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Die Casting Basics

Die is closed. Metal

is drawn in to tool(plunger).

Tool injects metal

into cavity.

Cavity continues

to fill. (fractionsof a second)

Metal solidifiesunder pressure.

Die is opened.Casting removed.

Machine recovers toinitial orientation

(cycle starts over)

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Properties: Pb-Sb vs. Zn-Al

L35Pb-Sb Alloy

Zamak 3Zn-Al Alloy

Density( g/cm^3) at 21C

11.04 6.6Solidificationshrinkage ( % ) 3.36 1.17Freezingrange ( C )

252-299 381 - 387Specific heat capacity( J/kg/C ) at 20 - 100 C

133.1 418.7Thermal expansion( um/m per C at 20-100 C)

27.8 27.4Thermal conductivity( cal/cm2/cm/ C/sec at 70-140C )

.073 .27Viscosity (poise) .032 .01

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Stage III:

Temperature Monitoring Overview

Nozzle temperature

Nozzle freezing

Die temperature

Thermal expansions

Holding pot temperature

Temperature gradients

Excess superheat

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Results:

Injection Pressure

Monitor weight as afunction of pressure

Decreasing Pressure: Reduces flashing

Decreases machineerrors

Weight variation foreach setting < 1%

*Tolerance (41.24 43.80g)

Pressure Dependency Analysis

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Molten metalleakingthrough thegap is calledflashing.

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Assume the upper and low er mold pieces have opposing faces w hich are perfectly smooth, but

w ith a gap of thickness 103

m .. Consider if a Zn melt is pressurized to 3 105

Pa while

atmospheric pressure is 1 10

5

Pa . The viscosity of the Zn is 0.003 Pa s. Determine the steadystate volume flow rate through the mold gap if the circumference of the cylindrical mold is 6 m

and the distance from the inside of the mold to the outside of the mold is 0.2 m. Assume laminar

f low . Hint: think about f low between parallel plates.

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Upper Mold

Lower Mold

PressurizedMoltenMetal

Atmospheric

Pressure

Mold gap

VolumeFlowRate2

3

P

L

3 W

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Solution : The volume f low rate is given by the average velocity multiplied by the cross section of

f low , w hich is the same as the velocity prof ile integrated over the gap thickness multiplied by the

w idth of the gap, W

VolumeFlowRate2

3

P

L

3 W

w here is the half thickness of the gap and W is the w idth (circumference in this case)

0.5103

m P 2 105

Pa 0.003Pa s W 6 m L 0.2 m

VolumeFlowRat e2

3

P

L

3 W

VolumeFlowRat e 0.167m3

s

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Upper Mold

Lower Mold

PressurizedMolten Metal

AtmosphericPressure

Mold gap

L(t1) L(t2) L(t3) L(t4)

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The above problem is concerned w ith steady state f low of molten metal through the mold gap.

Of greater interest is the time required for the molten alloy to reach the outside of the mold

after it is first injected into the mold. We can estimate that time by letting L be the distance

betw een the melt and the tip of the f low through the mold gap and assuming that the rate of

change of the of L is the average rate of f low between parallel plates. Note that this is an

approximate solution to this problem.

AverageFlowRate1

2

yP

2 L t( )

2y

2

d

t

L t( )d

d

Af ter integration with respect to y

tL t( )

d

d

P

3 L t( )

2

Solving the diff erential equation by integration

0

L

LL t t( )( )d P

3 2

0

t

1d

1

2L

2

P

3

2 t

The approximation that theliquid:air interface velocity isequal to the average velocityof the steady stae profile wasintroduced by E. W.Washburn, Physical Review,vol. 17, pp. 213-283, 1921.

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1

2L

2

P

3

2 t

The solutions f or L from the Mathcad symbolic solver ('symbolics' on

the tool bar, then variable and then solve) are

1

3 6

1

2

P t

1

2

1

3 6

1

2 P t

1

2

Picking the positive one

L1

3 6

1

2 P t

1

2

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t 0 0.001 0.01

L t( )1

3 6

1

2 P t

1

2

0 0.005 0.01

0

0.2

0.4

L t( )

t

m

s

Looks like it w ill take about 0.005 seconds for the molten metal to begin f lashing through

the mold w all gap given the parameters defined above.

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AirFluid

Further discussion of the planar interfaceapproximation.

Flow profile is disturbed at the fluid air interface

Average velocity must be equal for incompressible fluid

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Represent the surface tensions of a multi-phase junction as vectors drawn parallel to the respective surfacesThe surface energies for the for the solid/liquid, the solid/vapor and the liquid/vapor interfaces are sl, sv, lv

sl

sv

lv

The contact angle is a measureof the magnitude of the solid

liquid interface energy comparedto the solid vapor and liquid

vapor energies.

Complications regarding the shape of the moving solid vapor interface.Meniscus Formation

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Youngs Equation

Represent the surface tensions of a multi-phase junction as vectors drawn parallel to the respective surfaces

The surface energies for the for the solid/liquid, the solid/vapor and the liquid/vapor interfaces are sl, sv, lv.

The vectors representing these surface energies must balance at the three phase triple junction. This equationrepresenting this balance is known as Youngs equation

sl

sv

lv

lvslsv )cos(

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sl

sv

lv

sl

sv

lv

Large sl, non-wetting

Large

Large sv, wetting small

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Liquid Vapor

Liquid Vapor

Interface shapes for wetting and non-wetting contact angles