1. FUNDAMENTALS OF METAL CASTING
Overview of Casting Technology
Heating and Pouring
Solidification and Cooling

2. Casting
Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity
The term casting also applies to the part made in the process
Steps in casting seem simple:
Melt the metal
Pour it into a mold
Let it freeze

3. Capabilities and Advantages of Casting
Can create complex part geometries
Can create both external and internal shapes
Some casting processes are net shape; others are near net shape
Can produce very large parts
Some casting methods are suited to mass production

4. Disadvantages of Casting
Different disadvantages for different casting processes:
Limitations on mechanical properties
Poor dimensional accuracy and surface finish for some processes; e.g., sand casting
Safety hazards to workers due to hot molten metals
Environmental problems

5. Parts Made by Casting Big parts
Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, pump housings
Small parts
Dental crowns, jewelry, small statues, frying pans
All varieties of metals can be cast, ferrous and nonferrous

6. Overview of Casting Technology
Casting is usually performed in a foundry
Foundry = factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting
Workers who perform casting are called foundrymen

7. The Mold in Casting
Contains cavity whose geometry determines part shape
Actual size and shape of cavity must be slightly oversized to allow for shrinkage of metal during solidification and cooling
Molds are made of a variety of materials, including sand, plaster, ceramic, and metal

8. Open Molds and Closed Molds
Figure Two forms of mold: (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity.

9. Two Categories of Casting Processes
Expendable mold processes – uses an expendable mold which must be destroyed to remove casting
Mold materials: sand, plaster, and similar materials, plus binders
Permanent mold processes – uses a permanent mold which can be used over and over to produce many castings
Made of metal (or, less commonly, a ceramic refractory material

10. Advantages and Disadvantages
More intricate geometries are possible with expendable mold processes
Part shapes in permanent mold processes are limited by the need to open the mold
Permanent mold processes are more economic in high production operations

11. Sand Casting Mold
Figure 10.2 (b) Sand casting mold.

12. Sand Casting Mold Terms
Mold consists of two halves:
Cope = upper half of mold
Drag = bottom half
Mold halves are contained in a box, called a flask
The two halves separate at the parting line

13. Forming the Mold Cavity
Mold cavity is formed by packing sand around a pattern, which has the shape of the part
When the pattern is removed, the remaining cavity of the packed sand has desired shape of cast part
The pattern is usually oversized to allow for shrinkage of metal during solidification and cooling
Sand for the mold is moist and contains a binder to maintain its shape

14. Use of a Core in the Mold Cavity
The mold cavity provides the external surfaces of the cast part
In addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of part
In sand casting, cores are generally made of sand

15. Gating System
Channel through which molten metal flows into cavity from outside of mold
Consists of a downsprue, through which metal enters a runner leading to the main cavity
At the top of downsprue, a pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue

16. Riser
Reservoir in the mold which is a source of liquid metal to compensate for shrinkage of the part during solidification
The riser must be designed to freeze after the main casting in order to satisfy its function

17. Heating the Metal
Heating furnaces are used to heat the metal to molten temperature sufficient for casting
The heat required is the sum of:
Heat to raise temperature to melting point
Heat of fusion to convert from solid to liquid
Heat to raise molten metal to desired temperature for pouring

18. Pouring the Molten Metal
For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying
Factors that determine success
Pouring temperature
Pouring rate
Turbulence

19. Solidification Time Solidification takes time
Total solidification time TTS = time required for casting to solidify after pouring
TTS depends on size and shape of casting by relationship known as Chvorinov's Rule
where TST = total solidification time; V = volume of the casting; A = surface area of casting; n = exponent with typical value = 2; and Cm is mold constant.

