CASTING TECHNOLOGY

GATING SYSTEM
Gating System
 Pouring basin: A small funnel shaped cavity at the
top of the mould into which the molten metal is
poured.

 Sprue: The passage through which the molten metal,

from the pouring basin, reaches the mould cavity. In
many cases it controls the flow of metal into the
mould.
 Runner: A runner is commonly a horizontal channel
which connects the sprue with gates, thus allowing the
molten metal to enter the mould cavity. The runners
are of larger cross-section and often streamlined to
slow down and smooth out the flow, and are designed
to provide approximately uniform flow rates to the
various parts of the mould cavity. Runners are
commonly made trapezoidal in cross-section.
 Ingate: A channel through which the moltenmetal
enters the mould cavity.
 Vent: Small opening in the mould to facilitateescape
of air and gases.
Types of Gate or In-gate
Top gate: Causes turbulence in the mould cavity, it is prone
to form dross, favourable temperature gradient towards the
gate, only for ferrousalloys.

Bottom gate: No mould erosion, used for very deep moulds,

higher pouring time, Causes unfavourable temperature
gradients.
Parting Gate: most widely used gate, easiest and most
economical in preparation.
Step Gate: Used for heavy and large castings, size of ingates
are normally increased from top tobottom.
 To minimize turbulence to avoid trapping gasses into
the mold
 To get enough metal into the mold cavity before the
metal starts to solidify
 To avoid shrinkage
 Establish the best possible temperature gradient in the
solidifying casting so that the shrinkage if occurs must
be in the gating system not in the required cast part.
 Incorporates a system for trapping the non-metallic
inclusions.
The gating systems are of two types:

 Pressurized gating system

 Un-pressurized gating system

 Pressurized Gating System

 The total cross sectional area decreases towards the

mold cavity
 Back pressure is maintained by the restrictions in the
metal flow
 Flow of liquid (volume) is almost equal from all gates
 Back pressure helps in reducing the aspiration as the
sprue always runs full
 Because of the restrictions the metal flows at high
velocity leading to more turbulence and chances of
mold erosion.
 Un-Pressurized Gating System

 The total cross sectional area increases towards the

mold cavity

 Restriction only at the bottom ofsprue

 Flow of liquid (volume) is different from all gates

 Aspiration in the gating system as the system never
runs full

 Less turbulence.
 Sprue Design

 Sprue: Sprue is the channel through which the molten

metal is brought into the parting plane where it enters the
runners and gates to ultimately reach the mould cavity.
 The molten metal when moving from the top of the cope to
the parting plane gains in velocity and some low-pressure
area would be created around the metal in the sprue.
 Since the sand mould is permeable, atmospheric air would
be breathed into this low-pressure area which would then
be carried to the mouldcavity.
 To eliminate this problem of air aspiration, the sprue is
tapered to gradually reduce the cross section as it moves
away from the top of the cope as shown in Figure below (b).
Sistem saluran
(gating system)

FAKTOR TUJUAN PRINSIP DASAR

• Logam • memperkirakan waktu • Hukum kontinuitas

• Bahan cetakan penuangan aliran fluida • Hukum bernoulli
• Metode pembuatan lewat saluran tegak dan
cetakan mendatar
• Distribusi logam ke
rongga cetak
• Laju sesuai
• Menghindari
penurunan
temperature drastic
• Menghindari turbulensi
• Menghindari adanya
gas, slag dan dros yg
terperangkap
Hukum kontinuitas

• Menyatakan bahwa dinding yang impermiable (tidak

tembus udara) jika diisi dengan fluida yang tidak
termampatkan maka:
𝑄 = 𝐴1 𝑉1 = 𝐴2 𝑉2 = 𝐴 𝑉= C

Dimana:
Q = Kapasitas aliran
A = Luas penampang
V = Kecepatan aliran
The exact tapering can be obtained by the equation of
continuity. Denoting the top and choke sections of The sprue by
the subscripts’t’ and 'c' respectively, weget

