Feeders are meant to even out the solidification shrinkage of a casting to avoid shrinkage porosity. Number, location, shape and dimension of feeders are the design parameters of feeders.Feed path and feeding distance
Feed path and feeding distance influence the location and number of feeders. The solidification starts from the end regions and ends at the last freezing points. The feed metal follows the opposite direction i.e., from regions at a higher temperature to the contiguous solidifying locations. This entire path is referred to as feed path and in this entire path any intermediate point should have only a single adjacent point with a higher temperature with the exception of the hot spot, which remains the local temperature maxima.
The hot spot must be inside the feeder to make the casting defect free and the distance from a feeder to the farthest point along the feed path is the feeding distance. Researchers like Pellini and Bishop have related section thickness and feeding distance for simple shaped steel metal castings in sand mold.
For the same, the total feeding distance is given by 4.5 t (the feeder effect is 2 t and the end effect is 2.5 t). Some other researchers have evinced a relationship between feeding distance and modulus. In some other metals the feeding distance is not directly related to section thickness. However, in complex shaped castings, the modulus of the simple shaped regions is calculated and that with higher modulus is assumed to feed its adjacent region.Feeding types
The feeding depends on thermal gradient and temperature. The gradient must be greater than a minimum critical value for feeding to take place. The critical value is influenced by casting shape and the quality requirements. Depending on the temperature and gradient at any point along the feed path there could be mass feeding, inter-dendritic feeding and solid feeding. If both temperature and gradient are high near the feeder then mass feeding takes place.
If temperature is high with a lower gradient near the centre of thick sections it results in inter- dendritic feeding and its reverse would be solid feeding. Improper feeding in these three zones results in macro porosity; micro porosity and surface sink respectively.Feeder design
The design of the feeder depends on the number of hot spots. If there is only one major hot spot, the feeder must be connected closest to it. Two or more means multiple feeders and for several hot spots with different solidification time design must be intended to cater to the hottest one first followed by analysis as to if it can feed any other hot spot. Then the next feeder is allocated for the next hot spot.
Minor hot spots are eliminated by chills. The use of exothermic sleeves & covers or insulating is a cardinal parameter in the design of feeder. They substantially enhance the modulus and hence a smaller feeder can be used still with a higher yield. The feed aids are available in standard shapes/size.Feeder types
Feeders can be top or side feeders. Top feeders are above the hot spots and have the advantage of gravity but needs a core for producing an under cut at its neck. The side feeders can do away with the core. There are also open feeders (risers) with the top open to the atmosphere and the one, which are not, are the blind ones. The location of the feeder must provide for fettling and grinding off the feeder mark. The ideal shape can be spherical, cylindrical or even cruciform. In practice, cylindrical feeders with or without spherical bottoms or top are used.