1. Manufacturing Processes
Metal Casting I
Manufacturing Processes

2. Outline Introduction Metal Solidification Fluid Flow
Fluidity of Molten Metal
Heat Transfer
Casting Defects

3. Introduction
Casting
Pouring molten metal into a mold shaped after the part to be produced, allowing it to harden, and removing it from the mold

4. Introduction

5. Introduction
Can be used to create complex internal and external part geometries
Some casting processes can produce parts to net shape (no further manufacturing operations are required)
Can produce very large parts (cast parts weighing over 100 tons have been made)
Can be used with any metal that can be heated to its liquid phase
Some types of casting are suited to mass production

6. Examples of Cast Parts
Crank handle formed by casting; some areas were machined and assembled after casting

7. C-clamps formed by casting (left) and machining (right)
Examples of Cast Parts
C-clamps formed by casting (left) and machining (right)

8. Examples of Cast Parts Complex part formed by casting
Courtesy of Toth Industries

9. Forms of Casting and Terminology

10. Introduction Requirements: Mold cavity with desired shape and size
Melting process to provide molten metal
Pouring process to introduce the metal into the mold
Solidification process controlled to prevent defects
Ability to remove the casting from the mold
Cleaning, finishing and inspection operations

11. Casting Terminology Flask The box containing the mold Cope
The top half of any part of a 2-part mold
Drag
The bottom half of any part of a 2-part mold
Core
A shape inserted into the mold to form internal cavities
Core Print
A region used to support the core

12. Casting Terminology Mold Cavity
The hollow mold area in which metal solidifies into the part
Riser
An extra cavity to store additional metal to prevent shrinkage
Gating System
Channels used to deliver metal into the mold cavity
Pouring Cup
The part of the gating system that receives poured metal
Sprue
Vertical channel
Runners
Horizontal channels

13. Casting Terminology Parting Line / Parting Surface
Interface that separates the cope and drag of a 2-part mold
Draft
Taper on a pattern or casting that allows removal from the mold
Core Box
Mold or die used to produce cores
Casting
The process and product of solidifying metal in a mold

14. Metal Solidification
Pure Metals / Alloys
Cooling Rate

15. Pure Metals / Alloys
Pure metals solidify at a constant temperature; alloys solidify within a temperature range

16. Metal Solidification
A nucleating agent (inoculant) is a substance that induces grains to nucleate and form at the same time throughout the structure.

17. Cooling Rate Rapid cooling produces equiaxed (roughly round) grains
Slow cooling towards the interior forms long columnar grains that grow towards the center

18. Metal Solidification Dendrites
Tree-like structures that form during the solidification of alloys
Slow cooling rates produce dendrites with larger branch spacing; faster cooling rates produce finer spacing; very fast cooling rates produce no dendrites or grains

19. Metal Solidification

20. Metal Solidification

21. Metal Solidification

22. Fluid Flow Metal is poured through a pouring cup
Risers hold and supply metal to prevent shrinking during solidification
Gates are designed to prevent contaminants from reaching the mold cavity

23. Fluidity of Molten Metal
The capability of a molten metal to fill mold cavities
Viscosity
Higher viscosity decreases fluidity
Surface tension
Decreases fluidity; often caused by oxide film
Inclusions
Insoluble particles can increase viscosity, reducing fluidity
Solidification pattern
Fluidity is inversely proportional to the freezing temperature range

24. Fluidity of Molten Metal
Mold design
The design and size of the sprue, runners, and risers affect fluidity
Mold material and surface
Thermal conductivity and roughness decrease fluidity
Superheating
The temperature increment above the melting point increases fluidity
Pouring
Lower pouring rates decrease fluidity because of faster cooling
Heat transfer
Affects the viscosity of the metal

25. Fluidity of Molten Metal

26. Heat Transfer
The metal that solidifies first is at the wall of the mold; this solid layer thickens as time passes
Shrinkage during cooling can change the part dimensions and sometimes cause cracking; it is caused by the metal’s thermal expansion properties and the phase change between liquid and solid.

