Basic layouts
There are three basic runner system layouts typically used for a multi-cavity system.
These layouts are illustrated in Figure 1 below.
1.) Standard (herringbone) runner system
2.) “H” bridge (branching) runner system
3.) Radial (star) runner system
FIGURE 1. Basic runner system layouts
Balanced layouts
The “H” (branching) and radial (star) systems are considered to be naturally balanced. The naturally
balanced runner provides equal distance and runner size from the sprue to all the cavities, so that
each cavity fills under the same conditions.
Unbalanced layouts
Although the herringbone is naturally unbalanced, it can accommodate more cavities than its
naturally balanced counterparts, with minimum runner volume and less tooling cost. An unbalanced
runner system can be artificially balanced by changing the diameter and the length of the runner.
Automatic balancing
Runner System Layout
Runner balancing can be accomplished automatically with C-MOLD runner balancing analysis.
Helping the melt flow
A runner system directs the melt flow from the sprue to the mold cavities. Additional pressure is required to push the melt through the runner system. Shear (frictional) heat
generated within the melt while the material is flowing through the runner raises the melt
temperature, also facilitating the flow.
Common designs
There are several common runner cross-sectional designs. They are illustrated in Figure
l.) Full-round runner
2.) Trapezoidal runner
3.) Modified trapezoidal runner (a combination of round and trapezoidal runner)
4.) Half-round runner
5.) 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
Factors you’ll need to consider when design the runner:-
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 byincreasing 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?
1.) According 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.
2.) Using Figure 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
3.) Multiply 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
Runner size considerations
Although properly sizing a runner to a given part and mold design has a tremendous pay-off, it is often overlooked since the basic principles are not widely understood.
Pros and cons of large runners
While large runners facilitate the flow of material at relatively low pressure requirements, they require a longer cooling time, more material consumption and scrap, and more clamping force.
Pros and cons of small runners
Designing the smallest adequate runner system will maximize efficiency in both raw material use and energy consumption in molding. At the same time, however, runner size reduction is
constrained by the molding machine’s injection pressure capability.
Optimal runner size
C-MOLD Runner Balancing is an excellent software tool for computing the optimal runner size that conveys a balanced filling pattern with a reasonable pressure drop.
Payoffs of good runner design
A runner system that has been designed correctly will:-
l.) Achieve the optimal number of cavities
2.) Deliver melt to the cavities
3.) Balance filling of multiple cavities
4.) Balance filling of multi-gate cavities
5.) Minimize scrap
6.) Eject easily
7.) Maximize efficiency in energy consumption
8.) Control the filling/packing/cycle time.
There are three basic runner system layouts typically used for a multi-cavity system.
These layouts are illustrated in Figure 1 below.
1.) Standard (herringbone) runner system
2.) “H” bridge (branching) runner system
3.) Radial (star) runner system
Balanced vs. unbalanced layouts
Balanced layouts
The “H” (branching) and radial (star) systems are considered to be naturally balanced. The naturally
balanced runner provides equal distance and runner size from the sprue to all the cavities, so that
each cavity fills under the same conditions.
Unbalanced layouts
Although the herringbone is naturally unbalanced, it can accommodate more cavities than its
naturally balanced counterparts, with minimum runner volume and less tooling cost. An unbalanced
runner system can be artificially balanced by changing the diameter and the length of the runner.
Automatic balancing
Runner System Layout
Runner balancing can be accomplished automatically with C-MOLD runner balancing analysis.
Helping the melt flow
A runner system directs the melt flow from the sprue to the mold cavities. Additional pressure is required to push the melt through the runner system. Shear (frictional) heat
generated within the melt while the material is flowing through the runner raises the melt
temperature, also facilitating the flow.
Common designs
There are several common runner cross-sectional designs. They are illustrated in Figure
l.) Full-round runner
2.) Trapezoidal runner
3.) Modified trapezoidal runner (a combination of round and trapezoidal runner)
4.) Half-round runner
5.) 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. Equivalent hydraulic diameters
Factors you’ll need to consider when design the runner:-
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 byincreasing 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?
1.) According 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.
2.) Using Figure 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
3.) Multiply 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
Runner size considerations
Although properly sizing a runner to a given part and mold design has a tremendous pay-off, it is often overlooked since the basic principles are not widely understood.
Pros and cons of large runners
While large runners facilitate the flow of material at relatively low pressure requirements, they require a longer cooling time, more material consumption and scrap, and more clamping force.
Pros and cons of small runners
Designing the smallest adequate runner system will maximize efficiency in both raw material use and energy consumption in molding. At the same time, however, runner size reduction is
constrained by the molding machine’s injection pressure capability.
Optimal runner size
C-MOLD Runner Balancing is an excellent software tool for computing the optimal runner size that conveys a balanced filling pattern with a reasonable pressure drop.
Payoffs of good runner design
A runner system that has been designed correctly will:-
l.) Achieve the optimal number of cavities
2.) Deliver melt to the cavities
3.) Balance filling of multiple cavities
4.) Balance filling of multi-gate cavities
5.) Minimize scrap
6.) Eject easily
7.) Maximize efficiency in energy consumption
8.) Control the filling/packing/cycle time.
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