Structural points of injection molding cooling system
The injection molding cooling system is a crucial component of the mold. Its structural design must meet the requirements for uniform, efficient, and reliable cooling to ensure product quality and production efficiency. Key structural features of the cooling system include water channel layout and cavity distance, water inlet and outlet methods, sealing performance, exhaust function, and coordination with other components. Each of these key points directly affects the cooling effect and requires special attention during the design process.
The distance between the water channel layout and the mold cavity is a key consideration in cooling system structural design. A reasonable distance ensures uniform cooling and prevents localized overheating or overcooling of the part. Generally speaking, the distance between the water channel and the cavity surface should be 1 to 2 times the part wall thickness, with a minimum of no less than 8 mm and a maximum of no more than 30 mm. For parts with uniform wall thickness, the water channels should be evenly distributed and spaced consistently. For parts with uneven wall thickness, the water channels in thicker areas should be appropriately spaced closer together, with the spacing shortened to 0.5 to 0.7 times that in thinner areas. For example, a plastic part may have two wall thicknesses: 3 mm and 6 mm. The water channels in the thinner areas are 10 mm from the cavity and spaced 30 mm apart. The water channels in the thicker areas are 7 mm from the cavity and spaced 20 mm apart. This differentiated layout ensures consistent cooling rates across all areas. Furthermore, the water channels should avoid interference with mold components such as ejectors and guide posts. If necessary, offset or curved channels can be used to bypass these components.
The design of the water inlet and outlet directly affects the flow rate and pressure distribution of the cooling medium. Common water inlet and outlet methods include series, parallel, and hybrid. The series inlet and outlet method routes the cooling medium through each channel sequentially, making it suitable for small molds or short channel lengths. Its advantage is its simple structure, but its disadvantage is a large temperature difference between the inlet and outlet of the medium, resulting in uneven cooling. The parallel inlet and outlet method connects each channel to both the inlet and outlet mains, resulting in even flow distribution and a small temperature difference between the inlet and outlet. This makes it suitable for large or multi-cavity molds, but requires precise calculation of the resistance of each channel to ensure balanced flow. The hybrid inlet and outlet method combines the advantages of series and parallel connections, with the main channels connected in parallel and the branch channels in series. This makes it suitable for complex molds. For example, a multi-cavity mold uses a parallel inlet and outlet method. The water channels of each of the six cavities are connected to a 20mm diameter inlet main. A flow control valve is installed at the inlet of each cavity to ensure that the cooling medium flow rate within each cavity does not vary by more than 10%.
Sealing performance is crucial for reliable cooling system operation. Poor sealing can lead to cooling medium leakage, impairing cooling efficiency and even damaging mold components. The sealing structure primarily involves sealing the water channel interfaces and the water channel itself. O-rings are typically used at the interfaces. The sealing ring material should be heat- and oil-resistant nitrile rubber or fluororubber. The cross-sectional diameter is determined by the joint clearance and is generally 1.5 to 3 mm. The sealing of the water channel itself requires precise machining. Drilled water channels must be deburred, with an inner wall roughness of Ra1.6μm or less to prevent leakage caused by roughness. For curved or spliced water channels, the joint surfaces must be surface-ground to a roughness of Ra0.8μm and secured with bolts to ensure a tight seal. For example, a mold’s spliced water channel utilizes a 10mm -wide sealing groove, fitted with a 2mm -diameter fluororubber O -ring. This is secured with 8.8- grade bolts at a torque of 25 N · m and tested to a pressure of 1.5 MPa with no leakage.
The exhaust function of a cooling system is often overlooked, but its impact on cooling efficiency cannot be ignored. As the cooling medium flows, it dissolves some air. When the medium pressure drops, this air is released, forming bubbles that adhere to the inner walls of the water channel and reduce the heat transfer coefficient. Therefore, the cooling system should be equipped with an exhaust device, typically an exhaust valve at the highest point of the water channel to regularly release air bubbles. For large molds, exhaust screws can be installed at the end of each water channel. During mold installation and commissioning, these screws can be opened and closed after the coolant has steadily flowed out, ensuring that no air remains in the water channel. For example, an automotive instrument panel mold features automatic exhaust valves at the four highest points in the cooling system. These valves automatically open when the air pressure in the system reaches 0.1 MPa, improving heat transfer efficiency by 15% and reducing cooling time by 8 seconds.
The coordination between the cooling system and other mold components is an important consideration in structural design, and conflicts with the ejection mechanism, core pulling mechanism, heating system, etc. must be avoided. In the early stages of mold design, an overall layout plan should be carried out to determine the position and spatial dimensions of each component. For molds with core pulling, the water channel should avoid the movement trajectory of the core pulling slider. If necessary, a follow-up water channel should be used, that is, the water channel moves with the slider and is connected to a fixed water channel through a telescopic joint. For molds with heating devices, such as hot runner molds, the distance between the cooling water channel and the heating element should be no less than 15mm to avoid heat interference. For example, the distance between the cooling water channel and the hot runner nozzle of a hot runner mold is maintained at 20mm. The water channel is made of stainless steel pipes, and the outer wall is wrapped with thermal insulation cotton, which effectively prevents the cooling efficiency from decreasing due to heat transfer.