27. Heat Transfer

28. Heat Transfer

29. Casting Defects Metallic Projections Cavities Discontinuities
Defective surface
Incomplete Casting
Incorrect dimensions or shape
Inclusions

30. Casting Defects

31. Casting Defects Porosity may be caused by shrinkage and/or gases
Thin sections solidify faster than thick sections; therefore the molten metal cannot be supplied to thick regions that are solidifying
Gases become less soluble in a metal as it cools and solidifies, causing it to be expelled and sometimes form or expand porosity

32. Casting Defects

33. Casting Defects Chills
Pieces of material placed in the mold to speed up heat transfer in thicker areas of the part to prevent shrinkage porosity
Internal chills are left within the cast part; external chills are removed

34. Chills

35. Summary
Casting involves melting metal and allowing it to solidify in the desired shape
Casting allows the creation of parts that would be difficult or uneconomical to make by machining

37. Manufacturing Processes
Metal Casting II
Manufacturing
Processes

38. Outline Sand Casting Shell Mold Casting Composite Molds
Expendable Pattern Casting
Plaster Mold Casting
Ceramic Mold Casting
Investment Casting
Pressure Casting
Vacuum Casting
Die Casting
Centrifugal Casting
Squeeze Casting and Semisolid Metal Forming
Casting Single Crystals
Rapid Solidification
Melting
Design Considerations

39. Examples of Cast Parts

40. Examples of Cast Parts

41. Typical Casting Metals
Aluminum
Aluminum-silicon alloy
Aluminum-copper
Brass
Gray cast iron
Copper
Lead
Steel

42. Casting Processes

43. Sand Casting
Uses a mold made of compressed sand; after the metal solidifies, the sand is broken away

44. Sand Casting Pattern Full sized model of the part Core
Full sized model of the interior surfaces of the part
Sand Silica (SiO2)
90% sand
3% water
7% clay

45. Sand Casting

46. Sand Casting

47. Example of a Sand Casting Mold

48. Sand Casting Advantages:
Almost no limit on size, shape, weight or complexity; low cost; almost any metal
Limitations:
Relatively poor tolerances and surface finish; machining often required; low production rate
Common metals:
Cast irons, steel, stainless steel, casting alloys of aluminum and copper, magnesium and nickel

49. Sand Casting Size limits: 1 oz – 6000 lb Thickness limits:
As thin as 3/32 in, no maximum
Tolerances:
1/32 in for the first 6 in, .003 in for each additional inch; additional increment across the parting line
Draft allowance:
1 - 3°
Surface finish:
µin

50. Shell Casting
Casting process in which the mold is a thin shell (typically 3/8 inch) made of sand held together by a thermosetting binder

51. Shell Casting

52. Shell Casting Advantages:
Higher production rate than sand casting; high dimensional accuracy and smooth finish
Limitations:
Requires expensive metal patterns; resin adds to cost; part size is limited
Common metals:
Cast irons, casting alloys of aluminum and copper

53. Shell Casting Size limits:
1 oz minimum; usually less than 25 lb; mold area usually less than 500 in2
Thickness limits:
1/16 – ¼ in depending on material
Tolerances:
.005 in/in
Draft allowance:
¼ - ½°
Surface finish:
50 – 150 µin

54. Composite Molds Made from 2 or more different materials
Good for complex shapes such as turbine blades

55. Expendable Pattern Casting
Polystyrene pattern vaporizes on contact with molten metal

56. Foam Pattern of an Engine Block

57. Plaster Mold Casting
Uses a mold made of plaster (gypsum) with talc and silica, which is broken away after the metal solidifies
The mold has a relatively low thermal conductivity; a somewhat uniform grain structure can be produced

58. Plaster Mold Casting Advantages:
High dimensional accuracy and smooth finish; can make net- or near-net-shaped parts
Limitations:
Lower temperature nonferrous metals only; long molding time; mold material is not reusable; maximum size limited
Common metals:
Primarily aluminum and copper

59. Plaster Mold Casting Size limits: 1 oz – 15 lb Thickness limits:
As thin as .025 in
Tolerances:
.005 in on the first 2 in; .002 in per additional inch
Draft allowance:
½ - 1°
Surface finish:
µin

60. Ceramic Mold Casting
Uses a mold made of refractory ceramic materials which can be used for high-temperature applications

61. Ceramic Mold Casting

62. Ceramic Mold Casting Advantages:
Intricate detail, close tolerances, smooth finish
Limitations:
Mold material is expensive and not reusable
Common metals:
Ferrous and high-temperature nonferrous metals are most common; can be used with alloys of aluminum, copper, magnesium, titanium and zinc

