Injection molding balanced arrangement
Balanced injection molding is a key design principle for ensuring uniform filling of all cavities in multi-cavity molds. By optimizing runner layout, cavity position, and size, the melt reaches each cavity simultaneously, achieving the same filling pressure and time, thereby ensuring consistent quality across all parts. Balanced molding effectively reduces part weight variations, dimensional deviations, and cosmetic defects caused by uneven filling, improving production efficiency and material utilization. It is a core technology in multi-cavity mold design.
A symmetrical runner layout is the foundation of a balanced arrangement. The main runner and its branch runners must form a symmetrical structure, such as an H-shaped, X-shaped, or circular shape, to ensure that the melt travels an equal length from the main runner to each cavity (with a deviation of ≤5mm). The cross-sectional dimensions of the branch runners must be consistent, typically using circular or trapezoidal cross-sections. The circular diameter is determined by the number of cavities, ranging from 8-10mm for a 4-cavity mold and 10-12mm for an 8-cavity mold. The cross-sectional dimension deviation should be ≤0.2mm. For example, an 8-cavity bottle cap mold utilizes X-shaped symmetrical runners, with each branch runner being 80mm long and 10mm in diameter. Mold flow simulations show that the filling time deviation between cavities is ≤0.1 seconds, and the weight difference is ≤0.5g, far superior to the 1.2g difference observed with an asymmetrical arrangement. Runner bends should have rounded corners (R ≥3mm) to reduce flow resistance and prevent differential pressure loss.
The positional distribution of the cavities must match the runner layout, ensuring that each cavity is equidistant from the main runner and arranged in a centrally or axisymmetric manner on the mold plane. For rectangular molds, the cavities can be arranged in a matrix with equal row and column spacing; for circular molds, a radial arrangement can be used, with adjacent cavities at the same angle. The dimensional accuracy of the cavities must be strictly controlled, with deviations of key dimensions (such as diameter and height) of each cavity ≤0.01mm and consistent surface roughness (Ra ≤ 0.8μm) to avoid varying filling speeds due to cavity differences. For example, a 16-cavity connector mold uses a 4×4 matrix arrangement with 50mm cavity spacing. Three-coordinate measurement shows that the aperture deviation of each cavity is ≤0.005mm. After mold trialing, the difference in insertion force between each product is ≤2N, meeting interchangeability requirements.
Gate design and balancing are crucial complements to a balanced arrangement. Multi-cavity molds typically utilize gates of the same type and size, such as point gates with a diameter of 0.8-1.2mm and side gates with a width of 2-5mm, with a dimensional deviation of ≤0.05mm. When mold flow analysis indicates unbalanced filling, fine-tuning the gate dimensions can be used to achieve balance. For example, the gate size of slower-filling cavities can be increased by 0.05-0.1mm at a time until the fill time difference between cavities is ≤0.2 seconds. For example, after a six-cavity shell mold trial, it was found that cavity 1 filled 0.5 seconds longer than the others. Increasing the gate width from 3mm to 3.1mm reduced the fill time difference to 0.1 seconds. Gates should be located in the same location in the cavities (e.g., at the midpoint of the long side) to ensure consistent melt flow paths across all cavities.
Coordinated optimization of process parameters can enhance the effectiveness of balanced injection molding. A multi-stage injection speed setting is required: a low initial speed ( 30-50 mm/s) to prevent melt fracture at the gate, followed by a high speed (80-120 mm/s) during the filling phase to ensure rapid filling of all cavities. The holding pressure during the holding phase is adjusted based on the part’s wall thickness (for thick-walled parts, the holding pressure is 60%-70% of the injection pressure). The holding time for each cavity must be consistent to avoid dimensional deviations caused by varying holding pressures. For example, a four-cavity toy mold was able to reduce the pressure drop between cavities from 15 bar to 5 bar by adjusting the injection speed profile, switching the filling phase speed from a single 80 mm/s to a two-stage speed of 50 to 100 mm/s. This reduced the pressure drop between cavities from 15 bar to 5 bar, and improved part weight stability by 30%. Furthermore, uniform barrel and nozzle temperatures must be maintained to prevent temperature fluctuations that could affect melt flow.
Verification and adjustment of the balanced arrangement require a combination of trial mold data and mold flow analysis. During the trial mold, the weight, dimensions, and appearance of each cavity’s product must be inspected. Weight variation should be ≤1%, dimensional deviation ≤0.02mm, and no significant differences in appearance (such as gloss or weld mark location) should be observed. If imbalance is detected, mold flow analysis software is used to identify the cause, such as differences in runner resistance or uneven cavity venting. Targeted adjustments can be made, such as polishing high-resistance runners (Ra reduced from 1.6μm to 0.8μm) or adding venting grooves (depth 0.01-0.02mm) to cavities with insufficient venting. For example, after a trial mold, the tooth thickness of the product in cavity #3 of a 12-cavity gear mold was too small. Mold flow analysis revealed high pressure loss in this cavity. By increasing the corresponding runner diameter by 0.2mm, the tooth thickness deviation was corrected from -0.05mm to ±0.01mm. During batch production, it is necessary to regularly inspect each cavity product, once every 500 molds, to ensure a stable balance state. When deviations occur, the process parameters or molds should be adjusted in a timely manner.
