Class5 Gating System

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Mechanics of Solidification -

Proper Design of Gatingsystem



Solidification

Type of mold

Geometry of shape

Inter phase heat transfer coefficient Type of metal flow ± Turbulence

Solidification: first skin forms and later solidification

progresses inwards

Thickness (t) E square root of time For twice time thickness = 1.41 or 41 % more2



5 s 1 min 2 min 6 min

Solidified skin on casting

The remaining metal is poured out at times indicated in the figure

Hollow ornamental and decorative objects are made by a process

called slush casting based on this principle.



Fluid Flow

Gating system design strongly depends on

knowledge of fluid flow

Sprue-Runner, Pouring basin, risers

Problems

Premature cooling

Turbulence Gas entrapment

Frothing and impurity

Reaction with mold producing gas



Fluidity of molten metal

Fluidity = capability of the molten metal to fill the moldcavities

Factors influencing fluidity

Viscosity: Increasing viscosity and sensitivity totemperature reduces fluidity

Surface tension: High surface tension reduces fluidity

Inclusions:Insoluble particles reduce fluidity



Solidification Pattern

Fluidity E 1/(Freezing Range)

Pure metals act with good fluidity (lower freezingrange)

Casting parameters also affect fluidity

1. Mold design

2. Mold material (heat mold for good fluidity)3. Degree of superheat helps better fluidity

4. Rate of pouring

5. Heat transfer which affects viscosity



Measuring fluidity A test for measuring

fluidity using a spiral

mold

The fluidity index of the

material is the length of

the solidified metal in

the spiral passage

The greater the lengthof the solidified metal

greater is the fluidity



Two Principles of fluid flow

Bernoulli¶s theorem

Law of mass continuity

Will help in design of gatingsystem

Bernoulli¶s Theorem

h + (P/ Vg) + (v2/2g) = constant

h: elevation from reference plane

P: pressure at the elevation

Vdensity of fluidv: velocity of the fluid

g: gravitational constant

F : friction

h1 + (P1/ Vg) + (v12/2g) = h2 +

(P2/ Vg) + (v2

2

/2g) + f



Laws of Continuity

For incompressible liquids the rate of flow is constant

Q = A1V1 = A2V2

Q: rate of flow m3/s

A: cross sectional area

V: velocity if fluid flow

Factors Affecting

Permeability

Gas loss



Design of Sprues

1

2

2

1

h

h

A

A!

As the liquid flows down the cross section of the

fluid decreases. So the taper is provided in thesprue

Liquid loses contact if sprue is straight-causes

µAspiration¶



hc

ht

h2

P1 = P3, Level 1 is

constant

V1 = 0, assume nofrictional loss

ght = (v3)2 /2

tgh23 !vTime to fill mold

Tf = V/ AgV3

Ag : cross sectional area

V: volume of mold cavity



Flow Characteristics

Important characteristics in fluid flow is Turbulence asopposed to Laminar Flow

Reynolds number

Re = vD V/

vvelocityD: diameter

Vdensity

: viscosity

Re is usually between 2000 and 20000



For Re above 20000

µdross¶ formations occur caused by air and gases Scum on top can get mixed with alloys

Elimination techniques

Avoid sudden changes in fluid flow Avoid sudden changes in cross section

µdross¶ can be reduced by filters ( ceramic, mica)

Also with proper pouring basin and gating system



Heat Transfer: Chvorinovs Rule

Solidification time is proportional to volume of casting

and its surface area

C: constant reflects mold metal properties

2

¹¹ º

¸©©ª

¨!

a surfaceare

volumeC time



Example

5.2

Q: Three pieces being cast have the same volume but different shapes.One is a

sphere,one a cube and the other a cylinder(Height = Diameter). Which piece will

solidify the fastest and which the slowest.

Solidification time E 1/(surface area)2

Assume volume to be unity

Sphere V = (4/3) Tr 3, r=(3/4T)1/3 and A = 4Tr 2 = 4T(3/4T)2/3 = 4.84

Cube V = a3, a=1, A= 6a2 = 6

Cylinder V = Tr 2h = 2Tr 3, r=(1/2T)1/3, A = 2Tr 2 + 2Trh = 6Tr 2 = 6T(1/2T)2/3 = 5.54

Thus respective solidification times are

Tsphere = 0.043 C Tcube = 0.028 C

Tcylinder = 0.033 C

C is a constant



Open riser Vent Pouring basin(cup)

Flask

Sand

Parting

line

Sand

gate

choke

Runner

Mold

CavityDrag

CopeBlind Riser

Core

Sand

Various features of a sand mold



Gating System

Pouring basin, Sprue, choke, runner, ingate

Function Trap contaminants

Regulate flow of molten metal

Control turbulence

To establish directional solidification



Design of gating system

Pouring cup

Cut into cope

Large enough to keep the sprue full Skim core to provide clean metal



Gating and Riser design

h1

h2

Aspiration at point 2

Vena cotracta

Vacuumgeneration

Molten metal

mold

Prevents vacuum

Pseudo

Vena

contracta



Risering

Risering is a process designed to prevent shrinkagevoids that occur during solidification contractions

Aluminum ± 6.6%

Steel ± 2.5 to 4%

Criteria for Riser design

Riser must remain molten until casting is completelysolidified

Riser should have enough liquid metal to feed casting Riser should be kept at proper distance from the

casting



PlatesThick objects



Risering

Solidification time for steel

castings different shapes in

green sand

Cylinder is insulated at ends

Risering a cube and plate

Both castings have equal freezing times yet the riser

which is adequate to feed the cube is not adequate to

feed the plate

Riser is 4´ in dia. Cube is 4´ side and plate is 8´x8´x2´

Cube Plate



Riser design: Caine¶s Method

V olumeCasting

V olume Riser

_

_

V olume Riser Area Riser

V olumeCasting AreaCasting

_

_

_

_

Relative riser and casting geometry to obtain sound steel castings



Defective

Sound

Vr/Vc

(l+w)/h

Riser

4.5 t

t

t

4 t

Riser on Plates and large A/V casting



Increasing riser efficiency

1. Blind riser

± Good for narrow freezing range

± Create a partial vacuum in the casting due to shrinkage

that can draw liquid metal from riser

Smaller riser - better yield

2. Add exothermic compound on riser

3. Use insulating sleeves on the sides of riser

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Class5 Gating System