At all runner intersections, the primary runner should overrun the secondary runner by a minimum distance equal to one diameter, as shown in the Recommended Design of a Cold Slug Well illustration. This produces a feature known as a melt trap or cold slug well.

Cold slug wells improve the flow of the polymer by atching the colder, higher-viscosity polymer moving at the forefront of the molten mass and allowing the following, hot, lower-viscosity polymer to flow more readily into the mold-cavity. The cold slug well thus prevents a mass of cold material from entering the cavity and adversely affecting the final properties of the finished part.