Vc
A t Vt = A c Vc At = A c
Vt
FLUID FLOW Bernoulli Equation

𝑣12 𝑃1 𝑣22 𝑃2
ℎ1 + + = ℎ2 + +
2𝑔 𝑤 2𝑔 𝑤

Potential Kinetic Pressure

energy energy

Dimana:
Hubungan antara tekanan, h = ketinggihan, ft atau in
kecepatan dan ketinggian v = kecepatan, ft/sec atau in/sec
dari sistem aliran g = percepatan grafitasi, 32,2 ft/sec2
p = tekanan, lb/ft2 atau lb/in3
w = berat jenis fluida, lb/ft3 atau
lb/in3
E1 = E2 + Elost (1-2) E1 = E2
Catatan:
Tekanan atmosfer : 14,71 lb/in2
Berat jenis besi cair: 0,26 lb/in3
 Since the velocities are proportional to the square of
the potential heads, as can be derived from
Bernoulli's equation,

hc
At = A c
ht

Where H = actual
sprue height
and ht = h + H
 Gating ratio is defined as: Sprue area: Runnerarea:
Ingate area.
 For high quality steel castings, a gating ratio of 1: 2: 2 or
1: 2: 1.5 will produce castings nearly free from erosion,
will minimize oxidation, and will produceuniform
flow.
 A gating ratio of 1: 4: 4 might favour the formationof
oxidation defects.
 Pouring, Gating design
A good gating design should ensure proper distribution of molten metal without
excessive temperature loss, turbulence, gas entrapping and slags.

If the molten metal is poured very slowly, since time taken to fill the mould
cavity will become longer, solidification will start even before the mould is
completely filled. This can be restricted by using super heated metal, but in this
case solubility will be a problem.

If the molten metal is poured very faster, it can erode the mould cavity.

So gating design is important and it depends on the metal and molten metal
composition. For example, aluminium can get oxidized easily.

Gating design is classified mainly into two (modified: three) types:

Vertical gating, bottom gating, horizontal gating

 A Ghosh and A K Mallik,
Manufacturing Science

Vertical gating: the liquid metal is poured vertically, directly to fill the mould
with atmospheric pressure at the base end.

Bottom gating: molten metal is poured from top, but filled from bottom to top.
This minimizes oxidation and splashing while pouring.

Horizontal gating is a modification of bottom gating, in which some horizontal

portions are added for good distribution of molten metal and to avoid
turbulence
 Analysis of pouring and filling up mould
(a) Vertical gating

For analysis we use energy balance equation like Bernoulli’s equation

2 2
p1 v1 v
+ + F1 = h3 + 3 + 3 + F3
p
h1 +
g 2g g 2g
Assuming p1 = p3 and level at 1 is maintained constant, so
v1 = 0; frictional losses are neglected.

The energy balance between point 1 and 3 gives,

gh = v 2 / 2 v3 = 2ght
t 3

Here v3 can be referred as velocity at the sprue base or

say gate, vg

Continuity equation: Volumetric flow rate, Q = A1v1 =A3v3

Above two equations say that sprue should be tapered.

As the metal flows into the sprue opening, it increases in velocity
and hence the cross-sectional area of the channel must be reduced

Otherwise, as the velocity of the flowing molten metal increases

toward the base of the sprue, air can be aspirated into the liquid and
taken into the mould cavity.

To prevent this condition, the sprue is designed with a taper, so that

the volume flow rate, Q = Av remains the same at the top and
bottom of the sprue.

The mould filling time is given by, t = V = V

f
Q Agv 3
Ag = cross-sectional area of gate; V = volume of mould

Note: This is the minimum time required to fill the mould cavity. Since the analysis
ignores friction losses and possible constriction of flow in the gating system; the
 Contoh

• Dengan dimensi coran 5 x 10 x 20 in dan

saluran masuk 1 x 1 dengan h = 5 in, maka
dengan mengabaikan kerugian gesekan
didapat:
𝑉 𝑉
𝑡= =
𝑄 𝐴 .𝑣
Dengan asumsi E1 = E2, dan karena p1 =
p2 dan w1 = w2
1000 𝑖𝑛3
𝑡= = 16,6 𝑑𝑒𝑡𝑖𝑘
𝑣12 𝑝1 𝑣22 𝑝2 1 𝑖𝑛2 𝑥 27,7 5 𝑖𝑛/𝑑𝑒𝑡𝑖𝑘
ℎ1 + + = ℎ2 + +
2𝑔 𝑤 2𝑔 𝑤

