Gating Ratio

The term gating ratio is used to describe the relative cross-sectional areas of the components of a gating system. It is defined as the ratio of sprue area to the total runner area to the total gate area. A gating system having sprue of 1sq.cm CS, a runner of sq.cm CS, and three gates, each of 1sq.cm CS, !ill have a gating ratio of 1"". #ating ratios are grouped in t!o classes, vi$., pressurized and unpressurized systems. In the the  pressurized  systems,   systems, the proportions of sprue, runner and gate areas are so arranged that bac%   pressure is maintained on the gating system by a fluid film restriction at the gates. This requires that the total gate area is not greater than the area of the sprue. #ating ratios such as 1"&.'("&.(, 1")"1 and )"1"1 !ill therefore produce a pressuri$ed system. A pressuri$ed pressuri$ed system %eeps itself full of metal. The danger of  metal pulling a!ay from the !alls !ith consequent air aspiration is thus minimi$ed. As this system is small in volume for a given metal flo! rate, it results in a smaller loss of metal and greater yield. *n the other hand, high metal velocities in a pressuri$ed system may end to cause severe turbulence at the  +unctions and corners and in mould cavity. This system is generally suitable for ferrous metals and brass. The pressuri$ed system encourage turbulence at ingate and hence not used for aluminum. Cast irons are generally cast by pressuri$ed gating. In the unpressurized  systems  systems of gating produces lo!er metal velocities and permits greater flo! rates. It reduces turbulence in the gating system and spurting in the mould cavity. *n the other hand, this system requires careful design to ensure complete filling, and large si$ed runners and gates, !hich reduce the yield and increase the !astage of metal. urther, equal flo! in the case of multiple gating is difficult to achieve. This system is generally adopted for metals such as aluminum and magnesium. The ratios used are 1")"), 1"" etc.