Nov6-Casting - ME 250 Nov 6 2007 Casting Casting A 3-step...

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Unformatted text preview: ME 250 Nov 6, 2007 Casting Casting A 3-step process in which molten metal flows (by gravity or other force) into a mold. The metal solidifies in the shape of the mold cavity The metal part is then removed from the mold and is referred to as a casting Casting classification • Expendable mold • Pattern reused • Sand casting Å Sec 11.1 • Shell & vacuum molding Å 11.2.1-11.2.3 • Pattern destroyed • Investment casting Å 11.2.4 • Plaster mold / Ceramic mold casting Å 11.2.5 • Permanent Mold (11.3.1-11.3.2) • Die casting Å 11.3.3 • Centrifugal casting Å 11.3.4 Casting – Basic Terminology • Casting ¾ The “process” of forming objects by putting liquid or viscous material into a prepared mold. Object formed by allowing molten material to solidify • Pattern ¾ Full size model of the part used to create the mold • Mold ¾ Contains the cavity whose geometry replicates the shape of the cast part Casting- Large and small, complex parts (with internal features) can be created (using wide range of ferrous and non- ferrous metals)- Some casting processes can be used for mass production Poor surface finish & dimensional tolerances Mechanical properties not as good Parts not fully dense; porosity is an issue (surface and internal) Advantages Disadvantages Parts Made by Casting • Big parts: engine blocks for automotive vehicles, railway wheels, machine frames, pump housings, pipes, etc. • Small parts: dental crowns, jewelry, and a variety of small items Casting Schematic (fig 11.4) Chaplet Cast part Sand casting – Most widely used casting process – Sand (silica: SiO 2 ) mold is broken to retrieve the part – Can work all metals, particularly ones with high melting point (steel, nickel, titanium) – Produces low tolerance/surface finish: need subsequent machining Expendable mold; pattern reused Sand casting – Mold Sand casting steps Sand Casting – Gating System / Riser Acts as a reservoir and vent ¾ Must be big enough to supply all needed molten metal ¾ Metal in riser must be hot and last to solidify ¾ Cylindrical to reduce heat loss rate Fig. 5.4, Manufacturing Processes, by Amstead, Ostwald, Begeman, Wiley Sand casting – Pattern (a) (b) (c) (d) – Full-size model of part enlarged to account for shrinkage and post-casting machining allowance – Made of wood, plastics, metals – Pattern types: solid, split, match plate, cope/drag Sand Casting – Chills • Metal pieces placed in mold to speed up solidification • Can be internal or external ¾ Internal chills must be same material as the casting Fig. 5.17, Manufacturing Processes for Engineering Materials, by S. Kalpakjian, Addison Wesleyby S....
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Nov6-Casting - ME 250 Nov 6 2007 Casting Casting A 3-step...

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