20. Mold Constant in Chvorinov's Rule
Mold constant Cm depends on:
Mold material
Thermal properties of casting metal
Pouring temperature relative to melting point
Value of Cm for a given casting operation can be based on experimental data from previous operations carried out using same mold material, metal, and pouring temperature, even though the shape of the part may be quite different

21. What Chvorinov's Rule Tells Us
A casting with a higher volume‑to‑surface area ratio cools and solidifies more slowly than one with a lower ratio
To feed molten metal to main cavity, TST for riser must greater than TST for main casting
Since mold constants of riser and casting will be equal, design the riser to have a larger volume‑to‑area ratio so that the main casting solidifies first
This minimizes the effects of shrinkage

22. Example Problem 1
(SI units) When casting low carbon steel under certain mold conditions, the mold constant in Chvorinov's rule = 4.0 min/cm1.9 and exponent value is Determine how long solidification will take for a rectangular casting whose length = 30 cm, width = 15 cm, and thickness = 20 mm.
Solution:
Volume V = 30 x 15 x 2 = 900 cm3
Area A = 2(30 x x x 2) = 1080 cm2
Chvorinov’s rule: TTS = Cm (V/A)1.9 =
4(900/1080)1.9 = min

23. Shrinkage in Solidification and Cooling
Figure (2) reduction in height and formation of shrinkage cavity caused by solidification shrinkage; (3) further reduction in height and diameter due to thermal contraction during cooling of solid metal (dimensional reductions are exaggerated for clarity).

24. Shrinkage Allowance
Patternmakers account for solidification shrinkage and thermal contraction by making mold cavity oversized
Amount by which mold is made larger relative to final casting size is called pattern shrinkage allowance
Casting dimensions are expressed linearly, so allowances are applied accordingly

25. Riser Design
Riser is waste metal that is separated from the casting and remelted to make more castings
To minimize waste in the unit operation, it is desirable for the volume of metal in the riser to be a minimum
Since the geometry of the riser is normally selected to maximize the V/A ratio, this allows riser volume to be reduced to the minimum possible value

26. Example Problem 2
(USCS units) Determine the shrink rule to be used by pattern makers for magnesium. Using the shrinkage value in Table 10.1, express your answer in terms of decimal fraction inches of elongation per foot of length compared to a standard one-foot scale.
Solution:
For magnesium shrinkage is 2.1% from Table 10.1.
Thus, linear contraction = 1.0 – =
Shrink rule elongation = (0.979)-1 =
L = (1 ft) = ft
1 ft = 12 inch , L = (12) = in
Elongation per foot of length = in

27. METAL CASTING PROCESSES
Sand Casting
Investment Casting
Permanent Mold Casting Processes
Casting Quality
Product Design Considerations

28. Two Categories of Casting Processes
Expendable mold processes - mold is sacrificed to remove part
Advantage: more complex shapes possible
Disadvantage: production rates often limited by time to make mold rather than casting itself
Permanent mold processes - mold is made of metal and can be used to make many castings
Advantage: higher production rates
Disadvantage: geometries limited by need to open mold

29. Overview of Sand Casting
Most widely used casting process, accounting for a significant majority of total tonnage cast
Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium
Castings range in size from small to very large
Production quantities from one to millions

30. Steps in Sand Casting Pour the molten metal into sand mold
Allow time for metal to solidify
Break up the mold to remove casting
Clean and inspect casting
Separate gating and riser system
Heat treatment of casting is sometimes required to improve metallurgical properties

31. Sand Casting Production Sequence
Figure Steps in the production sequence in sand casting.
The steps include not only the casting operation but also pattern‑making and mold‑making.

32. Core in Mold
Figure (a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.

33. Buoyancy in Sand Casting Operation
During pouring, buoyancy of the molten metal tends to displace the core, which can cause casting to be defective.
Weight of molten metal displaced
Weight of the core
Resultant upward force
FR/2
Core
Resultant upward force (FR) = Weight of molten metal displaced - Weight of the core
FR= Fb= buoyancy force

34. Example Problem 3
(SI units) An aluminum‑copper alloy (92% Al, 8% Cu) casting is made in a sand mold using a sand core that has a mass of 15 kg. Determine the buoyancy force in Newtons tending to lift the core during pouring.
Note: Sand density = 1.6 g/cm3 = lb/in3
Solution:
Sand density = 1.6 g/cm3 = kg/cm3
Core volume V = 15/ = 9375 cm3
Density of aluminum-copper alloy = 2.81 g/cm3 = kg/cm3 (Table 11.1)
Mass of displaced Al-Cu W = 9375( ) = kg
1 kg (mass) = 9.81 N (weight)
Fb = Wm - Wc = 9.81 ( ) = N

