For injection molding, filling unbalance often occurs in one mold with multiple cavities, and filling balance must be considered in the design of multi-cavity molds. In multi-cavity injection molding molds, runner systems are often designed in an “H” shape. Because the runners are geometrically perfectly symmetrical, they are also referred to as “geometrically balanced” or “naturally balanced” runner system designs. However, in actual production, there will be an unbalance in filling.

Because the shear heat generation has a clear influence on the melt temperature distribution in the flow channel, the unbalanced temperature distribution is the root cause of the unbalanced filling. At low speed, the melt flows through the runner for a relatively long time, and the resin conducts and loses more heat along the runner wall, so the overall temperature value is lower than that of high-speed injection, and the melt temperature distribution shows the characteristics of gradually decreasing from the high temperature area of the core to the runner wall. In the case of low-speed filling, the high-temperature area of the melt is biased towards the upper mold wall, so the upper mold cavity is filled faster.

During high-speed injection, the melt flows through the flow channel for a shorter time, the heat loss along the flow channel wall is reduced, and the shear heat generation effect is obvious, so the temperature distribution value is higher overall, and the closer to the flow channel wall, the larger the shear rate, the more significant the shear heat generation is. The temperature value near the flow channel wall rises to form a crest shape, and the core is the relatively low temperature area of the basin topography. When filling at high speed, the high temperature area of the melt is biased towards the lower mold wall, so the lower mold cavity is filled faster.

To solve the problem of unbalanced filling of multi-cavity injection mold, the fundamental thing is to improve or eliminate the asymmetry of melt temperature distribution in the flow plane in the split channel.

(1) The wall thickness of the product is uniform, and the gate position is as far away as possible from the thin-walled position of the product;
(2) The designed runner takes into account the geometric balance and rheological balance, and the gate position and runner layout are optimized by CAE software simulation analysis.