8 Runner System Design
8.2 Runner System Design Principles
8.2.1 Benefits of Good Runner Design
A mold and runner system that has been designed correctly will:
• Have an optimal number of cavities
• Achieve balanced filling of multiple cavities
• Achieve balanced filling of multi-gated cavities
• Minimize scrap
• Eject easily
• Not control the cycle time
8.2.2 Runner Design Philosophy
Traditionally runners have been thought of simply as a means of getting plastic into the cavity.
On this basis the size of the runner was not critical, as long as it was big enough to fill the cavity. With the Moldflow philosophy, the design of the runner is crucial because the combination of the position of the gate and the size of the runner controls the filling pattern within the cavity. The runner is used as a flow control device.
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8.2.3 Flow Balancing
Runner systems must be designed so each cavity (or portion of a cavity) fills at the same time and pressure. When a runner system is used to balance flow, the total fill pressure of runner plus cavity pressure drop must be equal. It is not sufficient only to balance the runners without considering the cavity. Changing the runner system will alter the cavity pressure drop because the flow rate and frictional heating will change.
The more uniform the balance between cavities and within cavities (multigated part), the higher the part quality will be and the easier the parts will be to mold.
8.2.4 Flow Control
Controlling the flow, or balancing, should be done by runners, not by gates.
8.2.4.1 Gates
Gates are very poor flow control devices because:
• The pressure drop over the gate can be heat-transfer dominated, so any small change in molding conditions gives a large change in filling pattern
• Gates are very prone to hesitation effects
• Entrance and exit losses, which tend to be very unstable, form a high proportion of the total pressure drop
• Machining errors and wear have a major effect on pressure drops
Figure 8.3 shows an example of how gates should not be used to balance the flows paths in a mold. In Figure 8.3 (a), all three parts have the same gate size of 0.50 x 0.50 mm (0.020 x 0.020 in). The gate land is very long. Before the use of flow analysis, balancing of the mold was done by changing the gate size. Since only the left cavity lags significantly behind the other two, the gate size of the left part is opened to 0.75 mm (0.030in) square. The resulting filling pattern is shown in Figure 8.3 (b). Now the left part fills before the other two. There is also a noticeable hesitation between the middle and right parts. Because the middle part is lagging the most, that gate is adjusted next as shown in Figure 8.3 (c). The gate is opened 0.10 mm (0.004 in) in thickness and width. The balance has significantly changed again.
Assuming this process continued until all the parts are filling at about the same time, the molding window would be very small. Any change in the process would have a noticeable influence on the balance between the cavities. This would include natural variation (noise) that occurs on the production floor.
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Figure 8.3 Gates make poor flow control devices
8.2.4.2 Runners
Runner systems are very good flow control devices because:
• A runner system is much larger than a gate, and therefore less sensitive to hesitation and thermal effects
• They have a fully developed and stable flow pattern
• Runners are easier to machine accurately
(a)
(b)
(c)
Gate closest to sprue hesitates significantly Original gate sizes All gates 0.50 x 0.50 mm (0.020 x 0.020 in)
Gate closest to sprue opened 0.75 x 0.75 mm (0.030 x 0.030 in) Outer cavities fill significantly after the first Noticeable hesitation in middle cavity
Middle gate opened 0.60 x 0.60 mm (0.024 x 0.024 in) Noticeable hesitation in outer cavity
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8.2.5 Frictional Heating in Runners
In addition to controlling flow, runners can be used to give controlled frictional heating. The concept of frictional heating in the runners is of major importance. As shown previously in Chapter 2, the residual stress level is lowered proportionately to the raising of the melt temperature. Simply raising the barrel temperature will reduce stress levels, but will also give severe degradation problems. This occurs because the plastic is then subject to a high temperature in the barrel for several machine cycles, a time measured in minutes.
In contrast, running the barrel at a lower temperature and relying on frictional heating in the runner will give the same effect of lower stress levels, but without degradation of the material.
This is a result of the plastic only being subject to the higher melt temperature from the time it enters the runner system until it starts to cool, a time measured in seconds.
Using smaller runners generates shear heat, which lowers stress levels in the part and produces a part of higher quality. This allows the barrel melt temperature to be lower while still having the hotter material in the part.
3
Frictional heating is generally associated with cold runners; however, there may be some frictional heating in hot runners as well.8.2.6 Thermal Shutoff
Runners should be designed to allow for proper filling and packing of the parts without controlling the cycle time. During the compensation phase, molecules are being forced into the cavity as the material is freezing and shrinking. The combination of flowing and freezing at the same time locks in high levels of orientation and residual stress. Larger runners stay open too long allowing more flowing and freezing to occur in parts.
Cold runners should also be small enough so that they do not limit cycle time of the molding machine. The runners do not have to be frozen at ejection, but they must be able to withstand ejection forces. As a maximum, the freeze time of the runner system should be two to three times that of the part.
8.2.7 System and Runner Pressures
In general, the higher the runner pressure drop, the better will be the flow control. The additional frictional heating will lower residual stress levels in the cavity, which should make better parts. The available pressure from the injection machine sets a limit for the maximum total filling pressure. Since normally some safety factor normally is used, the runners are designed so that the total pressure drop, (cavity plus runners), is 70 to 75% of the maximum available injection pressure. However, sink marks always have to be considered.
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There is always a conflict between sink marks and stress levels. Stress levels, which effect warping, are minimized by using high runner pressures and high melt temperatures. However, if the runners are made too small, they will freeze off before adequate compensating flow has occurred. Runners, therefore, have to be designed on a cooling time basis to ensure that they freeze off just after the holding pressure is dropped.
8.2.8 Constant Pressure Gradient
Runners should be designed using the constant pressure gradient principle. This will produce the lowest possible volume for a given pressure drop. Runners sized using a constant pressure gradient will get smaller as the runner splits. Figure 8.4 shows an example of a runner system balanced with constant pressure gradient. The model shown is one quarter of the mold. Each time the runner splits, the diameter changes a little.
Figure 8.4 Constant pressure gradient used to size and balance the runners
8.2.9 Cold Slug Wells
A cold slug well is an extension of a runner system past the last branch. The purpose of a slug well is to capture the cold slug of polymer that may form in the nozzle between shots. Most of the time, if the slug exists is trapped at the bottom of the sprue. Just in case it is not, the
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2.108[mm]
2.286[mm]
2.667[mm]
2.794[mm]
2.946[mm]
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runners are extended slightly. The amount they should extend past the branch is 1.0 to 1.5 diameters, as shown in Figure 8.5.
Figure 8.5 Cold slug well
8.2.10 Easy Ejection
Runner design must provide for easy ejection and easy removal from the molded part with proper cross-sectional and draft angle. For most materials, the runner surface must be polished to facilitate flow and part ejection. Extended runner systems should have multiple sprue pullers and ejection locations.