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Runner balancing

Posted by admin on Aug 3rd, 2012

Importance of balanced flow

Balanced flow into the cavities is a prerequisite for a quality part. This can be achieved by changing the runner size and length. Changing the gate dimension may give a seemingly balanced filling. However, it affects the gate freeze-off time greatly, which is detrimental to part uniformity. Whenever possible, a naturally balanced runner system should be used to balance the flow of material into the cavities. If a naturally balanced runner is not possible, then the runner system should be artificially balanced, as shown below. Using C-MOLD’s Runner Balancing tool to artificially balance a runner system not only saves time and expense, but also greatly improves the uniform filling of the part.

FIGURE 1. A family-mold part with artificially balanced runners

Reducing runner diameter

To balance a runner system, encourage flow to the cavities farthest from the sprue by reducing the diameter of runners feeding the other cavities. Note that decreasing the runner diameter too much may cause it to freeze prematurely, causing a short shot. On the other hand, increased frictional shear heating may actually reduce the resin’s viscosity, and thus, resistance to flow and fill the cavity even faster. Keep in mind that non-standard runner diameters will increase mold manufacturing and maintenance costs.

Using tighter process controls

An artificially balanced runner system designed for one material may not work for others. Further, an artificially balanced runner system requires tighter process controls. A small variation in the process control will alter the filling pattern of the mold, leading to consistently unbalanced filling.In the course of finalizing a runner design, C-MOLD analysis can help identify the sensitivity of the design to flow rate and the appropriate process window.

Varying the injection speed

For example, if you use a standard (herringbone) runner system with various injection rates, then various filling patterns will result. Generally speaking, a slow injection rate will first fill parts farther out onto the runner, while a faster injection rate will first fill the parts closest to the sprue. This is because at a slower injection rate, the melt tends to hesitate at the restricted gate it first encounters. It moves out to fill the remaining runner system. By the time all the runner branches are filled, the melt at the first, upstream gates may have already become more resistant than the downstream gates, due to solidification. Varied injection speed will result in filling patterns between these two extremes, as illustrated in Figure 2.

FIGURE 2. Filling patterns resulting from various injection rates, in an unbalanced runner system.

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