This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Venting of Moulds”.
1. Venting is a process which allows molten metal to compensate for its liquid and solidification shrinkage.
a) True
b) False
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Explanation: The compensation for liquid and solidification shrinkage of the molten casting is basically done by setting up risers in the casting system. Venting is a process which allows the escaping of gases from the molten metal during the solidification. For the venting process, a vent is provided in the casting system through which gases escape out.
2. Which of the following effects is the effect caused by the ignition of oxygen due to high pressure and temperature of melt?
a) Otto effect
b) Dual effect
c) Diesel effect
d) Rankine effect
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Explanation: Proper mould venting is very important and necessary for producing quality casting parts. During the moulding process, the gases included in the mould needs a way to escape, or else the molten metal will compress and trap those gases in the mould cavity. A combination of high melt pressure and temperature will then ignite the oxygen and will cause an effect that is called a diesel effect.
3. Improper venting during moulding process causes a diesel effect which further results in stress crack defects in the casting.
a) True
b) False
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Explanation: Improper or poor venting during moulding process causes a diesel effect that mainly occurs due to the combination of melt pressure and high temperature which ignite the oxygen and causes burnings in the mould which is also called as carbonization process that further results in marks and crack defects in the finished casting components.
4. For proper venting, the pouring time is increased which allow easy escape out of gases from the mould without inducing any defects.
a) True
b) False
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Explanation: For maintaining the proper venting, the pouring time cannot be increased because that will cause other problems in the casting process such as early solidification which will form the incomplete sections in the casting and several defects in the casting components like cracks and shrinkages.
5. High viscosity of molten metal can help in removing of gases but will cause defects in the casting.
a) True
b) False
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Explanation: For keeping the high viscosity of molten metal, the temperature of the melt should be kept low, but greater than its melting point. High viscosity generally reduces the pouring speed which takes more time to complete the process. This may be done for venting but it will cause defects in the casting such as cold cracks, incomplete sections, shrinkages etc.
6. Porous sintered materials can be inserted into the mould cavity to act as a vent.
a) True
b) False
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Explanation: Porous sintered materials can be placed in the mould cavity to act as a vent. It mainly depends on the thickness of the porous material used. More the thickness, then less will be the venting capacity. So venting passages should be machined properly to collect the gases, proper cleaning of vents is also important as the gas deposits can block the passages.
7. Vacuum technology for venting process is very economical and widely used in the casting process.
a) True
b) False
View Answer
Explanation: Vacuum technology for venting process includes removing of gases from the cavity before the pouring of molten metal. This actually generates a vacuum inside the cavity for which the cavity must be sealed completely. And although vacuum technology may solve gases problems, it is not effective in dealing with gas produced during pouring, so this process is not economical and cannot be used conventionally.
8. In the injection moulding process, an overflow system is used for the venting process.
a) True
b) False
View Answer
Explanation: An overflow system is an area which is connected to the mould cavity through a channel that is mainly created by machining an exit gate that joins the cavity’s last filling point with a cold runner that ends in a venting area. A groove is machined at the vent to collect all the gases and allow them to escape out from the mould.
9. For the venting process, the casting yield is generally high with the overflow system in the mould.
a) True
b) False
View Answer
Explanation: An overflow system includes extra material or metal which is wasted after every shot, overflow cutting operation and additional space requirements. This decreases the material or casting yield in the production of casting components. This further increase the cost involved in material handling.
10. In the injection moulding process, venting at the runner side can reduce the air pressure.
a) True
b) False
View Answer
Explanation: Venting at the runner side can decrease the air pressure because the air inside the sprue is ejected instead of compressing it into the mould cavity. Bernoulli’s principle states that an increase in velocity of fluid occurs simultaneously with a decrease in pressure. According to this law, the velocity of air flowing through the runner results in air suction from the mould cavity to the runner.
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