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

Engr 241

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

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

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

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

Engr 241

Defects Metallic projections. Cavities. Discontinuities. 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

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

Molds Engr 241

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

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

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

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

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

Examples of products Engine blocks Pump housings Cylinder heads Engr 241

Engr 241

Sands Silica based (SiO2), inexpensive, resistance to high temperature. Engr 241

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

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

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

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

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

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

Engr 241

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

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

Engr 241

Engr 241

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

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

Engr 241

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

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

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

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

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

Expendable-Pattern Casting Also known as Lost Foam, Evaporative-pattern, or Lost Pattern Casting Engr 241

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

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

Engr 241

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

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

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

Engr 241

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

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

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

Engr 241

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

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

Cold chamber piston not submerged higher melting alloys Engr 241

Dies Engr 241

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

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

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

Engr 241

Engr 241

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

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

Engr 241

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

Induction Furnace Engr 241

Crucible Furnace Engr 241

Cupola Engr 241

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