How to achieve feed balance
Feed balance is key to ensuring consistent quality across multiple-cavity injection molds. It ensures that the melt fills all cavities simultaneously and evenly, keeping deviations in weight, dimensions, and mechanical properties within acceptable limits. Unbalanced feed can lead to underfilling (flash) in some cavities, overfilling (flash) in others, or significant differences in shrinkage between products, severely impacting production efficiency and yield. Achieving balanced feed requires systematic optimization of mold design, process parameters, and raw material control. By precisely controlling the melt’s flow in the runners and cavities, consistent filling across all cavities is achieved.
Symmetrical design of the mold’s runner system is fundamental to achieving balanced feed flow and must adhere to the principles of “equal distance, equal cross-section, and equal resistance.” The runner layout should be symmetrical with the cavity distribution to ensure consistent flow path lengths from the main runner to each cavity. For example, a four-cavity mold uses a cross-shaped runner, while an eight-cavity mold uses a radial runner. The length tolerance of each branch runner should not exceed 1mm. Runner cross-sectional dimensions must be identical. The diameter tolerance of circular runners should be controlled within ±0.05mm, and the depth and width of trapezoidal runners should not exceed 0.03mm to avoid variations in flow resistance due to cross-sectional variations. Runner bends should use the same fillet radius (no less than 1.5 times the runner diameter) to minimize localized resistance variations. The connection between the branch runners and the gate should use the same transition form to avoid differential pressure loss due to structural abrupt changes. For example, a six-cavity mold experienced weight variations of up to 8% due to asymmetrical runner lengths (the longest and shortest differed by 5mm). After optimizing to symmetrical runners, this variation was reduced to less than 2%.
Consistent gate design is crucial for feed balance. The position, size, and form of each gate must be identical. Gates should be located symmetrically across all cavities to ensure consistent melt flow paths within the cavities. For example, for symmetrically arranged box-shaped parts, the gates should be located at the center of the long side. If the cavity structure is asymmetrical, gate positions should be adjusted through CAE simulation to ensure that filling time differences between cavities do not exceed 5%. Gate dimensions (such as diameter and length) must be strictly consistent. The diameter tolerance for point gates should be controlled within ±0.02mm, and the width and depth tolerance for side gates should not exceed 0.03mm to avoid dimensional variations that may lead to unequal flow rates. Gate forms should be uniform to avoid using point gates in some cavities and side gates in others, as different gate types can significantly vary in flow resistance and pressure holding performance. For example, in an 8-cavity bottle cap mold, one gate diameter was 0.05mm larger, resulting in a 6% heavier part weight compared to the other cavities. Replacing the gate eliminated the deviation.
Precise control of process parameters is crucial for achieving balanced feed flow. This requires optimizing injection speed, pressure, and temperature to minimize melt flow variations. A segmented injection process is employed: low injection speed in the initial stage ensures smooth melt flow into the runners, avoiding uneven distribution caused by turbulence. High injection speed in the intermediate stages shortens filling time differences between cavities. Low speed and pressure hold in the final stage ensure uniform shrinkage across cavities. Injection pressure must be high enough to ensure sufficient melt pressure upon reaching the furthest cavity, typically 10%-20% higher than in single-cavity molding. However, excessive pressure should be avoided, preventing flash. The melt temperature must be controlled within an appropriate range to ensure adequate fluidity while avoiding degradation or viscosity fluctuations caused by excessive temperatures. For example, when processing ABS, the melt temperature should be maintained at a stable 220-230°C, with fluctuations of no more than ±2°C. The mold temperature must be evenly distributed, with the temperature difference between cavities controlled within ±3°C. This can be achieved through independent temperature-controlled water circuits to prevent differential melt cooling rates due to temperature variations.
CAE flow simulation technology is an effective tool for optimizing feed balance. It can predict melt flow behavior before mold manufacturing, proactively identifying and resolving imbalances. The simulation analysis requires the establishment of a precise 3D model. The plastic’s PVT (pressure-volume-temperature) characteristics, rheological parameters, and molding process parameters are input to simulate the melt filling process in the runners and cavities, obtaining data such as filling time, pressure distribution, and temperature field for each cavity. Based on the simulation results, cavities with fill time variations exceeding 5% are identified. Adjustments to runner dimensions, gate locations, or process parameters are then made to optimize the fill time within 3%. For example, a simulation of a 12-cavity connector mold revealed that the fill time of the two furthest cavities was 8% longer than that of the others. By increasing the diameter of the corresponding branch runners by 0.2mm, the fill time difference was reduced to 2%. During the mold trial phase, the weight of each cavity should be measured by gravimetric weighing, in conjunction with the simulation results. Any weight deviation exceeding 3% requires further adjustment to ensure stability during mass production.
Stable raw material properties are a prerequisite for balanced feedstock. The melt flow rate (MFR), moisture content, and impurity content of raw materials must be strictly controlled to avoid flow variations caused by raw material fluctuations. The MFR variation within a batch of raw materials must be controlled within ±5%, and the MFR variation between batches must not exceed 10%. Mixing should be performed as necessary to ensure uniform raw material properties. Hygroscopic plastics must be thoroughly dried to a moisture content below 0.05% to avoid viscosity fluctuations caused by moisture. Dried raw materials must be sealed to prevent reabsorption of moisture. The impurity content in raw materials must be below 0.1% to prevent impurities from clogging runners or gates, leading to feed imbalance. For example, the MFR of a batch of PP raw material fluctuated by 15%, resulting in a 7% weight variation among parts in a multi-cavity mold. After replacing the batch with a more stable raw material, the variation was reduced to within 3%. Furthermore, the proportion of recycled material added must be stable, not exceeding 20%, and the recycled material must be screened and crushed to ensure a uniform particle size to avoid affecting melt flow.
Maintenance and monitoring of multi-cavity molds are key to maintaining long-term feed balance. Regular inspections of runners, gates, and temperature control systems are necessary to promptly identify and eliminate factors affecting balance. Before each production run, plastic debris should be cleaned from the runners and gates, and burnt materials should be removed with a copper brush or specialized cleaning tool to prevent uneven feed caused by blockage. Runner and gate dimensions should be measured regularly. When wear causes dimensional deviations exceeding 0.05mm, they should be repaired or replaced. Mold temperature sensors and heating tubes should be inspected to ensure accurate temperature control in each cavity, with deviations not exceeding ±3°C. During production, the weight, dimensions, and appearance of each cavity product should be regularly inspected at least once per hour. Any weight deviation exceeding 5% requires shutdown and inspection to eliminate problems such as runner blockage, gate wear, or process parameter fluctuations. Through continuous maintenance and monitoring, feed balance can be effectively maintained, ensuring the stability and consistency of multi-cavity production.
