Introduction to Manufacturing

1. Introduction to Manufacturing Chapters 10 & 11: Metal Casting Processes and Equipment

2. 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. • 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

3. 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

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

5. Solidification of Metals • Pure metals have single cooling temperature. • Chill zone- (skin) small equiaxed grains. • Columnar zone- crystal growth inward. • Homogeneous nucleation- grains grow upon each other. Engr 241

6. Cooling Rates • Slow- course dendrites, large spacing. • Moderate- fine with small dendrite spacing. • Fast- amorphous structure. Engr 241

7. Cooling Rates (Cont.) • When grain size decreases • strength and ductility increase. • microporosity decreases. • cracks decrease. • Lack of uniformity in grain size gives anisotropic properties Engr 241

8. Alloy Segregation • Microsegregation. • higher concentration of alloying elements at surface. • Normal Segregation. • higher concentration of alloying elements at center. Lower melting alloys forced to center. Engr 241

9. Alloy Segregation (Cont.) • Inverse Segregation • Metals with high concentrations of alloy elements -lower concentration of alloying elements at center. Dentrites shrink, alloys fill • Gravity Segregation • high density inclusions or compounds sink, lighter elements float. • Inoculation • heterogeneous nucleation. Engr 241

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

11. Fluid Flow (Cont.) • Bernoulli's Theorem. • pressure, velocity, elevation at any location, and frictional losses. • Continuity Law • rate of flow, permeability. • aspiration (non tapered sprue) • Flow Characteristics • laminar or turbulent. Engr 241

12. Fluidity • Capability of molten metal to fill the mold. • Metal Characteristics. • Casting Parameters. • (see p. 249) Engr 241

13. Heat Transfer During Cooling • Temperature Distribution. • flow, premature chilling- (Fig. 10-9) • Solidification Time (shapes). • ratio of volume to surface area. • Shrinkage- (Table 10.1). • contraction of the metal when cooled. • Grey iron expands. Engr 241

14. Defects • Metallic projections. • flash, fins, swells. • Cavities. • blow holes, pinholes, shrinkage. • Discontinuities. • cracks, cold or hot tearing, cold shuts. Engr 241

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

16. Porosity • Caused by shrinkage or gasses. • Chills are used to increase the rate of solidification Engr 241

17. Molds • Expendable molds. • made of sand, plaster, or ceramics (mixed with bonding agents/binders). • broken up to remove casting. • Permanent molds • used repeatedly. • made from metals which maintain strength at high temperatures. Engr 241

18. Other (Molding) Methods • Composite molds. • two or more types of materials. • used to improve mold strength, cooling rates, cost of process. Engr 241

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

20. Sands • Silica based (SiO2), inexpensive, resistance to high temperature. • Naturally bonded (bank sands). • Synthetic (lake sands). • Grain size (permeability- heat transfer/gases out, collapsibility- sand breaks down easily). • Sand is typically conditioned Engr 241

21. Sands (Cont.) • Mulling- mixing sand with additives. • Additives: • Clay (bentonite)- for bonding/strength • Zircon, Olivine, and Iron silicate- to lower thermal expansion • Chromite- for high heat transfer Engr 241

22. Types of Sand Molds • Green Sand: • sand, clay, and water. • least expensive. • Cold-box: • organic and inorganic binders. • greater dimensional accuracy. • greater cost. Engr 241

23. Types of Sand Molds (Cont.) • No-bake: • synthetic liquid resin mixed with sand. • Cold-setting process- bonding of mold takes place without heat Engr 241

24. Mold Components (Fig. 11-4, p.265) • Flask. • Cope/Drag. • Pouring basin or pouring cup. • Sprue. • Runner and gates. • Risers (blind and open). • Cores. • Vents. Engr 241

25. Mold Patterns • Used to create sand molds • Made of wood, aluminum, steel, plastic, cast iron. • 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. Mold Components • Cores- inner part of mold • Chaplets- anchors, supports for cores • Chill- insert for preferential cooling Engr 241

27. Shell Mold Casting • Uses binder which hardens in CO2 • Shell is formed from injected/poured sand over a mold • Close tolerance good surface finish, low cost. • Thin walled-low permeability Engr 241

28. Expendable-Pattern Casting • Also known as Lost Foam, Evaporative-pattern, or Lost Pattern Casting • Polystyrene beads, bonded by hot die • Flask formed vaporizes during pouring Engr 241

29. Expendable Foam • Advantages (P. 275) • simple process, no parting lines, or riser system. • inexpensive flasks, minimum finishing and cleaning. • polystyrene is cheap and gives good detail. • economical for long production runs (pattern mold cost). • can be automated. Engr 241

30. Plaster-Mold Casting • Plaster of paris with talc and silica flour. • Water mix- pour as a slurry. • Low permeability (gas cannot escape). • Good surface and details. • Cools slowly. • “Lower” temperature alloys (Mg, Al, Zn) Engr 241

31. Ceramic-Mold Casting • Also referred to as cope and drag investment casting. • Ceramic slurry is poured over a pattern, removed, and baked • Slurry: refractory mold materials (fine grained zircon, aluminum oxide, fused silica). • Good dimensional accuracy and surface finish, but expensive. Engr 241

32. Investment Casting (P. 278) • Lost-Wax Process • Consists in coating a pattern, made of wax or plastic, with a refractory material. Once the coating agent has dried, the mold is heated to remove the wax. • Superb finishing • Trends (RP). Engr 241

33. Vacuum Casting • Counter-gravity low pressure process. • Sand and urethane molded over metal die. • Gate is on the bottom. • Immersed into molten metal, which is drawn into mold cavity. • Thin wall, complex shapes, uniform properties, high volume, low cost. Engr 241

34. Die Casting • Molten metal is forced into a permanent mold (die) at high pressure. • Hot chamber and Cold chamber pressure casting • High production rates, high quality parts, good dimensions, complex shapes, good surface (net shape). Engr 241

35. Other Processes • Permanent Mold Casting. • Slush Casting. • Centrifugal Casting. • Squeeze Casting • Semi-solid Metal Forming. Engr 241

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