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Metal Casting Processes:
Permanent Mold Casting

Centrifugal Casting

In this group of processesthe molten metal is forced to distribute into the mold cavity due to centrifugal acceleration. Centrifugal casting processes can be classified as true centrifugal casting, which, when the molten metal is poured into the cavity, forces it against the mold walls where it solidifies into a hollow cylinder; semicentrifugal casting, which differs from true centrifugal casting in that the mold cavity is completely filled with molten metal so that the core, which is later removed, is subjected to low pressure and is where air and inclusions are trapped and the centrifuging method in which molten metal is poured into a number of mold cavities which are connected to a central down sprue and which rotate around the central axis of the sprue, which is shifted from the central axis of the castings and causes the mold cavities to be filled under high pressure.

Continuous Casting

In continuous casting, a stream of molten metal comes out of a water-cooled orifice and forms a continuous strip or rod which is then cut by a circular saw. One of the newer techniques, usually referred to as rotary continuous casting, involves the water-cooled orifice (mold) oscillating and rotating at about 120 revolutions per minute during the casting process. Continuous casting has a very high metal yield, about ninety-eight percent compared to eighty-seven percent in conventional ingot-mold practices, excellent quality of cast, controlled grain size and the ability of casting special cross-sectional shapes.

Die Casting

Die casting processes force the molten metal into the cavity of a steel mold, called a die, under very high pressure, from 1000 to 30,000 psi. Classification of die casting involves the types of machine used, the two primary types being hot-chamber machines and cold-chamber machines.

Hot-Chamber Machines:

The main components of the hot-chamber machine are a steel pot with molten metal in it and a pumping system consisting of a pressure cylinder, a plunger, a gooseneck passage and a nozzle. When the plunger is in the up position molten metal flows by gravity through the intake ports into the hot-chamber. When the power cylinder pushes down the plunger it shuts off the intake ports and forces the metal through the gooseneck passage and nozzle into the die cavity. High pressure is maintained to allow the casting to completely solidify, then the two halves of the die are pushed apart and the ejector pin knocks out the casting. Finally the die cavity is cleaned and lubricates and the cycle begins again. The advantages to hot-chamber casting include: high production rates, improved conductivity, superior surface finish, close tolerances and the ability to produce intricate shapes with thin walls. It does have limitations, however. Only low melting point alloys like zinc, tin and lead can be used because the parts of the pumping system are in direct contact with the molten metal for long periods of time. The process is also usually only suitable for making small castings that weigh less than ten pounds.

Cold-Chamber Machines:

For cold-chamber machines the molten metal reservoir is separate from the casting machine and just enough metal for one shot is ladled on every stroke. The metal is poured into the pouring hole of the shot chamber while the two halves of the die are locked together. Then the plunger moves forward, closing the pouring hole and forcing the molten metal into the die cavity. After solidification the die is opened and the casting is ejected from the die. As the chamber and plunger are in contact with the molten metal for shorter times, metals with higher melting points, such as aluminum, magnesium and brass, can be used in the process. It is also possible to produce large parts weighing up to fifty pounds by this process, but it does have a longer cycle than hot-chamber die casting.

Slush Casting

Slush casting is a process for making hollow articles by inverting the mold after partial freezing on the surface in order to drain out the still liquid metal at the center. Solidification begins at the walls because they are relatively cool and works inward, so the thickness of the shell is controlled by the amount of time allowed before the mold is drained. This is a relatively inexpensive process, however only low melting point alloys with narrow freezing ranges can be used and it is a slow method, requiring close temperature control of the liquid metal.

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