In the injection molding process, auxiliary runners serve as a critical structure connecting the main runners to the mold cavity. Their design directly impacts the mold quality and production efficiency of the plastic part. The auxiliary runners’ primary function is to smoothly and evenly deliver the molten plastic to each cavity, while also taking into account factors such as pressure loss, temperature distribution, and ease of demolding. The design process begins with determining the runner’s cross-sectional shape. Common shapes include circular, trapezoidal, and U-shaped. A circular cross-section is the optimal choice due to its minimal surface-to-volume ratio and reduced heat loss, but it also comes at the expense of relatively high processing costs. Trapezoidal cross-sections are easier to process and are widely used in the production of small and medium-sized plastic parts.
The design of auxiliary runner dimensions must be determined based on the part’s volume, wall thickness, and plastic fluidity. For highly fluid plastics like polyethylene and polypropylene, the runner diameter can be appropriately reduced, typically between 3-6mm. For less fluid engineering plastics like polycarbonate and polyoxymethylene, the runner diameter should be increased to 6-10mm to avoid underfilling due to insufficient pressure. Furthermore, runner lengths should be minimized and the layout symmetrical to ensure that the molten plastic reaches all cavities simultaneously, minimizing part dimensional deviations caused by varying filling times.
Flow channel layout
There are two types of layouts: flat and unbalanced. The balanced layout requires that the length and diameter of each auxiliary runner are exactly the same. This makes it suitable for multi-cavity molds, ensuring consistent molding conditions for each part and particularly well-suited for mass production of high-precision parts. The unbalanced layout flexibly adjusts runner parameters based on cavity position. While this simplifies the mold structure, it can lead to uneven filling of each cavity, requiring compensation through gate size adjustments. In actual design, the balanced layout is preferred, especially in applications requiring high precision, such as automotive parts and electronic housings.
The design of the connection between the auxiliary runner and the gate is equally critical. A circular arc should be used at the transition to avoid right or sharp angles that increase plastic flow resistance and localized overheating. Furthermore, a cold slug well should be provided at the end of the runner to store plastic that has lost fluidity due to cooling, preventing it from entering the mold cavity and impacting part quality. The size of the cold slug well should be determined based on the runner diameter, typically with a depth of 1-1.5 times the runner diameter. It must also be coordinated with the ejection mechanism to ensure smooth ejection of the cold slug.
With the advancement of injection molding technology, auxiliary runner design is gradually becoming more intelligent and sophisticated. CAE simulation software allows for the simulation of plastic flow, pressure distribution, and temperature changes within the runner during the design phase. This allows for early detection and optimization of design flaws, reducing mold trials and lowering production costs. For example, for complex plastic parts, a one-to-one correspondence between runners and cavities can be employed. By adjusting the bend angles and cross-sections of the runners, smooth plastic filling can be achieved, improving the part’s pass rate. In the future, with the application of 3D printing technology in mold manufacturing, auxiliary runner design will become even more flexible, meeting the needs of personalized, small-batch production.
