This set of Foundry Multiple Choice Questions & Answers (MCQs) focuses on “Carbon Dioxide Mould”.
1. The use of CO2 was to __________ the sand mould.
Explanation: The idea of hardening sand moulds by gassing it with carbon-di-oxide was conceived. However, the CO2 molding has been adopted for a wide range and scale for modern core making.
2. ________________ is the binder used in the CO2 moulding.
a) Silica sand
b) Sodium Silicate
c) Sodium Carbonate
Explanation: The CO2 moulding is an extremely simple procedure. The highly flow able mixture of pure dry silica and sodium silicate binder is rammed or blown into the mould or cores box. Due to sodium silicates excellent binding properties with silica sand it is used as binders.
3. CO2 used in CO2 molding is at high pressure.
Explanation: Carbon-dioxide gas is used at a low pressure usually about 1.5 kg/cm2 is diffused through a mixture (of sand and sodium silicate) to initiate the hardening reaction which takes from a few seconds to a few minutes depending upon the size of the core or mold.
4. If gas is passed longer than required, the bond becomes ____________
d) Does not affect much
Explanation: When the gas is passed at the required pressure, the compression strength of a given sand mixture reaches its maximum at some critical amount of gas passed. If gassing is continued beyond the critical point, the strength of the bond is impaired.
5. Clay is used in the CO2 molding process.
Explanation: The clay is used alongside sodium silicate as a binder. The clay used is usually in small quantities. The clay and starch (often used) are added to improve the green strength of the mix prepared.
6. _____________ increases SiO2-Na2O ratio that increases the bond.
a) Sulphuric acid
b) Acetic acid
c) Carbonic acid
d) Hydrochloric acid
Explanation: Passage of carbon-dioxide through the sand containing sodium silicate produces carbonic acid in the aqueous solution. This causes a rise in the SiO2-Na2O ratio and formation of a colloidal silica gel, which hardens and forms the bond between sand grains.
7. The additives in sodium silicate are ____________
Explanation: The sodium silicate my contain additives such as sugars to help the breakdown of the core in casting that is to improve the collapsibility of the core. Molasses is the by-product of the sugar industry hence it is not used.
8. Overgasing increases the core strength.
Explanation: Overgasing usually decreases the strength of the core which is responsible for functioning in the CO2 molding. It causes the production of the defective castings.
9. Rough patterns are difficult to withdraw from CO2 molding.
Explanation: Since gassing hardens the sand mass, a pattern having a rough surface may become difficult to withdraw from the mould. However, this can be overcome if the pattern has been rubbed with graphite before ramming the sand around it.
10. Short gassing time is highly effective in CO2 molding.
Explanation: Short gassing times (15-30 seconds) have proved to be effective in producing strong cores. The short gassing times increase the efficiency of the cores used in this process.
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