35. Investment Casting (Lost Wax Process)
A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal
"Investment" comes from a less familiar definition of "invest" - "to cover completely," which refers to coating of refractory material around wax pattern
It is a precision casting process - capable of producing castings of high accuracy and intricate detail

36. Investment Casting
Figure Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree

37. Investment Casting
Figure Steps in investment casting: (3) the pattern tree is coated with a thin layer of refractory* material, (4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid.
*The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in
the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium (lime). Fire clays are
also widely used in the manufacture of refractories.

38. Investment Casting
Figure Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies

39. Investment Casting
Figure Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue

40. Advantages and Disadvantages
Advantages of investment casting:
Parts of great complexity and intricacy can be cast
Close dimensional control and good surface finish
Wax can usually be recovered for reuse
Additional machining is not normally required ‑ this is a net shape process
Disadvantages
Many processing steps are required
Relatively expensive process

41. Permanent Mold Casting Processes
Economic disadvantage of expendable mold casting: a new mold is required for every casting
In permanent mold casting, the mold is reused many times
The processes include:
Basic permanent mold casting
Die casting
Centrifugal casting

42. The Basic Permanent Mold Process
Uses a metal mold constructed of two sections designed for easy, precise opening and closing
Molds used for casting lower melting point alloys are commonly made of steel or cast iron
Molds used for casting steel must be made of refractory material, due to the very high pouring temperatures

43. Permanent Mold Casting
Figure Steps in permanent mold casting: (1) mold is preheated and coated

44. Permanent Mold Casting
Figure Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.

45. Advantages and Limitations
Advantages of permanent mold casting:
Good dimensional control and surface finish
More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger
Limitations:
Generally limited to metals of lower melting point
Simpler part geometries compared to sand casting because of need to open the mold
High cost of mold

46. Applications of Permanent Mold Casting
Due to high mold cost, process is best suited to high volume production and can be automated accordingly
Typical parts: automotive pistons, pump bodies, and certain castings for aircraft and missiles
Metals commonly cast: aluminum, magnesium, copper‑base alloys, and cast iron

47. Die Casting
A permanent mold casting process in which molten metal is injected into mold cavity under high pressure
Pressure is maintained during solidification, then mold is opened and part is removed
Molds in this casting operation are called dies; hence the name die casting
Use of high pressure to force metal into die cavity is what distinguishes this from other permanent mold processes

48. Die Casting Machines
Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity
Two main types:
Hot‑chamber machine
Cold‑chamber machine

49. Advantages and Limitations
Advantages of die casting:
Economical for large production quantities
Good accuracy and surface finish
Thin sections are possible
Rapid cooling provides small grain size and good strength to casting
Disadvantages:
Generally limited to metals with low metal points
Part geometry must allow removal from die

50. Centrifugal Casting
Centrifugal casting refers to several casting methods in which the mold is rotated at high speed so that centrifugal force distributes the molten metal to the outer regions of the die cavity. The group includes
(1) true centrifugal casting,
(2) semicentrifugal casting,
(3) centrifuge casting,

51. True Centrifugal Casting
In true Centrifugal casting, molten metal is poured into a rotating mold to produce a tubular part, such as pipes, tubes, bushing, and rings.
For this casting, the so-called G-factor (GF) is the ratio of centrifugal force divided by weight:
R = inside radius of the mold, m (ft)
N = rotational speed, rev/min.
g = acceleration of gravity, 9.81 m/s2 (32.2 ft/s2)
GF = 60 to 80 are appropriate for horizontal centrifugal casting

52. Example Problem 4
(USCS units) A horizontal true centrifugal casting operation is performed to make cast iron pipe. The pipes have a length = 72.0 in, outside diameter = 6.0 in, and wall thickness = in. (a) If the rotational speed of the pipe = 750 rev/min, determine the G‑factor. (b) Is the operation likely to be successful? (c) If not, what should the rotational speed be to achieve a G-factor of 60?
Solution:
(a) Using outside wall of casting, R = 0.5(6)/12 = 0.25 ft
GF = (R( N/30)2)/g = (0.25( 750/30)2)/32.2 = 47.9
(b) Since the G-factor is less than 60, the rotational speed is not sufficient, and the operation is likely to be unsuccessful.
(c) Using a G-factor = 60, N = (30/π)(32.2 x 60/0.25)0.5 = 9.55(7728)0.5 = 840 rev/min