63. Ceramic Mold Casting Size limits: Several ounces to several tons
Thickness limits:
As thin as .05 in, no maximum
Tolerances:
.005 in on the first inch; .003 in per additional inch
Draft allowance: 1°
Surface finish:
µin

64. Investment Casting
Uses a wax pattern which is coated with refractory materials to form a mold; the wax is then melted out and the mold cavity is filled with metal
Can be used for high precision complex shapes from high melting point metals that are not readily machinable

65. Investment Casting

66. Example of a Wax Injection Mold

67. Example of a Wax Pattern

68. Example of a Coated Pattern

69. Example of Finished Castings

70. Investment Casting Advantages:
Excellent surface finish; high dimensional accuracy; nearly unlimited intricacy; almost any metal; no flash or parting line
Limitations:
Expensive patterns and molds; high labor costs; limited size
Common metals:
Mainly aluminum, copper and steel; also used with stainless steel, nickel, magnesium and precious metals

71. Investment Casting Size limits:
As small as 1/10 oz; usually less than 10 lb
Thickness limits:
As thin as .025 in, less than 3 in
Tolerances:
.005 in on the first inch; .002 in per additional inch
Draft allowance:
none required
Surface finish:
µin

72. Pressure Casting
Pressure casting forces the metal up into the mold chamber by applying a small amount of pressure

73. Vacuum Casting

74. Permanent Mold Casting (Pressure/Vacuum)
Advantages:
Good surface finish and dimensional accuracy; metal mold causes rapid cooling and fine grain structure; molds can be used up to times
Limitations:
High initial mold cost; shape, size and complexity are limited; mold life is very limited with metals with high melting points
Common metals:
Alloys of aluminum, magnesium and copper most common; iron and steel can be used in graphite molds; alloys of lead, tin and zinc also used

75. Permanent Mold Casting (Pressure/Vacuum)
Size limits:
Several ounces to about 150 lb
Thickness limits:
Minimum depends on material but generally thicker than 1/8 in; maximum about 2 in
Tolerances:
.015 in for the first inch and .002 in for each additional inch; .01 in added across the parting line
Draft allowance:
2 - 3°
Surface finish:
µin

76. Die Casting
Another form of permanent mold casting; molten metal is forced into the mold cavity at pressures ranging from .7 MPa MPa

77. Die Casting

78. Die Casting

79. Example of a Die Casting Mold

80. Centrifugal Casting
Uses a rotating mold to form hollow cylindrical parts such as pipes, gun barrels and lamp posts

81. Vertical Centrifugal Casting

82. Centrifugal Casting Advantages:
Can produce a wide range of cylindrical parts; good dimensional accuracy and cleanliness
Limitations:
Limited shape; spinning equipment may be expensive
Common metals:
Iron, steel, stainless steel, alloys of aluminum, copper and nickel

83. Centrifugal Casting Size limits:
Up to 10 ft in diameter and 50 ft in length
Thickness limits:
Wall thickness .1 – 5 in
Tolerances:
Outer diameter within .1 in; inner diameter within about .15 in
Draft allowance:
1/8 in / ft
Surface finish:
µin

84. Semicentrifugal Casting
Uses a rotating mold to form parts with radial symmetry, such as wheels with spokes

85. Squeeze Casting
A combination of casting and forging; a die applies pressure as the metal solidifies

86. Casting Single Crystals
Uses a slow crystal-growth solidification procedure to produce parts made of a single crystal with no grain boundaries
A helical constriction only allows one crystal of favorable orientation to grow into and fill the mold chamber

87. Casting Single Crystals

88. Rapid Solidification
Cools metal rapidly at rates as high as 106 K/s so that it cannot crystallize and instead forms an amorphous glasslike structure

89. Melting Furnaces
Cupola
Crucible Furnace
Induction Furnace

90. Melting Furnaces Cupola
A vertical cylindrical furnace used for melting cast iron

91. Melting Furnaces Crucible furnace
Melts metal without direct contact with a burning fuel mixture

92. Melting Furnaces Induction furnace
Uses an alternating magnetic field to heat the metal

93. Design Considerations

94. Design Considerations

95. Design Considerations

96. Design Considerations

97. Casting Alloys

98. Summary
A variety of casting processes are available for different applications
Design considerations must be taken to prevent casting defects