This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “CO2 Process”.
1. Which of the following moulds or moulding is also known as sodium silicate process.
a) Shell moulding
b) Permanent moulding
c) Slush moulding
d) Co2 moulding
Explanation: Carbon dioxide moulding is also known as sodium silicate process is one of the widely used processes for preparing of moulds and cores. Basically, sodium silicate is used as a binder, but it binds the sand particles only in the presence of Co2. Co2 is passed through the mould to tighten it, as shown in below figure.
2. Co2 moulding is mainly used for the increase in strength and hardness of the moulds and cores.
Explanation: Mould is prepared by adding sodium silicate binder. Co2 gas is supplied to the mould for sufficient time. It reacts with sodium silicate and produces silica gel which has better bonding properties. Due to this strength and hardness of the large sized moulds and cores can be increased.
3. How much percentage of sodium silicate (Na2SiO3) is added to the sand mixture in Co2 moulding?
a) 0 to 2 %
b) 2 to 6 %
c) 6 to 10 %
d) 10 to 14 %
Explanation: No other things like clay, water and resins are added to the sand except sodium silicate, which reacts with the sand mix to increase the flow-ability to fill up corners and intricate contours. Then the Co2 is supplied to the mould and reaction takes place as follows:
Na2SiO3 + Co2 —–> SiO2 + Na2Co3
Where, SiO2 is having gelling action (Silica gel).
4. Continuous passing of Co2 through the mould can decrease the strength of mould and results in a weak mould.
Explanation: Continuous passing of Co2 always results in a strong and hard mould. More the passing of Co2 more will be the strength of the mould. Only the drawback is the decrease in permeability and collapsibility of the mould. Much passing of Co2 gas results into difficult breathing of the mould.
5. How much time (in a minute) is usually required for the passing of Co2 through the mould?
Explanation: In Co2 moulding, mould of sand is hardened by passing of Co2 for about one minute. The Co2 gas forms a weak acid that hydrolyzes the sodium silicate (Na2O, SiO2) solution to form amorphous silica gel which acts as the bond and sodium silicate itself also provides some bonding action.
6. Additives can be used for the non-ferrous application to avoid the poor collapsibility of the mould.
Explanation: In Co2 moulding, poor collapsibility of the mould is a major disadvantage. Although some additives are used for the improvement in the properties of ferrous metal castings, these additives which are added to the sand mixture, cannot be applicable for the non-ferrous applications.
7. Sometimes over-gassing can cause the sticking of pattern to the mould in Co2 moulding.
Explanation: In Co2 moulding, over-gassing and under-gassing can affects the properties of cured sand adversely. When the continuous passing of Co2 is done for long time, this may results into sticking of pattern with the mould which cannot be removed easily, so proper care should be taken while passing of Co2 gas.
8. It is very safe to use Co2 in Co2 moulding, so it eliminates the use of any shielding apparatus.
Explanation: In Co2 moulding, the process uses relatively safe carbon dioxide gas and it does not cause any of the sand disposal problems or any kind of odour while mixing and pouring. So the using of carbon dioxide gas is very safe to human operators. And so, no any equipment is required for the shielding.
9. Which of the following additives are added to the sand in Co2 moulding for the improvement in collapsibility of the sand?
a) Copper oxide
b) Wood flour
c) Aluminium oxide
Explanation: After the preparation of the sand mixture by adding sodium silicate to the sand, the next step is to add additive, in which mainly aluminium oxide is added and sometimes molasses are also added to impart the favorable properties and for the improvement in the collapsibility of the sand mix.
10. Co2 moulding process is an ideal process, where speed and flexibility is the prime requirement.
Explanation: In Co2 moulding, moulds and cores of varied sizes and shapes can be formed, this is mainly achieved by the using of binder sodium silicate that can binds the sand in any shape and size, and that is further tightened by the passing of Co2 gas. This makes the moulding process very fast and flexible, so this process is more economical, where speed and flexibility is the prime requirement.
11. Moisture from the mould can be removed by using of dryer or heater in Co2 moulding.
Explanation: In Co2 moulding, water is never added while preparing a sand mixture. Instead of water, sodium silicate is added to act as a binder. Hence the moisture is completely eliminated from the mould. So, this process provides great dimensional tolerances and high accuracy in the production of castings.
12. Co2 moulding is a kind of sand moulding, but it is not economical than the green sand moulding.
Explanation: Better surface finish of castings with close tolerances are produced by Co2 moulding, but it is still not economical than the green sand moulding because this process uses expensive devices like Co2 cylinder, hoses, regulator, hand held applicator gun or nozzle etc., and the processes in green sand moulding are simple and no expensive devices are required, due to this, green sand moulding is more economical.
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