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Casting
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Casting since about 4000 BC
Ancient Greece; bronzestatue casting circa 450BC
Iron works in early Europe,e.g. cast iron cannons fromEngland circa 1543
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Outline
Sand Casting, Investment Casting, Die Casting
Basics and countermeasures
Phase Change, ShrinkageHeat Transfer
Pattern Design
Variations & DevelopmentsEnvironmental Issues
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Casting
Readings;
1. Kalpakjian, Chapters 10, 11,
12
2. Booothroyd, Design for DieCasting
3. Flemings Heat Flow in
Solidification
Note: a good heat transfer reference can be found by
Prof John Lienhard onlinehttp://web.mit.edu/lienhard/www/ahtt.html
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Casting Methods
Sand CastingHigh Temperature Alloy,Complex Geometry,Rough Surface Finish
Investment CastingHigh Temperature Alloy,Complex Geometry,Moderately Smooth SurfaceFinish
Die CastingHigh Temperature Alloy,Moderate Geometry,Smooth Surface
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SandCasting
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Sand Casting
Description: Tempered sand is packedinto wood or metal pattern halves,removed form the pattern, andassembled with or without cores, andmetal is poured into resultant cavities.
Various core materials can be used.Molds are broken to remove castings.Specialized binders now in use canimprove tolerances and surface finish.
Metals: Most castable metals.
Size Range: Limitation depends on
foundry capabilities. Ounces to manytons.
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Sand Casting Mold Features
Vents, which are placed inmolds to carry off gasesproduced when the molten
metal comes into contact withthe sand in the molds and core.They also exhaust air from themold cavity as the molten metalflows into the mold.
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See Video from Mass Foundry
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Production sand casting
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InvestmentCasting
The investment-castingprocess, also called thelost-waxprocess, was firstused during the period4000-3500 B.C. The patternis made of wax or a plastic
such as polystyrene. Thesequences involved ininvestment casting areshown in Figure 11.18. Thepattern is made by injectingmolten wax or plastic into ametal die in the shape ofthe object.
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Investment Casting
Description: Metal mold makes wax orplastic areplica. There are sprued,then surrounded with investmentmaterial, baked out, and metal ispoured in the resultant cavity. Moldsare broken to remove the castings.
Metals: Most castable metals.
Size Range: fraction of an ounce to150 lbs..
Surface Finish:63-125RMS
Minimum Draft Requirements: None
Normal Minimum Section Thickness:.030
(Small Areas).060
(Large Areas)
Ordering Quantities:Aluminum: usually under 1,000Other metals: all quantities
Normal Lead Time:Samples: 5-16 weeks (depending
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Die CastingCold-Chamber Casting
Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, moltenmetal is poured into the chamber; (2) ram forces metal to flow into die,maintaining pressure during the cooling and solidification; and (3) ram iswithdrawn, die is opened, and part is ejected. Used for higher temperature metals
eg Aluminum, Copper and alloys
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Die CastingHot-Chamber Casting
Cycle in hot-chambercasting: (1) with dieclosed and plungerwithdrawn, moltenmetal flows into thechamber; (2) plunger
forces metal inchamber to flow intodie, maintainingpressure duringcooling andsolidification; and (3)plunger is withdrawn,die is opened, and
solidified part isejected. Finished partis shown in (4).
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Die CastingDescription: Molten metal is injected, under pressure, intohardened steel dies, often water cooled. Dies are opened,and castings are ejected.
Metals: Aluminum, Zinc, Magnesium, and limited Brass.
Size Range: Not normally over 2 feet square. Some foundriescapable of larger sizes.
Tolerances:
Al and Mg .002/in.Zinc .0015/in.Brass .001/in.
Add .001to .015across parting line depending onsize
Surface Finish: 32-63RMS
Minimum Draft Requirements:Al & Mg: 1to 3Zinc: 1/2to 2
Brass: 2to 5Normal Minimum Section Thickness:
Al & Mg: .03Small Parts: .06Medium PartsZinc: .03Small Parts: .045Medium PartsBrass: .025Small Parts: .040Medium Parts
Ordering Quantities:Usually 2,500 and up.
Normal Lead Time:Samples: 12-20 weeksProduction: ASAP after approval.
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High Melt Temperature
Chemical ActivityHigh Latent HeatHandlingOff-gassing
3000C
0C
1000C
2000C
Tungsten Carbide, WC,Silicon Carbide, SiC
Molybdenum
Alumina Al2O3
Platinum, PtTitanium, TiIronFE, Plain Carbon Steels, low alloy, stainlessNickel, NiNickel Allows
Cubic Zirconia, ZrO2
Silicon, Si
Copper, Cu, Bronze, Brass
AluminumMagnesiumZinc, ZnPTFE (Teflon)Tin, SnHDPE
NylonAcetal
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Mold Filling
Bernoulis Equation:
Reynolds Number:
Short filling times
Potential Turbulence
(see p. 273 Kalpakjian
.2
2
Constg
v
pg
ph
vDPRe
h
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Mold Filling Example(1 of 2)
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Mold Filling Example(2 of 2)
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Phase
Change &Shrinkage
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Solidification of a binary alloy
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Composition change duringsolidification
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Solidification
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Cast structures
Schematic illustration of three caststructures solidified in a square mold:(a) pure metals; (b) solid solutionalloys; and structure obtained byusing nucleating agents. Source: G. W.Form, J. F. Wallace, and A. Cibula
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Pop quiz; If you top fill the
mold below, what will the partlook like after solidification?
