The function of the sprue is to guide metal down from the sprue cup into the sprue, inner sprue or directly into the cavity. Provide enough pressure, so that the metal liquid under the action of gravity can overcome various flow resistance, in the specified time to fill the cavity. The sprue is often made into a conical shape, a column of equal section and a conical shape. For aluminum and magnesium alloy castings, serpentine, sheet and slot straight runner are also used.
The sprue is the first channel through which the metal liquid enters the mold cavity, and it is also the primary part of pressure transmission, so its size will affect the flow speed and filling time of the metal liquid.