Metal Casting Process in which molten metal

Metal Casting • Process in which molten metal is poured into a mold (shaped after the part to be manufactured), then allowed to cool and solidify. Engr 241 0

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Metal Casting (cont. ) • After solidification occurs the part is removed from the mold to cool further. • The main objective is to produce parts free of defects and with the desired properties. Engr 241 2

Process Characteristics • Complex shapes that may have internal cavities • Large or small parts. • Can use materials which are otherwise hard to shape. • Economical. • Near net shape manufacturing. Engr 241 3

Considerations • Solidification – mold design and material effect cooling rate (Heat transfer). • Metal flow into mold cavity – Flow. Engr 241 4

Fluid Flow • Basic Casting System • Pouring basin (cup). • Sprue, runners- channels • Gate- entry point for mold • Riser- reservoir Engr 241 5

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Defects • Metallic projections. • flash, fins, swells. • Cavities. • blow holes, pinholes, shrinkage. • Discontinuities. • cracks, • cold or hot tearing – constrained from shrinking freely • cold shuts. - interface from two streams of liquid meeting Engr 241 7

Defects (Continued) • Defective surface. • folds, laps, scars, adhering sand, oxide. • Incomplete casting. • misruns, insufficient metal, runout. • Incorrect dimensions or shape. • Inclusions. • non-metallic - slag Engr 241 8

Molds Engr 241 9

Expendable molds. • made of sand, plaster, or ceramics (mixed with bonding agents/binders). • broken up to remove casting. Engr 241 10

Permanent molds • used repeatedly. • made from metals which maintain strength at high temperatures. Engr 241 11

Composite molds. • two or more types of materials. • used to improve mold strength, cooling rates, cost of process. Engr 241 12

Sand Casting • Consists in placing a pattern in sand to make an imprint, incorporating a gating system, filling the cavity with molten metal, letting it cool, breaking the mold to remove the casting. Engr 241 13

Characteristics • Traditional casting method. • Loose tolerances. • “poor” surface finish. • low cost. Engr 241 14

Examples of products • Engine blocks • Pump housings • Cylinder heads Engr 241 15

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Sands • Silica based (Si. O 2), inexpensive, resistance to high temperature. Engr 241 17

Sands (Cont. ) • Mulling- mixing sand with additives - conditioning • Clay used as cohesive agent to aid in bonding sand particles Engr 241 18

3 Types of Sand Molds • 1. Green Sand: • sand, clay, and water. • least expensive. • Can dry mold surfaces – called “skin dried” Engr 241 19

2 Cold-box: • organic and inorganic binders added. • greater dimensional accuracy. • greater cost. Engr 241 20

3 No-bake: • synthetic liquid resin mixed with sand. • Cold-setting process- bonding of mold takes place without heat • Petrobond Engr 241 21

Mold Components • Flask. • Cope (top) • Drag (bottom) • Parting line - seam • Pouring basin or pouring cup. • Sprue – molten metal flows downward. Engr 241 22

Mold components (cont. ) • • Runner and gates. Risers (blind and open). Cores – form hollow regions Vents – vent gasses Engr 241 23

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Patterns • Used to create sand molds • Made of wood, aluminum, steel, plastic, cast iron. Engr 241 25

Types of mold patterns • One piece (loose pattern). • simple shapes, low quantity production. • Split pattern • Two piece patterns, complex shapes. • Match plate • Split patterns secured to plate Engr 241 26

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Mold Components • Cores- provide internal cavities • Chaplets- anchors, supports for cores • Chill- insert for preferential cooling • Core prints – recesses to support core Engr 241 29

Shell Mold Casting • • • Pattern coated with sand binder Cured in oven Shell removed Left with hollow shell for filling Close tolerance good surface finish, low cost. Engr 241 30

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Shell molding: • Close tolerances • Good surface finish • Low cost Engr 241 32

Plaster-Mold Casting • • Plaster of paris with talc and silica flour. Mixed with water Poured over pattern Plaster sets – pattern removed Engr 241 33

Plaster–mold casting(cont. ) • Mold dried in oven • Poured in vacuum or under pressure due to low permeability • Low permeability (gas cannot escape) Engr 241 34

Plaster mold casting characteristics: • Plaster can only withstand about 2200 deg F. • Good surface and details. • “Lower” temperature alloys (Mg, Al, Zn) Engr 241 35

Ceramic-Mold Casting Similar to plaster mold casting but utilizes refractory mold materials of zircon, aluminum oxide and fused silica • Good dimensional accuracy and surface finish, but expensive. Engr 241 36

Expendable-Pattern Casting • Also known as Lost Foam, Evaporativepattern, or Lost Pattern Casting Engr 241 37

Process: • Polystyrene pattern coated with refractory slurry • Polystyrene pattern buried in silica sand within flask • Sprue extending out of sand Engr 241 38

Process (cont. ) • Flask vibrated to settle sand around pattern • Molten metal poured over sprue • Polystyrene vaporizes as metal fills cavity Engr 241 39

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Expendable Foam Advantages • • simple process, no parting lines, or risers inexpensive flasks, minimum finishing polystyrene is cheap and gives good detail. economical for long production runs (pattern mold cost). • can be automated. Engr 241 41

Investment Casting Process • Inject wax (plastic) in to metal die (pattern shape) • Wax pattern dipped to coat in refractory slurry – dries to form hard shell Engr 241 42

Investment casting (cont. ) • Wax melted out of hardened shell • Molds connected to tree with central sprue • Molten metal poured in • Mold destroyed to expose solidified parts Engr 241 43

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Characteristics • • High quality Good surface finish Good detail Expensive process Engr 241 45

Vacuum Casting • Counter-gravity low pressure process. • Sand urethane molded over metal die. • Gate is on the bottom. Engr 241 46

Vacuum Casting (cont. ) • Immersed into molten metal, which is drawn into mold cavity. • Thin wall, complex shapes, uniform properties, high volume, low cost. Engr 241 47

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Die Casting • Type of permanent die casting • Molten metal is forced into a permanent mold (die) at high pressure. • Dies are steel or graphite • Two types of die casting Engr 241 49

Hot chamber • piston submerged in molten metal • lower melting point metals Engr 241 50

Cold chamber • piston not submerged • higher melting alloys Engr 241 51

Dies Engr 241 52

Characteristics • • High production rates high quality parts Close dimensional tolerances complex shapes, good surface (net shape). Engr 241 53

Slush casting • Metal poured in • Allowed to cool until outer skin solidifies and thickens to desired size • Excess metal poured out Engr 241 54

Centrifugal casting • • Produces hollow cylindrical parts Street lamp posts Gun barrels Etc. Engr 241 55

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Squeeze casting • Solidification of molten metal under high pressure • Combination of casting and forging Engr 241 58

Melting Furnaces • Electric Arc • Fast - high production rates – low pollution Engr 241 59

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Induction Furnace • Electrical Furnace • Heat applied by induction heating of metal through coils • Well controlled heating • Common in foundries today • Low pollution Engr 241 61

Induction Furnace Engr 241 62

Crucible Furnace Engr 241 63

Cupola Engr 241 64

Economics of Casting L - low, M - medium, H - high Engr 241 65