This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Precision Investment Casting”.
1. Which of the following is used as the pattern material in precision investment casting?
b) Molten wax
Explanation: An expendable pattern, around which the mould is prepared. For the preparation of the pattern for every casting to be made, molten wax is used as the pattern material, which is injected under pressure of about 2.5 MPa into a metallic die, which has the cavity of the casting to be made. After the solidification of wax, it would produce the pattern.
2. Which of the following liquids is used for the formation of slurry in investment casting?
a) Ammonium hydroxide
b) Carbon disulfide
c) Sodium silicate
Explanation: To make the mould, the prepared pattern is dipped into slurry made by suspending fine ceramic materials in a liquid called sodium silicate or sometimes another liquid ethyl silicate is also used for the preparation of slurry. The excess liquid is allowed to drain off from the pattern to have an appropriate thickness of the mould.
3. The process of dipping and stuccoing is performed continuously to achieve the desired thickness of the mould in the investment casting.
Explanation: In the process of stuccoing, dry refractory grains such as zircon or silica are stuccoed on the liquid ceramic coating. A small shell is formed around the pattern, which is to be cured and then the process of dipping and stuccoing is continued with ceramic slurries to achieve a well desired thickness of the mould.
4. Molten metal is poured into the mould; meanwhile it removes the wax by melting it and vapourization of wax takes place simultaneously in the investment casting.
Explanation: Firstly the pattern is removed completely from the mould then the process of pouring of molten metal occurs in the investment casting. For the removal of pattern from the mould, heating is done to melt the pattern. Then the melted wax is completely drained through the sprue by inverting the mould.
5. Which of the following materials takes part in the removal of remnants wax from the mould?
b) Phosphoric acid
Explanation: After the removal of wax by melting and inverting the mould, some particles of the wax still stucked at the inside corners of the mould, and removal of them is very necessary to have castings free from defects. So, hot vapours of trichloro-ethylene are dissolved with the wax remnants in the mould to remove the same.
6. Pre-heating of the mould is done to generate adhering characteristics of the moulding surface.
Explanation: The moulds are pre-heated to a temperature of 100 to 1000 ⁰C, depending on the complexity, size and the metal of the casting. This is mainly done to reduce any last traces of wax left off and permit proper filling of all mould sections, which are too thin to be filled in a cold mould.
7. Investment casting is not suitable for producing complex shapes of castings.
Explanation: Complex shapes which are difficult to produce by any other method are possible in the precision investment castings since the pattern is withdrawn by melting it. And the thin sections with complexities can also be produced because the mould is heated before pouring of molten metal.
8. If mercury is used as a pattern material in place of wax, then for the preparation of mould, mercury is kept at temperature_____
a) below -38 ⁰C
b) 0 ⁰C
c) 0 to 38 ⁰C
d) -38 to 0 ⁰C
Explanation: If mercury is used as a pattern material in place of wax, then the process is known as mercast, the mercury is kept under -57 ⁰C where the mercury is frozen. And the complete mould preparation is to be undertaken at a temperature below -38 ⁰C to keep the mercury in solid state.
9. Which of the following is the main problem or disadvantage of using mercury as a pattern material?
a) Variation in density of pattern
b) Less formability
c) Keeping of pattern at too low temperature
d) Less strength
Explanation: When the mercury is used in place of wax as a pattern material, it has an advantage that it does not expand when changed from solid to liquid state as wax. But the main disadvantage or drawback is keeping the pattern at such low temperature, which is mainly responsible for its less useful.
10. Castings produced by investment casting do not require any further machining for finishing of the surface.
Explanation: As control of grain size and its orientation is possible in investment castings, so it makes the casting ready for use without any requirement of machining. This is particularly useful for those hard to machine materials such as nimonic alloys. So this process saves time and money both, which is usually spent on machining for other processes.
11. Investment casting is a cheap process with no limits on size and mass of the casting.
Explanation: Investment casting is a very expensive process for producing castings because of larger manual labour involved in the preparation of the pattern and the mould. And this process is also limited by the size and mass of the casting. The maximum limit on the mass of casting may be of the order of 5 kg.
12. Which of the following products is mostly produced by precision investment casting?
a) Aluminium billets
c) Connecting rod and crankshaft
d) Jewellery and surgical instruments
Explanation: As in investment casting, less mass and size of casting with high complexities can be produced easily. So the jewellery and surgical instruments are produced easily in the investment casting. Other products like vanes and blades for a gas turbine, shuttle eyes for weaving, wave guide for radars and impellers for turbo chargers can also be produced.
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