Chapter 12,Designing runner cross sections(Textbook for plastic injection mold design)

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Common designs
There are several common runner cross-sectional designs. They are illustrated in Figure
1.
l   Full-round runner
l   Trapezoidal runner
l   Modified trapezoidal runner (a combination of round and trapezoidal runner)
l   Half-round runner
l   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
Runner Cross Sections
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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