Gating System

A network of passage of molten metal from pouring basin to mould cavity is known as gating system. It consists of pouring basin, downsprue, skim bob, runner, gate, riser, etc. A good gating system should have the following properties:

• Metal should enter the mould cavity with low turbulence.
• Erosion of passageway should be avoided.
• There should be provision of directional solidification of metal. The solidification should progress from mould surfaces to the hottest metal so that there is always hot metal available to compensate for shrinkage.
• Slag or other foreign particles should be prevented from entering the mould cavity using skim bob.
• There should not be aspiration problem due to poor design of downsprue.

Types of Gates

• Parting line gate: Metal enters the cavity at parting plane (Figure 20.16a).
• Top gate: Metal enters at top of the cavity (Figure 20.16b).
• Bottom gate: Metal enters from bottom of the cavity (Figure 20.16c).
• Side gate: metal enters from side of the cavity (Figure 20.16d).

Figure 20.16 Types of Gating Systems

Risers

Risers are provided in mould to feed molten metal into the cavity to compensate the shrinkage. In the initial stages of pouring, it allows the air, steam, and gases to go out of the mould. They should be large in section, so as to remain molten as long as possible and should be located near heavy sections that will be subject to heavy shrinkage and are last to freeze.

Risers used in casting may be either open type or blind type. The open riser is open to atmosphere. The blind risers are dome shaped either on the top or side of a casting. It is surrounded by the moulding sand from all sides. A vent may be provided at its top. This riser may be located in either the cope or the drag. It derives its feeding force from the force of gravity on the liquid metal in it. Blind risers provide molten metal more hot in comparison to open risers.