A gating system refers to the channels through which the molten metal flows to the die cavity. Its key objective is to ensure its smooth and complete flow from the ladle to the mold cavity. To achieve perfect castings, it is important to have a gating system that is well designed.
Components of the Gating System
The main elements of the system comprise the pouring basin, the sprue, the well, the runner as well as the ingate. It can be classified on the basis of the parting plane’s position, and that of the ingate.
- Horizontal System: This type of gating system is suitable for flat casting, achieved by filling the mold cavity with gravity. It is generally applied in ferrous metals’ sand casting as well as in non-ferrous metals’ die casting process.
- Vertical System: This type of system is suitable for tall casting. It is used in high-pressure sand casting, shell mold casting, and die casting.
- Top Gating System: It is applied in processes where the metal in molten form is poured from the intended casting’s top space. It promotes directional solidification. The system suits merely flat casting to limit the metal’s damage, during the filling process.
- Bottom Gating System: It is applied in tall casting. In this system, the metal slowly enters the cavity from the bottom.
- Middle Gating System: As the name implies, this one has the features of both the bottom and top gating systems.
Gating System Design
It is designed to fill the cavity in the prescribed time by maintaining a constant molten metal level in its basin. This is in order to achieve a controlled rate of the molten metal flow through the choke – the cross section, which regulates the flow rate in the system.
The shape and dimensions of various elements in it are arrived at, by taking into account the following.
- Sprue: The circular cross section that minimizes heat loss and turbulence is sprue, and the area of it is quantified from the choke area as well as the gating ratio. Preferably, sprue has to be small at the bottom and big at the top.
- Sprue Well: It is also designed to limit the free molten metal fall, by directing the metal in a correct angle to the runner. The sprue well aids in minimizing the turbulence and aspiration. Ideally, it has to be cylindrical in shape having the diameter, two times than that of the sprue exit, and depth twice as that of the runner.
- Runner: It primarily slows down the flow speed of the molten metal, during its free fall from the above-mentioned channel to the ingate. The runner cross section has to be not just bigger than the sprue exit but also allow filling the molten metal, before letting it enter the ingates. For a gating system where there is more than an ingate, it is recommended the cross section area should be lowered after every ingate connection to make sure the smooth flow of the molten metal.
- Ingate: This is the component, which directs the liquid to the die cavity. Die casters recommend ingate be designed to minimize the metal velocity; the design has to facilitate easy fettling, should not lead to hot spot, and the molten metal flow from the ingate has to be proportional to the casting area’s volume.
The position of ingate(s) is extremely vital in designing the system. It is also important to use sufficient ingates that ensure the distance of the flow between the ingate, and any point that is filled by it, is less than the molten metal fluidity distance. The sprue usually directs its flow from the basin to both the runners as well as the ingates. The sprues’ location depends on the factors mentioned below.
- Flow Distance: It has to minimize the overall flow distance, with the gating channel present to offer minimum heat loss and maximum yield.
- Heat Concentration: It should be located away from the hot spots.
- Mold Layout: It has to be positioned to reduce the size of the box that encloses the whole casting.
- Hick Sections: It lets the molten metal flow to other channels with minimal cooling, thereby reducing breakage of the ingates during fettling.
Optimal Filling Time
The mold filling time is extremely crucial since it affects the final quality of the output. A slow fill leads to mis-runs and cold shuts, whereas a quick fill can bring about gaseous and solid inclusions. The optical mold filling time can be computed. The final function of the product, the pouring temperature die casters use, the weight and minimum section thickness of the casting, as well as the velocity of the molten metal play a role in determining the filling time.