Three major requirements that are to be satisfied by a feeder designed for a given hot spot are as follows:
Solidification Time: The solidification of the feeder must take place later than the nearest hot spot, expressed by the criterion:
Here the modulus of the feeder is given by M f, the modulus of the casting region around the hot spot is given by M h and the feeder design factor, usually more than 1 (more than 1.1 for ductile iron casting, and more than 1.2 for aluminum and steel casting) is given by k f. A larger factor might be needed (1.4 or more), if there is an intermediate section of casting between the feeder and the hot spot. It is to be noted that after connecting the feeder the modulus of the hot spot region will increase because the heat transfer area corresponding to the feeder neck would be reduced. Thus the feeder size must be further increased to take this into account.
Feed Path: Between the feeder and the hot spot there must be a clear feed path. For the liquid metal to flow from the feeder to the hot spot, sufficient gradients must exist. If the connection between the feeder and the casting is through a neck then the design should be such that the following criteria are satisfied:
In the equation M n represents the modulus of the feeder neck. If instead of being connected to the casting face near the hot spot, the feeder is connected to another intermediate section i with modulus M i, then the criterion would be modified as:
- In a way there has to be gradual increase in the modulus from the hot spot to the intermediate section to feeder neck to feeder, where its value must be the highest.
- For metals such as aluminum and steel castings that show shrinkage in the volume at the time of solidification, this relation works and each kf can have the minimum value of 1.1.
- However for metals which expand towards the end of solidification, like grey iron and ductile iron the value of k f should be less than 1.0 percent to prevent the 'reverse feeding' from casting to feeder through the neck.
- Feed Metal Volume: The solidification shrinkage of the hot spot region must be compensated for by the feeder.
- The satisfaction of this requirement is done by the criterion : n f V f = a (V c +V f)
In this equation the volume of the feeder and the casting are represented by V f and V c respectively; the feeder efficiency (ratio of volume of available feed metal to feeder volume) is denoted by n; and volumetric shrinkage of the caste metal is denoted by a. V c corresponds to the volume of the region fed by a particular feeder when multiple feeders are used. Since the feeder itself is solidifying and all of its volume is not available for feeding the casting therefore the feeding efficiency comes into picture. The feeder shape, type (open or blind) and application of feed aids (insulation or exothermic) are the factors on which the efficiency depends. The efficiency would be 14% for an open cylindrical feeder with height=1.5 times diameter. For the feeders with exothermic or insulated sleeves and pads the efficiency can be much higher (50% or more). The range of the volumetric shrinkage extends from zero for irons to 3-4% for steels to 6-7% for aluminum alloys. For large castings with thin sections and when the same feeder is connected to multiple castings, the feed metal volume check is likely to fail.
The following steps are followed by the feeder design:
- The modulus of the region around the hot spot is estimated.
- On the basis of the solidification time criterion the feeder modulus is determined.
- Based on its modulus the feeder shape, aspect ratio, and then its dimensions are selected.
- On the basis of the feed path criterion the feeder neck is designed.
- The modulus of the hot spot region (because of neck) is recalculated and the feeder redesigned.
- The feeder dimensions are increased if necessary after checking the feed metal volume criterion.
Feed Aids : When progressive directional solidification cannot be achieved by feeders alone then feed aids (including chills, insulation, and exothermic are used. The local solidification characteristics are altered when feed aids are kept in contact with a particular face of the casting or feeder.
The local solidification time is reduced by the chills which increase the local rate of heat transfer (compared to other surfaces of the casting in contact with mold). The solidification time of the local section is increased by both the insulating material (which reduce the rate of heat transfer) and exothermic materials (which add heat).
Copper, iron/steel, or graphite is usually used to make chills. To match the casting surface (form chills), they are in the forms of rectangular blocks or cylinders. Application to feeders is usually done by the insulation or exothermic materials that are in the shape of sleeves or covers.
- Three major considerations have to be taken into account in feed aid design. They are-the distance to which the feed aid must be effective, the initial rate of heat transfer required and the actual amount of heat transferred. These can be explained by taking the example of the chill.
- Effective Distance: The thermal conductivity of the casting material determines the distance to which a chill is effective assuming that the chill is not undersized (a small chill that gets saturated with heat is less effective).
- It has been shown by the experimental investigations that in iron castings (K=73 J/mKs) the chill is visible for a distance equal to 1-1.5 times the section thickness while it is visible for a distance up to 4 times the section thickness for aluminum castings (K=238 J/mKs). There is no change in local cooling rate or solidification time beyond this distance.
- Heat Transfer Rate: Thermal conductivity K of the chill material and the area of contact A determine the heat transfer rate. The conduction of heat of iron chill(K=73 J/mKs) is several order of magnitude higher than the sand mold and that of a copper chill is 5 times more than the iron chill.
- There is a reduction in rate as the chill keeps on becoming hotter.
- Heat Absorption: The size of the chill is largely dependent on this factor, in order to ensure it does not get saturated with heat. The dependence of the heat absorbed by the chill is on the specific heat C and the mass of the chill.
- The actual heat transfer of either the iron or copper chill (of same size) is twice as much as that of sand mold given the specific heats of sand, iron and copper ( 1130,456,and 386J/KgK respectively) and their densities (1500,7800,8900Kg/m3, respectively). In other words the effective modulus of the casting section is reduced to half of the original modulus by the chill. Experimentally it has been proven.
- On the basis of extension factor of the modulus (MEF) a simplified approach to estimate the effect of the feed aid on the solidification characteristics of a casting is done. The characteristic values of MEF for the materials which are insular or exothermic are 1.4 and 1.8 respectively.
- Put in other words in place of a larger feeder (without insulation) a smaller feeder would be required for the same solidification time, thereby improving the yield. An effective chill would have an MEF of 0.5.
Here the feeder modulus (without feed aid) is given by M f and the effective modulus is given by M f-effective.