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Tài liệu 04 casting processes

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ADVANCED MANUFACTURING ENGINEERING 4: Casting Processes and Equipment December 2016 2 Introduction • Casting is a manufacturing process where a solid is melted, heated to proper temperature (sometimes treated to modify its chemical composition), and is then poured into a cavity or mould, which contains it in the proper shape during solidification. • In a single step, simple or complex shapes can be made from any metal that can be melted. • Cast parts range in size from a few of mm and grams (such as the individual teeth on a zipper), to over 10 metres and many tonnes (such as the huge propellers). 3 Introduction • Casting has marked advantages in the production of complex shapes, parts having hollow sections or internal cavities, parts that contain irregular curved surfaces (except those made from thin sheet metal), very large parts and parts made from metals that are difficult to machine. • Due to these obvious advantages, casting is one of the most important of the manufacturing processes. • http://youtu.be/7BI4v0Gqc7k 40 secs • http://youtu.be/M95bhPrDwA0 7 mins 4 Parts Made by Casting • •Big parts: engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, and pump housings • •Small parts: dental crowns, jewellery, small statues, and frying pans • •All varieties of metals can be cast, ferrous and nonferrous 5 Typical Cast Parts (a) Typical gray-iron castings used in automobiles, including the transmission valve body (left) and the hub rotor with disk-brake cylinder (front). (b) A cast transmission housing. (c) The Polaroid PDC-2000 digital camera with a AZ191D die-cast high-purity magnesium case. (d) A two-piece Polaroid camera case made by the hot-chamber die-casting process. 6 Casting products Mymachining.com 7 Casting 8 Casting • Process in which molten metal flows by gravity or other force into a mould where it solidifies in the shape of the mould cavity • The term casting also applies to the part made in the process • Steps in casting seem simple: • 1. Melt the metal • 2. Pour it into a mould • 3. Let it freeze 9 Temperature & Density for Castings • Temperature as a function of time for the solidification of pure metals. Note that freezing takes place at a constant temperature. (b) Density as a function of time. 10 Capabilities and Advantages of Casting • Can create complex part geometries • Can create both external and internal shapes • Some casting processes are net shape; others are near net shape • Can produce very large parts • Some casting methods are suited to mass production 11 Disadvantages of Casting • Limitations on mechanical properties • Poor dimensional accuracy and surface finish for some processes; e.g., sand casting • Safety hazards to workers due to hot molten metals • Environmental problems 12 Casting technology • Today, it is nearly impossible to design anything that cannot be cast by one or more of the available casting processes. • However, as in all manufacturing techniques, the best results and economy are achieved if the designer understands the various options and tailors the design to use the most appropriate process in the most efficient manner. 13 Casting technology • The various processes differ primarily in the mould material (whether sand, metal, or other material) and the pouring method (gravity, vacuum, low pressure, or high pressure). • All of the processes share the requirement that the materials solidify in a manner that would maximise the properties, while simultaneously preventing potential defects, such as: • shrinkage voids, • gas porosity, and • trapped inclusions. 14 Classification of casting processes 15 Classification of Casting Processes 16 Casting processes • Green Sand Moulding • Dry Sand moulding • Precision casting methods • Chemically Bonded Sand Moulding Processes 17 The Mould in Casting • Contains cavity whose geometry determines part shape • Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling • Moulds are made of a variety of materials, including sand, plaster, ceramic, and metal. 18 Temperature distribution • Temperature distribution at the mould wall and liquid- metal interface during solidification of metals in casting 19 Green sand moulding • Green sand is by far the most diversified moulding method used in current metal casting operations. • The green sand process utilises a mould made of compressed or compacted moist sand packed around a wood or metal pattern. • The term "green" denotes the presence of moisture in the moulding sand, and indicates that the mould is not baked or dried. • http://youtu.be/QFLArEHFFHo 4/8 mins 20 Sand casting https://www.youtube.com/watch?v=JwjJ0DlE3fA#action=share
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