Cooling of injection molds
The injection mold’s cooling system is a key factor in determining molding efficiency and product quality. Its function is to quickly dissipate the heat released by the solidifying melt, allowing the product to cool and set within the specified time. Cooling time typically accounts for 50%-70% of the molding cycle. Optimizing the cooling system can shorten the cycle by over 30%. The cooling system design should adhere to the principle of “local cooling and uniform cooling.” Water channels should be located close to high-temperature areas in the mold cavity (such as thick walls and ribs) and at a distance of 1.5-2 times the channel diameter from the cavity surface. For example, a 10mm diameter water channel should be 15-20mm from the cavity surface. This ensures cooling efficiency without weakening the mold’s strength. Large molds require zoned cooling, with independent flow control in different zones to ensure consistent temperatures across all areas.
The structural form of the cooling water channel should be selected based on the product shape. Common types include straight-through, wrap-around, and stepped. Straight-through water channels are easy to process and are suitable for flat products. They use parallel straight holes and have an inlet and outlet temperature difference of ≤5°C. Wrap-around water channels are arranged along the contour of the cavity and are suitable for round or special-shaped products. For example, bottle cap molds use circular water channels to ensure uniform circumferential temperature. Stepped water channels are used for uneven cavity surfaces. Water channels of varying depths maintain a consistent cooling distance. For example, in the concave and convex areas of automobile dashboard molds, the water channel depth adjusts to the undulations of the cavity surface. Water channel bends require rounded corners (R ≥ 5mm) to avoid increased flow resistance and localized turbulence caused by right-angle turns, which can affect cooling efficiency.
The choice of cooling medium and parameter control significantly impact cooling efficiency. Water is the most commonly used cooling medium, offering advantages such as high specific heat capacity and low cost, making it suitable for cooling most plastics. For applications requiring rapid cooling (e.g., thick-walled products), an ice-water mixture (0-5°C) can be used, but condensation on the mold surface must be prevented. For high-temperature molded plastics (e.g., PEEK), hot oil (100-150°C) is required to control the mold temperature to avoid sudden temperature drops that can cause cracking in the product. The cooling water flow rate is determined based on heat calculations and is generally 5-15 L/min with an inlet pressure of 0.2-0.4 MPa. A flow meter is used to monitor the flow rate in each circuit to ensure stable flow. The water must be treated to a hardness of ≤100 mg/L to prevent scale deposits from clogging the waterways.
The sealing and exhaust of the cooling system are important details to ensure the cooling effect. The water channel interface uses a standard water nozzle (such as DME and HASCO specifications) and is sealed with an O-ring to prevent water leakage from affecting the performance of the mold. The water nozzle should be installed away from the moving parts of the mold to facilitate pipe connection and maintenance. An exhaust valve must be installed at the end of each water channel to exhaust the air in the system and avoid uneven cooling caused by air blockage. The exhaust valve must be manually opened and deflated when used for the first time until a continuous flow of water is discharged. For deep-hole water channels, an exhaust plug must be installed in the middle to ensure that the entire length of the water channel can be filled with cooling water. For example, for water channels with a length of more than 300mm, 1-2 exhaust plugs should be added in the middle to improve cooling uniformity.
Maintenance and optimization of the cooling system are crucial for continuously improving production efficiency. Water channels should be regularly cleaned of scale and impurities, flushed monthly with a citric acid solution (5%-10% concentration) for 30 minutes, and ultrasonic cleaning (40kHz frequency) performed annually to remove stubborn dirt adhering to the pipe walls. The mold surface temperature distribution is monitored using an infrared thermometer. When the local temperature difference exceeds 10°C, the water channels should be checked for blockage or insufficient flow, and auxiliary water channels should be added if necessary. For new product molds, CAE simulation can be used to analyze the cooling effect, and the location and size of the water channels can be adjusted based on the simulation results. For example, if the simulation shows slow cooling in a certain part of the product, an 8mm diameter branch water channel can be added to that area to shorten the cooling time. Regularly evaluate the performance of the cooling system, and continuously optimize cooling parameters based on product quality and production cycle to achieve efficient and stable production.
