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Casting


Casting
2.810 Prof. Timothy Gutowski

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Casting since about 4000 BC…

Ancient Greece; bronze statue casting circa 450BC

Iron works in early Eur

ope, e.g. cast iron cannons from England circa 1543
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Outline
Sand Casting, Investment Casting, Die Casting
Basics and countermeasures Phase Change, Shrinkage Heat Transfer Pattern Design

Variations & Developments
Environmental Issues

2

Casting
Readings;
1. 2. 3. Kalpakjian, Chapters 10, 11, 12 Booothroyd, “Design for Die

Casting”
Flemings “Heat Flow in Solidification”

Note: a good heat transfer reference can be found by Prof John Lienhard online http://web.mit.edu/lienhard/www/ahtt.html
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Casting Methods

? Sand Casting

High Temperature Alloy, Complex Geometry, Rough Surface Finish

? Investment Casting
High Temperature Alloy, Complex Geometry, Moderately Smooth Surface Finish

? Die Casting

High Temperature Alloy, Moderate Geometry, Smooth Surface

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Sand Casting

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Sand Casting
Description: Tempered sand is packed into wood or metal pattern halves, removed form the pattern, and assembled with or without cores, and metal is poured into resultant cavities. Various core materials can be used. Molds are broken to remove castings. Specialized binders now in use can improve tolerances and surface finish. Metals: Most castable metals. Size Range: Limitation depends on foundry capabilities. Ounces to many tons. Tolerances: Non-Ferrous ? 1/32? to 6? Add ? .003? to 3?, ? 3/64? from 3? to 6?. Across parting line add ? .020? to ? .090? depending on size. (Assumes metal patterns) Surface Finish: Non-Ferrous: 150-350 RMS Ferrous: 300-700RMS

Minimum Draft Requirements: 1° to 5° Cores: 1° to 1 1/2°
Normal Minimum Section Thickness: Non-Ferrous: 1/8? - 1/4? Ferrous: 1/4? - 3/8? Ordering Quantities: All quantities Normal Lead Time: Samples: 2-10 weeks Production 2-4 weeks A.S.A.

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Sand Casting Mold Features

Vents, which are placed in

molds to carry off gases produced when the molten metal comes into contact with the sand in the molds and core. They also exhaust air from the mold cavity as the molten metal flows into the mold.

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See Video from Mass Foundry

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Production sand casting

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Investment Casting
The investment-casting process, also called the lost-wax process, was first used during the period 4000-3500 B.C. The pattern is made of wax or a plastic such as polystyrene. The sequences involved in investment casting are shown in Figure 11.18. The pattern is made by injecting molten wax or plastic into a metal die in the shape of the object.

10

Investment Casting
Description: Metal mold makes wax or plastic replica. There are sprued, then surrounded with investment material, baked out, and metal is poured in the resultant cavity. Molds are broken to remove the castings. Metals: Most castable metals. Size Range: fraction of an ounce to 150 lbs.. Tolerances: ? .003? to 1/4? ? .004? to 1/2?, ? .005? per inch to 3? ? .003? for each additional inch Surface Finish: 63-125RMS Minimum Draft Requirements: None Normal Minimum Section Thickness: .030? (Small Areas) .060? (Large Areas) Ordering Quantities: Aluminum: usually under 1,000 Other metals: all quantities Normal Lead Time: Samples: 5-16 weeks (depending on complexity) Production 4-12 weeks A.S.A. (depending on subsequent operations). Talbot Associates Inc.

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Die Casting –

Cold-Chamber Casting

Cycle in cold-chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber; (2) ram forces metal to flow into die, maintaining pressure during the cooling and solidification; and (3) ram is withdrawn, die is opened, and part is ejected. Used for higher temperature metals eg Aluminum, Copper and alloys

12

Die Casting –
Cycle in hot-chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber; (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification; and (3) plunger is withdrawn, die is opened, and solidified part is ejected. Finished part is shown in (4).

Hot-Chamber Casting

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Die Casting
Description: Molten metal is injected, under pressure, into hardened 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 foundries capable of larger sizes. Tolerances: Al and Mg ? .002?/in. Zinc ? .0015?/in. Brass ? .001?/in. Add ? .001? to ? .015? across parting line depending on size Surface Finish: 32-63RMS Minimum Draft Requirements: Al & Mg: 1° to 3° Zinc: 1/2° to 2° Brass: 2° to 5° Normal Minimum Section Thickness: Al & Mg: .03? Small Parts: .06? Medium Parts Zinc: .03? Small Parts: .045? Medium Parts Brass: .025? Small Parts: .040? Medium Parts Ordering Quantities: Usually 2,500 and up. Normal Lead Time: Samples: 12-20 weeks Production: ASAP after approval.

