JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II MANUFACTURING PROCESS Previous Question Paper (November/December - 2020 - R16)

A crucible furnace is a primary melting unit used in foundries for melting non-ferrous metals such as brass, bronze, aluminum, and magnesium. It consists of a refractory-lined furnace chamber containing a crucible made of graphite or silicon carbide, which holds the metal charge.

The working principle involves heating the crucible indirectly using fuel combustion (oil, gas, or coke) or electrical resistance. The furnace is typically cylindrical, with the crucible placed on a pedestal at the center. The heat generated in the annular space between the crucible and the furnace wall transfers to the metal through the crucible walls via conduction and radiation.

The operation follows these steps:

1. Charging: The metal scrap and ingots are placed inside the crucible. Fluxes may be added to prevent oxidation and remove impurities.
2. Heating: The burner is ignited, and the temperature is raised until the metal reaches its molten state. The furnace lid is kept closed to maintain heat efficiency.
3. Refining: Once molten, the metal is skimmed to remove dross or slag from the surface.
4. Pouring: The crucible is lifted out of the furnace using tongs or a mechanical hoist and tilted to pour the molten metal into the prepared sand molds.

The furnace design includes a vent/flue for exhaust gases to escape and a refractory lining to minimize heat loss. The main advantage of this furnace is the high purity of the molten metal, as the charge does not come into direct contact with the fuel or combustion gases, preventing contamination.

Centrifugal casting is a casting process where molten metal is poured into a rotating mold, utilizing centrifugal force to distribute the metal against the mold walls. This process is primarily used for producing hollow cylindrical parts like pipes, cylinder liners, and bushings.

The process involves a permanent mold mounted on a horizontal or vertical axis, which is rotated at high speeds. As the mold spins, the molten metal is forced toward the inner surface of the mold cavity by centrifugal force. This force ensures that the metal fills the mold uniformly and results in a dense, high-quality casting with minimal impurities, as lighter inclusions are pushed toward the inner diameter where they can be machined away.

The key features of the setup include:

1. A cylindrical mold made of metal or graphite, mounted on rollers or a spindle.
2. A pouring basin or funnel to deliver the molten metal into the rotating mold.
3. A drive mechanism to rotate the mold at a calculated speed, typically measured in revolutions per minute (RPM), to generate sufficient centrifugal force (often 60 to 100 times the force of gravity).

Advantages include the elimination of cores for cylindrical shapes, high material yield due to the absence of risers, and superior mechanical properties due to the high-pressure solidification. Disadvantages include the limitation to symmetrical shapes and the high initial cost of the rotating machinery.