A mould cavity must be filled with clean metal in a controlled manner to ensure smooth, uniform and complete filling, for the casting to be free of discontinuities, solid inclusions and voids. This can be achieved by a well-designed gating system. The first step involves selecting the type of gating system and the layout of gating channels: the orientation and position of sprue, runner and ingate(s). The most critical design decision is the ideal filling time, based on which the gating channels are designed.

The main objective of a gating system is to lead clean molten metal poured from ladle to the casting cavity, ensuring smooth, uniform and complete filling. Clean metal implies preventing the entry of slag and inclusions into the mould cavity, and minimizing surface turbulence. Smooth filling implies minimizing bulk turbulence. Uniform filling implies that all portions of the casting fill in a controlled manner, usually at the same time. Complete filling implies leading molten metal to thin and end sections with minimum resistance.


Major elements of a gating system
The major elements of a gating system include pouring basin, sprue, well, runner and ingate, in the sequence of flow of molten metal from the ladle to the mould cavity (Fig.4.2). The pouring basin or bush or cup is a circular or rectangular pocket that accepts the molten metal from the ladle. The sprue or downsprue, usually circular in crosssection, leads molten metal from the pouring basin to the sprue well. The sprue well or base changes the direction of molten metal by right-angle and sends it to the runner. The runner takes the metal from the sprue to close to the casting. Finally, the ingate leads the metal to the mould cavity. Another major element is filter or slag trap, usually placed in the runner or between the runner and ingate, meant for filtering out slag and other inclusions.

The sprue is always vertical. The well, runner and ingate are usually located in the parting plane. Depending on the orientation of the parting plane, the gating systems can be classified as horizontal and vertical gating systems. Thus in horizontal gating systems, the sprue is perpendicular to the parting plane, whereas in vertical gating systems, the sprue is parallel to the parting plane.

Gating systems can be classified depending on the orientation of the parting plane (which contains the sprue, runner and ingates), as horizontal or vertical. Depending on the position of the ingate(s), gating systems can be classified as top, parting and bottom.

Horizontal gating systems are suitable for flat castings filled under gravity. They are widely used in sand casting of ferrous metals, as well as gravity diecasting of non-ferrous metals.


Layout of vertical gating system with top, side and bottom ingates
Vertical gating systems are suitable for tall castings. They are employed in high-pressure sand mould, shell mould and diecasting processes, where the parting plane is vertical.

Top gating systems, in which hot molten metal enters at the top of the casting, promote directional solidification from bottom to top of the casting. These are however, suitable only for flat castings to limit the damage to metal as well as the mould by free fall of the molten metal during initial filling.

Bottom gating systems have the opposite characteristics: the metal enters at the bottom of the casting and gradually fills up the mould with minimal disturbances. It is recommended for tall castings, where free fall of molten metal (from top or parting gates) has to be avoided.

Middle or side or parting gating systems combine the characteristics of top and bottom gating systems. If the gating channels are at the parting plane, they are also easier to produce and modify if necessary, during trial runs.

The most widely used system is the horizontal gating with ingates at the parting plane. In vertical gating systems, ingates may be positioned at top, bottom and side.