ME364 MET KS Ch11 - Chapter 11 Metal-Casting Processes 1...

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1 Chapter 11 Metal-Casting Processes
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2 METAL CASTING PROCESSES Classification of casting processes are related to: 1. Mold materials, 2. Molding processes, 3. Methods of feeding the mold with molten material. Expendable (non-permanent) Molds Typically made of sand, plaster, ceramics and mixed with binding materials. Typical sand mold consists of 90% sand, 7% clay and 3% water. These materials can withstand high temperature of molten metals. After the casting solidified, mold is broken to remove the casting. Permanent Molds Molds are made of metals that maintain their strength at high temperatures. They are used repeatedly, designed for fast removal of casting. Metal molds are better conductor of heat, therefore solidified casting is subjected to fast cooloing which affect its microstructure and grain size. Composite Molds Made of two or more materials combining advantages of each. Have permanent and non-permanent parts. Used to improve mold strength, control cooling rate and optimize economy.
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3 Typical Cast Parts Figure 11.1 (a) Typical gray-iron castings used in automobiles, including the transmission valve body (left) and the hub rotor with disk-brake cylinder (front). Source : Courtesy of Central Foundry Division of General Motors Corporation. (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. Source: Courtesy of Polaroid Corporation and Chicago White Metal Casting, Inc. (a) (b) (c) (d)
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4 Production Steps in Sand-Casting Figure 11.2 Outline of production steps in a typical sand-casting operation.
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5 Sand Mold Figure 11.3 Schematic illustration of a sand mold, showing various features.
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6 Factors Affecting Sand Selection Grain Size: Small grain sand forms a smooth surface, but reduces mold permeability Permeability: Factor affecting escape of gases and steam from the mold. Fine grains reduces permeability. Collapsibility: Mold should have good collapsibility to allow for casting to shrink while cooling, thus avoid hot tearing and cracking. The most coomonly used mold material is GREEN MOLDING SAND . Some sand molds with synthetic binders are oven dried. They are stronger than green sand molds, have better surface finish and dimensional accuracy. However they are more expensive, and suceptible to hot tearing.
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7 Pattern Plate Patterns are used to mold the sand mixture into the shape of the casting and
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ME364 MET KS Ch11 - Chapter 11 Metal-Casting Processes 1...

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