Different sized cavities use different sized gates
In the design of multi-cavity injection molds, when there are differences in cavity size, using gates of different sizes is a key measure to ensure uniform quality of plastic parts in each cavity. The gate is the key part connecting the runner and the cavity, and its size directly affects the melt filling speed, pressure loss, and pressure holding effect. If large and small cavities use the same size gate, the melt will preferentially fill the small cavity, while the large cavity will have defects such as material shortage and shrinkage due to insufficient filling, or dimensional deviation due to uneven pressure holding. Therefore, reasonably matching the gate size according to the cavity volume, plastic part structure and material properties is the core technology for achieving balanced multi-cavity molding.
The design of gate size differences needs to be based on the proportional relationship of the cavity volume. Generally, the gate cross-sectional area is proportional to the cavity volume, that is, a larger gate corresponds to a larger cavity to ensure sufficient melt flow and pressure transmission. For example, when the volume ratio of the two cavities is 3:1, the gate cross-sectional area ratio can be designed to be 2.5:1 (taking into account the slightly greater flow resistance of the large cavity), which can be adjusted by the correction factor. The diameter of the circular gate and the width and thickness of the rectangular gate need to be scaled according to this ratio to ensure that the melt fills different cavities in the same time. At the same time, the length of the gate also needs to be adapted. The gate of the large cavity can be appropriately shortened by 0.5-1mm to reduce pressure loss, while the gate of the small cavity can be slightly longer to avoid flash defects caused by over-fast filling.
The core goal of gate size design is to ensure consistent filling times across different cavities. The flow time of the melt in the runners and gates must be precisely calculated to ensure that the filling time difference between cavities is within 5%. For crystalline plastics (such as PP and PE), large differences in filling time can lead to varying degrees of crystallinity in parts with different cavities, resulting in dimensional and performance deviations. For amorphous plastics (such as PC and ABS), varying cooling rates can lead to uneven internal stress distribution. By adjusting the gate size, the melt flow rate can be altered. For example, a 1.2mm diameter circular gate can be used for a large cavity, while a 0.8mm gate can be used for a small cavity, ensuring that the filling time of both cavities is controlled within 2-3 seconds, achieving simultaneous filling.
The difference in gate size also needs to take into account the structural complexity of the plastic part. Even if the cavity volume is similar, if one of the plastic parts contains thin walls, deep cavities or complex ribs, the corresponding gate must be appropriately enlarged to overcome the additional flow resistance. For example, a mold contains two cavities of the same volume, one is a simple flat plastic part, and the other is a shell with multiple sets of reinforcing ribs. In this case, the gate diameter of the shell cavity needs to be 0.2-0.3mm larger than that of the flat cavity to ensure that the melt can smoothly fill the rib area. In addition, the selection of the gate position needs to be coordinated with the size design. The gate of the large cavity should be set as close to the thick wall of the plastic part or at the midpoint of the melt flow path as possible. The gate of the small cavity can be close to the edge. The filling effect is optimized by coordinating the size and position.
The final determination of gate size requires CAE simulation and mold trial verification. Flow simulations are performed using software such as Moldflow for different gate size options. The pressure distribution, temperature field, and fill time within each cavity are analyzed to initially identify a reasonable size range. During mold trials, focus is placed on inspecting the weight deviation (should be controlled within 3%), dimensional accuracy, and surface quality of the molded parts within each cavity. If sink marks are observed in large-cavity parts, the gate diameter can be increased by 0.1mm. If flash is observed in small cavities, the gate size should be reduced. For molds with large production runs, gate wear must also be considered. For large-cavity gates, more wear-resistant materials (such as SKD61) can be used, and appropriate margins in the initial size can be added to extend the mold’s stable production cycle. Through scientific design and repeated verification, matching gate sizes for different cavities can significantly improve production efficiency and product consistency for multi-cavity molds.
