机械制造专业英语第7章SAND CASTING(砂型铸造)

SAND CASTINGMost metal castings are made by pouring molten metal into a prepared cavity and allowing it to solidify. The process dates from antiquity. The largest bronze statue in existence today is the great Sun Buddha in Nara, Japan. Cast in the eighth century, it weighs 551 tons (500 metric tons) and is more than 71 ft (21m) high. Artisans of the Shang Dynasty in China (1766 - 1222B.C.) created art works of bronze with delicate filigree as sophisticated as anything that is designed and produced today.There are many casting processes available today, and selecting the best one to produce a particular part depends on several basic factors, such as cost, size, production rate, finish ,tolerance, section thickness, physical-mechanical properties, intricacy of design, machinability, and weldability.Sand casting, the oldest and still the most widely used casting process, will be presented in more detail than the other processes since many of the concepts carry over into those processes as well.Green SandGreen sand generally consists of silica sand and additives coated by rubbing the sand grains together with clay uniformly wetted with water. More stable and refractory sands have been developed, such as fused silica, zircon, and mullite, which replace lower-cost silica sand and have only 2% linear expansion at ferrous metal temperatures. Also, relatively unstable water and clay bonds are being replaced with synthetic resins, which are much more stable at elevated temperatures.Green sand molding is used to produce a wide variety of castings in sizes of less than a pound to as large as several tons. This versatile process is applicable to both ferrous and nonferrous materials.Green sand can be used to produce intricate molds since it provides for rapid collapsibility; that is, the mold is much less resistant to the contraction of the casting as it solidifies than are other molding processes. This results in less stress and strain in the casting.The sand is rammed or compacted around the pattern by a variety of methods, including hand or pneumatic-tool ramming, jolting (abrupt mechanical shaking),squeezing (compressing the top and bottom mold surfaces), and driving the sand into the mold at high velocities (sand slinging). Sand slingers are usually reserved for use in making very large stings where great volumes of sand are handled.For smaller castings，a two-part metal box or flask referred to as a cope and drag is used. First the pattern is positioned on a mold board, and the drag or lower half of the flask is positioned over it. Parting powder is sprinkled on the pattern and the box is filled with sand. A jolt squeeze machine quickly compacts the sand. The flask is then turned over and again parting powder is dusted on it. The cope is then positioned on the top half of the flask and is filled with sand, and the two-part mold with the pattern board sandwiched in between is squeezed.PatternsPatterns for sand casting have traditionally been made of wood or metal. However, it has been found that wood patterns change as much as 3% due to heat and moisture. This factor alone would put many castings out of acceptable tolerance for more exacting specifications. Now, patterns are often made from epoxies and from cold-setting rubber with stabilizing inserts. Patterns of simple design, with one or more flat surface, can be molded in one piece, provided that they can be withdrawn without disturbing the compacted sand. Other patterns may be split into two or more parts to facilitate their removal from the sand when using two-part flasks. The pattern must be tapered to permit easy removal from the sand. The taper is referred to as draft. When a part does not have some natural draft, it must be added. A more recent innovation in patterns for sand casting has been to make them out of foamed polystyrene that is vaporized by the molten metal. This type of casting, known as the full-mold process, does not require pattern draft.Sprues, Runners, and Gates.Access to the mold cavity for entry of the molten metal is provided by sprues, runners, and gates, as shown in Fig.7-1. A pouring basin can be carved in the sand at the top of the sprue, or a pour box, which provides a large opening, may be laid over the sprue to facilitate pouring. After the metal is poured, it cools most rapidly in the sand mold. Thus the outer surface forms a shell that permits the still molten metal nearthe center to flow toward it. As a result, the last portion of the casting to freeze will be deficient in metal and, in the absence of a supplemental metal-feed source, will result in some form of shrinkage. This shrinkage may take the form of l shrinkage (large cavities) or the more subtle microshrinkage (finely dispersed porosity). These porous spots can be avoided by the use of risers，as shown in Fig.7-1，which Provide molten metal to make up for shrinkage losses.Fig.7-1 Sectional view of a casting moldCoresCores are placed in molds wherever it is necessary to preserve the space