𝑣2 = 27,7 ℎ in/det
 (b) Bottom gating
2 2
p1 v1 v
+ + F1 = h3 + 3 + 3 + F3
p
h1 +
g 2g g 2g
Apply Bernoulli’s eqn. between points 1 and 3 and between 3 and
4 is equivalent to modifying V3 equation in the previousgating.

v g = v3 = 2g(h t − h)
Between 3 and 4:
Assume:
• V4 is very small
• All KE at 3 is lost after the liquid metal
Effective head enters the mould

Assuming in the mould the height moves up by ‘dh’ in a time ‘dt’; Am and Ag are mould
area and gate area, then

A m d h = A g v g dt A
1 dh
=
g
dt
Combining above two eqns., we get 2g ht − h Am

hm tf Am 1
1 dh Ag

=  dt tf = 2( ht − ht − hm )
2g
 ht −h Am 0
Ag 2g
0
(Check integration)
Find the filling time for both the mould types. Area of C.S. of gate = 5cm2
A Ghosh and A K Mallik, Manufacturing Science

Answer:
tf = 21.86 sec; 43.71 sec.

R.Ganesh Narayanan, IITG

 Aspiration effect
Aspiration effect: entering of gases from baking of organic compounds present in
the mould into the molten metal stream. This will produce porous castings. Pressure
anywhere in the liquid stream should not become negative.

Free falling liquid

Metal flow with aspiration effect

A tapered sprue without aspiration effect

Case 1: straight Vs tapered sprue

Pressure anywhere in the liquid stream

should not become negative.
 Points 2 & 3
v22 v2
gh 2 +  2 + = 3 +
p p 3
ρm = density of molten metal
m 2 m 2
Let in the limiting case, p2 = p3, then from above equation

v 32 v2 2
= gh 2
+
2 2
A3
We know that, v 2 = v 3 = Rv 3
A2
R
 v32
2 2
v 2 gh 2
Combining above two eqns., = h2 + 3
R = 1−
2
2g 2g v 32

We know that between points 1 and 3, gh = v 2 / 2

t 3

h2 h c R =
A3
= hc
Put this in R2 eqn, we get, R = 1−
2
=
ht ht A2 ht
 Mekanisme masuknya udara (aspirasi)
Ideal -> impermeable namun kenyataannya cetakan tembus udara (misal udara, uap air
atau hasil uraian material organic sebagai bahan perekat) -> sehingga harus
dipertimbangkan

Untuk penyederhanaan, kondisi di (1) dan (3) dalam tekanan

atmosphere (14,7 lb/in2) dan density besi cor 0,26 lb/in3, sehingga:

𝑣12 𝑝1 𝑣32 𝑝3
ℎ𝑡 + + = ℎ3 + +
2𝑔 𝑤 2𝑔 𝑤
𝑣3 = 27,7 ℎ𝑡 in/detik (gesekan diabaikan)

Pada titik (2) dengan ada perbedaan dengan titik (3) pada tekanan sebagai kompensasi
beda ketinggihan sehingga

𝑣22 𝑝2 𝑣32 𝑝3 𝑣22 𝑝2 𝑣32 14,7

ℎ𝑡 + + = ℎ3 + + ℎ𝑡 + + = ℎ3 + + p2 = 14,7 − ℎ2. 𝑤
2𝑔 𝑤 2𝑔 𝑤 2𝑔 𝑤 2𝑔 𝑤

Catatan: terlihat bahwa tekanan pada titik (2) lebih kecil dari tekanan
atmosfer sebanding dengan h2.w -> sehingga tekanan rendah itu
menyebabkan masuknya udara ke dalam logam
 Pencegahan aspirasi
# Cara pertama adalah dengan membuat tekanan pada posisi (2) dan posisi
(3) dibuat sama yaitu sama dengan 1 atmosfir
𝑣22 𝑝2 𝑣32 𝑝3
ℎ2 + + = ℎ3 + + Penyelesaian 1
2𝑔 𝑤 2𝑔 𝑤 Jika h2 = 6 in, ht = 8
in dan hc = 2 in, maka
𝑣22 𝑣32 perbandingan luas
ℎ2 + =
2𝑔 2𝑔 penampang saluran
𝐴3 turun pada titik 2
𝐴2𝑣2 = 𝐴3. 𝑣3 𝑣2 =.𝑣 dan 3 untuk
𝐴2 3 mencegah aspirasi
𝐴3
Bila, R= 𝐴2 maka, 𝑣2 = 𝑅. 𝑣3 adalah

