1) Proper design of the gating system is important for solidification and depends on factors like mold geometry, metal flow, and heat transfer. 2) The gating system includes components like the sprue, runner, and ingate and is designed based on principles of fluid flow like Bernoulli's theorem and the law of continuity. 3) Risers are used to prevent shrinkage voids and their design must ensure the riser remains molten until solidification is complete to feed the casting. The size and placement of risers depends on properties of the casting like its shape and surface area to volume ratio.

1. Mechanics of Solidification -
Proper Design of Gating
system

2. 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
2

3. 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.

4. 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

5. 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
• Inclusions:Insoluble particles reduce fluidity

6. • Solidification Pattern
Fluidity a 1/(Freezing Range)
• Pure metals act with good fluidity (lower freezing
range)
• 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

7. 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

8. Two Principles of fluid flow
• Bernoulli’s theorem
• Law of mass continuity
• Will help in design of gating
system
Bernoulli’s 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 : friction
h1 + (P1/rg) + (v1
2/2g) = h2 +
(P2/rg) + (v2
2/2g) + f

9. 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

10. 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 the
sprue
• Liquid loses contact if sprue is straight-causes
‘Aspiration’

11. hc
ht
h2
P1 = P3, Level 1 is
constant
V1 = 0, assume no
frictional loss
ght = (v3)2 /2
t
gh
2
3 
v
Time to fill mold
Tf = V/ AgV3
Ag : cross sectional area
V: volume of mold cavity

12. Flow Characteristics
• Important characteristics in fluid flow is Turbulence as
opposed to Laminar Flow
• Reynolds number
Re = vDr/η
v: velocity
D: diameter
r: density
η: viscosity
• Re is usually between 2000 and 20000

13. 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

14. Heat Transfer: Chvorinovs Rule
• Solidification time is proportional to volume of casting
and its surface area
C: constant reflects mold metal properties
2









a
surfaceare
volume
C
time

15. 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, a=1, 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

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

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

18. Design of gating system
Pouring cup
• Cut into cope
• Large enough to keep the sprue full
• Skim core to provide clean metal

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

20. 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

21. Plates
Thick objects

22. 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

23. Riser design: Caine’s Method
Volume
Casting
Volume
Riser
_
_
Volume
Riser
Area
Riser
Volume
Casting
Area
Casting
_
_
_
_
Relative riser and casting geometry to obtain sound steel castings

24. Defective
Sound
Vr/Vc
1.00
0.00
8 16 24 32
(l+w)/h
Riser
4.5 t
t
t
4 t
Riser on Plates and large A/V casting

25. 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