Example of flow balance in a multi-cavity injection mold
Flow balance in a single-mold, multi-cavity injection molding process involves rationally designing the mold structure and optimizing the injection molding process parameters to ensure that the plastic melt fills each cavity in a multi-cavity mold at the same pressure and speed within the same timeframe, thereby ensuring that the dimensional accuracy, weight consistency, and physical properties of the molded parts in each cavity are essentially the same. Unbalanced flow can lead to defects such as underfill, flash, sink marks, and weld marks in the molded parts, impacting product quality and production efficiency. In actual production, achieving flow balance in a single mold requires comprehensive consideration of multiple factors, including cavity layout, runner design, gate size, and process parameters. Optimization is achieved through a combination of theoretical calculations and actual mold trials. The following example illustrates how to achieve flow balance in a single-mold, multi-cavity mold.
For example, a mold with eight cavities for a plastic gear was constructed from POM, with a module of 2mm, 20 teeth, and a thickness of 5mm. Requirements were that the weight deviation of the gears within each cavity should not exceed ±0.5g, and the tooth profile tolerance should be within ±0.02mm. Initial mold trials revealed significant variations in the filling time of the gears in each cavity. The cavities closest to the sprue filled in 2.5 seconds, while those farther from the sprue filled in 4.0 seconds. This resulted in underfill and flash on some gears, and the weight deviation reached as high as 1.2g, failing to meet quality requirements. Analysis revealed that the flow imbalance was primarily due to improper runner design: consistent gate dimensions but varying distances from the sprue to each cavity. This resulted in significant variations in melt pressure loss and flow time within each runner. Therefore, optimization of the mold’s runner and gate design, along with adjustments to the injection molding process parameters, was necessary to achieve flow balance.
First, the mold’s runner system was optimized. The original mold employed a symmetrical rectangular layout, with eight cavities divided into four groups, each with two cavities symmetrically located on either side of the sprue. However, the distances from each cavity to the sprue varied, resulting in significant variations in melt flow path length. The optimized runner system employs an “H”-shaped runner design. The sprue is located in the mold’s center. A primary runner divides the melt into four paths, each of which is then connected to two cavities via a secondary runner. This ensures that the total runner length from each cavity to the sprue is essentially equal (within ±5mm). Furthermore, the cross-sectional dimensions of the runners were adjusted based on runner length and melt flow characteristics. The primary runners closer to the sprue were appropriately reduced in cross-section (8mm diameter), while the secondary runners further away from the sprue were appropriately increased in cross-section (10mm diameter) to balance melt pressure losses within each runner. Furthermore, rounded transitions (5mm radius) were used at the runner corners to reduce melt flow resistance and pressure loss.
Secondly, gate dimensions were differentiated. Although the runner lengths of each cavity were essentially the same, slight variations in melt flow could occur due to factors such as runner roughness and the number of bends. Therefore, gate dimensions needed to be fine-tuned to achieve consistent fill times across cavities. The original mold used a 1.0mm diameter circular gate for each cavity. After optimization, based on trial mold results, the gate diameter was appropriately increased (e.g., to 1.1mm) for slower-filling cavities and decreased (e.g., to 0.9mm) for faster-filling cavities. By adjusting the flow-limiting effect of the gate, the fill time variation across cavities was controlled within ±0.2 seconds. Furthermore, the gate was positioned at the edge of the gear’s inner hole, allowing the melt to enter the cavity radially, reducing flow resistance within the cavity and the formation of weld marks.
Finally, flow balance was further achieved by optimizing the injection molding process parameters. Based on the optimization of the mold runners and gates, the injection molding process parameters were adjusted. First, the injection speed was adjusted, using a segmented injection method. A lower speed (30 mm/s) was used in the initial stage (filling the runners) to avoid excessive shearing of the melt within the runners. As the melt approached the gate, the injection speed was increased (60 mm/s) to ensure rapid and uniform melt entry into all cavities. Later in the filling phase (when the cavities were 90% full), the speed was reduced (20 mm/s) to minimize flash. Next, the holding pressure parameters were adjusted. The holding pressure was set to 70% of the injection pressure, and the holding time was fine-tuned based on the filling status of each cavity. For cavities that filled slightly slower, the holding time was extended by 0.2-0.3 seconds to ensure adequate shrinkage in all cavities. Furthermore, the barrel temperature was increased by 5°C (from 190°C to 195°C) to increase melt fluidity and reduce flow variations between runners. The mold temperature was maintained at 60°C to ensure consistent cooling conditions across all cavities. After the above optimization, the mold was tested again. The filling time of the gears in each cavity was controlled within the range of 3.0±0.1 seconds, the weight deviation was reduced to ±0.3g, and the tooth size tolerance was controlled within ±0.015mm. The flow balance of one mold with eight cavities was achieved, meeting the quality requirements of the product.
