China mold maker and supplier

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.