This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Low Pressure Die Casting”.
1. Which of the following methods of casting is best suited for casting aluminium and magnesium based alloys?
a) Centrifugal casting
b) Low-pressure die casting
c) Shell moulding
d) Investment casting
Explanation: Low-pressure die casting has been adopted generally for the casting of aluminium and magnesium based alloys. Usually a permanent mould or sand moulds are also possible, but this method makes the casting totally free from shrinkage defects and also ensures proper solidification naturally.
2. What should be the pressure range (in bar) of the gas used in the low-pressure die casting?
a) 0.3 to 1.5
b) 2 to 10
c) 10 to 30
d) 30 to 50
Explanation: In low-pressure die casting, compressed gas is basically used at a pressure typically ranging from 0.3 to 1.5 bars to force the molten metal to rise slowly through the ceramic riser tube that is connected to the mould. Pressure should be below atmospheric pressure to easily pull the molten metal into the cavity.
3. In low-pressure die casting, mould is placed over the furnace containing the molten metal.
Explanation: Low-pressure die casting is not new; some changes are adapted to the permanent mould to generate new characteristics and better properties to the casting. So in this process, the permanent mould and the filling system are placed over the furnace containing the molten alloy in it.
4. The residual molten metal cannot flow back to the crucible and it remains in the riser tube.
Explanation: When the mould cavity is filled with molten metal, the pressure in the crucible is removed. This allows the molten metal to flow back to the crucible through the tube. Then after solidification, the sides die opens and the top die is raised vertically. And the casting moves with the top die and ejected onto a transfer tray.
5. Which of the following types of cooling is used for the cooling of top and bottom dies in the low-pressure die casting?
a) Water cooling
b) Liquid nitrogen cooling
c) Cooling with fins
d) Air jets cooling
Explanation: The cooling rate can affect the quality of the casting, so care should be taken to see that the casting is properly cooled before ejecting from the die. For the proper cooling of the top and bottom dies of the casting system, air jets are used which circulated through the dies and takes away the heat.
6. It is not necessary to consider the cooling circuits in designing of mould due to less cycle time.
Explanation: Proper cooling is very essential in low-pressure die casting to produce castings of high strength. So, care needs to be exercised during designing of the mould to provide proper cooling circuits, so that the heavier sections of the casting are located close to the feeding path to make it act as a riser.
7. In low-pressure die casting, metal enters the mould very slowly as compared to the die casting.
Explanation: In low-pressure die casting, due to the application of riser tube, molten metal flows upward in direction against the gravity. This makes the metal to flow into the mould slowly as compared to the die casting with less turbulence, so the casting quality is also improved by eliminating the defects.
8. Which of the following terms is used for the presence of excess metal in the parting plane?
Explanation: Flash is a term which is used for the excess metal forced into the parting plane. Before using the castings, it is very necessary to remove flash together with any overflow present in the casting. This problem can be solved by implementing low-pressure die casting method, where the condition of excess metal cannot be possible due to upward flow of molten metal.
9. Which of the following products is most suitable to cast by low-pressure die casting method?
a) Crank cases
b) Blades of turbine
c) Surgical instruments
d) Connecting rods
Explanation: Crank cases are very easy to cast by low-pressure die casting, as the upward flow of molten metal reaches to every small corner of the mould and makes the homogenous nature of castings. Many other components like magnetos, handle bar housing and other decorative items of automobile can also be casted by low-pressure die casting method.
10. The die has longer life for casting of materials but less for aluminium alloys.
Explanation: As the metallic die is used which has a long life, it is of the order of 300,000 pieces for zinc and similar alloys. But for the aluminium alloys, it is approximately up to the 150,000 pieces of it. The reason is higher melting temperature of aluminium alloys that can destroy the mould with the production of castings.
11. Surface finish of the castings is not better in the low-pressure die casting method as compared to other method of casting.
Explanation: Very good surface finish of the order of 1 micron can be obtained in low-pressure die casting method because of application of metallic dies. The surfaces generated by this method of casting can be directly electroplated without any further processing. This method is economical for moderate or large scale production.
12. Low-pressure die casting method is not economical for small-scale production.
Explanation: Low-pressure die casting method is not economical for small-scale production because the dies and machines are very expensive, and the other parts like riser tube and the device used for compressing gas make the casting process very expensive. So the economy in production is possible only when large quantities are produced.
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