2𝑔ℎ2
R2= 1 + ℎ2 = ℎ𝑡 − ℎ𝑐 𝑣3 = 2𝑔ℎ𝑡 𝐴3 2 1
𝑣3
=𝑅= =
ℎ𝑐 𝐴2 8 2
𝑅=
ℎ𝑡
 # Cara kedua perbandingan luas didekati dengan Gerakan jatuh bebas dari pouring cup
(cawan tuang) atau posisi

𝑣2 = 2𝑔ℎ𝑐

𝑣3 = 2𝑔ℎ𝑡
Karena Q2 = Q3 -> A2 v2 = A3 v3 sehingga

𝐴3 𝑣2 ℎ𝑐
= =
𝐴2 𝑣3 ℎ𝑡

Catatan:kedua metode menghasilkan perbanding luas yang sama namun pada cara kedua
bentuk saluran antara 2 dan 3 disesuakan dengan bentuk aliran sehingga tekanan pada setiap
titik tetap 1 atm. Sehingga tidak terjadi aspirasi. Bentuk ideal nya adalah hiperbolik namun
untk penyederhanaan biasanya dibuat dalam bentuk tirus.
 Ideal and actual
profiles of sprue

(a) (b)
Approximating tapered spure using choke mechanism
(a) Choke core, (b) Runner choke
In many high production casting systems, tapered sprue
will not be provided. Instead it is compensated by
having chokes at the end of sprue or runner.
 Case 2: sudden change in flow direction

runner

d’/d = 1.3

A sharp change in flow direction is avoided by designing the mould to fit vena
contracta.
 Preventing impurities and turbulence in casting

The items provided in the gating system to avoid impurities and turbulence are:
Pouring basin:
This reduces the eroding force of the liquid metal poured from furnace.This
also maintains a constant pouring head. Experience shows that pouring
basin depth of 2.5 times the sprue entrance diameter is enough for smooth
metal flow. Radius of 25R (mm) is good for smooth entrance of sprue.
 P Rao, Manufacturing Technology: Foundry, Forming And Welding

Delay screen/Strainer core:

A delay screen is a small piece of perforated screen placed on top of the sprue. This
screen actually melts because of the heat from the metal and this delays the entrance
of metal into the sprue, maintaining the pouring basin head. This also removes dross
in the molten metal.
Strainer core is a ceramic coated screen with many small holes and used for same
purpose.

Splash core: provided at the end of the sprue length which reduces the eroding force
of the liquid metal

Skim bob: this traps lighter and heavier impurities in the horizontal flow
 Gating ratios
Gating ratio: sprue area : runner area : gate area
Non-pressurized:
has choke at the bottom of the sprue base, has total runner area and gate
areas higher than the sprue area. No pressure is present in the system and
hence no turbulence. But chances of air aspiration is possible. Suitable for
Al and Mg alloys.
In this, Gating ratio = 1 : 4 : 4
Pressurized:
Here gate area is smallest, thus maintaining the back pressure throughout
the gating system. This backpressure generates turbulence and thereby
minimizes the air aspiration even when straight sprue is used.
Not good for light alloys, but good for ferrous castings.
In this, Gating ratio = 1 : 2 : 1
Gating ratios used in practice
The flow rate of liquid metal into the downsprue of a mold = 1 liter/sec. The cross- sectional area at the
top of the sprue = 800 mm2 and its length = 175 mm. What area should be used at the base of the sprue
to avoid aspiration of the molten metal?

