2. Gating System
It refers to all the sections through which the molten
metal passes while entering into the mold cavity.
ELEMENTS OF GATING SYSTEM :
1. Pouring cup
2. Sprue
3. Sprue well
4. Runner
5. Ingates (Gates)
6. Riser
3. FUNCTIOS OF GATING SYSTEM
Fill the mould cavity completely before freezing.
Minimizing turbulence.
Avoiding erosion.
Removing inclusions.
Regulate flow of molten metal.
Consume least metal-less scrap.
Trap contaminants.
Establish directional solidifications.
7. Use of Anti-Swirl bars to reduce turbulence
Anti-Swirl bars
• Anti-Swirl bars avoid the
swirling of the liquid metal
and allow it to flow straight,
hence reduce turbulence.
11. Law of continuity of mass
The rate of flow of mass of the fluid is constant at any
cross-section.
m=dA1V1=dA2V2=dA3V3
where m = rate of flow of mass
d = density of liquid metal
A1 = area of cross-section at (1)
A2 = area of cross-section at (2)
A3 = area of cross-section at (3)
12. V1 = velocity of liquid metal at (1)
V2 = velocity of liquid metal at (2)
V3 = velocity of liquid metal at (3)
Q =A1V1=A2V2= A3V3
Q = volume rate of flow
13. A2V2 = A3V3
𝑉2 = 2𝑔ℎ1 𝑉3 = 2𝑔ℎ2
𝐴2
𝐴3
=
ℎ2
ℎ1
As the liquid flows down, the cross-section of the fluid decreases.
So the taper is provided in the sprue.
Liquid loses contact if the sprue is straight which could cause‘
aspiration’.
15. Choke area
The smallest area that occurs at the bottom of the sprue is
known as‘choke area’.
(1)
(2)
choke area
Choke area is designed based on Bernoulli’s theorem.
16. Bernoulli's theorem
It is based on principle of conservation of energy and
relates pressure, velocity and elevation.
Bernoulli’s Equation :
𝒑
𝒅𝒈
+
𝑽𝟐
𝟐𝒈
+ 𝒁 − 𝜟𝑭 = 𝑯
𝑝
𝑑𝑔
is the FLOW ENERGY per unit weight
(p – pressure, d = density)
𝑉2
2𝑔
is the KINETIC ENERGY of the fluid per unit weight
17. 𝒑
𝒅𝒈
+
𝑽𝟐
𝟐𝒈
+ 𝒁 − 𝜟𝑭 = 𝑯
Z is the POTENTIAL ENERGY of the fluid per unit
weight
Δ𝐹 is the FRICTIONAL LOSS
H is theTOTAL ENERGY of the fluid per unit weight
which is always CONSTANT along the same streamline
18. Applying Bernoulli’s theorem between 1 and 3,
𝐴𝑐 =
𝑊
𝑑𝑡𝑐 2𝑔𝐻
Where,
𝐴𝑐 = choke area at section (3)
W = casting weight
d = density of liquid metal
19. 𝐴𝑐 =
𝑊
𝑑𝑡𝑐 2𝑔𝐻
t = Pouring time (seconds)
c = Efficiency factor
g =Acceleration due to gravity
H = Effective height of metal head
20. Calculation of pouring times
1. Gray-Iron Castings < 1000 pounds
Pouring time,
𝑡 = 𝐾(0.95 +
𝑇
0.853
) 𝑊
Where,
K is the FLUIDITY FACTOR ,depends on temperature
and composition of the molten metal (1.0 to 1.6).
21. W is the weight of the casting in pound
T is the average thickness of the casting in inches.
t is the pouring time in seconds.
2. Gray-Iron Castings > 1000 pounds
Pouring time,
𝑡 = 𝐾(0.95 +
𝑇
0.853
)
3
𝑊
23. 3. Shell moulded ductile iron
pouring time,
𝑡 = 𝐾 𝑊
Where
K is about 1.4 for castings of thinner section
K is about 1.8 for castings of medium section
K is about 2.0 for castings of heavier section
24. 4. Steel Castings
Pouring time,
𝑡 = 𝐾 𝑊
Where
K is about 1.2 for castings of weight 100 pounds.
K is about 1.8 for castings of weight 100,000 pounds.
