This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Shell Moulding”.
1. Which of the following synthetic resins are used for mixing with sand in shell moulding?
a) Thermosetting resins
b) Epoxy resins
c) Fiber glass resins
Explanation: The synthetic resins used in shell moulding are essentially thermosetting resins, which get hardened by heat. Phenol formaldehyde resins are mostly used, which combines with sand to form a proper mixture for shell moulding. The mixture formed has high strength and resistance to heat.
2. Grain size of the sand particles has no affects on the surface finish of the casting.
Explanation: Generally, for the preparation of shell moulding sand, dry and fine sand that is completely free of the clay is used. The grain size to be chosen depends on the surface finish required on the casting. Too much fine a grain size requires a large amount of resin, which makes the mould expensive.
3. In shell moulding sand preparation, the resin is combined with a catalyst (hexa) to develop the_____
a) thermoplastic characteristics
b) toughness of the resin
c) thermosetting characteristics
d) viscosity of the resin
Explanation: The phenolic resins used in shell moulding have excess of phenol and acts like a thermoplastic material. During coating with sand the resin is mixed with a catalyst such as hexa methylene tetramine (hexa) in a proportion of 14 to 16 % so as to develop the thermosetting characteristics.
4. Which of the following compositions of the shell sand is used for avoiding of thermal crack during pouring?
a) Silica sand
b) Zircon sand
Explanation: Generally, additives are added to the sand mixture to improve the surface finish and also for the avoiding of any thermal crack during pouring. Some of the additives used are coal dust, manganese dioxide, calcium carbonate, pulverized slag and ammonium boroflouride.
5. Which of the following lubricants is added to the resin sand mixture for better flowability of the sand?
a) Lead stearate
b) Magnesium stearate
c) Sulphur stearate
d) Zinc stearate
Explanation: Some lubricants such as zinc stearate and calcium stearate are added to the resin sand mixture to improve the flowability of the sand, which helps in covering of maximum part of the metallic pattern efficiently and also permit easy release of the shell from the pattern.
6. Which of the following devices is used for the mixing or complete formation of the shell sand mixture?
c) Semi circular roller
d) Semi cylindrical roller
Explanation: Sand, hexa and additives, which are all dry, are mixed inside a Mueller for a period of 1 minute, then the liquid resin is added and mixing is done for another 3 minutes. Then by introducing cold or warm air into the Mueller, mixing is continued till all the liquid is removed from the mixture and coating of the grains is achieved to the desired degree.
7. Which of the following metals is used for the preparation of patterns in shell moulding?
c) Grey cast iron
Explanation: Generally, metal patterns with the associated gating are used. The metal used for preparing patterns is grey cast iron; it is mainly used because of its easy availability and good stability at the temperatures involved in the process. Sometimes, additional risers are also required as the cooling in shell moulds is slow.
8. Which of the following devices is used for the formation of shell mould in the shell moulding procedure?
a) Air roller
b) Re-circulating ball roller
d) Dump box
Explanation: A silicone release agent is sprayed on the heated metallic pattern which is fixed to a dump box, wherein the coated sand of sufficient amount is filled in. Then the dump box is rotated so that the coated sand falls on the heated pattern and the heat liberating from the pattern melts the resin and causes sand mixture to adhere to the pattern.
9. At the last of shell moulding procedure, the Dump box is rotated backward for adhering of as much as the possible amount of sand to the metallic pattern.
Explanation: When after the achieving of a desired thickness of the shell, the dump box is rotated backward by 180 degrees so that the excess of sand falls back into the box, leaving the formed shell intact with the pattern. Thickness of the shell depends on the temperature of the pattern and the coating time of sand in contact with the heated pattern.
10. Over-curing or under-curing of the shell mould, leads to the generation of sophisticated problems in the shell mould.
Explanation: For the curing of the shell mould, it is kept in an electric or gas fired oven. The curing of the shell should be done as per requirements only because over-curing can cause the mould to break down as the resin would burn out. And under-curing may result in blow holes in the casting or the shell may break during handling.
11. There is no need for any external support for the shells at the time of pouring of molten metal.
Explanation: Since the shells are thin, and pouring of molten metal into the shells may cause an imbalance which further leads to failure of the castings. So to avoid this, it may require some outside support to withstand the pressure of the molten metal and to form the castings efficiently with proper balance.
12. Which of the following types of clamping is mostly performed for the joining of shells?
Explanation: The prepared shells are joined together by mechanical clamping or sometimes adhesive bonding is also preferred for joining of shells. The resin used as an adhesive may be applied at the parting plane before mechanical clamping and then allowed for 20 to 40 seconds to achieve necessary bonding. Then the shell mould becomes ready for pouring of molten metal.
Sanfoundry Global Education & Learning Series – Casting, Forming and Welding.
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