Advanced Casting Questions on “Riser Design – 2”.
1. The main function of a riser is to maintain the escaping of gases from the casting during solidification.
a) True
b) False
Answer: b
Clarification: 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.
2. Which of the following materials can have the negative volumetric shrinkage of the casting?
a) Aluminium
b) Steel
c) Zinc
d) Grey cast iron
Answer: d
Clarification: 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.
3. There is no requirement of any risers for the casting of grey cast iron.
a) True
b) False
Answer: a
Clarification: 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.
4. Improper risering has always resulted in the formation of voids inside the castings.
a) True
b) False
Answer: a
Clarification: 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.
5. The cooling characteristics of casting can be represented by the_____
a) volume to surface area ratio
b) product of volume and surface area
c) surface area to volume ratio
d) surface area to perimeter ratio
Answer: c
Clarification: 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.
6. Which of the following relations is given by chvorinov for the solidification time of a casting?
a) ts = k(V/SA)2
b) ts = k(V/SA)
c) ts = k(V*SA)2
d) ts = k(V/SA)3
Answer: a
Clarification: 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.
7. A sphere can be used as a riser because of its freezing ratio, which is greater than unity.
a) True
b) False