a 6mm diameter water channel is five times that of a 10mm diameter channel (Darcy’s equation). A multi-cavity mold using 6mm water channels experienced a terminal pressure loss of 0.5 MPa, a flow rate drop to 1.2 m/s (laminar flow), and a 40% decrease in cooling efficiency. Switching to 8mm water channels reduced the pressure loss to 0.2 MPa, maintained a flow rate of 2 m/s, and restored cooling uniformity. For complex water channel systems, a variable diameter design is required, with a main channel diameter of 10-12 mm and branch channels of 8 mm to ensure a flow rate deviation of less than 10% between branches. For an automotive instrument panel mold, through variable diameter optimization, the cooling time difference between different zones was reduced from 3 seconds to 0.5 seconds. The upper limit of flow rate is limited by the water pump capacity and mold sealing, usually not exceeding 3m/s. Too high a flow rate will lead to increased wear of the seals. In one case, when the flow rate was 3.5m/s, the life of the seal ring was reduced from 10,000 molds to 3,000 molds, and the overall cost actually increased.
Flow distribution must adhere to the “equal flow” principle. Diversion design ensures sufficient cooling medium for each cooling zone, which is particularly important for large molds. A washing machine drum mold (1200×600mm) utilizes four independent cooling circuits, each designed for a flow rate of 2.5 m³/h. Flow control valves maintain a deviation of ±0.1 m³/h, and the temperature difference between zones is less than 3°C. If the flow rate in a particular zone drops to 1.8 m³/h, the temperature in the corresponding zone rises by 5°C, resulting in noticeable shrinkage in the plastic part. Flow monitoring can be achieved using turbine flowmeters installed at each branch outlet, displaying real-time flow values and comparing them to setpoints. This approach has enabled one production line to control flow fluctuations to within ±5%, improving cooling time stability to 95%. For hot runner molds, a separate cooling circuit for the hot nozzle is designed, with a flow rate of 0.5-1 m³/h, to ensure nozzle temperature stability. Insufficient nozzle cooling flow (0.3 m³/h) on a PET preform mold caused bottle neck deformation. Increasing the flow rate to 0.7 m³/h eliminated the defect.
Maintaining stable turbulent flow requires consideration of the cooling medium’s physical properties, including temperature, viscosity, and cleanliness. Increasing water temperature reduces viscosity (water at 25°C is 30% more viscous than at 5°C). At the same flow rate, the Reynolds number increases with increasing water temperature. For example, in summer (water temperature 30°C), a mold can achieve a Re = 5000 at a flow rate of 2 m/s, while in winter (water temperature 10°C), a flow rate of 2.2 m/s is required to achieve the same Reynolds number. Therefore, the pump frequency must be adjusted seasonally. Impurities in water (such as rust and scale) increase roughness, leading to increased resistance along the flow path. In one mold, scale buildup (0.5 mm thick) in the water channel caused the flow rate to drop from 2 m/s to 1.5 m/s at the same flow rate. Regular cleaning with citric acid (every three months) is required to restore the flow rate to the designed value. For high-precision molds, the use of deionized water or antifreeze is recommended to reduce scale formation. For an optical lens mold, the water channel maintenance cycle was extended from three months to one year after using deionized water.
Optimizing flow rate and flow rate requires a combination of mold trials and simulations to find the right balance between energy consumption and cooling efficiency. For a mold, the initial flow rate was set at 2.5 m/s (flow rate 4 m³/h) and a cooling time of 25 seconds. Simulation analysis revealed that a flow rate of 2 m/s (flow rate 3.2 m³/h) could meet the cooling requirements, increasing cooling time by only 1 second while reducing pump energy consumption by 20%. For multi-cavity molds, a “zoned flow control” strategy can be employed, increasing the flow rate to 2.5 m/s in thick-walled areas and maintaining it at 2 m/s in thin-walled areas. This approach reduced the cooling time difference between cavities in an 8-cavity connector mold from 1.5 seconds to 0.3 seconds, with product weight variation of < 0.5%. Flow rate and flow rate can be verified by capturing the mold surface temperature distribution with an infrared thermal imager. Turbulent flow results in a uniform temperature distribution (variation < 3°C), while laminar flow results in distinct hot spots (variation > 8°C). In one case, thermal imaging revealed a hidden blockage in a waterway, which was promptly cleared to restore stable turbulent flow.
