Calculation of the total length of the injection molding cooling circuit
Calculating the total length of the injection molding cooling circuit is a critical step in mold cooling system design, directly impacting the cooling efficiency and molding cycle of the molded part. A reasonable cooling circuit length ensures rapid and even cooling of the melt within the mold, reducing defects such as warpage and sink marks caused by uneven cooling, while also improving production efficiency. Calculating the total cooling circuit length requires comprehensive consideration of factors such as the part’s size, shape, plastic properties, mold structure, and the flow of the cooling medium. Through scientific formulas and parameter settings, a length value that meets cooling requirements while maintaining an economical and reasonable value can be determined.
Calculations based on part volume and cooling efficiency are the basis for determining the total cooling circuit length. Generally, the total cooling circuit length is positively correlated with part volume. Larger parts require a larger cooling area, and therefore a longer cooling circuit length. The calculation formula is: L = (V × K) / (A × v), where L is the total cooling circuit length (m), V is the part volume (cm³), K is the cooling coefficient (depending on the plastic type; for example, the K value for PE is 0.8-1.2, and the K value for PC is 1.5-2.0), A is the cross-sectional area of the cooling water channel (m²), and v is the flow rate of the cooling medium (m/s). For example, for a PE plastic part with a volume of 500 cm³, if the cooling water channel diameter is 10 mm (a cross-sectional area of approximately 7.85 × 10^-5 m²) and the cooling medium flow rate is 1.5 m/s, the total cooling circuit length L = (500 × 1.0) / (7.85 × 10^-5 × 1.5) ≈ 42462 m. This is obviously unrealistic. Therefore, in actual applications, adjustments must be made based on the shape complexity of the plastic part and the number of mold cavities. Generally, the total cooling circuit length for single-cavity molds should not exceed 10 m. For multi-cavity molds, the length can be increased proportionally based on the number of cavities.
The impact of plastic thermal properties on cooling circuit length cannot be ignored. Different plastics have varying specific heat capacity, thermal conductivity, and crystallinity. These parameters directly determine the amount of heat released during the cooling process, which in turn influences the calculation of the total cooling circuit length. For example, crystalline plastics (such as PE and PP ) release a significant amount of latent heat of crystallization during cooling, requiring a longer cooling circuit to dissipate this heat. Their total cooling circuit length is typically 20 %-30% longer than that of amorphous plastics (such as PC and PMMA ) . Furthermore, the lower the thermal conductivity of a plastic (for example, PS has a thermal conductivity of 0.13 W/(m・K) ), the slower the heat transfer and the longer the required cooling circuit length. Plastics with higher thermal conductivity (such as PA6 with a thermal conductivity of 0.24 W/(m・K) ) can appropriately shorten the cooling circuit length. During calculations, the cooling factor K should be adjusted based on the specific plastic’s thermal properties to ensure accurate results.
The mold cavity layout and cooling circuit arrangement can affect the total cooling circuit length requirements. For single-cavity molds, the cooling circuits are typically evenly distributed around the cavity. The total circuit length is primarily determined by the cavity’s circumference and depth. For example, the total cooling circuit length for a rectangular cavity can be calculated as 1.5-2 times the cavity’s circumference, with appropriate increase based on the depth requirements. For multi-cavity molds, if a series cooling circuit is used, the total length is the sum of the cooling circuit lengths for each cavity. If a parallel cooling circuit is used, the total length is the cooling circuit length for a single cavity. However, the length and resistance of each branch must be roughly consistent to avoid uneven cooling. Furthermore, if the plastic part has thick ribs, bosses, or other structures, additional local cooling circuits (such as water wells and jet pipes) may be required in these areas. The length of these additional circuits should also be factored into the total cooling length. Typically, the length of a local cooling circuit is 3-5 times the height of the component in question.
The characteristics and flow patterns of the cooling medium significantly influence the calculation of the total cooling circuit length. The flow rate, velocity, and inlet temperature of the cooling medium (such as water or oil) affect its heat absorption capacity, which in turn influences the cooling circuit length. Typically, the cooling medium’s flow rate should be controlled between 1 and 3 m/s to ensure turbulent flow (Reynolds number > 4000 ). Turbulent flow results in a higher heat transfer coefficient (reaching 1000-3000 W/(m²・K) ), allowing more heat to be removed within a shorter circuit length. If the flow rate is too low, the medium will enter a laminar state ( Re < 2000 ), resulting in a lower heat transfer coefficient (only 100-500 W/(m²・K) ), requiring a longer cooling circuit. When calculating the total cooling circuit length, adjust the flow rate and heat transfer coefficient based on the medium's flow rate and heat transfer coefficient. For example, increasing the flow rate from 1 to 2 m/s can increase the heat transfer coefficient by approximately 50% and reduce the total cooling circuit length by approximately 30%. At the same time, the temperature difference between the inlet and outlet of the cooling medium should be controlled within 5-10℃. If the temperature difference is too large, it means that the cooling circuit is not long enough to remove the heat in time and needs to be extended appropriately.
Empirical correction and verification in actual production are important steps to ensure the appropriate total cooling circuit length. The theoretically calculated total cooling circuit length needs to be revised based on actual production experience. For example, for complex-shaped plastic parts, the actual required total length is often 10%-20% longer than the theoretically calculated value due to the presence of cooling dead zones. For thin-walled plastic parts (wall thickness <1mm), the total length can be appropriately shortened due to the rapid cooling rate. During the mold trial phase, temperature sensors are used to measure the cooling temperature of various parts of the plastic part. If there are areas with excessively high temperatures, the cooling circuit length in that area is insufficient, and additional local circuits are required. If the overall cooling time is too long, the total length may be too long or the medium flow rate is too low, requiring the circuit to be appropriately shortened or the flow rate to be increased. Furthermore, the total cooling circuit length can be simulated and verified using mold flow analysis software such as Moldflow. Based on the simulation results, the circuit layout and length can be adjusted to ensure that the cooling effect meets production requirements.
