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Factors you’ll need to consider
The diameter and length of runners influence flow resistance. The higher the flow resistance in
the runner, the higher the pressure drop will be. Reducing flow resistance in runners by
increasing the diameter will use more resin material and cause a longer cycle time if the runner
has to cool down before ejection. First design the diameter by using empirical data or the
following equation. Then fine-tune the runner diameter using C-MOLD to optimize the delivery
system.

Formula
Following is the formula for runner dimension design:
where
D= runner diameter (mm)
W= part weight (g)
L= runner length (mm)

Example: using empirical data to calculate runner
dimensions
Figures 1 and 2 provide empirical data that you can use to calculate runner dimensions. For example, what
should the runner diameter be for an ABS part of 300 grams, with a nominal thickness of 3 mm, and a
runner length of 200 mm?
1According to Figure 1, take the point of 300 grams of the ordinate, draw a horizontal line and meet the
line of nominal thickness = 3 mm, draw a vertical line through the intersection point and meet the abscissa
at 5.8 mm.
This is the reference diameter.
2UsingFigure 2, take the point of 200 mm of the coordinate, draw a horizontal line and meet the curve,
draw a vertical line through the intersection point and meet the abscissa at 1.29.
This is the length coefficient.
Determining Runner Dimensions
3Multiply 5.8 mm by 1.29 to calculate the runner diameter.
The diameter is 7.5 mm.

FIGURE 1. Runner diameter chart for several materials. G=weight (g); S=nominal thickness (mm); D=reference
diameter (mm).

FIGURE 2. Effect of runner length and length coefficient on diameter.
Typical runner diameters
Typical runner diameters for unfilled materials are listed in Table 1.
TABLE 1. Typical runner diameters for unfilled generic materials