Full Casting

8/11/2019 Full Casting

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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

Full Casting

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