What is gating system? Gating system is the process of deciding the flow of the molten metal during casting by keeping the appropriate amount in the basin so that the level of the molten metal going inside the cast remains in a constant volume. This molten metal will go through a small segment called "choke". This plays an important roll in the gating system because; this is which allows the metal to go in a constant volume in an appropriate amount, which is required. This choke has the smallest ratio in the gating system when compared to all other segments. The choke area is given by.
In which W is the Gross weight including feeders and gating channels. ?c the metal densities is the gross filling time and the velocity of the choke is depicted by Vc.
The choke velocity is given by:
When this formula is concerned the metabolic pressure head is depicted by H.this is nothing but the distance of the molten metal between from it pours (cup) and to the centerline of the choke. Pouring velocities value would be given (VP) as Non-zero when the liquid is poured from a vertical distance or if the bottom pouring labels are used. The friction factor in this gating system is totally based on the finish of the surface and the it's geometry and it varies between 0.6 to 0.9.
The weight of the gating system sis unpredictable during the calculation of the mold filling time and the choke area. We can know the gross casting mass after the gating d design process and the repeating the calculation.
When the filling happens the merabolicstatic pressure head slowly losses its strength after the molten metal increases above the coke level, which leads to the slow filling. Thus the cross sectional area of sprue exit, ingates and runners is determined by the choke area, number of individual elements and gating ratio.
The sectional area, shape and size of the elements are determined as follows:
The Sprue :The circular cross section of the sprue at the exit or bottom is designed based on the choke area and gating ratio. This in turn reduces the turbulence and heat loss.
The formula for the calculation of the sprue top using the mass and the energy balance equations is as follows,
Where,
H1 and H2 are the metallostatic pressure head at the top and bottom of the sprue
A1 and A2 being the respective cross-sectional areas.
it is normally cylindrical, for which the diameter is twice the sprue top and the depth twice the runner. It must be designed in such a way that it reduces the instability and air aspiration. Its work is to arrest the free fall of the molten metal and making the metal to flow towards the runner. Thus, the molten metal is transferred smoothly between the well and the runner.
The Runner :Runner slows down the fall of the molten metal, whose velocity is very high when it falls through the sprue. The total cross-sectional area of the runner must be greater than the sprue exit. Generally 1:2 ratio is suggested because 1:4 ratio may lead to flow separation. The runner must be filled completely with the molten metal before it runs through the ingates to guarantee uniform flow.
The Ingate :The molten metal reaches the mold cavity from the gating system through ingate. The ingate will have following characteristics:
- The ingate area must be bigger than the sprue exit, so that the flow of the metal is slow.
- The ingate must have a smaller cross-section, so that it is easy to handle.
- The ratio of volume to cooling surface area of the ingate must be smaller than that of the connected section, in order to avoid the local hot spot.
- The molten metal, which flows through the ingate, must be related to the volume of the region that is connected to the casting.
Hence the ingates must be properly designed in such a way that it satisfies the above requirements.
Thus the process of gating system works.