Beware of the “Naturally Balanced” Runner System

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For years now the standard method designing a melt delivery (runner) system has been the naturally balanced runner system. A naturally balanced runner system is a runner configuration where there is an even flow length from the tip of the sprue to the end of each cavity with symmetrical cross-sectional shapes. While the naturally balanced runner system looks good from a visual standpoint, it is what lies underneath that leads to molding issues. This tech tip will explain the pitfalls of the naturally balanced runner system and how to avoid them.

Figure 1 – Naturally balanced runner system

Figure 1 – Naturally balanced runner system

The naturally balanced runner was a tremendous improvement over the previous standard technique of designing a runner system, which was the ladder / fishbone / parts on a stick method. See Figure 2.

Figure 2 – Ladder or fishbone style runner system

Figure 2 – Ladder or fishbone style runner system

The main problem with the ladder runner system is obvious; the parts closer to the sprue will fill first simply due to their close proximity. Ideally, a molder wants all cavities of a mold to fill at the same time. These different flow length’s lead to flashing issues, packing inconsistencies, short shots, variation between cavities, etc. These complications caused the industry to move toward using the naturally balanced runner system-a runner with even flow lengths to each part. This was and still is a big improvement over the ladder style runner.

The injection molding industry, as a whole, had to learn the hard way that even the naturally balanced runner system still causes variation between the cavities. This variation is primarily due to shear heating effects. Shear heating happens as plastic moves thru the nozzle, sprue, runners, gates, and cavities. The majority of the shear heating happens along the outer perimeter of the runner system (Figure 3).

Figure 3 – Cross section of runner channel showing difference in temperature due to shear heating Red = hotter plastic due to shear heating    Blue = Cooler, yet still molten, plastic

Figure 3 – Cross section of runner channel showing difference in temperature due to shear heating
Red = hotter plastic due to shear heating Blue = Cooler, yet still molten, plastic

Due to the fact that plastic is a laminar flowing material, when it reaches an intersection, it splits. So at the next runner branch, one side of the runner now has a higher sheared, less viscous, easier flowing material laminant the other. The cavities that receive this higher sheared material not only fill first, but also fill with a hotter material, causing even further differences between cavities. These difference will lead to issues such as short shots, flash, narrow process windows, dimensional issues, warpage inconsistencies, etc.

Figure 4 – Filling imbalance due to shear heating with the naturally balanced runner system

Figure 4 – Filling imbalance due to shear heating with the naturally balanced runner system

The naturally balanced runner system has no way of controlling these shear heating variations, nor will process changes or mold alterations be able to help the problem. Instead, the only method for achieving a complete balance between cavities is by using a rheologically balanced runner system or what is most commonly known as a MeltFlipper® runner system.

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