机械制造专业英语第7章SAND CASTING(砂型铸造)

SAND CASTING

Most 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 Sand

Green 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.

Patterns

Patterns 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 near