Factors affecting runner design in injection molding
As a critical channel connecting the main runner and the gate, the rationality of the injection molding runner’s design directly impacts the melt’s flow characteristics, part quality, and production efficiency. The runner’s primary function is to evenly distribute the melt from the main runner to each cavity while minimizing pressure loss and melt degradation. Numerous factors influence runner design, including part material properties, the number and arrangement of cavities, and injection molding machine parameters. These interrelated factors require comprehensive consideration to achieve an optimal design. For example, in the production of ABS multi-cavity parts, improper runner design can lead to uneven filling of each cavity, part weight deviations exceeding 5%, and even defects such as material shortages or overflow. Therefore, in-depth analysis of the factors influencing runner design is crucial for improving the stability and economic efficiency of injection molding production.
The properties of the plastic part material are key factors influencing runner design. Different materials have significantly different requirements for runner cross-sectional shape, dimensions, and surface quality. Materials with high melt viscosity (such as PC and PMMA) require larger runner cross-sectional dimensions to reduce flow resistance and avoid excessive pressure loss that can lead to underfilling. For example, the runner diameter for PC is typically 8-12mm, while that for PE, which has good flow properties, is only 5-8mm. Material thermal stability is also crucial. For easily degradable plastics (such as PVC and POM), runners should be as short and smooth as possible to minimize melt residence time within the flow channel and prevent material degradation due to shear heating. Furthermore, fiber-reinforced plastics (such as glass-fiber-reinforced PA) require runners with circular or trapezoidal cross-sections to prevent fiber breakage and uneven orientation. Surface roughness should also be controlled to Ra ≤ 0.8μm to minimize fiber wear. For example, a square runner cross-section in glass-fiber-reinforced PC can cause fiber breakage at corners, affecting the mechanical properties of the part.
The number and arrangement of cavities directly determine the layout and length of the runners. Runner design for single-cavity molds is relatively simple, typically employing a straight or L-shaped design. Multi-cavity molds require branching runners tailored to the number and arrangement of cavities, ensuring consistent flow path lengths across all cavities. For example, in a four-cavity mold with a symmetrical arrangement, the runners should be arranged in a cross pattern, with each branch length error no more than 5%. An eight-cavity mold with a circular arrangement should utilize radial runners, extending evenly from the main runner outward. The total runner length should be kept as short as possible to minimize melt pressure loss and cooling time, generally within a range of 50-200mm. Excessive runner lengths can result in excessive melt temperature drop and poor flow properties. For molds with more than 16 cavities, a two-stage runner design can be employed. This involves the main runner first connecting to a primary runner, which then branches off into secondary runners for each cavity, ensuring even melt distribution. For example, in a 32-cavity mold employing a two-stage runner design, the fill time difference between cavities can be kept to within 0.1 seconds.
The cross-sectional shape and dimensions of the runner are key design parameters, directly affecting melt flow resistance and pressure loss. Common cross-sectional shapes include circular, trapezoidal, U-shaped, and semicircular. A circular cross-section offers the lowest flow resistance and the most uniform melt flow, making it the optimal choice. However, it is challenging to process and requires a split mold for molding. A trapezoidal cross-section is simple to machine and suitable for small and medium-sized parts. Its vertex angle is typically 50°-60°, and its depth-to-width ratio is 1:1.5-1:2. Runner dimensions are determined based on part weight and material flowability. The calculation formula is: Circular runner diameter d = k × √W, where k is the material coefficient (0.8 for PE, 1.0 for ABS, and 1.2 for PC), and W is the weight of the single-cavity part (g). For example, when producing a 10g ABS part, the runner diameter d = 1.0 × √10 ≈ 3.2mm. In actual designs, 4mm is used. The size of the runner should not be too large or too small. If it is too large, the melt cooling time will be prolonged, which will increase the molding cycle. If it is too small, the flow resistance will be large, and the injection pressure will need to be increased, which will increase the risk of overflow.
Injection molding machine parameters and production processes significantly constrain runner design. The maximum injection pressure and flow rate of the injection molding machine limit the maximum length and minimum cross-sectional dimensions of the runner. If the injection molding machine pressure is insufficient, the runner diameter must be increased to reduce resistance. For example, when producing PC parts on a small injection molding machine (with a clamping force below 500kN), the runner diameter needs to be 1-2mm larger than on a larger machine. Injection speed also influences runner design. High-speed injection allows for smaller runner dimensions, while low-speed injection requires larger dimensions to ensure filling speed. Furthermore, runner design must be coordinated with the cooling system. Cooling water channels should be located around the runners, 15-25mm from the runner surface, to prevent overcooling of the melt within the runners. For example, the cooling water channel diameter for a PE runner should be 8-10mm, with a water flow rate of 0.5-1m/s to maintain the melt temperature within the runners at 50-80°C above the melting point. By comprehensively considering these factors, the runner design can control the melt pressure loss within 10%-15%, and improve the filling uniformity of each cavity by more than 40%.
