This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Vacuum Die Casting”.
1. Which of the following problems arises in die casting that can be solved by having vacuum die casting?
a) Overheating
b) Interruption in progressive solidification
c) Air in the cavity
d) Difficulty in cooling of casting
View Answer
Explanation: The major problem with die casting is the air left in the cavity after closing of die. Since the air cannot escape out so it remains inside the casting. This result in the generation of blisters, which basically appears on the surface of the castings, when the casting is heat treated. This problem can be solved by implementing a vacuum die casting method.
2. In vacuum die casting, the molten metal does not experience any back pressure while injecting it into the die cavity.
a) True
b) False
View Answer
Explanation: In vacuum die casting, before the injection of molten metal, all the air presents in the die cavity is evacuated and then the die is closed. So by performing this operation, the molten metal does not experience any back pressure while injecting it into the die cavity, this makes the process easier.
3. In vacuum die casting, the molten metal enters into the cavity slowly as compared to the die casting.
a) True
b) False
View Answer
Explanation: The removal of all the gases from the die cavity makes a vacuum inside the cavity, so there is no pressure or force inside the cavity which could be opposite to the injecting pressure of the molten metal. So, the molten metal enters much faster into the die, thereby decreasing the fill time.
4. The castings made by vacuum die casting have more strength than the castings made by die casting.
a) True
b) False
View Answer
Explanation: In vacuum die casting, the components which are casted do not experience any porosity due to the removal of all the air from the cavity. The castings made have negligible porosity as compared to the castings made by die casting. So, the castings are of high strength in the vacuum die casting.
5. Which of the following helps in reducing the oxidation of the material in vacuum die casting?
a) Mould thickness
b) Mould material
c) Tight tolerances
d) Mould coating
View Answer
Explanation: In vacuum die casting, the part is exposed to atmospheric air only after solidification, and oxidation of the material is avoided as much as possible. This can be achieved by processing of parts with tight tolerances and also by faster cooling rate which results into fine microstructure nature of castings.
6. Rapid solidification rates of casting tend to have a reduction in oxidation of the material.
a) True
b) False
View Answer
Explanation: Rapid solidification rates of the casting can avoid the oxidation of the material in the vacuum die casting (VDC). This is accomplished by providing very thin walls (1 to 12 mm) of mould to the casting with close tolerances. Then the properties of casting approach to that of wrought product, and with relatively short cycle times.
7. Which of the following helps in removing of casting from the ejector die?
a) Tie bar
b) Plunger
c) Platen
d) Ejector pins
View Answer
Explanation: The ejector pins, which move through the moving die to free the casting from the ejector die. The number of ejector pins should be sufficient so as to remove the hot casting without any distortion. And the position of ejector pins should be like so that the pins marks remain on the casting are not objectionable.
8. The moving cores are used in the vacuum die casting, which are parallel to the movement of a die.
a) True
b) False
View Answer
Explanation: Fixed cores and moving cores, both of metal are used in this method of casting. Moving cores are not parallel with the die movement and hence are to be removed before the casting is ejected from the die. And other is fixed cores, which are fixed to the die halves and parallel to the die movement.
9. Which of the following parts is fixed to the stationary platen of the casting machine?
a) Cover die
b) Ejector die
c) Sprue
d) Runner
View Answer
Explanation: Mainly two parts of the casting machine, one is a cover die which is fixed to the stationary platen and another one is ejector die which is fixed to the moving platen of the casting machine. Cover die also makes contact with the nozzle of gooseneck or with the shot chamber in the casting machine.
10. Which of the following is used to remove the metal defect of cold shuts from the casting?
a) Ejector pins
b) Overflows
c) Metal cores
d) Biscuit/Sprue
View Answer
Explanation: Overflows are basically used for the small components, to provide a sufficient amount of heat to the die material, so that no cold shuts occur in the castings. Also, the overflows can be utilized for the positioning of ejector pins so that no objectionable ejector pin marks appear on the casting.
11. Overflows should be avoided as much as possible in the designing of a die.
a) True
b) False
View Answer
Explanation: Overflows are provided in the parting plane for the first metal which is usually cold, enters in the die cavity, to solidify. The overflow is a luxury and cannot be acceptable in every casting, so it should be avoided as much as possible in the designing of the die. It is primarily provided for small components only.
12. Short cycle time leads to having a high temperature of the dies in the vacuum die casting.
a) True
b) False
View Answer
Explanation: If the cycle time is very small, then the dies would readily get heated. Particularly in the sections such as sprues, or heavy sections of the casting, the temperature may be too high. To maintain the required and uniform temperature, water is circulated through the hot regions of the die.
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