The purpose of the runner is to transport the melt from the sprue to the gates. There are three basic parameters for runner geometry:

The cross-sectional shape
The diameter
The cavity layout

The most efficient runner section is the full round and this should be used wherever possible.For three-plate tools the trapezoidal runner has to be used.The semi circular or half round runner severely restricts flow and should be avoided
Although it is frequently seen in production mold tools. Square section and rectangular section runner should never be used.
The melt stream will always revert to a circular cross-section, leaving areas of slow-moving melt in the corners. However, the trapezoidal runner is the only choice for certain applications such as three-plate molds.
The rectangular, square and semicircular cross-sections present similar problems with melt flow efficiency and should never be used. Even though many existing runner systems employ these undesirable designs they are the illogically inefficient and wasteful on material and energy.
The size of the runner should be based on the thickness of the molding wall section. It must be large enough to provide adequate pressure to all the cavities. This ensures that there will be no packing pressure shortfall and thus permit adequate control over the molding conditions to achieve satisfactory moldings. An alternative method is to calculate the runner cross-section based on an appropriate pressure drop along the length of the runner.
All runner intersections should have a cold slug well beneath each intersection and preferably have an ejector pin beneath them. The cold slug well helps the flow of material through the runner system and into the cavity. The length of the well is usually equal to the runner diameter. A cold slug well must also be placed at the end of each runner after it intersects with another runner. This cnyurtrs that, as the runner cools, the advancing cool front is trapped at each stage.

Link to this article：Runner Cross-Section and Layout

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