27. Types of gating system
1. Pressurized Gating System
The total cross-sectional area gradually
DECREASES from choke to ingates.
2. Un-pressurized Gating System
The total cross-sectional area gradually
INCREASES from choke to ingates.
28. Typical Gating Ratios
Pressurized gating system :
AC : AR : AG = 1 : 1.3 : 1.1 ( for grey cast iron)
AC : AR : AG = 1 : 2 : 1 ( for aluminum)
AC : AR : AG = 1 : 2 : 1.5 ( for steel)
Un-pressurized gating system :
AC : AR : AG = 1 : 4 : 4 ( for grey cast iron)
AC : AR : AG = 1 : 3 : 3 ( for aluminum)
AC : AR : AG = 1 : 3 : 3 ( for steel)
C- Choke ,R – Runner ,G- Gate
29. Comparison of gating system
Pressurized Gating Un-pressurized gating
Total cross-sectional area decreases
toward the mold cavity.
Total cross-sectional area
increases toward the mold
cavity.
More turbulence and chances of
mold erosion.
Less turbulence.
Flow of liquid metal is almost equal
from all ingates.
Flow of liquid metal is different
from each ingate.
Casting yield is more. Casting yield is less.
Complex and thin sections can be
successfully cast.
Complex and thin sections may
not be successfully cast.
30. Design and location of ingates
Multiple ingates are often preferable for large castings.
A fillet should be used where an ingates meets a casting
(Produce less turbulence).
The minimum ingates length should be three to five
times the ingate’s width, depending on the metal being
cast.
Curved ingates should be avoided, as far as possible
32. Primary function of a Riser
It should act as a reservoir of molten metal in the mold
to compensate for shrinkage during solidification.
33. Guidelines for Riser Design and
Location
The riser must not solidify before the casting.
The volume of riser(s) must be large enough to feed the
entire shrinkage of the casting.
The pressure head from the riser should enable complete
cavity filling.
Risers must be placed so that then liquid metal can be
delivered to locations where it is most needed.
36. Riser Design : Modulus Method
Modulus od solidification :
Modulus of solidification of casting (or riser) is defined
as the ratio of its volume and surface area.
𝑀𝑜𝑑𝑢𝑙𝑢𝑠 𝑜𝑓 𝑠𝑜𝑙𝑖𝑑𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 =
𝑉𝑜𝑙𝑢𝑚𝑒
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎
Modulus method is based on Chvorinov’s Rule.
37. Chvorinov’s Rule
𝑻𝑺𝑻 = 𝑪𝒎(
𝑽
𝑨
)𝒏
Where,
TST = total solidification time.
V = Volume of the casting.
A = Surface area of casting.
n = exponent usually taken as 2.
𝐶𝑚 = Constant depends on mold material
38. What Chvorinov’s Rule Tells Us?
𝑻𝑺𝑻 = 𝑪𝒎(
𝑽
𝑨
)𝒏
A casting with a higher modulus (volume to surface area
ratio) cools and solidifies more slower than the one with a
lower modulus.
To feed molten metal to the casting,TST of the riser must be
greater thanTST of the casting.
Since mold constants of riser and casting will be equal,
design riser to have a larger modulus so that the main casting
solidifies first.
39. Requirement of the riser to feed the casting.
𝑴𝑹 = 𝟏. 𝟐 × 𝑴𝑪
where,
𝑀𝑅 = Modulus of casting.
𝑀𝐶 = Modulus of riser.
40. Feeding Distance
Casting without a riser
- influence of END EFFECT
Casting with riser
- influence of END
and RISER EFFECT
Feeding distance for steel castings (plates/bars) :
• End effect - promotes a distance of 2.5 *t
• Riser effect – promotes a distance of 2*t
Total feeding distance = 4.5*t
41. Feeding distance with two Riser
Steel plate casting with two riser
Influence of END and RISER EFFECT
Total feeding distance is about 13*t
42. Feeding Distance (Large steel casting) :
Shape Factor, SF = (L +W)/T
where,
L denotes the length
W denotes the width
T denotes the thickness
(Note : L>W>T)
45. Fluidity of molten metal
It is the capability of the molten metal to flow and fill
into the mould cavities.
pure metals act with good fluidity.
