Mechanics of Solidification Proper Design of Gating system

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Mechanics of Solidification - Proper Design of
Gating system
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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) a square root of time
• For twice time thickness 1.41 or 41 more

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Solidified skin on casting
1 min
2 min
6 min
5 s

• 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.

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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

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• Fluidity of molten metal
• Fluidity capability of the molten metal to fill
  the mold cavities
• Factors influencing fluidity
• Viscosity Increasing viscosity and sensitivity
  to temperature reduces fluidity
• Surface tension High surface tension reduces
  fluidity
• InclusionsInsoluble particles reduce fluidity

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• Solidification Pattern
• Fluidity a 1/(Freezing Range)
• Pure metals act with good fluidity (lower
  freezing range)
• Casting parameters also affect fluidity
• Mold design
• Mold material (heat mold for good fluidity)
• Degree of superheat helps better fluidity
• Rate of pouring
• Heat transfer which affects viscosity

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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 length of the solidified metal
  greater is the fluidity

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• Two Principles of fluid flow
• Bernoullis theorem
• Law of mass continuity
• Will help in design of gating system
• Bernoullis Theorem
• h (P/rg) (v2/2g) constant
• h elevation from reference plane
• P pressure at the elevation
• r density of fluid
• v velocity of the fluid
• g gravitational constant
• F frinction

h1 (P1/rg) (v12/2g) h2 (P2/rg) (v22/2g)
f
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• 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

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Design of Sprues

• As the liquid flows down the cross section of the
  fluid decreases. So the taper is provided in the
  sprue
• Liquid loses contact if sprue is straight-causes
  Aspiration

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P1 P3, Level 1 is constant V1 0, assume no
frictional loss ght (v3)2 /2
hc
h2
ht
Time to fill mold Tf V/ AgV3 Ag cross
sectional area V volume of mold cavity
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• Flow Characteristics
• Important characteristics in fluid flow is
  Turbulence as opposed to Laminar Flow
• Reynolds number
• Re vDr/?
• v velocity
• D diameter
• r density
• ? viscosity
• Re is usually between 2000 and 20000

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• For Re above 20000
• dross 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
• dross can be reduced by filters ( ceramic,
  mica)
• Also with proper pouring basin and gating system

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Heat Transfer Chvorinovs Rule

• Solidification time is proportional to volume of
  casting and its surface area
• C constant reflects mold metal properties

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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 a 1/(surface area)2
• Assume volume to be unity
• Sphere V (4/3) pr3, r(3/4p)1/3 and A 4pr2
  4p(3/4p)2/3 4.84
• Cube V a3, a1, A 6a2 6
• Cylinder V pr2h 2pr3, r(1/2p)1/3, A 2pr2
  2prh 6pr2 6p(1/2p)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

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Various features of a sand mold
Pouring basin(cup)
Vent
Open riser
Core Sand
Flask
Blind Riser
Sand
Cope
Parting line
choke
Mold Cavity
Drag
Sand
Runner
gate
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Gating System

• Pouring basin, Sprue, choke, runner, ingate
• Function
• Trap contaminants
• Regulate flow of molten metal
• Control turbulence
• To establish directional solidification

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Design of gating system

• Pouring cup
• Cut into cope
• Large enough to keep the sprue full
• Skim core to provide clean metal

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Gating and Riser design
Vena cotracta
Molten metal
h1
h2
mold
Vacuum generation
Aspiration at point 2
Pseudo Vena contracta
Prevents vacuum
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Risering

• Risering is a process designed to prevent
  shrinkage voids that occur during solidification
  contractions
• Aluminum 6.6
• Steel 2.5 to 4
• Criteria for Riser design
• Riser must remain molten until casting is
  completely solidified
• Riser should have enough liquid metal to feed
  casting
• Riser should be kept at proper distance from the
  casting

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Plates Thick objects
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Risering
Cube
Plate

• 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
  8x8x2

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Riser design Caines Method
Relative riser and casting geometry to obtain
sound steel castings
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Riser on Plates and large A/V casting
Riser
1.00
Sound
4.5 t
Vr/Vc
t
Defective
0.00
8 16 24 32
(lw)/h
4 t
t
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Increasing riser efficiency

• 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
• Add exothermic compound on riser
• Use insulating sleeves on the sides of riser