Balanced layout of injection runners
The key to balanced injection molding runner layout is to achieve even melt distribution across all cavities, ensuring that all parts are filled and held simultaneously. Therefore, the principle of “equal distance and equal cross-section” must be adhered to. In a multi-cavity mold, the runners should extend as evenly as possible from the center of the sprue to each cavity, with a maximum length difference of no more than 3mm. For example, in a six-cavity mold with six cavities arranged in a regular hexagon, the runners should extend 120mm from the center sprue to each cavity to ensure consistent melt flow paths. When the cavity layout cannot achieve absolute equal distances, the runner cross-sectional dimensions should be adjusted to compensate. Long runners should have larger cross-sectional areas, while short runners should have smaller cross-sectional areas. For example, a 150mm long runner should have a 10mm diameter, while a 120mm long runner should have an 8mm diameter. This increases the melt flow in the long runners, offsetting the pressure loss caused by the distance differences. The cross-sectional shape of the runners must be consistent to avoid uneven flow resistance caused by shape changes. For example, all of them should be circular or all of them should be trapezoidal. The surface roughness of each runner should be uniformly controlled below Ra1.6μm to ensure that the flow characteristics of the melt in each runner are the same.
The layout of the runners must match the cavity distribution. Common balanced layouts include radial, dendritic, and matrix, each suitable for different cavity numbers and arrangements. A radial layout is suitable for molds with a small number of cavities (2-8 cavities) and a symmetrical distribution. For example, a circular part with four cavities in a mold has four cavities arranged in a cross-shaped pattern around the main runner. Runners radiate from the center, each with identical length and diameter. After entering the main runner, the melt is evenly distributed radially to each cavity, keeping the fill time difference within 0.2 seconds. A dendritic layout is suitable for molds with a large number of cavities (10-32 cavities), such as a small connector part with sixteen cavities in a mold. The main runner extends from the main runner and connects the cavities through secondary runners. The cross-sectional dimensions of the primary and secondary runners decrease in accordance with flow requirements (main runner diameter: 12mm, secondary runner diameter: 8mm). Runners within the same level are identical in size to ensure balanced melt distribution. The matrix layout is suitable for molds with rectangular cavities, such as a mobile phone keypad plastic part with 24 cavities in one mold. The runners are distributed in a grid pattern along the horizontal and vertical directions. The horizontal and vertical runners have the same size and length, and the isobaric distribution of the melt is achieved through the grid nodes.
The connection between the runner and the gate must ensure a smooth transition of melt flow from the runner to the gate, avoiding disruption of equilibrium due to improper connection methods. The runner-to-gate connection should utilize a circular arc with a transition radius of 0.5 times the runner diameter. For example, when connecting an 8mm diameter runner to the gate, the transition radius should be 4mm to prevent eddy currents and pressure loss in the melt. Gate dimensions for each cavity must be identical, including gate width, thickness, and length, with tolerances within ±0.02mm. For example, pinpoint gates should have a uniform diameter of 1.2mm, and fan gates should have a uniform width of 5mm. This ensures consistent melt flow at each gate. For high-viscosity plastics (such as PC), a small pressure relief groove (1-2mm smaller in diameter and 5-8mm long) can be placed near the gate at the end of the runner to stabilize melt pressure before entering the gate, reducing filling variations caused by pressure fluctuations. In addition, the positions of the gates need to be symmetrically distributed on the plastic part. For example, the gates of each plastic part are set at the center or the same edge position to ensure that the filling path of the melt in the cavity is consistent.
Optimizing the balanced layout of runners using CAE mold flow analysis is a key tool in modern mold design. Computer simulations can predict melt flow conditions in advance and allow for adjustments. During mold flow analysis, a 3D model is created and the plastic’s material parameters (such as viscosity, specific heat capacity, and thermal conductivity) and process parameters (such as melt temperature and injection pressure) are input. The flow velocity, pressure distribution, and temperature changes in the runners are simulated. If a cavity fills more than 0.5 seconds longer than others, the corresponding runner diameter or length is adjusted. For example, increasing the runner diameter for the slower-filling cavity from 6mm to 6.5mm can reduce the fill time difference to less than 0.2 seconds. Mold flow analysis can also predict pressure drop in the runners, ensuring that the pressure drop difference between runners does not exceed 5%. For example, if the pressure at the end of the main runner is 120 MPa, the pressure at the end of each runner should be within the range of 114-126 MPa. Through multiple simulation optimizations, runner balancing accuracy can be improved by over 30%, reducing mold trials and mold modification costs. For complex multi-cavity molds, mold flow analysis is an indispensable tool that can effectively solve multi-variable balance problems that are difficult to handle with traditional empirical design.
The verification and debugging of balanced runner layouts require rigorous and meticulous testing, with final optimization based on feedback from mold trial data. During mold trial, the molded parts in each cavity should be numbered, and the weight, dimensions, and wall thickness of each part should be measured and the deviation calculated. If the weight of a particular part deviates by more than 3% from the average, it indicates an imbalance in the melt distribution in the corresponding runner, and the runner dimensions should be adjusted. For example, if, after a mold trial of an eight-cavity mold, the weight of cavity 3 is found to be 5% lighter than the average, the runner diameter corresponding to cavity 3 should be increased from 7mm to 7.3mm, and the mold should be retested for verification. For parts requiring high dimensional accuracy, such as gears, critical dimensions (such as tooth thickness and pitch) of each part should be measured. If dimensional deviations exceed 0.03mm, the runner length or gate dimensions should be adjusted. During mold trial, the melt filling sequence should also be observed. This can be achieved by placing color markers at the gate or using high-speed video recording to ensure that the fill time difference between cavities does not exceed 0.3 seconds. Through mold trial verification and optimization, the actual effect of the balanced layout of the runners can be made consistent with the design goals, ensuring the quality stability of all plastic parts during mass production.
