【正文】 ning, and fabrication. Introduction The first metal castings were made during the period from 4000 to 3000 ., using stone and metal molds for casting copper. Various casting processes have been developed over time, each with its own characteristics and applications (see also Fig. ), to meet specific engineering and service requirements (Table ). A large variety of parts and ponents are made by casting, such as engine blocks, crankshafts, automotive ponents and power trains (Fig. ), agricultural and railroad equipment, pipes and plumbing fixtures, power tools, gun barrels, frying pans, office equipment, and very large ponents for hydraulic turbines. Two trends have had a major impact on the casting industry .The first is the mechanization and automation of the casting process, which has led to significant changes in the use of equipment and labor. Advanced machinery and automated processcontrol systems have replaced traditional methods of casting. The second major trend has been the increasing demand for highquality castings with close dimensional tolerances. This chapter is anized around the major classifications of casting practices (see Fig.Ⅱ .2 in the Introduction to Part Ⅱ ). These classifications are related to mold materials, molding processes, and methods of feeding the mold with molten metal. The major categories are as follows: 1. Expendable molds, which typically are made of sand, plaster, ceramics, and similar materials and generally are mixed with various binders (bonding agents) for improved properties. A typical sand mold consists of 90% sand, 7% clay, and 3% water. As described in Chapter 8, these materials are refractories (that is, they are capable of withstanding the high temperatures of molten metals). After the casting has solidified, the mold is broken up to remove the casting. 2. Permanent molds, which are made of metals that maintain their strength at high temperatures. As the name implies, they are used repeatedly and are designed in such a way that the casting can be removed easily and the mold used for the next casting. Metal molds are better heat conductors than expendable nonmetallic molds (see Table )。 hence, the solidifying casting is subjected to a higher rate of cooling, which in turn affects the microstructure and grain size within the casting. 3. Composite molds, which are made of two or more different materials (such as sand, graphite, and metal) bining the advantages of each material. These molds have a permanent and an expendable portion and ate used in various casting processes to improve mold strength, control the cooling rates, and optimize the overall economics of the casting process. The general characteristics of sand casting and other casting processes are given in Table . Almost all mercially used metals can be cast. The surface finish obtained is largely a function of the mold material。 in the United States alone, about 15 million tons of metal are cast by this method each year. Typical applications of sand casting include machine bases, large turbine impellers, propellers, plumbing fixtures, and numerous ponents for agricultural and railroad equipment. The capabilities of sand casting are given in Table . Basically, sand casting consists of (a) placing a pattern (having the shape of the desired casting) in sand to make an imprint, (b) incorporating a gating system, (c) removing the pattern and filling the mold cavity with molten metal, (d) allowing the metal to cool until it solidifies, (e) breaking away the sand mold, and (f) removing the casting(). Sands. Most sandcasting operations use silica sand (SiO2) as mold material. Sand is inexpensive and is suitable as mold material because of its hightemperature characteristics and high melting point. There are two general types of sand: naturally bonded (bank sand) and synthetic (lake sand). Because its position can be controlled more accurately, synthetic sand is preferred by most foundries. For proper functioning, mold sand must be clean and preferably new. Several factors are important in the selection of sand for molds, and it involves certain tradeoffs with respect to properties. Sand having fine, round grains can be packed closely and, thus, forms a smooth mold surface. Although finegrained sand through pores. Good permeability of molds and cores allows gases and steam evolved during the casting to escape easily. The mold also should have good collapsibility to allow for the casting to shrink while cooling and, thus, to avoid defects in the casting, such as hot tearing and cracking (see ). Types of sand molds. Sand molds () are characterized by the types of sand that prise them and by the methods used to produce them. There are three basic types of sand molds: greensand, coldbox, and nobake molds. The most mon mold material is gre