In analogy to gate size calculation, ratio between the cross-sectional areas from the downsprue to the runner and the sum of all gates.
In gravity casting, e.g. sand casting or gravity die casting, the ratio of Adownsprue : Arunner : Agate should be 1:1:1 to 1:2:2 or 1:4:4. The latter applies to metals exhibiting a high propensity to foaming and slag formation and readily oxidizing metals.
When designing channel cross sections, it must be ensured that the sum of all gate areas is at least equal to or better greater than the narrowest downsprue area. Spraying effects or turbulences within the melt flowing into the mold cavity must be avoided; this is referred to as an unpressurized gate system (Fig. 1, right).
Generally, the runner area is chosen to be slightly greater than the smallest downsprue area. There is a 90° deflection in flow direction at the junction between the vertical sprue channel and the horizontal runner. The flowing metal is slowed down, i.e. its flow rate decreases. Therefore, the runner area must be widened in order to avoid backflow. Friedrich Nielsen suggested a 1.4-fold expansion of the channel following the downsprue (the precise value without losses would be the factor √2, see Downsprue).
In order to ensure optimum filling of the mold for cast iron and heavy metal casting materials (e.g. casting of fittings), the sum of all gate areas is designed to be less than the narrowest downsprue area (pressurized gate, pressurized gating system, pressurized casting). These systems are particularly preferred in pressure die casting; here, the total area is slightly less than the narrowest downsprue area (see Gate size calculation, Casting time diagram). Based on an appropriate downsprue-runner gating ratio, the minimum downsprue area required can also be determined.
In conclusion: in many cases, the casting quality is defined by the selection of the right casting system and the correct area ratios. There is a general distinction made between pressurized and unpressurized casting systems. The main difference is the position of the narrowest cross-sectional area within the runner system, namely:
Pressurized casting system:
The sum of all gate areas is the narrowest cross-sectional area, thus being critical for the mold filling time. For cast iron, this is the most common casting system. In case of high heads, the following should be noted: the high casting pressure and throttling result in high flow rates and therefore turbulences at the gate!
Unpressurized casting system:
The smallest cross-sectional area is located in front of the gates inside the runner or at the end of the runner. This casting system is used for molds used more than once and casting materials which are sensitive to oxidation and could lead to oxide and gas inclusions when subject to high turbulences in the gate.
Flow rate in the gate