46. Factors affecting fluidity
a) Related to molten metal :
1.Viscosity 4.Surface tension
2.Inclusion 5.Freezing range of alloy
3.Latent heat of fusion 6.Thermal diffusivity
of alloy.
b) Related to casting parameters :
Mould material – thermal conductivity, surface
characteristics, etc.
47. Method for determining the fludity
1) Fluidity spiral tube method
2) Suction tube method
48. Measuring Fluidity
• 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.
50. Cooling curve for a pure metal during
casting
A pure metal solidifies at a constant temperature equal to its
freezing point (same as melting point)
51. `
a) Phase diagram for a copper-nickel alloy system
b) Cooling curve for a 50 % Cu alloy during casting
Most alloy freeze over a temperature range rather than at a single
temperature
52. Solidification of casting
Solidification involves two steps that are
NUCLEATION and GROWTH.
NUCLEATION :
It refers to the process in which tiny solid particles
called,‘Nuclei’, are formed when liquid metal cools below
its liquidous temperature.
53. Two types of Nucleation :
a) HOMOGENEOUS NUCLEATION :
It occurs without the help of foreign particles.
b) HETEROGENEOUS NUCLEATION :
It occurs with the help of foreign particles
(such as the mould material, impurities, and added
nucleation materials.)
54. Homogeneous nucleation
Critical nucleus :
The nucleus grows to give rise to the solid structure only
when its size is greater than certain size which is known as
‘Critical Nucleus’.This requires supercooling.
The critical nucleus radius r is given by,
r = -2σ/ΔFV
σ= specific surface energy
ΔFV= change in free energy
55. Heterogeneous Nucleation
Solid phase crystallizes on a foreign particle – larger
nucleus.
Degree of super cooling needed for solidification is
smaller.
Heterogeneous nucleation is the dominating mechanism
in the early stage of solidification.
Scarcity of foreign particles in the central part od the
casting – results in homogeneous nucleation.
56. Grain Structure
At the surface, heterogeneous nucleation takes place for
few layers.These grains are known as‘Equiaxed Grains’.
Inside absence of sand particles leads to homogeneously
nuckeated grains.Their orientation will be from the
surface to the center.These grains are known as the
‘Columnar Grains’.
58. When columns forms side arms, it is known as a‘Dendrite’
and the grain structure is known as the‘Dendritic grain
structure’.
New nuclei resist the growth of neighboring nuclei. Hence
an equiaxed grain structure is produced at the center.
Columnar and dendritic grain structure are coarse and
directional-undesirable in most situation.
This can be changed in practice by adding‘Nucleating
Agents’, which produce an equiaxed grain structure in the
entire casting.
59. Nucleating agents for different alloys :
Metals Nucleating Agents
Al alloys Ti compounds(TiAl3,TiB2,TiC)
Plain carbon steel Al compounds(AlN,Al2O3)
Stainless steel Ca and Mg cynides
Mg alloy ZrC, ZrN, Zr oxides
Cast iron Sulphur compounds
60. Three basic types of cast structure:
A) Columnar dendritic
B) Equiaxed dendritic
C) Equiaxed nondendritic
61. Cooling Shrinkages
When liquid metal cools from its pouring temperature
in a casting , it undergoes the following types of shrinkages
a) Liquid shrinkage : It occurs during cooling up to
the liquidous temperature. It is about 0.5 % of the
room temperature volume.
b) Solidification shrinkage : It occurs during cooling
from liquidous to solidus temperatures.
62. c) Solid shrinkage : It occurs during cooling from
solidus temperature to room temperature. It is a function
of the coefficient of thermal expansion of material.
The liquid and the solidification shrinkage are taken care of
by a riser
63. Solidification casting for important cast
metals/alloys
Sr
No.
Metal or Alloy Solidification
Shrinkage (% vol)
1 Aluminum 7.1
2 Zinc 6.5
3 Gold 5.5
4 Copper 4.9
5 Magnesium 4.2
6 Lead 3.2
7 Al-4.5 % Cu 6.3
64. Sr.No. Metal or Alloy Solidification
shrinkage (%
volume)
8 Al-12 % Si 3.8
9 Brass (70-30) 4.5
10 90 % Cu- 10 % Al 4
11 Carbon Steel 2.5 to 4
12 White Iron 4.5 to 5
Liquid shrinkage is about 0.5% by volume with normal
superheat.
