This method employs a cup-shaped, refractory-lined, metal furnace which is normally heated by gas or oil and sometimes by electrical resistance or induction. It has an inner crucible to hold the metal charge. The crucible is made of either a clay-silicon-carbide or a claygraphite mixture. The furnace can either tilt for pouring or the crucible can be lifted out. Fig. 1A3 illustrates a tilting type with a lift-out crucible. The crucible method is used to melt brass, bronze, aluminum, and magnesium for sand castings. Except for induction heating, ferrous metals are not usually
melted in this kind of furnace.
(Handbook of Manufacturing Processes, James G. Bralla, page 2)
Air Furnace (reverberatory) Melting
air furnace (reverberatory) melting has similarities to open-hearth melting. Fig. 1A4 shows
a typical air furnace. Oil or pulverized coal is burned in one chamber and the charge is placed in
another. Heat from the burning fuel passes over and is absorbed by the charge, melting it. There is no direct contact between the metal and the fuel, allowing carbon content to be closely controlled. Oil or finely pulverized bituminous coal are used as fuels. Some smaller furnaces use natural gas. This type of furnace is used in the production of castings from malleable and gray cast iron, brass, and bronze.
(Handbook of Manufacturing Processes, James G. Bralla, page 2)
Induction Melting
With this method, alternating electric current in a coil creates a magnetic field that induces corresponding secondary electrical currents in the metal charge. The resistance of the metal in the charge causes its temperature to rise to the melting point. Melting can be very rapid and there is no pollution or contamination from the heat source. Induction melting is used for steel, brass, bronze, aluminum, and magnesium.
(Handbook of Manufacturing Processes, James G. Bralla, page 2)
Open-Hearth Melting
This method, used in the production of steel and cast iron, is also used to supply molten metal for casting operations. Foundry open-hearth furnaces are usually smaller than those found in steel mills. Fig. 1A6 illustrates a typical open-hearth furnace which is both reverberatory
and regenerative. Metal in the furnace is heated by a flame passing over the charge. The
flame comes from the combustion of gas, oil, tar, or pulverized coal. The low roof of the furnace
reflects heat downward to the metal in the furnace. Both fuel and air are fed from one side into the central area where the flame and heating take place.The chambers on the opposite side are heated by the flame and exhaust gases moving through them. The pool of molten metal in the furnace is shallow, which provides the maximum area for heat transfer per unit volume of metal. After a period of time, the direction of flow is reversed. The chambers heated from the previous cycle, in turn, heat the incoming fuel and air. Most open hearth furnaces are chemically basic (rather than acidic) as determined by the material of the brick furnace lining. The basic furnaces remove sulfur, silicon, carbon, and manganese from the charge metal. The charge used in making structural steel includes iron ore, limestone, scrap, and, later, molten pig iron. Additions can be made to the steel to produce the desired composition. Oxygen may be added to the furnace combustion area to reduce the process time and the amount of fuel required. Finished metal is removed from a hole in the rear of the furnace and transferred to a ladle.
(Handbook of Manufacturing Processes, James G. Bralla, page 4)
Pouring
Metal is usually tapped from the melting furnace into either a ladle from which it is poured by gravity into the mold, or into one that is used to transfer a quantity of metal to a pouring ladle. Such transfer ladles are usually covered to reduce heat loss during transfer. Pouring ladle capacities range from about 60 lb (27 Kg) up. Ladles are frequently transported by overhead
cranes. There are three basic types of ladles, as illustrated in Fig. 1A7: open-lip ladles that pour by tilting, “teapot” ladles that also pour by tilting but which avoid pouring slag, and bottom-pour ladles which also avoid pouring slag. Tilting ladles often utilize worm-gear tilting systems to provide better control and prevent the ladle from tipping too much or too fast. Numerous automatic pouring systems, designed to accurately meter the amount of molten metal poured, are also used. Some consist of mechanized or robotic dip-and-pour ladles. Others pour
directly from a larger holding pot, using either stopper rods as shown in Fig. 1A7, or sliding gate valves. Some pouring vessels are fitted with electrical heating apparatus to maintain the metal at the proper pouring temperature. (The ideal pouring temperature involves a “superheat”, a metal temperature sufficiently high to ensure that all parts of the mold are fully filled before solidification starts.) Other pouring systems include machine vision to sense when the mold is full, or weight controls to pour a prescribed amount, by weight, into the mold.
(Handbook of Manufacturing Processes, James G. Bralla, page 4)
Dry Sand Casting
In this process, the green sand mold is dried or baked before it is filled with molten metal. Typically, the mold is heated to 300°F (150°C) or higher, by baking or forced hot air until
most of the moisture is evaporated. This approach produces a stronger mold and there is less gas
(steam) generated when the molten metal is poured into the mold. One or more coatings of refractory material - silica, zircon, or graphite - are usually applied to the mold surface in a water or solvent carrier. Dry-sand molds can withstand more handling and longer storage, and have better resistance to the pressure of molten metal. The dry-sand process is normally used for medium-size to very large, multi-ton castings where greater mold strength is needed to withstand the mass of the molten metal. Dried sand gives a better surface finish but is more costly than green-sand molding because of energy, space, and equipment costs.
(Handbook of Manufacturing Processes, James G. Bralla, page 6)
Skin-Dried Casting
To reduce the lengthy drying time of dry-sand casting, the drying is often limited to a depth of only about 1/4 to 1/2 in (6 to 13 mm). The patterns are usually first coated with a wash of refractory material. Heat for drying is supplied from a torch, from infra-red lamps, or from hot air. The mold is then referred to as a skin-dried mold. The dried skin is backed up with a mixture of green and dry sand. The approach is used extensively for the casting of steel, which
involves higher pouring temperatures, and has also largely replaced dry sand molding for other applications. The casting surface is improved by elimination of the moisture in the facing sand, which could cause pin holes in the casting. Shake out is also facilitated. Additional binders such as linseed oil, corn flour or molasses may be used in the facing sand to improve the strength of the dried mold skin.
(Handbook of Manufacturing Processes, James G. Bralla, page 6)
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