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Can you explain these
features?
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Heat TransferSand Casting
2
A
Vts
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Heat TransferDie Casting
1
A
Vts
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Steady State Conduction Heat Transfer
Figure 1
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Steady State Conduction Heat Transfer
Figure 2
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Thermal Conductivitykof VariousMaterials for Parts and Molds (W/m K)
Copper 394
Aluminum 222
Iron 29
Sand 0.61
PMMA 0.20
PVC 0.16
dx
dTkq
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Film Coefficients W/m2K
Typical die casting 5,000
Natural convection 1 - 10
Flowing air 10 - 50
Thq
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Transient Heat Transfer
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Sand Casting (see Flemings)
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Sand Casting (see Flemings)
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Solidification Time
enthapy
Use Flemingsresult here
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Solidification Time (cont.)
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Cooling Time; thin slab
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Cooling time;intersection
1
1
2
11
2
h
Lh
A
V
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Pattern Design suggestions
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More
PatternDesign
suggestions
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And more
Figure 7.2.32Omit outsidebosses and theneed for cores.(Courtesy ofMeehanite MetalCorp.)
Figure 7.2.35Avoid using ribs
which meet atacute angles.(Courtesy ofMeehanite MetalCorp.)
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Die Casting
SolidificationTime
s
Time to formsolid part
A
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Time to cool part to the ejectiontemperature. (lumped parameter model)
mCpdT
dt Ah T To
let TTo
tf
tip
dtmC
Ahdi
f
i= Ti + Tsp- Tmold
Tsp= H/Cp
f= Teject- Tmold
tmCp
Ahln
f
i
Integration yields
twCp
2hln
Tinject TspTmold
Teject Tmold
Or for thin sheets of thickness w,
sp means superheat
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Pattern Design Issues (Alum)
Shrinkage Allowance .013/1
Machining Allowance 1/16
Minimum thickness 3/16
Parting Line
Draft Angle 3 to 5%
Uniform Thickness
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Pattern Design
Table 12.1
Normal Shrinkage Allowance forSome Metals Cast in Sand Molds
Metal PercentGray cast iron 0.83 1.3White cast iron 2.1Malleable cast iron0.78 1.0Aluminum alloys 1.3Magnesium alloys 1.3Yellow brass 1.3 1.6Phosphor bronze 1.0 1.6
Aluminum bronze 2.1High-manganese steel 2.6
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Variations and Developments
Continuous casting
Lost foam molding
3D Printing of Investment tooling
Direct printing with metal droplets
Uniform metal spray
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Continuous casting ref AISI
Steel from the electric or basic oxygen furnace is tapped into a ladle and taken tothe continuous casting machine. The ladle is raised onto a turret that rotates the ladleinto the casting position above the tundish. Referring to Figure 2,liquid steel flows outof the ladle (1) into the tundish (2), and then into a water-cooled copper mold (3).Solidification begins in the mold, and continues through the First Zone (4) and Strand Guide (5).In this configuration, the strand is straightened (6), torch-cut (8), then discharged (12)for intermediate storage or hot charged for finished rolling.
3D Printing
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3D Printingof Investment cast tooling
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Shell and part (Turbine blade)
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Microcasting of droplets
CMUMIT
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Environmental Issues
Smelting
Energy
Off-gassing see AFS webpage on green sand emissions;http://www.afsinc.org/environmental.html
Cooling water
Waste sand disposalOff shore locations
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Metal Smelting; reducing oxides andsulfides to metal..
http://www.steel.org/learning/howmade/blast_furnace.htm
E i t l l d b
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Environmental loads bymanufacturing sector
Carbon Dioxide and Toxic Materials per Value of Shipments
0
0.5
1
1.5
2
2.53
3.5
4
4.5
5
Chem
icals
Petro
leum
and
Coal
Plasticsand
Rubber
Primary
Metal
Fabricated
Me
tal
Mach
inery
Elect
ronic
Transpor
tation
Manufacturing industries
Weight/Dollars
CO2 (metric ton/$10,000)
Toxic Mat'ls (lb/$1000)
EPA 2001, DOE 2001
Th i d i l f f i
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The estimated environmental performance of various
mfg processes (not including auxiliary requirements)
*Energy per wt. normalized
by the melt energy
** total raw matl normalized
by the part wt.
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Summary
Sand Casting, Investment Casting,
Die Casting
Basics and countermeasures
Phase Change, Shrinkage
Heat Transfer
Pattern DesignVariations and Developments
Environmental Issues