Explanation: Basically, the main objective of the riser is to feed the casting during solidification so that no any kinds of shrinkage cavities are formed in the casting. Type of metal poured and the complexity of the casting is responsible for the requirement of risers in the casting system to produce castings.

Explanation: Different materials have different volumetric shrinkage, in which grey cast iron is the material which sometimes may have a negative shrinkage. This mainly occurs because of high carbon and silicon contents in the grey cast iron, which results in occurring of graphitization which increases the volume.

Explanation: In grey cast iron, while solidification the process of graphitization occurs, this increases the volume of the casting and counteracts the metal shrinkage, so risers can be eliminated for the casting of grey cast iron. But for some metals like aluminium and steel, the volumetric contraction is very high, so elaborate risering is required.

Explanation: The main reason for the formation of voids inside the castings is that the liquid metal in the centre which solidifies in the end is not fed during the solidification of the castings; hence the liquid shrinkage ends up as a void. This can be avoided by proper design of the riser in the casting system.

Explanation: Solidification is basically occurred by losing heat from the surfaces and the amount of the heat is given by the volume of the casting, and the cooling characteristics of casting are given by the surface area to volume ratio. Riser is also similar to the casting in its solidification nature; hence the riser characteristics can also be given by the ratio of its surface area to volume.

Explanation: Chvorinov scientist has shown that the solidification time of a casting is directly proportional to the square of the ratio of volume-to-surface area of the casting and the constant of proportionality is known as mould constant. His relation is given by: ts = k(V/SA)2, where ts = solidification time, V = volume, SA = surface area, k = mould constant.

Explanation: Freezing ratio greater than unity is always preferred for the ability to feed the casting. Sphere could be used as a riser as its surface area to volume ratio is very low. But in sphere, the hottest metal being at centre, it is very difficult to use it for feeding to the casting, so a cylindrical type is always preferred.

Explanation: For the finding of optimum riser size, modulus method is very easy to take in action, and empirically it is also established that if the modulus of the riser exceeds the modulus of the casting by a factor of 1.2, then the feeding during the solidification part of the casting would be satisfactory.

Explanation: For the chunky type castings like a cube, the volume component can be neglected, but for the rangy castings, like as plate-like casting, the influence of the volume part becomes increasingly significant. So, it is very useful to have a parameter called ranginess factor (R), to define the casting type.

Explanation: In the casting of massive ring shaped bodies, the bore area is very small, so the bore experiences very high temperature that can damage or destroy the bore material, so in maximum cases, the designs are made to eliminate the use of cores or the highly refractory core materials are used to resist the high temperature.

Explanation: In the casting system, the risers should be located at the heaviest sections to achieve a better casting with fewer defects, and because the heavier sections, themselves act as feeders for the thin sections. And variations in volume of riser must be done if the size of casting sections also varies.

Explanation: At the time of solidification, the shrinkages are produced in the castings which are eliminated by the use of a riser. The metal in riser should be in liquid form for a long period of time as to feed the casting till it solidifies. So feeding aids are used for this purpose, they can be either exothermic materials or insulators.