Ans: A= 540 mm2

• convert Q in lit/sec to mm3/sec
• Find v = 2 gh

• Base area, A = Q/v

Molten metal can be poured into the pouring cup of a sand mold at a steady rate of
1000 cm3/s. The molten metal overflows the pouring cup and flows into the
downsprue. The cross-section of the sprue is round, with a diameter at the top = 3.4
cm. If the sprue is 25 cm long, determine the proper diameter at its base so as to
maintain the same volume flow rate.
Ans: D = 2.4 cm
• Find velocity at base, v = 2 g h
• find area at base, A = Q/v
• Find D = √4A/π
During pouring into a sand mold, the molten metal can be poured into the downsprue at a
constant flow rate during the time it takes to fill the mold. At the end of pouring the sprue is filled
and there is negligible metal in the pouring cup.

The downsprue is 6.0 in long. Its cross-sectional area at the top = 0.8 in2 and at the base = 0.6
in2.

The cross-sectional area of the runner leading from the sprue also = 0.6 in2, and it is 8.0 in long
before leading into the mold cavity, whose volume = 65 in3.

The volume of the riser located along the runner near the mold cavity = 25 in3. It takes a total of
3.0 sec to fill the entire mold (including cavity, riser, runner, and sprue). This is more than the
theoretical time required, indicating a loss of velocity due to friction in the sprue and runner.

Find: (a) the theoretical velocity and flow rate at the base of the downsprue; (b) the total volume of
the mold; (c) the actual velocity and flow rate at the base of the sprue; and (d) the loss of head in
the gating system due to friction.

Ans: (a) 68.1 in/sec, 40.8 in3/sec; (b) 99 in3; (c) 33 in3/sec, 55 in/sec; (d) 2.086 in
 Effect of friction and velocity distribution
The velocity of the liquid metal in the sprue and gate are assumed constant. This
depends on the nature of flow and shape of the channel.
Moreover no frictional losses are considered. In real cases, friction losses are always
present, specifically when there is sudden contraction and expansion in cross-
sections.

The non-uniform velocity distribution is accounted for by modifying the KE term in the
energy balance equation by replacing (v)2 by v 2 where β is a constant and v is the
average velocity. 

For circular conduit, β is equal to 0.5 for laminar flow and 1 for turbulent flow.
4 flv 2
The energy loss due to friction in a circular channel (per unit mass) is given by, E f 1 =
2d
Here l and d are length and diameter of channel. The value of f (friction factor)
depends on the nature of flow and channel smoothness. This Ef1 should be added to
energy at point 2 (say there are two points 1 and 2 discussed earlier).

For smooth channel: f =16 / Re where Re < 2000 for laminar flow
1
= 4 log 10 (Re f ) −0.4 for turbulent flow (Re > 2000)
f
f = 0.079(R e) −0.25 5
for the range 2100<Re<10 (simplified from above eqn.)
Frictional losses also occur due to sudden change in flow direction like in 90°
bends. In such cases, proper (l/d) ratio should be considered in Ef1 equation.

The energy loss due to sudden contraction and enlargement of flow area (per unit mass),
2
v e . Here
E =
f2 f v is the average velocity of the fluid in smaller CS region and
2
ef is the friction loss factor and it depends on the ratio of flow area and Re . In this ef
depends on sudden expansion or sudden contraction as shown in figure.
Sudden contraction
Sudden expansion

R.Ganesh Narayanan, IITG

 2 2
p1 v1 p3 3v

h1 + + + F1 = h3 + + + F3
g 2g g 2g
The energy balance eqn. between points 1 and 3, after accounting for sudden
contraction loss at 2 is given by,

p1 v32 p3
+ 0 + ght = + + Ef1 +Ef2
m m 2 
2 2
4 flv v e
By having P1 = P3, and using equations E f1
= and E f 2
= f , we get
2d 2

v3 = C D 2gh t where C D = ( 1 + e f + 4 f l ) −1/ 2

 d
CD = dischargecoefficient

If the sprue has got a bend or fitting,

1 l L
CD ={ +ef +4 f[ + ( ) eq ]} −1/ 2
 d D

Here l and d are length and diameter of channel (like sprue), (L/D) eq is for the bend.