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.
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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.
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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.
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5. Considerations
Solidification – mold design and material effect cooling rate (Heat transfer).
Metal flow into mold cavity – Flow.
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6. Fluid Flow Basic Casting System Pouring basin (cup).
Sprue, runners- channels
Gate- entry point for mold
Riser- reservoir
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8. 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
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9. 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
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10. Molds
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11. Expendable molds.
made of sand, plaster, or ceramics (mixed with bonding agents/binders).
broken up to remove casting.
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12. Permanent molds used repeatedly.
made from metals which maintain strength at high temperatures.
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13. Composite molds. two or more types of materials.
used to improve mold strength, cooling rates, cost of process.
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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.
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15. Characteristics Traditional casting method. Loose tolerances.
“poor” surface finish.
low cost.
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16. Examples of products Engine blocks Pump housings Cylinder heads
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18. Sands
Silica based (SiO2), inexpensive, resistance to high temperature.
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19. Sands (Cont.) Mulling- mixing sand with additives - conditioning
Clay used as cohesive agent to aid in bonding sand particles
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20. 3 Types of Sand Molds 1. Green Sand: sand, clay, and water.
least expensive.
Can dry mold surfaces – called “skin dried”
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21. 2 Cold-box: organic and inorganic binders added.
greater dimensional accuracy.
greater cost.
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22. 3 No-bake: synthetic liquid resin mixed with sand.
Cold-setting process- bonding of mold takes place without heat
Petrobond
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23. Mold Components Flask. Parting line - seam
Cope (top)
Drag (bottom)
Parting line - seam
Pouring basin or pouring cup.
Sprue – molten metal flows downward.
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24. Mold components (cont.)
Runner and gates.
Risers (blind and open).
Cores – form hollow regions
Vents – vent gasses
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26. Patterns Used to create sand molds
Made of wood, aluminum, steel, plastic, cast iron.
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27. 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
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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
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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.
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33. Shell molding:
Close tolerances
Good surface finish
Low cost
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34. Plaster-Mold Casting Plaster of paris with talc and silica flour.
Mixed with water
Poured over pattern
Plaster sets – pattern removed
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35. Plaster–mold casting(cont.)
Mold dried in oven
Poured in vacuum or under pressure due to low permeability
Low permeability (gas cannot escape)
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36. Plaster mold casting characteristics:
Plaster can only withstand about 2200 deg F.
Good surface and details.
“Lower” temperature alloys (Mg, Al, Zn)
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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.
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38. Expendable-Pattern Casting
Also known as Lost Foam, Evaporative-pattern, or Lost Pattern Casting
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39. Process: Polystyrene pattern coated with refractory slurry
Polystyrene pattern buried in silica sand within flask
Sprue extending out of sand
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40. Process (cont.) Flask vibrated to settle sand around pattern
Molten metal poured over sprue
Polystyrene vaporizes as metal fills cavity
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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.
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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
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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
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46. Characteristics High quality Good surface finish Good detail
Expensive process
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47. Vacuum Casting Counter-gravity low pressure process.
Sand and urethane molded over metal die.
Gate is on the bottom.
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48. Vacuum Casting (cont.)
Immersed into molten metal, which is drawn into mold cavity.
Thin wall, complex shapes, uniform properties, high volume, low cost.
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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
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51. Hot chamber piston submerged in molten metal
lower melting point metals
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52. Cold chamber
piston not submerged
higher melting alloys
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53. Dies
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54. Characteristics High production rates high quality parts
Close dimensional tolerances
complex shapes, good surface (net shape).
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55. Slush casting Metal poured in
Allowed to cool until outer skin solidifies and thickens to desired size
Excess metal poured out
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56. Centrifugal casting Produces hollow cylindrical parts
Street lamp posts
Gun barrels
Etc.
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59. Squeeze casting Solidification of molten metal under high pressure
Combination of casting and forging
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60. Melting Furnaces Electric Arc
Fast - high production rates – low pollution
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62. Induction Furnace Electrical Furnace
Heat applied by induction heating of metal through coils
Well controlled heating
Common in foundries today
Low pollution
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63. Induction Furnace
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64. Crucible Furnace
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65. Cupola
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66. Economics of Casting
L - low, M - medium, H - high
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