53. Additional Steps After Solidification
Trimming
Removing the core
Surface cleaning
Inspection
Repair, if required
Heat treatment

54. General Defects: Misrun
A casting that has solidified before completely filling mold cavity
Figure Some common defects in castings: (a) misrun

55. General Defects: Cold Shut
Two portions of metal flow together but there is a lack of fusion due to premature freezing
Figure Some common defects in castings: (b) cold shut

56. General Defects: Cold Shot
Metal splatters during pouring and solid globules form and become entrapped in casting
Figure Some common defects in castings: (c) cold shot

57. General Defects: Shrinkage Cavity
Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze
Figure Some common defects in castings: (d) shrinkage cavity

58. Sand Casting Defects: Sand Blow
Balloon‑shaped gas cavity caused by release of mold gases during pouring
Figure Common defects in sand castings: (a) sand blow

59. Sand Casting Defects: Pin Holes
Formation of many small gas cavities at or slightly below surface of casting
Figure Common defects in sand castings: (b) pin holes

60. Sand Casting Defects: Penetration
When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metal
Figure Common defects in sand castings: (e) penetration

61. Sand Casting Defects: Mold Shift
A step in cast product at parting line caused by sidewise relative displacement of cope and drag
Figure Common defects in sand castings: (f) mold shift

62. Foundry Inspection Methods
Visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flaws
Dimensional measurements to insure that tolerances have been met
Metallurgical, chemical, physical, and other tests concerned with quality of cast metal

63. Product Design Considerations
Corners on the casting:
Sharp corners and angles should be avoided, since they are sources of stress concentrations and may cause hot tearing and cracks
Generous fillets should be designed on inside corners and sharp edges should be blended

64. Product Design Considerations
Draft Guidelines:
In expendable mold casting, draft facilitates removal of pattern from mold
Draft = 1 for sand casting
In permanent mold casting, purpose is to aid in removal of the part from the mold
Draft = 2 to 3 for permanent mold processes
Similar tapers should be allowed if solid cores are used

65. Draft Minor changes in part design can reduce need for coring
Figure Design change to eliminate the need for using a core: (a) original design, and (b) redesign.

66. Product Design Considerations
Dimensional Tolerances and Surface Finish:
Significant differences in dimensional accuracies and finishes can be achieved in castings, depending on process:
Poor dimensional accuracies and finish for sand casting
Good dimensional accuracies and finish for die casting and investment casting

67. Product Design Considerations
Machining Allowances:
Almost all sand castings must be machined to achieve the required dimensions and part features
Additional material, called the machining allowance, is left on the casting in those surfaces where machining is necessary
Typical machining allowances for sand castings are around 1.5 and 3 mm (1/16 and 1/4 in)

68. Homework Assignment 1- Reading Assignment Chapters 10, and 11
2- Written Assignment
(USCS units) A disk‑shaped part is cast out of aluminum. Diameter of the disk = 25 in and thickness = 0.6 in. If n = 2, and the mold constant = min/in2 in Chvorinov's rule, how long will it take the casting to solidify?
(SI units) Determine the shrink rule to be used by mold makers for die casting of zinc. Using the shrinkage value in Table 10.1, express your answer in terms of decimal mm of elongation per 300 mm of length compared to a standard 300-mm scale.
(USCS units) A sand core has a volume of 57.0 in3. It is located inside a mold cavity used to cast a cast iron pump housing. Determine the buoyancy force that tends to lift the core during pouring.
(SI units) Copper tubes are produced by horizontal true centrifugal casting. The tube lengths = 2.0 m, outside diameter = 16.0 cm, and inside diameter = 15.0 cm. If the rotational speed of the pipe = 900 rev/min, determine the G‑factor.