Initial Runner Sizing

8.5.1 Determining Sprue Dimensions

Sprues or sprue bushings, are typically standard off the shelf items. For a flow analysis, there are typically three required dimensions:

• The orifice diameter, O

• The length, L

• The included angle

See Figure 8.10. The orifice diameter is determined by the injection-molding machine’s nozzle orifice diameter. The sprue orifice diameter must be slightly larger than the nozzle’s diameter so that there is no sharp corner for the polymer to flow over creating an excessive amount of shear. Typically, the sprue orifice is 0.5 mm or 1/32 in (0.031 in) larger than the sprue.

Table 9: Typical standard sprue orifice sizes

The length of the sprue is the flow length which is measured from the bottom of the spherical radius to the bottom of the sprue.

Metric Sizes English Sizes

2.5 mm 3/32 in. (0.094 in.)

3.0 mm 5/32 in. (0.156 in.)

2.5 mm 7/32 in. (0.219 in.)

3.0 mm 9/32 in. (0.281 in.)

4.0 mm 11/32 in. (0.344 in.)

4.5 mm 6.5 mm

142 Runner System Design

The included angle ranges from 1 to 3º, with most sprues with English dimensions having an included angle of 1/2" per foot or 2.38º included angle.

Figure 8.10 Typical sprue dimensions

8.5.1.1 Which Size Sprue to Use

Typically the smallest possible sprue should be used. The smallest diameter is determined by the pressure requirements for the entire tool and the bottom diameter of the sprue compared to the main runner. The pressure drop of the entire tool should be no more than about 75%

of the machine capacity. In some cases, sprues can have a pressure drop through them equal to the rest of the tool. In rare cases, the shear rate in the sprue is higher than the gates. This could happen when the tool has many cavities, for example 32. The gate diameter may be small, and the sprue will have a flow rate 32 times that of the gates, assuming one gate per part.

The bottom sprue diameter should not be smaller than the diameter of the primary runner it feeds.

8.5.1.2 Nonstandard Sprue Sizes

In some cases, the sprue size is not standard. In these cases, the size of the sprue is determined by the orifice diameter in the same way that standard sprue sizes are, and the diameter of the primary runner the sprue feeds. This is done in cases when using a standard sprue, the bottom diameter of the sprue is much larger than the diameter of the primary runner. When this happens, the sprue normally becomes the limiting factor in determining the cycle time. If a custom sprue is going to be used, the included angle must be great enough to allow for easy ejection of the sprue. Although using a custom sprue may be beneficial, it is rarely done in practice.

8.5.2 Designing Runner Cross Sections

8.5.2.1 Common Designs

There are several common runner cross-sectional designs. They are illustrated in Figure 8.11.

Included angle L

O

Initial Runner Sizing 143

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 semicircular 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. The shape of the trapezoid is critical.

Figure 8.12 shows the proper shape of a trapezoidal runner compared to a round cross section. The depth of the trapezoid is equal to the diameter of the runner, and the angled sides are tangent to a circle. The included angle is normally between 10 to 20º, or the taper angle is half the included angle.

Modified Trapezoidal Runner: This cross section is a combination of round and trapezoidal shapes. The bottom of the runner is fully round and extends to the parting line at the included angle of the trapezoid.

Figure 8.11 Commonly used runner cross sections Full-round

144 Runner System Design

Figure 8.12 Trapezoidal runner shape

8.5.2.3 Hydraulic Diameter and Flow Resistance

To compare runners of different shapes, 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:

(8.5) where

Figure 8.13 illustrates how to use the hydraulic diameter to compare different runner shapes.

Figure 8.13 Equivalent hydraulic diameters

Initial Runner Sizing 145

8.5.3 Determining Runner Diameters

A flow analysis is the best place to determine or optimize the runner diameter. However, where is a good starting point? The pressure drop over the runner is related to:

• Viscosity of the material

• Flow length in the runner

• Volumetric flow rate of the polymer

As any of the items listed above increase, so does the pressure requirement.

8.5.3.1 Typical Runner Diameters

Over the years, guidelines for runner sizes have been developed by many different organizations. Generally, they are all about the same. Most give a wide range of sizes for a given material type. This can be used as a good starting point. Table 10 lists typical runner diameters for unfilled materials.

8.5.3.2 Branched Runners

In geometrically balanced runner systems, it is common for the runners to reduce in size from the sprue to the gates. The change in size would occur when the runners split or branch.

Figure 8.14 shows an example of changing the runner size. It is best to calculate the runner sizes using the constant pressure gradient principle using Moldflow runner balancing analysis.

However, the sizes can be approximated using the following formula:

(8.6)

where:

= the diameter of the runner feeding the branch.

= the diameter of the runner branch.

N = the number of branches.

3

In a geometrically balanced runner system, the number of branches will always be two.

For the model in Figure 8.14, the diameter of the runner at the gate is 3.0 mm and is the starting point for the calculations. The number of cavities is eight, so there are two branches in the runner system. To calculate the secondary runner:

(8.7)

In document Mold Flow (Page 161-165)