Injection molding unbalanced arrangement
An unbalanced arrangement for injection molding refers to a layout in which the distances between each cavity and the main runner in a multi-cavity mold are inconsistent. The melt enters the cavity through branch runners of varying lengths, resulting in differences in filling time and pressure. Compared to a balanced arrangement, an unbalanced arrangement is more flexible in mold structure design. It is particularly suitable for scenarios with a large number of cavities, complex part shapes, or the need to use a standard mold base. For example, for a 16-cavity special-shaped connector mold, the use of an unbalanced arrangement can reduce the mold size by 20%, reducing manufacturing costs. The core challenge of an unbalanced arrangement is how to balance the filling state of each cavity through runner design and process adjustments to avoid dimensional differences or quality fluctuations in the plastic parts due to uneven filling. This layout requires the combination of CAE mold flow analysis technology to accurately predict the filling behavior of each cavity. Dynamic balance can be achieved by optimizing runner dimensions, setting throttle valves, and other methods to ensure the quality consistency of multi-cavity plastic parts.
The structural characteristics and applicable scenarios of an unbalanced injection molding arrangement determine its advantages for specific production needs. Structurally, the cavity distribution of an unbalanced arrangement is not restricted by symmetry, allowing for flexible arrangement based on part shape and mold base size. This makes it particularly suitable for irregularly shaped parts or those with a large number of cavities. For example, using an unbalanced arrangement to arrange 12 small parts of varying sizes within a 400mm×400mm mold base can save 30% of mold base space compared to a balanced arrangement. In terms of mold cost, an unbalanced arrangement can reduce total runner length and ease mold processing difficulty. For example, an unbalanced mold with eight cavities has a runner length 15%-20% shorter than a balanced mold, reducing processing time by 10%. Suitable applications include combination molds for small-batch production of multiple products, economical molds using standard mold bases, and multi-cavity molds with widely varying part sizes. However, an unbalanced arrangement also has limitations. When the number of cavities exceeds 32 or when extremely high part dimensional accuracy is required (tolerance <±0.02mm), balancing becomes significantly more challenging, making a balanced arrangement more suitable.
Unbalanced runner design is key to achieving balanced filling across cavities. Dimensional optimization and local adjustments are necessary to compensate for differences in runner lengths. First, the diameter of the branch runners is adjusted based on the distance from each cavity to the main runner, following the principle of “large diameter for long runners, small diameter for short runners.” For example, a cavity 100mm from the main runner corresponds to an 8mm branch runner diameter, while a cavity 200mm from the main runner corresponds to a 10mm diameter. This increases the cross-sectional area of the runners to reduce pressure losses in long runners. Circular or trapezoidal runner cross-sectional shapes are preferred. Circular runners minimize surface area (surface area/volume), reduce heat loss, and facilitate melt flow. Trapezoidal runners are also easy to process and suitable for economical molds. For cavities with large distance differences (e.g., the ratio of the farthest to the closest distance is greater than 2:1), a throttle valve or flow limiter must be installed near the gate to increase the flow resistance by reducing the local flow channel diameter and balance the filling time of each cavity. For example, a throttling section with a diameter reduction of 2mm is installed in the flow channel of the cavity near the main channel. This can extend the filling time of the cavity by 0.3-0.5 seconds, synchronizing it with the distant cavity.
Process parameter adjustments assist in balancing unbalanced molds, optimizing filling performance through the coordinated control of temperature, pressure, and speed. Melt temperature should be adjusted appropriately based on runner length variations. For cavities with long runners, the melt temperature can be increased by 5-10°C to reduce melt viscosity and minimize flow resistance. For example, the standard melt temperature for PP is 190°C, but for cavities with long runners, this can be increased to 195-200°C. Injection speed is controlled in stages: initially, a low speed (20-30 mm/s) ensures smooth melt entry into the runners. As the melt approaches the farthest cavity, the speed is increased (50-70 mm/s) to minimize fill time differences. Holding pressure parameters should be set individually for each cavity (some high-end injection molding machines support multi-stage holding). For slower-filling cavities, the holding time can be extended by 1-2 seconds to compensate for shrinkage differences. In addition, the mold temperature is precisely controlled by the mold temperature controller, so that the temperature deviation of each cavity area is controlled within ±2°C to avoid the filling imbalance aggravated by uneven temperature. For example, a heating rod is added near the far cavity to ensure that its temperature is consistent with that of the near cavity.
Balance verification and optimization of unbalanced arrangements require mold flow analysis and trial mold adjustments to ensure consistent part quality across cavities. During the design phase, CAE software such as Moldflow is used to simulate the filling time, pressure distribution, and weld mark location of each cavity. The fill time difference between cavities is required to be ≤0.5 seconds and the pressure difference ≤5 MPa. Otherwise, the runner dimensions or layout must be readjusted. During the trial mold, pressure sensors are placed in each cavity to monitor pressure changes during the filling process in real time, recording the weight, dimensions, and appearance of the part. If parts in a particular cavity exhibit material shortages or flash, the runner diameter or throttling parameters for that cavity are adjusted accordingly. For example, during trial molds of a six-cavity unbalanced mold, the farthest cavity filled 1.2 seconds longer than the nearest cavity. By increasing the runner diameter from 8mm to 9mm and adding a 0.5mm throttling section to the near cavity’s runner, the fill time difference was reduced to 0.3 seconds, and part weight deviation was controlled within ±1%. After production begins, the dimensions and mechanical properties of each cavity plastic part must be regularly inspected to ensure long-term production stability. When quality fluctuations occur, priority should be given to checking whether the flow channel is blocked or the temperature is abnormal, and timely maintenance and adjustments should be made.
