Flow efficiency in runners increases as the cross section approaches a circular shape. The round runner configuration shown in Figure 15 requires machining both halves of the mold across the parting line. Typically, a compromise is reached with the half-round approach. Trapezoidal and rectangular runner systems are not optimum, as most of the flow takes place in the circular channels (dark shaded on the diagram), and the rest of the runner cross section is not used efficiently.
Runner Design Guidelines
Usually a 5 degree draft angle on the flat sides of the runner to ensure good ejection. The bottom of the runner should be fully radiused.
When provided with full-round runner sizes and tool must use another cross section the simplest method for determining the dimensions for the alternate shape is to circumscribe a circle of the recommended diameter with the alternate shape.
Half-Round Runner Design