it occupies in the mold as a void in the resulting castings. As shown in Fig.7-11 the core will be put in place after the pattern is removed. To ensure its proper location, the pattern has extensions known as core prints that leave cavities in the mold into which the core is seated. Sometimes the core may be molded integrally with the green sand and is then referred to as a green-sand core. Generally, the core is made of sand bonded with core oil, some organic bonding materials, and water. These materials are thoroughly blended and placed in a mold or core box. After forming, they are removed and baked at 350°to 450°F (177°to 232°C). Cores that consist of two or more parts are pasted together after baking.CO2 CoresCO2 cores are made by ramming up moist sand in a core box. Sodium silicate is used as a binder, which is quickly hardened by blowing CO2 gas over it. The CO2 system has the advantage of making the cores immediately available.Pouring the MetalSeveral types of containers are used to move the molten metal from the furnace tothe pouring area. Large castings of the floor-and-pit type are poured with a ladle that has a plug in the bottom, or, as it is called, a bottom-pouring ladle. It is also employed in mechanized operations where the molds are moved along a line and each is poured as it is momentarily stopped beneath the large bottom-pour ladle.Ladles used for pouring ferrous metals are lined with a high alumina-content refractory. After long use and oxidation, it can be broken out and replaced. Ladles used in handling ferrous metals must be preheated with gas flames to approximately 2600° to 2700°F (1427° to 1482 °C) before filling. Once the ladle is filled, it is used constantly until it has been emptied.For nonferrous metals, simple clay-graphite crucibles are used. While they are quite susceptible breakage, they are very resistant to the metal and will hold up a long time under normal conditions. They usually do not require preheating, although care must be taken to avoid moisture pickup. For this reason they are sometimes baked out to assure dryness.The pouring process must be carefully controlled, since the temperature of the melt greatly affects the degree of liquid contraction before solidification, the rate of solidification, which in turn affects the amount of columnar growth present at the mold wall, the extent and nature of the dendritic growth, the degree of alloy burnout, and the feeding characteristics of the risering system.Finishing OperationsAfter the castings have solidified and cooled somewhat, they are placed on a shakeout table or grating on which the sand mold is broken up, leaving the casting free to be picked out. The casting is then taken to the finishing room where the gates and risers are removed. Small gates and risers may be broken off with a hammer if the material is brittle. Larger ones require sawing, cutting with a torch, or shearing. Unwanted metal protrusions such as fins, bosses, and small portions of gates and risers need to be smoothed off to blend with the surface. Most of this work is done with a heavy-duty grinder and the process is known as snagging or snag grinding. On large castings it is easier to move the grinder than the work, so swing-type grinders are used. Smaller castings are brought to stand- or bench-type grinders. Hand and pneumaticchisels are also used to trim castings. A more recent method of removing excess metal from ferrous castings is with a carbon-air torch. This consists of a carbon rod and high-amperage current with a stream of compressed air blowing at the base of it. This oxidizes and removes the metal as soon as it is molten. In many foundries this method has replaced nearly all chipping and grinding operations.New Wordscasting n.铸造，铸件cavity n.空腔，型腔solidify vt.凝固antiquity n.古代Buddha n.佛Nara n.奈良市artisan n.工匠filigree n.精细之作finish n.光洁度tolerance n.公差intricacy n.复杂machinability n.(可)切削性weldability n.(可）焊接性silica n.石英additive n.添加剂clay n.粘土refractory a.难熔的，耐火的fuse vt.使熔化zircon n.锆石mullite n.富铝红柱石ferrous a.铁的resin n.树脂，松香molding n.铸型，造型nonferrous a.非铁的intricate a.复杂的collapsibility n.退让性contraction n.收缩ram vt.夯实pattern n.模型，木模pneumatic a.气动的jolt vi.振动，摇动sling n.抛（砂）flask n.砂箱cope n.上砂箱drag n.下砂箱sprinkle vt.撒epoxy n.环氧树脂（胶）taper n.锥度，起模斜度draft n.起模斜度foamed a.泡沫的polystyrene n.聚苯乙烯sprue n.直浇口runner n.内浇口，横浇口basin n.浇口杯deficient a.不足的，缺乏的shrinkage n.收缩subtle a.细微的porosity n.多孔，缩松porous a.多孔的void n.空间integrally ad.整体地bonding n.粘结剂sodium n.钠silicate n.硅酸盐plug n.塞ladle n.浇勺，铁水包alumina n.氧化铝line v.做内衬susceptible a.容易的columnar a.柱状的dendritic a.树枝状的burnout n.熔蚀risering n.冒口protrusion n.凸出物fin n.周缘翅边boss n.表面凸出部分snag n.毛刺，凸出物；vt.清除（毛刺，浇口等）chisel n.凿子，凿刀chipping n.修整，清理Phrases and Expressionsbe applicable to (sb/sth)适用于be referred to as被称为gross shrinkage缩孔make up for 补偿be put in place放置在该放的位置上be susceptible to易于Notes1.Green sand generally consists of silica sand and additives coated by rubbing the sand grains together with clay uniformly wetted with water.型砂通常含有石英砂和添加剂，通过砂粒与用水均匀溅湿的粘土的搅拌，使砂粒及添加剂表面包复一层粘结薄膜。