CALCULATIONS FOR DESIGN OF FEEDING SYSTEM
The feeder must satisfy both of the following two requirements:
Heat transfer criteria
MC : MN : MF = 1 : 1.1 : 1.2
where MF, MC and MN are modulus of feeder, modulus of casting, and modulus of the neck of feeder at the junction of casting respectively.
For calculation of modulus of an object, see Fig.1 and Fig. 2.
Feed volume criterion
VF ≥ α VC / (ε – α)
where ε is the efficiency of the feeder, α is the solidification shrinkage, and VF and VC are volume of feeder and casting respectively.
For efficiency of differently shaped feeders, see Fig. 3.
The higher of the two values of feeder volume given by equations (1) and (2) satisfies both requirements and thus taken as the actual volume of the feeder.
DETERMINATION OF FEEDER SHAPE
According to Chvorinov, solidification time increases as the volume to area ratio or the modulus, M, increase. Thus
where, t is solidification time, and C is constant.
According to eq.(3), a sphere, having the smallest modulus per unit volume, is the ideal shape for a feeder. But a spherical feeder practically too difficult to mould and consequently cylindrical feeders are mostly used.
For a relationship between casting shape (M) and its solidification time (t) for various metals and alloys, see Fig. 4.
3. FEEDING DISTANCE
In normal conditions, there is a limit to how far a feeder can feed along a flow path. Up to this distance from feeder, the casting will be sound. Beyond this distance the casting will exhibit porosity. For
feeding distance rule, see Fig. 5.
4. INCREASING FEEING EFFICIENCY
There are number of ways by which the efficiency of a feeder can be increased. Some of the most common ways are:
1. Use feeder head with a higher feeding efficiency
2. Feeder insulation
3. Use of exothermic materials