Metal Casting Process: From Mold to Product

1. 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

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3. 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

4. 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

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

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

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8. 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

9. 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

10. Molds Engr 241

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

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

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

14. 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

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

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

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

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

20. 3 Types of Sand Molds • 1. Green Sand: • sand, clay, and water. • least expensive. • Can dry mold surfaces – called “skin dried” Engr 241

21. 2 Cold-box: • organic and inorganic binders added. • greater dimensional accuracy. • greater cost. Engr 241

22. 3 No-bake: • synthetic liquid resin mixed with sand. • Cold-setting process- bonding of mold takes place without heat • Petrobond Engr 241

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

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

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

27. 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

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

31. 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

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

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

35. 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

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

37. 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

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

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

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

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42. 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

43. 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

44. 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

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

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

48. 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

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50. 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