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Designing runner cross sections

Posted by admin on Aug 1st, 2012

Common designs

There are several common runner cross-sectional designs. They are illustrated in Figure 1.

Full-round runner

Trapezoidal runner

Modified trapezoidal runner (a combination of round and trapezoidal runner)

Half-round runner

Rectangular runner

Recommended cross-sectional designs

The first three runner cross-sectional designs listed above are generally recommended.

Full-round runner

The full-round runner is the best in terms of a maximum volume-to-surface ratio, which minimizes pressure drop and heat loss. However, the tooling cost is generally higher because both halves of the mold must be machined so that the two semi-circular sections are aligned when the mold is closed.

Trapezoidal runner

The trapezoidal runner also works well and permits the runner to be designed and cut on one side of the mold. It is commonly used in three-plate molds, where the full-round runner may not be released properly, and at the parting line in molds, where the full-round runner interferes with mold sliding action.

FIGURE 1. Commonly used runner cross sections

Hydraulic diameter and flow resistance

To compare runners of different shapes, you can use the hydraulic diameter, which is an index of flow resistance. The higher the hydraulic diameter, the lower the flow resistance. Hydraulic diameter can be defined as:

Figure 2 illustrates how to use the hydraulic diameter to compare different runner shapes.

FIGURE 2. Equivalent hydraulic diameters



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