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High Melt Temperature
?Chemical Activity ?High Latent Heat ?Handling ?Off-gassing
3000° C
Tungsten Carbide, WC, Silicon Carbide, SiC Molybdenum Cubic Zirconia, ZrO2

2000° C

Alumina Al2O3

Platinum, Pt Titanium, Ti IronFE, Plain Carbon Steels, low alloy, stainless Nickel, Ni Silicon, Si Nickel Allows

1000° C

Copper, Cu, Bronze, Brass

0° C

Aluminum Magnesium Nylon Zinc, Zn Acetal PTFE (Teflon) Tin, Sn HDPE

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Mold Filling
Bernouli’s Equation: p v2 h? ? ? Const. pg 2 g
Reynold’s Number: h

Re ?

vDP

?

?Short filling times ?Potential Turbulence

(see p. 273 … Kalpakjian
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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 during solidification

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Solidification

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Cast structures
Schematic illustration of three cast structures solidified in a square mold: (a) pure metals; (b) solid solution alloys; and ? structure obtained by using 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 part look like after solidification?

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Can you explain these features?

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Heat Transfer – Sand Casting
?V ? ts ? ? ? ? A?
2

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Heat Transfer – Die Casting
?V ? ts ? ? ? ? A?
1

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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 Conductivity “k” of Various Materials for Parts and Molds (W/m °K)
Copper Aluminum Iron Sand PMMA 394 222 29 0.61 0.20

PVC

0.16

dT q ? ?k dx
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Film Coefficients W/m2°K
Typical die casting Natural convection Flowing air 5,000 1 - 10 10 - 50

q ? ?h??T ?
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Transient Heat Transfer

32

Sand Casting

(see Flemings)

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Sand Casting

(see Flemings)

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Solidification Time

enthapy

Use Flemings result here

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Solidification Time (cont.)

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Cooling Time; thin slab

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Cooling time;intersection

? ? ?? V h ? 1 ? 1 ?? ?? ? ?1 ? ? A 2 ? 2 ? L ? 1 ?? ? ? ? ? h ?? ? ?
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Pattern Design suggestions

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More Pattern Design suggestions

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And more…
Figure 7.2.32
Omit outside bosses and the need for cores. (Courtesy of Meehanite Metal Corp.)

Figure 7.2.35

Avoid using ribs which meet at acute angles. (Courtesy of Meehanite Metal Corp.)

41

Die Casting Solidification Time
s

Time to form solid part

A

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Time to cool part to the ejection temperature. (lumped parameter model)
mC p dT ? ? Ah ?T ? To ? dt

Integration yields…

let ? ? T ? T o
tf Ah ? d? ? dt ? ? ??ti ?? f ? m Cp ?? ?

t?

?mCp Ah

ln

?? f ?? i

?i

Or for thin sheets of thickness “w”,
??i = Ti + ?Tsp - Tmold ?Tsp = H/Cp
t? w ?C p 2h ??T inject ??T sp ? T mold ?? ln ?? ?? T ? T eject mold ?? ??

??f = Teject - Tmold

“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 for Some Metals Cast in Sand Molds Metal Percent Gray cast iron 0.83 – 1.3 White cast iron 2.1 Malleable cast iron0.78 – 1.0 Aluminum alloys 1.3 Magnesium alloys 1.3 Yellow brass 1.3 – 1.6 Phosphor bronze 1.0 – 1.6 Aluminum bronze 2.1 High-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 to the continuous casting machine. The ladle is raised onto a turret that rotates the ladle into the casting position above the tundish. Referring to Figure 2, liquid steel flows out of 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. 47

3D Printing of Investment cast tooling

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Shell and part (Turbine blade)

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Microcasting of droplets

CMU

MIT
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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 disposal Off shore locations
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Metal Smelting; reducing oxides and sulfides to metal…..

http://www.steel.org/learning/howmade/blast_furnace.htm
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Environmental loads by manufacturing sector
Carbon Dioxide and Toxic Materials per Value of Shipments
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

Weight/Dollars

CO2 (metric ton/$10,000)

Toxic Mat'ls (lb/$1000)

Primary Metal

Fabricated Metal

Manufacturing industries

Transportation

Plastics and Rubber

Petroleum and Coal

Chemicals

Machinery

Electronic

EPA 2001, DOE 2001

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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 mat’l normalized 54 by the part wt.

Summary
Sand Casting, Investment Casting, Die Casting Basics and countermeasures

Phase Change, Shrinkage
Heat Transfer

Pattern Design
Variations and Developments Environmental Issues
55


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