me_350_-_lect_9_-_ch_11.20090923.4abaf6b12f8890.02303255

me_350_-_lect_9_-_ch_11.20090923.4abaf6b12f8890.02303255 -...

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Unformatted text preview: ME 350 Lecture 9 Chapter 11 METAL CASTING PROCESSES 1. Sand Casting 2. Other Expendable Mold Casting Processes 3. Permanent Mold Casting Processes 4. Casting Quality 5. Product Design Considerations Making the Sand Mold The cavity in the sand mold is formed by packing sand around a pattern, then separating the mold into two halves and removing the pattern The mold must also contain gating and riser system If casting is to have internal surfaces, a must be included in mold A new sand mold must be made for each part produced Patterns Pattern a model of the part, slightly Types of patterns used in sand casting: (a) solid pattern, (b) split pattern, (c) matchplate pattern (d) cope and drag pattern Core Fullscale model of surfaces of part May require supports to hold it in position in the mold cavity during pouring, called (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity. Buoyancy in Sand Casting Operation During pouring, buoyancy of the molten metal tends to displace the core, which can cause casting to be defective Force tending to lift core = weight of displaced liquid less the weight of core itself F b = W m W c where F b = buoyancy force; W m = weight of molten metal displaced; and W c = weight of core Desirable Mold Properties Strength to maintain shape and resist Permeability to allow to pass through voids in sand Thermal stability to resist cracking on contact with molten metal Collapsibility ability to give way and allow casting to shrink without cracking the casting Reusability can sand from broken mold be reused to make other molds? Foundry Sands Silica (SiO 2 ) or silica mixed with other minerals Good refractory properties capacity to endure high Small grain size yields better During pouring, large grain size is more Irregular grain shapes strengthen molds due to interlocking, compared to round grains Disadvantage: interlocking tends to reduce Types of Sand Mold Greensand molds - mixture of sand, clay, and water or oil; Green" means mold contains moisture at time of pouring Drysand mold instead of clay it uses organic binders such as phenolic resins For strength, mold usually must be: Skindried mold - drying mold cavity surface of a greensand mold Typical depth of 10 to 25 mm, using torches or heating lamps 1. Shell Molding Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder Steps in shellmolding: (1) a matchplate or copeanddrag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.with thermosetting resin....
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