A gating system is usually made up of :
(i) Pouring basin
(ii) Sprue well
(iii) Runner
(iv) Flow-off gate
Basic requirements expected from a gating system are to minimize turbulence and the aspiration of air and gases, to avoid dross formation to decrease the velocity of metal. So as to minimize mould erosion, to avoid premature freezing of metal and assist in developing proper temperature gradient.
The are of sprue base
As = Ra(d√2gH)
where
R = Adjusted pouring rate, d = density of molten metal
H = Sprue height
Here Ra = R R = Pouring rate
R.C
K = Metal fluidity
C = Friction coefficient
The pouring rate R for ferrous metals and copper base alloys is given by the empirical formula
R = (w)P
(0.95+t/0.853) W = Weight of casting
t = Critical casting thickness
and P a quotient whole value depends upon the weight of the casting.
The value of P for different castings is as given:
Weight of Casting Value of P
Up to 500 Kg 0.50
500 - 5000 Kg 0.67
5000 - 15000 Kg 0.70
For light metal castings : R = b √w
Where the value of b relies on the wall thickness
Weight of Casting Value of P
Below 6 mm 1.48
6 - 12 mm 1.25
Above 12 mm 0.70